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The Way into the Bridge A New Bonding Mode of Tertiary Phosphanes Arsanes and Stibanes.

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Reviews
H. Werner
Bridging Ligands
The Way into the Bridge: A New Bonding Mode of
Tertiary Phosphanes, Arsanes, and Stibanes
Helmut Werner*
Keywords:
As ligands · bridging ligands · coordination chemistry · P ligands ·
Sb ligands
Dedicated to Professor Ernst Otto Fischer
on the occasion of his 85th birthday
Angewandte
Chemie
938
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200300627
Angew. Chem. Int. Ed. 2004, 43, 938 – 954
Angewandte
Chemie
Bridging Ligands
Until recently, tertiary phosphanes, arsanes, and stibanes
were considered to bind to transition-metal centers only in a
terminal coordination mode. Investigations on the reactivity
of square-planar trans-[RhCl(=CRR’)(L)2] compounds
revealed that compounds in which L = SbiPr3 can be
converted upon heating into dinuclear complexes [Rh2Cl2(mCRR’)2(m-SbiPr3)] with the carbene and stibane ligands in
bridging positions. Although attempts to replace the stibane
in these complexes with a tertiary arsane or phosphane
failed, substitution of the chloro ligands for acetylacetonates
followed by bridge–ligand exchange allowed the preparation
of the phosphane- and arsane-bridged compounds
[Rh2(acac)2(m-CRR’)2(m-PR3)] and [Rh2(acac)2(mCRR’)2(m-AsMe3)]. The acac ligands can be replaced by
anionic Lewis bases to give either monomeric [Rh2X2(mCRR’)2(m-ER3)] or dimeric chain-like [XRh(m-CRR’)2(mER3)Rh(m-X)2Rh(m-CRR’)2(m-ER3)RhX] molecules.
From the Contents
1. Introduction
939
2. The First Steps into a New Field
940
3. The Rich Chemistry of the Stibane-Bridged
Complexes
942
4. The Unexpected Entry into a Family of
Mixed-Valence Rhodium Complexes
944
5. The Breakthrough: Tertiary Phosphanes in
Semibridging and Doubly Bridging Positions
945
6. The Youngest Members of the {Rh(m-PR3)Rh}
Family
948
7. The Final Goal: Trimethylarsane as a
Semibridging and a Doubly Bridging Ligand
950
8. Theoretical Studies
951
9. Concluding Remarks
952
1. Introduction
Tertiary phosphanes PR3 with R = alkyl or aryl groups
belong, together with CO, to the most well-known ligands in
the chemistry of low-valent transition-metal complexes. With
regard to CO, not only numerous metal compounds with
terminal but also with doubly bridging carbonyl ligands have
been reported, in contrast it is generally accepted that tertiary
phosphanes (as well as tertiary arsanes and stibanes) behave
exclusively as terminal coordinated ligands.[1, 2]
In spite of this clear statement, there were some hints in
the literature that a doubly bridging bonding mode for
phosphane ligands could be achieved. By investigating the
chemistry, including the molecular structure, of platinum–
gold clusters, Braunstein and co-workers observed that in
compound 1 the AuPPh2 distances are relatively short (see
Scheme 1).[3] Owing to this structural feature, and by virtue of
the isolobal analogy between Au(PPh3) and H, the authors
predicted “that polynuclear systems containing a bridging
phosphane ligand might exist”.[3]
Shortly after this paper appeared, Pasquali, Pregosin et al.
described the molecular structure of the dinuclear palladium
complex 2, in which the tBu2PH···Pd unit has an agostic-like
interaction.[4] Although the PH···Pd hydrogen atom was not
found in the X-ray crystal structure analysis, one- and twodimensional NMR spectroscopy revealed different electronic
environments for the two phosphido ligands. Preliminary
calculations based on heteronuclear Overhauser experiments
suggested a PH distance of about 1.9 7, which places the
PH proton between the palladium and the bridging phosphide unit.[4, 5] Thus, 2 has a similar structural feature to that
Angew. Chem. Int. Ed. 2004, 43, 938 – 954
Scheme 1.
[*] Prof. Dr. H. Werner
Institut f r Anorganische Chemie der Universit$t W rzburg
Am Hubland, 97074 W rzburg (Germany)
Fax: (+ 49) 931-888-4623
E-mail: helmut.werner@mail.uni-wuerzburg.de
DOI: 10.1002/anie.200300627
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
939
Reviews
H. Werner
found in compound 1, which underlies the prediction made by
Braunstein and co-workers.
One year after the publication of the work by Pasquali,
Pregosin and co-workers, the group of van Leeuwen reported
the preparation and molecular structure of the palladium(i)
dimer 3, in which one phosphorus atom of each 1,3bis(diphenylphosphino)propane (dppp) ligand occupies an
asymmetrically bridging position.[6] The X-ray crystal-structure analysis also suggests an interaction between the ipsocarbon atom of a phenyl group at a bridging PPh2 moiety and
a palladium atom which, according to the 13C NMR spectrum,
persists in solution. As not only this agostic-like interaction
but also the bridging or side-on PdP bond is rather weak, the
dimer is easily split by donor groups such as CO or CH3CN to
give monomeric species.[6]
A compound somewhat related to 3 was described
recently by Kurosawa and co-workers.[7] During investigations
aimed to create at least two vacant coordination sites on a
PdI–PdI framework, these authors observed that the dication
[Pd2(CH3CN)4(PPh3)2]2+ easily loses two acetonitrile ligands
to give the stable dipalladium complex 4, in which each of the
two vacant sites is occupied by an ipso-carbon atom of a h1bound phenyl ring. Since according to the X-ray crystalstructure analysis there is a strong PdP bond to one metal
center and a weak PdP bond to the second, the triphenylphosphane can be formally considered as a four-electron
donor similar to the HPtBu2 unit in compound 2. For this
reason, the bonding scheme of the bridging phosphane
neither in 2 nor in 4 is comparable to that of a bridging CO
group in a {M(m-CO)M} moiety.
The nearest analogy between the two-electron donor
capabilities of CO and a molecule of the general composition
PR3 was found by Balch et al.[8] In attempting to obtain a
dinuclear palladium compound with {Pd(m-PF3)Pd} as the
core unit, these authors reported in 1990 the preparation and
structural characterization of a cationic Pd3 complex, 5, which
consists of a nearly equilateral triangle of palladium atoms
bridged at the edges by bis(diphenylphosphino)methane
ligands and capped by the triply bridging phosphorus atom
of PF3. This situation is reminiscent of that of various
carbonylmetal clusters, in which molecular building blocks
of the general type {M3(m-CO)} exist.[9] However, from a
comparison of complex 5 with the other compounds shown in
Helmut Werner was born in 1934 at Mhlhausen (Thringen) and studied chemistry
in Jena and Munich. He did his PhD with
E. O. Fischer and was a postdoctoral fellow
with J. H. Richards at Caltech in Pasadena.
After his Habilitation at the Technical University in Munich (1966), he joined the University of Zrich (1968). In 1975 he was
appointed Professor and Head of the Institute of Inorganic Chemistry at the University
of Wrzburg where he became Professor
Emeritus (2002). He has gained numerous
international awards and in 2001 received
an honorary doctorate from the University of
Zaragoza (Spain).
940
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Scheme 1, because of its strong p-acceptor character, PF3 can
be considered as an atypical phosphane as it is closer to CO
than to trialkyl- or triarylphosphanes.
2. The First Steps into a New Field
The detective story entitled “How to find a route to
generate a phosphane-bridge {M(m-PR3)M} as an analogue to
a {M(m-CO)M} bridge” began far away from the real topic.
