New polyhedral zinc siloxanes: Synthesis and x-ray crystal structures of Zn8Me7(dioxane)2(O3SiR)3 and [Zn7Me2(THF)5(O3SiR) 4] [R = (2,6-i-Pr2C6H3)N(SiMe 3)]

Article abstract of DOI:10.1021/om049863h

In continuation of our previous work on the synthesis of zinc siloxanes to understand the influence of stoichiometry in the assembly of polyhedral zinc siloxanes, we have carried out two different reactions between RSi(OH) ₃ [R = (2,6-i-Pr₂C₆H₃)N(SiMe ₃)] and ZnMe₂ in molar ratios of 1:3 and 1:1.75. In these reactions, we have isolated an octanuclear Zn₈Me₇(dioxane) ₂(O₃SiR)₃ (5) and a heptanuclear zinc siloxane [Zn₇Me₂(THF)₅(O₃SiR)₄] (6), respectively. Compound 5 is a complex cage structure and can be understood as being a derivative of a drum-shaped species containing a Zn ₇Si₃O₉ core. In contrast, 6 consists of a Zn₄Si₄O₁₂ unit with zinc and silicon atoms occupying alternate corners of the cube; three contiguous faces of this cube are capped by zinc atoms, leaving the other three faces open.

Full text of DOI:10.1021/om049863h

Organometallics 2004, 23, 2251-2256
2251
New P olyh ed r a l Zin c Siloxa n es: Syn th esis a n d X-r a y
Cr ysta l Str u ctu r es of Zn 8Me7(d ioxa n e)2(O3SiR)3 a n d
[Zn 7Me2(THF )5(O3SiR)4] [R ) (2,6-i-P r 2C6H3)N(SiMe3)]†
Ganapathi Anantharaman,‡ Vadapalli Chandrasekhar,§ Umesh N. Nehete,‡
Herbert W. Roesky,*,‡ Denis Vidovic,‡ and J o¨rg Magull‡
Institut fu¨r Anorganische Chemie der Universita¨t Go¨ttingen, Tammannstrasse 4,
D-37077 Go¨ttingen, Germany, and Department of Chemistry, Indian Institute of Technology,
Kanpur-208016, India
Received February 25, 2004
In continuation of our previous work on the synthesis of zinc siloxanes to understand the
influence of stoichiometry in the assembly of polyhedral zinc siloxanes, we have carried out
two different reactions between RSi(OH)3 [R ) (2,6-i-Pr2C6H3)N(SiMe3)] and ZnMe2 in molar
ratios of 1:3 and 1:1.75. In these reactions, we have isolated an octanuclear Zn8Me7(dioxane)2(O3-
SiR)3 (5) and a heptanuclear zinc siloxane [Zn7Me2(THF)5(O3SiR)4] (6), respectively.
Compound 5 is a complex cage structure and can be understood as being a derivative of a
drum-shaped species containing a Zn7Si3O9 core. In contrast, 6 consists of a Zn4Si4O12 unit
with zinc and silicon atoms occupying alternate corners of the cube; three contiguous faces
of this cube are capped by zinc atoms, leaving the other three faces open.
There has been considerable interest in recent years
in the synthesis of polyhedral metallasiloxanes. This
emanates from many reasons. Soluble metallasiloxanes
can serve as model compounds for many complex metal
silicate frameworks that either occur naturally or have
been synthesized in the laboratory.1-18 Thus, we have
recently shown the utility of molecular titanasiloxanes
toward understanding the spectroscopic features of the
titanium-containing silicates TS-1 and TS-2.19,20 Second,
lipophilic metallasiloxanes are also useful models for
silica-supported transition metal catalysts.1-6,21,22 Other
applications include the utility of some metallasiloxanes
as single-sourceprecursors for metalsilicatematerials.23-26
More recently the potential of metallasiloxanes in
homogeneous catalysis is also being evaluated and
investigated vigorously.6,27,28
† Dedicated to Professor C. N. R. Rao on the occasion of his 70th
birthday.
* Corresponding author. Fax: +49-551-393373. E-mail: hroesky@
We have been utilizing the N-bonded silanetriol RSi-
(OH)3 [R ) (2,6-i-Pr2C6H3)N(SiMe3)] (1) for the prepara-
tion of various polyhedral metallasiloxanes with a high
M/Si ratio.1-3 In view of the importance of zinc-
exchanged zeolites (Zn/H-ZSM-5) in catalysis as well as
the naturally occurring zinc silicates such as willemite
and hemimorphite, we have embarked on a program for
assembling soluble zinc siloxanes.29-33 However, the
mismatch of the number of functional groups on the two
gwdg.de.
