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-But 2-4,6)Me2] (13), [CpTi(OC6H2{C10H9}-2-But 2-4,6)Me2] (14), [Cp*Ti(OC6HPh2-2,6-But 2-3,5)Me2] (15), [CpTi(OC6Ph4-2,3,5,6-Br-4)Me2] (18), [CpTi(OC6H2Me2-2,6-Br-4)Me2] (20), [CpTi(OC6H3Pri 2-2,6)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-d 8 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.

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The trichloride compounds [Ta(OC 6 HPh 2 -2,6-R 2 -3,5)Cl 3 ] (1: R = H a, Ph b, Me c, Pr i d or Bu t e) have been obtained by treating [Ta 2 Cl 10 ] with the corresponding 3,5-disubstituted-2,6-diphenylphenols Ia-Ie. The solid-state structures of 1c and 1d show a square-pyramidal structure with an axial aryloxide ligand. The reaction of 1 with LiCH 2 SiMe 3 (3 equivalents) led to the isolation of the tris(alkyls) [Ta(OC 6 HPh 2 -2,6-R 2 -3,5) 2 (CH 2 SiMe 3 ) 3 ] (4a-4d) except in the case of the 3,5-di-tert-butyl derivative 1e which generated the alkylidene compound [Ta(OC 6 H 3 Ph 2 -2,6-Bu t -3,5) 2 (᎐ ᎐ CHSiMe 3 )(CH 2 SiMe 3 )] 6e. The alkylidenes 6a-6d can be produced by photolysis of the corresponding tris(alkyls) 4a-4d. The alkylidenes 6a-6d undergo intramolecular cyclometallation of the aryloxide ligand (addition of an aromatic C᎐H bond to the tantalum alkylidene) at a rate which is extremely dependent on the meta substituents on the phenoxide nucleus. Kinetic studies show that conversion of 6a-6d into monometallated 7a-7d is first order with the phenyl, methyl and isopropyl substituents slowing the ring closure down by factors of 20, 90 and 360 respectively. The tert-butyl substituent completely shuts down cyclometallation of the adjacent phenyl ring. It is argued that bulky substituents inhibit rotation of the ortho-phenyl ring into a conformation necessary for C᎐H bond activation. Structural analysis of the torsion angles between ortho-phenyl and phenoxy rings has been carried out. The use of 1 H NMR chemical shifts has been demonstrated to be a valuable tool to probe the average conformations adopted in solution.

A series of dimethyl compounds of Ti(IV) have been isolated containing both cyclopentadiene and ortho-arylphenoxide ligation; reaction with [B(C 6 F 5 ) 3 ] generates corresponding cationic methyl species which eliminate methane and form [Cp(ArO)Ti{CH 2 B(C 6 F 5 ) 2 }(C 6 F 5 )] derivatives.

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