Stereospecific sp³ C–H oxidation with m-CPBA: A Co^III Schiff base complex as pre-catalyst vs. its Co^IIICd^II heterometallic derivative

The mono- and binuclear Schiff base complexes [CoL₃]·DMF (1) and [CoCdL₃Cl₂]·0.5H₂O (2) were facilely synthesized using zerovalent cobalt and cadmium chloride (for 2) as starting materials and the pre-formed pro-ligand HL (2-methoxy-6-[(methylimino)methyl]phenol, the product of condensation of o-vanillin and methylamine) in air. The compounds were characterized by single crystal X-ray diffraction analysis and spectroscopic methods in solution and in the solid state. Both complexes demonstrate a profound catalytic activity in the stereoselective oxidation of cis-1,2-dimethylcyclohexane (model substrate) with m-CPBA (m-chloroperbenzoic acid) under mild conditions in the presence of promoters of various acidity (HNO₃, TFA and HOAc). The heterometallic binuclear Co^IIICd^II pre-catalyst (2) was more active than the mononuclear Co^III one (1), exhibiting higher products yields up to 51% and excellent stereospecificity (up to 99.2% retention of stereoconfiguration). This result could be associated with a synergistic effect of two different metals in 2. Based on the large obtained kinetic isotope effect and H₂¹⁸O labeling studies, the overall reaction mechanism was proposed to proceed without the participation of free alkyl radicals. The acidity of the promoter was shown to influence catalytic parameters for both 1 and 2 so that the better parameters are achieved with the acid possessing lower pK_a values (a stronger acid). The comparison of the catalytic behaviours of 1 and 2 is discussed in detail considering relevant examples from the literature.