‡ Universita¨t Go¨ttingen.
§ Indian Institute of Technology.
(1) Murugavel, R.; Voigt, A.; Walawalkar, M. G.; Roesky, H. W.
Chem. Rev. 1996, 96, 2205-2236.
(2) Murugavel, R.; Chandrasekhar, V.; Roesky, H. W. Acc. Chem.
Res. 1996, 29, 183-189.
(3) Murugavel, R.; Bhattacharjee, M.; Roesky, H. W. Appl. Orga-
nomet. Chem. 1999, 13, 227-243.
(4) Duchateau, R. Chem. Rev. 2002, 102, 3525-3542.
(5) Feher, F. J .; Budzichowski, T. A. Polyhedron 1995, 14, 3239-
3253.
(6) Abbenhuis, H. C. L. Chem. Eur. J . 2000, 6, 25-32.
(7) Baerlocher, C.; Meier, W. M.; Olson, D. H. Atlas of Zeolite
Framework Types, 5th ed.; Elsevier: Amsterdam, 2001.
(8) Montero, M. L.; Uso´n, I.; Roesky, H. W. Angew. Chem. 1994, 106,
2198-2200; Angew. Chem., Int. Ed. Engl. 1994, 33, 2103-2104.
(9) Montero, M. L.; Voigt, A.; Teichert, M.; Uso´n, I.; Roesky, H. W.
Angew. Chem. 1995, 107, 2761-2763; Angew. Chem., Int. Ed. Engl.
1995, 34, 2504-2506.
(19) Winkhofer, N.; Voigt, A.; Dorn, H.; Roesky, H. W.; Steiner, A.;
Stalke, D.; Reller, A. Angew. Chem. 1994, 106, 1414-1416; Angew.
Chem., Int. Ed. Engl. 1994, 33, 1352-1354.
(20) Voigt, A.; Murugavel, R.; Chandrasekhar, V.; Winkhofer, N.;
Roesky, H. W.; Schmidt, H.-G.; Uso´n, I. Organometallics 1996, 15,
1610-1613.
(21) Gouzyr, A. I.; Wessel, H.; Barnes C. E.; Roesky, H. W.; Teichert,
M.; Uso´n, I. Inorg. Chem. 1997, 36, 3392-3393.
(22) Winkhofer, N.; Roesky, H. W.; Noltemeyer, M.; Robinson, W.
T. Angew. Chem. 1992, 104, 670-671; Angew. Chem., Int. Ed. Engl.
1992, 31, 599-601.
(10) Veith, M.; Schu¨tt, O.; Blin, J .; Becker, S.; Freres, J .; Huch, V.
Z. Anorg. Allg. Chem. 2002, 628, 138-146.
(11) Veith, M.; Vogelgesang, H.; Huch, V. Organometallics 2002, 21,
380-388.
(12) Walawalkar, M. G. Organometallics 2003, 22, 879-881.
(13) Veith, M.; J arczyk, M.; Huch, V. Angew. Chem. 1997, 109, 140-
142; Angew. Chem., Int. Ed. Engl. 1997, 36, 117-119.
(14) Driess, M.; Merz, K.; Rell, S. Eur. J . Inorg. Chem. 2000, 2517-
2522.
(23) Su, K.; Tilley, T. D.; Sailor, M. J . J . Am. Chem. Soc. 1996, 118,
3459-3468.
(24) Terry, K. W.; Lugmair, C. G.; Gantzel, P. K.; Tilley, T. D. Chem.
Mater. 1996, 8, 274-280.
(25) Murugavel, R.; Shete, V. S.; Baheti, K.; Davis, P. J . Organomet.
Chem. 2001, 625, 195-199.
(15) Merz, K.; Block, S.; Schoenen, R.; Driess, M. Dalton Trans.
2003, 3365-3369.
(26) Murugavel, R.; Davis, P.; Shete, V. S. Inorg. Chem. 2003, 42,
4696-4706.
(16) Hambrock, J .; Rabe, S.; Merz, K.; Birkner, A.; Wohlfart, A.;
Fischer, R. A.; Driess, M. J . Mater. Chem. 2003, 13, 1731-1736.
