The electrochemical hydrogen evolution reaction is catalyzed most effectively by the Pt group metals. As H2 is considered as a future energy carrier, the need for these catalysts will increase and alternatives to the scarce and expensive Pt group catalysts will be needed. We analyze the ability of different metal surfaces and of the enzymes nitrogenase and hydrogenase to catalyze the hydrogen evolution reaction and find a necessary criterion for high catalytic activity. The necessary criterion is that the binding free energy of atomic hydrogen to the catalyst is close to zero. The criterion enables us to search for new catalysts, and inspired by the nitrogenase active site, we find that MoS2 nanoparticles supported on graphite are a promising catalyst. They catalyze electrochemical hydrogen evolution at a moderate overpotential of 0.1−0.2 V.

Biomimetic Hydrogen Evolution: MoS2 Nanoparticles as Catalyst for Hydrogen Evolution

Copper is an essential trace element in living systems, present in the parts per million concentration range. It is a key cofactor in a diverse array of biological oxidation-reduction reactions. These involve either outer-sphere electron transfer, as in the blue copper proteins and the Cu{sub A} site of cytochrome oxidase and nitrous oxide redutase, or inner-sphere electron transfer in the binding, activation, and reduction of dioxygen, superoxide, nitrite, and nitrous oxide. Copper sites have historically been divided into three classes based on their spectroscopic features, which reflect the geometric and electronic structure of the active site: type 1 (T1) or blue copper, type 2 (T2) or normal copper, and type 3 (T3) or coupled binuclear copper centers. 428 refs.

Multicopper Oxidases and Oxygenases

For present purposes, a protein-bound metal site consists of one or more metal ions and all protein side chain and exogenous bridging and terminal ligands that define the first coordination sphere of each metal ion. Such sites can be classified into five basic types with the indicated functions: (1) structural -- configuration (in part) of protein tertiary and/or quaternary structure; (2) storage -- uptake, binding, and release of metals in soluble form: (3) electron transfer -- uptake, release, and storage of electrons; (4) dioxygen binding -- metal-O{sub 2} coordination and decoordination; and (5) catalytic -- substrate binding, activation, and turnover. The authors present here a classification and structure/function analysis of native metal sites based on these functions, where 5 is an extensive class subdivided by the type of reaction catalyzed. Within this purview, coverage of the various site types is extensive, but not exhaustive. The purpose of this exposition is to present examples of all types of sites and to relate, insofar as is currently feasible, the structure and function of selected types. The authors largely confine their considerations to the sites themselves, with due recognition that these site features are coupled to protein structure at all levels. In themore » next section, the coordination chemistry of metalloprotein sites and the unique properties of a protein as a ligand are briefly summarized. Structure/function relationships are systematically explored and tabulations of structurally defined sites presented. Finally, future directions in bioinorganic research in the context of metal site chemistry are considered. 620 refs.« less

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Structural and Functional Aspects of Metal Sites in Biology

Even though the well-established Haber–Bosch process has been the major artificial way to “fertilize” the earth, its energy-intensive nature has been motivating people to learn from nitrogenase, which can fix atmospheric N2 to NH3 in vivo under mild conditions with its precisely arranged proteins Here we demonstrate that efficient fixation of N2 to NH3 can proceed under room temperature and atmospheric pressure in water using visible light illuminated BiOBr nanosheets of oxygen vacancies in the absence of any organic scavengers and precious-metal cocatalysts The designed catalytic oxygen vacancies of BiOBr nanosheets on the exposed {001} facets, with the availability of localized electrons for π-back-donation, have the ability to activate the adsorbed N2, which can thus be efficiently reduced to NH3 by the interfacial electrons transferred from the excited BiOBr nanosheets This study might open up a new vista to fix atmospheric N2 to NH3 through the less energy-demanding photochemical process

http://pubs.acs.org/doi/pdf/10.1021/jacs.5b03105

Efficient Visible Light Nitrogen Fixation with BiOBr Nanosheets of Oxygen Vacancies on the Exposed {001} Facets

The high resolution three-dimensional x-ray structure of the metal sites of bovine heart cytochrome c oxidase is reported. Cytochrome c oxidase is the largest membrane protein yet crystallized and analyzed at atomic resolution. Electron density distribution of the oxidized bovine cytochrome c oxidase at 2.8 A resolution indicates a dinuclear copper center with an unexpected structure similar to a [2Fe-2S]-type iron-sulfur center. Previously predicted zinc and magnesium sites have been located, the former bound by a nuclear encoded subunit on the matrix side of the membrane, and the latter situated between heme a3 and CuA, at the interface of subunits I and II. The O2 binding site contains heme a3 iron and copper atoms (CuB) with an interatomic distance of 4.5 A; there is no detectable bridging ligand between iron and copper atoms in spite of a strong antiferromagnetic coupling between them. A hydrogen bond is present between a hydroxyl group of the hydroxyfarnesylethyl side chain of heme a3 and an OH of a tyrosine. The tyrosine phenol plane is immediately adjacent and perpendicular to an imidazole group bonded to CuB, suggesting a possible role in intramolecular electron transfer or conformational control, the latter of which could induce the redox-coupled proton pumping. A phenyl group located halfway between a pyrrole plane of the heme a3 and an imidazole plane liganded to the other heme (heme a) could also influence electron transfer or conformational control.

https://science.sciencemag.org/content/sci/269/5227/1069.full.pdf

Structures of metal sites of oxidized bovine heart cytochrome c oxidase at 2.8 A

The tetrathiometalates [NbS 4 ] 3- and [TaS 4 ] 3- , previously known only in the form of intractable solids prepared at high temperatures, have been synthesized in the reaction system M(OEt) 5 /(Me3Si) 2 S/LiOMe (1:4:3) in acetonitrile solution at ambient temperature. The species [VS 4 ] 3- was prepared similarly from VO(OMe) 3 .

