X-ray spectroscopy of enzyme active site analogues and related molecules: Bis(dithiolene)molybdenum(IV) and -tungsten(IV,VI) complexes with variant terminal ligands

The X-ray absorption spectra at the molybdenum and selenium K-edges and the tungsten L-2,L-3-edges are acquired for a set of 14 Mo(IV) and W(IV,VI) bis(dithiolene) complexes related to the active sites of molybdo- and tungstoenzymes. The set includes square pyramidal [(MoL)-L-IV(S2C2Me2)(2)](-) (L = O2-, R3SiO-, RO-, RS-, RSe-) and [W-IV(OR)(S2C2Me2)(2)](-), distorted trigonal prismatic [Mo-IV(CO)(SeR)(S2C2Me2)(2)](-) and [W-IV(CO)L(S2C2Me2)(2)](-) (L = RS-, RSe-), and distorted octahedral [(WO)-O-VI(OR)(S2C2Me2)(2)](-). The dithiolene simulates the pterin-dithiolene cofactor ligand, and L represents a protein ligand. Bond lengths are determined by EXAFS analysis using the GNXAS protocol. Normalized edge spectra, non-phase-shift-corrected Fourier transforms, and EXAFS data and fits are presented. Bond lengths determined by EXAFS and X-ray crystallography agree to less than or equal to0.02 Angstrom as do the M-Se distances determined by both metal and selenium EXAFS. The complexes [Mo-IV(QR)(S2C2Me2)(2)](-) simulate protein Ligation by the DMSO reductase family of enzymes, including DMSO reductase itself (Q = O), dissimilatory nitrate reductase (Q = S), and formate dehydrogenase (Q = Se). Edge shifts of these complexes correlate with the ligand electronegativities. Terminal Ligand binding is clearly distinguished in the presence of four Mo-S(dithiolene) interactions. Similarly, five-coordinate [ML(S2C2Me2)(2)](-) and six-coordinate [M(CO)L(S2C2Me2)(2)](-)are distinguishable by edge and EXAFS spectra. This study expands a previous XAS investigation of bis(dithiolene)-metal(IV,V,VI) complexes (Musgrave, K. B.; Donahue, J. P.; Lorber, C.; Holm, R. Fl.; Hedman, B.; Hodgson, K. O. J. Am. Chem. Sec. 1999, 121, 10297) by including a larger inventory of molecules with variant physiologically relevant terminal ligation. The previous and present XAS results should prove useful in characterizing and refining metric features and structures of enzyme sites.