Three 3-amino-1,2,4-triazole-based cobalt(II) complexes incorporating with different carboxylate coligands: synthesis,crystal structures,and magnetic behavior

Three 3-amino-1,2,4-triazole (Hatz)-based paramagnetic metal complexes, {[Co₂(Hatz)₂(nip)₂]·H₂O}_n (1), [Co(atz)(nb)]_n (2), and {[Co₃(H₂O)₄(Hatz)₆(btc)₂]·11.5H₂O} (3) (nip^2? = 5-nitroisophathalate, nb^– = p-nitrobenzolate, and btc^3– = 1,2,4-benzenetricarboxylate), were respectively prepared by introducing differently carboxylate-containing rigid coligands. All these complexes were structurally and magnetically characterized by single-crystal and powder X-ray diffractions, elemental analysis, FT-IR spectra, thermogravimetric and magnetic measurements. Complex 1 has a four-connected CdSO₄-type framework with binuclear subunits periodically extended by ditopic μ-N1, N3-Hatz and carboxylate-containing linkers. Tetrahedral Co^II ions in 2 are periodically interlinked into an undulated layer by anionic μ₃-N1,N2,N3-atz^? connectors with deprotonated nb^– spacers located on the both sides. By contrast, complex 3 is a centrosymmetric trinuclear molecule aggregated by six neutral μ-N1, N3-Hatz molecules. The structural difference of 1–3 is significantly due to the flexible binding modes adopted by triazolyl and carboxylate groups. Additionally, 1 exhibits a field-induced metamagnetic transition from an antiferromagnetic ordering to a weak ferromagnetic state resulting from the magnetic competition between triazolyl and carboxylate mediators. Instead, comparable antiferromagnetic couplings transmitted by cyclic triazolyl groups are observed in the nearest neighbors of 2 and 3.