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 (lambda(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(t)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, Rh2(0,II)(dfpma)3Cl2CN(t)Bu (9) (dfpma = bis(difluorophosphino)methylamine, MeN(PF2)2). The halogen migration is captured in Ir2(I,I)(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 H2 by dirhodium dfpma complexes in homogeneous solutions of hydrohalic acids.

Reversible dihydrogen uptake by a two-electron mixed-valence di-iridium complex is examined with nonlocal density-functional calculations. Optimized metrics compare favorably with crystal structures of isolated species, and the calculated activation enthalpy of acetonitrile exchange is accurate within experimental error. Dihydrogen attacks the Ir(2) core at Ir(II); the Ir(0) center is electronically saturated and of incorrect orbital parity to interact with H(2). Isomeric eta(2)-H(2) complexes have been located, and harmonic frequency calculations confirm these to be potential energy minima. A transition state links one such complex with the final dihydride; calculated atomic charges suggest a heterolytic H(2) bond scission within the di-iridium coordination sphere. This investigation also establishes a ligand-design criterion for attaining cooperative bimetallic reactivity, namely, that the supporting ligand framework has sufficient mechanical flexibility so that the target complex can accommodate the nuclear reorganizations that accompany substrate activation.

Photolysis of the equilibrium mixture (silox)3NbPMe3 (1) + H2 (1-3 atm) right arrow over left arrow (silox)3Nb(Heq)2 (2e, tbp)/(silox)3Nb(Ht)2 (2t, pseudo-Td) + PMe3 causes PC bond cleavage. Depending on conditions, various amounts of (silox)3Nb=CH2 (3), (silox)3Nb=PH (5-H), (silox)3Nb=PMe (5-Me), (silox)3Nb=P(H)Nb(silox)3 (9, precipitated if N2 is present; X-ray), (silox)3NbH (4, active only through equilibration with 2e,t), and CH4 are produced. Addition of PH3 to 1 provides an independent route to 5-H; its deprotonation gives [(silox)3NbP]Li (6), whose methylation yields 5-Me. Early conversion 3:5-H ratios of approximately 3:1 suggest that initial PC bond activation is slow relative to subsequent PC bond cleavages. Addition of HPMe2 and H2PMe to 1 generates (silox)3HNbPMe2 (7) and (silox)3HNbPHMe (8), respectively, and both degrade faster than PMe3. A mechanism based around sequential PC or CH oxidative addition, followed by 1,2-elimination events, is proposed. The limiting step in the decomposition of all PMe3 is a slow hydrogenation of 3 to regenerate 2e,t and produces CH4. Hydrides 2e,t are likely to be the photolytically active species.