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 obt ained 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 structure s of 6 and 3 Li(TMEDA) (TMEDA = N,N,N',N'-tetramethylethylenediamine) highlights the effects of charge and delocalization on the heterocycli c framework. The reaction of 3 . Li(THF)(2) with CpZrCl3 (Cp* = C5Me5 ) and CpZrMe2Cl affords the novel complexes (AnB-Ph)Cp*ZrCl2(4) (AnB- Ph = 9-phenyl-9-borataanthracene) and (AnB-Ph)CpZrMe2 (5), respective ly. The crystallographically determined molecular structure of 4 resem bles a bent metallocene with a tetrahedral disposition of ligands arou nd zirconium. The borataanthracene ligand bends significantly (approx 16 degrees) to avoid steric contacts with the CpZrCl2 core. The angle of the exocyclic phenyl substituent relative to the anthracene unit r emains nearly invariant at 62 degrees for 6, 3 Li(TMEDA), and 4, sugge sting that the steric envelope around boron prevents an optimum orient ation for orbital overlap between boron and the pi-system of the pheny l ring. Treatment of 5 with B(C6F5)(3) gives [(AnB-Ph)CpZrMe] [MeB(C6 F5)(3)] (7). Reaction of 4 with methylaluminoxane (MAO) and 1 atm of e thylene produces a mixture of low molecular weight l-alkenes, 2-alkene s, and 2-alkyl-1-alkenes, whereas the reaction of 7 with ethylene give s low molecular weight polyethylene. Reactions of 4/MAO and 7 with 1-t ridecene were carried out to determine the fate of l-alkenes generated using 4/MAO/C2H4 and to delineate the role of the activator. The comp lex (C5H5B-Ph)CpZrCl2 (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.