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

Abstract

[EN] 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 pcu 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)<middle dot>2MeOH (1(Ag)<middle dot>2MeOH), Fe(3-dpyan)[Ag(CN)(2)](2)<middle dot><middle dot><middle dot>0.35NO(2)Bz<middle dot>MeOH (1(Ag)<middle dot>0.35NO(2)Bz<middle dot>MeOH), Fe(3-dpyan)[Au(CN)(2)](2)<middle dot>NO(2)Bz (1(Au)<middle dot>NO(2)Bz), and Fe(bpmdan)[Ag(CN)(2)](2)<middle dot>CH(3)Bz (2(Ag)<middle dot>CH(3)Bz) (MeOH = methanol, NO(2)Bz = nitrobenzene, CH(3)Bz = toluene). Our structural studies have revealed that 1(Ag)<middle dot>2MeOH and 1(Ag)<middle dot>0.35NO(2)Bz<middle dot>MeOH exhibit isomorphous doubly interpenetrated 3D structures strongly differing from the unusual noninterpenetrated ones exhibited by 1(Au)<middle dot>NO(2)Bz and 2(Ag)<middle dot>CH(3)Bz. 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 (1(Au)<middle dot>NO(2)Bz) to gradual and incomplete multistepped SCO centered at 120 K (1(Ag)<middle dot>0.35NO(2)Bz<middle dot>MeOH). This assorted SCO properties are discussed and correlated to the acquired chemical and structural information.