Hydroxyl-terminated dendrimers with sulfonimide linkers as binders for metals of industrial significance

First- and second-generation hydroxyl-terminated dendrimers were prepared starting from a 1,3-diaminopropane core and sulfonimide linkers. A first-generation mesitylene-derived dendrimer was also prepared with the same terminals. The dendrimers were then reacted with Fe3+, Al3+, and UO22+ separately in order to apply the dendrimers for binding these metals, which have important industrial applications and pose environmental problems simultaneously. The prepared dendrimers were also shown to bind Fe3+ selectively from mixtures with Al3+.

One of the main methods to extract Al from natural kaolin uses HCl [19].However, this dissolves Fe leaving the Al solution contaminated with iron.Different methods are used to remove Fe from Al such as solvent extraction which uses phosphates [20], amines [21], and carboxylic acids [22].
The interest in recovering uranium from various sources has increased to meet the growing demand for energy.Moreover, radioactive contamination caused by U is an environmental concern.The most common processes for recovering uranium from minerals such as phosphates are extraction [23], ion exchange [24], and sorption [25].Organic solvents are hazardous, while exchange processes lack selectivity [24].
Since Al 3+ , Fe 3+ , and UO 2 2+ are hard Lewis acids and thus strongly bind hard bases like O-donors, they are expected to have good affinities for oxygen-terminated dendrimers [26][27][28].Therefore, we prepared oxygen-terminated dendrimers for binding these ions.The composition and structures of the dendrimers and their metal complexes were proved by spectroscopic methods as well as elemental and thermal analysis.

Materials and methods
All chemicals were purchased off the analytical grade.4-bromomethylbenzenesulfonyl chloride, 1,3-diaminopropane, 4-nitrobenzene-sulfonyl chloride, and Et 3 N from Sigma-Aldrich (USA).All solvents from Tedia (USA).Tris, FeCl 3 , AlCl 3 , and K 2 CO 3 from Merck (Germany).Uranyl nitrate from BDH (England) and the uranium nitrate standard solution (1000 μg/mL U in 2%-5% aqueous HNO 3 ) from AccuStandard, (USA). 1 H-NMR and 13 C-NMR were done on a 400 MHz Bruker instrument using DMSO as a solvent.The infrared spectra were recorded on a Tensor II FT-IR spectrometer with an ATR attachment from Bruker.UV-Vis spectra were recorded using a SPECORD 200 PLUS spectrophotometer, Analytik-Jena (Germany).Elemental analysis was performed using a FLASH 2000 CHNS/O Analyzer, Thermo-Scientific (USA).Thermal gravimetric analysis (TGA) was observed at a rate of 10 °C/min up to 900 °C under N 2 in alumina crucibles using a Netzsch TG 209F1 instrument.The sample mass range was 4.74-13.55mg.

Synthesis of the metal complexes
The complexes were synthesized by stirring the metal salt and the dendrimer in 40 mL DMF at 20 °C for 2 h then the solvent evaporated and the solid was washed with ethanol and Et 2 O and then dried in a vacuum.

Selective binding of iron from aluminum solutions
Three separate solutions of Fe 3+ and Al 3+ were prepared in 30 mL water by mixing a dendrimer with 3.8 × 10 -4 mol of both FeCl 3 .6H 2 O (0.104 g) and AlCl 3 .6H 2 O (0.093 g).The solutions were stirred for 20 h at 20 ºC then tested for both metals (see A, B).Solution 1. Dendrimer added: L1, (0.10 g, 8.6 × 10 -5 mol).Solution 2. Dendrimer added: L2, (0.139 g, 4.75 × 10 -5 mol).Solution 3. Dendrimer added: L3, (0.065 g, 1.25 × 10 -4 mol).Test A) A 10 mL sample of the filtered solution was diluted to 100 mL using 0.002 M NaSCN.The absorbance of the resulting FeSCN 2+ was measured at 447 nm and the free Fe 3+ was determined using standard Fe 3+ solutions.Test B) To a 1 mL sample of the solution, drops of 6 M NH 3 are added until the solution is basic and Al 3+ precipitates as Al(OH) 3 (s).To confirm the presence of Al 3+ , 3 drops of 0.1% (wt/ V) Aluminon solution are added with shaking.Aluminon, the ammonium salt of aurin tricarboxylic acid, adsorbs onto the surface of Al(OH) 3 giving it a pink-red color.The solution was then centrifuged producing a red precipitate.

