Internal Fluorocarbon Coordination as a Tool for the Protection of Active Catalytic Sites: Experimental Characterization of the Zr···F−C Interaction in the Group 4 Metallocene(butadiene)/B(C₆F₅)₃ Betaine Ziegler Catalyst Systems

Abstract

The Lewis acid tris(pentafluorophenyl)borane adds to the (butadiene)group 4 metallocenes 1a−d (metallocene = Cp2Zr, Cp2Hf, (MeCp)2Zr, (Me3CCp)2Zr) to give the metallocene−(μ-C4H6)−borate−betaine complexes 2a−d. (Isoprene)zirconocene (1e) and (2-phenylbutadiene)zirconocene (1f) add the B(C6F5)3 reagent regioselectively at the carbon atom C-4 to give the complexes 2e and 2f, respectively. The complexes 2 all show a pronounced M···F−C interaction with one of the six ortho-B(C6F5)3 fluorine atoms. The resulting metallacyclic structures were characterized by X-ray diffraction of the complexes 2c and 2e (Zr···F ≈ 2.40 Å, angle Zr−F−C ≈ 140°). The bridging fluorine atom of the complexes in solution is characterized by an extreme upfield shift of its 19F NMR resonance (δ ≈ −210 to −220 ppm) relative to the signals of the remaining five o-F resonances of the B(C6F5)3 moiety (average δ ≈ −135 ppm). The 19F NMR spectra of the complexes 2 are dynamic even in the noncoordinating solvent toluene-d8. All six o-fluorine signals equilibrate with coalescence temperatures around 240 K at 564 MHz to give a single resonance signal at high temperature. This fluorine equilibration process of the −B(C6F5)3 end of the metallocene−borate−betaine complexes 2 is very likely to proceed via a rate determining cleavage of the coordinative M···F−C interaction. From the activation barrier of this process, obtained from the dynamic fluorine NMR spectra, Zr···F bond dissociation energies of ca. 8.5 kcal/mol were estimated for the complexes 2. This magnitude of the M···F−C bond dissociation energy makes the internal fluorocarbon coordination a very suitable tool for protecting active electrophilic metal catalyst centers. The Zr···F−C bond of the complexes 2 is cleaved by the addition of the donor solvent THF with formation of acyclic 1,2-η2-allyl metallocene complexes.