Tetra- and Binuclear Complexes of Hydroxy-Rich Ligands

Tetra- and Binuclear Complexes of Hydroxy-Rich Ligands: Supramolecular Structures, Stabilization of Unusual Water Clusters, and Magnetic Properties Yongshu Xie,*,† Jia Ni,§ Fakun Zheng,‡ Yong Cui,‡,| Quanguo Wang,† Seik Weng Ng,⊥ and Weihong Zhu†

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 118–126

Key Laboratory for AdVanced Materials and Institute of Fine Chemicals, East China UniVersity of Science & Technology, Shanghai 200237, P. R. China, State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, PR China, Center Laboratory, Shantou UniVersity, Shantou, 515063, P. R. China, School of Chemistry and Chemical Technology, Shanghai Jiao Tong UniVersity, Shanghai 200240, China, and Department of Chemistry, UniVersity of Malaya, 50603 Kuala Lumpur, Malaysia ReceiVed December 8, 2007; ReVised Manuscript ReceiVed October 8, 2008

ABSTRACT: Three tetranuclear complexes [Cu4(H2L1)4] · 10H2O (1), [Ni4(H2O)3(H2L1)3(OH)]NO3 · 6H2O (2) and [Zn4(H2L1)4] · 2MeOH · 2H2O (3) and two binuclear complexes [Cu2(H3L1)2(NO3)2] (4) and [Cu2(H3L2)2Cl2](ClO4)2 · 2H2O (5) have been synthesized from two potentially pentadentate hydroxyl-rich ligands H4L1 and H3L2 (H4L1 ) tris(hydroxymethyl)(2-hydroxybenzylamino)methane, and H3L2 ) tris(hydroxymethyl)(2-pyridylamino) methane). X-ray analyses reveal that complexes 1, 2, and 3 have cubane core structures. For 1, ten lattice water molecules are linked by twelve hydrogen bonds, forming an extremely unusual adamantanoid (H2O)10 cluster, which is linked to two tetranuclear moieties by octadruple hydrogen bonds. For 2, four water dimers are bridged by two nitrates, giving a [(H2O)8(NO3)2]2- cluster. 4 has a bis(µ2-phenoxo)-bridged dicopper(II) structure with the binuclear species linked by multiple hydrogen bonds, affording a ladder-like structure. 5 has a bis(µ2-Cl)-bridged dicopper(II) structure with the binuclear moieties linked by π-π interactions and hydrogen bonds, affording a 2D network. In these complexes, the hydroxyl-rich ligands coordinate in dianionic, monoanionic or neutral forms, with one of the alkoxyl groups of each ligand left noncoordinated and open for intermolecular interactions, resulting in the stabilization of various water clusters and the formation of various supramolecular assemblies. Magnetic measurements reveal that, for 1 and 2, antiferromagnetic and ferromagnetic couplings coexist, thus resulting in interesting nonzero spin ground state. The ferromagnetic behavior of 1 can be ascribed to the nearly perpendicular arrangement of dx2-y2 magnetic orbitals of Cu(II) centers with the dihedral angle of 84.06(3)° between the Cu(II) equatorial planes. 4 shows a strong antiferromagnetic interaction. On the other hand, 5 shows a ferromagnetic interaction between copper(II) centers. Magneto-structural correlations have been discussed. Introduction In past few decades, polynuclear complexes have caused extensive interest due to their relevance to the active sites of metalloenzymes,1 potential applications as optical,2 electronic, and molecule-based magnetic materials,3 and aesthetic appeals. In the synthesis of these complexes, structures of products are dependent on many factors, such as the structure of chelating ligands,4 metal centers, anions, and solvents.5 One of the most important strategies is to use low-dentate chelating ligands with suitable disposed endogenous bridging groups. When M(II) atoms coordinate to these ligands in 1:1 mode, the coordination sphere of M(II) remains unsaturated, and the resulting mononuclear coordination moieties can further oligomerize with the aid of endogenous bridging groups affording polynuclear complexes. Thus, many bidentate6 and tridentate4b,7 ligands have been designed to synthesize polynuclear complexes and magnetostructural correlations have been studied. In contrast to success of bidentate and tridentate ligands for the synthesis of polynuclear complexes, polydentate ligands have received less attention in this respect. Recently, it has been shown that polydentate ligands also can be used for this purpose and the noncoordinated groups presented in the resulting * Corresponding author. E-mail: [email protected]. † East China University of Science & Technology. ‡ Fujian Institute of Research on the Structure of Matter. § Shantou University. | Shanghai Jiao Tong University. ⊥ University of Malaya.

complexes can be further employed to obtain more complicated and exended structures.8 Along this way, we herein report the synthesis of a series of novel tetranuclear cubane and binuclear complexes starting from two potentially pentadentate hydroxylrich ligands. Actually, hydroxyl-rich ligands containing amino groups are interesting not only for the possibility of synthesizing polynuclear complexes,9 but also for potential applications as biomimetic model systems of metalloproteins and metalloenzymes,10 and for the versatility in obtaining supramolecular coordination assemblies.11 Tris(hydroxymethyl)aminomethane (TRIS) is a typical hydroxyl-rich ligand containing reactive amino and hydroxyl groups, which can be readily further functionalized. Despite of this fact and the extreme importance of TRIS in biochemical systems as a commonly used buffer compound in addition to its interesting interactions with biomacromolecules,12 its derivatives as ligands for the synthesis of metal complexes have received little attention.13 In this work, starting from two derivatives of TRIS, H4L1 and H3L2 (H4L1 ) tris(hydroxymethyl) (2-hydroxybenzylamino)methane, and H3L2 ) tris(hydroxymethyl) (2-pyridylamino)methane) (Scheme 1), we synthesized three tetranuclear complexes with M4O4 cubane cores, [Cu4(H2L1)4] · 10H2O (1), [Ni4(H2O)3(H2L1)3(OH)]NO3 · 6H2O (2), and [Zn4(H2L1)4] · 2MeOH · 2H2O (3). In addition, two binuclear complexes, [Cu2(H3L1)2(NO3)2] (4) and [Cu2(H3L2)2Cl2](ClO4)2 · 2H2O (5), were obtained by the variation of reaction conditions. 4 and 5 have a bis(µ2-phenoxo)-bridged dicopper(II) and a bis(µ2-Cl)-bridged dicopper(II) core, respectively. For

