Square planar versus square pyramidal copper(II) complexes containing N3O moiety: Synthesis, structural characterization, kinetic and catalytic mimicking activity

Abstract

In quest of copper complexes having [CuN3O] cores, [Cu(phen)(l–Val)(H2O)]NO3 (1) and [Cu(bpy)(l–Val)]ClO4 (2) complexes have been synthesized and structurally characterized (phen = 1,10–phenanthroline; bpy = 2,2′–bipyridine). Complex 1 possesses a distorted square-pyramidal, whereas 2 has a distorted square-planar coordination geometry. Structures of 1 and 2 have supramolecular networks formed via inter- and intra-molecular hydrogen bonding interactions. The kinetics and mechanism of ligand substitution of 1 and 2 by thiourea (TU) were studied in detail and showed a biphasic process in which an initial fast reaction is followed by a slower one. The activation parameters for the fast reaction: ΔH# = 68 ± 4 and 73 ± 5 kJ mol−1, ΔS# = 43 ± 10 and 54 ± 9 J K−1 mol−1 for 1 and 2, respectively, supports a dissociative substitution mechanism. Whereas for the slow reaction: ΔH# = 33 ± 6 and 43 ± 3 kJ mol−1, ΔS# = −77 ± 10 and −56 ± 9 J K−1 mol−1 for 1 and 2, respectively, support an associative substitution mechanism. It is concluded from the activation parameters that the difference in structure does not affect the mechanism. Complexes 1 and 2 have also been evaluated as functional models for the catechol oxidase enzyme and phenoxazinone synthase. The model complexes 1 and 2 show catecholase activity of Kcat = 10.9 × 103 and 11.4 × 103 h−1 and phenoxazinone synthase activity of Kcat = 2.1 × 103 and 4.3 × 103 h−1, respectively. Compared to the enzyme itself (Kcat = 8.3 h−1), the model complexes 1 and 2 are promising candidates as functional mimics for catechol oxidase and phenoxazinone synthase.