Synthesis, X-ray Structures, and Magnetic Properties of Copper(II) Pyridinecarboxylate Coordination Networks Chapman,†
Ayyappan,†
Mary E. Ponnaiyan Gordon T. Yee,*,‡ and Wenbin Lin*,†
Bruce M.
Foxman,†
CRYSTAL GROWTH & DESIGN 2001 VOL. 1, NO. 2 159-163
Department of Chemistry, Brandeis University, Waltham, Massachusetts 02454, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309 Received October 17, 2000
ABSTRACT: The three-dimensional Cu(II) coordination polymers [Cu(isonicotinate)2][EtOH] (1) and Cu(nicotinate)2 (2) were synthesized by treating copper nitrate with isonicotinic acid and nicotinic acid under hydro(solvo)thermal conditions, respectively. X-ray single-crystal structure determinations reveal that the Cu(II) centers in 1 adopt a slightly distorted square pyramidal geometry, while the Cu(II) centers in 2 exhibit a coordination geometry intermediate between square pyramid and trigonal bipyramid. The Cu(II) centers in 1 and 2 coordinate to both the pyridyl and carboxylate functionalities of the isonicotinate and nicotinate bridging ligands, respectively, to result in complicated 3-D framework structures. Compound 1 exhibits rhombic open channels that are occupied by removable ethanol molecules. Magnetic measurements indicated that 1 is a simple paramagnet with only very weak antiferromagnetic interactions, while 2 exhibits more pronounced antiferromagnetic interactions with J/k ) -6.4 K. Crystal data for 1: monoclinic, space group Cc, a ) 5.033(1) Å, b ) 24.855(5) Å, c ) 11.176(2) Å, β ) 99.35(3)°, and Z ) 4. Crystal data for 2: monoclinic, space group P21/n, a ) 10.693(2) Å, b ) 9.589(2) Å, c ) 12.535(3) Å, β ) 112.09(3)°, and Z ) 4. Introduction
Experimental Section
Metal-directed self-assembly processes have recently provided a powerful tool for the synthesis of extended metal-organic coordination networks with diverse potential applications as zeolitic, catalytic, magnetic, and second-order nonlinear optical materials.1 Most research effort in this area has so far been focused on coordination polymers with either neutral donor ligands (e.g., 4,4′-bipyridine and polynitriles)2,3 or strictly anionic groups (e.g., terephthalic acid and trimesic acid).4 We have recently become interested in synthesizing coordination polymers with multifunctional ligands in which both neutral and anionic donor groups are present and can coordinate to metal centers, potentially to result in neutral polymeric structures. By introducing electronic asymmetry into the conjugated bridging ligands, we have demonstrated the rational synthesis of polar neutral coordination networks of transparent d10 metals exhibiting interesting second-order nonlinear optical properties.5 More recently, we have studied structuremagnetic property relationships of manganese(II) pyridinecarboxylate coordination networks that appear to be good examples of 1-D Heisenberg antiferromagnetic chains.6 In this work, we examine the synthesis of Cu(II) coordination networks with both isonicotinate and nicotinate bridging ligands. We report here the synthesis, X-ray single-crystal structures, and magnetic properties of the two Cu(II) coordination polymers [Cu(isonicotinate)2][EtOH] (1) and Cu(nicotinate)2 (2).
