Effects on the magnetic interaction caused by molecular recognition in complexes of 1,2-azole-based oxamate and [Cu(bpca)]+ units

Abstract

This work explores the magneto–structural relationships and crystal engineering in five copper(II) complexes of formulas [Cu(bpca)(H₂L¹)]_n·n(CH₃)₂SO·3nH₂O (1), [Cu(bpca)(H₂L¹)]_n·3nCH₃OH (2), [Cu(bpca){Cu(bpca)(H₂L¹)}(H₂L¹)]_n·0.5n(CH₂OH)₂·3.5nH₂O (3) and [Cu(bpca){Cu(bpca)(H₂L¹)}]_n(NO₃)_n·2n(CH₂OH)₂·nH₂O (4) and [{Cu(bpca)}₂L²]·2H₂O·0.33(CH₂OH)₂ (5) [Hbpca = bis(2-pyridylcarbonyl)amide, H₃L¹ = 4-(1H-pyrazole-4-yl)phenylene-N-(oxamic acid) and H₂L² = 4-(1,2-oxazol-4-yl)phenylene-N-(oxamic acid)]. They are constituted by [Cu(bpca)]⁺ units and two oxamate-based ligands, one of them containing pyrazole substituents (1–4) and the other one an isoxazole group (5). Compounds 1–4 are one-dimensional coordination polymers. 1 and 2 are coordination isomers with different solvates and the first examples of monodentate oxamate ligands. 3 exhibits a novel carboxylate(oxamate) coordination mode in the syn–anti conformation and the oxamate group in 4 lies between the previous coordination modes, adopting a bidentate/monodentate bridging form. They all have the pyrazole group as a probe for molecular recognition, especially recognizing the deprotonated carboxylate portion of the oxamate via hydrogen bonds. The new coordination modes of the oxamate groups in 1–3 result from the presence of pyrazole and the synthesis in different solvent mixtures. 5 is a dicopper(II) complex where the oxamate group exhibits the bis(bidentate) bridging mode. There is no possibility of molecular recognition in 5 because of the lack of hydrogen bond donors and it can be considered as a counterpoint to investigate the results of L¹-based complexes. An insight based on crystal engineering concerning the nature of the ligands and their synthetic routes was provided. Cryomagnetic measurements on 1, 3, and 5 in the temperature range 1.9–300 K revealed the occurrence of weak antiferromagnetic interactions [J = −0.86 (1), −0.60 (3) and −1.25 cm⁻¹ (5)]. Orbital overlap considerations based on their crystal structures originating from the different molecular recognition of the pyrazole/isoxazole groups were used to account for these small magnetic couplings.