Design, structural, spectral, DFT and analytical studies of novel nano-palladium schiff base complex

A novel nano-palladium (II) Schiff base complex (C1) is synthesized by the reaction between palladium chloride and the Schiff base N, N’-1, 2-phenylene) bis (3 -aminobenzamide (A1). The prepared compounds were characterized by elemental analysis, Ultraviolet–Visible spectroscopy (UV–Vis), Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and Thermogravimetric Analysis (TGA). A combined solvent sublation-ICP OES methodology has been studied for the preconcentration, separation and determination of trace palladium (II) in media of diverse origin using the Schiff base ligand (A1). The different experimental variables that affect the sublation efficiency (S, %) were thoroughly investigated viz.: pH of sample solution; amounts of A1, Pd (II) and TBAB; type and amounts of surfactants, types of organic solvent, temperature and stirring time. The method involves the determination of trace palladium (II) after selective separation by solvent sublation, thus eliminating the effect of foreign ions and increasing the sensitivity. Also, palladium is determined directly in the organic phase, which decreases the determination time and its loss during determination. At optimum conditions, the linear range of Pd (II) was 10.0–100.0 ngmL−1. The coefficient of determination, the limit of detection (LOD) and limit of quantification (LOQ) were 0.9943, 21.29 ngL−1 and 64.5 ngL−1, respectively. This sublation method was applied to real samples and recoveries of more than 95% were obtained in the spiked samples with a preconcentration factor of 100. The mechanism of solvent sublatation of the TBA.[PdII-(A1)2] ion pairs is discussed. The computational studying was estimated to approve the geometry of the isolated solid compounds.


Instruments.
• For the solvent sublation technique, two types of flotation cells were used 31 . Type 1 was a cylindrical graduated glass tube of 16 mm inner diameter and 290 mm length with a stopper at the top and a stopcock at the bottom. This cell was used for adjusting the factors affecting the efficiency of the sublatation process. The second one was a cylindrical tube of 6.0 cm inner diameter and 45 cm length with a stopcock at the prime and was used for the separation of the studied analytes from the quite large volumes. • Elemental analyses (C, H, N, M) were performed using a Costech ECS-4010-Analyzer.
• The Fourier transform-infrared (FT-IR) spectra were recorded with a JASCO FT/IR-460 spectrophotometer with the use of KBr tablets in the range from 400 to 4000 cm −1 at room temperature. • Ultra violet visible (UV-Vis) spectra were gained for the prepared ligand (A1) and the complex (C1) by using a PerkinElmer 550 spectrophotometer in 1 cm quartz cell in ethanol over a range of 200-900 nm. • Traces of Pd (II) were detected by inductivity coupled plasma-optic emission spectroscopy (ICPOES) through a Varian spectrometer Model Varian Vista Pro, CCD Simultaneous. The optimum parameters are shown in (Table S1).

Preparations. Synthesis of the Schiff base ligand (N, N'-1, 2-phenylene) bis (3-aminobenzamide) (A1). The
Schiff base (A1) is prepared according to our recently published work 30 . The Schiff base (A1) was produced from refluxing a 3-aminobenzoic acid (3-ABA) (2.0 mmol, 0.2743 g) dissolved in 10 ml ethanol with 5 ml of 1, 2-phenylenediamine (Phen) (1.0 mmol, 0.1081 g) dissolved in ethanol in the presence of 2 drops of glacial acetic acid (1 M) for 2 days at 75 °C according to reaction synthesis 32,33 , Fig. 1. During the reaction, the color of the prepared solution was changed from the pale yellow to the formed reddish-brown precipitate. This formed precipitate was filtered off, washed several times with water/ ethanol, and afterward dried under a vacuum.
