Complexes with Linear Tritopic Ligands - American Chemical Society

St. John's, Newfoundland A1B 3X7, Canada. Received March 27, 2003. The structures and magnetic properties of self-assembled copper(II) clusters and gr...
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Inorg. Chem. 2004, 43, 1874−1884

Octanuclear and Nonanuclear Supramolecular Copper(II) Complexes with Linear “Tritopic” Ligands: Structural and Magnetic Studies Victoria A. Milway, Virginie Niel, Tareque S. M. Abedin, Zhiqiang Xu, Laurence K. Thompson,* Hilde Grove, David O. Miller, and Stewart R. Parsons Department of Chemistry, Memorial UniVersity of Newfoundland, St. John’s, Newfoundland A1B 3X7, Canada Received March 27, 2003

The structures and magnetic properties of self-assembled copper(II) clusters and grids with the “tritopic” ligands 2poap (a), Cl2poap (b), m2poap (c), Cl2pomp (d), and 2pomp (e) are described [ligands derived by reaction of 4-R-2,6-pyridinedicarboxylic hydrazide (R ) H, Cl, MeO) with 2-pyridinemethylimidate (a−c, respectively) or 2-acetylpyridine (d, R ) Cl; e, R ) H)]. Cl2poap and Cl2pomp self-assemble with Cu(NO3)2 to form octanuclear “pinwheel” cluster complexes [Cu8(Cl2poap-2H)4(NO3)8]‚20H2O (1) and [Cu8(Cl2pomp-2H)4(NO3)8]‚15H2O (2), built on a square [2 × 2] grid with four pendant copper arms, using “mild” reaction conditions. Similar reactions of Cl2pomp and 2pomp with Cu(ClO4)2 produce pinwheel clusters [Cu8(Cl2pomp-2H)4(H2O)8](ClO4)8‚7H2O (3) and [Cu8(2pomp-2H)4(H2O)8](ClO4)8 (4), respectively. Heating a solution of 1 in MeOH/H2O produces a [3 × 3] nonanuclear square grid complex, [Cu9(Cl2poap-H)3(Cl2poap-2H)3](NO3)9‚18H2O (5), which is also produced by direct reaction of the ligand and metal salt under similar conditions. Reaction of m2poap with Cu(NO3)2 produces only the [3 × 3] grid [Cu9(m2poap-H)2(m2poap-2H)4](NO3)8‚17H2O (6) under similar conditions. Mixing the tritopic ligand 2poap with pyridine-2,6-dicarboxylic acid (picd) in the presence of Cu(NO3)2 produces a remarkable mixed ligand, nonanuclear grid complex [Cu9(2poap-H)4(picd-H)3(picd-2H)](NO3)9‚9H2O (7), in which aromatic π-stacking interactions are important in stabilizing the structure. Complexes 1−3 and 5−7 involve single oxygen atom (alkoxide) bridging connections between adjacent copper centers, while complex 4 has an unprecedented mixed µ-(N−N) and µ-O metal ion connectivity. Compound 1 (C76H92N44Cu8O50Cl4) crystallizes in the tetragonal system, space group I4h, with a ) 21.645(1) Å, c ) 12.950(1) Å, and Z ) 2. Compound 2 (C84H88N36O44Cl4Cu8) crystallizes in the tetragonal system, space group I4h, with a ) 21.2562(8) Å, c ) 12.7583(9) Å, and Z ) 2. Compound 4 (C84H120N28O66Cl8Cu8) crystallizes in the tetragonal system, space group I41/a, with a ) 20.7790(4) Å, c ) 32.561(1) Å, and Z ) 4. Compound 7 (C104H104N46O56Cu9) crystallizes in the triclinic system, space group P1h, with a ) 15.473(1) Å, b ) 19.869(2) Å, c ) 23.083(2) Å, R ) 88.890(2)°, β ) 81.511(2)°, γ ) 68.607(1)°, and Z ) 2. All complexes exhibit dominant intramolecular ferromagnetic exchange coupling, resulting from an orthogonal bridging arrangement within each polynuclear structure.