After we had investigated in detail the chemistry of squareplanar vinylidene- and allenylidene-rhodium(i) complexes of
type B and C,[10] we also became interested in the preparation
of the corresponding rhodium carbenes A, which represent
the missing link in the series of Rh=C double-bond systems
A-B-C (see Figure 1). However, the reaction of the dimer
Figure 1. A series of square-planar rhodium(i) complexes containing
rhodium-carbon double bonds (L = PiPr3).
[{Rh(m-Cl)(PiPr3)2}2], used as the starting material for the
synthesis of compounds B and C, with either CH2N2,
PhCHN2, or Ph2CN2 did not lead to the formation of the
required carbene complexes of type A but gave instead olefinor diazoalkane-rhodium(i) derivatives.[11] Since we were
convinced, owing to the composition of the products and
their reactivity towards diazoalkanes, that rhodium carbenes
were involved in these reactions, we attempted to achieve the
original goal by varying the coordination sphere around the
rhodium(i) center.
The key to success was the use of triisopropylstibane,
instead of triisopropylphosphane, as the supporting (formally
innocent) ligand. Treatment of trans-[RhCl(C2H4)(SbiPr3)2]
(6) with Ph2CN2, other diaryldiazoalkanes, or with
PhC(CF3)N2 afforded complexes of type A (L = SbiPr3) in
good to excellent yields.[12] Although these compounds are
quite stable and can even be handled in air for short periods of
time, they are highly reactive and undergo a variety of ligand
substitution reactions with either neutral or anionic Lewis
bases. As illustrated in Scheme 2, which uses compound 7 as
an example, it is possible to exchange the chloride, the
stibane, or both for other donor ligands without breaking the
metal–carbene bond.[12, 13]
However, the most interesting reaction of 7 was discovered when we investigated the thermal behavior of the
compound. We observed that slightly below the decomposition point of 61 8C, the compound changed from dark green to
red. A repetition of the process in benzene at 60 8C led to the
formation of the dinuclear complex 13 (Scheme 3) which,
after chromatographic workup, was isolated in 81 % yield.
While the square-planar precursors 11 and 12 behaved
similarly and afforded in nearly quantitative yields compounds 14 and 15, respectively, under the same conditions the
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Angewandte
Chemie
Bridging Ligands
Scheme 2.
Scheme 3.
tions of the aryl groups are present. Since the intensity of the
resonances for the carbene carbon atoms of the three isomers
remain unchanged upon stirring a solution of 15 in C6D6 for
12 h at 25 8C, the conclusion is that under these conditions the
isomeric mixture is already at equilibrium.[14]
The X-ray crystal-structure analysis of 13 (Figure 3)
established the hitherto unknown capability of trialkylstibanes to behave as bridging ligands and to form two metal–
antimony bonds. Quite surprisingly, there is almost no
influence of the bridging function of the stibane on the
SbC bond lengths and, despite the increase in the coordination number of antimony from three to five, on the C-Sb-C
bond angles. The relatively short RhRh distance in 13
(2.5349(5) 7) is indicative of a direct metal–metal interaction.
Noteworthy are the larger RhSb distances in 13 (2.6868(5)
and 2.6695(5) 7) compared with those in 7 (2.5843(5) and
2.5633(5) 7), which is in agreement with the bridging function
of the stibane ligand. The arrangement of the Cl-Rh-Rh-Cl
moiety is nearly linear, the somewhat slight bending away
from the antimony atom being possibly due to the steric
repulsion between the chloro ligands and the bulky isopropyl
groups.[14]
Studies that concerned the ability of compound 7 to
undergo CC coupling reactions with various substrates
offered an alternative route to dinuclear rhodium complexes
with a bridging stibane ligand. While attempts to connect the
CPh2 unit of 7 with CH2 or CH(CO2Et) by using CH2N2 or
bis(phosphane) derivatives 8 a–c were inert and after a
solution of these complexes had been stirred in benzene
for two days, the starting material was recovered
unchanged. Since during the thermal reactions of 7, 11,
and 12 the formation of free triisopropylstibane was
observed, we originally thought that one SbiPr3 ligand
from the precursors had been eliminated and two of the
resulting 14-electron species [RhCl(=CRR’)(SbiPr3)] had
formed a dimer with either chloro or diarylcarbene
bridges. Although this does not occur, it is nevertheless
conceivable that the supposed dimer [RhCl(=CRR’)(SbiPr3)]2 with two bridging carbene ligands is
generated as an intermediate from which subsequent loss
Figure 3. Molecular structure (SCHAKAL representation) of compound 13 (hydrogen
of one stibane affords the final product.[14]
atoms omitted for clarity).
The dinuclear compounds 13–15 are red, slightly airsensitive solids that, remarkably, decompose only above
180 8C. With regard to the spectroscopic data, a characteristic feature is that, in contrast to those of 7, the phenyl
CH(CO2Et)N2 as a carbene source did not generate the
groups of complex 13 are spectroscopically nonequivalent.
corresponding olefin but gave a mixture of products, treatMoreover, the 13C NMR spectrum of the less symmetrical
ment of 7 with (p-Tol)2CN2 or PhC(p-Tol)N2 under the same
complex 15 reveals that all three possible isomers syn-syn,
conditions led to the formation of the more unsymmetrical
syn-anti, and anti-anti (see Figure 2) with different orientadirhodium complexes 16 and 17 in about 75 % yield
(Scheme 4). Compounds 16 and 17 are also accessible by
conproportionation of equimolar amounts of 13 and 14, or of
13 and 15, respectively. It is remarkable that although
complexes 13–15 are probably very similar in structure and
bonding, after a solution of 13 and 14 had been stirred in
diethyl ether for 5 h at room temperature, instead of an
equilibrium mixture of 13, 14, and 16 in the ratio of 1:1:2, the
Figure 2. Newman projections of the configurational isomers of comless symmetrical product 16 was obtained nearly quantitapound 15 viewed along the Cl-Rh-Rh-Cl axis.
Angew. Chem. Int. Ed. 2004, 43, 938 – 954
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
941
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H. Werner
Scheme 4.
Scheme 5.
tively. Moreover, in contrast to 7 the dinuclear complex 13 is
inert toward (p-Tol)2CN2 and does not generate either 14 or
one of the olefins R2C=CR02 (R, R’ = Ph, p-Tol).[14]
rhodium atoms. In the case of the symmetrical complexes 13,
18, and 19 the corresponding signal is a triplet.
The X-ray crystal structure analysis of 20 revealed that
owing to the asymmetry of the dinuclear molecule, the stibane
and one of the diphenylcarbene ligands are linked to the two
rhodium centers in an unsymmetrical fashion.[16] The bond
lengths SbRh1 and SbRh2 differ by about 0.35 7 and those
between one of the carbene carbon atoms and Rh1, and
between C2 and Rh2 differ by approximately the same value.
In 20 the SbRh1 bond is about 0.19 7 shorter and the
SbRh2 about 0.18 7 longer than the corresponding bonds
found in the starting material 13; the longer SbRh bond
being directed to the lower-coordinated metal center. It is also
noteworthy that the acetylacetonate is coordinated in an
unsymmetrical mode, such that the shorter RhO bond is
opposite to the shorter RhC(carbene) bond. The RhRh
distance in 20 is about 0.15 7 longer than that in 13, but the
value still lies within the range of distances of other dinuclear
rhodium(i) complexes with a metal–metal bond.[17]
From the reaction of 13 with an excess of either M(acac)
or M(acac-f3) (M = Na, Tl) the bis(acetylacetonato) compounds 23 or 24 were obtained in excellent yields.[15, 16] If
under the same conditions the reaction of 13 or 22 was carried
out with Na(dpm), a mixture of products was isolated with 25
as the dominating species. While it was possible to separate 25
from 22 and other by-products by column chromatography,
attempts to remove small amounts of Na(dpm), used in
excess, failed. Therefore, compound 25 was characterized
spectroscopically. Quite remarkably, the symmetrical complexes 23 and 24 are thermally significantly less stable than
the unsymmetrical counterparts 20 and 21, which might be
because of the steric requirements imposed by the chelating
ligands (see Section 5). In this context it is worth mentioning
that all attempts to obtain dirhodium compounds with a
{Rh(m-CR2)2(m-SbiPr3)} framework and one or two hexafluorinated acetylacetonates remained unsuccessful.
