A summary of the key crystallographic information for complexes 1 and 2 was given in Supporting Information (SI) Table S1 and Table S5, respectively. The bond lengths and bond angles of complexes 1 and 2 by X-ray diffractions are listed in SI Table S2, S3, S6, S7. The structural units of two complexes are made up of host [Cu2(phen)2(H2PO4)3(H2O)]1+ and [Cu2(phen)2(H2PO4)3]1+ units, and guest two (H2PO4)1− and five H2O molecules for 1 (Figure 1 ), and host four [Cu(phen)2(H2PO4)]+ and guest one [(H3PO4)4(HPO4)2]4− and six H2O molecules for 2 (Figure 2), respectively. Cu atom in complexes 1 and 2 have five-coordination configurations. According to the determination of five-coordination environmental configuration in literature reported before [10], the τ value of complex 1 is 0.033, while τ value of complex 2 is 1.500. Thus, the coordination configuration of the central copper atom in complex 1 is best described as a slightly distorted tetragonal pyramid geometry. And, complex 2 is best described as a twisted triangular bipyramidal geometry. The Cu-N bond lengths 1.96(2)Å ~ 2.03(2)Å for 1, 2.00(2)Å ~ 2.12(3)Å for 2, and N-Cu-N bond angles 80.4(9)°~83.7(8)° for 1, 79.4(1)° ~ 96.5(1)° for 2, are in good agreement with the similar structures we reported before [11].
In complex 1, the four Cu atoms can be thought of as consisting of two dispersed binuclear copper groups. Cu1 and Cu2 coordinate with an o-phenanthroline ligand to form a five-member chelating ring. Then, Cu1 and Cu2 are bridged by two P-O bonds of one (H2PO4)1− and two O atoms of the other two (H2PO4)1− (Figure S2 in SI). Cu3 and Cu4 also form a five-member chelating ring with o-phenanthroline, and they are bridged by two O atoms in two (H2PO4)1−. But the coordination group o-phenanthroline opposite ligand is H2O molecule for Cu3 and is (H2PO4)1− a group for Cu4, respectively (Figure S3 in SI). The four Cu atoms all form a tetragonal pyramid geometry and the equatorial position is occupied by bridged O atoms in (H2PO4)1−. The Cu-O bond length are all within reasonable limits. The longest Cu-O bonds are located at the vertices of the tetragonal pyramid, and the bond lengths are Cu1-O11 2.26(3)Å, Cu2-O6 2.31(1)Å, Cu3-O14 2.32(1)Å and Cu4-O22 2.37Å, respectively. Therefore, the main part of complex 1 is best represented by the molecular formula {[Cu2(phen)2(H2PO4)3][[Cu2(phen)2(H2O)(H2PO4)3]}2+, two (H2PO4)1− anion equilibrium charge, five solvate water molecules.
The host of complex 2 consists of four dispersed distort trigonal bipyramid [Cu(phen)2 (H2PO4)]1+ groups. The two planes of phenanthroline are almost perpendicular, with an angle of 80.02ºto 81.39ºbetween them. [H2PO4]1− is located at the triangular plane and the Cu-O bond lengths are 1.96(3)Å for Cu1-O1, 1.91(2)Å for Cu2-O8, 1.97(3)Å for Cu3-O12, 1.91(2)Å for Cu4-O13, respectively. Since complex 2 has 10 phosphorus atoms in its formula, and neither phenanthroline ligand nor water molecules is charged, the positively charged Cu2+ should be in balance with the negatively charged phosphoric acid. According to crystal resolution, 10 phosphorus atoms should be four electrically neutral H3PO4, four (H2PO4)1− anion, and two (HPO4)2− anion. According to the length of P-O bond and the coordination strength of hydroxyl oxygen and carbonyl oxygen, complex 2 should be composed of four [Cu(phen)2(H2PO4)]1+, two (HPO4)2−, four neutral H3PO4 and six H2O molecules. Therefore, complex 2 is best represented by [Cu(phen)2(H2PO4)]4·4H3PO4·2HPO4·6H2O.
