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. 2023 Feb 24;8(9):8530–8540. doi: 10.1021/acsomega.2c07686

Synthesis, Characterization, Crystal Structures, and Supramolecular Assembly of Copper Complexes Derived from Nitroterephthalic Acid along with Hirshfeld Surface Analysis and Quantum Chemical Studies

Seadat Suliddin Hasanova †, Emina Agil Yolchueva †, Aliyeva Qudrat Mashadi ‡, Shabbir Muhammad §, Muhammad Ashfaq ∥,*, Movsumov Elman Muhammed †, Khurram Shahzad Munawar ⊥, Muhammad Nawaz Tahir ∥, Abdullah G Al-Sehemi §, Saleh S Alarfaji §
PMCID: PMC9996808  PMID: 36910960

Abstract

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Two new Cu(II) carboxylate complexes, Cu-NTA and Cu-DNTA, were prepared by treating 2-nitroterephthalic acid with CuSO4·5H2O at room temperature. The synthesized complexes were characterized by elemental (CHN), FT-IR, and thermogravimetric analysis. The crystal structures of both complexes were explored by single crystal X-ray diffraction analysis, which inferred that the coordination geometry is slightly distorted octahedral and square pyramidal in Cu-NTA and Cu-DNTA, respectively. The non-covalent interactions that are the main feature of the supramolecular assembly were investigated by Hirshfeld surface analysis for both complexes. The propensity of each pair of chemical moieties involved in crystal-packing interactions was determined by the enrichment ratio. Quantum chemical computations were performed to optimize the molecular geometry of complex Cu-NTA and compared it with the experimental single crystal structure, which was found to be in sensible agreement with the experimental structure of the complex. The DFT method was used to see the potential of the selected Cu-NTA complex for linear and nonlinear optical properties. The static NLO polarizability <γ> of complex Cu-NTA was calculated to be 86.28 × 10–36 esu at M06 functional and 6-31G*/LANL2DZ basis set, which was rationally large to look for NLO applications of complex Cu-NTA. Additionally, the molecular electrostatic potential and frontier molecular orbitals were also computed with the same methodology to see electronic characteristics and ground-state electronic charge distributions.

1. Introduction

Over the past 25 years, a great deal of research has been done on metal complexes based on benzene dicarboxylic acid to better understand their structural characteristics and diverse properties.1 Dicarboxylates are frequently employed in the construction of coordination polymers with desirable topologies from a synthetic perspective due to their abundant coordination modes and potential to behave like hydrogen bonding acceptors and donors.2,3 Terephthalic acid, which has two carboxylate groups at the trans position, can be helpful in the preparation of coordination polymers through complete and partial deprotonation due to the evenly spaced carboxylic groups, the stiffness of the phenyl skeleton, and particularly its various bridging modes.4−6 Additionally, research has demonstrated that the functionalization of the organic linker with either nitro or amino groups can successfully modify the band gap of effective semiconductor supramolecular species.7 Hence, nitroterephthalic acid is preferred over simple terephthalic acid. There are now more than twenty 2-nitroterephthalic acid complexes that have been identified to have different coordination features.8−12

The ability to produce polymers with a larger range of structural characteristics has been made feasible by using mixed ligands rather than a single kind of ligand. As a result, adding auxiliary ligands to the reaction systems, such as aromatic N-donors, can lead to a wide range of structural diversities.13 The major problem in the process of synthesizing coordination polymers is to control the dimensions. Lower dimensionalities are typically the consequence of the coordination of H2O or molecules of other solvents by multifunctional ligands that cause steric hindrance around the central metal atoms or ions.14

Cueto et al. reported that the copper terephthalate trihydrate complex was one of the first metal terephthalate complexes to be described as porous MOFs, and it was further investigated for conductivity and magnetic properties.15 Mori et al. announced the discovery of the first copper terephthalate with a significant surface area.16,17 It was thought that this first copper terephthalate complex has a paddlewheel-type structure, even though no structural information was ever provided.18

Therefore, based on the aforementioned factors and our experience with metal carboxylates,19−24 in this study, we have described the synthesis of two Cu(II) complexes, Cu-NTA and Cu-DNTA, from CuSO4·5H2O, nitro-substituted terephthalic acid, and N-donor ligand. The method applied for the synthesis of complexes is very simple, as there is no vigorous heating or reflux needed. The reaction was conducted in aqueous media, which is considered a green and cheap solvent. Additionally, quantum chemical approaches were applied to study the electronic structure and optical and nonlinear optical response properties of the synthesized Cu-NTA complex.