3. The Rich Chemistry of the Stibane-Bridged
Complexes
Since 13 is a 26-electron compound and appears not only
electronically but also coordinatively unsaturated, it is not
surprising that it is highly reactive toward Lewis bases. While
attempts to convert the mononuclear precursors trans[RhX(=CPh2)(SbiPr3)2] (X = Br, I) to the corresponding
dinuclear complexes 18 and 19 remained unsuccessful, the
reactions of 13 with an excess of NaBr or NaI in acetone
affords the required dibromo and diiodo derivatives 18 and 19
in virtually quantitative yield (Scheme 5).[15] The 1H and
13
C NMR spectra of 18 and 19 are quite similar to those of the
dichloro counterpart 13 and thus there is no doubt that the
proposed structure is correct. Attempts to replace the chloro
ligands in 13 with fluoride, by using either NaF or CsF as
fluoride source, failed.
The reactions of 13 with equimolar amounts of the sodium
salts of acetylacetone (acacH), trifluoroacetylacetone (acacf3-H), and dipivaloylmethane (dpmH) led to the formation of
the unsymmetrical dinuclear rhodium(i) compounds 20–22 in
92–95 % yield of the isolated products.[15] Although in our
initial studies we used Tl(acac) as the acac source, [16] for the
preparation of 20, Na(acac) is the preferred substrate. The
bis(di-p-tolylcarbene) complex [Rh2(acac)2{m-C(p-Tol)2}2(mSbiPr3)] is also accessible by using the same route. With
respect to the spectroscopic data of 20–22, the remarkable
feature is that the resonance for the 13C nuclei of the bridging
carbene atoms appears in the 13C NMR spectra of 20 and 21 as
a multiplet but in the spectrum of 22 as a doublet of doublets
owing to the coupling of this carbon atom to two different
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angewandte
Chemie
Bridging Ligands
cally identical to that of the dichloro analogue 13, the RhSb
bond lengths in 29 (2.6902(2) and 2.7126(2) 7) are slightly
longer than in 13. The fact that most of the structural data for
the core units {Rh2(m-CPh2)2(m-SbiPr3)} of 13 and 29 are quite
similar is noteworthy insofar as the coordination number of
the rhodium atoms in the two molecules is different. It should
also be emphasized that while in the
bis(acac) complexes [Rh2(k2-acac)2(mCPh2)2(m-CO)] and [Rh2(k2-acac)2(mCPh2)2(m-PMe3)] (Section 5) the two
planes containing the chelating systems
are twisted (like a propeller), the two
rhodium atoms and the four oxygen
atoms of 29 lie in the same plane.
However, even more remarkable
than the smooth exchange of the anionic
units in 13 and 23 is the fact that the
bridging stibane ligand of these compounds can be substituted by a variety of
Lewis bases without breaking the {Rh(mCPh2)Rh} bridges. Whereas the dichloro
derivative 13 is inert towards ethene and
terminal alkynes, it reacts with CO in
Scheme 6.
benzene at room temperature to afford a
brick-red solid that is practically insoluble in all common organic solvents. Since the elemental
amount of acetic acid led to a mixture of 27 (ca. 80 %), 29 (ca.
analysis of the solid is consistent with the composition
10 %), and the starting material 23 (ca. 10 %), which could not
[RhCl(CPh2)(CO)], we assume that the (presumably polybe separated. However, compound 23 reacted unequivocally
with an excess of CH3CO2H to give the bis(acetato) complex
meric) product is identical to a compound obtained by
Sonogashira and co-workers from [{Rh(m-Cl)(CO)2}2] and
29 in 85 % yield. The {Rh2(O2CCF3)2} analogue, 28, which had
also been prepared from 13 and TlO2CCF3,[16] was obtained in
diphenylketene.[18] Although it is conceivable, as suggested by
the same way. From the IR data, it can be seen that the
the Sonogashira group, that two CO groups bridges two
trifluoroacetate behaves as a monodentate ligand in comadjacent rhodium centers, we think that the structure 31
pound 26, whereas it is coordinated in a chelating fashion in
shown in Scheme 7 is more likely. An argument in favor of
28.
this proposal is that 31 reacts with NaC5H5 to give [(h5The bidentate coordination mode of the acetate units in
C5H5)2Rh2(m-CPh2)2(m-CO)] and with pyridine to afford
complex 29 was confirmed crystallographically (Figure 4).[15]
[Rh2Cl2(py)2(m-CPh2)2(m-CO)], respectively.[17a]
The coordination geometry around each of the rhodium
In contrast to 31, the product 32 obtained from the
centers can be best described as distorted square-pyramidal
bis(acac) complex 23 and CO is monomeric and this has been
with the antimony atom in the apical position. The respective
substantiated by an X-ray crystal-structure analysis.[16b] In a
metal atom lies above the plane formed by the oxygen atoms
similar way to 29, the coordination geometry around the
and the carbene carbon atoms of the bridging CPh2 ligands.
rhodium centers is best described as square-pyramidal with
the carbene carbon atom in the apical position. The molecule
Whereas the Rh1Rh2 distance (2.5429(3) 7) in 29 is practiis highly symmetric and in contrast
to 20 also the two acetylacetonato
ligands are symmetrically linked to
the metal centers. The bond length
RhCO in 32 is slightly shorter
than the bond lengths between
rhodium and the carbene carbon
atoms which is in agreement with
the data for the above-mentioned
compounds
[(h5-C5H5)2Rh2(mCPh2)2(m-CO)]
and
[Rh2Cl2(py)2(m-CPh2)2(m-CO)].
With tBuNC, compounds 13
and 23 react in an analogous fashion and afford the two isocyanideFigure 4. Molecular structure (SCHAKAL representation), including a side-view, of compound 29
bridged complexes 34 and 35 in
(hydrogen atoms omitted for clarity).
Not only 13 but also the bis(acac) complex 23 has been
used as a starting material for the preparation of new
dinuclear rhodium(i) derivatives with a bridging SbiPr3
ligand. Treatment of 23 with CF3CO2H in the molar ratio of
1:1.1 gave the compound 26 in almost quantitative yield
(Scheme 6). In contrast, the reaction of 23 with an equimolar
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H. Werner
decomposes at 242 8C! The mono(acac) and bis(acac) derivatives 20 and 23 behave in a similar way as 13 and upon
treatment with SbEt3 or Sb(CH2Ph)3 form the products 39–42
in excellent yields.[15, 16] We note, however, that while it is easy
to substitute the bridging SbiPr3 ligand in 13, 20, and 23 for
trialkylstibanes, all attempts to prepare a dinuclear rhodium
complex with SbPh3 in the bridging position failed.
4. The Unexpected Entry into a Family of MixedValence Rhodium Complexes
Scheme 7.
nearly quantitative yields. A clean and quick bridge–ligand
exchange also occurs between 13 and SbMe3, SbEt3, and
Sb(CH2Ph)3 to give the dirhodium counterparts 36–38
(Scheme 8).[14, 15] Quite remarkably, the SbMe3-bridged compound is even more stable than the precursor 13 and
Scheme 8.
