Unusual Metal–organic Multicomponent Ni(II) and Mononuclear Zn(II) Compounds Involving Pyridine dicarboxylates: Supramolecular Assemblies and Theoretical Studies

Abstract

In the present work, we reported the synthesis and characterization [single crystal X-ray diffraction technique, spectroscopic, etc.] of two new Ni(II) and Zn(II) coordination compounds, viz. [Ni(2,6-PDC)₂]₂[Ni(en)₂(H₂O)₂]₂[Ni(en)(H₂O)₄]·4H₂O (1) and [Zn(2,6-PDC)(Hdmpz)₂] (2) (where 2,6-PDC = 2,6-pyridinedicarboxylate, en = ethylene-1,2-diamine, and Hdmpz = 3,5-dimethyl pyrazole). Compound 1 is found to crystallize as a multicomponent Ni(II) compound with five discrete complex moieties, whereas compound 2 is isolated as a mononuclear Zn(II) compound. A deep analysis of the crystal structure of 1 unfolds unusual dual enclathration of guest complex cationic moieties within the supramolecular host cavity stabilized by anion–π, π-stacking, N–H⋯O, C–H⋯O, and O–H⋯O hydrogen bonding interactions. Again, the crystal structure of compound 2 is stabilized by the presence of unconventional C–H⋯π(chelate ring) interactions along with C–H⋯O, C–H⋯N hydrogen bonding, π-stacking, and C–H⋯π(pyridyl) interactions. These non-covalent interactions were further studied theoretically using density functional theory (DFT) calculations, molecular electrostatic potential (MEP) surfaces, non-covalent interaction (NCI) plot index, and quantum theory of atoms in molecules (QTAIM) computational tools. The computational study displays that π-stacking or H bonds greatly tune the directionality of compound 1, although non-directional electrostatic forces dominate energetically. For compound 2, a combined QTAIM/NCI plot analysis confirms the presence of unconventional C–H⋯π(chelate ring) interactions along with other weak interactions obtained from the crystal structure analysis. Further, the individual energy contributions of these weak yet significant non-covalent interactions have also been determined computationally.

1. Introduction

Organic ligand-based synthesis of metal complexes wherein supramolecular interactions are involved are at the focal point of the research community due to their stunning structural topologies as well as countless applications in a wide range of fields [1,2,3,4]. The wise choosing of the metal and ligand and careful play of the reaction conditions are of utmost importance in generating these compounds with fascinating supramolecular architectures [5,6,7]. The artist behind the formation of these supramolecular assemblies from the crystal structures of the compounds is non-covalent interactions, which are at the heart of supramolecular chemistry [8,9,10]. Though these non-covalent interactions are weaker in nature, their collective accumulation has the ability to direct and energetically influence the self-assembly processes of crystal structures and their properties too [11,12]. Therefore, it is desirable to synthesize these kinds of metal–organic compounds and study their self-assemblies involving non-covalent interactions to further understand and for breakthroughs in supramolecular chemistry [13,14]. Different kinds of non-covalent interactions studied in supramolecular chemistry include hydrogen bonding, aromatic π-stacking, metal ion coordination [15,16,17,18,19], etc. Moreover, unconventional σ-hole and π-hole, C–H⋯π, anion–π, and lone pair (lp)–π interactions have gained interest in recent times [20,21,22,23,24,25]. Particularly, the anion–π interaction, an attractive non-covalent contact between electron-poor aromatic rings and anions, has gained tremendous research interest recently due to its diverse applications in areas like molecular recognition, catalysis, sensing, and also in the design of new selective anion receptors [26]. On a similar line, C–H⋯π interactions (a weak attraction between the π system and C–H bond) have also captivated researchers for their ability to guide crystal packing, impact in biology, and molecular recognition processes [27,28,29]. However, hydrogen bonding and aromatic π-stacking are still the most common non-covalent interactions that guide and also energetically influence the formation of supramolecular assemblies of metal–organic compounds [30,31,32]. The tactful utilization of these non-covalent interactions can lead to new crystal systems with desired physical and chemical properties, leading to quite a number of practical applications [33,34,35]. Multicomponent compounds, classified into co-crystals, salts, and polymorphs, are crystalline substances comprising two or more components (ions, atoms, or molecules) within the same crystal lattice. They have garnered astounding focus in crystal engineering mainly because of the advancing role of solid-state chemistry in different scientific domains [36,37]. It has now been proven that both organic and metal–organic multicomponent compounds can pave the way for novel solid formulations of active pharmaceutical ingredients with enhanced features like improved solubility, chemical stability, and other mechanical features [38,39].

The utilization of N- and O-donor ligands has gained wide recognition in the supramolecular chemistry of metal–organic compounds owing to the possibility of structural variations along with a wide range of applications [40,41,42]. Notably, pyridine 2,6-dicarboxylic acid or dipicolinic acid, C₅H₃N(COOH)₂ (2,6-PDCH₂), is a well-established ligand known for its multidentate and bridging ligation [43,44,45,46,47]. It generates stable chelates with metal ions, which can be utilized for the formation of different unconventional non-covalent contacts involving the chelate ring, like C–H⋯π(chelate) interactions [48]. Pyrazole-derived ligands, like 3,5-dimethyl pyrazole (Hdmpz), are two-nitrogen-atom-containing ligands that generally bind to metals through pyridine-type N atoms [49,50,51]. The –NH moiety of the Hdmpz ligand allows for hydrogen bond formation with nearby donor and acceptor units that can impact the creation and stability of the overall self-assembly [52]. Metal complexes of pyrazole display a wide range of excellent activities, including anti-cancer [53], catalysis [54,55], luminescence [56,57], etc. Moreover, coordination complexes involving chelating ligands, like ethylene diamine, have also gained importance in the development of metal–organic complexes, mostly from the viewpoint of generating intriguing structural topology and applications [58,59,60].

