Di-, Tetra-, and Hexanuclear Hydroxy-Bridged Copper(II) Cluster

Di-, Tetra-, and Hexanuclear Hydroxy-Bridged Copper(II) Cluster Compounds: Syntheses, Structures, and Properties Xing Li, Deyi Cheng, Jianli Lin, Zhifeng Li, and Yueqing Zheng* State Key Laboratory Base of NoVel Functional Materials & Preparation Science, Faculty of Materials Science & Chemical Engineering, Ningbo UniVersity, Ningbo, Zhejiang 315211, P. R. China

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 8 2853–2861

ReceiVed NoVember 23, 2007; ReVised Manuscript ReceiVed January 31, 2008

ABSTRACT: Self-assembly reactions of copper (II) ions, dicarboxylate (adipate or terephthalate) and terminal ligands (4,4′-bipyridine or phenanthroline) yielded three hydroxy-bridged Cu(II) cluster complexes: [Cu(phen)(OH)(H2O)]2 · (C8H4O4) · 8H2O (1), [Cu4(bpy)4(OH)4(H2O)2] · (C8H4O4)2 · 6H2O (2) and [Cu6(bpy)6(OH)6(H2O)2] · (C6H8O4)3 · 23H2O (3) (phen ) phenanthroline, bpy ) 2,2′-bipyridine, C8H6O4 ) terephthalic acid, C6H10O4 ) adipic acid). Single crystal X-ray diffraction analyses reveal that complex 1 is a discrete dinuclear Cu2O2 motif, 2 presents a discrete chairlike tetranuclear copper(II) Cu4O4 core formed by two Cu2O2 units, and 3 assumes a discrete hexameric copper(II) Cu6O6 cluster consisting of three Cu2O2 units. The polynuclear structures are generated from the Cu(II) ions linked by the µ2- and/or µ3-OH groups. In 1-3, all Cu(II) ions show square pyramidal coordination geometry, and the dicarboxylate ions act as counteranions to keep the whole structure neutral. Thermogravimetric analyses (TGA) and magnetic properties of 1-3 were studied, respectively. Introduction Design and assembly of single molecule-based magnets are currently is of the most important focuses of intensive research in coordination chemistry, supramolecular chemistry and materials science,1–3 in which polynuclear systems play a significant role due to allowing multifold interaction among different metal centers for energy transfer and magnetic exchange.4–6 Thus, considerable effort has been devoted to design and syntheses of new building blocks and to assembly of required products with special properties and novel topological features.7 However, preparation of polynuclear clusters with special topological features has to be confronted with many great challenges because crystal structures are affected by multiple factors such as pH value, metal ions, organic ligands, reaction conditions and so on.1 Extensive investigations showed that various building motifs featuring metal-oxygen polyhedra are accessible through self-assembly of metal cations with carboxylate anions. But these metal-oxygen polyhedra are easily interconnected by bridging organic spacers into 1D polymeric chains,8 2D layers9 and 3D frameworks.10 For example, 1D chainlike polymeric metal oxide connectivity in [Co(H2O)4(C4H4O4)]n can be visualized as from substitution of the linear trimer of three edge-sharing {CoO6} octahedra.11,12 A series of hydrothermally prepared cobalt hydroxo succinate polymers such as {[Co4(OH)2(H2O)2(C4H4O4)3] · 2H2O}n, [Co5(OH)2(C4H4O4)4]n and {[Co6(OH)2(C4H4O4)5] · H2O}n exhibit 2D metal oxide connectivity into layers pillared by dicarboxylate ligands.13 Recently, a new 3D metal oxide connectivity of {NiO6} octahedron was reported for an open Ni(II) succinate {[Ni7(C4H4O4)6(OH)2(H2O)2] · 2H2O}n.14 To the best of our knowledge, no fine theoretical mode was found to fit the magnetic behaviors for the infinite architectures. Studies on magnetic properties of single molecule-based magnets are performed commonly by certain building motifs acting as basic units of magnetic exchange to simulate the compound properties. Therefore, the discrete polynuclear metal clusters are ideal candidates for the investigation on the properties of molecule-based magnets. The most important motifs with oxygen atom as bridging ligand are * To whom correspondence should be addressed. E-mail: zhengyueqing@ nbu.edu.cn. Fax: +86-574-87600734.

shown in Scheme 1: (I) dinuclear, (II) steplike or chairlike, (III) cubane-like, (IV) double-open cubane-like, and (V) steplike of hexanuclear metal cluster, of which the dinuclear motif can be viewed as basic building unit to form tetranuclear or hexanuclear clusters. Otherwise, copper(II) ions are frequently selected by preference for assembly of the discrete polynuclear metal clusters15,16 due to the following: (a) A copper(II) ion has various coordination geometries: linetype, planar trigon, trigonal pyramid, planar quadrilateral, tetrahedron, trigonal bipyramid, tetragonal pyramid, octahedron and so on (Scheme 2). (b) Polynuclear copper clusters with different structural blocks are accessible with appropriate bridged linkers (Scheme 1). (c) The copper(II) ion is paramagnetic, and its compounds often exhibit rich chemical-physical properties. In the past decades, many discrete dinuclear copper(II) compounds were prepared and their magnetic behaviors were interpreted well;17 and the discrete tetranuclear copper(II) clusters are not so prevalent compared with the discrete dinuclear copper(II) compounds. The first cubane-like tetranuclear copper(II) compound with Cu4O4 core bridged by hydroxy species was {[Cu(2,2′-bipy)(OH)]4 · (PF6)4}.18 Mathews and Manohar reported the first steppedcubane structure with Cu4O4 core, [Cu4L2(bipy)4(µ3-OH)2][ClO4]4 (HL ) 5-hydroxy-6-methylpyridine-3,4-dimethanol, bipy ) 2,2′-bipyridine).19 our group reported the first chairlike tetranuclear copper(II) structure, in which the Cu4(OH)4 core contained both µ2-OH and µ3-OH spacer.20 In the discrete tetranuclear clusters, the cubane-like and double-opencubane (Scheme 1, III and IV) are correspondingly abundant and their magnetic properties are known;15,21 nevertheless, the chairlike or steplike Cu4O4 tetramers (Scheme 1, II) are scant and the investigation on their magnetic properties is in trouble by the formula available.16,19 Strangely, until now only one hexanuclear copper(II) compound was reported,22 which may be attributed to the limitation of synthetic techniques. In fact, a lot of new complexes are found by chance, though the directed syntheses were carried out through the control and modification of reaction parameters. One of our research interests has focused on a systematic study of the discrete polynuclear copper(II) compounds. By employing the dicarboxylate ligands and copper(II) ions along with terminal ligands (phen or bpy), we have successfully

