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

: The new coordination polymers (CPs) [Zn(tr 2 ad)Cl 2 ] n , {[Cu(tr 2 ad)Cl]Cl · 4H 2 O} n , [Cd 2 (tr 2 ad)Cl 4 ] n , {[Cu(tr 2 ad)(NO 3 )](NO 3 )} n and {[Cd(tr 2 ad)(NO 3 )](NO 3 ) · H 2 O} n 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 (tr 2 ad). Investigation of the thermal behaviour assessed the thermal stability of these CPs, with [Cd 2 (tr 2 ad)Cl 4 ] n starting to decompose only around 365 ◦ C. As retrieved by powder X-ray di ﬀ raction, while [Zn(tr 2 ad)Cl 2 ] n 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 tr 2 ad 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(tr 2 ad)Cl 2 ] n is the ﬁrst Zn II -based CP containing the tr 2 ad spacer.


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][3][4][5], including the subclass of metal-organic frameworks (MOFs) [6][7][8][9][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 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][18][19] and biomedicine [20]. The successful preparation of CPs has generally relied on organic ligands from the class of poly(carboxylic) acids [21][22][23], pyrazines and bipyridines [21][22][23][24], phosphonic acids [25] and poly(azoles) [26][27][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 1 ,N 2 -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][32][33][34][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.
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

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.

Synthesis and Preliminary Characterization
A detailed description of the synthesis of the tr 2 ad 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 tr 2  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). as a consequence of the ligand coordination to the metal ions ( Figure 1). The medium-intensity broad bands centered around 3400 cm −1 in the IR spectra of compounds  The IR spectrum of the tr 2 ad ligand ( Figure S1, Supplementary Materials) shows a strong absorption band at 1517 cm −1 , 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 −1 ), as a consequence of the ligand coordination to the metal ions ( Figure 1). The medium-intensity broad bands centered around 3400 cm −

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(tr 2 ad)Cl 2 ] n is stable up to 350 • C, temperature at which a slow decomposition begins. In the temperature range 30-120 • C, {[Cu(tr 2 ad)Cl]Cl·4H 2 O} n 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%).

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]n 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}n 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%).   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 Cd II 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.

Crystal and Molecular Structures
Tr2ad crystallizes in the monoclinic space group P21/n. The asymmetric unit contains one tr2ad molecule in general position. Figure S2a shows the Ortep drawing at 40% probability level. Due to the 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 After solvent loss, no further weight loss is observed up to the decomposition onset at 325 • C. Upon heating, compound {[Cu(tr 2 ad)(NO 3 )](NO 3 )} n 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 tr 2 3 )·H 2 O} n 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 Cd II 2-D coordination polymers containing the tr 2 ad 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.

Crystal and Molecular Structures
Tr 2 ad crystallizes in the monoclinic space group P2 1 /n. The asymmetric unit contains one tr 2 ad molecule in general position. Figure S2a shows the Ortep drawing at 40% probability level. Due to the lack of conventional hydrogen-bond donors, the crystal structure of tr 2 ad 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 tr 2 ad 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]. 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].
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    The hydrate ligand tr 2 ad·3H 2 O crystallizes in the orthorhombic space group Pnma. The asymmetric unit contains half of a tr 2 ad molecule and half of a H 2 O 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 tr 2 ad 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 H 2 O···H-O-H···OH 2 linked to four triazole-N sites. Overall, the crystal structures of tr 2 ad and tr 2 ad·3H 2 O reveal the potentiality of triazole-N 1 ,N 2 atoms as efficient hydrogen-bond acceptors.
Compound [Zn(tr 2 ad)Cl 2 ] n crystallizes in the orthorhombic space group P2 1 2 1 2 1 . The asymmetric unit contains one Zn II ion, one tr 2 ad ligand and two chloride anions, all in general positions. The metal centre shows a ZnCl 2 N 2 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 tr 2 ad ligands. The ligands are exo-bidentate (µ 2 -κN 1 :κN 1 ) and bridge neighbouring Zn II 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].   Figure S3b). No empty volume is envisaged [43].   (Figure 6c). This structural motif is analogous to that found in {[Cu(tr 2 ad)Cl]Cl·4H 2 O} n (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]. lying on mirror planes (Wyckoff letter h). The metal centre is hexa-coordinated in trans-CuN4O2 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-κO 1 :κO 2 ) nearby metal centres 3.54(2) Å apart, while the other nitrate anion is not coordinated. The tr2ad ligand is exo-tetradentate (μ4-κN 1  . The structural motif is analogous to that found in the Cu II and Cd II compounds described above. No empty volume is observed [43].  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 Cu II and Cd II 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 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 Cu II and Cd II compounds described above. No empty volume is observed [43].  (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 tr 2 ad 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 Cu II and Cd II 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 tr 2 ad ligands. No empty volume is observed [43].

