Involvement of a Binuclear Species with the Re−C(O)O−Re Moiety in CO_2 Reduction Catalyzed by Tricarbonyl Rhenium(I) Complexes with Diimine Ligands: Strikingly Slow Formation of the Re−Re and Re−C(O)O−Re Species from Re(dmb)(CO)_3S (dmb = 4,4‘-Dimethyl-2,2‘-bipyridine, S = Solvent)

Abstract

Excited-state properties of fac-[Re(dmb)(CO)_3(CH_3CN)]PF_6, [Re(dmb)(CO)_3]_2 (where dmb = 4,4‘-dimethyl-2,2‘-bipyridine), and other tricarbonyl rhenium(I) complexes were investigated by transient FTIR and UV−vis spectroscopy in CH_3CN or THF. The one-electron reduced monomer, Re(dmb)(CO)_3S (S = CH_3CN or THF), can be prepared either by reductive quenching of the excited states of fac-[Re(dmb)(CO)_3(CH_3CN)]PF_6 or by homolysis of [Re(dmb)(CO)_3]_2. In the reduced monomer's ground state, the odd electron resides on the dmb ligand rather than on the metal center. Re(dmb)(CO)_3S dimerizes slowly in THF, k_d = 40 ± 5 M^(-1) s^(-1). This rate constant is much smaller than those of other organometallic radicals which are typically 10^9 M^(-1) s^(-1). The slower rate suggests that the equilibrium between the ligand-centered and metal-centered radicals is very unfavorable (K ≈ 10^(-4)). The reaction of Re(dmb)(CO)_3S with CO_2 is slow and competes with the dimerization. Photolysis of [Re(dmb)(CO)_3]_2 in the presence of CO_2 produces CO with a 25−50% yield based on [Re]. A CO_2 bridged dimer, (CO)_3(dmb)Re−CO(O)−Re(dmb)(CO)_3 is identified as an intermediate. Both [Re(dmb)(CO)_3]_2(OCO_2) and Re(dmb)(CO)_3(OC(O)OH) are detected as oxidation products; however, the previously reported formato-rhenium species is not detected.