10.1021/cg701150q CCC: $40.75  2008 American Chemical Society Published on Web 07/15/2008

2854 Crystal Growth & Design, Vol. 8, No. 8, 2008

Li et al. Scheme 1

Scheme 2

prepared a series of the discrete structures: mononuclear compound,23 [Cu(phen)2(C6H8O4)] · 4.5H2O; dinuclears,24 [Cu2(phen)2(H2O)2(C7H10O4)2] · 4H2O, [Cu2(phen)2(H2O)2(OH)2] · (C4H4O4) · 6H2O, [Cu2(phen)2(H2O)2(C5H6O4)2], [Cu2(phen)2(C9H14O4)2] · 6H2O; tetranuclears,20 [Cu4(bpy)4(OH)4Cl2] · Cl2 · 6H2O and [Cu4(phen)4(OH)4(H2O)2] · Cl4 · 4H2O. These discrete units were further extended into supramolecular framework by hydrogen bonds and π · · · π stacking interactions. Our previous work spurred us on to systematically study metaldicarboxylate coordination chemistry with discrete polynuclear cluster units and to investigate the correlation between structures and properties. It has been well-documented that the introduction of terminal ligands into a reaction system can result in the formation of low-dimensional networks as the terminal ligands reduce the available metal binding sites and restrict the polymeric growth.25 Therefore, the terminal ligand 2,2′-pyridine (bpy) or phenanthroline (phen) was introduced into a reaction system containing copper(II) ions and the dicarboxylate ligands with the hope of isolating discrete high polynuclear complexes with special chemical-physical properties. As an ongoing part of our investigation on multicopper cluster compounds, we report here three new hydroxy-bridged copper(II) complexes, [Cu(phen)(OH)(H2O)]2 · (C8H4O4) · 8H2O (1), [Cu4(bpy)4(µ2OH)2(µ3-OH)2(H2O)2] · (C8H4O4)2 · 6H2O (2) and [Cu6(bpy)6(µ2OH)2(µ3-OH)4(H2O)2] · (C6H8O4)3 · 23H2O (3). Complex 1 is a discrete dinuclear Cu2O2 structure; 2 presents a chairlike tetranuclear copper(II) with Cu4O4 core formed by two Cu2O2 units; 3 assumes a steppedlike hexameric copper(II) cluster Cu6O6, which could be visualized as from trimerization of Cu2O2 motifs via four out-of-plane Cu-O bonds. To the best of our knowledge, such a steppedlike hexanuclear copper(II) [Cu6(µ2OH)2(µ3-OH)4] core has never been reported in the literature. Experimental Section Synthesis of [Cu(phen)(OH)(H2O)]2 · (C8H4O4) · 8H2O (1). Dropwise addition of 1.0 mL (1 M) of Na2CO3 to a stirred aqueous solution of CuCl2 · 2H2O (0.17 g, 1.00 mmol) in 5.0 mL of H2O gave a blue precipitate, which was separated by centrifugation and washed with

water until no Cl- anions were detectable in the supernatant. The collected blue precipitate was transferred to a mixture solution of methanol and water (1:1 V/V, 50 mL), to which phenanthroline (0.198 g, 1.00 mmol) and sodium terephthalate (0.166 g, 1.00 mmol) were added successively. The mixture solution was adjusted with NH3 · H2O to pH ) 11.1 and allowed to stand at room temperature. Blue blocklike crystals were grown by slow evaporation for over 1 day (yield: ca. 75% based on the initial CuCl2 · 2H2O input). Anal. Calcd for C32H42Cu2N4O16 (%): C, 44.39; H, 4.89; N, 6.47. Found: C, 44.32; H, 4.86; N, 6.52. IR (cm-1): 3225vw, 2905w, 1695m, 1639m, 1564vs, 1552vs, 1521s, 1429vs, 1381vs, 1300m, 1218m, 1200m, 1139m, 11061m, 875m, 846s, 725s, 634m, 496m. Synthesis of [Cu4(bpy)4(µ2-OH)2(µ3-OH)2(H2O)2] · (C8H4O4)2 · 6H2O (2). Dropwise addition of 2.0 mL (1 M) of NaOH to a stirred aqueous solution of CuCl2 · 2H2O (0.17 g, 1.00 mmol) in 5.0 mL of H2O gave a fine blue precipitate, which was separated by centrifugation and washed with water until no Cl- anions were detectable in the supernatant. The collected blue precipitate was added to a methanolic solution of 2,2′-bipyridine (0.16 g, 1.00 mmol) in 20 mL of CH3OH and stirred for 15 min to give a blue solution, to which an aqueous solution of sodium terephthalate (0.21 g, 1.26 mmol) in 20 mL of H2O was added. The mixture was refluxed for 2 h. After filtration, the blue filtrate (pH ) 9.5) was maintained at room temperature and greenish blue crystals were grown in 3 days (yield: ca. 43% based on the initial CuCl2 · 2H2O input). Anal. Calc.d for C56H60Cu4N8O20 (%): C, 47.35; H, 4.23; N, 7.89. Found: C, 47.28; H, 4.15; N, 7.83. IR (cm-1): 3385s (broad), 3109vw, 3057vw, 3034vw, 1653m, 1601s (sharp), 1447s (sharp), 1354s, 1254vw, 1161w, 1030m, 1018m, 853m (sharp), 770s (sharp), 731s (sharp). Synthesis of [Cu6(bpy)6(µ2-OH)2(µ3-OH)4(H2O)2] · (C6H8O4)3 · 23H2O (3). Dropwise addition of 1.0 mL (1 M) of Na2CO3 to a stirred aqueous solution of CuCl2 · 2H2O (0.10 g, 0.64 mmol) in 5.0 mL of H2O yielded a fine blue precipitate, which was separated by centrifugation and washed with water until no Cl- anions were detectable in the supernatant. The fresh precipitate was then added to a stirred aqueous solution of adipic acid (0.094 g, 0.64 mmol) in 10.0 mL of H2O, producing a green suspension, to which an ethanolic solution of 2,2′bipyridine (0.10 g, 0.64 mmol) in 10 mL of C2H5OH was dropped. The mixture was further stirred vigorously, and the resulting blue solution (pH ) 6.0) was then adjusted with NaOH to pH ) 8.6 and allowed to stand at room temperature. Blue platelike crystals were grown by slow evaporation for 1 week (yield: ca. 85% based on the initial CuCl2 · 2H2O input). Anal. Calcd for C78H128Cu6N12O43 (%): C, 40.68; H, 5.60; N, 7.30. Found: C, 40.53; H, 5.72; N, 7.38. IR (cm-1): 3379vs (broad), 1655m, 1603s (sharp), 1564vs (sharp), 1447s (sharp), 1404s (sharp), 1321w, 1167vw, 1123vw, 1036m (doublet), 773vs (sharp), 729s (sharp), 658w, 492m. X-ray Crystallography. Suitable single crystals were selected under a polarizing microscope and fixed with epoxy cement on respective fine glass fibers which were then mounted on a Bruker P4 diffractometer with graphite-monochromated Mo KR radiation (λ ) 0.71073 Å) for cell determination and subsequent data collection for 1-3. The reflection intensities with 2θmax ) 55° were collected at 293 K using the θ-2θ scan technique and corrected for Lp and absorption effects. SHELXS-97 and SHELXL-97 programs26,27 were used for structure solution and refinement. The structures were solved by using direct

