**New Coordination Polymers of Zinc(II), Copper(II) and Cadmium(II) with 1,3-Bis(1,2,4-triazol-4-yl)adamantane**

#### **Nertil Xhaferaj 1,2, Aurel Tăbăcaru 3,\* , Marco Moroni <sup>4</sup> , Ganna A. Senchyk <sup>5</sup> , Kostiantyn V. Domasevitch <sup>5</sup> , Claudio Pettinari 2,6 and Simona Galli 4,7,\***


Received: 16 October 2020; Accepted: 30 October 2020; Published: 6 November 2020

**Abstract:** The new coordination polymers (CPs) [Zn(tr2ad)Cl2]*n*, {[Cu(tr2ad)Cl]Cl·4H2O}*n*, [Cd2(tr2ad)Cl4]*n*, {[Cu(tr2ad)(NO3)](NO3)}*<sup>n</sup>* and {[Cd(tr2ad)(NO3)](NO3)·H2O}*<sup>n</sup>* were obtained in the form of air- and moisture-stable microcrystalline powders by the solvothermal reactions of zinc(II), copper(II) and cadmium(II) chlorides or nitrates with the ligand 1,3-bis(1,2,4-triazol-4-yl)adamantane (tr2ad). Investigation of the thermal behaviour assessed the thermal stability of these CPs, with [Cd2(tr2ad)Cl4]*<sup>n</sup>* starting to decompose only around 365 ◦C. As retrieved by powder X-ray diffraction, while [Zn(tr2ad)Cl2]*<sup>n</sup>* features 1-D chains along which the metal centre shows a tetrahedral geometry and the spacer is exo-bidentate, the other CPs contain 2-D double-layers in which the metal ions possess an octahedral stereochemistry and the linker is exo-tetradentate. A comparative structural analysis involving known coordination compounds containing the tr2ad ligand enabled us to disclose (i) the versatility of the ligand, as far as the coordination modes are concerned; (ii) the variability in crystal structure dimensionality, ranging from 1-D to 3-D; (iii) the fact that, to the best of our knowledge, [Zn(tr2ad)Cl2]*<sup>n</sup>* is the first ZnII-based CP containing the tr2ad spacer.

**Keywords:** coordination polymers; poly(azolate) spacers; 1,3-bis(1,2,4-triazol-4-yl)adamantane; zinc; copper; cadmium; crystal structure

#### **1. Introduction**

Since the discovery that metal ions and organic ligands can act as connectors and spacers, respectively, to generate infinite frameworks [1], the chemistry of coordination polymers (CPs) [2–5], including the subclass of metal–organic frameworks (MOFs) [6–10], has recorded a rapid growth, due to the plethora of functional properties they were found to possess. One of the main advantages of CPs

and MOFs is the possibility to modulate their chemical composition, crystal structure and functional properties through a modification of the metal ion and/or the organic spacer. In view of their potential applications, CPs and MOFs appear as interesting platforms which may offer sustainable solutions in fields of major economical, technological and environmental importance, e.g., gas storage and separation [11], catalysis [12], luminescence [13,14], conductivity [15], magnetism [16], sensing [17–19] and biomedicine [20]. The successful preparation of CPs has generally relied on organic ligands from the class of poly(carboxylic) acids [21–23], pyrazines and bipyridines [21–24], phosphonic acids [25] and poly(azoles) [26–28].

Among the nitrogen-donor ligands from the class of poly(azoles), attention has been paid also to 1,2,4-triazolyl derivatives, due to their electron-donating ability and rich coordination chemistry. As a representative example, they can provide the *N<sup>1</sup>* ,*N<sup>2</sup>* -bridging between two adjacent metal ions [29] in the same manner as pyrazolates do [30]. Based on the coordination modes they can adopt, 1,2,4-triazolyl ligands have been exploited in building up polynuclear and polymeric coordination compounds [31–35]. This is also the case of the ditopic ligand 1,3-bis(1,2,4-triazol-4-yl)adamantane (tr2ad, Scheme 1) which, although at present less explored, provides an attractive platform for crystal engineering. ∙ ∙

**Scheme 1.** Molecular structure of 1,3-bis(1,2,4-triazol-4-yl)adamantane (tr2ad).

Aiming at enlarging and diversifying the library of tr2ad-based coordination frameworks, we report hereafter on the synthesis, thermal behavior and structural characterization of the five new compounds [Zn(tr2ad)Cl2]*n*, {[Cu(tr2ad)Cl]Cl·4H2O}*n*, [Cd2(tr2ad)Cl4]*n*, {[Cu(tr2ad)(NO3)](NO3)}*<sup>n</sup>* and {[Cd(tr2ad)(NO3)](NO3)·H2O}*n*. The crystal and molecular structures of the anhydrous and trihydrate tr2ad ligand are also described.

#### **2. Results and Discussion**

#### *2.1. Synthesis and Preliminary Characterization*

∙ ∙ A detailed description of the synthesis of the tr2ad ligand, including analytical details on the intermediates never reported before, is provided in the Supporting Information.

∙

∙ Several screening reactions, involving the adoption of synthetic conditions differing in solvent, metal-to-ligand ratio, temperature, and/or time, were carried out in order to successfully obtain microcrystalline batches of the tr2ad-based CPs [Zn(tr2ad)Cl2]*n*, {[Cu(tr2ad)Cl]Cl·4H2O}*n*, [Cd2(tr2ad)Cl4]*n*, {[Cu(tr2ad)(NO3)](NO3)}*<sup>n</sup>* and {[Cd(tr2ad)(NO3)](NO3)·H2O}*n*. Scheme 2 shows the synthetic conditions fruitfully used for their isolation.

− − Compounds [Zn(tr2ad)Cl2]*<sup>n</sup>* and {[Cu(tr2ad)Cl]Cl·4H2O}*<sup>n</sup>* were isolated by carrying out a solvothermal reaction among zinc(II) chloride dihydrate and anhydrous copper(II) chloride, respectively, and tr2ad in the 2:1 molar ratio (DMF, 150 ◦C, 24 h). Also, the formation of compounds [Cd2(tr2ad)Cl4]*n*, {[Cu(tr2ad)(NO3)](NO3)}*<sup>n</sup>* and {[Cd(tr2ad)(NO3)](NO3)·H2O}*<sup>n</sup>* required the application of solvothermal conditions, reacting anhydrous cadmium(II) chloride, copper(II) nitrate hemipentahydrate and cadmium(II) nitrate tetrahydrate, respectively, with tr2ad in the 1:1 molar ratio (methanol, 100 ◦C, 24 h). All the compounds were isolated, in reasonable yields (55–70%), in the form of air- and moisture-stable microcrystalline powders, insoluble in water and in most common organic solvents (see Section 3.2).

−

∙

∙

∙

−

**Scheme 2.** Synthetic paths for the formation of the tr2ad-based coordination polymers (CPs) described in this work.

The IR spectrum of the tr2ad ligand (Figure S1, Supplementary Materials) shows a strong absorption band at 1517 cm−<sup>1</sup> , which is assigned to the stretching vibration of the triazolyl ring [36]. In the case of the title CPs, this absorption is shifted towards higher wavenumbers (1551–1539 cm−<sup>1</sup> ), as a consequence of the ligand coordination to the metal ions (Figure 1). The medium-intensity broad bands centered around 3400 cm−<sup>1</sup> in the IR spectra of compounds {[Cu(tr2ad)Cl]Cl·4H2O}*<sup>n</sup>* and {[Cd(tr2ad)(NO3)](NO3)·H2O}*<sup>n</sup>* (Figure 1), characteristic of the O–H stretching vibration, witness the

− ∙ ∙ ∙ **Figure 1.** IR spectra of [Zn(tr2ad)Cl<sup>2</sup> ]*<sup>n</sup>* (black), {[Cu(tr2ad)Cl]Cl·4H2O}*<sup>n</sup>* (green), [Cd<sup>2</sup> (tr2ad)Cl<sup>4</sup> ]*<sup>n</sup>* (red), {[Cu(tr2ad)(NO<sup>3</sup> )](NO<sup>3</sup> )}*<sup>n</sup>* (blue) and {[Cd(tr2ad)(NO<sup>3</sup> )](NO<sup>3</sup> )·H2O}*<sup>n</sup>* (fuchsia).

