Reactions of H_2, silanes, and olefins with superelectrophilic cationic rhenium complexes: heterolytic cleavage of H_2 and relation to the structure and function of hydrogenases

摘要

The reaction of cis-Re(CO)_4(PR_3)Me (R=Ph, Cy) with the Lewis acid B(C_6F_5)_2 was studied by NMR spectroscopy, and was found to provide an equilibrium mixture of the solvent-coordinated complex [cis-Re(CO)_4(PR_3)(ClCH_2Cl)][MeB(C_6F_5)_3] and the reactions. Reaction of cis-Re(CO)_4(RP_3)Me with HX (X=H, SiEt_3) in the presence of B(C_6H_5)_3 at low temperature yielded the #sigma#-bonded complexes [cis-Re(CO)_4(PR_3)(#eta#~2-HX)][MeB(C_6F_5)_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(PR_3)]_2(#mu#-H){MeB(C_6F_5)_3}. The unstable H_2 binding and cleavage on this and other highly electrophilic organometallic complexes that contain strong #pi#-acceptor CO ligands can be related to the structure and function of metalloenzymes such as Fe-containing hydrogenases that catalyze H_2 <-> 2H~+ + 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_2 formation, and electrophilic fragments such as [Re(CO)_4(L)]~+ strongly prefer to form bridging rather than terminal hydrides. The reaction of olefins and Et_3SiH with the [cis-Re(CO)_4(PR_3)(ClCH_2Cl)][BAr_F] (BAr_F=B[3,5-(CF_3)_2(C_6H_3)_4~-]) system was also investigated, and the X-ray crystal structure of the dimer {[cis-Re(CO)_4(PPh_3)]_2(#mu#-H)} {BAr_F} was determined.