Crystal Engineering with 2-Aminopurine Containing a Carboxylic Acid

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Crystal Engineering with 2- Aminopurine Containing a Carboxylic Acid Pendant Balaram Mohapatra, V Venkatesh, and Sandeep Verma Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg501142v • Publication Date (Web): 28 Aug 2014 Downloaded from http://pubs.acs.org on September 1, 2014

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Crystal Growth & Design

Crystal Engineering with 2-Aminopurine Containing a Carboxylic Acid Pendant Balaram Mohapatra,1 V. Venkatesh,1 Sandeep Verma*,1, 2 1

Department of Chemistry and 2DST Thematic Unit of Excellence on Soft Nanofabrication,

Indian institute of Technology Kanpur, Kanpur 208016 (UP), India

Abstract: This article reports synthesis, design and luminescent properties of a series of structurally interesting coordination frameworks prepared from a modified purine ligand, 3(2-amino-9H-purin-9-yl) propanoic acid (L). Corresponding transition metal complexes reported in this study were unambiguously characterized by X-ray crystallography to reveal an array of diverse crystallographic signatures reflecting crystal design around varying coordination geometries of a central metal ion. While silver complex 1 [C16H18Ag2N10O5] affords formation of coordination framework with embedded dimeric, tetrameric and pentameric metallacycles, corresponding copper complexation results in a discrete dimer 2 [C32H46Cl2Cu2N20O14]. Changing the counteranion from strongly coordinating chloride ion to weakly coordinating perchlorate anion, resulted in the formation of a 1D coordination polymer

3

[C18H26Cl2CuN10O14].

Cobalt

complexes

4

[C16H32CoN10O12]

and

5

[C16H30CoN12O16] yielded 2D grid-type assembly and a discrete dimer, respectively. Change in pH offered an interesting effect on the structural outcome of cadmium complexes: acidic and neutral conditions lead to the formation of 1D coordination polymer 6 [C8H12CdCl2N6 O6] and 7 [C16H24Cd2N12O14], while basic conditions yielded an unusual porous metal organic framework 8 [C9H15CdN5O5.5].

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Introduction: 2-aminopurine (2AP), a constitutional isomer of naturally available purine nucleobase adenine,1 possesses fluorescent properties making it an important marker in nucleic acid research and in mutagenesis experiments, made possible due to its pairing ability with pyrimidine nucleobases.2 Addition of 2AP to E. Coli growth medium3 leads to transition mutations and forces frequent incidences of frameshift mutations.4 Notably, red-shift in 2AP absorption maxima, compared to other natural nucleobases, offers selective excitation of nucleic acids containing this modified base. Altered position of exocyclic amino group, from C6 to C2, leads to different hydrogen bonding possibility, supporting non-cognate base pairing. From coordination chemistry point of view, this arrangement improves metal ion binding ability of N7 imino nitrogen, while obliterating metal ion interactions at N3 position possibly due to steric hindrance offered by exocyclic amino group.5 Consequently, marked metal ion coordination preferences have been observed for modified purine derivatives 9-[2-phosphonomethoxy)ethyl]-2-aminopurine and 9-[2-phosphonomethoxy)ethyl]-adenine.6 In the formation of huge supramolecular motifs, carboxylic acids having both hydrogen donor and acceptor properties are known to be efficient functional group7 imparting strength as well as direction for self association through its hydrogen bonds.8 Their self-association through dimer forms homosynthons whereas heterosynthons are the result of the hydrogen bonding between the hydroxyl group of carboxylic acid and heterocyclic nitrogen atom.9 Our contribution in designing diverse metal-nucleobase supramolecular architectures was significant.10 We have utilized silver-adenine interaction for designing fascinating coordination framework with interesting photophysical properties.11 The inherent fluorescent property of 2-aminopurine and its scarcely studied metal complexes induced us to design a luminescent supramolecular framework. In order to overcome problems in its solubility, N9


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position of 2-amino purine has been substituted with different groups and their complexation behaviour with different metal ions have been studied. As an outcome, we have recently reported the synthesis and luminescent properties of 2-amino-9-propyl purine metal complexes.12

Carboxylate-metal

interactions

are

known

for

designing

complex

supramolecular architectures. Previously we have reported the coordination complexes of 6aminopurine and guanine ligand bearing a carboxylic acid pendant arm

13

In this report we

have introduced a carboxylic acid group at N9 position of 2-amino purine and explored its possible coordination mode of binding with different transition metal ions and reported their luminescent properties. Experimental:

Scheme1. Schematic representation of synthesis of ligand L Synthesis of methyl 3-(2-amino-6-chloro-9H-purin-9-yl)propionate (a): 6-chloroguanine (2 g, 11.79 mmol) was dissolved in dry DMF followed by addition of anhydrous potassium carbonate (1.956 g, 14.15 mmol), the reaction mixture was allowed to stir at 0°C under N2 atmosphere for 2 h. Methyl 3-bromopropionate (1.58 mL, 14.15 mmol) was added dropwise into the reaction mixture. The reaction was continued for 24 h at room temperature. Then the



