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) bi s(dithiolene) complexes related to the active sites of molybdo- and tungsto enzymes. The set includes square pyramidal [(MoL)-L-IV(S2C2Me2)(2)](-) (L = O2-, R3SiO-, RO-, RS-, RSe-) and [W-IV(OR)(S2C2Me2)(2)](-), distorted trig onal 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 rep resents a protein ligand. Bond lengths are determined by EXAFS analysis usi ng the GNXAS protocol. Normalized edge spectra, non-phase-shift-corrected F ourier transforms, and EXAFS data and fits are presented. Bond lengths dete rmined by EXAFS and X-ray crystallography agree to less than or equal to0.0 2 Angstrom as do the M-Se distances determined by both metal and selenium E XAFS. 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 electronega tivities. Terminal Ligand binding is clearly distinguished in the presence of four Mo-S(dithiolene) interactions. Similarly, five-coordinate [ML(S2C2M e2)(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. 199 9, 121, 10297) by including a larger inventory of molecules with variant ph ysiologically relevant terminal ligation. The previous and present XAS resu lts should prove useful in characterizing and refining metric features and structures of enzyme sites.