Cyclic Organohydroborate Complexes of Metallocenes. VII. Synthesis, Structure, and Fluxional Behavior of Cp₂ZrX{(μ-H)₂BC₈H₁₄} (X = H, D, Cl)

Abstract

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.