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reaction mixture was concentrated under reduced pressure and further purified by column chromatography using DCM: MeOH (98:2) and the yield obtained was (2.01g, 70%). HRMS: (M+H)+ calculated: 255.0523, observed: 255.0523. 1H NMR: (500 MHz, DMSO-d6, 25 °C, TMS) δ (ppm) 2.88-2.91 (t, 2H, -CH2-), 3.54 (s, 3H, O-CH3), 4.23-4.26 (t, 2H, -CH2-), 6.90 (s, 2H, N2-H), 8.05 (s, 1H, C8-H).13C NMR (125 MHz, CDCl3, 25 °C, TMS); δ (ppm) 33.46, 39.52, 52.17, 123.80, 143.80, 149.82, 154.53, 160.27, 171.50. Synthesis of methyl 3-(2-amino-9H-purin-9-yl)propionate (b): The compound a (1 g, 3.91 mmol) was dissolved in dry methanol followed by the addition of ammonium formate (1.33 g, 15.65 mmol) and catalytic amount (0.025 g) palladium-charcoal . The reaction was continued for 4 h, and then reaction mixture was filtered through cellite for the decolourisation of charcoal. The solvent was evaporated under reduced pressure. White crystalline powder of b was separated through column chromatography (0.850 g, 85 %). HRMS: (M+H)+ calculated: 222.0991, observed: 222.0994. 1H NMR: (500 MHz, DMSO-d6, 25 °C, TMS) δ (ppm) 2.88-2.91 (t, 2H, CH2), 3.54 (s, 3H, -OCH3), 4.24-4.26 (t, 2H,-CH2-), 6.49 (s, 2H, N2-H), 7.98 (s, 1H, C8-H), 8.52 (s, 1H, C6-H).

13

C NMR: (125 MHz, CDCl3,

25°C, TMS); δ (ppm) 33.46, 39.52, 52.17, 123.80, 143.80, 149.82, 154.53, 160.27, 171.50. Synthesis of 3-(2-amino-9H-purin-9-yl) propanoic acid (L): Ligand L was synthesized through the base hydrolysis of b; the methyl ester (0.500 g, 2.26 mmol) was suspended in 30 mL methanol and 1N NaOH solution (500µL) was added drop wise, and stirred for one hour. Then it was neutralised using 1N HCl until neutralized, wherein, the white precipitate obtained was filtered and dried. The HRMS: (M+H)+ calculated: 208.0834, observed: 208.0834. Elemental analysis: calcd. (%) for C8H9N5O2: C, 46.38; H, 4.38; N, 33.80; Found C, 46.29; H, 4.41; N, 33.67; 1H NMR: (500 MHz, DMSO-d6, 25 °C, TMS) δ (ppm) 2.79-2.81 (t, 2H, CH2), 3.54 (s, 3H, O-CH3), 4.19-4.22 (t, 2H. -N-CH2-), 6.50 (s, 2H, N2-H), 7.97 (s,


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1H, C8-H), 8.51(s, 1H, C8-H),

13

C NMR: (125 MHz, CDCl3, 25°C, TMS); δ (ppm) 33.84,

39.08, 127.36, 143.36, 149.49, 153.42, 160.99, 172.68. Synthesis of Complex 1 [C16H18Ag2N10O5]: In a 25 ml round bottom flask, Ligand L (0.020 g, 0.10 mmol) was dissolved in 5 mL of methanol, AgNO3 (0.049 g, 0.29 mmol) was slowly added into the reaction mixture and stirred for 1 h. Finally, the white precipitate obtained was dissolved in a mixture of 25% aqueous ammonia solution (1 mL) and acetonitrile (3 mL) solution. The clear solution obtained was kept for crystallisation at room temperature. After three days, colourless needled shaped crystals were collected. HRMS: [L+H]+: 208.0836 (Calcd.: 208.0834), [2L+Ag]+: 521.0546, (Calcd.: 521.0563). Elemental analysis: calcd. (%) for C16H18Ag2N10O5: C, 29.74; H, 2.81; N, 21.68; Found C, 29.44; H, 2.89; N, 21.53; IR (KBr, v/cm-1): 3375, 3193, 1634, 1578, 1526, 1479, 1428, 1359, 1291, 790, 634, 612. Synthesis of Complex 2 [C32H46Cl2Cu2N20O14]: In a 25 mL round-bottomed flask, Ligand L (0.020 g, 0.10 mmol) was dissolved in 3mL methanol, 2 mL aqueous solution of CuCl2.2H2O (0.032 g, 0.19 mmol) was added into it. Then the solution was allowed to stir for 2 h, few drops 1N HCl was added for dissolving the suspension and the clear solution obtained was kept for crystallisation at room temperature. After a week, block shaped blue coloured crystals were grown by slow evaporation method. (yield 34 %). HRMS: [L+H] +: 208.0824 (Calcd.: 208.0834), [L+Cu+O]+: 286.0978 (Calcd.:286.0001). HRMS: [2L+Cu] +: 477.0719 (Calcd.: 477.0809). Elemental analysis: calcd. (%) for C32H46Cl2Cu2N20O14: C, 33.93; H, 4.09; N, 24.73; Found C, 33.81; H, 4.13; N, 24.62; IR (KBr, v/cm-1): 3413, 3218, 1717, 1633,1596,1436,1327, 1241, 1198, 977, 784, 624. Synthesis of Complex 3 [C18H26Cl2CuN10O14]: In a 50 mL round bottom flask, ligand L (0.020 g, 0.10 mmol) and Cu(ClO4)2.6H2O (0.071 g, 0.19 mmol) was dissolved in 12 mL of MeOH/ H2O (1:3, v/v) and then allowed to stir for 2 h. One drop of 1N HCl was added to the reaction mixture for dissolving suspended particles, obtained clear solution was kept for



