COUPLING H-2 TO ELECTRON-TRANSFER WITH A 17-ELECTRON HETEROBIMETALLICHYDRIDE - 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'-bi s(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 characte rized by a NIR absorption at 912 nm (epsilon = 486 M(-1) cm(-1)) assig ned to an intervalence transfer band and a series of atom transfer rea ctions yielding the even electron derivatives [Cp(dppf)RuXH]PF6 (X = H, Cl, Br, I). A crystallographically determined Fe-Ru distance of 4.3 83(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-valenc e bimetallic complex containing the widely used dppf ligand. The abili ty of 1 to serve as a heterobimetallic catalyst for the reduction of m ethyl viologen with H-2 makes it a unique functional model of [NiFe] h ydrogenase enzymes.