Formation of two drastically different MOFs based on Mn(ii)–benzoate and pyrazine with a change in seasonal temperature: structural analysis and magnetic study

Abstract

Two new stoichiometrically and structurally very different Mn(II) coordination compounds {[Mn(pyz)(C₆H₅COO)₂](C₂H₅OH)}_n (1) and [Mn₅(pyz)(C₆H₅COO)₁₀(H₂O)₂]_n (2) (where pyz = pyrazine) have been synthesized by the reaction of Mn(C₆H₅COO)₂·4H₂O and pyz in 1 : 1 molar ratio in different seasons of the year. The structural analyses reveal that complex 1, which is synthesized in summer when the temperature remains around 28–40 °C, features a 1D molecular ladder formed by joining of syn–syn benzoate-bridged dimeric Mn(II) units by pyz. Complex 2, which is obtained in winter (temperature around 10–20 °C), is an unprecedented 2D rectangular sheet motif based on a 1D column constructed from syn–syn benzoate bridged repetitive Mn₅ units which are further interconnected by the pyz spacer. The compounds are interconvertible in solution: when an ethanol solution of 1 is allowed to evaporate slowly in winter or in a refrigerator (5–6 °C) it transformed into 2 whereas crystallization of 2 from ethanol medium in summer (28–40 °C) yields 1. Variable-temperature (1.8–300 K) magnetic susceptibility measurements showed presence of weak antiferromagnetic coupling in 1 that has been fitted with the van Vleck equation. The interaction in benzoate-bridged Mn₂ unit was characterized to be J = −0.586(1) cm⁻¹, where the spin-Hamiltonian is defined as H = −2JS₁S₂. The interaction in 2 was also characterized with the modified van Vleck equation to be weakly antiferromagnetic.