Reactivity of Tetrabutylammonium Iodide with a Heteronuclear

Article pubs.acs.org/IECR

Reactivity of Tetrabutylammonium Iodide with a Heteronuclear 6Copper(II)−4Na(I) Complex: Selective Recognition of Iodide Ion Niraj Kumari,† Md. Amin Hasan,† Benzamin D. Ward,‡ and Lallan Mishra*,† †

Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005, India Department of Chemistry, University of Cardiff, Cardiff CF10 3AT, U.K.

‡

S Supporting Information *

ABSTRACT: A complex of type [Cu6Na4(μ-O)4(NTA)2(bpy)6(H2O)4(NO3)2]·3H2O 1 was allowed to react separately with tetrabutylammonium salts (TBAX), X = F−, Cl−, Br−, I−, AcO−, NO3−, and H2PO4−. The color of complex 1 changed immediately on the addition of TBAI (tetrabutylammonium iodide) only. However, keeping the corresponding solution for 2 months at room temperature, four complexes of compositions [Cu 6 (μ-OH) 4 (NTA) 2 (bpy) 6 ]2I − ·5H 2 O 2, [Cu2(bpy)2(OH)2(I)]2(NO3)2(H2O) 3, [Cu(bpy)2I]I3− 4, and [Cu(bpy)2I]I·TBAI 5 (bpy = 2,2′-bipyridyl, H3NTA = nitrilotriacetic acid) together with some residues of TBAI and complex 1 were isolated in one-pot synthesis. These complexes were characterized using elemental analysis, spectroscopic (IR, UV−vis) and single crystal X-ray diffraction techniques. Colorimetric response of complex 1 with iodide anions was further supported by the changes in its UV−vis spectra recorded in methanol. The results indicated that 1 exhibits a strong affinity for iodide anions.

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INTRODUCTION The coordination chemistry of anions is continuously expanding with the development of new synthetic molecules capable of recognizing anions especially of environmental and biological relevance.1−4 An earlier report that Na+/I− symporter (NIS) is an important plasma membrane glycoprotein which mediates active I− transport in the thyroid gland makes this kind of research more attractive.5 Thus, iodide plays a vital role in many biological activities including neurological activity and thyroid function. Therefore, development of receptors for the detection of iodide ions is considered worthy owing to its larger ionic radius, low charge density, and low hydrogen-bonding ability. Moreover, most of the sensors based on hydrogen bonding for iodide anions detection are found incompatible with aqueous systems. With regard to iodide ions only a few reports with sensors can be found in the literature.6−11 Therefore, developing receptors that can bind iodide ions selectively is strongly desired. Thus, different strategies for selective binding of anions at the molecular level have been adopted. These are based either on noncovalent interactions (hydrogen bonding, π−π donor−acceptor, electrostatic, hydrophobic, and hydrophilic) or on coordination-based metal− ligand interactions. If the receptor is designed to carry out a chromophore, the binding event is hopefully monitored by changes in color of the receptor or changes in its fluorescence pattern.12−16 Optical detection based methods are largely preferred, as they are usually very sensitive, are of low cost, and are easily performed in terms of equipment requirements.17,18 In aqueous conditions, anion receptors based on intrinsically weak noncovalent interactions can suffer from the competition of H2O with the anion. This problem is reduced with the synthesis of receptors generally based on stronger metal−anion covalent interactions, and this kind of receptor is more suitable for anion recognition in this medium.12−17 In particular, coordinatively unsaturated copper(II) complexes can be very © 2013 American Chemical Society

useful for this purpose because they normally display an absorption in the visible region of the spectrum which can be tuned by strong binding of anionic species to the metal center. They can offer the possibility of selective colorimetric detection of anions even by the naked eye.18,19 Thus, in this precedence, it was thought worthwhile to exploit one of our earlier reported copper(II) complexes,20 for the recognition of halide ions, as it contained four Na cations in its skeleton besides its good solubility in common solvents. The color of complex 2 ( [Cu6(μ-OH)4(NTA)2(bpy)6]2I−·5H2O) matched well with the color observed after reaction of complex 1 with tetrabutylammonium iodide (TBAI). With the lapse of time, several complexes of lower nuclearity without Na atom/atoms in their composition were isolated in a one-pot reaction. Thus, complex 1 ([Cu6Na4(μ-O)4(NTA)2(bpy)6(H2O)4(NO3)2]·3H2O)was found to be an interesting new class of receptor which changes its color instantaneously after reaction of TBAI only. The formation of these complexes was found as a result of systematic breaking of a hexanuclear Cu(II) complex in low nuclearity complexes [2, [Cu2(bpy)2(OH)2(I)]2(NO3)2(H2O) 3, [Cu(bpy)2I]I3− 4, and [Cu(bpy)2I]I·TBAI 5 (bpy = 2,2′bipyridyl, H3NTA = nitrilotriacetic acid))].

