§Department of Chemistry, Supercomputing Institute, and Chemical Theory Center, University of Minnesota, Minneapolis, Minnesota 55455, United States.
Inorg Chem. 2015 May 4;54(9):4445-55. doi: 10.1021/acs.inorgchem.5b00248. Epub 2015 Apr 13.
The self-assembly of uranyl peroxide polyhedra into a rich family of nanoscale cage clusters is thought to be favored by cation templating effects and the pliability of the intrinsically bent U-O2-U dihedral angle. Herein, the importance of ligand and cationic effects on the U-O2-U dihedral angle were explored by studying a family of peroxide-bridged dimers of uranyl polyhedra. Four chemically distinct peroxide-bridged uranyl dimers were isolated that contain combinations of pyridine-2,6-dicarboxylate, picolinate, acetate, and oxalate as coordinating ligands. These dimers were synthesized with a variety of counterions, resulting in the crystallographic characterization of 15 different uranyl dimer compounds containing 17 symmetrically distinct dimers. Eleven of the dimers have U-O2-U dihedral angles in the expected range from 134.0 to 156.3°; however, six have 180° U-O2-U dihedral angles, the first time this has been observed for peroxide-bridged uranyl dimers. The influence of crystal packing, countercation linkages, and π-π stacking impact the dihedral angle. Density functional theory calculations indicate that the ligand does not alter the electronic structure of these systems and that the U-O2-U bridge is highly pliable. Less than 3 kcal·mol(-1) is required to bend the U-O2-U bridge from its minimum energy configuration to a dihedral angle of 180°. These results suggest that the energetic advantage of bending the U-O2-U dihedral angle of a peroxide-bridged uranyl dimer is at most a modest factor in favor of cage cluster formation. The role of counterions in stabilizing the formation of rings of uranyl ions, and ultimately their assembly into clusters, is at least as important as the energetic advantage of a bent U-O2-U interaction.
多酸铀氧八面体通过自身组装形成了丰富的纳米笼状簇，这种组装被认为是由阳离子模板效应和固有弯曲的 U-O2-U 二面角的柔韧性共同作用的结果。在此，通过研究一系列过氧桥联的铀氧多面体二聚体，探讨了配体和阳离子效应对 U-O2-U 二面角的影响。分离出了四个化学性质不同的过氧桥联铀氧二聚体，它们包含吡啶-2,6-二甲酸酯、吡啶酸酯、乙酸盐和草酸盐作为配位配体的组合。这些二聚体与各种抗衡离子合成，导致了 15 种不同的铀酰二聚体化合物的晶体结构特征，其中包含 17 个对称的不同二聚体。11 个二聚体的 U-O2-U 二面角在预期的 134.0 到 156.3°范围内；然而，有 6 个二聚体的 U-O2-U 二面角为 180°，这是首次在过氧桥联铀氧二聚体中观察到这种情况。晶体堆积、抗衡阳离子键合和π-π堆积的影响会改变二面角。密度泛函理论计算表明，配体不会改变这些体系的电子结构，并且 U-O2-U 桥非常柔韧。只需不到 3 kcal·mol(-1) 的能量就可以将 U-O2-U 桥从其最低能量构象弯曲到 180°的二面角。这些结果表明，弯曲过氧桥联铀氧二聚体的 U-O2-U 二面角的能量优势最多只是有利于笼状簇形成的适度因素。抗衡离子在稳定铀离子环的形成以及最终将其组装成簇中的作用至少与弯曲的 U-O2-U 相互作用的能量优势一样重要。