Synthesis of the nano-palladium (II) Schiff base Complex (C1). Nano-palladium Schiff base complex (C1) was synthesized via refluxing PdCl 2 .2H 2 O (1.0 mmol, 0.2134 g) dissolved in 10 ml distilled water (DW) with ligand (A1) for 2 days at 75 °C. Afterwards the system was refluxed. During the reaction, the color of the solution was changed from reddish-brown to a dark red precipitate. The precipitate was filtered off, washed several times with water/ ethanol, and finally dried under a vacuum.
Molecular modeling. Geometry optimizations and other DFT calculations were performed on the Schiff base (A1) ligand and its palladium complex (C1) using B3LYP level of theory, Becke's three parameter (B3) non local exchanges with the correlation functional of Lee, Yang, and Parr (LYP) 34

Solvent sublation of palladium (II).
In an Erlenmeyer flask, 2 mL of 1.88 × 10 −5 mol L −1 (A1) solution was added to an aliquot containing 0.94 × 10 −6 molL −1 of Pd (II); the pH was adjusted by 0.5 mol L −1 HCl and or 0.5 mol L −1 NaOH and the solution was mixed completely. A red color of C1 developed instantaneously. Two ml of 4 × 10 −4 molL −1 of TBAB solution were added and the mixture was shaken well for 2 min to allow complete development of the TBA.[Pd II -(A1) 2 ] ion pairs. All contents were quantitatively transferred into a flotation cell (type 1) and the volume was adjusted to 10 mL. Then, 2 mL of 2 × 10 −4 mol L −1 HOL was added. The flotation cell was shaken upside down for 2 min. Vigorous shaking of the flotation cell in the presence of the surfactant (HOL) created bubbles in the solution which enhanced the floatability of the TBA.[Pd II -(A1) 2 ] ion pairs. 5 mL of MIBK were added to the solution surface and the flotation cell was shaken upside down by hand. The red TBA.
[Pd II -(A1) 2 ] ion pairs were quantitatively extracted into the organic layer on the solution surface. The aqueous phase was run off through the bottom of the cell. After complete sublatation (10 min) the scum layer was evaporated to half volume by heating in a water bath. It was then made up to 10 mL in a volumetric flask using a solution containing 20:80, 1.0 mol L −1 HNO 3 /10% v/v methanol and analyzed by ICP OES to measure the concentration of Pd(II) at λ max 340.4 nm with axial viewing.
The solvent sublation efficiency (S %) of Pd (II) was determined from the following relationship: Here, C o and C i denote the concentration of Pd (II) in the organic and in the initial aqueous layers, respectively. Alternatively, Pd (II) was determined directly by ICP OES in the aqueous mother liquor. The separation efficiency (S %) of the analyte was calculated from its concentration in the mother liquor according to the relation where c i and c f denote the initial and final concentrations of the analyte, respectively.

Results and discussion
Ligand, metal-complex and ion pairs. Both A1 and C1 are stable colored compounds, slightly soluble in DMF and soluble in DMSO. The most important obtained physical and microanalytical data for the prepared compounds are summarized in Table 1. Melting point is ~ 174.1 °C for A1 and > 300 °C for C1; the yields of A1 and C1 are 96.5 and 82.1%, respectively.
