Four-electron photochemistry of dirhodium fluorophosphine compounds

The singly bonded dirhodium(II) complex, Rh-2(dfpma)(3)Br-4 (dfpma = bis(difluorophosphine)methylamine, CH3N(PF2)(2)), photoreacts when irradiated with UV or visible light. The mixed-valence LRh0-(RhX2)-X-II species, Rh-2(dfpma)(3)Br-2(L), is obtained quantitatively when THF solutions containing Rh-2(dfpma)(3)Br-4 and excess L = dfpma or PR3 are photolyzed (lambda(exc) > 436 nm). The photoreaction quantum yield is similar to 10(-2) over the near-UV absorption manifold, decreasing slightly as the excitation wavelength is extended into the visible region (phi(P)(313-405) = 0.022(3), phi(P)(436-468) = 0.012(1)). Continued irradiation with excitation wavelengths coincident with the absorption manifold of the LRh(0)Rh(II)X2 complex results in a second two-electron elimination reaction to give the (LRhRhL)-Rh-0-L-0 dimer, Rh-2(dfpma)(3)L-2, again in quantitative yield but with an attenuated quantum efficiency (phi(P)(313-365) = 0.0035(3), phi(P)(405-436) = 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 phi(P) and the results of extended Huckel calculations are consistent with the photoreaction occurring from an excited state of d sigma* parentage. The ability to preserve the same d sigma* 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.