Structural and Spectroscopic Characteristics of NiII and CuII Complexes with Poly (Vinyl Alcohol-Nicotinic Acid) Copolymers for Photocatalytic Degradation of Indigo Carmine Dye

Poly-vinyl-alcohol (PVA) has been cross-linked chemically with nicotinic-acid (NA) in an aqueous medium. The copolymers were complexed with NiII and CuII ions. The complexes and copolymers were analyzed using FT-IR and UV–Visible spectroscopy, XRD and TGA, but copolymers were extra analyzed with nuclear magnetic resonance (1H NMR). FT-IR spectra of copolymer revealed the presence of C=O & C-N groups due to the esterification of PVA-NA. The Cu/NA-PVA formed via bidentate interaction of the pyridinyl and carboxyl of NA. EPR/UV-vis data shows the square-planar geometry for NiII and CuII complexes. The adsorption of IC dye onto CuII/NA-PVA complex was noticeably greater (90%) in 35 min than NiII/NA-PVA. The DFT\B3LYP with 6- 311G* quantum chemical calculations were carried out for tested compounds. The DFT was conducted to examine an interaction mode of the target compounds with the reaction system. The QSPR was calculated as: optimization geometries, (FMOs), chemical-reactivities and NLO for the copolymers. The (MEPs) were figured to predict the interaction behavior of the ligand and its complexes.


Introduction
Artificial dyes are most commonly used in fabric and skin treatment manufacturing [1]. Dyes cause harmful effects in the environment, even in low concentrations. In addition, there are other highly toxic compounds in the discharge of colored wastewater that increase environmental problems [2,3]. These wastes, released from fabric and skin treatment manufacturing, having 1 mg/L of dye, are sufficient to impart color to H2O, thus making it unpotable for everyday use [4,5]. Treatments used for color removal (chemical, physical, biological or hybrid) have been highly effective in removing color [3,6]. However, chemical, and physical treatments have the disadvantage of generating sludge that is difficult to handle, expensive, and requires large treatment areas [3,7] Coordination polymers (CPs) can be useful in several fields, for example, gas adsorption and packing, drug supply, various catalysis, electronic devices, magnetism, photoluminescence, and others [8][9][10][11]. To the best of our knowledge, the mixed-ligands strategy, incorporating the multicarboxylate and N-donor ligands, is beneficial to form new CPs and further construct multifunctional materials. Moreover, as a consequence of the interpenetration, CPs may be used as potential super-hard, porous, and magnetic materials [12].
Additionally, the barrier properties and thermal stability of PVA have increased by changed cross-links with boric acid mixture films [13,14]. Carlotti et al. obtained water soluble PVA-Lactic acid with no catalysts [15]. The mixture of PVA, chitosan and AgNO3 has been previously used for electrospun nanofibers with antibacterial activity [16].
Pyridyl carboxylic acids and their N-oxide products are particularly suitable in this regard. On the other hand, the mistreatment of the ligand with open-framework products remains insufficient [17,18]. Nevertheless, the actions of ligand vis CPs are still incomplete; the metal ion (MI) in CPs of N-oxides based on NA have been structurally studied [19]. Complexation of NA against several metals (Mn, Co, Ni, Cu & Zn) were reported [20].
Many researchers have carried out studies on CPs of transition metals with suitable ligands [1]. Various Cu(II) containing CPs with diverse geometries play an important role in catalytic applications [2]. Cu NPs based catalysts or Cu complexes immobilized on polymers can be obtained by different processes such as metallic or ionic Cu. In 2008, Orto et al. reported the fabrication and application of Cu(II)-polyampholyte as an effective catalyst for methyl orange (MO) degradation as a pollutant at room temperature viaH2O2 activation [3].
In this paper, we report synthesis and characterization of PVA copolymers using Nicotinic acid as monomer. Coordination polymers of NA-PVA with Metal ions (CuII and NiII) were prepared by incipient wetness impregnation method. CPs offer a good photocatalytic performance, non-toxicity, and low production costs. A new complex CuII-/or NiII/NA-PVA: we expected, the novelty CuII and NiII complex to provide a promising way for photocatalytic degradation of Indigo carmine dye (IC) as applications in wastewater treatment. Physico-chemical features for the investigated Cps and its complexes were analyzed using different spectral data (FT-IR, UV/Vis, NMR, ESR, TGA and X-ray-analysis).

