Synthesis and characterization of neutral, homo and heteronuclear clusters with unsubstituted germanium or tin atoms

Article abstract of DOI:10.1002/anie.200351907

The first, well-characterized, polyhedral cluster with two different heavier group 14 elements is prepared (see picture). Low-valent group 14 halides are used to synthesize species in which the majority of the group 14 atoms carry no organic substituents.

Full text of DOI:10.1002/anie.200351907

Angewandte
Chemie
Mixed-Metal Main-Group Clusters
Synthesis and Characterization of Neutral, Homo
and Heteronuclear Clusters with Unsubstituted
Germanium or Tin Atoms**
Anne F. Richards, Håkon Hope, and Philip P. Power*
Stable polyhedral clusters of the formula MnRm (n > m; M =
Si–Pb (tetrel; a tetrel is a group 14 element); R = organo or
related group) that contain several tetrels unsubstituted by
organic or related groups are rare.[1–4] The synthesis of such
species is of interest as it could allow the crystallization of
well-defined larger tetrel clusters of nanometer dimension-
ality that may have unusual electronic properties. Generally
speaking, most currently known tetrel clusters can be
2ꢀ
categorized as either Zintl anions,[5] for example M5 or
M9nꢀ (M = Ge, Sn or Pb; n = 2, 3 or 4), or as neutral clusters of
the formula MnRn (M = Si–Pb, n = 4, 6, 8 or 10)[6] where each
tetrel carries one substituent. In contrast, the substituent poor
polyhedral tetrel clusters are represented only by the com-
pounds Sn8{Si(SiMe3)3}6,[2] Sn8{C6H3-2,6-Mes2}4
and the
[3]
[4]
germanium species Ge8{N(SiMe3)2}6 that was recently
reported by Schnepf and co-workers. The tin compounds
[*] Prof. Dr. P. P. Power, Dr. A. F. Richards, Prof. Dr. H. Hope
Department of Chemistry
University of California Davis
One Shields Avenue, Davis, CA 95616 (USA)
Fax: (+1)530-752-8995
E-mail: pppower@ucdavis.edu
[**] This work was supported by the Donors of the Petroleum Research
Fund Administered by the American Chemical Society.
Angew. Chem. Int. Ed. 2003, 42, 4071 –4074
DOI: 10.1002/anie.200351907
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4071

Communications
were obtained either by thermolysis[2] or by reduction/ligand
stripping of an aryl tin(ii) halide precursor Sn(Cl)C6H3--2,6-
Mes2.[3] The more recent synthesis of Ge8{N(SiMe3)2}6 repre-
sented a new departure in that it employed germanium(i)
bromide trapped at low temperature as the germanium
source.[4] This approach is analogous to that employed by
Schnöckel and co-workers[7] for the synthesis of numerous
aluminum and gallium clusters with unsubstituted metals
through the use of low oxidation state AlI and GaI halides. It is
believed that the disproportionation of these generally
unstable species provides a pathway through which clusters
involving unsubstituted metal atoms are formed and trapped
by ligands attached to the cluster surface. The reduction of the
low valent aryl tin(ii) halide Sn(Cl)C6H3-2,6-Mes2 near room
[3]
temperature to give the cluster Sn8{C6H3-2,6-Mes2}4 sug-
gested to us that it might be possible to use the low valent
tetrel(ii) halides themselves to introduce unsubstituted tetrel
atoms into a cluster. We now describe a new strategy for the
synthesis of tetrel clusters of this type and show that it can be
used to synthesize species in which the majority of tetrel
atoms carry no organic substituent. Furthermore, we show
that this approach allows the synthesis and the first detailed
structural characterization of a stable species in which two
different tetrels comprise the cluster framework.[8]
Figure 2. Structure of 2; H atoms are not shown. Representative bond
lengths [Š] and angles [8]: Ge1-Sn1 2.763(2), Ge1-Sn2 2.714(2), Ge1-
Sn3 2.728(2), Ge1-Sn4 2.704(2), Sn1-Sn2 3.098(2), Sn1-Sn4 3.135(2),
Sn2-Sn3 3.115(2), Sn3-Sn4 3.143(2), Ge2-C3(1) 1.98(1), Sn1-Ge1-C11
128.3(4), Sn3-Ge1-C11 128.3(4), Sn1-Ge1-Sn2 69.10(5), Sn2-Ge1-Sn3
69.85(5), Sn2-Sn1-Sn4 90.19(4), Sn1-Sn2-Sn3 90.60(4), Sn2-Sn3-Sn4
89.72(4), Sn1-Sn4-Sn3 89.41(4).