(1)  www.nature.com/scientificreports/ A1 is dissociated to A1 − and H + and the unshared electron pairs of -O-and = N-in A1 takes part in a coordination bond with metal ions. Anionic metal complex is formed in the presence of excess A1; and the anionic complex forms ion pairs with TBA + ion by the addition of tetra-n-butyl ammonium bromide solution 43 . Characterization. Elemental analysis. The elemental analyses of the Schiff base A1, the Pd(II)-A1 complex (C1) and the [TBA-Pd(II)-A1] ion pairs (C2), are presented in Table 1 FT-IR Spectra. The FTIR spectra of the ligand A1, the complex C1 in aqueous solution, TBA.[Pd (A1)] (C2) ion associate in organic layer are presented in Fig. 2a,b,c,d. The most significant frequencies of A1, C1 and C2 are presented in Table 1. As it has already been mentioned earlier, the IR of A1 (Fig. 2a) exhibits an absorption broad band in the range between 3635-3278 cm −1 which can be recognized to the υ(OH) of the alcohol, υ(OH), υ(NH) of keto-enol form and υ(NH 2 ) 44,45 . The band at 2935 cm −1 is assigned to υ(CH)-sp3. The bands between 1730-1440 cm −1 can be attributed to υ(C = O), υ(C = N) and ν(C = C) and the broad bands due to the formation of the two forms of keto and enol structures. The band that appeared at 1385 cm −1 is due to ν(C-N) groups. The bands between 1330 and 736 cm 1 are due to aromatic CH 46 . The IR spectrum of the C1 complex, Fig. 2b, proves the complexation of the ligand A1 with the metal ion. In contrast to the spectrum of the free ligand A1, the IR of the complex C1 shows a significant shift of υ(OH),ν(NH), υ(C = N) and υ(C-N) vibrations. While, the υ(C = O) and υ(C = C) vibrations stay at more or less at the same place, which designates that (C = C) does not contribute in coordination with the metal ion. So, A1 acts as a neutral bidentate coordination site via υ (OH), (C-N), (C = O), and azomethine nitrogen (C = N) atoms. This observation is additionally supported by new bands that were found at 671 and 436 cm −1 that can be assigned to υ (Pd-O) and υ(Pd-N), respectively 47 .
The IR spectrum of the C2 ion pair in aqueous solution, Fig. 2c, is different from the IR spectra of A1 and C1, Fig. 3a,b, reflecting that there is chemical contribution between TBAB and the C1 complex as -NH 2 stretching band disappeared and new -NH stretching band appeared. This means that protonation occurred by the  UV-vis spectra. The electronic absorption spectra of A1 and C1were showed in Fig. 3. From Fig. 3, it was recognized that the A1 (Fig. 3a) exposes three absorption bands at 226, 296, and 440 nm, which were agreed to intra ligand charge transfer (n-π* and π-π*). While, the C1 exposes four absorption bands at 236, 268, 274, and 510 nm, which were agreed to the ligand-metal charge transfers transitions (LMCT) and owing to the intraligand charge transfers (n-π* and π-π*) [48][49][50][51][52] .
The absorption spectra of the C1 in the aqueous solution and in the organic layer are completely different from that of A1. It is observed that: 1) the λ max of C1 exhibits a red shift (70 nm) from that of A1 and 2) the absorbance of C1-HOL-MIBK system exhibits nearly 4-folds that of C1 in aqueous solution. This proves that the species are highly concentrated in the presence of HOL and MIBK.   53 . The HOMO is electron donor and LUMO the electron acceptor sites and the molecular hardness and softness of a compound can be predicted from the HOMO-LUMO gap 54 . The Global Reactivity Parameters 39-41 such as ionization potential (I P ), electron affinity (E A ), hardness ( η) , softness ( σ ) and electronegativity ( χ )] can be determined from the HOMO and LUMO orbital energies through Koopman's theorem 55 . The ionization potential (I P ) and electron affinity (E A ) are defined as the negative of HOMO and LUMO energies, respectively (I P = − E HOMO and E A = − E LUMO ). The chemical hardness ( η) is a measure of resistance of an atom against the charge transfer 56 . It can be computed using the I P and E A energies: The softness ( σ ), which is the reciprocal of hardness ( σ = 1 η ), is a property of its capability to accept the electron.