Materials & Instruments
The chemical used are listed in Supplementary Materials.

Synthesis of Copolymer
PVA-CPs based in NA as monomer were prepared, as previously described elsewhere [12]. PVA-NA was prepared via the substance reaction of PVA and NA in equal molar ratio. PVA (0.3 g, 6.80 mmol of OH) and NA (0.6 g, 4.76 mmol of COOH) were presented in a 50 mL glass container fixed with an automatic stirrer, a nitrogen inlet, a condensation side-arm joined to a condenser and other arm for receiving flask. Temperature was increased from 383 to 453 K. The end of the reaction was characterized by the full recuperation of the theoretically required amount of water generated by the esterification reaction.

Synthesis of M/PVA-NA Complexes
The incipient wetness impregnation method was used for preparation of M/PVA-NA (M = Ni II , Cu II ). The desired metal ions and CP(PVA-NA) were utilized to obtain the acquired distribution of MI (1:1 molar-ratio). Alternatively, metal ions were dissolved in distilled water; then PVA-NA was added, whilst stirring for additional 7 h at 80 °C. The obtained final mixtures were filtered, then washed-away using a distilled H2O and dried for 12 h at 62.5 °C.

Decolorization in Air (Adsorption)
Testing decolorization of IC dye on M/PVA-NA in the absence of ultraviolet irradiation were carried out in a batch method, as reported in supplementary material.

Photocatalytic Evaluation
All the tests were carried out by a horizontal cylinder annular batch apparatus. IC was designated and preformed as a classical mode [19].
The removal-efficiency% (IC) was calculated by equation; where Co is IC-content and C is IC-retained in solution.

Molecular Modeling Study & computational Model
The quantum chemical computations were performed, using MOPAC16 package, then the Gaussian 09W program package, as mentioned in Supplementary Materials.

IR Spectra
FT-IR spectra of PVA, NA and PVA-NA are illustrated in Figure 2A. The 3424 & 3411 cm −1 bands owing to stretching OH vibration related uncombined PVA and NA, respectively. Bands located at 1646 & 1417 cm −1 attributed to the bending mode for OH due to presence of H2O molecules in parent PVA and NA, respectively. The 2924, 2852, 1454 cm −1 bands for the parent PVA are due to different modes of CH2 vibrations (symmetric, anti-symmetric & bending), in PVA carbon chain [21]. The 3071 & 3163 cm −1 bands, the one explained by C-H stretching of NA aromatic ring. The stretching mode for C-O of free PVA appeared at 1037 cm −1 [22]. The C-O group in original NA appeared at 1298 cm −1 in stretching vibration mode, while the 1709 & 1322 cm −1 bands are related to stretching C=O and υ(C-N), respectively. IR spectra of Ni II /and Cu II /PVA-NA formulation was interpreted in Figure 2B. It is well-known that the Cu II /PVA-NA were formed through bi-dentate contact for the nitrogen atom of NA subsequent in a υ(C-N) shift from 1326 cm −1 to a greater range of frequency at 1384 cm −1 . The spectrum of Cu-complex was shifted due to bidentate interaction of the pyridinyl ring of NA with COOH group. The low intensity band at 459 cm −1 is ascribed to Cu-O vibration [23].  Figure 3 showed the UV-visible spectra of Ni II and Cu II complexed with NA-PVA. Band at 286 nm assigned to a π→ π* transition: although band at 342 nm associated n→π*. This transition is due to C=O group connected with (=C=C=) bonds of (-CH=CH)-CO. The presence of carbonyl functionalities is due to PVA and NA interaction. Spectra of Ni/NA-PVA complex demonstrated broad band 367 nm is related to the 3 A2g (F)→ 3 T2g with charge transfer transitions, respectively. Thus, the Ni/NA-PVA complex has square planar geometry [24]. On the other hand, Cu II/ Cp complex shows a relatively broad band at 342 nm with a low intensity. This band could be ascribed to ( 2 Eg→ 2 T2g) M→ L charge transfer transitions, which is consistent with square planar geometry [25]. Figure 4 showed Ni II /PVA-NA & Cu II /PVA-NA EPR spectra. The spectra of EPR for Cu II /PVA-NA showed axial signals with an isotropic component, gll = 2.163 and g⊥ = 2.179. So, the unpaired electron located in the dz 2 orbital to obtain 2 A1g as the ground state with g⊥ > g|| > 2. Thus, Cu II /PVA-NA has square planar geometry around the copper (II) ion [26,27]. No signal appeared at half field in the spectrum, where the possibility of dimeric form of Cu II /CP [28]. The Ni II /NA-PVA shows gII = 2.009 and g⊥ = 0 in Figure 4. From examination of the ESR signals at 3460.410 G, it was deduced that gll > g⊥ > 2.0 and possess a lengthened octahedral geometry for the CP complexes, the unpaired electron mostly sits in the dx2-y2 orbital, with maybe some dz 2 character due to the slight symmetry, which supports a square planar for Ni II /NA-PVA structure [29,30].

