Directing the Formation of Adenine Coordination Polymers from

Article pubs.acs.org/crystal

Directing the Formation of Adenine Coordination Polymers from Tunable Copper(II)/Dicarboxylato/Adenine Paddle-Wheel Building Units Sonia Pérez-Yáñez, Garikoitz Beobide, Oscar Castillo,* Javier Cepeda, Antonio Luque,* and Pascual Román Departamento de Química Inorgánica, Facultad de Ciencia y Tecnología, Universidad del País Vasco, UPV/EHU, Apartado 644, E-48080 Bilbao, Spain S Supporting Information *

ABSTRACT: Coordination polymers containing paddle-wheel shaped building units of general formula [Cu2(μ-adeninato)2(μcarboxylato)2] (1−3) and [Cu2(μ-carboxylato)4(methyladenine)2] (4−6) are reported. The copper(II) centers of the compounds {[Cu2(μ3-adeninato)2(μ-Hglut)2]·2H2O}n (1), {[Cu2(μ3-adeninato)2(μ-Hadip)2]}n (2), and {[Cu2(μ3-adeninato)2(μ-Hpime)2]}n (3) (where glut: glutarato; adip: adipato; and pime: pimelato) are bridged by tridentate N3,N7,N9-adeninato ligands to give a similar covalent three-dimensional network in which the dicarboxylate anions act as bidentate μ-κO1:κO2 ligands with a free hydrogencarboxylic group placed within the channels present in the crystal structures. In 2−3, the −COOH group of the pendant aliphatic chain is hydrogen bonded to the Watson−Crick face (N6H/N1) of an adenine nucleobase placed at the opposite side of the channel, whereas in 1, the shorter aliphatic chain precludes this interaction and crystallization water molecules are placed between the hydrogencarboxylic group and the nucleobase. Compounds {[Cu2(μ4-glut)2(3Meade)2]·4H2O}n (4), {[Cu2(μ4-glut)2(9Meade)2]}n (5), and {[Cu2(μ4-pime)2(9Meade)2]·2H2pime}n (6) (where 3Meade: 3-methyladenine and 9Meade: 9-methyladenine) contain neutral chains where the paddle-wheel motifs are doubly bridged by tetratopic dicarboxylate anions. The supramolecular architecture of 4 and 5 is essentially knitted by hydrogen bonding interactions between the Watson−Crick faces of adjacent adenines, whereas compound 6 shows the inclusion of guest pimelic molecules which are anchored to the polymeric chains through fork-like hydrogen bonding interactions between one of the carboxylic groups and the peripheral adenine moieties, affording a supramolecular layered structure. The magnetic data of all the compounds show the occurrence of an antiferromagnetic behavior which is dominated by the orbital complementarity of the adenine and carboxylato bridging ligands in compounds 1−3.

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varying the metal cores, the bridging moieties, or the X-ligands.4 This functional versatility of the dinuclear PW motifs makes them particularly suitable as secondary building units (SBUs) for the design and synthesis of numerous crystalline materials ranging from zero-dimensional (0D) species to three-dimensional (3D) coordination polymers with interesting properties in areas such as magnetism, medicine, catalysis, and gas storage.2,5,6 One of our main research lines takes advantage of the synthetic control over the three dicopper paddle-wheel entities built up from the adenine nucleobase and carboxylato ligands (Figure 1). The first building unit, [M2(μ-adenine)4], in which the metal centers are bridged by the adenine nucleobase acting as the

INTRODUCTION Design of coordination frameworks via deliberate selection of metals and multifunctional ligands, including biological relevant molecules such as nucleobases,1 is one of the most attractive topical areas of chemistry due to their fascinating structural diversity and their development as new materials with tunable properties.2 An essential part of coordination polymer design, and the wider field of crystal engineering, is the use of building blocks that combine the flexibility and necessary interconnection capability to achieve the required dimensionality, but also enough strength to permit a predictable core which maintains their structural integrity throughout the construction of the solid. In this sense, [M2(μ-L)4X2] entities have been known for a long time since the crystal structure of the [Cu2(μacetato)4(H2O)2] compound was reported.3 The attractiveness of the paddle-wheel (PW) motif is that structural and functional changes can be achieved almost at will by simply © 2012 American Chemical Society