Materials and Methods. All chemicals were purchased from Aldrich and were used as received without further purification. The IR spectra were recorded as KBr pellets on a Paragon 1000 FT-IR spectrometer. Magnetic measurements were taken on a Quantum Design MPMS-7 SQUID magnetometer at the National Institute of Standards and Technology in Boulder, CO. Synthesis of Bis(isonicotinato)copper(II)-Ethanol (1). Copper(II) nitrate hemipentahydrate (0.0581 g; 0.25 mmol) and isonicotinic acid (0.062 g, 0.5 mmol) were mixed in a heavywalled Pyrex tube with ethanol (0.14 mL), acetonitrile (0.16 mL), and deionized water (0.50 mL). The tube was frozen with liquid nitrogen and sealed under vacuum. The tube was then heated at 75 °C for 24 h, at 110 °C for 72 h, and then at 130 °C for 24 h. After these different stages of heating, small bright blue crystals were obtained. The crystals were washed with 2 mL of ethanol and then twice with 2 mL of ethyl acetate. Yield: 0.050 g (57%). IR (cm-1): 1604 (s), 1550 (m), 1503 (w), 1416 (m), 1374 (s), 1231 (w), 1058 (m), 1032 (w), 882 (w), 849 (w), 775 (m), 706 (m). Synthesis of Bis(nicotinato)copper(II) (2). Copper nitrate hemipentahydrate (0.0581 g, 0.25 mmol) and nicotinic acid (0.062 g, 0.5 mmol) were placed in a heavy-walled Pyrex tube with ethanol (0.10 mL), acetonitrile (0.20 mL), and deionized water (0.50 mL). The tube was frozen with liquid nitrogen and sealed under vacuum. The tube was then heated at 105 °C for 48 h and at 130 °C for 24 h. The blue product was washed three times with 2 mL of water and once with 2 mL of ethanol. Yield: 0.072 g (98%). IR (cm-1): 1654 (m), 1629 (s), 1570 (w), 1426 (m), 1385 (s), 1306 (w), 1189 (m), 1156 (w), 1092 (m), 1050 (m), 971 (w), 850 (m), 762 (m), 717 (w), 694 (m), 717 (w), 694 (m), 650 (w), 586 (w). Magnetic Data. The magnetic properties of 1 and 2 were investigated by SQUID magnetometry. The ∼20 mg samples were packed between two cotton plugs and placed into gelatin capsules. Two samples of each compound, from separate preparations, were measured, and the results were essentially the same for each pair. The applied field was 1000 G, and diamagnetic corrections were determined from Pascal’s constants and from the average gram susceptibilites of empty
* To whom correspondence should be addressed. E-mail for W.L.:
[email protected]. † Brandeis University. ‡ University of Colorado.
10.1021/cg005519l CCC: $20.00 © 2001 American Chemical Society Published on Web 01/19/2001
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Table 1. Data for the X-ray Diffraction of 1 and 2a chem formula a, Å b, Å c, Å β, deg V, Å3 Z fw space group T, °C λ(Mo KR), Å Fcalcd, g/cm3 m, cm-1 (Mo KR) min and max residual density, e/Å3 R1 wR2 goodness of fit Flack param
1
2
CuC14H14N2O5 5.033(1) 24.855(5) 11.176(2) 99.35(3) 1379.4(5) 4 353.82 Cc (No. 9) 20(2) 0.710 73 1.71 16.0 -0.62, 0.51
CuC12H8N2O4 10.693(2) 9.589(2) 12.535(3) 112.09(3) 1191.0(5) 4 307.75 P21/n (No. 14) 20(2) 0.710 73 1.72 18.4 -0.38, 0.73
0.033 0.103 1.14 0.04(2)
0.041 0.130 1.09 nab
a R1 ) ∑||F | - |F ||/∑|F |; wR2 ) [∑[w(F 2 - F 2)2]/∑[w(F 2)2]]1/2; o c o o c o GOF ) [∑[w(Fo2 - Fc2)2]/((no. of reflections) - (no. of param1/2 b eters))] . na ) not applicable.
