PARAMAGNETIC ALKYLCHROMIUM COMPOUNDS AS HOMOGENEOUS CATALYSTS FOR THE POLYMERIZATION OF ETHYLENE

A class of homogeneous model compounds for chromium-based ethylene polymerization catalysts has been prepared. Abstraction of halide from dimers [Cp*Cr(III)(mu-Cl)R]2 in the presence of Lewis bases gave cationic half-sandwich complexes [Cp*Cr(III)(L)2R]+X- (R = Me, Et; L = py, 3,5-lut, PMe3, dppe dmpe, MeCN, THF; X- = PF6-, BPh4-). The crystal structures of [Cp*Cr(III)(py)2Et]+PF6- (10) and [Cp*Cr(III)(THF)2Me]+BPh4- (14) were determined by X-ray diffraction. 10 crystallized in the monoclinic space group P2(1)/c with a = 8.810 (2) angstrom, b = 15.425 (3) angstrom, c = 17.792 (3) angstrom, and beta = 89.34 (9) degrees with Z = 4. 14 crystallized in the monoclinic space group P2(1)/c with a = 10.718 (3) angstrom, b = 16.209 (4) angstrom, c = 21.880 (6) angstrom, and beta = 103.63 (2) degrees with Z = 4. [Cp*Cr(dmpe)Me]+PF6- (7) exhibited a reversible reduction wave at -1.88 V (vs Fc+/Fc). Chemical reduction of 7 yielded Cp*Cr(II)(dmpe)Me (16), a paramagnetic (mu-eff = 2.79-mu-B) alkylchromium(II) complex with a 16-electron configuration. 16 was structurally characterized; it crystallized in the orthorhombic space group Pbca with a = 14.223 (2), b = 16.108 (3), and c = 17.391 (3) angstrom with Z = 8. 14 catalyzed the polymerization of ethylene at ambient temperature and pressure. The mechanism of the catalysis involved dissociation of THF from 14, thus yielding coordinatively unsaturated ]Cp*Cr(III)(THF)Me[+. The polyethylene was characterized by GPC and C-13 NMR. It exhibited narrow molecular weight distributions (M(w)/M(n) = 1.61-4.60) and little branching. Reaction of 14 with styrene gave Cp*Cr(II)(eta-6-Ph-BPh3) (17), a model for the catalyst deactivation process. The X-ray crystal structure of 17 showed it to crystallize in the monoclinic space group P2(1)/c with a = 11.486 (4) angstrom, b = 11.732 (4) angstrom, c = 20.716 (6) angstrom, and beta = 100.25 (3) degrees with Z = 4. One of the phenyl groups of the tetraphenylborate anion was coordinated to chromium. A direct comparison of the reactivity of divalent and trivalent chromium with ethylene was made. While [Cp*Cr(III)(dmpe)Me]+PF6- (7) polymerized ethylene at 90-degrees-C, Cp*Cr(II)(dmpe)Me (16) reacted with ethylene at room temperature; however, facile hydrogen beta-elimination prevented the formation of polymer.