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EXPERIMENTAL SECTION Materials and Measurements. Chemicals of AR grade purchased from Sigma-Aldrich and Merck were used without further purification. Elemental analysis was carried out using a Carbo-Erba elemental analyzer 1108. IR spectra were recorded as KBr pellets using a Varian 3100 FT-IR spectrometer. A Shimadzu UV-1701 spectrophotometer was used to record

Received: Revised: Accepted: Published: 15007

April 30, 2013 September 27, 2013 October 3, 2013 October 3, 2013 dx.doi.org/10.1021/ie401373m | Ind. Eng. Chem. Res. 2013, 52, 15007−15014

Industrial & Engineering Chemistry Research

Article

Figure 1. UV−vis spectra of complexes 1−5 recorded in DMSO solution (10−4 M).

H, 2.92; N, 8.88. Found: C, 39.02; H, 2.31; N 8.73. IR data: 3324, 1640, 1602, 1458, 1023, 782, 654, 535. UV−vis (dimethyl sulfoxide (DMSO), 10−4 M): λmax/(nm) (εmax × 104 M−1 cm−1) 287 (3.58), 313 (3.53), 660 (0.016). The light blue colored crystals of 3 were obtained in 12% yield. Elemental analysis calcd (%) for C40H36Cu4N10O10I2: C, 36.27; H, 2.74; N, 10.57. Found: C, 36.31; H, 2.89; N 10.38. IR data: 1605, 1452, 1378, 1286, 860. UV−vis (DMSO, 10−4 M): λmax/(nm) (εmax × 104 M−1 cm−1) 311 (3.33), 664 (0.016). The third lot of green crystals of complex 4 was obtained in 10% yield. Elemental analysis calcd (%) for C20H16CuN4I4: C, 27.19; H, 1.83; N, 6.34. Found: C, 27.31; H, 2.01; N 6.17. IR data: 3419, 1608, 1506, 1023, 782, 654, 535. UV−vis (DMSO, 10−4 M): λmax/(nm) (εmax × 104 M−1 cm−1) 289 (3.21), 313 (3.19), 356 (0.57), 742 (0.016). Last, light green colored crystals of complex 5 were picked up in 21% yield: Elemental analysis calcd (%) for C36H52CuN5I3: C, 43.28; H, 5.25; N, 7.01. Found: C, 42.94; H, 5.01; N 6.84. IR data: 3419, 1604, 1506, 1023, 782, 654, 535. UV−vis (DMSO, 10−4 M): λmax/(nm) (εmax × 104 M−1 cm−1) 289 (3.21) 340 (1.053), 748 (0.016). The remaining parts in the solution were identified as starting materials. X-ray Crystallographic Studies. The crystals of complexes suitable for X-ray measurement were grown directly from their corresponding reaction mixtures in DMF/MeOH (6:1 by volume) at room temperature. X-ray diffraction data were collected using an Oxford diffraction XCALIBUR-EOS diffractometer by mounting a single crystal of the sample on glass fibers. Monochromated Mo Kα radiation (λ = 0.710 73 Å) was used for the measurements. The crystal structures were solved by direct methods using the SHELXS-97 Program23 and have been refined by full-matrix least-squares SHELXL-97.24 Drawings were carried out using MERCURY,25 and special computations were carried out using PLATON.26

UV−vis spectra. The titration experiments were performed at room temperature by maintaining the concentration of metal complexes constant at 10 μM while the concentration of tetrabutylammonium (TBA) salts of F−, Cl−, Br−, I− AcO−, NO3−, and H2PO4− was varied within 0−40 μM. An equal quantity of tetrabutylammonium salt was also added to the reference solution to eliminate absorption by the anion. The binding constants were calculated from absorbance data obtained from the titration curve at a specified wavelength using eq 1.21 Aobs = (A 0 + A∞K[G]T )/(1 + K[G]T )