In recent times, the enclathration of guest molecules in the cavities of supramolecular hosts has gained a lot of attention because of their potential application in gas storage, catalysis, etc. [61,62,63,64]. However, examples where both host and guest are metal complexes are scarce [65,66]. Even rarer is the report of dual enclathration of complex moieties inside the self-assembled host cavities of complexes formed via non-covalent interactions.

In this work, we describe the syntheses and crystal structures of two new coordination complexes of Ni(II) and Zn(II) involving 2,6-PDC moieties and further characterize them using spectroscopic (FT-IR and electronic spectroscopy), elemental, and thermogravimetric (TG) analyses. The crystal analysis of compound 1 unveils uncommon dual enclathration of guest complex cationic moieties within the supramolecular host cavity governed by anion–π, π-stacking, N–H⋯O, C–H⋯O, and O–H⋯O hydrogen bonding interactions. The presence of various non-covalent interactions involving aromatic π-systems and chelate rings such as C–H⋯π(chelate ring), C–H⋯π(pyridyl), and π–π interactions along with C–H⋯O and C–H⋯N hydrogen bonding interactions stabilize the crystal structure of compound 2. The structure-directing role of some non-covalent interactions was further analyzed theoretically using density functional theory (DFT) calculations, non-covalent interaction (NCI) plot index, and quantum theory of atoms in molecules (QTAIM) computational tools. The unconventional C–H⋯π(chelate ring) interaction has also been established theoretically along with other interactions and their energetic contributions to the stability of the crystal structures of compounds 1 and 2.

2. Results and Discussion

2.1. Syntheses and General Aspects

[Ni(2,6-PDC)₂]₂[Ni(en)₂(H₂O)₂]₂[Ni(en)(H₂O)₄]·4H₂O (1) was prepared by the reaction between one equivalent of Ni(Ac)₂·4H₂O, one equivalent of disodium salt of 2,6-PDC, and one equivalent of en at room temperature using de-ionized water as a solvent. Similarly, [Zn(2,6-PDC)(Hdmpz)₂] (2) was synthesized by taking one equivalent of Zn(Ac)₂·2H₂O, one equivalent of disodium salt of 2,6-PDC, and two equivalents of Hdmpz at room temperature in a de-ionized water medium. Both compounds 1 and 2 are well soluble in water as well as in common organic solvents. Compound 1 exhibits a room-temperature magnetic moment of 2.83 BM, suggesting the presence of two unpaired electrons in the octahedral Ni(II) centers [67].

2.2. Crystal Structure Analysis

Figure 1 showcases the molecular structure of 1, which crystallizes in a triclinic crystal system with a P$\overline{1 }$ space group. Selected bond lengths and bond angles containing the Ni(II) centers are summarized in Table S1. Compound 1 comprises five mononuclear hexa-coordinated Ni(II) centers (Ni1–Ni5). A crystal structure analysis of the compound reveals that the Ni2 and Ni5 centers lie on inversion centers. The Ni1 center is hexa-coordinated with four coordinated water molecules and one bidentate en moiety. The coordination environment around the cationic Ni1 center is a distorted octahedron with the O1 water molecule and N1 atom of the en moiety occupying the axial sites and the N2 atom from the en moiety and the O2, O3, and O4 water molecules occupying the equatorial sites.

The cationic Ni2 and Ni5 centers of the compound are crystallographically unique with two water molecules and two bidentate en moieties, respectively. The coordination geometries around the Ni2 and Ni5 centers are ideal octahedrons with the water molecules (O5 and O5# for Ni2; O26 and O26# for Ni5) occupying the axial sites, and the N atoms (N3, N4, N3#, and N4# for Ni2; N9, N10, N9#, and N10# for Ni5) of en moieties are present at the equatorial sites. Similarly, anionic Ni3 and Ni4 centers are also hexa-coordinated with two tridentate 2,6-pyridinedicarboxylate moieties. The axial sites of the Ni3 center are occupied by N5 and N6 atoms, whereas the equatorial positions are occupied by O6, O8, O10, and O11 atoms from the two coordinated 2,6-PDC moieties. Similarly, the axial sites of the Ni4 center are occupied by N7 and N8 atoms, while the equatorial sites are occupied by O14, O16, O18, and O20 atoms from the 2,6-PDC moieties. Moreover, four lattice water molecules (O22, O23, O24, and O25) are also part of the asymmetric unit of 1. The average Ni–O and Ni–N bond lengths are in close proximity to reported similar Ni(II) complexes [68].

A detailed crystallographic analysis unveils that the neighboring anionic complex moieties of 1 are interlinked through anion–π, π–π, and C–H⋯O H bonding interactions along the crystallographic c axis to generate the 1D supramolecular chain (Figure 2). Anion–π interactions are observed between the carboxyl O atoms (O13 and O17) and π systems of the pyridyl ring of 2,6-pyridinedicarboxylate with the centroid(C20–C24, N7)⋯O13 and centroid(C13–C17, N6)⋯O17 separation distances of 3.36 and 3.49 Å, respectively. The corresponding angles between the respective anions (O13 and O17), centroids, and the planes of the aromatic rings are found to be 93.1° and 91.1°, respectively, which are nearby to the ideal value of 90°, thereby establishing the unique strength of anion–π interactions [69]. Aromatic π-stacking interactions are also located between the aromatic rings of 2,6-PDC moieties with centroid(N8, C27–C31)–centroid(N8′, C27′–C31′) and centroid(N5, C6–C10)–centroid(N5′, C6′–C10′) separations of 3.63 and 3.66 Å, respectively. The corresponding slipped angles (angles between the ring normal and the vector joining the two ring centroids) are measured to be 21.5 and 21.9°, respectively, thus closely matching the literature value of slipped π-stacking interactions [22]. Moreover, there are C–H⋯O hydrogen bonding contacts between the –CH moieties (–C21H21 and –C14H14) and carboxyl O atoms (O6 and O20) of coordinated 2,6-PDC with C14–H14⋯O20 and C21–H21⋯O6 distances of 2.73 and 2.51 Å, respectively.

