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A brief description about some selected model complexes in molybdenum and tungsten bioinorganic chemistry is provided. The synthetic strategies involved and their limitations are discussed. Current status of molybdenum and tungsten bioinorganic modeling chemistry is presented briefly and synthetic problems associated therein are analyzed. Possible future directions which may expand the scope of modeling chemistry are suggested.

An extensive series of heterometal-iron-sulfur single cubane-type clusters with core oxidation levels [MFe(3)S(3)Q](3+,2+) (M = Mo, W; Q = S, Se) has been prepared by means of a new method of cluster self-assembly. The procedure utilizes the assembly system [((t)Bu(3)tach)M(VI)S(3)]/FeCl(2)/Na(2)Q/NaSR in acetonitrile/THF and affords product clusters in 30-50% yield. The trisulfido precursor acts as a template, binding Fe(II) under reducing conditions and supplying the MS(3) unit of the product. The system leads to specific incorporation of a μ(3)-chalcogenide from an external source (Na(2)Q) and affords the products [((t)Bu(3)tach)MFe(3)S(3)QL(3)](0/1-) (L = Cl(-), RS(-)), among which are the first MFe(3)S(3)Se clusters prepared. Some 16 clusters have been prepared, 13 of which have been characterized by X-ray structure determinations including the incomplete cubane [((t)Bu(3)tach)MoFe(2)S(3)Cl(2)(μ(2)-SPh)], a possible trapped intermediate in the assembly process. Comparisons of structural and electronic features of clusters differing only in atom Q at one cubane vertex are provided. In comparative pairs of complexes differing only in Q, placement of one selenide atom in the core increases core volumes by about 2% over the Q = S case, sets the order Q = Se > S in Fe-Q bond lengths and Q = S > Se in Fe-Q-Fe bond angles, causes small positive shifts in redox potentials, and has an essentially nil effect on (57)Fe isomer shifts. Iron mean oxidation states and charge distributions are assigned to most clusters from isomer shifts. ((t)Bu(3)tach = 1,3,5-tert-butyl-1,3,5-triazacyclohexane)., (© 2011 American Chemical Society)

Four desoxo Mo(iv) bis(dithiolene) complexes, [Et(4)N][Mo(IV)(PPh(3))(SC(6)H(4)-p-Me)(mnt)(2)] () (mnt = maleonitrile dithiolate), [PNP][Mo(IV)(PPh(3))(SC(6)H(4)-o-COOH)(mnt)(2)].CH(3)CN.(i)PrOH () (PNP is [Ph(3)PNPPh(3)](+)), [Et(4)N][Mo(IV)(PPh(3))(NCS)(mnt)(2)] () and [Et(4)N](2)[Mo(IV)(NCS)(2)(mnt)(2)] () are synthesized and characterized structurally. Complexes and are found to release the pentacoordinated species, [Mo(IV)(SR)(mnt)(2)](1-) (R = -C(6)H(4)-o-COOH, -C(6)H(4)-p-Me) due to the dissociation of coordinated PPh(3) in solution and this pentacoordinated species can then reduce nitrate to nitrite. But unlike the model complex of nitrate reductase, [Et(4)N][Mo(IV)(PPh(3))(SPh)(mnt)(2)] () (A. Majumdar, K. Pal and S. Sarkar, J. Am. Chem. Soc. 2006, 128, 4196), these reactions remain incomplete. Such inefficiency of complexes and to mediate a complete reduction of nitrate is attributed to the steric bulk exerted by the para-methyl and ortho-carboxylic acid functionalities thereby retarding the formation of the nitrate bound Mo(iv) complex necessary for nitrate reduction. Complex does not release a pentacoordinated species in solution and hence is unable to reduce nitrate. Although dissociates one coordinated NCS ligand in solution to release a penta-coordinated species {[Mo(IV)(NCS)(mnt)(2)]}(1-), which remains inactive towards nitrate reduction indicating the necessity of thiolate coordination and hence the necessity of fine electronic tuning to effect nitrate reduction. Complexes , and can be converted to or depending on the amount of thiocyanate employed. Complexes and undergo a facile interconversion among themselves. Substitution of essential thiolate coordination rather than replacing the dissociable PPh(3) in and by thiocyanate resulted in inactivation (formation of ) towards nitrate reduction which is somewhat similar to the dead-end type inhibition often encountered in native systems. These results followed by theoretical calculations at DFT level establish the necessity of fine stereoelectronic tuning at the axial position of desosxo molybdenum bis(dithiolene) complexes to warrant nitrate reduction.