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Although we were convinced, from the time when we had
isolated the first stibane-bridged compound 13, that it could
be possible to prepare an analogue of 13 with {Rh(m-CPh2)2(mPR3)Rh} as the central core, the way to achieve this goal was
paved not only with several set-backs but also with a big
surprise. Since we speculated that, owing to the stability and
the ease of formation of 13, PiPr3 could be similar in its
coordination capabilities to SbiPr3, we first considered
triisopropylphosphane as an appropriate candidate for the
formation of a {Rh(m-PR3)Rh} bridge. However, the reaction
of 13 with one equivalent of PiPr3 gave the square-planar
carbene complex 8 a in about 25 % yield. With four equivalents of the phosphane, the mononuclear product was
obtained nearly quantitatively. Other tertiary phosphanes
such as PPh3 or PMePh2 behaved similarly (see Scheme 8).[14]
In contrast to 13, the bis(acac) compound 23 reacts with an
excess of PiPr3 much more slowly and affords, after four hours
at 60 8C in benzene, the novel dinuclear complex 43 in 68 %
yield of the isolated product (Scheme 9).[16] The reactions of
23 with PiPr2Ph, PiPrPh2, and PPh3 proceed analogously and
gave, after reaction times of 8, 24 and 48 h, respectively, the
related compounds 44–46 in equally good yields. By using the
same methodology, the bis(di-p-tolylcarbene) complex
[(PiPr3)Rh{m-C(p-Tol)2}2Rh(acac)2] is obtained from 30 and
excess triisopropylphosphane. Since the 1H and 13C NMR
spectra of 43–46 displayed two sets of signals for the protons
and carbon atoms of the OC(CH3) moieties of the acac
ligands, it was apparent that an unsymmetrical environment
around the central [Rh(m-CPh2)2Rh] core should exist.
This proposal was confirmed by the X-ray crystal-structure analysis of 43. Evidently, one of the chelating ligands has
migrated from one metal center to the other and its former
position is occupied by the phosphane. The stibane ligand has
been replaced, while the bridging diphenylcarbene units are
maintained. Not unexpectedly, the distances between the less
coordinated rhodium atom and the carbene carbon atoms are
shorter than the others. Moreover, in analogy to the mixed
{Rh2Cl(acac)} complex 20, the acetylacetonato ligands are not
coordinated in a symmetrical fashion and, therefore, in both
six-membered chelate rings the RhO bond lengths differ by
0.08 to 0.10 7. The unsymmetrical structure is, without any
doubt, also maintained in solution since the 31P NMR
spectrum of 43 displays a sharp doublet of doublets with a
large 1J(103Rh,31P) and a small 2J(103Rh,31P) coupling constant.[16]
However, the novel mixed-valence compounds are not
only interesting from a structural point of view but also with
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Bridging Ligands
5. The Breakthrough: Tertiary Phosphanes in
Semibridging and Doubly Bridging
Positions
Scheme 9.
respect to their reactivity toward Lewis bases. With the PiPr3
derivative as the starting material the corresponding dinuclear monoalkyne, bis(isocyanide) and tricarbonyl complexes
47 a–d, 48 and 49 can be obtained in good to excellent yields
(Scheme 9). As the X-ray crystal structure analysis of 47 a
revealed, the RhRh distance in 47 a is about 0.03 7 shorter
than in 43, while the RhC(carbene) and RhO bond lengths
differ only slightly to those of the PiPr3 counterpart. The axis
of the alkyne carbon atoms lies perpendicular to the Rh-C-CRh plane with the CH2OMe fragment bent away from the
nearby rhodium center. Notably, all attempts to rearrange the
coordinated alkyne to an isomeric vinylidene ligand either by
heating or photolyzing solutions of 47 a or 47 b failed.[16]
The reactions of 43 with CNtBu and CO did not only lead
to a displacement of the phosphane but also to an increase of
the coordination number at the low-valent rhodium center. It
is thus evident that the coordination sphere around Rh0 can
change and, depending on the size of the ligands, is quite
flexible. As confirmed crystallographically, in the bis(isocyanide) complex 48 the low-valent metal center is coordinated
in a considerably distorted square-planar fashion with C-RhC bond angles that deviate significantly from the ideal 908
value.[16] The coordination geometry around the higher-valent
metal center is distorted octahedral and quite similar to that
found in 43 and 47 a. A further increase in the coordination
number at Rh0 occurs if PiPr3 is replaced by CO as is
substantiated by the appearance of three CO stretching
modes in the IR spectrum of 49. This increase in coordination
number is accompanied by lability, as indicated by the
observation that compound 49 loses smoothly in solution
one carbonyl ligand and thus it can be stored unchanged only
under a CO atmosphere.
Angew. Chem. Int. Ed. 2004, 43, 938 – 954
All attempts undertaken between 1994 and 1999
to replace the bridging stibane in compounds such as
13 or 23 by a tertiary phosphane remained unsuccessful. The breakthrough occurred when we used
the unsymmetrical complex 20 as the starting material and the smaller trimethylphosphane as the
substrate, instead of the more bulky phosphanes
that were essential in our work on metallacumulenes.[10, 19] The treatment of a solution of 20 in either
pentane/diethyl ether or in dichloromethane at
78 8C with an equimolar amount of PMe3 gave the
substitution product 50 as a red-brown, slightly airsensitive solid in about 85 % yield of the isolated
product (Scheme 10). The 31P NMR spectrum of 50
displayed a doublet of doublets with two relatively
large 1J(103Rh,31P) coupling constants, thus indicating
that the phosphane is not linked to one of the metal
centers in a terminal mode.[20]
The X-ray crystal structure analysis of 50 established that the position of the PMe3 ligand can be best
described as semibridging (see Figure 5).[21] Apart
from the different bond lengths (Rh1P
(2.2406(15) 7) and Rh2P (2.8410(14) 7), characteristic features are in particular the Rh-Rh-P bond angles,
which are significantly smaller than 908. Because of the higher
coordination number of Rh1 compared with Rh2, not only the
phosphane but also the two diphenylcarbene ligands are
linked to the metal centers in an unsymmetrical fashion. The
bond lengths Rh2C1 and Rh2C2 are about 0.11 7 shorter
than those from Rh1C1 and Rh1C2, quite similar to the
situation found for the stibane-bridged compound 20. How-
Scheme 10.
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The coordination geometry of both rhodium
centers corresponds to a distorted square pyramid,
the difference being that for the polyhedron
around Rh1 the phosphorus atom and for the
polyhedron around Rh2 the carbene carbon atom
C1 is in the apical position. The two planar sixmembered rings formed by the metals and the acac
ligands lie not in the same plane but are twisted;
the dihedral angle between the two planes is 30.28.
In contrast with Rh1O1 and Rh1O2, the bond
lengths Rh2O3 and Rh2O4 are not the same, a
fact which reflects the loss of symmetry in the
Figure 5. Molecular structure (SCHAKAL representation), including a side-view, of compound
crystal.
50 (hydrogen atoms omitted for clarity).
Compound 51 is highly reactive toward carbon
monoxide and upon treatment in benzene affords
the dinuclear complex 52 in nearly quantitative
yield.[22] The shift of the PMe3 ligand from a bridging to a
ever, in contrast to 20 the acetylacetonate unit in 50 is
coordinated in a symmetrical mode. The three PC distances
terminal position is indicated by the splitting of the 31P NMR
are nearly the same and lie in the range of rhodium(i)
resonance, which is a doublet of doublets with one large and
compounds with a terminal PMe3 ligand. While the Rh1Rh2
one small J(103Rh,31P) coupling constant. The appearance of a
bond length of 2.5318(8) 7 is practically identical to that in
n(CO) stretching mode at ñ = 1829 cm1 in the IR spectrum of
13, it is significantly shorter than in the SbiPr3 counterpart 20.
52 leaves no doubt that the carbonyl ligand occupies a doubly
bridging position.
