Unravelling the Photochemical Transformations of Chromium(I) 1,3 Bis(diphenylphosphino), [Cr(CO)4(dppp)]+, by EPR Spectroscopy

UV-induced photochemical transformations of the paramagnetic [Cr­(CO)4(Ph2PCH2CH2CH2PPh2)]+ complex (abbreviated [Cr­(CO)4(dppp)]+) in dichloromethane was investigated by CW EPR spectroscopy. Room-temperature UV irradiation results in the rapid transformation of [Cr­(CO)4(dppp)]+ into trans-[Cr­(CO)2(dppp)2]+. However, low-temperature (77–120 K) UV irradiation reveals the presence of an intermediate mer-[Cr­(CO)3(κ1-dppp)­(κ2-dppp)]+ complex which photochemically transforms into trans-[Cr­(CO)2(dppp)2]+. The derived spin Hamiltonian parameters for these complexes were confirmed by DFT calculations. The photoinduced reaction is shown to be concentration-dependent, leading to a distribution of the three complexes ([Cr­(CO)4(dppp)]+, mer-[Cr­(CO)3(κ1-dppp)­(κ 2-dppp)]+, and trans-[Cr­(CO)2(dppp)2]+). A bimolecular photoinduced mechanism is proposed to account for the formation of mer-[Cr­(CO)3(κ1-dppp)­(κ2-dppp)]+ and trans-[Cr­(CO)2(dppp)2]+.