Cyclic organohydroborate complexes of metallocenes. VII. Synthesis, structure, and fluxional behavior of Cp2ZrX{(μ-H)2BC8H14} (X = H, D, Cl)

Article abstract of DOI:10.1021/om0206249

The reaction of K[H₂BC₈H₁₄] with Cp₂ZrCl₂ in a 1:1 molar ratio afforded Cp₂ZrCl{(μ-H)₂BC₈H₁₄}, 1, which when reacted with KH yielded Cp₂ZrH{(μ-H)₂BC₈H₁₄}, 2. These complexes were structurally characterized by single-crystal X-ray analysis. Partially deuterated 1 and 2, complexes Cp₂ZrCl{(μ-D)(μ-H)BC₈H₁₄}, 3, and Cp₂ZrD{(μ-H)₂BC₈H₁₄}, 4, were prepared. ²H NMR spectra revealed that in complex 3, which contains a terminal chloride on zirconium, no exchange of bridge and Cp hydrogens occurred. However, in complex 4, which includes a terminal hydrogen on zirconium, there was facile mixing of terminal, bridge, and Cp-hydrogens. Furthermore, exchange of terminal zirconium-hydrogen with bridge hydrogen was more rapid than exchange with the Cp-hydrogen. Additionally, this fluxional behavior occurred even at room temperature and in the solid state. Variable-temperature ¹H NMR studies are consistent with these results of dynamic hydrogen exchange in complexes 1 and 2. The complexes Cp₂Nb{(μ-H)₂BC₈H₁₄}, 7, and Cp₂Nb{μ-D)(μ-H)-BC₈H₁₄}, 8, have no terminal hydrogen on niobium, and there is no exchange between bridging hydrogens and Cp-hydrogens. The possible participation of the 9-BBN unit ({(μ-H)₂BC₈H₁₄}) hydrogens in dynamic exchange was examined for 2, the 9-BBN dimer ((C₈H₁₄B)(μ-H)₂(BC₈H₁₄ )), and the 9-BBN anion ([H₂BC₈H₁₄]) through the partially deuterated complexes Li[(D)(H)BC₈H₁₄], 5, and (C₈H₁₄B)(μ-D)(μ-H)(BC₈H₁₄), 6.