Thermal Analysis
Thermal-decomposition characters were examined using TG, DTG. PVA-NA, Ni II -, Cu II -/PVA-NA data at a heating rate of 10 °C/min starts 298-973 K and are revealed in Figure 5. TG curve for parent PVA-NA shows weight loss occurs with three-steps of temperatures in the variety 323 to 973 K. First stage: the weight loss of about 32.62% occurred at 323-523 K due to absent of H2O partials. This related to complete dehydration reaction of PVA-NA in the limited parts of its molecular chain in which the main PVA could be converted into poly-acetylene(-CH=CH-)n. Second step: the loss of weight with 47.44% corresponds to 523 to 673 K, due to the breakdown of polymeric network. Third step: the range of temperature between 673-973 K with weight loss (15.93%). The NA change of thermal stability in copolymers with PVA higher than parent PVA. From analysis of thermal curves of Ni II /PVA-NA & Cu II -/PVA-NA, one can propose three breakdown steps. First step: temperature ranged from 298-498 K exhibited weightlosses 22.44% for Ni II -and 6.31% for Cu II , that conform to loss of hydrated H2O molecules. Second step starts between 498 and 748 K, corresponding to loss of weight by 53.55%, 10.60% for Ni II &Cu II /PVA-NA, respectively, which related to the breakdown of the polymeric network. The high mass loss in this step is attributable to the release of the PVA-NA molecules. Third step: the mass loss 17.04%, 20.02% for Ni II &Cu II /PVA-NA at temperature range from 498-748 K, with as a result of continuous reduction of the lattice oxygen and preparation of carbon metals/metal oxides.
Thermodynamic activation properties of the structural assets for ligand with metal type through the thermal performance of the complexes, namely, Ea "activation-energy", ΔH* "enthalpy", ΔS* "entropy", ΔG* "Gibbs free energy variation in the breakdown" and the n "order", were examined beginning with graphically-thermograms for TG and DTG and the equations postulated by Coast-Redfern & Horowitz-Metzger [31,32]. The obtained kinetic parameters are tabled in (Tables 1 and 2). These observations are outlined below: ΔS* has negative value, which revealed that the activated Ni II &Cu II /PVA-NA process take place, and the higher values were ordered with inflexibly of the structure rather than the reactants and intermediate. Thus, the reactions are lower than usual, which is additionally supported by low values [33,34].
Gibbs' free energies were increasing against the following stage for obtained Ni II &Cu II /PVA-NA, which reflects that the degree of elimination of the attached ligand will be less than that of the standard ligand [35,36]. This may be due to the inflexibilitystructural of the residual Ni II &Cu II /PVA-NA when the explosion of ligands is related to the model complex and needing higher energy, TΔS*, for its earlier rearrangement, which is suffering from any structural change.
We noticed a significant gap for Ea values during the first stage at (98-148 kJ mol −1 ). From Tables 1 and 2, it is apparent that the Ea for Cu II &Ni-chelates are 124.46 and 147.24 kJ mol −1 for the removal of H2O particles during the hydration process against Cu II -and Ni-chelates, respectively. From these Ea values, we can observe that H2O with anion particles are simple to remove from the ternary-chelates, in the following order: Cu II -> Ni IIchelates.
Ni II -complex displays a higher thermal stability than Cu II /PVA-NA. This can be conferred by electron pairs repulsion in the valence-shell of the central-ion. Cu II has higher electronegativity than Ni II , which is related to a higher repulsive force through bondingpairs with Cu II valence-shell and decreasing stability [37][38][39].
The comparable values of ΔG* for the breakdown stage which relate the similar decomposing particles in the Ni II &Cu II /PVA-NA, demonstrating that the mechanism of collapse is the same, and the conclusion of the ligands is more noticeable than that of the divalent metals. Figure 6 exhibited XRD of Ni II -& Cu II -/PVA-NA copolymers. XRD of the parent PVA-NA copolymers shows a characteristic peak at 2θ = 19.99° and a shoulder at 2θ = 22.85°, which can be assigned to the combination of (101) reflection; this is associated with intermolecular-interference between PVA-NA chains (through intermolecular-H-bonding) and (200) crystalline-planes, respectively [40,41]. After complexation with Ni II or Cu with PVA-NA, the peak intensity has been reduced steadily, as can be seen from the XRD-patterns.

