Reactions of H2, silanes, and olefins with superelectrophilic cationic rhenium complexes:: heterolytic cleavage of H2 and relation to the structure and function of hydrogenases

The reaction of cis-Re(CO)(4)(PR3)Me (R = Ph, Cy) with the Lewis acid B(C6F5)(3) was studied by NMR spectroscopy, and was found to provide an equilibrium mixture of the solvent-coordinated complex [cis-Re(CO)(4)(PR3)(ClCH2Cl)][MeB(C6F5)(3)] and the reactants. Reaction of cis-Re(CO)(4)(PR3)Me with HX (X = H, SiEt3) in the presence of B(C6F5)(3) at low temperature yielded the sigma-bonded complexes [cis-Re(CO)(4)(PR3)(eta(2)-HX)][MeB(C6F5)(3)] which decomposed at room temperature via intramolecular heterolytic cleavage of the X-H bond to produce MeX and the hydride-bridged dimer [cis-Re(CO)(4)(PR3)](2)(mu-H){MeB(C6F5)(3)}. The unstable H-2 binding and cleavage on this and other highly electrophilic organometallic complexes that contain strong a-acceptor CO ligands can be related to the structure and function of metalloenzymes such as Fe-containing hydrogenases that catalyze H-2 <--> H+ + 2e(-). The latter contain dinuclear organometallic-like active sites with CO ligands, which would promote binding and heterolytic cleavage of molecular H-2 in biological systems. The Fe-Fe and Ni-Fe bonds in hydrogenases are likely sites for protonation to form a bridging hydride as the initial step in the mechanism of H, formation, and electrophilic fragments such as [Re(CO)(4)(L)](+) strongly prefer to form bridging rather than terminal hydrides. The reaction of olefins and Et3SiH with the [cis-Re(Co)(4)(PR3)(ClCH2Cl)][BArF] (BArF = B[3,5-(CF3)(2)(C6H3)(4)(-)]) system was also investigated, and the X-ray crystal structure of the dimer {[cis -Re(CO)(4)(PPh3)](2)(mu-H)}{BArF} was determined. (C) 1999 Elsevier Science S.A. All rights reserved.