*Article* **Supramolecular Assemblies in Pyridine- and Pyrazole-Based Coordination Compounds of Co(II) and Ni(II): Characterization, Hirshfeld Analysis and Theoretical Studies**

**Trishnajyoti Baishya 1, Rosa M. Gomila 2, Miquel Barcelo-Oliver 2, Diego M. Gil 3, Manjit K. Bhattacharyya 1,\* and Antonio Frontera 2,\***


**Abstract:** Two new Ni(II) and Co(II) coordination compounds, viz., [Ni(H2O)5(DMAP)](IPhth)·2H2O (**1**) and [Co(Hdmpz)4(H2O)2]Cl2 (**2**) (where DMAP = 4-dimethylaminopyridine, IPhth = Isophthalate, Hdmpz = 3,5-dimethylpyrazole),were synthesized and characterized using elemental analysis, TGA, spectroscopic (FTIR and electronic) and single-crystal X-ray diffraction techniques. Compound **1** crystallizes as a co-crystal hydrate of Ni(II), whereas compound **2** is a mononuclear compound of Co(II). The crystal structure analysis of compound **1** reveals the presence of various non-covalent interactions such as anion–π, π–π, C–H···π, C–H···C, etc., which stabilize the layered assembly of the compound. In compound **2**, enclathration of counter chloride ions within the supramolecular trimeric host cavity plays a crucial role in the stabilization of the compound. The non-covalent interactions observed in the crystal structures were further studied theoretically, focusing on the cooperative π-stacking interactions between the *DMAP* and *IPhth* counter-ions in **1**. To identify the non-covalent interactions of the compounds, Hirshfeld surfaces and their associated two-dimensional fingerprint regions were analyzed. Theoretical calculations confirm that H-bonding interactions combined with the π-stacking contacts are crucial synthons for the solid-state stability of compound **1**.

**Keywords:** co-crystal hydrate; supramolecular assemblies; enclathration; cooperative π-stacking; DFT calculations

## **1. Introduction**

Transition-metal-based coordination compounds involving organic ligands have attracted researchers because of their wide potential applications in catalysis, non-linear optics, semiconductor devices and in biology [1–4]. The intriguing self-assemblies of metal–organic compounds have also gained remarkable interest because of their interesting structural topologies and network architectures [5,6]. Metal–organic co-crystals, involving metal centers having a minimum of two components with lattice water molecules, have also received emphasis owing to their wide practical utilities in the pharmaceutical industry, electronic devices as well as in synthetic chemistry [7,8].

Over the last few years, various N-donors, especially with pyridine-based moieties, have been extensively studied to construct supramolecular assemblies of desired architectures and potential applications [9,10]. Similarly, organic moieties containing pyrazole rings have also been exploited to explore self-assembled architectures with fascinating properties [11,12]. Metal–organic compounds of pyrazole- and pyridine-based ligands have been reported to display interesting biological applications [13,14]. The multiple coordination modes of the carboxylate-based aromatic ligands such as terminal, bridging,

**Citation:** Baishya, T.; Gomila, R.M.; Barcelo-Oliver, M.; Gil, D.M.; Bhattacharyya, M.K.; Frontera, A. Supramolecular Assemblies in Pyridine- and Pyrazole-Based Coordination Compounds of Co(II) and Ni(II): Characterization, Hirshfeld Analysis and Theoretical Studies. *Crystals* **2023**, *13*, 203. https://doi.org/10.3390/cryst13020203

Academic Editor: Alexander Y. Nazarenko

Received: 10 December 2022 Revised: 19 January 2023 Accepted: 20 January 2023 Published: 23 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

chelating, syn-anti, anti-anti, etc., have resulted in the formation of metal–organic compounds with fascinating structural topologies [15–20]. Non-covalent interactions are the backbone of supramolecular chemistry which play interesting roles in various fields such as synthesis, catalysis, crystal engineering, design of pharmaceutical agents, molecular biology, etc. [21,22]. Non-covalent contacts involving aromatic rings play crucial roles in the solid-state stability of metal–organic compounds [23] and also in guiding structural topologies of novel functional materials [24,25]. Counter anions present in the crystal lattice of metal–organic compounds not only neutralize the positive charge of complex cationic moieties but are also involved in non-covalent interactions, adding new dimensions in crystal engineering [26,27].

In the present study; we aim to explore the synthesis, detailed structural investigations and supramolecular assemblies of two new Ni(II) and Co(II) compounds involving pyridine and pyrazole ligands. The compounds were characterized using FT-IR, electronic spectroscopic techniques, TGA and elemental analysis. The presence of various non-covalent contactsin the crystal structure of the compounds, such as anion–π, π–π, C–H···π, C–H···C, etc., stabilize the layered assemblies. In compound **2**, enclathration of counter chloride ions within the supramolecular trimeric host cavity plays an important role in the stabilization of the compound. The non-covalent interactions in the crystal structures have been studied using Hirshfeld surface analysis and density functional theory (DFT) calculations, focusing on the π-stacking interactions between the *DMAP* and *IPhth* counter-ions that are further assisted by strong H-bonds involving the water molecules. The supramolecular ternary assembly was further characterized using molecular electrostatic potential (MEP) surface analysis and combined quantum theory of atoms-in-molecules (QTAIM) and non-covalent interaction (NCI) plot computational tools.

#### **2. Materials and Methods**

The chemicals used in the present study, viz., nickel(II) chloride hexahydrate, cobalt(II) chloride hexahydrate, isophthalic acid, 4-dimethylaminopyridine and 3,5-dimethyl pyrazole, were purchased from commercial sources and used without further purifications. The elemental analyses of the compounds were carried out using Perkin Elmer 2400 series II CHNS/O analyzer. The FT-IR spectra of the compounds were recorded in the frequency region 4000–500 cm−<sup>1</sup> using Bruker Alpha (II) Infrared spectrophotometer. The electronic spectra of the compounds were recorded using Shimadzu UV-2600 spectrophotometer. For UV-Vis-NIR spectra, BaSO4 powder was used as reference (100% reflectance). Sherwood Mark 1 magnetic susceptibility balance was used to calculate the room-temperature magnetic moments of the compounds. Thermogravimetric studies were carried out under the flow of N2 gas using Mettler Toledo TGA/DSC1 STARe system at the heating rate of 10 ◦C min<sup>−</sup>1.

#### *2.1. Syntheses*
