**1. Introduction**

For decades, 4,4 -bipyridine (4,4 -bpy), its longer homologues and modified derivatives were widely used as bidentate-bridging ligands in the synthesis of coordination polymers, giving rise either to cationic polymeric arrays when combined with easy-leaving anions [1–4], or to neutral coordination networks when used in a partnership with strongly coordinated anions such as polycarboxylates [5–8]. Although ditopic amino-ligands are weaker bases than the five- and six-membered heterocyclic N-bases, the presence of terminal amino groups gives them the advantage of simultaneous coordination with the metal and participation in different hydrogen bonds and weak interactions reinforcing the crystal lattice [9–11]. Moreover, the NH2-group in organic luminescent materials is

**Citation:** Craciun, N.; Chisca, D.; Melnic, E.; Fonari, M.S. Unprecedented Coordination Compounds with 4,4 -Diaminodiphenylethane as a Supramolecular Agent and Ditopic Ligand: Synthesis, Crystal Structures and Hirshfeld Surface Analysis. *Crystals* **2023**, *13*, 289. https:// doi.org/10.3390/cryst13020289

Academic Editor: Waldemar Maniukiewicz

Received: 24 January 2023 Revised: 3 February 2023 Accepted: 5 February 2023 Published: 8 February 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/).

regarded as an electron-donating group suitable for binding the electron-accepting metal ions. Compounds containing such amino ligands show luminescence through intraligand p–p\* transitions [12].

So far, a significant number of coordination compounds, including silver and cadmium coordination polymers, were documented [13] for 4,4 -diaminodiphenylmethane [10–12,14]. Silver and cadmium coordination polymers were also obtained with the more extended ditopic ligand, 4,4 (1,4-phenylenediisopropylidene)bis(aniline) [15,16]. The 3,3 -diaminobiphenylsulfone was reported to trap heavy metals (Cu(II), Hg(II)) in the form of coordination compounds [17,18]. On the other hand, no coordination compounds with 4,4 -diaminodiphenylethane (dadpe, also known under the names 4,4 -diaminobibenzyl, 4,4 -ethylenedianiline), the closest homologue of 4,4 -diaminodiphenylmethane, were yet reported. The survey of CSD (Version 5.43, November 2022 updates) only disclosed the dadpe in the form of hydrate [19], and as a guest in the inclusion compound with beta-cyclodextrin (β-CD) [20]. In the crystal of the hydrate, a dadpe molecule was registered in an extended trans-conformation. Alternatively, the same molecule exhibited one extended and two bent conformations in the inclusion complex with β-CD [20]. The expected and registered conformational flexibility of dadpe was an additional benefit for its use in the synthesis of flexible coordination networks. The flexible CPs reveal some advantages and attractive applications compared with the rigid frameworks. For example, they show pore opening and significantly increased adsorption capacity [21]. In this pioneering work, we report the synthesis, IR spectroscopic characterization and crystal structures for seven coordination compounds obtained from the different metal salts, Cd(ClO4)2·2H2O, Cd(NO3)2 **.** 4H2O, NiSO4·7H2O, Co(BF4)2·6H2O, Zn(NO3)2 **.** 6H2O and a ditopic dadpe ligand. New coordination compounds include mononuclear complexes [Cd(2,2 -bpy)3](ClO4)2](dadpe)(4,4 bpy) (**1**), [Ni(dadpe)2(H2O)4](SO4) **.** 2H2O (**2**), one-dimensional (1D) coordination polymers {[Zn(NO3)(dadpe)(dmf)2](NO3)}n (**3**), {[Cd(2,2 -bpy)2(dadpe)](ClO4)2}n (**4**), and twodimensional (2D) coordination polymers, {[Cd(4,4 -bpy)2(H2O)2](ClO4)2(dadpe)(EtOH)2}n (**5**), {[Co(4,4 -bpy)2(H2O)2](BF4)2(dadpe)(EtOH)2}n (**6**) and {[Cd(adi)(dadpe)](H2adi)}n (**7**). The distribution of intermolecular interactions in compounds **1**, **2**, **3** and **7**, which reveal different structural functions of dadpe ligand (as a neutral guest, a monodentate terminal ligand, and a bidentate-bridging ligand), was evaluated by Hirshfeld surface analysis.

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

#### *2.1. Materials and Measurements*

The starting salts, organic ligands and solvents were obtained from commercial sources (Sigma-Aldrich, St. Louis, MO, USA) and were used without further purification. The IR(ATR) spectra were recorded on a FTIR Spectrum-100 Perkin Elmer spectrometer in the range of 4000–650 cm<sup>−</sup>1. Elemental analysis was performed on a Vario EL III Element Analyzer.

#### *2.2. Synthesis*

#### 2.2.1. [Cd(2,2 -bpy)3](ClO4)2](dadpe)(4,4 -bpy) (**1**)

Compound **1** was prepared with the hydrothermal method. Dadpe 0.04 g (0.2 mmol) was dissolved in 7 mL EtOH. In this solution, 0.03 g (0.1 mmol) Cd(ClO4)2·2H2O, 0.045 g (0.3 mmol) 2,2 -bpy and 0.045 g (0.3 mmol) 4,4 -bpy, were added successfully. The obtained solution was placed in a 20 mL Teflon-lined stainless steel autoclave, which was then sealed and heated to 100 ◦C for 20 h. The solution was filtered and left for slow evaporation at room temperature. After 72 h, colorless crystals were filtered and dried in the air (yield 57%). Anal. Calc. for C42H36CdCl2N8O8 (%): C, 52.32; H, 3.76; N, 11.62. Found: C, 52.98; H, 3.49; N, 11.04. IR-ATR (cm−1): 3439 m, 3355 m, 1611 m, 1515 s, 14,382, 1278 m, 1193 w, 1083 s, 1024 m, 970 w, 819 s, 762 s, 735 w, 648 w, 621 m, 527 s, 488 w, 411 m.
