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Formation of Neutral and Cationic Methyl Derivatives of Titanium Containing Cyclopentadienyl and Aryloxide Ancillary Ligation

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posted on 26.04.2004, 00:00 authored by Andrew E. Fenwick, Khamphee Phomphrai, Matthew G. Thorn, Jonathan S. Vilardo, Christine A. Trefun, Brigitte Hanna, Phillip E. Fanwick, Ian P. Rothwell
The series of dimethyltitanium compounds [CpTi(OAr)Me2], ligated by one cyclopentadienyl (Cp) and one 2,6-disubstituted aryloxide (OAr), have been prepared by the reaction of [CpTi(OAr)Cl2] with 2 equiv of LiMe or by the addition of parent phenol (HOAr) to a cold ether solution of [CpTiMe3]. The compounds are stable, except for those containing less bulky o-methyl substituents; the compounds [CpTi(OC6H2Me2-2,6-X-4)Me2] (X = H (19), Br (20)) undergo ligand exchange to produce [CpTi(OC6H2Me2-2,6-X-4)2Me] (X = H (22), Br (23)) and [CpTiMe3]. X-ray crystal structures have been obtained for the dimethyl compounds [CpTi(OC6H2Np-2-But2-4,6)Me2] (13), [CpTi(OC6H2{C10H9}-2-But2-4,6)Me2] (14), [Cp*Ti(OC6HPh2-2,6-But2-3,5)Me2] (15), [CpTi(OC6Ph4-2,3,5,6-Br-4)Me2] (18), [CpTi(OC6H2Me2-2,6-Br-4)Me2] (20), [CpTi(OC6H3Pri2-2,6)Me2] (21), and the monomethyl species 23. Reaction of the dimethyl compounds with [B(C6F5)3] generates the corresponding cationic methyl species [CpTi(OAr)Me][MeB(C6F5)3]. The compound [CpTi(OC6HPh4-2,3,5,6)Me][MeB(C6F5)3] (27) was studied via solution VT-NMR spectroscopy, and the free energy of activation for methyl exchange was estimated to be 14.4(5) kcal mol-1 at 10 °C. These thermally unstable cationic derivatives readily eliminate methane at room temperature, affording compounds of the type [CpTi(OAr)(C6F5){CH2B(C6F5)2}]. A kinetic study of the conversion of 27 to [CpTi(OC6HPh4-2,3,5,6)(C6F5){CH2B(C6F5)2}] (31) was undertaken. Toluene-d8 solutions of 27 were found to cleanly convert to 31 and methane, as monitored by 1H NMR spectroscopy. A first-order rate constant of [7.6(2)] × 10-4 s-1 was measured at 25.0(5) °C. The solid-state structure of [CpTi(OC6HPh2-2,6-Me2-3,5)(C6F5)(CH2B{C6F5}2)] (28) confirms this formulation and reveals a trigonal-planar boron atom exhibiting no interaction with the adjacent Ti−C6F5 unit.