Ligand K-Edge and Metal L-Edge X-ray Absorption Spectroscopy and Density Functional Calculations of Oxomolybdenum Complexes with Thiolate and Related Ligands: Implications for Sulfite Oxidase Yasuo Izumi, ²,§ Thorsten Glaser, ² Kendra Rose, ² Jonathan McMaster, ⊥ Partha Basu, ⊥ John H. Enemark,* ,⊥ Britt Hedman,* ,²,‡ Keith O. Hodgson,* ,²,‡ and Edward I. Solomon* ,² Contribution from the Department of Chemistry and Stanford Synchrotron Radiation Laboratory, Stanford UniVersity, Stanford, California 94305, and Department of Chemistry, UniVersity of Arizona, Tucson, Arizona 85721 ReceiVed February 5, 1999 Abstract: X-ray absorption spectra have been measured at the S K-, Cl K-, and Mo L 3 - and L 2 -edges for the d 0 dioxomolybdenum(VI) complexes LMoO 2 (SCH 2 Ph), LMoO 2 Cl, and LMoO 2 (OPh) (L ) hydrotris(3,5- dimethyl-1-pyrazolyl)borate) to investigate ligand-metal covalency and its effects on oxo transfer reactivity. Two dominant peaks are observed at the S K-edge (2470.5 and 2472.5 eV) for LMoO 2 (SCH 2 Ph) and at the Cl K-edge (2821.9 and 2824.2 eV) for LMoO 2 Cl, demonstrating two major covalent contributions from S and Cl to the Mo d orbitals. Density functional calculations were performed on models of the three Mo complexes, and the energies and characters of the Mo 4d orbitals were interpreted in terms of the effects of two strong cis-oxo bonds and additional perturbations due to the thiolate, chloride, or alkoxide ligand. The major perturbation effects are for thiolate and Cl - π mixed with the d xz orbital and σ mixed with the d z 2 orbital. The calculated 4d orbital energy splittings for models of these two major contributions to the bonding of thiolate and Cl ligands (2.47 and 2.71 eV, respectively) correspond to the splittings observed experimentally for the two dominant ligand K-edge peaks for LMoO 2 (SCH 2 Ph) and LMoO 2 Cl (2.0 and 2.3 eV, respectively) after consideration of final state electronic relaxation. Quantification of the S and Cl covalencies in the d orbital manifold from the pre-edge intensity yields, ∼42% and ∼17% for LMoO 2 (SCH 2 Ph) and LMoO 2 Cl, respectively. The Mo L 2 -edge spectra provide a direct probe of metal 4d character for the three Mo complexes. The spectra contain a strong, broad peak and two additional sharp peaks at higher energy, which are assigned to 2p transitions to the overlapping t 2g set and to the d z 2 and d xy levels, respectively. The total peak intensities of the Mo L 2 - edges for LMoO 2 (OPh) and LMoO 2 Cl are similar to and larger than those for LMoO 2 (SCH 2 Ph), which agrees with the calculated trend in ligand-metal covalency. The theoretical and experimental description of bonding developed from these studies provides insight into the relationship of electronic structure to the oxo transfer chemistry observed for the LMoO 2 X complexes. These results imply that anisotropic covalency of the Mo- S cys bond in sulfite oxidase may promote preferential transfer of one of the oxo groups during catalysis. 1. Introduction Mo-containing enzymes are essential for all forms of life. With the exception of nitrogenase, most of these enzymes catalyze reactions in which there is a net transfer of an oxygen atom between substrate and water, as shown in eq 1. 1 Prior to 1995, structural information about the Mo centers in these enzymes was deduced principally from EXAFS at the Mo K-edge and EPR spectroscopy of their transient Mo(V) states. 1 Since then, several protein crystal structures have been reported. 2-11 These protein structures confirm that there are three distinct structural families for the Mo centers: the xanthine oxidase, DMSO reductase, and sulfite oxidase families. 1 In all three structural families, the Mo center is coordinated by S donors from the cis-ene-1,2-dithiolate (dithiolene) of one (or two) novel pyranopterin (molybdopterin) units. 1,12 This ligand system is unique to Mo and W in metalloproteins, and it appears that coordination by this ligand (and in some cases an additional S ligand) is essential for the catalytic function of these enzymes. * To whom correspondence should be addressed. ² Stanford University. ‡ Stanford Synchrotron Radiation Laboratory, SLAC. ⊥ The University of Arizona. § On leave from Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Yokohama, Japan. 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H.; Rees, D. C. Cell 1997, 91, 973- 983. X + H 2 O h XO + 2H + + 2e - (1) 10035 J. Am. Chem. Soc. 1999, 121, 10035-10046 10.1021/ja9903678 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/13/1999