Paramagnetic Oxotungsten(V) Complexes Containing the Hydrotris(3,5-dimethylpyrazol-1-yl)borate Ligand

Sky-blue Tp*WOCl₂ has been synthesized from the high-yielding reaction of Tp*WO₂Cl with boron trichloride in refluxing toluene. Dark-red Tp*WOI₂ was prepared via thermal decarbonylation followed by aerial oxidation of Tp*WI(CO)₃ in acetonitrile. From these precursors, an extensive series of mononuclear tungstenyl complexes, Tp*WOXY [X = Cl-, Y = OPh-, SPh-; X = Y = OPh-, 2-(n-propyl)phenolate (PP-), SPh-, SePh-; XY = toluene-3,4-dithiolate (tdt²−), quinoxaline-2,3-dithiolate (qdt²-), benzene-1,2-diselenolate (bds²-); Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate], was prepared by metathesis with the respective alkali-metal salt of X-/XY²- or (NHEt₃)₂(qdt). The complexes were characterized by microanalysis, mass spectrometry, electrochemistry, IR, electron paramagnetic resonance (EPR), and electronic absorption spectroscopies, and X-ray crystallography (for X = Y = OPh-, PP-, SPh-; XY = bds²-). The six-coordinate, distorted-octahedral tungsten centers are coordinated by terminal oxo [W≡O = 1.689(6)-1.704(3) Å], tridentate Tp*, and monodentate or bidentate O/S/Se-donor ligands. Spin Hamiltonian parameters derived from the simulation of fluid-solution X-band EPR spectra revealed that the soft-donor S/Se ligand complexes had larger g values and smaller 183W hyperfine coupling constants than the less covalent hard-donor O/Cl species. The former showed low-energy ligand-to-metal charge-transfer bands in the near-IR region of their electronic absorption spectra. These oxotungsten(V) complexes display lower reduction potentials than their molybdenum counterparts, underscoring the preference of tungsten for higher oxidation states. Furthermore, the protonation of the pyrazine nitrogen atoms of the qdt²- ligand has been examined by spectroelectrochemistry; the product of the one-electron reduction of [Tp*WO(qdtH)]+ revealed usually intense low-energy bands.