The first niobasilsesquioxanes

Article abstract of DOI:10.1021/om0606587

Synthetic routes leading to the first polyhedral oligosilsesquioxane derivatives incorporating niobium are reported. Tetrasilanol (c-C ₆H₁₁)₆Si₆₆O ₇(OH)₄ (2), for which an improved laboratory-scale synthesis is reported, cleanly reacts with Nb(OEt)₅ to afford the dinuclear niobasilsesquioxane [(c-C₆H₁₁) ₆Si₆O₁₁-NbOEt]₂ (3), which slowly converts to the tetranuclear μ-oxo species 4 in the presence of moisture. A structurally different μ-ethoxy-bridged dinuclear niobasilsesquioxane, [(c-C₆H₁₁)₇Si₇O ₁₂NbOEt(μ-OEt)]₂ (6), is readily accessible from (c-C₆H₁₁)₇Si₇O₉(OH) ₃ (5) and Nb(OEt)₅. The molecular structures of 2, 3, 4, and 6 have been determined by X-ray diffraction.

Full text of DOI:10.1021/om0606587

5922
Organometallics 2006, 25, 5922-5926
The First Niobasilsesquioxanes§
Volker Lorenz,† Steffen Blaurock,† Helmar Go¨rls,‡ and Frank T. Edelmann*,†
Chemisches Institut der Otto-Von-Guericke-UniVersita¨t Magdeburg, UniVersita¨tsplatz 2, D-39106
Magdeburg, Germany, and Institut fu¨r Anorganische und Analytische Chemie, August-Bebel-Strasse 2,
D-07743 Jena, Germany
ReceiVed July 21, 2006
Synthetic routes leading to the first polyhedral oligosilsesquioxane derivatives incorporating niobium
are reported. Tetrasilanol (c-C6H11)6Si6O7(OH)4 (2), for which an improved laboratory-scale synthesis is
reported, cleanly reacts with Nb(OEt)5 to afford the dinuclear niobasilsesquioxane [(c-C6H11)6Si6O11-
NbOEt]2 (3), which slowly converts to the tetranuclear µ-oxo species 4 in the presence of moisture. A
structurally different µ-ethoxy-bridged dinuclear niobasilsesquioxane, [(c-C6H11)7Si7O12NbOEt(µ-OEt)]2
(6), is readily accessible from (c-C6H11)7Si7O9(OH)3 (5) and Nb(OEt)5. The molecular structures of 2, 3,
4, and 6 have been determined by X-ray diffraction.
in 19947 and Sullivan in 1997.8 Also known in the literature
are some niobium species containing the bulky tBu3SiO- ()
silox) ligand. Especially remarkable is the PC bond cleavage
of (silox)3NbPMe3 under dihydrogen leading to (silox)3Nbd
CH2, (silox)3NbdPH or (silox)3NbP(H)(silox)3Nb, and CH4.9
This unusual reactivity clearly demonstrates the need for further
work on niobium siloxides. Here we report a facile synthetic
route to niobasilsesquioxanes starting from commercially avail-
able Nb(OEt)5.
Introduction
Incompletely condensed silsesquioxanes1 form an interesting
class of Si-O cage compounds representing partial structures
of the silica surface used to support heterogeneous catalysts.
Metallasilsesquioxanes2 derived from these species have been
found to effectively catalyze the polymerization and epoxidation
of olefins.2d The “Periodic Table of Metallasilsesquioxanes”2e
already covers numerous metallic elements ranging from alkaline
metals2,3 through early2,4 and late transition metals2,5 to the
lanthanide elements.2,6 Surprisingly, niobium represents one of
the elements for which no metallasilsesquioxane derivatives are
known to date, although the first silanediolates and disilox-
anediolates of niobium have already been reported by Roesky
Experimental Section
General Comments. Preparation of the niobium compounds and
handling of reagents were performed under an atmosphere of dry
nitrogen or argon using standard Schlenk techniques or a glovebox
where the O2 and H2O levels were usually kept below 1 ppm. All
glassware was oven-dried at 140 °C for at least 24 h, assembled
while hot, and cooled under high vacuum prior to use. Toluene
was dried over Na/benzophenone and freshly distilled prior to use.
