EPR studies of oxo-molybdemun(V) complexes with sulfur donor ligands: Synthesis, X-band EPR spectra and approaches to single crystal host lattices.

Abstract

A series of well characterized mononuclear oxo-molybdenum(V) complexes with bidentate sulfur donor ligands, which possess the minimal structural feature for the Mo(V) state of the proposed molybdenum cofactor (Moco) have been investigated by X-band electron paramagnetic resonance (EPR) spectroscopy in fluid and frozen solutions. The complete EPR parameters (anisotropic g(i) and A(^(95,97)Mo) hyperfine splittings) were obtained by rigorous spectral simulations using the QPOW program. Comparison of the EPR parameters of LMoO(bdt) and LMoO{S(CH₂)ₓ)S}, x = 2, 3, 4 complexes reveals similar anisotropic g and A(^(95,97)Mo) values, and indicates that it is the sulfur ligation rather than the chelate skeleton that is the primary factor dictating these parameters. Fenske-Hall calculations show that the oxo-molybdenum(V) complexes of the type LMoO{S(C)ₓS}, x = 1,2 have similar electronic structures and indicate that the first electronic transition will be a charge transfer (CT) transition from Sπ to d(xy). Such a low lying charge transfer band accounts for one of the g(i) values being greater than the free electron value (gₑ = 2.0023) for complexes LMoO(bdt) and LMoO{S(CH₂)ₓ)S}, x = 2, 3, 4. The EPR spectrum of the low and high pH forms of sulfite oxidase have been reinvestigated due to its qualitative similarities with our well characterized model compounds. Complete EPR parameters, including anisotropic A(^(95,97)Mo) hyperfine splittings were reported for the first time for the low and high pH forms of the sulfite oxidase. Comparison of the EPR parameters for the model compounds to those of the low and high pH forms of sulfite oxidase reveals that these minimal structural models for Moco mimic the anisotropic g values and A₁ values, but the A₂ and A₃ components in the models are distinctly different from those in the enzyme. Preliminary single crystal EPR studies on LMoO(bdt) doped into the diamagnetic LMo(NO(bdt) and LMo(NO)(tdt) host lattices are reported. A series of mononuclear and binuclear molybdenum complexes of the neutral facially coordinating hydrotris(3,5-dimethyl-1-pyrazolyl)methane (L*) ligand have been prepared and characterized by X-ray crystallography and spectroscopic techniques. An unusual feature of the chemistry of L*, compared to other related facially coordinating tridentate nitrogen ligands, is the facile formation of novel unsymmetrical di-μ-oxo dimers such as L*Mo₂O₄Cl₂, which contain both five-coordinate and six-coordinate oxo-molybdenum centers.