Received: April 2, 2012 Revised: April 27, 2012 Published: April 30, 2012 3324

dx.doi.org/10.1021/cg300446d | Cryst. Growth Des. 2012, 12, 3324−3334

Crystal Growth & Design

Article

the axial positions of small molecules or ions, such as water molecules or halides but not of more sterically hindered molecules. In this way, the polymerization of the [M2(μ-adenine)4] entity is promoted by means of the deprotonation of the nucleobase, but as far as the coordination of an adjacent PW entity to the axial position is forbidden, the presence of a second less hindered metal center becomes crucial for the polymerization to succeed.7,8 With the well-known [M2(μ-carboxylato)4] entities, the resulting crystal structure can be directed by means of the corrrect selection of the polycarboxylato ligand toward a great variety of architectures ranging from polynuclear discrete entities to 3D crystal structures. In these entities, the axial positions, usually occupied by solvent molecules, can be replaced by nucleobase ligands, thus increasing the ability of the systems to establish molecular recognition processes.2 In the [M2(μ-adenine)2(μ-carboxylato)2] unit, both approaches are possible: (i) polymerization through the deprotonation of the nucleobase and its further coordination, and (ii) polymerization by using a polycarboxylato ligand. Our first approach to this system gave rise to a 3D coordination polymer containing the adenine nucleobase as an anionic N3,N7,N9-bridging ligand, {[Cu2 (μ-adeninato)4 (H2 O)2 ][Cu(ox)(H2O)]2}n, by means of the polymerization of the [Cu2(μ-adeninato)4] entity through the deprotonation of the adenine and its coordination to less sterically hindered [Cu(ox)(H2O)] units. The resulting structure contains one-dimensional (1D) tubular channels with a diameter of about 13 Å.7 Thereafter, we obtained discrete [M2 (μ-adenine) 2 (μ-mal) 2 (H 2 O) 2 ]· xH2O [M = Ni and Co] compounds in which the metal(II) atoms are bridged by two adenine ligands coordinated by their N3 and N9 nitrogen atoms and two chelating malonate anions with a μ-κ2O1,O2:κO1 coordination mode.9 In this case, the [M2(μ-adenine)2(μ-carboxylato)2] entity did not polymerize as expected because neither the nucleobase nor the carboxylato ligand acted as linker between the dimeric entities. Therefore, we decided to use longer aliphatic dicarboxylato ligands to avoid the aforementioned chelating effect, as a consequence, providing the desired interconnection of the dimeric entities. Following this strategy we have obtained three compounds, {[Cu2(μ3-adeninato)2(μ-Hglut)2]·2H2O}n (1), {[Cu2(μ3-adeninato)2(μ-Hadip)2]}n (2), and {[Cu2(μ3-adeninato)2(μ-Hpime)2]}n (3), with a covalent 3D network built up of PW entities that are axially connected by means of N3,N7,N9tridentate adeninato bridging ligands. Unexpectedly, the dicarboxylato ligands act as bidentate μ-κO1:κO2 ligands and they do not join the dimeric fragments as predicted. In the light of these results, we decided to use N3- and N9-methylated adenine molecules to decorate the axial positions of the [M2(μ-carboxylato)4] unit. Methylation at N3/N9 inhibits the PW assembling through bridging adenine molecules and promotes the formation of 1D metal−organic architectures maintained mainly through the dicarboxylato bridges. In this way, we have obtained the compounds {[Cu2(μ4-glut)2(3Meade)2]·4H2O}n (4), {[Cu2(μ4-glut)2(9Meade)2]}n (5), and {[Cu2(μ4-pime)2(9Meade)2]·2H2pime}n (6) that contain chains of interconnected PW entities in which the methylated nucleobases exhibit their usual monodentate N7-coordination pattern and the dicarboxylato ligands show the expected μ-κO1: κO2:κO3:κO4 binding mode to the copper centers.

Figure 1. Paddle-wheel entities for the metal/carboxylato/adenine system.

