A Photocycle for Hydrogen Production from Two-Electron Mixed-Valence Complexes

Dihydrides of the formula Rh₂^II,II(tfepma)₃H₂Cl₂ (tfepma = (bis[bis(trifluoroethoxy)phosphino]methylamine, MeN(P[OCH₂CF₃]₂)₂), have been prepared by the addition of H₂ to the two-electron mixed-valence complex, Rh₂^0,II(tfepma)₃Cl₂ (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 (λ_max = 600 nm) and a stoichiometric quantity of H₂, as determined by Toepler pump and isotopic labeling experiments. The blue photoproduct was identified as a Rh₂^I,I complex resulting from the reductive elimination of H₂, 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 M₂^I,I(tfepm)₃Cl₂ (5, M = Rh and 6, M = Ir) have been isolated. The d⁸···d⁸ dimer of 5 converts to Rh₂^0,II(tfepm)₃Cl₂CN^tBu (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, Rh₂^0,II(dfpma)₃Cl₂CN^tBu (9) (dfpma = bis(difluorophosphino)methylamine, MeN(PF₂)₂). The halogen migration is captured in Ir₂^I,I(tfepm)₃(μ-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 H₂ by dirhodium dfpma complexes in homogeneous solutions of hydrohalic acids.