Reciprocating thermal behaviour in a series of 7-coordinate Mn(ii) and Co(ii) complexes

Abstract

Solvothermal reactions of tetradentate ligands, H₂L¹–H₂L³, with hydrated Mn(ClO₄)₂ and hydrated Co(NO₃)₂ afforded the mononuclear neutral compounds [Mn^II(L¹)(H₂O)₃]·3H₂O (1), [Mn^II(L²)(H₂O)₃]·3H₂O (2), [Mn^II(L³)(H₂O)₂(CH₃CH₂OH)]·3H₂O·CH₃CH₂OH (3), and [Co^II(L¹)(H₂O)₃]·3H₂O (4) (H₂L¹ = 1,10-phenanthroline-2,9-dicarboxylic acid, H₂L² = 2,2′-bipyridine-6,6′-dicarboxylic acid, and H₂L³ = 6,6′-bis(1H-tetrazol-5-yl)-2,2′-bipyridine), respectively, in moderate yields. All the compounds were characterized by X-ray crystallography and all the metal centres were coordinated by a tetradentate ligand and three solvent molecules in a distorted 7-coordinate pentagonal bipyramidal geometry. Through magnetic measurements, 1–4 show interesting slow magnetic relaxation under an external direct current (dc) field. It is noted that the temperature dependence of the relaxation time (τ) in all the compounds exhibits the ‘reciprocating thermal behaviour’ (RTB), where the weak inter-molecular interactions in the solid state are responsible for this strange phenomenon.