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: 1. Among the CPs retrieved in the literature (  Figure S4) that the novel materials share distances comparable with those of the literature CPs. 4. Apart from [Zn(tr2ad)Cl2]n, all the other compounds studied in this work are 2-D coordination polymers characterized by the same structural motif (see . At variance, Table 1 shows that in the known compounds the dimensionality ranges from 1-D to 3-D. Interestingly, the structural motif observed in the title Cu II and Cd II derivatives, with rhombic cavities within 2-D strands, is shown also by COVFIE, COVFOK, KEHDEI, KEMLEV and TUGSIY, containing Cu II or Cd II ions, and UZAKIQ, containing the Mo II ion.

Comparative Structure Analysis
A search in the Cambridge Structural Database (v 2020.1) for coordination compounds containing the tr 2 ad ligand has revealed the existence of 22 coordination polymers. Table 1 collects key structural aspects (coordination sphere and geometry at the metal ion, tr 2 ad ligand hapticity, polymer dimensionality) of these compounds. The following observations can be carried out:

1.
Among the CPs retrieved in the literature (Table 1), 14 contain Cu II or Cd II , while the others feature Cu I , Ag I , Mo II or Fe II . Hence, to the best of our knowledge, [Zn(tr 2 ad)Cl 2 ] n is the first example of Zn II -based coordination compound containing the tr 2 ad ligand.

2.
As regards the stereochemistry at the metal ion, apart from [Cd 3 (tr 2 ad) 2 I 6 ], in which one of the two independent Cd II ions shows a tetrahedral geometry, in all the known Cd II CPs the metal centre adopts an octahedral geometry, as in  (Table 1).

3.
Upon comparing the values of the M-N distances (M = Cu II or Cd II , N = tr 2 ad nitrogen atom), it appears ( Figure S4) that the novel materials share distances comparable with those of the literature CPs.

4.
Apart from [Zn(tr 2 ad)Cl 2 ] n , all the other compounds studied in this work are 2-D coordination polymers characterized by the same structural motif (see . At variance, Table 1 shows that in the known compounds the dimensionality ranges from 1-D to 3-D. Interestingly, the structural motif observed in the title Cu II and Cd II derivatives, with rhombic cavities within 2-D strands, is shown also by COVFIE, COVFOK, KEHDEI, KEMLEV and TUGSIY, containing Cu II or Cd II ions, and UZAKIQ, containing the Mo II ion.

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 (tr 2 ad) 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 The X-ray diffraction data of tr 2 ad (colorless prism with dimensions of 0.27 × 0.22 × 0.20 mm) and tr 2 ad·3H 2 O (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 2 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 tr 2 ad·3H 2 O.
Crystal data for tr 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 tr 2 ad 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 tr 2 ad: C ad / tz -C ad = 1.55 Å, C tz -N tz , N tz -N tz = 1. ). 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 [B iso (M)] was refined for the metal centres; the isotropic thermal factor of lighter atoms was calculated as B iso (L) = B iso (M) + 2.0 (Å 2 ). 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(tr 2 ad)Cl 2 ] n and {[Cu(tr 2 ad)(NO 3

Conclusions
In this work, we have described the synthesis and solid-state characterization of the novel coordination polymers (CPs) [Zn(tr 2 ad)Cl 2 ] n , {[Cu(tr 2 ad)Cl]Cl·4H 2 O} n , [Cd 2 (tr 2 ad)Cl 4 ] n , {[Cu(tr 2 ad)(NO 3 )](NO 3 )} n and {[Cd(tr 2 ad)(NO 3 )](NO 3 )·H 2 O} n [tr 2 ad = 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(tr 2 ad)Cl 2 ] n features 1-D chains, the other compounds contain 2-D double-layers. A comparative structural analysis involving known CPs built up with the tr 2 ad 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.