Supramolecular Architectures and Magnetic Behavior of Coordination

CRYSTAL GROWTH & DESIGN

Supramolecular Architectures and Magnetic Behavior of Coordination Polymers from Copper(II) Carboxylates and 1,2-Bis(4-pyridyl)ethane as a Flexible Bridging Ligand

2007 VOL. 7, NO. 6 1069-1077

Rosa Carballo,*,† Berta Covelo,*,†,‡ M. Salah El Fallah,§ Joan Ribas,§ and Ezequiel M. Va´zquez-Lo´pez† Departamento de Quı´mica Inorga´ nica, Facultade de Quı´mica and Unidade Determinacio´ n Estructural e Proteo´ mica, CACTI, UniVersidade de Vigo, 36310-Vigo, Spain and Departament de Quı´mica Inorga` nica, Facultat de Quı´mica, UniVersitat de Barcelona, 08028-Barcelona, Spain ReceiVed September 15, 2006; ReVised Manuscript ReceiVed March 1, 2007

ABSTRACT: The combination of the flexible bpetha ligand with different copper(II) carboxylates in a 1:1 molar ratio leads to the formation by self-assembly of three new one-dimensional (1D) polymeric compounds: {[Cu(L1)2(bpetha)(H2O)]‚2H2O}n (1), {[Cu(L2)2(bpetha)]‚5H2O}n (2), and [Cu(L3)2(bpetha)]n (3), where bpetha ) 1,2-bis(4-pyridyl)ethane, L1 ) acetato, L2 ) lactato, and L3 ) 2-methyllactato. These compounds were characterized by elemental analysis, infrared and electronic absorption spectroscopy, and single-crystal X-ray diffraction. X-ray structural analysis revealed that the polymeric compounds created by the bridging bpetha ligand present different 1D structural motifs: linear chains for 1, zigzag chains for 2, and ladderlike chains for 3. Weak interactions such as hydrogen bonding, π‚‚‚π stacking and/or C-H‚‚‚π interactions join the polymeric chains to generate three-dimensional networks. Variable temperature magnetic susceptibility measurements reveal very weak antiferromagnetic coupling between the copper(II) centers in the three compounds. Introduction The rational design of solid-state architectures constructed from metal-based coordination has recently become an area of increasing interest owing to their novel and diverse topologies and potential applications in the fields of molecular absorption, catalysis, host-guest chemistry, electrical conductivity, and molecular magnetic materials.1-7 Self-assembly based on molecules has emerged as an attractive approach for the fabrication of these new materials, and considerable effort has been directed toward supramolecular networks assembled by covalent and hydrogen bonds or other molecular interactions.8-12 Remarkable progress has been made in this new field of chemistry and materials science, but it is still difficult to prepare metal-organic frameworks with predictable topologies,13 although some fundamental aspects of coordination chemistry, such as the nature, oxidation state, and coordination preference of the metal center or the relative flexibility of the organic linker, can nonetheless be utilized to direct the product architecture. A very common strategy to design and construct desirable frameworks is the selection and employment of mutidentate N- or O-donor ligands, flexible dipyridyl ligands with certain spacers, such as 1,2-bis(4-pyridyl)ethane (bpetha) and its analogues, that are particularly good candidates to produce unique structural motifs with beautiful aesthetics and useful functional properties owing to their flexibility and conformational freedom.14-20 Another common feature of this kind of metal framework is the tendency to contain crystallization water molecules as isolated units or as aggregates21-24 that fill the empty voids or channels generated in the crystal packing. We report here the preparation by self-assembly of three new mixed-ligand polymers of copper(II) in which one component is the flexible spacer bpetha and the other a carboxylato ligand, L1 ) acetato, L2 ) lactato, or L3 ) 2-methyllactato (Scheme * To whom correspondence should be addressed. E-mail: [email protected] (R.C.); [email protected] (B.C.). † Departamento de Quı´mica Inorga ´ nica, Facultade de Quı´mica. ‡ Unidade Determinacio ´ n Estructural e Proteo´mica, CACTI. § Universitat de Barcelona.

