Extended Tapes of Cyclic Water Hexamers in an Unusual Oxalate-Bridged One-Dimensional Copper(II) Complex Xue-Yan Song,† Li-Cun Li,*,†,‡ Dai-Zheng Liao,† Zong-Hui Jiang,† and Shi-Ping Yan† Department of Chemistry, Nankai UniVersity, Tianjin 300071, China, and State Key Laboratory of Sructural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 7 1220-1222
ReceiVed January 7, 2007; ReVised Manuscript ReceiVed May 18, 2007
ABSTRACT: Extended tapes of cyclic water hexamers are formed in a one-dimensional copper(II) complex bridged by oxalate with a rare mono, bidentate coordination mode. The water tapes are stabilized by the hydrogen bond interactions between uncoordinated oxalate oxygen atoms and water molecules. Magnetic analysis reveals weak ferromagnetic interactions mediated by the oxalate bridge with the exchange parameter J ) 0.36 cm-1. Water has raised more interest than other compounds because of its fundamental importance in many biological and chemical processes.1 In this context, water clusters have been extensively studied both theoretically and experimentally, because they can provide a description and insights into the structures and characteristics of liquid water or ice.2,3 A variety of water clusters such as trimers, tetramers, pentamers, and larger oligomers have been investigated. Among those, hexamer water clusters are more interesting.4 Recent studies indicate that this cluster is the smallest possible unit that can exhibit some of the properties found in bulk water.5,6 However, on the basis of how the discrete water clusters form chains, tapes, and layers and how they are stabilized in crystal hosts, much work is required to synthesize new species with some special frameworks. On the other hand, the oxalate ligand as old but evergreen bridging ligand has played a key role in the field of molecular magnetism because of its remarkable ability to mediate magnetic exchange interactions between transition metal centers.7-9 By employing this ligand, a great diversity of homo- and heterometallic compounds with interesting structures (one-, two-, and threedimensional)10-13 and physical properties14 have been prepared. In general, the adoption of the bis(chelating) coordination mode of the oxalate bridge to form five-membered rings with the metal centers is extensively dominant, and so far the number of compounds structurally characterized in which one or two oxygen atoms of the oxalate are uncoordinated is very rare. The magnetostructural studies on this kind of oxalato-bridged metal complexes are limited. Herein, we report a ferromagnetic one-dimensional oxalatebridged copper(II) complex [Cu(L)(ox)]n‚3nH2O(1), (L ) bidentate Schiff base ligand; ox ) oxalate anion), in which the oxalate ligands bridge the metal ions through an unusual mono, bidentate coordination mode. Cyclic water hexamers with chair conformation selfassemble into unprecedented one-dimensional tapes, which are stabilized by uncoordinated oxalate oxygen atoms because of the hydrogen bond interactions. Deep green crystals of 1 were obtained by slow evaporation of Cu(NO3)2‚3H2O, [(NH4)3Fe(ox)3)]‚3H2O, and L in water/methanol for 1 month.15 Single-crystal X-ray analysis16 reveals that complex 1 is built of [Cu(L)]2+ units bridged by oxalate anions to form zigzag chains parallel to the a-axis (Figure 1). The Cu(II) ion is pentacoordinate, by virtue of two oxygen atoms (Cu(1)-O(1), 1.964(3) Å; Cu(1)-O(2), 1.951(3) Å) from one oxalate ligand and two nitrogen atoms (Cu(1)-N(1), 2.031(4)Å; Cu(1)-N(2), 1.998(4)Å) of bidentate Schiff base ligand to form the basal plane and * Corresponding author. E-mail:
[email protected]. † Nankai University. ‡ Chinese Academy of Sciences.
Figure 1. Perspective view of a fragment of the resulting chain structure of 1. Color code: C, gray; N, blue; O, red; Cu, cyan.
Figure 2. 1D chains locked through π-π interactions between adjacent chains in complex 1.
the coordination sphere is completed by an apical oxygen atom from another oxalate with a longer bond (Cu(1)-O(4A), 2.325(3)
10.1021/cg070017u CCC: $37.00 © 2007 American Chemical Society Published on Web 05/31/2007
Communications
Crystal Growth & Design, Vol. 7, No. 7, 2007 1221
Figure 3. (a) Chair cyclic water hexamer. (b) Self-assembly of the water hexamers into extended tape. Table 1. Geometrical Parameters of Hydrogen Bonds (Å, deg) in Complex 1a D-H‚‚‚A
D‚‚‚A
∠D-H-A
O(8)-H(8′′)‚‚‚O(3) O(8)-H(8′)‚‚‚O(7)#1 O(7)-H(7′)‚‚‚O(6)#2 O(7)-H(7′′)‚‚‚O(8)#3 O(6)-H(6′)‚‚‚O(8)#4
2.845 2.874 2.843 2.883 3.342
127.98 113 170.78 119.56 143.12
a Symmetry codes: #1 -x, y + 1/2, -z + 1/2; #2 -x + 1, -y + 1, -z + 1; #3 x, -y + 1/2, z + 1/2; #4 -x + 1, y + 1/2, -z + 1/2.
Å). The coordination geometry at the copper center can be described as slightly distorted square pyramid with a parameter τ ) 0.1[τ ) |β - R|/60, where β and R are the bond angles involving the trans donor atoms in the basal plane, τ ) 0 and 1 for ideal square pyramidal and trigonal bipyramidal geometries, respectively.17,18 The shortest intrachain copper-copper distance across the oxalate ligand is 6.003 Å, which is longer than those polynuclear complexes with bis-bidentate oxalate bridges (
