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Structures of Organo Alkali Metal Complexes and Related Compounds.

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Volume 32
-
Number 11
November 1993
Pages 1501- 1670
International Edition in English
Structures of Organo Alkali Metal Complexes and Related
Compounds
By Erwin Weiss"
Dedicated to Projgssor Ernst Otto Fischer on the occasion of his 75th birthday
The investigation of the reactivity and structure of organometallic compounds of alkali metals
has experienced a blustering development in the last decades. This class includes compounds
that are especially important for our understanding of chemical bonding and also quite simple,
for example methyl alkali metal complexes, whose structures have been unequivocally determined. Organometallic compounds of alkali metals (and also magnesium) generally exist as
ion aggregates whose properties can be significantly modified through solvation by, for example. ether or amines. Important advances in the synthesis of new compounds, especially
those of the heavier alkali metals, have been based on these results. It was long believed that
the alkali metals had little tendency to undergo coordination and that their coordination
chemistry would offer few surprises. This picture has now changed completely. Results from
crystal structure investigations have revealed a variety of often surprising structure types
(rings, heterocubanes, chains, layers, etc.) not only with the organometallic compounds but
also with the amides, imides, alkoxides, phenoxides, enolates, and even halides. A comparison
reveals interesting similarities between compounds that appear to be so different and leads to
a general classification of the structure types possible with C, N, 0, and halo ligands.
1. Introduction
Organo alkali metal complexes were first reported in 1914,
14 years after the discovery of the Grignard compounds, in
the pioneering work of Wilhelm Schlenk.['] His name also
remains associated with the development of inert gas techniques for the handling of reactive compounds. A comprehensive monograph published in 1937f21showed that only
few advances had been achieved in the study of organo alkali
metal compounds up to the 1930s. A revival in organolithium chemistry occurred soon afterwards, however, which was
particularly promoted through the work of Karl Ziegler,
Georg Wittig, and Henry Gilman. The application of lithium
compounds in preparative chemistry was the main area of
[*] Prof Dr. E. Weiss
lnstitut fur Anorganische und Angewandte Chemie der Universitit
Marlin-Luther-King-Plztz 6. D-20146 Hamburg (FRG)
A n g m . . C h n . Inl. Ed. Engl. 1993. 32. 1501 -1523
c)
interest, and this class of reagents soon achieved an importance similar to that of the Grignard compounds.
Structure investigations were first performed in the 1960s
and have since then yielded many new and unexpected results on the structures and coordination modes of alkali
metal and magnesium compounds involving not only carbanions but also amides, alkoxides, and other anions. Since
these reagents are typically used in solution, the nature of
their solvent adducts is an obvious area of interest. Structure
information enables a better understanding of the reaction
mechanisms and is therefore useful in synthetic chemistry.
The important structure types will be presented and compared in this contribution. Because of the tremendous volume of published results and space limitations, the list of
references is unfortunately rather incomplete. For this reason the reader is referred to further review articles and
monographs[3- 91 as well as to annual reports."']
V C H V e r l u g s ~ ~ s ~ N s rmhH.
h u f ~ 0.69451 Wrinherm, 1993
10.00f.25 0
0570-0833/93~1II1-150l~
1501
Crystal structure analysis (X-ray diffraction and, more
recently, neutron and synchrotron X-ray diffraction) is unquestionably the most important method of investigation.
As such, these results will be emphasized in this report. Some
of the knowledge gained from crystal studies may also allow
conclusions to be drawn about the structures in solution;
however, direct methods for the investigation of structures in
solution are also available (NMR and IR spectroscopy, and
measurements of colligative properties).
2. Simple, Unsolvated Organo Alkali Metal
Compounds
2.1. Methyllithiurn: Structure and Bonding
The investigation of the structure of methyl alkali metal
compounds began in 1964 with methyllithium.[""l Since
single crystals were not available, X-ray powder methods
had to be applied. Shortly before this and independently,
the structure of ethyllithium had been reported by H .
DietrichI"] as the first organo alkali metal compound (single-crystal methods). Both compounds form tetrameric aggregates having a Li,C, heterocubane framework. The tetramers in (LiCH,), have ideal & symmetry and form a
body-centered cubic lattice (Fig. 1). A structure refine-
Fig. 1. Space-filling model of the body-centered cubic unit cell of
(LiCH,),[l1,18]. The Li radii were chosen to be 75 pm. in accord with approximately 85% ionic character (normal ionic radius of 65 pm).
mentr"bl also provided the approximate positions of the H
atoms and thus evidence for pyramidal methyl groups (C,"
symmetry). In this structure the C atom of each methyl
group has three short contacts to three of the Li atoms of its
tetramer as well as a further contact only about 5 - 10 pm
longer to a neighboring tetramer located on a space diagonal
of the body-centered cubic unit cell (coordination number of
7 for C!).
Each tetramer interacts with its eight neighbors. The very
low volatility and the nonmelting character of methyllithium
are direct consequences of this three-dimensional network.
The strength of these interactions is drasticaIIy reduced for
ethyllithium and falls off almost entirely for organolithium
compounds having larger organic residues. The compounds
become increasingly more volatile and have lower melting
points o r are even liquid as with (LinBu),. It is misleading to
try to deduce covalent bonding from these properties as was
often done earlier. The physical properties result from the
overall structures that form from approximately spherical
aggregates, which are lipophilic on the outside but can be
strongly polar on the inside.
A few remarks on bonding relationships are in order at
this point. These are of general interest for the organolithium
compounds, and particularly for methyllithium, and have
been the topic of numerous and sometimes controversial
discussions. Since organolithium compounds contain only
light atoms, they are well suited for theoretical studies, especially a b initio calculations. Many authors, but above all P.
von R. Schleyer et aI.,[,. ' 31 have carried out these types of
calculations. In the first publication on (LiCH,),[""] we interpreted the bonding in the tetramer based on a qualitative
MO model. These ideas have been refined many times by
other authors, and (LiCH,), has even been mentioned in
textbooks as an illustration of LiC, two electron-four center bonds.
Initially, the covalent contribution to the Li-C bond was
considerably overestimated based on the observation of
7Li-13C couplings in the N M R
Streitwieser et
a].,[' however, had already shown in the late 1970s that the
tetrameric structure of methyllithium can also be explained
with a purely electrostatic point charge model. This model
was extended and generalized by describing the atoms as
hard spheres (Hard Sphere Electrostatic Model).['61 Although the ionic contribution to the M-C bond is not easily
determined for M = Li, recent calculations[4*1 3 .
estimate
it to be about 80-88% and to be even greater with the
heavier alkali metals. According to the current viewpoint,
therefore, the alkali metal-carbon bond should be considered predominantly ionic, and the results of numerous structure investigations presented in the following discussions fully support this. The aggregates often observed are, in fact,
ion aggregates having only a small covalent bonding contri-
Erwin Weiss, born in 1926 in ArzberglOberfranken, studied chemistry at the Technische Hochschule in Munich and received his Ph.D. in 1956 for his work with Walter Hieher on the structure
of metal carbonyls and arene complexes. After conducting postdoctoral research (Massachusetts
Institute of Technology, Cambridge, U SA , 1956), he worked in research laboratories in Europe
(European Research Institute, Brussels, 1957- 1961, Cyanamide European Research Institute,
Geneva, 1961 -1965) and was than was appointed to the faculty of'the University of Hamburg,
where he remained until 1991. His research interests ,focus on the synthesis and structure of'
organometallic compounds wiih main group and suhgroup metals. This review ~ i a written
s
in part
during a research sabbatical in Paris which was associated with the French Alexander-von-Humholdt-Preis 1992.
1502
A n p i . Chrm
hi.Ed Engl. 1993, 32. 1501- 1523
bution. This approach can also be reflected in the nomenclature where, for example, the methyl alkali metal compounds
would be denoted as alkali metal methanides."'. 251 In this
report, however, the more conventional "-yl" nomenclature
will be used.
We have recently further refined the structure of methyllithium from neutron diffraction data and have thereby also
found that no phase transition occurs down to 1.5 K.""
Since the incoherent scattering of 'H nuclei causes a high
level of background noise in the diffraction diagram,
powder samples of the deuterated compound (LiCD,), were
examined. These investigations furnished the first precise structure data for a trigonal-pyramidal methyl carbanion (we will return to this in Section 2.4). These studies
also confirmed a staggered orientation of the methyl carbanion relative to the three neighboring Li atoms of the
tetramer.
2.2. Methylpotassium, Methylrubidium, and
Methylcesium
Since the structures of the methyl compounds of the heavier alkali metals are simpler, their descriptions will be treated
before that of methylsodium. Methylpotassium was first
described by Schlenk and Holtzl'cl as a black coating on
potassium metal obtained upon its reaction with dimethylmercury. These samples were not suitable for X-ray investigations because of substantial amalgam impurities. Our synthesis of nearly colorless and pure methylpotassium from a
metal-exchange reaction between methyllithium and potassium tert-butoxide allowed its structure to be determined."''
The First preparation and investigation of the highly reactive
compounds methylrubidium and methylcesium was possible
in an analogous
All three compounds display the NiAs structure type and
contain CH; ions with trigonal-prismatic coordination to
six K ions. Although the diffraction data, which was once
again obtained from powder samples, could not furnish the
exact H atom positions, IR spectroscopic investigations suggested pyramidal CH; ions.['" Later, a more precise neutron diffraction analysis was carried out on the deuterated
compound KCD, (powder samples, measurements a t 290
and 1.4K).["]
A significant result from this structure refinement was the
experimental verification of pyramidal methyl anions. Their
orientations alternate within the lattice whereby, rather than
the hexagonal cell previously measured, a larger orthorhombic cell results. The relation between the two unit cells is
illustrated in Figure 2. Each methyl group is surrounded by
six K ions in a distorted trigonal prism. This distortion is a
consequence of the CH; structure and also causes smaller
deviations of the K ions from their ideal positions in the
NiAs structure. The three K ions surrounding the lone pair
of electrons on CH; have shorter K-C distances (295 and
302 pm) than those of the three K atoms associated with the
H atoms (330 and 344 pm). A space-filling model of the unit
cell of methylpotassium is shown in Figure 3. Methylrubidium and methylcesium crystallize in the same orthorhombic
structure type, as a recent reexamination'221 of the earlier
powder datac2'] proved.
Fig. 2. Crystal structure of KCH, (ball-and-stick model) illustrating the relationship between the orthorhombic (Pmm) and hexdgondl unit cells (honhorh
zz
1/3 ah,,) I2 1I.
Fig. 3. Space-filling model of the unit cell of KCH, (K radius chosen to be
145 pm)[21].
2.3. Methylsodium
2.3.1. Methyflithium-Containing Methylsodium
(NaCH,),. f (LiCH,),]=, x 5 0.333
In earlier work[231we described the synthesis and structure of methylsodium containing variable amounts of
methyllithium. Finely crystalline samples, were obtained
from LiCH, and sodium tert-butoxide by a metal-exchange
reaction [Eq.(a)], and various amounts of LiCH, (mol ratios
of NaCH,:LiCH, from ca. 3 : 1 to 36: 1 ) could be incorporated depending on the composition of the solvent.
In these experiments, NaOtBu was added to a solution of
LiCH, . The structure of the product (Fig. 4), as determined
from the powder data, is a giant cubic cell in which 24
tetrameric (NaCH,), aggregates build a zeolite-like host lattice. Although it is similar to the methyllithium lattice, the
arrangment of the (NaCH,), units is more complicated.
