Ground- and Excited-State Oxidation-Reduction Chemistry of (Triphenyltin)- and (Triphenylgermanium)tricarbonyl(l,10-phenanthroline)-rhenium and Related Compounds

Optical absorption and emission spectroscopy and the photochemistry and electrochemistry are reported for complexes of the general formula R3EM(CO)3L (R = Ph or Me; E = Ge or Sn; = Mn or Re; L = 1,10-phenanthroline, 2,2'-bipyridine, or 2,2'-biquinoline). The lowest excited state in each system results from charge-transfer, (E-M)σb -πL, absorption. Several of the Re complexes (R = Ph; E = Ge or Sn; L = 2,2'-bipyridine or 1,10-phenanthroline) exhibit optical emission from the lowest excited state at 298 K in fluid solution; emission lifetimes under such conditions for these complexes are ~10-6 s. These excited complexes can be quenched by both electron-donor quenchers and by electron-acceptor quenchers. Detailed quenching studies of Ph3SnRe(CO)3(phen) (phen = 1,10-phenanthroline) have been carried out, and quenching obeys Stern-Volmer kinetics. Electron donors, Q, for which E°(Q+/Q) is more negative than ~+0.2 V vs. SCE quench at an essentially diffusion-controlled rate. Electron acceptors, P+, for which E°(P+/P) is more positive than -1.0 V vs. SCE also quench at nearly a diffusion-controlled rate. Cyclic voltammetry of the complexes in CH3CN/0.1 M [n-Bu4N]C104 typically shows a one-electron, reversible reduction in the -1.1 to -1.7 V vs. SCE range associated with the population of the lowest available π* orbital principally localized on L. An irreversible oxidation current peak is observed in the range +0.5 to +0.8 V vs. SCE. The M-containing oxidation product is/uc-[(CH3CN)M(CO)3L]+. Consistent with the ground state electrochemistry, quenching by reversible electron-donor quenchers (e.g., N,N,N',N'-tetramethyl-p-phenylenediamine) results in no net photoredox reaction (<10~3) whereas quenching by reversible electron-acceptor quenchers (e.g., TV,TV-dimethyl-4,4'-bipyridinium) results in net redox chemistry to reduce the quencher and to form/uc-[(CH3CN)M(CO)3L]+ from the complex. The data are consistent with primary formation of R3E and the 16-valence electron [M(CO)3L]+ from cleavage of the [R3EM(CO)3L]+ formed by excited-state electron transfer. Rate of [R3EM(CO)3L]+ cleavage is similar to the dissociative E-M bond cleavage induced by the (E-M)ab → π*L optical excitation.