Hydroxy-Bridged Copper(II) Cluster Compounds

Crystal Growth & Design, Vol. 8, No. 8, 2008 2855

Table 1. Crystal Data and Structure Refinement Parameters for Compounds 1-3 compounds

1

2

3

empirical formula formula mass crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) volume (Å3) Z Dcalc (g cm–3) F(000) µ (mm–1) θ range (deg) total no. of data collected no. of obsd data (I g 2σ(I)) R1, wR2 [I g 2σ(I)]a R1, wR2 (all data)a GOF on F2 no. of variables largest diff peak and hole (e Å–3)

C32H42Cu2N4O16 865.78 triclinic P1j 9.388(2) 10.613(2) 11.299(2) 114.74(3) 112.84(3) 94.48(3) 897.7(3) 1 1.601 448 1.265 2.21–27.50 3904 3311 0.0430, 0.1039 0.0543, 0.1106 1.040 291 0.420, –0.928



a R1 ) ∑(|Fo| – |Fc|)/∑|Fo|, wR2 ) [∑w(Fo2 – Fc2)2/∑w(Fo2)2]1/2, w ) [σ2(Fo2) + (aP)2 + bP]–1, where P ) (Fo2 + 2Fc2)/3. For 1: a ) 0.0570, b ) 0.7456. For 2: a ) 0.0531, b ) 0.3091. For 3: a ) 0.0579, b ) 1.6815.

Table 2. Selected Interatomic Distances (Å) and Bond Angles (deg) for 1a Cu-O(1) Cu-O(1)#1 Cu-O(2) O(1) /Cu/O(1)#1 O(1)/Cu/N(1) O(1)/Cu/N(2) O(2)/Cu/O(1) O(2)/Cu/O(1)#1 N(1)/Cu/O(1)#1

1.938(2) 1.935(2) 2.402(3) 82.86(9) 95.50(9) 166.2(1) 99.6(1) 99.5(1) 168.2(1)

Cu-N(1) Cu-N(2) Cu-Cu#1 N(1)/Cu/O(2) N(2)/Cu/O(2) N(2)/Cu/N(1) N(2)/Cu/O(1)#1 Cu/O(1)/Cu#1

2.015(2) 2.010(2) 2.904(1) 92.4(1) 94.1(1) 82.48(9) 96.31(9) 97.14(9)

a Symmetry transformations used to generate equivalent atoms: #1 ) -x + 1, -y, -z.

Table 3. Selected Interatomic Distances (Å) and Bond Angles (deg) for 2a Cu(1)-O(1) Cu(1)-O(2) Cu(1)-O(3) Cu(1)-N(1) Cu(1)-N(2) O(1)/Cu(1)/O(2) O(1)/Cu(1)/O(3) O(1)/Cu(1)/N(1) O(1)/Cu(1)/N(2) O(2)/Cu(1)/O(3) O(2)/Cu(1)/N(1) O(2)/Cu(1)/N(2) O(3)/Cu(1)/N(1) O(3)/Cu(1)/N(2) N(1)/Cu(1)/N(2) O(1)/Cu(2)/O(2) O(1)/Cu(2)/O(2)#1

1.930(2) 1.963(2) 2.250(2) 1.999(2) 2.010(2) 81.99(7) 97.24(8) 169.89(8) 95.37(8) 102.41(8) 98.16(8) 156.73(8) 92.61(9) 100.86(9) 80.47(8) 82.20(7) 100.66(7)

Cu(2)-O(1) Cu(2)-O(2) Cu(2)-O(2)#1 Cu(2)-N(3) Cu(2)-N(4) O(1)/Cu(2)/N(3) O(1)/Cu(2)/N(4) O(2)/Cu(2)/O(2)#1 O(2)/Cu(2)/N(3) O(2)/Cu(2)/N(4) O(2)#1/Cu(2)/N(3) O(2)#1/Cu(2)/N(4) N(3)/Cu(2)/N(4) Cu(1)/O(1)/Cu(2) Cu(1)/O(2)/Cu(2) Cu(2)/O(2)/Cu(2)#1

1.933(2) 1.951(2) 2.322(2) 2.029(2) 1.988(2) 162.61(8) 95.96(8) 83.37(7) 100.76(8) 177.61(7) 96.70(7) 98.49(8) 80.55(9) 98.00(8) 96.30(7) 96.63(7)

a Symmetry transformations used to generate equivalent atoms: #1 ) -x + 1, -y + 1, -z; #2 ) -x + 1, -y + 1, -z - 1; #3 ) -x, -y + 2, -z + 1.

methods and followed by successive Fourier and difference Fourier syntheses. All hydrogen atoms of dicarboxylate anions were geometrically generated. The hydroxyl and aqua hydrogen atoms were located from the successive difference Fourier syntheses. All nonhydrogen atoms were refined with anisotropic displacement parameters by full-matrix least-squares technique and all hydrogen atoms with isotropic displacement parameters. Detailed information about the crystal data and structure determination is summarized in Table 1. Selected interatomic distances and bond angles are given in Table 234.