10.1021/cg7012073 CCC: $40.75  2009 American Chemical Society Published on Web 12/02/2008

Hydroxy-Rich Ligands

Crystal Growth & Design, Vol. 9, No. 1, 2009 119 Scheme 1

hydroxyl groups in these ligands, they can coordinate either in a neutral or in a deprotonated form; they also can remain neutral and noncoordinated, open for intermolecular hydrogen bonds, affording various supramolecular assemblies. For complex 1, 10 lattice water molecules are linked by twelve hydrogen bonds, forming an adamantanoid (H2O)10 cluster. The clusters are further linked with the tetranuclear moieties by octadruple hydrogen bonds, affording a complicated 3D network. Structural and theoretical investigation of hydrogen-bonded water clusters has caused intensive attention due to their importance in many chemical and biological systems. A variety of water clusters within the range of (H2O)3 and (H2O)45 has been reported.14 Examples of various topologies of (H2O)10 clusters are known.15 However, adamantanoid (H2O)10 clusters are extremely rare. Only two such examples have been reported.16 For 2, four water dimers are bridged by two nitrates, giving a [(H2O)8(NO3)2]2cluster, which is anchored onto the tetranuclear moieties. The binuclear moieties of complex 4 are linked by multiple hydrogen bonds, affording a ladder-like structure. The coordination moieties of 5 are linked by π-π interactions of pyridyl rings and hydrogen bonds of hydroxyl groups, affording a 2D network. Variable temperature magnetic measurements reveal that for complexes 1 and 2, antiferromagnetic and ferromagnetic couplings coexist, resulting in interesting nonzero spin ground state. The ferromagnetic behavior of 1 can be ascribed to the nearly perpendicular arrangement of dx2-y2 magnetic orbitals of Cu(II) centers. Complex 4 shows a strong antiferromagnetic interaction between Cu(II) centers mediated by bridging phenoxo oxygens. Complex 5 shows a ferromagnetic interaction between copper(II) centers, mediated by the dichloro-bridge. Magnetostructural correlations have been discussed. Experimental Section Materials. All chemicals were of reagent grade quality and were used as received from commercial sources. Physical Measurements. IR spectra were recorded on a Bruker Vector-22 spectrometer (KBr Disc.). UV-vis spectra were obtained on a Shimadzu UV-vis 2401 PC spectrophotometer. Microanalyses of C, H and N were carried out with a GmbH VarioEL elemental analyzer. 1H NMR spectra were recorded on a JEOL AL300BX Spectrometer in DMSO-d6 with tetramethylsilane as internal standard. Variable temperature magnetic susceptibility data were collected with a Quantum Design MPMS7 SQUID magnetometer between 2 and 300 K. The data were corrected for the diamagnetism of the sample holder and for diamagnetic contributions with Pascal’s constants; a value of 60 × 10-6 cm3 mol-1 was used for the TIP of the Cu(II) ion. Preparation. Caution! Perchlorate salts of metal complexes with organic ligands are potentially explosiVe. Only a small amount should be used and it should be handled with extreme care. H4L1. This ligand was synthesized by a procedure modified form that reported earlier.17 To a methanol solution of tris(hydroxymethyl) aminomethane (2.42 g, 20.0 mmol) was added salicylaldehyde (2.44 g, 20.0 mmol). After stirring for 2 h, KBH4 (1.62 g, 30.0 mmol) was added in small portions to the yellow solution, which gradually faded to nearly colorless. Stirring was continued for 6 h. To decompose excessive KBH4, pH of the solution was adjusted with hydrochloric acid to about 4, and then to about 9 with NaOH dissolved in methanol. After the solvent was removed in Vacuo, the residue was added to a