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crystallisation at room temperature. Blue coloured crystals were grown by slow evaporation method over a period of a week time. (Yield 30 %). HRMS: [L+H] +: 208.0766 (Calcd.: 208.0834), [2L+Cu]+: 477.0801 (Calcd.: 477.0809). [L+Cu+O]+: 286.0912 (Calcd.: 286.0001). Elemental analysis: calcd. (%) for C18H26Cl2CuN10O14: C, 29.18; H, 3.54; N, 18.90; Found C, 29.05; H, 3.59; N, 18.97; IR (KBr, v/cm-1): 3448, 3111, 1669, 1536, 1450, 1357, 1207, 1108, 955, 793,780, 639, 624. Synthesis of Complex 4 [C16H32CoN10O12]: In a 25 ml round bottom flask, ligand L (0.020 g, 0.10 mmol) was dissolved in 3 mL of methanol, CoCl2.6H2O (0.068 g, 0.29 mmol) was dissolved in 1 mL water and added into it, then the reaction was continued for 4 hours at 45°C. The clear solution obtained was kept for crystallisations resulting in block shaped pink coloured crystals. (Yield, 42 %). HRMS: HRMS: [L+H] +: 208.0830 (Calcd.: 208.0834) [LH+Co+2Cl]+:

334.9110

(Calcd.:

334.9387).

Elemental

analysis:

calcd.

(%)

for

C16H32CoN10O12: C, 31.23; H, 5.24; N, 22.76; Found C, 31.12; H, 5.13; N, 22.66. IR (KBr, v/cm-1): 3429, 3282, 1714, 1642, 1601, 1532, 1431, 1359, 1302, 1192, 1044,776, 614. Synthesis of Complex 5 [C16H30CoN12O16]: Synthesis of complex 5 was similar to that of complex 4, where Co(NO3)2.6H2O (0.056 g, 0.19 mmol) was used instead of CoCl2.6H2O. HRMS: [L+H] +: 208.0875 (Calcd.: 208.0834), [LH+Co+H2O+1]+: 286.0272 (Calcd.: 286.0350). Elemental analysis: cald. (%) for C16H30CoN12O16: C, 27.24; H, 4.29; N, 23.83; Found C, 27.05; H, 4.38; N, 23.68; IR (KBr, v/cm-1): 3379, 3168, 1723, 1666, 1594, 1532, 1357, 1200, 946, 826, 776, 632. Synthesis of Complex 6 [C8H12CdCl2N6O6]: In a 25 mL round bottom flask, Ligand L (0.020 g, 0.10 mmol) was dissolved in 3 mL of methanol, Cd(NO3)2.4H2O (0.089 g, 0.29 mmol) was added into it. Then the reaction mixture was stirred for 3 h at room temperature. The white coloured precipitate obtained was redissolved in methanol followed by 4 drops of 1N HCl was added to get a clear solution which was kept for crystallization at room


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temperature yielding block shaped colourless crystals in a week. HRMS: [L+H] +: 208.0838 (Calcd.: 208.0834), [L+Cd]+: 320.0044 (Calcd.:320.9790)

and [2L-H+Cd]+: 526.0557

(Calcd.: 526.0390). Elemental analysis: calcd. (%) for C8H12CdCl2N6O6: C, 20.38; H, 2.57; N, 17.82; Found C, 20.43; H, 2.61; N, 17.71; IR (KBr, v/cm-1): 3210, 1762, 1654, 1631, 1595, 1484, 1383, 1283, 789, 714, 670. Synthesis of Complex 7 [C16H24Cd2N12O14]: In a 25 mL round bottom flask, Ligand L (0.020 g, 0.10 mmol) was dissolved in 3 mL of methanol, Cd(NO3)2.4H2O (0.089 g, 0.29 mmol) was added into it. Then the reaction mixture was stirred for 4 h at 80°C. On cooling, white coloured precipitate was obtained, which was redissolved in DMF: MeOH (1:2) ratio. The resultant clear solution was kept for slow evaporation yielding needle shaped colourless crystals at room temperature. HRMS: [L+H] +: 208.0838 (Calcd.: 208.0834), [2L-2H+Cd]+: 526.0557 (Calcd.: 526.0390) and [2L+Cd+H2O]+: 546.0325 (Calcd.: 546.0652). Elemental analysis: calcd. (%) for C16H24Cd2N12O14: C, 23.06; H, 2.90; N, 20.17; Found C, 22.91; H, 2.87; N, 20.05; IR (KBr, v/cm-1): 3329, 3241, 1633, 1592, 1426, 1359, 1289, 1201, 1184, 989, 792, 642. Synthesis of Complex 8 [C9H15CdN5O5.5]: In a 25 mL round bottom flask, Ligand L (0.020 g, 0.10 mmol) was dissolved in 3 mL methanol, Cd(NO3)2.4H2O (0.089 g, 0.29 mmol) was added into it. To basify the solution, 25% ammonia solution (1 mL) was added and stirred for 4 h at 120 °C. The clear solution was filtered and kept in freeze (4 °C) for 5 hours resulting in block shaped crystals. HRMS: [L+H]+: 208.0823 (Calcd.: 208.0834), [L-H+Cd+2NO3] : 443.9516 (Calcd.:443.9468) and [2L+Cd+NO3] : 590.0367 (Calcd.:590.0424). Elemental