Results and discussion
The dendrimers were prepared by the divergent method.L1 and L2 have 1,3-diaminopropane cores and contain sulfonimide linkers.The 1 st -generation dendrimer L1 was derived from 4-toluenesulfonyl chloride while the 2 nd -generation dendrimer L2 was derived from 4-nitrobenzenesulfonyl chloride.4-Bromomethylbenzenesulfonyl was then used to extend the branches.Unlike these two dendrimers, the 1 st -generation dendrimer L3 has a mesitylene core.The terminals were derived from tris and act as tridentate ligands to each metal via O atoms.These hard atoms are suited to bind hard metals with high oxidation states such as Al 3+ , Fe 3+ , and UO 2

2+
. The off-white ligands were slightly soluble in the polar solvents water, DMF, and DMSO, and insoluble in Et 2 O and benzene reflecting their high polarity.The dendrimers were characterized using IR, 1 H-NMR, and 13 C-NMR spectroscopy.Elemental analysis confirmed the composition of the ligands.

Dendrimers derived from 4-toluenesulfonyl chloride, L1
The dendrimer L1 was prepared by reacting 1,3-diaminopropane with excess 4-bromomethyl-benzenesulfonyl chloride, in the presence of Et 3 N as a base, resulting in the introduction of four 4-toluenesulfonyl groups on the two nitrogen atoms (Scheme 1).Evidence for full substitution on N comes from the IR data of the dendrimer, Compound 1, which does not show any NH stretching vibrations (Figure S1).The aromatic and sulfonyl vibrations appear at their usual positions and C-Br stretching vibrations appear at 463 cm -1 [29].The tetrabrominated product was then reacted with tris in the presence of K 2 CO 3 as a base and KI as a catalyst, causing the disappearance of the C-Br stretching vibration in L1 (Figure S2).Benzene ring vibrations appear at 1594, 1450, and 855 cm -1 .Stretching vibrations due to sulfonyl groups give rise to absorptions at 1364 and 1162 cm -1 .A broad band at 3335 was assigned to stretching vibrations of the alcoholic OH groups, for which C-O stretching and O-H deformation appear at 1296, 1080, and 1037 cm -1 .Finally, N-H stretching appears at 3285 cm -1 .
In the 1 H-NMR spectrum of L1 (Figure 1) aromatic protons appear at 7.2-7.8ppm, OH and NH protons appear as a broadened peak at 3.69, the methylene protons of Ar-CH 2 -N at 3.54 while those of NCH 2 O appear at 3.28 ppm.The 13 C-NMR spectrum (Figure S3) shows the aromatic carbons in the range of 128-145 ppm, the C-O carbon at 63, and the quaternary carbon at 60.4 ppm, while the Ar-CH 2 -N carbon appears at 57 ppm [29].
Compound 3 was then condensed with 4-bromomethylbenzenesulfonyl chloride causing the NH 2 features to disappear, while C-Br stretching appears at 600 cm -1 .No frequencies appear in the 3200-3300 cm -1 region, proving the attachment of two sulfonyl groups to each nitrogen atom of the primary amine (Figure S5).Reacting the product, Compound 4, with tris produced L2.The IR spectrum of L2 (Figure S6) shows a band at 3420 cm -1 due to OH stretching [29].Coupled C-O stretching and O-H deformation appear at 1085 cm -1 .Moreover, in the 1 H-NMR spectrum of L2 (Figure S7) the OH protons appear as a broadened peak at 4.34 ppm.The methylene protons of NCH 2 Ar appear at 4.49 and those of CH 2 O at 3.36 ppm.The 13 C-NMR spectrum (Figure S8) shows the aromatic carbons in the range 126-143 ppm, the C-O carbon at 61.7, and the quaternary carbon at 60.6 ppm.Ar-CH 2 -N carbons appear at 52.5 ppm.

Mesitylene-derived dendrimer, L3
Tris(bromomethyl)mesitylene was reacted with tris to afford L3 (Scheme 3).In the 1 H-NMR spectrum of L3 (Figure S9) the terminal OH protons appear broadened at 4.41 ppm.The methylene protons of Ar-CH 2 -N appear at 3.75, while those of CH 2 O at 3.47 ppm.The 13 C-NMR spectrum (Figure S10) shows the aromatic carbons at 134.9 and 135.6 ppm, the C-O carbon at 61.6, C-CH 2 -N at 40.8, and the CH 3 carbons at 15.2 ppm.The IR spectrum (Figure S11) shows a broad band attributed to OH stretching at 3355 cm -1 .Coupled C-O stretching and O-H deformation appear at 1042 and 1346 cm -1 .