Nb(OEt)5 (Strem) was used as received; (c-C6H11)6Si6O9 (1) and
(c-C6H11)7Si7O9(OH)3 (5) were prepared according to literature
procedures.1 IR spectra were recorded using KBr pellets on a Perkin-
Elmer FT-IR spectrometer system 2000 between 4000 and 400
§ Dedicated to Professor Erwin Weiss on the occasion of his 80th
birthday.
* Corresponding author. Phone: +49-391-67-18327. Fax: +49-391-67-
12933. E-mail: frank.edelmann@vst.uni-magdeburg.de.
† Magdeburg University.
‡ Jena University.
(1) (a) Feher, F. J.; Newman, D. A.; Walzer, J. F. J. Am. Chem. Soc.
1989, 111, 1741. (b) Feher, J. F.; Newman, D. A. J. Am. Chem. Soc. 1990,
112, 1931. (c) Feher, F. J.; Budzichowski, T. A.; Blanski, R. L.; Weller, K.
J.; Ziller, J. W. Organometallics 1991, 10, 2526. (d) Feher, F. J.; Terroba,
R.; Ziller, J. W. Chem. Commun. 1999, 2526.
1
cm-1. H (400 MHz), 13C (101 MHz), and 29Si (79.5 MHz) NMR
(2) For reviews see: (a) Feher, F. J.; Budzichowski, T. A. Polyhedron
1995, 14, 3239. (b) Murugavel, R.; Voigt, A.; Walawalkar, M. G.; Roesky,
H. W. Chem. ReV. 1996, 96, 2205. (c) Abbenhuis, H. C. L. Chem. Eur. J.
2000, 6, 25. (d) Lorenz, V.; Fischer, A.; Giessmann, S.; Gilje, J. W.; Gun’ko,
Y.; Jacob, K.; Edelmann, F. T. Coord. Chem. ReV. 2000, 206-207, 321.
(e) Duchateau, R. Chem. ReV. 2002, 102, 3525. (f) Hanssen, R. W. J. M.;
van Santen, R. A.; Abbenhuis, H. C. L. Eur. J. Inorg. Chem. 2004, 675.
(g) Lorenz, V.; Edelmann, F. T. Z. Anorg. Allg. Chem. 2004, 630, 1147.
(h) Lorenz, V.; Edelmann, F. T. AdV. Organomet. Chem. 2005, 53, 101.
(3) (a) Annand, J.; Aspinall, H. C.; Steiner, A. Inorg. Chem. 1999, 38,
1931. (b) Lorenz, V.; Giessmann, S.; Gun’ko, Y. K.; Fischer, A. K.; Gilje,
J. W.; Edelmann, F. T. Angew. Chem. 2004, 116, 4703; Angew. Chem.,
Int. Ed. 2004, 43, 4603.
(4) (a) Feher, F. J.; Budzichowski, T. A.; Rahimian, K.; Ziller, J. W. J.
Am. Chem. Soc. 1992, 114, 3859. (b) Crocker, M.; Herold, R. H. M.; Orpen,
A. G.; Overgaag, M. T. A. J. Chem. Soc., Dalton Trans. 1999, 3791. (c)
Wada, K.; Bundo, M.; Nakabayashi, D.; Itayama, N.; Kondo, T.; Mitsudo,
T. Chem. Lett. 2000, 628. (d) Chabanas, M.; Quadrelli, E. A.; Fenet, B.;
Coperet, C.; Thivolle-Cazat, J.; Basset, J. M.; Lesage, A.; Emsley, L. Angew.
Chem. 2001, 113, 4625; Angew. Chem., Int. Ed. 2001, 40, 4493.
(5) (a) Abbenhuis, H. C. L.; Burrows, A. D.; Kooijman, H.; Lutz, M.;
Palmer, M. T.; van Santen, R. A.; Spek, A. L. Chem. Commun. 1998, 2627.
(b) Quadrelli, E. A.; Davies, J. E. Johnson, B. F. G.; Feeder, N. Chem.
Commun. 2000, 1031. (c) Maxim, N.; Overweg, A.; Kooijman, P. J.; van
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P. A.; Bigham, W. S.; Hay, M. T. Inorg. Chim. Acta 2003, 345, 255.
spectra were recorded in THF-d8 solutions on a Bruker DPX 400
spectrometer at 25 °C. Chemical shifts are referenced to TMS.
Microanalyses of the compounds were performed using a Leco
CHNS 923 apparatus. Melting/decomposition points were measured
in sealed glass tubes with a Bu¨chi B-450 digital melting point
apparatus and are not corrected.