Thus, a structure becomes harder (softer) when its energy gap (ΔE (LUMO-HOMO) ) is getting wider (narrower). The electronegativity ( χ ) in terms of IP and EA are given by: The higher reactivity of the Pd-complex (C1) over the Schiff base ligand (A1) is explained in the light of energy gap, ΔE (LUMO-HOMO) , which measures the reactivity; as the energy gap decreases the reactivity increases and the amount of electronic charge transferring from (A1) to the central metal ion increases. Table 2 illustrates www.nature.com/scientificreports/ that ΔE (LUMO-HOMO) for Pd-complex (C1) was found to be more reactive than of the Schiff base ligand (A1). Hard molecules ( η) have a large energy gap, and soft molecules ( σ ) have a small energy gap 57,58 . A soft molecule is more reactive than a hard molecule because a soft molecule has a lower ΔE (LUMO-HOMO) 59 . From Table 2, C1 is softer than A1 and this confirms that C1 is more reactive than L1. The χ is a measure of power of atom(s) to attract the electrons 60 . A high value of electronegativity (χ) for C1 suggests strong ability to attract electrons from the A1, which leads to greater interaction to form the complex C1. Finally, the dipole moment (μ) is a factor that can also provide information about interaction between A1 molecule and Pd. The value (μ) of Pd-complex (C1) is higher than (μ) of the Schiff base ligand (A1); this suggests the stronger interactions between A1 and Pd to form the complex. It was observed that there is charge density transferred from Schiff base atoms to metal ion and there is electrons back-donation from the metal ion to the donating atoms on the Schiff base after complexation.   Method development. Before analysis of the standard and real samples by ICP OES, convenient conditions of the instrument should be obtained. Thus, different parameters influencing the intensity of signals in ICP OES such as radio frequency (rf), generator power, nebulizer pressure, viewing height of plasma, auxiliary gas flow rates and peristaltic pump rate were optimized. Then, to guarantee that maximum extraction efficiency was obtained, the parameters affecting solvent sublation efficiency including metal, surfactant and chelating agent concentration, shaking time and temperature, pH of solution and sample volume were optimized. The optimized conditions of ICP OES are given in Table S1.
Digital photographs. The digital photographs of the TBA.[Pd II -(A1)] ion pairs in the aqueous solution and in the organic layer are shown in Fig. 6. As it can be clearly seen, the ion pair has a clear homogenous red color in the aqueous solution (Fig. 6a). After the successful sublation of the ion pair, the red colored TBA.[Pd II -(A1)] ion pair is concentrated in the organic layer on the top of solution surface while the rest of the solution becomes colorless as it can be noticed in Fig. 6d.
Effect of experimental variables. Effect of pH. The formation of C1 complex was influenced by solution pH and Pd (II) ions can form hydroxide precipitates in a strongly basic solution. The completeness of complex formation was investigated by changing the pH from 2 to 7 (Fig. 7). The sublation of the Pd (II) was very low in the acidic range of pH 2-3 because the analyte could not be extracted into MIBK. This is due to the difficult formation of complexes because A1 is nearly non ionized in such an acidic solution. Hydroxide precipitates also were not formed in this range. A series of experiments was carried out to study the effect of pH on the sublation efficiency (S%) of Pd (II) (2 × 10 −6 mol L −1 ) with 5 ml of 1 × 10 −3 mol L −1 TBAB, 2 × 10 −3 mol L −1 HOL and 5 ml MIBK in the absence and in the presence of 2 × 10 − mol L −1 of A1. The data in Fig. 7, curve a, prove that in the absence of the ligand A1, the maximum separation efficiency doesn't exceed 30% at all pH values tested. Such separation percent is not satisfactory from the analytical point of view. On the other hand, Fig. 7, Fig. 8 show that excess ligand amount had no drastic effect on the sublation process.
To confirm the previous data, another series of experiments was carried out by fixing the ligand concentration and changing the metal concentration from 1.0 to 5.0 × 10 −6 mol L −1 ). The results obtained showed that  www.nature.com/scientificreports/ In the present study, the efficiency of sublation by the type of surfactant was evaluated using anionic (HOL), non-ionic TX-80 and the cationic (CTAB) surfactants. The TBA.[Pd II -(A1)] ion associate was not sublated at all upon using the cationic or the nonionic surfactants. On the other hand, the sublation efficiency (S %) reaches its maximum on using the HOL anionic surfactant. Such phenomenon can be interpreted to mean that excess TBA + around Pd II -(A1) − complex creates the ion pairs of positive charge. Therefore, the addition of the anionic surfactant improves the efficiency of sublation.