Secondary bonds play an important role in the construction of complexes [12]. Two complexes all exhibit a variety of types of secondary interactions between host and guest. For complex 1, the five H2O molecules are divided into two groups, one consisting of three water molecules, Ow35, Ow37 and Ow38, forming discrete chain (H2O)3, and the other consisting of Ow34 and Ow36, forming a hole chain with two (H2PO4)1− anion (Figure S4 in SI). In discrete (H2O)3 water cluster, the Ow···Ow distances are 2.686Å for Ow35···Ow37 and 2.828Å for Ow37···Ow38, these values all in agreement with those in ice Ih [13], which suggest a strong H-bonds action. The Ow35, Ow37 and Ow38 connects [Cu2(phen)2(H2PO4)3(H2O)]1+ in Cu3 and Cu4 binuclear copper groups with the donor-acceptor distances of 2.617Å, 2.849Å and 3.275Å, respectively. Three water molecules and [Cu2(phen)2(H2PO4)3(H2O)]1+ group forms ringlike hydrogen bond network along the b axis as shown in Figure 3. The Ow34 and Ow36 with two (H2PO4)1− anion connects [Cu2(phen)2 (H2PO4)3]1+ in Cu1 and Cu2 with the donor-acceptor distances of Ow34-O26 2.534Å, Ow34-O8 2.843Å, Ow34-O31 2.662Å, Ow36-O27 2.524Å, Ow36-O33 2.781Å and Ow36-O2 2.951Å, respectively. The unit of 2H2PO4·2H2O forms supramolecular large ring with (H2PO4)1− group of P8 being the connection point, and [Cu2(phen)2 (H2PO4)3]1+ groups are located on either side of the large supramolecular ring as shown in Figure 4.
In addition, the two sets of supramolecular assembly units, [Cu2(phen)2(H2PO4)3(H2O)]3H2O and [Cu2(phen)2(H2PO4)3]·2H2PO4·2H2O, are related, although there is no crossover or association between the two supramolecular large rings, [(H2PO4)4(H2O)](H2O)3 and [(H2PO4)2](H2O)2. Both binuclear copper host have three (H2PO4)1− ligands. These (H2PO4)1− ligand bind two sets of supramolecular assembly units, [(H2PO4)4(H2O)](H2O)3 and [(H2PO4)2](H2O)2, together by hydrogen bonding. There are the strong H-bonds actions between [Cu2(phen)2(H2PO4)3(H2O)] 3H2O and [Cu2(phen)2(H2PO4)3]·2H2PO4·2H2O, the donor-acceptor distances of 2.497Å and 2.551Å for O···O, respectively. Thus, the hydrogen bonding forces of complex 1 are formed into a three-dimensional network. This supramolecular force must have a certain effect on the physical and chemical properties of complex 1. For example, the thermal stability of complex 1 is good, and its magnetic changes are different from those of binuclear Cu (II) previously reported [14]. Multiple H-bonds interactions make Cu(1) and Cu(2) binuclear groups, Cu(3) and Cu(4) binuclear groups, water cluster (H2O)3 and hole chain [(H2PO4)(H2O)]n form an intricate 3D network structure, which makes the crystal structure very stable (See Figure 5).
For complex 2, there are six H2O molecules, four neutral H3PO4 and two (HPO4)2− anion in the guest. They form very complex three-dimensional structures by hydrogen bonding, with water molecules acting as donors as bridges connecting receptors (H3PO4) and (HPO4)2− to form large rings. First, Ow41, Ow42 and Ow45 bridge two (H3PO4) and two (HPO4)2− to form a small ring (Fig. 6a). Second, Ow41~Ow44 five water molecules bridge seven (H3PO4) and three (HPO4)2− to form a large ring with a diameter of about 14.346Å (Fig. 6b). Then, three H2O Ow42, Ow43 and Ow44, with six (H3PO4) and two (HPO4)2− form a large ring with a diameter of about 14.371Å. Ow43, Ow44 and Ow45 with eight (H3PO4) and two (HPO4)2− form a large ring with a diameter of about 14.660Å. Ow41 and Ow44 with six (H3PO4) and four (HPO4)2− form a large ring with a diameter of about 14.679Å (see Fig. 6c). These large rings interleave each other to form a cage structure with holes about 14Å in diameter (Fig. 7a). The Ow···O distances are 2.584Å to 3.173Å, which indicates that these hydrogen bond interactions are within the normal range. H3PO4, (H2PO4)1− and (HPO4)2− all have OH groups in their structures, which can act as both acceptors and donors to form strong hydrogen bonds. The O···O distances of 2.296Å to 2.734 Å are very strong hydrogen bonds interaction among H3PO4, (H2PO4)1− and (HPO4)2− groups. It can be seen that (H2PO4)1− is not only a ligand to form a coordinate bond with Cu atoms, but also participates in supramolecular assembly. The host, four [Cu(phen)2(H2PO4)] + ion, is located in the cage structure formed by supramolecular action and is enclosed in the pores by supramolecular chemical bonds (Fig. 7b). It should be noted that the water molecule wH2O(45) does not participate in the supramolecular assembly of the host and guest, and it is more than 3.5 Å away from other O atoms, indicating that it does not form hydrogen bond interactions with other O atoms, and is only crystal water located in the crystal structure. However, from the thermogravimetric analysis of complex 2, we found that the water of crystallization wH2O(45) was not significantly different from the other five water molecules, which were all lost at 108.1ºC. This shows that it may be deviation to judge the strength of hydrogen bond force only by the distance of donor-acceptor. This prompted us to further study the supramolecular forces of complexs 1 and 2 by means of Hirschfeld surface analysis. The combination of single-crystal X-ray with Crystal Explorer software enables us to analyze the internal structure and electron cloud configuration of molecules more clearly.
