DOI: 10.1021/cg100371w
Protonated Adenine and Cytosine Ribbons Stabilized by Dipicolinato Metal Frameworks
2010, Vol. 10 3242–3249
Babulal Das and Jubaraj B. Baruah* Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039 Assam, India Received March 20, 2010; Revised Manuscript Received May 24, 2010
ABSTRACT: Stabilization of protonated adenine (ade) and cytosine (cyt) ribbons through hydrogen bonding interactions in a series of bis-dipicolinato metal(II) (metal = manganese, copper, zinc at þ2 oxidation state) complexes are studied. Protonated assemblies of adenine as infinite chains in [1H, 9H-ade][3H, 7H-ade][MnL2] 3 3H2O and [1H, 9H-ade][3H, 7H-ade][CuL2] 3 3H2O (L = dipicolinato anion) are structurally characterized. The one-dimensional (1D) zigzag ribbons are comprised of alternate nonequivalent [1H, 9H-ade] and [3H, 7H-ade] adeninium cations. Protonated cytosinium cation aggregates with neutral cytosine to form pairs, and such pairs form tetrameric units with a length of 33.45 A˚ in the lattice of the seven coordinated manganese(II) complex [1H, 3H-cyt]2[MnL2(H2O)] 3 2cyt 3 6H2O. The discrete cations of cytosine stabilized by electrostatic, hydrogen bond, and stacking interactions within the inorganic layers build the closely packed organic-inorganic hybrid materials in the crystal lattice of the complexes [1H, 3H-cyt]2[CuL2] 3 5H2O and [1H, 3H-cyt]2[ZnL2] 3 5H2O.
Introduction The influence of metal ions on the behavior of nucleobases has growing attention in recent years.1,2 The studies of metalnucleobase bindings provide an understanding of the role of metal ions in the function of nucleic acids especially in genetic information transfer.3-5 Molecular recognition of these nucleobases, such as adeninium and cytosinium cation, in their most stable tautomeric forms by artificial receptors such as metaloxalato and metal-malonato coordination polymers has been reported in the literature.6 Hexachlorostannate anions stabilize cytosinium and adeninium cation in strong hydrogen bonded networks.7 Studies concerning these nucleobases are carried out in gas phase8 or aqueous media.9 Some studies on nucleobases are directed toward pharmaceuticals;10 construction of ligands;11 stabilization of noncanonical tautomers through interactions with metallic ions;12 development of artificial receptors for specific DNA/RNA base recognition; and for determination of therapeutic agents.13 We are interested in using individual nucleobases for construction of various suprastructures with the aid of metal carboxylate complexes. Dipicolinato metal frameworks have been widely used in the design and synthesis of inorganic/organic layered solids where the guest molecules/organic substituents are sandwiched between layers.14 This study is with an idea to stabilize different types of nonequivalent protonated structures of adenine and cytosine in the interstices of the dipicolinato metal frameworks. Neutral cytosine and/or its 1H, 3H-cytosinium aggregates have five different one-dimensional (1D) nucleobase patterns.15 Moreover, adenine forms a wide range of neutral tautomers and protonated dimers including the well-known Watson-Crick interaction observed in DNA16 due to its five available proton attachment sites. This has been the subject of numerous theoretical and experimental investigations.17 From density functional theory calculations, 1H, 9H-adeninium cation is more stable than 3H, 7H-adeninium cation by a energy difference of 0.46 kcal/mol.18 *To whom correspondence should be addressed. E-mail:
[email protected]. in. pubs.acs.org/crystal
Published on Web 06/08/2010
Figure 1. Self-assembly of protonated cytosine and adenine, and structure of dipicolinato anion.