−

− ∙ μ η − ∙ − − ∙ A deeper analysis of the IR spectra of {[Cu(tr2ad)(NO3)](NO3)}*<sup>n</sup>* and {[Cd(tr2ad)(NO3)](NO3)·H2O}*<sup>n</sup>* (Figure 1) allows to differentiate among the uncoordinated and coordinated forms of the nitrate anion. Indeed, the strong bands located at 1439 and 1282 cm−<sup>1</sup> for {[Cu(tr2ad)(NO3)](NO3)}*n*, and at 1478 and 1275 cm−<sup>1</sup> for {[Cd(tr2ad)(NO3)](NO3)·H2O}*n*, assigned to the asymmetric and symmetric stretching vibrations of the nitrate group, together with the presence of two very weak bands, at 1755 and

−

−

1733 cm−<sup>1</sup> for {[Cu(tr2ad)(NO3)](NO3)}*<sup>n</sup>* and at 1748 and 1717 cm−<sup>1</sup> for {[Cd(tr2ad)(NO3)](NO3)·H2O}*n*, suggest the presence of µ2:η <sup>2</sup> nitrate anions [37,38]. At variance, the bands observed at 1394 and 1346 cm−<sup>1</sup> for {[Cu(tr2ad)(NO3)](NO3)}*<sup>n</sup>* and at 1374 and 1339 cm−<sup>1</sup> for {[Cd(tr2ad)(NO3)](NO3)·H2O}*n*, together with the band centered at 1073 cm−<sup>1</sup> in {[Cu(tr2ad)(NO3)](NO3)}*<sup>n</sup>* and at 1076 cm−<sup>1</sup> in {[Cd(tr2ad)(NO3)](NO3)·H2O}*n*, can be ascribed to the asymmetric and symmetric stretching modes of uncoordinated nitrate anions [39]. − ∙ − − ∙

#### *2.2. Thermal Behaviour*

Thermogravimetric analyses (TGAs) were performed on the five compounds from 30 ◦C to 700 ◦C under a flow of nitrogen. The resulting TGA curves are gathered in Figure 2. Compound [Zn(tr2ad)Cl2]*<sup>n</sup>* is stable up to 350 ◦C, temperature at which a slow decomposition begins. In the temperature range 30–120 ◦C, {[Cu(tr2ad)Cl]Cl·4H2O}*<sup>n</sup>* undergoes a weight loss of ca. 15%, which reasonably corresponds to the evolution of four water molecules per formula unit (calculated weight loss 15.1%). ∙

∙ ∙ **Figure 2.** Thermogravimetric analysis (TGA) traces of [Zn(tr2ad)Cl<sup>2</sup> ]*<sup>n</sup>* (black), {[Cu(tr2ad)Cl] Cl·4H2O}*<sup>n</sup>* (green), [Cd<sup>2</sup> (tr2ad)Cl<sup>4</sup> ]*<sup>n</sup>* (red), {[Cu(tr2ad)(NO<sup>3</sup> )](NO<sup>3</sup> )}*<sup>n</sup>* (blue) and {[Cd(tr2ad)(NO<sup>3</sup> )] (NO<sup>3</sup> )·H2O}*<sup>n</sup>* (fuchsia).

μ ∙ ∙ ∙ After solvent loss, no further weight loss is observed up to the decomposition onset at 325 ◦C. Upon heating, compound {[Cu(tr2ad)(NO3)](NO3)}*<sup>n</sup>* does not undergo any weight loss up to 250 ◦C, the temperature at which decomposition starts. To the best of our knowledge, the only known 2-D coordination polymers containing the tr2ad ligand of which the thermal behavior have been studied are [CuII <sup>2</sup>(tr2ad)4](Mo8O26), [CuII <sup>4</sup>(µ4-O)(tr2ad)2(MoO4)3]·7.5H2O [40] and [Cu3(tr2ad)4(H2O)4](SiF6)3·16H2O [41], which decompose at 310 ◦C, 240 ◦C and 190 ◦C, respectively. Compound [Cd2(tr2ad)Cl4]*<sup>n</sup>* displays the highest thermal robustness, peaking up to 365 ◦C. Until this temperature, no weight loss is observed. Finally, compound {[Cd(tr2ad)(NO3)](NO3)·H2O}*<sup>n</sup>* undergoes a weight loss of ca. 3.5% in the range 30–150 ◦C, which reasonably corresponds to the release of one water molecule per formula unit (calculated weight loss 3.4%). After this event, no further weight losses are observed up to the decomposition onset at 330 ◦C. To the best of our knowledge, in no case the thermal behavior of the known CdII 2-D coordination polymers containing the tr2ad ligand has been investigated, so that a comparison cannot be carried out. For the title compounds, at the end of the heating process, black residues, possibly containing carbonaceous species, have been recovered.

#### *2.3. Crystal and Molecular Structures*

Tr2ad crystallizes in the monoclinic space group *P*21/*n*. The asymmetric unit contains one tr2ad molecule in general position. Figure S2a shows the Ortep drawing at 40% probability level. Due to the

∙∙∙

π π

∙∙∙π

∙

lack of conventional hydrogen-bond donors, the crystal structure of tr2ad only features a network of weak CH···N interactions, with shortest C···N distances of 3.350(2) Å. Both triazole and adamantane CH groups act as unconventional hydrogen bond donors, and most of these non-bonding interactions are directional. Two pairs of such CH···N interactions, together with a slipped π/π interaction among adjacent triazole rings (centroid-centroid distance 3.82 Å, slippage angle 6.6◦ ), concur to the formation of tr2ad centrosymmetric dimers (Figure 3a). Such self-association is reminiscent of the pairing of 1,3,5-triphenyladamantane molecules prompted by weak CH···π interactions [42]. ∙∙∙ ∙∙∙ ∙∙∙ ∙∙∙ ∙∙∙ ∙∙∙ ∙

∙ ∙∙∙ π π ∙∙∙ ∙∙∙ **Figure 3.** Representation of portion of the crystal structure of (**a**) tr2ad and (**b**) tr2ad·3H2O, showing the principal supramolecular motifs created by non-bonding interactions involving the triazole nitrogen atoms as acceptors: (**a**) multiple CH···N interactions (black dashed lines) concur with π/π stacking interactions (red dashed lines) to form tr2ad dimers; (**b**) OH···N and OH···O hydrogen bonds (black dashed lines) support the formation of 2-D supramolecular layers. Atoms colour code: C, grey; H, light grey; N, blue; O, red.

The hydrate ligand tr2ad·3H2O crystallizes in the orthorhombic space group *Pnma*. The asymmetric unit contains half of a tr2ad molecule and half of a H2O molecule, both situated across a mirror plane (Wyckoff letter *c*), and one water molecule in general position. Figure S2b shows the Ortep drawing at 30% probability level. The primary intermolecular interactions in the crystal structure are conventional OH···N hydrogen bonds (O···N = 2.896(3), 2.932(3) Å) involving all the triazole nitrogen atoms as acceptors. These interactions assemble the tr2ad and water molecules (in a 1:2 ratio) into 1-D strips along the crystallographic *a*-axis (Figure 3b). Additional water molecules establish bridges between the strips through pairs of symmetry-equivalent OH···O bonds (O···O = 2.764(2) Å) (Figure 3b). The 2-D hydrogen-bond connectivity comprises water trimers H2O···H–O–H···OH<sup>2</sup> linked to four triazole-N sites. Overall, the crystal structures of tr2ad and tr2ad·3H2O reveal the potentiality of triazole-*N*<sup>1</sup> ,*N*<sup>2</sup> atoms as efficient hydrogen-bond acceptors.

Compound [Zn(tr2ad)Cl2]*<sup>n</sup>* crystallizes in the orthorhombic space group *P*212121. The asymmetric unit contains one ZnII ion, one tr2ad ligand and two chloride anions, all in general positions. The metal centre shows a ZnCl2N<sup>2</sup> tetrahedral stereochemistry (Figure 4a; the Figure caption collects the values of the bond distances and angles at the metal ion), defined by two chloride anions and the nitrogen atoms of the triazole rings of two tr2ad ligands. The ligands are exo-bidentate (µ2-κ*N*<sup>1</sup> :κ*N*<sup>1</sup> ′ ) and bridge neighbouring ZnII ions along 1-D polymeric chains (Figure 4b) of pitch 11.120(4) Å parallel to the [001] crystallographic direction (this occurrence rationalizing the preferred orientation pole; see Section 3.3). The chains pack in the *ab* plane defining a rectangular motif (Figure 4c). Non-bonding interactions of the kind C–H···N (C···N 3.2 Å) and C–H···Cl involving both chloride anions (C···Cl 3.5–3.7 Å) are at work within the chains and between nearby chains, respectively. No empty volume is present [43]. μ κ κ ′ ∙∙∙ ∙∙∙ ∙∙∙ ∙∙∙

∙∙∙ **Figure 4.** Representation of the crystal structure of [Zn(tr2ad)Cl<sup>2</sup> ]*n*: (**a**) the coordination sphere of the ZnII ions; (**b**) portion of the 1-D polymeric motif running along the [001] crystallographic direction; (**c**) portion of the packing, viewed along the [001] crystallographic direction. Horizontal axis, *a*; Vertical axis, *b*. Highlighted in fuchsia the rectangular packing of the 1-D chains. Atoms colour code: C, grey; H, light grey; Cl, light green; N, blue; Zn, green. Main bond distances (Å) and angles (◦ ) at the metal ions: Zn–Cl 2.153(7), 2.316(7); Zn–N 1.944(9), 2.01(1); shortest intra-chain Zn···Zn 11.120(3); N–Zn–N 99.5(5); Cl–Zn–Cl 117.7(2); Cl–Zn–N 107.0(6), 107.1(5), 110.0(5), 113.7(7).