mixture of water (15 mL) and CH2Cl2 (15 mL), and stirred violently for several hours, the white precipitate was collected, and recrystallized from ethanol/water (2:1). Yield: 3.93 g, 86%. Mp: 161-163 °C. 1H NMR: 3.25-3.32 (d, 1 H), 3.45-3.67 (m, 4 H, CH2OH 1 CH2NH), 4.03 (br, 3 H, CH2OH), 4.45-4.55 (t, 1 H) and 6.68-6.72 (m, 2H, aromatic), 7.03-7.13 (m, 2H, aromatic). Anal. Calcd for C11H17NO4: C, 58.14; H, 7.54; N, 6.16. Found: C, 57.86; H, 7.73; N, 6.24. IR (KBr pellet, cm-1): 3334 br (H2O), 3286 m (N-H), 3129 m, 2948, 2923, 2878 w, 1612 s, 1455 s, 1403 m, 1361 m, 1350 m, 1278 m, 1249 m, 1153 m, 1139 m, 1104 s, 1066 s, 1022 s, 944 m, 915 m, 884 m, 828 m, 752 s, 549 w, 530 w, 496 w, 464 w, 443 w. H3L2. It was prepared by a procedure similar to that of H4L1. Pyridine-2-carboxaldehyde (2.14 g, 0.02 mol) was used in stead of salicylaldehyde. After removal of the solvent, the residue was added to H2O (15 mL), and extracted with CH2Cl2 (35 mL). The aqueous layer was left at room temperature, affording white precipitate, which was collected, and recrystallized from ethanol/water (2:1). Yield: 3.95 g, 93%. 1H NMR: 8.47 (t, 1 H, py), 7.71 (t, 1 H, py), 7.40 (t, 1 H, py), 7.20 (t, 1H, py), 3.77 (s, 2H, benzylic), 3.60-3.69 (m, 3H), 3.39-3.51 (m, 3H), 3.23 (s, 1H) ppm. Anal. Calcd for C10H16N2O3: C, 56.59; H, 7.60; N, 13.20. Found: C, 56.35; H, 7.82; N, 13.09. IR (KBr pellet, cm-1): 3390 br, 3313 br, 2916 m, 2857 m, 1595 s, 1571 m, 1476 s, 1436 s, 1355 w, 1318 w, 1274 w, 1249 w, 1103 s, 1049 s, 1003 w, 912 m, 756 s, 684 w, 632 w, 610 w, 541 w, 499 w. [Cu4(H2L1)4] · 10H2O (1). To a methanol solution containing H4L1 (0.091 g, 0.40 mmol) and Cu(ClO4)2 · 6H2O (0.148 g, 0.40 mmol) was added NaOH (0.20 mL, 4.0 M, 0.80 mmol) dropwise with stirring. After refluxing for 4 h, the resulting solution was concentrated in Vacuo to 5 mL, filtered, and left undisturbed at room temperature for several days, yielding platelike dark green single crystals of 1 suitable for X-ray diffraction analysis. Yield: 0.049 g, 37%. Anal. Calcd for C44H80Cu4N4O26: C, 39.63; H, 6.06; N, 4.20. Found: C, 39.33; H, 6.37; N, 3.83. IR (KBr pellet, cm-1): 3428 br (H2O), 3215 m, br (N-H), 2927 m, 2855 m, 1596 s, 1566 m, 1482 s, 1452 s, 1273 s (C-O), 1127 w, 1069 s, 1038 m, 1012 m, 949 w, 892 w, 865 w, 763 s, 733 w, 676 w, 581 m, 514 w, 470 w, 432 w. [Ni4(H2O)3(H2L1)3(OH)]NO3 · 6H2O (2). 2 was prepared by a procedure similar to that of 1 from the reaction of H4L1 (0.091 g, 0.40 mmol), Ni(NO3)2 · 6H2O (0.116 g, 0.40 mmol) and NaOH (0.20 mL, 4.0 M, 0.80 mmol). Yield: 0.038 g, 33%. Anal. Calcd for C33H64N4Ni4O25: C, 34.42; H, 5.60; N, 4.86. Found: C, 33.95; H, 5.79; N, 4.54. IR (KBr pellet, cm-1): 3422 br (H2O), 3278 m (N-H), 2923 m, 2871 w, 1595 s, 1561 m, 1478 s, 1448 s, 1384 m, 1268 s (C-O), 1132 w, 1110 w, 1065 s, 1034 s, 1005 s, 935 w, 894 w, 865 w, 762 s, 731 w, 671 w, 614 w, 574 w, 497 w, 447 w, 421 w. [Zn4(H2L1)4] · 2 MeOH · 2 H2O (3). 3 was prepared by a procedure similar to that of 1 from the reaction of H4L1 (0.091 g, 0.40 mmol), Zn(ClO4)2 · 6H2O (0.149 g, 0.40 mmol) and NaOH (0.20 mL, 4.0 M, 0.80 mmol). Yield: 0.054 g, 41%. Anal.Calcd for C46H72N4O20Zn4: C, 43.76; H, 5.75; N, 4.44. Found: C, 43.24; H, 6.29; N, 4.48. IR (KBr pellet, cm-1): 3400 br (H2O), 3223 m, br (N-H), 2921 m, 2887 m, 1597 s, 1567 m, 1482 s, 1451 s, 1275 s (C-O), 1131 m, 1066 s, 1036 s, 1008 s, 950 w, 927 w, 886 m, 854 m, 805 w, 761 s, 730 m, 659 w, 572 m, 488 w, 419 w. [Cu2(H3L1)2(NO3)2] (4). 4 was prepared by a procedure similar to that of 1 from the reaction of Cu(NO3)2 · 3H2O (0.097 g, 0.40 mmol), H4L1 (0.109 g, 0.40 mmol), and NaOH (0.10 mL, 4.0 M, 0.40 mmol). Yield: 0.089 g, 63%. Anal. Calcd for C22Cu2H32N4O14: C, 37.56; H, 4.58; N, 7.96. Found: C, 37.54; H, 4.66; N, 7.77. IR (KBr pellet, cm-1): 3441 br (H2O), 3362 br, 3238 br, 3172 s (N-H), 2930 m, 2889 w, 1600, 1486 s, 1453 s, 1384 s, 1341 s, 1273 s (C-O), 1251 s, 1160 w, 1130 w, 1111 w, 1069 m, 1036 m, 1003 s, 964 w, 938 w, 896 m, 868 m, 813 w, 757 s, 729 m, 673 m, 609 w, 580 w, 512 w, 472 w, 429 w. [Cu2(H3L2)2Cl2](ClO4)2 · 2H2O (5). 5 was prepared by a procedure similar to that of 1 from the reaction of CuCl2 · 2H2O (0.034 g, 0.20 mmol), Cu(ClO4)2 · 6H2O (0.074 g, 0.20 mmol), H3L2 (0.085 g, 0.40 mmol), and NaOH (0.05 mL, 4.0 M, 0.20 mmol). Yield: 0.091 g, 53%. Anal. Calcd for C20H36Cl4Cu2N4O16: C, 28.02; H, 4.23; N, 6.53. Found: C, 28.23; H, 4.01; N, 6.47. IR (KBr pellet, cm-1): 3458 br, 3213 m, 2954 w, 2923 w, 2853 w, 1615 m, 1573 m, 1485 m, 1452 m, 1417 w, 1436 w, 1374 w, 1284 m, 1239 w, 1099 br, vs, 1004 s, 943 m, 763 s, 724 m, 657 s, 624 s, 488 w, 414 w. X-ray Crystallography. X-Ray diffraction data were collected on a Bruker-AXS APEX or a Rigaku RAXIS-IV image plate area detector

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Table 1. Crystal Data and Structure Refinement for 1-5

empirical formula fw crystal system space group T (K) a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (Mg/m3) µ (mm-1) no. of reflns (I > 2σ(I)) final R1a [I > 2σ(I)] wR2b (all data) goodness of fit a