Here we observe stabilization of self-assembly of nonequivalent (1H, 9H) and (3H, 7H) protonated adenine and also cytosinium-cytosine assembly (Figure 1) by metal dipicolinato complexes. Furthermore, the role of crystallized water molecules in such complexes to act as a linker between the alternating organic and inorganic layer is studied. Results and Discussion It is observed that depending on the environment of the central divalent metal ion, the metal complexes of dianion of dipicolinic acid (abbreviated as L) can stabilize adeninium (adeH) and cytosinium (cytH) cations in different ways as illustrated in Scheme 1. In the reaction of dipicolinic acid and manganese(II) acetate tetrahydrate or copper(II) acetate monohydrate followed by reaction with adenine under ambient condition, two nonequivalent (1H, 9H) and (3H, 7H) adeninium cations are formed in the interstices of metal dipicolinato anion. These adeninium cations aggregate through hydrogen bonding to form an infinite 1D ribbonlike structure, whereas under similar reaction conditions dipicolinic acid reacting with manganese(II) acetate followed by treatment with cytosine forms a mixed cytosine-cytosinium 1D supramolecular ribbon through intermolecular hydrogen bonding in the lattice of a seven coordinated manganese(II) complex. Analogous reactions with copper(II) or zinc(II) acetate with cytosine as a nucleobase leads to discrete monomeric cytosinium cations. In all these complexes, the cationic nucleobases are associated with the metal dipicolinato anion through electrostatic and noncovalent interactions. The crystallized water molecules r 2010 American Chemical Society
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Scheme 1. Different Type Cationic Species of Cytosine and Adenine
occupy the interstitial spaces of the lattices and display hydrogen bond to the dipicolinato oxygen resulting lattice stabilization19 and also to maintain supramolecular assembly. The bis-dipicolinato complexes with adeninium cation, namely, [1H, 9H-ade][3H, 7H-ade][MnL2] 3 3H2O (1) and [1H, 9H-ade][3H, 7H-ade][CuL2] 3 3H2O (2), are isostructural and crystallizes in the P21/c space group. The crystals of manganese complex 1 are obtained as pale yellow crystals, whereas crystals of copper(II) complex 2 are blue. Each asymmetric unit of complex 2 consists of a distorted octahedral complex anion [CuL2]2-, two symmetrically nonequivalent [1H, 9H ] and [3H, 7H ] adeninium cations along with three crystallized water molecules (Figure 2a). The coordination polyhedron of the copper atom in [CuL2]2- is distorted octahedral; where the equatorial plane contains the nitrogen atom and axial positions are occupied by the carboxylato oxygen atoms of dipicolinato ligands. The representative compound consists of one 1H, 9H-adeninium cation and another adeninium cation having hydrogen atoms at N3 and N7, which is the second most stable protonated form of adenine.18 This is unprecedented in the adenine solid-state chemistry. The location of the H-atoms, except those attached to the -NH2 group of the protonated adenine are decided by the difference Fourier synthesis map and also by careful observation of the complementing nature of hydrogen bonding interactions.
Since complexes 1 and 2 have similar structural features, only the perspective view of complex 2 is shown in Figure 2a. Comparative bond parameters of the complex anions are shown in Table 1. Crystal packing of the complex 2 can be regarded as a lamellar inorganic-organic hybrid network built up of anionic sheets of metal dipicolinato anion and cationic nucleobase among them. Insertion of protonated nucleobase 1H, 9Hadeninium and 3H, 7H-adeninium cations between the inorganic layers are shown in Figure 2b, and they are sequentially arranged to form polymeric 1D zigzag ribbons running along the crystallographic c axis. The electrostatic and hydrogen bond interactions of the complex anion with the protonated adenine dimer are shown in Figure 2c. The crystallized water molecules occupy the interstitial spaces19 between the anionic complexes and display strong hydrogen bonding interactions among themselves and to the dipicolinato oxygen atom. The ribbon or layer arrangements of nucleobases, either in neutral form or in 1H, 9H or 3H, 7H-adeninium form are shown in mixed ligand complexes; recently,6,20 however, the ribbons consisting of two different 1H, 9H and 3H, 7H-adeninium cations, to our knowledge, have not been observed. The 1H, 9H and 3H, 7H-adeninium cations generate cationic ribbons running along the crystallographic c direction with the aid of intermolecular hydrogen bonding interactions between
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Figure 2. (a) Asymmetric unit of compound 2 with 50% thermal ellipsoid; (b) the inorganic-organic hydrogen bonding interactions along with crystallized water molecules in compound 2; (c) the hydrogen bonded infinite [1H, 9H] and [3H, 7H] adeninium ribbon with H-bond distances.
