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

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posted on 30.11.2005, 00:00 by Arthur J. Esswein, Adam S. Veige, Daniel G. Nocera
Dihydrides of the formula Rh2II,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, Rh20,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 (λmax = 600 nm) and a stoichiometric quantity of H2, as determined by Toepler pump and isotopic labeling experiments. The blue photoproduct was identified as a Rh2I,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 M2I,I(tfepm)3Cl2 (5, M = Rh and 6, M = Ir) have been isolated. The d8···d8 dimer of 5 converts to Rh20,II(tfepm)3Cl2CNtBu (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, Rh20,II(dfpma)3Cl2CNtBu (9) (dfpma = bis(difluorophosphino)methylamine, MeN(PF2)2). The halogen migration is captured in Ir2I,I(tfepm)3(μ-Cl)Cl (7), which is distinguished by the presence of a chloride that bridges the diiridium centers. Taken together, complexes 19 permit the construction of a complete photocycle for the photogeneration of H2 by dirhodium dfpma complexes in homogeneous solutions of hydrohalic acids.