Synthesis, Crystal Structures, and Properties of Molecular Squares

Apr 7, 2009 - Hitoshi Kumagai,*,†,¶ Motoko Akita-Tanaka,†,# Satoshi Kawata,‡ Katsuya Inoue,†,#. Cameron J. Kepert,§ and Mohamedally Kurmoo*,...
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Synthesis, Crystal Structures, and Properties of Molecular Squares Displaying Hydrogen and π-π Bonded Networks Hitoshi Kumagai,*,†,¶ Motoko Akita-Tanaka,†,# Satoshi Kawata,‡ Katsuya Inoue,†,# Cameron J. Kepert,§ and Mohamedally Kurmoo*,|

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 6 2734–2741

Institute for Molecular Science, Nishigounaka 38, Myoudaiji, Okazaki 444-8585, Japan, Department of Chemistry, Graduate School of Science, Osaka UniVersity, Toyonaka, Osaka 560-0043, Japan, School of Chemistry, UniVersity of Sydney, NSW 2006, Australia, and Laboratoire de Chimie de Coordination Organique, UMR7140-CNRS, UniVersite´ de Strasbourg, Institut Le Bel, 4 rue Blaise Pascal, 67000 Strasbourg Cedex, France ReceiVed December 17, 2008; ReVised Manuscript ReceiVed March 11, 2009

ABSTRACT: We report the hydrothermal syntheses, single-crystal structure determinations, and the thermal and magnetic properties of five binuclear complexes obtained from either a mixture of P and M 2,2′-biphenyldicarboxylate, (2,2′-bpdc)2-, or a mixture of cis- and trans-1,4-cyclohexanedicarboxylate, (1,4-chdc)2-, as bridging ligands, di-2-pyridylamine (dpa) as a capping terminal ligand, and a divalent transition metal (Cu, Co, and Ni). All five compounds, [Co2(2,2′-bpdc)2(dpa)2] (1), [Cu2(2,2′-bpdc)2(dpa)2] · 4H2O (2a), [Cu2(2,2′-bpdc)2(dpa)2] · 2(2,2′-bpdcH2) · H2O (2b), [Co2(cis-chdc)2(dpa)2] (3), and [Ni2(cis-chdc)2(dpa)2] (4), consist of neutral molecular squares of two metal centers bridged by two elbowed dicarboxylate, and each metal is capped by the chelating di-2pyridylamine. In 1, the Co(II) adopts a tetrahedral coordination from two oxygen atoms of monodentate 2,2′-bpdc2- and two nitrogen atoms from dpa. In 2a and 2b, the Cu(II) adopts distorted octahedral geometry from two bidentate 2,2′-bpdc2- and one bidentate dpa. 1, 2a, and 2b contain both P and M forms of 2,2′-bpdc2-. 3 and 4 contain octahedral metal coordination with only the chair form of cis-chdc2- as bridging ligands in bidentate mode and bidentate peripheral dpa. In each of these complexes, hydrogen bonds between the amine nitrogen and a carboxylate oxygen atom and π-π overlap between the pyridine rings of the dpa define the supramolecular interactions which contribute to the crystals’ stability. The magnetic properties are those of paramagnets with weak exchange interactions, as a consequence of the distant exchange pathway between nearest neighbor moment carriers. Introduction There has been considerable interest in synthesizing organicinorganic hybrid materials due to their intriguing structural diversity and potential dual or multiple functions, such as superconductivity, nonlinear optical activity, and magnetism.1 These materials also belong to an important class of compounds for their host-guest chemistry, catalysis, and gas sorption properties.2 Therefore, rational synthesis of 0D clusters, 1D, 2D, or 3D coordination networks with appropriate bridging ligands is fundamental in developing these materials. Among the synthetic techniques that have been developed and used in coordination chemistry, hydrothermal is among the most common. We have used hydrothermal synthesis in our study of magnetic transition metal carboxylate hybrid materials. In particular, we have focused on a number of points which include (a) the number of carboxylate groups attached to a benzene or cyclohexane ring ranging from one to six, (b) the position and distance between neighboring carboxylate groups, and (c) the stereochemistry and optical activity of the carboxylic acids.3 This study has resulted in a number of compounds having interesting magnetic and structural properties derived from the different modes of connection between the paramagnetic centers. The magnetic properties of the compounds can be classified in two groups, those displaying long-range ordering and those * To whom correspondence should be addressed. E-mail: kurmoo@ chimie.u-strasbg.fr. † Institute for Molecular Science. ‡ Osaka University. § University of Sydney. | Universite´ de Strasbourg. ¶ Present address: Toyota Central Research and Development Laboratories. Inc., Yokomichi 41-1, Nagakute, Aichi 480-1192, Japan. # Present address: Hiroshima University, 1-3-1 Kagamiyama, Higashi Hiroshima, Hiroshima 739-8526, Japan.