Crystallographic data (excluding structure factors) for complexes 1-3 in this paper have been deposited with Cambridge Crystallographic Data Centre as supplementary publications. CCDC-630632, 630633 and 630634 for 1-3 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, U.K.; fax (international), +44-1223/336-033; e-mail, [email protected]]. Physical Measurements. The C, H and N microanalyses were performed with a Perkin-Elmer 2400II elemental analyzer. The FT-IR spectra were recorded from KBr pellets in the range 4000-400 cm-1 on a Shimadzu FTIR-8900 spectrometer. Powder X-ray diffraction data for 3 were collected on Exstra 6300II diffractometer at room temperature. The magnetic susceptibilities were measured using a SQUID magnetometer (Quantum Design model MPMS-7) in the temperature range 2 e T e 300 K. Thermogravimetric measurements were carried out from room temperature to 800 °C for 1 and 2, to 500 °C for 3 on preweighed samples in nitrogen stream using a Seiko Exstar6000 TG/ DTA6300 apparatus with a heating rate of 10 °C/min.

Results and Discussion Syntheses of the Complexes. The coordination chemistry of dicarboxylate ligands with transition metals has been studied on a large scale in recent years.8–25 But systematical studies on the discrete copper(II) systems have received scant attention. Our aim is to investigate the coordination chemistry of Cudicarboxylate and hope to obtain novel discrete polymeric clusters with special properties as well as to study the effects of synthesis approaches, acidity and auxiliary ligands on the structure formation of polynuclear clusters. Our previous work indicated that the presence of R,ω-dicarboxylic acids is essential for formation of chairlike stepped cubane Cu4(OH)4 core containing both µ2-OH or µ3-OH. In order to gain deep insight into the role played by the dicarboxylate ions in nucleation of cluster cores, the present syntheses were carried out by using terephthalate salt as well as adipate salt since the linear adipate and terephthalate anions have comparable backbone in length between the terminal carboxylate groups. Through solution reactions of dicarboxylates, terminal ligands (e.g., bpy or phen) and copper(II) ions, three discrete copper complexes were successfully isolated.


Li et al.

Table 4. Selected Interatomic Distances (Å) and Bond Angles (deg) for 3a Cu(1)-O(1) Cu(1)-O(2) Cu(1)-O(4) Cu(1)-N(1) Cu(1)-N(2) O(1)/Cu(1)/O(2) O(1)/Cu(1)/O(4) O(1)/Cu(1)/N(1) O(1)/Cu(1)/N(2) O(2)/Cu(1)/O(3) O(2)/Cu(1)/N(1) O(2)/Cu(1)/N(2) O(4)/Cu(1)/N(1) O(4)/Cu(1)/N(2) N(1)/Cu(1)/N(2)

1.931(2) 1.968(2) 2.267(2) 2.005(2) 2.014(3) 82.7(1) 89.7(1) 173.6(1) 98.9(1) 91.9(1) 95.3(1) 159.6(1) 96.4(1) 108.5(1) 80.9(1)

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

1.929(2) 1.950(2) 2.415(2) 2.008(3) 2.008(2) 83.2(1) 98.4(1) 165.6(1) 97.8(1) 83.2(1) 96.7(1) 175.1(1) 96.0(1) 101.4(1) 81.0(1)

Cu(3)-O(2) Cu(3)-O(3) Cu(3)-O(4)#1 Cu(3)-N(5) Cu(3)-N(6) O(2)/Cu(3)/O(3) O(2)/Cu(3)/O(3)#1 O(2)/Cu(3)/N(5) O(2)/Cu(3)/N(6) O(3)/Cu(3)/O(3)#1 O(3)/Cu(3)/N(5) O(3)/Cu(3)/N(6) O(3)#1/Cu(3)/N(5) O(3)#1/Cu(3)/N(6) N(5)/Cu(3)/N(6)

2.383(2) 1.945(2) 1.964(2) 1.989(2) 2.027(2) 84.2(1) 108.3(1) 97.8(1) 95.1(1) 80.1(1) 177.5(1) 101.0(1) 97.8(1) 156.5(1) 80.5(1)

a Symmetry transformations used to generate equivalent atoms: #1 ) -x + 2, -y + 1, -z + 1; #2 ) -x + 3, -y + 1, -z + 2; #3 ) -x + 2, -y + 2, -z + 2.