(1)

where Aobs is the observed absorbance, A0 is the absorbance of the free complex, A∞ is the maximum absorbance induced by the presence of a given anionic guest, [G]T is the total concentration of the guest (halide ions), and K is the binding constant of the host−guest entity. Binding constants were performed in duplicate, and their average value is reported. Similar titration experiments have been done using sodium salts, such as NaF, NaCl, NaBr, NaI, NaNO3, NaH2PO4, and CH3COONa. The precursor complex of type Cu(2,2′bipyridyl)(NO3)2·H2O was prepared using a reported procedure.22 Synthesis of [Cu6Na4(NTA)2(μ-O)2(bpy)6(H2O)4(NO3)2]· 3H2O (1). The complex was synthesized and characterized using a reported procedure.20 Synthetic details are given in the Supporting Information (S1). One-Pot Synthesis of Complexes [Cu6(μOH)4(NTA)2(bpy)6]2I−·5H2O (2), [Cu2(bpy)2(OH)2(I)]2(NO3)2(H2O) (3), [Cu(bpy)2(I)]I3 (4), and [Cu(bpy)2I]I·TBAI (5). A methanolic solution containing an excess of tetrabutylammonium iodide was added slowly to a solution of complex 1 (0.426 g, 20.00 mmol) in dimethylformamide (DMF) with stirring. After complete addition, the solution was left as such. After 2 months, four types of crystals were picked up using a needle as they differed in color. The light green crystals of complex 2 were obtained in 17% yield. Elemental analysis calcd (%) for C72H64Cu6I2N14O28: C, 39.16;

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DENSITY FUNCTIONAL THEORY (DFT) All DFT calculations were performed using the Gaussian 03 program suite27 at the B3LYP (unrestricted) functional level 15008

dx.doi.org/10.1021/ie401373m | Ind. Eng. Chem. Res. 2013, 52, 15007−15014


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Table 1. Summary of Crystallographic Data of Complexes 2, 3, 4, and 5 parameter formula M crystal syst temp (K) space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dc (mg m3) abs coeff (mm−1) F(000) reflns collected/unique R(int) index ranges

refinement method final R indices [I > 2σ(I)] R indices (all data) GOF

2 C72H64Cu6I2N14O28 2208.47 triclinic 293(2) P1̅ 12.084(5) 12.583(5) 14.642(5) 89.324(5) 81.452(5) 78.217(5) 2154.8(14) 1 1.677 1.049 1082 40 064/11 892 0.0463 −16 ≤ h ≤ 16 −17≤ k ≤ 17 −19 ≤ l ≤ 20 R1 = 0.0767, wR2 = 0.2482 R1 = 0.1124, wR2 = 0.2889 1.066

3

4

C40H36Cu4I2N10O10 1324.75 triclinic 293(2) P1̅ 9.4674(13) 10.7587(10) 13.3802(14) 88.486(8) 76.392(10) 72.853(10) 1264.4(2) 1 1.782 2.946 664 11 083/5775 0.0253 −11 ≤ h ≤ 12 −14 ≤ k ≤ 14 −16 ≤ l ≤ 18 full-matrix, least R1 = 0.0800, wR2 = 0.2416 R1 = 0.1022, wR2 = 0.2648 1.094

C20H16CuI4N4 883.51 monoclinic 293(2) C2/c 26.376(12) 7.3661(16) 14.871(6) 90 119.77(6) 90 2507.9(16) 4 2.340 5.808 1620 4709/2129 0.0253 −32 ≤ h ≤ 22 −9 ≤ k ≤ 6 −18 ≤ l ≤ 16 squares on F2 R1 = 0.0312, wR2 = 0.0768 R1 = 0.0492, wR2 = 0.0824 0.943

5 C36H52CuI3N5 999.07 monoclinic 293(2) C21/c1 19.545(5) 15.866(5) 13.698(5) 90 109.619(4) 90 4001.2(6) 4 1.659 2.891 1964 28 323/3624 0.0315 −22 ≤ h ≤ 23 −19 ≤ k ≤ 16 −16 ≤ l ≤ 16 R1 = 0.1350, wR2 = 0.4275 R1 = 0.1672, wR2 = 0.4666 1.964

