Electron paramagnetic resonance studies of some supported organometallic catalysts

E.p.r. spectroscopy has been used to study catalysts obtained by supporting the following complexes on high surface area silica: Mo2(CH3CO2)4, M2[CH2Si(CH3)3]6(M = Mo or W), W(CO)6, W(CH3)6, Cr(CO)6, Cr[CH2C(CH3)3]4. The supported molybdenum complexes activated at sufficiently high temperatures give rise to e.p.r. signals identical to those observed previously from silica supported Mo(CO)6 and MoO3 catalysts, due to MoV in square pyramidal co-ordination, and O–2 formed by interaction of O2 with tetrahedrally coordinated MoV. The supported tungsten complexes give only an O–2 signal on exposure to O2. The spectrum of adsorbed CH3 radicals is detected during decomposition of supported W(CH3)6. The supported chromium complexes activated in vacuo and exposed to O2 give CrV signals identical to those observed from silica supported CrO3. The catalytic implications of these observations are discussed.