Chain growth in the reaction of ethylene with Cp*CrMe 2 (PMe 3 ) activated with methylaluminoxane is restricted by a fast Cr/Al transmetalation process.

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In contrast to the reactivity observed with the isoelectronic cyclopentadienyl salts, the reaction of CrCl 3 (THF) 3 with boratabenzene anions results in the formation of Cr(II) complexes. Thus, addition of Li(C 5 H 5 B-Me) to CrCl 3 (THF) 3 in THF gives (C 5 H 5 B-Me) 2 Cr (3). Similarly, Li(C 5 H 5 B-NMe 2 ) and CrCl 3 (THF) 3 yield (C 5 H 5 B-NMe 2 ) 2 Cr (4), while Li-(C 5 H 5 B-Ph) and CrCl 3 (THF) 3 provide (C 5 H 5 B-Ph) 2 Cr (5). Compounds 3-5 were characterized by single-crystal X-ray diffraction studies, and all possess typical sandwich structures. The reaction of borabenzene-ligand adducts with suitable Cr(III) starting materials provides boratabenzene-Cr(III) complexes. Addition of C 5 H 5 B-PMe 3 (Bb-PMe 3 ) to MeCrCl 2 (THF) 3 in benzene gives (C 5 H 5 B-Me)CrCl 2 (PMe 3 ) (6) in low yield. Treatment of MeCrCl 2 (THF) 3 with the pyridine adduct of borabenzene, C 5 H 5 B-NC 5 H 5 (Bb-Py), does not work effectively. The composition of one of the products from this reaction, the binuclear dimer [(C 5 H 5 B-Me)CrClMe] 2 (7), indicates Me/Cl redistribution processes. Treatment of CrCl 3 (THF) 3 with 3 equivalents MeMgBr in THF, followed by addition of Bb-Py gives (C 5 H 5 B-Me)CrMe 2 (Py) (8). Similarly, Ph 3 Cr(THF) 3 and Bb-Py afford (C 5 H 5 B-Ph)CrPh 2 (Py) (9). Compound 8 with the well-defined activators B(C 6 F 5 ) 3 and [Ph 3 C][B(C 6 F 5 ) 4 ] can polymerize ethylene with activities competitive with those of (C 5 H 5 B-Me)CrMe 2 (PMe 3 )/B(C 6 F 5 ) 3 (2/B(C 6 F 5 ) 3 ) and Cp*CrMe 2 (PMe 3 )/B(C 6 F 5 ) 3 . Methylaluminoxane (MAO) can also be used as an activator with the complexes containing a coordinated phosphine. The pyridine counterparts fail to give polymerization catalysts with MAO.

Addition of primary and secondary alcohols to C 5 H 5 B-PMe 3 (2) affords 1-alkoxyboracyclohexa-2,4-dienes in high yields. Deprotonation of these boracyclohexadienes, using NaH or lithium diisopropylamide, followed by the reaction with ZrCl 4 allows for the coordination of alkoxyboratabenzene ligands to zirconium. Thus, complexes of the type [C 5 H 5 B-OR] 2 ZrCl 2 (R ) Et, 1; Cy, 3; Ph, 4; and CH 2 Ph, 5) can be produced in 45-65% overall yield. The crystallographically determined molecular structure of 4 shows evidence for B-O π orbital overlap. The linked diols 1,2-trans-cyclohexanediol and binaphthol can be used to generate ansatype zirconium complexes 7 and 9, respectively. When 1, 3, 4, or 5 react with (AlMe 3 ) 2 the organometallic product is [C 5 H 5 BsMe] 2 ZrCl 2 (10). Cp*[C 5 H 5 B-OEt]ZrCl 2 (11, Cp* ) C 5 Me 5 ) and (AlMe 3 ) 2 give Cp*[C 5 H 5 BsMe]ZrCl 2 (13). The complex Cp*[C 5 H 5 B(OEt)(AlMe 3 )]ZrCl 2 (12) appears to be an intermediate in the conversion of 11 to 13. A comparison of the molecular structures of 11 and 12 shows that the B-O interaction weakens and the Zr-B distance contracts upon adduct formation. Complexes 1, 3, 4, 9, 10, and [C 5 H 5 B-Ph] 2 ZrCl 2 (14) react with excess methylaluminoxane (MAO) and ethylene (1 atm) to give a FloryShultz distribution of olefins. For 7/MAO, ethylene addition results in the formation of polyethylene. The overall activity toward monomer and selectivity for linear 1-alkenes of the catalyst solutions are determined by the exocyclic group of the alkoxyboratabenzene zirconium precursor.