A brief description about some selected model complexes in molybdenum and tungsten bioinorganic chemistry is provided. The synthetic strategies involved and their limitations are discussed. Current status of molybdenum and tungsten bioinorganic modeling chemistry is presented briefly and synthetic problems associated therein are analyzed. Possible future directions which may expand the scope of modeling chemistry are suggested. [ABSTRACT FROM AUTHOR]

Reactions directed at the synthesis of structural analogues of the active site of molybdenum-containing carbon monoxide dehydrogenase have been investigated utilizing [WO2S(bdt)]2- (1) and (WOS2(bdt)]2- (2) and slerically hindered [Cu(R)L1 or [Cu(SSiR'3)2] as reactants. All successful reactions 012 afford the binuclear WVI/CuI products [WO(bdt)(μ2-S)2Cu(L)]2-/- with L = carbene (3), Ar*S (4), Ar* (7), SSiR3 (R n Ph (5), Pri (6)). Similarly, [W(bdl)(OSiPh3)S2] leads 10 [W(bdt)(OSiPh3)(μ2-S)2Cu(SAr*)] (8). These complexes, with apical oxo and basal dithiolalo and sulfido coordination (exetuding 8), terminal thiolate ligation at Cu' (4-6, 8), and W-(μ2-S)-Cu bridging, bear a structural resemblance 10 the enzyme site. Differences include two bridges instead of one and the absence of basal oxo/hydroxo ligation. Complex 8 differs from the others by utilizing apical and basal sulfido ligands in bddge formation. Related reaction systems based on 1 gave 4 in small yield or product mixtures in which the desired monobridged complex [WO2(bdt)(μ2-S)Cu(R)]2 was nol detected. Mass spectrometHc analysis of the reaction system with L n carbene suggests that any monobridged species forms may converted to Ihe dibridged form by disproportionation. In these experiments, the use of WVI preserves Ihe structural integdty of MoVI, whose analogues of 1 and 2 have not been isolated. (Ar* = 2,6-bis(2,4,6-lriisopropylphenyt)phenyl, bdl n benzene-1 ,2-dithiolate(2-)). [ABSTRACT FROM AUTHOR]

Complexes analogous to the active site of dissimilatory nitrate reductase from Desulfovibrio desulfuricans are synthesized. The hexacoordinated complexes, [PPh4][MoIV(PPh3)(SR)(mnt)2] (R = -CH2CH3 (1), -CH2Ph (2)) released PPh3 in solution to generate the active model cofactor, {MoIV(SR)(mnt)2}1-, ready with a site for nitrate binding. Kinetics for nitrate reduction by the complexes 1 and 2 followed Michaelis-Menten saturation kinetics with a faster rate in the case of 1 (VMax = 3.2 × 10-2 s-1, KM = 2.3 × 10-4 M) than that reported earlier (VMax = 4.2 × 10-3 s-1, KM = 4.3 × 10-4 M) (Majumdar, A.; Pal, K.; Sarkar, S. J. Am. Chem. Soc. 2006, 128, 4196-4197). The oxidized molybdenum species may be reduced back by PPh3 to the starting complex, and a catalytic cycle involving [Bu4N][NO3] and PPh3 as the oxidizing and reducing substrates, respectively, is established with the complexes 1 and 2. lsostructural complexes, [Et4N][MoIV(PPh3)(X)(mnt)2) (X = -Br (3), -I(4)) did not show any reductive activity toward nitrate. The selectivity of the thiolate ligand for the functional activity and the cessation of such activity in isostructural halo complexes demonstrate the necessity of thiolate coordination. Electrochemical data of all these complexes correlate the ability of the thiolated species for such oxotransfer activity. Compounds 1 and 2 are capable of reducing substrates like TMANO or DMSO, but after the initial 15-20% conversion, the product trimethylamine or dimethylsulfide formed interacts with the active parent complexes 1 and 2 thereby slowing down further oxo-transfer reaction similar to feedback type reactions. From the functional nitrate reduction, the molybdenum species finally reacts with the nitrite formed leading to nitrosylation similar to the NO evolution reaction by periplasmic nitrate reductase from Pseudomonas dentrificans. All these complexes (1-4) are characterized structurally by X-ray, elemental analysis, electrochemistry, electronic, FT-IR, mass and 31P NMR spectroscopic measurements. [ABSTRACT FROM AUTHOR]

Abstract: The similarities and differences in the fundamental coordination chemistry of molybdenum and tungsten mainly in physiological oxidation states MIV–VI are examined in relation to the properties of enzyme sites that catalyze oxygen atom transfer reactions. The comparative aspects of dithiolene complexes, which as synthetic analogues simulate structural and electronic features of these sites, are emphasized. Analogue reaction systems of enzymes are summarized. The mechanism of reduction of the biological substrate Me2SO in one such system as elucidated with density functional calculations is presented as a case study. [Copyright &y& Elsevier]