Molybdenum enzymes ligand complexes

Mononuclear Mov complexes with sterically hindered monodentate thiolato ligands are of interest as models for special molybdenum enzymes (the Mo oxidases). Here dimer formation via thiolato bridges and sulfide formation by C—S bond cleavage do not occur. Such a ligand is TIPTH. The complex [Mo(CO)2(TIPT)3) (14), with essentially trigonal prismatic coordination about Mo and trans CO groups in the axial sites, can be obtained.91... 

The thermochromism in Scheme 1-54 represents valence tautomerism in molybdenum complexes and, at the same time, the inherent redox activity of a dithoilate ligand. This phenomenon may be significant with respect to the biological activity of various molybdenum enzymes. 

Mo(V) complex disproportionates as it dissociates to produce mononuclear Mo (IV) and Mo (VI). As Mo (IV) and Mo (VI) are directly interconvertible by an oxo transfer reaction, they are viable participants in catalytic cycles. A dinuclear Mo(V) species of this nature can thus supply either the oxidizing or reducing member of this couple and presents a mechanism by which molybdenum enzymes can channel reducing or oxidizing power. Several inorganic reactions have recently been explained using this scheme (80, 81). To date, however, Reaction 12 only applies when the ligand is a dithiocarbamate or dithiophosphate. Nevertheless, were there known dinuclear active sites in enzymes, this would be an important mechanism to consider. 

The most detailed spectroscopic and electronic structure studies of metallo-mono(dithiolenes) have focused on the nature of ligand-to-ligand charge transfer (LLCT) excitations in [M(diimine)(dithiolene)] complexes (112, 250-257, 262, 264, 295-301) and in monooxo molybdenum dithiolenes (19, 20, 22, 23) as models for pyranopterin molybdenum enzymes such as sulfite oxidase (SO). Since metallo-mono(dithiolenes) generally possess little or no symmetry, detailed spectrosopic and electronic structure studies of this class of metallo-dithiolenes have only recently begun to appear. The analysis of the spectroscopic data has been aided by the fact that the dithiolene-to-metal charge... 

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-... 

The photoelectron spectra of model complexes for these molybdenum enzyme active sites have been investigated to gain a better understanding of their basic electronic structure and the role of the ene-dithiolate ligand.For example, the metal coordination of model complexes such as Tp MoO(tdt) are similar to the molybdenum center of sulfite oxidase, which possesses the basic structural core of a terminal oxo group cis to a 1,2-dithiolate. 

As mentioned above, enzymes of the DMSOR family are distinguished from other molybdenum enzymes by the presence of a bis(MPT)Mo center. Based on the type of the additional ligands, the family can be further subdivided (see below). Generally, model compounds for the Mo(iv) oxidation level of the enzyme reaction centers can be synthesized by several procedures but model complexes for the Mo(vi) level are difficult to be accessed. 

Interest in the molecular structures of molybdenum and tungsten complexes, particularly in those containing N-donor ligands, is enhanced by the occurrence of molybdenum in the enzyme nitrogenase. It is possible that the stereochemistry of these metals may be of importance in understanding processes of in vivo and in vitro nitrogen-fixation. 

Complexes of molybdenum and tungsten with bidentate sulfur ligands have been investigated extensively. In recent years, the work in this field has been escalated by the impetus of designing models of such bioinorganic enzymes as nitrogenase and xanthine oxidase (125). The early work reviewed by Coucouvanis (1) dealt exclusively with the isolation of oxomolybdenum(V) and -(VI) species. 

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