A photocycle for hydrogen production from two-electron mixed-valence complexes

Dihydrides of the formula Rh2(II,II)(tfepma)(3)H2Cl2 (tfepma = (bis[bis(trifluoroethoxy)phosphinojmethylamine, MeN(P[OCH2CF3](2))(2)), have been prepared by the addition of H-2 to the two-electron mixed-valence complex, Rh-2(0.11)(tfepma)(3)Cl-2 (1). Three isomeric forms with hydrides in syn (2), anti (3), and cis (4) conformations have been characterized by X-ray diffraction. Photolysis of 2 results in prompt formation of a short-lived blue photoproduct (Am = 600 nm) and a stoichiometric quantity of H-2, as determined by Toepler pump and isotopic labeling experiments. The blue photoproduct was identified as a Rh2(1.1) complex resulting from the reductive elimination of H-2, as determined from the examination of bimetallic cores coordinated by tfepm (tfepm = (bis[bis(trifluoroethoxy)phosphino]methane, CH2(P[OCH2CF3](2))(2)), for which complexes of the formula M-2(1.1)(tfepM)(3)Cl-2 (5, M = Rh and 6, M = Ir) have been isolated. The d8(...)d8 dimer of 5 converts to Rh-2(0.11)(tfepm)(3)Cl2CN/Bu (8) upon the addition of 1 equiv of tert-butylisonitrile, a result of halogen migration and disproportionation of the valence symmetric core of 5, which is structurally compared to its two-electron mixed-valence analogue, Rh-2(0.11)(dfpma)(3)(Cl2CNBu)-Bu-/ (9) (dfpma = bis(difluorophosphino)-methylamine, MeN(PF2)(2)). The halogen migration is captured in lr(2)(1.1)(tfePM)(3)(mu-Cl)Cl (7), which is distinguished by the presence of a chloride that bridges the diiridium centers. Taken together, complexes 1-9 permit the construction of a complete photocycle for the photogeneration of H-2 by dirhodium dfpma complexes in homogeneous solutions of hydrohalic acids.