| To date, enzymes containing mononuclear molybdenum or tungsten active sites are all found to be chelated with one or two pyranopterindithiolate (S2pd) cofactors. Chapter 1 provides an introductory background of these molybdo- and tungstoenzymes as well as the S2pd ligand. In research related to these enzymes, the ene-1,2-dithiolate ligands, Me 2C2S22- and C6H 4S22- (bdt), were selected as representations of S2pd in synthetic analogues of protein sites. Investigation of these dithiolene-containing Mo and W complexes provides both structural and functional approaches to the enzyme sites.; Structural. In Chapters 2 and 3, it is shown that ligand substitution of the bis(dithiolene) complexes [M(CO)2(S2C 2Me2)2] eliminates both carbonyl ligands, leading to square pyramidal [MIV(QR)(S2C2Me 2)2]1-,{09}trigonal prismatic [M IV(O2CR) (S2C2Me2) 2]1-, and distorted trigonal prismatic [W VIO(QR)(S2C2Me2)2] 1- (M = Mo, W; Q = O, S) complexes were synthesized to represent the active sites of arsenite oxidase/arsenate reductase; selenate reductase, nitrate reductase, and (likely) perchlorate reductase. In Chapter 4, the synthesis and structures of the mono(dithiolene) complexes [WVIOS(OSiR 2R')(bdt)]1- and [WVIO2S(bdt)] 2- are described. These are the first and only synthetic molecules that contain a cis-WVIOS structure and a dithiolene ligand. These five-coordinate square pyramidal complexes are synthetic analogues of active sites in the xanthine oxidase family.; Functional. In Chapter 3, five-coordinate [M IV(QR)(S2C2Me2)2] 1- complexes are shown to be biomimetic models of oxotransferases, accepting an oxygen atom from SeO42- or other substrates XO = N-oxides and S-oxides to form six-coordinate [MVIO(QR)(S2C2Me 2)2]1- complexes in acetonitrile and to complete second-order oxo transfer reactions whose negative entropies of activation implicate associative transition states. Furthermore in Chapter 2, in addition to classic oxo transfer in the reaction [MIV(OC 6H4-p-X')(S2C2Me 2)2]1- + Ph3AsQ → [M VIQ(OC6H4-p-X')(S2C 2Me2)2]1- + Ph3As, Ph3AsS was introduced to complete the first quantitative examination of metal-mediated sulfido transfer reactions. Kinetics comparisons with oxo transfer reactions for M = Mo and W are presented. Results of oxo transfer reactions of [MoVIO2(SR)(bdt)]1- to substrates are also discussed in Chapter 4. |
