Nickel(II) complexes of monofunctionalized pyridine-azamacrocycles: synthesis, structures, pendant arm "on-off" coordination equilibria, and peroxidase-like activity.

Novel nickel(II) complexes of pyridine-azamacrocycles (PyMACs) with pendant arms have been synthesized using simple, direct, and selective mono-N-functionalization of PyMACs. These complexes have been characterized by spectroscopy and X-ray crystallography. Nickel(II)-PyMAC complexes with a flexible pendant arm bearing a tertiary amine, a carboxylic acid, or an amide group exhibit structural and color changes due to "on-off" arm coordination to the metal center. Five- or six-coordinate complexes with the arm bound to the nickel(II) center are high-spin, while their four-coordinate "arm-off" counterparts are low-spin. Synergistic axial coordination of acetonitrile and the amide group from the pendant arm was observed. Coordination to the nickel(II) center lowers the pK(a) of the functional group attached to the macrocycle via a propylene linker by up to 4-5 orders of magnitude. Varying hydrogen bonding and proton-donating properties of the pendant arm affects the peroxidase-like activity of Ni(II)-PyMAC complexes in the oxidation of ABTS with hydrogen peroxide.