which do not. The most interesting and unusual ones are those behaving as ferrimagnets with Curie temperatures of up to 60 K and coercivity of 20 kOe and the metamagnets exhibiting remanant magnetization and coercivity in excess of 50 kOe at 2 K. From the structural point of view, we have previously attempted to introduce chirality in these materials by employing 2,2′-biphenyldicarboxylic acid (also known as diphenic acid and here abbreviated as 2,2′-bpdcH2) because it contains two sterically hindered carboxylate groups and adopts two optically active geometries (M and P) and 1,4-cyclohexanedicarboxylic acid (chdcH2) in which cis- and trans-isomers are present (Scheme 1).4-7 The structural characterizations show the formation of one-dimensional metallo-helicates for the Co(II) and Ni(II) with 2,2′-bpdc but metallo-ladders for Cu(II). Each helicate of Co(II) or Ni(II) is made up of only one isomer, P or M, and the crystals contain an equal number of chains of both helicities. On the other hand, every metallo-ladder of Cu(II) contains both P and M isomers. Consequently, all of the structures are centrosymmetric and have no optical activity.4 Several other coordination polymers using 2,2′-bpdc have been reported.8 Hydrothermal reaction of Co(II) and Ni(II) with cisand trans-mixture of chdc yields porous magnets consisting of linear chain or layered metal hydroxide connected by either the cis- or the trans-dicarboxylate.6 Complete segregation of the isomers has been observed so far for these two metals. The compounds not only are magnetic at low temperature but also exhibit reversible guest exchange and dynamic behavior of the framework for Co(II).6 Interestingly, we observed the transformation between ferrimagnetism for the hydrate form to ferromagnetism for the dehydrate form for the Ni(II) compounds.7 These studies prompted us to search for a way to limit the propagation of the 3D framework formation in order to create well-defined magnetic clusters. Therefore, we used a capping

10.1021/cg801369u CCC: $40.75  2009 American Chemical Society Published on Web 04/07/2009

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Scheme 1. Molecular Structure of the P and M Forms of Diphenic Acid and of the cis- and trans-Form of 1,4-Cyclohexanedicarboxylic Acid (R ) COOH): Representation of the Expected Assembled Metal Complexes

agent such as dipyridylamine, which usually coordinates to metals in a bidentate fashion. From a different point of view, coordination-driven selfassembly of building blocks into discrete molecular clusters is a growing area at the forefront of modern supramolecular chemistry.9 Metallacycles, such as triangles, squares, and polyhedra, have attracted much attention.10,11 For the design of metallacycles, one must take into account the shape, rigidity, number, and disposition of coordination sites of the donor organic ligand as well as the oxidation state, geometry, and coordination number of the metal center. Among metal-organic supramolecular systems, the construction of molecular squares based on metal corners with ∼90° angles and linear bridging ligands has proven to be a very reliable strategy. The majority of molecular squares are based on Pt(II) or Pd(II) ions which normally adopt the square-planar geometry and rigid ligands such as 4,4′-bipyridine, and thus, the resulting assemblies are ionic. Another category of bridging group is the cyanide ion, which has been widely used as a linker to prepare magnetic multinuclear molecules.12 Novel square architectures consisting of flexible ligands, which have conformational and geometrical degrees of freedom, may provide further insights into supramolecular isomerism as well as in the design of multifunctional materials. Therefore, much work is required to extend existing knowledge of the structural types and to establish rational synthetic strategies to the desired architectures using flexible building modules. Here, we encountered macrocyclic complexes or molecular squares with the use of 2,2′-bpdc and chdc as bridging ligands and dpa as a terminal ligand with a divalent transition metal. From geometrical considerations, we anticipate both the P and M forms will be involved in the formation of metallo-square in the M(II)/2,2′-bpdc/dpa system. On the other hand, only the cis-form of chdc can yield a square complex due to the geometry of the bridging ligand in M(II)/cis-chdc/ dpa system, as shown in Scheme 1.8,13 The dpa ligand not only chelates to the metal ion as a terminating ligand to prevent the formation of polymeric species but also can form intermolecular hydrogen bonds through the noncoordinating amino group, thus facilitating the possible formation of extended hydrogen bonded networks. The predictable self-organization of molecules into one-, two-, or three-dimensional framework is of the utmost importance in crystal engineering. For such rational design, hydrogen bonding of conventional OH · · · N and NH · · · O motifs

