3.1. Crystal Structure Description
Compound
1 crystallizes in the noncentrosymmetrical monoclinic
Pn space group. X-ray determination of the crystal structure reveals the formation of an interpenetrated neutral three-dimensional coordination polymer (
Figure 1 and
Figure 2). The asymmetric unit of
1 consists of 3 copper centres, 2 fully deprotonated ligand (L
1)
3− molecules, and a total of 16 water molecules; out of these, 8 act as terminal ligands and 8 are present in the lattice. In both (L
1)
3− molecules, each of the three carboxylate groups coordinate to each of the three Cu centers. Additionally, in both ligands, the three amido groups exist in a
trans conformation and all of them are in
anti conformation (
Figure 3). Cu1 is coordinated to six atoms and exhibits a distorted octahedral geometry (s/h = 1.04, φ = 52.9° [
43]). The equatorial positions of this octahedron are occupied by four carboxylate oxygen atoms deriving from two different ligands. Cu2 and Cu3 are each coordinated to five atoms and exhibit a square pyramidal geometry (τ = 0.06 for Cu2, 0.17 for Cu3 [
44]). In the coordination environment of both metal centers, the basal plane consists of two oxygen atoms from two different ligand molecules, as well as two oxygen atoms from terminal water molecules. An oxygen atom from another terminal water molecule occupies the apical position in both cases. Selected bond lengths are listed in
Table 1. The Cu–Cu distances between the metal centers range from 11.400(3) to 14.176(3) Å. Furthermore, the crystal structure of
1 is stabilized by strong intermolecular O–H···O hydrogen bonds, which involve the oxygen atoms of all 16 water molecules as donors. The atoms involved as acceptors in these bonds are oxygen atoms of either water molecules or the carbonyl group of the amide.
3.2. Topological Analysis
The complicated structure of compound
1 can be simplified into a net considering each ligand as a three-connected node and each metal center as a two-connected node; therefore, the two-connected nodes are not further considered for the classification. The final outcome of the topological analysis of the three-dimensional coordination polymer
1, with the use of the TOPOS software [
45] and the standard representation methodology, is a three-connected, 12-fold interpenetrated symmetric
ths net (
Figure 4). According to a literature survey in the TOPOS and CCDC databases, compounds EJISAS [
46] and KOBFEN [
47] can be also represented as 12-fold
ths nets; however, this simplification derives when a standard representation is selected. The topological analysis of the latter two compounds using the standard cluster representation and considering the Cu(O
2)Cu as nodes [
48], yields a 12-fold interpenetrated diamond (
dia) net, therefore compound
1 represents the first example of a standard 12-fold interpenetrated
ths net.
3.3. TGA and IR Studies
To examine the thermal behavior and stability of
1, TGA was carried out between room temperature and 800 °C under N
2 atmosphere. This analysis (
Figure S1) shows that the first mass loss is continuous, as it begins in the region of 50°C and is completed at approximately 150 °C. This is attributed to the loss of eight lattice and eight ligated water molecules, in good agreement with the theoretical value (calc.: 23.60%, theor.: 23.25%). The remaining framework is then relatively stable up to ~310 °C, where it is subjected to a further mass loss due to decomposition to CuO (calc.: 63.14%, theor.: 65.14%). The reported peaks in the IR spectrum of
1 (
Figure S2) are in good agreement with the crystallographic data. A broad absorption peak is found at 3253 cm
−1 and is attributed to the stretching vibration of the O–H bonds. The peak at 1622 cm
−1 is due to the presence of the noncoordinated carbonyl group of the amide, in good agreement with previously reported values for similar [
23] compounds. Furthermore, peaks at 1557 and 1403 cm
−1 can be attributed to the
ν(CO
2)
as and symmetric
ν(CO
2)
s bands of coordinated carboxylate groups, respectively. Finally, some peaks related possibly to C–H bending vibrations appear at 909 and 733 cm
−1.