(17) Merz, K.; Hu, H. M.; Rell, S.; Driess, M. Eur. J . Inorg. Chem.
2003, 51-53.
(27) Fujiwara, M.; Wessel, H.; Park, H.-S.; Roesky, H. W. Tetrahe-
dron 2002, 58, 239-243.
(28) Fujiwara, M.; Wessel, H.; Park, H.-S.; Roesky, H. W. Chem.
Mater. 2002, 14, 49-75.
(18) Schmidbaur, H. Angew. Chem. 1965, 77, 206-216; Angew.
Chem., Int. Ed. Engl. 1965, 4, 201-211.
(29) Biscardi, J . A.; Meitzner, G. D.; Iglesia, E. J . Catal. 1998, 179,
192-202.
10.1021/om049863h CCC: $27.50 © 2004 American Chemical Society
Publication on Web 04/15/2004

2252 Organometallics, Vol. 23, No. 10, 2004
Anantharaman et al.
1051, 1043, 1017, 945, 919, 869, 837, 807, 753, 729, 685, 647,
611, 582, 547, 519, 480, 464, 445 cm-1. Anal. Calcd for
reactants, viz., RSi(OH)3 (three hydroxyl groups) vis-a`-
vis ZnR2 (two alkyl groups), provided a synthetic chal-
lenge and at the same time an unique opportunity.
Thus, unlike in other instances that we had investigated
earlier, the reaction between these two substrates was
subject to considerable stoichiometric control. The sen-
sitivity of the eventual nuclearity of the zinc siloxane
is governed by a subtle change in the molar ratio of the
reactants. Previously we have isolated and structurally
characterized two tetranuclear and octanuclear zinc
siloxanes (2-4).34-36 In a further demonstration of the
modulation of the nuclearity and topology of the zinc
siloxanes in terms of stoichiometric control, herein we
report the synthesis and X-ray crystal structures of Zn8-
Me7(dioxane)2(O3SiR)3 (R ) (2,6-i-Pr2C6H3)N(SiMe3)) (5)
and [Zn7Me2(THF)5(O3SiR)4] (R ) (2,6-i-Pr2C6H3)N-
(SiMe3)) (6). While isolating compound 6 we have also
obtained [Zn4(THF)4(MeZn)4(O3SiR)4] (R ) (2,6-i-Pr2-
C6H3)N(SiMe3)) (7) as another product from the same
reaction. The structure of 7 was similar to an octa-
nuclear zinc siloxane that we have reported earlier.35
C
71H131N3O15Si6Zn8 (1958.44): C, 43.54; H, 6.74; N, 2.15.
Found: C, 43.3; H, 6.8; N, 2.2.
Syn t h esis of Zin c Siloxa n es [Zn 7Me2(THF )5(O3SiR )4]
(6) a n d [Zn 4(THF )4(Zn Me)4(O3SiR)4] (7). A solution of
ZnMe2 (4.6 mL of a 2.0 M solution in toluene, 9.2 mmol) was
slowly added to a suspension of RSi(OH)3 (1.8 g, 3.87 mmol)
in THF/hexane (10 mL, 40 mL) at room temperature. After
the evolution of methane gas had ceased the resulting solution
was further stirred overnight at room temperature. A white
precipitate was obtained and was separated by filtration from
the mother solution. The precipitate was dissolved in toluene
and kept for crystallization. Colorless crystals of 6 were
obtained from this solution. The original filtrate from the
reaction mixture was concentrated to 15 mL and kept at room
temperature for crystallization. Colorless crystals of 7 were
obtained from this solution.
1
6: Yield: 0.8 g, 25%. Mp > 300 °C. H NMR (300.13 MHz,
C6D6): δ 0.12, 0.02, (s, 6H, Zn(CH3)), 0.06 (m, 36H, Si(CH3)3),
1.11 (m, 48H, CH(CH3)2), 1.75 (m, 16H, OCH2CH2), 3.64 (m,
24H, OCH2, CH(CH3)2), 6.93 (m, 12H, aromatic). 29Si NMR
(99.36 MHz, C6D6): δ -53.62, -61.10 (SiO3), 5.17, 4.20 (Si-
(CH3)3). IR (Nujol): ν˜ ) 1245, 1185, 1106, 1074, 1052, 1041,
1023, 967, 937, 916, 872, 837, 802, 755, 732, 680 cm-1. Anal.
