Cyclometalated Tantalum Diphenolate Pincer Complexes: Intramolecular C−H/M−CH_3σ-Bond Metathesis May Be Faster than O−H/M−CH_3 Protonolysis

A diphenol linked at the ortho positions to a benzene ring was metalated with TaCl_2(CH_3)_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−CH_3 σ-bond metathesis, leading to cyclometalation of the linking ring and finally protonation of the cyclometalated group by the pendant phenol.