Selective recognition and extraction of iodide from pure water by a tripodal selenoimidazol(ium)-based chalcogen bonding receptor

Summary A selenium-based tripodal chalcogen bond (ChB) donor TPI-3Se is demonstrated for the recognition and extraction of I− from 100% water medium. NMR and ITC studies with the halides reveal that the ChB donor selectively binds with the large, weakly hydrated I−. Interestingly, I− crystallizes out selectively in the presence of other halides supporting the superiority of the selective recognition of I−. The X-ray structure of the ChB-iodide complex manifests both the μ1 and μ2 coordinated interactions, which is rare in the C–Se···I chalcogen bonding. Furthermore, to validate the selective I− binding potency of TPI-3Se in pure water, comparisons are made with its hydrogen and halogen bond donor analogs. The computational analysis also provides the mode of I− recognition by TPI-3Se. Importantly, this receptor is capable of extracting I− from pure water through selenium sigma-hole and I− interaction with a high degree of efficiency (∼70%).


INTRODUCTION
Majority of the reported abiotic anion receptors are found to be operative in organic solvents and most of the receptors have been designed by employing non-covalent interactions, such as hydrogen bonding (HB), 1,2 halogen bonding (XB), 3,4 and anionÀp interactions. 5Due to the large hydration energies of anions, the extraction of anions in pure water is one of the key challenges in supramolecular chemistry.Iodide (I À ) is among one of the most important anions that regulate the neurological activities and thyroid gland functions of the human body. 65][16][17][18][19] Despite this, it is essential to develop a remediation process to effectively remove I À from contaminated environments.Various conventional methods for ion capture and environmental remediation include chemical precipitation, solid-liquid as well as liquid-liquid extraction, ion exchange resins, and adsorption onto activated carbon. 20,21But among these methods chemical precipitation is considered one of the most effective and mature method for anion extraction due to its unique advantages in terms of selectivity, simplicity, cost-effectiveness, and scalability.In particular, for wastewater treatment in industry, this effective tool has been widely used.However, it may have some limitations, especially in dealing with complex sample matrices and the potential for contamination.Notwithstanding, anion extraction is a critical step, so the choice of extraction methods should depend on the specific requirements of the application and careful consideration of the advantages and limitations.
3][24][25][26][27][28] Concretely, the s-hole interactions are mainly composed by the halogen (HX), chalcogen (ChB), pnictogen (PnB), 29,30 and tetrel (TrB) bonding 31 associated with the groups 17, 16, 15, and 14 elements, respectively.Such, highly directional non-covalent interactions indeed occur between the electropositive region (s-hole) of the atoms described previously and an electron-rich atom (Lewis base).The potency of s-hole can also be regulated by increasing the polarizability and decreasing the electronegativity of these atoms.Sustained research related to these interactions is attracting growing prominence and has been contextualized in different fields, including supramolecular chemistry, 22 materials science, 32,33 and biochemistry, 34 among others.
6][37][38] The presence of two highly directional s-holes (opposite to two covalent bonds) in the ChB donor represents the strong ChB interaction compared to HB and in contrast to XB, ChB donor atoms exhibit a greater electropositivity, as well as possess contrasting steric and geometric diversity for host-guest interactions. 38,39Thus, the ChB non-covalent interaction provides high binding opportunities as well as fine-tuning options to enable more precise three-dimensional spatial regulation for anion characterized by different spectroscopic methods such as 1 H, 13 C, 19 F, 77 Se-NMR, and HRMS (see the supplemental information) (Scheme S1 and Figures S1-S11).In this work, further analogous HB (TPI-3H) and XB (TPI-3I) receptors are also prepared according to the reported method 65 followed by anion exchange with aqueous AgOtf (Figures S12-S17).