Calcd for C89H159N4O17Si8Zn7 (2239.65) (6‚C7H8): C, 47.7; H,
7.2; N 2.5. Found: C, 47.1; H, 7.1; N, 2.5.
Exp er im en ta l Section
Gen er a l In for m a tion a n d Ma ter ia ls. All manipulations
were performed on a vacuum line or in the glovebox under a
purified N2 atmosphere. Solvents were distilled from Na/
benzophenone ketyl prior to use. Zinc dimethyl (2 M solution
in toluene) (Fluka) and zinc diethyl (1.1 M solution in toluene)
(Fluka) were purchased and used as received. Caution: Zinc
alkyls are highly pyrophoric. They should be handled in an
efficient fume hood by the use of rigorous Schlenk techniques.
RSi(OH)3 was prepared according to the procedure reported
previously.19 Elemental analyses were performed by the Ana-
lytisches Labor des Instituts fu¨r Anorganische Chemie, der
Universita¨t Go¨ttingen. NMR spectra were recorded on an AM
200 Bruker instrument. Chemical shifts are reported in ppm
with reference to TMS (1H and 29Si). IR spectra were recorded
on a Bio-Rad FTS-7 spectrometer as Nujol mulls. Melting
points were measured in a sealed glass tube and are not
corrected.
7: Yield: 0.41 g, 15.0%. Mp > 200 °C. 1H NMR (300.13 MHz,
CDCl3): δ -0.15, -0.03, 0.06, 0.07, 0.09 (s, 12H, ZnCH3), 0.15,
(m, 36H, Si(CH3)3), 0.37 (m, 48H, CH(CH3)2), 1.81 (m, 16H,
OCH2CH2), 3.73 (br, 24H, OCH2, CH(CH3)2), 6.95 (m, 12H,
aromatic). IR (Nujol): ν˜ ) 1318, 1257, 1246, 1182, 1106, 1052,
1042, 1031, 966, 937, 917, 872, 835, 801, 753, 728, 693, 683,
600, 599, 546, 501 cm-1. Anal. Calcd for C80H148N4O16Si8Zn8
(2169.85): C, 44.28; H, 6.87; N, 2.58. Found: C, 45.26; H, 6.89;
N, 2.31.
X-r a y Cr ysta llogr a p h y. Single crystals of 5-7 obtained
from the above reactions were subjected to X-ray diffraction
studies. The X-ray cell parameters are given in Table 1. 7 has
a structure similar to that of compound 4, which we have
reported earlier, except that it crystallizes with an additional
THF molecule. The structural parameters of 7 are similar to
that found for 4 except that the cell parameters of 7 are
different. Because of this, X-ray structure cell parameters for
this compound are given in Table 1. Diffraction data were
collected on a IPDS II Stoe image-plate diffractometer with
graphite-monochromated Mo KR radiation (λ ) 0.71073 Å).
The structure was solved by direct methods (SHELX-97)37 and
refined against F2 on all data by full-matrix least squares with
SHELX-97.38 The heavy atoms were refined anisotropically.
Hydrogen atoms were included using the riding model with
Uiso tied to the Uiso of the parent atoms.
Syn th esis of [Zn 8Me7(d ioxa n e)2(O3SiR)3] (R ) (2,6-i-
P r 2C6H3)N(SiMe3)) (5). A solution of ZnMe2 (4.5 mL of a 2.0
M solution in toluene, 9.0 mmol) was slowly added to a
suspension of RSi(OH)3 (1.0 g, 3.05 mmol) in dioxane/hexane
(5 mL, 40 mL) at room temperature. After the evolution of
methane gas had ceased the resulting solution was stirred for
a further period of 16 h at room temperature. It was then
concentrated to 10 mL and kept for crystallization at room
temperature to yield colorless crystals of 5.
1
5: Yield: 1.53 g, 80%. Mp > 200 °C. H NMR (200.13 MHz,
Resu lts a n d Discu ssion
CDCl3, TMS): δ -0.73, (s, 21H, ZnCH3), 0.07 (s, 27H, Si-
(CH3)3), 1.14 (d, 18H, CH(CH3)2), 1.23 (d, 18H, CH(CH3)2), 3.61
(br, 22H, dioxane protons, CH(CH3)2), 7.16 (m, 9H, aromatic)
29Si NMR (99.36 MHz, CDCl3): δ -62.2 (SiO3), 6.18 (SiMe3).
