Fine-tuning synthesis of a novel CuI/CuII mixed-valence coordination polymer

Abstract

The chemistry of mixed-valence coordination polymers (CPs) and metal–organic frameworks (MOFs) is of great interest mainly due to their limited numbers and intriguing properties. The scarcity of these types of compounds can be attributed to the lack of effective methods for their synthesis. In this study, we report on the fine-tuning synthesis and structure of a novel Cu^I/Cu^II mixed-valence coordination polymer [Cu₃Cl(4-ptz)₂(OH)₂]_n (1), based on the reaction of a 5-(4-pyridyl)tetrazole (4-H-ptz) ligand and CuCl₂·2H₂O in a molar ratio of 1 : 3 under hydrothermal conditions. X-ray crystallography analysis of 1 and bond valence method calculations confirmed the mixed-valence characteristics of the MOF, where Cu(1) presents a +1 charge but Cu(2) presents a +2 charge. The structure is unique, in which the Cu^I center adopts a T-shaped geometry and Cu^II exists in a square-pyramidal arrangement. The Cu^I/Cu^II mixed-valence compound 1 was formed by the partial reduction of Cu^II salts in the presence of a 4-H-ptz ligand, which also serves as a reducing agent. In addition, the tetrazolate and pyridine groups of the deprotonated 4-ptz ligand in an appropriate spatial arrangement and coordination sites perfectly fit the requirements for the successful assembly of the final mixed-valence product. Furthermore, a univalence Cu^I-based compound [Cu₂Cl(4-ptz)]_n (2) was obtained by using a molar ratio of L : M = 1 : 2 (L = 4-H-ptz, M = CuCl₂·2H₂O) under 160 °C hydrothermal conditions. The isolation of this univalent 2 highlights the fact that the fine-tuning synthesis of the Cu^I/Cu^II mixed-valence compound 1 is possible. These results provide an excellent example of the design, synthesis, and crystal engineering of mixed-valence compounds.