Two-Dimensional Coordination Polymers Exhibiting Antiferromagnetic Gd(III)-Cu(II) Coupling Hui-Zhong Kou,* Yun-Bo Jiang, and Ai-Li Cui Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 1 77-79
Received March 22, 2004
ABSTRACT: A series of carboxylate-bridged 4f-3d complexes Cat+[Cu(nta)Cl]2Ln(H2O)4 (Cat+ ) Na+ or K+, Ln3+ ) Gd3+ or La3+, and nta3- ) nitrilotriacetate) have been synthesized by self-assembly of CuCl2‚4H2O, H3nta, and LnCl3‚6H2O in the presence of stoichiometric amounts of CatOH. X-ray diffraction analysis revealed that these complexes are isostructural and have a novel layered structure. The Ln3+ ion is coordinated by nine oxygen atoms of four [Cu(nta)Cl]2- and four water molecules, while each [Cu(nta)Cl]2- anion connects two Ln3+ using carboxylate oxygen atoms giving rise to a rhombus-like layered structure. The alkaline metal ions Cat+ are situated between the layers and coordinated to the oxygen atoms of [Cu(nta)Cl]2- generating a quasi-three-dimensional network. Magnetic studies indicate the presence of apparent antiferromagnetic coupling between Cu(II) and Gd(III) in KCu2Gd and NaCu2Gd. The magnetic susceptibilities of KCu2La were simulated by using a quadratic model, which suggests a weak antiferromagnetic Cu(II)-Cu(II) coupling. The mechanism of magnetic coupling between 3d and 4f metal ions is complicated and is far from clear elucidation. For example, the magnetic exchange between Cu2+ and Gd3+ has been found to be either ferromagnetic1-41 or antiferromagnetic42-47 although the former situation is more commonly observed. Therefore, the intrinsic ferromagnetic Cu(II)-Gd(III) coupling was doubted.43-45 Very recently, a state of the art quantum chemical calculation has been used for the investigation of the mechanism of ferromagnetic Cu(II)-Gd(III) coupling and has shown that the magnetic properties are associated with what orbital symmetries are taken. The conclusions have been drawn that the usual ferromagnetic coupling is correlated with the large occurrence of approximate pseudo-C2v geometry, while the antiferromagnetic interactions occur when the molecular symmetry is lowered.48 The carboxylate-bridged Gd(III)-Cu(II) complexes have been investigated magnetically, and more frequently, weak antiferromagnetic intermetallic coupling was observed.46,47 Because of the tendency to form complicated carboxylatebridged Cu(II)-Gd(III) linkages, the net antiferromagnetic character may partly be a result of the presence of an intra-/intermolecular Cu(II)-Cu(II) antiferromagnetic interaction.39,40 Recently, [Cu(nta)Cl]2- has been utilized to synthesize one-dimensional chainlike and two-dimensional (2D) honeycomb-like Cu-La complexes [LnCuCl(nta)(H2O)6](ClO4)‚nH2O (Ln ) La and Gd, n ) 1; Ln ) Er, n ) 3).41,49 The different molecular structure of these complexes results from the synergetic effects of the coordinated Cland the charge-compensating ClO4- anions as well as the radii of Ln3+ ions. Interested in the effect of the counteranion in structure, we synthesized, in the absence of ClO4anion, a series of new Cu-nta-Ln complexes Cat+[Cu(nta)Cl]2Ln(H2O)4 [abbreviated as KCuLa (1), KCuGd (2), and NaCuGd (3)] and investigated their magnetic properties. Blue single crystals suitable for X-ray structure analysis were grown at room temperature by the slow evaporation of an aqueous solution of CuCl2‚4H2O, H3nta, LnCl3‚6H2O, and KOH or NaOH (molar ratio ) 1:1:1:3). The initial precipitate was dissolved by gentle heating of the mixture; yield, 70%.50 X-ray crystallography51 revealed that the structure of the three complexes is isomorphous (Figure 1) and consists of an anionic infinite nonplanar sheet with the repeating unit of {[Cu(nta)Cl]2Ln(H2O)4}- and one alkaline metal ion Cat+
Figure 1. Structure of complex 2 (K+ and the hydrogen molecules are not shown for clarity).
