Coordination Polymers in Selective Separation of Cations and Anions

Organic Chemistry, Indian Association for the Cultivation of Science (IACS), 2A & 2B Raja S C Mullick Road, Jadavpur Kolkata − 700032, West Beng...
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Coordination Polymers in Selective Separation of Cations and Anions: A Series of Rarely Observed All Helical Three-Dimensional Coordination Polymers Derived from Various Chiral Amino Acid Based Bis-pyridyl-bis-amide Ligands Subhabrata Banerjee and Parthasarathi Dastidar* Department of Organic Chemistry, Indian Association for the Cultivation of Science (IACS), 2A & 2B Raja S C Mullick Road, Jadavpur Kolkata 700032, West Bengal, India

bS Supporting Information ABSTRACT: A series of CuII coordination polymers [{Cu(μL1)2Br2} 3 2H2O]∞ CP1, [{Cu(μL2)2 3 H2O 3 Br} 3 Br 3 4H2O]∞ CP2, [{Cu(μL3)2Br2} 3 2H2O]∞ CP3, [{Cu(μL3)2Cl2} 3 2H2O]∞ CP4, derived from bis-pyridyl-bis-amide ligands having a natural L-amino acid backbone have been synthesized and characterized by single crystal X-ray diffraction. Environmentally relevant cations (CuII) and anions (Br ) have been separated selectively from complex mixtures of cations (CuII, CoII, and ZnII) and anions (Br , NO3 , SO42 , ClO4 and BF4 ) via an in situ crystallization technique in separate experiments, respectively. Such separation of cations by in situ crystallization of coordination polymers represents the first example in the literature.

’ INTRODUCTION Significant progress has been made in the field of metal organic frameworks (MOF) or coordination polymers (CPs).1a This class of materials is known for their various potential applications viz. gas storage,1b,c catalysis,1d anion separation,1e,f magnetic property,1g drug delivery,1h battery materials,1i etc. Toxic metal separation is of immense importance to environmental cleanup. Selective removal of toxic metal ions from aqueous solutions has been mainly achieved through solvent extraction and solid sorbents. The solid sorbents include ligand-grafted organic polymers, silica gel surface coated with ligands, layered metal phosphonates, mesoporous materials coated with monolayers of ligands, and redox-recyclable ion-exchange materials.2 CuII is an important toxic metal ion present in trace amounts in the environment. Its permissible limit is 2 mg/L in drinking water. If it exceeds the permissible limit, it becomes a carcinogen causing lung cancer. CuII is liberated in the environment mainly through mines, brass manufacture, electroplating industries, agriculture, water treatments, etc.3 Therefore, removal of CuII from an aqueous medium is a potential target in the context of environmental cleanup. The affinity of a ligand site to coordinate preferentially a metal center may be exploited in selective separation of a metal cation by in situ crystallization of coordination polymers from a complex mixture of cations. However, to the best of our knowledge, no such efforts have been reported in the literature. On the other hand, various anions scattered in the nature play crucial roles in biological processes.4 Certain anions present in r 2011 American Chemical Society

excess have an adverse effect on the environment.5 Therefore, selective removal of anions is important. For example, SO42 removal is important for the cleaning of nuclear waste tank;1f F should be removed from drinking water because it causes several health hazards such as dental and skeletal fluorosis and osteosarcoma.6 Br is also an important toxic anion in the environment causing various health hazards. Drinking water resources near costal areas has high Br concentration due to seawater intrusion; during disinfection by chlorination, chloroamination or ozonization, etc. disinfection byproducts (DBPs) such as trihalomethanes (THM), haloacetic acids (HAAs), etc. are generated that create serious health hazards.7 The ability of CP to exchange anion is long known.8 Soon after this remarkable discovery, efforts have been made to discover new coordination networks having the ability to exchange anions. Since selectivity is the ultimate goal of any separation, it is of paramount importance to develop coordination networks that have a high affinity toward a particular anion within a complex mixture.1e However, development of an anion-specific coordination network needs profound understanding of the anionexchange/separation mechanism. The most well recognized proposal supports a solid state ion-exchange mechanism. The acceptability of this proposal takes its cue from the insoluble Received: September 5, 2011 Revised: October 12, 2011 Published: October 13, 2011 5592

