Mechanistic insights into dehydrocoupling of amine boranes using dinuclear zirconocene complexes

Abstract

Catalytic dehydrocoupling of H₃B·NMe₂H using the in situ system Cp₂Zr(Cl)(μ-Me₃SiC₃SiMe₃)Zr(Cl)Cp₂ (1)/MeLi was studied as a model for previously reported dehydropolymerisation of H₃B·NMeH₂. NMR and UV-vis spectroscopic monitoring of the precatalyst activation sequence as well as a series of stoichiometric experiments showed that formation of a zirconocene dimethyl complex (2) is not relevant for activation of the precatalyst. Instead, deprotonation of H₃B·NMe₂H and reaction of thus formed Li[NMe₂BH₃] is proposed to result in the formation of Zr amidoborane and hydride species. DFT analysis using such complexes as active species showed a pathway for formation of H₂ and H₂BNMe₂. ¹H NMR spectroscopic monitoring and stoichiometric control experiments revealed the formation of unusual diamagnetic dinuclear complexes Cp₂Zr(C₂SiMe₃)(μ-R)ZrCp₂ (R = CH₂SiMe₃, 7; R = H, 9) formed by activation of the allenediide unit of the precatalyst 1. Such species can be regarded as rare single-component catalysts for the dehydrocoupling of amine boranes.