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The (CF3C(O)NH)(C6H5CH2NH)2P(O) phosphoric triamide as a novel carrier with excellent efficiency for Cu(ii) in a liquid membrane transport system

Transport of Ag(i), Cd(ii), Co(ii), Cu(ii), Ni(ii), Pb(ii) and Zn(ii) cations across a bulk liquid membrane (BLM) containing N,N′-dibenzyl-N′′-(2,2,2-trifluoroacetyl)-phosphoric triamide (PTC) as a new carrier is studied by atomic absorption spectrometry. The results show selective and efficient transport of the copper(ii) cation from aqueous solution in the presence of the other cations. Various factors are optimized in order to obtain maximum transport efficiency. The PTC ligand is characterized by single crystal X-ray diffraction analysis, IR, NMR (19F, 31P, 1H, 13C) and mass spectroscopy. The complex formation reaction between copper(ii) and PTC is studied by a conductometric method, which shows the 1 : 1 stoichiometry for ligand and copper(ii).


Introduction
Copper is one of the essential micronutrients that participates in a wide range of metabolic processes. At least thirty Cucontaining enzymes are known, as redox catalysts (e.g. cytochrome oxidase, nitrate reductase) or dioxygen carriers. As copper is not biodegradable in natural conditions and accumulates in living organisms, its excess, as well as deciency, can lead to diseases and biological disorders. Hence, its determination and elimination is very important for environmental protection. [1][2][3][4] Towards this aim, various separation techniques are well-known, such as plasma atomic emission spectrometry, 5,6 liquid chromatography, 7-9 cloud point extraction, 10 and ion transport methods using a bulk liquid membrane (BLM) system that has attracted attention in recent years. [11][12][13][14][15] The use of a BLM containing specic metal ion carriers offers an alternative to the solvent extraction processes for the selective separation and concentration of metal ions from aqueous solutions.
Phosphoramides are studied due to their interesting structural and spectroscopic features, [16][17][18] and owing to their extensive applications in coordination chemistry, catalysis and medicine. [19][20][21][22] According to data from Cambridge Structural Database (CSD), 23 thirty copper-phosphoric triamide complexes have been studied by X-ray diffraction analysis. Therefore, the potential use of a phosphoric triamide as a carrier in transport of copper can be expected through formation of related complex.
Oen in transport processes, the transport rate of Cu(II) ion reduces when Zn(II) exists in the system. This inconvenience is minimized in the present work using a new synthetic phosphoric triamide carrier under optimized parameters of the transport process. In this competitive transport, a selective and highly efficient transmission of Cu(II) cation is achieved, in the presence of Ag(I), Cd(II), Co(II), Ni(II), Pb(II) and especially Zn(II

Instruments and methods
The pH meter (Metrohm 691) with a glass electrode sensitive to the H 3 O + ion was used, and calibrated with standard buffer solutions in pH ¼ 4 and 7. Atomic absorption spectroscope (Hitachi Z-2000) with seven hallow cathode lamps (HCL) was used for analysis of metal cations concentrations. In all of the transport experiments, a water circulator was used around the cell, in order to control and ne adjust temperature. The samples were shaken on a shaker (Stuart CB-162). The conductance measurements were performed on a digital Metrohm conductivity apparatus (model 712) in a thermostated water-bath with a conductance temperature maintained within AE0.01 C. The electrolytic conductance was measured using a cell consisting of two platinum electrodes to which an alternating potential was applied. A conductometric cell with a cell constant of 0.950 cm À1 was used throughout the studies. For Xray diffraction experiment of PTC, a suitable single crystal was selected and mounted on a glass ber. The experiment was carried out in a four-circle diffractometer Gemini of Oxford Diffraction, Ltd., equipped with a CCD detector Atlas S1, and using Cu Ka radiation (l ¼ 1.5418Å) with a mirror monochromator. The crystal structure was solved using the charge ipping algorithm implemented in the program Superip. 24 Hydrogen atoms bonded to carbon atoms were kept in ideal positions, while the positions of hydrogen atoms attached to nitrogen atoms were rened using bond distance restrains of 0.87Å. Anisotropic atomic displacement parameters were introduced for all non-hydrogen atoms, whereas for hydrogen atoms isotropic U iso was evaluated as 1.2U eq of the corresponding parent atom. Final models were rened with the program package JANA2006. 25 1 H-, 13 C-, 19 F-and 31 P-NMR spectra were recorded on a Bruker Avance DRX 400 spectrometer. Chemical shis were determined relative to TMS for 1 H and 13 C and relative to CFCl 3 for 19 F, and 85% H 3 PO 4 for 31 P as the external standards. Infrared (IR) spectrum was recorded on a Buck 500 scientic spectrometer using a KBr disc. The mass spectrum was scanned on a Varian Mat CH-7 instrument at 70 eV.