Introduction Polytopic ligands can be designed and synthesized with preprogrammed coordination information “stored” in the coordination pockets, such that when they react with a transition metal ion it interprets this information according to its own coordination algorithm. If the ligand pockets do not provide all the necessary donors, self-assembly can occur such that homoleptic coordination clusters form with the * To whom correspondence should be addressed. E-mail address: [email protected]. Fax: 1-709-737-3702.

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nuclearity being a function of the polytopic nature of the ligand and the preferred metal ion coordination number. This approach has had a significant measure of success with linear “polytopic” ligands based on pyridazine bridging subunits, which have produced molecular [2 × 2] CuI41 (“ditopic” ligand) and [3 × 3] AgI92 (“tritopic” ligand) grids, by a strict self-assembly process. The metal ions fit neatly into the (1) Youinou, M.-T.; Rahmouni, N.; Fischer, J.; Osborn, J. A. Angew. Chem., Int. Ed. Engl. 1992, 31, 733. (2) Baxter, P. N. W.; Lehn, J.-M.; Fischer, J.; Youinou, M.-T. Angew. Chem., Int. Ed. Engl. 1994, 33, 2284.

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Cu(II) Complexes with Linear “Tritopic” Ligands Chart 1

tetrahedral pockets created by the assembly of pairs of ligands, which provide N2 donor groupings above and below the planar arrangement of metal centers. Mixing similar ditopic and tritopic ligands can lead to a mixed [2 × 3] rectangular Ag6 grid, along with small amounts of homoligand Ag4 and Ag9 grid complexes.3 [2 × 2] homo- and heterometallic grids4,5 of six-coordinate metal ions have been produced with ligands based on pyrimidine bridging subunits, where the donor groupings arise from the assembly of pairs of ligands that combine N3 donor pockets. With Co(II) and Ni(II) salts magnetically coupled [2 × 2] grids form with such ligands, exhibiting intramolecular antiferromagnetic exchange,6,7 despite long distances between metal centers (6.5 Å for Co). Evidence from NMR and ES mass spectrometry data has shown that with appropriate extension of these pyrimidine-based ligands [4 × 4] Pb16 square grids can also be produced.8 Tritopic ligands (e.g. 2poap, Cl2poap; Chart 1), containing a linear arrangement of coordination pockets, self-assemble in high yield to form M9 (M ) Mn(II), Fe(III), Co(II), Ni(II), Cu(II)) grid structures, with a [M9(µ-O)12] central core.9-15 Each ligand binds to three metal ions with an alkoxide oxygen bridging between metal centers (Chart 2). The large M-O-M bridge angles lead to antiferromagnetic exchange interactions with Mn(II), Co(II), and Ni(II) complexes, but in the case of Cu(II) a preferred orthogonal magnetic bridging arrangement throughout the grid leads to dominant intramolecular ferromagnetic exchange in all cases. The high-yield syntheses of these homoleptic M9 grids (usually >80%) indicates that the highly symmetric square (3) Baxter, P. N. W.; Lehn, J.-M.; Kneisel, B. O.; Fenske, D. Angew. Chem., Int. Ed. Engl. 1997, 36, 1978. (4) Hanan, G. S.; Volkmer, D.; Ulrich, S. S.; Lehn, J.-M.; Baum, G.; Fenske, D. Angew. Chem., Int. Ed. Engl. 1997, 36, 1842. (5) Bassani, D. M.; Lehn, J.-M.; Fromm, K.; Fenske, D. Angew. Chem., Int. Ed. 1998, 37, 2364. (6) Waldmann, O.; Hassmann, J.; Mu¨ller, P.; Hanan, G. S.; Volkmer, D.; Schubert, U. S.; Lehn, J.-M. Phys. ReV. Lett. 1997, 78, 3390. (7) Waldmann, O.; Hassmann, J.; Mu¨ller, P.; Hanan, G. S.; Volkmer, D.; Schubert, U. S.; Lehn, J.-M. Phys. ReV. B 1998, 58, 3277. (8) Garcia, A. M.; Romero-Salguero, F. J.; Bassani, D. M.; Lehn, J.-M.; Baum, G.; Fenske, D. Chem. Eur. J. 1999, 5, 1803. (9) Zhao, L.; Matthews, C. J.; Thompson, L. K.; Heath, S. L. J. Chem. Soc., Chem. Commun. 2000, 265. (10) Zhao, L.; Xu, Z.; Thompson, L. K.; Heath, S. L.; Miller, D. O.; Ohba, M. Angew. Chem., Int. Ed. 2000, 39, 3114. (11) Waldmann, O.; Koch, R.; Schromm, S.; Mu¨ller, P.; Zhao, L.; Thompson, L. K. Chem. Phys. Lett. 2000, 332, 73. (12) Waldmann, O.; Zhao, L.; Thompson, L. K. Phys. ReV. Lett. 2002, 88, 066401-1-4. (13) Zhao, L.; Xu, Z,; Thompson, L. K.; Miller, D. O. Polyhedron 2001, 20, 1359. (14) Xu, Z.; Thompson, L. K.; Miller, D. O. Polyhedron 2002, 21, 1715. (15) Thompson, L. K.; Zhao, L.; Xu, Z.; Miller, D. O.; Reiff, W. M. Inorg. Chem. 2003, 42, 128.