Solution phase halides binding by NMR studies
The anion-binding affinities of halides have been assessed by 1 H, 13 C, and 77 Se-NMR titration experiments with TPI-3Se in D 2 O.The gradual addition of sodium salt of halides, the resulting downfield chemical shifts corresponding to the peak Se-CH 3 (H b ) and one imidazole proton (H e ) are observed in 1 H-NMR studies.Relatively higher downfield shift Dd = 0.029 ppm of H b proton is observed upon the addition of $8 equivalents of I À (Figure S18) compared to Cl À (0.011 ppm) and Br À (0.010 ppm).In the case of H e proton 0.056, 0.017, and 0.022 ppm Dd are observed for I À , Br À , and Cl À , respectively (Figures S18-S20).This study indicates that the H b and H e protons are engaged in substantial hydrogen bonding interactions with I À as compared to Cl À and Br À .A well-fitted 1:1 and 1:2 binding isotherm in a Bindfit analysis (Figures S21-S26) provides the association constants of halides with the TPI-3Se (Tables 1 and S1).The high association constant for I À reveals that the I À binds to the tris-imidazolium cleft selectively through Se-CH 3 units of TPI-3Se ChB donor.
In contrast with the halides, the addition of other hydrophobic anions ($8 equivalents) such as ReO 4 À , ClO 4 À , and PF 6 À have shown insignificant protons shift in 1 H-NMR titration, which implies that the receptor TPI-3Se forms the strongest complex with I À only (Figures S27-S29).
Again, for additional evidence to support the proposed selective I À binding in water, we have performed the 1 H-NMR titrations in D 2 O:CD 3 CN (1:1) binary solvent (Figures S30-S35).The results clearly exhibit the same selectivity trend as observed in pure water (Table 1).Moreover, to check the superiority of the recognition of I À through TPI-3Se over its HB (TPI-3H) and XB (TPI-3I) congeners, analogous qualitative 1 H-NMR titrations are also performed.In the case of HB, after the addition of $8 equivalents of halide ions in D 2 O:CD 3 CN (1:1) solvent, a prominent downfield perturbation is observed in the acidic proton (H a' ) of the azolium ring, while for XB, imidazole protons shows significant chemical shifts (Figure S36-S47).Therefore, by monitoring the proton chemical shifts of both the receptors with halide anions concentration, 1:1 Bindfit analysis elicits that the binding constant of Br À is higher compared to Cl À and I À (Figures S39-S41 and S45-S47, and Table 1).This notable result suggests that the ChB receptor TPI-3Se is the most potent receptor for selective I À recognition in pure water compared to its XB and HB analogs.
It is noteworthy that, during 13 C-NMR titration of TPI-3Se (72.84 mM) with I À (87.40 mM) in 0.45 mL D 2 O, immediate precipitation of the host-guest adduct is noticed.However, such precipitation is not at all observed even upon the addition of 10 equivalents (excess) of Cl À or Br À to the TPI-3Se.Similar, observation is also perceived during 77 Se-NMR titration in D 2 O.Such accidental finding of selective precipitation of I À with TPI-3Se in water leads us to develop an easier pathway for the extraction of I À through precipitation (vide infra).Hence, 13 C and 77 Se-NMR investigations are carried out in 1:1 D 2 O:CD 3 CN binary solvents.In the 13 C-NMR titration of TPI-3Se (53.72 mM) with the addition of I À (357.49mM), a large downfield chemical shift Dd = 0.54 ppm of Se-CH 3 carbon is observed (Figure S48).In cases of Br À and Cl À , very low chemical shifts Dd = 0.13 and À0.02 ppm respectively are accounted upon the addition of $8 equivalents of respective halides (Figures S48-S50).This depicts that the Se-CH 3 subunits of TPI-3Se exert significant influence on the I À binding.Such interaction with I À is further established by 77 Se-NMR experiments, which show a large downfield chemical shift (Dd = 1.14 ppm) in the case of I À compared to Cl À (Dd = 0.15 ppm) and Br À (Dd = 0.29 ppm) upon addition of $8 equivalents of halides (Figures S51-S53).Alongside, in the presence of iodide-interfering pseudohalide CN À ($8 equivalents) no obvious shift in the Se peak is observed (Figure S54).Therefore, in the solution phase, these observations are consistent with the involvement of the ChB interaction between the s-hole of the selenium and I À .Furthermore, a comparison of the halide binding selectivity between the ChB receptor TPI-3Se and previously reported tripodal-based halogen and hydrogen systems reveals the fundamental differences in sensitivity to anion basicity. 65,66,68The halide binding propensity for TPI-3Se in water medium is found to be in the order Cl À < Br À < I À , which could be due to the intrinsic preference of selenium's s-holes toward the softer, more lipophilic, and easily desolvated higher homolog of halides. 39,60,61,69

