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

The reaction of (Cp2ZrCl2)-Zr-x (Cp-x = Cp, Cp*) with ammonia borane in presence of n-butyllithium yielded Cp2Zr(Cl)NH2BH3 and (Cp2Zr)-Zr-x(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 beta-agostic ethyl ligand or any other agostic alkyl group.