has been the most commonly observed supramolecular cements as well as π-π interactions.14 Here, we report the preparations, crystal structures, and characterizations of metallo-square complexes of Co, Ni, or Cu of cis-chdc or 2,2′-bpdc ligand and dpa. Experimental Section All chemicals were obtained from Wako, Aldrich, or Fluka and used without purification. The syntheses were carried out in home-built Teflon-lined cylindrical stainless steel pressure bombs of maximum capacity of 120 mL. Thermogravimetric analyses (30-500 °C) of 1 and 2 were performed on a Seiko SSC5200 TG-DTA system. Infrared spectra were recorded on a Perkin-Elmer spectrometer BX FT-IR system by transmission through KBr pellets containing ca. 1% of the compounds. The temperature and field dependence of the magnetization of the complexes were performed on a Quantum Design MPMS-XL SQUID operating in the temperature range of 2-300 K and fields up to 5 T. Preparation of [Co2(2,2′-bpdc)2(dpa)2] (1). Cobalt nitrate hexahydrate (2.47 g), diphenic acid (2.05 g), di-2-pyridylamine (1.45 g), and NaOH (0.34 g) were dissolved in distilled water (30 mL). The mixture was placed in the Teflon liner of an autoclave, sealed, and heated to 120 °C for 2 days. It was allowed to cool to room temperature in a water bath. Violet plate crystals were obtained. The crystals were washed with water and acetone and dried in air (yield 60% based on cobalt). Calcd for 1 (C24CoH17N3O4): C, 61.29; H, 3.64; N, 8.93. Found: C, 61.08; H, 3.60; N, 8.68. Selected IR data (ν/cm-1): 477, 530, 540, 663, 680, 712, 729, 760, 774, 1016, 1159, 1237, 1276, 1373, 1432, 1486, 1534, 1567, 1581, 1605, 3036, 3143, 3202, 3322. Preparation of [Cu2(2,2′-bpdc)2(dpa)2] · 4H2O (2a) and [Cu2(2,2′bpdc)2(dpa)2] · 2(2,2′-bpdcH2) · H2O (2b). Copper(II) nitrate trihydrate (2.47 g), diphenic acid (2.05 g), and di-2-pyridylamine (1.45 g) were dissolved in distilled water (30 mL). The mixture was placed in the Teflon liner of an autoclave, sealed, and heated to 120 °C for 2 days. The bomb was allowed to cool to room temperature in a water bath. Blue crystals were obtained. Crystals of 2a and 2b could not be separated as the color and shapes of the crystals were the same. The crystals were washed with water and acetone and dried in air (yield 65% based on copper). Calcd for 2a (C48Cu2H42O12N6): C, 56.41; H, 4.14; N, 8.22. Calcd for 2b (C76Cu2H56O17N6): C, 62.85; H, 3.89; N, 5.79. Found: C, 58.10; H, 3.78; N, 7.42. Calcd for 75% 2a and 25% 2b (C55Cu2H45.50O13.25N6): C, 58.48; H, 4.06; N, 7.44. Selected IR data (ν/cm-1): 421, 454, 532, 583, 663, 690, 718, 768, 862, 910, 1016, 1159, 1235, 1395, 1435, 1480, 1545, 1578, 1600, 1653, 3029, 3087, 3138, 3385, 3649. Preparation of [Co2(cis-chdc)2(dpa)2] (3). Cobalt nitrate hexahydrate (2.47 g), 1,4-cyclohexanedicarboxylic acid (1.46 g), di-2-

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Table 1. Summary of Crystallographic Data compound

1

2a

2b

3

4

formula molar mass color, habit system a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å3) space group Z value Dcalc (g/cm3) µ(Mo KR) (cm-1) R1, Rw (all data)

CoC24H17N3O4 470.35 violet, plate monoclinic 10.439(7) 18.93(1) 11.010(7)

Cu2C48H42N6O12 1021.98 blue, plate Triclinic 10.620(5) 10.896(5) 11.143(5) 74.781(7) 79.485(8) 62.072(7) 1096.7(9) P1j (No. 2) 1 1.547 10.44 0.053, 0.127

Cu2C76H56N6O17 1452.40 blue, plate triclinic 10.924(1) 11.066(1) 14.973(2) 76.329(2) 88.690(2) 74.958(2) 1697.0(3) P1j (No. 2) 1 1.421 7.03 0.087, 0.190

Co2C36H38N6O8 800.60 orange, plate monoclinic 9.7473(15) 13.607(2) 13.235(2)

Ni2C36H38N6O8 800.16 green, plate monoclinic 9.880(1) 13.482(1) 13.140(1)

