PHOTOCHEMISTRY OF trans‐[Rh(BIS(DIPHENYLPHOSPHINO)ETHANE)₂X₂][PF₆]: TRANSIENT ABSORBANCE KINETIC STUDIES OF METAL‐HALOGEN BOND HOMOLYSIS

Abstract

Abstract— Laser flash photolysis of trans‐[Rh(dppe)2X2][PF6] (X=Br and I; dppe=bis(diphenylphosphino)ethane) in CH2Cl2 or CH3CN produces the d7 Rh(II) radicals, [Rh(dppe)2X]+, and halogen atoms. The kinetics of the disappearance of [Rh(dppe)2X]+ radicals in CH2Cl2 or CH3CN were mixed order: H‐atom abstraction from solvent to produce the rhodium hydrides, [RhH(dppe)2X][PF6], and Rh/X recombination. In the poor H‐atom donor solvent, benzonitrile, Rh/Br recombination was observed to be uncomplicated by competing H‐atom abstraction. The hydride complexes [RhH(dppe)2X][PF6], formed by H‐atom abstraction were completely characterized by 31P{1H}‐NMR, 1H‐NMR, and mass specrometry. Cyclohexene was used as an effective trap for photogenerated Br atoms and yielded bromocyclohexane and 3‐bromocyclohexene in a relative yield, 1:9. The photochemical mechanism is discussed in light of the transient absorbance and trapping studies.