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Comprehensive Summary: Regulating spin crossover (SCO) behavior, especially controlling the spin transition steps, is an important scientific issue, mainly because people aim to control spin bistability and multistability. Presently, SCO bistability can be regulated by changing the ligand‐modifying species, non‐coordinated anions, guest molecules, and metal‐ion dopant. However, the control of multistability is extremely challenging, especially in Fe(III) SCO compounds. Here, we report that [FeIII(H‐5‐Br‐thsa)(5‐Br‐thsa)]·H2O (5‐Br‐thsa = (5‐bromo‐2‐hydroxybenzylidene)hydrazinecarbothioamide), a compound we have reported before, exists in two polymorphic forms: polymorph 1 exhibits three‐step SCO, and polymorph 2 shows five‐step SCO, with multi‐step SCO behavior effectively regulated by polymorphism. According to single‐crystal and powder X‐ray diffractometry, polymorphs 1 and 2 crystallize in different space groups during their spin transitions, with two‐step symmetry breaking observed (Pbcn → Pnc2 → Pbcn for polymorph 1; P21/n → Pn → P21/n for polymorph 2). We realized that the behavior of these two polymorphs depends significantly on the structure, including (i) the average Fe—N bond distance, (ii) deformation of octahedral FeIII atoms, and (iii) distinct crystal packing, which account for the large differences observed in magnetic properties. [ABSTRACT FROM AUTHOR]

Polymetallic complexes with covalently bridged metal centers that interact magnetically are important in the molecular magnetism field, with binuclear compounds receiving special attention because they represent the simplest type of multinuclear species with covalently bridged metal centers. Herein, we report the synthesis and properties of two new binuclear FeII complexes, namely, {[Fe(abpt-TPE)(NCS)2]2(bpym)}·2MeOH·2MeCN (1) and {[Fe(abpt-TPE)(NCS)2]2(bpym)}·2CH2Cl2 (2) (bpym = 2,2′-bipyrimidine). The crystal structure is analyzed at different temperatures, and its properties are analyzed by variable-temperature magnetic susceptibility and variable-temperature fluorescence emission spectroscopy tests. Variable-temperature magnetic susceptibility measurements of two binuclear compounds show different types of magnetic behavior. Complex 1 exhibits two-step spin transition behavior with an intermediate state near 150 K (Tc1 = 191 K, Tc2 = 111 K); 1 undergoes an [LS–LS] ↔ [LS–HS] ↔ [HS–HS] spin transition during thermal induction. On the other hand, complex 2 exhibits intramolecular antiferromagnetic coupling, with J = −0.47 cm−1. The analysis of correlations between the structural characteristics and different types of magnetic behaviors for two binuclear complexes, revealed that the different magnetic behaviors shown by the two complexes are attributable to different degrees of [FeN6] octahedral distortion caused by different lattice solvents, ligand strain and crystal stacking. [ABSTRACT FROM AUTHOR]

The reaction of Fe(OAc)2 and Hbpypz with neutral TCNQ results in the formation of [Fe2(bpypz)2(TCNQ)2](TCNQ)2 (1), in which Hbpypz=3,5‐bis(2‐pyridyl)pyrazole and TCNQ=7,7′,8,8′‐tetracyano‐p‐quinodimethane. Crystal packing of 1 with uncoordinated TCNQ and π–π stacking of bpypz− ligands produces an extended two‐dimensional supramolecular coordination assembly. Temperature dependence of the dc magnetic susceptibility and heat capacity measurements indicate that 1 undergoes an abrupt spin crossover (SCO) with thermal spin transition temperatures of 339 and 337 K for the heating and cooling modes, respectively, resulting in a thermal hysteresis of 2 K. Remarkably, the temperature dependence of dc electrical transport exhibits a transition that coincides with thermal SCO, demonstrating the thermally induced magnetic and electrical bistability of 1, strongly correlating magnetism with electrical conductivity. This outstanding feature leads to thermally induced simultaneous switching of magnetism and electrical conductivity and a magnetoresistance effect. [ABSTRACT FROM AUTHOR]