2. Results and Discussion

2.1. SC-XRD Description of Cu-NTA and Cu-DNTA

The crystal structure of both compounds is determined by the SC-XRD technique. Experimental facts are given in Table 1, whereas important bond lengths (Å) and bond angles (°) are given in Table 2.

Table 1. SC-XRD Experimental Details of Cu-NTA and Cu-DNTA.

crystal parameters Cu-NTA Cu-DNTA
chemical formula C16H16CuN2O16 C18H15CuN3O7
CCDC 2192298 2192299
Mr 555.86 231.67
crystal system, space group monoclinic, I2/a monoclinic, C2/c
temperature (K) 100 100
a, b, c (Å) 18.27331 (13), 6.99598 (5), 15.67276 (12) 15.0291 (13), 5.8680 (5), 21.9408 (18)
α, β, γ (°) 90, 96.6485 (7), 90 90, 104.359 (3), 90
V (Å3) 1990.13 (3) 1874.5 (3)
Z 4 4
radiation type Cu Kα Mo Kα
μ (mm–1) 2.438 1.213
crystal size (mm) 0.43 × 0.29 × 0.24 0.25 × 0.20 × 0.03
diffractometer Bruker Kappa APEXII CCD Bruker Kappa APEXII CCD
absorption correction multi-scan (SADABS; Bruker, 2007) multi-scan (SADABS; Bruker, 2007)
no. of measured, independent, and observed [I > 2σ(I)] reflections 8342, 1852, 1848 16,281, 3432, 2427
Rint 0.022 0.113
(sin θ/λ)max (Å–1) 0.611 0.759
R[F2 > 2 σ(F2)], wR(F2), S 0.025, 0.069, 1.15 0.051, 0.104, 1.05
no. of reflections 1852 3432
no. of parameters 176 150
no. of restraints 8  
H-atom treatment H-atoms are treated by a mixture of independent and constrained refinement H-atoms are treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å–3) 0.34, −0.37 0.53, −0.74
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Table 2. Important Geometrical Parameters of Cu-NTA and Cu-DNTAa.

bond lengths (Å) of Cu-NTA bond lengths (Å) of Cu-DNTA
Cu1–O1 1.9754 (11) Cu1–O1 2.190 (3)
Cu1–O1i 1.9754 (11) Cu1–O2 1.9754 (16)
Cu1–O2i 2.5164 (11) Cu1–O2ii 1.9755 (16)
Cu1–O2 2.5164 (11) Cu1–N1 2.009 (2)
Cu1–O3 1.9219 (11) Cu1–N1ii 2.009 (2)
Cu1–O3i 1.9219 (11) O2–C6 1.271 (3)
O3–C1 1.2673 (19) C7–C9iii 1.386 (3)
selected bond angles (°) of Cu-NTA selected bond angles (°) of Cu-DNTA
O1–Cu1–O1i 180.0 O2–Cu1–O2ii 168.27 (11)
O1–Cu1–O2 91.34 (4) O2–Cu1–N1 88.61 (7)
O1i–Cu1–O2 88.66 (4) O2ii–Cu1–N1 91.15 (7)
O3–Cu1–O2 85.72 (4) O2–Cu1–N1ii 91.15 (7)
O3i–Cu1–O2 94.28 (4) O2ii–Cu1–N1ii 88.61 (7)
O3–Cu1–O3i 180.0 N1–Cu1–N1ii 177.63 (13)
O3–Cu1–O1 87.34 (5) O2–Cu1–O1 95.86 (6)
O3i–Cu1–O1 92.66 (5) O2ii–Cu1–O1 95.86 (6)
O3–Cu1–O1i 92.66 (5) N1–Cu1–O1 91.18 (7)
O3i–Cu1–O1i 87.34 (5) N1ii–Cu1–O1 91.18 (7)
Cu1–O1–H1A 108.3 (14) C8–C7–C9iii 118.3 (2)
Cu1–O1–H1B 108.2 (14) C9iii–C7–C6 120.4 (2)
H1A–O1–H1B 110.1 (19) C8–C9–C7iii 120.0 (2)
H2A–O2–H2B 105.6 (19) C7iii–C9–H9 120.0
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a

Symmetry codes: (i) −x + 1/2, −y + 5/2, −z + 1/2; (ii) −x + 1, y, −z + 1/2; (iii) −x + 1, −y + 1, −z.

