Encapsulation of Unusual Nitrate−Water Cluster [(NO₃)₄(H₂O)₆]⁴⁻ Anions into Cages of a Three-Dimensional Cationic Coordination Polymer

Acidification (pH = 3) of a solution containing AgNO₃ with 1,4-benzenedicarboxylic acid (1,4-H₂BDC) and 1,2-bis(4-pyridyl)ethylene (bpe) (molar ratio = 1:1:1) with 6 M HNO₃ followed by hydrothermal treatment at 170 °C afforded a one-dimensional (1D) coordination polymer {[Ag₂(bpe)₂](1,4-BDC)·(1,4-H₂BDC)}_n (1). Photochemical [2 + 2] cycloaddition reaction of 1 through UV irradiation followed by dissolving the photolysis product in 6 M HNO₃ and hydrothermal treatment at 170 °C led to the formation of a three-dimensional (3D) coordination polymer {[Ag₄(4,4′-tpcb)₃][(NO₃)₄(H₂O)₆]}_n (4,4′-tpcb = tetrakis(4-pyridyl)-cyclobutane) (2). Compounds 1 and 2 were characterized by elemental analysis, IR, and single crystal X-ray diffraction. Compound 1 consists of a 1D linear chain extending along the [011] plane in which Ag(I) ions are bridged by bpe molecules. Compound 2 contains the 2-fold interpenetrating 3D cationic [Ag₄(4,4′-tpcb)₃]_n⁴ⁿ⁺ net with the Schlafli symbol of (6³)₄(6²8⁴)₃. Within the cages of this net occupy the uncommon nitrate-water cluster [(NO₃)₄(H₂O)₆]⁴⁻ anions. The formation of such a cluster anion may provide some insight into the nature of hydration of nitrate in H₂O.