Description:
<jats:title>Abstract</jats:title><jats:p>The iron‐molybdenum cofactor (FeMoco) is responsible for dinitrogen reduction in Mo nitrogenase. Unlike the resting state, E<jats:sub>0</jats:sub>, reduced states of FeMoco are much less well characterized. The E<jats:sub>2</jats:sub> state has been proposed to contain a hydride but direct spectroscopic evidence is still lacking. The E<jats:sub>2</jats:sub> state can, however, relax back the E<jats:sub>0</jats:sub> state via a H<jats:sub>2</jats:sub> side‐reaction, implying a hydride intermediate prior to H<jats:sub>2</jats:sub> formation. This E<jats:sub>2</jats:sub>→E<jats:sub>0</jats:sub> pathway is one of the primary mechanisms for H<jats:sub>2</jats:sub> formation under low‐electron flux conditions. In this study we present an exploration of the energy surface of the E<jats:sub>2</jats:sub> state. Utilizing both cluster‐continuum and QM/MM calculations, we explore various classes of E<jats:sub>2</jats:sub> models: including terminal hydrides, bridging hydrides with a closed or open sulfide‐bridge, as well as models without. Importantly, we find the hemilability of a protonated belt‐sulfide to strongly influence the stability of hydrides. Surprisingly, non‐hydride models are found to be almost equally favorable as hydride models. While the cluster‐continuum calculations suggest multiple possibilities, QM/MM suggests only two models as contenders for the E<jats:sub>2</jats:sub> state. These models feature either i) a bridging hydride between Fe<jats:sub>2</jats:sub> and Fe<jats:sub>6</jats:sub> and an open sulfide‐bridge with terminal SH on Fe<jats:sub>6</jats:sub> (<jats:bold>E<jats:sub>2</jats:sub>‐hyd</jats:bold>) or ii) a double belt‐sulfide protonated, reduced cofactor without a hydride (<jats:bold>E<jats:sub>2</jats:sub>‐nonhyd</jats:bold>). We suggest both models as contenders for the E<jats:sub>2</jats:sub> redox state and further calculate a mechanism for H<jats:sub>2</jats:sub> evolution. The changes in electronic structure of FeMoco during the proposed redox‐state cycle, E<jats:sub>0</jats:sub>→E<jats:sub>1</jats:sub>→E<jats:sub>2</jats:sub>→E<jats:sub>0</jats:sub>, are discussed.</jats:p>