Zirconium Complexes of 9-Phenyl-9-borataanthracene. Synthesis, Structural Characterization, and Reactivity

Lithium 9-phenyl-9-borataanthracene ( 3 ·Li(THF) x, x = 2 or 3) is obtained quantitatively from the deprotonation of 9-phenyl-9,10-dihydro-9-borataanthracene ( 6 ) with LiTMP (TMP = 2,2,6,6-tetramethylpiperidide) in THF. A comparison of the crystallographically determined structures of 6 and 3 ·Li(TMEDA) (TMEDA = N, N, N ‘, N ‘-tetramethylethylenediamine) highlights the effects of charge and delocalization on the heterocyclic framework. The reaction of 3 ·Li(THF) 2 with Cp*ZrCl 3 (Cp* = C 5 Me 5 ) and Cp*ZrMe 2 Cl affords the novel complexes (AnB-Ph)Cp*ZrCl 2 ( 4 ) (AnB-Ph = 9-phenyl-9-borataanthracene) and (AnB-Ph)Cp*ZrMe 2 ( 5 ), respectively. The crystallographically determined molecular structure of 4 resembles a bent metallocene with a tetrahedral disposition of ligands around zirconium. The borataanthracene ligand bends significantly (approx 16°) to avoid steric contacts with the Cp*ZrCl 2 core. The angle of the exocyclic phenyl substituent relative to the anthracene unit remains nearly invariant at 62° for 6, 3 ·Li(TMEDA), and 4, suggesting that the steric envelope around boron prevents an optimum orientation for orbital overlap between boron and the π-system of the phenyl ring. Treatment of 5 with B(C 6 F 5 ) 3 gives [(AnB-Ph)Cp*ZrMe][MeB(C 6 F 5 ) 3 ] ( 7 ). Reaction of 4 with methylaluminoxane (MAO) and 1 atm of ethylene produces a mixture of low molecular weight 1-alkenes, 2-alkenes, and 2-alkyl-1-alkenes, whereas the reaction of 7 with ethylene gives low molecular weight polyethylene. Reactions of 4 /MAO and 7 with 1-tridecene were carried out to determine the fate of 1-alkenes generated using 4 /MAO/C 2 H 4 and to delineate the role of the activator. The complex (C 5 H 5 B-Ph)Cp*ZrCl 2 ( 8 ) is obtained by reaction of Cp*ZrCl 3 with Li[C 5 H 5 B-Ph]. A comparison of the reactivity of 8 /MAO/C 2 H 4 against that of 4 /MAO/C 2 H 4 highlights, for the first time, the effect of sterics on the stability of catalysts supported by boratacyclic ligands.