the Hoogsteen face of a 1H, 9H-adeninium cation and the protonated N3 and unprotonated N9 atoms of an adjacent 3H, 7H-adeninium cation. This is followed by a hydrogen bond between the protonated N1 site of 1H, 9H-adeninium nucleobase and the unprotonated N1 site of the other nucleobase (Figure 2c). These planar protonated dimers are perpendicularly accommodated in the ribbon with one another. Formation of these different types of protonated adenine dimer may be attributed to simultaneous hydrogen bonded interactions of protonated adenine with the carboxylato oxygen of metal dipicolinato anion as well as with neighboring nonequivalent protonated adenine. The anionic complex
layers surrounding these cationic entities reduce the electrostatic repulsive forces between them and stabilize the ribbon to some extent. Selected hydrogen bond parameters for the adeninium ribbon are summarized in Table 2. The bis-dipicolinato manganese(II) complex (3) obtained as colorless crystals, with a composition of (1H,3H-cyt)2[MnL2(H2O)] 3 2cyt 3 6H2O consists of two crystallographically independent cytosinium cations and two neutral cytosine molecules along with six water molecules of crystallization in a seven coordinated system (Figure 3a). Besides two tridentate dipicolinato dianions, there is an aqua-ligand occupying the seventh coordination site. This makes a pentagonal
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Table 1. Comparative Bond Distances (A˚) and Bond Angles () in Complexes 1 and 2 bond parameters
for 1; M = Mn
for 2; M = Cu
bond parameters
for 1; M = Mn
for 2; M = Cu
M(1)-N(1) M(1)-N(2) M(1)-O(5) M(1)-O(1) M(1)-O(3) M(1)-O(7) — N(2)-M(1)-N(1) — N(2)-M(1)-O(5) — N(1)-M(1)-O(5) — N(2)-M(1)-O(1)
2.144(9) 2.142(9) 2.159(8) 2.179(7) 2.232(7) 2.253(8) 166.1(4) 74.2(4) 113.0(3) 118.4(3)
1.939(2) 1.950(2) 2.213(2) 2.109(2) 2.175(2) 2.184(2) 175.64(11) 76.97(9) 107.18(9) 102.15(9)
— O(1)-M(1)-O(7) — O(5)-M(1)-O(7) — N(1)-M(1)-O(7) — N(2)-M(1)-O(7) — O(1)-M(1)-O(3) — O(5)-M(1)-O(3) — N(1)-M(1)-O(3) — N(2)-M(1)-O(3) — O(5)-M(1)-O(1) — N(1)-M(1)-O(1)
93.5(3) 144.6(3) 102.4(3) 71.4(4) 145.7(3) 95.6(3) 72.1(3) 95.9(3) 95.4(3) 73.8(3)
93.93(8) 154.72(7) 98.10(9) 77.74(9) 156.10(8) 90.88(8) 77.33(10) 101.52(8) 91.25(8) 79.30(10)
Table 2. H-Bond Parameters of Protonated Adenine Ribbon in Complex 2 bond (symmetry)
dD-H(A˚)
dH 3 3 3 A(A˚)
dD 3 3 3 A(A˚)
N(9)---H(9N) 3 3 3 N(6) [1 þ x, 1/2 - y, 1/2 þ z] N(7)---H(7B) 3 3 3 N(10) [-1 þ x, 1/2 - y, -1/2 þ z] N(3)---H(3N) 3 3 3 N(8) [x, y, z]
0.84(4) 0.86 0.81(3)
2.10(4) 2.12 2.18(3)
2.899(5) 2.959(4) 2.971(4)
— D-H 3 3 3 A() 160(3) 165 163(4)
Figure 3. (a) Asymmetric unit of compound 3 with 50% thermal ellipsoid (hydrogen atoms on oxygen atoms of water molecules are not located); (b) the inorganic-organic hydrogen bonding interactions along with crystallized water molecules in the packing diagram of compound 3 along the a axis; (c) the tetrameric cytosinium-cytosine ribbon terminated by metal dipicolinato anion; (d) tetrameric hydrogen bonded ribbon of cytosine.
