Multiresponsive spin crossover driven by rotation of tetraphenylborate anion in an iron(III) complex

Abstract

Dynamic molecular materials, which involve mechanical motions in crystals, usually exhibit tunable properties related to conformational polymorphs. Herein, we describe a solvent-free molecular crystal [{FeIII(salten)}2(TPB)](BPh4)2 (I) developed by associating the rotatable tetraphenylborate anion with the spin crossover (SCO) binuclear iron(III) cation. The solvent-free phase I can undergo a temperature-induced phase transformation to phase II, during which rotation of the BPh4− anion interplays with the SCO component, giving rise to fascinating variations in SCO (from gradual to abrupt) and dielectric properties. Most importantly, an unexpected polymorphic transformation from II to III or IV, with increasing magnetic hysteresis, is realized by exposing II to water or ethanol vapor; the transition is reversible by heating. This rare successive transformation is further rationalized by theoretical calculations. Hence, this multiresponse system provides a new way for modulating synergistic effects and designing dynamic SCO materials by integrating intramolecular motions in the crtystal lattice.