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Novel Compounds of Elements of Group 14 Ligand-Stabilized Clusters with УNakedФ Atoms.

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Highlights
Carbon-Group Elements
Novel Compounds of Elements of Group 14: LigandStabilized Clusters with “Naked” Atoms
Andreas Schnepf*
Keywords:
cluster compounds · germanium · group 14 elements ·
nanostructures · tin
M
olecular cluster compounds of the
heavier elements of group 14 (Si–Pb)
have been known for some time. They
can be divided roughly into two large
classes (Scheme 1): the Zintl anions,
such as [M5]2 or [M9]n (M = Si, Ge,
Sn, Pb, n = 2–4),[1] and the ligand-stabilized cluster compounds of the general
formula [MnRn] (M = Ge, Si, n = 4,6,8;
M = Sn, n = 6,8,10)[2] which contain solely ligand-bearing metal atoms. Some
Zintl anions can be solubilized from
the respective Zintl phases with ethylenediamine and subsequently crystallized with cryptand molecules as isolated molecular units. The ligand-stabilized
cluster compounds are usually prepared
by reductive dehalogenation of the
corresponding REX3 halogen compounds
(e.g.
n RMX3 + 3n Na!
(RM)n + 3 nNaX).
In addition to these well-defined
compounds, which are mainly characterized by crystal-structure analysis, numerous silicon and germanium nanoparticles are known. These species exhibit interesting optical properties and
are of considerable interest for the
investigation of quantum size effects.[3]
The nanoparticles are prepared by controlled pyrolysis of SiH4 and GeH4,
respectively.[4] Germanium nanoparticles are also formed in the reduction of
SinGe1nO2 mixed oxides with elemental
hydrogen.[5] These nanoparticles are
obtained, however, as a mixture with a
[*] Dr. A. Schnepf
Institut f0r Anorganische Chemie
Universit3t Karlsruhe (TH)
Engesserstrasse, Geb.30.45
76128 Karlsruhe (Germany)
Fax: (+ 49) 721-608-4854
E-mail: schnepf@aoc2.uni-karlsruhe.de
664
Scheme 1. Selected structural formulae of group 14 ligand-stabilized cluster compounds (left)
and Zintl anions (right).
certain size distribution and consequently are structurally only poorly characterized. Particularly for small particles
(< 2 nm) no experimental information
on their structure is available,[6] although these particles exhibit especially
good photoluminescence properties.[5b]
Light could be shed on this structural darkness by cluster compounds of
the general formula [MmRn], where n <
m, which, in addition to ligand-bearing
metal atoms also contain metal atoms
which are only bonded to other metal
atoms, and which are referred to as
“naked” metal atoms in the following.[7]
Group 14 compounds of this type were,
until recently, unknown. First results
came from tin chemistry reported by
Sita and Bickerstaff. They were successful in preparing for the first time a
ligand-stabilized cluster containing naked tin atoms ([Sn5R6], R = 2,6-Et2C6H3 ;
Scheme 2 a).[8] In this cluster the number
of metal atoms is still smaller than that
of the ligands, but even in this case it was
demonstrated that such compounds exhibit interesting bonding situations.
Above all the question whether a bond
exists between the two bridgehead
atoms in the propellane [Sn5R6]—and
Scheme 2. Structural formulae of the tin compounds a) [Sn5R6], b) [Sn8R*6 ], c) [Sn8R04]. R = 2,6Et2C6H3 ; R* = SitBu3 ; R’ = 2,6-Mes2C6H3, Mes = 2,4,6-Me3C6H2.
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200301706
Angew. Chem. Int. Ed. 2004, 43, 664 –666
Angewandte
Chemie
if yes, how strong it is—has been discussed in depth.[9] A few years later
Wiberg et al.[10] and Power et al.[11] synthesized the first tin compounds which
fulfilled the aforementioned criteria,
namely [Sn8(SitBu3)6] and [Sn8R4](R =
2,6-Mes2C6H3, Mes = 2,4,6-Me3C6H2),
respectively (Scheme 2 b and c). Structurally these two tin clusters may be
derived from the cubic compound
[Sn8R8][12] by elimination of two and
four ligands, respectively. In [Sn8(SitBu3)6] the cubic geometry is hardly
distorted, whereas in the [Sn8R4] compound a strongly distorted {Sn8} cube is
present with two additional SnSn contacts, 310 pm in length (broken line in
Scheme 2 c).[11]
In the case of germanium, which is of
considerable interest in nanotechnology
several syntheses of corresponding cluster compounds have been developed in
recent years. Sekiguchi et al. synthesized
the cationic germanium cluster compound [Ge10(SitBu3)6I]+ (1) containing
three
naked
germanium
atoms
(Scheme 3) by mild thermolysis (50 8C,
several weeks) of a mixture of [Ge3(SitBu3)3I] and KI/K+TTFBP (TTFBP =
B(C6F4H)4) in toluene which gave 1 in
37 % yield.[13] The occurrence of naked
germanium atoms can be attributed to a
reductive elimination of SitBu3I which is
obtained as a byproduct of the reaction.
