Low-dose electron diffraction has been instrumental in determining the crystal structures of two compounds with metal-radical coordination frameworks {[Mn^II₂(NITIm)₃]CF₃SO₃·CH₃OH}_n (1) and {[Mn^II₂(NITImMe₂)₃]ClO₄}_n (2) that could never be grown to a crystal size large enough for single-crystal X-ray diffraction characterization. The compounds crystallize as nanocrystals upon addition of triflate (1) and perchlorate (2) anions and coordination of manganese(II) with bis-chelate nitronyl nitroxide radicals NITImH (1) and NITImHMe₂ (2) which are respectively 2-(2-imidazolyl)- and 2-(4,5-dimethylimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-3-oxide-1-oxyl. The two compounds have layered crystal structures in which cationic 2D metal-radical coordination polymers {[Mn^II₂(NITIm)₃]⁺}_n (1) and {[Mn^II₂(NITImMe₂)₃]⁺}_n (2) are separated by layers of triflate (1) or perchlorate (2) anions. Magnetic measurements evidence a ferrimagnetic behavior within the 2D metal-radical sheets due to alternating antiferromagnetically coupled spins (S_Mn²⁺ = 5/2 and S_radical = 1/2). Both compounds exhibit a long-range 3D ordering of weak-ferromagnetic type due to spin canting with Curie temperatures T_c = 45 K (1) and 40 K (2). This is associated with a field-induced metamagnetic transition from antiferromagnetic to ferromagnetic coupling of 2D metal-radical sheets. Studies of the crystal structures allows to rationalize how the molecular structure of nitronyl nitroxide radicals and of the counter-anions along with crystal packing affect the magnetic behavior related to interlayer distance and framework flexibility. These results are striking evidence that electron crystallography is a unique tool to solve structures of metal–organic compounds crystallizing as nanocrystals even with nitronyl nitroxide radical components too sensitive to typical electron doses. Overcoming the crystal size barrier, it allows the validation of chemical synthesis and the establishment of magneto-structural relationships fostering new advances in the design of molecule-based magnets.

中文翻译：

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电子衍射揭示了两个分子基磁体的二维金属自由基框架


低剂量电子衍射有助于确定两种具有金属自由基配位框架的化合物的晶体结构 {[Mn^II₂（NITIm）₃]CF₃SO₃·CH₃OH}_n （1） 和 {[Mn^II₂（NITImMe₂）₃]ClO₄}_n （2） 永远无法生长到足够大的晶体尺寸以进行单晶 X 射线衍射表征。在添加三氟磺酸酯 （1） 和高氯酸盐 （2） 阴离子以及锰 （II） 与双螯合物硝基亚硝基脱氧自由基 NITImH （1） 和 NITImHMe₂ （2） 的配位后，化合物结晶为纳米晶体，它们分别是 2-（2-咪唑基）- 和 2-（4,5-二甲基咪唑-2-基）-4,4,5,5-四甲基-4,5-二氢-1H-咪唑-3-氧化物-1-氧基。这两种化合物具有层状晶体结构，其中阳离子二维金属自由基配位聚合物 {[Mn^II₂（NITIm）₃]⁺}_n （1） 和 {[Mn^II₂（NITImMe₂）₃]⁺}_n （2） 由三氟磺酸 （1） 或高氯酸盐 （2） 阴离子层隔开。 磁性测量证明，由于交替的反铁磁耦合自旋（S_Mn²⁺ = 5/2 和 S_自由基 = 1/2），二维金属自由基片内的亚铁磁性行为。由于居里温度 T_c = 45 K （1） 和 40 K （2） 的自旋倾斜，这两种化合物都表现出弱铁磁型的长程 3D 有序。这与 2D 金属自由基片材从反铁磁到铁磁耦合的场诱导超磁转变有关。对晶体结构的研究可以合理化硝基基氮氧化物自由基和反阴离子的分子结构以及晶体堆积如何影响与层间距离和框架柔韧性相关的磁行为。这些结果是惊人的证据，表明电子晶体学是解决结晶为纳米晶体的金属有机化合物结构的独特工具，即使硝基氮氧化物自由基成分对典型电子剂量过于敏感。它克服了晶体尺寸的障碍，允许验证化学合成和建立磁结构关系，从而促进了基于分子的磁体设计的新进展。