(6) (a) Herrmann, W. A.; Anwander, R.; Dufaud, V.; Scherer, W. Angew.
Chem. 1994, 106, 1338; Angew. Chem., Int. Ed. 1994, 33, 1285. (b) Annand,
J.; Aspinall, H. C.; Steiner, A. Inorg. Chem. 1999, 38, 3941. (c) Annand,
J.; Aspinall, H. C. J. Chem. Soc., Dalton Trans. 2000, 1867. (d) Arnold, P.
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Chem. 2001, 113, 1319; Angew. Chem., Int. Ed. 2001, 40, 1279.
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Soc., Dalton Trans. 1997, 3363.
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42, 6204. (b) Hirsekorn, K. F.; Veige, A. S.; Marshak, M. P.; Koldobskaya,
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10.1021/om0606587 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 11/07/2006

The First Niobasilsesquioxanes
Organometallics, Vol. 25, No. 25, 2006 5923
Table 1. Crystallographic Data of (c-C6H11)6Si6O7(OH)4 (2), [(c-C6H11)6Si6O11NbOEt]2 (3),
[(c-C6H11)6Si6O11Nb2(OEt)(µ-OEt)(µ-O)(THF)]2 (4), and [(c-C6H11)7Si7O12NbOEt(µ-OEt)]2 (6)
2
3
4
6
formula
C38H75O11.5Si6
884.52
C78H142.5Nb2O24.5Si12
1995.32
C52H100Nb2O18Si6
1367.68
C46H87NbO14Si7
1153.70
fw
temperature (K)
cryst syst
space group
unit cell dimens (Å, deg)
183(2)
210(2)
180(2)
293(2)
monoclinic
P21/n
a ) 12.6692(2)
b ) 19.5522(3)
c ) 20.8522(5)
â ) 103.511(1)
tetragonal
P42/n
a ) 26.642(4)
b ) 26.642(4)
c ) 14.580(3)
triclinic
P1h
triclinic
P1h
a ) 14.677(3)
b ) 14.949(3)
c ) 17.199(3)
R ) 104.56(3)
â ) 96.05(3)
γ ) 108.18(3)
3399.9(12)
a ) 11.848(2)
b ) 16.503(3)
c ) 16.665(3)
R ) 100.03(3)
â ) 106.87(3)
γ ) 94.21(3)
3043.9(11)
2, 1.258
volume (Å3)
5022.36(16)
4, 1.170
0.216
1916
1.95 to 27.12
-16 e h e 15,
-23 e k e 25,
-24 e l e 26
31 575/11 047
[R(int) ) 0.0516]
θ ) 27.12, 99.6%
none
10349(3)
4, 1.281
0.424
4226
Z, calcd density (g cm-3
)
2, 1.336
0.504
1444
absorp coeff (mm-1
)
0.390
1228
F(000)
θ-range for data collection (deg)
2.07 to 25.95
-32 e h e 32,
-32 e k e 32,
-17 e l e 17
13 7493/10 097
[R(int) ) 0.1178]
θ ) 25.95, 99.8%
numeric
3.52 to 30.51
-20 e h e 20,
-21 e k e 20,
-22 e l e 24
32 557/20 258
[R(int) ) 0.0444]
θ ) 30.51, 97.5%
numeric
3.09 to 26.83
-14 e h e 14,
-19 e k e 20,
-21 e l e 21
35 694/12 858
[R(int) ) 0.0588]
θ ) 26.83, 98.5%
numeric
limiting indices
no. of reflns collected/unique
completeness to
absorp corr
max. and min. transmn
0.9589 and 0.8488
0.8239 and 0.7867
0.9620 and 0.9260
refinement method
full-matrix least-squares on F2
no. of data/restraints/params
11 047/4/511
1.047
10 097/0/556
0.751
20 258/0/722
0.983
12 858/0/865
1.089
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R1 ) 0.0797,
wR2 ) 0.2141
R1 ) 0.1140,
wR2 ) 0.2388
2.940 and -0.941
R1 ) 0.0395,
wR2 ) 0.0966
R1 ) 0.0905,
wR2 ) 0.1048
0.779 and -0.315
R1 ) 0.0568,
wR2 ) 0.1598
R1 ) 0.0855,
wR2 ) 0.1752
2.985 and -1.075
R1 ) 0.0488,
wR2 ) 0.1027
R1 ) 0.0674,
wR2 ) 0.1095
0.463 and -0.653
R indices (all data)
