*Article* **2D Layer Structure in Two New Cu(II) Crystals: Structural Evolvement and Properties**

**Jia-Jing Luo 1, Xiang-Xin Cao 1, Qi-Wei Chen 1, Ying Qin 1, Zhen-Wei Zhang 1,\*, Lian-Qiang Wei 2,\* and Qing Chen <sup>1</sup>**


**Abstract:** Two new Cu(II) crystals, {[Cu(dtp)]·H2O}n (**1**) and [Cu(Hdtp)(bdc)0.5]n (**2**) (H2dtp = 4 -(3,5 dicarboxyphenyl)-2,2 :6 ,2 -terpyridine, H2bdc = 1,4-benzenedicarboxylic acid) were synthesized under hydrothermal conditions. X-ray single-crystal structural analysis revealed that the 5-connective Cu(II) is in a distorted tetragonal-pyramidal coordination sphere for both compounds. Crystal **1** shows a "wave-shaped" 2D layer in the structure, while **2** bears a 1D coordination chain structure and a supermolecular 2D layer structure with a thickness of 7.9 Å via 1D chain stacking. PXRD and TGA measurements showed that **1** and **2** are air stable, with thermal stabilities near 300 ◦C.

**Keywords:** crystals; metal-organic frameworks; Cu(II) ion; 2D layer; structural evolvement

#### **1. Introduction**

In the past few decades, Metal-Organic Framework (MOF) material has been a hot topic in the field of chemistry and materials. It is a kind of hybrid material with a highly ordered network formed by the coordination bond connection between metal ions and organic ligands [1]. It has aroused great interest because of its structural diversity and its wide applications in catalysis [2,3], light-emitting sensors [4–8], gas adsorption/separation [9], magnetism [10], biomedicine [11] and other advanced materials [12]. MOF-based 2D nanosheet materials arouse great interest [13] due to the application of gas separation [14] and molecular sieving membranes [15]. Even though the "top-down" and the "bottom-up" methods have been developed to fabricate this 2D material [16], the understanding of the structure and the consequential tuning of the 2D MOF layer growth are still the key issues at this point.

The 4 -(3,5-dicarboxyphenyl)-2,2 :6 ,2 -terpyridine (H2dtp) ligand is a ditopic nearplane shape linker with *m*-dicarboxylic and tribipyridine groups [17–21], which is a good candidate for the construction of 2D MOFs structures [22]. In order to fulfill the purpose of the 2D layer configuration, it's important to further govern the coordination when a metal ion is coordinated to the tribipyridine group of H2dtp, as one can expect that a lower coordination number of the metal ion will reduce the possibility of the 3D network extending. Besides six, the coordination number of Cu(II) can be varied to five [23] or four [24], which makes it a potential low-coordinative ion to serve as a node in the linking of the 2D coordination network.

In this paper, two new MOFs with different kinds of 2D structure are synthesized, namely {[Cu(dtp)]·H2O}n (**1**) and [Cu(Hdtp)(bdc)0.5]n (**2**) (H2bdc=1,4-benzenedicarboxylic acid). The coordination number of the Cu(II) is five in both compounds. Besides this, **1** shows a "wave-shaped" 2D layer in the structure, as we expected. As for **2**, the introduction of bdc2<sup>−</sup> into the Cu-H2dtp coordination system reduces the 2D network into

**Citation:** Luo, J.-J.; Cao, X.-X.; Chen, Q.-W.; Qin, Y.; Zhang, Z.-W.; Wei, L.-Q.; Chen, Q. 2D Layer Structure in Two New Cu(II) Crystals: Structural Evolvement and Properties. *Crystals* **2022**, *12*, 585. https://doi.org/10.3390/ cryst12050585

Academic Editors: Mingyang Chen, Jinbo Ouyang, Dandan Han and Andrey Prokofiev

Received: 6 April 2022 Accepted: 20 April 2022 Published: 22 April 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 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/).

the 1D coordination chain; however, a unique supermolecular 2D layer with a thickness of 7.9 Å is also found, which is formed by the orderly array of the 1D chain. A detailed structural analysis shows how the network evolution comes about, and the PXRD and TGA measurements help in understanding the air and thermal stability of the compounds.

