ELECTRONIC SPECTRAL STUDIES OF MOLYBDENYL COMPLEXES - IMPLICATIONS FOR OXOMOLYBDENUM ENZYMES

被引:94
作者
CARDUCCI, MD
BROWN, C
SOLOMON, EI
ENEMARK, JH
机构
[1] STANFORD UNIV, DEPT CHEM, STANFORD, CA 94305 USA
[2] UNIV ARIZONA, DEPT CHEM, TUCSON, AZ 85721 USA
关键词
D O I
10.1021/ja00105a028
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
Magnetic circular dichroism spectroscopy (MCD) has been used to assign the optical spectra in high- and low-symmetry molybdenyl complexes. High-resolution single crystal polarized absorption spectra of [PPh(4)][MoOCl4] and [AsPh(4)][MoOCl4(H2O)] show two ligand field transitions whose polarization, MCD, and vibronic fine structure are compatible with the previously accepted assignments (2)E <-- B-2(2) (15000 and 13000 cm(-1), respectively) and B-2(1) <-- B-2(2) (23000 cm(-1) in both). In the case of [AsPh(4)][MoOCl4(H2O)], the splitting of the (2)E by spin-orbit coupling is clearly visible and a spin-orbit coupling constant of 810(2) cm(-1) has been measured. At higher energies, four charge transfer transitions arising from nonbonding chloride orbitals to the metal d(xy) orbital have been identified. The absorption and MCD spectra of LMoOX(2) complexes (where L is hydrotris(3,5-dimethyl-1-pyrazolyl)borate and X = O, Cl, or S) which possess only C-s symmetry can be interpreted within the same framework of ligand field and ligand pi to metal charge transfer transitions adopted for [AsPh(4)][MoOCl4(H2O)]. Although remarkably consistent in other respects, the MCD spectra of the lowest energy ligand field transition of the LMoOX(2) compounds show that the ordering of the d orbitals that are pi antibonding with respect to the molybdenyl group depends upon the equatorial donor ligands. For LMoO(2+) complexes of (SCH2CH2S-)-S-- (saturated chelate skeleton) and toluene-3,4-dithiolate (aromatic chelate skeleton), the features between similar to 9000 and 16000 cm(-1) are assigned to charge transfer transitions from filled sulfur out-of-plane pi orbitals to d(xy) that overlap with the lowest lying Ligand field bands. The pattern of intense MCD peaks between 17000 and 35000 cm(-1) (nearly identical in these compounds) is assigned to charge transfer transitions from filled out-of-plane sulfur pi orbitals to the oxomolybdenum pi-antibonding orbitals, d(xzyz). On the basis of the results of these molybdenyl model complexes, an interpretation is proposed for the recently reported MCD spectra of DMSO reductase. The spectroscopic studies of LMoO(SCH2CH2S) and LMoO(toluene-3,4-dithiolate) also provide insight into the general role of sulfur coordination in oxo-molybdenum enzymes. The filled sulfur pi-donor orbitals are close in energy to the molybdenum d orbitals;; pi donation from these filled sulfur orbitals competes with pi donation from the terminal oxo groups and weakens the Mo=O bond, thereby facilitating oxygen atom transfer chemistry. The filled sulfur pi orbitals also provide an effective low energy super exchange pathway for the one-electron transfers from d(xy) that interconvert the Mo(IV,V,VI) states during the catalytic cycle.
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页码:11856 / 11868
页数:13
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