*Article* **Eight-Fold Interpenetrating Diamondoid Coordination Polymers for Sensing Volatile Organic Compounds and Metal Ions**

**Venkatesan Lakshmanan 1, Yi-Ting Lai 1, Xiang-Kai Yang <sup>1</sup> , Manivannan Govindaraj <sup>1</sup> , Chia-Her Lin 2,\* and Jhy-Der Chen 1,\***


**Abstract:** Reactions of divalent metal salts with 4,4-oxybis(N-(pyridine-4-yl)-benzamide), **L**, and naphthalene-1,4-dicarboxylic acid (1,4-H2NDC) in various solvents gave [Zn(**L**)(1,4-NDC)·H2O]n, **1**, [Cd(**L**)(1,4-NDC)(H2O)·MeOH]n, **2**, and [Co(**L**)(1,4-NDC)(H2O)0.5·MeOH]n, **3**, which have been structurally characterized. Complexes **1**–**3** show eight-fold interpenetrating frameworks with the **dia** topology, which exhibit porosities substantiated by CO2 adsorption, whereas **1** and **2** manifest stability in aqueous environments and show high selectivity toward sensing of mesitylene molecules and Fe3+ ions with low detection limits and good reusability up to five cycles.

**Keywords:** coordination polymer; crystal structure analysis; entanglement; luminescence

#### **1. Introduction**

Coordination polymers (CPs) have shown potential applications in luminescent sensing, gas storage and separation, drug delivery, catalysis, ion exchange and magnetism [1–5]. Metal ions are linked together by organic ligands to form extended frameworks, which are often porous and comprised into infinite arrays. In addition, polycarboxylates play a vital role in the organization of diverse CPs with differential dimensionalities, which show multiple potential coordination sites that are highly active to coordinate with metal centers. Linear and angular types of polycarboxylate ligands have been used extensively in the preparation of CPs in a mixed system [6,7]. Entanglement involving interpenetration, polycatenation and self-catenation is a very interesting phenomenon, which is also a major factor contributing to the diversities of CPs due to their aesthetic and intriguing topological interests [8]. Despite many such interesting CPs being reported, the manipulation of the structural diversity is still a challenge.

Considerable effort has been invested into understanding the entanglement of CPs containing bis-pyridyl-bis-amide (bpba) ligands [9–13]. Two highly interpenetrated Zn(II) and Cd(II) CPs with eight- and nine-fold interpenetrating diamondoid modes, respectively, have been reported by using the long flexible *N*,*N* -di(4-pyridyl)adipoamide ligand and the short rigid 1,4-benzenedicarboxylic acid [13]. Construction of a high-fold interpenetrating diamondoid framework with linear bidentate ligand may thus be ascribed to the length and flexibility of the spacer ligands and the geometrical arrangements of the metal centers that may modulate pore size and shape in a single network. While the entanglement may reduce the solvent accessible volume required for the molecules/metal ions adsorptions, the modulated pore size and shape may enhance the selectivity. Moreover, the tunable porosity of luminescent CPs has a great impact on the sensitivity and selectivity for the detection of volatile organic compounds (VOCs) and metal ions that are harmful to the environment and public health [14].

**Citation:** Lakshmanan, V.; Lai, Y.-T.; Yang, X.-K.; Govindaraj, M.; Lin, C.-H.; Chen, J.-D. Eight-Fold Interpenetrating Diamondoid Coordination Polymers for Sensing Volatile Organic Compounds and Metal Ions. *Polymers* **2021**, *13*, 3018. https://doi.org/10.3390/ polym13183018

Academic Editor: Piotr Dobrzynski

Received: 2 August 2021 Accepted: 5 September 2021 Published: 7 September 2021

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In this study, three CPs constructed from the angular ligand 4,4-oxybis(N-(pyridine-4 yl)-benzamide (**L**), 1,4-H2NDC and divalent metal salts, namely, [Zn(**L**)(1,4-NDC)·H2O]n, **1**, [Cd(**L**)(1,4-NDC)(H2O)·MeOH]n, **2**, and [Co(**L**)(1,4-NDC)(H2O)0.5·MeOH]n, **3**, are reported, which show eight-fold interpenetrating diamondoid networks. Complexes **1** and **2** exhibit different emission intensity enhancement for distinct VOCs, and show a high sensitivity toward detection of Fe3+ ion in aqueous systems. The synthesis, structural characterization and the detections of VOCs and metal ions of these CPs form the subject of this report.