bipyramid geometry around the metal ions. The bond parameters are shown in Table 3. The interaction of the nucleobase takes place by electrostatic as well as by hydrogen bond
interactions with the carboxylato anion of the metal complex. Another weak interaction involved in stabilizing complex 3 is the π-π interaction. The centroid-to-centroid distances of the
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aromatic rings of dipicolinato ligand with the neutral and protonated cytosine entities are 3.99 and 4.30 A˚, respectively. This suggests very weak stacking interactions. Furthermore, there are three more π-π staking interactions between the neutral and protonated cytosine molecules. In these cases the centroid to centroid distances are observed as 3.60, 3.63 and 3.75 A˚, respectively. These separations indicate that the later set of π-π staking interactions to be comparatively stronger than the former. The crystallized water molecules exhibit hydrogen contact to both the metal dipicolinato anion and the protonated one-dimensional ribbon of cytosine. The locations of the hydrogen atoms in the protonated cytosine are justified by difference Fourier synthesis map. It is also determined by complementary hydrogen bonding interactions between the cationic and neutral cytosine molecule in the asymmetric unit. As the neutral and protonated cytosine molecules form a dimer in the crystal lattice through intermolecular hydrogen bonding, it is likely that these protons are disordered over two different nitrogen atoms.21 This is also evident from the relatively longer -Nþ-H bond distances, 0.97 and 0.99 A˚, respectively, observed in the two symmetrically independent protonated cytosine molecules (Table 4). The compound 3 displays a supramolecular three-dimensional lamellar structure built up of metal dipicolinato anion and planar hydrogen bonded tetrameric 1D ribbon of protonated and neutral cytosine molecules (Figure 3b). The mixed cytosinecytosinium base pairs self-assemble to 1D aggregates connected by three hydrogen bonds, two of which involve the N4 hydrogen bond donors and O2 hydrogen bond acceptors, while the third is formed between the protonated N3 atom of one base and the unprotonated N3 site of the other base, which resembles to a pseudo-Watson-Crick pattern. Furthermore, the adjacent CytHcytþ pairs are held together by double N1-H 3 3 3 OdC22 hydrogen bonds leading to two different types of 1D planar supramolecular tetrameric ribbons with alternating neutral and protonated cytosine entities. The hydrogen bond parameters for this tetrameric cytosine ribbon are summarized in Table 4. It is to be stressed that this self-assembled tetrameric ribbon with a dimension of 33.45 A˚ finally gets terminated through electrostatic Table 3. Selected Bond Distances (A˚) and Bond Angles () in Complex 3 bond parameters
for 3
Mn(1)-N(1) 2.234(4) Mn(1)-N(2) 2.271(4 Mn(1)-O(5) 2.456(4) Mn(1)-O(1) 2.214(4) Mn(1)-O(3) 2.395(4) Mn(1)-O(7) 2.419(4) Mn(1)-O(9) 2.221(5) — N(2)-Mn(1)-N(1) 141.10(17) — N(2)-Mn(1)-O(5) 67.33(15) — N(1)-Mn(1)-O(5) 76.06(15) — N(2)-Mn(1)-O(1) 98.76(15) — O(9)-Mn(1)-N(1) 90.45(16) — O(9)-Mn(1)-O(3) 76.36(16)
bond parameters
for 3
— O(1)-Mn(1)-O(7) — O(5)-Mn(1)-O(7) — N(1)-Mn(1)-O(7) — N(2)-Mn(1)-O(7) — O(1)-Mn(1)-O(3) — O(5)-Mn(1)-O(3) — O(1)-Mn(1)-O(9) — N(1)-Mn(1)-O(3) — N(2)-Mn(1)-O(3) — O(5)-Mn(1)-O(1) — N(1)-Mn(1)-O(1) — O(9)-Mn(1)-N(2) — O(9)-Mn(1)-O(7)
78.33(15) 134.68(14) 139.25(16) 69.34(15) 140.89(14) 76.38(15) 102.88(18) 69.38(15) 112.37(16) 95.59(16) 71.52(14) 128.52(14) 70.02(15)
and strong hydrogen bonding interactions of protonated nucleobase with the carboxylato anion (N1-H 3 3 3 O = C; dH 3 3 3 A, 1.91 A˚, dD 3 3 3 A, 2.77 A˚; — D-H 3 3 3 A, 175) of the metal dipicolinato anion at both the ends (Figure 3c,d). This triple hydrogen bonded self-assembled hydrogen bonding pattern between the neutral and protonated cytosine molecules is similar to the canonical W-C base pairing of guanine-cytosine in DNA.23 The water molecules of crystallization in the complex 3 are confined mainly in the manganese(II) dipicolinato anionic layers; they are involved in extensive hydrogen bond interactions among themselves and with the carboxylato group of metal complex. They bridge the anionic frameworks through hydrogen bond with the coordinated water molecule in the manganese(II) complex anion. This leaves the neutral and protonated cytosine molecule unperturbed, and they do not involve hydrogen bond formation with the crystallized water molecules except to form self-assemblies selectively among themselves as layer or ribbon arrangements. Furthermore, the seven coordination geometry of manganese makes its packing pattern different from other six coordinated complexes. The bis-dipicolinato complexes of copper(II) and zinc(II) with 1H,3H-cytosinium cation, namely, [1H, 3H-cyt]2[CuL2] 3 5H2O (4) and [1H, 3H-cyt]2[ZnL2] 3 5H2O (5) appeared as colorless and blue crystals, respectively. These two complexes are also isostructural and crystallize in the P1 space group. The complex has two equivalent cytosinium cations per [ML2]2- complex anion along with five water molecules of crystallization (Figure 4a). Apart from electrostatic interactions, complexes 4 and 5 are also stabilized by hydrogen bonding interactions of crystallized water molecules with both the complex anion and the protonated cytosine nucleobase. The other noncovalent forces stabilizing the complexes are the face-to-face π-π interactions between the planar protonated cytosinium cation and the aromatic ring of dipicolinato anion. The two cytosinium cations are stacked parallel to the aromatic ring of dipicolinato counterpart of the [ML2]2-. The stacks are comprised of partially slipped aromatic rings with centroid-to-centroid π-π distances measured as 3.98 and 4.06 A˚, respectively. Since the complexes 4 and 5 have similar structural features, only the structure of complex 5 is shown in Figure 4a. Comparative bond length and bond angles of the two complexes are listed in Table 5. The crystal packing of complexes 4 and 5 along the crystallographic c axis reveals a hybrid inorganic-organic lamellar structure containing layers of metal dipicolinato anionic complexes and hydrogen bonded discrete cationic cytosine heterocycles (Figure 4b). The crystallized water molecules not only act as space fillers, but also act as a donor/acceptor bridge between the anion/ cation or the cation/cation entity. The discrete cytosine cations are stabilized through hydrogen bonds with crystallized water molecules as well as with the carboxylato oxygen of metal dipicolinato frameworks before it can self-aggregate leading to a cationic ribbon as shown in complex 3.
