Pang Yunjie, Bjornsson Ragnar
College of Chemistry, Beijing Normal University, 100875, Beijing, China.
Max-Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany.
Phys Chem Chem Phys. 2023 Aug 9;25(31):21020-21036. doi: 10.1039/d3cp01106b.
Hydrides are present in the reduced states of the iron-molybdenum cofactor (FeMoco) of Mo nitrogenase and are believed to play a key mechanistic role in the dinitrogen reduction reaction catalyzed by the enzyme. Two hydrides are present in the E4 state according to H ENDOR and there is likely a single hydride in the E2 redox state. The 2-hydride E4 state has been experimentally observed to bind N and it has been speculated that E3 may bind N as well. However, the E3 state has not been directly observed and very little is known about its molecular and electronic structure or reactivity. In recent computational studies, we have explored the energy surfaces of the E2 and E4 by QM/MM modelling, and found that the most stable hydride isomers contain bridging or partially bridging hydrides with an open protonated belt sulfide-bridge. In this work we systematically explore the energy surface of the E3 redox state, comparing single hydride and two-hydride isomers with varying coordination and bridging terminal sulfhydryl groups. We also include a model featuring a triply protonated carbide. The results are only mildly dependent on the QM-region size. The three most stable E3 isomers at the rSCAN level of theory have in common: an open belt sulfide-bridge (terminal sulfhydryl group on Fe6) and either 2 bridging hydrides (between Fe2 and Fe6), 1 bridging-1-terminal hydride (around Fe2 and Fe6) or a dihydrogen ligand bound at the Fe2 site. Analyzing the functional dependency of the results, we find that functionals previously found to predict accurate structures of spin-coupled Fe/Mo dimers and FeMoco (TPSSh, B97-D3, rSCAN, and B3LYP*) are in generally good agreement about the stability of these 3 E3 isomers. However, B3LYP*, similar to its parent B3LYP method, predicts a triply protonated carbide isomer as the most stable isomer, an unlikely scenario in view of the lack of experimental evidence for carbide protonation occurring in reduced FeMoco states. Distinguishing further between the 3 hydride isomers is difficult and this flexible coordination nature of hydrides suggests that multiple hydride isomers could be present during experimental conditions. N binding was explored and resulted in geometries with 2 bridging hydrides and N bound to either Fe or Fe with a local low-spin state on the Fe. N binding is predicted to be mildly endothermic, similar to the E2 state, and it seems unlikely that the E3 state is capable of binding N.
氢化物存在于钼固氮酶的铁钼辅因子（FeMoco）的还原态中，并且被认为在该酶催化的二氮还原反应中起关键的机制作用。根据H ENDOR，在E4状态下存在两个氢化物，并且在E2氧化还原状态下可能存在单个氢化物。实验观察到具有两个氢化物的E4状态会结合N，并且推测E3也可能结合N。然而，尚未直接观察到E3状态，并且对其分子和电子结构或反应性了解甚少。在最近的计算研究中，我们通过QM/MM建模探索了E2和E4的能量表面，发现最稳定的氢化物异构体包含具有开放质子化带硫化物桥的桥连或部分桥连氢化物。在这项工作中，我们系统地探索了E3氧化还原状态的能量表面，比较了具有不同配位和桥连末端巯基的单个氢化物和两个氢化物异构体。我们还包括一个具有三重质子化碳化物的模型。结果仅对QM区域大小有轻微依赖性。在理论的rSCAN水平下，三个最稳定的E3异构体的共同之处在于：一个开放的带硫化物桥（Fe6上的末端巯基）和要么2个桥连氢化物（在Fe2和Fe6之间）、1个桥连-1个末端氢化物（围绕Fe2和Fe6）要么一个二氢配体结合在Fe2位点。分析结果的函数依赖性，我们发现先前发现能够预测自旋耦合的Fe/Mo二聚体和FeMoco（TPSSh、B97-D3、rSCAN和B3LYP*）准确结构的函数，在这3种E3异构体的稳定性方面总体上意见一致。然而，B3LYP*与其母体B3LYP方法类似，预测三重质子化碳化物异构体是最稳定的异构体，鉴于在还原的FeMoco状态下缺乏碳化物质子化的实验证据，这是不太可能的情况。进一步区分这3种氢化物异构体很困难，并且氢化物的这种灵活配位性质表明在实验条件下可能存在多种氢化物异构体。我们探索了N的结合，结果得到的几何结构中有2个桥连氢化物，并且N与Fe或Fe结合，Fe上具有局部低自旋状态。预计N的结合是轻微吸热的，类似于E2状态，并且E3状态似乎不太可能能够结合N。
