Tailored design, synthesis, and catalytic aspects of mononuclear cis-dichloro copper(ii) complexes with simple DPA-derived tridentate ligands and their biomimicking activities

Abstract

The activation of molecular oxygen remains a persistent challenge to the scientific community owing to its poor selectivity, especially towards organic substrates. In biological systems, phenoxazinone synthase and catechol oxidase are two copper-based metalloenzymes that have been extensively studied because of their catalytic properties in the formation of a phenoxazinone core and quinones, which subsequently produce value-added compounds. Inspired by the design of a native catechol oxidase metalloenzyme, two copper(II) complexes (1 and 2) with dipicolylamine (DPA)-derived tridentate ligands L¹ (4-methyl-N,N-bis(pyridin-2-ylmethyl)aniline) and L² (2,4,6-trimethyl-N,N-bis(pyridin-2-ylmethyl)aniline) are synthesized. The tridentate ligands provided similar environmental facilities for attachment with three histidine moieties found in nature and our goal is to understand their biomimicking activity. On the amine nitrogen center, basicity is varied by systematic substitutions via methyl groups, thus affecting the bond strength of the apical chloride atom, which results in increased lability. The ligands were thoroughly characterized using different spectroscopic methods, i.e., FT-IR, ¹H NMR, UV-vis spectroscopy, and ESI mass spectrometry. The molecular structures of 1 and 2 were elucidated using single-crystal X-ray diffraction, which confirms their distorted square pyramidal geometries with two distinct chlorines attached in a cis fashion. The investigation of their catalytic abilities was performed under ambient aerobic conditions in methanol with both o-aminophenol and 3,5-ditertbutyl catechol, which provided kinetic values that correlated with their structural modifications. Toluene-appended 1 (k_cat 525 h⁻¹ for o-aminophenol and 612 h⁻¹ for 3,5-ditertbutyl catechol) showed almost twice the activities of mesitylene-derived 2 (k_cat 255 h⁻¹ for o-aminophenol and 343 h⁻¹ for 3,5-ditertbutyl catechol). Mass spectroscopy supported their probable mechanisms for both substrates and confirmed the involvement of molecular oxygen in the reaction. EPR studies further confirmed radical generation during adduct formation in the case of catechol oxidation. This study offers detailed information regarding metal–ligand cooperation and their biomimicking activity, which directs the importance of designing suitable bioinspired catalysts.