Chart 2

arrangement, in which there is an exact match between the preferred coordination requirements of the metal ions and the available coordination pockets in the ligand, is highly favored thermodynamically. Conditions for the formation of these M9 grid systems usually involve heating in aqueous solvent mixtures. However in the case of copper(II) milder reaction conditions (e.g. room temperature), the use of solvent media with weaker donor character (e.g. CH3CN, CH3CN/CH3OH mixtures), and in some cases using anionic groups with significant donor capacity (e.g. acetate) can lead to the formation of linear trinuclear species,16 in which the diazine nitrogen atoms act as bridges between adjacent metal centers (Chart 2). This dramatically different ligand conformation leads to very different magnetic behavior, dominated by antiferromagnetic exchange.16 Extension of the basic hydrazide ligand to include terminal oxime groups leads to expansion of the coordination capacity of the ligand (e.g. dpocco, Chart 2) and the formation of a novel Cu36 cluster, with diazine N2 bridging, exhibiting antiferromagnetic exchange.17 In an unusual reaction in which 2poap was reacted with Cu(NO3)2‚3H2O in the presence of Gd(NO3)3 in CH3OH/ CH3CN a new oligomeric cluster/grid arrangement was obtained, in which four ligands self-assemble via half of each ligand to form a central square [Cu4(µ-O)4] core, with four appended alkoxide-bridged Cu(II) centers in a “pinwheel” arrangement.18 This system also behaves as an intramolecular ferromagnet. The Gd(III) ion was originally assumed to have a precoordination effect in the self-assembly process and contribute to the formation of the octanuclear complex. However it has now been shown that with different ligands, e.g. Cl2poap, Cl2pomp, and 2pomp (Chart 1), the presence of Gd(III) is not necessary and that Cu8 clusters can be produced independently by suitable choice of solvent and (16) Zhao, L.; Thompson, L. K.; Xu, Z.; Miller, D. O.; Stirling, D. R. J. Chem. Soc., Dalton Trans. 2001, 1706. (17) Abedin, T. S. M.; Thompson, L. K.; Miller, D. O.; Krupicka, E. J. Chem. Soc., Chem. Commun. 2003, 708. (18) Xu, Z.; Thompson, L. K.; Miller, D. O. J. Chem. Soc., Chem. Commun. 2001, 1170.