transition at ∼550−660 nm. Therefore, complexes 2 and 3 showing bands at λmax 664 and 669 nm, respectively, were assigned square pyramidal geometry. Complex 2 possesses four of its CuII centers in a distorted octahedral geometry, and the remaining two CuII centers possess a square pyramidal geometry. However, distinction between the two geometries could not be made owing to the broadness of bands. A trigonal bipyramidal geometry was assigned33 to complexes 4 and 5 on the basis of broad bands observed at λmax 742 and 748 nm, respectively. The bands observed at λmax 289 and 356 nm in the UV−vis spectrum of 4 were assigned to arising from I3− ion in consistency with the earlier report.34 The formation of I3− could be considered by the oxidation of I− ion using HNO3 obtained initially by the deprotonation of complex 1 leading to the formation of I2. The I2 in turn further reacted with another I− ion and formed I3− ion. Structural Description of Complexes. Structural refinement parameters of the complexes are given in Table 1. The selected bond distances (angstroms) and bond angles (degrees) data are given in the Supporting Information (S3). Complex 2 crystallizes in a triclinic crystal system with the P1̅ space group. The crystal structure of 2 consists of a unique hexanuclear (Cu6) motif where two cuboidal [Cu3O4] units were fused together through a μ-hydroxo group along with five cocrystallized water molecules and two iodide ions as depicted in Figure 2. The complex is centrosymmetric in which Cu(1) and Cu(2) are bonded to two μ-hydroxo groups while Cu(3) is bonded to carboxylate oxygens. Cu(2) is bonded to two μ3hydroxo groups and has two elongated Cu−oxygen bonds (i.e., (Cu(2)−O(11)#1, 2.503(6) Å, and Cu(2)−O(13), 2.613(7) Å) and four contracted (two Cu−oxygen and two Cu− nitrogen) distances of Cu(2)−O(11), 1.949(3) Å; Cu(2)− O(10), 1.905(3) Å; Cu(2)−N(3), 2.005(3) Å; and Cu(2)− N(4), 2.012(3) Å. In contrast, Cu(1) is connected to only one

with the 6-31G(d,p) basis set for copper and nitrogen atoms, the LANL2DZ basis set (with pseudo core potentials) for the iodine, and 6-31G for all remaining centers. The geometries were optimized without symmetry constraints and the nature of the stationary point was identified (minimum vs saddle point) by frequency calculations, which indicated no imaginary frequencies.

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RESULT AND DISCUSSION Synthesis. Complex 1 was prepared using the method reported by us and in the Supporting Information (S1).20 Although the molecular structure of complex 1 has been reported, for a ready reference the structure is depicted in the Supporting Information (S2). On addition of methanolic solution of tetrabutylammonium iodide in a solution (DMF) of complex 1, the color changed instantly from blue to green. After keeping the solution for 2 months, four types of crystals were isolated in one pot. These complexes were found thermally stable and soluble in DMF and DMSO. They were characterized by their elemental analyses, Fourier transform infrared (FT-IR) spectra, and UV−vis spectra, and authenticated by their single crystal X-ray crystallography. In IR spectra of the complexes, peaks observed in the region ∼3400 to ∼3200 cm−1 were assigned to υ(H2O) while the υ(bpy) vibration appeared at 1602, 1605, 1608, and 1604 cm−1 in the IR spectra of complexes 2, 3, 4, and 5, respectively. Complexes 2 and 3 showed a strong band at ∼3441 cm−1 and a weak band at ∼858 cm−1, which were assigned to stretching and deformation vibrations of hydroxo bridges.28,29 Complexes 2 and 3 showed a sharp band of medium intensity around 560− 570 cm−1 for the characteristic T2 mode of the Cu4O core.30 The UV−vis spectra of the complexes recorded in DMSO (10−4 M) are displayed in Figure 1. In view of earlier reports,31,32 square pyramidal complexes normally show 15009



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Figure 3. Coordination environment around Cu(II) ion in 3 drawn at 40% probability level. All hydrogen atoms are omitted for clarity.