Thermodynamic contributions to halide ions binding
Having demonstrated the importance of the ChB donor for selective halide recognition, isothermal titration calorimetric (ITC) studies are carried out to offer further insights into the thermodynamic enthalpic and entropic contributions behind the halides binding with TPI-3Se.Titration of sodium halides (I À , Br À , and Cl À ) with TPI-3Se in H 2 O:CH 3 CN (1:1) illustrates smooth and clear exothermic heat changes which are well fitted to a sequential 1:3 receptor-anion binding mode (Figures S55-S57).Notable to mention that, in the case of TPI-3H and TPI-3I in the same binary solvent led to the precipitation problem for all halides; therefore, we are unable to quantify the ITC studies.For TPI-3Se, inspection of Table 2 divulges that the binding of halides is a high enthalpy (DH) driven process.Moreover, the calculated 1:3 cumulative association Table 1.Association constants of halides (K I /M À1 ) for TPI-3Se, TPI-3I, and TPI-3H constants (b 3 ) for I À , Br À , and Cl À become 22.1 x 10 11 , 8.96 x 10 11 , and 4.16 x 10 11 M À3 , respectively, suggesting that the ascertains of higher binding affinity toward I À .The larger association constant for I À is primarily driven by a higher enthalpic contribution for the exothermic binding phenomenon, which can be attributed to selenium's s-holes' inherent affinity for softer I À .

Solid state structural study of TPI-3Se$$$l interactions
Further insight into the host-guest complexation through ChB interactions between I À and tri-cationic host is demonstrated by solid-state structural studies.Monoclinic crystal with P21/n space group of the iodide complex TPI-3Se-I (Table S2 and Figure S58) is grown by slow diffusion of diethyl ether and methanol binary solvent.The structure reveals that the three s-hole donor Se atoms are participating in intermolecular C-Se$$$I ChB interactions with three iodide anions.One iodide (I2) is coordinated with the s-hole located on the prolongation of the C Me -Se bond (Se3-I2) with m 1 coordination, and another two iodides (I1) are coordinated by the two Se (Se1 and Se2) from the neighboring molecules, providing a m 2 coordination, whereas I3 occluded in the crystal structure as a counter anion for the overall charge neutrality (Figure 1).In the case of m 2 coordination, the ChB interactions happen through the s-holes present along with the C Imd -Se bond.Therefore, in the unit cell, I2 is fully contributed to the one host molecule, but I1 is equally distributed to the adjacent two host molecules through two m 2 coordination, which unambiguously indicates the 1:2 host-guest stoichiometry in the solid state.The ChB distances between Se$$$I À are found to be 3.59 to 3.86 A ˚(Table S3) that are shorter than 4.10 A ˚, the sum of van der Waals radius of Se (1.90A ˚) and ionic radius I À (2.20 A ˚). [70][71][72] The ChB angles (formed with I, Se, and C) are found to be 169.18 to 171.69⁰, i.e., almost linear (Figure S58B and Table S3).These distance and angle parameters clearly suggest the formation of a strong Se-based chalcogen bonded I À complex in solid state through both m 1 and m 2 coordination. 47,57Further, the m 2 coordination with Se$$$l$$$Se bond angle of 85.12⁰ resulted a zigzag 1D extended supramolecular network through chalcogen bonding interactions (Figure S59).In addition, to evaluate the selective I À binding superiority over a mixture of halides (Cl À , Br À , and I À ) by the TPI-3Se, we investigate the fate of the solution of 1:1 mixture of TPI-3Se and sodium salt of halides in 100% aqueous medium.From this solution, we have found that TPI-3Se crystalizes out I À selectively through ChB interaction to form TPI-3Se-MI host-guest complex which is confirmed by single-crystal X-ray crystallography.The crystallographic details are given in Table S4 and Figure S60.Therefore, the solution state selectivity studies along with the aforementioned solid-state analysis manifest the superiority of I À toward TPI-3Se in the presence of competitive halide ions.