IR (Nujol): ν˜ ) 1317, 1291, 1249, 1183, 1169, 1124, 1106, 1066,
As described by us earlier, an equimolar (4:4) reaction
between RSi(OH)3 and ZnEt2 afforded cage 2 (Scheme
1).36 In compound 2 all the cage zinc atoms are dealky-
lated, whereas every silicon atom has one unreacted
hydroxyl group. These hydroxyl groups can be depro-
tonated in a facile manner in a reaction involving RSi-
(OH)3, ZnMe2, and MeLi in a 4:4:4 stoichiometry to
afford the hetero-trimetallic cage 3. On the other hand
the reaction between RSi(OH)3 and ZnMe2 in a 4:8 ratio
afforded the octanuclear zinc cage 4. It is of interest to
note that both 3 and 4 can be prepared independently
starting from 2.35,36
(30) Circolo, M. F.; Norby, P.; Hanson, J . C.; Corbin, D. R.; Grey, C.
P. J . Phys. Chem. B 1999, 103, 346-356.
(31) Murray, D. K.; Howard, T.; Goguen, P. W.; Krawietz, T. R.;
Haw, J . F. J . Am. Chem. Soc. 1994, 116, 6354-6360.
(32) Murray, D. K.; Chang, J . W.; Haw, J . F. J . Am. Chem. Soc.
1993, 115, 4732-4741.
(33) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Clarendon
Press: Oxford, 1984.
(34) Anantharaman, G.; Reddy, N. D.; Roesky, H. W.; Magull, J .
Organometallics 2001, 20, 5777-5779.
(35) Anantharaman, G.; Roesky, H. W.; Magull, J . Angew. Chem.
2002, 114, 1274-1277; Angew. Chem., Int. Ed. 2002, 41, 1226-1229.
(36) Anantharaman, G.; Roesky, H. W.; Schmidt, H.-G.; Noltemeyer,
M.; Pinkas, J . Inorg. Chem. 2003, 42, 970-973.
(37) Sheldrick, G. M. Acta Crystallogr. Sect. A 1990, 46, 467-473.
(38) Sheldrick, G. M. SHELX-97, Program for Crystal Structure
Refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997.

New Polyhedral Zinc Siloxanes
Organometallics, Vol. 23, No. 10, 2004 2253
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e Refin em en t Deta ils for 5, 6, a n d 7
5‚C7H8‚C4H8O2
6‚2 C7H8
7‚C4H8O‚2 C7H8
empirical formula
fw
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
C
71H131N3O15Si6Zn8
C
96H166N4O17Si8Zn7
C
102H172N4O17Si8Zn8
1958.29
triclinic
P1
2330.64
triclinic
P1h
2418.08
triclinic
P1h
12.893(3)
13.297(3)
15.071(3)
68.66(3)
69.74(3)
79.88(3)
2254.4(8)
133(2)
14.512(3)
15.663(3)
28.867(3)
96.43(3)
100.64(3)
114.54(3)
5734(2)
133(2)
14.436(3)
16.042(3)
27.288(6)
82.35(3)
89.79(3)
66.49(3)
5735(2)
133(2)
T (K)
λ (Å)
0.71073
1
1.442
2.226
1022
0.71073
2
1.350
1.581
2456
0.71073
2
1.40
1.785
2544
Z
Fcalcd (Mg m-3
)
µ (mm-1
F(000)
)
θ (deg)
no. of reflns colld
R (int)
1.52 to 24.83
34 851
0.0584
1.46 to 24.85
120 336
0.0937
1.49 to 24.84
43 024
0.0796
no. of data
13 876
19 778
19 382
no. of restraints
no. of params
3
958
0.992
0.0395, 0.0947
0.0431, 0.0959
0.743 and -0.414
1
1
1123
1.018
0.0326, 0.0664
0.0505, 0.0679
0.993, and -0.457
1170
0.922
0.0455, 0.1111
0.0656, 0.1177
1.182 and -0.698
goodness-of-fit on F2
R1 [I > 2σ (I)]a
wR2 all data
largest diff peak and hole (e Å-3
)
a
2
2
2
R1 ) ∑||Fo| - |Fc||/∑|Fo|. wR2 ) [∑w(|Fo | - |Fc |)2/∑w|Fo| ]1/2
.