In Cu-NTA (Figure 1 and Table 1), the asymmetric unit contains half of the molecule, while the remaining half molecule is created by symmetry (i) – x + 1/2, −y + 5/2, −z + 1/2. The Cu ion is hexa coordinated using four water molecules and two non-chelating 4-carboxy-2-nitrobenzoate molecules to complete the coordination sphere surrounding the Cu-atom. The bond lengths range from 1.9219 (11) to 2.5164 (11) Å and the bond angles range from 87.34 (5) to 180.0° (Table 2) in the coordination sphere, which indicates that the coordination geometry is slightly distorted octahedral. The phenyl ring A makes the dihedral angles of 41.9 (1), 5.9 (2), and 47.3 (9)° with the first carboxylate group B (C1/O4/O5), second carboxylate group C (C8/O5/O6), and nitro group C (N1/O7/O8), respectively. The molecules are interconnected to generate dimers via O–H···O bonding (Figure S1 and Table 3). The C–H···O bonding, which is comparatively weaker than O–H···O bonding, also plays an important role in crystal packing and interlinking of the dimers. One of the O-atoms of the nitro group behaves like a H-bond acceptor for ortho CH of the benzene ring, whereas the other O-atom of the nitro group is not involved in H-bonding. Two H-atoms of H2O molecules behave like H-bond donors for the O-atom of the carboxylate group. One of the water molecules also acts as a H-bond acceptor. The packing of the crystal is further stabilized by C–O···π and off-set π···π stacking interactions (Figure 2). The O···π distance for C–O···π interaction is 3.04 Å (Table 3). For off-set π···π stacking interactions, the inter-centroid separation is 3.69 Å (Figure 2) and the ring off-set ranges from 1.483 to 3.520 Å.

Figure 1.

Figure 1

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ORTEP diagram of Cu-NTA that is drawn at a 50% probability level. The hydrogen atoms are represented as tiny circles with variable radii.

Table 3. Hydrogen-Bond Geometry (Å, °) for Cu-NTA and Cu-DNTA Together with C–O···π and C–O···π Interaction in Cu-NTA and Cu-DNTA, Respectivelya.

Cu-NTA D–H···A D–H H···A D···A <(D–H···A)°
O1–H1A···O4i 0.83 (2) 1.95 (2) 2.7076 (16) 151 (2)
O1–H1B···O5ii 0.83 (2) 1.91 (2) 2.7315 (16) 169 (2)
O2–H2A···O4iii 0.85 (2) 1.90 (2) 2.7504 (16) 177 (2)
O2–H2B···O6iv 0.84 (2) 2.32 (2) 3.0335 (16) 143 (2)
O6–H6A···O2ii 0.86 (1) 1.85 (1) 2.7005 (16) 177 (2)
C4–H4···O8v 0.95 2.40 3.278 (2) 154
C–O···π C–O O···π C···π <(C–O···π)°
C8–O5···Cg1vi 1.211 (2) 3.0395 (13) 3.3058 (15) 91.73 (9)
Cu-DNTA D–H···A D–H H···A D···A <(D–H···A)°
O1–H1A···O3vii 0.87 (3) 1.81 (3) 2.673 (3) 172 (4)
C2–H2···O2viii 0.95 2.57 3.449 (3) 154
C5–H5···O3ix 0.95 2.60 3.492 (3) 157
N–O···π N–O N···π N···π <(N–O···π)°
N2A–O5A···Cg1 1.227 (6) 3.439 (6) 4.419 (5) 137.1 (4)
N2A–O4A···Cg2x 1.227 (6) 3.502 (4) 4.627 (5) 155.0 (4)
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a

Symmetry codes: (i) – x + 1/2, −y + 5/2, −z + 1/2; (ii) – x, −y + 2, −z; (iii) x, y + 1, z; (iv) x + 1/2, −y + 2, z; (v) x, −y + 1/2 + 1, +z-1/2; (vi) -x,1-y, −z; (vii) – x + 1, y – 1, −z + 1/2; (viii) x – 1/2, y + 1/2, z; (ix) x, y – 1, z; (ix) x, y + 1, z.

Figure 2.

Figure 2

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Graphical illustration of C–O···π and off-set π···π stacking interactions. Hydrogen atoms are left out for simplicity. The measurements are given in Å.