The concentration of HOL is an important parameter; up to a limit the separation percentage increases as the concentration of the surfactant increases. The effect of HOL concentration on the sublation efficiency of TBA.
[Pd II -(A1)] ion pairs were investigated. The results obtained indicated that the sublation efficiency of TBA.[Pd II -(A1)] ion pair reaches its maximum (100%) over a wide range of HOL concentrations (1 × 10 −3 to 1 × 10 −4 mol L −1 ) until its critical micelle concentration (CMC) is reached. At a higher HOL concentration, there will be a concentration, at which the surfactant molecules gather together to form a microball, called micelle. Micelles compete for the ion pairs and since they stay in solution, they reduce the effectiveness of separation. The concentration of the surfactant also changes the bubble size with the size getting smaller as the surfactant increases. This makes creamier foam. A suitable concentration (2X10 −3 mol L −1 ) of HOL was selected throughout this work.
Effect of temperature and shaking time. To study the role of temperature on the Pd (II) separation, Pd (II), surfactant and A1 solutions were either heated or cooled to the same temperature. A1, TBAB and HOL are quickly poured into the Pd (II) solution at zero time. The solution was introduced into flotation cell Jacketed with 1 cm thick fiberglass insulation. The sublation steps were preceded as previously mentioned at pH≈4. The results obtained, Fig. 2S, indicated that the sublation of TBA.[Pd II -(A1)] is not affected by raising the temperature up to 40 °C above which the sublation efficiency starts to decrease. The decrease in the sublation efficiency upon increasing the temperature may be attributed to the instability of the ion pairs TBA.[Pd II -(A1)] as well as the weak binding between the HOL surfactant and the ion associate species. A finding, that helps to explain the mechanism of sublation of the TBA.[Pd II -(A1)] ion associate species by the HOL surfactant and MIBK. The whole work has been carried out at room temperature (25 ± 1 °C).
A proper shaking is required for complete complexation of A1 with palladium ions, which forms hydrophobic complex after around 2.0 min and remains constant until 48.0 h, as shown in Fig. S3. For the following work, a shaking time of 60.0 s followed by a rest period of 3.0 min was used.
Influence of sample volume. A series of experiments were conducted to sublate a fixed concentration of the analyte (1.0 × 10 −6 mol L −1 of palladium) from different aqueous volumes (50-1500 ml) using suitable large flotation cells under the recommended conditions. It is clear from the data in Fig. 4S that, 10 −6 mol L −1 of the analyte can be quantitatively separated from different aqueous volumes up to 1 L using 10 mL of the organic layer. The enrichment factor (preconcentration factor) 61 , defined as the ratio of the volumes before and after solvent sublation, was 100.
Influence of ionic strength. Table 2S summarizes the effect of ionic strength on the separation efficiency of Pd (II) under the recommended conditions. Sodium, magnesium and calcium as chloride and sulfate were added during the sublation separation of the analyte at the recommended conditions. It was found that these salts, even up to 0.5 mol L −1 concentration have no effect on the solvent sublation efficiency of the analyte.
Influence of different ions. In the view of the high selectivity provided by ICP-OES, the interferences studied were related to the pre-concentration step. The effect of foreign metal ions on the solvent sublation of Pd (II) with A1 has been studied in detail. The obtained results reveal that in spite of the high tendency of A1 to form complexes with different transition metal ions, fortunately, most of these complexes are not sublated with the Pd (II) complex at pH 3.5-4.0. The experimental data showed that although some foreign ions have little interfering effects (∼3%), Table 3, all of these interferences were completely controlled by adding excess A1 (2 × 10 −4 mol L −1 ). Hence, one can predict that the interfering effects may be due to complex formation which is accompanied by a decrease in the ligand concentration. Consequently, masking of the interfering effects by adding excess A1 offers a highly selective procedure for the separation and determination of microamounts of Pd (II) in various complex materials, like environmental and biological samples.  Table 5.