Further analysis of 1 discloses unusual dual enclathration of two cationic Ni5 complex moieties inside the supramolecular host octameric cavity formed by Ni1, Ni3, and Ni4 complex moieties (Figure 3). The supramolecular host cavity is formed with the help of the aforementioned non-covalent anion–π and π-stacking interactions along with N–H⋯O and O–H⋯O hydrogen bonding interactions. N–H⋯O hydrogen bonding interactions are observed between the –NH moiety (–N2H2A′) of coordinated en and carboxyl O atoms (O14) of 2,6-pyridinedicarboxylate with an N2–H2A′⋯O14 distance of 1.97 Å. In addition, O–H⋯O hydrogen bonding interactions are also observed between the coordinated water molecules and carboxyl O atoms of 2,6-PDC with O2–H2A⋯O15, O3–H3A⋯O11, and O4–H4A⋯O12 distances of 1.87, 1.90, and 1.83 Å, respectively. However, the two enclathrated guest complex cationic moieties are stabilized within the supramolecular host cavity via N–H⋯O, C–H⋯O, and O–H⋯O H bonding interactions. Moreover, –NH moieties (–N9H9A and–N10H10A) of en from the two guest complex cationic moieties participated in N–H⋯O H bonding interactions with the O12 and O19 atoms of 2,6-PDC with N9–H9A⋯O12 and N10–H10A⋯O19 distances of 2.26 and 2.36 Å, respectively. The C–H⋯O hydrogen bonding contact is also located between the –C1H1D moiety of en and the O26 atom of 2,6-PDC with a C1–H1D⋯O26 distance of 3.00 Å. O–H⋯O hydrogen bonding interactions are visible between the carboxyl O19 atom of 2,6-pyridinedicarboxylate and the coordinated water molecule (O26) with an O26–H26A⋯O19 distance of 1.93 Å. All of the above-mentioned non-covalent contacts are analyzed with the help of theoretical methods (vide infra). These enclathrated dual guest complex cationic moieties strengthen the layered architecture of 1 along the crystallographic ac plane (Figure 4).

The cationic complex moiety (Ni5) of compound 1 is further enclathrated within another supramolecular host cavity assisted by anionic complex moieties (Ni4) and lattice water molecules (O24 and O25) (Figure 5). The supramolecular host cavity is stabilized by O–H⋯O H bonding and supramolecular C–H⋯C interactions. Lattice water molecules (O24 and O25) are involved in the O–H⋯O hydrogen bonding interactions with the carboxyl O atoms (O17 and O21) of coordinated 2,6-PDC of Ni4 complex moieties with O25–H25B⋯O17, O25–H25A⋯O24, and O24–H24B⋯O21 separations of 1.84, 1.92, and 1.89 Å, respectively. In addition, non-covalent C–H⋯C interactions are observed between the –C28H28 moieties and C27 atoms from 2,6-PDC of two adjacent anionic Ni4 units with a C28–H28⋯C27 distance of 3.48 [C(sp²)–H28⋯C(sp²); C28⋯C27 = 3.94 Å]. The guest Ni5 complex cationic moieties are interacting with the supramolecular host cavities through C–H⋯O, O–H⋯O, and N–H⋯O H bonding interactions. The –N9H9B moieties from the coordinated en of guest complex cationic moieties (Ni5) are involved in N–H⋯O hydrogen bonding interactions with the carboxyl O18 atoms from 2,6-PDC of two adjacent Ni4 complex units with an N9–H9B⋯O18 separation distance of 2.16 Å, whereas the –CH (–C34H34B) moieties of en are involved in C–H⋯O hydrogen bonding interactions with the carboxyl (O17) atom from 2,6-PDC of two neighboring Ni4 complex units with a C34–H34B⋯O17 distance of 2.72 Å. Moreover, the coordinated water molecules (O26) of the guest moiety are involved in O–H⋯O H bonding interactions with the carboxyl (O19) atoms of 2,6-pyridinedicarboxylate of adjacent host anionic moieties with an O19–H19B⋯O26 distance of 1.93 Å.

These enclathrated cationic moieties inside the supramolecular host cavities along with C–H⋯O and O–H⋯O H bonding interactions strengthen the 2D assembly of the compound along the crystallographic bc plane (Figure 6). C–H⋯O hydrogen bonding interactions are observed between the –CH moieties and carboxyl O6 atom of 2,6-PDC with a C22–H22⋯O6 separation of 2.93 Å. Lattice water molecules (O22, O23, and O24) along with the carboxyl (O7, O13, and O20) O atoms of 2,6-pyridinedicarboxylate are engaged in O–H⋯O H bonding interactions (O22–H22A⋯O13 = 1.82 Å; O22–H22B⋯O20 = 1.83 Å; O23–H23A⋯O22 = 1.89 Å; O23–H23B⋯O7 = 2.04 Å; and O24–H24A⋯O7 = 1.98 Å), further strengthening the assembly.

Figure 7 describes the molecular structure of 2. Selective bond lengths and bond angles are displayed in Table S1. The Zn(II) metal center lies on a 2-fold axis of symmetry. Compound 2 grows in a monoclinic crystal system that has a C2/c space group. In compound 2, the Zn(II) metal center is penta-coordinated with two monodentate Hdmpz moieties and one tridentate 2,6-PDC moiety. The coordination environment around the Zn1 center in compound 2 appears to be distorted square pyramidal, where the apical site is taken up by an N1 atom from the Hdmpz moiety, and the equatorial sites are constituted by coordinated O8, O8#, and N12 atoms from the 2,6-PDC moiety and an N1# atom from another Hdmpz unit. The equatorial atoms of the Zn1 center are displaced from the mean equatorial plane with the mean r.m.s. deviation of 0.1120 Å. The average zinc–oxygen and zinc–nitrogen bond lengths are almost parallel to earlier documented Zn(II) complexes [70].

