Coupling H₂ to Electron Transfer with a 17-Electron Heterobimetallic Hydride: A “Redox Switch” Model for the H₂-Activating Center of Hydrogenase

A meta-stable heterobimetallic mixed-valence ion, [Fe(II),Ru(III)], is formed by the one-electron oxidation of Cp*(dppf)RuH {1, dppf = 1,1‘-bis(diphenylphosphino)ferrocene, Cp* = pentamethylcyclopentadienide}. A remarkable stability toward one-electron oxidation is revealed by the cyclic voltammetry of 1 which contains two reversible oxidations at +0.073 and +0.541 V and a quasireversible oxidation at +0.975 V (vs NHE) assigned to Ru(III/II), Ru(IV/III), and Fe(III/II), respectively. The isolable Ru(III) metal hydride, [Cp*(dppf)RuH]PF6 (1+), is characterized by a NIR absorption at 912 nm (ε = 486 M-1 cm-1) assigned to an intervalence transfer band and a series of atom transfer reactions yielding the even electron derivatives [Cp*(dppf)RuXH]PF6 (X = H, Cl, Br, I). A crystallographically determined Fe−Ru distance of 4.383(1) Å in 1 is consonant with the classification of 1+ as a weakly coupled, Type II mixed-valence ion (Hab = 627 cm-1, α2 = 3.3 × 10-3). This is the first reported example of a mixed-valence bimetallic complex containing the widely used dppf ligand. The ability of 1 to serve as a heterobimetallic catalyst for the reduction of methyl viologen with H2 makes it a unique functional model of [NiFe] hydrogenase enzymes

Coupling H₂ to Electron Transfer with a 17-Electron Heterobimetallic Hydride: A “Redox Switch” Model for the H₂-Activating Center of Hydrogenase

A meta-stable heterobimetallic mixed-valence ion, [Fe(II),Ru(III)], is formed by the one-electron oxidation of Cp*(dppf)RuH {1, dppf = 1,1‘-bis(diphenylphosphino)ferrocene, Cp* = pentamethylcyclopentadienide}. A remarkable stability toward one-electron oxidation is revealed by the cyclic voltammetry of 1 which contains two reversible oxidations at +0.073 and +0.541 V and a quasireversible oxidation at +0.975 V (vs NHE) assigned to Ru(III/II), Ru(IV/III), and Fe(III/II), respectively. The isolable Ru(III) metal hydride, [Cp*(dppf)RuH]PF6 (1+), is characterized by a NIR absorption at 912 nm (ε = 486 M-1 cm-1) assigned to an intervalence transfer band and a series of atom transfer reactions yielding the even electron derivatives [Cp*(dppf)RuXH]PF6 (X = H, Cl, Br, I). A crystallographically determined Fe−Ru distance of 4.383(1) Å in 1 is consonant with the classification of 1+ as a weakly coupled, Type II mixed-valence ion (Hab = 627 cm-1, α2 = 3.3 × 10-3). This is the first reported example of a mixed-valence bimetallic complex containing the widely used dppf ligand. The ability of 1 to serve as a heterobimetallic catalyst for the reduction of methyl viologen with H2 makes it a unique functional model of [NiFe] hydrogenase enzymes

Synthesis and Characterization of <i>trans</i>-[Os(en)₂py(H)]²⁺ and Related Studies

trans-[Os(en)2pyH](Otf)2, 2, is recovered from an acidic solution of trans-[Os(en)2py(H2)](OTf)2, 1, which has been subjected to one electron oxidation. The structures of both 1 and 2 have been determined by single crystal X-ray analysis. In cyclic voltammetry, 2 shows a one electron oxidation wave at 0.95 V and a one electron reduction wave at −1.2 V, neither accompanied by a signal for the reverse process. Reduction of 2 by Zn/Hg in methanol solution leads to quantitative formation of [Os(en)2(py)H2)]2+, predominantly in the trans-form. In aqueous solution, species 2 reacts rapidly with N-methylacridium ion, [MAH]+, by hydride transfer. One electron chemical oxidation of 2 to the corresponding Os(IV) is slower than that of 1 to 2 owing to the increase in coordination number when Os(IV) is produced. Treatment of 1, or the cis-form, 1‘, in DMSO by sodium t-butoxide produces mainly the corresponding isomers of the monohydrides of OsII, that derived from 1‘ is deep red in color while the trans-monohydride is colorless. Both react with [MAH]+ to form [MAH]2, and both disappear rapidly in acetone or acetonitrile, presumably by reducing the solvents. Reaction of trans-[Os(NH3)4(H2)H2O](BPh4)2, 4, in acetone-d6 as solvent with either CH3CHO or styrene leads to hydrogenation of the substrate. Reactions which compete with trans-[Os(en)2(η2-H2)(CF3SO)3]CF3SO3 release of substrate from the trans-complex before isomerization to the cis-form, required for hydrogenation to occur, result in the trans-derivative of the added solute. When H2CCHCH2SCH3 is the substrate, binding takes place at sulfur. Complete conversion to the cis-substrate isomer is observed, without hydrogenation occurring even though contact between dihydrogen and the double bond is possible