No abstract

The effects of redox-inactive metal ions on dioxygen activation were explored using a new FeII complex containing a tripodal ligand with 3 sulfonamido groups. This iron complex exhibited a faster initial rate for the reduction of O2 than its MnII analog. Increases in initial rates were also observed in the presence of group 2 metal ions for both the FeII and MnII complexes, which followed the trend NMe4+ < BaII < CaII = SrII. These studies led to the isolation of heterobimetallic complexes containing FeIII-(μ-OH)-MII cores (MII = Ca, Sr, and Ba) and one with a [SrII(OH)MnIII]+ motif. The analogous [CaII(OH)GaIII]+ complex was also prepared and its solid state molecular structure is nearly identical to that of the [CaII(OH)FeIII]+ system. Nuclear magnetic resonance studies indicated that the diamagnetic [CaII(OH)GaIII]+ complex retained its structure in solution. Electrochemical measurements on the heterobimetallic systems revealed similar one-electron reduction potentials for the [CaII(OH)FeIII]+ and [SrII(OH)FeIII]+ complexes, which were more positive than the potential observed for [BaII(OH)FeIII]+. Similar results were obtained for the heterobimetallic MnII complexes. These findings suggest that Lewis acidity is not the only factor to consider when evaluating the effects of group 2 ions on redox processes, including those within the oxygen-evolving complex of Photosystem II.

The M-cluster is the [(homocitrate)MoFe7S9C] active site of nitrogenase that is derived from an 8Fe core assembled via coupling and rearrangement of two [Fe4S4] clusters concomitant with the insertion of an interstitial carbon and a ‘9th sulfur’. Combining synthetic [Fe4S4] clusters with an assembly protein template, here we show that sulfite can give rise to the ‘9th sulfur’ that is incorporated in the catalytically important belt region of the cofactor after the radical SAM-dependent carbide insertion and the concurrent 8Fe-core rearrangement have already taken place. Based on the differential reactivity of the formed cluster species, we also propose a new [Fe8S8C] cluster intermediate, the L*-cluster, that is similar to the [Fe8S9C] L- cluster but lacks the ‘9th S’ from sulfite. This work provides a semi-synthetic tool for protein reconstitution that could be widely applicable for the functional analysis of other FeS systems.

The Mo nitrogenase catalyzes the ambient reduction of N to NH at its M-cluster site. A complex metallocofactor with a core composition of [MoFe S C], the M-cluster, can be extracted from the protein scaffold and used to facilitate the catalytic reduction of CN , CO, and CO into hydrocarbons in the isolated state. Herein, we report the synthesis, structure, and reactivity of an asymmetric M-cluster analogue with a core composition of [MoFe S ]. This analogue, referred to as the Mo-cluster, is the first synthetic example of an M-cluster mimic with Fe and Mo positioned at opposite ends of the cluster. Moreover, the ability of the Mo-cluster to reduce C substrates to hydrocarbons suggests the feasibility of developing nitrogenase-based biomimetic approaches to recycle C waste into fuel products.

Solvent-extracted nitrogenase cofactors can reduce C substrates (CN, CO and CO) to hydrocarbons in reactions driven by a strong reductant, SmI (E = -1.55 V vs SCE). Here we show that a synthetic [EtN][FeS(SEt)] cluster (designated the Fe-cluster), which mimics the homometallic [FeSC] core of the nitrogenase cofactor (designated the L-cluster), is capable of conversion of C substrates into hydrocarbons in the same reactions. Comparison of the yields and product profiles between these homometallic clusters and their heterometallic counterparts points to possible roles of the heterometal, interstitial carbide and belt sulfur-bridged iron atoms in catalysis. More importantly, the observation that a "simplified", homometallic cofactor mimic can perform Fischer-Tropsch-like hydrocarbon synthesis suggests future biotechnological adaptability of nitrogenase-based biomimetic compounds for recycling C substrates into useful chemical and fuel products.