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

#### *2.1. Materials and General Methods*

The reagents and solvents were commercially available and used as received. All of the other starting materials were of analytical grade, and were used as received, without further purification. The powder X-ray diffraction (PXRD) data were recorded on a Rigaku Miniflex 600 diffractometer (Rigaku, Tokyo, Japan) using Cu Kα radiation (λ = 1.54056 Å), with a scan speed of 4◦ min−<sup>1</sup> in the 2*θ* = 5–45◦ region. Thermogravimetric analyses (TGA) were carried out on a Netzsch STA449 F5 analyzer (Netzsch, Serb, Germany), with the heating of the crystalline samples from room temperature to 800 ◦C at a rate of 10 ◦C min−<sup>1</sup> in a nitrogen atmosphere.

## *2.2. Synthesis of the Complexes*

Regarding the synthesis of {[Cu(H2dtp)]·H2O}n (**1**), a mixture of 0.1 mmol Cu(NO3)2·3H2O (0.024 g), 0.05 mmol H2dtp (0.020 g), 1.3 mL H2O and 3.6 mL DMA (N, N -dimethylacetamide), together with 3.6 mL methanol, was sealed in a 15 mL capped vial. The vial was heated at 150 ◦C for 48 h under autogenous pressure, and then cooled slowly down to room temperature. Light-green crystals with a regular cuboid structure were obtained. The yield was 35% based on H2dtp. The elemental analysis (%) was calculated for C23H15CuN3O5: C 57.92 H 3.17 N 8.81; it found: C 57.20 H 3.35 N 8.42. IR (cm−1): 3420 (vs), 1618 (s), 1560 (s), 1443 (s), 1388 (s), 1227 (m), 1178 (m), 1002 (w), 890 (w), 763 (m), 710 (vw), 665 (vw).

Regarding the synthesis of [Cu(H2dtp)(bdc)0.5]n (**2**), the synthesis of compound **2** was similar to that of **1**. In addition to using 15 mL H2O instead of the solvent condition, 0.05 mmol H2bdc (8 mg) and 0.25 mmol NaOH (10 mg) were added into the system. The mixture was sealed in a 25 mL capped vial, heated at 140 ◦C for 48 h under autogenous pressure, and then cooled gradually down to room temperature. Similarly, dark-green, slender bulk crystals were obtained. The yield was 32% based on H2dtp. Elemental analysis (%) was calculated for C27H16CuN3O6: C 59.83 H 2.98 N 7.75; it found: C 59.20 H 3.25 N 8.01. IR (cm<sup>−</sup>1): 3444 (vs), 1620 (s), 1554 (s), 1444 (s), 1398 (s), 1250 (m), 1174 (m), 1002 (w), 878 (w), 796 (m), 770 (m), 658 (m).

#### *2.3. Crystal Structure Determination*

Single crystals of **1** and **2** with the proper dimensions were chosen under an optical microscope and coated with high vacuum grease (Dow Corning Corporation) before being mounted on a glass fiber for the data collection. X-ray crystallography data of **1** and **2** were gathered on a Bruker Apex Smart CCD diffractometer (Bruker, Bremen, Germany) at 293 K with graphite-monochromated Mo Kα (*λ* = 0.71073 Å) or Cu Kα (*λ* = 1.54184) radiation by using the *ω*-2*θ* scan mode. The intensity data were corrected for Lorentz and polarization effects (SAINT), and empirical absorption corrections based on equivalent reflections were applied (SADABS) [25]. The structures ware solved by direct methods, and were refined by the full-matrix least-squares method on *F*<sup>2</sup> with the SHELXTL program package [26]. All of the non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms were calculated and refined as a riding model. The hydrogen atoms of carboxylic groups and water molecules were located from difference maps. The disordered guest molecules were treated by a solvent mask with the Olex2 program [27]. Crystallographic data for **1** and **2** are given in Table 1. The hydrogen-bonding parameters and selected bond lengths and angles for **1** and **2** are listed in Tables S1–S4 (see Supplementary Materials).


**Table 1.** Crystal data and structure refinement for **1** and **2**.

*<sup>R</sup>*<sup>1</sup> <sup>=</sup> <sup>∑</sup>*F*o|−|*F*c/∑|*F*o|, *wR*<sup>2</sup> = [∑*w*(|*F*o|−|*F*c|)2/∑*w*|*F*o|2] 1/2.