Complex 1 with Cu2(OH)2 dimeric core was obtained from reactions of Cu(II) ion, sodium terephthalate and phenanthroline in the solution of methanol and water (1:1 V/V, pH ) 11.1) in 75% yield, in which two Cu(II) ions are bridged by µ2-OH form a quadrilateral Cu2O2 unit (Scheme 1, I) and the coordination geometry of Cu(II) ion is tetragonal pyramid (Scheme 2 g). Utilization of 2,2′-bipyridine in place of phenanthroline resulted in formation of complex 2 at slightly lower pH value (pH ) 9.5) in 43% yield. Complex 2 exhibits chairlike structure with a Cu4(OH)4 core visualized as from association of two almost planar di(µ-hydroxy)-copper(II) dimers held together by long out-of-plane Cu-OH bonds (Scheme 1, II). Interestingly, substitution of adipate for terephthalate in the reaction system in 2 led to isolation of compound 3 in 85% yield (pH ) 8.6) with a steplike hexanuclear copper(II) Cu6(OH)6 cores consisting of five Cu2O2 units (Scheme 1, V), rather than the previous chairlike stepped cores. Phase of purity of the bulk materials of 3 was confirmed by comparison of their powder X-ray diffraction patterns with simulated ones based on the single crystal samples. The successful isolation of 1-3 prompted us to extend our work to higher polymeric copper cluster series. However, when we carried out similar reactions of copper(II) ions with different dicarboxylates and terminal organic ligands under hydro/ solvothermal condition, only uncharacterized precipitates or infinite structures were obtained.28 The above synthetic results imply that low pH value may be propitious to formation of high polynuclear cluster. Accordingly, we tried to lower the pH value to 4 or 3, and hoped to obtain discrete higher polynuclear structures with a steplike motif (e.g., octanuclear or decanuclear); unfortunately, only single crystal of dicarboxylic acid itself was isolated, which may arise from the fact that dicarboxylic acid was not deprotonated in such low pH values and the Cu(II)dicarboxylate system is found very difficult for growing single crystals suitable for X-ray diffraction. The formation of the discrete structure but not the infinite network for 1-3 may result from the following matters: (a) all central Cu(II) ions display square pyramidal coordination geometry (4 + 1); (b) the hydroxyl group uses µ2- or µ3-bridge to link Cu(II) ion in the different coordination environments, respectively; (c) the dicarboxylate ions act as free anions in the structural lattice but not as multidentate ligands coordinated to metal ions; (d) the coordinating bpy or phen ligands reduce the binding sites in the available Cu(II) ions and restrict the unit’s growth in that direction; additionally, the π · · · π stacking interaction between bpy or phen aromatic rings within a molecule enhances the stability of the unit; (e) water molecules act as terminal ligands coordinating to Cu(II) ions in the unit and restrain the dinuclear,

Figure 1. ORTEP view of coordination environments of Cu(II) ion with 50% displacement ellipsoids for 1. Guest water molecules are omitted for clarity.

tetranuclear or hexanuclear from growing into one-dimensional chain structure. In this reaction system, the introduction of EtOH or MeOH solvents makes it possible to obtain single crystals well suitable for X-ray diffraction, but they put no crucial role on the crystal structures. It should be noted that Cl- anion was removed from reaction mixtures for 1-3 so that OH- group acting as µ-bridge coordinates to the Cu(II) ion, which may be significant to form polynuclear hydroxy-bridged copper(II) cluster compounds due to the anion template effect. Otherwise, we also tried to prepare discrete polynuclear cluster compounds with an odd number of metal ions such as trinuclear, pentanuclear or heptanuclear; it is a pity that no desired products were isolated, but a few hydroxo coordination polymers are available, indicating seemingly a great difficulty to synthesize discrete polynuclear cluster compounds. Description of the Crystal Structures. [Cu(phen)(µ2-OH)(H2O)]2 · (C8H4O4) · 8H2O (1). The title compound is composed of dinuclear cation [Cu(phen)(OH)(H2O)]22+, anion (C8H4O4)2and lattice water molecules. The local coordination environment around copper(II) ion is shown in Figure 1. Cu(II) ion takes a tetragonal pyramidal coordination geometry of (4 + 1) type: two nitrogen atoms of bpy and two hydroxy oxygen atoms are located in the equatorial plane with Cu-N1 ) 2.015, Cu-N2 ) 2.010, Cu-O1 ) 1.938, Cu-O1A (-x + 1, -y, -z) ) 1.935 Å, and an aqua oxygen O2 occupies the axial position with Cu-O2 ) 2.402 Å, which is elongated due to the Jahn-Teller effect. The interplanar distance 3.17 Å between the neighboring aromatic rings of different [Cu(phen)(OH)(H2O)]22+ units indicates significant π · · · π stacking interactions making a contribution to formation of two-dimensional layers (Figure 2). In the terephthalate anion, four oxygen atoms are in a common plane and six carbon atoms reside in the other plane, the dihedral angle between them being 26.5°. Different kinds of hydrogen


Figure 2. Two-dimensional layer structure of 1.

Figure 3. ORTEP view of coordination environments of Cu(II) ion with 50% displacement ellipsoids for 2. Guest water molecules and terephthalate anion are omitted for clarity.

bonding are observed in the structure of 1: (a) hydrogen bonding among uncoordinated water molecules (O · · · O distances: 2.721-2.834 Å); (b) hydrogen bonding of uncoordinated water molecules/coordinated water molecules (O · · · O distances: 2.772-3.120 Å); (c) hydrogen bonding of uncoordinated water molecules/carboxylateoxygenatoms(O · · · Odistance:2.716-2.827 Å). Through extensive hydrogen bonds between the carboxylate oxygen atoms of free terephthalates, µ2-OH-, guest and coordination water molecules, along with π · · · π stacking interactions, complex 1 exhibits a three-dimensional supramolecular framework. [Cu4(bpy)4(µ2-OH)2(µ3-OH)2(H2O)2] · (C8H4O4)2 · 6H2O (2). Single crystal X-ray diffraction reveals that the structural unit of complex 2 comprises a chairlike tetranuclear Cu4O44+ cation, four OH-, two terephthalate C8H4O42- anions, four bpy, two coordination water and six guest water molecules. There are two types of coordination environments around the Cu2+ ions. As shown in Figure 3, the Cu1 atom displays tetragonal pyramidal geometry (4 + 1): two nitrogen atoms N1 and N2 from bpy, one oxygen atom O1 from µ2-OH and one oxygen atom O2 from µ3-OH define an equatorial plane (Cu1-N1 ) 1.999, Cu1-N2 ) 2.010, Cu1-O1 ) 1.930 and Cu1-O2 ) 1.963 Å), and an aqua ligand O3 occupies the axial position (Cu1-O3 ) 2.250 Å). Cu2, similar to Cu1, is also in a tetragonal pyramidal geometry: the N3, N4, O1 and O2 in the equatorial plane (Cu2-N3 ) 2.029, Cu2-N4 ) 1.988, Cu2-O1 ) 1.933 and Cu2-O2 ) 1.951 Å), and O2#1 (-x + 1, -y + 1, -z) from µ3-OH in the axial position (Cu2-O2#1 ) 2.322 Å). It can be seen that the average bond distances of Cu-O in the axial position are longer than those in the equatorial plane due to the Jahn-Teller effect. The