Compound 50 reacts not only with Tl(acac)[20] but also
Compounds 21 and 24 that have one or two acac-f3 ligands
with Na(acac) in the molar ratio of 1:4 at room temperature
by replacement of the chloro for the acetylacetonato ligand to
coordinated to the rhodium centers (instead of one or two
give the dinuclear complex 51 (see Scheme 10). An alteracac ligands), behave similarly to 20 and 23, and react with an
native procedure to obtain 51 involves the substitution of
equimolar amount of PMe3 to give the dinuclear complexes 53
SbiPr3 in compound 23 for PMe3 ; in both cases the yields are
and 54 (Scheme 11). Also in this case, the unsymmetrical
compound 53 is considerably more stable than the symexcellent.[22] As has already been observed for 20 and 23, the
metrical counterpart. In agreement with the different bonding
symmetrical complex 51 is thermally considerably less stable
mode (semibridging versus doubly bridging), the 31P NMR
than the unsymmetrical counterpart 50. Regarding the
spectroscopic data of 51, it is important to note that, in
contrast to 50, the 31P NMR spectrum does not display a
doublet of doublets but a sharp triplet, thus illustrating that in
solution the phosphane ligand is coordinated to both rhodium
centers in an identical mode.
The result of the X-ray crystal structure analysis of 51 is
shown in Figure 6. Although the two RhP bond lengths are
not exactly the same, the difference of about 0.3 7 is only half
of that in 50. Since both the 31P and the 13C NMR spectra
suggest that the core unit {Rh2(m-CPh2)2(m-PMe3)} in 51 is
symmetrical, it is conceivable that the deviation from the
ideal symmetry for the sterically hindered molecule found in
the crystal is due to steric reasons or simply packing effects.
Scheme 11.
Figure 6. Molecular structure (SCHAKAL representation), including a side-view, of compound
51 (hydrogen atoms omitted for clarity).
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spectrum of 53 displays a doublet of doublets
while that of 54 shows a sharp triplet with a
1 103
J( Rh,31P) coupling constant that is virtually
identical to that of 51.
In contrast to PMe3, the more bulky triisopropylphosphane reacts with 20 to give a mixture
of products, among which a species with a
bridging PiPr3 unit could not be detected. Thus,
we decided to study the reactivity of those
phosphanes toward 20 that, owing to Tolman's
cone angles,[23] are of a size between PMe3 and
PiPr3. Treatment of 20 with PEt3 under the same
conditions as used for the preparation of 50
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Bridging Ligands
affords the analogous dinuclear compound 55 as a dark brown
solid in 81 % yield of the isolated product (Scheme 12).[20] The
somewhat larger PnBu3 behaves similarly, although in this
case besides the required complex 56, some by-products were
also obtained. Both the 31P NMR spectra of 55 and 56 display
Scheme 12.
the expected doublet of doublets and thus there is no doubt
that the PEt3 as well as the PnBu3 ligand occupies a
semibridging position.
An interesting situation arises if
the symmetrical complex 23 is treated
with PEt3 or PnBu3. If the reaction of
23 with PEt3 is carried out in diethyl
ether at 30 8C to 0 8C, after lowtemperature crystallization from Et2O
the symmetrical phosphane-bridged
complex 57 is isolated in 61 % yield
(see Scheme 12). The reaction of 23
with PnBu3, under exactly the same
conditions, affords a mixture of products, among which the analogue of 57
is the dominating species. If the reaction of 23 with PnBu3 in C6D6 in an
NMR tube is monitored by NMR
spectroscopy and not stopped after
[Rh2(acac)2(m-CPh2)2(m-PnBu3)] has
been generated, a subsequent slow
Scheme 13.
rearrangement to isomer 61 can be
observed. On a preparative scale, this
complex is obtained from 23 with a
twofold excess of PnBu3 in benzene at room temperature in
nearly quantitative yield. The PEt3 analogue, 59, is accessible
either by using the same route or by stirring a solution of 57 in
benzene for 15 h at 25 8C. The 31P NMR spectra of the mixedvalence compounds 59 and 61 display the typical doublet of
doublet resonance, with one resonance being very large (265–
Angew. Chem. Int. Ed. 2004, 43, 938 – 954
270 Hz) and one very small (5 Hz) J(103Rh,31P) coupling
constant.
The dominating role, played by the size of the reacting
phosphane, is also evident in the reactions of 23 with PMe2Ph
and PMePh2. While the smaller ligand PMe2Ph generates
exclusively the phosphane-bridged complex 58, the more
bulky ligand PMePh2 produces the unsymmetrical compound
60 in high yield. In contrast to the rearrangement of 57 to 59,
attempts to convert 58 to the corresponding Rh0–RhII isomer
failed.
To get an insight into the mechanism of the isomerization
of the PR3-bridged complexes to the mixed-valence counterparts, the kinetics of the conversion of 57 to 59 were
investigated. In C6D6 at 293.5 K the reaction is strictly first
order with a Gibbs free energy of activation DG* =
94.5(5) kJ mol1. Thus one can assume that an intramolecular
rearrangement occurs and that the migration of the phosphane from the bridging into a terminal position is accompanied by the migration of an acac ligand from one metal
center to the next.
The reactivity of the phosphane-bridged {Rh2(acac)2}
compound 51 towards Brønsted acids HX, such as
CH3CO2H, CF3CO2H, and phenol, is quite similar to that of
the stibane-bridged analogue 13 (see Scheme 6). Independent
of whether only one acac ligand or both are replaced by the
anion of the acid, the core unit {Rh2(m-CPh2)2(m-PMe3)}
remains unchanged. While even with an excess of phenol the
substitution of the remaining acac unit in 62 could not be
achieved, the starting material 51 reacts cleanly with excess
acetic or trifluoroacetic acid to give the symmetrical complexes 64 and 65 in excellent yields (Scheme 13). In agreement with the proposed structure, the 31P NMR spectra of 64
and 65 show in each case a sharp triplet with coupling
constants that are very similar to those of 51. The fact, that the
1 103
J( Rh,31P) values for the 31P NMR resonances (doublets of
doublets) for 62 and 63 differ only slightly, supports the
proposal that in 63 the trifluoroacetate is coordinated as a
monodentate ligand.
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H. Werner
6. The Youngest Members of the {Rh(m-PR3)Rh}
Family
After the X-ray structure analysis of 51 revealed that in
the crystal the two acetylacetonates are not symmetrically
arranged to each other, thus leading to the conclusion that this
asymmetry could be the reason for the difference in the bond
lengths between the two rhodium atoms and the phosphorus
atom of the bridging trimethylphosphane, attempts were
made to reconvert the {Rh(acac)} to {RhX} units (X = Cl, Br,
I). This change of the terminal ligands should not only reduce
the coordination number but also the steric crowding around
the metal centers.
Treatment of 51 with an excess of Me3SiCl in benzene at
room temperature led indeed to a smooth replacement of the
acac ligands with chloride and gave a red solid, 66, correctly
analyzed as [Rh2Cl2(CPh2)2(PMe3)], in 91 % yield.[24] If the
reaction of 51 with Me3SiCl was carried out in the molar ratio
of 1:1, the unsymmetrical complex 50, originally prepared
from 20 by bridge-ligand exchange (see Scheme 10), was
obtained. While we anticipated, owing to the 1H and
13
C NMR spectra, that compound 66 has a structure analogous to the stibane-bridged complex 13, the X-ray crystal
structure analysis showed that in the lattice two dinuclear
moieties are connected by two bridging chlorides to give a
Rh4 species with a chain-like {ClRh2(mCl)2Rh2Cl} core
(Figure 7). Moreover, the midpoint of the planar {Rh(mCl)2Rh} unit is a center of symmetry. The coordination
geometry around Rh1 is distorted tetrahedral, whereas that
around Rh2 is best described as square-pyramidal with the
phosphorus atom in the apical position. The structure of the
fragment {ClRh(m-CPh2)2(m-PMe3)RhCl2} is thus similar to
that of the unsymmetrical molecule 50. Besides the RhRh
bond length of about 2.50 7 (there are two independent
molecules in the unit cell), which differs only slightly to that of
the stibane-bridged compound 13, the most important
structural features of 66 are the RhP bond lengths; they
are 2.3625(6) and 2.4826(6) 7 in the first molecule (Figure 7)
and 2.3890(6) and 2.4173(6) 7 in the second molecule. Most
noteworthy, the difference between the two RhP bond
lenghts in each case is much less than for the bis(acac)
complex 51, thus indicating that—at least in
the crystal lattice—the type of anionic ligands
bonded to rhodium influences significantly
the position of the bridging phosphane unit.