Table 2. Selected Bond Distances (Å) and Bond Angles (deg) for 1 and 2a 1
Scheme 1
2
Cu1-O2 Cu1-O4 Cu1-O3C Cu1-N2A Cu1-N1B
1.978(4) 2.332(3) 1.952(3) 2.029(4) 2.013(4)
Cu1-O2 Cu1-O4 Cu1-N1C Cu1-O3A Cu1-N2B
1.965(4) 1.925(4) 2.040(4) 2.300(4) 2.046(4)
O2-Cu1-O4 O2-Cu1-O3C O2-Cu1-N2A O2-Cu1-N1B O3C-Cu1-O4 O4-Cu1-N2A O4-Cu1-N1B O3C-Cu1-N2A O3C-Cu1-N1B N1B-Cu1-N2A
92.24(14) 170.99(14) 91.66(15) 84.51(15) 96.63(13) 86.39(13) 93.35(13) 90.49(14) 93.36(14) 176.14(15)
O2-Cu1-O4 O2-Cu1-N1C O2-Cu1-O3A O2-Cu1-N2B O4-Cu1-N1C O3A-Cu1-O4 O4-Cu1-N2B O3A-Cu1-N1C N1C-Cu1-N2B O3A-Cu1-N2B
151.31(16) 90.96(15) 84.00(15) 89.86(15) 90.69(15) 124.69(15) 91.93(15) 86.15(16) 173.01(17) 87.03(16)
Symmetry operations for 1: (A) x, 1 - y, 1/2 + z; (B) -1/2 + x, - y, -1/2 + z; (C) ) -1 + x, y, z. Symmetry operations for 2: (A) 1 - x, -y, 2 - z; (B) -1/2 + x, 1/2 - y, -1/2 + z; (C) 3/2 - x, -1/2 + y, 3/2 - z. a
3/
Figure 1. Coordination environment in 1. The asymmetric unit (except included ethanol) is shown with ellipsoids at 50% probability.
2
capsules and cotton wool. A small correction was applied to the raw data to account for the presence of a ferromagnetic impurity. In no case was this greater than that equivalent to 30 ppm saturated iron. Linearity of the inverse χ vs T plot was used as the criterion for determining this correction. X-ray Structure Determinations. Single crystals of 1 and 2 were mounted with epoxy on Pyrex fibers affixed to brass pins and transferred to an Enraf-Nonius CAD4-Turbo diffractometer equipped with Mo KR radiation. Data were collected using the Nonius EXPRESS program.7 Of the 1463 (2505) reflections measured, 1452 (2338) reflections with I > 2σ(I) were used in structure solution and refinement for 1 (2). The structure was solved using direct methods (SHELXS-97) and refined using the SHELXS-TL software package by full-matrix least squares using anisotropic displacement parameters for all non-hydrogen atoms.8 All the hydrogen atoms were located in an electron density difference map and refined isotropically. Final refinement gave R1 ) 0.033 (0.041), wR2 ) 0.103 (0.130), and a goodness of fit of 1.14 (1.09) for 1 (2). The choice of the Cc space group for 1 has been confirmed by PLATON using the ADDSYM routine.9 Experimental details for X-ray data collections of 1 and 2 are tabulated in Table 1. Selected bond distances and angles for 1 and 2 are listed in Table 2.
Results and Discussion Synthesis. When isonicotinic acid and nicotinic acid were reacted with Cu(NO3)2‚2.5H2O in 2:1 molar proportions under hydrothermal conditions, two novel Cu(II) coordination networks, [Cu(isonicotinate)2][EtOH] (1) and Cu(nicotinate)2 (2), were obtained (Scheme 1). The presence of isonicotinate groups in 1 and the nicotinate group in 2 is suggested from their infrared spectra, which exhibit strong peaks at ∼1380 and ∼1420 cm-1 assignable to carboxylate CdO symmetric and antisymmetric stretches, respectively.10 Furthermore, an obvious lack of peaks in the 1700-1720 cm-1 region indicates the absence of free carboxylic acid groups. The fact that both 1 and 2 are insoluble in water, ethanol, and common organic solvents suggests that they are polymeric in nature. Single-Crystal X-ray Structure of 1. Compound 1 crystallizes in the acentric space group Cc. The asymmetric unit of 1 contains one Cu atom, two isonicotinate groups, and one included ethanol molecule. The Cu1 center adopts a slightly distorted square pyramidal coordination geometry (Figure 1); the bond angles around the Cu1 center only deviate slightly from 90 or 180° (Table 2). In the equatorial plane, the Cu1 atom is bonded to two pyridyl nitrogen atoms and to two oxygen atoms of one monodentate carboxylate group (O2) and one bridging carboxylate group (O3C). The Cu1 center is bonded to the other oxygen atom (O4) of the bridging carboxylate group in the axial position to complete the distorted-square-pyramidal coordination geometry. The carboxylate bridge adopts a syn-anti conformation with a Cu-Cu separation of 5.03(1) Å. As expected, the axial
Cu(II) Pyridinecarboxylate Coordination Networks
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Figure 2. View of the carboxylate-bridged Cu-Cu chain in 1. Cu atoms are represented with ellipsoids, while O, N, and C atoms are shown as circles with decreasing sizes.
