Session: Organometallic Chemistry: Catalysis:

The atom-efficient catalytic formation of bonds between main group elements such as boron and nitrogen is of great interest in the fields of organometallic and materials chemistry as it could provide an entry point to new main group compounds and materials. Up to now, a plethora of early and late transition metal complexes are known to serve as catalysts for the dehydrocoupling of primary and secondary amine boranes. Coordinatively and electronically unsaturated early transition metal complexes, especially those of group 4 metals are very interesting in this context as these are known to possess unique, high reactivities with N-H and B-H bonds. Examples for the use of group 4 metallocene systems that consist of metallocene dihalide and organolithium reagents were reported in the past, however, in these systems the elucidation of the mechanistic details is complicated by the presence of the strong base.

Catalytic dehydrocoupling of H₃BNMe₂H using the in situ system Cp₂Zr(Cl)(µ-Me₃SiC₃SiMe₃)Zr(Cl)Cp₂ (1)/MeLi was studied as a model for previously reported dehydropolymerisation of H₃BNMeH₂. NMR and UV-Vis spectroscopic monitoring of the activation of the precatalyst with MeLi shows that this reaction proceeds slowly and stepwise via a singly methylated species. NMR spectroscopic monitoring of the catalytic reaction as well as speciation experiments indicate that dinuclear Zr(III) alkyl and hydride complexes, formed by decomposition of the allenediide ligand in 1, are important intermediates in the catalytic process. Overall, our findings suggest that the in situ system 1/MeLi comprises several interconvertible Zr species that catalytically convert the amine borane via different pathways and all contribute to the macroscopically observed activity of the system.

Fabian Reiss

Appearances