1

2

3

4

5

C44H80Cu4N4O26 1335.28 tetragonal I4(1)/a 293(2) 18.5961(9) 18.5961 15.5820(7) 90.00 90.00 90.00 5388.5(3) 4 1.646 1.648 2699 0.0398 0.1192 1.102

C33H64N4Ni4O25 1151.72 monoclinic P21/n 293(2) 12.4222(7) 16.7114(9) 22.2596(12) 90.00 90.5950(10) 90.00 4620.7(4) 4 1.656 1.696 7054 0.0741 0.1926 1.053

C46H72N4O20Zn4 1262.56 tetragonal I4(1)/a 291(2) 18.4470(10) 18.447 16.0520(10) 90.00 90.00 90.00 5462.4(5) 4 1.535 1.812 2548 0.0565 0.1674 1.114

C22H32Cu2N4O14 703.60 triclinic P1j 293(2) 7.4912(7) 9.4741(8) 10.1818(9) 66.034(2) 83.844(2) 80.625(1) 650.83(10) 1 1.795 1.716 2594 0.0432 0.1110 1.059

C20H36Cl4Cu2N4O16 857.41 triclinic P1j 293(2) 6.2741(7) 10.6662(7) 13.7275(8) 68.08(3) 87.39(3) 74.19(3) 818.37(25) 1 1.740 1.701 3327 0.0458 0.1410 1.097

R ) Σ|Fo| - |Fc|/Σ|Fo|. b Rw ) Σ|Fo| - |Fc|w1/2/Σ|Fo|w1/2.

diffractometer at 293 K utilizing Mo KR radiation (λ ) 0.71073 Å). The structures were solved by direct methods and refined with fullmatrix least- squares technique. Anisotropic thermal parameters were applied to all non hydrogen atoms. All of the hydrogen atoms in these structures are located from the differential electron density map and constrained to the ideal positions in the refinement procedure. All calculations were performed using SHELX-97 software package.18 A summary of crystal data is presented in Table 1.

Results and Discussion Synthesis. The ligands were synthesized by reduction of corresponding Schiff bases. After removal of the reaction solvent, the residue was stirred violently with water and CH2Cl2. Due to the high polarity of products with multiple hydroxyl groups, they cannot be readily extracted from water by CH2Cl2 as in usual cases, however, CH2Cl2 can extract some impurities, which favors the crystallization of products from water in the presence of salts. Recrystallization of crude products from EtOH/ H2O affords analytically pure ligands. All tetranuclear complexes were synthesized from H4L1 in MeOH under basic conditions to promote deprotonation and subsequent coordination of the phenoxyl and alkoxyl groups. For Zn(II), complex 3 was obtained no matter Zn(ClO4)2, ZnCl2, or Zn(NO3)2 was used. For Cu(II), the products are somewhat dependent on anions. When Cu(ClO4)2 or CuCl2 was used, tetranuclear complex 1 was obtained, however, when Cu(NO3)2 was used, the binuclear complex 4 was obtained. This may be related to the fact that NO3- can coordinate at the axial position of Cu(II), which prevents the binuclear moiety from further dimerization. This binuclear structure with long axial coordination bond can be stabilized by Jahn-Teller effect of Cu(II). For Ni(II), when Ni(ClO4)2 was used, an unidentified complex was obtained. Whereas, when Ni(NO3)2 was used, tetranuclear complex 2 was obtained. In contrast to the presence of four (H2L1)2- ligands in 1, complex 2 contains only three (H2L1)2- ligands, which may be related to the possible severe steric hindrance between the ligands due to shorter coordination bonds, as compared with Cu(II) and Zn(II). In all the tetranuclear complexes, H4L1 coordinates in a dianionic form, with the phenoxy group and one of the alkoxyl groups deprotonated, which is favorable for charge balance. In binuclear complex 4, the ligand coordinates in a monoanionic (H3L1)- form, with phenoxyl group deprotonated. The positive charge of Cu(II) is further balanced by a coordinated nitrate, which prevents the further dimerization of the binuclear moieties. In contrast to the phenol group in H4L1,

H3L2 has one pyridyl group, which cannot be deprotonated. Therefore, it is unfavorable for charge balance in forming tetranuclear complexes. Thus, no tetranuclear complex could be obtained from H3L2 even though we tried various metal salts. Interestingly, a binuclear complex 5 was obtained when CuCl2 and Cu(ClO4)2 were combined. In 5, H3L2 coordinates in a neutral form. The positive charge of Cu(II) is balanced by Cland ClO4-. Structural Results. Complexes 1-3 have similar cubanelike M4O4 cores, with four M(II) and four O occupying alternative vertices of the cube. Cu4 Complex Stabilizing a (H2O)10 Cluster. Crystal structure of 1 consists of a discrete [Cu4(H2L1)4] tetramer (Figure 1a), with an M: L molar ratio of 1:1. It can be understood as two dimers joined together by Cu-O bridging. Thus, each ligand coordinates in a tetradentate mode as an (H2L1)2- dianion (Scheme 2a), with the phenoxyl and one of the alkoxyl groups deprotonated. The latter O atom bridges three Cu(II) centers. For remaining two neutral alkoxyl groups, one is weakly coordinated, and the other is left noncoordinated. Each Cu(II) atom has an elongated octahedral geometry. The phenoxo O, amino N, and two bridging alkoxyl O atoms coordinate in the equatorial plane, with bond lengths varying in a range of 1.929(1)-1.999(1) Å. Axial positions are occupied by one bridging alkoxo O and one neutral alkoxyl O, with Cu-O bond lengths of 2.481(1) and 2.548(1) Å, respectively. The final average coordination bond length is 2.148 Å. The axial O-Cu-O angle is 157.30(7)°, indicating a servere distortion of the octahedral configuration. Cu-O-Cu angles cover a range of 89.45(4)-106.70(4)°. The dihedral angle for the equatorial planes of Cu1 and Cu1ii is 37.53(6)°. Whereas, the value for Cu1 and Cu1i is 84.06(3)°, which suggests a nearly perpendicular arrangement of the dx2-y2 magnetic orbitals of the metal centers. The tetranuclear moieties are linked by intermolecular hydrogen bonds between the alkoxyl groups, affording a 2D network approximately along the ab plane (Figure 1b). Related H · · · O and O · · · O distances are 1.94 and 2.790(1) Å, respectively, with the bond angle of 175°. Surprisingly, there are ten lattice water molecules in addition to the tetranuclear moiety. It is interesting that all the water molecules are linked by twelve hydrogen bonds, affording a (H2O)10 cluster with an adamantanoid structure. Geometry of the cluster is quite similar to that in Ic-type ice. Adjacent O · · · O distances lie in a range of


