The presence of a trimethylsilyl substituent in place of one of the methyl groups of each of the cyclopentadienyl ligands of decamethyltitanocene enhances the thermal stability of the resulting complex, [Ti II {η 5 -C 5 Me 4 (SiMe 3 )} 2 ] (1), and controls the products formed in thermolysis of its methyl derivatives. Titanocene 1 was found to be stable in toluene solution up to 90 °C, while under vacuum at 140 °C it liberated hydrogen to give the asymmetrical doubly tucked-in titanocene [Ti II {η 3 :η 4 -C 5 Me 2 (SiMe 3 )(CH 2 ) 2 }-{η 5 -C 5 Me 4 (SiMe 3 )}] (3). The mono-and dimethyl derivatives of 1, the complexes [Ti III Me{η 5 -C 5 Me 4 -(SiMe 3 )} 2 ] (5) and [Ti IV Me 2 {η 5 -C 5 Me 4 (SiMe 3 )} 2 ] (6), undergo thermolysis at lower temperature than do the corresponding permethyltitanocene derivatives and eliminate hydrogen from their trimethylsilyl group. Thus, the known [Ti III {η 5 :η 1 -C 5 Me 4 (SiMe 2 CH 2 )}{η 5 -C 5 Me 4 (SiMe 3 )}] (4) was obtained from 5, and compound 6 afforded [Ti II {η 6 :η 1 -C 5 Me 3 (CH 2 )(SiMe 2 CH 2 )}{η 5 -C 5 Me 4 (SiMe 3 )}] (7) at only 90 °C, both with liberation of methane. Crystal structures of 3, 5, and 7 were determined. DFT calculations for titanocene 1 revealed that the metal-cyclopentadienyl bonding is accomplished via a three-centerfour-electron orbital interaction. An auxiliary long-range Si-C bond interaction with the Ti center was also established, providing a reason for the enhanced thermal stability of 1. The molecular orbitals participating in the exo methylene-titanium bonds for 3 and 7 are also three-centered and are compatible with the assignment of their activated ligands to η 3 :η 4 -allyldiene and η 6 -fulvene structures, respectively. Qualitatively, the much higher thermal stability of 3 and 7 compared to that of 1 is due to the exploitation of four d orbitals in the bonding molecular orbitals for 3 and 7 versus only two d orbitals for 1.
The trialkyl complexes [M(iPr-trisox)(CH2SiMe2R)3] (R = Me, M = Y, (1), R = Ph, M = Lu (2a),
R = Me, M = Lu (2b), Tm (3), Er (4), Ho (5), and Dy (6)) were prepared from 1,1,1-tris[(S)-4-isopropyloxazolinyl]ethane (iPr-trisox) and the corresponding trialkyl precursors [M(CH2SiMe2R)3(THF)
n
].
Their molecular structures all display a highly distorted octahedral geometry, with the angles subtended
at the metal center significantly deviating from the ideal 90°, which is attributed to the steric demands
imposed by the large CH2SiMe2R ligands, both with each other and with the isopropyl groups of the
iPr-trisox ligand. Active catalysts for the polymerization of α-alkenes (n-hexene, n-heptene, and n-octene)
were generated in situ by reaction of the trialkyl precatalyst with 2 equiv of trityl tetrakis(pentafluorophenyl)borate. In all cases polyolefins with M
w/M
n values of between 1.58 and 2.08 and
isotacticities of 80−95% were obtained. The polymerization activity increases from lutetium to thulium
and then subsequently decreases with increasing ionic radius of the metal due to a combination of activation
with increasing ionic radius and decreasing catalyst stability.
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