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Model Compounds for Metal Oxides on SiO2 Surfaces.

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Model Compounds for Metal Oxides on Si02 Surfaces
By Frank 7: Edelmann*
Numerous important processes in the petrochemical industry are catalyzed by metal oxides immobilized on SiO,
supports."] Highly selective catalysts of this type play a significant role in the oxidation of hydrocarbons12] and the
oxidative coupling of methane,l3] as well as in the metathesisr4Iand polymerizationL5]of olefins and alkynes. Despite
highly refined methods of analysis, mechanistic studies and
structural investigations of immobilized compounds are still
extremely difficult. The exact nature of the SiO, surfaces
themselves is often a subject of controversy.[61Thus, much
current research is concerned with the synthesis and characterization of compounds which may model the specific surface morphologies of SiO, supports and their interactions
with metal oxides on the molecular level.
One compound that is much better suited for modeling the
SiO, surface than Me,SiOH or Ph,SiOH, trisilanol 1, was
prepared by Feher et al.[71Compound 1 is formed in the
kinetically controlled hydrolysis of cyclohexyltrichlorosilane
[Eq.(a)]. Under the specified reaction conditions, complete
R = C-C~H,
Polyhedral oligometallasesquisiloxanes (POMSS) are accessible by the reaction of 1 with suitable metal compounds.
For example a typical POMSS, 2, is formed in the reaction
of 1 with [(nPrO),VO], [(Me,SiCH,),VO], or VOCI,.r91A
monomer/dimer equilibrium between 2 and 3 exists in solution [Eq.(b)]. Compound 2 can realistically be considered a
hydrolysis of c-C,H, ,Si(OH), requires about three years!
However, preparatively useful quantities of 1 can be "harvested" after only a few weeks.[71The comparison of the
molecular structure of 1 (A) with the (111) octahedral face of
the cristobalite modification of SiO, (B) reveals an astonishing geometrical relationship. Common to both structural
units is a concave silicon-oxygen cavity, whose outer rim is
formed by six silicon and six oxygen atoms. The arrangement of potential 0-donor centers is also quite similar.
model for a vanadium-containing heterogeneous catalysts,
since it is capable of polymerizing ethylene and 1,3-butadiene. In contrast, the comparable vanadiumsiloxide
[(Ph,SiO),VO] is inactive under identical reaction conditions !
Dr. F. T. Edelmann
Institut fur Anorganische Chemie der Universitat
Tammannstrasse 4, D-W-3400 Gottingen (FRG)
$ \
Verlagsgesellschafl mbH, W-6940 Weinheim. 1992
Only a few of the wealth of results are highlighted here.
Parts of zeolite structures['0] as well as the silylation of silica["] can also be modeled by 1. The reaction of
[Mo,(OtBu),] with two equivalents of 1 in the absence of
Lewis base results in the cleavage of all the OtBu ligands and
formation of the dinuclear complex 4; the central Mo-Si
8 3SO+ .2Sj0
Angew. Chem. Int. Ed. Engl. 31 (1992) No.S
framework of 4 is shown in C.Here an intact Mo=Mo unit
is fixed to both SiO ligands.[121
Feher's trisilanol 1 is formally a heptameric condensation
product of c-C,H,,Si(OH), . The direct synthesis of a model
compound for metal oxides on SiO, surfaces from a stable
silantriol was first described by Roesky et a].[' 31 Controlled
hydrolysis of tBuSiC1, in the presence of aniline furnishes
tBuSi(OH), , which does not undergo condensation, in 94 %
yield. Undulating layers are found in the crystal in which the
silantriol units are interconnected by hydrogen bonds. tBu%(OH), reacts with Re,O, in a molar ratio of 1 : l with
condensation and substitution giving rise to the tetranuclear
rhenium siloxide shown in Figure 1.
4 tBuSi(OH),
+ 4 Re,O,
+ 2 Re,O,
. H,O
+ 4 H,O
Compound 5 is the first well-defined, structurally characterized transition-metal complex synthesized from a silantri01. Its synthesis demonstrates a new approach to preparing
soluble, metal-containing silicates.
