Synthesis, Characterization, and Reactivity of Chromium Boratabenzene Complexes

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.