Relation between the Catalytic Efficiency of the Synthetic Analogues of Catechol Oxidase with Their Electrochemical Property in the Free State and Substrate-Bound State

A library of 15 dicopper complexes as synthetic analogues of catechol oxidase has been synthesized with the aim to determine the relationship between the electrochemical behavior of the dicopper­(II) species in the absence as well as in the presence of 3,5-di-tert-butylcatechol (3,5-DTBC) as model substrate and the catalytic activity, kcat, in DMSO medium. The complexes have been characterized by routine physicochemical techniques as well as by X-ray single-crystal structure analysis in some cases. Fifteen “end-off” compartmental ligands have been designed as 1 + 2 Schiff-base condensation product of 2,6-diformyl-4-R-phenol (R = Me, tBu, and Cl) and five different amines, N-(2-aminoethyl)­piperazine, N-(2-aminoethyl)­pyrrolidine, N-(2-aminoethyl)­morpholine, N-(3-aminopropyl)­morpholine, and N-(2-aminoethyl)­piperidine. Interestingly, in case of the combination of 2,6-diformyl-4-methylphenol and N-(2-aminoethyl)­morpholine/N-(3-aminopropyl)­morpholine/N-(2-aminoethyl)­piperidine 1 + 1 condensation becomes the reality and the ligands are denoted as L21–3. On reaction of copper­(II) nitrate with L21–3 in situ complexes 3, 12, and 13 are formed having general formula Cu2(L21–3)2(NO3)2. The remaining 12 ligands obtained as 1 + 2 condensation products are denoted as L11–12, which produce complexes having general formula Cu2(L11–12)­(NO3)2. Catecholase activity of all 15 complexes has been investigated in DMSO medium using 3,5-DTBC as model substrate. Treatment on the basis of Michaelis–Menten model has been applied for kinetic study, and thereby turnover number, kcat, values have been evaluated. Cyclic voltametric (CV) and differential pulse voltametric (DPV) studies of the complexes in the presence as well as in the absence of 3,5-DTBC have been thoroughly investigated in DMSO medium. From those studies it is evident that oxidation of 3,5-DTBC catalyzed by dicopper­(II) complexes proceed via two steps: first, semibenzoquinone followed by benzoquinone with concomitant reduction of CuII to CuI. Our study reveals that apparently there is nearly no linear relationship between kcat and E° values of the complexes. However, a detailed density functional theory (DFT) calculation sheds light on this subject. A very good correlation prevails in terms of the energetics associated with the CuII to CuI reduction process and kcat values, as revealed from the combined theoretical and experimental approach.