Spin Crossover, Polymorphism and Porosity to Liquid Solvent in Heteroleptic Iron(III) {Quinolylsalicylaldimine/Thiosemicarbazone-Salicylaldimine} Complexes

Heteroleptic iron(III) complexes of formula [Fe(qsal)(thsa)]⋅solvent have been synthesized: [Fe(qsal)(thsa)]⋅0.4 BuOH (1), [Fe(qsal)(thsa)]⋅0.5 MeCN (2) and [Fe(qsal)(thsa)]⋅0.5 THF, (3). The latter two show partial solvent loss at room temperature to yield [Fe(qsal)(thsa)]⋅0.1 MeCN (2') and [Fe(qsal)(thsa)]⋅0.1 THF (3'), respectively. This family maintains a structural integrity which is analogous over different degrees of solvation, a rare occurrence in discrete molecular species. Uniquely, removal of MeCN from compound 2 leads to retention of crystallinity yielding the isostructural, fully desolvated compound [Fe(qsal)(thsa)] (2'') and a new high spin polymorph, 4. To the best of our knowledge, this is the first compound that forms polymorphs through a desolvation process. The desolvated mixture, 2'' and 4, is porous and can reabsorb MeCN and give rise to 2' again. This illustrates the reversible single-crystal-to-single-crystal transformation of two polymorphs back to a purely original phase, 2''+4↔2'. The structural, magnetic and Mőssbauer features of the various samples are described in terms of spin crossover.