Band Gap
The optical-energy band-gap (BG) was measured by the Tauc equation [42]. The Kubelka-Munk function is normally useful for changing the diffuse-reflectance into an equivalent-absorption coefficient and generally utilized for evaluating the powder-samples [43]. The Kubelka-Munk-function F(R) was helpful for estimating the energy of Ni II -&Cu II /PVA-NA BG. Thus, a quantity F(R), equal to the coefficient of absorption. The value of α for the Tauc-equation is related with F(R), and interpreted as shown in Equations (3) [44]. The promising property in this case was directed linking of grain-size-nanocrystals with optical BG. The energy of the optical-band was increased slightly and related to the reduced crystallite-size, which promotes a weak quantum-size-effect [45][46][47][48]. The difference of the BG with increasing atomic-size is a promising aspect for the photocatalytic-reduction-application of Cu II /NA-PVA composite. Figure 7 shows the UV-vis absorption spectra of the IC dye solutions irradiated by UV light in the presence of Cu II /NA-PVA catalyst. IC itself, in the absence of a catalyst, was photochemically inert (blank), as pointed out by no change in the absorption spectrum. The maximum wavelength for IC dye was determined to be 608 nm. This peak accounts for the blue color of solutions and can be attributed to the n→π* (transition of the non-bonding electrons to the anti-banding π) group orbital of the double-bond system, and it was used to monitor the mineralization of the dye. Increasing the mineralization efficiency of IC on Cu II /NA-PVA is accounted for oxidative degradation of the dye. The calculated amount of CO2 evolved from the reaction that was captured by an aqueous solution of barium hydroxide and determined gravimetrically, as precipitated barium carbonate measured 55% of the dye carbon atoms, calculated theoretically. The sulphate ions, on the other hand, determined as barium sulphate using barium chloride solution, measured a lower value than expected, based on stoichiometric ratios; this is possibly due to the adsorption of some sulphate ions on the catalyst surface. These results indicate that the oxidative degradation of IC on Cu II /NA-PVA can be proposed.

Photocatalytic Activity
The photocatalytic-reactions of indigo carmine on NA-PVA, Ni II -& Cu II /NA-PVA were carried out in order to evaluate the activities of the prepared photocatalysts. No significant decolorization IC dyes were detected through stirring the solution contaminated with dye and with a catalyst for over 120 min. This obviously designated the activity of the photocatalytic as a catalyst without any adsorption of the dye over copolymers. Blank experiments were carried out in the absence of a catalyst, in the presence of the copolymer with H2O2 and UV irradiation (about 77.7% degradation was noticed over 60 min) ( Figure  8). The photocatalytic activities of indigo carmine were observed with the presence of Ni II /NA-PVA and Cu II /NA-PVA; approximately 93.7% degradation was detected with Cu II -chelates after 35 min. As shown in Figure 8, the absorption of indigo carmine solution was observably decreased just after the beginning of visible light-irradiation, when Cu II /NA-PVA was used as the photocatalyst. Increasing of the light-irradiation time results in a further decrease in the absorption. Conversely, a decrease in the photocatalytic activity of the catalyst was detected on complexation with Ni II /PVA-NA. Nearly 75% conversion was obtained after 35 min and approximately 90% after 60 min. The kinetics of de-coloration by immersion in Co/Ct was plotted vs. t (min), which designates straight lines, where the slopes indicate the factors of the reaction rate. It can be clearly seen that the Cu II /PVA-NA has a better photocatalytic property than Ni II /PVA-NA. These lines indicate that this degradation reaction mimics is the 1st kinetic-reaction with k = 0.03 min −1 and R 2 = 0.988. From the experimental data, we can conclude that the increasing rate of the degradation for Cu II /PVA-NA relevant to reduced BG with high dispersion (showed in XRD).