Table 4. H-Bond Parameters of Cytosine-Cytosinium Tetrameric 1D Ribbon in Complex 3 bond (symmetry)
dD-H (A˚)
dH 3 3 3 A (A˚)
dD 3 3 3 A (A˚)
N(3)-H(3A) 3 3 3 O(13) [-1 þ x, y, 1 þ z] N(4)-H(4N) 3 3 3 N(12) [-1 þ x, y, 1 þ z] N(14)-H(14A) 3 3 3 O(10) [1 þ x, y, -1 þ z] N(8)-H(8A) 3 3 3 O(12) [1 - x, 1 - y, -z] N(6)-H(6N) 3 3 3 N(9) [1 - x, 1 - y, -z] N(11)-H(11A) 3 3 3 O(11) [1 - x, 1 - y, -z] N(5)-H(5N) 3 3 3 O(10) [1 - x, -y, 2 - z] N(10)-H(10N) 3 3 3 O(13) [1 - x, 1 - y, -z] N(13)-H(13N) 3 3 3 O(12) [1 - x, 1 - y, -z]
0.86 0.97(5) 0.86 0.86 0.99(7) 0.86 0.86 0.86 0.86
2.00 1.87(6) 1.99 1.94 1.89(7) 2.08 1.95 2.03 1.92
2.847(7) 2.814(7) 2.842(7) 2.795(7) 2.850(7) 2.938(7) 2.805(7) 2.869(7) 2.774(7)
— D-H 3 3 3 A () 167 165(5) 175 178 164(6) 171 170 166 171
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The thermogravimetric analysis of each compound shows loss of water molecules of crystallization and nucleobases on heating. Weight loss for each complex below 350 C tallies with the number of water molecules and the neutral nucleobases which leads to form H2[ML2] through thermal decomposition (for TG curves, please refer to Supporting Information). For example, complex 1 loses weight for such processes at the range of 125-320 C; complex 2 at 150-320 C, whereas complexes 3 and 4 lose their crystallized water molecules
Figure 4. (a) The structural unit of 5 (drawn with 50% thermal ellipsoids); (b) the inorganic-organic hydrogen bonding interactions along with crystallized water molecules in the packing diagram of compound 5 along the c axis.
