FT-EPR Study of the Photochemical Homolysis of Re− and Ru−Alkyl Bonds in Re(R)(CO)3(α-Diimine) and Ru(I)(R)(CO)2(α-Diimine) Complexes. Evidence for the Reactive σ(M−R)π* State

Abstract

A Fourier transform EPR (FT-EPR) study was made of the photochemistry of Re(R)(CO){sub 3}({alpha}-diimine) and Ru(I)(R)(CO){sub 2}({alpha}-diimine) complexes, where R = benzyl, 2-propyl, or ethyl and {alpha}-diimine = 4,4`-dimethyl-2, 2`-bipyridine or N,N`-diisopropyl-1,4-diazabutadiene. The FT-EPR technique was used to monitor the formation and decay of alkyl radicals generated by metal-R bound homolysis. The spectra showed pronounced chemically induced dynamic electron polarization (CIDEP) effects with polarization patterns that depend strongly on metal ion, alkyl substituent, and solvent. To varying degrees, all spectra shows a contribution form ST{sub 0} radical pair mechanism (RPM) CIDEP. From the low-field-emission/high-field-adsorption polarization pattern given by this mechanism, it can be deduced that the free radicals are formed via a triplet-state precursor, confirming conclusions reached in earlier spectroscopic studies. In addition to the RPM contribution, some spectra also show a net-absorptive CIDEP contribution while others show a net-emissive contribution. These net spin polarization contributions are attributed tentatively to triplet mechanism and spin-orbit coupling CIDEP. From the fact that the alkyl radical spectra exhibit strong spin polarization, it is concluded that metal ion atomic orbital contributions to the electronic states of precursor triplet and radical pair must be small. 17 refs., 4 figs.