Scheme 1

Scheme 2

1). In a previous work, we studied the supramolecular structures of copper(II) carboxylates mixed-complexes with no bridging N-donors ligands.25-29 The structures of each of these new polymers present different one-dimensional (1D) structural motifs: linear chains for 1, zigzag chains for 2, and ladderlike chains for 3 (Scheme 2). The compounds were characterized by elemental analysis, infrared (IR) and electronic absorption spectroscopy, single-crystal X-ray diffraction, and variable

10.1021/cg060616l CCC: $37.00 © 2007 American Chemical Society Published on Web 04/19/2007

1070 Crystal Growth & Design, Vol. 7, No. 6, 2007

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Table 1. Crystal Data and Structure Refinement Details for 1-3

empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) volume (Å3) Z, Fcalc (g.cm-3) F(000) crystal size (mm3) absorption coefficient (mm-1) θ range (°) maximum/minimum transmission reflections collected independent reflections (Rint) final R indices [I > 2σ(I)] final R indices (all data)

1

2

3

C16H24N2O7Cu 419.91 triclinic P1h 8.439(2) 9.908(3) 12.070(3) 79.046(5) 79.766(5) 70.424(6) 926.4(4) 2, 1.505 438 0.19 × 0.11 × 0.10 1.219 1.73-28.00 1.0000-0.8844 5825 4045 (0.0247) R1 ) 0.0397, wR2 ) 0.0856 R1 ) 0.0525, wR2 ) 0.0905



temperature magnetic measurements. The different water cluster patterns [(H2O)3 and (H2O)10 for compounds 1 and 2, respectively] and their role in the cohesiveness of the resulting supramolecular architectures are analyzed. Experimental Section Materials and Methods. All reagents and solvents were obtained commercially: (()-lactic acid from Probus, 2-methyllactic acid from Fluka, 1,2-bis(4-pyridyl)ethane from Aldrich, and Cu(AcO)2‚H2O from Fluka. All were used as supplied. Microanalyses (C, H, N) were carried out with a Fisons EA-1108 elemental analyzer. Melting points (mp) were determined with a Gallenkamp MBF-595 apparatus. Fourier transform infrared spectra were recorded from KBr pellets (4000-400 cm-1) or polyethylenesandwiched Nujol mulls (500-100 cm-1) on Bruker Vector 22 and Bruker IFS66v spectrophotometers, respectively. A Shimadzu UV3101PC spectrophotometer was used to obtain the electronic spectra (diffuse reflectance) in the region 250-900 nm. Magnetic susceptibility measurements for all the compounds were determined on polycrystalline samples at the Servei de Magnetoquı´mica of the Universitat de Barcelona with a Quantum Design SQUID MPMS-XL susceptometer working in the range 2-300 K under a magnetic field of approximately 10 000 G. Diamagnetic corrections were estimated from Pascal Tables. The electron paramagnetic resonance (EPR) spectra were recorded on an X-band Bruker Spectrometer (ESR 300E) working with an Oxford liquid helium cryostat for variable temperature. Synthesis of the precursors [Cu(L2)2(H2O)]‚H2O and [Cu(L3)2(H2O)2]. The precursors [Cu(L2)2(H2O)]‚H2O and [Cu(L3)2(H2O)2] were obtained by the reaction of Cu(AcO)2‚H2O and the corresponding R-hydroxycarboxylic acid (HL2 lactic or HL3 2-methyllactic acid) in 1:2 molar proportions in water. The resulting blue solutions were heated for 15 min and stirred at room temperature for several days. Blue crystalline products were obtained by slow concentration of the solution. These compounds were previously studied by Prout et al.30 Synthesis of {[Cu(L1)2(bpetha)(H2O)]‚2H2O}n (1). A colorless solution of 1,2-bis(4-pyridyl)ethane (46 mg, 0.25 mmol) in ethanol (2 mL) was slowly added to a blue solution of Cu(AcO)2‚H2O (50 mg, 0.25 mmol) in water (3 mL) in a test tube. The reagents were allowed to mix slowly by diffusion to form blue crystals. Yield: 81%. mp 216 °C. Anal. Calcd. for C16H24CuN2O7: C, 45.76; H, 5.76; N, 6.67%. Found: C, 45.59; H, 5.88; N, 6.64%. IR (KBr, cm-1): 3446 (vs), 1627 (sh), 1608 (vs), 1500 (m), 1430 (s), 1050 (w), 836 (m), 425 (w), 320 (w). UV-vis (λmax, nm): 650. Synthesis of {[Cu(L2)2(bpetha)]‚5H2O}n (2). Compound 2 was synthesized using a procedure similar to that for 1 except that [Cu(L2)2(H2O)]‚H2O (70 mg, 0.25 mmol) was used instead of Cu(AcO)2‚ H2O. Yield: 27%. mp >250 °C. Anal. Calcd. for C18H32CuN2O11: C, 41.90; H, 6.25; N, 5.43%. Found: C, 41.64; H, 5.98; N, 5.60%. IR