The clusters Ge6Ar20 (1) or Sn4(GeAr’)2 (2) (Ar’ = C6H3-
2,6-Dipp2; Dipp = C6H3-2,6-iPr2) were synthesized by the
reduction of a 1:1 proportion of Ge(Cl)Ar’[9] and GeCl2·diox-
ane or SnCl2 with three equivalents of KC8 in THF at room
temperature. The Ge6 derivative 1 (Figure 1) was isolated as
orange crystals, and the Sn4Ge2 cluster 2 (Figure 2) was
obtained as red crystals in about 40% and 20% yields,
respectively. Both compounds were characterized by X-ray
crystallography;[10] the structures are broadly similar with
distorted octahedral hexatetrel cores in which two of the six
vertices are substituted by Ar’ groups. The unsubstituted Ge4
(1) and Sn4 (2) atom sets comprise almost perfectly square
ꢀ
ꢀ
arrays which have Ge Ge and Sn Sn separations that
ꢀ
average 2.86(1) Š and 3.12(1) Š. The average Ar’Ge Ge
ꢀ
and Ar’Ge Sn distances in 1 and 2 are 2.50(2) in 2.72(2) Š.
The orientation of the central aryl rings of the terphenyl
substituents is coplanar with respect to each other in 1 and
almost perpendicular in 2 but this is probably a packing effect.
1H, 13C NMR and UV/Vis data for 1 and 2 are unremarkable.
The solution 119Sn NMR spectrum of 2 displayed a downfield
signal at d = 1583.
The observation of unsubstituted germanium or tin atoms
in the structures clearly arises from the inclusion of GeCl2 or
SnCl2 in the reaction mixtures as similar experiments involv-
ing the reduction of Ge(Cl)Ar’ or Sn(Cl)Ar’ or Sn(Cl)Ar’
without GeCl2 or SnCl2 give the alkyne analogues Ar’MMAr’
(M = Ge[11] or Sn[12]) exclusively. The distorted octahedral
core of 1 or 2 is their most conspicuous structural feature. It is
plausible to view this finding to be in agreement with Wade's
Rules[13] which predict a closo structure for the hypothetical
Ge62ꢀ or Ge2Sn42ꢀ clusters since they have n+1 electron pairs
(n = no. of vertices) available for cluster bonding in 1 and 2.
Thus, each unsubstituted tetrel provides two electrons and
each substituted moiety provides three electrons to afford a
total of seven electron pairs. Recent calculations by King and
co-workers for Ge62ꢀ have predicted that a regular octahedral
ꢀ
structure with a Ge Ge bond lengths of 2.687 Š as the global
2ꢀ [14]
minimum for Ge6
.
This value can be compared to the
ꢀ
average of 2.63 Š for the twelve Ge Ge bond lengths in 1.
The substitution of organic groups at two of the six
germanium atoms in 1 introduces large distortions to the
octahedron. The separation of the four unsubstituted germa-
Figure 1. Structure of 1; H atoms are not shown. Representative bond
lengths [Š] and angles [8]: Ge1-Ge2 2.546(1), Ge1-Ge3 2.498(2), Ge1-
Ge2A 2.532(1), Ge1-Ge3A 2.503(2), Ge2-Ge3 2.883(2), Ge2-Ge3A
2.886(2), Ge1-C1 1.974(6), C1-Ge1-Ge3 125.0(2), C1-Ge1-Ge2 126.0(2),
Ge2-Ge1-Ge3 68.70(5), Ge3-Ge2-Ge3A 88.66(5), Ge2-Ge3-Ge2A
91.34(5)8.