∙ Compound {[Cu(tr2ad)Cl]Cl·4H2O}*<sup>n</sup>* crystallizes in the monoclinic space group *P*21/*m*. The asymmetric unit is composed by half of a metal centre (on an inversion centre, Wyckoff letter *b*), half of a tr2ad ligand, two halves of a chloride anion and two halves of a water molecule (all on mirror planes, Wyckoff letter *e*), as well as one water molecule (in general position). The CuII ions are hexa-coordinated in *trans*-CuCl2N<sup>4</sup> octahedral geometry defined by the nitrogen atoms of four tr2ad ligands and one

μ

of the two independent chloride anions (Figure 5a; the main bond distances and angles at the metal ions are reported in the Figure caption). µ-coordination by triazole rings and coordinated chloride anions brings about the formation of 1-D helices of metal ions (Figure 5b) with pitch 3.5863(2) Å (half of the *b*-axis) running along the crystallographic direction [010]. The tr2ad ligands, which are overall exo-tetradentate (µ4-κ*N*<sup>1</sup> :κ*N*<sup>2</sup> :κ*N*<sup>1</sup> ′ :κ*N*<sup>2</sup> ′ ), connect the helices along the crystallographic direction [001], bringing about the formation of 2-D double-layers parallel to the *bc* crystallographic plane (Figure 5c). The layers pack, staggered, along the *a*-axis. The second independent chloride anion and one of the four independent water molecules occupy the rhombic cavities formed by the ligands within the double-layers (Figure 5c) and are involved in a HO–H···Cl non-bonding interaction (O···Cl 2.58(3) Å; Figure S7a). The other three water molecules are located in the inter-layer space and, by means of hydrogen bonds (O···O 2.44(2), 2.91(2), 3.07(2) Å; Figure S3a), define a 1-D supramolecular chain running parallel to the *b*-axis. Finally, the double layers are reinforced by C–H···Cl interactions (C···Cl 3.3–3.4 Å; Figure S3b). No empty volume is envisaged [43]. μ κ κ κ ′ κ ′ ∙∙∙ ∙∙∙ ∙∙∙ ∙∙∙ ∙∙∙

∙ ∙∙∙ ∙∙∙ **Figure 5.** Representation of the crystal structure of {[Cu(tr2ad)Cl]Cl·4H2O}*n*: (**a**) the coordination sphere of the metal ions; (**b**) portion of the 1-D helix; (**c**) portion of the packing, viewed along the [010] crystallographic direction. Horizontal axis, *c*; vertical axis, *a*. For the non-bonding interactions quoted in the text the reader is addressed to Figure S3. Atoms colour code: C, grey; H, light grey; Cl, light green; Cu, green; N, blue; O, red. Main bond distances (Å) and angles (◦ ) at the metal ions: Cu–Cl 2.632(7); Cu–N 1.969(4), 2.009(6); shortest Cu···Cu intra-chain 3.5863(2); shortest Cu···Cu inter-chain 10.9798(7); N–Cu–N 80.4(3), 99.6(3), 180; Cl–Cu–Cl 180; Cl–Cu–N 88.1(3), 89.7(3), 90.3(3), 91.9(3).

μ κ κ κ ′ Compound [Cd2(tr2ad)Cl4]*<sup>n</sup>* crystallizes in the triclinic space group *P*-1. The asymmetric unit equals the formula unit, i.e., it contains two cadmium(II) ions, four chloride anions and one tr2ad ligand, all in general positions. Both independent metal centres are hexa-coordinated and show an octahedral stereochemistry, though of different kind, namely: *cis*-CdN2Cl<sup>4</sup> and CdNCl<sup>5</sup> (Figure 6a; the main bond distances and angles at the metal ions are reported in the Figure caption). Three of the four chloride anions bridge adjacent metal centres, while the fourth one behaves as a terminal ligand. The tr2ad spacer is exo-tridentate (µ3-κ*N*<sup>1</sup> :κ*N*<sup>2</sup> :κ*N*<sup>1</sup> ′ ). The reciprocal disposition of cations and anions brings about the formation of 1-D polymeric strands (Figure 6b) running along the [100] crystallographic direction. The tr2ad linkers bridge nearby strands leading to the formation of 2-D double-layers parallel to the (01-1) plane and packing, staggered, along the [011] direction (Figure 6c). The reciprocal disposition of the spacers within a layer brings about the formation of intra-layer rhombic cavities, in which the terminal chloride anions are directed (Figure 6c). This structural motif is

∙∙∙ ∙∙∙

μ κ κ

analogous to that found in {[Cu(tr2ad)Cl]Cl·4H2O}*<sup>n</sup>* (see above). Weak intra- and inter-layer C–H···Cl interactions (C···Cl 3.3–3.7 Å) are present. No empty volume is observed [43]. μ κ κ κ ′ κ ′ μ

∙∙∙ ∙∙∙ **Figure 6.** Representation of the crystal structure of [Cd<sup>2</sup> (tr2ad)Cl<sup>4</sup> ]*n*: (**a**) the coordination sphere of the metal ions; (**b**) portion of the 1-D polymeric strands; (**c**) portion of the packing, viewed along the [100] crystallographic direction. Horizontal axis, *b*; vertical axis, *c*. Atoms colour code: C, grey; H, light grey; Cl, light green; Cd, green; N, blue. Main bond distances (Å) and angles (◦ ) at the metal ions: Cd1–Cl 2.59(2), 2.62(2), 2.75(2), 2.82(2), 2.87(2); Cd1–N 2.42(3); Cd2–Cl 2.58(2), 2.59(1), 2.62(2), 2.78(2); Cd2–N 2.32(2), 2.32(3); shortest intra-strand Cd···Cd 3.842(8)–4.241(8); shortest inter-strand Cd···Cd 7.526(7) Cl–Cd1–N 84(1)–165.0(1); Cl–Cd1–Cl 82.5(5)–174.2(5); Cl–Cd2–N 77(1)–165(1); Cl–Cd2–Cl 85.6(5)–173.5(6); N–Cd2–N 92.2(8).

∙∙∙ ∙∙∙ ∙ Compound {[Cu(tr2ad)(NO3)](NO3)}*<sup>n</sup>* crystallizes in the orthorhombic space group *Pnma*. The asymmetric unit is composed by one CuII ion, two halves of nitrate anions and half of a tr2ad spacer, all lying on mirror planes (Wyckoff letter *h*). The metal centre is hexa-coordinated in *trans*-CuN4O<sup>2</sup> stereochemistry (Figure 7a; the main bond distances and angles at the metal ions are reported in the Figure caption), defined by four tr2ad linkers and one of the two independent nitrate anions. The latter bridges (µ2-κ*O*<sup>1</sup> :κ*O*<sup>2</sup> ) nearby metal centres 3.54(2) Å apart, while the other nitrate anion is not coordinated. The tr2ad ligand is exo-tetradentate (µ4-κ*N*<sup>1</sup> :κ*N*<sup>2</sup> :κ*N*<sup>1</sup> ′ :κ*N*<sup>2</sup> ′ ). µ2-bridging of the nitrate anions and triazole rings brings about the formation of 1-D chains running along the [100] direction (Figure 7b).

μ κ κ

μ κ κ κ ′ κ ′ μ

∙∙∙ ∙∙∙

∙∙∙ ∙∙∙ **Figure 7.** Representation of the crystal structure of {[Cu(tr2ad)(NO<sup>3</sup> )](NO<sup>3</sup> )}*n*: (**a**) the coordination sphere of the metal ions; (**b**) portion of the 1-D polymeric chain; (**c**) portion of the packing, viewed along the [100] crystallographic direction. Horizontal axis, *b*; vertical axis, *c*. Atoms colour code: C, grey; H, light grey; Cu, green; N, blue; O, red. Main bond distances (Å) and angles (◦ ) at the metal ions: Cu–N 2.03(1), 2.148(9); Cu–O 2.09(2), 2.26(2); intra-chain shortest Cu···Cu 3.54(2); inter-chain shortest Cu···Cu 10.7226(7); N–Cu–N 76.4(6), 97.01(3), 173.5(5); O–Cu–O 180(1); O–Cu–N 68.9(5), 70.3(6), 109.8(6), 111.0(6).

Nearby chains are connected along [010] by the tr2ad spacers within 2-D polymeric double-layers parallel to the (001) plane and packing, staggered, along the [001] direction (Figure 7c; this occurrence explains the preferred orientation pole—see Section 3.3). The reciprocal disposition of the tr2ad linkers within a layer brings about the formation of intra-layer rhombic cavities, in which the not coordinated nitrate anions are located (Figure 7c) and involved in C–H···O non-bonding interactions (C···O 2.6–3.2 Å). The structural motif is analogous to that found in the CuII and CdII compounds described above. No empty volume is observed [43].