N3,N9-bridging ligand, is obtained in the absence of the carboxylic ligand in the reaction media. The second one, [M2(μ-carboxylato)4], where the metal centers are bridged exclusively by carboxylato ligands, is originated in the absence of adenine or when the nucleobase is functionalized in the N3 or N9 positions. The last one, [M2(μ-adenine)2(μ-carboxylato)2], appears with the simultaneous presence of adenine and carboxylic acid in the reaction media. Each of these dimeric entities can further polymerize to get extended systems. There are two main options to achieve this purpose: to make use of polycarboxylato ligands that are able to connect the PW motifs through the equatorial positions and/or to make use of the axial positions of the PW motifs. The latter option is not always available because of the steric hindrance exerted by the equatorial ligands over the axial position. Taking into account the van der Waals radii, we have made a simple estimation of the closest axial-approach of a pyridine ligand that led us to assess the accessibility of the axial position of each PW motif (Figure 2). The results indicate that the axial positions are only

Figure 2. Accessibility of the axial position of each PW motif.

available for pyridine like ligands in the case of [M2(μadenine)2(μ-carboxylato)2] and [M2(μ-carboxylato)4] entities. The [M2(μ-adenine)4] moiety only allows the coordination to 3325



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Table 1. Crystallographic Data and Structure Refinement Details of Compounds 1−6 empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z ρcalcd (g cm−3) μ (mm−1) reflections collected unique data/parameters Rint goodness of fit (S)a R1b/wR2c [I > 2σ(I)] R1b/wR2c [all data]

1

2

3

4

5

6

C20H28Cu2N10O10 695.59 tetragonal I41/a 15.403(1) 15.403(1) 22.617(3) 90 90 90 5366.1(9) 8 1.717 1.659 5377 2665/185 0.0593 1.131 0.0552/0.1077 0.1379/0.1227



C22H34Cu2N10O12 757.66 triclinic P1̅ 7.758(2) 8.341(2) 11.773(2) 100.982(2) 97.439(2) 94.185(2) 737.8(3) 1 1.687 1.520 4617 2528/199 0.0842 1.083 0.0504/0.1081 0.0789/0.1173



S = [∑w(F02 − Fc2)2/(Nobs − Nparam)]1/2. bR1 = ∑||F0| − |Fc||/∑|F0|. cwR2 = [∑w(F02 − Fc2)2/∑wF02]1/2; w = 1/[σ2(F02) + (aP)2 + bP] where P = (max(F02,0) + 2Fc2)/3 with a = 0.0558 (1), 0.0210 (2), 0.0557 (3), 0.0552 (4), 0.0470 (5), 0.0227 (6), and b = 19.4945 (3). a

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EXPERIMENTAL SECTION

Physical Measurements. All chemicals were of reagent grade and were used as commercially obtained. Elemental analyses (C, H, and N) were performed on an Euro EA (EuroVector) elemental analyzer. Metal content was determined by absorption spectrometry performed on a Perkin-Elmer Analyst 100. The IR spectra (KBr pellets) were recorded on a FTIR Shimadzu spectrometer in the 4000−400 cm−1 spectral region. Magnetic measurements were performed on polycrystalline samples of the compounds taken from the same uniform batches used for the structural determinations with a Quantum Design SQUID susceptometer covering the temperature range 2−300 K at a magnetic field of 1000 G. Synthesis. Green single crystals of compounds 1−3 were prepared by slow diffusion of a methanolic solution (5 mL) of 0.2 mmol of the corresponding dicarboxylic acid [glutaric acid (0.0267 g), adipic acid (0.0295 g) and pimelic acid (0.0327 g) for compounds 1, 2, and 3, respectively] into an aqueous solution of 0.2 mmol of Cu(NO3)2·3H2O (0.0483 g, 5 mL) and 0.2 mmol of adenine (0.0273 g, 20 mL). Yields: 70, 80, and 80% based on metal, for compounds 1, 2, and 3, respectively. Polycrystalline samples of these compounds can be obtained by direct mixing of the reagents, even with the use of higher stoichiometries of the dicarboxylic acid. Green single crystals of compounds 4−6 were prepared by slow diffusion of a methanolic solution (3 mL) of 0.1 mmol of the corresponding dicarboxylic acid [glutaric acid (0.0133 g) for compounds 4 and 5, pimelic acid (0. 0163 g) for compound 6] into an aqueous solution of 0.1 mmol of Cu(NO3)2·3H2O (0.0242 g, 3 mL) and 0.1 mmol of the methylated nucleobase [3-methyladenine (0.0152 g) for compound 4, and 9-methyladenine (0.0154 g) for compounds 5 and 6, in 10 mL]. Yields: 60, 55, and 65% based on metal, for compounds 4, 5, and 6, respectively. Various attempts have been made to obtain polycrystalline samples of these compounds by direct mixing of the reagents, but none yielded the expected products, resulting in amorphous or unidentified products. Anal. Calcd for {[Cu2(μ3-adeninato)2(μ-Hglut)2]·2H2O}n (1),. C20H28Cu2N10O10 (695.59 g/mol): C, 34.53; H, 4.06; N, 20.14; Cu, 18.27%. Found: C, 34.36; H, 4.18; N, 20.27; Cu, 18.03%. IR (cm−1, KBr pellet): 3410s for ν(O−H); 3350m for (ν(NH2) + 2δ(NH2)); 3210m for ν(C8−H + C2−H + NH2); 2940m for ν(CH3); 1710m, 1655s for νas(O−C−O); 1615m for (ν(CC) + δ(NH2)); 1580s for ν(C4−C5); 1538m for ν(N3−C4−C5); 1466w, 1452w for (δ(C2−H +