largest diff peak and hole (e Å-3
)
Scheme 1
g (63.6%). Decomposition > 112 °C. IR (KBr, ν (cm-1)): 2975
m, 2923 s, 2850 s, 1609 w, 1558 w, 1448 m, 1379 w, 1355 w,
1268 m, 1244 m, 1195 m, 1108 vs, 1067 vs, 986 s, 924 s, 892 s,
848 m, 825 m, 807 m, 762 w, 641 w, 530 m, 514 m, 463 m, 411
m. Anal. Calc for C76H142Nb2O24Si12 (1962.78 g mol-1): C 46.51,
1
H 7.29. Found: C 46.14, H 7.55. H NMR (400 MHz, [D8]THF,
25 °C): δ 4.35 (br s, 4H; CH2, OC2H5), 2.00-1.50 (br m, 60H;
CH2, c-C6H11), 1.40-1.00 (br m, 60H; CH2, c-C6H11 + 6H, CH3,
OC2H5), 0.80-0.55 (br m, 12H, CH, c-C6H11) ppm. 13C NMR (101
MHz, [D8]THF, 25 °C): δ 74.05-70.01 (br, O-CH2-CH3), 28.89,
28.81, 28.64, 28.49, 28.40, 28.35, 28.26, 28.19, 28.09, 28.01, 27.94,
27.89, 27.82, 27.75, 27.67 (CH2, c-C6H11), 25.63, 25.61, 24.94,
24.67, 24.34, 24.14(SiCH, c-C6H11), 19.29-17.68 (br, O-CH2-CH3)
ppm. 29Si NMR (79.5 MHz, [D8]THF, 25 °C): δ -56.95, -61.84,
-64.07, -64.91, -67.75, -68.18 ppm.
Improved Laboratory-Scale Preparation of (c-C6H11)6Si6O7-
(OH)4 (2). A solution of (c-C6H11)6Si6O9 (1, 10.0 g, 12 mmol) and
20% aqueous NEt4OH (11.9 g, 12 mmol) in THF (120 mL) was
stirred at 25 °C for 30 min and then neutralized with dilute aqueous
HCl. Evaporation of the volatiles, dissolution in Et2O, drying over
MgSO4, and concentration afforded (c-C6H11)6Si6O7(OH)4 as a white
solid. Impurities of (c-C6H11)7Si7O9(OH)3 were removed by re-
crystallization from pentane. Yield: 6.5 g (64%). Decomposition
> 230 °C. Anal. Calc for C36H70O11Si6 (847.45 g mol-1): C 51.02,
Preparation of [(c-C6H11)7Si7O12NbOEt(µ-OEt)]2 (6). Nb(OEt)5
(0.327 g, 1.027 mmol) was added to a solution of 1 g (1.03 mmol)
of (c-C6H11)7Si7O12(OH)3 (5) in toluene (50 mL; the initially formed
suspension of (c-C6H11)7Si7O12(OH)3 was slightly heated until a
clear solution had formed). The reaction mixture was stirred for
24 h, followed by refluxing for an additional 2 h. One-third of the
solvent was evaporated in vacuum, and cooling of the solution to
5 °C for 1 week afforded [(c-C6H11)7Si7O12NbOEt(µ-OEt)]2 as
colorless crystals. Yield: 0.85 g (71.5%). Decomposition > 120
°C. IR (KBr, ν (cm-1)): 2923 s, 2850 s, 1448 m, 1379 w, 1355 w,
1269 m, 1244 m, 1196 m, 1107 vs, 1038 m, 1026 m, 1005 s, 951
w, 914 m, 892 s, 849 m, 826 w, 742 w, 647 w, 557 w, 513 m, 469
m, 411 m. Anal. Calc for C92H174Nb2O28Si14 (2307.38 g mol-1):
1
H 8.33. Found: C 50.55, H 8.48. H NMR (400 MHz, [D8]THF,
25 °C): δ 7.29 (br s, 4H, OH), 1.73 (vbr m, 30H; CH2, c-C6H11),
1.23 (vbr m, 30H; CH2, c-C6H11), 0.75 (vbr m, 6H, CH, c-C6H11)
ppm. 13C NMR (101 MHz, [D8]THF, 25 °C): δ 27.57, 27.48, 26.88,
26.64 (CH2, c-C6H11), 23.69, 23.14 (SiCH, c-C6H11) ppm. 29Si NMR
(79.5 MHz, [D8]THF, 25 °C): δ -59.4, -68.8 ppm. IR (KBr, ν
(cm-1)): 3272 m(br), 2923 vs, 2850 s, 1448 m, 1356 vw, 1269 w,
1197 m, 1111 vs, 1038 m, 1027 m, 999 w, 913 m, 895 s, 848 m,
825 w, 757 w, 741 w, 516 m, 494 w, 461 w, 417 w.
