We report aseries of meltable Fe II complexes,which, depending on the length of aliphatic chains,d isplaya brupt forwardl ow-spin to high-spin transition or unprecedented melting-triggered reverse high-spin to low-spin transition on temperature rise.T he reverse spin transition is perfectly reproducible on thermal cycling and the obtained materials are easily processable in the form of thin film owing to their soft-matter nature.W ef ound that the discovered approach represents ap otentially generalizable new avenue to control both the location in temperature and the direction of the spin transition in meltable compounds. Scheme 1. Synthetic pathway for the pyN-n and pm2-n.

Hysteretic spin crossover in coordination complexes of 3d-metal ions represents one of the most spectacular phenomena of molecular bistability. In this paper we describe a self-assembly of pyrazine (pz) and Fe­(BH3CN)2 that afforded the new 2D coordination polymer [Fe­(pz)2(BH3CN)2]∞. It undergoes an abrupt, hysteretic spin crossover (SCO) with a T 1/2 of 338 K (heating) and 326 K (cooling) according to magnetic susceptibility measurements. Mössbauer spectroscopy revealed a complete transition between the low-spin (LS) and the high-spin (HS) states of the iron centers. This LS-to-HS transition induced an increase of the unit cell volume by 10.6%. Meanwhile, a modulation of multiple [C–Hδ+···Hδ−–B] dihydrogen bonds stimulates a contraction in direction c (2.2%). The simplicity of the synthesis, mild temperatures of transition, a pronounced thermochromism, stability upon thermal cycling, a striking volume expansion upon SCO, and an easy processability to composite films make this new complex an attractive material for switchable components of diverse applications.

Here we describe the elaboration and investigation of composites prepared from the spincrossover (SCO) complex [Fe(pyrazine){Au(CN) 2 } 2 ] and poly(vinylidene fluoride) matrix, with different contents of the active phase (10 -35 wt%). Optical measurements demonstrated that all composites preserve temperature induced hysteretic spin transitions. Tensile mechanical analysis showed a non-linear change of the Young's modulus upon increase of the complex content.Thermomechanical analysis upon a constant strain demonstrated pronouncable alterations of the applied stress in the SCO region. Composites with smaller loads require less stress to attain a given strain in the high-spin state, however for those with high loads the behaviour is more sofisticated. The modelling of stress vs. temperature behaviour revealed that this may be interpreted through a miscellaneous interplay of expansion of both the polymer and the complex, the contribution of both components to the elastic properties of composites, and the effects of SCO on these properties.

We report aseries of meltable Fe II complexes,which, depending on the length of aliphatic chains,d isplaya brupt forwardl ow-spin to high-spin transition or unprecedented melting-triggered reverse high-spin to low-spin transition on temperature rise.T he reverse spin transition is perfectly reproducible on thermal cycling and the obtained materials are easily processable in the form of thin film owing to their soft-matter nature.W ef ound that the discovered approach represents ap otentially generalizable new avenue to control both the location in temperature and the direction of the spin transition in meltable compounds. Scheme 1. Synthetic pathway for the pyN-n and pm2-n.