The neighboring monomeric units of compound 2 are joined via non-covalent C–H⋯O, C–H⋯N H bonding interactions in conjunction with π-stacking, C–H⋯π(chelate ring) [48], and CH···π(pyridyl) interactions to create the supramolecular 1D chain along the crystallographic c axis (Figure 8). The C–H⋯O H bonding interactions are located between the –C5H5 moiety of Hdmpz and the uncoordinated O10 atom of coordinated 2,6-PDC of an adjacent monomeric unit with a C–H⋯O distance of 2.84 Å. In a similar manner, C–H⋯N hydrogen bonding interactions are observed between the –C4H4A moiety of Hdmpz with the N1 atom of another Hdmpz of an adjacent complex unit with a C–H⋯N distance of 2.64 Å. In addition to it, π-stacking interactions involving an aromatic ring are observed between Hdmpz rings from neighboring units with a Cg-Cg (where Cg is the centroid formed by the atoms C3, C5, C6, N1, and N2) distance of 4.03 Å and a slipped angle (angle between the ring normal and the vector joining the ring centroids) of 15.9°. C–H∙∙∙π(pyridyl) interactions have also been observed between the –CH₃ group of Hdmpz and the pyridyl ring of 2,6-PDC coordinated to the metal center of adjacent monomeric units with C7-Cg₁ and H7B-Cg₁ distances of 3.67 and 2.93 Å, respectively (Cg₁ is the aromatic ring centroid defined by N12, C11, C13, C14, C13′, and C11′). The concerned C–H⋯π angle is 140.3°, thus revealing the high strength of the C–H⋯π interaction [71]. Interestingly, C–H∙∙∙π(chelate ring) interactions have been equally observed between the –CH₃ moiety of Hdmpz and the chelate ring formed by coordinated 2,6-PDC to the adjacent monomeric Zn(II) metal centers with C4-Cg₂ and H4A-Cg₂ distances of 3.58 and 2.93 Å, respectively (Cg₂ is the aromatic ring centroid formed by Zn1, O8, C9, C11, and N12). A dimeric unit of this 1D chain stabilized by the aforementioned non-covalent interactions was taken for the theoretical study to prove its existence and energetic significance in the crystal packing (vide infra).

Further introspection reveals that the adjacent 1D chains of 2 are utilized in the formation of a layered architecture along the crystallographic ab plane supported by non-covalent C–H⋯O and N–H⋯O H bonding interactions (Figure 9). The C–H⋯O interactions are located between the –CH₃ group of Hdmpz and the uncoordinated O10 atom of coordinated 2,6-pyridinedicarboxylate of a neighboring monomeric unit belonging to the adjacent layer with a C7–H7A⋯O10 distance of 2.63 Å. Similarly, N–H⋯O interactions, equally responsible for crafting the layered architecture, are observed between the –N2H2 unit of Hdmpz and the uncoordinated O10 atom of 2,6-PDC with an N2–H2⋯O10 distance of 1.96 Å.

For compound 2, the formation of a layered assembly along the crystallographic ac plane, aided by non-covalent C–H⋯π(pyridyl) and C–H⋯C interactions, is observed (Figure 10). The –CH moiety (–C7H7B) of 2,6-PDC is engaged in C–H⋯π interactions with the π-system of the pyridyl ring of 2,6-PDC with a centroid (N12, C11, C11′, C13, C13′,C14)⋯H7B distance of 2.93 Å. The angle between C7, H7B, and the centroid of the pyridyl ring is 132.2°, which depicts a strong interaction [72]. Moreover, supramolecular C–H⋯C interactions [72,73] are observed between the –CH moieties (–C7H7A and –C7H7B) and C5 atoms of Hdmpz with C7–H7A⋯C5 and C7–H7B⋯C5 distances of 3.62 and 3.06 Å, respectively [C(sp³)–H7A⋯C5(sp²); C7⋯C5 = 3.633 Å]. In addition, the –C5H5 moiety of Hdmpz is also utilized in C–H⋯C interactions with the C atoms (C5′ and C7) of Hdmpz having C5–H5⋯C5′ and C5–H5⋯C7 distances of 3.29 and 3.17 Å, respectively [C(sp³)–H7A⋯C5(sp²); C7⋯C5 = 3.633 Å; C(sp²)–H5A⋯C(sp²); C5⋯C5′ = 3.617 Å]. From this assembly, a dimer (responsible for chain propagation) formed by two complex moieties through two C–H⋯π interactions was established further by theoretical analysis (vide infra). Selective hydrogen bonds and their corresponding distances (Å) and angles (°) for both 1 and 2 are tabulated in Table 1.

2.3. Spectral Studies

2.3.1. Fourier-Transform Infrared (FT-IR) Spectroscopy

The FT-IR spectra of 1 and 2 were determined within the frequency range of 4000–500 cm⁻¹ (Figure S1). The broad absorption band in the spectrum of compound 1 in the region of 3200–3600 cm¹ can be ascertained to the stretching O–H vibrations of the lattice and coordinated water molecules [74,75]. Compound 1 also exhibits absorption bands owing to ρ_r (H₂O) (710 cm⁻¹) and ρ_w (H₂O) (670 cm⁻¹), supporting the coordination of water molecules [76]. The bands close to 1616 and 1593 cm⁻¹ in the FTIR spectra of 1 and 2, respectively, can be designated as the asymmetric stretching vibrations of the carboxylate groups of 2,6-pyridinedicarboxylate moieties, while the bands for symmetric stretching vibrations of 2,6-PDC of both compounds occur at 1381 and 1351 cm⁻¹. The asymmetric and symmetric stretching vibrations of the carboxylate groups of 2,6-pyridinedicarboxylate were separated by 235 and 242 cm⁻¹ for compounds 1 and 2, respectively, reflecting the monodentate coordination of the –COO⁻ groups to the respective metal centers [77]. The absorption peaks at 1593, 1108, and 774 cm⁻¹ in the FTIR spectrum of 1 are due to the –NH stretching, C–N stretching, and –CH₂ rocking vibrations of coordinated en moieties, respectively [78,79]. The band near 3235 cm⁻¹ in the spectrum of 2 can be referred to as the stretching N–H vibrations of the coordinated Hdmpz moiety [80,81] present in the compound. The C–H stretching vibrations corresponding to the coordinated Hdmpz moieties are observed in the region of 2970–2770 cm⁻¹ [82]. The peaks at 1425, 1273, and 1163 cm⁻¹ in 2 are due to the C–N, N–N, and C=N stretching bands of Hdmpz rings, respectively [83].