N atoms, a μ2-OH atom, and a μ3-OH atom, and the apical sites are occupied by an iodide ion and a μ3-OH group. Interplanar distances between 2,2′-bipyridine rings coordinated to Cu(II) ion on the same sides are averaged to 3.48 Å. This suggested that tetranuclear complex cations are stabilized by significant intracationic π−π stacking interactions. The complex cation is crystallographically imposed by symmetry of center at the centroid of the middle Cu2O2 ring. Within 3, the counterbalancing nitrate anions display considerable disorder: they were hydrogen bonded to the water of the lattice and formed hydrogen-bonded anionic layers parallel to the 001 plane. The resulting anionic layers are sandwiched by the supramolecular cationic layers parallel to 001 (Supporting Information, S6). The hydrogen-bonding interactions between anionic and cationic layers significantly stabilize the structure of 3. Complex 4 crystallized in a monoclinic crystal system with the C12/c1 space group possessing a CuN4I coordination core in a distorted trigonal bipyramidal geometry. The molecular structure of complex 4 consists of one Cu(II) ion, two 2,2′bipyridine rings, together with an iodide anion, and cocrystallized with an I3− anion as depicted in Figure 4. The in-plane, Cu(1)−I(1) distance of 2.633(8) Å together with the N(2)− Cu(1)−N(2)# angle of 173.98(9)° shows almost a linear arrangement. The coordination geometry around the Cu center is distorted trigonal bipyramidal with τ = 0.689. Complex 5 was crystallized in a monoclinic crystal system with the space group C12/c1. Its molecular structure, as shown in Figure 5, contains a [Cu(bpy)2I]I unit together with a cocrystallized tetrabutylammonium iodide molecule. It possesses a CuN4I coordination core in a distorted trigonal bipyramidal geometry with τ = 0.689. The perfect square pyramidal and trigonal bipyramidal geometries provide τ = 0 and 1, respectively.35 Thus, isolation of four complexes by the reaction of TBAI with complex 1 that contains four sodium atoms suggested that initially complex 1 is getting destabilized most likely by the formation of sodium iodides. The destabilized complex could have been then further dissociated into lower nuclearity copper complexes as evidenced by the formation of a tetranuclear complex followed by the formation of dinuclear and mononuclear complexes (Cu6, Cu4, Cu2, Cu1).


μ3-oxo group. Out of three copper centers, Cu(2) and Cu(3) adopt distorted octahedral geometry whereas Cu(1) possesses distorted square pyramidal geometry with τ = 0.123 (τ = |β − α|/60°, where β and α are the two largest angles around the central atom). The perfect square pyramidal and trigonal bipyramidal geometries provide τ = 0 and 1, respectively.35 The coordination cores around Cu(1), Cu(2), and Cu(3) are CuN2O(μ3-OH)(μ2-OH), CuN2O(μ3-OH)2(μ3-OH), and CuN3O3, respectively. Out of three carboxylate groups of the NTA ligand, one carboxylate group acted in a tridentate fashion whereas the remaining two carboxylate groups and a nitrogen atom acted in a monodentate fashion. Thus, a NTA molecule acted as hexadentate ligand bridging three metal centers (Cu(1), Cu(2), Cu(3)). Two iodide ions remain outside the cavity, satisfying the charge requirement of the complex, and also interact with cocrystallized water molecules. The supramolecular structure showing encapsulation of the iodide anion is depicted in the Supporting Information (S4). Its chemical structure is also shown in the Supporting Information (S5). Complex 3, a triclinic crystal system with the P1̅ space group, possesses a distorted square pyramidal geometry. A perspective view of its molecular structure is shown in Figure 3. In 3, the building unit is a tetranuclear [Cu4(bpy)4(I)2(μ2-OH)2(μ3OH)2]2+ complex cation. The complex cation may originate from dimerization of a dinuclear [Cu2(bpy)2(μ2-OH)2] system through μ3-OH and exhibits a nonplanar rectangular fourmembered Cu2O2 ring in the center together with two coplanar inner and outer Cu2O2 rings. The μ3-OH bridged copper centers are separated by 3.175 Å whereas μ2-OH bridged inner and outer ring copper centers are separated by 2.913 Å respectively and were found comparable with Cu···Cu distances reported for the dinuclear [Cu2(μ2-OH)2] complex.36 Each Cu(II) ion is pentacoordinated and possesses a square pyramidal geometry. The basal plane is defined by two pyridyl 15010