Compound {[Cd(tr2ad)(NO3)](NO3)·H2O}*<sup>n</sup>* crystallizes in the monoclinic space group *C*2/*c*. The asymmetric unit contains two halves of CdII ions (one on an inversion centre, Wyckoff position *b*, the other one on a two-fold axis, Wyckoff position *e*), two nitrate anions, one tr2ad ligand and one water molecule—all in general positions. The metal centres are hexa-coordinated in *trans*-CdN4O<sup>2</sup> octahedral stereochemistry defined by four ligands and two nitrate anions (Figure 8a; the main bond distances and angles at the metal ions are reported in the Figure caption). One of the two independent nitrate anions bridge (µ2-κ*O*<sup>1</sup> :κ*O*<sup>2</sup> ) neighbouring metal centres, while the other one is not coordinated. The tr2ad spacer is exo-tetradentate (µ4-κ*N*<sup>1</sup> :κ*N*<sup>2</sup> :κ*N*<sup>1</sup> ′ :κ*N*<sup>2</sup> ′ ). µ2-Coordination of the nitrate anions and triazole rings is responsible for the formation of 1-D chains parallel to the [001] crystallographic direction (Figure 8b). Adjacent chains are bridged along the [010] direction to yield 2-D double-layers parallel to the *bc* plane and packing, staggered, along the *a*-axis (Figure 8c; this occurrence explains the preferred orientation pole—see Section 3.3). The reciprocal disposition of the tr2ad linkers within a layer brings about the formation of intra-layer rhombic cavities, in which the not coordinated nitrate anions are located (Figure 8c). The structural motif is analogous to that found in the CuII and CdII compounds described above. The water molecules are located in the inter-layer space (Figure 8c) and are involved in C–H···O non-bonding interactions (C···O 2.9–3.2 Å) with adjacent tr2ad ligands. No empty volume is observed [43].

∙ ∙∙∙ ∙∙∙ **Figure 8.** Representation of the crystal structure of {[Cd(tr2ad)(NO<sup>3</sup> )](NO<sup>3</sup> )·H2O}*n*: (**a**) the coordination sphere of the metal ions; (**b**) portion of the 1-D polymeric chains; (**c**) portion of the packing, viewed in perspective along the [001] crystallographic direction. Horizontal axis, *b*; vertical axis, *a*. Atoms colour code: C, grey; H, light grey; Cu, green; N, blue; O, red. Main bond distances (Å) and angles (◦ ) at the metal ions: Cd1–N 2.24(1), 2.30(1); Cd1–O 1.54(3); Cd2–N 2.17(1), 2.23(1); Cd2–O 2.79(4); intra-chain Cd···Cd 3.8724(3); inter-chain Cd···Cd 11.39(2); N–Cd1–N 80.2(5)–176.3(5); O–Cd1–O 168(3); N–Cd1–O 72(1)–105(1); N–Cd2–N 88.7(6), 91.3(6), 180; O–Cd2–O 180; N–Cd2–O 68(1)–110.1(8).

#### *2.4. Comparative Structure Analysis*

A search in the Cambridge Structural Database (v 2020.1) for coordination compounds containing the tr2ad ligand has revealed the existence of 22 coordination polymers. Table 1 collects key structural aspects (coordination sphere and geometry at the metal ion, tr2ad ligand hapticity, polymer dimensionality) of these compounds. The following observations can be carried out:


**Table 1.** Main structural properties of the known coordination polymers containing the tr2ad ligand. Abbreviations: M = metal ion; OC = octahedral; SP = square planar; SQP = square pyramidal; TB = trigonal bipyramidal; TD = tetrahedral; TP = trigonal planar; H3btc = 1,3,5-benzenetricarboxylic acid; H4adtc=1,3,5,7-adamantane-tetracarboxylic acid; a M–F.


#### *Inorganics* **2020**,*8*, 60


**Table 1.** *Cont.*

#### **3. Materials and Methods**

#### *3.1. General*

All reagents and solvents were purchased from Sigma-Aldrich (Darmstadt, Germany) and used as received, without further purification. The ligand 1,3-bis(1,2,4-triazol-4-yl)adamantane (tr2ad) was synthesized by the acid-catalyzed condensation reaction of 1,3-diaminoadamantane and *N*,*N*-dimethylformamide azine, according to an already reported method [46]. A detailed description regarding the preparation of the intermediates (Scheme S1), on which no details have ever been reported before, is provided in the Supplementary Materials. NMR spectra (DMSO-*d*6, δ, ppm) were recorded on a Bruker 400 MHz spectrometer. The IR spectra were recorded from 4000 to 650 cm−<sup>1</sup> with a PerkinElmer Spectrum 100 instrument (Perkin-Elmer, Shelton, CT, USA) by attenuated total reflectance on a CdSe crystal. Elemental analyses (carbon, hydrogen, and nitrogen %) were performed with a Fisons Instruments 1108 CHNS-O elemental analyzer (Thermo Scientific, Waltham, MA, USA). Before the analytical characterization was carried out, all the samples were dried under vacuum (50 ◦C, ~0.1 Torr) until a constant weight was reached. Thermogravimetric analyses (TGAs) were carried out under a N<sup>2</sup> flow (25 mL/min), in the temperature range 30–700 ◦C and with a heating rate of 5 ◦C/min, using a PerkinElmer STA 6000 simultaneous thermal analyzer (Perkin-Elmer, Shelton, CT, USA).

#### *3.2. Synthesis of the Tr2ad-Based CPs*

#### 3.2.1. Synthesis of [Zn(tr2ad)Cl2]*<sup>n</sup>*

Tr2ad (0.054 g, 0.2 mmol) was dissolved in *N*,*N*-dimethylformamide (DMF) (5 mL) and the obtained solution was left under stirring at room temperature for 5 min. Then, ZnCl2·2H2O (0.017 g, 0.1 mmol) was added, and the resulting solution was introduced into a high-pressure glass tube and heated at 150 ◦C for 24 h. Slow cooling of the solution to room temperature, followed by partial slow evaporation of the solvent, afforded a white solid which was filtered off, washed twice with DMF, dried under vacuum and identified as [Zn(tr2ad)Cl2]. Yield: 55%. [Zn(tr2ad)Cl2] is insoluble in alcohols, acetone, acetonitrile, chlorinated solvents, DMF, dimethylsulfoxide (DMSO) and water. Elem. anal. calc. for C14H18Cl2N6Zn (FW = 406.65 g/mol): C, 41.35; H, 4.46; N, 20.67%. Found: C, 40.95; H, 4.23; N, 20.35%. IR (cm−<sup>1</sup> ): 3160(w), 3110(m) ν(C–Haromatic), 2930(m), 2870(m) ν(C–Haliphatic), 1539(s) ν(C=N), 1387(m), 1329(m), 1192(vs), 1110(m), 1029(vs), 973(w), 883(m), 934(m), 786(w), 728(m), 680(m), 656(vs).

#### 3.2.2. Synthesis of {[Cu(tr2ad)Cl]Cl·4H2O}*<sup>n</sup>*

Tr2ad (0.054 g, 0.2 mmol) was dissolved in DMF (5 mL) and the obtained solution was left under stirring at room temperature for 5 min. Then, CuCl<sup>2</sup> (0.013 g, 0.1 mmol) was added, and the resulting solution was introduced into a high-pressure glass tube and heated at 150 ◦C for 24 h. Slow cooling of the solution to room temperature, followed by slow partial evaporation of the solvent, afforded a light blue solid which was filtered off, washed twice with DMF, dried under vacuum and identified as [Cu(tr2ad)Cl]Cl·4H2O. Yield: 65%. [Cu(tr2ad)Cl]Cl·4H2O is insoluble in alcohols, acetone, acetonitrile, chlorinated solvents, DMF, DMSO and water. Elem. anal. calc. for C14H26Cl2CuN6O<sup>4</sup> (FW = 476.91 g/mol): C, 35.26; H, 5.49; N, 17.62%. Found: C, 34.85; H, 5.33; N, 17.25%. IR (cm−<sup>1</sup> ): 3500–3200(br) ν(H–O), 3200–3000(w) ν(C–Haromatic), 2921(m), 2864(w) ν(C–Haliphatic), 1543(s) ν(C=N), 1346(s), 1207(vs), 1086(vs), 1045(w), 1021(w), 883(m), 842(w), 786(w), 728(m), 680(m).

#### 3.2.3. Synthesis of [Cd2(tr2ad)Cl4]*<sup>n</sup>*

Tr2ad (0.054 g, 0.2 mmol) was dissolved in methanol (5 mL) and the obtained solution was left under stirring at room temperature for 5 min. Then, CdCl<sup>2</sup> (0.036 g, 0.2 mmol) was added, and the resulting solution was introduced into a high-pressure glass tube and heated at 100 ◦C for 24 h. The white precipitate which was formed was filtered off, washed three times with hot methanol, dried under vacuum and identified as [Cd2(tr2ad)Cl4]. Yield: 65%. [Cd2(tr2ad)Cl4] is insoluble in

alcohols, acetone, acetonitrile, chlorinated solvents, DMF, DMSO and water. Elem. anal. calc. for C14H18Cd2Cl4N<sup>6</sup> (FW = 637.00 g/mol): C, 26.40; H, 2.85; N, 13.19%. Found: C, 26.33; H, 2.92; N, 12.85%. IR (cm−<sup>1</sup> ): 3143(w), 3121(m) ν(C–Haromatic), 2915(m), 2858(w) ν(C–Haliphatic), 1737(m), 1534(m) ν(C=N), 1371(m), 1253(m), 1191(vs), 1105(m), 1069(s), 1046(vs), 990(m), 883(m), 866(m), 850(m), 791(w), 730(m), 681(m), 658(m).

