EPR studies of photochemical reactions of [MeCpMo(CO) 3] 2 with quinones

Abstract

The photochemical reactions of hexacarbonylbis[(1,2,3,4,5-η)-1-methyl-2,4-cyclopentadien-1yl]dimolybdenum ( 2) and o-quinones, 1,2-diketones and p-benzoquinones have been investigated by EPR spectroscopy. The reactions proceed in the same way as in the case of hexacarbonylbis(η 5-2, 4-cyclopentadien-1-yl)dimolybdenum ( 1) as previously reported, but the observed paramagnetic products differ in characteristic features from the complexes derived from 1. As in the case of 1, the paramagnetic products of 2 with o-quinone or 1,2-diketone ligands show appreciable changes of g values and Mo hyperfine couplings with the ligands, reflecting the changes in the spin distribution between the metal fragment and the ligands but in general in complexes from 2 the unpaired spin tends to delocalize more onto the metal fragment. The presence of the isomers (which have different orientation of the methyl group in the cyclopentadienyl ring) was inferred from the line width broadening in the EPR spectra. The methyl group in the cyclopentadienyl ring affects the preference of the carbonyl group which coordinates to the metal to a greater extent in the coordination of substituted p- benzoquinones. A remarkable temperature dependence of the EPR spectra has been observed in the complexes of 2 with p-benzoquinones. The intramolecular motions and the most stable conformation of the quinone ligand in such a complex are discussed based on the spectra.