Large cavities result which firmly and selectively incorporate
(LiCH,), units u p to a structurally specified NaCH,: LiCH,
1503
Fig. 4. The host-guest compound (NaCH,),.[(LiCH,),],
(u I 0.333)[23].
The host lattice (cubic cell. u = 2020 pm with 24 (NaCH,), units) contains large
cavities that can incorporate a maximum of 8 (LiCH,), aggregates.
ratio of 3: 1. To our knowledge this is the first example of an
organometallic supramolecular compound. Previously obtained methyl/ethyl- and ethylltert-butyllithium comp o u n d ~ [most
~ ~ ]probably are similar types ofclathrates with
complicated structures.
2.3.2. Methylsodium: Synthesis and Structure
Only later[2s1did we realize that pure methylsodium has a
different, more compact structure. It is obtained when the
reaction following Equation (a) is performed in the opposite
sequence; that is, the LiCH, solution must be added to the
NaOtBu solution. Thus, the tetrameric methyllithium aggregates present in the ether solutionLz6]
envelop the (NaCH,),
tetramers that form upon addition of NaOtBu, and thereby
a special clathrate lattice is constructed. The formation of
the NaCH,/LiCH, inclusion compound is therefore template directed.
Methylsodium crystallizes as a new and complicated
structure type (orthorhombic, 1222, 2 = 16). The structure
solution was in no way trivial, for it was only possible by a
combination of the complimentary methods of neutron diffraction and X-ray synchrotron diffraction on NaCD, powder
In this new approach the Na and C positions
were determined from the synchrotron data, and the D positions were subsequently obtained from the neutron diffraction data. Measurements performed at 1.5 and 293 K indicated that no phase transformation took place. Representations of the structure are shown in Figures 5 and 6 (and as
stereoplots in ref.1251).
In this structure, half of the ions form (NaCH,), tetramers
that have slightly distorted Td symmetry but are very similar
to those in methyllithium. The remaining eight Na and eight
methyl ions connect the tetramers through Na-C contacts.
Depending on the orientation of the methyl ion relative to
the cation, both short and long M-C distances (253 to
291 pm) are observed here, too. The distances are short within the tetramers and longer to the immediately adjoining ions
(“backsides” of the methyl groups). The ions located between the tetramers also show this type of coordination :
each CH; ion is surrounded by three N a ions at shorter
distances and one Na at a longer distance in a distorted
tetrahedron. The structure of methylsodium is thus related
to that of methyllithium, and its lower symmetry is a result
1504
Fig. 5. Ball-and-stick model of the methylsodium lattice (Na red, C gray);
adjoining unit cells ofmethylsodium[25]. A row of two teframeric aggregates is
found at each edge and in the middle of the picture. Dashed hnes denote Na-C
contacts less than 270 pm. Thus the methyl groups locared between the tetramers also appear to be coordinated to three Na ions.
Fig 6. Space-filling model of the crystal structure of methylsodium (orientation as in Fig. 5 ) . The N a radii have been chosen to be 110 pm in accord with
approximately 90% ionic character (standard ionic radius 97 pm).
of the different cation radii. All of the methyl ions are trigonal pyramidal with C-H bond lengths of 109 pm and H-CH bond angles of 106” (at 1.5K).
These results filled the gap in our knowledge on the structures of the methyl alkali metal compounds. Methylsodium
takes on a median position between methyllithium and the
methyl compounds of the heavier alkali metals, because its
structure contains both tetrameric aggregates as well as
single ions. An optimal packing in the ionic lattice is
achieved through its special structure.
Angew. Chem. I n t . Ed. Engl. 1993, 32. 1501 -1523
2.4. A Comparison of the Structures of CH;, SiH;,
and GeH;
The exact structure determinations performed with neutron diffraction that were discussed in the previous sections
provide good data for the CH; ion in crystalline solids.[Z81
It is always pyramidal (average H-C-H bond angle 107 ”) and
isostructural with NH, (H-N-H bond angle 107.6”,Table I),
but the inversional bending modes known for ammonia are
not possible for the CH; ion in a rigid crystal lattice.
Table I . ComparisonofCH,;. SiH;,andGeH;
analogues from the fifth main group.
witheachotherand withtheir
EH:-’
Distance E-H [pm]
Angle H-E-H
CH.;
107.2
109.4 (ave.)
109 6 (ave.)
106
108.2
106.2 (ave.)
105.1 (ave.)
107.6
94(4)
95
93.5
93(4)
91.8
NH,
SIH;
143
141
PH,
GeH;
-
ASH
152
[7
Determined for
LiCD, (1.5 K [lS])
NaCD, (1.5 K 1251)
KCD, (1.5 K [21])
(2 K W1)
KSiH, (NMR [30])
KSiH, [32]
KGeH, (NMR [30])
Fig. 7. The layered structure ofethylsodium (rhombohedra1space group R h ;
Na radii chosen to be 110 pm).
We had previously examined a number of alkali metal
silanides and germanides: KSiH,, RbSiH,, CsSiH,,[”]
KGeH,, RbGeH,, and C S G ~ H , . ’ ~We
~ . found
~ ’ ~ that with
the exception of CsGeH, all the compounds crystallize at
room temperature in the NaCl structure type. The H-E-H
bond angles for the silanide and germanide ions were determined by solid-state NMR spectroscopy and are listed for
comparison in Table 1 .
For the understanding of the structure of KCH, (distorted
NiAs type), a comparison with the room-temperature modification of KSiH, (NaCI type)[291is especially informative.
The trigonal-prismatic coordination of the methyl carbanions to six alkali metal ions is a consequence of the trigonal
CH; structure. In contrast, the SiH; ion is more spherical
on account of the larger radius of Si. Thus, steric factors are
less important than electrostatic factors in the alkali metal
silanides (and germanides), and octahedral coordination
(NaC1 structure type) results, at least in the high-temperature
modifications.‘ 321
well as the methoxides and ethoxides (discussed in Section S), have related structures. As a result of their layered
structures, they are neither volatile nor soluble in nonpolar
solvents, and so, once again, structure investigations could
only be performed on microcrystalline powder samples. The
alkali metal acetylides M C r C M (M = Li, Na, K) have also
been investigated (Fig. 8).f361A distorted antifluorite structure is shown by Na,C,,’36b1 and the K analogue is isostruct~raI.[~~~’
2.5. Ethyllithiurn, Ethylsodium, Alkali Metal Alkynides
and Acetylides, and Dilithiomethane
As already mentioned, ethyllithium forms tetramers that
are similar to those of methyllithium but of lower symmetry.
Very different, however, is the layered structure found for
ethylsodium (Fig. 7).i341This structure is a consequence of
the larger radius of the cation and clearly indicates ionic
interactions. An ABC arrangement of three sequentially
staggered but identical double layers lies perpendicular to
the long c axis of the crystal. The Na ions are located within
the double layers, and the ethyl ions are oriented perpendicular to the layers with their methyl groups pointing outward
(Na-C 263 and 268 pm).
The alkali metal ethynides MC-CH (M = Na, K, Rb)13’]
and propynides MC=CCH, (M = Na, K)[35a1(Fig. 8), as
Li2C2
Fig. 8. Space-filling model of the layered structures of several alkynides; the
structure type and the metal ions that lead to isotypic structures are given i n
parentheses. NaCECH (tetragonal; K. Rb, Cs), NaC=CCH, (tetragonal: K).
Li,C, (tetragonal)[36a], Nd,C, (tetragonal[36 b]; K [36c]).
Polylithiated hydrocarbons were first structurally investigated in the early 1 9 8 0 ~ .The
[ ~ ~theoretical
~
proposition[381
that the cis planar structure of gas-phase dilithiomethane
Li,CH, is more stable than the tetrahedral structure has
1505
drawn great attention. Unfortunately, the only experimental
information available to date is for the solid phase. Following earlier, still incomplete powder X-ray diffraction investig a t i o n ~ , ’ ~the
~ ] structure was finally completely determined
by neutron diffraction.[40’ Here, bent CH, anions are coordinated in a complicated way to six Li ions in a strongly
distorted antifluorite structure.
3. The Significance of Solvation: Base Adducts
of Organolithium Compounds
3.1. Solvation
Solvation has great impact on the physical and chemical
properties of organolithium compounds.[8, 91 Organolithium
compounds are generally employed as ether solutions, and
Grignard reagents only exist as ether adducts since dismutation to MgR, and MgX, occurs otherwise.[411Solvation increases solubility and can also cause a drastic increase in
reacti~ity.’~’’
Thus, base adducts have found important applications in synthetic chemistry. The metalation of benzene
by nBuLi, for example, is only possible with the addition of
tmeda or tert-butoxide (“LICKOR s u p e r b a ~ e ” ) . [ ~ ~ ]
Solvation often lowers the degree of aggregation ; nevertheless, organolithium compounds remain mostly associated
even in polar solvents, as the examples in Table2 show.
Here, molar mass determination^^^^' and especially N M R
have provided important results. Since
solid-state investigations are the primary source of detailed
structural information, we will report exclusively on these
results. Crystalline ether adducts are only slightly thermally
stable and thus easily degraded. Therefore, more stable adducts with amines and especially those with the respective
bidentate and tridentate chelating ligands tmeda
(Me,NCH,CH,NMe,) and pmdta (Me,NCH,CH,N(Me)CH,CH,NMe,) are usually examined. Non-N-methylated
amines such as ethylenediamine are easily metalated and
therefore seldom used as donor ligands. Even ethers, in particular thf, can be attacked and cleaved,[451which can lead to
problems with the more reactive organometallic compounds
of the heavier alkali metals.
Often, but not always, the structure of an amine adduct
can be used to infer that of the corresponding ether adduct.
Many structure investigations have revealed the specific influence of not only the carbanion but also the donor ligand
on the total structure. Examples of aggregated organolithium compounds both in solution and in the solid state are
listed in Table 2. Also included are some unsolvated
tetramers and hexamers like [Li(l - n ~ r b o r n y l ) ] , [ ~and
~~
[ L i ( c y ~ l o h e x y l ) ] ,(Fig.
~ ~ ~ ~9).
3.2. Base Adducts of Organolithium Compounds
3.2.1. Methyllithium and Its Derivatives
The LiCH, tetramer is relatively stable and cannot be
cleaved by either ethers o r amines. In its tmeda adduct
[(LiCH,),(tmeda),],.[531 the tmeda molecules d o not act as
1506
Table 2. Degree ofaggregation for some organolithium compounds in solution
and in the crystals[d]
~
Compound
a) 1JI S < / l U i ; O n [9, 101
LiMe
LiEt
LiEt
LinBu
LinBu
LiiBu
LifBu
LiPh
LiCH,Ph
LiC,H,
~~
Degree of
association
Solvent
o r ref.
tetramer
tetramer
hexamer
hexamer
tetrdmeridimer
tetramer
dimer ‘monomer
dimer
>monomer
>monomer
Et,O. T H F
Et,O, T H F
hydrocarbon
hydrocarbon
THF
hydrocarbon
EtlO. T H F
Et,O, T H F
Et,O. T H F
Et,O. T H F
h) I n mwd~
tet ramer
(LiMe),
tetramer
(LJE~),
[Li(l -norbornyl)],
tetramer
(LiiBuj,
tetramer
(LinBu),
hexamer. cluster
[Li(cyclohexyl)],
hexamer. cluster
[Li(tetramethylcy~lopropylmethyl)]~hexamer. cluster
trimer. ring
[Li :2.6-(NMe,),C6H,:1,
ILi(CH ,SiMe,)],
hexamer. ring
dimer
[LiC(SiMe,),l,
c ) Busr u~Id~ic/.s
3D net of tetramers
monomer
dimer
ion pair. ate complex
[(LiMej,(tmeda),],
[Li:CH(SiMe,),)(pmdta)]
[LitBu(Et,O)],
[Li(thfj,l[L1(C(siMe,),)21
[iiCH,CH,CH,OMe],
[LiCH,CH,CH,NMe,],
[LiPh(Et,O)],
[LiPh(Me,S)],
[LiPh(tmeda)IZ
[LiPh(pmdtd)]
[Li(2.4.6-rBu3C,H, j(tmpda j]
[LiC=CH(en)].