#### 3.2.4. Synthesis of {[Cu(tr2ad)(NO3)](NO3)}*<sup>n</sup>*

Tr2ad (0.054 g, 0.2 mmol) was dissolved in methanol (5 mL) and the obtained solution was left under stirring at room temperature for 5 min. Then, Cu(NO3)2·2.5H2O (0.037 g, 0.2 mmol) was added, and the resulting solution was introduced into a high-pressure glass tube and heated at 100 ◦C for 24 h. A green precipitate was formed, which was filtered off, washed three times with hot methanol, dried under vacuum and identified as [Cu(tr2ad)(NO3)](NO3). Yield: 70%. [Cu(tr2ad)(NO3)](NO3) is insoluble in alcohols, acetone, acetonitrile, chlorinated solvents, DMF, DMSO and water. Elem. Anal. calc. for C14H18CuN8O<sup>6</sup> (FW = 457.95 g/mol): C, 36.72; H, 3.96; N, 24.47%. Found: C, 36.35; H, 3.73; N, 24.19%. IR (cm−<sup>1</sup> ): 3136(m), 3016(vw) ν(C–Haromatic), 2916(m), 2856(w) ν(C–Haliphatic), 1755(vw), 1733(vw), 1551(s) ν(C=N), 1439(s) νasym(coordinated NO3), 1394(vs) νasym(uncoordinated NO3), 1346(vs) νsym(uncoordinated NO3), 1329(vs), 1282(vs) νsym(coordinated NO3), 1210(s), 1180(m), 1119(w), 1089(s), 1073(m), 1054(m), 1019(s), 867(vs), 826(m), 787(w), 738(m), 716(w), 681(m).

#### 3.2.5. Synthesis of {[Cd(tr2ad)(NO3)](NO3)·H2O}*<sup>n</sup>*

Tr2ad (0.054 g, 0.2 mmol) was dissolved in methanol (5 mL) and the obtained solution was left under stirring at room temperature for 5 min. Then, Cd(NO3)2·4H2O (0.047 g, 0.2 mmol) was added, and the resulting solution was introduced into a high pressure glass tube and heated at 100 ◦C for 24 h. The white precipitate which was formed was filtered off, washed three times with hot methanol, dried under vacuum and identified as [Cd(tr2ad)(NO3)](NO3)·H2O. Yield: 60%. [Cd(tr2ad)(NO3)](NO3)·H2O is insoluble in alcohols, acetone, acetonitrile, chlorinated solvents, DMF, DMSO and water. Elem. Anal. calc. for C14H20CdN8O<sup>7</sup> (FW = 524.83 g/mol): C, 32.04; H, 3.84; N, 21.35%. Found: C, 31.87; H, 3.55; N, 20.98%. IR (cm−<sup>1</sup> ): 3418(br) ν(H–O), 3098(w), 3043(w) ν(C–Haromatic), 2916(m), 2869(w), 2853(w) ν(C–Haliphatic), 1748(vw), 1717(vw), 1633(w), 1543(m) ν(C=N), 1478(s) νasym(coordinated NO3), 1374(vs) νasym(uncoordinated NO3), 1339(vs) νsym(uncoordinated NO3), 1301(vs), 1275(vs) νsym(coordinated NO3), 1208(vs), 1174(m), 1110(w), 1076(m), 1037(vs), 998(vs), 851(m), 828(m), 789(w), 731(s), 683(s).

#### *3.3. X-ray Di*ff*raction Structural Analysis*

#### 3.3.1. Structural Analysis of tr2ad and tr2ad·3H2O

The X-ray diffraction data of tr2ad (colorless prism with dimensions of 0.27 × 0.22 × 0.20 mm) and tr2ad·3H2O (colorless prism with dimensions of 0.33 × 0.16 × 0.13 mm) were collected at 173 K on a Bruker APEXII area-detector diffractometer (Bruker, Billerica, MA, USA) equipped with a sealed X-ray tube (Mo-Kα radiation, λ = 0.71073 Å). The data were corrected for Lorentz-polarization effects and for the effects of absorption (multi-scans method). The crystal structures were solved by direct methods and refined against *F* <sup>2</sup> using the programs SHELXS-97 or SHELXL-2018/1 [54,55]. The non-hydrogen atoms were assigned anisotropic thermal displacement parameters. All the hydrogen atoms were located in difference Fourier maps and then refined freely with isotropic thermal displacement parameters and with soft similarity restraints applied to O–H bond lengths in the structure of tr2ad·3H2O.

Crystal data for tr2ad, FW = 270.34 g mol−<sup>1</sup> : monoclinic, *P*21/*n*, *a* = 8.9372(4) Å, *b* = 8.7877(5) Å, *c* = 16.8366(8) Å, β = 97.888(2)◦ , *V* = 1309.79(11) Å<sup>3</sup> , *Z* = 4, ρ = 1.371 g cm−<sup>3</sup> , *F*(000) = 576, *R*1 = 0.039, w*R*2 = 0.095 [*I* > 2σ(*I*)] and *R*1 = 0.053, w*R*2 = 0.1032 (all data) for 2656 data and 253 parameters in the 4.9–52.8◦ 2θ range. CCDC No. 2034960.

Crystal data for tr2ad·3H2O, FW = 324.39 g mol−<sup>1</sup> : orthorhombic, *Pnma*, *a* = 9.6759(10) Å, *b* = 16.3052(8) Å, *c* = 10.0229(11) Å, *V* = 1581.3(3) Å<sup>3</sup> , *Z* = 4, ρ = 1.363 g cm−<sup>3</sup> , *F*(000) = 696, *R*1 = 0.049, w*R*2 = 0.087 [*I* > 2σ(*I*)] and *R*1 = 0.110, w*R*2 = 0.104 (all data) for 1656 data and 164 parameters in the 4.8–52.8◦ 2θ range. CCDC No. 2034961.

#### 3.3.2. Structural Analysis of the Coordination Polymers

Powdered samples (~50 mg) of the five CPs were deposited in the cavity of a silicon free-background sample-holder 0.2 mm deep (Assing S.r.l., Monterotondo, Italy). Powder X-ray diffraction (PXRD) data acquisitions were carried out with a Bruker AXS D8 Advance vertical-scan θ:θ diffractometer (Bruker, Billerica, MA, USA), equipped with a sealed X-ray tube (Cu-Kα, λ = 1.5418 Å), a Bruker Lynxeye linear position-sensitive detector, a filter of nickel in the diffracted beam and the following optical components: primary beam Soller slits (aperture 2.5◦ ), fixed divergence slit (aperture 0.5◦ ), anti-scatter slit (aperture 8 mm). The generator was set at 40 kV and 40 mA. Preliminary PXRD analyses to unveil the purity and crystallinity of the samples were performed in the 2θ range 3.0–35.0◦ , with steps of 0.02◦ and time per step of 1 s. PXRD acquisitions for the assessment of the crystal structure were performed in the 2θ range 5.0–105.0◦ , with steps of 0.02◦ and time per step of 10 s. After a standard peak search, enabling us to assess the maximum position of the 20–25 lower-angle peaks, indexing was performed applying the Singular Value Decomposition approach [56] implemented in TOPAS-R V3 [57]. The space groups were assigned based on the systematic absences. The crystallographically independent portion of the tr2ad ligand and nitrate anion were described using rigid bodies built up through the z-matrix formalism, assigning average values to the bond distances and angles (For tr2ad: Cad/tz–Cad = 1.55 Å, Ctz–Ntz, Ntz–Ntz = 1.36 Å, C–H = 0.95 Å; Triazole Internal and External Bond Angles = 108 and 126◦ ; Angles at the Cad Atoms = 109.5◦ . For the Nitrate Anion: N–O = 1.30 Å; O–N–O = 120◦ .). The structures were solved working in the real space with the Simulated Annealing approach [58], as implemented in TOPAS-R V3. Structures refinement was carried out with the Rietveld method [59], as implemented in TOPAS-R V3. The background was modelled through a polynomial function of the Chebyshev type. An isotropic thermal factor [Biso(M)] was refined for the metal centres; the isotropic thermal factor of lighter atoms was calculated as Biso(L) = Biso(M) + 2.0 (Å<sup>2</sup> ). The peak profile was modelled trough the Fundamental Parameters Approach [60]. The anisotropic shape of the peaks was modelled with the aid of spherical harmonics in all the cases. A correction was applied for preferred orientation adopting the March-Dollase model [61] in the case of [Zn(tr2ad)Cl2]*<sup>n</sup>* and {[Cu(tr2ad)(NO3)](NO3)}*<sup>n</sup>* (along the [001] pole), as well as of {[Cd(tr2ad)(NO3)](NO3)·H2O}*<sup>n</sup>* (along the [100] pole). The final Rietveld refinement plots are shown in Figures S5–S9.

Crystal data for [Zn(tr2ad)Cl2]*n*, FW = 406.65 g mol−<sup>1</sup> : orthorhombic, *P*212121, *a* = 14.6240(3) Å, *b* = 10.1054(2) Å, *c* = 11.1204(1) Å, *V* = 1643.40(5) Å<sup>3</sup> , *Z* = *Z*' = 4, ρ = 1.64 g cm−<sup>3</sup> , *F*(000) = 832.0, *<sup>R</sup>*Bragg <sup>=</sup> 0.051, *<sup>R</sup>*<sup>p</sup> <sup>=</sup> 0.057 and *<sup>R</sup>*wp <sup>=</sup> 0.078, for 4801 data and 46 parameters in the 9.0–105.0◦ <sup>2</sup>θ range. CCDC No. 2038425.