analysis: calcd. (%) for C9H15CdN5O5.5: C, 27.46; H, 3.84; N, 17.79; Found C, 27.21; H, 3.60; N, 17.91; IR (KBr, v/cm-1): 3343, 3222, 1617, 1583, 1433, 1384, 1252, 1203, 947, 858, 792, 642.



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Crystal Structure Determination and Refinements: Single Crystal of 1–8 were coated with light hydrocarbon oil and mounted in the 100 K dinitrogen stream of a Bruker SMART APEX CCD diffractometer equipped with CRYO Industries low-temperature apparatus and intensity data were collected using graphitemonochromated Mo Kα radiation. The data integration and reduction were processed with the SAINT software.14 An absorption correction was applied.15 Structures were solved by the direct method using SHELXS-97 and refined on F2 by a full-matrix least-squares technique using the SHELXL-97 program package.16 Non-hydrogen atoms were refined anisotropically. In the refinement, hydrogens were treated as riding atoms using the SHELXL default parameters. Crystal structure refinement parameters are given in Table S1. H-atoms of lattice solvent molecules in complex 2 and the H-atoms of coordinated water molecule in complex 2 and 8 could not be located in difference Fourier maps. The contribution of all solvent atoms has been incorporated in both empirical formulas and formula weights of the complexes. Squeeze refinement was performed for 4 and 8 using PLATON. For 4, calculation shows that there were 119 electron per unit cell (Z = 2). This electron density was assigned to 6 aqua molecules. For 8, it was found to be 96 electron per unit cell (Z = 4). This electron density was assigned to one methanol molecule and half aqua molecule.


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Figure 1: Different coordination modes utilized by carboxylate anion in the complexes 1, 2, 3, 4, 7 and 8.

Results and discussion:

Figure 2: Crystal lattice of complex 1, where dimeric, tetrameric and pentameric metallo cycles are shown in the different colours. Crystal structure description of complex 1: Silver complex 1 was synthesized by reacting L (0.10 mmol) with silver (I) nitrate (0.29 mmol) in aqueous methanol. The silver complex formed as a white precipitate was solubilised in a mixture of 25% ammonia solution and



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acetonitrile (1:3) solution. The clear solution obtained was kept for crystallization at room temperature. Colourless crystals suitable for X-ray diffraction were grown by slow evaporation method in a week. Crystal refinement data suggested that the crystal belonged to triclinic (P-1) space group. The asymmetric unit of the crystal contained two ligand molecule and two silver ions. Ligand L was involved in two different types of coordination modes with silver ion; one being the linear N7-Ag-N7 coordination and the other, five coordinated distorted trigonal bipyramidal geometry with N1-Ag-O coordination (Figure 2). The observed bond distance in complex 1 was in the range of Ag-N7 = 2.119(3)-2.124(3) Å, Ag-O = 2.319(2) -2.344(3) Å. Ligand presented three major silver ion binding sites, N1, N7 of 2-amino purine and a pendant carboxylate anion.

Figure 3: Dimeric, tetrameric and pentameric metallacycles present in the crystal lattice of complex 1 (a-d) along with Ag...π interaction present in the crystal lattice.


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The carboxylate anion shows both monodendate as well as bidendate coordination mode with silver ion. Closer investigation shows that the carboxylate anion was involved in monodendate coordination modes (µ2:η1) and (µ3:η1) wherever N1 nitrogen was involved in silver coordination (Figure 1). The crystal lattice of 1 contains dimeric, tetrameric and pentameric metallacycles as shown in Figure 2. The coordination polymer 1 was stabilized by intermolecular hydrogen bonding present in the crystal lattice. Hydrogen bonding between purine moieties were involved through N2-H...N3, the observed hydrogen bonding distance was 2.334 Å (Figure S1). Hydrogen bonding arrangement in complex 1 conformed to an unusual guanine-guanine GG4 pattern.17 Complex 1 was further stabilized by Ag...Ag and Ag…π interactions (Figure 3d). The argentophilic interactions was observed in the range of (Ag...Ag) = 3.0374(8) Å, which is smaller than the sum of their Van der waals radii (3.40 Å), 18

and presence of Ag...π interaction with adjacent purine rings at a distance of 3.32 Å.19

