Group 4 metal polyhydrides: Synthesis, structure, and reactivity of tetrakis[(pentamethylcyclopentadienyl)hafnium dihydride chloride]

Article abstract of DOI:10.1021/om00095a003

The polyhydride [Cp*HfH₂Cl]₄² was prepared by high-pressure hydrogenation of Cp*Hf(C₆H₁₀)Cl(C₆H₁₀ = 2,3-dimethyl-1,3-butadiene) or Cp*Hf(COT)Cl (COT = cyclooctatetraene). The structure of [Cp*HfH₂Cl]₄ was determined by X-ray diffraction. The compound crystallizes in the tetragonal space group P42₁m with a = 21.454 (3) ?, c = 10.525 (3) ?, and Z = 4. The structure was refined to R = 0.030 and R_w = 0.039 for 303 parameters and 2839 reflections with I > 2.5σ(I). The molecule has four Hf atoms in a butterfly arrangement with alternating Cp* and Cl ligands (Hf-Ct = 2.15, 2.23, and 2.27 ?; Hf-Cl = 2.397 (3), 2.438 (3) and 2.432 (3) ?). Hydride positions could not be found due to disorder in one of the Cp* ligands probably caused by the presence of a benzene solvate molecule. IR and NMR studies indicate the presence of two sets of four equivalent hydrides bridging the edges of a Hf₄ butterfly arrangement. Bridging hydride ligands account for the low reactivity of the complex.