1
C 47.89, H 7.60. Found: C 47.34, H 7.39. H NMR (400 MHz,
[D8]THF, 25 °C): δ 4.60-4.20 (br s, 8H; CH2, OC2H5), 1.90-
1.60 (br m, 70 H; CH2, c-C6H11), 1.40-1.10 (br m, 70 H; CH2,
c-C6H11 + 12H, CH3, OC2H5), 0.80-0.60 (br m, 14H, CH, c-C6H11)
ppm. 13C NMR (101 MHz, [D8]THF, 25 °C): δ 69.53 (br, O-CH2-
CH3), 28.42, 28.28, 28.09, 27.74, 27.72, 27.65, 27.58 (CH2,
c-C6H11), 25.25, 24.25, 24.21 (SiCH, c-C6H11), 18.35 (br, O-CH2-
CH3) ppm. 29Si NMR (79.5 MHz, [D8]THF, 25 °C): δ -67.74,
-68.67, -69.47 ppm.
Preparation of [(c-C6H11)6Si6O11NbOEt]2 (3). Nb(OEt)5 (0.51
g, 1.60 mmol) was added to a solution of 1.33 g (1.57 mmol) of
(c-C6H11)6Si6O7(OH)4 (2) in toluene (50 mL; the initially formed
suspension of (c-C6H11)6Si6O7(OH)4 was slightly heated until a clear
solution had formed). The reaction mixture was stirred for 15 min,
followed by refluxing for an additional 1 h. One-third of the solvent
was distilled off, and cooling of the solution to 5 °C for 1 week
afforded [(c-C6H11)6Si6O11NbOEt]2 as colorless needles. Yield: 0.98

5924 Organometallics, Vol. 25, No. 25, 2006
Lorenz et al.
Figure 1. Perspective ORTEP view of the molecular structure of 2. Thermal ellipsoids are drawn to encompass 50% probability. Selected
distances [Å] and angles [deg]: Si-O 1.607(3)-1.627(3), Si-C 1.845(4)-1.861(4), intermolecular O-O (hydrogen bonds) O1A-O11B
2.729(4), O4A-O8B 2.700(5), O8A-O4B 2.700(5), O11A-O1B 2.729(4), O-Si-O 107.70(15)-110.8(2), Si-O-Si 142.06(19)-157.1-
(2), O-Si-C 107.51(18)-112.86(18).
Scheme 2
Results and Discussion
One of the silsesquioxane starting materials, the tetrasilanol
derivative (c-C6H11)6Si6O7(OH)4 (2), is prepared by cage-
opening of closo-(c-C6H11)6Si6O9 (1) in the presence of tetra-
ethylammonium hydroxide. Compound 2 was first reported by
Feher et al. and isolated in minor quantities (<0.2 g).10 We
found that the preparation of 2 according to Scheme 1 can be
scaled up by a factor of about 100, leading to synthetically useful
amounts of this valuable precursor.
An X-ray crystal structure determination of 2 revealed the
presence of hydrogen-bonded dimers in the solid state (Figure
1). Closely related structures have been reported earlier for the
corresponding tetraphenyl derivative11 and a cycloheptyl-
substituted tetrasilanol, although the structure of the latter could
not be fully refined due to severe disorder problems.1d
Commercially available niobium(V) pentaethoxide, Nb(OEt)5,
was found to be the reagent of choice for the straightforward
preparation of niobasilsesquioxane derivatives. Nb(OEt)5 cleanly
reacts with 2 in a 1:1 molar ratio in toluene solution under
elimination of 8 equiv of ethanol (Scheme 2). The resulting
dinuclear niobasilsesquioxane derivative [(c-C6H11)6Si6O11-
NbOEt]2 (3) was isolated in 64% yield in the form of colorless,
moderately moisture-sensitive needle-like crystals.
Crystal Data Collection, Structure Solution, and Refinement.
