Halocarbonyltungsten(II) Complexes Containing Tripodal Tris(pyrazolyl)borate Ligands

The halo tricarbonyl complexes LWX(CO)₃ (X = I, Br, Cl) are formed when NEt₄[LW(CO)₃] is reacted with I₂, <i>N</i>-bromosuccinimide, and <i>N</i>-chlorosuccinimide (or PhICl₂), respectively, while reaction of NEt₄[L^PrW(CO)₃] and I₂ yields L^PrWI(CO)₃ [L = hydrotris(3,5-dimethylpyrazol-1-yl)borate, L^Pr = hydrotris(3-isopropylpyrazol-1-yl)borate]. Reaction of NEt₄[LW(CO)₃] with Br₂ yields mixtures of LWBr(CO)₃ and L^BrWBr(CO)₃ [L^Br = hydrotris(4-bromo-3,5-dimethylpyrazol-1-yl)borate] as a result of competitive bromination of both tungsten and ligand L. The complexes generally exhibit three ν(CO) bands, one at ca. 2025 cm^-1 and two between 1930 and 1900 cm^-1, and NMR spectra consistent with fluxional carbonyl-capped octahedral (<i>C</i><i>_s</i>) structures. Crystals of L^PrWI(CO)₃·MeOH are orthorhombic, space group <i>Pbca</i>, with <i>a</i> = 11.779(2) Å, <i>b</i> = 15.975(4) Å, <i>c</i> = 29.476(3) Å, and <i>V</i> = 5547(2) ų for <i>Z</i> = 8. The seven-coordinate complex exhibits a 3:3:1 carbonyl-capped octahedral structure. In hot acetonitrile or tetrahydrofuran, the tricarbonyl complexes are converted into brown, paramagnetic (μ_eff ca. 1.15 μ_B) dicarbonyl species, LWX(CO)₂ and L^BrWBr(CO)₂, which exhibit two ν(CO) bands (ca. 1930 and 1840 cm^-1) and isotropically shifted NMR spectra consistent with a mononuclear structure with <i>C</i><i>_s</i> symmetry. Further decarbonylation of L^BrWBr(CO)₂ in refluxing acetonitrile results in the formation of L^BrWBr(MeCN-κ²<i>N,C</i>)(CO), which crystallizes in monoclinic space group <i>C</i>2/<i>c</i>, with <i>a</i> = 22.245(4) Å, <i>b</i> = 19.077(4) Å, <i>c</i> = 15.639(3) Å, β = 128.61(2)°, and <i>V</i> = 5286(2) ų for <i>Z</i> = 8. The seven-coordinate complex features a “side-on” bonded, four-electron-donor acetonitrile ligand and a completely 4-brominated L^Br ligand.