Determination of Pd (II) in natural water samples. In order to investigate the applicability to a natural-water samples, the recoveries of known amounts of Pd (II) added to bidistilled, river and sea water samples were examined by such a procedure. To 20 ml aliquots of clear uncontaminated, filtered water samples 1, 3 and 5 μg of Pd (II) were added and the pH was adjusted by HCl to pH 4.0. The as previously mentioned solvent sublation -ICP OES methodology was applied. The recoveries obtained, Table 6, were in the range of 98.00-99.42%.

Mechanism of solvent sublation of [TBA + [Pd II -(A1) 2 − ] ion-associate complex. According to
Sebba 63 , the separation mechanism in the solvent sublation technique is very simple. As the gas bubbles pass through the liquid mass they collect the colligend-collector species, which is then transferred to the organic phase on the upper surface of the liquid mass. In studies concerning separation via sublation, the role of surfactant is very important. The nature of the interaction between oleic acid surfactant and the formed TBA + Pd II -(A1) 2 − ion-pair must be studied to approach the actual mechanism of sublation. An ion pair is a pair of oppositely charged ions held together by Coulomb attraction without formation of a covalent bond. The proposed mechanism may proceed through: i) a physical interaction through Van der Waal; ii) by forming a hydrogen bond between the hydrophilic part of the surfactant and the active sites in the ligand complex or iii) by an interaction between oleic acid and the ion pair, formed in solution through a coordinate bond, forming a self-floatable [TBA +. [Pd II -(A1) − -HOL) species. In all cases, the hydrophobic part of the surfactant attaches to air bubbles and floats separating the analyte-containing species. As it has been mentioned earlier in this study, A1 forms anionic complex with Pd (II) and the addition of sufficient amount of TBAB turned this anionic complex into a positively charged ion pair that was easily made hydrophobic by the addition of HOL, floated by vigorous shaking of the flotation cell and extracted into the MIBK.
In the present investigation, the mechanism of sublation is proposed to be physical in nature. This suggestion may be attributed to the following findings: 1) The sublated species in the organic layer has the same colour (red) as that obtained in the aqueous solution (especially in high reactants concentrations). 2) The sublation efficiency was greatly affected by temperature. Raising the temperature above 40 (Fig. 2c) and organic layer, (Fig. 2d), are identical denoting that the process of sublation could be physical in nature.
In conclusion, all of the mentioned studies suggest the first proposal mechanism.
Comparison with published reports. A comparison between the analytical performances of the present approach and other previous reports in the kinds of literature for the determination of palladium is presented in Table 7. The analytical figures of merits of our work are comparable or better than previously reported reports.

Conclusion
In this paper, nanopalladium(II) complex of the 1-N, N'-1, 2-phenylene)bis(3-aminobenzamide) Schiff base ligand (A1) was synthesized and characterized by elemental analysis, spectroscopy techniques and physical measurements. The results showed that the A1acted as a neutral bidentate ligand. Trace Pd(II) in several water samples of adjusted pH 4.0 were determined by a solvent sublation using an ion pair of Pd II (A1) − anion and tetrabutylammonium ion for the utilization of a synergistic effect. The hydrophobic ion pairs created by the addition of oleic acid were floated and extracted into MIBK by vigorous shaking of the flotation cell. This procedure was applied to the analysis of several real water samples and recoveries of more than 95% were obtained in spiked samples of given amounts of analytes. The RSD of less than 3% was obtained for standard spiked samples.
Such results show that this is a fairly accurate and reproducible method and can be applied to similar samples. The results show that the solvent sublation pretreatment is a sensitive, rapid, simple and safe method for the separation/preconcentration of palladium. Also, DFT calculations were done to predict the host-gust interaction between the Schiff base and Pd (II) cations.