1,2,4-Triazolyl-4-acetate: a ditopic ligand combining soft and hard donor sites in homometallic (AgI) and heterometallic (AgI/UVI) coordination polymers

Abstract

1,2,4-Triazol-4-yl-acetic acid (trGly-H) has been used as a heterobifunctional ligand for preparing homometallic (Ag^I) and heterometallic (Ag^I/U^VI) coordination polymers under hydrothermal conditions. Four coordination compounds, [Ag₄(trGly)₄]·2H₂O (1), [Ag₄(trGly)₂(trGly-H)₂](NO₃)₂·2H₂O (2), [(UO₂){Ag₄(trGly)₄(NO₃)₂(H₂O)}]·4H₂O (3) and [(UO₂)₂{Ag₆(trGly)₁₀}]·7H₂O (4), have been synthesized and structurally characterized. Compounds 1 and 2 are similar and consist of tetranuclear [Ag₄(tr)₄(COO)₂] secondary building units (SBUs) supported by four [–N–N–] triazole bridges in the {Ag₄}-cluster plane and two carboxylate functions in the axial positions, which link the rhomboid units together into a thick layer. Addition of uranyl ions in the reaction system leads to compounds 3 and 4, where tr- and –COO⁻-functional groups are differentiated by their coordination role: tr binds Ag⁺ ions, while –COO⁻ interacts with UO₂²⁺. The crystal structure of 3 includes tetranuclear [Ag₄(tr)₄(NO₃)₂] units linked by four [–N–N–] triazoles and two nitrate anions above and below the cluster core. In 3, uranium in a pentagonal bipyramidal {UO₇} arrangement forms “cation–cation” interactions with Ag^I centers (the Ag⋯OUO distance is short, 2.600 Å). The 3D framework of compound 4 is constructed from hexanuclear [Ag₆(tr)₁₀] SBUs in a form of hexagonal twisted prisms that are further stacked into a column motif through argentophilic (Ag⋯Ag) interactions of 3.06 Å. To explore the AgNO₃/trGly-H/UO₂(NO₃)₂·6H₂O system, the composition space diagram approach was employed. The defined crystallization fields of the compounds show that complex 3 is the dominant product, while 4 appears only in a high ligand concentration range. The thermal behavior and solid-state photoluminescence properties of the compounds have been investigated.