3.4. Synthetic Aspects
Our initial efforts for the synthesis of
1 involved experiments in various ratios of water/alcohol media, based on our previous experiences with H
3L
1 [
23] as well as the related literature [
30]. However, no crystals were obtained in this case. The protocol was therefore modified with various techniques and ratios in order to facilitate crystallization. After extensive screening, liquid diffusion in acetone was found to be the only effective technique amongst the tested ones. The use of other suitable secondary crystallization solvents (e.g., acetonitrile) led, instead, to amorphous material. It is worth noting that the water/alcohol mix seems to be critical for the pure synthesis of
1, as a similar experiment in H
2O also yielded crystals of the organic ligand; however, no MeOH molecules were found in the structure, despite their potential participation in H-bonding.
While the topology of
1 has not been observed before, the afforded compound is not the only structure which contains a Cu(II) source and the H
3L
1 ligand. In fact, a search in the CCDC [
49] revealed a variety of structures, but all of these show a different topology. To shed more light into this as well as attempt to rationalize the synthesis, we opted to perform a more systematic search in the literature for similar tripodal pseudopeptidic ligands. This narrowed our results to a total of 28 reported coordination compounds, with 3 different ligands depending on the varying amino acid: either Glycine (H
3L
1),
l-Alanine (H
3L
2), or D-Alanine (H
3L
3) (
Scheme 3). To provide a full insight, we included a full list of factors that could point towards the resulting differences. These parameters included the metal ion, the synthetic conditions including solvent and temperature, and the presence of a base or a second organic linker. These are listed in detail in
Table 2.
In regard to the H3L1 glycine-based ligand, a comparison between our result (Entry 1) and the rest of the reported Cu(II) compounds (entries 2–5) already revealed major influences of these parameters. Compound 1 was synthesized using Cu(NO3)2·2.5H2O, while, in the rest of the relevant entries, CuCl2·2H2O was used as the metal source. The role of the metal ion in the resulting structure has already been reported, especially for Cu(II) sources in similar pseudopeptidic ligands. Therefore, our result further confirmed this effect. The rest of the parameters revealed additional interesting information: a comparison of entries 2–4 showed that the presence and amount of base (and as a consequence, the tuning of pH) led to different structures; in the case of entry 4, the base (pyridine) actually coordinated to the metal center, which led to a 2D coordination polymer instead of a 3D, and to a less exciting topology. Regarding the synthetic conditions between these entries, a possible temperature effect over time could be observed. Efforts to obtain a crystal structure using CuCl2·2H2O and the synthetic method of 1, or Cu(NO3)2·2.5H2O and solvothermal conditions were unfortunately unsuccessful. However, we could obtain a wider scope for conclusions by bringing also the glycine-based compounds with other metals (entries 6–20) into the comparison. Through this, it is worth noting the following: (a) the compounds of entries 2 and 6–9 had a general formula of [M(L1)(H2O)3]2[M(H2O)6]·(H2O)3 regardless of the synthetic method; (b) our attempts to utilize our synthetic method with other metals (Co, Zn, Mn) resulted in the same crystal structures in entries 2 and 6–9, conclusively proving that the synthetic procedures were not the prevalent factor in order to get structures with the 12-fold topology; (c) as expected, the presence of a second organic linker led to even more unpredictable structures. Interestingly, a comparison between entries 5 and 12 (Cu- and Co-based respectively), in which the same linker (bpp) and similar synthetic methods were employed, revealed significant differences in the resulting products, further pointing to the lesser importance of the conditions compared to the choice of metal; (d) a comparison between Ca(II)-based compounds [Ca6(L1)4(H2O)14](H2O)3 and [Ca2(HL1)2(μ-H2O)(H2O)5]·3H2O (entries 18 and 19 respectively), which were synthesized under very similar methods but with a different Ca(II) source (chloride for entry 18, nitrate for 19), further pointed towards the metal ion influence; (e) only the metals with flexibility in their coordination environment and geometry (copper, alkaline earth metals, lanthanides) provided any cases of structural variety. Interestingly, the largest variety of compounds was observed when Cu(II) sources were employed.
