Synthesis, characterization, and reactivity of chromium boratabenzene complexes

In contrast to the reactivity observed with the isoelectronic cyclopentadie nyl 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 CrC l3(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(TH F)(3) provide (C5H5B-Ph)(2)Cr (5). Compounds 3-5 were characterized by sing le-crystal X-ray diffraction studies, and all possess typical sandwich stru ctures. The reaction of borabenzene-ligand adducts with suitable Cr(III) st arting materials provides boratabenzene-Cr(III) complexes. Addition of C5H5 B-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 compositio n of one of the products from this reaction, the binuclear dimer [(C5H5B-Me )CrClMe](2) (7), indicates Me/Cl redistribution processes. Treatment of CrC l3(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 (C 5H5B-Ph)CrPh2(Py) (9). Compound 8 with the well-defined activators B(C6F5)( 3) and [Ph3C][B(C6F5)(4)] can polymerize ethylene with activities competiti ve with those of (C5H5B-Me)CrMe2(PMe3)/B(C6F5)(3) (2/B(C6F5)(3)) and Cp*CrM e2(PMe3)/B(C6F5)(3). Methylaluminoxane (MAO) can also be used as an activat or with the complexes containing a coordinated phosphine. The pyridine coun terparts fail to give polymerization catalysts with MAO.