The all-iron (FeFe) nitrogenase utilizes a complex iron–sulfur cluster to catalyze the reduction of dinitrogen to ammonia (N2R). Recently, it has been found that methylthio-alkane reductase (mar), which reduces methyl thioalkanes to methanethiol and hydrocarbon fragments, has a structurally similar active site. In this report, we explore the reactivity of a trisphosphine boratrane iron complex (Fe), an N2R catalyst, with thioalkanes as a functional model system. Reacting the neutral iron dinitrogen complex (Fe(N2)) with dimethyl sulfide (Me2S) yields a 1:1 mixture of iron methyl (FeMe) and iron methyl thiolate (Fe(SMe)) products. Subsequent protonation forms methane and methanethiol, the products of Me2S reduction by mar. Kinetic studies, including kinetic isotope effects (KIE), pre-equilibrium Van’t Hoff parameters, and Eyring analysis, as well as the reactivity with a broad scope of thioalkanes, point to Fe(SMe2), formed via an uphill pre-equilibrium with Fe(N2) as a key intermediate. In Fe(SMe2), the C–S bond strength decreases by 44 kcal mol–1, priming it for bond homolysis. We propose that homolytic C–S bond cleavage occurs with the released alkyl radical being trapped by a second equivalent of iron. Comparisons of N2 and thioalkane reduction by Fe suggest similar modes of substrate activation at Fe, namely, backbonding into a π* orbital of N≡N and a C–S σ* orbital of Me2S, setting the substrates up for N–H bond formation and homolytic C–S bond cleavage, respectively.
Johansen et al. (Mon,) studied this question.