[LiC=CPh(tmpda)],
[(LiCrCPhj,(tmhda),].
[(LiC=CiBu),(thf),]
[(LiC=CtBuj,,(thf),]
[Li(allyl)(tmeda)],,
[Li(q2-allyl)(pmdta)]
[Li(~3-1.3-diphenylallyl)(Et,0)].
[Li{q’-1.3-bis(trimethyls1Iyl)allyl)(tmeda)]
[Li(benzylj(dabco)].
[Li(benz~J)(Et,O)l,~
[Li(benzyl)(tmeda)(thf)l
[Li/CH(SiMe,)Ph)(tmeda)]
[Li(CPh,)(tmeda)]
[Li(CPh,)(Et,O),l
[Li[l 2]crown-4),][CHPh21
[Li[ 12]crown-4j,][CPh ,]
tetramer
tetramer
tetramer
tetramer
dimer
monomer
monomer
double chain
(ladder strucrure)
dimer
chain of tetramers
(double helix)
tetramer
stack
chain
monomer
chain
monomer
chain
chain
monomer
monomer
monomer
monomer
ion pair
[a] Abbreviations: tmeda = tetramethylethylenediamine, tmpdd = tetramethyl1.3-propylenediamine. tmhda = tetramethyl-l .6-hexylenediamine. pmdta =
pentamethyldiethylenetriamine,dabco = 1,4-diardbicyclo[2.2.2]octane.
chelating ligands but rather as bridges between the tetramers
and effect the formation of a three-dimensional network.
Recent investigations have shown that the tetramer (LitBu),
is transformed into the dimer [LitBu(Et,O)], upon solvation
with ether.[471
A monomer, [Li{CH(SiMe,),)(pmdta)],[541is formed under the large steric requirements of, on the one hand, the
highly substituted methyl carbanion CH(SiMe,); and, on
the other hand, the tridentate base pmdta. Still greater
shielding can lead to solvent-separated ion pairs as observed
Angrw. Cliem. h i . Ed. En,qI. 1993. 32. I501 - 1523
Fig. 9 Structures of [LI(1 -norbornyl)], (top) and [Li(cyclohexyl)], (bottom,
hall-and-\tick and space-filling models).
in the ate complex [Li(thf),][Li{C(SiMe3)3)z].[55
Crown
1
ethers likewise lead to ion pairs such as [Li([l2]crown-4),]+
with the counterions [CHPh,]- and [CPh,]-.[731"Internal
solvation" occurs when an appropriately functionalized carbanion is employed, as in [LiCH,CH,CH,0Me],[561 and
diamine (en), ethynyllithium forms the polymeric compound
[LiC=CH(en)],, whose crystal structure reveals double
strands connected in a ladder-like fashion by ethylenediamine bridges.
Phenylethynyllithium is solvated by the longer chained
diamines tmpda (Me,N(CH,),NMe,) and tmhda (Me,N(CH,),NMe,). With the former, the dimer [LiCZCPh(tmpda)], having nearly symmetric alkynyl bridges res u l t ~ , [and
~ ~ ]with the latter, tetramers are formed that are
linked by pairs of bridging tmhda ligands to make a double
helix, [(LiC=CPh),(tmhda),], .I6,]
Depending on the conditions for crystallization, the
thf adducts of tevt-butylethynyllithium will form as
either tetramers, [(LiC=CtBu),(thf),],
or dodecamers,
[(LiC=CtBu),,(thf),] (Fig. 1
These especially interesting examples illustrate an often observed structural principle: the stacking of dimers. Thus, the stacking of two
dimers produces a cubane structure and that of six dimers a
dodecamer. Other degrees of aggregation are also conceivable, and examples are found among the alkali metal amides
(Section 7.2).
I
[LiCH,CH,CH,NMe,],.Ls71
3.2.2. Phenyllitliium
This compound is a prime example illustrating the influence that the donor ligand has on the structure: with Et,O
or MezS the respective heterocubane-type tetramers
[LiPh(Et,0)],[58"1 and [LiPh(MezS)]4[58b1
are generated,
with tmeda the dimer [LiPh(tmeda)], having somewhat unsymmetric phenyl bridges,'591and with the tridentate pmdta
ligand the monomer [LiPh(pmdta)lf6'] (Fig. 10). Since the Li
ion is always four coordinate, it is understandable that the
degree of aggregation decreases as the hapticity of the
ligands increases. Recent solid-state I3C NMR investigations have also verified this.[741
Fig. 11. Structures of [(LiC=CtBu),(thf),] (bottom) and [(LiC=CtBu),,(thf),] (top) can he described as stacks of cyclic dimers.
3.2.4. Allyllithiurn
Fig. 10. The influence of the solvent on the aggregation of phenyllithium.
From left to right: [LiPh(pmdta)][60]. [LiPh(tmeda)],[59], and [LiPh(Et,0)],[58]. The TI electron system of the benzene rings is not indicated.
3.2.3. Alkynyllithium Compounds
Ethynyllithium is only stable as a base adduct. Our
investigations["]
have shown that with ethylene-
The allyl ion is particularly interesting since it is one of the
simplest nelectron systems. The first crystal structure investigation on a tmeda adduct revealed a chain structure ILi(allyl)(tmeda)],, in which solvated Li ions act as bridges between the terminal CH, groups of the allyl ions as
(. . . Li(allyl)Li(allyl)~.
A Li ion coordinated to the ally1
rrelectron system was first found in the pmdta adduct [Li(q2allyl)(pmdta)][671which appears as a monomer as expected
because of the greater hapticity of the tripod ligand. In agree-
ment with the results from model calculation^,[^] the carbanion is no longer planar as a result of its (predominantly ionic)
interaction with the cation. The unexpected q2-coordination
is a consequence of the very strong shielding of the Li ion by
the pmdta ligand, which occupies three coordination sites on
the Li. These factors are to a large extent not applicable in
the compound [Li(q3-l,3-diphenylallyl)(Et20)1,[68”1,and the
expected q3-coordination is observed (polymer chains).
with [Na(CPh,)(tmeda)].[’61 In a valuable variation of this
metalation reaction, [NanBu(tmeda)], an adduct soluble in
hydrocarbon solvents and readily synthesized from nBuLi
and NaOtBu, is employed.[”. 781 The analogous potassium
compound, [KnBu(tmeda)]. is likewise soluble and also an
effective metaiating
4.2. Structures of Solvated Organometallic Compounds of
the Higher Alkali Metals
3.2.5. Benzyl- and Tviphenylmethyllithium
4.2.1. Phenylsodium and Biphenylylsodium
The early X-ray structure analyses of [Li(q2-benzy1)(N(CH,CH,),N),],
(polymer chains)169] and [Li(benzyl)(Et,O)], (polymer
are representative of the
structures of the numerous benzyllithium compounds. The
benzyl ligands are planar in these chain structures, and the Li
atoms lie in approximately symmetrical positions above and
below the benzylic carbon atoms. The monomers [Li(ben~ y l ) ( t r n e d a ) ( t h f ) ] [ and
~ ~ ~ ~[Li{CH(SiMe3)Ph)(tmeda)1[70”
are rare examples showing benzyl C atoms with pyramidal
coordination.
The monomeric triphenylmethyl compounds [Li(CPh,)(tmeda)][”] and [Li(CPh,)(Et,0)2]~721contain central C
atoms with planar coordination. Additional alkali metal
compounds having this carbanion will be discussed in the
next section.
4. Base Adducts of Organometallic Compounds
of the Heavier Alkali Metals
4.1. Synthesis of Organosodium and Organopotassium
Compounds
The organometallic compounds of the heavier alkali
metals are more reactive and less soluble than their lithium
analogues, which makes their syntheses particularly problematic. The strongly polar solvent (ether) required in these
reactions is readily attacked by the reactive organometallic
compounds, and even with cooling this process can be
slowed but not halted. For a long time only a few representatives were known; only those derived from strong C-H
acids like cyclopentadiene or alkynes were significant.
Based on the fundamental work of L. Lochmann et
al.,[4zd.h1
we began in 1968 to use metal-exchange reactions
between organolithium compounds and soluble alkali metal
alkoxides (sodium and potassium tert-butoxide, see Section 8) to successfully prepare various compounds of the
heavier alkali metals; important examples of these compounds were discussed in Sections 2.2 and 2.3). We also described the synthesis of unsolvated phenylsodium and
phenylpotas~ium.[’~~
whose structure determinations are
still pending.
In an alternative preparative route to these compounds,
n-butyllithium and sodium rert-butoxide are used to prepare
n-butylsodium, whose C-H acidity is strong enough to serve
in the metalation of hydrocarbons. Aromatic hydrocarbons
as well as amines and other compounds can be used in these
reactions. Solvated organosodium compounds can be prepared in the presence of donor ligands. as was first shown
1508
In contrast to the monomeric lithium compound [LiPh(pmdta)],[601the adduct formed from phenylsodium and
pmdta. [NaPh(pmdta)],. is a dimer in the solid state
(Fig. 12).[”01 The difference is a consequence of the different
Fig. 12. Dimeric amine adducts of phenylsodium (left) and biphenylylsodium
(right).
cation radii: the larger Na ion has a coordination number of
5 and thus the dimer structure is possible. A similar structure is also exhibited by the Na derivative of biphenyl
having bidentate tmeda ligands, [Na(C,H,Ph)(tmeda)],
(Fig. 12).[811Here, the two benzene rings of the bridging
biphenylyl ligands are no longer coplanar, a result also
observed in the corresponding lithiurn compound
[Li(C,H,Ph)(tmeda)], .1821
4.2.2. Benzyl, DiphenyImethyl, and T‘iphenylmethyI
Compounds
In the foIlowing, examples containing the phenyl-substituted methyl anions benzyl, diphenylmethyl, and triphenylmethyl anion will be presented. Depending on the steric requirements of both the carbanion and the donor ligand, a
number of different structure types result.
Bmzyl anion: Both the benzyl and the 0-xylyl carbanion
form cyclic tetramers with the tmeda-solvated Na ion in the
respective complexes [Na(ben~yI)(tmeda)],~’~~
and [Na(o~ylyl)(tmeda)],[~~]
(Fig. 13 left). In contrast, with the tridentate pmdta ligand, a polymeric structure results, [Na(q’-ben~yl)(pmdta)],,,[’~~
in which only the ips0 carbon atoms of the
planar benzyl anions are coordinated to the Na ions and
form the zigzag chains (Fig. 13 right). Similar polymer
chains are also formed in the lithium compound [Li(q2-ben~yl)(OEt,)],,[’~”~
(see Section 3.2.5) and in the recently reA ~ ~ L ‘CWh ~. n ?In!.
. Ed. EnfI. 1993, 32. 1501- 1523
-..._.-.._
I
.
Fig. 13. Aggregation of [Na(o-xylylj(tmedajI, (left) and [Na(benzyl)(pmdta)].,
fright)
r,
'?
ported potassium compound [ K ( y b e n ~ y l ) ( p m d t a ) I , , [ ~al~~I
though the hapticity is different.
