The mechanisms of the photochemical reactions of M2(CO)10 (M = Mn, Re) with ortho- and para-quinones: a combined EPR and FTIR study

Article abstract of DOI:10.1016/0022-328X(91)86228-I

A study has been made of the photolysis of M2(CO)10 (M = Mn, Re) in the presence of ortho- and para-quinones in coordinating (THF, 2-MeTHF, 10^-1 M pyridine in toluene) and in inert (toluene, CH2Cl2, 2-chlorobutane) solvents.For comparison, the same reactions have been studied for M2(CO)8(bpy') (bpy' = 4,4'-dimethyl-2,2'-bipyridine).Both EPR and FTIR techniques have been used to monitor the photochemical reactions under various conditions.A comprehensive picture of the overall reaction mechanism has been established.Both primary photoproducts, M(CO)5 and M2(CO)9, were found to react with quinones.Oxidative addition to M(CO)5 produces unstable M(I) complexes that contain either a p-semiquinone (p-SQ) or monodentate o-semiquinone (o-SQ) ligand.The former species are stabilized by polar solvents.M(CO)5(p-SQ), M(CO)5(o-SQ), Mn(CO)3L2(p-SQ) (L = THF, py) and Re(CO)5-n(THF)n(p-SQ) have been identified as photoproducts by EPR and IR spectroscopy.The o-semiquinone complexes undergo a thermal or photochemical intramolecular substitution of CO by the second o-semiquinone oxygen atom, producing complexes that contain a chelating o-semiquinone radical anionic ligand O,O-o-SQ.The cis-CO ligand of these complexes is loosely bound and can therefore easily be substituted, thermally as well as photochemically, by a Lewis base (L) to give M(CO)3L (O,O-o-SQ) complexes as final products.Both quinone isomers also coordinate to the primary photoproducts M2(CO)9.The resulting M2(CO)9 (quinone) species are unstable and react with CO to regenerate the parent compounds M2(CO)10.In the case of the o-quinones, they undergo thermal intramolecular electron transfer reaction resulting in M-M bond splitting, leading again to M(CO)3L(O,O-o-SQ) species.Apparently, EPR-active radical products can also be formed by an entirely non-radical process, since the quinones first coordinate to M2(CO)9 before the electron transfer takes place.This result shows that the occurrence of spin-trapping cannot unambiguously be deduced from the observation of EPR-active products only.The photochemical reactions with M2(CO)8(bpy') produce for both quinone isomers the novel complexes M(CO)3(bpy')(SQ), which have been characterized by IR and EPR spectroscopy.