ꢀ
nium atoms is over 0.3 Š greater than the Ar’Ge Ge bond
lengths, and although the angles within the plane formed by
Ge(2), Ge(3), Ge(2A) and Ge(3A) are near 908, the
corresponding Ge(2)-Ge(1)-Ge(2A) and Ge(3)-Ge(1)-
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Angewandte
Chemie
3
CH(CH3)2), 2.84 (sept, JH,H = 6.8 Hz, 18H; o-CH(CH3)2)), 6.979 (d,
Ge(3A) angles associated with the substituted germanium
3JH,H = 7.2 Hz, 4H; m-C6H3), 7.1535 (d, JH,H = 8.0 Hz, 8H; m-Dipp),
3
ꢀ
atoms have opened out to 107.75(3) and 106.33(3)8. The Ge
Ge distances may be compared with those observed in the
7.174 (t, 3JH,H = 7.2 Hz, 4H; p-C6H3), 7.234 ppm (t, 3JH,H = 8.0 Hz, 4H;
p-Dipp); 13C{1H} NMR (C6D6, 100.59 MHz, 258C): d = 24.18 (o-
CH(CH3)2), 24.42 (o-CH(CH3)2), 34.71 (o-CH(CH3)2), 120.56 (m-
Dipp), 122.67 (p-C6H3), 137.41 (m-C6H3), 141.08 (p-Dipp), 146.66 (i-
Dipp), 148.21 (o-Dipp), 149.92 ppm (o-C6H3), (i-C6H3 carbon was not
observed); 119Sn NMR (C6D6), d = 1583.5 ppm.
substituted clusters Ge6R6 (R = Dipp[15] or CH(SiMe3)2[16]);
ꢀ
Ge Ge range 2.465(1)–2.584(2) Š), [PPh4]2[Ge6{M(CO)5}6]
[17]
ꢀ
(M = Cr, Mo, or W, Ge Ge = 2.541(1)–2.592(4) Š, and in
the species Ge8[N(SiMe3)2]6 where bond lengths of about 2.67
ꢀ
ꢀ
and 2.50 Š were observed for the R2NGe Ge(NR2) and Ge
Ge(NR2) (R = SiMe3) germanium bonds. A similar pattern of
distortion is seen in the structure of 2 where the distances
between the unsubstituted tin atoms average 3.12(2) Š and
Received: May 15, 2003 [Z51907]
Keywords: cluster compounds · germanium · terphenyls · tin
.
ꢀ
the Ge Sn distances are in the range 2.701(2)–2.746(2) Š.
The latter distances are longer than the two center-two-
electron Ge Sn bonds in compounds of the type R3MM’R3
[1] Two neutral tin clusters, Sn5Ar6 and Sn7Ar8 (Ar= C6H3-2,6-Et2)2,
that contain two unsubstituted tin atoms are known, but the
number of organic substituents exceeds that of the metal atoms
as some metal atoms carry two organic substituents. They were
obtained from the thermolysis of (SnAr2)3. See; L. R. Sita, Adv.
Organomet. Chem. 1995, 38, 187 for an account of this and
related work. In addition, a cationic germanium cluster contain-
ing unsubstituted germanium atoms [(tBu3Si)6Ge10I]+ has been
structurally characterized: A. Sekiguchi, Y. Ishida, Y. Kabe, M.
Ichinohe, J. Am. Chem. Soc. 2002, 124, 8776.
ꢀ
(M = Ge; M = Sn; R = organic group; range 2.573(2)–
2.652(2) Š.[18] The unsubstituted Ge Ge and Sn Sn bond
lengths in 1 and 2 lie at the longer end of the ranges generally
found in Zintl anions and approach the distances seen in the
ꢀ
ꢀ
square M4 moieties of the [M9]4ꢀ Zintl species[19,20] or the
[3]
ꢀ
“long” Sn Sn contacts (3.107(2) Š) in Sn8(C6H3-2,6-Mes3)4.
The weak M M contacts within the M4 squares receive
ꢀ
support from the 119Sn NMR chemical shift which lies well
downfield toward a region most commonly associated with
two coordinate tin(ii) species.[21] The NMR data suggest that
writing the structures of 1 or 2 with the bonds between the
unsubstituted germanium or tin atoms omitted also is a
plausible representation of their bonding environment. Work
to isolate a wider range of stable group 14 clusters with larger
numbers of unsubstituted atoms (nanosized clusters) and the
extension of the range of stable heteronuclear neutral clusters
to other heavier main group elements is in hand.