In addition, we have been able to
prepare the cluster compounds [Ge8{N(SiMe3)2}6] (2)[14] and [Ge9{Si(SiMe3)3}3]
Scheme 3. Structural formulae of the cluster
compounds [Ge10(SitBu3)6I]+ (1),
[Ge8{N(SiMe3)2}6] (2), [Ge9{Si(SiMe3)3}3] (3),
[Ge6R2](4; E = Ge) and [Ge2Sn4R2] (5; E = Sn)
where R = 2,6-Dipp2C6H3 ; Dipp = 2,6-iPr2C6H3).
Angew. Chem. Int. Ed. 2004, 43, 664 –666
(3)[15] with two and six naked germanium
atoms, respectively, by disproportionation of a subvalent GeI halide (GeBr).
Recently Power et al. introduced a
further, highly promising possibility for
the synthesis of cluster compounds with
naked metal atoms of group 14 which
gave the cluster compounds [Ge6R2] (4)
and [Ge2Sn4R2] (5) (R = 2,6-Dipp2C6H3 ;
Dipp = 2,6-iPr2C6H3).[16] In this approach RGeCl was reductively coupled
in the presence of GeCl2 or SnCl2 with
the aid of C8K. The naked germanium or
tin atoms originate from the dihalides
since without their addition to the
reaction mixture only the compound
[(GeR)2] was isolated.[17] At the first
glance the yields of 4 (40 %) and 5
(20 %) appear low, but considering the
complex reaction course taking place
during the formation of the octahedral
compounds 4 and 5 from monomeric
units the yields are surprisingly high.
With these compounds prepared by
Power the family of ligand-stabilized
germanium cluster compounds with
naked germanium atoms already includes four members (1–4).
Common to the cluster compounds
1–4 is that some of the GeGe bonds in
the cluster core are much longer than a
normal GeGe single bond (ca.
244 pm[18]). For example, the GeGe
separation between the naked germanium atoms in 1 is 325 pm,[13] and consequently the presence of a significant
bonding interaction appears questionable. Quantum chemical calculations on
the model compound [Ge10H7]+ (1’)
show, however, that in spite of the very
large GeGe separation a three-center,
two-electron bond between the three
naked germanium atoms is present. The
occurrence of a bonding interaction in 1’
emerges from a comparison with the
model compound [Ge10H10], without
naked metal atoms, for which an unambiguous nonbonding GeGe separation of 379 pm was calculated. Furthermore, the quantum chemical calculations showed that a homoaromatic system is present in 1’ since an aromatic
stabilization energy of 19.2 kJ mol1
and a nucleus-independent chemical
shift (NICS) of 26.4 ppm were calculated.[13]
The occurrence of multicenter bonding components could also be demonstrated for compounds 2 and 3. Thus
www.angewandte.org
quantum chemical calculations on the
model compounds [Ge8(NH2)6] (2’)and
[Ge9H3] (3’) gave three-center bonding
components with an SEN (shared electron number) of 0.13 for 2’ and 0.32 for
3’.[14, 15] However, the bonding situations
in this case are more complicated than in
1 since no isolated multicenter bond (as
in 1’) is found. Rathermore, molecular
orbitals are present which extend over
the whole cluster core and thus can be
classified as delocalized cluster MOs.[19]
These findings confirm that owing to
the additional electrons which are available to the cluster core through the
naked germanium atoms delocalization
of the bonding electrons occurs in the
cluster core.[20] The bonding situation in
the clusters 1–4 thus resembles that in
the Zintl anions, in which the bonding
electrons are also delocalized. In the
case of the Zintl anions it is also found,
however, that the germanium atoms
with the higher coordination number
form, as expected, longer GeGe contacts. In contrast—in the compounds 3
(triply capped trigonal prism) and 4
(octahedral) which are structurally most
closely related to the Zintl anions—the
germanium atoms with the higher coordination number form shorter GeGe
bonds. Thus a bonding situation is
presented which has not been previously
observed in germanium cluster compounds.
The bonding situation found in this
new group of complexes can also be
interpreted as follows: starting from the
fully substituted germanium clusters
[(GeR)n], the bonding situation of
which can be described by localized
two-center two-electron bonds in the
cluster core. The formal introduction of
germanium atoms causes delocalization
of the bonding electrons in the cluster
core (analogous to 1–4). Further insertion of germanium atoms leads next to
nanoparticles and finally to the solid
phase of elemental germanium in which
the bonding situation for the germanium
atoms can again be described simply by
localized
two-center
two-electron
bonds.
What effects this interaction between localized and delocalized bonding
electrons has for the physical and chemical properties of the compounds have to
be explored in future experimental and
theoretical studies. Moreover, the syn-
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
665
Highlights
thesis strategies introduced need to be
extended to prepare larger cluster systems.[21]
[1] J. D. Corbett, Angew. Chem. 2000, 112,
682; Angew. Chem. Int. Ed. 2000, 39,
670.