The intensity data of 2 were collected on a Nonius Kappa CCD
diffractometer using graphite-monochromated Mo KR radiation (λ
) 0.71073 Å). Data were corrected for Lorentz and polarization
effects, but not for absorption effects. The data for compound 2
were collected with the Bruker COLLECT program using ω-scans.
The diffraction data for the compounds 3, 4, and 6 were measured
on a Stoe IPDS 2T diffractometer with Mo KR radiation. The data
for compounds 3, 4, and 6 were collected with the Stoe XAREA
program using ω-scans. Numeric absorption correction was applied.
The space groups were determined with the XPREP program, and
the structures were solved by direct methods (SHELXS-97) and
refined with all data by full-matrix least-squares methods on F2
using SHELXL-97. Crystallographic data for the structures reported
in this paper have been deposited with the Cambridge Crystal-
lographic Data Centre as supplementary publication nos. CCDC-
617586 (2), CCDC-617587 (3), CCDC-617588 (4), and CCDC-
617589 (6). These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ,
UK; fax: (+44)1223-336-033; or deposit@ccdc.cam.ac.uk). Data
collection parameters are given in Table 1.
1
A H NMR spectrum of 3 showed the presence of only one
remaining ethoxide ligand per niobium, while the 29Si NMR
spectrum displayed six signals at δ -56.95, -61.84, -64.07,
-64.91, -67.75, and -68.18 ppm. This pattern clearly indicated
a different chemical environment for each of the six cage Si
(10) Feher, F. J.; Terroba, R.; Ziller, J. W. Chem. Commun. 1999, 2153.
(11) Feher, F. J.; Schwab, J. J.; Soulivong, D.; Ziller, J. W. Main Group
Chem. 1997, 2, 123.

The First Niobasilsesquioxanes
Organometallics, Vol. 25, No. 25, 2006 5925
Figure 2. Perspective ORTEP view of the molecular structure of 3. Thermal ellipsoids are drawn to encompass 50% probability. Only the
ipso-carbon atoms of the cyclohexyl groups are shown, and the hydrogen atoms are omitted for clarity. Selected distances [Å] and angles
[deg]: Nb(1)-O(1) 1.849(3), Nb(1)-O(1) 1.890(3), Nb(1)-O(9) 1.920(3), Nb(1)-O(5) 2.161, Si-O 1.617(3)-1.659(3), Si-C 1.840-
(4)-1.860(4), O(9)-Nb(1)-O(10) 91.41(12), O(5)-Nb(1)-O(2) 88.01(10), O(10)-Nb(1)-O(5) 163.77(10), O(9)-Nb(1)-O(2) 172.47-
(11), O-Si-O 107.55(15)-110.83(16), Si-O-Si 136.12(18)-156.82(19), Nb(1)-O(10)-Si(3) 139.07(17), Nb(1)-O(9)-Si(4) 175.77(17).
Figure 3. Perspective ORTEP view of the molecular structure of 4. Thermal ellipsoids are drawn to encompass 50% probability. Only the
ipso-carbon atoms of the cyclohexyl groups are shown, and the hydrogen atoms are omitted for clarity. Selected distances [Å] and angles
[deg]: Nb(1)-O(2) 1.865(2), Nb(1)-O(4) 1.899(3), Nb(1)-O(6) 1.912(2), Nb(1)-O(3) 1.947(3), Nb(1)-O(1) 2.110(2), Nb(1)-O(5) 2.110-
(2), Nb(1)-Nb(2) 3.2211(18), Nb(2)-O(1#) 1.775(2), Nb(2)-O(9) 1.915(3), Nb(2)-O(8) 1.941(3), Nb(2)-O(6) 1.952(2), Nb(2)-O(5)
2.113(2), Nb(2)-O(7) 2.338(3), Si-O 1.609(3)-1.629(3), O(2)-Nb(1)-O(4) 95.49(12), O(2)-Nb(1)-O(6) 96.27(11), O(4)-Nb(1)-
O(6) 98.54(12), O(2)-Nb(1)-O(3) 91.12(12), O(4)-Nb(1)-O(3) 99.59(12), O(6)-Nb(1)-O(3) 159.65(11), O(2)-Nb(1)-O(1) 175.82-
(12), O(4)-Nb(1)-O(1) 85.75(11), O(6)-Nb(1)-O(1) 87.48(10), O(3)-Nb(1)-O(1) 84.73(10), O(2)-Nb(1)-O(5) 94.61(11), O(4)-
Nb(1)-O(5) 168.09(10), O(6)-Nb(1)-O(5) 74.07(10), O(3)-Nb(1)-O(5) 86.48(11), O(1)-Nb(1)-O(5) 84.62(9), O-Si-O 108.15(15)-
110.64(15).
atoms. This was verified by the X-ray diffraction analysis of 3,
which revealed the presence of a dinuclear niobasilsesquioxane.