Diphenylmethyl anion: The diphenylmethyl ion is also planar in its sodium compounds. Its adduct with the tridentate
base pmdta, [Na(CHPh,)(pmdta)], is, as expected, monomeric. With tmeda, however, it forms a cyclic tetramer.
[Na(CHPh,)(tmeda)],[841 (Fig. 14); isotypic compounds
[M(CHPh,)(pmdta)], (M = K, Rb, C S ) [ ' ~ ~are
] polymers.
A
\ A T .-
I N ' .
q..:;'
Na
.:...
...... ...
.*.
,
' . *... ' ':~
.
.,:,
I
Fig. 14. The influence of the solvent on the aggregation of diphenylmethylsodium. [Na(CHPh,)(pmdta) (leftj, [Na(CHPh,)(tmeda)], (right). The asymmetric unit of [Na(CHPh,)(pmdta)] contains two very similar molecules.
Tr@henylmethyl anion: The aliphatic H atom in triphenylmethane is easily replaced by a metal atom. It was already
noticed in the first examples, [Li(CPh,)(tmeda)]["] and
[Na(CPh,)(tmeda)],,[761that the propeller-shaped carbanion
contains a central planar sp2 carbon atom. The cation takes
up an asymmetrical position relative to this carbon atom in
order to accommodate a maximum number of M-C contacts. Additional weak intermolecular contacts are present in
the sodium compound leading to polymer chains with zigzag
structures (Fig. 15 top). Since these early reports, triphenylmethyl compounds of all the alkali metals have been examined. In these, the position of the cation relative to the CPh;
anion can vary greatly such that it is shifted towards the
peripheral phenyl groups as the cation radius increases. We
have recently investigated three potassium compounds with
various bases (including ethers)[s61 and found that
[K(CPh,)(prndta)] is monomeric, [K(CPh,)(thf)], a two-dimensional network, and [K(CPh,)(diglyme)], a polymer
chain (Fig. 15 middle and bottom). The large counterions K ,
Rb, and Cs allow not only greater hapticity but are also
particularly well suited for $-coordination to the phenyl
groups, as recently observed in the polymeric chain compounds [Rb(CPh,)(pmdta)], and [Cs(CPh,)(pmdta)], as well
as the monomeric compound [K(CPh,)(prndta)(thf)].[871
Fig. 15. Structures ofsome adducts of triphenylmethylsodium and -potassium.
[Na(CPh,)(tmeda)], (top) [76], [K(CPh,j(pmdta)] (middle left) [861.
[K(CPh,)(thf)], (middle right) [86]. [K(CPh,)(diglyme)], (bottom) [86].
All the compounds described in this section have been
shown to be either ion pairs or complicated aggregates of
carbanions and solvated counterions. The structure is determined by a combination of electrostatic interactions and
steric effects (structure of the carbanion and donor ligand,
cation radius). The following examples also emphasize this
principle.
The metal-C distances can vary tremendously even within one compound. In such cases, the number of M-C contacts, in other words the coordination number of the cation,
often cannot be definitively established. The coordination
number of sodium is always greater than 4, that of potassium, rubidium, and cesium often around 8. The minimum
distance for a proposed Na-C contact is roughly 265 pm; in
the figures distances up to 300 pm are typically indicated
with dashed lines. In general, the larger the cation radius, the
greater the number of M-C contacts.
4.2.3. Ally1 Compounds
The metalation of allylbenzene by butylsodium in the
presence of pmdta yields the monomeric compound
"a( 1-phenylallyl)(pmdta)][881with the expected v3-coordination (Scheme 1 ;compare to [Li(g2-allyI)(pmdta)]1671).The
analogous metalation of 1 -phenylcyclohexene also produces
a carbanion having an ally1 system[s81(Scheme 1). Surprisingly, here the solvated Na ion is not coordinated to the ally1
part of the carbanion but instead is centered q6 over the
phenyl group. This arrangement is analogous to that in a
Na-benzene complex in which the Na ion is sandwiched between the amine and the carbanion. What is the explaination
for this unusual coordination? The N a ion is so strongly
shielded by its solvation shell that the ally1 system of the
1509
+ IlBUN<1. pmdta
(tmeda)],[lOO1(polymer chains with yl/$ coordination). In
the structures shown in Figure 17 it is not surprising that
[Na($-indenyl)(pmdta)] forms as a monomer. Also, because
of the larger radius of the potassium cation, the compounds
[K($-indenyl)(tmeda)I, and [K(q’-indenyl)(pmdta)l, form
slightly angulated polymer chains (multidecker).[’o’l A detailed discussion of the factors that are structure determining
in the indenyl complexes is given in ref.[l00] and in the work
cited therein.
Na( pmdta)
Scheme 1. Slnthesis ofphenylallyl compounds with ri2-allyl and ilh-phenyI coordination [%I.
cyclohexenyl ion can no longer approach it (hindrance by the
CH, groups of the carbanion). In addition. the negative
charge on the carbanion is also distributed over the phenyl
part by conjugation. Thus, in consideration of both steric
and electrostatic interactions, the energetically most favorable position for the solvated cation is centered above the
phenyl group.
Additional examples in which the alkali metal has q6-coordination at the center of phenyl ligand are known, for example, [Na(tetraphenylallyl)(Et,0)]r*91(distorted dibenzene
sodiuni sandwich complex) and the triple contact ions in
[Na2(tetraphenylethene)(Et20)2][901 and “a,( I ,I .4.4-tetraphenylbutane-I .4-diyl)(Et,0),].[911
The compound [Ma( 1-phenylethyl)(tmeda)1,, obtained by
the sodium inetalation of ethylbenzene, is also worth mentioning in this context.[841 Here, the carbanions lie in a
bridged rigzag chain and are coordinated through their
CH,CH pro~ipsa s &ell :is through the phenyl rings (q*- and
I
)
?
-
AN,
Y
4.2.4. Cyclopentadienyl, Indenyl, and Fluorenyl Compounds
Sohated Cycfopentadienj.1Alkali Metcil Compounds: The
structures of these compounds have been repeatedly described and summarized.[92i A number of important examples with lithium and sodium have been known for quite
some time now and include [Li(qs-C,H,SiMe,)(tmeda)][931
(monomer), [Li(q5-C,H,Me)(tmeda)]1941 (monomer), and
[Na(~~-Cp)(tmeda)],,[~’~
(zigzag chain) (Fig. 16 left). In contrast, almost no crystal structures of the higher homologues
have been described till now. Recent examples are the potassium compounds [K{q5-C,(CH,Ph),)(thf),119hl (“pianostool geometry”) and [K($-Cp)(OEt,)],,[971 (polymer chain)
(Fig. 16 right). In the latter. the polymer chain is only slightly
angled as a result of the larger radius of potassium, and the
structure thus corresponds well to a “niultidecker structure”.
The first structurally characterized cyclopentadienyl alkali
metal compound that is not solvated is [K(q5-C,H,SiMe,)],,
(chains with weak interactions between the strands).[981
Solvuted Indenyl Alkrili Metal Compounds : Examples of
structurally characterized indenyl compounds include
[Li(~l~-indenyI)(tmeda)][~~~
(monomer) and [Na(indenyl)*O
Fig. 16. Solvated cyclopentadienyl alkali metal compounds. [Na(qS-Cp).
(tineda)],,(left), [K(qr-Cp)(OEt2)l,,(right).
1510
Y
Fig. 17. Solvated indenyl alkali metal compounds[lOl]. [Na(q’-indenyl)(pmdta)](top left). [K(il5-indenylj(pmdta)j,(top right). [K(ti’-indenylj(tmeda)l,
(bot tom).
coordination).
A O *
4s.
Solvated Fluorenyl Alkali Metal Compounds: Three different structures are found for fluorenylsodium and result from
a simple variation in the donor ligands:[’021 [Na(fluorenyl)L], with L = pmdta. tmeda. and tmpda. The compound is monomeric with the sterically demanding tridentate pmdta ligand, forms polymer chains with the smaller
tmeda, and exists as rings constructed from four monomer
units with tmpda, despite the fact that the chain lengths of
tmeda and tmpda differ by only one C atom. These examples
are an impressive illustration of the significance of steric
effects. Monomers, polymers, and cyclic tetramers comprise
the most important structure types for solvated organosodium compounds. In contrast to these, [K(fluorenyl)(pmdta)l,
(Fig. 18) is a cyclic dimer. no doubt as a result of the larger
radius of the
In the complicated structure of
[K(fluorenyl)(tmeda)], , [ I o 3 ] the tmeda can no longer coordinate the large potassium ion in a chelating fashion. and so it
bridges between two cations. Chains with “square waves’’
are formed in [Rb(q5-fluorenyl)(pmdta)],, .[‘04]
*
\
Fig. 18. Structure of [K(~uorenyI)(pmdta)I,[loll
A t ? , q w . C h ~ mIn!.
. Ed. Engl. 1993. 32. 1501 1523
~
4.2.5. The Influence of the Metal Ions
on the Structure of the Carbanions
It has been repeatedly shown both experimentally and
t h e ~ r e t i c a l l y41' ~that
~ the structures, reactivities, and energies of organometallic compounds are strongly influenced by
the metal cations. Some carbanions, for example C,H;, are
only stable in the presence of a c o ~ n t e r i o n . [ ~Numerous
1
examples (see for example refs.[67,69,100]) illustrate the influence the cation has on the structure of the carbanion. It is
noticeable that the phenyl groups bound to metal centers
have regular distortions of the six-membered rings. The C2C1-C6 angle is always less than 120" (down to ca. 112"), and
the angles at C2 and C6 atoms are widened to ca. 124". Of the
many examples known, we mention here only [LiPh(pmdta)], [NaPh(pmdta)], , and [MgPh,(tmeda)]. A qualitative
correlation exists between the endocyclic angles and the electronegativity of the metal.1611
Fig. 20. Space-filling model of [ M ~ M C ? ](body-centcred
,~
orthorhomhic unit
cell; Mg radius chosen. to be 70 pm).
5. Organomagnesium Compounds
In this section. as in the preceding, our own investigations
will be highlighted. The redder is referred to the literature
citations given in ref.[5] for additional examples, and to the
discussion of organomagnesates in Section 6.4.
Fig. 21. Space-filling model of [MgEt,],, (tetragonal unit cell: high-temperature
modification; M g radii chosen to he 70 pm).
5.1. Diorganomagnesium Compounds
Four unsolvated diorgdnomagnesium compounds are especially important: MgMe,, MgEt, , MgPh,, and MgCp,
(Figs. 19-21), Bis(g5-cyclopentadienyl)magnesium (magnesocene) was the first structurally characterized organomagnesium compound; its sandwich structure was first determined in 19SS['05"l and later refined[*05h1.Dimethylmagnesium and diethylmagnesium are obtained as finecrystalline powders by drying their thermally labile ether adducts. Their polymeric structures, [MgMe,],i'061 and
[MgEt,],,1'071 were discovered by us quite some time ago.
Recent investigations have shown that diphenylmagnesium
is also polymeric."
As expected, these compounds are not
volatile and only sparingly soluble in hydrocarbon solvents.
Ether and other polar solvents break the polymer chains by
formation of solvated monomers.
The magnesium ions in the polymer chain are each
surrounded by a tetrahedron of four carbanions which
form pairs of p 2 bridges between the M g ions similar
to that found in d i m e t h y l b e r y l l i ~ m . ~ Unsolvated
'~~~
monomers are obtained with more bulky carbanions as in
.. .