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independently in the range of 65-130 C and the nucleobase molecules are lost at a higher temperature. Decomposition of coordinated water molecule in complex 3 occurs at 166175 C, the cytosine at 275-320 C, whereas the loss of cytosine in complex 4 occurs in a very narrow temperature range (290-310 C). These small differences in the temperature range on loss of relatively volatile molecules from these complexes occur due to the difference in the packing patterns and variation of electrostatic interactions by change of cations (metal or nucleobase). Various protonated self-assemblies influence the conducting properties of the complexes in aqueous solution. A low molar conductance value of 30 S cm2 mol-1 is observed for complex 3, whereas the complexes 1 and 2 exhibit a molar conductance of 205 and 201 S cm2 mol-1, respectively, in water. The later two values are slightly on the higher side for 1:2 electrolytes. This result supports that some hydrogen bonded cytosine-cytosinium tetrameric assembly observed in complex 3 is maintained in the solution state also;24 that reduces the ionic mobility of the charged species and hence lowers the molar conductance. The ESI mass spectra of the complex is 3 recorded to know the signature of the dimeric and tetrameric assemblies of nucleobase in complex 3, and in fact we could observe them. The ESþ mass ion peak (m/e) at 221.5 and at 442.5 are attributed to originate from the dimer and tetramer respectively (refer to Supporting Information) also showing the existence of the cyt-cytHþ assembly. On the other hand, 1D adeninium ribbons are comparatively weaker than the cytosine tetramer, and hence the self-assembly does not exist in the solution state. This point is also supported by the high molar conductance of the complex. The complexes 4 and 5 exhibit the molar conductance values corresponding to 1:2 electrolytes. To see the effect of the size of anion in the stabilization of the cationic nucleobases, we studied two similar coordination complexes with heavy metal ion cerium(IV) having dipicolinato anion with adenine and cytosine as cations, namely, [1H,9H-ade][3H,7H-ade][CeL3] 3 4H2O (6) and [1H, 3H-cyt]2[CeL3] 3 2H2O (7). The crystal structure of compound 6 consists of complex anion [CeL3]2-, two nonequivalent [1H, 9Hade] and [3H, 7H-ade] adenine cations, and crystallization water molecules linked together by an electrostatic and intricate network of hydrogen bonds (refer to Supporting Information). The cerium(IV) ion is at the center of a distorted tricapped trigonal prismatic coordination polyhedron made up of three dipicolinato anions coordinating in the tridentate chelating mode. It is observed that the two nonequivalent adenine cations form hydrogen bonded self-assembly independently between two cerium(IV) trisdipicolinato anions. We did not observe assembly formation among two differently protonated adenine cations which was observed previously for complexes 1 and 2. This clearly suggests that the size of the central metal cation and their coordination ability
Table 5. Comparative Bond Distances (A˚) and Bond Angles () in Complexes 4 and 5 bond parameters
for 4; M = Cu
for 5; M = Zn
bond parameters
for 1; M = Cu
for 2; M = Zn
M(1)-N(1) M(1)-N(2) M(1)-O(5) M(1)-O(1) M(1)-O(3) M(1)-O(7) — N(2)-M(1)-N(1) — N(2)-M(1)-O(5) — N(1)-M(1)-O(5) — N(2)-M(1)-O(1)
1.9580(12) 1.9279(12) 2.1215(12) 2.2819(12) 2.2227(12) 2.1275(12) 175.78(5) 78.71(5) 94.86(5) 101.11(5)
2.0168(19) 2.0199(19) 2.1478(15) 2.1292(14) 2.2109(15) 2.2487(14) 171.16(6) 77.14(7) 107.56(6) 110.09(7)
— O(1)-M(1)-O(7) — O(5)-M(1)-O(7) — N(1)-M(1)-O(7) — N(2)-M(1)-O(7) — O(1)-M(1)-O(3) — O(5)-M(1)-O(3) — N(1)-M(1)-O(3) — N(2)-M(1)-O(3) — O(5)-M(1)-O(1) — N(1)-M(1)-O(1)
91.06(5) 157.70(5) 104.37(5) 79.05(5) 153.38(4) 93.87(5) 77.40(5) 105.24(5) 97.92(5) 76.52(5)
92.72(6) 151.87(7) 100.53(6) 75.20(6) 152.72(7) 94.06(6) 75.23(6) 97.19(6) 91.98(6) 77.58(7)
