Coupling H-2 to electron transfer with a 17-electron heterobimetallic hydride: A ''redox switch'' model for the H-2-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 (epsilon = 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) Angstrom in 1 is consonant with the classification of 1(+) as a weakly coupled, Type II mixed-valence ion (H-ab = 627 cm(-1), alpha(2) = 3.3 x 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 H-2 makes it a unique functional model of [NiFe] hydrogenase enzymes.