Crystal structure description of complex 2: Complex 2 was synthesised by reaction of L (0.01 mmol) with copper (II) chloride (0.19 mmol) in aqueous methanol. The crystals suitable for X-ray diffraction were grown in acidified aqueous methanol by slow evaporation method. Refinement data revealed that complex 2 crystallized in triclinic (P-1) space group. The asymmetric unit consists of two ligand molecules, one copper ion, one chloride ion, one coordinated water molecule along with two non-coordinated water molecules, the copper (II) involved in distorted square pyramidal coordination geometry (Figure 4b). Careful investigation of the crystal structure revealed that the oxygen atom of carboxylic acid and N7 nitrogen atom was involved in metal coordination, whereas the carboxylate anion coordinates with the copper (II) through monodendate coordination mode (µ1 η1). The complex forms M2L2 dimer as shown in Figure 4a, which further extends to a polymeric chain through purine-purine hydrogen bonding (Figure 4c) and another stabilizing interaction is present between amino group of purine with carboxylate oxygen atom O1A (N2B-H2BA...O1A)



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(Figure S2). The bond distance between N2B-H2BA...O1A is 3.005(7) Å. The two water molecules play an important role in the crystal packing by extending the polymeric structure (Figure S2). In addition, complex 2 exhibits face-to-face π-π interactions between the two purine rings with a distance of 4.01Å.

Figure 4: (a) Dimeric unit of complex 2 (Hydrogen atoms are omitted for clarity); (b) Coordination environment

of Copper ion in complex 2; (c) Crystal packing through

hydrogen bonding of complex 1 viewed along ‘a’ axis form a 1D polymeric chain. Crystal structure description of complex 3: To probe the effect of the counter anion, we replaced chloride ion in complex 2 with a weakly coordinating perchlorate counter anion. The complex was prepared by treating L (0.10 mmol) with copper (II) perchlorate (0.19 mmol) in acidified aqueous methanol. The single crystal suitable for X-ray diffraction studies were grown by slow evaporation at room temperature.


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Figure 5: (a) Dimeric unit of complex 3 (hydrogen atoms are omitted for clarity); (b) Coordination environment of copper ion in complex 3 (c) Crystal packing of complex 3 viewed along ‘a’ axis forming a 1D polymeric chain. (d) 2D polymeric structure formed through hydrogen bonding interaction between N1-H...O. The refinement data revealed that complex 3 crystallized in monoclinic (P 2/n) space group. The asymmetric unit consisted of one ligand, one copper ion and two perchlorate counter ions along with one non-coordinated methanol molecule. In complex 3, appended carboxylic acid is deprotonated, whereas N1 nitrogen of ligand L was protonated. Crystal structure showed



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that N7 nitrogen and carboxylate oxygen atom was involved in metal coordination, whereas in this case carboxylate anion utilized the bidendate coordination mode (µ1 η2). In complex 3, copper (II) coordinates to four carboxylate oxygen and two N7 nitrogen of ligand L, as a result, copper (II) was involved in octahedral coordination geometry (Figure 5b). Closer inspection of the crystal lattice revealed that cyclic metalla-dimers results in the formation of a 1D coordination polymer (Figure 5c). A 2D polymeric structure is formed through hydrogen bonding interaction present between N1-H...O (Figure 5d). In this complex, unlike complex 2, purine-purine interaction was not observed. Crystal structure description of 4: Cobalt complex 4 was prepared by reacting L (0.10 mmol) with cobalt (II) chloride (0.29 mmol) in aqueous methanol. The needle shaped pink coloured crystals were grown over a period of a week by slow evaporation. The crystal refinement parameters showed that 4 crystallized in the triclinic space group (P-1). The asymmetric unit consisted of two ligand molecules, two cobalt ions, and two coordinated water molecules. Cobalt ion showed octahedral coordination geometry (Figure 6d), with two N7 nitrogens and two carboxylate anions of two different ligand molecules along with two coordinated water molecules. Interestingly, the crystal lattice revealed that four ligand molecules were coordinated to four metal ions resulting in the formation of metallaquartets (Figure 6a). Closer inspection of the crystal lattice showed 2D sheet formation (Figure 6c), whereas the carboxylate anion utilized monodendate coordination mode (µ1:η1). The observed bond distance in 4 was Co-N7 = 2.136(3) Å and Co-O=2.104(3) Å. In complex 4, the crystal lattice was stabilised by π-π interactions with the centroid-centroid distance between the two purine rings were 3.50 Å (Figure S3). It was further stabilised by


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Table 1: Crystal structure refinement parameters for Complexes 1-8.

