Electronic Structure Studies of Iron-Sulfur Clusters: from Model Compounds to the Active Site of Nitrogenase

Iron-sulfur (FeS) clusters are omnipresent in nature, where they are involved in a variety of tasks, such as electron transfer, DNA repair, Fe storage, and substrate activation. The enzyme nitrogenase contains one of the largest known biological FeS clusters. Nitrogenase is responsible for the conversion of inert N 2 gas to bioavailable ammonia, a reaction that is copied by the industrial Haber-Bosch process. However, despite decades of research, little is known about the molecular mechanism behind enzymatic N reduction. The active site of nitrogenase is the iron-molybdenum cofactor (FeMoco, [MoFe 7 S 9 C]) in the MoFe protein, which contains a unique μ 6 -C 4- center. The complex electronic structure of FeMoco pushes state-of-the-art quantum mechanical methods to their limits, therefore, smaller model compounds are often explored with high-level methods. The present work studies FeS clusters ranging from monomeric and dimeric FeS model compounds up to the FeMoco cluster, which has eight metal centers. The results are based on quantum chemical calculations and put into context of experimental observables. The study is divided into four parts, each focusing on different characteristics of FeS clusters: (i) S and Se are compared as bridging ligands in [Fe 2 X 2 ] 2+,1+ (X = S,Se) synthetic model compounds. The electronic structure is calculated with the complete active space self-consistent field method (CASSCF). S→Se substitution is shown to reduce the antiferromagnetic coupling strength between the Fe centers, which can be related to a decrease in Heisenberg exchange and vibronic coupling and an increase in double exchange. Se-bridged Fe centers are relevant to nitrogenase, because selective replacement of S with Se is used to probe the electronic structure in FeMoco with element-specific techniques. (ii) Cysteine (Cys) and serine (Ser) are compared as terminal ligands for the [Fe 2 X 2 ] 1+ ferredoxins from Clostridium pasteurianum ( Cp ) and Aquifex aeolicus ( Aae ). According to density functional theory ...