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.