Mode of ChB$$$l interaction by DFT calculation
Further, DFT calculations of TPI-3Se and its iodide complex (TPI-3Se-I) are performed to correlate the experimental results.The DFT-optimized structures of TPI-3Se and TPI-3Se-I are shown in Figure S61.The optimized geometry of the TPI-3Se-I shows ChB interactions having C-Se$$$I distances in the range of 2.95-3.04A ˚with (C-Se-I) angles $180⁰ (Table S5), which represents significant chalcogen bonding interactions. 735][76] The single crystal X-ray structure also supports such anion-p interaction (Figure S63).The electrostatic potential calculation of TPI-3Se clearly defines the strong electron-deficient region (s-holes) on each of the Se atom, whereas one of the s-holes is centered on C Imd -Se and another is on C Me -Se bond with large V s,max values (Figure 2 and Table S6).70][71][72] The nature of non-covalent interactions between iodide and TPI-3Se are further analyzed by the noncovalent interaction-reduced density gradient (NCI-RDG). 77The plot of reduced electron density gradient RDG(s) vs. sign(l 2 )r(r) shows three spikes of sign(l 2 )r in the low-density and low-gradient region (Figure S64).The spikes shifted toward a more negative sign(l 2 )r region, suggesting the presence of strong noncovalent interaction between I À and TPI-3Se.It can also be confirmed from the NCI isosurfaces, where the deep blue disc-like embodies are observed due to efficient s-hole-based ChB interactions between Se and I À (Figure S65).
Additionally, the NBO analysis reveals that the lone pair (LP) electrons of I À transfer to the s* orbital of the ChB donor (C-Se).9][80] The high value of E(2) exhibits the strong s-hole interaction between Se and I À .

Preliminary iodide extraction study
During 13 C and 77 Se-NMR studies of TPI-3Se with I À in D 2 O immediate precipitation of the host-guest adducts was observed which prompted us to extract I À from the aqueous medium through the precipitation method.Therefore, to establish the removal of I À from 100% water medium a typical experiment is carried out by treating 17.6 mM aqueous solution of TPI-3Se with 0.52 M NaI ($30 equivalents) at pH 7.After that immediate formation of host-guest adducts (TPI-3Se-I) as a white precipitate is collected and characterized by different techniques.
Firstly, XPS analysis is employed on the precipitated adducts (TPI-3Se-I) to confirm the I À uptake from the solution as well as to determine the binding affinity.In the survey spectrum, two new peaks are located for TPI-3Se-I, which correspond to I3d 3/2 and I3d 5/2 with binding energies 617.78 and 629.27 eV, respectively (Figure 3C).Further analysis of Se3d, high-resolution spectra of TPI-3Se and TPI-3Se-I (Figures 3A  and 3B) reveal that the binding energies of Se3d 5/2 and Se3d 3/2 peaks are shifted from 55.91 to 53.76 eV and 56.79 to 55.93 eV, respectively. 81,824][85][86] Therefore, it clearly culminates that I À uptake is occurred by TPI-3Se through the ChB interaction.
Furthermore, the TEM analysis of TPI-3Se and TPI-3Se-I, is performed to obtain the elemental mapping which clearly demonstrates that iodide is uniformly distributed in the sample of TPI-3Se-I (Figures 4 and S66-S69).
In addition, the FTIR spectrum of both TPI-3Se and TPI-3Se-I, clearly showed that the significant stretching frequency of triflate ion at 1036 cm À1 and 641 cm À1 almost disappeared in TPI-3Se-I (Figure S70), which is indicative of I À uptake by the TPI-3Se in conjugation with anion exchange mechanism. 87hus, based on the aforementioned studies indeed confirms the extraction of I À from 100% water medium by precipitation through ChB interaction as a complex of TPI-3Se-I.