92.602(3)

92.977(2)

1753.5(5) P21/n (No. 14) 2 1.516 10.08 0.0298, 0.0895

1748.0(3) P21/n (No. 14) 2 1.520 11.39 0.065, 0.0900

109.77(1) 2047(2) P21/n (No. 14) 4 1.526 9.38 0.107, 0.161

pyridylamine (1.45 g), and NaOH (0.34 g) were dissolved in distilled water (30 mL). The mixture was placed in the Teflon liner of an autoclave, sealed, and heated to 170 °C for 2 days. The bomb was allowed to cool to room temperature in a water bath. Orange red crystals were obtained. The crystals were washed with water and acetone and dried in air (yield 40% based on cobalt). Calcd for 3 (Co2C36O8N6H38): C, 54.01; H, 4.78; N, 10.50. Found: C, 53.26; H, 4.77; N, 10.21. Selected IR data (ν/cm-1): 585, 647, 704, 790, 835, 851, 912, 936, 965, 1007, 1152, 1201, 1237, 1267, 1300, 1345, 1411, 1423, 1450, 1482, 1535, 1578, 1597, 1650, 2930, 3022. Preparation of [Ni2(cis-chdc)2(dpa)2] (4). Nickel nitrate hexahydrate (2.47 g), 1,4-cyclohexanedicarboxylic (1.46 g), di-2-pyridylamine (1.45 g), and NaOH (0.34 g) were dissolved in distilled water (30 mL). The mixture was placed in the Teflon liner of an autoclave, sealed, and heated to 170 °C for 2 days. The bomb was allowed to cool to room temperature in a water bath. Green crystals of 4 and a small amount of blue crystals were obtained. The crystals were washed with water and acetone and dried in air. The crystals were manually separated under a microscope. Calcd for 4 (Ni2C36O8N6H38): C, 54.04; H, 4.79; N, 10.50. Found: C, 53.91; H, 4.74; N, 10.35. Selected IR data (ν/cm-1): 586, 644,708,789, 840, 909, 938, 1007, 1164, 1203, 1242, 1266, 1301, 1340, 1413, 1448, 1487, 1522, 1552, 1586, 1654, 2922, 3020. X-ray Crystallography and Structure Solution. Selected single crystals were glued on glass fibers. Diffraction data for the complexes were collected on a Bruker SMART 1000CCD area detector and SMART APEX CCD area detector employing ω-scan mode at room temperature. The diffractometers were equipped with a graphite monochromated Mo KR (0.7107 Å) radiation. The data were corrected for Lorentz and polarization effects. The structures were solved by direct methods and expanded using Fourier techniques. The non-hydrogen atoms were refined anisotropically. The final cycle of full-matrix leastsquares refinement was based on the number of observed reflections and n variable parameters. They converged (large parameter shift was σ times its esd) with agreement factors of R ) ∑|Fo| - |Fc|/∑|Fo|, Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo|2]1/2. No extinction corrections have been applied. Details of crystallographic data are collected in Table 1. The crystal data have been deposited at CCDC, Cambridge, UK, and given the reference numbers CCDC 697428-697432.

Results and Discussion The carboxylate groups of 2,2′-bpdc are in the ortho positions and thus are sterically hindered, preventing free rotation about the central single C-C bond. This restricted rotation gives rise to perpendicular dis-symmetric planes and, consequently, two forms (P and M).15 Upon coordination to metals, the structures of the complexes can be predicted from simple geometrical considerations of the ligands and if segregation of P and M occurs different enantiomeric forms of crystals can be generated. From homochiral ligands, helical chain complexes are obtained. On the other hand, racemic ligands give discrete molecules or zigzag achiral chain (Scheme 1). For the synthesis, we employed a commercially available racemic mixture of two enantiomers of 2,2′-bpdc and dpa as a terminal ligand.