(KBr, cm-1): 3412 (m,b), 1614 (vs), 1506 (m), 1455 (m), 1336 (m), 1071 (w), 1032 (m), 813 (m), 431 (w), 312 (w). UV-vis (λmax, nm): 680. Synthesis of [Cu(L3)2(bpetha)]n (3). Compound 3 was synthesized using a procedure similar to that for 1 and 2 but in this case [Cu(L3)2(H2O)2] (61 mg, 0.20 mmol) was used. Yield: 25%. mp 240 °C. Anal. Calcd. for C20H26CuN2O6: C, 52.91; H, 5.77; N, 6.17%. Found: C, 52.21; H, 5.18; N, 6.18%. IR (KBr, cm-1): 3458 (m); 1650 (s), 1604 (vs), 1507 (w), 1431 (m), 1391 (m), 1188 (s), 1030 (m), 818 (m), 432 (w), 296 (w). UV-vis (λmax, nm): 670. X-ray Diffraction. Crystallographic data were collected on a Bruker Smart 1000 charged-coupled device diffractometer at CACTI (Universidade de Vigo) at 20 °C using graphite monochromated Mo KR radiation (λ ) 0.71073 Å), and were corrected for Lorentz and polarization effects. The frames were integrated with the Bruker SAINT31 software package and the data were corrected for absorption using the program SADABS.32 The structures were solved by direct methods using the program SHELXS97.33 All non-hydrogen atoms were refined with anisotropic thermal parameters by full-matrix least-squares calculations on F2 using the program SHELXL97.34 Hydrogen atoms were inserted at calculated positions and constrained with isotropic thermal parameters except for the hydrogen atoms of the hydroxyl group and of water molecules, which were located from a Fourier-difference map and refined isotropically (in some cases the O-H distances were restrained with DFIX 0.9). Because of the high degree of hydration and thermal motion, hydrogen atoms could not be located for the crystallization water molecules in 2; hydrogen bonding was inferred by considering the O‚‚‚O close contacts of water molecules involved (O‚‚‚O distance less than the sum of van der Waals radii 3.04 Å22). Drawings were produced with SCHAKAL9935 and MERCURY.36 Special computations for the crystal structure discussions were carried out with PLATON.37 Crystal data and structure refinement parameters are summarized in Table 1. Selected bond distances and angles are listed in Table 2. Hydrogen-bond distances and angles are listed in Table 3. The structural data have been deposited with the Cambridge Crystallographic Data Centre (CCDC) with reference numbers 615582, 615583, and 615584 for compounds 1, 2, and 3, respectively.

Results and Discussion Selected interatomic distances and angles, as well as the hydrogen bonds, for 1-3 are listed in Tables 2 and 3. Drawings of the coordination environments of the copper atoms are shown in Figures 1a-c together with the atom numbering schemes used. The structure of 1 is based on polymeric neutral complexes [Cu(L1)2(bpetha)(H2O)], where L1 is a monodentate acetate ligand, and crystallization water molecules. In compound 1, the copper atom has an almost perfect square pyramidal environ-

Copper(II) Carboxylates and 1,2-Bis(4-pyridyl)ethane

Crystal Growth & Design, Vol. 7, No. 6, 2007 1071

Table 2. Selected Bond Lengths (Å) and Angles (°) for 1-3a bond length (Å)

angles (°)

1 Cu-O11 Cu-N1 Cu-O1 O11-Cu-O21 O21-Cu-N1 O21-Cu-N2 O11-Cu-O1 N1-Cu-O1

1.9697(19) 2.023(2) 2.346(2) 176.49(8) 90.30(8) 89.05(8) 90.66(8) 93.55(9)

Cu-O21 Cu-N2

1.9708(18) 2.029(2)