The crystal structure of Cu-DNTA (Figure 3 and Table 1) is a one-dimensional polymeric compound that extends along the c-axis. The central Cu-atom is penta coordinated by the O-atoms of the non-chelating 2-nitroterephthalate ligand, the nitrogen atoms of the pyridine rings, and the O-atom of water. The value of the geometry index τ5 is calculated to find the exact coordination geometry. The τ5 value may be zero or one for the square pyramidal and trigonal bipyramidal geometries, respectively. For Cu-DNTA, the value of the geometry index is 0.00066, which indicates that the geometry is square pyramidal and the oxygen atom (O1) occupies the axial site. The Cu-atom is deviated by 0.122 Å from the basal plane defined by (N1/N1i/O2/O2i) atoms. The polymer chains are interlinked through O–H···O and moderately weak C–H···O bonding (Figure S2 and Table 3). Both hydrogen atoms of water act as H-donor for one of the oxygen atoms of the carboxylate group. The O-atom of water and the N-atom of the pyridine ring do not play their role as a H-bond acceptor. The packing of the crystal is further stabilized by the N–O···π interaction within a polymeric chain as well as among the polymeric chains (Table 3). One of the O-atoms of the nitro group is involved in the N–O···π interaction within the polymeric chain, whereas the other O-atom of the nitro group is engaged in the N–O···π interaction among the neighboring polymeric chains (Figure S3). In addition to the N–O···π interaction, very weak off-set π···π stacking interactions are present as the inter-centroid separation ranges from 4.8842 (16) to 5.8682 (14) Å.

Figure 3.

Figure 3

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ORTEP diagram of Cu-DNTA that is depicted at a 50% probability level. The H-atoms are represented as tiny circles with variable radii. Only the major parts of the disordered nitro group and disordered hydrogen attached to a carbon atom (C8) are shown for clarity.

2.2. Hirshfeld Surface Analysis

Nowadays, the non-covalent interactions in the single crystals can be explored comprehensively with the help of Hirshfeld surface analysis (HSA) using Crystal Explorer version 21.5.25 The Hirshfeld surface (HS) can be plotted by using various properties like normalized distances (dnorm), shape index, etc. The HS plotted over dnorm provides the exploration of H-bonding interactions by assigning a different color for short and long contacts. Short contacts are the representatives of the H-bonding interactions.26−31

On the HS, the short and long connections are represented by red and blue patches, respectively. The white dots on the HS indicate the connections where the distance between the interacting atoms is smaller than the total of the atoms’ van der Waals radii.32−34Figure 4a,b shows the HS over dnorm for Cu-NTA and Cu-DNTA, respectively. The red spot on the HS of Cu-NTA near carboxylate O-atoms, water molecules, and ortho CH of the benzene ring indicates that these atoms are involved in the H-bonding interactions. For Cu-DNTA, only one red spot is found on the HS that is around the Cu-atom, which represents the coordination of the Cu-atom with the symmetry-related O-atom of the 2-nitroterephthalate ligand. The π···π stacking interactions can be visualized by plotting the HS over the shape index. The consecutive red and blue triangular areas around the aromatic ring on the HS plotted over the shape index for Cu-NTA (Figure 4c) and Cu-DNTA (Figure 4d) designate the existence of π···π stacking interactions in Cu-NTA and Cu-DNTA.

Figure 4.

Figure 4

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HS plotted over dnorm for (a) Cu-NTA in the range of −0.6920 to 1.2737 a.u. and (b) Cu-DNTA in the array of −1.2743 to 1.1532 a.u. The HS plotted over the shape index in the range of −1 to 1 a.u. for (c) Cu-NTA and (d) Cu-DNTA.

The short, as well as the comparatively longer, contacts can be quantified by two-dimensional (2D) fingerprint plot analysis.35−38 2D fingerprint plots are designed by using the HS as an input. Figure 5a,e shows the 2D plots for the overall interactions for Cu-NTA and Cu-DNTA, respectively. The central sky blue region represents the π···π stacking interactions, which are larger for Cu-NTA (Figure 5a) as compared to Cu-DNTA (Figure 5b), which indicates that the π···π stacking interactions are stronger in Cu-NTA as compared to Cu-DNTA. For both compounds, the most important contacts are O···H, H···H, and C···H, having percentage contributions of 50.9, 18.3, and 5% in Cu-NTA and 41.8, 20.5, and 16.5% in Cu-DNTA. The remaining 2D fingerprint plots that are comparatively less important are shown in Figures S4 and S5 for Cu-NTA and Cu-DNTA, respectively. The combination of chemical species has a unique tendency for crystal-packing interactions. The propensity of one pair is different from other pairs. The enrichment ratio provides the propensity by dividing the proportion of the actual contacts in the crystal by the theoretical proportion of the random contacts.39−41Tables S1 and S2 list the enrichment ratio of the pair of chemical moieties in Cu-NTA and Cu-DNTA, respectively. The contacts with an enrichment ratio greater than one have a higher propensity to generate crystal-packing interactions relative to other contacts. For both compounds, C···C and O···H contacts have a higher propensity than other contacts, with enrichment ratios of 3.74 and 1.45, respectively, in Cu-NTA and 2.28 and 1.53, respectively, in Cu-DNTA. C···H contacts have a higher propensity in Cu-NTA as compared to Cu-DNTA. The N···O contacts have a higher propensity in Cu-DNTA as compared to in Cu-NTA, which is due to the fact that the N–O···π interaction is present in Cu-DNTA, which is absent in Cu-NTA.

