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*ZrCl3 (Cp* = C5Me5) and Cp*ZrMe2Cl affords the novel complexes (AnB-Ph)Cp*ZrCl2(4) (AnB-Ph = 9-phenyl-9-borataanthracene) and (AnB-Ph)Cp*ZrMe2 (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 degrees) to avoid steric contacts with the Cp*ZrCl2 core. The angle of the exocyclic phenyl substituent relative to the anthracene unit remains nearly invariant at 62 degrees 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 pi-system of the phenyl ring. Treatment of 5 with B(C6F5)(3) gives [(AnB-Ph)Cp*ZrMe] [MeB(C6F5)(3)] (7). Reaction of 4 with methylaluminoxane (MAO) and 1 atm of ethylene produces a mixture of low molecular weight l-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 l-alkenes generated using 4/MAO/C2H4 and to delineate the role of the activator. The complex (C5H5B-Ph)Cp*ZrCl2 (8) is obtained by reaction of Cp*ZrCl3 with Li[C5H5B-Ph]. A comparison of the reactivity of 8/MAO/C2H4 against that of 4/MAO/C2H4 highlights, for the first time, the effect of sterics on the stability of catalysts supported by boratacyclic ligands.