Excited-state properties of fac-[Re(dmb)(CO)3(CH 3CN)]PF6, [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 CH3CN or THF. The one-electron reduced monomer, Re(dmb)(CO)3S (S = CH3CN or THF), can be prepared either by reductive quenching of the excited states of fac-[Re(dmb)-(CO) 3(CH3CN)]PF6 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, kd = 40 ± 5 M-1 s-1. This rate constant is much smaller than those of other organometallic radicals which are typically 109 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 CO2 is slow and competes with the dimerization. Photolysis of [Re(dmb)(CO) 3]2 in the presence of CO2 produces CO with a 25-50% yield based on [Re]. A CO2 bridged dimer, (CO) 3(dmb)Re-CO(O)-Re(dmb)(CO)3 is identified as an intermediate. Both [Re(dmb)(CO)3]2(OCO2) and Re(dmb)(CO)3(OC(O)OH) are detected as oxidation products; however, the previously reported formato-rhenium species is not detected.
ASJC Scopus subject areas
- Colloid and Surface Chemistry