Crystal Growth & Design, Vol. 9, No. 1, 2009 121

Figure 1. (a) View of complex 1, and (b) 2D network along the ab plane linked by intermolecular hydrogen bonds between noncoordinated alkoxyl groups. Lattice waters and hydrogen atoms attached to carbons are omitted for clarity. Symmetry operations: (i) y - 1/4, -x + 5/4, -z + 1/4; (ii) -x + 1, -y + 3/2, z; (iii) -y + 5/4, x + 1/4, -z + 1/4.

Scheme 2. Coordination Modes of the Ligands in the Complexes: (a) Complexes 1-3; (b) Complex 4; (c) Complex 5

2.618(1)-2.943(1) Å, with an average of 2.727 Å, which is similar to that of 2.74 Å in Ic-type ice.19 O · · · O · · · O angles at the bridgeheads lie in a range of 88.64(4)-124.71(5)° with an average value of 108.8, whereas those at other corners are 111.77(5) ° and 105.18(4)°, with an average value of 108.5°. The distance across the decamer is 6.361(2) Å, which is slightly longer than the value of 6.35 Å in ice Ic. These clusters are further linked with tetranuclear complex moieties by octadruple hydrogen bonds with NH and OH from (H2L1)2- ligands, forming a chain-like structure along the c axis (Figure 2). For these hydrogen bonds, H · · · O distances are 2.19 and 2.23 Å, respectively; bond angles are 167° and 141°, respectively; N · · · O and O · · · O distances are 3.031(1) and 2.939(1) Å, respectively. Finally, a complicated 3D hydrogen-bonding network is formed by the linkage of all these hydrogen bonds. Ni4 Complex with a [(H2O)8(NO3)2]2- Cluster. Complex 2 has a [Ni4(H2O)3(H2L1)3(OH)]+ cation (Figure 3a) containing a cubane core. Different from 1, complex 2 has an M:L ratio of 4:3, with one of the vertices of the cube occupied by a µ3OH-, and other vertices are occupied alternatively by four Ni(II) atoms and three deprotonated alkoxo groups from three (H2L1)2ligands. Positive charge of the cation is balanced by a noncoordinated nitrate. The complex cation has a low symmetry, containing four crystallographically independent nickel(II) atoms. Ni1 and Ni2 have similar coordination environments provided by one amino N, one deprotonated phenoxy, one hydroxyl, two deprotonated bridging alkoxo, and one neutral alkoxy O atom. Ni3 has a coordination environment similar to

that of Ni1 and Ni2 except that one deprotonated alkoxo O is coordinated in stead of the hydroxyl O atom. Ni4 is coordinated to three water molecules, one bridging hydroxyl, and two deprotonated bridging alkoxo O atoms. Thus, all nickel(II) atoms have octahedrally coordinated environment, with Ni(II)-O and Ni(II)-Nbondlengthsvaryinginasmallrangeof2.017(3)-2.169(4) Å, resulting in an average value of 2.060 Å, which is significantly smaller than that of complex 1. Ni(II)-O-Ni(II) angles vary in a range of 92.58(14)-100.50(15)°. In addition to the tetranuclear moiety, there are 6 lattice waters, which are linked by hydrogen bonds, affording discrete dimers. Two of such dimers are bridged by two nitrates, giving a macrocycle with two additional dimers attached to it. Thus, a [(H2O)8(NO3)2]2cluster was formed (Figure 3b). In the macrocycle, intradimeric O · · · O distances are 2.697(1) Å. For dimers out of the macrocycle, corresponding distances are 2.800(1) Å. This cluster is anchored onto neighboring tetranuclear moieties, affording a very complicated 3D hydrogen-bonded network. Zn4 Complex. Complex 3 consists of a discrete [Zn4(H2L1)4] moiety containing a Zn4O4 cubane core (Supporting Information, Figure S1). Coordination bond lengths vary in a range of 1.999(1)-2.376(1) Å with an average value of 2.133 Å, which is intermediate between those in complexes 1 and 2. The tetranuclear moieties are linked by intermolecular hydrogen bonds between the alkoxyl groups, affording a 2D network (Supporting Information, Figure S2) similar to that of complex 1. Related H · · · O and O · · · O distances are 2.03 and 2.851(1) Å, respectively, with the bond angle of 173°. In addition to the tetranuclear moiety, there are two disordered lattice methanol molecules and two lattice water molecules. Copper complex with a Cu2(OAr)2 core. Complex 4 has a bis(µ2-phenoxo)-bridged dicopper(II) structure (Figure 4). Each copper(II) atom is coordinated to one (H3L1)- ligand (Scheme 2b), which acts in a tetradentate mode, with amino N, deprotonated phenoxo O atom and two neutral alkoxyl O atoms coordinated. The remaining alkoxyl O atom of (H3L1)- is left noncoordinated in a neutral form. Two phenoxo groups bridge two Cu(II) atoms giving the binuclear structure, containing an exactly planar Cu2O2 core owing to the crystallographic inversion symmetry. A nitrate is weakly coordinated at one of the axial positions, with a Cu-O bond length of 2.5772(1) Å, resulting in an octahedral geometry around Cu(II). The average axial bond length is 2.535 Å, which is significantly longer than that of 1.964 Å in the equatorial plane. The axial bond angle


Xie et al.

