Biological nitrogen fixation is mediated by the nitrogenase enzyme system that catalyses the ATP dependent reduction of atmospheric dinitrogen to ammonia. Nitrogenase consists of two component metalloproteins, the MoFe-protein with the FeMo-cofactor that provides the active site for substrate reduction, and the Fe-protein that couples ATP hydrolysis to electron transfer. An overview of the nitrogenase system is presented that emphasizes the structural organization of the proteins and associated metalloclusters that have the remarkable ability to catalyse nitrogen fixation under ambient conditions. Although the mechanism of ammonia formation by nitrogenase remains enigmatic, mechanistic inferences motivated by recent developments in the areas of nitrogenase biochemistry, spectroscopy, model chemistry and computational studies are discussed within this structural framework.

Structural Basis of Biological Nitrogen Fixation.

π-Bonding and the Lone Pair Effect in Multiple Bonds between Heavier Main Group Elements

Structural models for the nitrogenase FeMo-cofactor and P-clusters are proposed based on crystallographic analysis of the nitrogenase molybdenum-iron (MoFe)-protein from Azotobacter vinelandii at 2.7 angstrom resolution. Each center consists of two bridged clusters; the FeMo-cofactor has 4Fe:3S and 1Mo:3Fe:3S clusters bridged by three non-protein ligands, and the P-clusters contain two 4Fe:4S clusters bridged by two cysteine thiol ligands. Six of the seven Fe sites in the FeMo-cofactor appear to have trigonal coordination geometry, including one ligand provided by a bridging group. The remaining Fe site has tetrahedral geometry and is liganded to the side chain of Cys alpha 275. The Mo site exhibits approximate octahedral coordination geometry and is liganded by three sulfurs in the cofactor, two oxygens from homocitrate, and the imidazole side chain of His alpha 442. The P-clusters are liganded by six cysteine thiol groups, two which bridge the two clusters, alpha 88 and beta 95, and four which singly coordinate the remaining Fe sites, alpha 62, alpha 154, beta 70, and beta 153. The side chain of Ser beta 188 may also coordinate one iron. The polypeptide folds of the homologous alpha and beta subunits surrounding the P-clusters are approximately related by a twofold rotation that may be utilized in the binding interactions between the MoFe-protein and the nitrogenase Fe-protein. Neither the FeMo-cofactor nor the P-clusters are exposed to the surface, suggesting that substrate entry, electron transfer, and product release must involve a carefully regulated sequence of interactions between the MoFe-protein and Fe-protein of nitrogenase.

https://science.sciencemag.org/content/sci/257/5077/1677.full.pdf

Structural models for the metal centers in the nitrogenase molybdenum-iron protein

A series of two-coordinate complexes of iron(II) were prepared and studied for single-molecule magnet behavior. Five of the compounds, Fe[N(SiMe3)(Dipp)]2 (1), Fe[C(SiMe3)3]2 (2), Fe[N(H)Ar′]2 (3), Fe[N(H)Ar*]2 (4), and Fe(OAr′)2 (5) feature a linear geometry at the FeII center, while the sixth compound, Fe[N(H)Ar#]2 (6), is bent with an N–Fe–N angle of 140.9(2)° (Dipp = C6H3-2,6-Pri2; Ar′ = C6H3-2,6-(C6H3-2,6-Pri2)2; Ar* = C6H3-2,6-(C6H2-2,4,6-Pri2)2; Ar# = C6H3-2,6-(C6H2-2,4,6-Me3)2). Ac magnetic susceptibility data for all compounds revealed slow magnetic relaxation under an applied dc field, with the magnetic relaxation times following a general trend of 1 > 2 > 3 > 4 > 5 ≫ 6. Arrhenius plots created for the linear complexes were fit by employing a sum of tunneling, direct, Raman, and Orbach relaxation processes, resulting in spin reversal barriers of Ueff = 181, 146, 109, 104, and 43 cm−1 for 1–5, respectively. CASSCF/NEVPT2 calculations on the crystal structures were performed to explore the influence of deviations from rigorous D∞h geometry on the d-orbital splittings and the electronic state energies. Asymmetry in the ligand fields quenches the orbital angular momentum of 1–6, but ultimately spin–orbit coupling is strong enough to compensate and regenerate the orbital moment. The lack of simple Arrhenius behavior in 1–5 can be attributed to a combination of the asymmetric ligand field and the influence of vibronic coupling, with the latter possibility being suggested by thermal ellipsoid models to the diffraction data.

Slow magnetization dynamics in a series of two-coordinate iron(II) complexes

Publisher Summary This chapter discusses the current patterns and perspectives in organolithium chemistry, including the structures of organonitrogenlithium (N-Li) compounds. The chapter explains the compounds with N-Li bonds. These are chiefly lithium imides [iminolithiums (RR’C=NLi) n ] and their complexes with added Lewis bases (L), and lithium amides [amidolithiums (RR’NLi) n ] and their complexes. For the lithium amide species, particularly, only those whose R,R’ groups that do not contain additional functionalities are described—that is, the R,R’ groups remain largely uninvolved with lithium centers. These species are termed “simple” lithium amides. N-Li compounds and, particularly, lithium amides (R 2 NLi) are widely used both in organic and in organometallic syntheses. For the former, these strong bases are employed as proton abstractors to generate new organolithiums. Although synthetic uses have dominated the interest in N-Li compounds, the chapter focuses on the structures. Most of the physical properties of organolithium compounds (the marked exception being conductance) arise, because of the overall size and shape of the units making up these materials and the nature of the peripheries of these units. The basic structural building block of any organolithium is an ion pair, R – L + . The identities and structures of complexed organolithiums are of particular importance.

Structures of Organonitrogen—Lithium Compounds: Recent Patterns and Perspectives in Organolithium Chemistry

The Neutral Transition Metal Thiolates [M(SAr)2]2 (M Mn, Fe or Co, Ar 2,4,6-t-Bu3C6H2)

Series of Two-Coordinate and Quasi-Two-Coordinate Transition-Metal Complexes: Synthesis, Structural, and Spectroscopic Studies of Sterically Demanding Borylamide Ligands -NRBR′2 (R: Ph, R′: Mes, Xyl; R = R′: Mes), Their Lithium Salts

Synthesis and characterization of the isoelectronic d10 species bis[bis(methyldiphenylsilyl)amido]cuprate(1-) and -zinc

Synthesis and x-ray crystal structures of the mono- and binuclear arylmanganate complexes [{Li(Et2O)2}2Mn2Ph6], [Li(THF)4]2[Mn2Ph6], and [Li(THF)4][MnMes3]

Steven C. Shoner

Papers

The Neutral Transition Metal Thiolates [M(SAr)2]2 (M Mn, Fe or Co, Ar 2,4,6-t-Bu3C6H2)

Series of Two-Coordinate and Quasi-Two-Coordinate Transition-Metal Complexes: Synthesis, Structural, and Spectroscopic Studies of Sterically Demanding Borylamide Ligands -NRBR′2 (R: Ph, R′: Mes, Xyl; R = R′: Mes), Their Lithium Salts

Synthesis and characterization of the isoelectronic d10 species bis[bis(methyldiphenylsilyl)amido]cuprate(1-) and -zinc

Synthesis and x-ray crystal structures of the mono- and binuclear arylmanganate complexes [{Li(Et2O)2}2Mn2Ph6], [Li(THF)4]2[Mn2Ph6], and [Li(THF)4][MnMes3]

A Compound with a Boron–Arsenic Double Bond