Intramolecular activation of aromatic C-H bonds at tantalum(v) metal centers: evaluating cyclometallation 'resistant' and 'immune' aryloxide ligation

The trichloride compounds [Ta(OC6HPh2-2,6-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 [Ta2Cl10] with the corresponding 3,5-disubstituted-2,6-diphenylphenols Ia-Ie. The solid-state structures of Ic and Id show a square-pyramidal structure with an axial aryloxide ligand. The reaction of 1 with LiCH2SiMe3 (3 equivalents) led to the isolation of the tris(alkyls) [Ta(OC6HPh2-2,6-R-2-3,5)(2)(CH2SiMe3)(3)] (4a-4d) except in the case of the 3,5-di-tert-butyl derivative Ie which generated the alkylidene compound [Ta(OC6H3Ph2-2,6-Bu-t-3,5)(2)(=CHSiMe3)(CH2SiMe3)] 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 H-1 NMR chemical shifts has been demonstrated to be a valuable tool to probe the average conformations adopted in solution.