Crystal data for {[Cu(tr2ad)Cl]Cl·4H2O}*n*, FW <sup>=</sup> 476.91 g mol−<sup>1</sup> : monoclinic, *P*21/*m*, *a* = 14.5644(9) Å, *b* = 7.1726(4) Å, *c* = 10.9798(6) Å, β = 122.820(3)◦ , *V* = 963.9(1) Å<sup>3</sup> , *Z* = 4, *Z*' = 2, ρ = 1.64 g cm−<sup>3</sup> , *F*(000) = 494.0, *R*Bragg = 0.019, *R*<sup>p</sup> = 0.028 and *R*wp = 0.040, for 4951 data and 71 parameters in the 6.0–105.0◦ 2θ range. CCDC No. 2038423.

Crystal data for [Cd2(tr2ad)Cl4]*n*, FW = 637.00 g mol−<sup>1</sup> : triclinic, *P*-1, *a* = 6.9425(2) Å, *b* = 12.2352(3) Å, *c* = 12.6513(2) Å, α = 115.621(1)◦ , β = 90.837(2)◦ , γ = 101.165(2)◦ , *V* = 944.74(4) Å<sup>3</sup> , *Z* = *Z*' = 2, ρ = 2.24 g cm−<sup>3</sup> , *F*(000) = 616.0, *R*Bragg = 0.050, *R*<sup>p</sup> = 0.049 and *R*wp = 0.068, for 4901 data and 68 parameters in the 7.0–105.0◦ 2θ range. CCDC No. 2038421.

Crystal data for {[Cu(tr2ad)(NO3)](NO3)}*n*, FW <sup>=</sup> 457.95 g mol−<sup>1</sup> : orthorhombic, *Pnma*, *a* = 7.0648(4) Å, *b*=10.7226(5) Å, *c*=22.495(1) Å, *V* =1704.0(2) Å<sup>3</sup> ,*Z*=8,*Z*'=4, ρ=1.79 g cm−<sup>3</sup> , *F*(000)=940.0,*R*Bragg =0.025, *R*<sup>p</sup> = 0.037 and *R*wp = 0.048, for 4951 data and 73 parameters in the 6.0–105.0◦ 2θ range. CCDC No. 2038424.

Crystal data for {[Cd(tr2ad)(NO3)](NO3)·H2O}*n*, FW = 524.83 g mol−<sup>1</sup> : monoclinic, *C*2/*c*, *a* = 23.181(1) Å, *b* = 11. 3867(2) Å, *c* = 15.486(1) Å, β = 108.956(5)◦ , *V* = 3866.1(3) Å<sup>3</sup> , *Z* = *Z*' = 8, ρ = 1.80 g cm−<sup>3</sup> , *F*(000) = 2112.0, *R*Bragg = 0.047, *R*<sup>p</sup> = 0.079 and *R*wp = 0.110, for 4901 data and 53 parameters in the 7.0–105.0◦ 2θ range. CCDC No. 2038422.

#### **4. Conclusions**

In this work, we have described the synthesis and solid-state characterization of the novel coordination polymers (CPs) [Zn(tr2ad)Cl2]*n*, {[Cu(tr2ad)Cl]Cl·4H2O}*n*, [Cd2(tr2ad)Cl4]*n*, {[Cu(tr2ad)(NO3)](NO3)}*<sup>n</sup>* and {[Cd(tr2ad)(NO3)](NO3)·H2O}*<sup>n</sup>* [tr2ad = 1,3-bis(1,2,4-triazol-4-yl)adamantane], isolated as airand moisture-stable microcrystalline powders by means of solvothermal reactions. As assessed by thermogravimetric analysis, the five CPs show an appreciable thermal stability. As retrieved by powder X-ray diffraction, while [Zn(tr2ad)Cl2]*<sup>n</sup>* features 1-D chains, the other compounds contain 2-D double-layers. A comparative structural analysis involving known CPs built up with the tr2ad ligand unveiled the coordination modes versatility of the ligand and the crystal structure dimensionality variability. Work can be anticipated in the functional characterization of these CPs as heterogeneous catalysts for cutting-edge organic reactions.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2304-6740/8/11/60/s1. Detailed synthesis of the tr2ad ligand (Scheme S1). FTIR spectrum of tr2ad (Figure S1). Ortep drawings for tr2ad and tr2ad·3H2O (Figure S2). Further representation of the crystal structure of {[Cu(tr2ad)Cl]Cl·4H2O}*<sup>n</sup>* (Figure S3). Comparison of bond distances at the metal ion (Figure S4). Graphical result of the final structure refinement carried out on [Zn(tr2ad)Cl<sup>2</sup> ]*n*, {[Cu(tr2ad)Cl]Cl·4H2O}*n*, [Cd<sup>2</sup> (tr2ad)Cl<sup>4</sup> ]*n*, {[Cu(tr2ad)(NO<sup>3</sup> )](NO<sup>3</sup> )}*n*, and {[Cd(tr2ad)(NO<sup>3</sup> )](NO<sup>3</sup> )·H2O}*<sup>n</sup>* (Figures S5-S9). The CIF files of tr2ad, tr2ad·3H2O and the five CPs, and the checkCIF output files of tr2ad and tr2ad·3H2O.

**Author Contributions:** Conceptualization, C.P. and S.G.; investigation: N.X., A.T., S.G., M.M., K.V.D. and G.A.S.; resources: K.V.D., C.P. and S.G.; writing—original draft preparation: A.T. and S.G.; writing—review and editing, A.T. and S.G.; supervision, C.P. and S.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** S.G. acknowledges Università dell'Insubria and C.P. acknowledges Università di Camerino for partial funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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

© 2020 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 (http://creativecommons.org/licenses/by/4.0/).

### *Article* **Adsorption Properties of Ce5(BDC)7.5(DMF)<sup>4</sup> MOF**

#### **Cesare Atzori <sup>1</sup> , Jayashree Ethiraj <sup>2</sup> , Valentina Colombo <sup>3</sup> , Francesca Bonino 1,\* and Silvia Bordiga <sup>1</sup>**


Received: 7 November 2019; Accepted: 21 January 2020; Published: 26 January 2020

**Abstract:** In this article we report on the spectroscopic and adsorptive studies done on Ce(III)-based MOF possessing, upon desolvation, open metal sites, and a discrete surface area. The Ce-based MOF was synthesized from terephthalic acid linker (H2BDC) and Ce3<sup>+</sup> cations by the classical solvothermal method. Preliminary powder X-ray diffraction analysis showed that the obtained materials corresponded to the ones reported by other authors. Spectroscopic techniques, such as XAS and in situ FTIR with probe molecules were used. In situ FTIR spectroscopy confirmed the successful removal of DMF molecules within the pore system at temperatures above 250 ◦C. Moreover, the use of CO as a probe molecule evidenced the presence of a Ce3<sup>+</sup> open metal sites. Detailed volumetric and calorimetric CO<sup>2</sup> adsorption studies are also reported.

**Keywords:** cerium; MOF; terephthalic acid; spectroscopic characterization; adsorption; calorimetry; carbon dioxide

#### **1. Introduction**

Cerium is the most abundant lanthanide element present in the earth crust [1,2] and the ores that are mined for the extraction of more rare and precious rare earth elements (REEs) are also rich in Ce; thus, its cost is relatively low. Its oxide, CeO2, commonly named ceria, is particularly relevant for redox chemistry, being a catalyst for oxidation and reduction reactions [3], for example, combustion catalysis [4] and photocatalysis [3].

Ce-based MOFs have recently created interest in the scientific community. General features that can be drawn from looking at the current published literature are the following: (i) both Ce3<sup>+</sup> and Ce4<sup>+</sup> oxidation states can be used in the synthesis of MOFs [5–9]; (ii) synthetic conditions for Ce3+-containing MOFs tends to be harsher than Ce4<sup>+</sup> [5,6,10]; (iii) usually, Ce4<sup>+</sup> starting reagents may be reduced to Ce3<sup>+</sup> during the synthesis [11,12]; (iv) Ce3<sup>+</sup> materials more frequently have peculiar structures, while Ce4<sup>+</sup> tends to give rise to MOFs with the same structure as other 4+ cations (e.g., Zr4<sup>+</sup> or Hf4+) [5–7,13,14]. Their thermal stability is generally lower than their Zr4<sup>+</sup> counterparts [7,8,14,15]. As a possible application of Ce MOFs as redox catalysts, Smolders et al. [8] reported the successful use of Ce4+-UiO-67 in the aerobic oxidation of benzylic alcohol to benzaldehyde mediated by TEMPO (2,2,6,6-Tetramethylpiperidinyloxyl). Furthermore, Ethiraj et al. [5] reported the use of a Ce3+-based MOF for the selective capture and storage of CO2, obtaining high figures of merit of capacity and separation.

The present work reports on the synthesis, spectroscopic characterization, and adsorption properties of Ce3+-based MOFs with terephthalic acid (H2BDC) as the linker. This material has been already introduced in the literature by D'Arras et al. [11], who discovered it and suggested its crystal structure in the as-synthesized form, together with the characterization of the thermal properties. However, the porosity of the material has not been studied, indeed, here we report on its adsorption properties, examined through spectroscopic techniques (XAS and in situ FTIR) and adsorption isotherms.