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Crystal Growth & Design Scheme 1. Bis-pyridyl-bis-amide Ligands (L1, L2, L3) in Their Anti-Syn Conformation

nature of CPs to common solvents and the similarity between the initial and final structures. However, this solid state single-crystalto-single-crystal (SCSC) anion exchange mechanism is not beyond criticism; in certain cases, it was undoubtedly proven that the anion exchange process in CPs is solvent mediated and involves dissolution and crystallization.9 Encouraged by the second alternative mechanism which involves dissolution and competitive crystallization, Custlecean et al. showed that in situ synthesis of CPs by the crystallization technique could itself be exploited as an anion separating tool.1e In this process, CPs are selectively crystallized out from a mixture of metal salts having various counteranions and a suitable ligand. Depending on the shape, the size, and the nature of interactions of the anions within the network structures, an anion of interest may be selectively crystallized out from the complex mixture of anions as the corresponding CP. This process is advantageous over the solidstate SCSC exchange process because it excludes the common problem of slow solid state ion exchange kinetics. In this paper, a series of bis-pyridyl-bis-amide ligands (L1, L2, and L3, Scheme 1) derived from naturally occurring amino acids such as L-phenyl alanine, L-alanine, and L-leucine have been exploited to separate CuII cations from a complex mixture of cations (CuII, CoII, and ZnII) and L1 and L3 were used to separate Br from a complex mixture of anions (Br , SO42 , ClO4 , NO3 , BF4 ) in separate experiments. The synthesis, characterization, and single crystal X-ray structures of the coordination polymers, namely, • [{Cu(μL1)2Br2} 3 2H2O]∞ CP1 • [{Cu(μL2)2 3 H2O 3 Br} 3 Br 3 4H2O]∞ CP2 • [{Cu(μL3)2Br2} 3 2H2O]∞ CP3 • [{Cu(μL3)2Cl2} 3 2H2O]∞ CP4 have been discussed in the context of the above-mentioned problem of cation and anion separation. The effect of amino side chain on the resultant supramolecular structures of the CPs has also been investigated.

’ RESULTS AND DISCUSSION Crystal Structures of the CPs. In the recent past,10 we have exploited the ligands L1 and L2 to generate intriguing CPs wherein the effect of amino acid side chain was attributed to the resultant supramolecular structures; while L1 resulted in an unprecedented all helical intertwined network when reacted with CuCl2, the same reaction with L2 produced rarely observed three-dimensional (3D), non-interpenetrated octahedral structure. To extend the study further in order to understand the role (if any) of the amino acid side chains and counteranions, we reacted L1, L2, and L3 with CuBr2; L3 was also reacted with

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CuCl2 in order to compare the resultant structure with that of earlier published structures involving L1 and L2.10 Since all the CPs described herein (Table 1) are isomorphous displaying an identical space group P3221, the description of only one CP, namely, CP4 is presented herein. Following a slow evaporation process, CP4 is synthesized as a blue block shaped crystal by layering an aqueous methanolic solution of L3 over an ethanolic solution of CuCl2 taken in a 1:2 (metal/ligand) molar ratio. In the asymmetric unit, a half occupied copper(II) atom (located on a 2-fold symmetry), one molecule of L3, one chlorine atom, and a solvate water molecule are located. Ligand moiety displays an anti-syn conformation. Surprisingly, instead of being a one-dimensional (1D) looped chain structure expected from a ditopic ligand having anti-syn conformation, a 3D coordination polymeric network is formed due to dendritic propagation of metal ligand assembly in the crystal structure of CP4 (Scheme 2). Steric bulk of the side chain of L-leucine (an isobutyl group) might be preventing these ligands from assembling in a 1D looped chain topology thereby promoting the dendritic propagation. The ligand has a bent geometry (with an angle of 45.7(4) involving the amide moieties and 24.3(1) involving the pyridyl rings; the corresponding angles in CP1 and CP3 are 53.7, 43.9(4), and ∼23.7, 21.9(1), respectively; e.s.d.s. for the values associated with CP1 are not provided as it was refined isotropically) displaying an angular ligating topology. The propagation of the metal ligand coordination leads to the formation of a helical polymeric chain that runs along the 32 screw axis. To complete one helical turn, three amino acid units are required displaying a helical pitch of ∼40 Å (Figure 1). The helical chains are originated from each octahedral metal center, and there are two metal centers within a helical turn. As a result, the overall supramolecular structure of CP4 can be best described as an all helical 3D intertwined network (Figure 2). The solvate water molecules are located in the channels that run along the helical 32 screw axis and are involved in various hydrogen bonding interactions; the amide moiety bound to the N-terminal of the leucine moiety makes hydrogen bonding contacts with the solvate water molecule and the metal-bound chloride [O 3 3 3 O = 2.877(4) Å, — O H 3 3 3 O = 160.0(3); N 3 3 3 Cl = 3.315(3) Å), — N H 3 3 3 Cl = 149.3(5)]. Solvate water molecule make hydrogen bonding with carbonyl oxygen attached to the C-terminal of leucine [N 3 3 3 O = 2.807(4)Å, — N H 3 3 3 O = 169.8 (1). The solvate water also makes a hydrogen bonding contact with the metal bound chloride [O 3 3 3 Cl = 3.194(3) Å, — O H 3 3 3 Cl = 175.0(4)]. Although CP2 is isomorphous with the rest of the CPs, it has a marked structural difference both in its ligand bent geometry and coordination environment of the metal center; the ligand bent geometry in CP2 is characterized with an angle of ∼83.3 involving the amide moieties and ∼51.00 involving the pyridyl rings which are widely different from the values obtained in CP1, CP3, and CP4 (vide supra; e.s.d.s. are not provided because of the disordered nature of the amide backbone in CP2). Two halides were found to be coordinated to the metal center in all the CPs except in CP2 wherein one of the halides (Br ) was occluded as a counteranion in the channel space of the helical 3D network. Thermogravimetric analyses correspond well with the single crystal structures of the CPs (Supporting Information). Cation Separation. For the reasons stated above (see Introduction), separation of CuII from a mixture of cations was undertaken 5593

