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 {A2+[Pt(mnt)2]2-} (A2+ ) 2,2¢-,4,4¢-bipyridinium or cycloalkylated biimidazolium dication; mnt2- ) 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 acceptors with a first reduction potential more positive than -0.6 V back electron transfer prevails. When the potential is more negative however, A¥+ reduces O2 by pseudo-first-order kinetics to generate O2 ¥-, 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 (ð,ð*) state (334 nm) than to the metal-to-ligand charge-transfer state (437 or 580 nm). In the latter cases competitive radiationless deactivation Via metalcentered states occurs. The corresponding Ni and Pd complexes do not exhibit any reactivity due to their very short excited state lifetimes. Formation of O2 ¥- 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 °C. The activation energy of 108 ( 10 kJ/mol as obtained for the thermal electron transfer from [Pt(mnt)2]2- to A2+ corresponds well to the value calculated from the Hush-Marcus model