Synthesis of PTC
The (F 3 CC(O)NH)P(O)Cl 2 reagent was prepared according to the procedure described in the literature, from the reaction between PCl 5 and CF 3 C(O)NH 2 in dry CCl 4 in a reux condition and then the treatment of HCOOH at ice bath temperature. 26,27 The synthesis and melting point of (F 3 CC(O)NH)(C 6 H 5 CH 2 -NH) 2 P(O) (PTC) was also reported 27 and the procedure described here for PTC is similar to the literature method with a few modications (in solvent and the reaction temperature). Moreover, we further study this compound with IR, NMR ( 31 P, 1 H, 13 C, 19 F) and single crystal X-ray diffraction. For the synthesis of PTC, to a solution of (F 3 CC(O)NH)P(O)Cl 2 (2.06 mmol) in dry CH 3 CN, a solution of benzylamine (8.24 mmol) in the same solvent was added dropwise at 273 K. Aer stirring for 4 h, the solvent was evaporated in vacuum and the residue was washed with distilled water. Single crystals were obtained in CH 3 OH/CH 3 CN (4 : 1 v/v) aer slow evaporation at room temperature.

Construction of BLM
The transport experiment was carried out by a double-standard concentric cell in which the source aqueous phase (10 ml) and the receiving aqueous phase (30 ml) were separated by an organic membrane phase (50 ml). The experimental set-up 28,29 was a double jacket cylindrical glass cell (4.5 cm diameter) holding a glass tube (with a diameter of 2.25 cm) for separating the two aqueous phases. In a BLM, a relatively thick layer of immiscible uid is used to separate the source and receiving phase. Actually, there is no means of support for the membrane phase and it is kept apart from the external phases only by means of its immiscibility. The glass concentric cell was enclosed with a water jacket, at the temperature setting in 25 C. The details of the cell (BLM) are shown in Fig. 1.
The source aqueous phase consisted of a buffer solution at pH ¼ 4.9 (acetic acid/sodium acetate) containing an equimolar mixture of seven metal cations, each at the concentration of 1 Â 10 À2 mol L À1 . The receiving phase consisted of a buffer solution at pH ¼ 3.0 (formic acid/sodium hydroxide). The membrane phase contained the PTC carrier (1 Â 10 À3 mol L À1 ) in organic solvent (CHCl 3 , DCM, 1,2-DCE, NB). In each experiment, the organic phase was stirred for 24 h at 60 rpm. Samples of source and receiving phases were analyzed by atomic absorption spectroscopy aer each transport run. A series of standard solutions, which were made similarly, were also analyzed by atomic absorption spectroscopy. With analysis of cations in the source and receiving phases, the amounts of the metal cations within the membrane phase were measured. According to the measured values of the metal cations in liquid organic membrane, source aqueous phase and recipient aqueous phase, the speed and percentage of transport for the cations were calculated. The reported results are the averages from three experiments.

Conductometric method
The experimental procedure to determine the stability constants of complexes has been done according to a previously reported method. 30 Briey as follows: to a solution of metal salt (1 Â 10 À4 M) in a titration cell, the PTC solution (2 Â 10 À3 M) has been added. Then the solution was carried out to the titration cell by using a microburette, rapidly. The conductances of the solutions were measured initially and aer each transfer in the desired temperature.

X-ray crystal structure description
The achiral (CF 3 C(O)NH)(C 6 H 5 CH 2 NH) 2 P(O) phosphoric triamide (with chemical structure as shown in Scheme 1) crystallizes in the chiral space group P2 1 2 1 2 1 , with Z ¼ 4. Crystallographic data and structure renement parameters are listed in Table 1. Fig. 2 shows a thermal ellipsoid plot of one complete molecule, which is present in the asymmetric unit, and selected bond lengths and angles are given in the caption of the gure.
The P]O bond length is standard for phosphoramide compounds, 31    In the crystal packing, adjacent molecules are linked via ((C 6 H 5 CH 2 )N-H) 2 /O]P and CF 3 C(O)/HNC(O)CF 3 hydrogen bonds, forming a linear arrangement along the a axis (Fig. 3). This arrangement includes non-centrosymmetric R 2 1 (6) and R 2 2 (10) graph-set motifs, together with a C4 chain motif. The hydrogen-bond parameters are listed in Table 2.

NMR study
The 31 P and 19 F signals appear at 6.61 and À74.54 ppm, respectively that conrm the chemical structure and purity. The chemical shis are comparable with the phosphorus signal at 9.68 ppm for (C 6 H 5 NH)(C 6 H 5 CH 2 NH) 2 P(O) 32 and uorine signal at À74.67 ppm for (CF 3 C(O)NH)P(O)(NH) 2 C 3 H 4 (CH 3 ) 2 . 33 The NH proton of the C(O)NHP(O) segment is revealed as a broad signal centred at the high frequency of 10.34 ppm, due to its mobility caused by the electronegative CF 3 group, which leads to its acidic character, and also due to possibility of the tautomeric equilibrium in solution (as illustrated in Fig. 4).
The NH protons of the C 6 H 5 CH 2 NH groups appear at 5.34 ppm as a doublet of triplets ne structure due to H-H and P-H couplings.
In the 13 C NMR spectrum, the signals of CF 3 (at 115.30 ppm) and CO (at 157.33 ppm) both appear as a qd pattern, due to the couplings with uorine and phosphorus nuclei. The ipsocarbon atom of benzyl group at 140.72 ppm shows a three-bond separation phosphorus-carbon coupling constant ( 3 J P-C ¼ 5.4 Hz).

