Exploring CH-Activation Pathways in Bifunctional Zirconocene/Borane Systems

The dimethylsilanediyl-bridged ansa-zirconocene dichloride 1, that contains a pendent allyl
substituent at a Cp-ring, adds HB(C6F5)2 to the vinyl group to yield the bifunctional zirconocene/borane
complex 2. Substituted benzimidazoles were added to the strongly electrophilic borane moiety as protective
groups, which allowed subsequent chloride versus −CH2SiMe3 exchange at zirconium to take place by
treatment with the respective alkyllithium reagent. Alternatively, the introduction of active σ-ligands at
zirconium is carried out first, followed by the hydroboration reaction. This route was followed for the synthesis
of the diphenyl-ansa-zirconocene/borane complex 12. Complex 12 reacts slowly in solution by intramolecular
electrophilic attack of the borane at its adjacent Cp-ring, followed by deprotonation using a [Zr]−Ph group
to yield the zwitterionic complex 14 featuring a borata-tetrahydroindenyl moiety as part of the ansa-metallocene framework. Complex 14 was characterized by X-ray diffraction. It adds PMe3 at zirconium to
yield 15. Thermolysis of 12 with excess PMe3 leads to the formation of the (aryne)zirconocene complex
18, which is stabilized by PMe3 coordination to zirconium and PMe3 addition to boron. N-Methylbenzimidazole
adds to the −B(C6F5)2 unit of 12 to give the 1:1 adduct 19. Thermolysis of 19 at 80 °C in benzene solution
in the presence of one additional equivalent of N-methylbenzimidazole results in deprotonation of the
substrate to yield the σ-N-methylbenzimidazolyl zirconium complex 20 (as a mixture of two diastereoisomers).
An additional N-methylbenzimidazole ligand is bonded to the B(C6F5)2 unit in this product.