2.3.2. Electronic Spectroscopy

The electronic spectra (both solid and aqueous phases) of the compounds were determined (Figures S2 and S3). The spectra of the compounds support the presence of distorted octahedral Ni(II) and distorted square pyramidal Zn(II) centers in compounds 1 and 2, respectively [84,85,86,87,88,89]. The absorption peaks for the π→π* transition of the aromatic ligands are found at the desired positions [90,91]. The similarity in the absorption bands in both phases of the spectra of compounds 1 and 2 reflects lesser deformation in the structural aspects of the compounds in both phases [92,93].

2.4. Thermogravimetric Study

Thermogravimetric data of the compounds 1 and 2 were determined in the temperature range of 25–800 °C in N₂ atmosphere, keeping the heating rate at 10 °C/min (Figure S4). For comp. 1, firstly, the lattice and coordinated water molecules were decomposed within the temperature range of 35–148 °C with the recorded weight loss of 13.44% (calc. = 14.30%) [94,95]. Finally, the coordinated en moieties and all the PDC moieties are lost within the temperature range of 170–345 °C (obs. = 68.09%; calc. = 66.94%) [96,97]. In the case of compound 2, the first decomposition step is found at 70–405 °C, corroborating to a mass loss of 44.51% (calcd. = 45.48%) for two coordinated Hdmpz moieties [98]. Finally, at 410–580 °C, a mass loss of 40.95% (calcd. = 39.26%) is incurred, corresponding to the loss of the PDC moiety [99,100].

2.5. Theoretical Study

The theoretical DFT study is focused on the study of several supramolecular assemblies observed in the solid state of compounds 1 and 2 that are relevant to understanding their crystal packing. Compound 1 can be regarded as a salt composed of dicationic and dianionic Ni(II) complexes. Therefore, non-directional electrostatic forces are dominant. However, the orientation of the molecules is finely tuned by other weaker interactions like π-stacking or H bonds. As a representative assembly, we have computed the tetramer shown in Figure 11 of two cations and two anions. The formation energy is quite large (–72.4 kcal/mol) due to the Coulombic attraction between the ion pairs. The combined QTAIM/NCI plot analysis of the tetramer is represented in Figure 11 and shows that the π-stacking interaction is characterized by three bond critical points (CPs, represented as small red spheres) and bond paths (orange lines) interconnecting three C atoms of the aromatic ligands. The anion···π interactions, described above in Figure 2 and Figure 3, are not revealed by the QTAIM analysis (absence of CPs connecting the O atom to the ring). However, the NCI plot method, which is very useful for revealing interactions in real space, shows a large RDG (reduced density gradient) isosurface that extends to the region between the carboxylate group and the aromatic ring, thus supporting the existence of anion(O)···π interactions. The combined QTAIM/NCI plot analysis also reveals the existence of a network of H bonding contacts. In particular, there are six N–H⋯O, two O–H⋯O, and four C–H⋯O hydrogen bonds with a total contribution of –31.9 kcal/mol, which was evaluated using the QTAIM by means of the potential energy density (V_r) at the bond CPs that characterize the H bonds (see Section 3.3). It should be mentioned that this energy is free from the influence of ion pair effects. The strongest H bond corresponds to the O–H⋯O due to the enhanced acidity of the Ni-coordinated water protons and the anionic nature of the acceptor.

In compound 2, we studied the π-stacking, C–H⋯π, and hydrogen bonding interactions observed in its solid-state architecture. First, we computed the MEP surface (Figure 12) of compound 2, showing that the maximum is located at the -NH group of the coordinated pyrazole ligand (+48 kcal/mol). The MEP is also large and positive at the H atoms of the methyl groups (+23 kcal/mol). The MEP minimum is located at the non-coordinated O atom of the carboxylic group (−54 kcal/mol) of the 2,6-PDC ligand. The MEP is positive over the center of the pyrazole ring (+10 kcal/mol) and small and negative over the aromatic ring of the 2,6-PDC ligand (−5 kcal/mol). The positive MEP value at the methyl H atoms and negative MEP value over the aromatic ring of the 2,6-PDC ligand showcase the feasibility of interactions like C–H⋯π.

