Electronic spectral studies of molybdenyl complexes. 2. MCD spectroscopy of [MoOS4](-) centers

Magnetic circular dichroism (MCD) and absorption spectroscopies have been u sed to probe the electronic structure of [PPh4][MoO(p-SC6H4X)(4)] (X = H, C l, OMe) and [PPh4][MoO(edt)(2)] complexes (edt = ethane-1,2-dithiolate). Th e results of density functional calculations (DFT) on [MoO(SMe)(4)](-) and [MoO(edt)(2)](-) model complexes were used to provide a framework for the i nterpretation of the spectra. Our analysis shows that the lowest energy tra nsitions in [(MoOS4)-O-V] chromophores (S-4 = sulfur donor ligand) result f rom S --> Mo charge transfer transitions from S valence orbitals that he cl ose to the ligand field manifold. The energies of these transitions are str ongly dependent on the orientation of the S lone-pair orbitals with respect to the Mo atom that is determined by the geometry of the ligand backbone. Thus, the lowest energy transition in the MCD spectrum of [PPh4][MoO(pSC(6) H(4)X)(4)] (X = H) occurs at 14 800 cm(-1), while that in [PPh4][MoO(edt)(2 )] occurs at 11 900 cm(-1). The identification of three bands in the absorp tion spectrum of [PPh4][MoO(edt)(2)] arising from LMCT from S pseudo-sigma combinations to the singly occupied Mo 4d orbital in the xy plane suggests that then is considerable covalency in the ground-state electronic structur es of [MoOS4] complexes. DFT calculations on [MoO(SMe)(4)](-) reveal that t he singly occupied HOMO is 53% Mo 4d(xy) and 35% S p for the equilibrium C- 4 geometry. For [MoO(edt)(4)](-) the steric constraints imposed by the edt ligands result in the S pi orbitals being of similar energy to the Mo 4d ma nifold. Significant S pseudo-sigma and pi donation may also weaken the Mo e quivalent toO bond in [MoOS4] centers, a requirement for facile active site regeneration in the catalytic cycle of the DMSO reductases. The strong dep endence of the energies of the bands in the absorption and MCD spectra of [ PPh4][MoO(p-SC6H4X)(4)] (X = H, Cl, OMe) and [PPh4][MoO(edt)(2)] on the lig and geometry suggests that the structural features of the active sites of t he DMSO reductases may result in an electronic structure that is optimized for facile oxygen atom transfer.