Figure 3. Dicopper paddle-wheel SBU of compound 1. C8−N9) + ν(C8−H)); 1400m for δ(N1−C6−H6); 1385w, 1344w for (ν(C5−N7−C8) + δ(CH3)); 1316m, 1280w, 1238w for (ν(N9−C8 + N3−C2) + δ(C−H) + νs(O−C−O)); 1205m, 1155m for (δ(C8−H) + ν(N7−C8)); 1037w, 1014w, 991w for τ(NH2); 921w, 899w, 866w for (ν(N1−C6) + τ(NH2)); 792m, 740w for δ(O−C−O); 649 m, 600 m for ring deformation; 578m, 466w, 433w for ν(M−O + M−N). Anal. Calcd for {[Cu 2 (μ 3 -adeninato) 2 (μ-Hadip) 2 ]} n (2),. C22H26Cu2N10O8 (685.62 g/mol): C, 38.54; H, 3.82; N, 20.43; Cu, 18.67%. Found: C, 38.33; H, 4.02; N, 20.67; Cu, 18.54%. IR (cm−1, KBr pellet): 3440s for ν(O−H); 3360s for (ν(NH2) + 2δ(NH2)); 3170s for ν(C8−H + C2−H + NH2); 2960m, 2910m for ν(CH3); 1717m, 1664s for νas(O−C−O); 1620s for (ν(CC) + δ(NH2)); 1578s for ν(C4−C5); 1538m for ν(N3−C4−C5); 1466m, 1451m for (δ(C2−H + C8−N9) + ν(C8−H)); 1397m for δ(N1−C6−H6); 1382m, 1340m for (ν(C5−N7−C8) + δ(CH3)); 1313m, 1278m, 1213m for (ν(N9−C8 + N3−C2) + δ(C−H) + νs(O−C−O)); 1184m, 1155m, 1079w for (δ(C8−H) + ν(N7−C8)); 1046w, 989w for τ(NH2); 935w, 898w, 875w, 842w for (ν(N1−C6) + τ(NH2)); 793m, 773m, 740m, 3326



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Table 2. Selected Bond Lengths (Å) of Compounds 1−3a 1 Cu1−N3 Cu1−N7a Cu1−N9b Cu1−O11 Cu1−O12b Cu1···Cu1b Cu1···Cu1c

1.994(5) 2.168(5) 1.955(5) 2.000(4) 2.008(4) 2.871(1) 6.002(1)

Cu1−N3a Cu1−N7b Cu1−N9 Cu1−O11 Cu1−O12a Cu1···Cu1a Cu1···Cu1c

2

3

2.030(6) 2.179(6) 1.980(7) 1.986(5) 1.975(5) 2.875(2) 6.062(1)

2.032(4) 2.188(4) 1.994(4) 1.981(3) 2.004(3) 2.865(1) 6.075(1)