Figure 5.

Figure 5

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2D fingerprint plots showing significant interatomic contacts and overall interactions (a–d) for Cu-NTA and (e, f) for Cu-DNTA.

2.3. Void Analysis

The voids are directly associated with the mechanical properties of the single crystals. If a single crystal has a very small number of voids, then it means that it will have good mechanical properties like a response to applied stress, melting point, etc. From this perspective, we have calculated voids in Cu-NTA and Cu-DNTA by assuming that all the atoms are spherically symmetric and by adding the electronic density of all the atoms existing in the crystal structure.42−45Figure S6 shows the voids present in the crystal packing of Cu-NTA and Cu-DNTA, respectively. The study inferred that the percentage space occupied by voids is 7.5 and 8.9% in Cu-NTA and Cu-DNTA, respectively, which indicates that in both compounds, the molecules are strongly packed without significant cavities.

2.4. FT-IR Spectral Investigations

According to the literature,46 the FT-IR spectra of copper complexes displayed absorption bands in their definite sections. The most noticeable difference was the disappearance of wider bands at 3400 cm–1 in the Cu-DNTA spectra, which are thought to be due to COOH vibrations of the free ligand. The appearance of additional medium-intensity bands around 550 cm–1 for both complexes owing to Cu–O bonding further confirmed the existence of coordinated deprotonated carboxylate ligands in the spectra of the complexes. Asymmetric and symmetric COO– vibrations in Cu-NTA showed prominent bands at 1658 and 1425 cm–1, respectively, with a Δv value of 233, indicating a monodentate mode of carboxylate ligand binding. The two strong bands were seen for the COO– moiety’s asymmetric (1665 cm–1) and symmetric (1430 cm–1) vibrations in Cu-DNTA, with a Δv value of 235, which also guarantees the monodentate mode of coordination of the carboxylate ligand. The SC-XRD data further confirms the mode of coordination of the ligands. The medium-intensity bands at 650 cm–1 due to the newly formed Cu–N bond represent the coordination of pyridine to the copper core in Cu-DNTA. The coordinated water molecules show their presence as a broad band from 3300 to 2800 cm–1 in both complexes.

2.5. Thermogravimetric Analysis

Thermolysis of both complexes occurs in four stages at different temperature intervals: In the first stage of Cu-NTA at a temperature range of 180–200 °C, four H2O molecules are released, accompanied by a weight loss of 12.95%. In the second stage, at a temperature range of 200–365 °C, thermolysis is stable. Apparently, the monomeric structure becomes dimeric, which is characteristic of Cu(II) complexes with benzoic acid derivatives. In the third stage, at 370–495 °C, the breakdown of dimeric molecules occurs, along with the burning out of carbon hydrate residues with a deep exo-effect. In the fourth stage, Cu(II) carbonate is formed (690 °C) and finally decomposes to metal oxides at 780 °C.

In the first stage of Cu-DNTA at a temperature range of 175–210 °C, coordinating one molecule of water and two molecules of pyridine are released. The remaining three stages of thermolysis proceeded in the same manner as that of Cu-NTA.

