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 [CpTiMe 3 ]. The compounds are stable, except for those containing less bulky o -methyl substituents; the compounds [CpTi(OC 6 H 2 Me 2 -2,6-X-4)Me 2 ] (X = H ( 19 ), Br ( 20 )) undergo ligand exchange to produce [CpTi(OC 6 H 2 Me 2 -2,6-X-4) 2 Me] (X = H ( 22 ), Br ( 23 )) and [CpTiMe 3 ]. X-ray crystal structures have been obtained for the dimethyl compounds [CpTi(OC 6 H 2 Np-2-Bu t 2 -4,6)Me 2 ] ( 13 ), [CpTi(OC 6 H 2 {C 10 H 9 }-2-Bu t 2 -4,6)Me 2 ] ( 14 ), [Cp*Ti(OC 6 HPh 2 -2,6-Bu t 2 -3,5)Me 2 ] ( 15 ), [CpTi(OC 6 Ph 4 -2,3,5,6-Br-4)Me 2 ] ( 18 ), [CpTi(OC 6 H 2 Me 2 -2,6-Br-4)Me 2 ] ( 20 ), [CpTi(OC 6 H 3 Pr i 2 -2,6)Me 2 ] ( 21 ), and the monomethyl species 23 . Reaction of the dimethyl compounds with [B(C 6 F 5 ) 3 ] generates the corresponding cationic methyl species [CpTi(OAr)Me][MeB(C 6 F 5 ) 3 ]. The compound [CpTi(OC 6 HPh 4 -2,3,5,6)Me][MeB(C 6 F 5 ) 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)(C 6 F 5 ){CH 2 B(C 6 F 5 ) 2 }]. A kinetic study of the conversion of 27 to [CpTi(OC 6 HPh 4 -2,3,5,6)(C 6 F 5 ){CH 2 B(C 6 F 5 ) 2 }] ( 31 ) was undertaken. Toluene- d 8 solutions of 27 were found to cleanly convert to 31 and methane, as monitored by 1 H 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(OC 6 HPh 2 -2,6-Me 2 -3,5)(C 6 F 5 )(CH 2 B{C 6 F 5 } 2 )] ( 28 ) confirms this formulation and reveals a trigonal-planar boron atom exhibiting no interaction with the adjacent Ti−C 6 F 5 unit.