Structure of 1. The molecular structure of 1 with the atomic numbering scheme is shown in Figure 1a, and selected bond distances and angles are given in Table 2. The key feature of the structure is the pseudosquare unit consisting of two flattened tetrahedral Co(II) ions and two bridging dicarboxylate (Figure 2). Dipyridylamine chelates to the metal as terminal ligand to give discrete molecules. The molecules interact with each other through two intermolecular interactions. One is a hydrogen bonding and the other is a π-π stacking interaction. The former occurs between the noncoordinated oxygen atom of the carboxylate and the hydrogen atom of the amino group of the dpa. The intermolecular hydrogen bond distance between N(2) and O(4′) is 2.765(5) Å. The intermolecular π-π stacking interaction is found between two planar pyridyl groups of adjacent molecules, C-C distances of 3.391(7)-3.581(8) Å. These hydrogen bonds and π-π stacking interactions result in the formation of a one-dimensional chain structure along the a-axis. These chains are packed into layers along the ac-plane. The two Co(II) ions are symmetry related, thus the coordination geometries around metal centers are crystallographically equivalent. The metal centers adopt a distorted tetrahedral geometry with a chelating dpa and two carboxylate oxygen atoms to give CoN2O2 coordination environment. The carboxylate groups coordinate to the metal centers in a monodentate fashion, and the Co-O distances lie in the range 1.941(3)-2.011(4) Å and angles of 92.7(2)-119.1(2)°. The distorted tetrahedral environment of Co(II) sites are related to the steric demand of the dpa ligand and characterized by N-Co-N chelate angle of 92.7(2)°. The Co · · · Co distance within a molecule is 4.60 Å with Co-OC6O-Co connectivity, and C-C distance within a square is 8.26 Å. The size of the cavity within the molecular square is therefore small for accommodating any solvent or gas. The molecules are not packed well in the solid, and thus the density is slightly low. The torsion angle between two phenyl groups [C(12)-C(17)-C(18′)-C(23′)] is 74.8(6)° and appears like the wings of a butterfly. The torsion angle results in a distorted molecular square as compared to those based on a rigid ligand such as 4,4′-bpy and with square-planar complex employed as a rigid metal corner. The carboxylate groups are slightly out of the plane of the phenyl rings. Two dihedral angles between CO2 groups and the C6 phenyl rings are different. One is 1.9° and the other is 43.2°. There is a crystallographic inversion center, and this particular structure contains an equal number of M and P form of 2,2′-bpdc within the molecule (Figure 2). Therefore, no optical activity is expected for compound 1. This structural feature of the dianion in the present compound is different than that found in a one-dimensional helical structure in which only the P or M form is present within one chain.

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Figure 2. Projection of the structure of 1 along the b-axis showing the π-π overlap of the pyridine in dpa and the hydrogen bonds (broken) between H of dpa and O of 2,2′-bpdc. Table 2. Selected Bond Distances and Angles for 1 atom-atom

distance (Å)

atom-atom

distance (Å)

Co(1)-O(1) Co(1)-N(1) atom-atom-atom O(1)-Co(1)-O(3) O(1)-Co(1)-N(3) O(3)-Co(1)-N(3)

1.950(3) 2.011(4) angle (°) 114.0(2) 115.2(2) 112.5(2)

Co(1)-O(3) Co(1)-N(3) atom-atom-atom O(1)-Co(1)-N(1) O(3)-Co(1)-N(1) N(1)-Co(1)-N(3)

1.941(3) 2.000(4) angle (°) 101.4(2) 119.1(2) 92.7(2)

Table 3. Selected Bond Distances and Angles for 2a

Figure 1. Atom labeling for the three different molecular squares observed in (a) 1, (b) 2a and 2b, and (c) 3 and 4.

Structure of 2a. The hydrothermal reaction of Cu(II), 2,2′bpdc, and dpa yields a mixture of single crystals of 2a and 2b irrespective of the conditions used. The color and shapes of the crystals are very similar, and an attempt to separate the crystals was not successful. The difference in the crystal structures of

atom-atom

distance (Å)

atom-atom

distance (Å)

Cu(1)-O(1) Cu(1)-O(3′) Cu(1)-N(1) atom-atom-atom O(1)-Cu(1)-O(2) O(1)-Cu(1)-O(4′) O(1)-Cu(1)-N(3) O(2)-Cu(1)-O(4′) O(2)-Cu(1)-N(3) O(3′)-Cu(1)-N(1) O(4′)-Cu(1)-N(1) N(1)-Cu(1)-N(3)

1.955(2) 1.987(2) 1.976(3) angle (°) 58.13(9) 115.93(9) 157.3(1) 144.59(8) 99.24(10) 153.0(1) 100.28(9) 91.6(1)

Cu(1)-O(2) Cu(1)-O(4′) Cu(1)-N(3) atom-atom-atom O(1)-Cu(1)-O(3′) O(1)-Cu(1)-N(1) O(2)-Cu(1)-O(3′) O(2)-Cu(1)-N(1) O(3′)-Cu(1)-O(4′) O(3′)-Cu(1)-N(3) O(4′)-Cu(1)-N(3)

2.465(2) 2.677(2) 1.960(3) angle (°) 89.78(9) 96.63(10) 90.74(9) 114.9(1) 53.86(8) 92.5(1) 83.10(10)

