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Nitrogen heterocycles are a class of organic compounds with extremely versatile functionality. Imidines, HN[C(NH) R ] 2 , are a rare class of heterocycles related to imides, HN[C(O) R ] 2 , in which the O atoms of the carbonyl groups are replaced by N—H groups. The useful synthesis of the imidine compounds succinimidine and glutarimidine, as well as their partially hydrolyzed imino–imide congeners, was first described in the mid-1950s, though structural characterization is presented for the first time in this article. In the solid state, these structures are different from the proposed imidine form: succinimidine crystallizes as an imino–amine, 2-imino-3,4-dihydro-2 H -pyrrol-5-amine, C 4 H 7 N 2 ( 1 ), glutarimidine as 6-imino-3,4,5,6-tetrahydropyridin-2-amine methanol monosolvate, C 5 H 9 N 3 ·CH 3 OH ( 2 ), and the corresponding hydrolyzed imino–imide compounds as amino–amides 5-amino-3,4-dihydro-2 H -pyrrol-2-one, C 4 H 6 N 2 O ( 3 ), and 6-amino-4,5-dihydropyridin-2(3 H )-one, C 5 H 8 N 2 O ( 4 ). Imidine 1 was also determined as the hydrochloride salt solvate 5-amino-3,4-dihydro-2 H -pyrrol-2-iminium chloride–2-imino-3,4-dihydro-2 H -pyrrol-5-amine–water (1/1/1), C 4 H 8 N 3 + ·Cl − ·C 4 H 7 N 3 ·H 2 O ( 1 ·HCl). As such, 1 and 2 show alternating short and long C—N bonds across the molecule, revealing distinct imino (C=NH) and amine (C—NH 2 ) groups throughout the C—N backbone. These structures provide definitive evidence for the predominant imino–amine tautomer in the solid state, which serves to enrich the previously proposed imidine-focused structures that have appeared in organic chemistry textbooks since the discovery of this class of compounds in 1883. 

A new and growing library of 3D models that can be utilized to illustrate many important concepts in the field of crystallography is presented. These models are accessible in the classroom via computers and smartphones and offer significant advantages over 2D depictions found in crystallography textbooks. Through the use of Blender , a free 3D modeling and animation program, over 100 new models focusing on different aspects of crystallographic education have been created. To simplify distribution/access, all of these models have been uploaded to Sketchfab, a model hosting and viewing web site that works similarly to YouTube. The current set of models is also given as a list in the supporting information. All of these models are free to view in a web browser or through a smartphone application. Additionally, all of these models are freely downloadable through the supporting information and Sketchfab, and users are encouraged to download and modify these models to best suit their needs. This library of models is part of the authors' ongoing outreach program to provide 3D models for free for educational purposes, and the authors offer their services to create additional models and moderate this library as additional requests or critiques are provided. 

Iron-based extended metal atom chains (EMACs) are potentially high-spin molecules with axial magnetic anisotropy and thus candidate single-molecule magnets (SMMs). We herein compare the tetrairon( ii ), halide-capped complexes [Fe 4 (tpda) 3 Cl 2 ] ( 1Cl ) and [Fe 4 (tpda) 3 Br 2 ] ( 1Br ), obtained by reacting iron( ii ) dihalides with [Fe 2 (Mes) 4 ] and N 2 , N 6 -di(pyridin-2-yl)pyridine-2,6-diamine (H 2 tpda) in toluene, under strictly anhydrous and anaerobic conditions (HMes = mesitylene). Detailed structural, electrochemical and Mössbauer data are presented along with direct-current (DC) and alternating-current (AC) magnetic characterizations. DC measurements revealed similar static magnetic properties for the two derivatives, with χ M T at room temperature above that for independent spin carriers, but much lower at low temperature. The electronic structure of the iron( ii ) ions in each derivative was explored by ab initio (CASSCF-NEVPT2-SO) calculations, which showed that the main magnetic axis of all metals is directed close to the axis of the chain. The outer metals, Fe1 and Fe4, have an easy-axis magnetic anisotropy ( D = −11 to −19 cm −1 , | E / D | = 0.05–0.18), while the internal metals, Fe2 and Fe3, possess weaker hard-axis anisotropy ( D = 8–10 cm −1 , | E / D | = 0.06–0.21). These single-ion parameters were held constant in the fitting of DC magnetic data, which revealed ferromagnetic Fe1–Fe2 and Fe3–Fe4 interactions and antiferromagnetic Fe2–Fe3 coupling. The competition between super-exchange interactions and the large, noncollinear anisotropies at metal sites results in a weakly magnetic non-Kramers doublet ground state. This explains the SMM behavior displayed by both derivatives in the AC susceptibility data, with slow magnetic relaxation in 1Br being observable even in zero static field.