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1 to I− was quite evident as it showed remarkable enhancement of the absorption intensity with a blue shift of λmax by 15 nm as depicted in Figure 7, while other competitive anions did not show any change in the spectrum of the complex. The addition of TBAI to complex 1 showed prominent changes in the spectrum of 1, whereas addition of a large excess of other competitive anions (F−, Cl−, Br−, AcO−, NO3−, and H2PO4−) showed no significant change in the spectrum (Supporting Information, S8). Thus, spectral study also supported the selective binding of complex 1 to I− ions. In this context, it had been reported that complementarity for halides is generally achieved by varying the size of the receptor binding site in view of the spherical shape of halide ions.7 The halide ions were found to bind in size order, with iodide producing the highest binding constant. The maximum binding constant with higher selectivity of 1 for iodide ions could be understood in view of an earlier report,8 as the distance between I− and CH2 protons at 3.107 Å and that between I− and bipyridine protons at 3.163 Å, together with the distance between I− and H2O protons at 3.284 Å, were found to be consistent with those reported therein. To verify the anionic specificity, some assays were performed with sodium salts, such as NaF, NaCl, NaBr, NaI, NaNO3, NaH2PO4, and CH3COONa. Under identical conditions (see the Supporting Information, S9), the color of all mixture solutions stay blue except the one which contains iodide ion changes to green. To explore the applicability of 1, UV−vis spectra were recorded in aqueous methanol (1:1 v/v) separately using a solution of sodium salt, such as NaF, NaCl, NaBr, NaI, NaNO3, NaH2PO4, and CH3COONa. Results revealed (Supporting Information, S10) that no significant spectral changes were observed on the addition of inorganic iodide to an aqueous solution (MeOH−H2O, 1: 1 v/ v) of complex 1, except that it showed a blue shift (∼5 nm) in its wavelength. It may happen most likely owing to the solvation of I− ions in water. These optical results supported that the complex 1 is selectively sensitive to iodide ions and is independent of the nature of the cations. Thus, complex 1 can


Colorimetric Response of Complex 1 to Halide Ions. Owing to the presence of Na ions in the molecular structure of complex 1, it was thought worthwhile to exploit them as receptors for halide ions. The anion-sensing abilities of 1 were studied only on a qualitative level by visual examination of the anion-induced color changes in its methanolic solution (1 × 10−5 M) before and after the addition of an anion. In order to avoid any complication of cations, tetrabutylammonium salts (TBAX) (X = F−, Cl−, Br−, I−, AcO−, NO3−, and H2PO4−) were used as guest molecules. As displayed in Figure 6a, dramatic change in the color of 1 from blue to green occurred only in the presence of tetrabutylammonium iodide. The color change was found visible up to 5.0 × 10−6 M concentration of complex 1 (Supporting Information, S7). This experiment suggested that 1 showed selective binding affinity with iodide anions. The interference by other anions was discarded, as displayed in Figure 6b; significant change in color occurred only when iodide anions were added. The response of complex

Figure 5. Coordination environment around Cu(II) ion in 5 drawn at 40% probability level. All hydrogen atoms are omitted for clarity. 15011



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Figure 6. Colorimetric response of complex 1 in MeOH (1 × 10−5 M) (a) on addition of four equivalents of salts of TBA together with (b) anions interfering in the response.

Figure 7. Changes in UV−vis spectrum of 1 (1 × 10−5 M) on addition of different TBA salts.

Figure 8. Changes in the UV−vis spectrum of 1 in MeOH (1 × 10−5 M) upon addition of different equivalents of the solutions of tetrabutyl ammonium iodide (inset: Job’s plot of complex 1 in presence of iodide ions).

detect both organic and inorganic iodide. This simple, rapid, and versatile methodology is based on the different electrostatic interactions and the conformational change of the cationic complex. In order to get quantitative insight into complex 1− anion interactions, spectrophotometric titrations were carried out with different anions (F−, Cl−, Br−, I− AcO−, NO3−, and H2PO4−) in methanol. The variation in the spectral pattern of complex 1 on the incremental addition of iodide anions is depicted in Figure 8. The receptor showed an absorption band at λmax 304 nm in the absence of iodide ions. The addition of I− to the receptor solution (MeOH) resulted in a gradual increase in the intensity while lowering the wavelength (blue shift of ∼15 nm), showing an isosbestic point at 272 nm (Figure 8). The color and spectral changes are perhaps due to the replacement of attached groups linked to copper ions by an added iodide anion. The 1:2 stoichiometry between the

complex and iodide anion was established using Job’s method. Using eq 1, the binding constant K for complex 1−anion interactions has been evaluated and the values are given in Table 2. The results revealed that binding constants measured in aqueous methanol are less than those observed in dry methanol. This retardation in binding between the host and the guest may occur owing to the competition between anions and H2O for receptor binding sites.37 It also showed a detection limit of 5.0 × 10−7 M which could be considered a larger detection range as compared to those reported earlier.38 The different types of complexes formed by the interaction of complex 1 with tetrabutylammonium iodide have already been discussed. However, to investigate the composition of complex formed instantaneously by addition of excess of TBAI to complex 1, the electrospray ionization mass spectrum (ESIMS) of the solution was recorded. As depicted in Figure 9, a 15012