3. Computational Results

3.1. Computational Methodology

Computational calculations were carried out using the Gaussian 16 program47 to understand the Cu-NTA complex’s electronic properties. The initial geometry of the Cu-NTA complex, which served as the starting configuration for the search for optimal geometry, was taken from the crystal structure. The optimization of the molecular structure was conducted with the help of the M06 method with a mixed basis set of 6-31G*/LanL2DZ. In some prior studies, the reliability of the Minnesota family functionals (M06 and M06-2X) has been evaluated and found to be better than other currently available conventional DFT functionals.48,49 For theoretical calculations of transition metal complexes, LanL2DZ relativistic pseudopotentials have proven to be more trustworthy and yield findings that are equivalent to experimental data. Both of these basis sets were applied to different atoms of the complex, as LanL2DZ was utilized for Cu metal and 6-31G* was for carbon, oxygen, nitrogen, and hydrogen. The calculations of dipole moment, linear polarizability, and third-order nonlinear polarizability of the Cu-NTA complex were performed by using the same level of theory. The GaussView 6.0.1650 was used to analyze the complex’s ground-state optimal geometry, frontier molecular orbital energy profile, and three-dimensional molecular electrostatic potential map. Further computational details can be seen in our previously published full computational studies papers.51−54

The dipole moment (μ) and linear polarizability (α°) can be calculated by using eqs 1 and 2:

3.1. 1
3.1. 2

The second hyperpolarizability is calculated by using eq 3:

3.1. 3

These tensors are reduced to only six components (by Kleinman symmetry) using eq 4:

3.1. 4

3.2. Computational Geometry Optimization for the Cu-NTA Complex

Nowadays, computational calculations are commonly used to describe the structural properties of many compounds. Among the above-synthesized compounds, we chose the Cu-NTA complex for studying its geometry and other electronic properties through computational tools because the molecular geometry of the Cu-DNTA complex has not been obtained due to its larger size and involvement of multiple metal atoms. The geometry optimization of the Cu-NTA complex has been carried out by employing the M06 functional. For comparison of geometrical parameters, experimentally (within parenthesis) and theoretically (without parentheses) observed bond lengths of the Cu-NTA complex are shown in Figure S7. The Cu-NTA complex consists of two ligands of 2-nitroterephthalic acid (NTA) and four solvent water molecules bound to the Cu(II) ion in an octahedral coordination environment. Cu(II) ion occupies a centrosymmetric inversion center. Two ligands of 2-nitroterephthalic acid occupy two trans axial positions in this coordination environment. In the equatorial plane, the Cu1–O33 and Cu1–O8 (oxygen atom of 2-nitroterephthalic acid) bond distance is 1.921 Å in the crystal structure and 1.938 Å in the calculated structure. A noticeable difference is observed between the crystal structure and theoretical bond lengths in the two axial bonds of Cu1–O2 and Cu1–O27. That is 1.975 Å experimentally, while it is appreciably elongated to 2.141 Å in computed geometry. The long bonds of Cu, including Cu1–O30 and Cu–O5, are seen at 2.516 Å in the XRD structure for a d9 system of Cu, while it is reduced to 2.234 Å in an optimized structure. Possible causes of this bond length reduction include phase changes because the XRD structure is analyzed in the solid phase and theoretical calculations are performed in the gas phase. There are no catastrophic disagreements between the experimental and theoretical bond distances among 2-nitroterephthalic acids, as manifested in Figure S7. This comparison offers confidence that the M06 functional can be used for further calculations.

3.3. Linear and Third-Order NLO Properties

Nonlinear optical materials play a vital role in recent technology development. The NLO materials are utilized in laser frequency doubling, telecommunication, and other digital data storage and processing. The third-order NLO response has been calculated through the calculation of second hyperpolarizability (gamma) amplitudes. Besides this, linear polarizability, which shows a linear response to the applied electric field, has also been calculated at the same level of theory. Linear polarizability also plays a key role while designing electro-optical materials. Table S3 shows the amplitudes of linear polarizability and third-order NLO polarizability with their individual components. The isotropic and anisotropic linear polarizabilities are calculated to be 41.23 × 10–24 and 41.03 × 10–24 esu, respectively. The nonzero polarizability shows the tendency of the molecules to polarize their electric cloud under an applied electric field. The isotropic and anisotropic amplitudes are similar, which indicates that there are no directional aspects and the molecule shows similar polarization regardless of its orientation.

Most importantly, the static <γ> amplitude of Cu-NTA is seen to be 86.28 × 10–36 esu. Besides this, we have also calculated the frequency-dependent second hyperpolarizability in the form of γ(−w;w,0,0) and γ(−2w;w,w,0) with an applied wavelength of 1060 nm. The frequency-dependent values of γ(−w;w,0,0) and γ(−2w;w,w,0) are found to be 118.56 × 10–36 and 256.49 × 10–36 esu, which are larger than the static values of the second hyperpolarizability. The calculated third-order NLO amplitudes are found to be reasonably large and also comparable to some previously reported metal complexes,55−57 which show its potential to consider Cu-NTA a fair contender for NLO applications.

