Publisher Summary This chapter presents a discussion on the bioinorganic chemistry of pterin-containing molybdenum and tungsten enzymes. The chapter focuses on the current results and the interplay of model and enzyme chemistry. Attention is directed to sulfite oxidase and xanthine oxidase, the archetypal examples of molybdenum enzymes, containing, respectively, dioxo-Mo(VI) and oxo-thio-Mo(VI) oxidized centers. Biochemical and model studies of molybdopterin, Mo-co, and related species are described in this chapter. A brief survey on the physical and spectroscopic techniques employed in the study of the enzymes is presented and their impact on the current understanding of the coordination about the molybdenum atom in sulfite oxidase and xanthine oxidase is discussed. Structural and spectroscopic models are also presented in the chapter. The chapter also explains the xanthine oxidase cycle and facets of intramolecular electron transfer in molybdenum enzymes. The pterin-containing tungsten enzymes and the evolving model chemistry are discussed. Current descriptions of the coordination environment of the molybdenum centers of the enzymes rest primarily upon the comparisons of the spectra of the enzymes with the spectra of well-characterized molybdenum complexes. The unexpected discovery that several hyperthermophilic organisms possess pterin-containing tungsten enzymes promises to stimulate the development of oxo-thio-tungsten chemistry.

Bioinorganic chemistry of pterin-containing molybdenum and tungsten enzymes

A novel tridentate schiff base dioxo-molybdenum (vi) complex: synthesis, crystal structure and catalytic performance in green oxidation of sulfides by urea hydrogen peroxide

Toward functional models of metalloenzyme active sites: analog reaction systems of the molybdenum oxo transferases

Synthesis, spectral and X-ray structural characterization of [cis-MoO2(L)(solv)] (L=salicylidene salicyloyl hydrazine) and its use as catalytic oxidant

In order to study the reactivity pattern of Mo−Ot bonds associated with anionic sulfur ligands, precursor complexes MoO2L·D (H2L = S-methyl 3-(2-hydroxyphenyl)methylenedithiocarbazate; D = CH3OH (1), H2O (2)) were synthesized. Complex 2 crystallizes in the orthorhombic space group P212121, with a = 6.079(1) A, b = 11.638(2) A, c = 17.325(2) A, V = 1225.7(4) A3, and Z = 4. In its reaction with PhNHOH, 1 forms a seven-coordinate oxaziridine compound [MoO(η2-ONPh)L·CH3OH] (3) by oxo-rearrangement (elimination−substitution) without a change in the molybdenum oxidation state. The crystal data for 3 are a = 9.573(3) A, b = 9.859(2) A, c = 10.604(3) A, α = 95.90(2)°, β = 95.81(2)°, γ = 112.12(2)°, V = 911.5(4) A3, Z = 2, and triclinic space group P1. In contrast to that of the precursor compound 1 (Mo−Ot = 1.700(4) A), the terminal Mo−Ot distance in 3 (1.668(2) A) is typical of Mo−O bonds of order 3, which drives the formation of an apparently unstable three-membered metallacycle via spectator oxo stabilization...

Reactivity of Mo−Ot Terminal Bonds toward Substrates Having Simultaneous Proton- and Electron-Donor Properties: A Rudimentary Functional Model for Oxotransferase Molybdenum Enzymes§

Molybdenum(VI) complexes of the type MoO 2 (L). where L is a tridentate dianionic Schiff base ligand, are reported. Infrared data suggest oligomerization via a Mo=0→Mo bridge which is cleaved by a wide variety of donors (D): aldehyde, amide, amine. N-oxide, sulfoxide, phosphine oxide, water, alcohol, and phosphinetroducing Mo0 2 (L)(D). The tendency of adduct formation by the molybdenyl systems increases in a particular way as L is changed. Mo0 2 (L)(D) usually shows two IR absorptions attributable to cis-Mo0 2 stretches, and on the average, species with oxygen donors vibrate at higher frequencies than those with nitrogen donors. On the basis of ligand displacement reactions D binding increases in the order CH 3 CHO 6 H 5 CHO 2 S0 2 (L)(D) is readily converted to Mo0 2 (L) in a single endothermic step. Thermal analysis data together with NMR data support the weak binding of the D ligands. The electron transfer characteristics of a number of the MoO 2 (L)(dmf) species are studied with use of voltarmmetric techniques. In general two to three reduction waves are obsened, all !mated at potentials more negative than —0.9 V vs. SCE. The chemistry and electrochemistry of a dinuclear molybdenum V) species resulting from hydrazine reduction of one Mo0 2 (L) is also briefly described.

O. A. Rajan

Papers

Molybdenum complexes. 1. Acceptor behavior and related properties of Mo(VI)O2(tridentate) systems