Identification Complex 1 Complex 2 code Empirical C32H46Cl2Cu2N20 C16H18Ag2N10O5 formula O14 Mr 646.14 1132.89 crystal system Triclinic Triclinic space group P-1 P-1 a(Ǻ) 8.0457(19) 7.049(2) b(Ǻ) 10.061(2) 12.588(4) c(Ǻ) 12.783(3) 13.896(4) α (deg) 81.511(4) 67.248(4) β (deg) 89.548(4) 78.509(5) γ (deg) 74.687(4) 88.131(6) Volume (Ǻ3) 986.6(4) 1113.0(6) Z 2 1 -3 Dx (Mg m ) 2.175 1.672 F(000) 636 570 -1 µ (mm ) 2.042 1.164 θ range for data 2.12-28.29 2.82-25.99 collection(deg) -10→h→10 -8→h→5, Limiting -12→k→13 -15→k→15, indices -17→l→14 -17→l→17 Reflections 7733 6764 collected unique 4832 4327 reflections R(int) 0.0189 0.0623 Completeness 28.29, 98.6 25.99, 99.0 to θ Tmax / Tmin 0.665/ 0.631 0.792 / 0.774 Data / restraints / 4832 / 0 / 301 4327 / 0 / 317 parameters GOF on F2 1.046 0.976 R1 and R2 0.0350, 0.0817 0.0666, 0.1447 [I>2σ(I)] R1 and R2 (all 0.0458, 0.0875 0.1325, 0.1710 data) Largest diff. 1.543 and peak and 0.731 and -0.431 0.825 hole(e.A-3) 1011368 1011369 CCDC NO

Complex 3

Complex 4

740.93 Monoclinic P 2/n 12.800(5) 8.333(5) 14.046(5) 90.000(5) 113.164(5) 90.000(5) 1377.4(11) 2 1.786 758 1.076

C16 H32 Co N10 O12 615.45 Triclinic P-1 7.1207(10) 13.2355(19) 14.725(2) 98.846(3) 101.901(3) 94.406(2) 1333.3(3) 2 1.264 522 0.690

2.4-25.99

2.30-26.00

-15→h→15, -4→k→10, -16→l→13

-8→h→8, -15→k→16, -15→l→18

4302

8934

2670

5208

0.0394

0.1340

25.99, 98.5

26.0, 99.4

0.824 /0.806

0.883/0.871

C18H26Cl2CuN10O14

2670 / 0 / 213

5208 / 0 / 303

1.049

1.044

0.0765, 0.1983

0.0612,0.1660

0.1030, 0.2198

0.0756, 0.1773

2.014 and -0.746 1011370


1.051 and -0.853 1011371


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Identification code Empirical formula Mr crystal system space group a(Ǻ) b(Ǻ) c(Ǻ) α (deg) β (deg) γ (deg) Volume (Ǻ3) Z Dx (Mg m-3) F(000) µ (mm-1) θ range for data collection(deg) Limiting indices Reflections collected unique reflections R(int) Completeness to θ Tmax / Tmin Data / restraints / parameters GOF on F2 R1 and R2 [I>2σ(I)] R1 and R2 (all data) Largest diff. peak and hole(e.A-3) CCDC NO

Complex 5 C16H30CoN12O16 705.45 Monoclinic P 21/c 8.6628(13) 21.603(3) 7.5795(11) 90 100.174(3) 90 1396.2(4) 2 1.678 730 0.712

Complex 6

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Complex 7

C8H12CdCl2N6 C16H24Cd2N12O14 O6 471.54 833.29 Triclinic Triclinic P-1 P-1 7.2795(18) 9.772(5) 9.856(3) 11.071(5) 10.962(3) 12.865(5) 82.736(4) 106.417(5) 81.059(4) 90.094(5) 78.575(4) 90.033(5) 757.8(3) 1335.1(10) 2 2 2.067 2.073 464 824 1.834 1.685

Complex 8 C9H15CdN5O5.5 393.65 Monoclinic P 2/c 12.8331(19) 15.626(2) 7.5450(11) 90 93.225(3) 90 1510.6(4) 4 1.542 684 1.458

2.39-25.50

2.70-25.49

2.08-26.50

2.06-26.50

-10→h→10, -25→k→26, -5→l→9,

-8→h→8, -11→k→9, -13→l→12

-12→h→12, -12→k→13, -16→l→13

-16→h→13 -19→k→16 -9→l→9

7861

4486

10374

11558

2598

2753

5524

3144

0.0357

0.0303

0.0250

0.0519

25.50, 99.9

25.49, 98.0

26.50, 99.7

26.50, 99.8

0.867/ 0.855

0.719/0.700

0.721 /0.707

0.758/0.744

2598 / 16 / 218

2753/ 1/ 216

5524 / 20 / 399

3144 / 12/ 165

1.105

1.023

0.0468, 0.1344

0.0834, 0.2496

0.0528, 0.1408

0.0951, 0.2597

3.468 and -2.414

3.446 and -4.046

1.104 0.0954, 0.2073 0.1076, 0.2148

1.117 0.0432, 0.1248 0.0457, 0.1287

1.842 and 1.002

2.222 and -1.737

1011372

1011373

1011374


1011375

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purine-purine interactions through the Watson-Crick face (Figure S3). Interestingly, water molecules were involved in secondary interactions with carboxylate oxygen and other water molecules. Important bond distances and bond angles are given in Table S2.