A neutral mononuclear FeIII complex [FeIII(H-5-Br-thsa-Me)(5-Br-thsa-Me)]⋅H2O ( 1; H2-5-Br-thsa-Me=5-bromosalicylaldehyde methylthiosemicarbazone) was prepared that exhibited a three-step spin-crossover (SCO) with symmetry breaking and a 14 K hysteresis loop owing to strong cooperativity. Two ordered intermediate states of 1 were observed, 4HS-2LS and 2HS-4LS, which exhibited reentrant phase-transition behavior. This study provides a new platform for examining multistability in SCO complexes. [ABSTRACT FROM AUTHOR]

The mononuclear ferric compound Fe(H-5-Cl-thsa-Me)(5-Cl-thsa-Me)·H2O (1) was synthesized and characterized using powder/single-crystal X-ray diffraction, Mössbauer spectroscopy, differential scanning calorimetry (DSC), and magnetic susceptibility measurements. This photo-responsive compound shows reversible, two-step spin-crossover behaviour. Moreover, dielectric anomalies were observed during the spin transitions, demonstrating the multifunctional properties of compound 1. [ABSTRACT FROM AUTHOR]

Spin doctor: A mononuclear ferric complex [Fe(H‐5‐Br‐thsa)(5‐Br‐thsa)]⋅H2O (1) (H2‐5‐Br‐thsa=5‐bromo‐2‐hydroxybenzylidene)hydrazinecarbothioamide) was synthesized and its magnetic properties and structure were investigated by DFT calculations. This complex shows unprecedented reversible, six/five‐step spin‐crossover behavior accompanied by symmetry breaking. More importantly, each step in the multi‐step transition was successfully characterized by single‐crystal X‐ray diffraction. [ABSTRACT FROM AUTHOR]

• Spin crossover (SCO) and valence tautomerism (VT) conductors are summarized. • Magnetic properties and conductivities of SCO and VT conductors are discussed. • Synergistic and cooperative properties of bistability and conductivity are presented. Multifunctional molecular materials exhibiting both magnetic bistability and electrical conductivity have excellent application potential in microelectronic devices. In the last two decades, research into conductive magnets and switching materials has continuously expanded and provided materials with practical application prospects. In this review, we present the recent progress on spin crossover and valence tautomerism conductors, focusing on syntheses, strategies, crystal structures, magnetic properties, and conductivities of such materials as well as synergistic and cooperative properties of bistability and conductivity. [ABSTRACT FROM AUTHOR]

B Thermally-induced simultaneous switching b of magnetism and electrical conductivity has been realized by a two-dimensional supramolecular coordination assembly composed of Fe SP II sp spin-crossover complexes and partially charged TCNQ (7,7',8,8'-tetracyano- I p i -quinodimethane) radicals. B Thermally-induced simultaneous switching b of magnetism and electrical conductivity has been realized by a two-dimensional supramolecular coordination assembly composed of Fe SP II sp spin-crossover complexes and partially charged TCNQ (7,7',8,8'-tetracyano- I p i -quinodimethane) radicals. Fe SP II sp spin-crossover units are directly linked to -stacked TCNQ columns, thus possessing strongly coupled magnetism (from localized d electrons) and electrical conductivity (from delocalized electrons). [Extracted from the article]

[Display omitted] • Synergistic luminescent spin crossover (SCO) materials are reviewed. • Strategies used to synthesize luminescent SCO materials are presented. • Challenges for achieving SCO-luminescence synergy in materials are discussed. The spin crossover (SCO) phenomenon has witnessed remarkable progress in applications since it was first discovered almost a century ago. Specifically, materials that exhibit SCO-luminescence synergy have garnered significant interest owing to their excellent potential applications in optoelectronic devices. Over the past two decades, the interplay and synergy between SCO and luminescence have led to the development of an extensively researched field. This review aims to evoke a concise discussion on luminescent SCO materials by presenting an overview that focuses on their luminescence and magnetic properties, strategies for their syntheses, and progress made over the years toward constructing such materials, from nanoscale to long-chain polymers. [ABSTRACT FROM AUTHOR]