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Milway et al. temperature. Also in one case the Cu8 pinwheel cluster complex can be converted to the more thermodynamically favored Cu9 grid simply by heating in a polar solvent. The structural and magnetic properties for [Cu8(Cl2poap2H)4(NO3)8]‚20H2O (1), [Cu8(Cl2pomp-2H)4(NO3)8]‚15H2O (2), [Cu8(2pomp-2H)4(H2O)8](ClO4)8 (4), [Cu9(Cl2poap-H)3(Cl2poap-2H)3](NO3)9‚18H2O (5), and [Cu9(m2poap-H)2(m2poap-2H)4](NO3)8‚17H2O (6) are now reported, in addition to a novel complex with a mixture of ligands, [Cu9(2poap-H)4(picd-H)3(picd-2H)](NO3)9‚9H2O (7), which highlights the importance of ligand-ligand interactions and ligand “bite” in the construction of the grid. Experimental Section Physical Measurements. Infrared spectra were recorded as Nujol mulls using a Mattson Polaris FT-IR instrument, and UV/vis spectra were obtained as Nujol mulls using a Cary 5E spectrometer. Microanalyses were carried out by Canadian Microanalytical Service, Delta, Canada. Variable-temperature magnetic data (2300 K) were obtained using a Quantum Design MPMS5S SQUID magnetometer using field strengths in the range 0.1-5 T. Background corrections for the sample holder assembly and diamagnetic components of the complexes were applied. Synthesis of Complexes. [Cu8(Cl2poap-2H)4(NO3)8]‚20H2O (1). Cl2poap was synthesized by adopting a published method starting from 4-chloro-2,6-pyridinedicarboxylic acid.9,10 Cl2poap (0.44 g, 1.0 mmol) was suspended in a solution of Cu(NO3)2‚3H2O (0.87 g, 3.0 mmol) in a mixture of 10 mL of acetonitrile and 10 mL of methanol at room temperature. The resulting mixture was stirred for a few minutes at room temperature, forming a clear deep green solution. Green crystals, suitable for a structural determination, formed from the filtrate after standing overnight at room temperature (yield 83.5%). IR (Nujol mull, cm-1): 3400, 3192 (br, w) (ν(H2O) and ν(NH2)); 1771 (w), 1749 (w), 1733 (w) (ν(NO3)). 1666 (ν(CdO)); 1028, 1020 (m) (ν(py)). Anal. Found: C, 29.20; H, 2.94; N, 20.00. Calcd for (C19H14N9O2Cl)4Cu8(NO3)8(H2O)20: C, 29.42; H, 3.10; N, 19.87. [Cu8(Cl2pomp-2H)4(NO3)8]‚15H2O (2) and [Cu8(Cl2pomp2H)4(H2O)8](ClO4)8‚7H2O (3). Cl2pomp was synthesized by reaction of 2-acetylpyridine with 4-chloropyridine-2,6-dicarboxylic hydrazide by standard procedures (yield 93%, mp >300 °C). Anal. Found: C, 57.53; H, 4.07; N, 22.27. Calcd for C21H18N7O2Cl: C, 57.91; H, 4.17; N, 22.52.9,10,16 Cu(NO3)2‚3H2O (0.21 g, 0.87 mmol) was dissolved in methanol (10 mL). Cl2pomp (0.10 g, 0.23 mmol) was added and dissolved rapidly on warming. A powdery green precipitate formed, which was redissolved by addition of water (10 mL) and warming. Green rectangular prismatic crystals, suitable for structural analysis, formed after 3 days (yield 83%). IR (Nujol mull, cm-1): 3403 (ν(NH2) and ν(H2O)); 1627 (ν(CdO)), 1601 (ν(CdN)); 1025 (w) (ν(py)). Anal. Found: C, 33.55; H, 2.87; N, 16.72. Calcd for (C21H16N7O2Cl)4Cu8(NO3)8(H2O)15 (2): C, 33.58; H, 3.16; N, 16.79. 3 was prepared in the same manner with Cu(ClO4)2‚6H2O and obtained as green crystals (yield 80%). Anal. Found: C, 30.40; H, 2.63; N, 12.09. Calcd for (C21H16N7O2Cl)4Cu8(ClO4)8(H2O)15: C, 30.57; H, 2.87; N, 11.89. [Cu8(2pomp-2H)4(H2O)8](ClO4)8 (4). 2pomp was synthesized by standard procedures by reaction of 2-acetylpyridine with pyridine-2,6-dicarboxylic hydrazide.9,10,16 2pomp (0.24 g, 0.50 mmol) was added to a solution of Cu(ClO4)2‚6H2O dissolved in methanol/water mixture (10 mL/5 mL) with stirring. A clear deep blue green solution formed, which was filtered after several hours

