Electrochemical cleavage of N=N bonds at a Mo2(μ-SMe)3 site relevant to the biological reduction of dinitrogen at a bimetallic sulfur centre

Abstract : The reduction of diazene complexes [Mo2Cp2(μ-SMe)3(μ- η2-H-N=N-R)]+(R=Ph (3a); Me (3b)) and of the hydrazido(2 -) derivative [Mo2Cp2(μ-SMe)3_μ- η1-N=N(Me)H_]+ (1b) has been studied by cyclic voltammetry, controlled-potential electrolysis, and coulometry in THF. The electrochemical reduction of 3a in the presence of acid leads to cleavage of the N=N bond and produces aniline and either the amido complex [Mo2Cp2(μ-SMe)3-(μ-NH2)] 4 or the ammine complex [Mo2Cp2(μ-SMe)3(NH3)(X)] 5, depending on the initial concentration of acid (HX=HTsO or CF3CO2H). The N=N bond of the methyldiazene analogue 3b is not cleaved under the same conditions. The ability of 3a but not 3b to undergo reductive cleavage of the N=N bond is attributed to electronic control of the strength of the Mo-N(R) bond by the R group. The electrochemical reduction of the methylhydrazido(2 -) compound 1b in the presence of HX also results in cleavage of the N=N bond, with formation of methylamine, 4 (or 5) and the methyldiazenido complex [Mo2Cp2(μ-SMe)3(μ- η1-N=N-Me)]. Formation of the last of these complexes indicates that two mechanisms (N=N bond cleavage and possibly H2 production) are operative. A pathway for the reduction of N2 at a dinuclear site of FeMoco is proposed on the basis of these results.