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Table 6. Crystal Parameters for Compounds 1-5 formula formula weight crystal size (mm3) crystal system space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z Fcalc (g cm-3) μ (mm-1) F(000) reflns collected reflns unique Rint R1 [I g 2σ(I)] wR2 [I g 2σ(I)] R1 (all data) wR2 (all data)
1
2
3
4
5
C24H24MnN12O11 711.49 0.28 0.22 0.12 monoclinic P21/c 9.707(2) 18.695(3) 17.370(3) 90.00 112.008(14) 90.00 2922.5(9) 4 1.617 0.534 1460 21547 5030 0.1134 0.0774 0.1803 0.1029 0.2491
C24H24CuN12O11 720.09 0.38 0.26 0.19 monoclinic P21/c 9.6497(4) 18.7034(7) 17.8872(6) 90.00 116.027(2) 90.00 2900.93(19) 4 1.649 0.836 1476 39237 5112 0.0802 0.0384 0.0829 0.0637 0.0937
C30H42MnN14 O19 957.62 0.35 0.20 0.14 triclinic P1 10.5913(4) 13.0090(4) 16.5020(5) 73.770(2) 79.787(2) 73.752(2) 2083.73(12) 2 1.507 0.410 970 23178 7043 0.0915 0.0606 0.2155 0.0892 0.2399
C22H28CuN8 O15 708.06 0.45 0.28 0.22 triclinic P1 9.4706(4) 11.8131(4) 13.4429(5) 77.635(2) 84.664(2) 82.454(2) 1453.07(10) 2 1.618 0.839 730 19655 7125 0.0437 0.0321 0.0890 0.0372 0.0916
C22H28N8O15 Zn 709.89 0.48 0.34 0.22 triclinic P1 9.4076(3) 11.8945(5) 13.4682(5) 77.712(2) 84.142(2) 82.466(2) 1455.55(9) 2 1.620 0.931 732 9965 4926 0.0492 0.0335 0.0923 0.0376 0.0950
governs the packing patterns which make different types of assemblies. In conclusion, we have demonstrated stabilization of a hydrogen bonded infinite chain of adeninium cations having two nonequivalent 1H, 9H and 3H, 7H-adeninium cations at alternating positions by anionic sheets of metal dipicolinato anions. Stabilization of a distinct tetrameric planar 1D ribbon formed by neutral and 1H, 3H-cytosinium cation by a seven-coordinated manganese(II) dipicolinato complex anion is established. The discrete cations of cytosine are stabilized by electrostatic, hydrogen bond, and stacking interactions within the inorganic layers. A novel method to generate new types of nucleobase assemblies with definite dimension and shape in confined environments under a near neutral condition is depicted. In addition, this study contributed to the molecular recognition processes of a particular type of cations of adenine or cytosine by an artificial receptor, namely, metal dipicolinato anion. Experimental Section Synthesis and Characterization. Complex [1H, 9H-ade][3H, 7Hade][MnL2] 3 3H2O (1) and [1H, 9H-ade][3H, 7H-ade] [CuL2] 3 3H2O (2). To a solution of dipicolinic acid (0.334 g, 2 mmol) dissolved in methanol (20 mL) a solution of manganese(II) acetate tetrahydrate (0.245 g, 1 mmol) or copper(II) acetate monohydrate (0.199 g, 1 mmol) was added. Depending upon the metal ion, different colored solids were obtained, for example, white or blue precipitate for manganese(II) or copper(II) respectively. The reaction mixture was stirred for half an hour, and 10 mL of solution of adenine (0.27 g, 2 mmol dissolved in 50% methanol) was added in small portions to the respective precipitate and stirred. The solution was left overnight at room temperature. A pale yellow (for manganese) or blue (for copper) precipitate obtained from the reaction mixture was filtered, dried, and crystallized from Milli Q water. For complex 1: Isolated yield, 45%. Elemental anal calcd for C24H24MnN12O11, C, 40.47; H, 3.37; found C, 40.50; H, 3.34. IR (KBr, cm-1): 3410 (bs), 1621(s), 1590(w), 1424 (w), 1374 (s), 1277 (w), 1085(w). Molar conductance: 205.0 S cm2 mol-1 in water, μeff at 298 K: 5.42 μB. Thermal analysis: decomposition range: ∼60139 C (evaporation of three water molecules of crystallization); further decomposition occurs at ∼165 C. For complex 2: Isolated yield: 48%. Elemental anal calcd for C24H24CuN12O11, C, 40.00; H, 3.33; found C, 39.78; H, 3.24. IR (KBr, cm-1): 3413 (bs), 1624 (s), 1591 (w), 1426 (w), 1377 (s), 1279 (w), 1088 (w). Molar conductance: 201.0 S cm2 mol-1 in water, μeff at 298 K: 1.65 μB vis (H2O) λmax: 772.0 nm; ε = 57.5 M-1 cm-1.
