Organic Ferrimagnetics Based on Charged Open-Shell Molecules

作为一种奇异的磁性材料，构建真正的有机分子基亚铁磁体是近几十年来材料科学和自旋化学领域的一个具有挑战性的问题。在构建基于有机分子的亚铁磁性体中的实际困难是具有不同自旋量子数S的两种分子的共结晶，例如，S=1，S=1/2。我们提出了一个超分子的方法来共结晶。离子电荷之间的非共价键合（例如氢键和库仑相互作用）可以是具有不同S的开壳分子结晶的驱动力，例如，我们设计并合成了以吡啶和苯胺为取代基的硝酰基和亚氨基氮氧自由基作为超分子铁磁体的S=1组分。合成了2，6-和3，5-取代吡啶和苯胺的中性双自由基。通过中性前体的N-甲基化制备阳离子双自由基。磁化率测量结果表明，中性双自由基和闭壳层阴离子盐的分子基态均为三重态（S=1）。因此，中性双自由基和阳离子都可以作为有机超分子铁磁体的结构单元，其中一种中性双自由基，硝基氮氧自由基的3，5-吡啶衍生物可以与对苯甲酸取代的单自由基共结晶。氢键复合物在5 K发生反铁磁相变。根据X射线晶体结构和取向单晶磁化率的测量，阐明了相变的性质和起源。

Construction of genuinely organic molecule-based ferrimagnets as an exotic magnetic material has been a challenging issue in materials science and spin chemistry during last decades. A practical difficulty in constructing organic molecule-based ferrimagnetics is co-crystallization of two kinds of molecules with different spin quantum numbers S's, e.g., S=1 and S=1/2. We propose a supramolecular approach to the co-crystallization. Noncovalent bonding such as hydrogen bonding and coulombic interaction between ionic charges can be a driving force for crystallization of open-shell molecules with differing S's, e.g., biradical (S=1) and monoradical (S=1/2).We have designed and synthesized nitronyl-and imino-nitroxide biradicals with pyridine and aniline substituents as S=1 components for the supramolecular ferrimagnets. The neutral biradicals of 2,6- and 3,5-substituted pyridines and anilines were synthesized. The cationic biradicals were prepared by N-methylation of the neutral precursors. The molecular ground states of of the neutral biradicals and the salts with closed-shell anions were found to be triplet (S=1) from magnetic susceptibility measurements. Therefore, both the neutral biradicals and the cations serve as building blocks for organic supramolecular ferrimagnets.One of the the neutral biradicals, the 3,5-pyridine derivative of nitronyl nitroxide was found to co-crystallize with p-benzoic acid substituted monoradical. The hydrogen-bonded complex undergoes an antiferromagnetic phase transition at 5 K. The nature and the origin of the phase transition have been clrified on the basis the X-ray crystal structure and magnetic susceptibility measurements on aligned single crystals.