Nathaniel S. Sickerman

Heterobimetallic complexes with MIII-(μ-OH)-MII cores (MIII = Fe, Mn, Ga; MII = Ca, Sr, and Ba): structural, kinetic, and redox properties

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.

Utilizing tautomerization of 2-amino-oxazoline in hydrogen bonding tripodal ligands

A tetradentate tripodal ligand containing 2-amino-oxazoline moieties has been developed. This system tautomerizes upon chelation of a metal ion, forming a flexible cavity capable of accommodating ligands via an intramolecular hydrogen bonding network.

Synthesis, Structure, and Physical Properties for a Series of Trigonal Bipyramidal MII–Cl Complexes with Intramolecular Hydrogen Bonds

A series of transition metal chloro complexes with the tetradentate tripodal tris(2-amino-oxazoline)amine ligand (TAO) have been synthesized and characterized. X-Ray structural analyses of these compounds demonstrate the formation of the mononuclear complexes [M(II)(TAO)(Cl)](+), where M(II) = Cr, Mn, Fe, Co, Ni, Cu and Zn. These complexes exhibit distorted trigonal-bipyramidal geometry, coordinating the metal through an apical tertiary amine, three equatorial imino nitrogen atoms, and an axial chloride anion. All the complexes possess an intramolecular hydrogen-bonding (H-bonding) network within the cavity occupied by the metal-bound chloride ion. The metal-chloride bond distances are atypically long, which is attributed to the effects of the H-bonding network. Nuclear magnetic resonance (NMR) spectroscopy of the Zn complex suggests that the solid-state structures are representative of that observed in solution, and that the H-bonding interactions persist as well. Additionally, density functional theory (DFT) calculations were carried out to probe the electronic structures of the complexes.

Structure and Reactivity of an Asymmetric Synthetic Mimic of Nitrogenase Cofactor

The Mo nitrogenase catalyzes the ambient reduction of N2 to NH3 at its M-cluster site. A complex metallocofactor with a core composition of [MoFe7 S9 C], the M-cluster, can be extracted from the protein scaffold and used to facilitate the catalytic reduction of CN- , CO, and CO2 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 [MoFe5 S9 ]. 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 C1 substrates to hydrocarbons suggests the feasibility of developing nitrogenase-based biomimetic approaches to recycle C1  waste into fuel products.

Synthetic Analogues of Nitrogenase Metallocofactors: Challenges and Developments

Nitrogenase is the only known biological system capable of reducing N2 to NH3 , which is a critical component of bioavailable nitrogen fixation. Since the discovery of discrete iron-sulfur metalloclusters within the nitrogenase MoFe protein, synthetic inorganic chemists have sought to reproduce the structural features of these clusters in order to understand how they facilitate the binding, activation and hydrogenation of N2 . Through the decades following the initial identification of these clusters, significant progress has been made to synthetically replicate certain compositional and functional aspects of the biogenic clusters. Although much work remains to generate synthetic iron-sulfur clusters that can reduce N2 to NH3 , the insights borne from past and recent developments are discussed in this concept article.

Synthesis, structure and reactivity of FeII/III–NH3 complexes bearing a tripodal sulfonamido ligand

Complexes [M(n)MST(NH3)](n-3) (M(n) = Fe(II), Fe(III), Ga(III)) were prepared and each contains an intramolecular hydrogen bonding network involving the ammonia ligand. Deprotonation of the Fe(III)-NH3 complex afforded a putative [Fe(III)MST(NH2)](-) species whose reactivity has been explored.

Activation of CO2 by Vanadium Nitrogenase

The reduction of CO2 into useful products, including hydrocarbon fuels, is an ongoing area of particular interest due to efforts to mitigate buildup of this greenhouse gas. While the industrial Fischer-Tropsch process can facilitate the hydrogenation of CO2 with H2 to form short-chain hydrocarbon products under high temperatures and pressures, a desire to perform these reactions under ambient conditions has inspired the use of biological approaches. Particularly, enzymes offer insight into how to activate and reduce CO2 , but only one enzyme, nitrogenase, can perform the multielectron, multiproton reduction of CO2 into hydrocarbons. The vanadium-containing variant, V-nitrogenase, displays especial reactivity towards the hydrogenation of CO and CO2 . This Focus Review discusses recent progress towards the activation and reduction of CO2 with three primary V-nitrogenase systems. These systems span both ATP-dependent and ATP-independent processes and utilize approaches with whole cells, isolated proteins, and extracted cofactors.

Nitrogenase Assembly: Strategies and Procedures

Nitrogenase is a metalloenzyme system that plays a critical role in biological nitrogen fixation, and the study of how its metallocenters are assembled into functional entities to facilitate the catalytic reduction of dinitrogen to ammonia is an active area of interest. The diazotroph Azotobacter vinelandii is especially amenable to culturing and genetic manipulation, and this organism has provided the basis for many insights into the assembly of nitrogenase proteins and their respective metallocofactors. This chapter will cover the basic procedures necessary for growing A. vinelandii cultures and subsequent recombinant transformation and protein expression techniques. Furthermore, protocols for nitrogenase protein purification and substrate reduction activity assays are described. These methods provide a solid framework for the assessment of nitrogenase assembly and catalysis.

Tracing the ‘ninth sulfur’ of the nitrogenase cofactor via a semi-synthetic approach

AbstractThe M-cluster is the [(homocitrate)MoFe7S9C] active site of nitrogenase that is derived from an 8Fe core assembled viacoupling and rearrangement of two [Fe4S4] clusters concomitant with the insertion of an interstitial carbon and a ‘ninth sulfur’. Combining synthetic [Fe4S4] clusters with an assembly protein template, here we show that sulfite can give rise to the ninth sulfur that is incorporated in the catalytically important belt region of the cofactor after the radical S-adenosyl-l-methionine-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, which is similar to the [Fe8S9C] L-cluster, but lacks the ninth sulfur 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 M-cluster in the active site of nitrogenase 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 ninth sulfur. Now, by combining synthetic [Fe4S4] clusters and assembly with a protein template, it has been shown that sulfite gives rise to the ninth sulfur that is inserted into the nitrogenase cofactor after the radical SAM-dependent carbide insertion and cofactor core rearrangement.

Ambient conversion of CO 2 to hydrocarbons by biogenic and synthetic [Fe 4 S 4 ] clusters

The Fe protein of nitrogenase contains a redox active [Fe4S4] cluster that plays a key role in electron transfer and substrate reduction. Here we show that the Fe protein of Methanosarcina acetivorans can reduce CO2 and CO to hydrocarbons under ambient conditions. Further, we demonstrate that this reactivity is inherent to [Fe4S4] clusters, showing the ability of a synthetic [Fe4S4] compound to catalyse the same ambient reaction in solutions. Theoretical calculations suggest a reaction mechanism involving an aldehyde-like intermediate that gives rise to hydrocarbon products upon proton-coupled electron transfer and concomitant removal of water molecules. These results provide a framework for mechanistic investigations of FeS-based activation and reduction of CO2 and CO while facilitating potential development of FeS catalysts capable of ambient conversion of CO2 and CO into fuel products.The Fe protein of nitrogenase contains a redox-active [Fe4S4] cluster that plays a key role in electron transfer and substrate reduction. Here, Hu and co-workers show that the Fe protein of Methanosarcina acetivorans can reduce CO2 and CO to hydrocarbons under ambient conditions.