Four-Electron Photochemistry of Dirhodium Fluorophosphine Compounds

The singly bonded dirhodium(II) complex, Rh2(dfpma)3Br4 (dfpma = bis(difluorophosphine)methylamine, CH3N(PF2)2), photoreacts when irradiated with UV or visible light. The mixed-valence LRh0RhΙΙX2 species, Rh2(dfpma)3Br2(L), is obtained quantitatively when THF solutions containing Rh2(dfpma)3Br4 and excess L = dfpma or PR3 are photolyzed (λexc > 436 nm). The photoreaction quantum yield is ∼10-2 over the near-UV absorption manifold, decreasing slightly as the excitation wavelength is extended into the visible region (φp313-405 = 0.022(3), φp436-468 = 0.012(1)). Continued irradiation with excitation wavelengths coincident with the absorption manifold of the LRh0RhΙΙX2 complex results in a second two-electron elimination reaction to give the LRh0Rh0L dimer, Rh2(dfpma)3L2, again in quantitative yield but with an attenuated quantum efficiency (φp313-365 = 0.0035(3), φp405-436 = 0.0017(3)). For photoreactions performed in THF, the bromine photoproduct is HBr, which may be trapped with 2,6-lutidine. NMR experiments reveal the production of 2 equiv of HBr for each two-electron transformation of the dirhodium photoreagent. The wavelength dependence of φp and the results of extended Hückel calculations are consistent with the photoreaction occurring from an excited state of dσ* parentage. The ability to preserve the same dσ* electronic structure across the four-electron series allows us to overcome the barriers traditionally associated with metal−halogen bond cleavage and consequently to design a four-electron photoreaction among discrete molecular species.