Chemistry of Constrained Dioxocyclam Ligands with Co(III):  Unusual Examples of C−H and C−N Bond Cleavage

The reactions between H2dc3 and Co(acac)3 have been studied in the presence and absence of base. In the presence of base, a complex with an intramolecular Co−C bond, Co(dc3-C-(8))(H2O), 1, is formed, presumably through heterolytic C−H bond activation. An X-ray crystallographic study demonstrates the presence of a Co−C bond and shows that the diazacyclooctane (daco) subunit adopts the chair−boat conformation with respect to the metal. The cobalt−carbon bond induces strain in the macrocycle as demonstrated by bond angles significantly deviating from tetrahedral. The 13C NMR resonance of the carbon atom bound to cobalt (−10.5 ppm) suggests significant ionic character in the cobalt−carbon bond. However, we were unable to cleave this bond in the presence of strong acid. In the absence of base, the reaction of Co(acac)3 with H2dc3 resulted in C−N cleavage of the ligand and the formation of a complex of dioxocyclam, Co(dc)(acac), 2. This complex has subsequently been prepared in high yield by the reaction of Co(acac)3 with dioxocyclam. An X-ray crystallographic study demonstrates that dioxocyclam adopts the heretofore unreported cis configuration, having folded along a N−Co−N axis that is perpendicular to the Co-acac plane.