Fig. 19. Polymer chains ofdimethylmagnesium (left). di&ylmagnesium (mid.
dle), and diphenylmagnesium (right).
A n ~ r i i . C/?mi.111,. Ed Engl. 1993, 32. 1501- 1523
[Mg{C(SiMe,),},]."
in which the magnesium ion has linear coordination.
As with the methyl alkali metal compounds, there was
some uncertainty as to the H positions in [Mg(CH,),],, and
[Mg(C,H,),], . Therefore, we recently carried out new structure determinations from neutron diffraction data on the
deuterated compounds [Mg(CD,),], and [Mg(C2D,),], at
293 and 1.5K.it111
These structures verified the earlier results but also showed a statistical or dynamic disorder in the
alkyl groups (rotation of the methyl groups). A phase transition only occurs with [MgEt,], (tetragonal G monoclinic at
ca. 260K). As a result of the bent ethyl groups. the chains are
chiral helices (right- and left-handed screw sense) in a statistical distribution, whereby a higher pseudosymmetry is attained."
5.2. Solvated Diorganomagnesium Compounds
As already mentioned, because the ether adducts of the
organomagnesium compounds are thermally labile, the
more stable adducts of the chelating amines were examined
in preference. Since improvements in instrumentation have
made low-temperature measurements experimentally more
accessible, the number of ether adducts investigated has increased. The structural characterization of the species involved in the Schlenk equilibrium of the Grignard compounds is especially challenging. The halo-bridged dimers
belong to this group of compounds but will not be discussed
here. Even though base adducts of the amines can also serve
as models for the ether adducts, they are interesting in their
own right. Similar to the alkali metal compounds, the reacIS11
tivity of the magnesium compounds can be increased
through solvation, whereby the character of the carbanion is
strengthened. The complexes formed from the addition of
tmeda to Grignard compounds have proved to be cocatalysts in the anionic polymerization of olefins, and some yield
syndiotactic polymers.[' 'I
Only monomeric adducts are listed in Table 3 . They contain magnesium atoms with various coordination modes as
has been hardly investigated till now. By their reaction with
C-H acidic hydrocarbons R'H (alkynes, cyclopentadiene,
indene, fluorene) in a 1 : 1 ratio, we have recently['251synthesized the adducts [MgMe(y3-Cp)(tmeda)], [MgMe(q3-indenyl)(tmeda)], [MgMe(~'-fluorenyl)(tmeda)],and [Mg,Et(CECPh),)(tmeda)],. It is remarkable that none of the x
carbanions exhibit $-coordination. On account of steric
hindrance of the methyl groups, at most q3-coordination can
be achieved. and with the fluorenyl carbanion only q'-coordination is attained.
Tdbie 3. Solvated monomeric organornagnesiuni compounds.
Compound
Mg
C.N. [a]
Ref.
Compound
Mg
Ref
C N. [d]
6. Organometalates: Compounds with
Two Different Metals
6
~~~
[a] C.N. = coordination number
illustrated in Figure 22 by selected examples. Tetrahedrally
coordinated magnesium can be found in the compounds
with tmeda : [MgMe,(tmeda)],[' I 3 I [MgEt,(tmeda)],[' 14] and
[MgPh,(tmeda)].['
The coordination number 5 (distorted
trigonal bipyramid) is also possible as shown by recent
14]
[MgEt,examples with pmdta-[MgMe,(pmdta)]."
(pmdta)]" I4]-and a few Grignard compounds. The "slender" alkynyl carbanions allow for even octahedral coordination in [Mg(C-CPh),(tmeda),][' 'I and [Mg(C-CtBu),(tmeda),]." 91
Fig. 22. Structures of[MgMe,(tmeda)] (top left), [MgEt,(tmeda)] (topmiddle).
and [MgPh,(tmeda)]. all of which have tetrahedral coordination at Mg; structures of [MgMe,(pmdta)] (bottom left) and [MgEt,(pmdta)j, which both have
distorted trigonal-bipyrdmidal coordination at Mg.
These compounds are among the organometallic compounds longest known and include, for example,
NaEt.ZnEt, .r1261 They are often obtained by simply mixing
together stoichiometric amounts of the components and
were initially considered as double compounds. The true
nature of these compounds was recognized only later when
G . Wittig termed them ate complexes.['27] In many cases
their true nature could only be completely elucidated by
X-ray structure analysis. We investigated compounds that
contain in addition to an alkali metal the following elements
in the organometalate: lithium, copper (as Cu'), beryllium,
magnesium, zinc, cadmium, boron, aluminum, gallium, indium, and thallium (Table 4; further examples in refsJ3.41 and
the work cited therein). In these compounds the coordination numbers 2 (Cu, linear), 3 (Zn, trigonal planar), 4 (tetrahedral), and 5 (Mg, trigonal bipyramidal) have been observed. Sometimes the organometalate ion is di- or even
trinuclear; these special cases will be discussed in more detail.
Although sometimes the partitioning of these compounds
into a complex organometalate ion and a counterion is only
formally possible, it is very useful as an ordering principle as
well as for the nomenclature. In most cases the relation of the
organic substituents to the central atom is clearly discernible
from the structures. The charge localization on the carbanion is always greatest for the cation with the highest charge
( M 3 + > M 2 + > M') and, for cations with the same charge,
the one with the smaller radius (Li' > Na'). This is then the
coordination center. As far as it is sterically possible, the
carbanions orient their most strongly negative polarized centers toward the coordination centers and form as many and
as short contacts as possible. In this way carbanions can also
become bridging ligands (e.g. p2-Me, p2-Ph), whereby both
symmetric and unsymmetric bridges are possible as the examples in Figure 23 show. Solvation of the counterion is
often observed, especially with the small lithium ion. Solvent-separated ion pairs are ultimately formed with sterically
demanding donor ligands.
5.3. Unsymmetric Diorganomagnesium Compounds
The base adducts that are soluble in hydrocarbon solvents, for example [MgR,(tmeda)] (R = Me, Et), are suitable starting materials for the preparation of unsymmetric
diorganomagnesium compounds, a class of compounds that
1512
6.1. Lithates
In 1958 Wittig et al. described a sodium diphenyllithate
("diphenyllithium sodium")[l 561 whose structure could not
Angeic. Cliein. In!. Ed. Ennl. 1993, 32. 1501-1523
M
Table ?. Some organometalates structurally characterized by X-ray crystallography
C.N.
of the
central
atom
Compound
Orgui?oiithiitcs
Tetraorganoltthate: [Na(tmeda)],[LiPh,]
Diorganolithate: [Li(thf),][Li.IC(SiMe,),)
Ref.
4
,I
z
[a]
Diorjioiio~iiprcllrs
2
[{Lt(Et,O))(CuPh,)l2
[Li,CuZMe,] (ring)
[Li,(€t,O),Cu(CrCPh),,1
2
4
~l.jilillfl/Jl~l.l~//liIPS
4
3
LiJBeMe,]
Li[BetBu ,]
Orgu~ioiiiujin~~.su~~~s
Triorganomagnesate: [Mg(neopentyl)([Z.l .l]cryptand)]lMg( neopentyl),]
Tetraorgaiiomagnesates: [Li(tmeda)l,[MgMe,]
~Na(pmdta)l,[MgPh,l
Hexaor~dnodimagnesates:[Li(tmeda)],[Mg,Ph,]
[MgEt([Z.l .l]cryptand)],[Mg,Et,I
[Mg(neopentyl)([2.1 .l]cryptand)l,[Mg,Et,1
Octaor~iiiotrirna~nesate:
[Mg,Me,(tacn),],IMg,Me,] [b]
Dimeric di;imine(triorgano)magnesates:
Li,]Mp(C~CPh),(tmeda)]~
Na,[Mp(C_C~Bu),(tnieda)]~
Na,[Mg(C~CtBu),(pmdta)],
[Li(trneda),][tmeda)Li(benzyl),Mg(benzyl),],
[a]
Fig. 23. Coordination modes for bridging methyl and phenyl groups
3
4
4
4
4
4
4
5
5
5
4
Orguiwziii( rites
Tetraorganozincates: LiZ[ZnMe,]
K,[Zn(C=CH),]
Tnorganoztncate: K[ZnMe,]
4
4
3
Orgun01 udimiii~
K,[Cd(C=CH),]
4
Fig. 24. [Na(tmeda)],[LiPh,], a tetraphenyllithate with three solvated counterions; amine ligands are not shown in the picture on the right.
6.6. Cuprates
OI.gui7fihorarP
[L~(BML.,)I,(ring)
Na[BMe,]. K[BMe,l. Rb[BMe,]. Cs[BMe,]
K[BPh,l
Rb[BPh,]
"Me,l[BPh,l
4
4 A Icl
4
4
4
OrgunoIiiiiiiiiurPs
Tetraorganoaluminates. K[AIMe,], Rb[AIMe,], Cs[AIMe,] 4 B [c]
[LiEt,AIElJ, (p2-bridged polymer)
4
Hydridotriorgdnoaluminate-K[AIHMe,]
4
Orgrmogullritrs
K[GaMe,]. Rb[GaMe,l
4 B Icl
Orjiri~ronlrllltrs
L~[lnMe,]. Na[InMe,]
K[lnMe,l. Rh[lnMe,]
Cs[lnMe,]
Li[InPh,]. Na[lnPh,]
4 C [cl
4 B [cl
4 D [cl
4
Many examples of lithium organocuprates are known." 571
They are important in organic synthesis and in many cases
even superior to the traditional Grignard and organolithium
reagents. These very labile compounds are generally introduced as ether solutions and usually decompose upon attempted isolation. For this reason the first example of a
simple ether-solvated lithium cuprate confirmed by X-ray
crystallography is of particular significance. This dimer,
[{Li(Et,O))(CuPh,)], (Fig. 25), is formed from two linear
diphenylcuprate units held together by solvated Li ions.['301
Ol.jiLlnotilnillirrs
4 C [El
4 B [cl
4
Na[TIMe,]
K[TIMe,]
Na[TlPh,]
[a] Solvent-separated ion pair. [b] tacn = N,N',N"-trimethyl-1,4,7-triazacyclononane. [c] The compounds listed together on the same line are isotypical
(structure types A-D).
Fig. 25. [{Li(OEt,)~(CuPh,)],, a dimeric lithium diphenylcuprate
be characterized. Under similar reaction conditions, but with
tmeda, we were able to isolate the compound [LiNa,Ph,(tmeda),].~'281It contains a tetrahedral tetraphenyllithate
ion surrounded by three solvated sodium ions[Na(tmeda)],[LiPh,] (Fig. 24). Also of note is [Li(thf),][Li{C(SiMe,),),], only the second example of an organolithate known to date. It is a solvent-separated ion pair in
which a diorganolithate having a central atom with linear
coordination is the anion.['291
Anjiiw.
Ciiiwi.
Int. Ed Eirgl. 1993. 32. 1501 - 1523
The compound [Li,(Et,O),Cu,(C=CPh),,1 is an example
of the even rarer, higher order organocuprates ("HO
cup rate^").['^^^ It contains a [Cu(C=CPh),13- tetrahedral
anion "crowned" by a cyclic [Li,Cu,(C=CPh),] unit. The
Li,Cu, atoms of this unit form a slightly bent six-membered
ring and are bridged by the C, atoms of the six phenylethynyl
ligands. The three remaining Li atoms are solvated by Et,O
and lie below the base of the anion.
