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, k_cat, 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 L2^1–3. On reaction of copper­(II) nitrate with L2^1–3 in situ complexes 3, 12, and 13 are formed having general formula Cu₂(L2^1–3)₂(NO₃)₂. The remaining 12 ligands obtained as 1 + 2 condensation products are denoted as L1^1–12, which produce complexes having general formula Cu₂(L1^1–12)­(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, k_cat, 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 Cu^II to Cu^I. Our study reveals that apparently there is nearly no linear relationship between k_cat 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 Cu^II to Cu^I reduction process and k_cat values, as revealed from the combined theoretical and experimental approach.