Tuning the Transition Temperature and Cooperativity of bapbpy-Based Mononuclear Spin-Crossover Compounds: Interplay between Molecular and Crystal Engineering

In this study, we show that 1) different isomers of the same mononuclear iron(II) complex give materials with different spin‐crossover (hereafter SCO) properties, and 2) minor modifications of the bapbpy (bapbpy=N6,N6′‐di(pyridin‐2‐yl)‐2,2′‐bipyridine‐6,6′‐diamine) ligand allows SCO to be obtained near room temperature. We also provide a qualitative model to understand the link between the structure of bapbpy‐based ligands and the SCO properties of their iron(II) compounds. Thus, seven new trans‐[Fe{R2(bapbpy)}(NCS)2] compounds were prepared, in which the R2bapbpy ligand bears picoline (9–12), quin‐2‐oline (13), isoquin‐3‐oline (14), or isoquin‐1‐oline (15) substituents. From this series, three compounds (12, 14, and 15) have SCO properties, one of which (15) occurs at 288 K. The crystal structures of compounds 11, 12, and 15 show that the intermolecular interactions in these materials are similar to those found in the parent compound [Fe(bapbpy)(NCS)2] (1), in which each iron complex interacts with its neighbors through weak NH⋅⋅⋅S hydrogen bonding and π–π stacking. For compounds 12 and 15, hindering groups located near the NH bridges weaken the NS intermolecular interactions, which is correlated to non‐cooperative SCO. For compound 14, the substitution is further away from the NH bridges, and the SCO remains cooperative as in 1 with a hysteresis cycle. Optical microscopy photographs show the strikingly different spatio‐temporal evolution of the phase transition in the noncooperative SCO compound 12 relative to that found in 1. Heat‐capacity measurements were made for compounds 1, 12, 14, and 15 and fitted to the Sorai domain model. The number n of like‐spin SCO centers per interacting domain, which is related to the cooperativity of the spin transition, was found high for compounds 1 and 14 and low for compounds 12 and 15. Finally, we found that although both pairs of compounds 11/12 and 14/15 are pairs of isomers their SCO properties are surprisingly different. A model of cooperativity: The spin‐crossover properties of different isomers of substituted spin‐crossover compounds [Fe{R2(bapbpy)}(NCS)2] are presented. Both the cooperativity and transition temperature critically depend on the position of the substituents R on the terminal pyridine rings of the ligand (see figure); a simple model based on hydrogen‐bonding networks is provided.