Crystal structure and spectroscopic characterization of a cobalt(II) tetraazamacrocycle: completing a series of first-row transition-metal complexes

The tetraazamacrocyclic ligand 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetra-decane (TMC) has been used to bind a variety of first-row transition metals but to date the crystal structure of the cobalt(II) complex has been missing from this series. The missing cobalt complex chlorido (1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane-k(4)N) cobalt(II) chloride dihydrate, [CoCl(C14H32N4)]Cl center dot 2H(2)O or [(CoCl)-Cl-II(TMC)]Cl center dot 2H(2)O, crystallizes as a purple crystal. This species adopts a distorted square-pyramidal geometry in which the TMC ligand assumes the trans-I configuration and the chloride ion binds in the syn-methyl pocket of the ligand. The Co-II ion adopts an S = 3/2 spin state, as measured by the Evans NMR method, and UV-visible spectroscopic studies indicate that the title hydrated salt is stable in solution. Density functional theory (DFT) studies reveal that the geometric parameters of [Co-II Cl(TMC)]Cl center dot 2H(2)O are sensitive to the cobalt spin state and correctly predict a change in spin state upon a minor perturbation to the ligand environment.