Electrochemical and spectroscopic studies of metal cyclam complexes with thiocyanate. Evidence for mixed ligand complex formation

Complexes of NiII, CoII and CuII containing the macrocyclic ligand, 1,4,8,11-tetraazacyclotetradecane (cyclam), and their ability to form mixed ligand complexes with thiocyanate have been studied. These complexes in a 1:2 mole ratio, exhibit new absorption peaks at 450, 538 and 512 nm respectively. Addition of thiocyanate to the nickel–cyclam complex (1:2:5 mole ratio) led to the formation of a purple complex, exhibiting three distinct new absorption peaks at 330, 455 and 662 nm. A purple complex (1:2:10 mole ratio) separated, having absorption peaks at 352, 503 and 693 nm in CHCl3. The CoII–cyclam complex with thiocyanate in the same mole ratio exhibits two absorption peaks at 437 and 519 nm without appearance of any precipitate. The CuII–cyclam complex with thiocyanate did not form a mixed ligand complex. Electrochemical studies also confirmed the complex formation of NiII–cyclam with the thiocyanate with the appearance of two new oxidation peaks close to 1.25 and 1.60 V versus Ag/AgCl in H2O and CHCl3. The CoII–cyclam complex with thiocyanate exhibited an oxidation peak at 1.2 V versus Ag/AgCl, while no peak was observed for the CuII–cyclam complex with thiocyanate. Based on spectroscopic and electrochemical studies the geometry of the complex has been evaluated.