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and allowed to stand at room temperature. Dark green crystals suitable for structural determination were obtained after 2 weeks (yield 70%). IR (Nujol mull, cm-1): 3600, 3450 (ν(H2O)), 1620 (ν(CdO)), 1602, 1571 (ν(CdN)), 1077 (ν(ClO4)). UV/vis (λmax (nm); water): 683. Anal. Found: C, 33.23; H, 2.70; N, 13.03. Calcd for (C21H16N7O2)4Cu8(ClO4)8(H2O)8: C, 33.11; H, 3.02; N, 12.88. [Cu9(Cl2poap-H)3(Cl2poap-2H)3](NO3)9‚20H2O (5). The reaction between Cl2poap and Cu(NO3)2‚3H2O was repeated using a different solvent mixture (10 mL of methanol and 10 mL of deionized water) with warming. An orange-red crystalline product formed, suitable for a structural analysis (yield 86%). IR (Nujol mull, cm-1): 3337 (w) (ν(NH2) and ν(H2O)); 1670 (ν(CdO)); 1015 (w) (ν(py)). Anal. Found: C, 33.57; H, 2.98; N, 21.31. Calcd for (C19H14N9O2Cl)3(C19H15N9O2Cl)3Cu9(NO3)9(H2O)20: C, 33.33; H, 3.12; N, 21.48. [Cu9(m2poap-H)2(m2poap-2H)4](NO3)8‚17H2O (6). m2poap14 (0.100 g, 0.231 mmol) was dissolved in methanol (10 mL) and added to a solution of Cu(NO3)2‚3H2O (0.170 g, 0.693 mmol) in deionized water (10 mL). The resulting green solution was warmed, refluxed for 15 min, and left to evaporate slowly at room temperature. Brown crystals suitable for structural analysis were obtained after 2 weeks (yield 30%). IR (Nujol mull, cm-1): 3337 (w) (ν(NH2) and ν(H2O)); 1667 (ν(CdO)); 1587 (ν(CdN)); 1012 (ν(py)). UV/vis (λmax (nm); Nujol mull): 395, 480 (sh), 743. Anal. Found: C, 36.09; H, 3.07; N, 22.00. Calcd for (C20H18N9O3)2(C20H17N9O3)4Cu9(NO3)8(H2O)17: C, 36.33; H, 3.56; N, 21.89. [Cu9(2poap-H)4(picd-H)3(picd-2H)](NO3)9‚9H2O (7). Cu(NO3)2‚ 3H2O (0.90 g, 3.7 mmol) was dissolved in MeOH/H2O (10 mL/5 mL). 2poap (0.40 g, 1.0 mmol) and pyridine-2,6-dicarboxylic acid (0.17 g, 1.0 mmol) were well mixed and added together to the metal ion solution with stirring. A clear green solution was formed on heating, which deposited dark green crystals, suitable for structural analysis, after several days (yield 75%). IR (Nujol mull, cm-1): 3337 (w) (ν(NH2) and ν(H2O)); 1670 (ν(CdO)); 1015 (w) (ν(py)). Anal. Found: C, 34.93; H, 2.80; N, 19.56. Calcd for (C19H16N9O2)4(C7H4NO4)3(C7H3NO4)Cu9(NO3)9(H2O)9: C, 35.07; H, 2.75; N, 19.25. Crystallographic Data and Refinement of the Structures. The diffraction intensities of a green prismatic crystal of 1 of dimensions 0.33 × 0.26 × 0.22 mm were collected with graphite-monochromatized Mo KR X-radiation (rotating anode generator) using a Bruker P4/CCD diffractometer at 193(1) K to a maximum 2θ value of 52.8°. The data were corrected for Lorentz and polarization effects. The structure was solved by direct methods.19,20 All atoms except hydrogens were refined anisotropically. Hydrogen atoms were placed in calculated positions with isotropic thermal parameters set to 20% greater than their bonded partners and were not refined. Neutral atom scattering factors21 and anomalous-dispersion terms22,23 were taken from the usual sources. All other calculations were performed with the teXsan24 crystallographic software package using a PC computer. The maximum and minimum peaks on the (19) (a) SHELX97: Sheldrick, G. M., 1997. (b) SIR97: Altomare, A.; Cascarano, M.; Giacovazzo, C.; Guagliardi, A. J. Appl. Crystallogr. 1993, 26, 343. (20) DIRDIF94: Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; de Gelder, R.; Israel, R.; Smits, J. M. M. The DIRDIF-94 program system; Technical Report of the Crystallography Laboratory, University of Nijmegen: Nijmegen, The Netherlands, 1994. (21) Cromer, D. T.; Waber, J. T. International Tables for X-ray Crystallography; The Kynoch Press: Birmingham, England, 1974; Vol. IV, Table 2.2A. (22) Ibers, J. A.; Hamilton, W. C. Acta Crystallogr. 1964, 17, 781. (23) Creagh, D. C.; McAuley, W. J. International Tables for Crystallography; Wilson, A. J. C., Ed.; Kluwer Academic Publishers: Boston, MA, 1992; Vol. C, Table 4.2.6.8, pp 219-222.

