Photochemical bond homolysis in a novel series of metal-metal bonded complexes Ru(E)(E′)(CO)2(iPr-DAB)

Article abstract of DOI:10.1016/S0020-1693(97)05530-8

Photochemistry of the complexes trans,cis-Ru(E)(E′)(CO)₂(iPr-DAB) (E = Cl, SnPh₃, PbPh₃, Mn(CO)₅, Re(CO)₅, Me; E′ (depending on E) = SnPh₃, PbPh₃, GePh₃, Mn(CO)₅, Re(CO)₅) was found to be strongly dependent on the combination and characters of the axial ligands E and E′. Except for Ru(Cl)(SnPh₃)(CO)₂(iPr-DAB) and Ru(Cl)(PbPh₃)(CO)₂(iPr-DAB) which are nearly unreactive, one of the Ru-E/E′ bonds is split homolytically upon irradiation into the lowest-energy absorption band of the complex. For Ru(SnPh₃)₂(CO)₂(iPr-DAB), this reaction occurs from a thermally equilibrated ³σπ* excited state with a rate constant of 2.3 × 10⁵ s^-1 and a temperature-dependent quantum yield (E_a = 1450 cm^-1). The unselective Ru-Ge (60%) and Ru-Sn (40%) bond homolysis of Ru(SnPh₃)(GePh₃)(CO)₂-(iPr-DAB) follows the same mechanism. On the other hand, bond homolysis is much faster (?10⁸ s^-1) for complexes which contain Ru-Me, Ru-Mn or Ru-Re bonds. Bond homolysis in these species is highly selective, since only Ru-Me, Ru-Mn and Ru-Re bond splitting was observed for Ru(Me)(SnPh₃)(CO)₂(iPr-DAB), Ru(SnPh₃)(Mn(CO)₅)(CO)₂(iPr-DAB) and Ru(SnPh₃)(Re(CO)₅)(CO)₂(iPr-DAB), respectively. The photoproduced [Ru(E)(CO)₂(iPr-DAB)]^· radicals were detected by time resolved UV-Vis spectroscopy on a timescale 10 ns-100 μs. The [Ru(SnPh₃)(CO)₂(iPr-DAB)]^· radical was also characterised by EPR in the form of its adduct with PPh₃. Depending on the solvent used, they either dimerise or abstract a chlorine atom from the solvent to produce Ru(Cl)(E)(CO)₂(iPr-DAB).