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)(kappa(1)-dppp)(kappa(2)-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)(kappa(1)-dppp)(kappa(2)-dpp)](+), and trans-[Cr(CO)(2)(dppp)(2)](+)). A bimolecular photoinduced mechanism is proposed to account for the formation of mer-[Cr(CO)(3)(kappa(1)-dppp)(kappa(2)-dppp)](+) and trans-[Cr(CO)(2)(dppp)(2)](+).