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experimental, 1.663 Å). Likewise, the structure of [Cu(bpy)2I] shows Cu−N bond lengths of 2.043 and 2.153 Å. As expected, the bond lengths are significantly lengthened in the Cu(I) complex in comparison to the Cu(II) analogue. Computing the energy change for the reaction of [Cu(bpy)2X]+ with X− (X = Cl, Br, I) to afford the corresponding X2 along with [Cu(bpy)2X] failed to provide any meaningful explanation of the apparent selectivity of the redox process with iodide, as opposed to with bromide or chloride; however, on comparison of the standard redox potentials some insight could be obtained. The standard redox potentials for 2Cu2+ + 2X− → 2Cu++ X2 are −1.04 V (Cl), −0.78 V (Br), and −0.23 V (I).39 From these data, it can be clearly seen that small perturbations of these values (e.g., by the incorporation of bpy ligands) is most likely to favor the iodide, whereas the bromide and chloride are more significantly disfavored and are likely to remain disfavored under the conditions described above.

Table 2. Values of the Binding Constant of 1 with Anions anion −

F Cl− Br− I− AcO− NO3− H2PO43−

K (M−1)a in methanol K (M−1)b in methanol:water (1:1, v/v) 1.12 2.02 3.14 5.65 2.35 − 3.65

× × × × ×

102 102 102 104 102

× 102

0.34 1.15 2.32 3.07 1.97 − 2.11

× × × × ×

101 101 101 103 101

× 102

a

Tetrabutylammonium salts are used as anion source. bSodium salts are used for anion source.

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CONCLUSIONS A heteronuclear complex of type [Cu6Na4(μO)4(NTA)2(bpy)6(H2O)4(NO3)2]·3H2O (1) binds iodide anions selectively as evidenced by naked eye detection without interference with other anions (F−, Cl−, Br−, AcO−, NO3−, and H2PO4−). Significant variation in the magnitude of λmax with iodide anions allows complex 1 to perform as a useful receptor for iodide anions in solution with a detection limit of 5.0 × 10−7 M. Complexes of lower nuclearity are formed after keeping the solution of 1 with TBAI for 2 months. These complexes are characterized using spectroscopic and X-ray diffraction techniques.

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Figure 9. ESI-MS of solution of 1 containing excess tetrabutylammonium iodide.

ASSOCIATED CONTENT

S Supporting Information *

peak observed at m/z 1900.47 was assigned to [Cu6(μOH)4(NTA)2(bpy)6]2+. This experiment clearly demonstrated that color change on addition of TBAI to complex 1 occurred due to the formation of an isolated complex 2. Density Functional Theory. To understand the mechanism of the selective binding of complex 1 with I−, initially, the formation of Cu(I) complex was thought to be an intermediate. Therefore, the structures of [Cu(bpy)2I]+ and [Cu(bpy)2I] were optimized using DFT method and are displayed in Figure 10. The Cu−N bond lengths of [Cu(bpy)2I]+ are in excellent agreement with the experimental values taken from the crystallographic data of 4 (calculated Cu−N, 1.991 and 2.105 Å; experimental, 1.989 and 2.099 Å), although the Cu−I distance is somewhat overestimated (calculated Cu−I, 2.732 Å;

CCDC reference nos. 792035, 833626, 927202, and 927203 contain the supplementary crystallographic data for the complexes 2, 3, 4, and 5, respectively. This data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/ retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K., fax (+44) 1223-336-033, or e-mail: [email protected]. Molecular structure of 1, packing diagram of complex 2, chemical structure of complex 2, and UV−vis spectra of 1 with iodide anion in the presence of excess of other halides in methanol. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank DST and CSIR New Delhi, India, for financial assistance.

+

Figure 10. Calculated structures of (a) [Cu(bpy)2I] [Cu(bpy)2I.

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DEDICATION Dedicated to Prof. Koji Araki, IIS, University of Tokyo, Komaba campus, on the occasion of his 65th birthday.

and (b) 15013



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Article

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Reactivity of Tetrabutylammonium Iodide with a Heteronuclear

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