Cu-O4 distance of 2.33(1) Å is significantly longer than the equatorial Cu-O2 and Cu-O3C distances of 1.98(1) and 1.95(1) Å. Along the a axis, adjacent Cu atoms are linked to each other via the bridging carboxylate groups to form infinite chains (Figure 2). In the bc plane, each Cu atom is linked to four adjacent Cu atoms through the isonicotinate groups to form a rhombohedral grid structure. These grids are further linked by the carboxylate bridges along the a axis to form a 3-D framework structure. The distances between Cu centers in the bc plane are 8.89(1) and 8.90(1) Å, respectively, while the separation between rhombohedral grids along the a axis is 5.03(1) Å. Interestingly, compound 1 exhibits regular diamond-shaped channels along the a axis which have been filled by included ethanol molecules (Figure 3). We believe that the crystals of 1 slowly lose included ethanol molecules at room temperature. In fact, the crystal structure of 1, which was taken several days after the harvest of the crystal, exhibits an occupancy factor of 0.53 for the ethanol molecules. Consistent with this, thermogravimetric analyses (TGA) on a fresher sample indicate a weight loss of 8.5% in the 20-60 °C temperature range, which corresponds to the removal of included ethanol molecules (10.2% expected for occupancy factor of 1 for ethanol). Despite the acentric nature of 1, we did not carry out second-harmonicgeneration measurements on 1 owing to the highly absorbing nature of the d9 Cu(II) centers. Single-Crystal X-ray Structure of 2. Compound 2 crystallizes in the monoclinic space group P21/n. The asymmetric unit of 2 contains one Cu atom and two nicotinate ligands. The Cu1 center adopts a coordination geometry intermediate between square pyramid and trigonal bipyramid, as evidenced by the O2-Cu1-O4 angle of 151.3(2)°, a value halfway between the 120° expected for a trigonal bipyramid and the 180° expected for a square pyramid (Figure 4). In the highly distorted equatorial positions, the Cu1 center is bonded to one monodentate carboxylate group with a Cu1-O2 bond distance of 1.97(1) Å and to the O4 atom of the bridging carboxylate group, as well as to two pyridyl nitrogen atoms of the two nicotinate groups. The Cu1-N1C and Cu1-N2B distances are 2.04(1) and 2.05(1) Å, respectively, while the N2B-Cu1-N1C angle is 173.0(2)°. The
Figure 3. Space-filling model of 1 down the a axis, showing the diamond-like channels that are occupied by included ethanol molecules (not shown).
Cu1 center is bonded to the other oxygen atom of the bridging carboxylate (O3A) in the axial position. The O3A and O4 atoms link the Cu1 center to an adjacent Cu center to form a doubly carboxylate bridged Cu(II) dimer. The Cu1-O3A distance is 2.30(1) Å, while the Cu1-O4 distance is 1.93(1) Å. Each of the two carboxylate bridges adopt a syn-syn conformation with a Cu-Cu separation of 4.19(1) Å. The exo-tridentate nicotinate groups (with bridging carboxylate groups) link doubly carboxylate bridged Cu(II) dimers into a pleated-sheet structure through the coordination of their pyridyl nitrogen atoms (Figure 5a). The exo-bidentate nicotinate groups link adjacent Cu centers to form zigzag chains along the a axis (Figure 5b) and thus cross-link the pleated sheets formed by the Cu centers and exo-tridentate nicotinate groups to result
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Figure 6. Plot of χT vs T measured at 1000 G for 1.
Figure 4. Coordination environment in 1. The asymmetric unit is shown in ellipsoids awitht 50% probability.