727m for δ(O−C−O); 647m, 608m for ring deformation; 581m, 572m, 559m for ν(M−O + M−N). Anal. Calcd for {[Cu 2 (μ 3 -adeninato) 2 (μ-Hpime) 2 ]} n (3),. C24H30Cu2N10O8 (713.66 g/mol): C, 40.39; H, 4.24; N, 19.63; Cu, 17.81%. Found: C, 40.52; H, 4.18; N, 19.44; Cu, 17.78%. IR (cm−1, KBr pellet): 3450s for ν(O−H); 3320s for (ν(NH2) + 2δ(NH2)); 3140s for ν(C8−H + C2−H + NH2); 2930m, 2860m for ν(CH3); 1707m, 1674s for νas(O−C−O); 1620s for (ν(CC) + δ(NH2)); 1573s for ν(C4−C5); 1532m for ν(N3−C4−C5); 1463m, 1415m for (δ(C2−H + C8−N9) + ν(C8−H)); 1390m for δ(N1−C6−H6); 1314m, 1274m for (ν(N9−C8 + N3−C2) + δ(C−H) + νs(O−C−O)); 1197m, 1152m, 1086w for (δ(C8−H) + ν(N7−C8)); 1044w, 989w for τ(NH2); 941w, 883w for (ν(N1−C6) + τ(NH2)); 792w, 736w for

a Symmetry codes: for 1 (a) y + 1/4, −x + 1/4, z + 1/4; (b) −x, −y, − z + 1; (c) y − 1/4, −x + 1/4, −z + 5/4; for 2 and 3 (a) −x, −y, −z + 1; (b) −y + 1/4, x + 1/4, −z + 5/4; (c) y − 1/4, −x + 1/4, −z + 5/4.

Figure 4. (a) Linkage between the PW units and (b) perspective view of the overall 3D structure of compound 1 along the crystallographic a-axis. 3327



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Figure 5. Comparison of hydrogen bonding interactions of the adenine Watson−Crick faces in 1 (red) and 2 (blue). δ(O−C−O); 647 m for ring deformation; 577w, 561w for ν(M−O + M−N). Anal. Calcd for {[Cu 2 (μ 4 -glut) 2 (3Meade) 2 ]·4H 2 O} n (4),. C22H34Cu2N10O12 (757.66 g/mol): C, 34.88; H, 4.52; N, 18.49; Cu, 16.77%. Found: C, 34.63; H, 4.33; N, 18.79; Cu, 16.94%. IR (cm−1, KBr pellet): 3450s for (ν(NH2) + 2δ(NH2)); 3290m for ν(C8−H + C2−H + NH2); 3140m for ν(CH3); 1680m for νas(O−C−O); 1620s for (ν(CC) + δ(NH2)); 1573m for ν(C4−C5); 1523w for ν(N3− C4−C5); 1466m for (δ(C2−H + C8−N9) + ν(C8−H)); 1410m for δ(N1−C6−H6); 1380m, 1346w for (ν(C5−N7−C8) + δ(CH3)); 1314w, 1271w, 1246w for (ν(N9−C8 + N3−C2) + δ(C−H) + νs(O−C−O)); 1217w, 1192s, 1108w for (δ(C8−H) + ν(N7−C8)); 1059w, 1016m, 1004w for τ(NH2); 965w, 930w, 915w, 904w, 881w for (ν(N1−C6) + τ(NH2)); 814m, 805m, 780m, 760m, 723m for δ(O−C−O); 673m, 664m, 652m, 628w for ring deformation; 580m, 564w, 540w, 441m for ν(M−O + M−N). Anal. Calcd for {[Cu2(μ4-glut)2(9Meade)2]}n (5),. C22H26Cu2N10O8 (685.60 g/mol): C, 38.54; H, 3.82; N, 20.43; Cu, 18.54%. Found: C, 38.78; H, 3.62; N, 20.01; Cu, 18.63%. IR (cm−1, KBr pellet): 3450s for (ν(NH2) + 2δ(NH2)); 3360s, 3320s for ν(C8−H + C2−H + NH2); 3150s for ν(CH3); 1672s for νas(O−C−O); 1623s, 1593s for (ν(C C) + δ(NH2)); 1579s for ν(C4−C5); 1486m for ν(N3−C4−C5); 1459m, 1437m for (δ(C2−H + C8−N9) + ν(C8−H)); 1413s for δ(N1−C6−H6); 1346w, 1322m for (ν(C5−N7−C8) + δ(CH3)); 1302m, 1271m, 1260w for (ν(N9−C8 + N3−C2) + δ(C−H) + νs(O−C−O)); 1233m, 1193w, 1157w for (δ(C8−H) + ν(N7−C8)); 1055m, 1015m, 1004w for τ(NH2); 953w, 911w, 888w, 864w for (ν(N1−C6) + τ(NH2)); 806m, 796m, 751m, 715m for δ(O−C−O); 666m, 639m, 621m for ring deformation; 564w, 534m, 440m for ν(M−O + M−N). Anal. Calcd for {[Cu2(μ4-pime)2(9Meade)2]·2H2 pime}n (6),. C40H58Cu2N10O16 (1062.04 g/mol): C, 45.24; H, 5.51; N, 13.19; Cu, 11.97%. Found: C, 45.56; H, 5.64; N, 12.99; Cu, 11.78%. IR (cm−1, KBr pellet): 3450s for ν(O−H); 3200s for (ν(NH2) + 2δ(NH2)); 1658s for νas(O−C−O); 1608s for (ν(CC) + δ(NH2)); 1580s for ν(C4−C5); 1541 m for ν(N3−C4−C5); 1463m for δ(C2−H + C8−N9); 1430m for ν(C8−H); 1402m for δ(N1−C6−H6); 1341w for ν(C5−N7−C8); 1280w for (ν(N9−C8 + N3−C2) + δ(C−H) + νs(O−C−O)); 1208w, 1151w, 1033w for (δ(C8−H) + ν(N7−C8)); 988w for τ(NH2); 941w, 897w, 852w for (ν(N1−C6) + τ(NH2)); 794w, 741w for δ(O−C−O); 669m, 644m, 625w for ring deformation; 575m, 456w for ν(M−O + M−N). X-ray Diffraction Data Collection and Structure Determination. Diffraction data of single crystals were collected at 293 K (1) and at 100 K (2, 5) on an Oxford Diffraction Xcalibur and at 100 K on

