Photochemical hydrogen atom transfer reactions of binuclear platinum complexes

Abstract

This thesis focuses on hydrogen atom (H-atom) transfer reactions of the lowest electronic excited state of the d8 dimer [Pt2(P2O5H2)4]4- (the ground state is abbreviated Pt2]; the lowest excited state is a long-lived triplet, abbreviated 3Pt2*). Factors that influence initial rates of reaction of the excited state with alcohol and hydrocarbon H-atom donors are examined in Chapter 2. Observation of a large kinetic deuterium isotope effect (kH/kD=4) for phosphorescence quenching by [alpha]-secphenethyl alcohol verifies the importance of bond strength in determining reaction rates. A plot of H-atom abstraction rates versus C-H bond strengths reveals that bond strength is not the sole determinant of abstraction rate, however. Faster rates are observed when an [alpha]-hydroxy functionality is present, and the size of the substrate also influences abstraction rates. Chapter 3 features characterization of a diplatinum dihydride complex (Pt2H2) that acts as a key intermediate in H-atom transfer reactions. 3Pt2* reacts with hydrogen-atom donors to give a complex characterized by strong absorption at 314 nm; NMR (1H and 31P) and IR spectroscopic studies show that the complex is an axial dihydride (Pt2H2). Reactions of Pt2H2 include photochemical release of H2, rapid thermal reduction of O2, and thermal reduction of HCl to H2. Chapter 4 describes synthesis and characterization of a derivative of Pt2 modified by substitution of electron-withdrawing BF2+ groups for ligand H+. The new compound exhibits photophysical properties (phosphorescence lifetime, quantum yield, absorption and emission maxima) virtually identical with those of 3Pt2*, but dramatically shifted ground-state electrochemical properties (peak potential for oxidation shifted +750 mV). Comparisons of thermal reactivity (preparation of axial dihalide and dihydride complexes) and photochemical reactivity (reductive quenching and H-atom transfer quenching) of the two platinum dimers, detailed in Chapter 5, provide further insight into the factors influencing H-atom transfer in binuclear d8-d8 compounds.