3.4. Frontier Molecular Orbital and Molecular Electrostatic Potential Diagram

The frontier molecular orbitals, which include HOMO–LUMO, serve as an example of the chemical reactivity and kinetic stability of the molecules. Figure 6 shows the frontier molecular orbital surfaces to better understand the bonding structure of the Cu-NTA complex. The highest occupied molecular orbital (HOMO) distribution indicates that the charge density is evenly distributed over the metal and its ligands. In distinction, the electronic density in the lowest unoccupied molecular orbital (LUMO) is concentrated on the NTA ligands exclusively. The electronic density transfers from the occupied d orbitals of the metals to the unoccupied antibonding π orbital of the ligand. The energy gap between HOMO and LUMO in Cu-NTA is 5.14 eV, showing its polarizability and higher chemical reactivity with lower kinetic stability.

Figure 6.

Figure 6

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Frontier molecular orbitals of the Cu-NTA complex calculated at the M06 functional.

The molecular electrostatic potential (MEP) postulates the nucleophilic and electrophilic sites over the total density surface of a complex.58−60 The MEP of the Cu-NTA complex is exhibited in Figure 7. A positive electrostatic potential is present at both ends of the NTA ligands on the oxygen atom attached to hydrogen in the carboxylic group, which is shown by the blue color. It provides a site for nucleophilic attack. It might be caused by a drop in electron density because electron density moves toward the doubly bounded oxygen atom of the carboxylic group indicated by the yellow color providing a site for electrophilic attack. Moreover, the yellow color is also present on the oxygen atom of the carboxylate group in the NTA ligand other than the oxygen that is linked to copper metal because the electron density of the double bond moves toward this oxygen and electrons in the d orbital of Cu metal transfer toward another oxygen atom. The oxygen atom of water molecules that are attached to Cu metal represents a slight blue color related to nucleophilic reactivity.

Figure 7.

Figure 7

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MEP diagram of the Cu-NTA complex, where the iso-surface value is 0.002 a.u. The blue and red colors indicate the positive and negative potential maxima, respectively.

4. Conclusions and Perspectives

The current research work presents the synthesis, crystal structure, spectroscopic characterization, and computational studies of two copper complexes derived from nitroterephthalic acid. The synthetic procedure is very simple and produces the complexes in a pure form with excellent yield. The SC-XRD study inferred that the coordination geometry is relatively different in Cu-NTA and Cu-DNTA. The HSA concludes that O···H, H···H, and C···H contacts have a higher contribution in the crystal packing as compared to other contacts, but C···H contacts have a higher propensity to form the crystal-packing interaction in Cu-NTA as compared to Cu-DNTA. The DFT method is used to study the selected Cu-NTA complex for its linear and nonlinear optical properties. The molecular geometry of complex Cu-NTA is optimized by quantum chemical computations and compared with its experimental single crystal structure, which is found to be in sensible agreement with the experimental structure of the complex. The static NLO polarizability <γ> of complex Cu-NTA is found to be 86.28 × 10–36 esu at the M06 functional and 6-31G*/LAN2DZ basis set. Additionally, we have also calculated the frequency-dependent second hyperpolarizability in the form of γ(−w;w,0,0) and γ(−2w;w,w,0) with an applied wavelength of 1060 nm. The frequency-dependent values of γ(−w;w,0,0) and γ(−2w;w,w,0) are found to be 118.56 × 10–36 and 256.49 × 10–36 esu, which are larger than the static γ amplitude. The calculated third-order NLO amplitudes are found to be reasonably large to consider Cu-NTA a fair contender for NLO applications. Moreover, the MEP and frontier molecular orbital are also computed with the same methodology to see electronic characteristics and ground-state electronic charge distributions.

5. Experimental Section

5.1. Materials and Methods

The analytical-grade compounds that are readily available commercially are employed in the current research work. The infrared spectra from 450 to 4000 cm–1 were measured using KBr pellets with a PerkinElmer Spectrum 100 FT-IR spectrophotometer. The NETZSCH STA-409 PC/PG derivatograph was used for thermal analysis. The DTA, TG, and DTG curves were measured at an increasing heating rate of 10 °C per min from ambient to 800 °C with the help of platinum crucibles under nitrogen atmosphere. The Costech ECS 4010 CHNSO analyzer was used for elemental analysis (CHN).

