Alternative Synthesis, Density Functional Calculations and Proton Reactivity Study of a Trinuclear [NiFe] Hydrogenase Model Compound

The trinuclear complex [('S 2 '){Ni(PMe 3 )} 2 Fe(CO)('S 2 ') 2 ] (1) {'S 2 -' 2- = 1,2-benzenedithiolate(2-)} has been reported as a structural as well as functional model for [NiFe] hydro-genases since it contains key structural features of the [NiFe] hydrogenase active site, and is oxidized by protons to give [1] + and H 2 . Complex 1 formed as an unexpected product from the reaction of [Fe(CO) 2 ('S 3 ')] 2 {'S 3 '2- = bis(2-mercaptophenyl) sulfide(2-)} and [Ni(PMe 3 ) 2 ('S 2 ')]. Both the iron and nickel centers of 1 are chelated by 'S 2 ' donors, but not with any 'S 3 ' ligand. In order to understand this reaction, the new precursor [Fe(CO) 2 ( si S 3 )] 2 (2) { si S 3 2- = bis(2-mercapto-3-trimethylsilylphenyl) sulfide(2-)} was synthesized. Compound 2 readily loses its CO ligand to give [Fe(CO)( si S 3 )] 2 (3), which consists of two 16 valence electron fragments and could be isolated in the solid state. Conversion of 3 to 2 is feasible with gentle bubbling of CO gas for about 2 min. Treatment of 2 with [Ni(PMe 3 ) 2 ('S 2 ')] resulted exclusively in the formation of complex 1, which confirms that all three 'S 2 ' ligands in 1 originate from [Ni(PMe 3 ) 2 ('S 2 ')]. Therefore, an alternative synthesis of 1, which does not involve any 'S 3 ' ligand, has been developed. Density functional theory (DFT) calculations suggest that the oxidation states of the metal centers are Fe II and Ni II and do not change upon oxidation of 1 to [1] + . The unpaired electron in [1] + is located to a large extent on the nickel atoms and the adjacent thiolate donor functions. The charge, however, is distributed over the whole cluster, main parts residing on the 'S 2 ' ligands. Preliminary constant potential coulometric measurements indicate that 1 mediates the reduction of protons to dihydrogen at a mild potential (-0.48 V vs. NHE). Based upon these experimental and theoretical results, plausible mechanisms for this reduction are briefly discussed.