The versatility of 1,4,8,11-tetraazacyclotetradecane (cyclam) in the formation of compounds of Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ with metal ions in and out of the cyclic ligand ring

Herein we report the results of preparing metal compounds (where the metal ions are Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺) with the cyclic ligand 1,4,8,11-tetraazacyclotetradecane [cyclam] under a variety of conditions of metal-ligand ratios and solvent media. In all cases, we used metal Cl₂·nH₂O salts (except for anhydrous CoCl₂), as specified. Outcome: we isolated species with a four-coordinate metal in the N4 cavity of the ligand alone, and also with either one or two additional axial ligands. Those axial ligands can be (a) a single chloride, leading to penta-coordinated moncationic products; (b) two chlorides, leading to octahedral-neutral compounds; (c) two waters, giving rise to hexa-coordinated [(cyclam)metal(H₂O)₂]²⁺ species. Finally, in the case of HCl added to the reaction medium, the cyclam can be di-protonated and appears as [(cyclam)H₂]²⁺ in the crystals. With such a variety of products, it is not surprising that since the metal coordination numbers vary, the cyclam ligand stereochemistries are thereby affected. Interestingly, the [(cyclam)metal] species are invariably hydrogen-bonded to one another in infinite strings of two kinds: (1) those for which the crystal's Z′ = 1 have single strings; (2) when Z′ = 2, there is a pair of homogeneous strings attached to one another by a variety of hydrogen-bonding linkages.