Facile C-H bond activation:: Synthesis of the N4C donor set pentadentate ligand 1,4-bis(2-pyridylmethyl)-1,4-diazacyclononane (dmpdacn) and a structural study of its alkyl-cobalt(III) complex [Co(dmpdacn-C)(OH2)](ClO4)2•H2O and its hydroxylated derivative [Co(dmpdacnOH-O)Cl](ClO4)2•C3H6O

The 1,4-bis(2-pyridylmethyl)-1,4-diazacyclononane (dmpdacn) ligand with a N4C donor set deprotonates at a CH2 gamma to an amine under extraordinarily mild conditions (pH 7) and binds as a pentadentate ligand to Co(III) as the [Co(dmpdacn-C)(OH2)](2+) complex. This complex was characterized by 1D and 2D NMR techniques, and a single-crystal X-ray structure is reported. In an alternative synthesis from Co(II), dmpdacn, and air, the same C-bonded complex is obtained along with a novel hydroxylated Co(III) complex [Co(dmpdacnOH-O)Cl](2+) which has been similarly characterized. Here the carbanion has been oxidized, a C- to O-bonded rearrangement has taken place, and the bound aqua group is replaced by Cl-. The base hydrolysis kinetics of the hydroxylated Co(III) complex are reported, and mechanisms for this and the unusually facile C-H cleavage and CH2 oxidation reactions are discussed.