German version: Angew. Chem. 1992, 104, 600
[l] Y. I. Yermakov, B. N. Kuznetsov, V. A. Zakharov, Catalysis by Supported
Complexes, Elsevier, New York, 1981; b) New Horizons in Catalysis (Eds. :
T. Setyama, K. Tanabe), Elsevier, New York, 1980; c) F. R. Hartley, Supported Metal Complexes. Reidel, Boston, 1985;d) Tailored Metal Catalysis
(Ed.: Y.Iwasawa), Reidel, Boston, 1986.
[2] G. I. Golodets, Heterogeneous Catalytic Reactions Involving Molecular
Oxygen, Elsevier, New York, 1983.
[3] a) J. A. Sofranko, J. J. Leonhard, C. A. Jones, J Catal. 1987, 103, 302;
b) C. A. Jones, J. J. Leonhard, J. A. Sofranko, ibid. 1987, 103,311; c) K. J.
Zhen, M. M. Khan, C. H. Mak, K. B. Lewis, G. A. Somorjai, ibid. 1985,
94, 501; d) M. M. Khan, G. A. Somorjai, ibid. 1985, 91, 263.
[4] a) V. Dragutan, A. T. Balkaban, M. Dimonie, Olefin Metathesis and Ring
Opening Polymerizations of Cycloolefns, Academic Press, New York,
1986; b) K. J. Ivin, Olefin Metathesis, Academic Press, New York, 1986.
[5] a) H. Sinn, W. Kaminsky, Adv. Organomet. Chem. 1980, 18, 99; b) M. P.
McDaniel, Adv. Catal. 1985, 33, 47.
[6] Kirk-Othmer Encyclopedia of Chemical Technology, (Eds.: M. Grayson, D.
Eckroth), 2nd ed., Wiley, New York, 1978, p. 397-400, and literature cited
[7] F. J. Feher, D. A. Newman, J. F. Walzer,J. Am. Chem. SOC.1989.1f1, 1741.
[8] K. Kihara, 2. Kristallogr. 1978, 148, 237.
[9] F. J. Feher, J. F. Walzer, R. L. Blanski, J. Am. Chem. Soc. 1991, 113, 3618.
[lo] J. J. Feher, K. J. Weller, Organometallics 1990, 9, 2638.
[ l l ] F. Feher, D. A. Newman, J. Am. Chem. Soc. 1990, 112, 1931.
[12] T. A. Budzichowski, S . T. Chacon, M. H. Chisholm, F, J. Feher, W. Streib,
J. Am. Chem. SOC.1991,113,688.
[13] N. Winkhofer, H. W Roesky, M. Noltemeyer, W. T. Robinson, Angew.
Chem. 1992, 104,670; Angew. Chem. Int. Ed. Engl. 1992,31, 599.
[14] H. Werner, Angew. Chem. 1990, 102, 1109; Angew. Chem. Int. Ed. Engl.
1990,29, 1077.
[15] a) J. S . Anderson, J. S . Ogden, J. Chem. Phys. 1969, 51, 4189; b) M.
Auwarter, Angew. Chem. 1975,87,227; Angew. Chem. Int. Ed. Engl. 1975,
14, 207.
[16] T. Mehner, R. Koppe, H. Schnockel, Angew. Chem. 1992,104,653,Angew.
Chem. I n t . Ed. Engl. 1992, 31, 638.
Fig. 1. Structure of 5 in the crystal
Angew. Chem. Int. Ed. Engl. 31 (1992) No. 5
Even the smallest of these model compounds can be considered a true highlight. Until recently metal complexes with
SiO ligands were completely
Complexes of
type [M(SiO)L,J could not be detected in matrix studies,
much less be isolated." A significant step in the direction
towards Si analogues of metal carbonyls was achieved by
Schnockel et al. with their IR spectroscopic proof of [PdSiO]
(6).['61 This complex is formed when SiO and palladium
atoms are deposited simultaneously on a helium-cooled copper surface. According to the spectroscopic data, 6 has a
linear, Si-bridged structure. The absorption band of the SiO
stretching vibration is at 1246.3 cm- '. The experimental and
theoretical results show that the bonding relationships in 6
are comparable to those in [PdCO], that is, that in SiO ligands the o-donor bond is also reinforced by n-acceptor contributions.
0 VCH ~rlagsgesellschaftmbH,
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oxide, mode, sio2, compounds, metali, surface
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