Figure 2. View of a 1D chain linked by intermolecular hydrogen bonds between (H2O)10 clusters and (H2L1)2- ligands in 1. Hydrogen atoms are omitted for clarity.

Figure 5. View of a ladder-like structure linked by multiple intermolecular hydrogen bonds in complex 4. The hydrogen atoms attached to carbons are omitted for clarity.

Figure 3. (a) View of cubane Ni(II) complex cation of 2, and (b) four water dimers linked with two nitrates by hydrogen bonds. Symmetry operation: (i) -x + 1, -y + 1, -z + 1; (ii) x + 1, y, z; (iii) -x + 2, -y + 1, -z + 1. Hydrogen atoms attached to carbons are omitted for clarity.

Figure 4. View of binuclear Cu(II) complex 4. Symmetry transformation: (i) -x, -y, -z.

O3-Cu1-O5 has a small value of 149.32(1)°, indicating a severe distortion of the octahedral configuration. The bridging O1 atom has almost equivalent bond lengths of 1.932(1) and 1.939(1) Å with Cu1 and Cu1i, respectively. Copper(II) centers are separated by 2.9846(1) Å, with Cu1-O1-Cu1i bridging angles of 100.91(1)°, falling within the normal range for diphenoxo-bridged copper(II) complexes.20

The binuclear moieties are linked with pairs of nitrates by hydrogen bonds. Each nitrate forms two hydrogen bonds with one alkoxyl group and one amino group, with O · · · H and N · · · H bond lengths of 1.94 and 2.23 Å, O · · · O and N · · · O distances of 2.754(1) and 3.079(1) Å, and bond angles of 169° and 167°, respectively. The binuclear moieties are also directly linked by hydrogen bonds between alkoxyl groups, with O · · · H, O · · · O distances and bond angles of 2.02 Å, 2.756(3) Å, and 149°, respectively. Finally, a ladder-like structure is formed by the linkage of these hydrogen bonds (Figure 5). Copper Complex with a Cu2Cl2 Core. Complex 5 has a binuclear structure containing a Cu2Cl2 core, which is planar owing to the crystallographic inversion symmetry (Figure 6a). Each copper(II) atom has a pseudooctahedral configuration, comprised of two bridging chlorine ligands and a neutral H3L2 ligand in a tetradentate form (Scheme 2c). The positive charge of each Cu(II) is further balanced by a perchlorate. The amino N, pyridyl N, neutral alkoxyl O1, and chlorine atom Cl1 coordinate in the equatorial plane of Cu(1), with an average in-plane bond length of 2.056 Å. Axial positions are weakly coordinated by chlorine Cl1i and a neutral alkoxyl O2, with bond lengths of 2.744(1) and 2.704(1) Å, respectively. Thus, each chlorine is strongly coordinated in the equatorial plane of one Cu(II), and weakly coordinated at the axial position of the other Cu(II) atom. Cu(II) center is displaced 0.68 Å from the equatorial plane toward the axial chlorine. Equatorial planes of Cu(II) atoms are parallel to each other, with the Cu1-Cl1-Cu1i bridging angle of 90.52(2)°. Intermolecular hydrogen bonds occur between neutral alkoxyl groups (Figure 6b), with H · · · O and O · · · O lengths of 2.07 Å and 2.624(4) Å, respectively, and bond angles of 125°. Furthermore, there are intermolecular slipped-parallel21 π · · · π interactions between pyridyl rings, with an interplane distance of 3.392(1) Å, an offset of 1.307(1) Å, and a centroid · · · centroid distance of 3.635(1) Å, indicative of strong π · · · π interactions. By these hydrogen bonds and π · · · π interactions, the binuclear moieties are linked to give a 2D network (Figure 6b). Magnetic Studies. In the crystal structures of complexes 1, 2, 4, and 5, the metal centers are bridged by alkoxo, phenoxo, and chloro atoms, respectively. To understand the magnetic interactions mediated by the bridges in these complexes, variable temperature magnetic susceptibilities were measured.


Crystal Growth & Design, Vol. 9, No. 1, 2009 123 Scheme 3. Magnetic Exchange Pathways for 1 with Longer Cu-O Bonds Shown by Dashed Lines and Where Arrows Indicate xy Planes of Cu(II)

Theoretically, there are six exchange interactions between four Cu(II) centers. Considering the structural symmetry, the magnetic data can be analyzed using the Heisenberg Hamiltonian according to a simple two-J model:

H ) -2J1(S1S1ii+S1iS1iii) - 2J2(S1S1i+S1iS1ii+S1iiS1iii+ S1iiiS1)

(1)

where 2J1 and 2J2 represents the exchange constants as shown in Scheme 3. The product χmT can then be fitted to the Heisenberg-van Vleck equation22 Figure 6. (a) View of dichloro-bridged binuclear Cu(II) complex 5, and (b) 2D network approximately along the ab plane linked by intermolecular hydrogen bonds and π · · · π interactions. The hydrogen atoms attached to carbons are omitted for clarity. Symmetry transformation: (i) -x, -y, -z.

Figure 7. Plot of products χmT vs temperature for complex 1. The solid line represents the best calculated fit (see text for the fitted parameters).

For complex 1, the temperature dependence of the magnetic susceptibility obeys the Curie-Weiss law χ ) C/(T - Θ) in the temperature range of 50-300 K. Fitting of the χ-1 vs T plot (Supporting Information, Figure S3) gives a Weiss constant of Θ ) 14.9(5) K, indicating moderate ferromagnetic coupling. The χmT product at room temperature is 1.83 cm3 · K · mol-1. Gradual increase of this product is observed as the temperature is decreased (Figure 7), reaching a maximum of 2.751 cm3 · K · mol-1 at 6 K, and the values are decreased at lower temperatures, which is also indicative of a nonzero spin groundstate contributed by both ferromagnetic and antiferromagnetic couplings between Cu(II) centers.