a STOE IPDS (3, 4, 6) diffractometers with graphite-monochromated Mo−Kα radiation (λ = 0.71073 Å). The data reduction was done with the CrysAlis RED10 and X-RED11 programs, respectively. Structures were solved by direct methods using the SIR92 program12 and refined by full-matrix least-squares on F2 including all reflections (SHELXL97).13 All calculations were performed using the WINGX crystallographic software package.14 Crystal data and details of the refinement parameters of the compounds are given in Table 1. During the resolution of compounds 2 and 3, the assignation of an occupation factor of 100% to the carbon atoms C5 and C6, and to the nitrogen atom N6 of the adenine molecule, led to an unusually high thermal displacement. In addition, nearby peaks appeared with high electron density. Thus, the adeninato ligand of compounds 2 and 3 was disordered into two coplanar arrangements with inverted orientation regarding the coordination mode (μ-κN3:κN9/μ-κN9:κN3). This disorder was modeled including the observed peaks in the Fourier difference map as carbon atoms C5B and C6B and nitrogen atom N6B, with free occupation factors common to each subgroup with the condition of a 100% for the sum of both.

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RESULTS AND DISCUSSION Crystal Structures of {[Cu2(μ3-adeninato)2(μ-OOC(CH2)y-COOH)2]·xH2O}n [y = 3, x = 2 (1); y = 4, x = 0 (2); y = 5, x = 0 (3)]. The X-ray crystal structure analysis revealed that the basic building unit of the compounds is a paddle-wheel entity (Figure 3) in which two copper(II) atoms are coordinated to the N3 and N9 nitrogen atoms of two adeninato ligands and to the carboxylate groups belonging to two dicarboxylato ligands, giving a Cu···Cu distance of around 2.9 Å. The square pyramidal N3O2 donor sets around the copper centers (Table 2) are completed by the apical coordination of the imidazole N7 atoms of adjacent paddle-wheel moieties with a Cu···Cu separation across the N9/N7 bridge of 6.002, 6.062, and 6.075 Å, respectively. The cross-linkage of the copper centers through the μ3-N3,N7,N9adeninato ligands (Figure 4) leads to a 4-connected uninodal net with lvt topology and a (42·84) point symbol, using the dinuclear building unit as node and the adenine nucleobase as connector.15 Although the covalent 3D nets of these compounds present the same connectivity, subtle differences are found within the channels generated by the metal-adeninato framework. In compounds 2 and 3, the pendant chain of the dicarboxylato ligand crosses the channels in such a way that its protonated carboxylate group points toward the Watson−Crick face 3328