#### **2. Results and Discussion**

Ce5(BDC)7.5(DMF)<sup>4</sup> MOF [16] optimized synthesis is reported in the Supplementary Materials. The Ce5(BDC)7.5(DMF)<sup>4</sup> PXRD pattern was coincident with the one reported by D'Arras et al. [11]. The hypothesized structure was taken from Reference [11] and it is reported in Figure 1, for clarity. The asymmetric unit of crystalline structure shows chains of five independent cerium atoms arranged linearly and surrounded by BDC2<sup>−</sup> and DMF molecules (see Figure 1a). The two terminal atoms of the group are coordinated by eight oxygen atoms in a distorted square antiprismatic shape: six oxygen atoms belong to BDC2<sup>−</sup> and two oxygen atoms to DMF molecules, while the "central" three cerium atoms are coordinated with nine oxygens, all coming from the ligands, in an uncommon distorted shape. There are mono-dimensional or 1D channels formed in the structure parallel to the Ce chains along the crystallographic 110 direction. As depicted in Figure 1b, these micropores are mainly occupied by the coordinated DMF molecules, which protrude into the pores. The surface area and porosity of the material were clarified by the N<sup>2</sup> adsorption experiments (vide infra). − − 11ത0

11ത0 **Figure 1.** Structure of Ce<sup>5</sup> (BDC)7.5(DMF)<sup>4</sup> . (**a**) Depiction of the asymmetric unit, containing five Ce atoms, 18 BDC2−, and four DMF molecules. Cerium, carbon, nitrogen, and oxygen atoms are pale yellow, black, blue, and red respectively; hydrogen atoms are omitted for the sake of clarity; (**b**) a view through the 110 direction. Meaningful distances are highlighted.

The powder diffraction pattern of the as synthesized material, as shown in the synthesis development reported in the Supplementary Materials (Figures S1–S3), was compared to the calculated powder pattern obtained from the crystal structure reported by D'Arras et al. (Figure S3). SEM images of the synthesized powder are available in Figure S4.

A variable temperature powder X-ray diffraction (VTXRD) experiment in N<sup>2</sup> flow in the RT–600 ◦<sup>C</sup> temperature range (see Figure 2) showed that the material maintained the crystallinity until 475 ◦C, undergoing some changes in the XRD pattern, especially from 200 to 250 ◦C (highlighted in blue color), which could be due to the solvent removal, as strongly suggested by the TGA measurements reported in Figure S5.

The solution of the crystal structure of the desolvated material was out of the scope of the present work. At 500 ◦C the MOF started decomposing and at 525 ◦C the formation of broad diffraction peaks due to cerium dioxide was visible. The broadness of the peaks testifies that the particles were nanometric in size. Scherrer's equation [3], corrected by instrumental broadening using a Si standard from NIST, suggested a size for the cerium dioxide particles of 5 ± 1 nm.

**Figure 2.** Variable temperature powder X-ray diffraction (VTXRD) recorded in the RT–600 ◦C range in N<sup>2</sup> flow.

The 3+ oxidation state of Ce in the as-synthesized state and activated at 350 ◦C material was confirmed by means of XAS spectroscopy, in the XANES region comparison with Ce3<sup>+</sup> and Ce4<sup>+</sup> standards. XANES spectra are reported in Figure 3. The 3+ oxidation state was also maintained after activation at 350 ◦C in the He stream directly in the measurement cell, in agreement with the results reported by the XPS experiments of D'Arras et al. [11].

**Figure 3.** Ce L<sup>3</sup> edge XANES spectra of Ce<sup>5</sup> (BDC)7.5(DMF)<sup>4</sup> as synthesized and activated at 350 ◦C, compared with CeO<sup>2</sup> and Ce(NO<sup>3</sup> )3 ·6H2O.

N<sup>2</sup> adsorption volumetric isotherms were measured on the material in order to point out the specific surface area and porosity of the MOF. Thermal treatments were performed in vacuo for 3 h (a longer time in comparison with those ones performed in case of IR or XRD because of the bigger amount of sample) and were made in a consecutive way. From the isotherms reported in Figure 4 it is clear that upon the solvent loss (occurring in the 200–250 ◦C range) N<sup>2</sup> adsorption grew dramatically, showing microporosity (as the isotherm is a Type I) and higher surface area (more than 200 m<sup>2</sup> /g) due to the accessible pores.

BET and Langmuir adsorption models for the surface area were applied; the results are summarized in Table 1. Generally, the reported value can be quite modest for MOF materials, compared with typical MOF surface areas (thousands of m<sup>2</sup> /g) [17].

− **Figure 4.** <sup>N</sup><sup>2</sup> Adsorption isotherms at <sup>−</sup>−<sup>196</sup> ◦C measured on Ce<sup>5</sup> (BDC)7.5(DMF)<sup>4</sup> . at different activation temperatures (150 ◦C, 200 ◦C, 250 ◦C, 350 ◦C, 450 ◦C).


**Table 1.** BET and Langmuir surface area for Ce<sup>5</sup> (BDC)7,5(DMF)<sup>4</sup> different temperature treatments.

As synthesized Ce5(BDC)7.5(DMF)<sup>4</sup> showed the typical mid-IR spectrum for a solvated MOF (see Figure 5). The typical vibrational fingerprints due to DMF solvent molecules mainly inside the pores and to H2O molecules adsorbed from the atmosphere can be recognized: a broad band centered at 3400 cm−<sup>1</sup> due to the hydrogen-bonded H2O molecules and sharp features at frequencies lower than 3000 cm−<sup>1</sup> , in the range of the aliphatic C–H stretching mode, and a very intense band centered at 1670 cm−<sup>1</sup> , in the range of the carbonyl stretching mode, due to DMF molecules. Upon progressive outgassing, also by increasing the temperatures, vibrational signals due to DMF and H2O disappeared and the typical spectrum of a carboxylate-based MOF material was shown: very intense bands in the 1650–1250 cm−<sup>1</sup> range due to carboxylate stretching modes (both symmetrical and asymmetrical) and sharp features at frequencies higher than 3000 cm−<sup>1</sup> , in the range of aromatic C–H stretching mode due to BDC2−. − − − − − −

These data support the VTXRD, TGA (see Figure S5), and SSA experiments and therefore we can affirm the material is not destroyed even if activated at 450 ◦C, maintaining crystallinity and surface area, even if its crystal structure undergoes a phase transition that has not been determined in this study.

**Figure 5.** FTIR spectra of Ce<sup>5</sup> (BDC)7.5(DMF)<sup>4</sup> activated at different temperature in vacuo.

− − − − In situ CO adsorption FTIR spectra at −196 ◦C were recorded on a sample activated at 350 ◦C for 1 h (see Figure 6). A pressure of 5 mbar of CO was dosed (black spectrum). The lowest frequency peak, at 2131 cm−<sup>1</sup> , was assigned to the physi-sorbed CO in the pores, as the first one to be desorbed. The other two bands at higher frequencies (2161 cm−<sup>1</sup> and 2152 cm−<sup>1</sup> ) required more time for complete desorption, and for this reason they can be assigned to CO adsorbed on Lewis acidic sites.

Upon outgassing, the initial activated MOF spectrum was obtained due to the total reversibility of CO adsorption. Because the CO vibrational mode on the MOF open metal sites is intermediate between metals in oxides and metals grafted in different systems [18,19], the doublet can be assigned to CO adsorbed on Ce3<sup>+</sup> sites, since CO interacting with Ce4<sup>+</sup> is expected to give bands at frequencies

−

higher than 2156 cm−<sup>1</sup> [20,21], in agreement also with XANES results. With the crystal structure of the desolvated material unknown, we can only hypothesize the presence of at least two different Ce3<sup>+</sup> probed sites [20–22]. Only two out of the five different cerium atoms among the crystallographic asymmetric unit underwent the removal of DMF molecules during activation, however for an in-depth understanding of the desolvation process more investigation is needed. It is of relevance also the overall low intensity of the bands associated with CO adsorption could be due to a low accessibility of the uncoordinated sites. In case of FTIR CO adsorption on MOF-76-Ce [5], two quite intense bands at 2155 cm−<sup>1</sup> and 2149 cm−<sup>1</sup> , due, respectively, to the probe interacting with one Ce site and bridged on two close sites, were reported, testifying the completely different local structure of the metal in MOF-76-Ce and in the present Ce5(BDC)7.5(DMF)4. − − − − −

− **Figure 6.** Background-subtracted CO adsorption FTIR spectra at <sup>−</sup><sup>196</sup> ◦C on Ce<sup>5</sup> (BDC)7.5(DMF)<sup>4</sup> activated at 350 ◦C for 1 h.

CO<sup>2</sup> volumetric isotherms at various activation temperatures are reported in Figure 7. The same powder was heated in vacuo for three hours at the next activation temperature in a consecutive way.

