Solvent-induced single-crystal-to-single-crystal transformation and tunable magnetic properties of 1D azido-Cu(ii) chains with a carboxylate bridge

Abstract

By means of the solvent effect, three new azido-copper 1D coordination polymers, [Cu(4-aba)(N₃)] (1), [Cu(4-aba)(N₃)(CH₃OH)] (2), and [Cu(4-aba)(N₃)(C₂H₅OH)] (3) (4-aba = 4-azidobenzoic acid), were successfully prepared in the presence of Cu²⁺ ion, NaN₃ and 4-azidobenzoic acid. Interestingly, 1 can be employed as a precursor and transformed to 2 and 3via the coordination of methanol or ethanol, respectively. Meanwhile, the identical products of 1, namely 1a and 1b, could be obtained from both 2 and 3 by a dealcoholized process. As a result, the geometric configurations of Cu(II) ions vary from the tetracoordinated square-planar in 1 to the hexacoordinated octahedron in 2 or 3. Compound 1 displays a well-isolated 1D chain with dual-bridges of EO-azido and syn,syn-carboxylate, while isomorphic 2 and 3 are triple-bridged chain-like motifs containing EO-azido, syn,syn-carboxylate and μ2-alkanol. The structural transformations caused by the intervention of alkanol molecules modulate the intrachain Cu–Cu distances (3.570 for 1, 3.204 Å for 2 and 3.154 Å for 3) and Cu–N–Cu angles (127.3° for 1, 106.82° for 2 and 104.81° for 3). This modulation, however, further leads to different intrachain ferromagnetic interactions (J = 28.4 cm⁻¹ for 1, 67.6 cm⁻¹ for 2, 40.2 cm⁻¹ for 3) that are qualitatively demonstrated by theoretical calculation. More importantly, the significant scenarios of magnetic ordering and slow magnetic relaxation, which are infrequent in most of the reported azido-copper cases, are only observed in 2 and 3, due to the distinct interchain networks among 1–3. In addition, heat-capacity measurements highlight the characteristics of long-range ferromagnetic ordering in 2 and 3.