2a and 2b was only seen by the diffraction study, and it involves the presence of only water of crystallization for 2a but a free 2,2′-bpdcH2 molecule and one molecule of water for 2b. Figure 1b shows an ORTEP drawing of 2a with the atomic numbering scheme. Selected bond distances and angles with their estimated standard deviations are listed in Table 3. The structure is also composed of square units consisting of two copper ions, two chelating dpa, and two bridging dicarboxylate (Figure 3). Similar to compound 1, the molecules are connected to each other by hydrogen-bonding interactions. The hydrogen bonds occur between the oxygen atom of the carboxylate and the hydrogen atom of the amine of dpa to give a one-dimensional structure along the b-axis. The intersquare hydrogen bond distance between N(2) and O(4′) is 2.824(3) Å. These chains are arranged next to each other to form a layer in the bc-plane. The differences of the structures of 1 and 2a are (a) coordination geometries around the metal centers, (b) bridging mode of the dicarboxylate, and (c) the presence of the water molecules in the crystal lattice. While Co(II) ions show tetrahedral geometry, the Cu(II) ions exhibit distorted octahedral geometries composed of carboxylate

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Figure 3. Projection of the structure of 2a showing the π-π overlap of the pyridine in dpa and the hydrogen bonds (dotted) between H of dpa and O of 2,2′-bpdc. Yellow and purple are water molecules; the yellow ones are H-bonded to the molecular squares.

Figure 5. View of the dimeric 2,2′-bpdcH2 intercalated between the metal-organic layers of 2b showing hydrogen bonds (dotted) between the different 2,2′-bpdc and 2,2′-bpdcH2 moieties. Yellow atoms are oxygen from water molecules which are H-bonded to the molecular squares.

Figure 4. Structure of the metal-organic layer of 2b showing the π-π overlap of the pyridine in dpa and the hydrogen bonds (dotted) between H of dpa and O of 2,2′-bpdc. Yellow atoms are oxygen from water molecules which are H-bonded to the molecular squares. Table 4. Selected Bond Distances and Angles for 2b atom-atom

distance (Å)

atom-atom

distance (Å)

Cu(1)-O(1) Cu(1)-O(3) Cu(1)-N(1) atom-atom-atom O(1)-Cu(1)-O(2) O(1)-Cu(1)-N(1) O(2)-Cu(1)-O(3) O(2)-Cu(1)-N(3) O(3)-Cu(1)-N(3)

1.955(3) 1.967(3) 1.976(4) angle (°) 56.8(1) 97.3(2) 89.4(1) 94.7(1) 92.8(2)

Cu(1)-O(2) Cu(1)-O(4) Cu(1)-N(3) atom-atom-atom O(1)-Cu(1)-O(3) O(1)-Cu(1)-N(3) O(2)-Cu(1)-N(1) O(3)-Cu(1)-N(1) N(1)-Cu(1)-N(3)

2.515(4) 2.642(4) 1.960(4) angle (°) 91.8(1) 151.1(2) 117.0(1) 152.8(1) 91.6(2)

oxygen atoms and nitrogen atoms of the dpa ligand. An inversion center is present between the copper ions, and thus the two copper ions in the dimer are equivalent. Each copper atom has two long Cu-O bonds [2.465(2) and 2.677(2) Å] with the 2,2′-bpdc. The distortion from a regular octahedral geometry is seen by different bond distances and also by O-Cu-O angles of 53.86(8), 58.13(9), and 144.59(8)°. The former is imposed by the constraint of the bidentate carboxylate that is rigid with

O-C-O angle of 120°. The Cu · · · Cu distance within the dimer is 5.81 Å with Cu-OC6O-Cu connectivity and C-C distance within a square unit of 8.0 Å. The size of the cavity defined from the distances in the molecular square is rather small for solvents or gases. Figure S1 in the Supporting Information shows the space-filling model using van der Waals radii for the atoms of 1 and 2a. 2a shows a cavity larger than that of 1 due to the orthogonal angle between two phenyl groups. The geometry of 2,2′-bpdc in 2a is different than those found in the pure acid and in 1. The torsion angle of C(12)-C(17)-C(18)-C(23) is 91.4(6°) and is larger than that of 1. The carboxylate groups are slightly out of the plane of the phenyl rings. The two dihedral angles between the CO2 groups and the C6 phenyl rings are very similar (ca. 25°). The copper ions are bridged by both the P and M forms of 2,2′-bpdc. Thus two enantiomers are present within one molecule. Second, the difference of 2,2′-bpdc in 2a is the bidentate coordination mode of the two carboxylate groups, in contrast to the monodentate mode for 1. Third, another difference is the presence of water molecules [O(5) and O(6)]. Although we could not locate the hydrogen atoms of the water molecules, we found that O(5) sits on the top and bottom of the holes within the squares and forms weak H bonds with the carboxylate oxygen O(3), O(4′) of 2.918(4) and 3.064(4) Å, respectively. Water molecule O(6) is located in between the chains and has one H bond with carboxylate oxygen atom O(2) at 2.770(4) Å. Structure of 2b. The structure of 2b makes an interesting comparison to that of 2a. 2b differs from 2a in its content where two neutral 2,2′-bpdcH2 and a water molecule have replaced the four water molecules within the lattice of 2a. The square complex forming construction units of the two compounds are the same. Interestingly, the water molecules [O(9)] in 2b are in similar positions as in 2a with three H bonds of 2.762, 2.859, and 3.067 Å to O(4), O(3), and O(1), respectively. The geometries and arrangement of these molecular units into layers