Cu(II) Complexes with Linear “Tritopic” Ligands Table 1. Summary of Crystallographic Data for 1, 2, 4, and 7 param

1

2

4

7

empirical formula Mr cryst system space group a/Å b/Å c/Å R/deg β/deg γ/deg V/A3 Fcalcd/g cm-3 T/K Z µ/cm-1 reflcns collcd: tot., unique, Rint reflcns obsd (I > 2.00σ(I)) final R1,a wR2b

C76H92N44Cu8O50Cl4 3072.01 tetragonal I4h 21.645(1) 21.645(1) 12.950(1) 90 90 90 6067.0(6) 1.681 193(1) 2 15.68 20 830, 6234, 0.040 5606 0.043, 0.127

C84H88N36O44Cl4Cu8 2956.01 tetragonal I4h 21.2562(8) 21.2562(8) 12.7583(4) 90 90 90 5764.5(4) 1.703 193(1) 2 16.41 17 468, 58 73, 0.064 4743 0.055, 0.148

C84H120N28O66Cl8Cu8 3370.01 tetragonal I41/a 20.7790(4) 20.7790(4) 32.561(1) 90 90 90 14058.8(6) 1.592 193(1) 4 14.39 34 336, 7189, 0.025 7189 0.061, 0.203

C104H104N46O56Cu9 3466.15 triclinic P1h 15.473(1) 19.869(2) 23.083(2) 88.890(2) 81.511(2) 68.607(1) 6530.7(8) 1.763 193(1) 2 15.50 37 176, 26 239, 0.044 15 585 0.075, 0.211

a

R1 ) ∑|Fo| - |Fc|/∑|Fo|. b wR2 ) [∑[w(|Fo|2 - |Fc|2)2]/∑[w(|Fo|2)2]]1/2.