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Table 1. Crystallographic Parameters for CP1 CP4 CP1a

crystal parameters

CP2

CP3

CP4

CCDC no.

838097

838096

838098

838097

empirical formula

C40H40Br2CuN8O6

C28H38Br2CuN8O9

C34H44Br2CuN8O6

C34H44Cl2CuN8O6

formula weight

952.16

854.02

884.13

795.21

crystal size/mm

0.15  0.03  0.03

0.28  0.19  0.18

0.28  0.16  0.11

0.34  0.22  0.10

crystal system

trigonal

trigonal

trigonal

trigonal

space group

P3221

P3221

P3221

P3221

a/Å

13.0842(11)

13.6744(8)

13.0785(10)

12.9292(6)

b/Å c/Å

13.0842(11) 19.878(2)

13.6744(8) 15.0225(12)

13.0785(10) 20.083(3)

12.9292(6) 20.1039(11)

α/

90.00

90.00

90.00

90.00

β/

90.00

90.00

90.00

90.00 120.00

γ/

120.00

120.00

120.00

volume/Å3

2947.1(5)

2432.7(3)

2974.9(5)

2910.4(2)

Z

3

3

3

3

F(000)

1449

1299

1353

1245

μ MoKα/mm 1 temperature/K

0.515 298(2)

1.332 100(2)

0.844 298(2)

0.753 298(2)

Rint

0.0909

range of h, k, l

11/11,

0.0746 11/11,

17/16

16/17,

0.033 17/17,

18/18

15/15,

0.059 15/15,

23/23

13/13,

13/13,

θmin/max/

1.8/17.69

1.72/26.88

1.8/25.00

1.82/22.21

reflections collected/unique/

12892/1298/1172

25779/3503/2925

34640/3499/3277

21654/2451/2258

21/21

observed [I > 2σ(I)]

a

data/restraints/parameters

1298/0/115

3503/6/217

3499/0/241

2451/0/241

goodness of fit on F2 final R indices [I > 2σ(I)]

1.736 R1 = 0.0777

1.115 R1 = 0.0614

1.064 R1 = 0.0229

1.041 R1 = 0.0262

wR2 = 0.2137

wR2 = 0.1591

wR2 = 0.0579

wR2 = 0.0618

R indices (all data)

R1 = 0.0860

R1 = 0.0800

R1 = 0.0263

R1 = 0.0299

wR2 = 0.2216

wR2 = 0.1727

wR2 = 0.0600

wR2 = 0.0640

Owing to the poorly diffracting crystal (2θ = 35.5) of CP1, the model could not be treated aniosotropically.

