Pyrazinetetracarboxylic Acid as an Assembler Ligand in Uranyl−Organic Frameworks

The reaction of uranyl nitrate with pyrazinetetracarboxylic acid (H4PZTC) has been investigated under different experimental conditions, and the crystal structures of the resulting complexes have been determined. In all cases, the uranyl ion is chelated in the ONO tridentate site, as in the complexes with pyridine-2,6-dicarboxylic acid, but much variety arises from the increased number of potential donor atoms. Hydrothermal synthesis in the presence of NEt4Br led to the complex [UO2(PZDC)(H2O)] (1), from in situ decarboxylation of H4PZTC into pyrazine-2,6-dicarboxylic acid (H2PZDC) upon prolonged heating. Complex 1crystallizes as ribbons held by bridging carboxylate groups and hydrogen bonds. In the presence of NaOH and under hydrothermal conditions, two species could be obtained: [(UO2)2(PZTC)(H2O)]·2H2O (2) and [UO2Na2(PZTC)(H2O)4] (3). In these compounds, [UO2(PZTC)]n2n− linear chains with bis-chelating PZTC ligands are further assembled either into a two-dimensional assemblage by other, carboxylate-bound bridging uranyl ions or into a three-dimensional framework by bridging [Na2(H2O)4]2+ dimers. In contrast, in [UO2K2(H2PZTC)2(H2O)2]·2H2O (4) and the two isomorphous compounds [UO2K2(H2PZTC)2]·4H2O (5) and [UO2Rb2(H2PZTC)2]·4H2O (6), which were not obtained under hydrothermal conditions, the ligand retains two protons and is bound to only one uranyl ion. The [UO2(H2PZTC)2]2− units are then assembled into a three-dimensional framework by bridging, oxo-bonded potassium ions in 4 or in layers by the carboxylic/ate-bound alkali-metal ions in 5 and 6 (with formation of a three-dimensional framework in the case of Rb+ through weaker Rb−acid bonds). In the last case, narrow channels contain the hydrogen-bonded water molecules. These results evidence the remarkable assembling potential of the H4−xPZTCx− ligand, with all 10 donor atoms coordinated in complexes 2 and 3 (x = 4) and bonding to as many as 7 metal atoms in 6 (x = 2).

本研究在不同实验条件下探究了硝酸铀酰与吡嗪四羧酸（pyrazinetetracarboxylic acid, H4PZTC）的反应，并解析了所得配合物的晶体结构。在所有配合物中，铀酰离子（uranyl ion）均以类似吡啶-2,6-二羧酸配合物的方式螯合于ONO三齿配位位点，但由于潜在供体原子数量增加，体系呈现出更为丰富的结构多样性。在四乙基溴化铵（NEt4Br）存在下进行水热合成时，H4PZTC经长时间加热原位脱羧生成吡嗪-2,6-二羧酸（H2PZDC），进而得到配合物[UO2(PZDC)(H2O)]（1）。该配合物以桥连羧基与氢键维系形成带状晶体结构。在氢氧化钠（NaOH）存在的水热条件下，可得到两种产物：[(UO2)2(PZTC)(H2O)]·2H2O（2）与[UO2Na2(PZTC)(H2O)4]（3）。在这两种化合物中，由双螯合PZTC配体构建的[UO2(PZTC)]n2n−线性链，可进一步通过两种方式组装：一是通过其他羧基桥连的铀酰离子形成二维组装体，二是通过桥连[Na2(H2O)4]2+二聚体构建三维骨架。与之不同的是，[UO2K2(H2PZTC)2(H2O)2]·2H2O（4）以及同构的[UO2K2(H2PZTC)2]·4H2O（5）和[UO2Rb2(H2PZTC)2]·4H2O（6）并非通过水热法得到，此类配合物中的配体保留两个质子，且仅与一个铀酰离子配位。其中，[UO2(H2PZTC)2]2−单元可通过两种方式组装：在配合物4中，通过氧键合钾离子形成三维骨架；在配合物5和6中，则通过羧基/羧酸盐配位的碱金属离子形成层状结构（对于Rb+体系，较弱的Rb−acid键作用可进一步形成三维骨架）。在配合物6中，狭窄的孔道中填充有氢键结合的水分子。上述结果表明，H4−xPZTCx−配体具有优异的组装能力：在配合物2和3（x=4）中，配体的全部10个供体原子均参与配位；而在配合物6（x=2）中，该配体最多可与7个金属原子成键。