1-SILA-3-METALLACYCLOBUTANES, PRECURSORS FOR THE GENERATION OF HIGHLY ELECTROPHILIC GROUP-4 METALLOCENE ALKYL CATIONS - SPECTROSCOPIC AND STRUCTURAL EVIDENCE OF A WEAKLY BOUND THF LIGAND IN [(C5ME5)2ZR(CH2SIME3)(THF)][BPH4]

The protonation of L(n) activated M(CH2SiMe2CH2) (L(n) = (C5H5)2, SiMe2(C5H4)2 M = Ti, Zr; L(n) = (C5Me5)2 M = Zr) with [NEt3H][BPh4] in THF produces the corresponding (trimethylsilyl)methyl-substituted metal alkyl tetraphenylborate salts [L(n)M(CH2SiMe3)(THF)[BPh4] (1-5). These complexes have been characterized by elemental analysis and H-1 and C-13 NMR measurements. The molecular structure of [Cp*2Zr-(CH2SiMe3)(THF)][BPh4] (5) has been determined by X-ray diffraction methods. Complex 5 crystallizes in space group P2(1)/a with a = 26.911 (9) angstrom, b = 13.929 (4) angstrom, c = 12.569 (4) angstrom, beta = 101.73 (2)-degrees, V = 4613 (3) angstrom 3, and Z = 4. The steric bulk of the pentamethylcyclopentadienyl ligands controls the orientation of the THF molecule, placing it nearly ''parallel'' to the equatorial plane of the metallocene wedge. Attempts to prepare the THF-free cation [Cp*2Zr(CH2SiMe3)]+ by the reaction of Cp*2 activated Zr(CH2SiMe2CH2) with [N(n-butyl)3H][BPh4] in toluene led to the isolation of the previously reported zwitterion Cp*2Zr(+)-(m-C6H4)-B(-)Ph3 (6). Compounds 1-4 (in CH2Cl2) and compound 6 (in toluene) behave as ethylene polymerization catalysts, producing high-density polyethylene (HDPE) under mild conditions in the absence of an Al cocatalyst.