Neutral Molecular Iron(II) Complexes Showing Tunable Bistability at Above, Below, and Just Room Temperature by a Crystal Engineering Approach: Ligand Mobility into a Three-Dimensional Flexible Supramolecular Network

Room temperature (RT) bistable switching materials continue to fascinate the scientists since they can be utilized for a new class of molecular-based switches or memories. While the spin crossover (SCO) compound is categorized into these attractive materials, designing of a SCO system showing desirable bistability (i.e., wide hysteresis loop spanning RT) in a rational way is still a very challenging issue. We report herein a new family of neutral molecular iron­(II) complexes showing hysteretic SCO in a wide range of switching temperatures (239–409 K) and hysteresis widths (1–31 K) spanning RT. These materials were obtained as single crystals of two solvent-free compounds [Fe^II(ptm₂-dmpn)­(NCS)₂] (<b>1</b>; ptm₂-dmpn = <i>N</i>,<i>N′</i>-bis­[(1-phenyl-1<i>H</i>-1,2,3-triazol-4-yl)­methylene]-2,2-dimethylpropane-1,3-diamine) and [Fe^II(p-ttm₂-dmpn)­(NCS)₂] (<b>2</b>; p-ttm₂-dmpn = <i>N</i>,<i>N′</i>-bis­[(1-<i>p</i>-tolyl-1<i>H</i>-1,2,3-triazol-4-yl)­methylene]-2,2-dimethylpropane-1,3-diamine), and two solvatomorphs [Fe^II(p-ttm₂-etpn)­(NCS)₂]­·solvent (p-ttm₂-etpn = <i>N</i>,<i>N′</i>-bis­[(1-<i>p</i>-tolyl-1<i>H</i>-1,2,3-triazol-4-yl)­methylene]-1-ethylpropane-1,3-diamine, and solvent = 0.5H₂O and 0.5MeCN­·0.5MeOH­·H₂O for <b>3a</b> and <b>3b</b>, respectively). All compounds are constructed of a three-dimensional (3D) flexible supramolecular network by multiple weak CH···S hydrogen bonds incorporating an additional orderly dimensional structure by aromatic interactions (i.e., π–π and CH···π interactions) between adjacent aromatic rings of tetradentate ligands. These 3D networks can accommodate a number of molecular motions such as (1) NCS bending, (2) tetradentate ligand biting, (3) aromatic ring rotation, and (4) propylene fragment oscillation to various degrees depending on a slight modification of small alkyl substituents and lattice solvents. The present crystal engineering approach of introducing concerted multimolecular motions into a 3D flexible network can be a new designing concept for controlling switching temperature and cooperativity in a systematic way toward the discovery of unprecedented RT bistable SCO materials