Scheme 2. Dendritic Propagation of Metal Ligand Assembly in the CPs Reported Herein

by exploiting the in situ crystallization technique; for this purpose CPs, namely, CP1 CP3, were selected. To demonstrate this, we performed four sets of experiments under different conditions (Table 2); the CPs were originally synthesized by reacting the corresponding ligands with the metal salts in a 2:1 molar ratio (indicated “as synthesized” in Table 2). In condition I, two more metal salts (Co and Zn) were added in the same molar ratio. In condition II, the other metal salts (Co and Zn) were taken in twice the amount of Cu salt. In condition III, molar ratio of the ligand and all the competitive metal salts were kept in 3:1; in

Figure 1. Illustration of the crystal structure of CP4: from left to right — a fragment of metallo-double helical motif showing helical pitch, a right-handed helical chain displaying helical pitch, metallo-double helical motif, and part of the intertwined helical 3D network.

condition IV, the molar ratio of the ligand, Cu salt and other two metal salts were kept in 3:1:2. In all the experiments, the isolated crystalline compounds were characterized by FT-IR, powder X-ray diffraction (PXRD), and elemental analysis (see Supporting Information). The data 5594

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revealed that in each case the crystalline solid was the corresponding coordination polymer. For example, in Figure 3 the near superimposable FT-IR and PXRD pattern with the corresponding patterns of the as-synthesized CPs clearly indicated the separation of CuII as the corresponding CPs. Elemental analysis also supported these results. To study the efficiency of such cation separation, we have undertaken atomic absorption spectroscopy (AAS) of the supernatant liquid in each experimental condition. It is revealed that in all the cases the CuII removal is >98%. In a typical experiment, for example, condition I in synthesing CP1, 6.4 mg of CuBr2 which is equivalent to 1.8 mg of

Figure 2. Left to right: General C-direction view of the CPs displaying channels (amino acid side chain is removed for clarity); TOPOS diagram.11

Table 2. Competitive Experimental Conditions for CuII Separation (the Numerical Values Represent the Molar Ratios) conditions

ligand L1/L2/L3

CuBr2

as synthesized

2

1

I

2

1

CoBr2

ZnBr2

outcome

1

1

CP1/CP2/CP3

CP1/CP2/CP3

II

2

1

2

2

CP1/CP2/CP3

III

3

1

1

1

CP1/CP2/CP3

IV

3

1

2

2

CP1/CP2/CP3

Cu was taken in 10 mL of crystallizing solvent (MeOH/aq. EtOH). After removal of the crystals of CP1, the AAS data indicated the presence of only 0.0108 mg of Cu which amounts to 99% Cu removal. Both the hard soft-acid base (HSAB) principle and the Irving William series can be invoked to explain such CuII selectivity. CuII being borderline acid and softer than CoII and ZnII,12a it is expected to form a coordination bond with the pyridyl N, which is a soft base.12b On the other hand, the Irving Williams series [MnII < FeII < CoII < NiII < CuII > ZnII]13 is based on the stability of the corresponding complexes and holds true for a wide variety of ligands. It is clear that the stability of the CuII coordination compound is more than that of CoII and ZnII compounds derived from the same set of ligands. Thus, formation of CuII coordination compounds in the presence of CoII and ZnII as in the preset case is expected according to Irving William series. Such selective separation of cations by exploiting in situ crystallization of CPs is hitherto unknown to the best of our knowledge.14 Anion Separation. To demonstrate the selective separation of Br (which is an important toxic anion as discussed above) from a complex mixture of anions such as Br , SO42 , NO3 , ClO4 , and BF4 , in situ crystallization of CPs (e.g., CP1 and CP3) was undertaken under two different conditions; in condition I, the corresponding ligand was treated with CuBr2, CuSO4, CuNO3, Cu(ClO4)2, Cu(BF4)2 in a 2:1 metal/ligand ratio. In condition II, the ratio of metal salts other than CuBr2 was doubled. In the cases of L1 and L3, Br ion is separated exclusively as CP1 and CP3, respectively, in both conditions as supported by SXRD, PXRD, FT-IR, and elemental analysis (Figure 4). The preference for CuII for Br to the other anions to be separated as the corresponding crystalline CPs may be governed by the fact that CuII is a borderline acid and Br is a soft base compared to the other anions in the reaction mixture. Hofmeister bias15a enlists various anions as per their hydration energy; therefore, ease of extraction in a liquid liquid system follows the order ClO4 > I > SCN > NO3 > Br > Cl . SO42 > CO32 > PO43 .15b Thus, separation of Br over the two comparatively less hydrated

Figure 3. PXRD and FT-IR spectra patterns and elemental analysis data in various conditions of cation separation for CP1 (similar results are obtained for CP2 and CP3, see Supporting Information). 5595

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Figure 4. PXRD patterns, FT-IR spectra, and elemental analysis data in various conditions of Br separation for CP1 (similar results are obtained for CP3, see Supporting Information).

anions such as ClO4 and NO3 is remarkable. However, with respect to the more hydrated anion SO42 , separation of Br follows Hofmeister bias.