Ta ble 2. Selected Bon d Len gth s (Å) a n d Bon d
An gles (d eg) of 5
The formation of the octanuclear zinc cage 4 prompted
us to investigate the reaction of RSi(OH)3 with ZnMe2
further. The objective was to check if such high nucle-
arity products were formed in reactions involving other
stoichiometric ratios as well. A second point of interest
was to investigate the possibility of formation of slightly
lower nuclearity cages enroute to the formation of 4.
Accordingly the reaction of RSi(OH)3 with ZnMe2 in a
1:3 ratio was carried out, and an octanuclear zinc
siloxane [Zn8Me7(dioxane)2(O3SiR)3] (5) was isolated in
about 80% yield (Scheme 2). Analogous to other metal-
lasiloxanes prepared from RSi(OH)3, 5 also is lipophilic
and is soluble in various organic solvents such as
benzene, toluene, and THF. The 29Si NMR of 5 revealed
the presence of two resonances at -62.2 ppm (δ SiO3)
and 6.18 ppm (δ Si(CH3)3). However, an unambiguous
characterization of 5 could only be accomplished by its
single-crystal X-ray measurement (vide infra).
Zn(1)-O(2)
Zn(1)-O(6)
Zn(2)-O(7)
Zn(2)-O(6)
Zn(3)-O(3)
Zn(5)-O(5)
Zn(7)-C(300)
Zn(8)-O(4)
Si(11)-O(5)
Si(11)-O(4)
Si(12)-O(1)
1.970(4)
2.067(4)
1.942(4)
1.955(3)
1.992(4)
2.097(4)
1.969(6)
1.856(4)
1.619(4)
1.663(4)
1.636(4)
Zn(1)-C(41)
Zn(1)-O(4)
Zn(2)-C(61)
Zn(3)-C(400)
Zn(3)-O(9)
Zn(7)-O(9)
Zn(7)-O(2)
Zn(8)-C(60)
Si(11)-O(6)
Si(12)-O(3)
Si(12)-O(2)
1.977(6)
2.365(4)
1.943(6)
1.961(7)
1.995(4)
1.938(4)
1.985(3)
1.921(7)
1.626(4)
1.628(4)
1.636(3)
O(2)-Zn(1)-C(41) 131.6(2)
C(41)-Zn(1)-O(6) 120.3(2)
C(41)-Zn(1)-O(4) 105.1(2)
O(3)-Si(12)-O(1) 107.4(2)
O(2)-Zn(1)-O(6)
O(2)-Zn(1)-O(4)
O(6)-Zn(1)-O(4)
104.17(15)
106.93(14)
70.42(13)
O(3)-Si(12)-O(2) 108.78(19)
O(1)-Si(12)-O(2) 107.94(19) O(3)-Si(12)-N(1) 108.5(2)
O(1)-Si(12)-N(1) 111.2(2) O(2)-Si(12)-N(1) 112.8(2)
O(3)-Si(12)-Zn(6) 41.48(13) O(1)-Si(12)-Zn(6) 66.28(14)
O(2)-Si(12)-Zn(6) 126.46(14) N(1)-Si(12)-Zn(6) 118.66(16)
The possibility of the existence of other intermediate
products during the formation of 4 was probed by
carrying out the reaction of RSi(OH)3 with ZnMe2 in a
1:1.75 ratio. One of the products isolated was the
octanuclear cage 7, which was shown by X-ray crystal-
lography to be a polymorph of 4 (similar structure,
different cell parameters, see Table 1 and Supporting
Information).35 However, in addition to 7 another prod-
uct, the heptanuclear cage [Zn7Me2(THF)5(O3SiR)4] (6),
could be crystallized (Scheme 3). The 29Si NMR of 6
revealed two resonances in the high-field region. One
of these that resonates at -61.1 ppm (δ SiO3) could be
assigned to the silicon centers Si5 and Si7 (see Figure
2) on the basis of our earlier work. The other resonance
at -53.3 ppm (δ SiO3) is assigned to the silicon centers
Si1 and Si3 (see Figure 2), which are involved in binding
to three dealkylated zinc centers.
in Table 2. The molecular structure of 5 consists of a
Zn7Si3O9 core. This is a complex cage arrangement and
can be understood as being a derivative of a drum-
shaped structure. Thus, if two hexagons are fused to
each other, in a face-to-face manner, one obtains a drum
with the sides of the drum having six four-membered
rings. In the current structure the floor and roof of the
drum are represented by two Zn2SiO3 six-membered
rings [Zn1-O2-Si12-O1-Zn5-O4; Zn2-O6-Si11-
O5-Zn4-O7]. Two of the drum edges are broken; this
leaves only three of the possible six four-membered rings
intact (Figure 2).