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Figure 6. Projection of a chain within the structure of 4 showing the π-π overlap of the pyridine in dpa and the hydrogen bonds (dotted) between H of dpa and O of cis-chdc. Table 5. Selected Bond Distances and Angles for 3 and 4 atom-atom M(1)-N(1) M(1)-O(1) M(1)-O(3) atom-atom-atom O(1)-M(1)-O(2) O(1)-M(1)-O(4) O(1)-M(1)-N(3) O(2)-M(1)-O(4) O(2)-M(1)-N(3) O(3)-M(1)-N(1) O(4)-M(1)-N(1) N(1)-M(1)-N(3)

distance (Å) M ) Co (3)

distance (Å) M ) Ni (4)

2.050(1) 2.301(1) 2.150(1) angle (°) M ) Co (3) 59.60(4) 94.70(4) 152.69(4) 144.87(4) 93.22(4) 159.91(4) 98.16(4) 90.91(4)

2.007(2) 2.210(2) 2.071(2) angle (°) M ) Ni (4) 61.18(7) 100.06(7) 154.23(8) 153.55(7) 93.05(8) 161.10(8) 98.31(8) 91.18(8)

(ac-plane) in 2b are almost the same as in 2a (Table 4 and Figure 4). The formation of H-bonded, N(2)-O(4) of 2.801 Å, and π-π bonded (dpa-dpa) chains is found along the c-axis. The big difference between the two is that while in 2a the layers are intercalated by water molecules in 2b they are intercalated by neutral 2,2′-bpdcH2 H-bonded pairs through one carboxylic acid from each (Figure 5). The closest C-C distance between the phenyl rings of 3.64 Å results into parallel layers in the ac-plane as for the metal-organic layer above. The other carboxylic acid group further forms a H bond with the metalorganic layer, O(5)-O(2) of 2.662 Å. Structures of 3 and 4. Figure 1c shows the molecular structure of 4 with the atomic numbering scheme, and selected bond distances and angles of 3 and 4 are given in Table 5, respectively. Since the two compounds are isostructural, we will restrict our description to only the nickel compound, and only mention the pertinent points for cobalt when appropriate. Each molecule is composed of two Ni(II) ions, two bridging cischdc2-, and two terminal dpa. The molecules are connected to one another via two hydrogen bonds and π-π overlaps of pyridine rings to afford a one-dimensional chain structure (Figure 6). Hydrogen-bonding interactions occur between the oxygen atoms of the carboxylate and the hydrogen atom of dpa, N(2) and O(2′) is 2.802(3) Å. Similar interactions are seen in complexes 1 and 2a. The two Ni ions are symmetry related, thus the coordination geometries are equivalent. The metal centers adopt a severely distorted octahedral geometry with a chelating dpa and two chelating carboxylate groups to give the NiN2O4 coordination environment. The Ni-O distances lie in therangeof2.042(2)-2.209(2)Åandanglesof61.04(7)-161.05(8)°.

atom-atom M(1)-N(3) M(1)-O(2) M(1)-O(4) atom-atom-atom O(1)-M(1)-O(3) O(1)-M(1)-N(1) O(2)-M(1)-O(3) O(2)-M(1)-N(1) O(3)-M(1)-O(4) O(3)-M(1)-N(3) O(4)-M(1)-N(3)

distance (Å) M ) Co (3)

distance (Å) M ) Ni (4)

2.084(1) 2.088 (1) 2.100(1) angle (°) M ) Co (3) 90.40(4) 94.48(4) 92.51(4) 106.77(4) 61.98(4) 93.64(4) 111.02(4)

2.041(2) 2.070(2) 2.102(2) angle (°) M ) Ni (4) 87.93(7) 94.09(8) 96.10(7) 101.34(8) 62.88(7) 95.13(8) 104.11(8)