χmT ) (1 - F)(2Ng2β2/k){2 exp(2J1/kT) + exp[(4J12J2)/kT] + 5 exp[(4J1+2J2)/kT]}/{1 + 6 exp(2J1/kT) + exp[(4J1-4J2)/kT] + 3 exp[(4J1-2J2)/kT] + 5 exp[(4J1+ 2J2)/kT]} + (Ng2β2/k)F

(2)

where F is the molar fraction of mononuclear paramagnetic Cu(II) ions and all other symbols have usual meanings. The best fit gave F ) 0.0044(1), g ) 2.098(8), 2J1 ) -33.5(2) cm-1, 2J2 ) 67.0(3) cm-1, and R ) 8 × 10-5. The calculated g value agrees well with that obtained from ESR measurements (Supporting Information, Figure S4). The combination of a negative value of J1 and a positive value of J2 show the coexistence of relatively strong ferromagnetic and weak antiferromagnetic interactions, resulting in a nonzero spin ground state. Cubane complexes with a nonzero spin ground-state are of importance as new magnetic systems. However, in most of the cases, such complexes show a global antiferromagnetic interaction, with only few exceptions.8b,23 Nakano and co-workers reported a Cu4O4 cubane complex23d with the dihedral angles of 82.58° and 96.57° for the basal planes of copper atoms, which suggests that dx2-y2 magnetic orbitals of the metal centers are nearly perpendicular to each other thereby promoting a ferromagnetic interaction (2J ) 89.8 cm-1). Ramasesha and co-workers also reported cubane copper complexes with similar structural features, showing ferromagnetic couplings (2J ) 29.4 ∼ 77.0 cm-1).8b As concerned with the structure of complex 1, equatorial square planes of the copper atoms associated with the ferromagnetic coupling also show a nearly perpendicular arrangement (Scheme 3), with a dihedral angle of 84.06(3)°, which is in accordance with the ferromagnetic behavior. On the other hand, equatorial planes of the copper atoms associated with the antiferromagnetic coupling are nearly parallel. On the other hand, Cu-O-Cu bridging angle has been found to be the crucial geometrical parameter for the magnetic coupling between Cu(II) centeres in hydroxo- or alkoxo-bridged binuclear Cu(II) complexes.24 Ferromagnetism is associated with smaller


Xie et al.

2Ng2µB2 1 Nβ2g2 χmT ) (1 - F) + F k 2k 3 + e-2J/kT


angles, and antiferromagnetism with larger ones. However, complex 1 has a more complicated tetranuclear cubane core containing 4 short and 2 long Cu · · · Cu distances, which can be classified as the “4 + 2” class as defined by Eliseo Ruiz and coauthors.25 The presence of long Cu-O bond distances in all exchange pathways considerably complicates the application of the rules derived for simple binuclear complexes. Eliseo Ruiz and coauthors have studied the magnetostructural correlations in such tetranuclear Cu(II) complexes by DFT calculations.25 For complex 1, the bridging angles are 99.87° and 106.71°, respectively; the bridging distances are 1.967 and 2.482 Å, respectively. 2J1 values of -33.5(2) cm-1 seem to be too large and antiferromagnetic compared with the small ferromagnetic constants obtained from the calculations for similar Cu4 complexes, and 2J2 values of 67.0(3) cm-1 are larger than those obtained from the calculations based on complexes with similar small brdging angles and nearly perpendicularly arranged magnetic orbitals. This can be rationalized by the chelating nature of (L1)2- in complex 1 compared with the non chelated models applied in the calculations.25 The field-dependent magnetization of complex 1 measured at 2.0 K (Supporting Information, Figure S5) initially increases quickly to 3.41 Nβ in the low-field region, and above 20 kOe, it increases slowly with the field and reaches 4.34 Nβ per Cu4 unit at 50 kOe, which is slightly higher than the expected saturation value of 4 Nβ for four spin-only Cu(II) ions with S ) 1/2, and g ) 2.0. This confirms the ferromagnetic characteristics. The plot of χmT products vs temperature for complex 2 (Supporting Information, Figure S6) exhibit a trend similar to that of complex 1, indicating a similar nonzero spin ground state. Due to the low semmetry of complex 2, all nickel(II) atoms are crystallographically independent. Theoretically, the data fitting requires a total of 6 independent J values. To simplify the data fitting, we made an approximation by assuming that the J values are the same for the nickel pairs with similar structural data, thus we tried the three-J model26 to analyze the data, but no satisfactory results could be obtained. For complex 4, Supporting Information Figure S7 shows the plot of χ-1 vs T. The data cannot be fit to Curie-Weiss law in the whole temperature range, which may be ascribed to a rather strong antiferromagnetic interaction between Cu(II) centers. Gradual decrease in χmT is observed as the temperature is decreased (Figure 8), indicating an intramolecular antiferromagnetic interaction. The data were fitted to the Bleaney-Bowers equation (eq 3):27

(3)