Figure 13 shows two interesting assemblies (centrosymmetric dimers) retrieved from the solid state of compound 2. The first assembly (Figure 13a) is used to emphasize the existence of π-stacking between the pyrazole rings that is characterized by two bond CPs connecting the one carbon atom of one ring to the N atom of the adjacent ring and vice versa. It is further characterized by an extended green RDG isosurface, typical in π-stacking interactions. More remarkably, the combined QTAIM/NCI plot analysis shows the existence of a large RDG isosurface located between one methyl group of the Hdmpz moiety and the five-membered chelate ring(CR) involving the Zn center and coordinated 2,6-PDC moiety. This is further characterized by a bond CP and bond path connecting one H atom of the methyl group to the O atom of the chelate ring. Although the QTAIM only shows one CP connecting the methyl and one atom of the CR, the shape and size of the RDG isosurface clearly support the participation of all atoms of the CR, evidencing the C–H⋯π(chelate ring) interaction. Finally, the QTAIM/NCI plot also shows two additional contacts (C–H⋯O and C–H⋯N), each one characterized by a bond CP and bond path connecting two CH groups (aliphatic and aromatic) to the N or O atoms of the Hdmpz and 2,6-PDC ligands, respectively. The contributions of these H bonds were estimated using the V_r energy predictors, which are −1.19 kcal/mol and −0.54 kcal/mol for the C–H⋯N and C–H⋯O contacts, respectively. The total interaction energy of this self-assembled dimer is large and negative (−19.7 kcal/mol) due to this intricate combination of interactions. Figure 13b shows the other self-assembled dimer that is governed by two C–H⋯π(pyridyl) or CH₃···π interactions. This type of dimer is important for the formation of the 2D layered assembly in compound 2 (Figure 10 above). The QTAIM/NCI plot analysis shows that the CH₃···π interaction is characterized by a bond CP and bond path connecting one H atom of the methyl group to one C atom of the pyridine ring. Again, the NCI plot analysis better describes the π-nature of the interaction, showing an extended green RDG isosurface that embraces most of the π-cloud of the pyridine ring. The interaction energy is −8.8 kcal/mol, thus revealing that each CH₃···π interaction is −4.4 kcal/mol, in line with the positive MEP at the methyl groups of pyrazole and the negative MEP value over the 2,6-PDC ligand (see Figure 12).

3. Materials and Methods

The chemicals required during this work [Ni(Ac)₂·4H₂O, Zn(Ac)₂·2H₂O, ethylene diamine (en), 2,6-pyridinedicarboxylic acid (2,6-PDCH₂), and 3,5-dimethylpyrazole (Hdmpz)] were procured from Sigma Aldrich (St. Louis, MI, USA) and Merck (India) Ltd. (Maharashtra, India) and further used as they were. Elemental analyses were conducted with the help of a Perkin Elmer (Waltham, MA, USA) 2400 Series II CHN analyzer. A Bruker (Billerica, MA, USA) Alpha (II) infrared spectrophotometer (working range = 4000–500 cm⁻¹) was used to record the FT-IR spectra of the compounds. A Shimadzu (Kyoto, Japan) UV-2600 spectrophotometer was utilized for the purpose of recording the diffuse-reflectance electronic spectra for the compounds. For the solid-phase, UV-Vis-NIR spectra were carried out using BaSO₄ powder as a reference (100% reflectance). Room-temperature magnetic susceptibilities were determined at 300 K using a Sherwood Mark 1 Magnetic Susceptibility Balance following the Evans method. Thermogravimetric studies were conducted under N₂ gas flow, utilizing a Mettler Toledo TGA/DSC1 STAR^e system with a heating rate = 10 °C min⁻¹.

3.1. Syntheses

3.1.1. Preparation of [Ni(2,6-PDC)₂]₂[Ni(en)₂(H₂O)₂]₂[Ni(en)(H₂O)₄]·4H₂O (1)

In a round bottom flask, Ni(Ac)₂·4H₂O (0.248 g, 1 mmol) and disodium 2,6-PDC salt (0.211 g, 1 mmol) were dissolved in 10 mL of distilled water, and the solution was mechanically stirred for one hour at room temperature. To the obtained green-colored solution, en (0.06 mL, 1 mmol) was mixed slowly, and stirring continued for another two hours (Figure 14). The final solution was then kept untroubled at a lower temperature (2–5 °C) for crystallization. Green-colored block-type single crystals, useful for SCXRD studies, were found after several days. Yield: 1.105 g (88%). Anal. calcd. For C₃₄H₅₆N₁₀Ni₄O₂₆: C, 32.52%; H, 4.50%; N, 11.16%; Found: C, 32.41%; H, 4.42%; N, 11.04%. IR (KBr pellet, cm⁻¹): 3410 (br), 3304 (w), 2961 (sh), 2895 (sh), 2377 (w), 2002 (w), 1933 (w), 1616 (s), 1593 (m), 1426 (m), 1381 (s), 1278 (s), 1108 (m), 1073 (m), 1029 (s), 915 (m), 821 (w), 774 (s), 730 (m), 670 (m), 595 (w), 542 (w) (s, strong; m, medium; w, weak; br, broad; sh, shoulder).

3.1.2. Preparation of [Zn(2,6-PDC)(Hdmpz)₂] (2)

Zn(Ac)₂·2H₂O (0.219 g, 1 mmol) and disodium 2,6-PDC salt (0.211 g, 1 mmol) were dissolved together in 10 mL of de-ionized water and mechanically stirred in a round-bottomed flask at ambient conditions for one hour. To this colorless solution, Hdmpz (0.192 g, 2 mmol) was added slowly, and stirring continued for another two hours (Figure 14). The final solution was then put for crystallization in a refrigerator below 5 °C. Block-type colorless single crystals, useful for single-crystal X-ray diffraction studies, grew after a few days. Yield: 0.368 g (87%). Anal. calcd. For C₁₇H₁₉N₅O₄Zn: C, 48.30%; H, 4.53%; N, 16.57%; Found: C, 48.19%; H, 4.41%; N, 16.49%. IR (KBr pellet, cm⁻¹): 3440 (br), 3235 (sh), 3052 (w), 2969 (w), 2821 (w), 2750 (w), 1646 (s), 1593 (s), 1425 (m), 1351 (s), 1273 (m), 1163 (m), 1057 (m), 1027 (w), 898 (m), 867 (w), 806 (m), 722 (s), 570 (w) (s, strong; w, weak; m, medium; br, broad; sh, shoulder).

