Influence of reaction temperature and stoichiometry on the coordination mode of a multidentate pyridylpyrazole ligand in Co(ii) complexes: from a 0D mononuclear structure to 3D frameworks

Abstract

One-pot solvothermal reactions of CoCl₂·6H₂O with the semi-rigid ligand 3-(pyridin-4-yl)-1H-pyrazole-5-carboxylic acid under different reaction temperatures and stoichiometry conditions gave rise to three new complexes: [Co(Hppza)₂(H₂O)₂] (1), [Co(Hppza)₂]_n (2) and [Co(Hppza)(H₂O)Cl]_n (3), which were characterized by elemental analysis, FT-IR spectroscopy, UV-vis absorption spectroscopy, thermogravimetric analysis (TGA) and X-ray diffraction methods. A comparative study revealed the marked sensitivity of the structural chemistry of the above-mentioned ligand to the synthesis conditions. Complex 1, which exhibited a zero-dimensional (0D) structure of mononuclear cobalt(II), was formed at 120 °C with a metal-to-ligand molar ratio of 2 : 1. A further increase in the reaction temperature to 130 °C resulted in the two-dimensional (2D) structure of 2. Moreover, the three-dimensional (3D) structure of 3 was formed when the metal-to-ligand molar ratio was changed to 1 : 1 at 130 °C. The magnetic measurements of 3 showed weak ferromagnetic coupling with the decrease in temperature from 8 to 30 K and antiferromagnetic interaction among the Co^II ions. In addition, the luminescence and electrogenerated chemiluminescence (ECL) behavior of 1–3 were also studied.