197715-48-3Relevant academic research and scientific papers
Intramolecular activation of aromatic C-H bonds at tantalum(v) metal centers: Evaluating cyclometallation 'resistant' and 'immune' aryloxide ligation
Vilardo, Jonathan S.,Lockwood, Mark A.,Hanson, Linda G.,Clark, Janet R.,Parkin, Bernardeta C.,Fanwick, Phillip E.,Rothwell, Ian P.
, p. 3353 - 3362 (1997)
The trichloride compounds [Ta(OC6HPh2-2,6-R2-3,5)Cl3] (1: R = H a, Ph b, Me c, Pri d or But e) have been obtained by treating [Ta2Cl10] 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 LiCH2SiMe3 (3 equivalents) led to the isolation of the tris(alkyls) [Ta(OC6HPh2-2,6-R2-3,5)2(CH 2SiMe3)3] (4a-4d) except in the case of the 3,5-di-tert-butyl derivative 1e which generated the alkylidene compound [Ta(OC6H3Ph2-2,6-But-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 mono-metallated 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 1H NMR chemical shifts has been demonstrated to be a valuable tool to probe the average conformations adopted in solution.