The distortion from octahedral geometry is characterized by chelate angles of carboxylate groups [61.04(7) and 62.85(7)°]. The Ni · · · Ni distance within a dimer is 7.5 Å via Ni-OC6O-Ni, and the C(13)-C(15′) distance of the square is 5.3 Å. Again considering the van der Waals radii of the constituent atoms, the size of the cavity is nonaccessible for solvents or gases. These dimensions are only slightly different for the cobalt analogue to take account of the difference of the ionic radii of the cations. The two pyridyl groups of dpa are almost planar and very similar to those found in 1 and 2. In complex 1 and 2, both the P and M form are present within the molecule. However, only the cis-form is present in complexes 3 and 4. Thermogravimetry and Magnetic Properties. The thermogravimetric analyses were only run for 1 and 2 (Figure S2 in Supporting Information). They show different thermal behaviors. 1 is stable up to about 350 °C and decomposes to the expected oxide, Co3O4. The sample of Cu/2,2′-bpdc/dpa contained the two types of crystals, 2a and 2b, and a quantitative analysis was difficult. However, it is clear that there are two steps of weight loss: one starting at room temperature up to 110 °C (∼4%) and the other starting above 200 °C and finishing around 270 °C, which is supposed to be the decomposition of the organic moieties. The two steps may be the difference between the coordinated 2,2′-bpdc and the noncoordinated 2,2′bpdcH2. The loss of water from the crystal of 2a takes place over a wide temperature range, and this may be due to the presence of two types of water molecules, O(5) and O(6). There is a difference of 120 °C in the decomposition of the organics between the cobalt and copper complexes which may be due to the difference in bond strengths.

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Figure 7. Temperature dependence of the magnetic susceptibility (red), its inverse (green), and its product with temperature (blue) for a sample of (a) 1, (b) 2a and 2b, (c) 3, and (d) 4. Table 6. Summary of the Observed Magnetic Data

compound 1 2 3 4

temperature range (K) 2-300 2-300 100-300 2-300

Curie constant (emu K/mol)

Weiss constant (K)

µeff per cation (µB)

4.93(1) 0.81(1) 5.96(1) 2.23(1)

-1.2(2) -1.4(2) -1.4(2) -0.5(1)

4.44 1.80 4.88 2.99

The magnetic susceptibilities of the compounds were studied as a function of temperature in a fixed magnetic field of 1 kOe and the isothermal magnetization at 2 K in a field up to 50 kOe. The results are shown in Figure 7. Each plot shows the temperature dependence of the magnetic susceptibility, its inverse, and its product with temperature. In all cases, the data fit the Curie-Weiss function, and the results are given in Table 6. All the effective moments per cation are within the observed experimental range. A jump in the χT value of 3 was observed at 60 K, which is due to a very small amount of the ferrimagnetic impurity Co5(OH)8(trans-chdc) · 4H2O not visible by eye.6 The isothermal magnetizations also reach the expected saturation values. It is to be noted that we use the molar mass corresponding to 0.75 of 2a and 0.25 of 2b, as observed by chemical analyses for the same batch, for the calculation of the Curie and Weiss constants. Conclusion By the use of the commercially available mixtures of P and M diphenic acid or cis- and trans-1,4-cyclohexanedicarboxylic acid in the presence of a chelating agent, such as dipyridylamine,

molecular squares were obtained by the hydrothermal reaction. For the diphenic acid, both isomers are present in the complexes, while only the cis-isomer of chdc is found. Different coordination geometries, heavily distorted tetrahedral and octahedral, resulted due to the severe constraints imposed by the geometry of the coordinating groups. In all complexes, H bonds between the NH of the amine to the O of the carboxylate were observed. Some π-π interactions were also present in pairwise contacts between adjacent pyridine rings. An interesting finding is that the intercalated water molecules between the metal-organic layers in [Cu2(2,2′-bpdc)2(dpa)2] · 4H2O are replaced by the H-bonded pairs of neutral 2,2′-bpdcH2 in [Cu2(2,2′-bpdc)2(dpa)2] · 2(2,2′-bpdcH2) · H2O without major difference to the dimeric clusters. Due to the large separation and the long pathway between nearest neighbor moment carriers, these compounds are paramagnets. Acknowledgment. H.K. thanks the JSPS-Japan for a young scientist fellowship for his one-year stay in Strasbourg. M.K. thanks the CNRS (France) and the Royal Society of Chemistry (UK) for a travel grant. Supporting Information Available: Crystallographic CIF files of the structures, figures of space filling of the molecular squares, and thermogravimetric analyses. This material is available free of charge via the Internet at http://pubs.acs.org.

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