Cu4O4 unit can be regarded as a trimer consisting of three quadrate Cu2O2 through sharing Cu-O bond as the side with Cu1-Cu2 ) 2.915 Å and Cu2-Cu2#1 ) 3.200 Å, being similar with our previous report,20 and its magnetic properties were discussed by Chaudhuri et al.29 The closest distance (3.31 Å) between the neighboring bpy ligands within the [Cu4(bpy)4(OH)4(H2O)2]4+ unit suggests a strongly offset faceto-face π · · · π stacking interaction and enhances the stability of the unit, in favor of the formation of the chairlike tetranuclear structure (Scheme 1, II) but not cubane-like motif (Scheme 1, III). Additionally, the interactionic π · · · π stacking interactions between the adjacent ligands of different tetranuclear units lead to a two-dimensional network with large cavities (Figure 4). Several kinds of hydrogen bonding are observed in the structure of 2: (a) hydrogen bonding among uncoordinated water molecules (O · · · O distances: 2.989-3.166 Å); (b) hydrogen bonding of uncoordinated water molecules/coordinated water molecules (O · · · O distances: 2.766 Å); (c) hydrogen bonding of uncoordinated water molecules/carboxylate oxygen atoms (O · · · O distance: 2.793-2.873 Å); (d) hydrogen bonding of coordinated water molecule/carboxylate oxygen atom (O3 · · · O6 distance: 2.734 Å); (e) hydrogen bonding of hydroxyl oxygen/carboxylate oxygen atoms (O · · · O distance: 2.736-2.845 Å). Through the π · · · π stacking interactions and hydrogen bonds between terephthalate C8H4O42-, OH- and guest waters, complex 2 forms a 3D packing structure. [Cu6(bpy)6(OH)6(H2O)2] · (C6H8O4)3 · 23H2O (3). The structural unit cell of 3 contains a centrosymmetric cation [Cu6(bpy)6(OH)6(H2O)2]6+, three anions (C6H8O4)2-, and twentythree guest water molecules, of which the central Cu(II) ions show square pyramid CuN2O3 geometries (4 + 1) with appreciably different coordination environments. As shown in Figure 5, Cu1 is five-coordinated by two nitrogen atoms N1 and N2 from bpy, one oxygen atom O1 from µ2-OH and another (O2) from µ3-OH forming an equatorial plane (Cu1-N1 ) 2.005, Cu1-N2 ) 2.014, Cu1-O1 ) 1.931 and Cu1-O2 ) 1.968 Å), and an aqua oxygen O4 occupies the axial position (Cu1-O4 ) 2.267 Å); Cu2 is similar to Cu1 except for O3 (µ3-OH) in place of O4 (the coordination water) perching on the axial position with Cu2-O3 ) 2.415 Å. Similar to Cu1 and Cu2, Cu3 is also square pyramid geometry, where three coordination oxygen atoms are from µ3-OH, of which O2 lies in the axial position with Cu3-O2 ) 2.383 Å. Due to the Jahn-Teller effect, the distances of the Cu1-O4, Cu2-O3 and Cu3-O2 are all elongated in the unit [Cu6(bpy)6(OH)6(H2O)2]6+ for complex 3. In the structural unit, two copper(II) ions and two oxygen atoms of µ-OH form a tetragon (Cu2O2), and five such tetragons (Cu2O2) sharing Cu-O bond as the side afford


Figure 5. ORTEP view of coordination environments of Cu(II) ion with 50% displacement ellipsoids for 3. Guest water molecules and adipate anion are omitted for clarity.


a steplike discrete cluster Cu6O6 with ∠(O1-Cu2-O3) ) 98.37°, ∠(Cu2-O3-Cu3#) ) 107.06°, ∠(O3-Cu3#-O2#) ) 108.35°, ∠(Cu3#-O2#-Cu1#) ) 109.33°, Cu1-Cu2 ) 2.90 Å, Cu2-Cu3 ) 3.25 Å, Cu3-Cu3# ) 2.99 Å. In other words, the discrete cluster [Cu6(bpy)6(OH)6(H2O)2]6+ can be regarded as a hexamer consisting of square pyramid CuN2O3 by sharing the oxygen atom of µ-OH, in which a strongly offset face to face π · · · π stacking interaction between the adjacent aromatic rings of the bpy ligands with distance about 3.30 Å results in high stability for the steplike discrete unit. The π · · · π stacking interaction with distance about 3.40 Å between the different discrete units leads to formation of two-dimensional networks in the sheets parallel to (001) (Figure 6). In 3, all adipate (C6H8O4)2- ions assume the anti conformation with the inversion of the crystallographic centrosymmetry, and act as anions to keep the whole structure neutral. The oxygen atoms of the adipate, OH-, coordination water together with free water, yield diverse hydrogen bonding: (a) hydrogen bonding among uncoordinated water molecules (O · · · O distances: 2.576-2.964 Å); (b) hydrogen bonding of uncoordinated water molecules/ coordinated water molecules (O4 · · · O17 distances: 2.762 Å); (c) hydrogen bonding of uncoordinated water molecules/ carboxylate oxygen atoms (O · · · O distance: 2.668-2.905 Å); (d) hydrogen bonding of coordinated water molecule/carboxylate

Li et al.