Selective iodide extraction from mixed halide ions
We have also investigated the effect of I À uptake by TPI-3Se in the presence of high concentrations of co-existing other halides (Cl À and Br À ) (I À : Cl À /Br À = 1:100) in 100% water.Interestingly, it has been found that the iodide complex gets precipitated (TPI-3Se-MI) selectively from the mixed halides (Cl À , Br À , and I À ) solution which is confirmed by XPS, TEM-EDX, EDS-mapping, and PXRD data.The extracted solid from the mixed halides solution (TPI-3Se-MI) has also shown the I3d 3/2 , and I3d 5/2 peaks in the high-resolution XPS spectra with almost the same binding energies compared to TPI-3Se-I, and no peak correspond to Br and Cl are observed (Figure 3E).Not only that, the Se3d 5/2 and Se3d 3/2 peaks also show almost same binding energies (53.78 and 55.85 eV respectively) as in TPI-3Se-I (Figure 3D).
Furthermore, to determine the chemical compositions of TPI-3Se-MI, energy dispersive X-ray (EDX) measurements are performed.The results indicate only the presence of C, Se, and I in both TPI-3Se-I and TPI-3Se-MI samples (Figures 4, S68, S69, S71, and S72).Meanwhile, the EDS mapping images corresponding to the area in Figure 4 conclusively revealed the uniform distribution of them.In contrast, significantly negligible bromide or chloride sample distribution is observed in TPI-3Se-MI, which further supports the observation of selective I À uptake by the ChB donor TPI-3Se.
Additionally, the FTIR spectrometric analyses of TPI-3Se-MI indicated no obvious spectral changes w.r.t TPI-3Se-I (Figure S70), which also confirms the appeal interpretation.
Besides, Cl À and Br À , we have also precipitated the ChB-anion complex in the presence of a mixture of sodium salts (I À : Anions = 1:50) in 100% water (anion salts = NaF, NaCl, NaBr, NaI, NaH 2 PO 4 , NaCN, and NaAcO).The TEM EDX analysis of the precipitated ChB-anion complex clearly shows that only a significant amount of Se and I, have been distributed in the sample, rather than F, Cl, Br, P, and O (Figures S73 and  S74).In addition to the absence of stretching frequencies for -CN, and C=O (AcO À ) in the IR spectra, which manifest that ChB receptor shows fondness selectively to form ChB-anion complex with I À only (Figure S75).Therefore, all these studies clearly suggest that the TPI-3Se selectively binds and extracts the I À from the pure water medium even in the presence of various sodium salt anions as well as high concentration of Br À and Cl À .
In order to check the bulk purity of the extracted mass, we have further investigated the powder diffraction pattern of the extracted iodide complexes (TPI-3Se-I and TPI-3Se-MI) and have compared it with the simulated pattern.The resemblance of the experimental PXRD patterns with the simulated one indicated that the precipitate iodide complexes form with a high degree of bulk purity (Figure 5).Thus, we can endorse that the ChB receptor TPI-3Se can be used for pure extraction of I À from 100% water medium in the presence of competitive halides.
Furthermore, in order to generalize the superiority of ChB receptor TPI-3Se over their HB (TPI-3H) and XB analogs (TPI-3I), we have investigated the selective precipitation of the halides complex.Eventually, the precipitated halide complexes of TPI-3H and TPI-3I demonstrate all the halides (I À , Cl À , and Br À ) are concurrently precipitated, which is confirmed by the TEM-EDX and mapping data (Figures S76-S79).Therefore, it can conclusively reveal that only TPI-3Se is selectively able to remove I À but its analogs TPI-3H and TPI-3I fail to do so.

Extraction of iodide under batch experiments
To evaluate the removal efficiency of I À through the TPI-3Se, concentration-dependent extraction experiments are carried out.Therefore, we have performed batch experiments with different concentrations of I À which clearly revealed that the minimum $0.3 M I À concentration is required to precipitate the ChB-iodide host-guest complex.The detailed removal of I À procedures is discussed in the ESI.To determine, the remaining I À concentration in the solution UV-Vis spectroscopy is used. 88It is important to note that TPI-3Se shows maximum 70.4% extraction efficiency when the minimum concentration of I À becomes $0.5 M (Table 3).For the practical applicability, the pH-dependent (pH = 3, 7, and 10) I À removal efficiency is also evaluated and the results demonstrated there is no significant effect on efficiency by varying the pH of the solution (Table S8).In all the cases, the PXRD pattern of the precipitated ChB-iodide complexes along with TEM-EDX data have shown similar to that of the pure TPI-3Se-I complex (Figures 4 and S80-S85).This result manifests that in 100% aqueous medium pure I À extraction can be achieved by the ChB receptor TPI-3Se and also illustrates the exploration of the ChB bond as a functional material for real-world application.

Conclusions
In summary, this work demonstrates the first selenium-based tripodal ChB receptor which selectively extracts iodide from 100% water through the s-hole-based interactions.Thus, it highlights the genuine potential of a sigma-hole-based receptor for anion extraction from water.The integration of ChB donor motifs into hosts of different dimensionalities is currently underway in our laboratory for selective extraction and catalysis studies.S3).