Molecular Geometry
The optimization geometry was performed using DFT molecular orbital, as reported earlier [49]. All calculated energies are listed in (Table 3). The pyridine ring was arranged in parallel mode with the aliphatic chain to stabilize the PVA-NA. The pyridine as well as M-core for CuII & NiII copolymers were stabilized in coplanar arrangement with an aliphatic chain, which confirming by the tetrahedral angle (   We studied frontier molecular orbitals (FMOs), which play an important role in understanding the way the tested compounds interact with different chemical systems. FMOs' gap has been calculated to determine the chemical reactivity and kinetic stability of the molecule. The chemical interaction was stabilized inversely with the energy gap.
Stabilization interaction was improved by increasing EHOMO for one molecule and decreasing ELUMO energy in another [50]. The EHOMO and ELUMO have been estimated using DFT theory and are listed in Table 3 [51]. The increasing of the EHOMO indicated the elevated power of the practical in donating electron direction, which is related to detached electrons from the valance shell, exhibiting a high ability via oxidation, and vice versa [50,52]. The EHOMO for the ligand was exhibited at a higher value than its metal complexes, while the values for both Cu and Ni complexes were nearly the same. The HOMO region condensed PVA-NA over C=O of pyridine moiety (Figure 9). The HOMO in the case of Cu II -/Ni II -chelates is located in pyridine and metal rings (Figure 9). Furthermore, the LUMO zone condensed over the pyridine ring for PVA-NA and Ni-chalet. While, in Cucomplex, LUMO zone was situated on the pyridine and metal rings (Figure 9). EHOMOs and ELUMOs for the PVA-NA and M/PVA-NA have negative values. These values promote the high stability interaction with the dye, and migration of electrons from the aliphatic chain to pyridine and metal rings through an intramolecular transfer mechanism. In addition, FMOs are directly linked via softness and hardness. Thus, one can interpret the electrophiles and nucleophiles through these concepts. PVA-NA and M/PVA-NA exhibited low ΔG values, organized as ligand < Ni II -< Cu II -and demonstrated that: (i) an elevated chemical reactivity, (ii) growing softness and trended as PVE < Ni II < Cu II , (iii) The nucleophilicity increasing as Cu II < PVE < Ni II . While, PVE was presented the highest on the electrophilicity index (ωI =5.47 ev.), which easily explained the complexion with metal ions. The highest electron accepting ability "μ+" appeared in NA-PVA. Electron donating chemical potential increased by introducing Cu II over original polymer and Ni II ion. Intermolecular reactivity has been analyzed founded on group-philicty "ω ± ". This term is used when electrophile and nucleophile is hitting simultaneously for particles. The positive value of ω ± has been attributed to likely electrophile attacking with (ω − <ω + ) and charge shift as media →molecule, and vice versa. The ω ± for ligand and copolymers have a positive value arranged as Cu II < PVA-NA < Ni II , thus, the nucleophilicity is more likely for tested compounds. From (Table 3) one can deduce that these ligand and copolymers are promising nucleophiles with high ability for receiving electrons from dye (ω − = 2.51-6.37ev.). In addition, ΔNmax has -Ve values between −0.55 and −1.95 ev., and the electron cloud migrates as Ligand→Dye (Table 3) (ii) Molecular electrostatic potential map (EPM): The electrostatic potential map (EPM) can identify dye by the equilibrium between repulsive (+charge) and attractive (-charge) forces [53], and is related to nucleophilic electrophilic reactivities, respectively. The EPM was mapped for ligand and metal complexes ( Figure 10). The areas with high electron density were highlighted (orange, yellow, red). The positive efficiency region has colors toward blue, while the intermediate potential region is shaded by the color green. The positive charge was enclosed over pyridine in NA-PVA and CuII copolymer, while in Ni complex, +Ve tagged upon the metal ring. The difference between colors in EPM morphology demonstrated the electrostatic variation between potential values. Growth in the red zone for all compounds under investigation indicated the high ability of electrophilic process responsible to recognize the nucleophile groups in the dye. The local reactivity (LR) parameters were identified by the variation in electronic systems, through distortion on the electronic cloud at the exact atomic region related to accepting or donating of electrons [54,55]. LRs were accomplished at examining site-selectivity toward chemical reaction ( Figure 11). The " + ", signified by the red color corner, displayed the rise in electron density after gaining charge, while +Ve was represented by the blue color. In " − ", the red site revealed reducing electron density after donating electrons. In PVA-NA, the C=O group has the most sensitive site against earning or contributing electrons (" − and + "). This observation showed that the electron cloud migrated between the pyridine ring and C=O, which caused the stabilization interaction with metal. The negative charges in Cu II , Ni II copolymers are more condensed over M-cores for both cases − and + , thus, the electron deported as pyridine →M-cores and enhanced the chelation power with dye.
The variation in the density of charges where electrons were gained in the electronic system from dye "D" atom was represented by "ΔρKElec ". When this mechanism is reversed, it can be represented as "ΔρKNuc".

