Thermal and\nLight-Induced Spin Transitions in 3D Hofmann-type\nFrameworks Built on Nonlinear 3‑Substituted Pyridine and Pyrimidine\nPillaring Ligands

Integration of spin crossover (SCO) properties in 3D\nframeworks\nmade up of cyano-bimetallic layers connected through pillaring organic\nligands, the so-called Hofmann-type coordination polymers (HCPs)-\nrepresents an important source of multifunctional advanced materials.\nTypically, these 3D structures are constituted by 4-substituted pyridine-based\nlinear pillars which afford HCPs with regular <b>pcu</b> topology.\nHere, we have investigated the suitability of the 3-substituted pyridine\nand pyrimidine bis-monodentate ligands 2,5-di(pyridin-3-yl)aniline\n(3-dpyan) and 2,5-di(pyrimidin-5-yl)aniline (bpmdan) as alternative\nbuilding blocks to explore new structural topologies and functionalities.\nIn this context, we have prepared the compounds Fe(3-dpyan)[Ag(CN)<sub>2</sub>]<sub>2</sub>·2MeOH (<b>1</b><sup><b>Ag</b></sup><b>·2MeOH</b>), Fe(3-dpyan)[Ag(CN)<sub>2</sub>]<sub>2</sub>···0.35NO<sub>2</sub>Bz·MeOH (<b>1</b><sup><b>Ag</b></sup><b>·0.35NO</b><sub><b>2</b></sub><b>Bz·MeOH</b>), Fe(3-dpyan)[Au(CN)<sub>2</sub>]<sub>2</sub>·NO<sub>2</sub>Bz (<b>1</b><sup><b>Au</b></sup><b>·NO</b><sub><b>2</b></sub><b>Bz</b>), and Fe(bpmdan)[Ag(CN)<sub>2</sub>]<sub>2</sub>·CH<sub>3</sub>Bz (<b>2</b><sup><b>Ag</b></sup><b>·CH</b><sub><b>3</b></sub><b>Bz</b>) (MeOH = methanol, NO<sub>2</sub>Bz = nitrobenzene, CH<sub>3</sub>Bz = toluene). Our structural\nstudies have revealed that <b>1</b><sup><b>Ag</b></sup><b>·2MeOH</b> and <b>1</b><sup><b>Ag</b></sup><b>·0.35NO</b><sub><b>2</b></sub><b>Bz·MeOH</b> exhibit isomorphous doubly interpenetrated 3D structures strongly\ndiffering from the unusual noninterpenetrated ones exhibited by <b>1</b><sup><b>Au</b></sup><b>·NO</b><sub><b>2</b></sub><b>Bz</b> and <b>2</b><sup><b>Ag</b></sup><b>·CH</b><sub><b>3</b></sub><b>Bz</b>. Temperature-dependent magnetic susceptibility measurements have\nshown that all the reported compounds exhibit thermal-induced SCO\nproperties, and moreover, three of them display Light Induced Excited\nSpin State Trapping at low temperatures (LIESST effect). The studied\ncompounds show a wide diversity of SCO behaviors, ranging from abrupt\ncomplete one-step SCO centered at 253 K (<b>1</b><sup><b>Au</b></sup><b>·NO</b><sub><b>2</b></sub><b>Bz</b>) to gradual and incomplete multistepped SCO centered at\n120 K (<b>1</b><sup><b>Ag</b></sup><b>·0.35NO</b><sub><b>2</b></sub><b>Bz·MeOH</b>). This assorted\nSCO properties are discussed and correlated to the acquired chemical\nand structural information.