Joining of Trinuclear Heterometallic CuII2–MII (M = Mn, Cd) Nodes by Nicotinate to Form 1D Chains: Magnetic Properties and Catalytic Activities

Abstract

In the present work, four new heterometallic coordination complexes, {[(CuL)2Mn(nic)(H2O)2](ClO4)(0.5H2O)}n (1), {[(CuL)2Cd(nic)(H2O)2](ClO4)(H2O)}n (2), [(CuL)2Mn(nic)2]·2CH3OH (3), and [(CuL)2Cd(nic)2]·2CH3OH (4) (where H2L = N,N′-bis(α-methylsalicylidene)-1,3-propanediamine and nic = nicotinate ion), have been synthesized and characterized by single-crystal X-ray crystallography. In complexes 1 and 2, the nicotinate ion acts as a bifunctional linker (N,O donor) and joins the linear trinuclear nodes to form 1D polymeric chains. However, in complexes 3 and 4, the nicotinate ion uses only the oxygen atoms of the carboxylic acid (O donor) to bind to the metal centers, forming discrete linear trinuclear units, while the pyridyl nitrogen (N donor atom) remains free. The dc magnetic susceptibility measurements show that the CuII and MnII ions are antiferromagnetically coupled in both 1 and 3, with exchange coupling constants (JMn–Cu) of −20.57 ± 0.08 and −9.38 ± 0.08 cm–1, respectively. Among the four complexes, 1 and 3 show catechol oxidase and phenoxazinone synthase like catalytic activities. The turnover numbers (kcat) of complexes 1 and 3 for catecholase activity are 1121 and 720 h–1, respectively, at an optimum pH of 8.0 and for phenoxazinone synthase activity are 429 and 398 h–1, respectively, at an optimum pH of 9.7. The higher kcat values of 1 for both reactions are attributable to a water molecule coordinated to the central MnII atom that facilitates the substrate–catalyst binding. An ESI-mass spectral analysis indicates that trinuclear heterometallic species, e.g., [(CuL)2Mn(nic)(H2O)]+ for 1 and [(CuL)2Mn(nic)]+ for 3, are the active species that bind to the substrate, and on that basis, probable mechanisms through the formation of radical intermediates have been proposed.