Dalton Transactions PAPER Cite this: Dalton Trans., 2018, 47, 7013 Received 18th January 2018, Accepted 18th April 2018 DOI: 10.1039/c8dt00227d rsc.li/dalton Selecting the spin crossover profile with controlled crystallization of mononuclear Fe(III) polymorphs†‡ Ana I. Vicente, a,b Liliana P. Ferreira, b,c Maria de Deus Carvalho, a Vítor H. N. Rodrigues, c Marinela M. Dîrtu, d,e Yann Garcia, d Maria José Calhorda a,b and Paulo N. Martinho * a,b Two polymorphic species of the [Fe(5-Br-salEen) 2 ]ClO 4 compound were obtained, each of them being selectively recovered after evaporation of the solvent at a controlled rate. While polymorph 1a is formed during slow evaporation, fast evaporation favors polymorph 1b. The importance of the evaporation rate was recognized after detailed studies of the reaction temperature, solvent evaporation rate and crystalliza- tion temperature effects. The complex in the new polymorphic form 1a showed an abrupt spin crossover at 172 K with a small 1 K hysteresis window and over a narrow 10 K range. 57 Fe Mössbauer spectroscopy and differential scanning calorimetry, complemented by X-ray studies for both the high-spin and low- spin forms, were used to further characterize the new polymorphic phase 1a. Both polymorphs are based on the same Fe(III) complex cation hydrogen bonded to the perchlorate anion. These units are loosely bound in the crystals via weak interactions. In the new polymorph 1a, the hydrogen bonds are stronger, while the weak hydrogen and halogen bonds, as well as π–π stacking, create a cooperative network, not present in 1b, responsible for the spin transition profile. Introduction Polymorphism, or crystal isomerism, results from the capability of a given compound to produce two or more crystalline forms which differ in the packing arrangement of the molecules or ions in the crystal lattice. 1 Understanding polymorphism requires studies of crystallization and phase transition, among others, and is particularly relevant in materials synthesis and biomineralization, and in the manufacture of drugs. The role of crystal polymorphs is therefore important for the spin crossover (SCO) research community. This phenomenon can be observed for d 4 to d 7 3d transition metals in an octahedral ligand environment, 2 and is associated with switching spin states. It is highly sensitive to polymorphism since the different environ- ment in the crystal may lead to diverse SCO behaviors that can be analyzed in terms of intermolecular interactions. 3 Polymorphism associated with SCO may help to understand the relative contribution of intra- and intermolecular interactions in the spin transition features and consequently to define strat- egies to obtain and isolate each polymorphic phase. 4–6 Thermal spin conversion may drastically change between different polymorphs, going from a crossover (gradual spin- state conversion between high-spin (HS) and low-spin (LS)) to a first-order transition (discontinuous variation of the HS frac- tion). This has been observed for mononuclear Fe(II) complexes 7–10 with some cases where only one of the poly- morphs shows a thermal switching. 11–15 † In memory of Maria de Deus Carvalho. ‡ Electronic supplementary information (ESI) available: Additional figures, tables and crystallographic data. CCDC 1579737 and 1579738. For ESI and crys- tallographic data in CIF or other electronic format see DOI: 10.1039/c8dt00227d a Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal. E-mail: pnmartinho@fc.ul.pt b Biosystems and Integrative Sciences Institute (BioISI), Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal c Physics Department, University of Coimbra, 3004-516 Coimbra, Portugal d Institute of Condensed Matter and Nanosciences, Molecules, Solids and Reactivity (IMCN/MOST), Université Catholique de Louvain, Place L. Pasteur 1, 1348 Louvain- la-Neuve, Belgium e Faculty of Electrical Engineering and Computer Science & MANSiD Research Center, Stefancel Mare University, Suceava, Romania This journal is © The Royal Society of Chemistry 2018 Dalton Trans. , 2018, 47, 7013–7019 | 7013 Published on 18 April 2018. Downloaded by Universidade de Coimbra on 11/12/2018 10:16:55 AM. View Article Online View Journal | View Issue