APPROACHES TO HEXANE-SOLUBLE CATIONIC ORGANOMETALLIC LEWIS-ACIDS - SYNTHESIS, STRUCTURE, AND REACTIVITY OF TITANOCENE DERIVATIVES CONTAINING POLYSILYLATED CYCLOPENTADIENYL LIGANDS

A series of titanocene dichloride derivatives was synthesized where the total trimethylsilyl substitution on the two cyclopentadienyl rings was 0, 2, 3, 4, and 5. Treatment of these complexes with methyllithium afforded 88-93% yields of the dimethyl derivatives, while reaction of the dichlorides with silver tetrafluoroborate afforded 45-57% yields of the difluoro derivatives. However, 1,1',2,3',4-pentakis(trimethylsilyl)titanocene dichloride afforded complex reaction mixtures upon treatment with either methyllithium or silver tetrafluoroborate. Complexes containing three and four trimethylsilyl groups afforded moderately stable cationic intermediates. The cationic complex derived from 1,1',3,3'-tetrakis(trimethylsilyl)titanocene dichlonue and silver tetrafluoroborate was the most stable and was partially characterized by H-1, F-19, and C-13 NMR and IR spectroscopy. The barrier to cyclopentadienyl rotation was examined for 1,1',3,3'-tetrakis(trimethylsilyl)titanocene dichloride and 1,1',3,3'-tetrakis(trimethylsilyl)titanocene difluoride and was found to be 8.9 +/- 0.5 kcal/mol for both complexes. The X-ray crystal structures of 1,1',3,3'-tetrakis(trimethylsilyl) titanocene difluoride and 1,1',2,4-tetrakis(trimethylsilyl)titanocene difluoride were determined to probe for steric distortions. 1,1',3,3'-Tetrakis(trimethylsilyl)titanocene difluoride crystallizes in the orthorhombic space group Pbna with a = 8.788 (9) angstrom, b = 18.093 (2) angstrom, c = 18.585 (2) angstrom, V = 2954.8 (9) angstrom3, and Z = 4. 1,1',2,4-Tetrakis(trimethylsilyl)titanocene difluoride crystallizes in the triclinic space group P1BAR with a = 9.2476 (5) angstrom, b = 10.6838 (15) angstrom, c = 15.8842 (19) angstrom, alpha = 106.754 (10)-degrees, beta = 96.727 (8)-degrees, gamma = 99.291 (8)-degrees, V = 1456.9 (3) angstrom3, and Z = 2.