[2] A. Sekiguchi, H. Sakurai, Adv. Organomet. Chem. 1995, 37, 1.
[3] “Nanotechnology—Molecularly
Designed Materials”: M. S. El-Shall, S. Li,
D. Graiver, U. Pernisz, ACS Symp. Ser.
1996, 622.
[4] C. R. Gorla, S. Liang, G. S. Tompa, W. E.
Mayo, Y. Lu, J. Vac. Sci. Technol. A
1997, 15, 860; A. Bapat, C. R. Perrey,
S. A. Campbell, C. B. Carter, U. Kortshagen, J. Appl. Phys. 2003, 94, 1969.
[5] a) D. C. Paine, C. Caragianis, T. Y. Kim,
Y. Shigesato, Appl. Phys. Lett. 1993, 62,
2842; b) H. Yang, X. Wang, H. Shi, S.
Xie, F. Wang, X. Gu, X. Yao, Appl. Phys.
Lett. 2002, 81, 5144.
[6] D. K. Yu, R. Q. Zhang, S. T. Lee, Phys.
Rev. B 2002, 65, 245 417.
[7] “Naked” here does not mean isolated,
that is interaction-free, but is merely a
linguistic simplification to distinguish
the different types of metal atoms in
these clusters. Alternatively the metal
atoms could also be termed ligand-free.
However, this description is equally
inexact since bonded metal atoms can
also be classified as ligands.
666
[8] L. R. Sita, R. D. Bickerstaff, J. Am.
Chem. Soc. 1989, 111, 6454.
[9] By means of comprehensive ab initio
calculations on the model compounds
[E5H6] of the [1.1.1]propellane type (E =
C, Si, Ge, Sn) it was possible to show
that the bond strength between the two
bridgehead atoms decreases from carbon to tin. Furthermore, in these calculations only in the case of the carbon
compound can a bond critical point
between the two bridgehead atoms be
calculated, which is necessary for a
classical covalent bond. In contrast, in
the case of the higher homologues the
bonding electrons are not directly localized along the nuclear bonding axis.
M. S. Gordon, K. A. Nguyen, M. T. Carroll, Polyhedron 1991, 10, 1247; H.
GrNtzmacher, F. Breher, Angew. Chem.
2002, 114, 4178; Angew. Chem. Int. Ed.
2002, 41, 4006.
[10] N. Wiberg, H.-W. Lerner, S. Wagner, H.
NOth, T. Seifert, Z. Naturforsch. B 1999,
54, 877.
[11] B. E. Eichler, P. P. Power, Angew. Chem.
2001, 113, 818; Angew. Chem. Int. Ed.
2001, 40, 796.
[12] L. R. Sita, I. Kinoshita, Organometallics
1990, 9, 2865.
[13] A. Sekiguchi, Y. Ishida, Y. Kabe, M.
Ichinohe, J. Am. Chem. Soc. 2002, 124,
8776.
[14] A. Schnepf, R. KOppe, Angew. Chem.
2003, 115, 940; Angew. Chem. Int. Ed.
2003, 42, 911.
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
[15] A. Schnepf, Angew. Chem. 2003, 115,
2728; Angew. Chem. Int. Ed. 2003, 42,
2624.
[16] A. F. Richards, H. Hope, P. P. Power,
Angew. Chem. 2003, 115, 4205; Angew.
Chem. Int. Ed. 2003, 42, 4071.
[17] M. Stender, A. D. Phillips, R. J. Wright,
P. P. Power, Angew. Chem. 2002, 114,
1863; Angew. Chem. Int. Ed. 2002 41,
1785.
[18] A. F. Wells, Structural Inorganic Chemistry, 5th ed., Oxford, 1984, p. 1279.
[19] Such a delocalization of the bonding
electrons is also to be expected in the
compounds 4 and 5 described by Power
et al. since the GeGe and SnSn separations in the four-membered ring of
naked Ge/Sn atoms are very large at 286
and 312 pm, respectively.
[20] a) Thus no multiple bonds are formed
that would lead to a shortening of the
GeGe bond as in the series germane
(244 pm)—germene (230 pm in [Ge2R4];
R = 2,6-iPr2C6H3)[20b]—germine (228 pm
in [Ge2R2]; R = 2,6-Dipp2C6H3, Dipp =
2,6-iPr2C6H3).[17] b) J. Park, S. A. Batcheller, S. Masamune, J. Organomet.
Chem. 1989, 367, 39.
[21] A coupling of the individual clusters to
larger aggregates which has already led
to interesting compounds with the Zintl
anions would also be a plausible route.T. F. FPssler, Angew. Chem. 2001, 113,
4289; Angew. Chem. Int. Ed. 2001, 40,
4161.
Angew. Chem. Int. Ed. 2004, 43, 664 –666
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