Protic Oligozwitterions as a Means of Introducing Strong Hydrogen Bonding Interactions in Uranyl Ion Coordination Polymers

The neutral, dizwitterionic dicarboxylate 1,4‐bis(carboxylatomethyl)piperazine (cmpip) is used in the solvohydrothermal synthesis of three uranyl ion complexes. Both [(UO 2 ) 2 Cl 2 (pda)(cmpip) 2 ]·2H 2 O ( 1 ) and [(UO 2 ) 2 Cl 2 (chdc)(cmpip) 2 ] ( 2 ) are mixed‐ligand complexes involving additional dianionic dicarboxylates, 1,4‐phenylenediacetate (pda 2− ), or trans ‐1,4‐cyclohexanedicarboxylate (chdc 2− ), respectively, which crystallize as two‐stranded, ladder‐like monoperiodic coordination polymers. The chains in 1 and 2 are arranged into thick layers, adjacent chains being associated through strong NH…O reciprocal hydrogen bonding of cmpip ligands. The complex [UO 2 (cmpip) 2 (H 2 O)](NO 3 )Cl·2H 2 O ( 3 ) involves only cmpip as a carboxylate donor, and it crystallizes as a diperiodic assembly with the sql topology. The four cmpip ligands bound to uranyl are located on the same side of it so as to form a deep cavity, resulting in the heavily puckered geometry of the layers. The cmpip ligands in 3 are involved in extended hydrogen bonding with other cmpip ligands and chlorine and nitrate anions. Compound 2 is nonemissive, but 1 and 3 have a photoluminescence quantum yield of 7% in the solid state, with an emission spectrum showing the usual vibronic fine structure.