These three four-membered rings consist of two
ZnSiO2 and Zn2O2 rings [Zn1-O6-Si11-O4; Zn5-O4-
Si11-O5] and one Zn2O2 ring [Zn4-O1-Zn5-O5]. The
fusion of this incomplete drum with a Zn3SiO3 motif
[Zn7-O9-Zn3-O3-Zn6-O8-Si10] completes the cage.
One of the most interesting aspects of the structure of
5 is that one of the zinc centers (Zn8) is not part of the
X-r a y Cr ysta l Str u ctu r es of 5 a n d 6. The molecular
structure of 5 is shown in Figure 1. Selected metric
parameters obtained for this compound are summarized

2254 Organometallics, Vol. 23, No. 10, 2004
Anantharaman et al.
Sch em e 1
Sch em e 2
cage at all but is merely present as an exocyclic
substituent. Thus, the overall complex cage structure
of 5 consists of five six-membered Zn2SiO3 rings, three
ZnSiO2 four-membered rings, one Zn2O2 four-membered
ring, and one Zn4SiO3 eight-membered ring. An inter-
esting feature of this arrangement is the contiguous
fusion of the four six-membered Zn2SiO3 rings. It may
be noted that the mineral hemimorphite has a sheet-
like structure comprised of fused Zn2SiO3 rings.33 Of the
eight zinc centers present in 5, only one (Zn4) is
completely dealkylated. All the rest have one methyl
group each. In contrast, all the hydroxyl groups of the
silanetriol are completely deprotonated. This is consis-
tent with the stoichiometry of the reactants. Another
interesting aspect of the molecular structure of 5 is the
coordination environment around zinc atoms. Similar
to what was observed in the molecular structure of 4,
in 5 also various zinc centers adopt different coordina-

New Polyhedral Zinc Siloxanes
Organometallics, Vol. 23, No. 10, 2004 2255
F igu r e 1. ORTEP diagram of core 5 with 50% probability. Most of the substituents on silicon and zinc atoms are omitted
for the sake of clarity.
Sch em e 3
tion geometries. Thus, three of the zinc centers (Zn2,
Zn6, Zn7) are trigonal planar, three others (Zn1, Zn3,
Zn5) are in a tetrahedral geometry, and one (Zn4) has
a trigonal bipyramidal geometry.35 However, the eighth
zinc atom is dicoordinate and has nearly linear geom-
etry. Such a structural variation within a single mol-
ecule is quite remarkable and to the best of our
knowledge has only one precedent.35 The metric param-
eters of the zinc siloxane 5 are quite normal and are
analogous to those found earlier.34-36
The molecular structure of 6 is shown in Figure 3.
Selected metric parameters obtained for this compound
are summarized in Table 3. The molecular structure of
6 consists of a nearly cubic Zn4Si4O12 unit with zinc and
silicon atoms occupying alternate corners of the cube
[Zn2-Si3-Zn3-Si1-Zn5-Si5-Zn6-Si7]. In this re-
gard this framework is analogous to those found for
other metallasiloxanes of the type [RSiO3M‚S]4 (M )
F igu r e 2. Drum-shaped motif of 5. Two edges in this motif
are broken (shown by dashed lines, Si- - - O and Zn- - -O)
to afford compound 5.

2256 Organometallics, Vol. 23, No. 10, 2004
Anantharaman et al.
Si-OH units in the polyhedral cage 2. The uncapped
faces of the polyhedral cage 6 are composed of normal
Zn2SiO4 rings, because of capping by zinc atoms, while
the other faces undergo modification. Thus, capping by
Zn1 leads to the formation of four four-membered rings
(two ZnSiO2: Zn1-O1-Si1-O2 and Zn1-O4-Si3-O5;
two Zn2O2: Zn1-O1-Zn2-O5 and Zn1-O2-Zn3-O4),
while capping by Zn4 and Zn7 leads to two new six-
membered rings [Zn4-O3-Si1-O1-Zn2-O7; Zn7-
O6-Si3-O4-Zn3-O12]. Among the seven zinc atoms
present in 6, six are tetracoordinate, while one (Zn1) is
pentacoordinate. Five of the zinc atoms are completely
dealkylated, while the remaining two (Zn4, Zn7) retain
one methyl group each.