Because of the similarity of the RhRh and
RhP interatomic distances, the bond angles
of the Rh2P triangle are nearly the same and
deviate only marginally from the 608 value.
Although cryoscopic measurements with a
saturated solution of 66 in benzene confirm
that under these conditions the tetranuclear
compound is present, the 31P NMR spectrum
of 66 in C6D6 at room temperature is concentration-dependent. The spectrum of a nearly
saturated solution (4 mmol L1) exhibits a
somewhat broadened triplet at d =
24.6 ppm, which, after lowering the concentration to 0.1 mmol L1, transforms into a
Scheme 14.
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 7. Molecular structure (SCHAKAL representation), including a
side-view, of compound 66 (hydrogen atoms omitted for clarity).
sharp triplet with a chemical shift of d = 20.4 ppm. Since
the data in CD2Cl2 are quite similar, it is to assume that both
in benzene and dichloromethane a rapid equilibrium between
the Rh4 and the Rh2 species exists (see Scheme 14) and that at
low concentrations the Rh2 species 66’ dominates. At 80 8C
in [D8]toluene, the 31P NMR spectrum of 66 displays a doublet
of doublets at d = 30.4 ppm with J(103Rh,31P) coupling
constants of 128.4 and 95.4 Hz, thus indicating that under
these conditions the conversion of 66 to 66’ is inhibited on the
NMR time scale.[24]
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Bridging Ligands
The reactivity of the bis(acac) compound 51 with Me3SiBr
and Me3SiI is similar to that of 51 with Me3SiCl. With
equimolar amounts of the substrates, the unsymmetrical
complexes 67 and 68 are formed and isolated as deeply
colored, moderately air-stable solids in nearly quantitative
yields (Scheme 15). An alternative procedure consists of the
Scheme 15.
spectrum of 72 remains unchanged by lowering the temperature to 193 K, the corresponding spectrum of 71 changes and
at 193 K shows together with the triplet at d = 17.5 ppm a
doublet of doublets at d = 31.8 ppm. If we take the data
reported for 66 and 66’ into consideration, the appearance of
these two signals indicate that in the case of 71 at low
temperature, in addition to the presence of
dinuclear compound 71, the tetranuclear species
[BrRh(m-CPh2)2(m-PMe3)Rh(m-Br)2Rh(m-CPh2)2(m-PMe3)RhBr] is present. At 193 K, the equilibrium between the monomer and the dimer is
evidently very slow on the NMR time scale, thus
both the Rh2 and the Rh4 complexes can be
observed.
The assumption that the PMe3 ligand in 72
indeed occupies a truly symmetrical doubly bridging position was confirmed by an X-ray crystalstructure analysis.[27] As shown in Figure 8, the
molecule contains a C2 axis passing through the
phosphorus atom and the midpoint of the RhRh
bond. Therefore, the structure is quite similar to
that of the stibane-bridged compound 13. The IRh-Rh-I axis in 72 is nearly linear with bond
angles
Rh1-Rh1A-I1A = Rh1A-Rh1-I1 =
174.37(2)8. The RhP bond lengths are
2.412(3) 7 and thus lie exactly between those of
the bis(acac) derivative 51 (2.2707(7) and
2.5700(8) 7).
The reactions of 57 and 58 with an excess of
Me3SiCl also led to the displacement of the acac
ligands by chloride to afford the compounds 73
and 74 in excellent yields. Surprisingly, the
31
P NMR spectrum of 73 displays a sharp triplet,
which does not change either at higher concentrations of the
solution or at lowering the temperature to 193 K. Therefore, it
is assumed that in contrast to the PMe3-counterpart 66, under
the experimental conditions, only the dinuclear complex 73
exists. The 31P NMR spectrum of 74 shows at room temperature a broadened triplet at d = 17.0 ppm with
1 103
J( Rh,31P) = 101.7 Hz, which at 193 K becomes a doublet
of doublets with J(103Rh,31P) coupling constants of 139.8 and
68.6 Hz.[25] These data clearly indicate that in the case of the
PMe2Ph-bridged compound the dimer 74 dominates at low
temperature, whereas at room temperature the dimer is in
reaction of 50 with NaBr or NaI, which equally affords 67 and
68 in good to excellent yields.[25] Both 67 and 68 are thermally
less stable than 50 but can be stored at room temperature
under argon for weeks. The same is true for the {Rh2Cl(acac)}
compounds 69 and 70, which were obtained from 57 or 58 and
Me3SiCl in the molar ratio of 1:1. The 31P NMR spectra of 67
and 68 display, similarly to the spectrum of 50, one doublet of
doublets with J(103Rh,31P) coupling constants that differ for 67
by 93.3 Hz and for 68 by 111.7 Hz (for a comparison see 50:
DJ = 84.3 Hz). As for transition-metal complexes that contain
terminal phosphane ligands, the size of the coupling constant
1 103
J( Rh,31P) is, to a first approximation, inversely
proportional to the RhP bond length,[26] it is
conceivable that the difference in the distances
between the phosphorus atom of the semibridging
trimethylphosphane and the two rhodium centers is
larger in 67, and particularly in 68, than in 50.
The reactions of 51 with a twofold excess of
Me3SiBr or Me3SiI proceed quite slowly and afford
in toluene at room temperature the dibromo and
diiodo complexes 71 and 72 in about 90 % yield of
the isolated product. In contrast to that of 66, the
31
P NMR spectra of 71 and 72 are independent of
the concentration of the solution and display in
each case a sharp triplet at d = 18.3 (71) and
Figure 8. Molecular structure (SCHAKAL representation), including a side-view, of com20.8 ppm (72). However, while the 31P NMR
pound 72 (hydrogen atoms omitted for clarity).
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H. Werner
sparingly soluble, the arsane-bridged compound 77 precipequilibrium with the corresponding monomeric Rh2 species. It
itates and is isolated in excellent yield (Scheme 17).[24] The
should be mentioned that attempts to prepare 73 and 74 from
13 and PEt3 or PMe2Ph by ligand exchange failed.
light-brown solid is thermally more stable than the PMe3
After it had been confirmed that PMe3, PMe2Ph,
and PEt3 can behave as doubly bridging ligands, we
finally also succeeded with transferring the more
bulky triisopropylphosphane from a terminal into a
bridging position. It was already mentioned that
while the dichloro compound 13 reacts with PiPr3 to
give trans-[RhCl(=CPh2)(PiPr3)2], the bis(acac)
counterpart 23 affords upon treatment with PiPr3
the mixed-valence complex 43 exclusively (see
Scheme 9). Since we considered it as conceivable
that the latter reaction proceeds via the phosphanebridged isomer 75, we attempted to trap this species
by replacing the acac ligands with chloride. The
mixed-valence compound 43 was thus treated with
an excess of Me3SiCl resulting in a mixture of
Scheme 17.
products. A clean reaction occurred, however, when
a solution of HCl in benzene was dropped into a
vigorously stirred solution of 43 in the same solvent; after
analogue 51 and can be stored under argon at low temperworkup, the required complex was isolated as a red solid in
atures for weeks. The 1H and 13C NMR spectra of 77 suggest
91 % yield. In the same way, the PPh3 analogue, 76, could be
that the two acac as well as the two CPh2 ligands are
generated from 46 (see Scheme 16).[27] Since the 31P NMR
equivalent and thus it can be assumed that the trimethylarsane is symmetrically bound to the two rhodium centers.