[2] N. Wiberg, H.-W. Lerner, S. Wagner, H. Nöth, T. Seifert, Z.
Naturforsch. B 1999, 54, 877.
[3] B. E. Eichler, P. P. Power, Angew. Chem. 2001, 113, 818; Angew.
Chem. Int. Ed. 2001, 40, 796.
[4] A. Schnepf, R. Köppe, Angew. Chem. 2003, 115, 940; Angew.
Chem. Int. Ed. 2003, 42, 911.
[5] J. D. Corbett, Angew. Chem. 2000, 112, 682; Angew. Chem. Int.
Ed. 2000, 39, 670.
[6] A. Sekiguchi, H. Sakurai, Adv. Organomet. Chem. 1995, 37, 1.
[7] A. Schnepf, H. Schnöckel, Angew. Chem. 2002, 114, 3682;
Angew. Chem. Int. Ed. 2002, 41, 3532.
[8] The extraction of Zintl phases such as Sn9ꢀxEx4ꢀ (E = Ge or Pb;
x = 0–9) with ethylene diamine, and their study by 119Sn NMR
spectroscopy, have proven the existence of mixtures of anionic
heteronuclear tetrel clusters. No crystal structures have been
determined, however. See: R. W. Rudolph, W. L. Wilson, R. C.
Taylor, J. Am. Chem. Soc. 1981, 103, 2480.
Experimental Section
All manipulations were carried out under anaerobic and anhydrous
conditions.
1: Ar’GeCl (0.505 g, 1 mmol)[4] and GeCl2(dioxane), (0.232 g,
1 mmol) was added dropwise to a suspension of KC8 (0.111 g, K) in
THF (20 mL). After the reaction had been stirred for 16 h, the THF
was removed under reduced pressure and the resultant red solid was
extracted into hexanes (40 mL). The volume was reduced to incipient
crystallization and storage of the solution at about ꢀ58C for 48 h
[9] M. Stender, A. D. Phillips, R. J. Wright, P. P. Power, Angew.
Chem. 2002, 114, 1863; Angew. Chem. Int. Ed. 2002, 41, 1785.
[10] Crystal data for 1 and 2·hexane with MoKa (l = 0.7107 Š)
radiation at 90 K: 1, monoclinic, P2(1)/n, orange block, a =
10.9496(11), b = 19.2684(19), c = 13.7857(14) Š, b = 98.547(2)8,
V= 2876.2(5) Š3, Z = 2, R1 (obs data) = 0.0589, wR2 (all data) =
0.1523, GOF = 0.966; 2·hexane: monoclinic, P21/c, red block, a =
16.928(4), b = 15.442(4), c = 25.196(7) Š, b = 103.857(5)8, V=
6395(3) Š3, Z = 4, R1 (obs data) = 0.1001, wR2 (all data) =
0.2465, GOF = 1.146. CCDC-210326 (1) and CCDC-210327 (2)
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.cam.a-
c.uk/conts/retrieving.html (or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB21EZ, UK;
fax: (+ 44)1223-336-033; or deposit@ccdc.cam.ac.uk). The struc-
ture of 1 displayed disorder problems involving the unsubsti-
tuted Ge4 array such that three plausible Ge4 arrangements are
possible. The major (70%) array features an almost square Ge4
unit similar in structure to the Sn4 moiety in the tin counterpart.