1513
6.3. Beryllates
Organoberyllates are not very well investigated. The compound Li,[BeMe4],[1331prepared without solvent. contains
the tetrahedral tetramethylberyllate ion (see Fig. 28). In contrast, simple planar threefold coordination occurs with the
voluminous 1-butyl anion in L ~ [ B ~ ( ~ B u ) , ] . [ ' ~ ~ ~
6.4. Magnesates
Organomagnesates exist in many interesting structure
types and with various coordinations ( 3 , 4, 5 ) at the
magnesium centers (see Table 4). The compounds
[Li(tmeda)]2[MgMe,][13h1
and [Na(pmdta)],[MgPh4][1371
have tetrahedral coordination (Fig. 26 top), and the organic
groups form unsymmetric p, bridges between magnesium
and the solvated alkali metal atom.
,,\
Fig. 27. Organomdgnesates with pentacoordindted magnesium: Li,[Mg(C=CPh),(tmeda)j, and Na,[Mg(C-C~Bu),(pmdta)]~
Finally, solvent-separated ions are found in the benzylmagnesate [Li(tmeda),][(tmeda)Li(p2-benzyl),Mg(benzyl),], .11401 The anion has a complicated structure in which
two of the four benzyl groups of a strongly distorted tetrahedral Mg(benzyl), unit are bridged at their CH,groups by a
lithium atom.
6.5. Zincates and Cadmates
The zincates Li,[ZnMe,]['411 and K2[Zn(C=CH),],"421
and the cadmate K,[Cd(C-CH)4]['421 all contain tetrahedral anions. The simple substitution of potassium for lithium
in the methyl zincate causes a change to the rarely encountered trigonal-planar coordination as found in the triorganozincate K[ZnMe,] (Fig. 28).[1431
Fig. 26. Top: Tetraorgdnomagnesates with methyl and phcnyl bridges:
[Li(trneda)],[MgMe,] and [Na(pmdta)]JMgPh,]. Bottom: [Li(tmeda)lI[Mg2Ph,]. an example ofa hexaorganodiinagiiesate. The pairsof phenyl groups
form either symmetrical or unsyrninetrical bridges.
The two magnesium atoms in the dinuclear hexaorganodimagnesates [Li(tmeda)]2[Mg,Ph,][1381
(Fig. 26 bottom) and
[Mg(neopentyl)([2,1,I]~ryptand)],[Mg,Et,1"~~~
have distorted tetrahedral coordination. The (formally) [Ph,Mg(p-Ph),MgPh,]'- ion is, incidentally, isoelectronic and isostructural with the neutral compound AI,Ph,11581 and contains symmetric phenyl bridges between Mg and Mg but
unsymmetric bridges between Mg and Li (Fig. 26 bottom).
We recently found the first octaorganotrimagnesate with
the compound [Mg,Me,(ta~n),],[Mg,Me,1,"~~'which is obtained from the reaction of MgMe, with the cyclic triamine
tacn. Unprecedented here is also the occurrence of an
organomagnesium cation with a "triple-decker structure" in
which three p,-Me groups bridge the solvated magnesium
ions [LMg(p,-Me),MgL]+.
A few organomagnesates exist as dimers in which the pentacoordinate magnesium center is bound to three organic
substituents and one bidentate diamine ligand: Li,[Mg(CzCPh),(tmeda)], ,[1401 Na,[Mg(C=CiBu),(tmeda)l,
and Na,[Mg(C-CtBu),(pmdta)l,
The alkali metal ions
connect the two units of the dimer through contacts to the
alkyne groups as shown in Figure 27.
~
1514
Fig. 28. Methylzincates with tetrahedral and trigonal-planar coordination at
the zinc atom. Left: Li,[ZnMe,] (isotypic to Li,[BeMe,][l33]). Right:
K[ZnMe,] (monoclinic unit cell).
6.6. Borates, Aluminates, Gallates, Lndates,
and Thallates
All the compounds presented here are tetraorganometalates with the exception of the hydridometalate K[AIHMe,].[15i1Apart from [Li(BMe,)], (ring structure),L1441
they
all contain quite distinct MR, units and display a number of
crystallographically different structure types. Orthorhom bic
(type A), tetragonal (types B and D), and cubic (type C)
lattices have been observed for the methyl compounds
(Figs. 29 and 30). The isotypic representatives of each type
Angcw. Ckrm. Ini. Ed. Engl. 1993. 32, 1501 1523
~
ture of sodium amide we were also able to locate the H atoms
in good agreement with the neutron diffraction results
(Fig. 31).1971The structure can be understood as a three-dimensional network of dimeric (NaNH,), units.
structure type B
Fig. ZY Example 01‘ n tetramethylmetalate of structure type B (tetragonal).
K[InMc,]
Fig. 31. Structure of NaNHi. Projection is along (010) in the ccntrosynimetric
setting of Frlck1[97.159e].
structure type C
structure type D
Fig. 30. Examples of tetrdmethyhnetahtes of structure types C (cubic) and D
(tetragoniil). Nii[InMc,] ( C . cubic) and Cs[lnMe,] (D, tctragonal). In structure
type C hotli types of metal centers have tetrahedral coordination.
are summarized in Scheme 2. A few tetraphenylmetalates
have also been structurally characterized (Scheme 2).
Tip(>A
Na[BMc,]. K[BMe,], Rb[BMe,], Cs[BMe,]
TI./WB
K[AIMe,], Rb[AlMe,]. Cs[AIMe,], K[GaMe,], Rb[GaMe,], K[InMe,],
Rb[InMc,]. K[TIMe.,l
TL’IJC’ c‘
Li[lnMc,]. Na[lnMe,]. Na[TIMe,]
Tr./,e D
Cs[I n Me,]
~~i~ti~Jhi’il~~/i,lerrr(rrtt..r
K[BPh,l. Rb[BPh,]. [NMe,][BPh,],
Li[InPh,]. Na[InPh,]. Na[ThPh,]
Scheme 1. Structurally characterized methyl- and phenylborates. -aluminates,
-gallate\. -indales. and -thallates.
7. Alkali Metal Amides
Together with the organometallic compounds, the alkali
metal amides have established themselves as the most important metalating and deprotonating agents. For this reason a
great interest in their structures has developed.
7.1. Simple Alkali Metal Amides
The structures of the simple alkali and alkaline earth metal
amides M(NH,), (n = 1.2) have been known for a long time
from the investigations of R. Juza, H. Jacobs, and othe r ~ . [ ” The
~ ~ H atoms were located by either X-ray
(LiNH,“ 59h1) or neutron diffraction (NaND,[159e1).Because of the difference in the anion shapes, no structural
relationships exist between the methyl compounds MCH,
and the amides MNH,. In an X-ray refinement of the struc-
More recently a number of solvated alkali metal amides
derived from organic amines have been investigated; these
will be discussed in Section 7.2. In contrast. the study of the
nonsolvated compounds is only in its initial phases. Here.
potassium diethylamide is one of the first examples and contains (KNEt,), units which build rings in a complicated arrangement.[971An interesting “ladder structure” has been
found in [Na(2.3,4,5-tetramethylpyrrolyl)],, (azacyclopentadienyl ligand),[1601 in which the characteristic features of
amide bridges are combined with those of the x coordination
of the cyclopentadienyl ligands.
7.2. Base Adducts of Alkali Metal Amides and Imides
The structural chemistry of the solvated lithium amides
and imides having organic substituents on N has made great
progress. Numerous original papers and some excellent reviews16. 161. 1621 attest to the multitude of structure types
known for these compounds, including stacked rings and
ladder structures. In the following, the amides of the heavier
alkali metals, which have been less well investigated, will be
highlighted. In general, alkali metal amide adducts are very
often observed as dimers with four-membered M,N2 rings;
however, other structure types are also known (Table 5).
New, as yet unpublished examples of dimeric amides are
shown in Figure 32. Structural investigations on their base
adducts are also of interest here since the reactivity of amides
can be increased by such additives.11631
Alkali metal amides are generally obtained by the direct
reaction of amines with the metals. Often, hydrides and
organo alkali metal compounds such as the soluble adduct
[NanBu(tmeda)] are more suitable for metalation reactions.
since the synthesis can then take place in a homogeneous
medium. Many of the amides listed in Table 5 have been
prepared in this way. As a result of the high reactivity of
butylsodium, however, cleavage of tmeda under formation
of NaNMe, cannot be ruled
This product crystallizes in situ in the form of oligomeric aggregates. The interesting stacked structures of [Nai,(NMe,),o(tmeda),] and
[Na,,(NMe,),,(tmeda),]
were thus obtained in this way
(Fig. 33).[1671
These are decameric and dodecameric aggre1515
Table 5. Solvated amide and imide complexes of the heavier alkali metals
Degree of
aggregation
Compound
Ref.
Degree of
aggregation
Compound
Ref.
~~
[NaNMe,(pmdta)],
[NaNEt,(tmeda)],
[LiNPh,(tmedaj],
[NaNPh,(tmeda)],
[KNPh,(tmeda)],
[K,(NPh,),(tmedaj,l,
[Li(NMePh)(tmeda)],
[Na j NPh(2-pyridylJ(pmdta)l2
[NaN =C/BU,],[HN=CIBU,],
Stack
stack
stack
stack
dimer
dimer
dimer
dimer
dimer
dimer
dimer
dimer
dimer
dimer
chain [a]
dimer
dimer
cubane
[a] [K,(NPh2)(tmeda),], forms chains of [K(NPh,)(tmeda)], units bridged by tmeda
Thus, analogies to the organolithium compound
[ ( L i C ~ C t B u ) , , ( t h f ) , ] ~discussed
~~~
in Section 3.2.3 can
be drawn. The occurrence of [Na,,(NMe2),o(p-xylyl)2(pmdta),] indicates that carbanions can also be fitted into the
amide stack!1671
Related examples can be found under the amides listed in
Table 5 which are formed from the heterocycles pyridine,
carbazole, and indole. These amides also form bridges in the
dimers and can exhibit both o and n interactions. The amide
bridges are unsymmetric in [Na(indolyl)(tmeda)], and symmetric in [Na(indolyl)(pmeda)], and thus reflect the difference in the spatial requirements of the solvate bases.[1681
,
U
Fig. 32. Structures of dimeric base adducts of alkali metal amides.
Top: [NaNMe,(pmdta)],, [NaNEt,(tmeda)],; bottom: [LiNPh,(tmeda)],,
[NaNPh,(tmeda)], , and [KNPh,(pmdta)], [97].
8. Alkoxides, Aryloxides, Enolates, and
Acetylenediolates
Investigations oii the structures of alkali metal alkoxides
also began relatively late, as the appearance of a monograph
first in 1978 indicates.[171]A review on phenoxides is given
in ref.[172]. Considering the importance of these compounds, not many structures are known even today. Table 6
contains a few simple but important examples of some unsolvated compounds.
Table 6. Unsolvated metal alkoxides, thiolates, and acetylenediolates.
Compound
Degree of aggregation
LiOMe
NaOMe
KOMe
RbOMe. CsOMe
KOiPr. RbOrPr, CsOiPr
(NaOtBu),. (NaOtBu),
(KOIBU),. (RhOtBu),, (CsOlBuj,
(KOSiMe,),. (RbOSiMe,),
(CsOSiMe,),
(CuOtBu),
Ca(OMe),, Sr(OMe),. Ba(OMe),
LiSMe. NaSMe
KSMe
NaOCsCONa
KOC=COK
RbOCSCORb. CsOC=COCs
layered structure
layered structure (LiOMe-type)
layered structure
layered structure (KOMe-type)
layered structure (LiOMe-type)
hexamer, nonamer
tetramer. cubane
tetramer. cubane
.
