Non-Centrosymmetric Tetrameric Assemblies of Tetramethylammonium Halides with Uranyl Salophen Complexes in the Solid State

Ditopic salophen-UO2 receptors 1−4 and 7 co-crystallize with tetramethylammonium (TMA) chloride and fluoride salts producing good quality crystals amenable for X-ray diffraction characterization. The arrangement of the receptor and salt units in the crystal lattice is such that tetrameric ball-shaped assemblies are formed, where an inner cluster of four TMA cations are surrounded by an outer shell of four UO2-bound anions. These elaborate architectures, which occur in all cases, regardless of a certain degree of structural modification on the receptors, lead to lattices that belong to non-centrosymmetric (NCS) space groups. Interestingly, the tetragonal symmetry of the tetrameric ball-shaped assemblies is either retained (I4̅) or lost (R3c and I4̅3d) at the lattice level, without compromising the NCS nature of the crystal lattices. The principal X-ray investigation on TMAX (X = Cl/F) co-crystals, that is, 1-(TMA)Cl, 2-(TMA)Cl, 3-(TMA)Cl, 4-(TMA)Cl, 7-(TMA)Cl, and 7-(TMA)F, is accompanied by NMR and electrospray ionization (ESI) mass spectrometry studies to gather additional insight on the modality of formation of the solid state structures observed. The important role of cation-π interactions in the receptor-salt recognition process is renewed and strengthened by comparison with NMR titration data with a novel reference compound, the salophen-UO2 complex 8. Given the importance of NCS and polar crystalline solids in the development of functional materials, this study shows that this property can be introduced into elaborate host−guest systems, as those which assemble in the architectures described here, thus expanding its field of applicability.

双位点salophen-二氧化铀（UO2）受体1−4与7可与四甲基铵（TMA）氯化物、氟化物共结晶，得到适用于X射线衍射表征的高质量晶体。晶体晶格内受体与盐单元的排布模式可形成四聚体球状组装体：四个TMA阳离子构成内部簇，外围被四个结合二氧化铀（UO2）的阴离子壳层包裹。无论受体发生何种程度的结构修饰，这类复杂组装结构均普遍存在，最终形成的晶格均属于非中心对称（NCS）空间群。值得注意的是，四聚体球状组装体的四方对称性在晶格层面或得以保留（I4̅），或发生丢失（R3c与I4̅3d），但始终不破坏晶格的非中心对称特性。针对TMAX（X=Cl/F）共晶体——即1-(TMA)Cl、2-(TMA)Cl、3-(TMA)Cl、4-(TMA)Cl、7-(TMA)Cl与7-(TMA)F——开展的核心X射线研究，辅以核磁共振（NMR）与电喷雾电离（ESI）质谱分析，以进一步阐明所观测固态结构的形成机制。通过与新型参比化合物salophen-二氧化铀（UO2）配合物8的核磁共振滴定数据对比，受体-盐识别过程中阳离子-π相互作用的关键作用得到了重新验证与强化。鉴于非中心对称与极性晶态固体在功能材料开发中的重要性，本研究表明该特性可被引入此类复杂主客体组装体系，从而拓展了其应用领域。