3.2. Crystallographic Data Collection and Refinement

For compounds 1 and 2 (single crystals covered with Parabar 10320), respective crystal structures were ascertained on a D8 Venture diffractometer with a Photon III 14 detector, utilizing an Incoatec high brilliance IμS DIAMOND Cu tube equipped with Incoatec Helios MX multilayer optics at 100 K. Data reduction and cell refinements were performed using the Bruker APEX5 program [101]. For compound 1, scaling and absorption correction was carried out using TWINABS [102] to produce an HKLF5 file, and the structure was solved as a 2-component twin (twin law: (1.00389 −0.02976 −0.01993 0.06488 0.99697 0.01214 0.11722 −0.04498 0.99434); twin fraction 0.16777). Using Olex2, the structures of the compounds were solved with the XT structure solution program using intrinsic phasing and refined with the SHELXL refinement packages using least-squares minimization [103,104]. Refinement of all non-hydrogen atoms was carried out with anisotropic thermal parameters by full-matrix least-squares calculations on F2. Hydrogen atoms were inserted at calculated positions and refined as riders. The O−H hydrogen atoms were not located from the different Fourier maps and, hence, were fixed at nominal X-ray distances to the O atoms to ascertain their positions. In addition, there is disorder among the solvent molecules. The structural diagrams were drawn with Diamond 3.2 [105]. The crystallographic data of compounds 1 and 2 are provided in Table 2, with CCDC deposition numbers cited in Appendix A.

3.3. Computational Methods

The single-point calculations were carried out using the Turbomole 7.7 program [106] and the PBE0 [107] -D3 [108] /def2-TZVP [109] level of theory. The crystallographic coordinates were used to evaluate the interactions in compounds 1 and 2, since we are interested in studying the H-bond contacts as they stand in the solid state. The Bader’s “Atoms in molecules” theory (QTAIM) [110] and non-covalent interaction plot (NCI Plot) [111] were used to study the interactions discussed herein using the Multiwfn program [112] and represented using the VMD visualization software version 1.9 [113]. For the calculation of the H-bond energies, we used the equation proposed by Espinosa et al. (E = ½Vr) [114]. For the calculation of the binding energy, we used the supramolecular approach, where the sum of energies of the monomers was subtracted from the energy of the assembly. For the representation of the MEP surface, the 0.001 a.u. isosurface was used to emulate the van der Waals surface.

4. Conclusions

Two new Ni(II) and Zn(II) coordination compounds, viz. [Ni(2,6-PDC)₂]₂[Ni(en)₂(H₂O)₂]₂[Ni(en)(H₂O)₄]·4H₂O (1) and [Zn(2,6-PDC)(Hdmpz)₂] (2) (where 2,6-PDC = 2,6-pyridinedicarboxylate, en = ethylene-1,2-diamine, and Hdmpz = 3,5-dimethyl pyrazole) were synthesized and characterized using a single-crystal X-ray diffraction technique as well as spectroscopic (FT-IR, electronic), elemental, and thermogravimetric analyses. Compound 1 crystallizes as a multicomponent Ni(II) compound with five discrete complex moieties, whereas compound 2 is a mononuclear compound of Zn(II). The Zn(II) metal center is found to lie on a C₂ axis of symmetry. A crystal structure analysis of compound 1 reveals the unusual mono and dual enclathration of complex cationic moieties formed by anion–π, π-stacking, N–H⋯O, C–H⋯O, O–H⋯O, and C–H⋯C hydrogen bonding interactions. Cationic Ni1 and anionic Ni3, Ni4 complex moieties enclathrate two Ni5 cationic moieties, while the same single Ni5 cationic moiety is enclathrated by two anionic Ni4 anionic moieties and lattice water molecules in another supramolecular host cavity. The presence of unconventional C–H⋯π(chelate ring) interactions and C–H⋯O, C–H⋯N hydrogen bonding, π-stacking, and C–H⋯π(pyridyl) interactions stabilize the crystal structure of compound 2. These non-covalent interactions were further studied theoretically using density functional theory (DFT) calculations and a combination of MEP, QTAIM, and NCI plot methods. The computational study showcases that π-stacking or H bonds finely tune the directionality of compound 1 amidst the strong non-directional electrostatic forces. They also reveal the existence of energetically significant π-stacking, C–H⋯π(chelate ring), and C–H⋯π(pyridyl) interactions in compound 2. In addition, the individual energetic contributions of these weak yet notable non-covalent interactions were determined computationally.

Author Contributions

Conceptualization, A.F. and M.K.B.; methodology, A.F. and M.K.B.; software, A.F. and R.M.G.; formal analysis, A.F.; investigation, K.K.D., P.S., S.B. and R.M.G.; data curation, M.B.-O.; writing—original draft preparation, K.K.D.; writing—review and editing, M.K.B.; visualization, A.F.; supervision, M.K.B.; project administration, A.F. and M.K.B.; funding acquisition, A.F. and M.K.B. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study, in the collection, analysis, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.

Footnotes

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Figures and Tables

Figure 1. Molecular structure of [Ni(2,6-PDC)2]2[Ni(en)2(H2O)2]2[Ni(en)(H2O)4]·4H2O (1).

Figure 2. Formation of supramolecular 1D chain of compound 1 with the help of non-covalent anion–π, π-stacking, and C–H⋯O hydrogen bonding contacts along the crystallographic c axis.

Figure 3. Unusual dual enclathration of guest complex cationic moieties within the supramolecular host cavity of compound 1 stabilized by anion–π, π-stacking, N–H⋯O, C–H⋯O, and O–H⋯O hydrogen bonding interactions.

Figure 4. Layered architecture of compound 1 guided by dual enclathration of complex cationic moieties inside the self-assembled host cavity of 1 along the crystallographic ac plane.

Figure 5. Enclathration of cationic (Ni5) complex moiety within the non-covalent host cavity of compound 1 strengthened by N–H⋯O, O–H⋯O, and C–H⋯O hydrogen bonding interactions.

Figure 6. Layered assembly of compound 1 assisted by enclathration of cationic moieties inside self-assembled host cavities along the crystallographic bc plane.

Figure 7. Molecular structure of [Zn(2,6-PDC)(Hdmpz)2] (2).

Figure 8. 1D chain of compound 2 involving intermolecular C–H⋯O, C–H⋯N H bonding interactions and π-stacking, CH3···π(CR) and CH3···π(pyridyl) interactions along the crystallographic c axis.

