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

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(3) sigma pi* excited state with a rate constant of 2.3 x 10(5) 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 (much greater than 10(8) 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 mu s. 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). (C) 1997 Elsevier Science S.A.