Preparation and characterization of TPI-3H
An acetonitrile (40mL) solution of 1,3,5-tris(bromomethyl)-2,4,6-trimethylbenzene (1 equiv.,1.85 mmol) was introduced with 1-methyl imidazole (3.3 equiv., 5.9 mmol) under argon atmosphere with reflux condition.A thick precipitate was started to form and the reaction was continued till 1day.Afterward, the white solid was filtered off and washed by acetonitrile to remove excess 1-methyl imidazole and finally washed with diethyl ether for several times.The obtained bromide salt was dissolved in water and poured into a AgOTf solution of water resulting in thick white precipitate of AgBr.After that, the remaining solution was filtered off and evaporated to dryness (93 % yield).

Preparation and characterization of TPI-3I
An acetonitrile (40mL) solution of 1,3,5-tris(bromomethyl)-2,4,6-trimethylbenzene (1 equiv.) was introduced with 1-methyl-2-iodo imidazole (3.3 equiv.)under argon atmosphere with reflux condition.Precipitate was started to form and the reaction was continued till 24hr.Afterward, the yellow solid was filtered off and washed by acetonitrile to remove excess 1-methyl-2-iodo imidazole and finally washed with diethyl ether for several times.The dried mass was dissolved in water and poured into a AgOTf solution of water resulting in thick white precipitate of AgBr.
After that, the remaining solution was filtered off and evaporated to dryness (82 % yield).

NMR titration studies
1 H-NMR titrations of TPI-3Se with the sodium salts of guest anions were carried out in a 300 MHz NMR instrument at 298K.The measured amount of receptor was taken in the NMR tube and dissolved in 0.5 ml of D 2 O. Known volumes of anionic guest as their sodium salts in D 2 O were added and the spectra were recorded after each addition.In each case, Se-Me proton and one imidazole proton signal of the host was monitored.While for TPI-3H and TPI-3I, titration experiments could not be performed in pure D 2 O due to the precipitation problem.Instead, a binary 1:1 D 2 O and CD 3 CN solvent mixture was used in these cases.For TPI-3H and TPI-3I, acidic proton in the azolium ring and imidazole proton signal of the receptors were monitored respectively.The change in the chemical shift value for the host spectra were monitored as a function of guest concentration.The data was analysed to get the binding constant values from Bindfit software.(Figures S18-S47 and Table S1).

13
C NMR titrations with TPI-3Se were appeared to be difficult in recording in D 2 O because of the precipitation problem (after addition of 1equiv.NaI).Thus, we choose a 1:1 mixture of D 2 O and CD 3 CN as our experimental binary solvent for this titration experiment.Here also our host was dissolved in 0.5ml mixed solvent and known volumes of sodium halide guest in the same mixed solvent were added and the spectra were recorded.Here Se-Me carbon signal of the host was monitored.(Figures S48-S50).77   Se NMR titrations were also done using 1:1 mixture of D 2 O and CD 3 CN.In this case dimethyl diselenide (Me 2 Se 2 ) used as an external standard and the spectra of the host were recorded in the absence and presence of 8 equiv.amount of guest solution.The Se-peak corresponds to the host was monitored.(Figures S51-S54).

Isothermal titration calorimetric (ITC) studies
In a typical ITC experiment, a 1mM host TPI-3Se solution was titrated with 20mM of sodium salt of halides at 298 K.A 1:1 binary solution of acetonitrile and water(milli-Q) was used as solvent for ITC experiment.(Figures S55-S57).

Figure 1 .
Figure 1.X-ray crystallographic m 1 and m 2 coordinated structures of TPI-3Se-I ChB interaction represented with cyan dash line.Purple = Iodine, blue = Nitrogen, yellow = Selenium.All H atoms are removed for clarity.

Figure 2 .
Figure 2. Computed electrostatic potential surface of TPI-3Se; negative charge density (red), and positive charge density (blue) Yellow arrow represents the s hole.Scale unit: kcal/mol.

Figure 5 .
Figure 5. PXRD spectra of the iodide complexes and the simulated pattern

Table 2 .
Thermodynamic parameters of halides binding determine by fitting 1 : 3 sequential site model from ITC studies

Table 3 .
I À extraction efficiency of TPI-3Se at different concentrations of I À Under an argon atmosphere, compound P2 (1 equiv.,0.69 mmol), K 2 CO 3 (6 equiv.,4.14 mmol) and elemental selenium powder (6 equiv., 4.14 mmol) were taken into a two-necked R.B containing dry Methanol(25mL).The solution was left for reflux for 24 hr.with vigorously stirring.Next the hot solution was filtered by vaccum filtration through celite and washed by DCM for two times.