Hirshfeld Fingerprint
Hirshfeld fingerprint surface (HF) was utilized for examining the intermolecular interaction between neighbor molecular sites. The HF for the polymer and its complexes has been explained by "dnorm, de & di" terms, which are related to the interior or exterior distance between neighbor atomic sites, respectively ( Figure 12). The dnorm was calculated using the following equation (4): dnorm = (di − r vdW i)/r vdW i + (de − r vdW e)/r vdW e [10] (4) where r vdW i and r vdW e are Van der Waals radii for the internal and external of the closest atomic site via the molecular surface.
If dnorm has a positive value, it means r vdW is short, while long r vdW has a negative value. The 3D HS of ligand showed dnorm =−0.7823 to 6.6325 Å, di = 0.6105 to 6.3480 Å, de = 0.6085-7.4791 Å, shape-index = −1.00 to 1.00 Å, curvedness =−4.00 to 4.00 Å, and patch fragment = 0.00-12.00 Å, respectively. While HFs for Cu III  The dnorm illustrated that the weak O…H/H…OH binding represented with light blue shading, which contributed to the molecular structure of the Cu III polymer more than Ni III and PVE. The C…O/O…C interactions appeared as a green spike in di HF ( Figure 12). The low degree of curvedness in HF in all structures was due to the flat surface sites in all particles. The curvature sites are sharp due to the sharp curvedness degree. The surface was divided two folds, which comes back to the bonding between the closest particles. The concave zones located in the curvedness indicated the presence of weak stacking π…π interactions (red greenish concave).
The hyperpolarizability was demonstrated in trend as CuII > NiII > urea > PVE-NA, as represented in (Table 4). The increase in this characteristic related to the existence of conjugation π-electron systems. Furthermore, we used the reversal relation between ΔG and hyperpolarizability to explain the NLO's characteristic. The energy gap for ligand and related copolymers has negative values; this implies a significant, non-direct relationship with their optical characteristics [58]. This information upheld the intramolecular charge transfer (donating → accepting) electron through π-electron systems [59].

Conclusions
• PVA was successfully modified with nicotinic acid, yielding graft copolymers. • FT-IR and NMR spectroscopy confirmed the distribution of NA onto PVA, and the coordinating of copolymer with the M-ion.

•
The ESR spectra showed the distorted square planar structure in Ni II /PVA-NA and Cu II /PVA-NA complexes.
• Incorporation of Ni II and Cu II into PVA-NA plays an advantageous role in the thermal stability through ionic interaction. Thus, the Ni II and Cu II copolymers become better blending agents than PVA-NA itself.

•
The adsorption of IC dye onto Cu II /NA-PVA complex was noticeably greater (90%) in 35 min.

•
The simulated FMOs showed that the ligand and copolymers are promising nucleophiles with high ability for receiving electrons from dye is related to the distribution of M-ions into PVA-NA; this occurs due to electron transfer between aliphatic chain of PVA-NA and M-core.

•
The EPMs showed that the electrophilic site for these copolymers recognized the nucleophile region of the dye.

•
The ΔNmax for copolymers demonstrated that the charge migrates as Ligand→Dye.