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

In quest of copper complexes having [CuN3O] cores, [Cu(phen)(L-Val)(H2O)]NO3 (1) and [Cu(bpy)(L-Val)]CIO4 (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 intramolecular 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: Delta H-# = 68 +/- 4 and 73 +/- 5 kJ mol(-1) , Delta S-# = 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: Delta H-# = 33 +/- 6 and 43 +/- 3 kJ mol(-1) , Delta S-# = -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 K-cat= 10.9 x 10(3) and 11.4 x 10(3) h(-1) and phenoxazinone synthase activity of K-cat = 2.1 x 10(3) and 4.3 x 10(3) h(-1) , respectively. Compared to the enzyme itself (K-cat = 8.3 h(-1) ), the model complexes 1 and 2 are promising candidates as functional mimics for catechol oxidase and phenoxazinone synthase.