Organometallic complexes containing a Co-N-C three-membered ring: Factors affecting the dynamic of the ring closure

Treatment of [ClCH2CoIII(L1NH-py)(HL1NH-py)]ClO4, where HL1NH-py is the tridentate 2-((2-pyridylmethyl)amino)-3-butanone oxime and L1NH-py(-) its conjugated base with diluted NaOH, afforded the new complex [CH2CoIII(L1NH-py)(HL1-py)]ClO4, containing a three-membered metallacycle by a pathway involving the intramolecular nucleophilic addition of an equatorial nitrogen donor to the axial carbon. The X-ray analysis revealed a highly distorted structure with the distortion concentrated in the Co-C bond. The kinetics of formation of this complex and of the analogous complexes (previously synthesized) containing the 2-((2-(2-pyridyl)ethyl)amino)-3-butanone oxime ligand (HLNH-py) were studied. These studies revealed that the intramolecular process forming the metallacycle is quite rapid, whereas the carbon atom bonded to cobalt is inert toward nucleophilic intermolecular attack. The rapidity of the reaction was ascribed to a favorable entropy contribution and to the "proximity effect" imposed by the relatively rigid framework of the molecule, which holds together the reactive centers (C and N atoms) within a critical distance.