Thermal and Light-Induced Spin Transitions in 3D Hofmann-type Frameworks Built on Nonlinear 3-Substituted Pyridine and Pyrimidine Pillaring Ligands

Integration of spin crossover (SCO) properties in 3D frameworks made up of cyano-bimetallic layers connected through pillaring organic ligands, the so-called Hofmann-type coordination polymers (HCPs)- represents an important source of multifunctional advanced materials. Typically, these 3D structures are constituted by 4-substituted pyridine-based linear pillars which afford HCPs with regular <b>pcu</b> topology. Here, we have investigated the suitability of the 3-substituted pyridine and pyrimidine bis-monodentate ligands 2,5-di(pyridin-3-yl)aniline (3-dpyan) and 2,5-di(pyrimidin-5-yl)aniline (bpmdan) as alternative building blocks to explore new structural topologies and functionalities. In this context, we have prepared the compounds Fe(3-dpyan)[Ag(CN)₂]₂·2MeOH (<b>1</b>^<b>Ag</b><b>·2MeOH</b>), Fe(3-dpyan)[Ag(CN)₂]₂···0.35NO₂Bz·MeOH (<b>1</b>^<b>Ag</b><b>·0.35NO</b>_<b>2</b><b>Bz·MeOH</b>), Fe(3-dpyan)[Au(CN)₂]₂·NO₂Bz (<b>1</b>^<b>Au</b><b>·NO</b>_<b>2</b><b>Bz</b>), and Fe(bpmdan)[Ag(CN)₂]₂·CH₃Bz (<b>2</b>^<b>Ag</b><b>·CH</b>_<b>3</b><b>Bz</b>) (MeOH = methanol, NO₂Bz = nitrobenzene, CH₃Bz = toluene). Our structural studies have revealed that <b>1</b>^<b>Ag</b><b>·2MeOH</b> and <b>1</b>^<b>Ag</b><b>·0.35NO</b>_<b>2</b><b>Bz·MeOH</b> exhibit isomorphous doubly interpenetrated 3D structures strongly differing from the unusual noninterpenetrated ones exhibited by <b>1</b>^<b>Au</b><b>·NO</b>_<b>2</b><b>Bz</b> and <b>2</b>^<b>Ag</b><b>·CH</b>_<b>3</b><b>Bz</b>. Temperature-dependent magnetic susceptibility measurements have shown that all the reported compounds exhibit thermal-induced SCO properties, and moreover, three of them display Light Induced Excited Spin State Trapping at low temperatures (LIESST effect). The studied compounds show a wide diversity of SCO behaviors, ranging from abrupt complete one-step SCO centered at 253 K (<b>1</b>^<b>Au</b><b>·NO</b>_<b>2</b><b>Bz</b>) to gradual and incomplete multistepped SCO centered at 120 K (<b>1</b>^<b>Ag</b><b>·0.35NO</b>_<b>2</b><b>Bz·MeOH</b>). This assorted SCO properties are discussed and correlated to the acquired chemical and structural information.