Cyclometalated tantalum diphenolate pincer complexes:: Intramolecular C-H/M-CH3 σ-bond metathesis may be faster than O-H/M-CH3 protonolysis

A diphenol linked at the ortho positions to a benzene ring was metalated with TaCl2(CH3)(3). Deuterium labeling of the phenol hydrogens and of the linking 1,3-benzenediyl ring reveals an unexpected mechanism involving protonolysis of a methyl group, followed by C-H/Ta-CH3 sigma-bond metathesis, leading to cyclometalation of the linking ring and finally protonation of the cyclometalated group by the pendant phenol.