**Figure 7.** CO<sup>2</sup> Adsorption isotherms at 25 ◦C on Ce<sup>5</sup> (BDC)7.5(DMF)<sup>4</sup> at different activation temperatures (150 ◦C, 200 ◦C, 250 ◦C, 350 ◦C, 450 ◦C).

CO<sup>2</sup> uptake showed a clear increase with the activation temperature by reaching a plateau at the 350–450 ◦C temperature range, as reported in Figure 7 and Table 2.


**Table 2.** Summary of CO<sup>2</sup> uptake measurements at 1 bar and 25 ◦C.

Calorimetric data, reported in Figure 8, were recorded for the adsorption of CO<sup>2</sup> at 30 ◦C in the 0–90 mbar range. The adsorbed quantity and the heat released by the adsorption as a function of the pressure are plotted, respectively, in Figure 8a,b. Both these curves are nearly Henry-type, as confirmed by the first part of our CO<sup>2</sup> volumetric adsorption isotherms collected with a different instrument at 25 ◦C (Figure 7). The studied pressure range was too low to observe saturation of the adsorbing sites and the temperature difference of 5 ◦C between these two experiments was due to technical requirements. The differential heat of adsorption at low coverages was about 32–33 kJ/mol and it is quite typical for the interaction of CO<sup>2</sup> with an open-metal site, as in MOF-76-ds [5], HKUST-1 [23], Mg-MOF-74 [17]. It is worth noting that the differential heat curve (Figure 8c) is characterized by an abrupt diminishing with the adsorbed quantity; this can be ascribed to the overall low number of strongly coordinating sites present in the desolvated material, as highlighted by our CO FTIR experiment (Figure 6). The total reversibility of the adsorption of CO<sup>2</sup> upon outgassing at 30 ◦C was testified thoroughly by the perfect recovery of the adsorption properties between the primary and secondary adsorption cycles.

**Figure 8.** (**a**) Volumetric isotherm (**b**) calorimetric isotherm and (**c**) differential heat distribution of CO<sup>2</sup> adsorption measured at 30 ◦C on Ce<sup>5</sup> (BDC)7.5(DMF)<sup>4</sup> activated at 350 ◦C.

#### **3. Materials and Methods**

α β The variable temperature X-ray diffraction patterns (VTXRD) were collected with an X'Pert PRO MPD diffractometer from PANalytical (Almelo, The Netherlands), working in Bragg–Brentano geometry equipped with a Cu Kα source using about 10 mg of sample. Scattered photons were revealed by an X'celerator linear detector (PANalytical, Almelo, The Netherlands) equipped with a Ni filter to attenuate Kβ. A non-ambient chamber XRK900 from Anton Paar (Graz, Austria) with Be windows was used to collect diffractograms as a function of temperature in a flow of dry N<sup>2</sup> (20 mL/min). The temperature program was set to measure a pattern every 25 ◦C, waiting 25 min at the target temperature before collecting the data. The temperature was increased at a rate of 2 ◦C/min.

X-Ray absorption spectra at the Ce L<sup>3</sup> edge (5723 eV) were collected at the BM23 beamline of the European Synchrotron Radiation Facility (ESRF). Data were acquired up to the Ce L<sup>2</sup> edge (6164 eV), which limited the EXAFS signal down to k ≈ 10 Å−<sup>1</sup> .The acquisition step was set to 0.3 eV in the near-edge region and ∆k = 0.035 Å−<sup>1</sup> in the EXAFS part of the spectrum. We used three He/N2-filled ionization chambers as I0, I1, and I<sup>2</sup> detectors, placing chromium foil between I<sup>1</sup> and I<sup>2</sup> for energy calibration. XANES and EXAFS data were analyzed by using the Demeter 0.9.20 package. Sample treatment was carried out in an in situ cell under a flow of He (80 mL min−<sup>1</sup> ). During activation, the sample was heated to 350 ◦C after a ramp heating at 4 ◦C min−<sup>1</sup> , to then be cooled to 30 ◦C.

Adsorption isotherms were collected on an ASAP 2020 apparatus from Micromeritics (Norcross, GA, USA) using a liquid nitrogen bath at −<sup>196</sup> ◦C, albeit CO<sup>2</sup> isotherms were collected filling the same dewar vessel with water at 25 ◦C. About 150 mg of sample was heated in dynamic vacuum at 350 ◦C for 3 h prior to measuring the isotherm. Langmuir fit was made in the 0.05 < *p*/*p*<sup>0</sup> < 0.2 range, while the BET analysis was carried out in a very low-pressure region, as prescribed by the so-called Rouquerol rules [24], in order to obtain a positive C value.

FTIR spectra were collected on a Nicolet 6700 from Thermo Scientific (Waltham, MA, USA) equipped with an MCT detector in the 4000–400 cm−<sup>1</sup> range with a resolution of 2 cm−<sup>1</sup> . The sample was prepared by pressing a thin self-supporting pellet (10 mg of sample) and using a vacuum line and a jacketed IR cell of local construction capable of cooling down the sample with liquid nitrogen and permitting the dosing of probe molecules (i.e., CO). The experiment was run by activating the pellet in dynamic vacuum at 350 ◦C for 1 h, then dosing about 15 mbar of CO on the sample to record spectra during cooling to the liquid nitrogen temperature, desorption, and then heating back to RT.

Adsorption heats were measured simultaneously with the adsorption isotherms by means of a C80 Tian-Calvet microcalorimeter from Setaram (Caluire-et-Cuire, France) at a temperature of 30 ◦C, coupled with a glass vacuum line of local construction. The procedure is thoroughly described in references [25,26], and in this case required thermal activation under dynamic vacuum at 350 ◦C for 3 h, then an overnight outgassing at 30 ◦C in the calorimeter before measuring the primary and the secondary adsorption runs in order to determine the non-desorbable (irreversible) fraction.

#### **4. Conclusions**

The synthesis of Ce5(BDC)7.5(DMF)<sup>4</sup> was successful, starting from Ce(NO3)3·6H2O and H2BDC in solvothermal conditions in DMF at 140 ◦C. We obtained the same crystal structure reported by D'Arras et al. [11] using a Ce3<sup>+</sup> source directly, conversely to the previous contribution. The thermal stability (up to 475 ◦C in an inert atmosphere) previously observed by D'Arras et al. [11] was confirmed by our VTXRD and TGA measurements. Differently from D'Arras et al. [11] we found a discrete surface area due to microporosity through N<sup>2</sup> adsorption isotherms at −<sup>196</sup> ◦C (about 220 m<sup>2</sup> /g) after thermal activation in the 250–450 ◦C range. XAS confirmed the presence of Ce3<sup>+</sup> in the material also after desolvation. FTIR spectroscopy confirmed the successful removal of DMF within the pore system at temperatures above 250 ◦C, and by means of low-temperature CO adsorption evidenced the presence of a Ce3<sup>+</sup> open metal sites. The interaction of the desolvated material with CO<sup>2</sup> was characterized by volumetric and calorimetric measurements, finding a modest capacity of adsorption (about 3.5 wt % at 1 bar and 25 ◦C) but a relevant enthalpy (32–33 kJ/mol) for the very first dose, compatible with the presence of open metal sites. This work can open the way to a deep understanding and description of this MOF crystal structure and phase changes upon activation.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2304-6740/8/2/9/s1, Figure S1: Diffractograms of 1, 3, 4, and 6 batches: adopted solvent and metal to ratio (M:L) are reported, Figure S2: Diffractograms of 8, 12, 19 batches: reaction conditions are reported, Figure S3: Diffractograms of batch 19 and the MOF reported by D'Arras et al., Figure S4: SEM images of Ce<sup>5</sup> (BDC)7.5(DMF)<sup>4</sup> MOF. Part (b) reports a

magnification of a portion reported in part (a), Figure S5: TGA in N<sup>2</sup> (solid line) and dry (dash-dotted line) air flow of Ce<sup>5</sup> (BDC)7.5(DMF)<sup>4</sup> MOF, Figure S6: Magnitude of the Fourier transform of k<sup>2</sup> χ(k) EXAFS signal (phase uncorrected) at different temperatures of Ce<sup>5</sup> (BDC)7.5(DMF)<sup>4</sup> . 2.74 < k < 9.874 Å−<sup>1</sup> range for the transform is used, Figure S7: Background subtracted CO<sup>2</sup> adsorption FTIR spectra at RT on Ce<sup>5</sup> (BDC)7.5(DMF)<sup>4</sup> activated 350 ◦C for 1 h.

**Author Contributions:** Conceptualization, F.B. and J.E.; Methodology, F.B., J.E., and C.A.; Formal Analysis, C.A.; Investigation, C.A.; Resources, S.B.; Data Curation, C.A.; Writing—Original Draft Preparation, F.B. and C.A.; Writing—Review and Editing, all authors; Visualization, C.A. and F.B.; Validation, V.C.; Supervision, F.B. and S.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by The Italian Ministry of Education, University and Research (MIUR), grant number 2017KKP5ZR (PRIN project). The APC was funded by the same institution.

**Acknowledgments:** Claudia Barolo is acknowledged for the fruitful discussion during the synthetic procedure optimization. The autors thank Jenny G. Vitillo for the help in collecting the adsorption data and Kirill A. Lomachenko for XAS data collection.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 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 (http://creativecommons.org/licenses/by/4.0/).