Group 5 tetrathiometalates : simplified syntheses and structures

Bis[hydrotris(pyrazol-1-yl)borato]titanium(II): a stable Tp2M complex of singular reactivity

The structural coordination chemistry of the sterically hindered 3,3‘-bis(triphenylsilyl)-1,1‘-bi-2-phenoxide ligand ([TPSLO2]2-), an isosteric homologue of a chiral binaphthoxide ligand used in asymmetric induction, has been investigated on Fe(II), Fe(III), and Cr(II). The ligand diol (TPSL(OH)2, 2) can be prepared on a multigram scale from 2,2‘-dimethoxybiphenyl via a convenient, two-step ortho-metalation/silylation/deprotection sequence; the sodium salt of the ligand dianion (Na2[TPSLO2], 3) can be obtained by NaH deprotonation and was crystallographically characterized with two THF ligands bound to each cation. Protonolysis of Fe[N(SiMe3)2]2 or Cr[N(SiMe3)2]2(THF)2 with biphenol 2 in arene solvent gives [Fe(μ-TPSLO2)]2 (4) or (TPSLO2)Cr(THF)2 (9), respectively, while anion metathesis of FeCl2 or FeCl3/bipy with biphenoxide salt 3 in THF yields (TPSLO2)Fe(THF)2 (8) or ferric monomer (TPSLO2)FeCl(bipy) (10), respectively. Dimer 4 reacts with exogenous ligands to form monomeric ligand adducts of Fe(II): ...

3,3'-Bis(triphenylsilyl)biphenoxide as a sterically hindered ligand on Fe(II), Fe(III), and Cr(II).

The sterically hindered Fe(II) thiolate dimer Fe2(μ-STriph)2(STriph)2 (1; [STriph]− = 2,4,6-triphenylbenzenethiolate) reacts with primary amines (tBuNH2, aniline) and N2H4 to form the structurally characterized addition complexes Fe(STriph)2(NH2tBu)2, Fe2(μ-STriph)2(STriph)2(NH2Ph)2, and Fe2(μ-η1:η1-N2H4)2(N2H4)4(STriph)4 in high yield. Chemical and NMR spectroscopic evidence indicate that the binding of these nitrogen donors is labile in solution and multispecies equilibria are likely. With arylhydrazines, 1 catalytically disproportionates 1,2-diphenylhydrazine to aniline and azobenzene, and it rearranges 1-methyl-1,2-diarylhydrazines to give, after treatment with alumina, mononuclear, trigonal bipyramidal Fe(III) complexes of composition Fe(ISQ)2(STriph), where [ISQ]− denotes an appropriately substituted bidentate o-diiminobenzosemiquinonate ligand. Complex 1 shows no reaction with hindered 1,2-dialkylhydrazines (isopropyl or tert-butyl) or tetrasubstituted 1,2-dimethyl-1,2-diphenylhydrazine.

Reactivity of a sterically hindered Fe(II) thiolate dimer with amines and hydrazines.

Ligand K-edge X-ray absorption spectroscopy was used to study dimeric and tetrameric Cl-terminated Fe–S clusters with variable numbers of S2– substituted by NtBu2– (NtBu2– = tertbutylimido) ligands to gain insights into the functional role of the interstitial light atom in the iron–molybdenum cofactor (FeMoco) of nitrogenase. These studies are complemented by time-dependent density functional theory analysis to quantify the relative effects on Fe–S and Fe–Cl bonding. The results show that NtBu2– substitution dramatically affects the electronic structure of dimeric clusters, while the impact on tetrameric clusters is small. Strong agreement between experiment and theory merited extension of this analysis to hypothetical clusters with S2– substituted by N and C-atom donor ligands as well as FeMoco itself. These results show that very strong electron donors are required to appreciably modulate the electronic structure of tetrameric (or larger) iron sulfur clusters, pointing to a possible role of the central ...

Sonny C. Lee

Papers

Group 5 tetrathiometalates : simplified syntheses and structures

Bis[hydrotris(pyrazol-1-yl)borato]titanium(II): a stable Tp2M complex of singular reactivity

3,3'-Bis(triphenylsilyl)biphenoxide as a sterically hindered ligand on Fe(II), Fe(III), and Cr(II).

Reactivity of a sterically hindered Fe(II) thiolate dimer with amines and hydrazines.

Light-atom influences on the electronic structures of iron-sulfur clusters.