Antagonist elastic interactions tuning spin crossover and LIESST behaviours in FeII trinuclear-based one-dimensional chains

Abstract

A new 1D coordination polymer, [Fe₃(μ₂-bntrz)₆(bntrz)₂(μ₂-tcnsme)₂](tcnsme)₄·4H₂O (1·4H₂O), based on [Fe₃(μ₂-bntrz)₆] trinuclear units covalently linked by symmetric double cyanocarbanion coligands involving 12-membered metallacycles has been prepared and characterised. The crystal structure of 1·4H₂O revealed a trinuclear fragment, [Fe₃(μ₂-bntrz)₆(bntrz)₂(μ₂-tcnsme)₂]⁴⁺, composed of a central Fe^II ion (Fe1) and two external metal ions (Fe2), similar to that described for the discrete trinuclear complex [Fe₃(μ₂-bntrz)₆(tcnset)₆] (2) previously reported. Magnetic studies of 1·4H₂O showed a continuous gradual decrease of the magnetic signal, characteristic of the presence of a complete one-step gradual HS to LS transition (T_1/2 = 281 K) different from the abrupt one described for 2. This difference has been attributed to the presence of competing ferro- and anti-ferroelastic interactions, induced, respectively, by the intra- (μ₂-bntrz) and inter-trimer (μ₂-tcnsme) covalent links in 1·4H₂O. TGA and powder X-ray diffraction analyses revealed that 1·4H₂O shows a complete and reversible “dehydration (1)/hydration (1·4H₂O)” process that occurs, respectively, by heating 1·4H₂O at 370 K and by soaking in water the dehydrated sample (1). The magnetic behaviour of 1 exhibits a two-step SCO transition at ca. 245 K. Thanks to the combined magnetic and vibrational infrared data allowing correct extraction of the contribution of the two Fe^II active centres (one central Fe1 and two external Fe2 ions), the first step (370–245 K) was attributed to a concomitant HS/LS switching of the central Fe1 and a half of the Fe2 ions and the second one, occurring below 245 K, was attributed to the HS/LS transition of the remaining HS state among the randomly distributed Fe2 external ions. Photomagnetic experiments have been performed for both hydrated (1·4H₂O) and dehydrated (1) phases. As expected from their thermal transition temperatures, no photomagnetic response was observed for 1·4H₂O at 638 nm nor at 450 nm, while the dehydrated phase (1) shows an increase of the magnetic moment, reaching a maximum of 1.4 cm³ K mol⁻¹ at 22 K, followed by the thermally-induced HS to LS relaxation with T_LIESST = 41 K.