Figure 6: (a) Crystal packing of complex 4 forms a metallaquartets; b) 4; b) schematic representation of complex 4 c) crystal lattice form a 2D sheet type assembly; d) Coordination environment of the Co(II) ions in complex. Crystal structure description of 5: The cobalt complex 5 was prepared by reacting ligand L (0.10 mmol) with cobalt (II) nitrate (0.2 mmol) in aqueous methanolic solution. The single crystal suitable for X-ray diffraction was grown in methanol by slow evaporation. The refinement data of 5 belonged to monoclinic (P 21/c) space group. The asymmetric unit consisted of one ligand, half cobalt ion, three water molecules and one non-coordinated nitrate counter anion. The cobalt ion was involved in octahedral coordination geometry, where each cobalt ion was coordinated to two N7 nitrogen of ligand molecule and four



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oxygen atoms of water molecules (Figure 7a). Careful investigation showed that complex 5 exhibited cross self-dimerization via interaction between purine moiety and carboxylate group of the other purine moiety, resulting in the formation of a 2D polymeric sheet (Figure 7b). Four water molecules played an important role in stabilizing the crystal lattice. The two coordinated water molecules participated in hydrogen bonding with the non coordinating nitrate counter anion and water molecules leading to the formation of a polymeric chain-like structure (Figure 7c). Closer inspection of crystal lattice showed that O3W participated in hydrogen bonding with O3 and O5 to form a chair-like crystal packing structure (Figure 7c). All important bond angles and bond distances are given in Table S1.

Figure 7: (a) Dimeric unit of complex 5 (nitrate groups are omitted for clarity); b) Purinepurine hetro dimerization forming 2D sheet assembly; c) Hydrogen bonding between water and nitrate counter anion present in crystal lattice.


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Figure 8: Synthetic scheme for cadmium complexes Crystal structure description of 6: The cadmium complex 6 was prepared by reacting L (0.10 mmol) with cadmium (II) nitrate (0.29 mmol) under acidic condition results in formation of 1D polymeric chain (Figure 9a). The single crystal suitable for X-ray diffraction was grown in acidified methanol by slow evaporation. The refinement data of 6 belonged to triclinic (P-1) space group. The asymmetric unit consist of one ligand, two cadmium ions with 0.5 occupancy, two chlorine ions and one nitrate counter anion, protonation of N1 nitrogen of ligand L make the overall charge of the complex neutral. Both the cadmium ions were involved in octahedral coordination geometry. One cadmium ion was coordinated to two N7 nitrogen of ligand molecule and four bridged chloride ions, whereas the other cadmium ion was coordinated to four bridged chloride ions and two axial oxygen of water molecule. Closer inspection of crystal structure shows that the chloride bridging assists the formation of 1D polymer. The two axial oxygen of water molecules act as donor atom for secondary interaction to the acceptor oxygen of nitrate counter anion. Under acidic condition the carboxylic acid group was unavailable for metal coordination, so only N7 nitrogen was involved in metal coordination. The carboxylic acid group was involved in hydrogen bonding



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with carboxylic acid group of another ligand molecule and extending the polymeric structure as shown in (Figure 9b). In additional the lattice was further stabilized by N2-H...Cl and N1H...O5 hydrogen bonding as shown in Figure 9c. The observed bond distances in complex 6 are Cd-N7 = 2.33(2) Å; Cd-Cl=2.583(7)-2.617(7) Å.

Figure 9 a) Shows Cd-Cl-Cd bridge present in complex 6 facilitate the formation of 1D coordination polymer; b) O-H...O hydrogen bonding interaction makes hydrogen bonded 2D


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framework; c) N2-H...Cl and N1-H...O5 hydrogen bonding interactions present in crystal lattice. Crystal structure description of 7: To access the metal coordination of carboxylate oxygen we have reacted L (0.10 mmol) with cadmium (II) nitrate (0.29 mmol) under neutral condition. Block shaped crystals suitable for X-ray diffraction were grown over a period of a week time. Complex 7 crystallized in triclinic (P-1) space group. The asymmetric unit of 7 consist of two deprotonated ligand molecule, two cadmium ions, two nitrate counter anion along with two coordinated water molecule.

Figure 10: a) Part of the crystal lattice showing N1, N7 and carboxylate oxygen binding modes in complex 7; b) Octahedral coordination environment of Cadmium ions in complex 7; c) 1D polymeric crystal packing of complex 7. Interestingly the crystal lattice consist of two cadmium atoms, Each cadmium atom is coordinated to three different coordinating sites N1, N7 and carboxylate oxygens (O1, O2) of



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three different ligand molecules (Figure 10a). Such type of coordination mode was not observed in complexes 6 and 8. Each cadmium ion was involved in penta coordination geometry (Figure 10b), where carboxylate anion utilizes bidendate coordination mode (µ1:η2). Careful observation reveals that cyclic metalla dimer (Figure 10a) extends the lattice in to 1D coordination polymer (Figure 10c). The observed bond distances are CdN7A=2.277(4) Å; Cd-N1 = 2.411(4) Å-2.413(4) Å and Cd-O=2.350(5) Å-2.516(4) Å.

