EPR Studies of Oxo−Molybdenum(V) Complexes with Sulfur Donor Ligands:  Implications for the Molybdenum Center of Sulfite Oxidase

A series of six-coordinate compounds containing a chelating dithiolate coordinated to the [LMoVO]2+ unit (L = hydrotris(3,5-dimethyl-1-pyrazolyl)borate) have been characterized by EPR spectroscopy as models for the molybdenum centers of pterin-containing molybdenum enzymes. The structure of LMoO(bdt) (1) (bdt = 1,2-benzenedithiolate) has been determined by X-ray crystallography; the space group is P21/n with a = 10.727(1) Å, b = 14.673(2) Å, c = 15.887(2) Å, β = 100.317(4)° and Z = 4. Compound 1 exhibits distorted octahedral stereochemistry; the terminal oxo group and the sulfur atoms are mutually cis to one another. The MoO distance is 1.678(4) Å, and the average Mo−S distance is 2.373(2) Å. The EPR parameters for 1, determined from simulation of the frozen-solution spectrum, are g 1 = 2.004, g 2 = 1.972, g 3 = 1.934 and A 1(95,97Mo) = 50.0 × 10-4, A 2 = 11.4 × 10-4, A 3 = 49.7 × 10-4 cm-1. The EPR parameters for several LMoVO{S(CH2) x S} compounds (x = 2−4) with saturated chelate skeletons are similar to those of 1, indicating that it is the coordinated S atoms and not unsaturation of the chelate skeleton that gives rise to the large g values for 1. The presence of g components larger than the free-electron value is ascribed to low-energy charge transfer transitions from the filled sulfur π orbitals to half-filled Mo d orbitals. The EPR spectrum of [LMoVO{S2P(OEt)2}]+ shows an unusually large isotropic 31P hyperfine splitting of 66.1 × 10-4 cm-1 from the noncoordinated phosphorus atom. The frozen-solution EPR spectra of the low-pH and high-pH forms of sulfite oxidase have been reinvestigated in D2O and the anisotropic g and A(95,97Mo) parameters determined by simulation of the spectrum arising from the naturally abundant Mo isotopes (75% I = 0, 25% I = 5/2). The EPR parameters for the low-pH form are g 1 = 2.007, g 2 = 1.974, g 3 = 1.968 and A 1 = 56.7 × 10-4, A 2 = 25.0 × 10-4, A 3 = 16.7 × 10-4 cm-1. The EPR parameters for the high-pH form are g 1 = 1.990, g 2 = 1.966, g 3 = 1.954 and A 1 = 54.4 × 10-4, A 2 = 21.0 × 10-4, A 3 = 11.3 × 10-4 cm-1. These are the first determinations of the complete A(95,97Mo) hyperfine components for an enzyme that possesses an [MoVIO2]2+ core in its fully oxidized state.