final difference Fourier map corresponded to 1.38 and -0.36 e-/Å3, respectively. Abbreviated crystal data for 1 are given in Table 1. Diffraction data were collected for 2 (green prism; 0.29 × 0.09 × 0.08 mm), 4 (dark prism; 0.53 × 0.45 × 0.40 mm), 5 (red brown prism; 0.18 × 0.15 × 0.07 mm), 6 (brown prism; 0.74 × 0.34 × 0.33 mm), and 7 (yellow-brown prism; 0.40 × 0.20 × 0.15 mm) in a similar manner. A very weak data set for 5 (10% observed with an Rint ) 49%) led to poor refinement values and a rather low parameter/data ratio (20 K. The preliminary structure clearly reveals the Cu9 grid arrange(29) Xu, Z.; Thompson, L. K.; Miller, D. O. Inorg. Chem. 1997, 36, 3985. (30) Thompson, L. K.; Xu, Z.; Goeta, A. E.; Howard, J. A. K.; Clase, H. J.; Miller, D. O. Inorg. Chem. 1998, 37, 3217.

Cu(II) Complexes with Linear “Tritopic” Ligands

Figure 13. Plot of magnetic moment/mol as a function of temperature for 5. The solid line represents the best fit to eqs 2 and 3 with g ) 2.29(5), J1 ) 0.87(4) cm-1, J2 ) -17.9(7) cm-1, TIP ) 550 × 10-6 emu‚mol-1, and Θ ) -0.3 K (102R ) 0.91).

Figure 14. Magnetization data for 5 as a function of field at 2 K. The solid line is calculated for g ) 2.29 and S ) 7/2.

ment, and this is confirmed by the magnetic data. The magnetic data were fitted successfully to eqs 2 and 3, with g ) 2.29(5), J1 ) 0.87(4) cm-1, J2 ) -17.9(7) cm-1, TIP ) 550 × 10-6 emu‚mol-1, and Θ ) -0.30 K (102R ) 0.91). The solid line in Figure 13 was calculated with these parameters. Magnetization data at 2 K, as a function of field, show that 5 is almost saturated at 5.0 T, with an Nβ value consistent with an S ) 7/2 ground state. Figure 14 compares the experimental values with calculated values for g ) 2.29 and S ) 7/2 at 2 K. The slightly lower experimental profile may be associated with an intermolecular antiferromagnetic association, as suggested by the small negative Θ value. It is interesting to note that while J1 is comparable to J1 for the analogous grid complex [Cu9(2poap)6](NO3)12‚9H2O (J1 ) 0.52 cm-1, J2 ) -24.3 cm-1),10,11,13 |J2| is somewhat smaller. This could be rationalized in terms of the presence of electron-withdrawing chlorine atoms on the two central pyridine rings, which are bonded to the central copper. 6 exhibits similar magnetic properties, expected on the basis of the grid structure (Figure 6), with magnetic moment dropping from 5.84 µB (per mole) at 300 K to a minimum value of 5.16 µB at 26 K, followed by a sharp rise to 6.39 µB at 2 K. The magnetic data were fitted successfully to eqs 2 and 3 giving g ) 2.17(1), J1 ) 0.5(1) cm-1, J2 ) -23(1) cm-1, TIP ) 790 × 10-6 emu‚mol-1, and Θ ) 0 K (102R ) 1.85). J1 and J2 are comparable with the values observed for [Cu9(2poap)6](NO3)12‚9H2O.10,11,13 Magnetization data as

Figure 15. Plot of magnetic moment/mol as a function of temperature for 7. The solid line represents the best fit to eqs 2 and 3 with g ) 2.09(1), J1 ) 0.5(2) cm-1, J2 ) -29(2) cm-1, TIP ) 550 × 10-6 emu.mol-1, and Θ ) 0 K (102R ) 1.3).

