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

The series of dimethyltitanium compounds [CpTi(OAr)Me-2], ligated by one cyclopentadienyl (Cp) and one 2,6-disubstituted aryloxide (OAr), have been prepared by the reaction of [CpTi(OAr)Cl-2] 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)Me-2] (X H (19), Br (20)) undergo ligand exchange to produce [CpTi(OC6H2Me2-2,6-X-4)(2)Me] (X = H (22), Br (23)) and [CpTiMe3], X-ray crystal structures have been obtained for the dimethyl compounds [CpTi(OC6H2Np-2-BU2t-4,6)Me-2] (13), [CpTi(OC6H2{C10H9}-2-Bu-2(t)-4,6)Me-2] (14), [Cp*Ti(OC6H2Ph2-2,6-Bu-2(t)-3,5)Me-2] (15), [CpTi(OC6Ph4-2,3,5,6-Br-4)Me-2] (18), [CpTi(OC6H2Me2-2,6-Br-4)Me-2] (20), [CpTi(OC6H3Pr2i-2,6)Me-2] (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 degreesC. 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-d(8) solutions of 27 were found to cleanly convert to 31 and methane, as monitored by H-1 NMR spectroscopy. A first-order rate constant of [7.6(2)] X 10(-4) s(-1) was measured at 25.0(5) degreesC. The solid-state structure of [CpTi(OC6HPh2-2,6-Me-2-3,5)(C6F5)(CH2B{C6F5}(2))] (28) confirms this formulation and reveals a trigonal-planar boron atom exhibiting no interaction with the adjacent Ti-C6F5 unit.