5.2. Synthesis of Cu-NTA

The Cu(II) complex (Cu-NTA) was prepared by dissolving 2.11 g (10 mmol) of 2-nitroterephthalic acid in 25 mL of hot distilled water, to which an aqueous solution (25 mL) of 1.25 g (5 mmol) of CuSO4·5H2O was added with continuous stirring and then left at room temperature (Scheme 1). After a few days, bluish-colored prismatic crystals settled down. The crystals were filtered and dried in a desiccator under anhydrous conditions to a constant weight.

Scheme 1. Synthesis of Cu-NTA and Cu-DNTA.

Scheme 1

Open in a new tab

Cu-NTA: Yield 81%; Color Blue; Anal. Calcd for C16H16CuN2O16 (%): C, 34.57; H, 2.90; N, 5.04. Found %: C, 34.65; H, 2.86; N, 4.98; FT-IR (cm–1): 1658s ν(COO)asym, 1425s ν(COO)sym, 233 = Δν, 1365s ν(C=C), 690s ν(C–NO2), 550s ν(Cu–O), 3400b ν(Cu–H2O).

5.3. Synthesis of Cu-DNTA

2-Nitroterephthalic acid 2.11 g (10 mmol) was dissolved in 25 mL of hot distilled water, to which an aqueous solution (25 mL) of 1.68 g (20 mmol) of NaHCO3 was added drop by drop with continuous stirring. Then, an aqueous solution (25 mL) of 2.50 g (10 mmol) of CuSO4·5H2O was added to it (Scheme 1). After that, 2–3 drops of pyridine were added and the flask containing the reaction mixture was left as such for a few days until the blue needle-like crystals of Cu-DNTA appeared. The resulting crystals were filtered and dried in a desiccator over anhydrous CaCl2.

Cu-DNTA: Yield 80%; Color Blue; Anal. Calcd for C18H15CuN3O7 (%): C, 48.16; H, 3.37; N, 9.36. Found %: C, 48.13; H, 3.35; N, 9.38; FT-IR (cm–1): 1665s ν(COO)asym, 1430s ν(COO)sym, 235 = Δν, 1367s ν(C=C), 692s ν(C–NO2), 650s ν(Cu–N), 551s ν(Cu–O).

5.4. Single Crystal X-ray Diffraction Procedure

The single crystal XRD data of both complexes were collected on a Bruker Kappa Apex-II CCD diffractometer having APEX-II software. The structure solution is performed on SHELXT-201461 and structure refinement is done on SHELXL 2019/2.62 All non-hydrogen atoms are refined with the help of anisotropic displacement parameters, while hydrogen atoms are assigned by isotropic displacement parameters. All H-atoms are added in the refinement model by using the riding model except the H-atoms attached to O-atoms (O1/O2/O6) in the case of Cu-NTA and O-atom (O1) in the case of Cu-DNTA. The H-atoms attached to the above-mentioned O-atoms are refined freely for finding the correct orientation of these H-atoms. ORTEP-III,63 PLATON,64 and Mercury 4.065 are used for graphical illustrations.

Acknowledgments

The authors from King Khalid University extend their appreciation to the Ministry of Education in KSA for funding the research work through project number KKU-IFP2-DB-7.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c07686.

  • Figure S1: Packing diagram of Cu-NTA. Selected H-atoms are shown for clarity. Figure S2: Packing diagram of Cu-DNTA. Selected H-atoms are shown for clarity. Figure S3: Graphical representation of the N–O···π interaction in Cu-DNTA. Hydrogen atoms are omitted for clearness. Figure S4: Remaining 2D fingerprint plots of Cu-NTA. Figure S5: Remaining 2D fingerprint plots of Cu-DNTA. Figure S6. Graphical representation of voids in the crystal packing of (a) Cu-NTA and (b) Cu-DNTA. Figure S7. The geometrical comparison of experimental and calculated structures of Cu-NTA at the M06 functional. The values in parenthesis are from experimental single crystal data. Table S1. Enrichment ratio for the pair of chemical moieties in Cu-NTA. Table S2. Enrichment ratio for the pair of chemical moieties in Cu-DNTA. Table S3. The computed amplitudes of linear polarizability (α × 10–24) and NLO hyperpolarizability (γ × 10–36) for compound Cu-NTA (PDF)

Author Present Address

# Present address: Department of Chemistry, University of Mianwali, Mianwali 42200, Pakistan

The authors declare no competing financial interest.

Supplementary Material

ao2c07686_si_002.pdf (901.6KB, pdf)

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