’ CONCLUSIONS Thus, a series of CPs derived from bis-pyridyl chiral amino acid based ligands have been synthesized and characterized by single crystal X-ray diffraction. All the CPs displayed an all helical 3D coordination network having channels running down the caxis. This structural invariance perhaps indicates the amino acid side chains do not influence the overall supramolecular structures of the CPs. It was shown to be possible to separate a divalent cation, namely, Cu II , from a complex mixture of cations (CuII, CoII, and ZnII) by in situ crystallization of CPs (CP1 CP3) for the first time. It was also demonstrated that Br can be separated from a complex mixture of anions (ClO4 , NO3 , BF4 , and SO42 ) in the form of crystallized CPs (CP1 and CP3). The fact that Br is separated from an aqueous solution containing less hydrated ClO4 and NO3 indicates anti-Hofmeister behavior. However, the inability to separate SO42 in these experiments is in agreement with Hofmeister bias. The results clearly demonstrate that coordination polymers having multiple properties such as both selective cation and anion separation can indeed be rationally designed. ’ EXPERIMENTAL SECTION Materials and Methods. All chemicals were commercially available and used without further purification. The elemental analyses were carried out using a Perkin-Elmer 2400 Series-II CHN analyzer. FT-IR spectra were recorded using Perkin-Elmer Spectrum GX, and TGA analyses were performed on a SDT Q Series 600 Universal VA.2E TA Instruments. PXRD patterns were recorded on a Bruker AXS D8 Advance Powder (Cu Kα1 radiation, λ = 1.5406 Å) diffractometer. The mass spectra were recorded on QTOF Micro YA263. NMR spectra (1H and 13C) were recorded using 300 MHz Bruker Avance DPX200 spectrometer. Atomic absorption spectra were recorded on Shimadzu AA-6300.

Synthesis of Ligands. L1 and L2 were reported previously,10 and L3 was synthesized following a reported procedure.16 L3: m.p: 168 C.1H NMR (300 MHz, [D6] DMSO): δ = 10.36 ppm (s, 1H, N H), 9.05 ppm (s, 1H, Py-H), 8.89 8.86 ppm (d, J = 9 Hz, 1H, Py-H), 8.71 8.70 ppm (d, J = 3 Hz 1H, Py-H), 8.76 ppm (s,1H, Py-H), 8.3 ppm (s, 1H, Py-H), 8.22 8.25(d, ppm J = 9 Hz, 1H, Py-H), 7.52 7.47 (t, J = 6 Hz, 6 Hz, 1H, Py-H), 7.35 7.31 (t, ppm J = 6 Hz, 3 Hz, 1H, Py-H), 4.69 4.63 (t, J = 6 Hz,12 Hz, 1H, CH) ppm. 1.84 1.70 ppm (m, J = 6 Hz, 2H, CH2) 1.64 1.58 ppm (m,1H,-CH) 0.94 0.90 ppm (t, J = 6 Hz,6 Hz, 6H,-2CH3) 13C NMR (300 MHz, [[D6] DMSO 171.66, 165.14, 151.91, 148.63, 144.25, 140.94, 135.52, 129.36, 126.29, 123.27, 79.07, 52.61,24.46,21.34 ppm. IR (KBr pellet): 3265 (m, N H stretch), 2939 (m, aliphatic C H stretch), 1668 (m, amide CdO stretch), 1637 (s, amide CdO stretch), 1608 (m, amide N H bend), 1593 m, 1540s, 1481s, 1425s, 1292s, 704s cm 1. HRMS ESI (CH3OH): m/z (%): 312.98 [M + H]+, 334.95 [M + Na]+. CP1: Coordination polymer was synthesized by layering a methanolic solution of L1 (34.7 mg, 0.1 mmol) over an aqueous ethanolic solution of CuBr2 (11.56 mg, 0.05 mmol). After one week very tiny crystals were obtained. Yield: 50% (24 mg, 0.0252 mmol). Elemental analysis calcd for C40H40Br2CuN8O6 (%): C 50.46, H 4.23, N 11.77; found: C 50.01, H 3.73, N 10.62. FT-I.R (KBr pellet): 3413 (s, N H stretch), 3228 (s, aromatic C H stretch), 3068 (s, aliphatic C H stretch), 1705(s, amide CdO stretch), 1650 (s, amide N H bend), 1610s, 1556s, 1487s, 1423 m, 1361 m, 1307s, 1247w, 1199 m, 1108w, 1033w, 810 m, 734w, 696s, 675 m, 653w, 582w cm 1. TGA of a crystalline sample of CP1 showed a weight loss of 3.85%, which corresponds to the loss of two solvated water molecules (calc. weight loss = 3.78%, temperature range 28 208 C). These results matched well with the respective single crystal structure. CP2: Coordination polymer CP2 was synthesized by layering a methanolic solution of CuBr2 (11.56 mg, 0.050 mmol) over an aqueous ethanolic solution of L2 (27.0 mg, 0.100 mmol). After three weeks, X-ray quality crystals were obtained. Yield: 55.8% (24 mg, 0.028 mmol). Elemental analysis calcd for C28H38Br2CuN8O9 (%): C 39.38, H 4.48, N 13.12; found: C, 41.18, H 3.93, N 12.74. FT-IR (KBr pellet): 3365 (s, N H stretch), 3213 (s, aromatic C H stretch), 3155 (s, aliphatic C H stretch), 3066s, 1707 (s, amide CdO stretch), 1685 (m, amide CdO stretch), 1649 (s, amide N H bend), 1589 (s, amide N H bend), 1552s, 1429m, 1286s, 1166s, 1058w, 812m, 702s, 555w cm 1. 5596