The Hirshfeld surface (HS) is a technique currently used to analyze and verify the types of intermolecular interactions in a crystal lattice. The Hirshfeld surface analysis and finger pattern generated using the Crystal-Explorer software can be used to identify the types and regions of molecular interactions. Hirshfeld surface gives a detailed explanation of the immediate environment of a molecule in a crystal 15]. The 3D Hirshfeld surfaces have been mapped over dnorm, shape index, and curvedness for complex 1 and 2 as shown in Figure 8 and 9, respectively. The surfaces are shown as transparent to allow visualization of the molecular moiety around which they were calculated. The dnorm surface shows regions with red, blue, and white colors, which indicate contacts with smaller, larger, and closer distances to the sum of van der Waals radii, respectively. Red spots are observed in the dnorm surfaces, indicating the presence of close-contacts in the crystal structure, such as classical hydrogen bonds O–H∙∙∙O, O-H···N and non-classical interactions, such as C–H∙∙∙O and C–H∙∙∙H. The shape index surface indicates the presence of π∙∙∙π stacking interactions in the crystal structure of this compound, observed by the detached red and blue region [15b]. The curvedness is the measurement of “how much shape” the flat areas of the surface correspond to low values of curvedness, while sharp curvature areas correspond to high values of curvedness, indicating interactions between neighboring molecules. The large flat region indicated by a blue outline on the curvedness surface refers to the π···π stacking interactions of the molecule. The π···π stacking information conveyed by the shape index and curvedness plots are consistent with the crystal structure analyses. As can be seen from Fig. 8 and 9, both complexes 1 and 2 indicate strong hydrogen bonding in solid surface (red pots in dnorm and shape index), and significant π···π interactions between phenanthroline ring (blue outline in curvedness). This is consistent with crystal structure analysis.
Hirshfeld 2D fingerprint plots allow quick and easy identification of the significant intermolecular interactions map on the molecular surface [16]. Fingerprint plots of complexes 1 and 2 are represented in Fig. 10 and 11, respectively, and display the intermolecular contacts present in the crystalline solid. From Fig. 10, we can see the main contributions of various supramolecular forces to the crystal structures of the complex 1. The O∙∙∙H and H∙∙∙H interactions have the most contribution, with the O∙∙∙H interaction contributed of 49.4%, the H∙∙∙H interaction contributed 31.0%. The others interaction contributed are C···H 7.4%, π∙∙∙π 6.0%, C···N 2.9%, N∙∙∙H 1.0%, O···O 0.6%, C···O 0.5%, Cu···H 0.8% and Cu···C 0.4%, which are found in Figure S7 (Left). The main contributions of the supramolecular forces in complex 2 were also the O∙∙∙H and H∙∙∙H interactions, with O∙∙∙H 39.5% and H∙∙∙H 34.1%. The others interaction contributed are C···H 10.5%, π∙∙∙π 7.8%, C···N 2.1%, N∙∙∙H 2.1%, O···O 2.0%, C···O 1.9%, Cu···H 0.1% and Cu···C 0%, which are found in Figure S7 (Right).
To get more insight into the properties relative to supramolecular interaction for complexes 1 and 2, their dehydration behavior has been investigated using thermogravimetric analysis. Complex 1 and 2 all have water clusters in their structure. To the best of our knowledge, the losses of all water molecules in neutral water clusters are all below 140°C [17]. The losses of the coordinated water molecules in crystal we found so far the highest dehydration temperature was 225°C [18]. H2O molecules in anion water clusters are much stable, which can be existed at higher temperature. The TGA and DTG curves of complex 1, {[Cu2(phen)2(H2PO4)3][Cu2(phen)2(H2O) (H2PO4)3]}·2H2PO4·5H2O, are illustrated in Fig. 12a. These supramolecular interactions keep the light at higher thermal stability, it loses water molecules when they were heating to 117.9ºC under N2 atmospheric pressure. Complex 1 has four obvious weightlessness processes accompanied by sharp heat absorption peaks. In the first stage, there was an obvious heat absorption peak at 117.9ºC, and the loss of 4.75% indicated that five water molecules were lost. Combined with crystal structure analysis, it is speculated that five water molecules in the structure are lost completely. In the second stage, the weight loss was at 247.4ºC with an obvious heat absorption peak. The weight loss of 5.00% was presumed to be the removal of H3PO4 molecules (calc. 5.17%). In the third stage, the weight loss was 4.09%, and the heat absorption peak was 367.0ºC, indicating that two H3PO4 began to decompose or react, and the residual chemical formula was Cu4(phen)4(PO4)(H2PO4)5 (found 86.17% calc. 86.18%). In the fourth stage, it loses 11.2% weight with the absorption peak at 509.9ºC, indicating that the residue may be Cu4(phen)4 (PO4)2(H2PO4)2 (found 74.97% calc. 75.80%).