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Figure 7. Basic building block found in compound 5.

bonding ring. Nevertheless, in compound 1, the shorter chain of the glutaric ligand (y = 3) precludes the above-described interaction, so additional crystallization water molecules have to fill the voids giving rise to a different hydrogen bonding scheme (Figure 5). The Watson−Crick face of the nucleobase establishes hydrogen bonds with the crystallization water molecule through the N1 nitrogen atom and with the O21 oxygen atom of the free carboxylate groups through the N6 amino site which completes its hydrogen bonding interactions by an intramolecular linkage with the O11 coordinated oxygen atom of the glutarato ligand belonging to a neighboring paddle-wheel. The covalent 3D metal-adeninato net of the compounds 1−3 resembles that previously reported for the family of compounds 3D-[Cu 2 (μ 3 -adeninato) 2 (μ2 -OOC(CH 2 ) y CH 3 ) 2 ]·xH 2 O (y from 0 to 5), in which the paddle-wheel entities contain monocarboxylic acids instead of the dicarboxylate anions.16 In this family, the arrangement of the non-hydrogen bonded aliphatic chains and the self-assembling process directed by the metal-adeninato linkages give a 3D system of intersecting cavities which is filled by crystallization water molecules that are engaged themselves and anchored to the inner walls of the pores via hydrogen bonds involving the donor pyrimidinic N1 atom and the acceptor exocyclic N6 amino group of the Watson−Crick face of the adeninato ligands. The removal of the water molecules leads to robust open frameworks with interesting adsorptive properties.16,17 The tunable porosity of these monocarboxylato compounds and the similar structure of herein presented analogous dicarboxylato complexes verify the flexibility and recurrence of the 3D metal-adeninato framework, that is able to adapt itself to admit chains with different features and length without significant structural changes (Figure 6). In fact, the TGA and thermodiffractometric data of compound 1 confirm the stability of the crystal structure after the removal of the crystallization water molecules up to 200 °C (see Supporting Information). Crystal Structures of {[Cu2(μ4-glut)2(3Meade)2]·4H2O}n (4), {[Cu2(μ4-glut)2(9Meade)2]}n (5), and {[Cu2(μ4-pime)2(9Meade)2]·2H2pim}n (6). The basic building block of these compounds also shows a paddle-wheel arrangement, but the blades are only made up of four dicarboxylato ligands coordinated by means of one of their carboxylate groups (Figure 7).

Figure 6. Overlapping of the (a) 3D frameworks (the dicarboxylic ligands have been removed for clarity) and of the (b) dicopper basic building blocks of compounds 1−3. (c) Comparison of the crystal buildings of compound 1 with that corresponding to the analogous monocarboxylic butanoato ligand [Cu2(μ3-adeninato)2(μ2-OOC(CH2)2CH3)2]·xH2O.

(N6H/N1) of the adenine entity from a PW moiety placed at the opposite side of the channel to give a R22(8) hydrogen 3329



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Figure 8. One-dimensional chain in compound 6.

Table 3. Selected Bond Lengths (Å) of Compounds 4−6a Cu1−N7 Cu1−O11 Cu1−O12a Cu1−O21b Cu1−O22c Cu1···Cu1a

4

5

6

2.156(4) 1.975(4) 1.973(4) 1.975(4) 1.990(4) 2.691(2)

2.189(2) 1.962(2) 1.969(2) 1.959(2) 1.977(2) 2.672(1)