O11-Cu-N1 O11-Cu-N2 N1-Cu-N2 O21-Cu-O1 N2-Cu-O1

92.18(8) 88.31(8) 176.35(9) 91.66(8) 90.06(8)

2 Cu1-O11 Cu1-O13 Cu1-N1 O11-Cu1-O11i O11-Cu1-N1 O11-Cu1-N1i N1-Cu1-N1i O11-Cu1-O13i O11-Cu1-O13 N1-Cu1-O13i N1-Cu1-O13 O13-Cu1-O13i

1.980(5) 2.355(5) 2.020(5) 180.0 91.0(2) 89.0(2) 180.0 102.6(2) 77.4(2) 95.4(2) 84.6(2) 180.0

Cu2-O21 Cu2-O23 Cu2-N2 O21-Cu2-O21ii O21-Cu2-N2 O21-Cu2-N2ii N2-Cu2-N2ii O21-Cu2-O23ii O21-Cu2-O23 N2-Cu2-O23ii N2-Cu2-O23 O23-Cu2-O23ii

1.958(5) 2.349(5) 2.037(6) 180.0 89.5(2) 90.5(2) 180.0 104.2(2) 75.8(2) 90.3(2) 89.7(2) 180.0

3 Cu-O11 Cu-N1 Cu-O12iii O11-Cu-O21 O11-Cu-N2 O21-Cu-N2 O11-Cu-O12iii N2-Cu-O12iii

1.953(2) 2.038(2) 2.257(2) 170.23(8) 91.17(9) 89.21(9) 105.68(9) 97.32(9)

Cu-O21 Cu-N2

1.974(2) 2.029(2)

O11-Cu-N1 O21-Cu-N1 N2-Cu-N1 O21-Cu-O12iii N1-Cu-O12iii

88.33(9) 89.94(9) 172.08(10) 83.95(8) 90.43(9)

a Symmetry codes: i ) -x + 2, -y + 1, -z; ii ) -x + 1, -y, -z + 1; iii ) -x + 2, -y + 1, -z + 1.

Table 3. Parameters [Å, °] of Hydrogen Bond Interactions in 1-3a d(D-H)

d(H‚‚‚A)

d(D‚‚‚A)

∠(DHA)

1

O1-H11‚‚‚O21i O1-H12···O2w O2w-H2a···O1w O2w-H2b···O22ii O1w-H1b···O12iii

0.71(3) 0.88(4) 0.70(4) 0.86(4) 0.77(5)

2.11(3) 1.91(4) 2.17(4) 1.95(4) 2.02(5)

2.812(3) 2.792(3) 2.866(4) 2.810(4) 2.786(4)

169(3) 178(4) 174(4) 176(4) 177(5)

2

O13-H13···O22v O23-H23···O1w O1w···O3wi O1w···O12 O2w···O3w O2w···O4w O4w···O12 O4w···O4wiv O4w···O5w O4w···O5wiv

0.89(2) 0.91(2) -

1.77(3) 1.80(3) -

2.633(8) 2.685(9) 2.745(10) 2.738(7) 2.692(12) 2.681(15) 2.742(11) 2.93(2) 3.001(19) 3.03(2)

165(6) 161(7) -

3

O23-H23···O22 O13-H13‚‚‚O12vi O13-H13‚‚‚O12 C104-H104‚‚‚O13ii C107-H107‚‚‚O23vii

0.88(2) 0.82 0.82 0.93 0.93

1.93(6) 2.18 2.22 2.51 2.56

2.598(4) 2.598(4) 2.626(3) 3.408(4) 3.177(4)

131(7) 168.7 110.7 161.7 124.4

D-H‚‚‚A

a Symmetry codes: i ) -x + 1, -y + 1, -z + 1; ii ) x, y - 1, z; iii ) x + 1, y - 1, z; iv ) -x + 1, -y + 1, -z; v ) x + 1, y + 1, z; vi ) -x + 2, -y + 1, -z + 1; vii: -x + 1, -y + 1, -z + 2.