The TGA and DTG curves of the complex 2, [Cu(phen)2(H2PO4)]4·4H3PO4·2HPO4·6H2O, are illustrated in Fig. 12b. There are five stages of decomposition for complex 2. In the first stage, it loses 3.96% with an obvious heat absorption peak at 108.1ºC, which indicated that six water molecules were lost completely (calc. 3.89%). In the second stage, the weight loss was 7.01% with a heat absorption peak at 251.0ºC, which suggests the removal of two H3PO4 molecules (calc. 7.06%). In the third stage, the weight loss was 15.11%, and the heat absorption peak at 370.8ºC, indicating that H3PO4, (H2PO4)1− and (HPO4)2− group maybe began to decompose. In the fourth stage, it loses 6.21% weight with the absorption peak at 426.1ºC, and the residual chemical formula was Cu4(phen)8(P2O7)(O)2 (found 73.92% calc. 74.21%). The fifth stage, the absorption peak at 481.0ºC, weight loss of 6.56%, suggests that the residue may be [Cu4(phen)8](O)4 (found 61.16% calc. 69.10%). The large error of this formula indicates that the o-phenanthroline ligands may also decompose or lose at this temperature. At 898.9°C, the final residual weight was still 44.44%, indicating that o-phenanthroline had not been completely lost, and the final residual molecular formula was probably Cu4(phen)4(P2O7)(O)2 (calc. 45.39%).
The variable-temperature magnetic properties of complex 1 are shown in Figure 13 (Left) in the form of χMT versus T. At room temperature, the product of χMT is 0.85 emu·K·mol−1, corresponding to the theoretical value of two spin-only Cu(II) ions with S = 1/2 and g > 2 due to the spin-orbital contribution. Upon cooling, χMT increases very slowly and almost keep a constant from 300 to 10 K, and then quickly increase a little to 0.89 emu·K·mol−1 at 2 K, indicating the very weak ferromagnetic coupling between Cu(II) ions in the dinuclear complex. The magnetic properties can be fitted using Heisenberg-Dirac-van Vleck S1 = S2 = 1/2 spin-coupled dimer model (H = −2JS1·S2), where J is the coupling constant between two Cu(II) ions. The best fitting results gave: g = 2.123(2) and J = 0.15(2) cm−1 with R = ∑[(χMT)calc − (χMT)obs]2/∑(χMT)2obs = 8.2 × 10−5 as shown in Figure 13 (Left: solid line). The small positive J value indicates very weak ferromagnetic coupling between Cu(II) ions mediated by µ2-phosphate in complex 1. Factually, the weak ferromagnetic coupling between spins is easy to be understood. In the structure of 1, both of Cu1 and Cu2 locate in the square pyramidal coordination environment, so they have the same magnetic orbital of dx2−y2. Two µ2-phosphate ions bridge between Cu1 and Cu2 via axis direction of Cu1 and equatorial plane of Cu2 or axis direction of Cu2 and equatorial plane of Cu1. In addition, two equatorial planes for two copper ions are almost parallel. Such a structural arrangement leads to their magnetic orbitals to be orthogonal causing ferromagnetic coupling. At the same time, the coupling interaction is mediated through the lengthened axis because of the Jahn-Teller effect. Therefore, the coupling is very weak. In the unit cell of 1, there are two molecules with [Cu1Cu2] and [Cu3Cu4]. However, their structures are almost the same, so the magnetic properties of two molecules can be considered equivalent.
Complex 2 shows the very different magnetic properties from 1 because their structures are different. Complex 1 can be regarded as a mononuclear species relative to 2. Thus, complex 2 should shows the paramagnetic behavior. As expected, the variable-temperature magnetic susceptibilities in the form of χMT are almost a constant of 0.57 emu·K·mol−1 above 13 K. Below 13 K, χMT quickly decreases to 0.51 emu·K·mol−1 at 2 K. Using the mean-field theory as the molecular interaction zJ for fitting the magnetic properties obtained g = 2.45(1) and zJ = −0.078(1) cm−1 with R = ∑[(χMT)calc − (χMT)obs]2/∑(χMT)2obs = 1.4 × 10−6. The little negative value of zJ indicates the very weak molecular antiferromagnetic interaction in complex 2. It can be ascribed to the dipole-dipole interaction or π-π interaction between mononuclear molecules.
