The Chemistry of Transition Metal Complexes Containing Catechol and Semiquinone Ligands

Three- and Four-Coordinate Gold(I) Complexes.

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

The nuclear magnetic resonance properties of chromium, molybdenum and tungsten compounds

Chalcogenide-centred gold complexes are an important class of compounds in which a central chalcogen is surrounded by several gold atoms or gold and other metals. They have special characteristics such as unusual geometries, electron deficiency and properties such as luminescence or non-linear optical properties. The best known species are the trinuclear [E(AuPR3)3]+, ‘oxonium’ type species, that have high synthetic applicability, not only in other chalcogen-centred species, but in many other organometallic derivatives. The aurophilic interactions play an important role in the stability, preference for a particular geometry and luminescence properties in this type of derivatives (critical review, 117 references).

Chalcogenide centred gold complexes

The first example of a MoOS2+ complex, cis-[MoOS(C5H10NO)2][C5H10NO = piperidine N-oxido(1–)], prepared by reaction of the corresponding cis-MoO22+ complex with B2S3, has been crystallographically characterised. The title compound crystallises in the monoclinic space group P21/n, with a= 9.671(1), b= 22.401(2), c= 13.116(2)A, β= 95.51(1)°, and eight molecules, comprising two crystallographically distinct species, per unit cell. The structure has been determined from 4 033 unique observed diffractometer data and refined to R= 0.047. The two independent molecules are virtually identical, and possess Mo–O and Mo–S bonds of length 1.723(12) and 2.106(5)A respectively and an O–Mo–S interbond angle of 116.9(7)°. The dimensions and a selection of spectroscopic properties of the title complex are compared with those of the corresponding cis-[MoY2(C5H10NO)2](Y = O or S) complexes; cis-[MoS2(C5H10NO)2] may be prepared by treating the MoOS2+ complex with B2S3.

Crystal structure of the complex cis-oxobis[piperidine N-oxido(1–)-NO]thiomolybdenum(VI), containing a cis-MoOS2+ group

Synthesis of (1,2-benzenediolato(2-)-0,0')oxobis(1-piperidinolato(1-)-0,n)molybdenum( vi), [moo(c5h10no)2(c6h4o2)], and structure determination by correlated h-1-c-13 two-dimensional nmr-spectroscopy and x-ray crystallography

The preparation, spectroscopic properties, and X-ray crystal structures of two triphenylphosphine–gold–thiomolybdate clusters are reported. [MoS4(AuPPh3)2] crystallises in the triclinic space group P with a= 9.596(1), b= 10.630(1), c= 19.712(2)A, α= 89.06(1), β= 80.87(1), γ= 66.99(1)°, and Z= 2. [MoOS3(AuPPh3){Au(PPh3)2}] crystallises in the monoclinic space group P21/c with a= 21.251(2), b= 10.535(1), c= 23.167(2)A, β= 104.20(1)°, and Z= 4. The structures have been solved from 5 748 and 7 476 unique observed diffractometer data and have been refined to R= 0.031 and 0.036, respectively. The 95Mo n.m.r. spectra of these and related copper and silver tetrathiomolybdate(VI) clusters consist of a single resonance, whose chemical shift depends on the mole ratio of added PPh3 to Mo in the solution.

Preparations, crystal structures, and molybdenum-95 nuclear magnetic resonance spectroscopic studies of triphenylphosphine–gold–tetrathiomolybdate(VI) clusters

Cyclic voltammetry has been used to characterise the electrochemical reductions of the complexes cis-[MoY2(C5H10NO)2](Y = O and/or S, C5H10NOH = 1-hydroxypiperidine) in dimethylformamide. The complexes reduce at –2.50 (dioxo), –1.94 (oxothio), and –1.59 V (dithio), referenced to the ferrocenium–ferrocene couple. For the first time, electrochemical comparisons have been made within an homologous series of molybdenum(VI) complexes.The results are especially relevant to the chemical differences between the molybdenum centres of intact and desulpho xanthine oxidase and xanthine dehydrogenase.

Comparative electrochemistry of cis-MoOS2+, MoO22+, and MoS22+ centres in complexes of the type cis-[MoY2L2](Y = O and/or S, HL = 1-hydroxypiperidine). Comments on active and desulpho-forms of xanthine oxidase and xanthine dehydrogenase

The feasibility of using 95Mo n.m.r. spectroscopy to monitor the environment of this nucleus in biochemical systems has been established by the observation of the binding of [MoO4]2– and [MoS4]2– to the protein bovine serum albumin.

Molybdenum-95 n.m.r. spectroscopy as a probe of biological systems: the detection of tetraoxo- and tetrathio-molybdate(VI) bound to bovine serum albumin

Stuart Bristow

Papers

Crystal structure of the complex cis-oxobis[piperidine N-oxido(1–)-NO]thiomolybdenum(VI), containing a cis-MoOS2+ group

Synthesis of (1,2-benzenediolato(2-)-0,0')oxobis(1-piperidinolato(1-)-0,n)molybdenum( vi), [moo(c5h10no)2(c6h4o2)], and structure determination by correlated h-1-c-13 two-dimensional nmr-spectroscopy and x-ray crystallography

Preparations, crystal structures, and molybdenum-95 nuclear magnetic resonance spectroscopic studies of triphenylphosphine–gold–tetrathiomolybdate(VI) clusters

Comparative electrochemistry of cis-MoOS2+, MoO22+, and MoS22+ centres in complexes of the type cis-[MoY2L2](Y = O and/or S, HL = 1-hydroxypiperidine). Comments on active and desulpho-forms of xanthine oxidase and xanthine dehydrogenase

Molybdenum-95 n.m.r. spectroscopy as a probe of biological systems: the detection of tetraoxo- and tetrathio-molybdate(VI) bound to bovine serum albumin