The Si-O and Zn-O bond distances in 6 fall in the
ranges 1.585(2)-1.657(2) Å and 1.838(2)-2.170(2) Å,
respectively. A similar range of bond distances was also
observed in the case of other zinc siloxanes that have
been reported by us.34-36
To ascertain if compounds 5 and 6 retain their
structures in solution, we have attempted to carry out
electron spray mass spectra as well as vapor pressure
osmometric studies. However, because of the extreme
sensitivity of these compounds to air and moisture,
these experiments were not successful.
F igu r e 3. ORTEP diagram of core 6 with 50% probability.
Most of the substituents on silicon and zinc atoms are
omitted for the sake of clarity.
Ta ble 3. Selected Bon d Len gth s (Å) a n d Bon d
An gles (d eg) of 6
Zn(1)-O(13)
Zn(1)-O(4)
Zn(1)-O(2)
Zn(2)-O(5)
Zn(2)-O(15)
Zn(7)-O(6)
Zn(7)-O(11)
Si(1)-O(2)
Si(3)-O(6)
Si(3)-O(4)
Si(5)-O(8)
Si(7)-O(10)
1.966(3)
2.061(2)
2.100(2)
1.946(2)
2.066(2)
2.054(2)
2.116(2)
1.641(2)
1.619(2)
1.647(2)
1.638(2)
1.585(2)
Zn(1)-O(1)
Zn(1)-O(5)
Zn(2)-O(7)
Zn(2)-O(1)
Zn(7)-C(82)
Zn(7)-O(12)
Si(1)-O(3)
Si(1)-O(1)
Si(3)-O(5)
Si(5)-O(9)
Si(5)-O(7)
Si(7)-O(12)
2.053(2)
2.090(2)
1.880(2)
1.998(2)
1.968(4)
2.102(2)
1.609(2)
1.651(2)
1.628(2)
1.590(2)
1.646(2)
1.657(2)
Con clu sion
The products formed in the reaction of zinc alkyls with
silanetriol are dependent on the stoichiometric ratio of
these two reagents. This is possibly because of the
difference and mismatch in the number of functional
groups contained in these reagents. Thus, we have been
able to assemble five different zinc siloxanes with a Zn/
Si ratio varying from 4:4 to 8:3. The presence of reactive
Zn-Me units in these molecules renders these as attrac-
tive precursors for further elaboration.
O(13)-Zn(1)-O(1) 104.26(9) O(9)-Zn(6)-O(11) 138.47(10)
O(9)-Zn(6)-O(6) 119.74(10) O(11)-Zn(6)-O(6) 90.99(9)
O(9)-Zn(6)-O(17) 101.33(10) O(11)-Zn(6)-O(17) 91.59(9)
O(6)-Zn(6)-O(17) 110.66(10) O(13)-Zn(1)-O(4) 109.61(10)
O(1)-Zn(1)-O(4) 146.12(9) O(13)-Zn(1)-O(5) 120.62(10)
Ack n ow led gm en t. This work was supported by the
Deutsche Forschungsgemeinschaft (Graduiertenkol-
leg: Spektroskopie und Dynamik Molekularer Ag-
gregate, Ketten, Kna¨uel und Netzwerke) and the Go¨t-
tinger Akademie der Wissenschaften. V.C. thanks the
Alexander von Humboldt Foundation, Bonn, Germany,
for the Friedrich Wilhelm Bessel-Research award.
Al, Ga, In; S ) THF).1-3,7,8 Three of the contiguous faces
of the cube are capped by zinc atoms (Zn4, Zn1, Zn7),
while the other three faces are open. The presence of
the capping zinc units is reminiscent of the molecular
structure of the trimetallic derivative 3 (Scheme 1). In
this latter instance the four contiguous faces of the cubic
polyhedron are capped by the lithium ions. Thus, the
structural relationship between 3 and 6 is readily
established. The formation of both of these compounds
may be traced to the dehydroxylation of the residual
Su p p or tin g In for m a tion Ava ila ble: Single-crystal X-ray
structural data of compounds 5, 6, and 7 (CIF format). This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM049863H