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Crystal Growth & Design Thermogravimetric analysis revealed the weight loss of 9.275% in the first step which corresponds the above-mentioned solvated molecules (calc. weight loss = 10.05%) these results matched with single crystal structure. CP3: Coordination polymer was synthesized by layering a methanolic solution of CuBr2 (11.56 mg, 0.050 mmol) over an aqueous ethanolic solution of L3 (31.2 mg, 0.100 mmol). After three weeks X-ray quality crystals were obtained. Yield: 59.1% (26 mg, 0.0289 mmol) Elemental analysis calcd for C34H44Br2CuN8O6 (%): C 46.19, H 5.02, N 12.67; found: C, 46.08, H 4.59, N 12.48. FT-IR (KBr pellet): 3392 (s, N H stretch), 3238 (s, aromatic C H stretch), 3066 (s, aliphatic C H stretch), 1699 (s, amide CdO stretch), 1658 (m, amide CdO stretch), 1635 (s, amide N H bend), 1610 (m, amide N H bend), 1554s, 1487m, 1296s, 1199s, 812m, 696m, 653w cm 1. Thermogravimetric analysis revealed a weight loss of 5.95% in the first step which corresponds to the solvated two water molecules (calc. weight loss = 4.11%); these results matched with single crystal structure. CP4: Coordination polymer was synthesized by layering a methanolic solution of CuCl2 (8.5 mg, 0.05 mmol) over an aqueous ethanolic solution of L3 (31.2 mg, 0.100 mmol). After three weeks X-ray quality crystals were obtained. Yield: 57.5% (23 mg, 0.0294 mmol) Elemental analysis calcd for C34H44Cl2CuN8O6 (%): C 51.35, H 5.58, N 14.09; found: C, 51.18, H 5.44, N 13.97. FT-IR (KBr pellet): 3369 (s, N H stretch), 3228 (s, aromatic C H stretch), 3063 (w, aliphatic C H stretch), 1699 (s, amide CdO stretch), 1645 (s, amide CdO stretch), 1608 (s, amide N H bend), 1583 (m, amide N H bend), 1556s, 1487m, 1296s, 1199s, 812m, 696s, 653w cm 1. Thermogravimetric analysis revealed a weight loss of 6.05% in the first step which corresponds to the solvated two water molecules (calc. weight loss = 4.53%); these results matched with single crystal structure.

’ ASSOCIATED CONTENT

bS

Supporting Information. ORTEP diagram and hydrogen bonding parameters of compounds 1 3; TGAs of compounds 1 4; cation and anion separation: XRPD and IR plots at various conditions; experimental details regarding cation and anion separation; crystallographic information files. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: parthod123@rediffmail.com; [email protected].

’ ACKNOWLEDGMENT We thank Department of Science & Technology (DST), New Delhi, India, for financial support. S.B. thanks IACS for research fellowships. We also acknowledge Mr. Sanjib Naskar for his help in performing atomic absorption spectroscopy. Single crystal X-ray diffraction was performed at the DST-funded National Single Crystal Diffractometer Facility at the Department of Inorganic Chemistry, IACS.

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