Thiolates, molybdenum enzymes

These techniques are applicable only to paramagnetic Mo(V) centers, but the EPR parameters are extremely sensitive to coordination changes at the molybdenum center 17, 64). The molybdenum and ligand hyperfine splittings can provide additional information about the coordination environment of the molybdenum(V) species and the chemical reactions at the molybdenum center. EPR spectra from xanthine oxidase were first reported in 1959 by Bray et al. (65), and Bray and co-workers have continued to develop the application of EPR spectroscopy to molybdenum enzymes 17, 64). In 1966 it was shown (66) that mixing [Mo04] with dithiols produced EPR signals with (g) and (A( Mo)) values similar to those of xanthine oxidase. Only recently, however, have the structures of such thiolate complexes been determined (see Section IV.B.2.b). 39) and P (67) ENDOR spec-... 

It has long been a synthetic challenge to prepare mono(oxido) mono(sulfido) molybdenum centers that are analogous to the Mo(VI) state of enzymes in the xanthine oxidase family. Although studies in this area have met with limited success, it seems possible that the sulfido linkage of these centers could be stabilized by the 1,2-enedithiolate or by a molybdenum-bound thiolate ligand. 

Early Mo K-edge EXAFS data for sulfite oxidase showed that the oxidized resting state contains a [Mo 02] unit, whereas the fully reduced enzyme possesses a [Mo O] unit (8). Both units are ligated by two or three sulfur atoms and possibly additional N or O ligands. Two of the coordinated sulfur atoms are presumably provided by molyb-dopterin, as shown in 2. However, similar EXAFS results would be expected if the molybdenum atom were bound to thiolate groups of the protein itself... 

For (L-N3)MoO(SR)2 complexes the two thiolate groups must be cis to one another, as is required by the proposed structure for molybdop-terin (1). Several (L-N3)MoO(SR)2 complexes containing both monoden-tate and chelating thiolate ligands have been prepared (91,151,153). Their EPR parameters are similar to those for the Mo(V) states of enzymes. To date, (L-N3)MoO(tdt) (tdt, toluenedithiolate) (41) is the only example of a model compound that contains both an [Mo O] group and a single dithiolene ligand, a structural feature postulated for the molybdenum(V) state of Mo-co (2). 

An interesting series of molybdenum and tungsten hydride complexes with sterically hindered thiolate ligands has recently been established due to the interest in their possible relevance to hydrodesulfurization catalysis and to the active site of the enzyme nitrogenase. The complexes [MH(SAr)3(PMe2Ph)2l (M = Mo, W SAr = TIPT, TMT) were prepared as summarized in Eqs. (8) and (9). 

R. S. Pilato, K. A. Eriksen, M. A. Greaney, E. I. Stiefel, S. Goswami, L. Kilpatrick, T. G. Spiro, E. C. Taylor and A. L. Rheingold, Model complexes for molybdopterin-containing enzymes preparation and crystallographic characterization of a molybdenum ene-l-perthi-olate-2-thiolate (trithiolate) complex,/. Am. Chem. Soc., 1991, 113, 9372-9374. 

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