Thermal analysis: decomposition range: ∼50-136 C (loss of three water molecules of crystallization). Complexes [1H,3H-cyt]2 [MnL2(H2O)] 3 2cyt.6H2O (3), [1H,3Hcyt]2[CuL2] 3 5H2O (4) and [1H,3H-cyt]2[ZnL2] 3 5H2O (5). The dipicolinic acid (0.334 g, 2 mmol), manganese(II) acetate tetrahydrate (0.245 g, 1 mmol) or copper(II) acetate monohydrate (0.199 g, 1 mmol) or zinc(II) acetate dihydrate (0.219 g, 1 mmol) were reacted in 20 mL of methanol for half an hour. To the precipitate obtained in each case, 10 mL of cytosine (0.220 g, 2 mmol dissolved in 20% methanol) solution was added in small portions. The reaction was left overnight at room temperature. The white, blue, and white precipitates obtained were filtered, dried, and kept for crystallization in Milli Q water. White, blue, and colorless crystals of manganese, copper, and zinc complexes, respectively, were formed in 2-3 days. For complex 3: Isolated yield: 42%. Elemental anal calcd for C30H42MnN14O19, C, 37.59; H, 4.38; found C, 37.54; H, 4.40. IR (KBr, cm-1): 3370 (s), 3183 (s), 2923 (w), 2808 (w), 1661 (bs), 1651 (bs), 1497 (m), 1456 (s), 1374 (m), 1290 (m), 1235 (m), 791 (m). Molar conductance: 30.0 S cm2 mol-1 in water, μeff at 298 K: 2.1 μB. Thermal analysis: decomposition range: ∼65-130 C (loss of six water molecules of crystallization), further decomposition occurs at 166 C (loss of coordinated water molecule). Complex 4: Isolated yield: 74%. Elemental anal calcd for C22H28CuN8O15, C, 37.29; H, 3.95; found C, 37.22; H, 3.84. IR (KBr, cm-1): 3306 (b), 3102 (w), 2930 (w), 1729 (m), 1614 (s), 1581 (m), 1422 (m), 1370 (s), 1279 (w), 1228 (m), 774 (w). Molar conductance: 185.0 S cm2 mol-1 in water, μeff at 298 K: 1.60 μB Vis (H2O) λmax 774.0 nm; ε = 70.0 M-1 cm-1. Thermal analysis: decomposition range:∼ 66-115 C (loss of five water molecules of crystallization). Complex 5: Isolated yield: 72%. Elemental anal calcd for C22H28Zn N8O15, C, 37.18; H, 3.94; found C, 37.07; H, 3.79. IR (KBr, cm-1): 3304 (b), 3103 (w), 2924 (w), 1729 (m), 1620 (s), 1582 (w), 1427 (m), 1373 (s), 1284 (m), 1226 (m), 733 (w). 1H NMR (D2O, 400 MHz, ppm): 8.4 (2H, t, J = 6.8 Hz), 8.35 (4H, d, J = 7.2 Hz), 7.6 (2H, d, J = 7.6 Hz), 6.0 (2H, d, J = 7.2 Hz). Molar conductance: 181.0 S cm2 mol-1 in water. Physical Measurements. The IR spectra (KBr pellets) were recorded with a Perkin-Elmer Spectrum One FTIR spectrophotometer in the region 4000-400 cm-1 spectral region. Thermal analyses (TG/DTA) were performed on a Mettler Toledo TGA/ SDTA851e thermal analyzer in a synthetic air atmosphere (79% N2/ 21%O2) with a heating rate of 10 C. Visible spectra were recorded using Perkin-Elmer Lambda 750 UV-visible spectrophotometer. Elemental analyses were performed with a Perkin-Elmer 2400 series microanalytical analyzer. Molar conductance measurements of the complexes were calculated using Elico conductivity meter, model CM 180, and room temperature magnetic moments were measured using a Sherwood scientific magnetic susceptibility balance.
Article
Crystal Growth & Design, Vol. 10, No. 7, 2010
X-ray Structural Studies. The X-ray crystallographic data were collected at 296 K with Mo KR radiation (λ = 0.71073 A˚) using a Bruker Nonius SMART CCD diffractometer equipped with graphite monochromator. The SMART software was used for data collection and also for indexing the reflections and determining the unit cell parameters; the collected data were integrated using SAINT software.25 The structures were solved by direct methods and refined by full-matrix least-squares calculations using SHELXTL software. All the non-H atoms were refined in the anisotropic approximation against F2 of all reflections. The locations of the H atoms of the protonated adenine and cytosine of complexes 1, 4, and 5 were justified by a difference Fourier synthesis map, but the assignment of the protons has been unambiguously substantiated by particular hydrogen bonding interactions. The H-atoms attached to water molecules were located in the difference Fourier synthesis maps, and refined with isotropic displacement coefficients. Some of the hydrogen atoms attached to water molecules could not be located in the complexes 1, 2, and 3. It was also necessary to apply restraints to optimize the distances of some hydrogen atoms of water molecules. Crystal parameters and details of the final refinement parameters are summarized in Table 6.
Acknowledgment. The authors thank Department of Science and Technology (New-Delhi) India for financial support. Supporting Information Available: Crystallographic information files of all the compounds reported here; CIF files are also deposited to CCDC and have CCDC Nos. 756154, 756155, 756157, 756158, 756225, 777152, and 777153. Thermograms of the complexes 1-4 and packing diagram of the cerium complexes and mass spectra of 3. This material is available free of charge via the Internet at http:// pubs.acs.org.
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