Metal complexes of the dendrimers
The dendrimers were reacted at RT separately with the metal ions Fe 3+ , Al 3+ (as chlorides), and UO 2 2+ as the nitrate, in DMF as a solvent.L1 was also reacted with uranyl in HNO 3 to study its ability to bind uranium from acidic solutions.L1 was used in a 1:4 molar ratio to the metals, since it has a capacity of 4 ions/dendrimer molecule, L3 in a 1:3 ratio (capacity of 3), and L2, which has the highest capacity at 8, in a 1:8 ratio.The hard metals form coordinate bonds with the hard oxygen atoms on the periphery of the dendrimers (Figure 2).The Fe complexes of the dendrimers have brown colors, Al complexes offwhite, and UO 2 2+ complexes yellow, as expected from the coordination to OH [27].The complexes decomposed at 240-270 °C.The complexes were slightly soluble in the polar solvents DMF, DMSO, and water and insoluble in the less polar Et 2 O and ethyl acetate.Complexes with L2 were the least soluble in water, a direct result of the large size of the dendrimer.Complexation was studied by IR and UV-Vis spectroscopy as well as TGA.Elemental analysis confirmed the composition of the complexes.

Thermal gravimetric analysis
TGA data of the metal complexes are given in Table 1.The detailed fragmentation patterns and the assignments of the fragments lost from the peripheral groups, the branches, and the core, as well as the residues formed all comply with the proposed structures of the dendrimers and their metal complexes.Fragmentation starts with the loss of bound DMF followed by alcohol moieties from the OH terminals and then the amine branches.The loss of benzenesulfonyl fragments leaves the metal salt behind [30].
The complexes have bound DMF as indicated by the high temperature at which DMF leaves (from about 140 °C and up to 300 °C in some complexes) and the mass percent of the residues [31].Decomposition of all complexes started at 250-325 °C.Decomposition of the ligand in L1Fe 4 Cl 12 •4DMF (Figure S12) started with the loss of CH 3 OH from the terminals and (CH 3 ) 2 NH from the branches and continued till the formation of the residue, which forms 20% of the complex (calc.20.68%) [32].
A uranyl nitrate residue forms 47.9% of the complex L2U 8 O 16 (NO 3 ) 16 •8DMF (Figure 3).Decomposition of the ligand started by losing CH 3 OH from the termini and ended at 850 °C [32].Meanwhile, the ligand in L2Fe 8 Cl 24 •8DMF started decomposing by losing tris moieties and continued till an inorganic residue formed (Figure S13).The residue forms 34.0% of the complex L3Al 3 Cl 9 •3DMF (Figure S14) and 60.7% of L3U 3 O 6 (NO 3 ) 6 •3DMF, where the ligand started decomposing by losing terminal tris and ended with the loss of benzene from the interior (Figure S15).On the other hand, the decomposition of the ligand in L3Fe 3 Cl 9 •3DMF (Figure S16) started with the loss of terminal CH 3 OH and continued till the formation of FeCl 2 (29.5% of the complex).

IR spectra
The IR spectra of the Fe complexes (Figure 4, Figure S17) show shifts in the stretching vibration of the O ̶ H groups and the coupled O ̶ H deformation and C ̶ O stretching vibrations compared to the free dendrimers (Table 2).These shifts together with the appearance of a new peak in the complexes in the 500-600 cm -1 range are attributed to the newly formed Fe ̶ O bonds, proving that the binding of Fe takes place at the terminal OH groups of the dendrimers [33].
Diallo et al. observed significant binding of UO 2 2+ to PAMAM dendrimers in solutions containing up to 1.0 M HNO 3 and H 3 PO 4 [7].The IR spectra of the U complexes (Figure S18) prove the binding of UO 2 2+ ions from the solutions to OH with shifts to different frequencies for the OH stretching vibrations compared to the free dendrimers (Table 2).This is supported by the altered intensity and shift of the coupled C-O stretching and O-H deformation and the appearance of new peaks at 645-500 cm -1 due to UO 2

