Optical and thermal electron transfer activation of dioxygen by viologen dithiolene metalates

Photoinduced electron transfer activation of dioxygen by redoxactive charge-transfer ion pairs of the type {A(2+)[Pt(mnt)(2)](2-)} (A(2+)=2,2'-,4,4'-bipyridinium or cycloalkylated biimidazolium dication; mnt(2-)=maleonitriledithiolate) occurs through an optical electron transfer within an ion pair. This affords the primary redox products A(.+) and [Pt(mnt)(2)](-) as indicated by laser flash photolysis. Under argon the transients recombine by fast second-order kinetics. Under dioxygen a different behavior is observed. In the case of accepters with a first reduction potential more positive than -0.6 V back electron transfer prevails. When the potential is more negative however, A(.+) reduces O-2 by pseudo-first-order kinetics to generate O-2(.-), while [Pt(mnt)(2)](-) accumulates in the solution. Quantum yields increase with decreasing excitation wavelength. This suggests that internal conversion of the initially populated excited state to the photoreactive ion pair charge-transfer state is more efficient upon excitation to the interligand (pi,pi*) state (334 nm) than to the metal-to-ligand charge-transfer state (437 or 580 nm). In the latter cases competitive radiationless deactivation via metal-centered states occurs. The corresponding Ni and Pd complexes do not exhibit any reactivity due to their very short excited state lifetimes. Formation of O-2(.-) was proved by ESR spin-trapping techniques. Accumulation of [Pt(mnt)(2)](-) occurs also when instead of irradiating, the reaction is performed in the dark at about 160 degrees C. The activation energy of 108 +/- 10 kJ/mol as obtained for the thermal electron transfer from [Pt(mnt)(2)](2-) to A(2+) corresponds well to the value calculated from the Hush-Marcus model.