ment (Addison’s parameter τ38 is 0.002); two nitrogen atoms from two mutually trans-bpetha ligands occupy the basal positions around the copper(II) along with two oxygen atoms from the monodentate acetato ligands, and one water molecule occupies the apical position (Figure 1a). Recently, the study of another 1D polymer formed by the same tectons [copper(II) acetate and bpetha] has been reported but in this case the classic paddle-wheel “copper acetate” core is present.39,40 The copper atom in 1 is only slightly shifted from the N2O2 basal plane, being displaced by 0.0525 Å toward the axially coordinated water molecule. The Cu-O and Cu-N distances in the basal plane are very similar to those found in the polymeric compounds of copper(II) with 4,4′-bipyridine41,42 or bpetha39,40

Figure 1. Views of the copper atom environments with the corresponding labeling scheme (a) {[Cu(L1)2(bpetha)(H2O)]‚2H2O}n (1), (b) {[Cu(L2)2(bpetha)]‚5H2O}n (2), and (c) [Cu(L3)2(bpetha)]n (3); in this case the hydrogen atoms have been omitted for clarity. Ellipsoids are drawn at 30% probability level.

and acetato. The apical distance Cu-O1 [2.346(2) Å] is much longer than the basal Cu-O distances [Cu-O11, 1.9697(19) and Cu-O21 1.9708(18) Å]. The bpetha ligand bridges neighboring copper(II) ions to form polymeric linear chains due to the anti conformation of the ligand with a C-CH2-CH2-C


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Figure 2. Views of the different 1D frameworks of (a) {[Cu(L1)2(bpetha)(H2O)]‚2H2O}n (1), (b) {[Cu(L2)2(bpetha)]‚5H2O}n (2), and (c) [Cu(L3)2(bpetha)]n (3). Crystallization water molecules (in 1 and 2) and the hydrogen atoms (in 3) have been omitted for clarity.

dihedral angle of 178.37° (Figure 2a). However, in the related copper(II) acetato complex with 4,4′-bipyridine the monodentate bridging behavior of the acetato ligand creates a polymeric structure with a ladder motif.41,42 The pyridyl rings of the bpetha ligand form an angle of 38.12°. The Cu‚‚‚Cu distance across the bpetha ligand is 13.4427(26) Å. The linear chains are linked into a two-dimensional (2D) network through weak C-H‚‚‚π interactions43 (Figure 3a) involving methyl groups of the acetato ligands and pyridyl bpetha rings of neighboring chains (C‚‚‚centroid distance range 3.5-3.8 Å). The layers, which are parallel to the ac plane, generate a rectangular grid motif with two guest water molecules in each pore of dimensions 7 × 10 Å (Figure 3b). Different hydrogen bonds involving the crystallization water molecules, the coordinated water molecule, and the acetato ligand (Table 3) hold the layers in a three-dimensional (3D) network (Figure 3c). Every Cu(II) ion is bound to an acyclic water trimer (Figure 3d) with an average Ow‚‚‚Ow separation of 2.83 Å and an O-O-O angle of 128°. In this way, the water cluster is trapped in the host not only by hydrogen bonds but also by coordination interactions. Each O-atom of this trimer is hydrogen-bonded to the nearest carboxylate oxygens with an average Ow‚‚‚O distance of 2.80 Å. The structure of 2 consists of polymeric neutral complexes [Cu(L2)2(bpetha)], where L2 is a monoanionic lactato ligand, and solvated water molecules. There are two crystallographically independent copper atoms lying on inversion centers but these have very similar environments (Figure 1b, Table 2). The

monoanionic lactato ligands act as bidentate chelate systems through one carboxylate oxygen and the hydroxyl oxygen to form a five-membered ring. The coordinative behavior of lactato is the same as that found in the polymeric compound of copper(II) with 4,4′-bipyridine and lactato.44 The coordination geometry around each copper center is elongated octahedral with the two bpetha nitrogen atoms and two carboxylate oxygens in the equatorial plane and hydroxyl oxygens in the apical positions. Each pair of ligands is mutually trans. The Cu-O and Cu-N distances are very close to those found in the analogous compound with 4,4′-bipyridine.44 The bpetha ligand bridges between neighboring copper(II) atoms to form zigzag polymeric chains due to the gauche conformation of this ligand (Figure 2b) with a C-CH2-CH2-C dihedral angle of 70.78°. This situation is hindered in the related 4,4′-bipyridine polymer due to the rigid nature of the spacer ligand.44 The intrachain Cu‚‚ ‚Cu distance is shorter [9.029(2) Å] than the one found in related copper(II)-4,4′-bipyridine complex as a result of the folded gauche conformation adopted by the bpetha ligand. The pyridyl rings of the bpetha ligand form an angle of 53.33° in this case. Hydrogen bonds between one hydroxyl oxygen (O13) and a noncoordinated carboxy oxygen (O22) from a neighboring chain join the chains to form a 2D network with a pleated-sheet type structure (Table 3, Figure 4a,d). The π‚‚‚π interactions45 between pyridine rings (inter-ring distance range 3.47-3.67 Å) of the bpetha ligands join antiparallel sheets (Figure 4b,c) to create a 3D open framework with channels along the c-axis direction (10 × 18 Å). These channels are occupied by crystallization



