Characterization of β-B-Agostic Isomers in Zirconocene Amidoborane Complexes

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Abstract
The reaction of Cpx2ZrCl2 (Cpx = Cp, Cp*) with ammonia borane in presence of n-butyllithium yielded Cp2Zr(Cl)NH2BH3 and Cpx2Zr(H)NH2BH3. These derivatives are isoelectronic with the ethyl zirconocene chloride and hydride, respectively, and feature a chelating amidoborane ligand coordinating through a Zr−N bond and a Zr−H−B bridge. In solution, each of the complexes consists of an equilibrium mixture of two isomers differing in the orientation of the amidoborane ligand with respect to the Zr−X bond (X = H, Cl), while in the solid state, only one isomer was observed. Such isomers have not been characterized for any metal complexes containing the isoelectronic β-agostic ethyl ligand or any other agostic alkyl group.