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

The ability of platinum and gold catalysts to effect powerful atom-economic transformations has led to a marked increase in their utilization. The quite remarkable correlation of their catalytic behavior with the available structural data, coordination chemistry, and organometallic reactivity patterns, including relativistic effects, allows the underlying principles of catalytic carbophilic activation by π acids to be formulated. The spectrum of reactivity extends beyond their utility as catalytic and benign alternatives to conventional stoichiometric π acids. The resulting reactivity profile allows this entire field of catalysis to be rationalized, and brings together the apparently disparate electrophilic metal carbene and nonclassical carbocation explanations. The advances in coupling, cycloisomerization, and structural reorganization—from the design of new transformations to the improvement to known reactions—are highlighted in this Review. The application of platinum- and gold-catalyzed transformations in natural product synthesis is also discussed.

Catalytic Carbophilic Activation: Catalysis by Platinum and Gold π Acids

Zinc coordination, function, and structure of zinc enzymes and other proteins

By Electrochemical methods

Katalytische carbophile Aktivierung: Platin- und Gold-π-Säuren als Katalysatoren

The method of direct electrochemical synthesis consists of oxidizing a metal anode in a non-aqueous solution containing a ligand (or ligand precursor) to produce the appropriate inorganic or organometallic compound. In many cases, the product precipitates directly in the cell, making for easy isolation, so that the technique is both direct and simple, and in addition the product yields are very high. One advantage of the technique is that the products are often derivatives of a low oxidation state of the metal; \examples of this include chromium(III) bromide, tin(II) and lead(II) diolates and thiolates, hexahalogenodigallate(II) anions, thorium diiodide, copper(I) thiolate complexes, and indium(I) derivatives of thiols, dithiols, and diols. In some systems, the low oxidation state compound undergoes subsequent reaction; for example, in the synthesis of RInX2 the reaction sequence involves the oxidation of indium metal to give InX, which then reacts with RX to give RInX2. Another possible post-electrolysis process is disproportionation. Examples of these various preparative routes will be discussed.

Direct electrochemical synthesis of inorganic and organometallic compounds

A convenient electrochemical synthesis of various ML2, ML3, ML4 and MOL2 complexes (M = Ti, Zr,Hf, V,Cr, Mn,Fe, Co,Ni, Cu,In; L = acetylacetonate anion, 3-hydroxy-2-methyl-4-pyronate anion, 2-acetylpyrrolate anion, and other bidentate ligands) is described. The metal, as the anode of simple cell, is oxidised in the presence of the parent compound of the ligand (HL) in an organic solvent mixture. Gram quantities of complex can be produced in a few hours; other advantages of this method are discussed. Current efficiency measurements serve to identify the reaction mechanism.

Direct electrochemical synthesis of some metal chelate complexes

The coupling of organic groups by the electrochemical reduction of organic halides: Catalysis by 2,2′-bipyridinenickel complexes

The electrochemical oxidation of anodic copper or silver (= M) into non-aqueous solutions of RSH (R = alkyl, axyl) gives MISR as insoluble materials in high yield. In the presence of 1,10-phenanthroline (= L), the products are MISR•phen for M = Cu, but not Ag. Gold resists oxidation under such conditions, and AuISR (R = n-C4H9, C6H5) was obtained in only poor yield. The crystal structure of the solvated dimeric adduct [Cu(SC6H4CH3-o)•phen]2•CH3CN is triclinic, with a = 10.682(3) A, b = 11.729(4) A, c = 15.608(5) A, α = 76.87(2)°, β = 76.35(2)°, γ = 68.07(2)°, V = 1742(1) A3, Z = 2 and space group . The structure is based on a folded Cu2S2 ring with an unusually short Cu—Cu distance of 2.613(3) A Each copper atom has CuS2N2 pseudo-tetrahedral stereochemistry, with Cu—S = 2.337 A(av) and Cu—N = 2.10 A(av).

The direct electrochemical synthesis of thiolato complexes of copper, silver, and gold; the molecular structure of [Cu(SC6H4CH3-o)(1,10-phenanthroline)]2•CH3CN

Dennis G. Tuck

Papers

Direct electrochemical synthesis of inorganic and organometallic compounds

Direct electrochemical synthesis of some metal chelate complexes

The coupling of organic groups by the electrochemical reduction of organic halides: Catalysis by 2,2′-bipyridinenickel complexes

The direct electrochemical synthesis of thiolato complexes of copper, silver, and gold; the molecular structure of [Cu(SC6H4CH3-o)(1,10-phenanthroline)]2•CH3CN

The direct electrochemical synthesis of [(C6H5)3Ph]2[CoCl4]: An example of the use of anodic oxidation in the preparation of metal ion complexes