Figure 7. Plot of χT vs T measured at 1000 G for 2 (dark circles) and fit to the Bleaney-Bowers equation (solid line).
Figure 5. (a, top) View of the pleated sheet of Cu(II) dimers formed by the Cu(II) centers and exo-tridentate nicotinate groups down the a axis. (b, bottom) Zigzag chain formed by the Cu(II) centers and exo-bidentate nicotinate groups running along the a axis. Circles with decreasing sizes represent Cu, O, N, and C, respectively.
in a complex 3-D framework structure. In contrast to 1, compound 2 does not have any cavity accessible to solvent molecules. TGA indicates no weight loss until the onset of decomposition of 2 at 240 °C. Magnetic Properties. Compound 1 is a simple paramagnet over the entire temperature range from 1.8 to 250 K with only a slight downturn below 20 K, indicative of a small antiferromagnetic interaction. A representative plot is shown in Figure 6. These data are not surprising, given that the crystal structure shows that each Cu(II) complex exhibits square-pyramidal geometry with the carboxylate bridge in the axial position. Since the unpaired electron on Cu should be in the dx2-y2 orbital, which lies in the equatorial plane,
one would expect very inefficient antiferromagnetic coupling. The g value from a Curie-Weiss fit to the data gives g ) 2.40, slightly higher than is usual for Cu(II) (2.0-2.3).11 This could be due to the uncertainty regarding the degree of ethanol solvation (see above) in the lattice or to partial alignment of the magnetically anisotropic crystals. In contrast, compound 2 exhibits a much more pronounced downturn at low temperature (Figure 7). From the crystal structure, it is apparent that 2 consists, magnetically, of doubly carboxylate bridged Cu(II) dimers. The data can be fit, though not altogether satisfactorily, to the Bleaney-Bowers equation,11 which is derived from the Heisenberg Hamiltonian, H ) -2JS ˆ 1‚S ˆ 2:
χT )
2Ng2µ2B -2J 1 1 + exp 3k 3 kT
[
(
)]
-1
A nonlinear least-squares fit to the data yields g ) 2.29 ( 0.02 and J/k ) -6.4 ( 0.8 K. These data are consistent with a weak antiferromagnetic interaction mediated by the carboxylate bridges. This magnitude of antiferromagnetic interaction is significantly smaller than those of other Cu(II) dimers bridged by two synsyn carboxylate groups. Previously reported Cu(II) dimers bridged by two syn-syn carboxylate groups exhibit a J/k value of 43-62.5 cm-1.12 This difference in antiferromagnetic interactions can be readily explained on the basis of the structure of 2. All the previously reported doubly carboxylate bridged Cu(II) dimers adopt either square-planar or square-pyramidal
Cu(II) Pyridinecarboxylate Coordination Networks
coordination geometry with the oxygen atoms of the bridging carboxylate groups in the equatorial planes. These syn-syn carboxylate groups interact effectively with the unpaired electrons in the dx2-y2 orbitals of the Cu(II) ions and thus lead to efficient antiferromagnetic superexchange. In 2, only one of the oxygen atoms (O4) of the bridging carboxylate groups lies in the equatorial plane, while the other oxygen atom (O3A) lies in the axial position and thus does not interact effectively with the unpaired electron in the dx2-y2 orbital of the Cu(II) ion. As a result, compound 2 exhibits a much smaller value of J/k. In summary, we have synthesized two new 3-D Cu(II) coordination networks based on pyridinecarboxylate bridging ligands. The magnetic properties of both 1 and 2 have been satisfactorily explained on the basis of their X-ray structures. Acknowledgment. We acknowledge financial support from the National Science Foundation (Grant No. DMR-9875544 to W.L.; and Grant No. CHE-9727485 to G.T.Y.). We thank the National Institute of Standards and Technology for use of the SQUID magnetometer. W.L. is an Alfred P. Sloan Fellow, an Arnold and Mabel Beckman Young Investigator, and a Cottrell Scholar of the Research Corp. Supporting Information Available: A figure giving the TGA curves for 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org.
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(3)
(4)
(5)
(6) (7) (8) (9) (10) (11)
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CG005519L