Figure 9. Layered structure of compound 2 involving C–H⋯O and N–H⋯O hydrogen bonding interactions along the crystallographic bc plane.

Figure 10. Layered assembly of 2 involving non-covalent C–H⋯π and C–H⋯C interactions along the crystallographic ac plane.

Figure 11. QTAIM (bond CPs in red and bond paths as orange lines) and NCI plot analyses (RDG = 0.6, ρcut-off = 0.04, color range −0.04 a.u. ≤ (signλ2)ρ ≤ 0.04 a.u.) for the π-stacking and H-bonded assemblies of compound 1. The H bonding energies evaluated using the Vr energy predictor are indicated in parenthesis.

Figure 12. MEP surface of compound 2. The values at selected points of the surface are given in kcal/mol. Isovalue of 0.001 a.u.

Figure 13. QTAIM (bond CPs in red and bond paths as orange lines) and NCI plot analyses (RDG = 0.6, ρcut-off = 0.04, color range –0.04 a.u. ≤ (signλ2)ρ ≤ 0.04 a.u.) for the π-stacking (a) and CH3···π (b) assemblies of compound 2. The H bonding energies evaluated using the Vr energy predictor are indicated in parenthesis.

Figure 14. Preparation of compounds 1 and 2.

Table 1

Selected hydrogen bond distances (Å) and angles (deg.) for compounds 1 and 2.

D–H⋯A	d(D–H)	d(D⋯A)	d(H⋯A)	(DHA)
Compound 1
C14–H14⋯O20	0.95	3.58	2.73	150.8
C21–H21⋯O6	0.95	3.27	2.51	136.4
N2–H2A′⋯O14	0.91	2.86	1.97	167.4
O2–H2A⋯O15	0.88	2.67	1.87	151.5
O3–H3A⋯O11	0.87	2.75	1.90	163.6
O4–H4A⋯O12	0.88	2.65	1.83	155.3
N9–H9A⋯O12	0.91	3.16	2.26	170.1
N10–H10A⋯O19	0.91	3.19	2.36	153.3
C1–H1D⋯O26	0.99	3.65	3.00	123.4
O26–H26A⋯O19	0.87	2.77	1.93	163.2
O25–H25B⋯O17	0.87	2.69	1.84	167.6
O25–H25A⋯O24	0.87	2.78	1.92	177.6
O24–H24B⋯O21	0.87	2.74	1.89	162.4
N9–H9B⋯O18	0.91	3.05	2.16	169.1
C34–H34B⋯O17	0.99	3.58	2.72	145.6
O19–H19B⋯O26	0.87	2.77	1.93	163.3
C22–H22⋯O6	0.95	3.36	2.93	108.9
O23–H23A⋯O22	0.87	2.76	1.89	170.6
O23–H23B⋯O7	0.87	2.74	2.04	136.4
O24–H24A⋯O7	0.87	2.83	1.98	167.2
O22–H22A⋯O13	0.87	2.68	1.82	171.1
O22–H22B⋯O20	0.87	2.70	1.83	177.0
Compound 2
C5–H5⋯O10	0.95	3.63	2.84	142.2
C4–H4A⋯N1	0.98	3.54	2.64	153.0
C7–H7A⋯O10	0.97	3.51	2.63	149.9
N2–H2⋯O10	0.88	2.79	1.96	156.6

Table 2

Crystallographic data and structure refinement details for 1 and 2.

Parameters	1	2
Formula	C34H56N10Ni4O26	C17H19N5O4Zn
Formula weight	1255.72	422.74
Temp, [K]	100(2)	100(2)
Crystal system	Triclinic	Monoclinic
Space group	$\mathrm{P}\overline{1 }$	C2/c
a, [Å]	8.9375(7)	13.9543(16)
b, [Å]	12.8596(10)	9.4508(11)
c, [Å]	21.1687(17)	13.968(2)
α, [°]	85.416(3)	90
β, [°]	82.686(2)	111.638(2)
γ, [°]	89.928(3)	90
V [Å3]	2405.4(3)	1712.3(4)
Z	2	4
Absorption coefficient (mm−1)	2.641	2.315
F(0 0 0)	1300	872
D (calcd), [Mg/m3]	1.734	1.640
Index ranges	−10 ≤ h ≤ 10, −15 ≤ k ≤ 15, −0 ≤ l ≤ 25	−16 ≤ h ≤ 16, −11 ≤ k ≤ 11, −16 ≤ l ≤ 16
Crystal size, [mm3]	0.31 × 0.28 × 0.15	0.21 × 0.16 × 0.13
θ range, [°]	6.10 to 68.28	3.40 to 68.17
Independent Reflections	8443	1491
Reflections collected	8519	1489
Refinement method	Full-matrix least-squares on F2	Full-matrix least-squares on F2
Data/restraints/parameters	8519/0/692	1491/0/126
Goodness-of-fit on F2	1.085	1.175
Final R indices [I >2σ(I)]	R1 = 0.0559, wR2 = 0.1573	R1 = 0.0419, wR2 = 0.1139
R indices (all data)	R1 = 0.0563, wR2 = 0.1577	R1 = 0.0419, wR2 = 0.1139
Largest hole and peak [e·Å−3]	1.27 and −0.77	0.48 and −0.71

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/inorganics12100267/s1, Table S1: Selected bond lengths (Å) and bond angles (°) of Ni(II) and Zn(II) centers in the compounds 1 and 2 respectively; Figure S1: FT-IR spectra of compounds 1 and 2; Figure S2: (a) UV-Vis-NIR spectrum of 1, (b) UV-Vis spectrum of 1; Figure S3: (a) UV-Vis-NIR spectrum of 2, (b) UV-Vis spectrum of 2; Figure S4: Thermogravimetric curves of compounds 1 and 2. Refs. [84,85,86,87,88,89,90,91,92,93] were cited in Supplementary Materials.

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