Understanding the Origin of Metal−Sulfur Vibrations in an Oxo-Molybdenum Dithiolene Complex:  Relevance to Sulfite Oxidase

Abstract

X-ray crystallography and resonance Raman (rR) spectroscopy have been used to further characterize (Tp*)MoO(qdt) (Tp* is hydrotris(3,5-dimethyl-1-pyrazolyl)borate and qdt is 2,3-quinoxalinedithiolene), which represents an important benchmark oxomolybdenum mono-dithiolene model system relevant to various pyranopterin Mo enzyme active sites, including sulfite oxidase. The compound (Tp*)MoO(qdt) crystallizes in the triclinic space group, P1, where a = 9.8424 (7) A, b = 11.2323 (8) A, c = 11.9408 (8) A, alpha = 92.7560 (10) degrees, beta = 98.9530 (10) degrees, and gamma = 104.1680 (10) degrees. The (Tp*)MoO(qdt) molecule exhibits the distorted six-coordinate geometry characteristic of related oxo-Mo(V) systems possessing a single coordinated dithiolene ligand. The first coordination sphere bond lengths and angles in (Tp*)MoO(qdt) are very similar to the corresponding structural parameters for (Tp*)MoO(bdt) (bdt is 1,2-benzenedithiolene). The relatively small inner-sphere structural variations observed between (Tp*)MoO(qdt) and (Tp*)MoO(bdt) strongly suggest that geometric effects are not a major contributor to the significant electronic structural differences reported for these two oxo-Mo(V) dithiolenes. Therefore, the large differences observed in the reduction potential and first ionization energy between the two molecules appear to derive primarily from differences in the effective nuclear charges of their respective sulfur donors. However, a subtle perturbation to Mo-S bonding is implied by the nonplanarity of the dithiolene chelate ring, which is defined by the fold angle. This angular distortion (theta = 29.5 degrees in (Tp*)MoO(qdt); 21.3 degrees in (Tp*)MoO(bdt)) observed between the MoS2 and S-C=C-S planes may contribute to the electronic structure of these oxo-Mo dithiolene systems by controlling the extent of S p-Mo d orbital overlap. In enzymes, the fold angle may be dynamically modulated by the pyranopterin, thereby functioning as a transducer of vibrational energy associated with protein conformational changes directly to the active site via changes in the fold angle. This process could effectively mediate charge redistribution at the active site during the course of atom- and electron-transfer processes. The rR spectrum shows bands at 348 and 407 cm(-1). From frequency analysis of the normal modes of the model, [(NH3)3MoO(qdt)]1+, using the Gaussian03 suite of programs, these bands are assigned as mixed-mode Mo-S vibrations of the five-membered Mo-ditholene core structure. Raman spectroscopy has also provided additional evidence for an in-plane pseudo-sigma dithiolene S-Mo d(xy) covalent bonding interaction in (Tp*)MoO(qdt) and related oxo-Mo-dithiolenes that has implications for electron-transfer regeneration of the active site in sulfite oxidase involving the pyranopterin dithiolene.

Large Ligand Folding Distortion in an Oxomolybdenum Donor–Acceptor Complex

“…We have previously used this orthogonal enhancement spectroscopic signature to assign electronic transitions to the Mo(dx 2 −y 2 ) and Mo(dxz,yz) acceptor orbitals in oxomolybdenum complexes. 34,35 In MoO(SPh) 2 ( i Pr 2 Dt o ), resonance enhancement of the Mo-dithione S-Mo-S stretching mode is anticipated for transitions from filled thiol and Mo(dx 2 −y 2 ) based MOs to the dithione based LUMO, which also possesses in-plane Mo(dx 2 −y 2 ) character. Resonance enhancement of the Mo≡O stretch is anticipated for transitions to the higher energy Mo(dxz,yz) virtual orbital set since the Mo(dxz,yz) orbitals possess Mo≡O π* character and electron occupation of Mo≡O π* orbitals will result in large excited state distortions along the Mo≡O bond.…”

“…35,36 For these higher valent oxomolybdenum dithiolenes, dithiolene → Mo(dx 2 −y 2 ) LMCT character admixed into the ground state is responsible for dithiolene ligand folding (Figure 2). 35,38–41 …”

Electronic Control of the “Bailar Twist” in Formally d0-d2 Molybdenum Tris(dithiolene) Complexes: A Sulfur K-edge X-ray Absorption Spectroscopy and Density Functional Theory Study

“…17 Similar folding distortions have also been observed in bis(cyclopentadienyl)-ML n complexes with a single dithiolene ligand and other Mo mono-dithiolene complexes. 18,19 In addition to the different geometric structures of complexes with dithiolene ligands, it has also been found that dithiolene complexes have interesting redox properties, specifically the question of ligand-vs. metal-based oxidation (ie. non-innocent vs. innocent).…”

Pyranopterin Dithiolene Distortions Relevant to Electron Transfer in Xanthine Oxidase/Dehydrogenase

“…Additional support for this assignment is based on polarized rR spectra of the benchmark Mo-ditholene complex Tp*Mo(bdt) (bdt = benzene-1,2-dithiolate), where the S–Mo–S symmetric stretch and bend are observed at 393 and 362 cm –1 , respectively. 21,22 Although low-frequency rR data for pyranopterin molybdenum enzymes are sparse, vibrational data for dimethyl sulfoxide reductase (DMSOR) have been collected and analyzed. 23−27 In contrast to XO/XDH, DMSOR family enzymes possess two pyranopterin ditholenes bound to the Mo ion.…”