In contrast to 51, the related AsMe3 complex 77 is rather
labile and reacts not only with SbiPr3 but also with tertiary
phosphanes such as PMe2Ph, PEt3, and PnBu3 by bridgeligand exchange. While compounds 57 and 58 had originally
been prepared from 23 and PMe2Ph or PEt3 (see Scheme 12),
it was already mentioned that attempts to obtain 78 in an
analogous way resulted in the formation of the mixed-valence
isomer 61 (see Section 5). The successful procedure to isolate
the phosphane-bridged complex 78 consists of the treatment
Scheme 16.
of a solution of 77 in toluene at 50 8C with one equivalent of
PnBu3, followed by warming of the solution to 0 8C and
stirring of the reaction mixture at this temperature under
reduced pressure for 20 min. Under these conditions, the
spectra of both 75 and 76 display a sharp triplet at room
volatile trimethylarsane is removed and the product 78
temperature and below, there is no doubt that the proposed
isolated in 89 % yield.[25] The 31P NMR spectrum of 78
structure with PiPr3 and PPh3 in a doubly bridging position
between the two rhodium centers is correct.
displays a sharp triplet at d = 9.6 ppm with a J(103Rh,31P)
coupling constant that is identical to that of 58. Notably, both
57 and 58 are obtained from 77 in much better yields than
when 23 is used as the precursor.
7. The Final Goal: Trimethylarsane as a
In a similar way to the PMe3 analogue 51, compound 77
Semibridging and a Doubly Bridging Ligand
reacts with Me3SiCl in the molar ratio of 1:1.1 to give the
dinuclear complex 79, which probably contains the arsane in a
While the stibane-bridged compound 13 reacts with PiPr3
semibridging coordination mode (Scheme 18). As already
and PPh3 by bridge cleavage, it is inert toward AsiPr3 and
observed for the {Rh2Cl(acac)(m-SbiPr3)} and {Rh2Cl(acac)(mAsPh3. A slow reaction takes place when 13 is treated with
AsMe3, but in this case a mixture of products results, which
PR3)} derivatives, the unsymmetrical species 79 is significantly
could not be separated by fractional crystallization or column
more stable than the symmetrical precursor 77 and does not
chromatography.
decompose in benzene even after it had been stored for
The successful route to bind trimethylarsane in a bridging
3 days.[24]
position was similar to that found for PMe3. Treatment of the
Replacement of the remaining acac ligand of 79 by
bis(acac) complex 23 with AsMe3 in benzene leads to an
chloride is more difficult and, even with a large excess of
Me3SiCl, the formation of 80 takes place slowly at room
equilibrium between 23 and the AsMe3 analogue, which, even
in the presence of a large excess of the arsane, could not be
temperature. Nevertheless, after removal of the volatiles, the
completely shifted to the side of the required product.
dichloro compound could be isolated as a red-brown solid in
However, in hexane as the solvent, in which 77 is only
91 % yield. As shown by the X-ray crystal-structure analy-
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Bridging Ligands
Rh2Cl2 and Rh2Cl2A distances of the bridging unit
in the center of the molecule. In this respect it should
be mentioned that in the unit cell of 80 there are no
contacts either between the terminal chloro ligands
and the corresponding RhCl fragment of a neighboring Rh2 subunit or between these chloro ligands and
the benzene molecules incorporated in the crystal. The
1
H and 13C NMR spectra of 80 remain unchanged in
the temperature range between 193 and 333 K, which
indicates that under these conditions no dissociation of
the dimeric Rh4 to the monomeric Rh2 species,
analogous to the dissociation of 66 to 66’, takes place.
Scheme 18.
8. Theoretical Studies
sis,[24] the AsMe3-bridged complex 80 is isomorphous to 66
and also has the midpoint of the Rh(m-Cl)2Rh unit as a center
of symmetry (see Figure 9). The RhRh distance in each
dinuclear subunit is slightly larger than in 66, probably
Figure 9. Molecular structure (SCHAKAL representation), including a
side-view, of compound 80 (hydrogen atoms omitted for clarity).
because of the increase in the covalent radius of arsenic
compared with phosphorus. The two RhAs bond lengths in
each subunit differ somewhat, which reflects the nonequivalence of the “outer” and “inner” metal centers of the
{ClRh2(m-Cl)2Rh2Cl} chain. Since not only the arsane but
also the two diphenylcarbene ligands are linked to the two
rhodium atoms of each dinuclear subunit in an unsymmetrical
fashion, no mirror plane exists along the Cl-Rh-Rh axis. As
expected, the terminal Rh1Cl1 distance is shorter than the
Angew. Chem. Int. Ed. 2004, 43, 938 – 954
DFT calculations for dinuclear compounds of the general
composition [A1M1(m-CH2)2(m-EX3)M2A2] (M1, M2 = Co, Rh,
Ir; A1, A2 = monoanionic ligands) have been carried out by
Kaupp, Schinzel, and Straka by using the B3LYP functional
with the Gaussian 98 program.[28] The present results can be
summarized as follows:
a) For phosphanes PX3 as the bridging unit, a more symmetrical position between two rhodium centers is
expected along the series X = CH3 < H < F. With the
increase of the electronegativity of X, the p-acceptor
strength of PX3 increases and thus the bonding capability
becomes more similar to that of CO.
b) For the parent phosphane, arsane, and stibane EH3, the
formation of a symmetrical bridge is favored along the
series E = P < As < Sb. In a similar way to a), the polarity
of the EH bond increases from phosphorus to antimony
and thus also the p-acceptor strength increases in the
same order. Taking both points a) and b) into consideration, it follows that SbF3 should be the preferred
candidate for building a doubly bridging {M(m-EX3)M}
framework. This has to be proven by experiment.
c) For halides as anionic ligands A1 and A2, the most
symmetrical {Rh(m-PH3)Rh} bridge in the model compound [A1Rh1(m-CH2)2(m-PH3)Rh2A2] should be formed
in the case of A1 = A2 = I and the less symmetrical bridge
in the case of A1 = A2 = F. For A1 = Cl and A2 = F, Cl, Br,
I, the smallest difference in the two RhP distances should
result for A2 = Cl and the largest for A2 = F. It appears
that an increase in the difference of the electronegativity
of two halogens also leads to an increase in the difference
of the RhP bond lengths.
d) For A1 = Cl and A2 = k2-acac, the distance Rh2P in the
model compound [ClRh1(m-CH2)2(m-PH3)Rh2(k2-acac)]
should be smaller than the distance Rh1P, despite the
increase in the coordination number at Rh2. This is in
agreement with the results found for the isolated complex
50. Since Rh1 is linked to one chloride and Rh2 to two
oxygen atoms, the positive charge at Rh2 should be higher
than at Rh1 and this should strengthen the Rh2P bond.
e) By changing the metal in Group 9, the asymmetry of the
{Ir(m-EH3)Ir} bridge should decrease along the series E =
P > As > Sb following the trend observed for rhodium as
the metal center. In contrast, for M = Co the highest
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asymmetry is found for E = As and the smallest for M =
Sb. For PF3 as the bridging ligand, the difference in the
two MP distances (M = Co, Rh, Ir) is significantly
smaller than for PH3 while the ratio d(M1P)/d(M2P) is
about the same for all three elements of Group 9. The
calculations leave no doubt that rhodium is the preferred
candidate for forming a {M(m-EX3)M} bridge.
f) The bonding analyses for the model compound [ClRh(mCH2)2(m-PH3)RhCl)], based on the electron localization
functions (ELF), reveals an asymmetric attractor from the
phosphorus to the rhodium atoms being in agreement
with a delocalized three-center bond. The coordination
geometry around the phosphorus atom in the {Rh(mPH3)Rh} fragment corresponds to a distorted trigonal
bipyramid with one hydrogen atom in axial and two
hydrogen atoms in equatorial positions. The calculated
distance PHax is slightly larger than the distances PHeq,
which could be explained by the more efficient p back
bonding from a metal d orbital into the in-plane s* orbital
of the phosphane. However, by comparing the values of
the positive charge at the phosphorus atom in free PH3
with the doubly bridging PH3 in the respective model
compound, it is nevertheless apparent that s bonding is
much more dominant than p bonding, which is also found
for complexes that have terminal PH3 ligands.
