Synthesis, Structural Characterization, and Multifrequency Electron Paramagnetic Resonance Studies of Mononuclear Thiomolybdenyl Complexes

Reaction of Tp*MoVSCl2 with a variety of phenols and thiols in the presence of triethylamine produces mononuclear,
thiomolybdenyl complexes Tp*MoVSX2 [Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate; X = 2-(ethylthio)phenolate
(etp), 2-(n-propyl)phenolate (pp), phenolate; X2 = benzene-1,2-dithiolate (bdt), 4-methylbenzene-1,2-dithiolate (tdt),
benzene-1,2-diolate (cat)]. The complexes have been characterized by microanalysis, mass spectrometry, IR, EPR,
and UV−visible spectroscopic data, and X-ray crystallography (for the etp, pp, bdt, and cat derivatives). The
mononuclear, six-coordinate, distorted-octahedral Mo centers are coordinated by terminal sulfido (Mo⋮S = 2.123(1)−2.1368(8) Å), tridentate facial Tp*, and monodentate or bidentate O/S−donor ligands. Multifrequency (S-, X-,
Q-band) EPR spectra of the complexes and selected molybdenyl analogues were acquired at 130 K and 295 K
and yielded a spin Hamiltonian of Cs symmetry or lower, with gzz < gyy < gxx < ge and Az‘z‘ > Ax‘x‘ ≈ Ay‘y‘, and a
noncoincidence angle in the range of β = 24−39°. Multifrequency EPR, especially at S-band, was found to be
particularly valuable in the unambiguous assignment of the spin Hamiltonian parameters in these low-symmetry
complexes. The weaker π-donor terminal sulfido ligand yields a smaller SOMO−LUMO gap and reduced g-values
for the thiomolybdenyl complexes compared with molybdenyl analogues, supporting existing crystallographic and
EPR data for an apically coordinated oxo group in the active site of xanthine oxidase.