a function of field at 2 K show a typical profile with M rising steeply and approaching saturation at 5.0 T with M ) 7.1 Nβ. The positive ferromagnetic J1 observed for both 5 and 6 is directly associated with the orthogonal bridging arrangement between the eight copper atoms in the outer ring, while the negative, antiferromagnetic J2 is unusual but typical of these systems and has been rationalized in terms of a fluxional magnetic ground state associated with the unusual, tetragonally compressed (dz2 ground state) central copper. On the basis of the dz2 ground state one would nominally expect not to have antiferromagnetic exchange involving the central copper, but the clearly defined J2 indicates that on average this metal center interacts with one adjacent copper antiferromagnetically. This is indicative of a possible dynamic Jahn-Teller distortion at the central copper, or a mixed orbital ground-state situation. The variable-temperature magnetic data for 7 are very similar to those of 5 and 6, with magnetic moment dropping from 5.5 µB at 300 K to 4.9 µB at ∼30 K, followed by a sharp rise at lower temperatures to 6.1 µB at 2 K (Figure 15), behavior typical of a grid exhibiting both ferromagnetic and antiferromagnetic exchange. The data were fitted successfully to eqs 2 and 3 for g ) 2.09(1), J1 ) 0.5(2) cm-1, J2 ) -29(2) cm-1, TIP ) 550 × 10-6 emu‚mol-1, and Θ ) 0 K (102R ) 1.3). The solid line in Figure 15 is calculated with these parameters. Magnetization data as a function of field at 2 K indicate that the system is not fully saturated at 5.0 T (50 000 Oe) but approaches the expected S ) 7/2 ground state. This is probably associated with the quite distorted and more flexible nature of the grid core and the larger than usual antiferromagnetic J2 term. However, what is remarkable is the fact that the magnetic properties of 7 are typical of the usual homoleptic [Cu9(µ-O)12] grids, despite the fact that four oxygen bridges are from carboxylate oxygen atoms and not alkoxide oxygens and that the complex contains a mixture of ligands. Conclusion It has been demonstrated that two oligomers can be produced with the ligand Cl2poap in self-assembly reactions with copper nitrate, depending on solvent and temperature, Inorganic Chemistry, Vol. 43, No. 6, 2004

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Milway et al. and that the alkoxide bridged pinwheel Cu8 cluster can be converted to the more thermodynamically favored, alkoxidebridged Cu9 [3 × 3] grid. In the case of Cl2pomp conversion of the pinwheel cluster to the Cu9 [3 × 3] grid form does not appear to occur, at least under the conditions tested. On the other hand m2poap forms a Cu9 [3 × 3] with no tendency to form a Cu8 structure. Another pinwheel Cu8 cluster is observed with the ligand 2pomp, in which there are two different bridging groups, µ2-(N-N) and µ2-alkoxide, indicating again the coordinative flexibility of this class of ligands. With a mixture of 2poap and pyridine-2,6-dicarboxylic acid a remarkable [Cu9(µ-O)12] [3 × 3] grid results, with four pyridine dicarboxylate ligands effectively occupying the normal position of two 2poap ligands. This indicates that grid construction can be accomplished with different ligand mixtures, perhaps opening up a way to grids of different and varied dimensions. Magnetic properties are interpreted in terms of dominant ferromagnetic coupling within both types of self-assembled supramolecular system, which shows up clearly at low temperatures, and is associated with orthogonal bridging arrangements. The magnetic ground state for 1-3 (S ) 8/2) indicates that all interactions are

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ferromagnetic, but for 4 there is evidence for a weak intramolecular antiferromagnetic component. For 5-7 a putative fluxional magnetic ground state at the central copper leads to an overall S ) 7/2 spin ground state for the grids, resulting from the compensating effects of ferromagnetic and antiferromagnetic exchange components. Acknowledgment. We thank the NSERC (Natural Sciences and Engineering Research Council of Canada) and the Research Council of Norway (H.G.) for financial support and Dr. R. McDonald, University of Alberta, for structural data. Supporting Information Available: X-ray crystallographic data in CIF format for 1, 2, 4, and 7. This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic data for 1, 2, 4, and 7 have also been deposited with the Cambridge Crystallographic Data Center, CCDC Nos. 206673, 206674, 206676, and 223491. Copies of this information may be obtained free of charge from The Director, CCDC, 12, Union Road, Cambridge CB2 1EZ, U.K. (fax +44-1223-336033; E-mail [email protected]; http://www.ccdc.cam.ac.uk). IC030113U