Here all symbols have the usual meanings. The best fit gave F ) 0.0043(1), g ) 2.061(5), 2J ) -449(3) cm-1, and R ) 1 × 10-5. The calculated g value agrees well with that obtained from ESR measurements (Supporting Information, Figure S4). The field-dependent magnetization of complex 4 measured at 2.0 K (Supporting Information, Figure S8) increases slowly and reaches a small value of 0.023 Nβ per Cu2 unit at 50 kOe, far from the expected saturation value. This confirms the strong antiferromagnetic interaction. As mentioned above, linear correlations have been found between the Cu-O-Cu bridging angle (θ) and spin coupling (J) between metal centers for binuclear copper(II) complexes equatorially bridged by pairs of hydroxide or alkoxide groups. A similar linear correlation has also been proposed for binuclear copper(II) complexes bridged by a pair of phenoxides.20b However, the best fit -2J value of 449 cm-1 for complex 4 associated with the Cu-O-Cu bridging angle of 100.97(9)° is significantly smaller than the value of 764.0 cm-1 expected from the empirical linear relationship and those for the complexes with similar bridging angles.20a It has been observed that the decrease of antiferromagnetic interaction in diphenoxo-bridged dicopper(II) complexes can be attributed to (i) the increasing pyramidal character of bridging phenolate; and (ii) the tetrahedral distortion of the basal plane containing Cu(II) atoms.28 In complex 4, the sum of three angles around O(1) is 357.58°, indicating a very small pyramidal distortion. On the other hand, the Cu1 atom is displaced 0.042 Å from the mean basal plane defined by O1, O2, N1 and O1A. The average displacement of O1 and N1 from the mean plane is 0.65 Å, while that of O2 and O1A is -0.65 Å, indicating a moderate tetrahedral distortion of the basal plane. Thus, decrease of antiferromagnetic interaction between Cu(II) centers in complex 4 can be partly ascribed to the moderate distortion of the basal plane of Cu(II). In fact, even weaker antiferromagnetic interactions have been observed for diphenoxo-bridged dicopper(II) complexes with similar bridging angles but with more severely distorted Cu(II) basal planes.28,29 Decrease of antiferromagnetic interaction might also be related to the absence of alternative exchange pathways through the conjugated π-framework proposed for the diphenoxo-bridged dicopper(II) model complexes of macrocyclic ligands.20a For complex 5, the temperature dependence of the magnetic susceptibility obeys the Curie-Weiss law. Fitting of the χ-1 vs T plot (Supporting Information, Figure S9) gives a Weiss constant of Θ ) 2.1(1) K, indicating weak ferromagnetic coupling. The χmT value (Figure 9) at room temperature is approximately 0.83 cm3 · K · mol-1. As the temperature is decreased, it remains almost constant until 165 K. As the temperature is further decreased, it is gradually increased to a value of 1.1 cm3 · K · mol-1 at 2 K, indicative of a ferromagnetic coupling between the copper(II) centers. The best fit to equation 3 gave F ) 0.015(1), g ) 2.091(7), 2J ) 7.7(2) cm-1, and R ) 2 × 10-5. The field-dependent magnetization of complex 5 measured at 2.0 K (Supporting Information, Figure S10) initially increases quickly to 1.59 Nβ in the low-field region, and above 20 kOe, it increased slowly with the field and reaches 1.99 Nβ per Cu2 at 50 kOe, which is roughly equivalent to the expected saturation value of 2 Nβ for four spin-only CuII ions with S ) 1/2, and g ) 2.0. This confirms the weak ferromagnetic characteristics.


Crystal Growth & Design, Vol. 9, No. 1, 2009 125

These results provide further insight into the subtle structuredirecting factors for polynuclear and binuclear metal complexes, supramolecular coordination assemblies, and the stabilization of water clusters. This work is valuable for the design and synthesis of novel complexes of the relevant structural types. Further work on the complexes with novel extended structures and interesting properties from relevant polydentate hydroxyrich ligands is underway in our laboratory.


Three main types of dichloro-bridged binuclear Cu(II) complexes have been reported: (I) square pyramids sharing a basal edge with coplanar basal planes;30 (II) two square pyramids sharing a base-to-apex edge with parallel basal planes;31 and (III) square pyramids sharing one base-to-apex edge with the two bases nearly perpendicular to each other.32 In the case of complex 5, Cu(II) centers are octahedral, but it still can be included magnetically in type II according to the bridging geometries. For type II, the superexchange pathway with metal centers will take place mainly through an interaction between Cu(II) dx2-y2 and the apical pCl orbitals. For an ideal square-based geometry, there would be no magnetic coupling between Cu atoms. As a result, compared with type I and type III, all complexes of type II present very small J values (-10 < J < +10 cm-1), which are caused by small structural deviations from the ideal squared Cu(II) cores. Hatfield and coworkers found that the experimental coupling constant is correlated with the quotient φ/R0, where φ is the Cu-X-Cu angle and R0 is the longest Cu-X distance.33 It was found that for values of this quotient lower than 32.6 and higher than 34.8°/Å, the exchange interaction is antiferromagnetic. For values between these limits, the exchange interaction is ferromagnetic. For complex 5, the quotient is 33.0°/Å, just lying in the range of 32.6-34.8, and ferromagnetic interactions are observed, indicating that complex 5 well follow Hatfield’s trend for type II complexes. Conclusions Hydroxyl-rich ligands H4L1 and H3L2 have been used to synthesize tetranuclear cubane complexes 1-3 and binuclear complexes 4-5. Their structures have been demonstrated to be dependent on various factors, including the disposition of the functional groups in the chelating ligands, metal ions, and anions. In the complexes, H4L1 coordinates in a dianionic form or a monoanionic form. In contrast, H3L2 coordinates in a neutral form. In all these complexes, the chelating ligands coordinate in a tetradentate form, with one neutral alkoxyl O left noncoordinated, and open for intermolecular interactions, resulting in the stabilization of various water clusters and the formation of various supramolecular assemblies. Magnetic measurements reveal that in complexes 1 and 2, antiferromagnetic and ferromagnetic couplings coexist, resulting in unusual nonzero spin ground state. For complex 4, a strong antiferromagnetic interaction is mediated by bridging phenoxide oxygens. For complex 5, ferromagnetic interactions between copper(II) centers are mediated by the dichloro-bridge.

Acknowledgment. This work was financially supported by the Shanghai Pujiang Program (08PJ14037), East China University of Science and Technology, NSFC/China, Education Committee of Shanghai, Scientific Committee of Shanghai, the Program for New Century Excellent Talents in University (NCET), SRF for ROCS, and SEM. Supporting Information Available: Crystallographic information files (CIF format) for compounds 1-5, figures showing the molecular structure and 2D hydrogen-bonded network of complex 3, plot of the products χmT vs temperature for complex 2, ESR spectra, χm-1 vs temperature, and field dependent magnetization for 1, 4, and 5, and tables for hydrogen-bonding geometries. This material is available free of charge via the Internet at http://pubs.acs.org.

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CG7012073

Tetra- and Binuclear Complexes of Hydroxy-Rich Ligands

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