Influence of the secondary coordination sphere on the physical properties of mononuclear copper(II) complexes and their catalytic activity on the oxidation of 3,5-di-tert-butylcatechol

• Published in: Polyhedron 2018-02, Vol.141, p.30-36
• Main Authors: Posada, Nelly B.M., Guimarães, Marcus A., Padilha, Diego S., Resende, Jackson A.L.C., Faria, Roberto B., Lanznaster, Mauricio, Amado, Roberto S., Scarpellini, Marciela
• Format: Article
• Language: English
• Subjects: 3,5-DTBC; Catalyst; Coordination chemistry; Copper(II) complex; Oxidation
• ISSN: 0277-5387
• DOI: 10.1016/j.poly.2017.11.020

Three copper(II) complexes derived from 2-(2-aminoethyl)pyridine (aep-R, where R = H, C1; R = tiophene, C2 and R = benzene, C3) were prepared in order to understand the influence of the pendant groups on the electronic, redox and catalytic properties in the oxidation of 3,5-DTBC. The most efficient complex is C3 (E = 72.9 L mol−1 s−1) which presents first order related to the complex. [Display omitted] Three mononuclear copper(II) complexes (C1–C3) containing the ligand 2-(2-aminomethyl)pyridine (aep) and some modifications of it were designed to get insights on the influence of pendant second sphere groups (thiophene and benzene) on their physicochemical and catalytic properties. Complexes C1–C3 were synthesized reacting copper(II) chloride with the precursor ligand LP1, N-(2-(pyridin-2-yl)ethylidene)-1-(thiophen-2-yl)methanamine, and the ligands L2, 2-(pyridin-2-yl)-N-(thiophen-2-y-lmethyl)ethanamine, and L3, N-benzyl-2-(pyridin-2-yl)ethanamine, respectively. The complexes were isolated and characterized by several methods. Single crystal X-ray crystallographic studies revealed that C1 results from the cleavage of the Schiff base PL1, yielding the distorted square planar copper(II) complex coordinated to one molecule of aep and two chloride ions, which have been already published. All complexes present the same coordination sphere; C2 and C3 have also pendant arms. These groups induce singular crystal packing by different intermolecular interactions, which correlates with the decomposition temperatures of the complexes. In addition, electronic, redox and catalytic properties are also influenced. Complex C3 is the most efficient in the oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) and presents catalytic efficiency of 72.9 L mol−1 s−1. It is 4.5 and 11-fold greater than those determined for C1 and C2, respectively. Complexes reaction orders of 0.70 ± 0.04 for C1, 1.6 ± 0.1 for C2 and 0.913 ± 0.002 for C3 indicate different catalytic species in each case, with mechanisms involving more than one active species for C1 and C2. Different mechanisms can explain the catalytic behavior of these complexes.