Figure 11: a) Crystal lattice of 8 viewed along c axis; b) cadmium-oxo bridge present in complex 8; c) cadmium-carboxylate bridging modes present in 8 ; d) space filling model shows the solvent accessible porous void present in 8.


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Crystal structure description of 8: Interestingly, reaction of L (0.10 mmol) with cadmium (II) nitrate (0.29 mmol) under basic condition results in the formation of porous MOF. The needle shaped crystals were grown by slow evaporation technique. Complex 8 crystallized in monoclinic (P 21/n) space group. The asymmetric unit of the crystal lattice consists of one deprotonated ligand molecule, two cadmium ions and one oxo bridged atom. The overall charge was neutralised by O- anion present in the unit cell (carboxylate oxygen and oxido ligand). The carboxylate anion and N7 nitrogen was involved in metal coordination. The carboxylate anion was involved in bidendate (µ1:η2) and (µ2:η3) coordination mode. The Cd(II) exhibits both 6 and 8 coordination number within the crystal lattice. Closer inspection of the crystal lattice shows that metal carboxylate cluster (Figure 11c) act as a secondary building unit (SBU) with a Cd···Cd separation of 3.98 Å which further propagates to a 3D framework via N7 atoms of the ligand L (Figure 11a). Oxo ligand bridges between two cadmium ions are stabilised the complex 8 (Figure 11b). This porous frame work contains solvent accessible void of 455 Å3 as calculated by PLATON (Figure 11d). The observed bond distances are Cd-N7=2.395(9) Å; Cd-O = 2.386(6)-2.537(6) Å.

Photoluminescence Study: In recent years, fabrication of luminescent supramolecules is of great interest because of its contribution towards sensor technologies. The metal nucleobase interaction and the mode of self association can bring different fluorescence response.20 Investigations of luminescence in supramolecules reveal their photophysical nature and also make it possible to explore its application in various fields like photo chemistry, electroluminescent displays and chemical/biological sensors.21



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Figure 12: a) Solid state luminescence emission spectra complex 1-5 along with ligand L b) Solid state luminescence emission spectra complex 6-8 along with ligand L. We investigated the solid state luminescence of ligand L along with complexes 1-8 at room temperature (Figure. 12). Upon excitation with UV light of 310 nm, ligand L displayed a broad emission with peak maxima at 441 nm. This could be attributed to intra ligand π-π* emission.22 However, its complexes 1-8 exhibited emission maxima at 391, 413, 427, 419, 432, 388, 399, 379 nm respectively on same excitation. The observed blue shift in the emission maxima of complexes 1-8 with respect to the free ligand L could arise from the metal mediated crystal packing, affecting the π-π interaction.23 The complex 1 showed weak emission intensity compared to ligand L, which may be the heavy metal effect of silver atom.24 Moreover complexes 2-5 showed reduced emission intensity compared to ligand L. This observation could be ascribed to the presence of guest transition metals, which contributes towards quenching through spin-orbit coupling, electron transfer or ligand to metal energy transfer.25 In contrast, Figure 12b illustrated fluorescence enhancement of complexes 6-8 compared to free ligand L. This could be attributed to the d10 metal ion (Cd2+) that does not facilitate any low-energy metal-centred or charge-separated excited states in to the molecule. Thus the energy transfer or electron transfer process cannot occur easily.25


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Conclusion: In summary, we have constructed eight novel transition metal complexes by employing carboxylic acid appended 2-amino purine ligand. The diverse coordination mode of the carboxylate group resulted in a wide variety of interesting architectures. The role of counter anion and the effect of pH on the structural outcomes were probed. We have also systematically studied the luminescence property of ligand (L) and complex (1-8) and observed blue shifts in emission after metal complexation. Acknowledgments: We thank the single–crystal CCD X–ray facility at IIT–Kanpur, CSIR for fellowship (B.M) and UGC for fellowship (V.V). SV thanks DAE for DAE-SRC Outstanding Investigator Award. Supporting Information Available: X-ray crystallographic data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. The supplementary crystallographic data for this paper with deposition numbers of CCDC 1011368, 1011369, 1011370, 1011371, 1011372, 1011373, 1011374 and 1011375 have also been deposited with the Cambridge Crystallographic Data Centre. The coordinates can be obtained, upon request, from the Director, Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax þ44-1223/336-033; e-mail [email protected]). References: 1. Law, S. M.; Eritja, R.; Goodman, M. F.; Breslauer, K. J. Biochemistry 1996, 35, 12329-12337. 2. (a) Lee, B. J.; Barch, M.; Castner, E. W.; Volker, J.; Breslauer, K. J. Biochemistry 2007, 46, 10756-10766. (b) Sowers, L. C.; Fazakerley, G. V.; Eritja, R.; Kaplan, B. E.; Goodman, M. F. Proc. Natl. Acad. Sci. U.S.A. 1986, 83, 5434-5438. (c) Jean, J.



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Graphical Abstract:


Crystal Engineering with 2-Aminopurine Containing a Carboxylic Acid

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