Self-Assembly of Oxamide Ligand-Based Cu(II) Complexes into Mononuclear, Polymeric, and 16-MCCu(II)-8 Metallacrown Frameworks: Structural, Magnetic, and Catalytic Insights

Abstract

Three structurally diverse copper complexes in the form of mononuclear {[Cu(L)(H2O)2] (1)}, coordination polymer {[Cu(L)(OH)2Li2]n (2)}, and a 16-MCCu(II)-8 Metallacrown {[KCu8(L)4(μ2-OH)8Cl3(H2O)]·7DMF·H2O·0.5Cl (3)} derived from a benzothiazole-based oxalamide ligand were synthesized by optimizing the reaction conditions. The electrocatalytic activities of complexes 2 and 3 were examined for the hydrogen evolution reaction (HER). The heterometallic polymeric structure 2 shows a significantly lower Tafel slope (93 mV dec–1) and overpotential (212 mV), indicating its excellent electrocatalytic properties for HER. The higher oxidation potential of 2 compared to 3 obtained from the cyclic voltammetry study indicates that complex 2 is more readily oxidized, reflecting its greater propensity for electron transfer during the hydrogen evolution reaction. DFT calculations consisting of three complementary analyses (NPA, NBO, and ESP) suggest that incorporation of Li induces higher electronic polarization and charge redistribution within the Cu coordination framework, providing a consistent and correlative electronic-structure rationale for the experimentally observed enhancement in hydrogen evolution activity of the Cu–Li (2) catalyst. Owing to the structural resemblance to catechol oxidase, complex 3 was tested for the oxidation of catechol. In the presence of a catalytic amount of 3, 3,5-di-tert-butylcatechol oxidases to 3,5-di-tert-butylquinone with good catalytic efficiency, indicating the biomimetic ability of the complex. Variable-temperature magnetic susceptibility and field-dependent magnetization measurements evidence dominant antiferromagnetic interactions within the Cu8 framework of complex 3. The magnetic data are satisfactorily modeled using an isotropic Heisenberg spin Hamiltonian incorporating two exchange constants, affording coupling parameters of J1 = −65 cm–1 for the μ-O-bridged Cu–Cu pairs and a stronger J2 = −80 cm–1 for the mixed μ-O/μ-Cl pathways. The enhanced antiferromagnetic coupling associated with the dual-bridged motifs is rationalized in terms of cooperative superexchange routes and favorable magneto-structural parameters, highlighting that subtle variations in bridging mode and geometry can effectively tune magnetic exchange interactions.