A mononuclear iron(ii) complex: cooperativity, kinetics and activation energy of the solvent-dependent spin transitionElectronic supplementary information (ESI) available: Experimental details, magnetic data, DSC data, TGA data, IR spectroscopy data, crystallographic data, X-ray powder diffraction data, fitting of magnetic data. CCDC 1409266-1409268. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt03750f

The system [FeL 2 ](BF 4 ) 2 ( 1 )-EtOH-H 2 O (L is 4-(3,5-dimethyl-1 H -pyrazol-1-yl)-2-(pyridin-2-yl)-6-methylpyrimidine) shows a complicated balance between the relative stabilities of solvatomorphs and polymorphs of the complex [FeL 2 ](BF 4 ) 2 . New solvatomorphs, 1 LS · EtOH · H 2 O and β- 1 LS · x H 2 O , were isolated in this system. They were converted into four daughter phases, 1 A/LS , 1 D/LS , 1 E/LS · y EtOH · z H 2 O and 1 F/LS . On thermal cycling in sealed ampoules, the phases 1 LS · EtOH · H 2 O and β- 1 LS · x H 2 O transform into the anhydrous phase 1 A/LS . The hysteresis loop width for the 1 A/LS ↔ 1 A/HS spin transition depends on the water and ethanol contents in the ampoule and varies from ca. 30 K up to 145 K. The reproducible hysteresis loop of 145 K is the widest ever reported one for a spin crossover complex. The phase 1 A/LS combines the outstanding spin crossover properties with thermal robustness allowing for multiple cycling in sealed ampoules without degradation. The kinetics of the 1 A/LS → 1 A/HS transition is sigmoidal which is indicative of strong cooperative interactions. The cooperativity of the 1 A/LS → 1 A/HS transition is related to the formation of a 2D supramolecular structure of the phase 1 A/LS . The activation energy for the spin transition is very high (hundreds of kJ mol −1 ). The kinetics of the 1 A/HS → 1 A/LS transition can either be sigmoidal or exponential depending on the water and ethanol contents in the ampoule. The phases 1 D/LS and 1 F/LS show gradual crossover, whereas the phase 1 E/LS · y EtOH · y H 2 O shows a reversible hysteretic transition associated with the solvent molecule release and uptake. A mononuclear iron( ii ) pyrimidine-based complex shows remarkable spin crossover properties and unprecedented thermal robustness.