9. Concluding Remarks
The studies described in this Review opens the gate to a
new field in coordination chemistry. After it had been
supposed for decades that tertiary phosphanes, arsanes, and
stibanes behave exclusively as terminal ligands, it was only
recently that this postulate became weakened. The work by
the groups of Braunstein, Pasquali, van Leeuwen, and Balch
provided the hint that it might be possible to bind tertiary
phosphanes in a bridging position, whereby the d8 and d9
metal centers presumably play a crucial role. However, for the
preparation of compound 13—the first “outsider” according
to Caulton[29]—it was important to have not tertiary phosphanes but tertiary stibanes, in particular SbiPr3, as supporting ligands. This facet has to be emphasized insofar as
trialkylstibanes complexes of the late transition-metals are
quite rare,[2, 30] probably because the MSbR3 bond is weaker
than the MAsR3 and MPR3 bonds. Because of the reduced
bond energy, trialkylstibanes can thus be considered as good
leaving groups, which is not only in agreement with the
smooth bridge–ligand exchange of SbiPr3 to PR3 but also with
the fact that compound 13 is generated from trans-[RhCl(=CPh2)(SbiPr3)2] by partial elimination of SbiPr3 in high
yield. In contrast, trans-[RhCl(=CPh2)(PiPr3)2] is quite inert
and neither heating nor UV irradiation transforms this
complex into [Rh2Cl2(m-CPh2)2(m-PiPr3)] (75).
However, the stibane-bridged compounds (of which 25
have been prepared to date) are not only interesting in
themselves, but are even more interesting as starting materials
for the preparation of the phosphane- and arsane-bridged
analogues. Moreover, the bis(acac) compound 23 with bridg-
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ing triisopropylstibane is the precursor for a series of mixedvalence Rh0–RhII complexes in which the coordination
number of the low-valent metal center can vary between
three and five. At present, the chemistry of these complexes is
mainly unexplored and it would be particularly interesting to
find out whether they can be oxidized, possibly electrochemically, to RhII–RhII or RhI–RhIII species.
Despite these challenging perspectives, the present work
is, without any doubt, highlighted by the preparation and
characterization of a variety of di- and tetranuclear phosphane-bridged rhodium compounds. These compounds are
really exceptional not only from a structural point of view but
equally with respect to their reactivity. Various substitution
reactions with both hard and soft Lewis bases had been
carried out in the periphery of the molecules without
destroying the bridging framework. While the increase in
the coordination number from four to five at the metal
centers has, besides an increase in the steric crowd, no
significant effect, the decrease in the coordination number
from five to four favors in some cases a dimerization of the
Rh2 species and leads to the formation of chainlike
{XRh2X2Rh2X} complexes. Another interesting facet is that
in contrast to the initial observations, not only small
phosphanes such as PMe3, PMe2Ph, or PEt3 but also more
bulky analogues such as PnBu3, PiPr3, or PPh3 can be forced to
bind to two metal centers in a doubly bridging mode.
The question, of how large the bond energy of a tertiary
phosphane in a doubly bridging position is, cannot be
answered at present. From thermochemical studies it is
known that the bond energy of doubly bridged carbonyls is
about half of the bond energy of terminal CO ligands.[31]
Moreover, since the bond energy for MPPh3 (as shown for
M = Mo) is significantly smaller (ca. 120 kJ mol1) than the
bond energy for MCO,[32] one can assume that a similar
relation holds for the bond energy of an {M(m-PR3)M} and an
{M(m-CO)M} bridge. From a kinetic point of view, the
formation of an {M(m-CO)M} bond appears to be considerably more favored compared with an {M(m-PR3)M} bond, as
indicated by the course of the reaction of compound 51 with
CO. In this case (see Scheme 10) instead of the complex [(k2acac)Rh(m-CPh2)2(m-PMe3)Rh(CO)(k2-acac)], formed by the
addition of CO to one of the rhodium centers of the starting
material, the isomer 52 with bridging CO and terminal
coordinated trimethylphosphane ligands is formed. Since this
compound does not rearrange, neither by heating nor by
photolysis, into the {Rh(m-PMe3)Rh(CO)} isomer, we assume
that it is not only kinetically but also thermodynamically
preferred.
With regard to future studies, it would be interesting to
find out whether dinuclear compounds with {M(m-ER3)M}
(E = P, As, Sb) as the core fragment are also accessible with
other transition metals, in particular with the neighboring
elements of rhodium in the periodic table, that is, cobalt,
ruthenium, palladium, and iridium. From the previous investigations mentioned in the Introduction, palladium seems to
be the preferred candidate. The recent discovery by Reau and
co-workers[33] that the phosphorus atom of substituted phospholes is able to bridge two palladium atoms in a symmetrical
fashion supports this prediction. In a Microreview published
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Angew. Chem. Int. Ed. 2004, 43, 938 – 954
Angewandte
Chemie
Bridging Ligands
in 2001, Braunstein and Boag argued that compounds with an
{M(m-ER3)M} molecular building block should in general not
be considered as thermodynamically disfavored.[34] These
authors took the isolobal analogy between PR3 and SiR3 , as
well as the existence of various silyl-bridged transition-metal
complexes, into consideration.[35] Independent of whether this
expectation will be fulfilled or not, the successful steps
discussed in this article should encourage additional work in
this field.
Our own research described in this Review was mainly carried
out by three exceptionally able Ph.D. students: Peter Schwab
opened up this field by discovering the unique role that SbiPr3
played as a ligand in carbenerhodium chemistry, Ulrich Herber
discovered the ability of the Rh(m-SbiPr3)Rh compounds to
generate the new family of mixed-valence Rh0–RhII complexes,
and Thomas Pechmann made the breakthrough in preparing
the whole series of phosphane- and arsane-bridged Rh2 and
Rh4 species. I would also like to acknowledge the indispensible
contributions of other members of our group (Norbert Mahr,
Paul Steinert, Birgit Webernd>rfer, Kerstin Ilg and in particular
Carsten D. Brandt), who elucidated the crystal and molecular
structures of the dinuclear and tetranuclear complexes
reported. Moreover, I am very grateful to Justin Wolf who
not only contributed with numerous fruitful discussions but
also encouraged the younger students in continuing their
efforts to achieve the final goal. It was a great pleasure to
collaborate with Professor Martin Kaupp and his group on the
bonding capabilities of bridging EX3 ligands. The constant and
unbureaucratic financial support by the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie and the
University of W@rzburg is also gratefully acknowledged.
Finally, I thank Professor Luis A. Oro for his generous
hospitality during my time as a visiting professor at the
University of Zaragoza when this Review was written.
Received: August 21, 2003 [A627]
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