In regard to the alanine-based compounds (entries 21–29), the presence of an additional methyl group led to completely different compounds and topologies, as expected. However, the stark difference in entries 21 and 22 (in which Cu(NO3)2·3H2O and CuCl2·2H2O were employed respectively) once again strongly suggests a metal ion influence towards the resulting product. In summary, when exploring the coordination chemistry of this type of pseudopeptidic ligands, the choice of the metal ion seems to play an important role towards the resulting product and, as a consequence, in the resulting topology and interpenetration.
3.5. Catalytic Studies
The A
3 coupling (
Scheme 4) has been widely studied in recent years [
57,
58,
59,
60,
61], as the resulting propargylamines have been proposed as key intermediates in the synthesis of various N-containing biologically active compounds [
62,
63,
64,
65]. Even though many metal sources and compounds, including CPs [
66,
67,
68,
69], have been tested as catalysts for this reaction, Cu(II) CPs have been used very rarely [
34].
In order to test the possible catalytic activity of
1, initial studies were performed for the A
3 coupling of cyclohexane carboxaldehyde, pyrrolidine, and phenylacetylene. After extensive screening, optimal conditions were obtained when the mixture was stirred for 24 h in the presence of 2-propanol (iPrOH) [
70], at 90 °C, under air atmosphere, and by adding only 0.03 mmol of compound
1 (in 1 mmol reaction scale of aldehyde). To our delight, these conditions accounted for quantitative yields of the model propargylamine; this accumulated to a turnover number of 33.3 for the catalyst. Additionally, no reaction was observed in the absence of
1, result that further supports the activity of the catalyst in the studied multicomponent coupling.
We then employed a variety of aldehydes, amines, and alkynes as substrates in order to study the scope of the reaction. Amine screening, as presented in
Table 3, entries 1–6, indicated that cyclic secondary amines afford the corresponding propargylamine products in excellent yields, while acyclic secondary amines were found to be slightly less effective. Results of the aldehyde screening (entries 7–10) revealed that aromatic aldehydes show slightly lower reactivity. Furthermore, the reactivity and respective yields were affected by the presence of an electron-donating or electron-withdrawing group in the aldehyde. In comparison, saturated aliphatic aldehydes displayed high reactivity and afforded excellent yields. In regard to the alkyne selection, the employment of either phenylacetylene or 1-hexyne resulted to the corresponding propargylamines in excellent yields when the model aldehyde and amine substrates were also used. The relevant results can be found as entries 1 and 11.
The characterization by TGA and IR spectroscopy pointed towards a similar identity of this solid compared to bulk samples of
1 (
Figures S3 and S4). The compound showed no solubility in common organic solvents during our tests; therefore, the next step was to study the heterogeneous nature and capabilities of the recycled compound. The catalyst could be easily recovered by filtration after the end of the reactions and then be reused after treatment with acetone and diethyl ether to remove any reagents or product. The simulated and the “as is” synthesized compound powder XRD patterns were in good agreement, however the spectrum of the postcatalysis recovered solid (
Figure S5) appeared to be similar to the XRD pattern of the reported compounds with the general formula [M(L
1)(H
2O)
3]
2[M(H
2O)
6]·(H
2O)
3 (
Table 2, entry 2; CCDC entry SIDJIZ was selected for comparison). This indicates that a phase transition or structure change of compound
1 to the corresponding SIDJIZ probably took place during the catalytic procedure. This phenomenon could not to be detected by TGA and IR measurements because of the similarities in the general formula. Experiments carried out with the model reaction and the recovered material showed that it can be reused at least four times with only a slight decrease in the catalytic activity (
Table 3, entry 12). Because of the lack of porous channels within the structure of
1, as well as the similar performance of the transformed recovered material, we envisage that the observed catalytic activity was revealed on the surface of the coordination polymer.