The photochemistry of (CO) 5MnMn(CO) 3(α-diimine) and XMn(CO 3)(α-diimine) (X=halide) complexes

Abstract

The Mn(CO) 5 and Mn(CO) 3(α-diimine) radicals formed by irradiation of a (CO) 5MnMn(CO) 3(α-diimine) complex undergo interesting radical coupling and electron transfer (chain) reactions. The mechanisms of these reactions are discussed and attention is paid to the character of the reactive excited state. A relationship is proposed to exist with the corresponding halide complexes XMn(CO) 3(α-diimine) (X=Cl,Br,I) which similarly undergo homolysis of the Mn-X bond upon irradiation. Preliminary m.o. data of XMn(CO) 3(bpy) and resonance Raman results obtained for BrRe(CO) 3(pTol-DAB) confirm the presence of a lowest LLCT state at least for the halide complexes. The complexes (CO) 5MnMn(CO) 3(α-diimine) and XMn(CO) 3(α-diimine) show a striking similarity in their photochemical behaviour since they both produce the dimeric species Mn 2(CO) 6(α-diimine) 2 upon irradiation into their lowest-energy absorption band. It is therefore tempting to propose a reactive LLCT state for both types of complexes although the characters of their h.o.m.o.'s will of course be different. As mentioned in the Introduction the temperature and wavelength independence of the quantum yields does not support a two-level scheme for the metal-metal bonded complexes involving a non-reactive MLCT state and the reactive 3σσ * state of the metal-metal bond. Definite proof for a one-level scheme involving a reactive LLCT state is, however, still lacking for these complexes. Picosecond flash photolysis has already shown that the primary photoproduct is formed within 20 ps. However, only femtosecond flash photolysis can provide convincing evidence for such a one-level scheme and such experiments are planned for the near future.