Transport properties
Although the effect of various carriers on the transport of alkali and alkaline earth metal cations through liquid membranes has been reported so far, relatively few carriers offer a selective and efficient transport of transition or heavy metal ions, 34,35 however they are also important from biological, medicinal, environmental and industrial points of view. 36,37 Competitive transport of seven metal cations among four membrane phases of chloroform (CHCl 3 ), dichloromethane (DCM), 1,2-dichloroethane (1,2-DCE) and nitrobenzene (NB) was evaluated in the presence of the PTC carrier. A pH gradient was utilized to facilitate transport of the metal ions across the membrane that controls the return penetration of protons from the receiver aqueous phase (with pH ¼ 3) to the source aqueous phase (pH ¼ 4.9).
In the source-organic interface, the metal ion comes in contact with the protonated carrier, and forms a complex with deprotonated carrier. The complex penetrates from organic phase to the more acidic receiver phase, and the metal ions are replaced by protons. Then, protonated carrier returns to the organic phase and the cycle is repeated, leading to an increase in the concentration of transferred metal ions in the receiver phase. In fact, the transfer occurs from the source phase to the membrane and then from the membrane phase to the recipient phase. From the obtained results, the transfer mechanism of Cu(II) ion is suggested as the pH gradient driven. The amounts of ions in receiving and membrane phases and the rate of transport for seven mentioned metal cations in different organic membrane solvents (CHCl 3 , DCM, 1,2-DCE and NB) are listed in Table 3 and visualized in Fig. 5. Transports of Ag(I), Cd(II), Co(II), Cu(II), Ni(II), Pb(II) and Zn(II) cations in the absence of PTC (e.g. in CHCl 3 , DCM, 1,2-DCE and NB neat solvents) were studied by atomic absorption spectrometry. The results show that none of the cations is transferred to the received phase.

Effect of solvent
The solvent used in the membrane phase plays an important role in conveying and separating metal ions. 38,39 The solvent should have high distribution coefficient, should be immiscible with the aqueous phase, with low viscosity and volatility. 40 Thus, various parameters of a solvent in the organic liquid membrane inuence on the transport properties. Also, mobility coefficient of species in boundary layers and value of complex formation constant (K f ) between the cation and ligand in organic membrane phase 41 are effective on the transport. The boundary  a Symmetry codes: (i) x + 1, y, z; (ii) x À 1, y, z. Fig. 4 The possible tautomeric equilibrium in solution.
layer composition, and distribution of cations in the organic solvent, is affected by dielectric constant and polarity of the solvent. 42 In solvents with higher polarity, the rate of ion transfer is increased because of the higher ability to dissolve the complex formed between the carrier molecule and cation. For four solvents tested in this work (Table 3), it was found that the optimal solvent is NB, where the transport of Cu(II) ion from the aqueous source phase into the receiving phase aer 24 h is 99.69%, while other competing ions are transported to a very small extent.
The sequence of transport rates for Cu(II) cation in organic solvents is as NB > 1,2-DCE > DCM > CHCl 3 Table 4. With all solvents, the selectivity of PTC for Cu(II) cation is superior to that for the other cations.

The study of complex formation between PTC and Cu 2+
The complex formation between PTC and Cu(II) ion was investigated by conductometry experiments in NB at different temperatures, with measuring the molar conductivity (L m ) as a function of ligand to cation molar ratio ([L] t /[M] t ), Fig. 6. As seen in Fig. 6, with addition of PTC to cation solution, the molar conductivity increases which indicates that the PTC-Cu(II) complex is more mobile than free solvated Cu 2+ cation. Moreover, the breaking of the curves is nearly to 1 and the stoichiometry ratio about 1 : 1 was observed for the complex formed.
The stability constants of the PTC-Cu complex were calculated using GENPLOT program, 43 which yield log K f of 2.81 AE Table 3 The results of competitive metal-ion transport of the sevenmetal cations across different bulk liquid membranes using (CF 3 C(O) NH)(C 6

Conclusions
The competitive transport of Ag(I), Cd(II), Co(II), Cu(II), Ni(II), Pb(II) and Zn(II) metal cations through the various bulk liquid membranes containing (CF 3 C(O)NH)(C 6 H 5 CH 2 NH) 2 P(O) (PTC) as a new carrier was studied and this ligand was found to be very suitable carrier for competitive transport of Cu 2+ metal cation. Also, this study demonstrates the usefulness of the liquid membrane technique for combining extraction and stripping operations in a single process. The results of the simultaneous transport show that the rate and selectivity of the ion transport are strongly inuenced by the nature of the membrane solvent. The selectivity of the membrane systems for Cu 2+ cation in presence of ion mixture depends on the solvents as follows: NB > 1,2-DCE > DCM > CHCl 3 . The percentage of copper transport reached 99.69% in optimum membrane solvent of NB at temperature of 298 K aer 24 h.

Conflicts of interest
The authors have no conicts of interest regarding this work.