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

Abstract 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.