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

Abstract

The 1,4-bis(2-pyridylmethyl)-1,4-diazacyclononane (dmpdacn) ligand with a N(4)C donor set deprotonates at a CH(2) gamma to an amine under extraordinarily mild conditions (pH 7) and binds as a pentadentate ligand to Co(III) as the [Co(dmpdacn-C)(OH(2))](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 CH(2) oxidation reactions are discussed.