Figure 4. (a) View of the 2D structure by hydrogen bonds in 2. (b,c) Different views of π‚‚‚π interactions between chains in 2. (d) 3D open framework with the channels along c-axis occupied by crystallization water molecules in 2. (e) View of (H2O)10 water cluster in 2.

Figure 3. (a) View of the 2D structure by C-H‚‚‚π interactions in 1. (b) Crystallization water molecules inside pores of the 2D network in 1. (c) View of the 3D hydrogen bond network in 1. (d) View of the acyclic water trimer in 1.

water molecules (Figure 4d, Table 3) interconnected by hydrogen bonds to build a (H2O)10 water cluster. In the decamer (H2O)10 (Figure 4e), four water molecules (O4w, O5w, O4wiv and O5wiv) form a cyclic coplanar water tetramer that is further hydrogen-bonded by two chains of three water molecules connected to each O4w molecule. The average Ow...Ow distance

is 2.85 Å. This water cluster pattern is similar to the decamer observed in the polymeric host [Cu2(BDC)2(L)4(H2O)2]‚14H2O46 (BDC: 1,4-benzenedicarboxylate), but in 2 the average Ow‚‚ ‚Ow distance in the tetramer (2.99 Å) is longer than the corresponding value (2.84 Å) in the coordination polymer containing BDC. The (H2O)10 cluster is stabilized by hydrogen bonding to the oxygen atoms of the lactato ligands. The structure of 3 consists of chains of centrosymmetric copper(II) dimers in which one methyllactato ligand bridges between two copper(II) atoms through the two oxygen atoms of the carboxylate group with the configuration of this group being syn-anti (Figures 1c and 2c) and the Cu‚‚‚Cu distance 4.6062(9) Å. The coordination polyhedron around the copper(II) ion is an almost perfect square pyramid (Addison’s parameter τ38 is 0.031) in which one bridging carboxylate oxygen atom is in the apical position, and in the base there are


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Figure 5. (a) View of intrachain O-H‚‚‚O hydrogen bonds in 3. (b,c) Views of the different interchain C-H‚‚‚O hydrogen bonds in 3. (d) 3D hydrogen bond network of 3. Some hydrogen atoms have been omitted for clarity.

two mutually trans nitrogen atoms from two bpetha ligands and two carboxylate oxygen atoms, one from the monodentate methyllactato ligand and the other from the bridging methyllactato ligands. In this case, the copper atom is slightly shifted from the N2O2 basal plane and is displaced by 0.1533 Å toward the axially coordinated carboxylate oxygen. The Cu-O and Cu-N distances in the basal plane are very similar to those found in the analogous polymeric copper(II) compounds with 4,4′-bipyridine.44 The apical distance Cu-O12i [2.257(2) Å, i: -x + 2, -y + 1, -z + 1] is longer than the basal Cu-O distances [Cu-O11, 1.953(2) Å and Cu-O21 1.974(2) Å]. Bpetha ligands link adjacent dinuclear cores together to form polymeric ladderlike chains. The conformation of this ligand is anti with a C-CH2-CH2-C dihedral angle of 177.05°, and the pyridyl rings are nearly perpendicular to each other (angle of 83.97°, Figure 2c) although in the related copper(II) methyllactato complex with 4,4′-bipyridine the bidentate-chelate behavior of the methyllactato ligand creates a linear polymeric structure. The Cu‚‚‚Cu distance across the bpetha ligand is 13.3753(14) Å (i.e., similar to that found in 1).

