Synthesis, characterization, and reactivity of chromium boratabenzene complexes

In contrast to the reactivity observed with the isoelectronic cyclopentadienyl salts, the reaction of CrCl3(THF)(3) with boratabenzene anions results in the formation of Cr(II) complexes. Thus, addition of Li(C5H5B-Me) to CrCl3(THF)(3) in THF gives (C5H5B-Me)(2)Cr (3). Similarly, Li(C5H5B-NMe2) and CrCl3(THF)(3) yield (C5H5B-NMe2)(2)Cr (4), while Li-(C5H5B-Ph) and CrCl3(THF)(3) provide (C5H5B-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 C5H5B-PMe3 (Bb-PMe3) to MeCrCl2(THF)(3) in benzene gives (C5H5B-Me)CrCl2(PMe3) (6) in low yield. Treatment of MeCrCl2(THF)(3) with the pyridine adduct of borabenzene, C5H5B-NC5H5 (Bb-Py), does not work effectively. The composition of one of the products from this reaction, the binuclear dimer [(C5H5B-Me)CrClMe](2) (7), indicates Me/Cl redistribution processes. Treatment of CrCl3(THF)(3) with 3 equivalents MeMgBr in THF, followed by addition of Bb-Py gives (C5R5B-Me)CrMe2(Py) (8). Similarly, Ph3Cr(THF)(3) and Bb-Py afford (C5H5B-Ph)CrPh2(Py) (9). Compound 8 with the well-defined activators B(C6F5)(3) and [Ph3C][B(C6F5)(4)] can polymerize ethylene with activities competitive with those of (C5H5B-Me)CrMe2(PMe3)/B(C6F5)(3) (2/B(C6F5)(3)) and Cp*CrMe2(PMe3)/B(C6F5)(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.