oxygen atom (O4 · · · O9 distance: 2.642 Å); (e) hydrogen bonding of hydroxyl oxygen/carboxylate oxygen atoms (O · · · O distance: 2.744-2.847 Å); (f) hydrogen bonding of hydroxyl oxygen/uncoordinated water molecule (O · · · O distance: 2.858 Å), and through the hydrogen bonding and π · · · π stacking interaction form three-dimensional supramolecular framework with channels extending in the [[100] and [010] directions, respectively. In the previous studies, a few discrete hydroxy-bridged dinuclear copper(II) clusters were reported,24 and discrete chairlike hydroxy-bridged tetranuclear copper(II) compounds were not common such as the dinuclear. Interestingly, only a discrete hexanuclear copper(II) compound bridged by µ-OCH3 was reported up to now,22 though many infinite frameworks containing multinuclear cluster units were synthesized in the past.30 To the best of our knowledge, compound 3 is first discrete steplike hexanuclear copper(II) cluster bridged by µ-OH groups. Thermogravimetric Analysis (TGA) and Powder X-ray Diffraction (PXRD). Thermogravimetric analyses for complexes 1-3 have been measured under a flow of nitrogen gas from room temperature to 850 °C at a heating rate of 10 °C/ min. Differential thermal analyses (DTA) of complex 1 present one strong absorption heat peak at 96 °C, and two weak absorption peaks at 220 and 251 °C, respectively, which indicates that complex 1 took corresponding chemical or physical reactions. The first weight loss of 20.3% (calcd: 20.8%) from 60 to 100 °C corresponds to the loss of ten water molecules per formula unit [Cu(phen)(OH)(H2O)]2 · (C8H4O4) · 8H2O, and the weight was slowly lost with further raising the temperature. Upon 480 °C the final decomposed product presumably is Cu2CO3(OH)2, 30.1% (calcd: 25.5%, based on the remaining mass). For 2, DTA studies show two strong absorption heat reactions at 127 and 224 °C, respectively. The first weight loss of 9.9% (calcd: 10.1%) from 40 to 135 °C corresponds to the loss of eight waters per formula unit [Cu4(bpy)4(µ2-OH)2(µ3OH)2-(H2O)2] · (C8H4O4)2 · 6H2O, and hardly any weight was lost from 135 to 180 °C. The second distinct weight loss of 65.1% (calcd: 67.1%) from 180 to 600 °C corresponds to the loss of two C8H4O42- and four bpy ligands per molecular unit, and the final decomposed product presumably is CuO, 23.1% (calcd: 22.4%, based on the remaining mass). For 3, DTA pattern assumes four absorption heat reactions at 56, 101, 146 and 264 °C, respectively. The first weight loss of 18.9% (calcd: 20.1%) from 35 to 115 °C corresponds to the loss of twenty-five water molecules per formula unit [Cu6(bpy)6(µ2-OH)2(µ3-OH)4(H2O)2] · (C6H8O4)3 · 23H2O; the second weight loss of 17.8% (calcd: 18.6%) from 115 to 200 °C corresponds to the loss of three C6H8O42- per formula unit; the weight loss of 38.7% (calcd: 40.3%) from 220 to 400 °C corresponds to the loss of the bpy ligands per formula unit; the final decomposed product presumably is CuO, 24.6% (calcd: 20.7%, based on the remaining mass). The purity of compound 3 is confirmed by powder X-ray diffraction analyses, in which the experimental PXRD patterns are consistent with those obtained from the simulated PXRD ones based on the single crystal samples at room temperature. The temperature dependent PXRD measurements show the identity with TGA and DTA results for thermal stability (details in Figure 4 of the Supporting Information). Magnetic Properties. The temperature dependent susceptibilities of 1-3 were measured on a SQUID magnetometer in the 2-300 K temperature range. The χmT and 1/χm vs T plots for 1 are shown in Figure 7. The room temperature effective moment (µeff) of 1.90µB is slightly higher than the expected one (1.73µB) for two uncoupled Cu(II) ions; the experimental



law with θ ) 6.12 K in the measured temperature range. The room temperature effective moment (µeff) of 2.05µB (χmT ) 0.529 cm3 · mol-1 · K) is higher than the theoretical one (1.73µB) for two isolated ST ) 1 spin state Cu(II) ions, which suggests the fact that the dimers are practically correlated. Upon cooling, the value of χmT almost stays constant until at about 120 K, and then the value increases rapidly with decreasing temperature, reaching a maximum of 0.821 cm3 · mol-1 · K at 2 K, all of which indicate ferromagnetic exchange interactions between Cu(II) ions in 2. In light of the magnetic exchange behaviors between the CuII ions, the spin Hamiltonian is used for tetranuclear copper(II) compound (eq 2), in which the Hamiltonian can be interpreted by Scheme 3.32

Figure 7. Temperature dependent plots of χmT and 1/χm vs T for 1.

ˆ )-2J1(Sˆ1 · Sˆ3+Sˆ2 · Sˆ4) - 2J2Sˆ1 · Sˆ2-2J3(Sˆ1 · Sˆ4+Sˆ2 · Sˆ3) H 2J4Sˆ3 · Sˆ4 (2) The energy levels and the spin quantum numbers are as follows:

E1 ) -J1 - J2 ⁄ 2 - J3 - J4 ⁄ 2 E2 ) -J1 - J2 ⁄ 2 + J3 - J4 ⁄ 2 E3 ) (J2 + J4) ⁄ 2 + [(J2 - J4)2 + (J3 - J1)2]1⁄2 E4 ) (J2 + J4) ⁄ 2 - [(J2 - J4)2 + (J3 - J1)2]1⁄2 E5 ) J1 + J3 + (J2 + J4) ⁄ 2 + [4(J12 + J32) + J22 + J42 2J1(J2 + 2J3 + J4) - 2J2(J3 - J4) - 2J3J4]1⁄2 E6 ) J1 + J3 + (J2 + J4) ⁄ 2 - [4(J12 + J32) + J22 + J42 Figure 8. Temperature dependent plots of χmT and 1/χm vs T for 2.

2J1(J2 + 2J3 + J4) - 2J2(J3 - J4) - 2J3J4]1⁄2 The magnetic data were fitted to eq 3:

Scheme 3

χm ) Ng2β2 ⁄ κT × x/y

data well fit the Curie-Weiss law with θ ) 12.10 K in the measured temperature range. The χmT value increases with decreasing temperature, reaching a maximum of 0.578 cm3 · mol-1 · K at 5.5 K, and then decreases upon further cooling. This magnetic behavior is characteristic of ferromagnetic coupling, and at low temperature the curve χmT decreases may be ascribed to zero field splitting effect. The susceptibility expression derived from the spin pair 1/2-1/2, coupled through an isotropic exchange interaction J (the Hamiltonian is written ˆ ) -2J1Sˆ1Sˆ2)31 is proposed to describe the magnetic as H behavior of compound 1. The data was fitted to eq 1:

χm)2Ng2β2 ⁄ κT(1/(3 + exp(-2J ⁄ kT)))

(1)

