Intramolecular activation of aromatic C–H bonds at tantalum(V) metal centers: evaluating cyclometallation ‘resistant’ and ‘immune’ aryloxide ligation

Abstract

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.