Ref.
tetranier, ring
layered structure (Ca1,-type)
layered structure (LiOMe-type)
layered structure (KOMe-type)
Fig. 33. The stacked structures of the amine adducts of sodium dimethylamides: [Na,,(NMe,),,(tmedaj.I (top) and [Na,l(NMe2),,(tmeda).I (bottom).
8.1. Layered Structures: Methoxides, Ethoxides,
Propoxides
gates, which can be considered to be constructed from ringshaped dimers and bounded by terminal donor molecules.
The standard synthesis of alkali metal alkoxides from a
metal and an alcohol leads to solvent adducts which are
1516
Angiw. Chctn. Inr. Ed. Engl. 1993, 32, 1501 -1523
I
i
a=355pm
c = 8 3 1 pm
c=877pm
c=769pm
*
t
I
*
a=395 pm
*
i
J
a=364pm
often crystallizable; the solvent-free compounds usually
provide finely crystalline powder upon drying. Their X-ray
investigations are thus more difficult, and the determination
of the hydrogen atom positions is rarely possible. Disorder
arising from rotomers is likely; solid-state 'H NMR investigations['7"b1have shown that the alkyl groups rotate in crystalline samples.
All of the simple alkoxides investigated so far, MOMe
- ' ~ MOiPr
~~
(M = K-CS),"~" have
(M = L ~ - C S ) . ~ " ~ and
layered structures (Fig. 34). The methoxides of calcium,
strontium. and barium[ls21 as well as the thiolates MSMe
(M = Li-K)[1831are also to be included here. The alkyl
groups are always found on the outside of the layers, and the
cations are distributed within the layer over one (Li, Na, Ca,
Sr. Ba) or two (K, Rb, Cs) sites depending on the radius of
the ion. The compounds are nonvolatile and insoluble in
apolar solvents as a consequence of their layered structures.
8.2. Spherical and Cyclic Aggregates: tert-Butoxides,
Enolates
Layered structures are no longer possible with large bulky
alkyl groups (the lower boundary seems to lie by t-butyl),
and so spherical or cyclic aggregates are formed in these
cases. The compounds initially investigated, (MOtBu),
(M = K, Rb, Cs; Fig. 35),[1791 as well as the trimethylsilanolates (MOSiMe,), (M = K, Rb, CS)['*~'and the recent-
Fig. 34. The layered structures of (from left to right)
LiOMe, KOMe (tetragonal). and Ca(OMe)2 (hexagonal).
ly described imide (CsNHSiMe,),,[' 8 7 1 are all tetrameric and
form cubane structures in crystalline samples. The corners of
the distorted cube are occupied by metal atoms alternating
with 0 or N atoms.
The compounds are soluble and volatile as a result of their
spherical structures, whereby the outer surfaces have predominantly hydrocarbon character. Molar mass determinations in aprotic solvents1' s81 indicate similar degrees of
aggregation as found in the crystal. According to mass spectetramers-(KOtB~),,~~'~~~
trometric
investigations,
(MOSiMe,), (M = K, Rb, C ~ ) [ ' ' ~ ~ ~ - a n hexamersd
(LiOCMe,),['sg] and (MOSiMe,), (M = Li, Na)['89]-also
exist in the gas phase.
The application of these alcoholates in the synthesis of
organometallic compounds (see Sections 2.2,2.3. and 4.1) as
well as their versatility as synthetic building blocks and catalysts for organic reactions is ultimately based upon their
structure-dependent s o l ~ b i l i t y . ~ ' ~Soluble
~"1
alkoxides are
also valuable precursors for the preparation of metal oxides
under mild reaction conditions (sol-gel and MOCVD methods). The discovery of high-temperature superconductors
has recently sparked a lively interest in mixed oxide systems
and other ceramic materials and has thus contributed to a
renaissance in the chemistry of the alkoxides.['
The structure of sodium tert-butoxide (Fig. 36) is especially noteworthy. Within its unit cell, hexameric (NaOtBu),
Me&
I
CMe3
Fig. 36. The structures of (NaOtBu), (left) and (NaOtBu), (right). The central
six-membered ring in the nonamer is expdnded to the extent that only three
Na-0 contacts are possible here.
Fig. 35. The cubane structure of (KOrBu),; isotypic structures include (MOtBu), ( M = Kb. C s ) and (MOSiMe,), (M = K, Rb. C s ) .
and nonameric (NaOtBu), aggregates (1 : 1 ratio) lie in very
close proximity to one another, a rarity even from a crystallographic viewpoint.['781Just as a cubane structure can result from the stacking of two ring-shaped dimers, so can the
trigonal-prismatic structure of (NaOtBu), be regarded as a
stacking of two ring-shaped trimers. Although inadequate
crystal quality has prohibited a definitive investigation o n
1517
lithium /or/-butoxide, hexamers are probably also present in
its crystals.
Copper(1) can hardly be compared to the alkali metals. In
its complexes the metal center strives for linear coordination,
as the structures of the diorganocuprates show. Thus, copper
trrt-butoxide forms the four-membered ring structure
( C L I O ~ B U ) (Fig.
~ ~ ' ~37).
' ~ Although the very explosive compound methylcopper has not yet been crystallographically
investigated, it should have an analogous structure.
tural framework in [Ca,O,(OEt,),]~ 1 4 E t 2 0 is made
up of a stack of three Ca,O, rings,[2oo1 and [Ca,(OCH,CH,0Me),,(HOCH,CH,0Me),1'20'1 is the largest
alkoxo complex known.
It was already mentioned in Section 3.1 that the reactivity
of the organometallic compounds can often be drastically
increased by the addition of amines (tmeda) or tert-butoxides. This very important effect is based on the formation of
complexes from organometallic compounds and the base
("superbases"). The solubility of the organometallic compound usually increases parallel to this. Although crystal
structure investigations on appropriate compounds are only
in their initial stages at this point, the benzene-soluble tetramer [nBuLi. LiOtBu],[202"1and [Li(4.6-dimethyl-2-sodiomethylphen~xide)(tmeda)]~.~~~*~~
which contains a Li40,
cubane framework, are the first examples. The solubilization
of organosodium compounds by magnesium a l k ~ x i d e s [ * ~ ~ " ]
is also possible and, conversely, polymeric diphenylmagnesium forms benzene-soluble complexes with alkali metal
alkoxide~.'~~~~~
8.4. Acetylenediolates
Fig. 37 Thc ring structure of (CuOtBu),
The enolates are very important members of the group of
lithium compounds having 0 ligands. Their structures have
been well investigated and included in reviews over the last
decade.""'] It is sufficient to indicate here that the known
structure types (dimers, tetramers, hexamers, etc.) also occur
with these compounds. Most compounds have been investigated as their solvent adducts. The trimeric magnesiumacetylacetonate Mg,[MeC(O)CHC(O)Me], and its phosphiare solnoyl derivative Mg,[(Et,0),P(0)CHC(O)Me],["21
vent-free enolates. As a consequence of the octahedral coordination preferred by the dications, these compounds
contain face-sharing MO, polyhedra similar to those found
in [ Z n , ( a ~ a c ) , ] " ~ ~and
' [Ni,(aca~),].['~~~
8.3. Base Adducts of the Alkoxides and Phenoxides,
Complexes from Alkoxides and Organometallic
Compounds
Apart from the alkali metal enolates, there are very few
structurally characterized compounds with other, even simple, alkoxides. Two examples are the dimeric phenoxides
(phenolates) [Li(p-OC,H,Br)(tmeda)]21'gs1
and [Na(OPh)(pmdta)],1'"51 which contain central M,O, rings. A few
compounds with more highly substituted phenoxides also
have ring structures: lithium-2,6-di(t-butyl)phenoxide [Li(OC,H,~BU,)(E~,O)],['"~~
and sodium-2,4,6-tris(trifluoromethy1)phenoxide [Na(OR)(thf),],
In [Li j0-2,6(CH,NMe,),-4-MeC,H2J1,,
a cyclic trimer (LiOR), is
formed by "inner solvation" with a phenoxide having two
amine s ~ b s t i t u e n t s . ~ ' ~ ~ ~
Some recent investigations on soluble (oxo)alkoxides of
the heavier alkali earth metals, and especially those with
barium,['y91are worth mentioning with regard to the interest
in superconductors noted in the previous section. The struc1518
J. Liebig reported in 1834 on the formation of a black and
extremely reactive "potassium cdrbonyl" from the reaction
of CO and melted potassium in a 1 : 1 mol ratio.[Z041The
product was later formulated as K,C,O,, since small
amounts of hexahydroxybenzene (together with its oxidation products) could be obtained by careful hydrolysis (otherwise an explosive reaction occurs).
The reaction can also be performed with alkali metals
dissolved in liquid ammonia. Following the introduction of
C O into the blue solution of the metal, a colorless and likewise very reactive powder precipitates which, upon hydrolysis, yields small amounts of glyoxal together with other products. The "alkali metal cdrbonyk" prepared in this way were
the subject of our systematic investigations with W. Buchner
starting in 1963.11s4X-ray diffraction studies, among
others, proved these compounds to exist as dimers. The Na,
K , Rb, and Cs salts of these acetylenediolates were examined
and found to be isotypic (Fig. 38). The lithium compound is
Fig. 3X. Spacc-fillins model of the crystal structure of KOC=COK[185].
NaOCzCONa [ l X4]. R b O C d W R b . and CsOCeCOCs [I 861 are isolypic.
A R ~ P I I . C'hrm. In1 Ed Engl. 1993. 32, 1501 -1523
extremely shock sensitive and thus its structure determination has not been possible. Generally the compounds become more stable with increasing cation radius, since intermolecular reactions within the crystal are then minimized.
The following reaction steps apparently occur during the
reaction of CO with alkali metals (Scheme 3). First, CO is
M
+ C‘O
2CO’
~
+ CO’
-0ceco-
-,M +
Among the systematic investigations on solvent adducts of
alkali metal halides carried out recently, those by R. Snaith
et aI. and A. H. White et a1 deserve particular note. We have
also investigated several adducts including [(LiCl),(tmeda),]
(two solvated Li,CI, units connected by a tmeda bridge).I2
[NaI(tmeda)], (heterocubane; Fig. 39),[” 51 [NaI(tmeda),],
(c~ltena-~-IINa(trneda),],
polymer chain; Fig. 40),f21h1
and
[NaI(pmdta)], (dimer; Fig. 40).12’’I These structures compli-
Scheme 3. Formation of iicetylenediolate from the reaction of CO with an
alkali iiictal
reduced to the radical anion C 0 ’ - (isoelectronic with NO).
which immediately dimerizes to form the acetylenediolate
(along with other compounds[2051).If the reaction is carried
out in a potassium melt, then cyclotrimerization directly follows to form K,C,O,. Such cyclization reactions are well
known with the alkynes.
Fig. 39 The structureof[Nal(tmeda)],
(heterocubane).