The best fit to the experimental data gives J ) 28.1441 cm-1, g ) 2.13, R ) 3.0 × 10-5 (the agreement factor R ) Σ (χobsd - χcalcd)2/Σχobsd2), corresponding to the observed magnetic susceptibility as seen in Figure 7. The positive J value suggests that the interactions between CuII1 and CuII2 ions are ferromagnetic. The plots of χmT and 1/χm vs T for compound 2 are shown in Figure 8. The experimental data well fit the Curie-Weiss

(3)

where x ) 10 exp(-E1/κT) + 2 exp(-E2/κT) + 2 exp(-E3/ κT) + 2 exp(-E4/κT), y ) 5 exp(-E1/κT) + 3 exp(-E2/κT) + 3 exp(-E3/κT) + 3 exp(-E4/κT) + exp(-E5/κT) + exp(-E6/κT). The fit of the magnetic data was made basing on the fact that the g value for each copper(II) ion is identical, and the interaction parameters J3 and J4 were fixed to 0 as the long distance between Cu1-Cu2#1 and Cu1-Cu1#1 (dCu1-Cu2#1 ) 3.51 and dCu1-Cu1#1 ) 5.61 Å, respectively). The best fit of parameters was obtained with J1 ) 6.79 cm-1, J2 ) 0.83 cm-1, g ) 2.35, and R ) 2.3 × 10-4 (the agreement factor R ) ∑(χobsd - χcalcd)2/∑χobsd2). The positive J1 and J2 values indicate that the interactions between Cu1 and Cu2, Cu1#1 and Cu2#1, Cu2 and Cu2#1 are ferromagnetic, and the small J2 value shows the relatively weak ferromagnetic coupling between Cu2 and Cu2#1. In comparison with 1 and 2, compound 3 exhibits bulk antiferromagnetic interaction and the experimental data well fit the Curie-Weiss law with θ ) -5.82 K in the measured temperature range. The value of χmT decreases tardily with decreasing temperature from 300 to 120 K, and decreases sharply on further cooling, reaching the minimum value of 0.383 cm3 · mol-1 · K at 18 K, then the value increases gradually for 3 (shown in Figure 9). The room temperature effective moment (µeff) of 1.89µB (χmT ) 0.449 cm3 · mol-1 · K) is close to the theoretical one (1.73µB) for the isolated uncoupled Cu(II) ions. Herein, in order to interpret the magnetic behaviors of such a step-shaped hexanuclear copper(II) cluster it can be regarded as the structural unit composed of a tetranuclear copper(II) unit


Li et al.

vertex, and 3 is a steplike structure composed of six CuO5 by sharing hydroxy oxygen atom as vertex. The studies on magnetic properties of 1-3 showed that the different building units yielded a subtle effect on the properties of the compounds. By this token, higher polynuclear discrete clusters may result in novel chemical-physical properties. However, it is a pity that syntheses of higher polynuclear discrete clusters (e.g., octanuclear or decanuclear) are in a sorry plight at present in our laboratory. On the other hand, it is interesting that there are more and greater difficulties remaining to be overcome in the preparation of discrete polynuclear compounds such as trinuclear, pentanuclear or heptanuclear, compared with tetranuclear, hexanuclear and so on. Certainly, it also offers a challenge and great opportunity to prepare and investigate novel compounds for scientists. Further studies on the syntheses and properties of discrete high polynuclear clusters are still in progress in our laboratory. This work may offer a useful approach for design and syntheses of new single molecule-based magnets.

Figure 9. Plots of χmT and 1/χm vs T for 3.

Scheme 4

Acknowledgment. The project was sponsored by K. C. Wong Magna Fund in Ningbo University and was supported by the National Natural Science Foundation of China (20341006), the Expert Project of Key Basic Research of the Ministry of Science and Technology of China (2003CCA00800), the Zhejiang Provincial Natural Science Foundation (Z203067) and the Ningbo Science and Technology Bureau (2003A61014, 2003A62026, 2008A610048). Supporting Information Available: The TG-DTA patterns for 1-3, powder X-ray diffraction analyses for 3, packing structures for 1-3, and hydrogen bonding information for 1-3. This material is available free of charge via the Internet at http://pubs.acs.org.

and two external noncorrelated copper(II) ions (Scheme 4). The magnetic data of 3 from 40 to 300 K was fitted by eq 4, which derives from the susceptibility expression of tetranuclear copper(II) compound.

χm ) Ng2β2 ⁄ κT × x/y + Ng2β2 ⁄ (2κT)

(4)

where x ) 10 exp(-E1/κT) + 2 exp(-E2/κT) + 2 exp(-E3/ κT) + 2 exp(-E4/κT), y ) 5 exp(-E1/κT) + 3 exp(-E2/κT) + 3 exp(-E3/κT) + 3 exp(-E4/κT) + exp(-E5/κT) + exp(-E6/κT). Similar to complex 1, an identical g value for each copper(II) ion in 3 has been considered and the interaction parameters J3 and J4 were fixed to 0. As a result, an excellent fit was shown with J1 ) 3.52 cm-1, J2 ) -20.63 cm-1, g ) 2.28, R ) 3.1 × 10-5 (the agreement factor R is defined as ∑(χobsd - χcalcd)2/ ∑χobsd2). The small positive value J1 suggests a weak ferromagnetic interaction between two copper(II) ions, and the large negative J2 indicates antiferromagnetic interaction. The overall magnetic behavior seems to be dominated by the antiferromagnetic interaction. Conclusion In conclusion, by self-assembly reactions of Cu(II) ions, dicarboxylates, bpy or phen ligands under mild ambient conditions, three new discrete hydroxy-bridged copper(II) cluster compounds, dinuclear [Cu(phen)(OH)(H2O)]2 · (C8H4O4) · 8H2O (1), tetranuclear [Cu4(bpy)4(µ2-OH)2(µ3-OH)2(H2O)2] · (C8H4O4)2 · 6H2O (2), and hexanuclear [Cu6(bpy)6(µ2-OH)2(µ3OH)4(H2O)2] · (C6H8O4)3 · 23H2O (3), were successfully isolated, in which all Cu(II) ions show square pyramid coordination geometries (4 + 1). Compound 2 is a chairlike structure consisting of four CuO5 by sharing hydroxy oxygen atom as

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CG701150Q

Di-, Tetra-, and Hexanuclear Hydroxy-Bridged Copper(II) Cluster

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