as the counterion. In the crystal, present are four independent Gd(III) ions and eight independent Cu(II) ions although the coordination environments of the Gd(III) ions or Cu(II) ions are very alike. Each Gd(III) ion coordinates to four adjacent [Cu(nta)Cl]2- anions through four carboxylate bridges of four [Cu(nta)Cl]2-. Each Cu(II) ion connects two Gd(III) ions in a nearly trans fashion. The water molecules and the K+ ions are positioned between the layers. The Cat+ ion is surrounded by seven or eight oxygen atoms of the coordinated nta3- ligands and links two adjacent layers generating a three-dimensional (3D) network (see Supporting Information). The 2D layered anion can be described as a rhombuslike network formed by the cross-linking of alternating [Cu(nta)Cl]-Ln(H2O)4 chains (Figure 2). The magnetic susceptibilities of three complexes have been measured on a MagLab 2000 magnetometer in the temperature range of 2.0-300 K. A plot of χmT vs T for complex 1 is shown in Figure 3, where χm is the magnetic susceptibility per Cu2LaK unit. With the decrease of the temperature, χmT decreases smoothly down to ca. 20 K and then decreases rapidly, a phenomenon typical of the presence of an overall antiferromagnetic coupling between the paramagnetic Cu(II) ions. The magnetic susceptibility obeys the Curie-Weiss law with a negative Weiss constant θ ) -2.9 K and a Curie constant of 0.86 emu K mol-1. The small Weiss constant (absolute value) indicates a weak Cu(II)-Cu(II) magnetic coupling, which is understandable considering that the interacting paramagnetic ions are spaced out by diamagnetic La3+ ions. It should be noted that the 2D honeycomb-like Cu-nta-La complex exhibits weak intralayer ferromagnetic Cu(II)-Cu(II) coupling.49 From the magnetic viewpoint, the structure of the K+linked 3D complex (1) can be described as carboxylato-
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Figure 4. Temperature dependence of χmT for 2 and 3. Figure 2. View of the rhombus-like layered structure of 1-3.
Figure 3. Temperature dependence of χmT for 1 measured at 20 kOe.
bridged 2D layers connected by diamagnetic K+ ions. Within the layer, the magnetic coupling is approximated as identical. On this basis, the magnetic susceptibility data were fitted through Line’s high temperature series expansion for an S ) 1/2 antiferromagnetic quadratic layer,52 based on the exchange Hamiltonian H ˆ ) -2J ∑ij S ˆi ‚ S ˆ j, where the sum runs over all pairs of nearest-neighboring spins i and j (eq 1) in which φ ) kT/|J|S(S + 1), a1 ) 4, a2 ) 2.667, a3 ) 1.185, a4 ) 0.149, a5 ) -0.191, a6 ) 0.001, and N, g, and β have their usual meanings. The best-fit parameters are J ) -0.54(1) cm-1 and g ) 2.11(1).
∑
Ng2β2/χm|J| ) 3φ + (
an/φn-1)
(1)
The plot of χmT vs T (per Cu2GdK for 2 and per Cu2GdNa for 3) is characteristic of the presence of a net antiferromagnetic interaction between paramagnetic ions (Figure 4). The apparent increase of χmT at low temperatures is suggestive of the ferrimagnetic behavior of both complexes, which is due to the presence of the noncompensation of antiparallel SCu ) 1/2 and SGd ) 7/2 spins. The magnetic susceptibilities obey the Curie-Weiss law with negative Weiss constants of -0.70 K for 2 and -3.49 K for 3. The Curie constant is equal to 8.46 and 8.70 emu K mol-1 for 2 and 3, respectively, close to the expected spinonly value of 8.625 emu K mol-1 with g ) 2.0. The decrease of χmT at high temperatures sometimes results from the antiferromagnetic coupling between transition metal ions since the 4f-3d magnetic exchange is usually weak. To eliminate this possibility, a control experiment is performed, i.e., comparing the magnetic susceptibilities of Cu-Gd complexes with that of the Cu-
La analogue (1). It can be concluded that the Cu(II)Gd(III) coupling is antiferromagnetic because at high temperatures the slope of the curves for complexes 2 and 3 is more sharp than that for complex 1. No magnetic ordering is observed in the Cu(II)-Gd(III) species down to 1.8 K, which indicates that the carboxylate bridges cannot so effectively mediate magnetic interaction as the cyanide bridge.53-55 It is worth noting that most phenoloxo- or oxamidato-bridged Cu(II)-Gd(III) complexes exhibit ferromagnetic intermetallic coupling, whereas carboxylate-bridged Cu(II)-Gd(III) complexes are usually antiferromagnetic. According to the mechanism raised by Hirao et al.,48 this phenomenon can be tentatively attributed to the presence of low molecular symmetry of the species. In conclusion, three new carboxylate-bridged Cu(II)Ln(III) complexes have been synthesized via the fine tuning of the counteranion. They exhibit a novel rhombus-like layered structure. It has been clearly shown that the Cu(II)-Gd(III) magnetic coupling is antiferromagnetic. The present research affords new examples showing antiferromagnetic Cu(II)-Gd(III) coupling and might help elucidate the intrinsic magnetic coupling between the Cu(II) and Gd(III) ions. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Projects 20201008 and 50272034) and Fok Ying Tong Education Foundation (No. 91016). We are grateful to Prof. Song Gao for the magnetic measurements. Supporting Information Available: Crystal cell packing plots of complexes 1-3. An X-ray crystallographic file. This material is available free of charge via the Internet at http:// pubs.acs.org.
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