All of the classical hydrogen bonds (O-H‚‚‚O, Figure 5a and Table 3) found in 3 are intrachain and strengthen the ladderlike structure obtained by coordination bonds (Figure 2c). Evidence for π‚‚‚π stacking and/or C-H‚‚‚π interactions was not found, either intra- or interchain. The chains are only linked by weak C-H‚‚‚O hydrogen bonds involving bpetha C-H groups as donors and methyllactato hydroxyl groups as acceptors (Figure 5b,c, Table 3) generating a 3D network (Figure 5d). The structure of 3 has a Kitaigorodskii packing index47 of 66.2%, a figure higher than the usual value of around 65%, indicating the absence of voids or channels in the crystal to accommodate water molecules. When the same index is calculated for 1 and 2 without the water molecules, lower values are obtained (65.4 and 55.5% for 1 and 2, respectively). Magnetic Properties. The global feature of the χMT versus T curves for 1 and 2 is characteristic of very weak antiferromagnetic interaction. The value of χMT at 300 K is 0.446 cm3 mol-1 K for 1, which is as expected for one uncoupled copper(II) ion with g ) 2.18, and 0.403 cm3 mol-1 K for 2, which is as expected for one uncoupled copper(II) ion with g ) 2.07.



Figure 6. Plot of observed χMT product versus T and M/Nβ versus H (inset) for complex 1 (per one Cu).

Figure 7. Plot of observed χMT product versus T and M/Nβ versus H (inset) for complex 3 (per two Cu).

The χMT values are practically constant over a wide temperature range, but decrease to 0.400 cm3 mol-1 K for 1 and 0.316 cm3 mol-1 K for 2 at very low temperature (2 K). The shape of these curves from 300 to 20 or 25 K for 1 and 2, respectively, is typical for the Curie law for S ) 1/2. The plot of the magnetic data for 1 (as a representative example of the two very similar complexes) is shown in Figure 6. As previously commented, the structures of 1 and 2 consist of copper ions linked together by a bpetha ligand, giving 1D systems in which the distance between the Cu(II) atoms through the bpetha ligand is very long (13.4427 and 9.029 Å in 1 and 2, respectively) without any possibility of noticeable magnetic pathways. Thus, from the magnetic point of view, the Cu(II) ions are quasi-isolated. Considering this situation, and in order to quantify the magnetic behavior, the experimental data were fitted to expression 148

χM ) Ng2β 2S(S + 1)/3kT - zJS(S + 1)

(1)

where

θ ) zJS(S + 1)/3k The fit to the experimental data gave the parameters θ ) -0.54 cm-1 and g ) 2.08 for 1 and θ ) -0.23 cm-1 and g ) 2.18 for 2, indicating, as expected, a very weak coupling. This very weak exchange can be attributed either to intramolecular or intermolecular interactions, such as hydrogen bonds present in the two complexes. The field dependence of the reduced magnetization (M/Nβ) (0-5.0 T) versus H measured at 2 K for complexes 1 and 2 tends to a value slightly greater than 1.0 Nβ (1 electron), which is the expected value for an ion with S ) 1/2 and g > 2.00 (g value for the free-electron). The χMT value for 3 at 300 K is 0.846 cm3 mol-1 K for two copper(II) ions, which is as expected for two isolated copper(II) ions with g ) 2.12. The χMT values are almost constant until ca. 35 K, and then they decrease sharply, giving the minimum value of 0.467 cm3 K mol-1 at 2 K (Figure 7). The drop in χMT at low temperatures indicates the presence of a very weak antiferromagnetic coupling between the copper(II) ions. As shown in Figure 2c, the structure of 3 consists of copper ions linked by bpetha and 2-methyllactato (L3) ligands, giving a system with a ladderlike chain motif. Thus, two coupling parameters (J1, J2) must be considered to interpret the magnetic interaction in the complex. J1 corresponds to the double axialequatorial Cu-L3-Cu, and J2 to the Cu-bpetha-Cu bridges,

Figure 8. Schematic diagram representing the exchange interactions within complex 3 (top). The dimer entity in 3, showing the Cu(II) environment (bottom).