2+
-O single bonds [34].The absorption at 925-825 cm -1 is typical of UO 2 2+ [35].The peaks at 1500-1560 cm -1 and 1300-1355 cm -1 are due to coordinated nitrate [36].The nitric acid solution was evaporated over several days thus reflecting the stability of these dendrimers in acidic solutions and showing their potential for binding metals from acidic solutions.
The IR spectrum of the Al complex L1Al showed notable changes from the free ligand spectrum (Figure S19).Although C-O stretching and O-H deformation of the OH groups appear at 1298 and 1039 cm -1 , close to the ligand positions (1296 and 1037), the absorption at 3335 cm -1 disappears.Thus, the involvement of OH groups in Al binding cannot be excluded, although they may be deprotonated.This suggestion is supported by the appearance of a new peak at 592 cm -1 , which       is assigned to the Al-O stretching frequency.Finally, the spectrum indicates the presence of DMF in the complex due to the presence of absorptions at 1664 and 3073 cm -1 .The IR spectra of Al 3+ with L2 and L3 (Figure S20) do not show recognizable features that suggest Al binding to L2 or L3.UV-visible spectra UV-visible spectra were recorded for the complexed dendrimers in aqueous solutions and compared to those of the free ligands as well as those of the aqueous ions.No absorptions were observed for the free L1 and L3 (Figures S21 and 5, respectively).New ligand-to-metal charge transfer (LMCT) peaks were observed at 300 nm for the Al-complexed dendrimers L1 and L3.These absorptions were not observed in the spectra of the free ligands or Al 3+ ions.On the other hand, the Cl → Fe 3+ CT absorptions were shifted from 334 nm in FeCl 3 [37] to 301 nm upon complexation of Fe to L3, and 300 nm upon Fe binding to L1, which is in the range expected of the O → Fe 3+ charge transfer in Fe 3+ -OH moieties [38].The extinction coefficient reported here (ε, 3.16 × 10 3 M -1 cm -1 for L1Fe and 1.58 × 10 3 for L3Fe) is similar to previous reports [38].
The UV-vis spectrum of the dendrimer L2 has a peak with a maximum at 296 nm (ε, 6.21 × 10 3 ) due to n → π* transitions (Figure S22).The spectra of the Al and Fe complexes of L2 have peaks at 304 and 300 nm, respectively.These strong absorptions can be attributed to oxygen → metal LMCT.
O → U LMCT from the ligand-based orbitals σ u and π u to the metal-based orbitals δ u and φ u appear at 370 and 415 nm in free uranyl nitrate [39].The absorptions are not strong since they are Laporte-forbidden [40].These absorptions become much stronger and appear to shift to new positions at 300, 357, and 429 nm (Figure S21) upon forming L1U.Similar peaks were obtained for L2U at 297, 351, and 434 (Figure S22) and L3U at 302, 382, and 430 nm (Figure 5).
These results from the electronic spectra give further proof to conclusions drawn from the results obtained using the previous techniques that the metal ions are bound to the hydroxyl terminal groups of the dendrimers (Figure 2).

Selective binding of iron from aluminum solutions
The dendrimers were tested for their ability to separate Fe 3+ from Al 3+ by adding each dendrimer, separately, to solutions containing equal amounts of both metal ions.The metals were added such that the concentration of each one would be enough to fulfill the capacity of the dendrimer by itself in order to get a clear answer to the question of the dendrimers' selectivity toward Fe 3+ and Al 3+ .Fe 3+ concentration was determined spectrophotometrically using NaSCN as a complexing agent, whereas Al 3+ was tested using the Aluminon test.While the Aluminon tests were all positive and proved the presence of significant amounts of Al in all samples, the concentration of free Fe ions was found to be lowered to less than 1% of its original value in all three tests (Table 3).Solution 2 did not produce any detectible quantities of Fe using the same test.These experiments give proof of the selectivity of these OH-terminated dendrimers toward the Fe 3+ ions compared to the Al 3+ ions and therefore could work to bind Fe selectively from Al solutions.Moreover, the quantitative binding of Fe from the solution gives further proof of the loading capacity of these dendrimers, i.e.L1 binds 4 Fe 3+ ions, L2 binds 8, and L3 binds 3 ions.L2 in particular shows the most promise for separating Fe from Al because of the low solubility of its Fe complex in water which facilitates its separation from Al, and because of its higher loading capacity.

Conclusions
The dendrimers prepared form an addition to the family of dendritic molecules and have the ability to bind to many metals of industrial significance.The composition and structure of the products were proved by different spectroscopic methods, elemental analysis, and TGA.The TGA fragmentation patterns of the complexes and their assignments all comply with the proposed structures of the dendrimers and their complexes.The dendrimers bind the metals studied although they do not appear to bind Al strongly.The experiments performed with mixtures of Fe and Al show a strong preference of these dendrimers toward the Fe 3+ ions over the Al 3+ ions, and therefore could potentially separate Fe from Al solutions.The dendrimers also appear suitable for binding UO 2 2+ from acids and show high stability over several days.

Figure 2 .
Figure 2. Binding of the dendrimers to the metal ions.M = Fe, Al.

Figure 2 .
Figure 2. Binding of the dendrimers to the metal ions.M = Fe, Al.

Figure 4 .
Figure 4.The IR Spectrum of the Fe complex of L2.

Figure
Figure S1.IR spectrum of compound 1.

Table 1 .
TGA of the complexes.

Table 2 .
IR data of the uranium and iron complexes.

Table 3 .
Selective binding of Fe 3+ in solutions of Fe 3+ and Al 3+ .