Different magnetic responses observed in Co II4, Co II3 and Co II1-based MOFs

Abstract

Four magnetic MOFs with anisotropic Co^II ions, {[Co₅(H₂O)₂(μ₃-OH)₂(atz)₂(stp)₂]·1.5H₂O}_n (1), {[Co₅(H₂O)₂(μ₃-OH)₂(trz)₂(stp)₂]·1.3H₂O}_n (2), {[Co₅(H₂O)₆(μ₃-OH)₂(trz)₂(stp)₂]·2.5CH₃OH}_n (3) and {[Co₃(H₂O)₄(Hdatrz)₂(stp)₂]·3H₂O}_n (4) (stp³⁻ = 2-sulfoterephthalate, trz⁻ = 1,2,4-triazolate, atz⁻ = 3-amino-1,2,4-triazolate and Hdatrz = 3,5-diamino-1,2,4-triazole) were solvothermally isolated by varying the substituent groups appended on the N-heterocyclic triazole and structurally and magnetically characterized. Structural analyses indicate that the former two complexes are crystallographically isostructural, exhibiting pillared-layer frameworks with mixed triazolyl and carboxylate extended Co^II₄ + Co^II₁ layers supported by rigid stp³⁻ connectors. Complex 3 is built from butterfly-shaped Co^II₄ cluster-based layers, which are interconnected with single cobalt(II) octahedra by ditopic stp³⁻ bridges. By contrast, complex 4 consists of linear {Co₃(μ-N1,N4-Hdatrz)₂} subunits, which are extended by 3-connected stp³⁻ linkers into a stable 3D framework. Magnetically, 1 exhibits ferromagnetic ordering below 2.7 K due to the well-organized alignment of the non-compensated resultant moment from octahedra and tetrahedral cobalt(II) carriers, while 3 is in a non-zero paramagnetic state above 2.0 K resulting from the coexistence of intermetallic ferromagnetic and antiferromagnetic interactions. The magnetic competition between weak inter-subunit antiferromagnetic interactions and the external magnetic field makes 4 behave as a field-induced metamagnet with a critical field of 27.5 kOe. These interesting magnetostructural results suggest that the anisotropy of the moment carrier and the interlayer/intersubunit separations significantly dominate the magnetic responses in extended MOFs, providing an informative platform for the further development of interesting magnetic materials, both of academic and industrial interest.