A Tetranuclear Niobium Oxo Acetate Complex. Synthesis, X-ray Crystal Structure, and
Characterization by Solid-State and Liquid-State NMR Spectroscopy
Nathalie Steunou, Christian Bonhomme, and Cle ´ ment Sanchez*
Chimie de la Matie `re Condense ´e, UMR CNRS 7574, Universite ´ Pierre et Marie Curie,
75252 Paris, France
Jacqueline Vaissermann
Chimie des Me ´taux de transition, Universite ´ Pierre et Marie Curie, 75252 Paris, France
Liliane G. Hubert-Pfalzgraf
Chimie Mole ´culaire, Universite ´ de NicesSophia Antipolis, Parc Valrose, 06108 Nice Cedex 2, France
ReceiVed August 13, 1997
The compound Nb
4
O
4
(OAc)
4
(OPr
i
)
8
synthesized by reacting Nb(OPr
i
)
5
and acetic acid has been fully characterized
by single-crystal X-ray diffraction (a ) 11,634(5) Å, b ) 14,370(6)Å, c ) 16,380(6) Å, R) 79.90(4)°, )
71.78(4)°, γ ) 79.21(4)°, triclinic, P1 h, Z ) 2) and multinuclear NMR experiments performed in the solid state
and in solution. A simple possible mechanism that can account for its formation is presented.
13
C CP MAS
experiments, using the IRCP sequence (inversion recovery cross polarization), are used to perform straightforward
assignments of the resonances of the different isopropoxy ligands bonded to niobum.
17
O,
13
C, and
1
H NMR
spectroscopies show that the cluster structure is fully preserved in solution.
Introduction
Increasing interest has been devoted to the study of transition
metal oxo alkoxo clusters whose general formula is M
x
O
y
(OR)
z
(M ) Ti, Zr, Nb).
1-4
These metallo-organic compounds play
an important role in understanding and characterizing mixed
transition metal oxo based materials prepared by sol-gel
chemistry.
5-7
The sol-gel process is based on hydrolysis and
condensation reactions of organometallic precursors of general
formula M(OR)
n
. The macromolecular metal-oxo-based net-
works thereby developed are predominantly amorphous and
polydisperse, and the scarceness of structural data makes their
characterization particularly difficult. They can be characterized
using spectroscopic techniques,
8-10
which can provide accurate
information about the structure of the oxo polymers by
comparing them to well-calibrated defined references com-
pounds such as crystalline metal oxo based complexes. In most
cases, such metallo-organic complexes are obtained for small
hydrolysis ratios. Numerous metal oxo alkoxide clusters (metal
) Ti
IV
, Zr
IV
, Ce
IV
) synthesized through hydrolysis and con-
densations reactions, with or without the presence of a com-
plexing ligand, have received much attention and have been
structurally characterized.
1-3, 11-14
In contrast, much less work have been devoted to niobium
oxo alkoxide complexes. Nb
8
O
10
(OEt)
20
was first obtained
through controlled hydrolysis of niobium ethoxide, and its
structure was resolved by single-crystal X-ray diffraction.
15
Another chemical route based on condensation of NbO(OEt)
3
with the release of two ether molecules Et
2
O was proposed for
the synthesis of the same octamer Nb
8
O
10
(OEt)
20
.
16
Another
approach which consists of generating water in situ through
esterification reactions was used to produce various hexanuclear
(1) Day, V. W.; Eberspacher, T. A.; Klemperer, W. G.; Park, C. W. J.
Am. Chem. Soc. 1993, 115, 8469.
(2) Toledano, P.; In, M.; Sanchez, C. C. R. Acad. Sci Paris Ser. 2 1991,
313, 1247.
(3) Sanchez, C.; In, M.; Toledano, P.; Griesmar, P. Mater. Res. Soc. Symp.
Proc. 1992, 271, 669.
(4) Kickelbick, G.; Schubert, U. Chem. Ber. 1997, 130, 473.
(5) Campana, C. F.; Chen, Y.; Day, V. W.; Klemperer, W. G.; Sparks,
R. A. J. Chem. Soc., Dalton Trans. 1996, 691.
(6) Chen, Y. W.; Klemperer, W. G.; Park, C. W. Mater. Res. Soc. Symp.
Proc. 1992, 271, 57.
(7) Sanchez, C.; Ribot, F. New J. Chem. 1994, 1, 1007.
(8) Blanchard, J.; Barboux-Doeuff, S.; Maquet, J.; Sanchez, C. New J.
Chem. 1995, 19, 929.
(9) Day, V. W.; Eberspacher, T. A.; Klemperer, W. G.; Park, C. W.;
Rosenberg, F. S. In Chemical Processing of AdVanced Materials;
Hench, L. L., West, K., Eds.; Wiley & Sons: New York; 1992; Chapter
2, p 257.
(10) Bastow, T. J.; Smith, M. E.; Whitfield, H. J. J. Mater. Chem. 1992,
2, 989.
(11) Day, V. W.; Eberspacher, T. A.; Klemperer, W. G.; Park, C. W.;
Rosenberg, F. S. J. Am. Chem. Soc. 1991, 113, 8190.
(12) Day, V. W.; Eberspacher , T. A.; Chen, Y.; Hao, J.; Klemperer, W.
G. Inorg. Chim. Acta 1995, 229, 391.
(13) Schmidt, R.; Mosset, A.; Galy, J. J. Chem. Soc., Dalton Trans. 1991,
8, 1999.
(14) Toledano, P.; Ribot, F.; Sanchez, C. C. R. Acad. Sci Paris, Ser. 2
1990, 311, 1315.
(15) Bradley, D. C.; Hurthouse, M. B.; Rodesiler, P. F. Chem. Commun.
1968, 18, 1112.
(16) Kessler, V. G.; Turova, N. Y.; Yanovskii, A. I.; Belokon, A. I.;
Struchov, Y. T. Russ. J. Inorg. Chem. 1991, 36, 938.
901 Inorg. Chem. 1998, 37, 901-910
S0020-1669(97)01024-0 CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/10/1998