In this molecule each niobium atom adopts a highly distorted
octahedral coordination geometry with the silsesquioxane tet-
raanions acting as bridging ligands as shown in Figure 2. Each
of the two Nb atoms retains one ethoxide group as a terminal
ligand.
diethyl ether, and even toluene despite its high molecular mass
of ∼2000 g/mol. Upon prolonged standing of a sample of 3 in
THF-d8 in an NMR tube, the formation of well-formed crystals
was observed, which had a distinctly more compact appearance
than the needle-like crystals of 3. An X-ray structural analysis
of these crystals revealed the formation of an unprecedented
tetranuclear oxoniobium silsesquioxane cluster (Figure 3). In 4
two silsesquioxane cages are connected via a tricyclic Nb4O4-
(OEt)2 unit, resulting in a condensed inorganic 15-ring system.
Due to the presence of 12 cyclohexyl substituents on the
outside of the molecule, compound 3 freely dissolves in THF,

5926 Organometallics, Vol. 25, No. 25, 2006
Lorenz et al.
Figure 4. Perspective ORTEP view of the molecular structure of 6. Thermal ellipsoids are drawn to encompass 50% probability. Only the
ipso-carbon atoms of the cyclohexyl groups are shown, and the hydrogen atoms are omitted for clarity. Selected distances [Å] and angles
[deg]: Nb(1)-O(1) 2.108(2), Nb(1)-O(2-5) 1.884(2)-1.944(2), Si-O 1.610(2)-1622(2), O(2)-Nb(1)-O(5) 93.80(10), O(2)-Nb-
O(4) 86.28(9), O(5)-Nb(1)-O(4) 102.28(9), O(2)-Nb(1)-O(3) 173.83(9), O(3)-Nb(1)-O(1) 70.62(8), O-Si-O 107.92(12) - 109.73-
(13), Si-O-Si 143.03(15)-158.15(17), Nb(1)-O-Nb(1#) 109.38(8), O(1)-Nb(1)-O(1#) 70.62(8).
Scheme 3
Scheme 4
The formation pathway leading from dinuclear 3 to tetra-
nuclear 4 is not clear yet. However, it is reasonable to anticipate
slow diffusion of moist air into the NMR sample, causing partial
hydrolysis followed by condensation, as qualitatively illustrated
in Scheme 3. Although this reaction could not yet be reproduced
on a preparative scale, the structural result is remarkable, as it
demonstrates the potential structural diversity in niobasilses-
quioxane chemistry.
Apart from serendipity, structurally different niobasilsesqui-
oxanes can also be prepared deliberately by employing the
incompletely condensed trisilanol precursor 5. Treatment of 5
with 1 equiv of Nb(OEt)5 in toluene according to Scheme 4
resulted in smooth formation of the dinuclear niobasilsesqui-
oxane [(c-C6H11)7Si7O12NbOEt(µ-OEt)]2 (6), which was isolated
in 72% yield. The colorless crystalline solid is only slightly
moisture sensitive and freely soluble in THF and toluene. In
contrast to 3, the 29Si NMR spectrum of 6 shows only three
resonances at δ -67.74, -68.67, and -69.47 ppm in an
intensity ratio of 3:3:1.
According to an X-ray structure determination (Figure 4),
compound 6 is the result of capping of the silsesquioxane cage
in 5 by niobium accompanied by elimination of 3 equiv of
ethanol. Subsequent dimerization occurs though two bridging
ethoxide ligands to give a four-membered Nb2O2 ring as the
central structural motif. Each niobium atom retains one terminal
ethoxide ligand. These initial results show that different types
of niobasilsesquioxanes are accessible in a straightforward
manner using the simple, commercially available precursor Nb-
(OEt)5. The results also indicate a high structural diversity,
which should stimulate further investigations in this field.
Acknowledgment. This work was financially supported by
the Otto-von-Guericke-Universita¨t Magdeburg.
Supporting Information Available: CIF files giving X-ray
structural data for 2, 3, 4, and 6. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM0606587