2.191(2) 1.958(2) 1.964(2) 1.993(2) 1.981(2) 2.654(1)

in which the distance between the PW-centroids increases [7.76 Å (4) and 7.87 Å (5) for glutarato; 10.38 Å (6) for pimelato] with the increasing length of the aliphatic chain, whereas a similar intradimeric Cu···Cu distance of around 2.7 Å is maintained. The apical position of the square-pyramidal NO4 donor set of each copper atom (Table 3) is occupied by the imidazolic N7 atom of the N3/N9-methylated adenine molecules, which is the most common coordination mode for these ligands due to the steric hindrance of the methyl group. One of the carboxylate groups of the tetratopic bridge is coplanar with the aliphatic backbone, whereas the other one is perpendicular to it. The arrangement of the nucleobases is almost parallel to the Cu···Cu vector (ca. 4° and ca. 3° for compounds 4 and 6, respectively), except for compound 5,

a Symmetry codes: for 4: (a) −x + 2, −y, −z + 3; (b) x − 1, y, z; (c) −x + 3, −y,−z + 3. For 5: (a) −x + 1, −y + 2, −z + 1; (b) x + 1, y, z; (c) −x, −y + 2, −z + 1; for 6: (a) −x + 1, −y + 2, −z; (b) x − 1, y + 1, z; (c) −x + 2, −y + 1, −z.

The second COO group of each dicarboxylato ligand links an adjacent Cu2 core to give a lineal double-stranded chain (Figure 8)

Figure 9. View along the b axis of the crystal packing of compound 5 showing the noncovalent hydrogen bonding (dashed black lines) and the π−π (dotted red lines) interactions. 3330



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Figure 10. View along the a axis of the three-dimensional packing (a) in compound 4 and (b) in compound 5, showing the hydrogen bonding interactions among the Watson−Crick faces of the nucleobases.

that is coordinated to the metal center. These interactions lead to lamellar structures formed by layers of polymeric chains in both compounds (Figure 10). In the crystal structure of compound 4, tetrameric aggregates of water molecules are inserted between the layers and are attached to them by means of hydrogen bonding interactions with the imidazolic N9 atom of the nucleobase and the O12 atom of the glutarato ligand (see Supporting Information). In contrast, compound 5 is anhydrous and the packing among the layers appears to occur through weak van der Waals interactions involving the

where the mean plane of the 9-methyladenine ligand deviates ca. 15° from that vector. The polymeric chains of compounds 4 and 5 are cross-linked by R22(8) hydrogen bonding rings between the Watson−Crick faces of two nucleobases belonging to adjacent chains (a double N6···N1 interaction) and by offset face to face interactions between the aromatic rings of the nucleobases which are almost parallel stacked (Figure 9). Additionally, the remaining hydrogen atom of the exocyclic N6 group forms an intrachain hydrogen bond with an oxygen atom of the glutarato ligand 3331



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compounds 4−5, the inclusion of the pimelic aggregates precludes the direct double hydrogen bonding interaction between the Watson−Crick faces and the presence of π−π interactions among the nucleobases. However, these inclusion molecules act as double-sided adhesive tapes between the polymeric chains to give a lamellar structure in which the pimelic molecules are anchored by means of a R22(8) hydrogen bonding ring established between the carboxylic group and the Watson−Crick face of a 9-methyladenine nucleobase, a usual supramolecular recognition pattern found in both artificial18 and biological adenine-carboxylato systems.19 Finally, as it can be observed in Figure 11, the layers are linked through a R44(10) hydrogen bonding ring that also involves the pimelic acid molecules and the nucleobase (O41−H41···N1−C2− H···O41−H41···N1). Magnetic Properties. The thermal evolution of the molar magnetic susceptibility (χM) and the χMT product per copper(II) atom for compounds 1−6 are shown in the Supporting Information. In all cases, the experimental data reveal the presence of overall strong antiferromagnetic interactions. The magnetic behavior of compounds 1−3 is dominated by the presence of two magnetic exchange pathways: the interaction involving the dimeric core (the resulting from two Cu1−N3−C4−N9−Cu1b and two Cu1− O11−C11−O12−Cu1b bridges) and a pathway through the imidazole bridge that links together the dimeric entities via the axial positions (Cu1−N9−C8−N7−Cu1c). Taking into account that the Cu···Cu separation across this last bridge (>6.0 Å) is substantially longer than that involving the dimeric core (

Directing the Formation of Adenine Coordination Polymers from

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