The remaining 30% is divided between two rectangular Ge4
arrays that have “short” bond lengths in the range 2.71–2.73 Š
afforded
1
as orange crystals. m.p. 69–718C; 1H NMR (C6D6,
3
399.7 MHz, 258C): d = 0.94 (d, JHH = 6.8 Hz, 24H; o-C(CH3)2), 1.24
(d, 3JHH = 6.8 Hz, 24H; o-CH(CH3)2), 2.709 (sept, 3JH,H = 6.8 Hz,
3
18H; o-CH(CH3)2)), 6.98 (d, JH,H = 7.2, Hz, 4H; m-C6H3), 7.08 (d,
3JH,H = 8.0 Hz, 8H; m-Dipp), 7.20 (t, 3JH,H = 7.2 Hz, 4H; p-C6H3),
7.22 ppm (t, 3JH,H = 8.0 Hz, 4H; p-Dipp); 13C{1H} NMR (C6D6,
100.59 MHz, 258C): d = 24.31 (o-CH(CH3)2), 25.76 (o-CH(CH3)2),
31.06 (o-CH(CH3)2), 122.69 (m-Dipp), 123.01 (p-C6H3), 136.75 (m-
C6H3), 140.49 (p-Dipp), 146.57 (i-Dipp), 146.68 (i-C6H3), 147.22 (o-
Dipp), 150.76 ppm (o-C6H3); UV/Vis (hexane): lmax, (e) 500 nm
(69,800).
2: A mixture of Ar’GeCl (0.505 g, 1 mmol)[9] and SnCl2 (0.19 g,
1 mmol) in THF solution (20 mL) was added dropwise to
a
suspension of KC8 (0.117 g, K) in THF (20 mL). After the reaction
had been stirred for 16 h the THF was removed under reduced
pressure and the resultant red solid was extracted into hexanes
(40 mL). The solution was concentrated and stored at room temper-
ature for 2 weeks, which afforded deep red cyrstals of 2 in 17% yield.
m.p. 100–1108C, 1H NMR (C6D6, 399.7 MHz, 258C): d = 1.016 (d,
3JH,H = 6.8 Hz, 24H; o-CH(CH3)2), 1.137 (d, 3JH,H = 6.8 Hz, 24H; o-
ꢀ
and longer (probably non-bonded) Ge Ge separations in the
range 3.41–3.64 Š. The positions of the minority Ge atoms were
clearly shown on a difference electron-density map. Distances
between the main Ge atoms and minority atoms are too large to
allow a description based on large amplitudes of motion. The
Angew. Chem. Int. Ed. 2003, 42, 4071 –4074
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Communications
final R index for 2 is higher than normal for a correct structure.
The reason for this is an overlap of parts of the main reciprocal
lattice and an extraneous lattice, either from a twin component
or a misoriented crystal fragment. By removing reflections with
large Io/Ic discrepancies, it was possible to lower R and reduce
the magnitude of the largest difference peak. These operations
are purely cosmetic, however, as they resulted in no discernible
structural changes. Final refinement was therefore performed
with the offending reflections included.
[11] M. Stender, A. D. Phillips, R. J. Wright, P. P. Power, Angew.
Chem. 2002, 114, 1863; Angew. Chem. Int. Ed. 2002, 41, 1785.
[12] A. D. Phillips, R. J. Wright, M. M. Olmstead, P. P. Power, J. Am.
Chem. Soc. 2002, 124, 5930.
[13] K. Wade, Adv. Inorg. Chem. Radiochem. 1976, 18, 1.
[14] R. B. King, I. Silaghi-Dumitrescu, A. Kun, J. Chem. Soc. Dalton
Trans. 2002, 3999.
[15] A. Sekiguchi, T. Yatabe, C. Kabuto, H. Sakurai, J. Am. Chem.
Soc. 1993, 115, 5853.
[16] A. Sekiguchi, C. Kabuto, H. Sakurai, Angew. Chem. 1989, 101,
97; Angew. Chem. Int. Ed. Engl. 1989, 28, 55.
[17] G. Renner, P. Kirchen, G. Huttner, P. Rutsch, K. Heinze, Eur. J.
Inorg. Chem. 2001, 973.
[18] K. H. Pannell, L. Parkanyi, H. Sharma, F. Cervantes-Lee, Inorg.
Chem. 1992, 31, 522.
[19] C. H. E. Belin, J. D. Corbett, A. Cisar, J. Am. Chem. Soc. 1977,
99, 7163.
[20] J. D. Corbett, P. A. Edwards, J. Am. Chem. Soc. 1977, 99, 3313.
[21] B. Wrackmeyer, Annu. Rep. NMR Spectrosc. 1999, 38, 203.
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Angew. Chem. Int. Ed. 2003, 42, 4071 –4074