9. Solvated Alkali Metal Halides
Gas-phase alkali metal halide homo-complexes[2061like
(NaCI),, ( t i = 7-4) have been known for a long time. Their
structures usually represent sections of the crystal lattice, for
example the Na,CI, heterocubane, and they can be stabilized
by solvation. Appropriate bases containing N and 0 atoms
can form adducts, a few of which are even soluble in hydrocarbon solvents. The most commonly known complexes are
those formed from crown ethers, cryptands. and calixarenes
and are usually solvent-separated ion pairs.[2071
In contrast, simple adducts of the alkali metal halides with
amines. and particularly those with chelating amines, remain
less well known, even though they display special propertiesi20x1and despite the fact that their structure investigations were conducted as far back as the 1 9 6 0 ~ . [ ”Since
~ ~ the
small Li ion is easily solvated. most of the investigations have
been performed with Li derivatives. Adducts of the heavier
alkali metals. and especially their chlorides, are not very
soluble in amines. Apparently their solvation enthalpies and
entropies are often not sufficient to overcome the lattice
energy: many adducts cannot be synthesized by simply mixing the salts with the donor ligands. They are formed more
easily when the metal halides are as dry and as finely dispersed as possible. These conditions sometimes exist in the
preparation of organo alkali metal compounds in the presence of halide sources. This explains the often unexpected
formation of solvated alkali metal halides during the synthesis of organometallic compounds. Thus, solvated lithium
complexes having both carbanions and halide ions
in the same aggregate have been obtained. The compounds [Li,(~yclopropyl),Br,(Et,O)~]~~’~~
and [Li,Ph,Br(Et20),3].r21 which crystallize as heterocubanes. are examples.
Syntheses aimed at preparing water-free solvent adducts
of lithium halides of the form [LiX.rL], are based on the
reactions of riBuLi with T ? B u X / L [ ~o’r~ reactions
]
of nBuLi
(or LiH) with solid ammonium halides in the presence of
bases L.12131We have been able to prepare sodium compounds analogously by using nBuNa and RX/L.
Fig. 40. The structures of [NaI(pmdta)], (dimer) aod [Nal(tmeda),],,.A diiner
structure very similar t o that of [NaI(pmdta)], is found as it component i i i
the crystal of the mixed aggregate (NaI(pmdta)]i[Li(pindta)(OH~)l~~
[ZlX]. In
~.ure,?u-~~-I[Na(tmeda)~]
the Na ions have distortcd octahedral coordination
ment those described in the literature, and in many cases the
compounds were examined independently by several research groups. A surprising variety of structures is also
found with these compounds; the most important types include monomers, rings (dimers, trimers). stacks (cubaneb,
double cubanes), and polymer chains.
10. Closing Remarks
The structures of organometallic compounds presented in
this review display an unexpected and fascinating variety
with the most diverse C, N, 0, and X ligands. Many of the
physical and chemical properties of these ion aggregates first
became understandable through their structure determinations. For example, the solubility of many organometallic
compounds is a consequence of their structure and not of
“covalent bonding”. The application of the structure information to the study of reaction mechanisms and to the directed control of reactions is still in its infancy.
The largest structural diversity is found with the lithium
compounds. Here, almost all of the important structure
1519
types occur: monomers, dimers (rings), trimers (rings), tetramers (heterocubanes and rings), hexamers (pseudooctahedra), decamers, dodecamers (stacked rings), and polymers
(chains. double chains). The heavier alkali metal ions allow
for a greater hapticity, whereby not only the formation of
two- and three-dimensional ion aggregates is promoted, but
also multihapto contacts to aromatic ring systems. The important structure types are presented in simplified form in
Figure 41 (Lewis-basic soivent molecules are excluded).
dimer (ring)
trimer (ring)
tetramer (ring)
C. N. 0, Hal
C, N. 0, Hal
C. N, 0
@
cubane
C. N. 0. Hal
double cubane
N. 0, Hal
stack of
four-membered rings
ladder, stairs
C, N
N
hexamers: prism and pseudooctahedron
N, 0
polymer (chain)
C.N, Hal
C. N
nonamer
0
Fig. 41. Important idealized coordination types for ion aggregates of the alkali
The pseudooctahedron can
metals ( 0 ) with C. N , 0. and halogen anions (0).
also be considered a distorted hexagonal prism. The anions for which each
coordination type has been found are listed for each case.
The organic compounds of the main group metals continue to be of tremendous interest. At the moment it seems that
a rapid development in the compounds of the heavier alkali
metals is taking place. Of the elements of the second main
group, magnesium remains at the focus of interest. When
will the decisive breakthrough occur for the heavier alkaline
earth metals?[' 19] Conditional here are substantial advances
in the further development of low-temperature techniques.
Good progress has been made in crystal structure analysis
which has enabled an increasing number of routine investigations on thermally labile substances.[2201Further surprises
in this field of study may certainly be expected.
M y special thanks go to the many co-workers and colleagues who have carried out or supported our work over the
past nearly three decades. Since their large number prevents
mentioning ail their names, I can only refer to the literature
citations. I Itjould, hotc'ever, like to thank especialiy Prof: Dr.
U. Behrens and Priv.-Doz. Dr. J. Kopf,for continuous and
valuable help, particularly with diyfkult crystd structure
analyses, and also Dr. J. Cockcroft,for the neutron difraction
investigations. I have had the pieasure of working with many
very dedicated and imaginative co-workers who have contributed substantially to the results presented here. I am grute,ful to the Deutsche Forsclzungsgemeinschaft and the Fonds der
Chemischen Industrie.
Received: April 6. 1993 [A923IE]
German version: Angew. Chem. 1993, 105, 1565
Translated by Dr. J. R. Kennedy, Stuttgart
1520
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11361 T. Greiser, J. Kopf. D. Thoennes, E. Weiss. C h n . Bw. 1981. 114. 209.
11371 See ref. [119].
I1381 D. Thoennes, E. Weiss. Chem. Ber. 1978, 111. 3726.
11391 H. Viehrock. E. Weiss, unpublished.
11401 9. Schuhert. E. Weiss. Chcm. Ber. 1984, 1f 7 , 366.
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11421 E. Weiss. H. Plass. J. Orgunornet. Chern. 1968, 14, 21.
[143] H. Alsdorf. E. Weiss. unpublished; H. Alsdorf, Dissertation. Hamburg,
1969. Single-crystal data: monoclinic P2,/c. (I =780(1). h = 683(1),
c = 1322(20) pm, fi =117.8(2); 2 = 4, 1005 reflections (film method).
R = 0.09 (without refinement of H atoms): Zn-C 207(1) pm, ZnC,
group is approximately trigonal planar.
11441 W. E. Rhine. G . D Stucky, S. W. Peterson. J Am. Chem. Soc. 1975. 97.
6401
11451 H. Staeghch. E. Weiss. unpublished; H. Stdeglich. Dissertation, Hamburg. 1976. Single-crystal data for K[BMe,]: orthorhomhic Phcm,
a = 858.6(8). h = 876.5(9), c = 925.3(7) pm (273K). Z = 4. refined anisotropically (without H atoms). R = 0.007, B - C 166 pm (average). Powder data: Na[BMe,]. u = 886(2), h = 843(2), c = 924(3); RbIBMe,]:
a = 886(2), b = 904(1). c = 928(2); Cs[BMe,]: u = 898(3), h = 936(3).
c = 942(4) pm.
11461 Ya. 0201. S. Vtmha. A. levins, Sov. Ph,w CrwfuUogr. 1962. 7, 289.
[147] Yd. Ozol, S. Vimba, A. levins, Lufv. PSR Zinat. Akad. Vestis Kim. Ser.
1975. 51 7.
11481 K. Hoffmann. E. Weiss, J. Orgunomet. Chem. 1974.67. 221.
[I491 R. Wolfrum. G . Sauermann, E. Weiss. J. Orgunomet. Chem. 1969. 18. 27.
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[I521 K. Hoffmann, E. Weiss. J. Orgunornet. Chem. 1972, 37. I .
I1531 K. Hoffmann, E. Weiss. J. Orgunomef. Chc,m. 1973, 50. 17.
11541 K. Hoffmann. E. Weiss, J. Orgunomet. Chem. 1973. 50, 25.
[I551 U. Joergens. E. Weiss. unpublished; U. Joergens, Dissertation, Hamburg.
1976. Single-crystal data: a) Na[TIMe,]: cubic (P23 or Pz3m).
u = 569.2(2), Z = l , isotypic to Li[InMe,], T I - C 226(4). N a - C
267(4) pm, refinement without H atoms and absorption correction,
R = 0.08: b) K[TIMe,]: tetragonal (M,;umdf, Z = 4, isotypic to
K[AIMe,]. u = 996.44(6), r = 805.4(9) pm; TI- C 231. K - C 352 and
310 pm. R = 0.06: c ) Na[TlPh,]: tetragonal (P32,c). u = 1202.7(20),
c = 682.6(8) pm. Z = 2, isotypic to Li[InPh,]. TI-C 226(4), R = 0.09.
11561 a ) G . Wittig. R. Ludwig. R. Polster, Chm7. Ber. 1955, 88. 294: h) G.
Wittig. F. Bickelhaupt, ibid. 1958. 91. 865; c) G. Wittig. E. Benz, ihId.
1958, 9I. 873.
11571 a) G. van Koten, J. G. Noltes in Comprehensive Orgunometullic Cherni.sfrj,, &I/. 4 (Eds.: G. Wilkinson). Pergamon, New York. 1982, p. 709:
b) P. P. Power, Prog. lnorg. Chem. 1991, 39. 75.
11581 J. F Malone, W S. McDonald, J. Chem. Soc. Dalton Trons. 1972, 2646.
I1591 a) Review: R. Juza. Angeic. Cheni. 1964, 76. 290; Angew. Cheni. In!. Ed.
Enpl. 1964.3. 471 ; h) LiNH, (new structure determination): H. Jacobs,
R. Juza, Z . Anorg. A//g. Chen?.1972,391, 271 ; c) NaNH,: R. Juza, H. H.
Weber. K. Opp. ihid. 1956.284.73; d) NaNH,: A. palkin, D. H. Templeton, J. PI??..,.
Chi~m.1956. 60,821; e) NaND, (neutron diffraction): M.
Nagih, H. Kistrup, H . Jacobs. Aroiiikernenergie 1975, 26, 87.
I1601 N. Kuhn, G. Henkel. J. Kreutzherg, Angen. Chem. 1990. 102, 1179;
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11631 See for example M. C. Carre, G. Ndeheka, A. Riondel, P. Bourgasser, P.
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11641 D. Barr, W. Clegg, R. E. Mulvey. R. Snaith. J. Wright. J. Chem. Soc.
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I1651 P. C. Andrews. W. Clegg. R. E. Mulvey. Angeu.. Chrm 1990. 102, 1480,
Angel! Chen?. I n [ . Ed Engl. 1990. 29. 1440.
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11671 N. P. Lorenzen. J. Kopf, F. Olhrich. U Schumann. E. Weiss. Angrti.
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[168] K. Gregory. M. Bremer. W. Bauer. P. von R. Schleyer. N. P. Lorenzen.
J Kopf. E. Weiss, O ~ g U ~ i ~ l / i f e l 1990.
u ~ ~ i c9.. ~1485.
I1691 R. Hacker. E. Kaufmdnn. P. von R. Schleyer. W Mahdt. H. Dietrich,
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11711 D C. Bradley. R. C. Mehrotrd, D. P. Gaur. MetalAlko.\-idf~,,Academic
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[I741 a) E. Weiss, %. Anorg. A&. Chem. 1964. 332. 197: b) E. Weiss, W.
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11751 E. Weiss. H e h Chin?. Aciu 1963. 46. 2051.
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11971 S Brooker. F. T. Edelmdnn. T. Kottke. H. W. Roesky. G. M. Sheldrick,
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[209] Set. for example a ) [LiCl(en)], [LiBr(en)]: F. Durant. P. Piret, M. van
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Jamet-Delcroix. J. Chiin. Piiw. P h j : ~ . Chbii. Bid. 1967, 64. 601 :
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[210] H Schmidbaur. A. Schier, U. Schubert, Cheni. Err. 1983, 116, 1938.
12111 H . Hope. P. P. Power. J. A m . Chern. Sot..1982, 105, 5320.
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1523
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