respectively (Figure 8). Once again, the distance between the copper(II) atoms in the fragments Cu-bpetha-Cu is very long (13.375 Å) and, as considered from the experimental data for 1 and 2, the ligand does not couple the copper(II) ions, while the distance between the copper(II) atoms in the Cu-L3-Cu fragments is shorter (4.606 Å). Consequently, we assume that in 3 the very weak magnetic coupling observed is mainly propagated by the double axial-equatorial Cu-L3-Cu (Figure 8), which reduces the system to dinuclear units that are magnetically isolated. On the basis of this hypothesis, the magnetic susceptibility can be fitted with the Bleaney-Bowers49 eq 2 for a couple of S ) 1/2 spin

χM ) (2Ng2µB2/KT)[3 + exp(-J/KT)]-1

(2)

The parameters N, µB and K in the equation have their usual meaning. The best fit parameters from 300 down to 2 K were found to be J ) -2.3 cm-1 and g ) 2.11 with an error R ) 4.6 × 10-5 where R ) Σ[(χMT)exp - (χMT)calc]2/Σ[(χMT)exp].2 The very low value of the exchange parameter can be related to the poor overlap between the x2 - y2 (short distance: 1.954 Å) and z2 (long distance: 2.257 Å) orbitals that intervene in the magnetic pathway.


The magnetization measurements at 2 K up to an external field of 5.0 T confirm the very weak antiferromagnetic interaction. At higher field, the reduced magnetization in M/Nβ units tends to a value greater than 2.0 without saturation. The shape of the curve is practically the same as the curve obtained from the Brillouin function for two quasi-isolated ions with S ) 1/2 at 2 K. The very small differences seen in the curves are due to the very small antiferromagnetic coupling. It must be considered than the g value for a copper(II) ion is greater than 2.00. The EPR spectra of 1, 2, and 3 recorded in the X band at variable temperature are also very similar (Figure S1, Supporting Information). Considering than the three complexes do not have any noticeable magnetic coupling, it is expected the EPR spectra of quasi-isolated copper(II) ions. Indeed, in all the spectra two bands are observed that correspond to the transition ∆MS ) (1 with its two components, g|| and g⊥. The bands are located at g|| ) 2.27 and g⊥ ) 2.07 (3076 and 3377 G for ν ) 9.7881 GHz) for 1, at g|| ) 2.29 and g⊥ ) 2.08 (3044 and 3353 G for ν ) 9.7880 GHz) for 2 and at g|| ) 2.21 and g⊥ ) 2.07 (3151 and 3377 G for ν ) 9.78755 GHz) for 3. The observed g|| > g⊥ > 2.00 in the three complexes agree with the coordination environment of them: square planar or elongated octahedral. No hyperfine splitting has been observed in the EPR spectra. Conclusions Three new coordination polymers constructed by selfassembly of the bpetha ligand and copper(II) carboxylates have been structurally characterized. The bpetha ligand adopts different conformations in the three compounds, anti with the pyridyl rings almost coplanar in 1, gauche in 2, and anti with the pyridyl rings perpendicular to each other in 3. This structural analysis revealed that supramolecular interactions may play an important role in the observed 1D structural motif, which is different in the three cases despite the use of similar tectons (linear chains for 1, zigzag chains for 2 and ladderlike chains for 3). The bulkiness of the carboxylato ligands has an influence on the established supramolecular interactions and therefore on the final supramolecular architecture. In the three compounds, the weak interactions, such as hydrogen bonding, π‚‚‚π stacking, and/or C-H‚‚‚π interactions, join the polymeric chains to generate 3D networks. Variable temperature magnetic susceptibility measurements reveal very weak antiferromagnetic coupling between the copper(II) centers in the three compounds. Acknowledgment. Financial support from ERDF (EU) and DGI-MCYT (Spain) (research projects CTQ2006-03949, CTQ2006-01759, CTQ2006-05642/BQU, and PGIDIT06PXIB314373PR) is gratefully acknowledged. We thank Professor A. Castin˜eiras (University Santiago de Compostela, Spain) for facilities in the diffuse reflectance measurements, and Luis Lopes (http://www.l2.pt.to/) for the design of Scheme 2. Supporting Information Available: Crystallographic information in CIF format, and EPR spectra of 1, 2, and 3. This material is available free of charge via Internet at http://pubs.acs.org.

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CG060616L

Supramolecular Architectures and Magnetic Behavior of Coordination

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