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(ptm(2)-dmpn)(NCS)(2)] (1; ptm(2)-dmpn = N,N '-bis[(1-phenyl-1H-1,2,3-triazol-4-yl)methylene]-2,2-dimethylpropane-1,3-diamine) and [Fe-II(p-ttm(2)-dmpn)(NCS)(2)] (2; p-ttm(2)-dmpn = N,N '-bis[(1-p-tolyl-1H-1,2,3-triazol-4-yl)methylene]-2,2-dimethylpropane-1,3-diamine), and two solvatomorphs [Fe-II(p-ttm(2)-etpn)(NCS)(2)]center dot solvent (p-ttm(2)-etpn = N,N '-bis[(1-p-tolyl-1H-1,2,3-triazol-4-yl)methylene]-1-ethylpropane-1,3-diamine, and solvent = 0.5H(2)O and 0.5MeCN center dot 0.5MeOH center dot H2O for 3a and 3b, respectively). All compounds are constructed of a three-dimensional (3D) flexible supramolecular network by multiple weak CH center dot center dot center dot S hydrogen bonds incorporating an additional orderly dimensional structure by aromatic interactions (i.e., pi-pi and CH center dot center dot center dot pi 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.