Assembling metals (CoII and MnII) with pyridylcarboxylates in the presence of azide: synthesis, structural aspects and magnetic behavior of three coordination polymers

Abstract

The reaction between Co(NO₃)₂·6H₂O and substituted pyridylcarboxylic acid [nicotinic acid (Hnic) or trans-3-pyridylacrylic acid (Htpa)] in the presence of NaN₃ under hydrothermal conditions yielded [Co_1.5(nic)₂ (Hnic)(N₃)]_n (1) and [Co_1.5(tpa)₂ (N₃)(H₂O)]_n (2), respectively. Single crystal structure analyses reveal that both complexes are 3D complicated coordination polymers. The basic repeating units in both of the complexes are Co₃ trinuclear clusters containing syn–syn bridging carboxylate and end-on azido linker. A similar reaction using MnCl₂·4H₂O in presence of equimolar amounts of Htpa and NaN₃ yielded a 2D corrugated sheet [Mn(tpa)₂]_n (3) containing no azide. Complex 3 can also be synthesized under hydrothermal conditions using Natpa in the absence of NaN₃. Surprisingly, the same reaction at room temperature yielded a known mononuclear complex [Mn(tpa)₂(H₂O)₄]. Variable temperature magnetic studies down to 2 K revealed the dominant antiferromagnetic nature of the first two complexes with a ferrimagnetic type of behavior despite the facts that they are homometallic and homospin systems. The susceptibility data in both cases were analyzed by a Co₃ trinuclear model as well as considering inter-trimer interactions. Complex 3 is weakly antiferromagnetic in nature with an exchange parameter of J = −2 cm⁻¹ through the syn–anti bridging carboxylate pathway.