Employing An Unprecedented Ferromagnetic Molecular Building Block (MBB) of [Co2Ln(µ3-OH)(CO2)5(N3)]2 (Ln ) Tb, Gd, Dy, Eu, Sm) to Construct a 6-Connected r-Po Net Yu-ting Yang, Feng Luo, Yun-xia Che, and Ji-min Zheng*
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 10 3508–3510
Department of Chemistry, Nankai UniVersity, Tian Jin 300071, China ReceiVed April 26, 2008; ReVised Manuscript ReceiVed July 10, 2008
ABSTRACT: A series of heterometallic Co(II)-Ln(III) metal-azido complexes, Co2Ln(µ3-OH) (nic)5(N3)(1) (Hnic ) nicotinic acid) (Ln ) Tb/1, Gd/2, Dy/3, Eu/4, Sm/5), have been synthesized hydrothermally and characterized by X-ray diffraction analysis, which shows the 6-connected R-Po net, where the uncommon [Co2Ln(µ3-OH)(CO2)5(N3)]2 MBBs act as six-connected nodes and display ferromagnetic interactions. In the ongoing quest for the design of functional materials, metalorganic materials (MOMs, e.g., metal-organic polyhedra, MOFs, or coordination polymers) have been identified as potential platforms for their exploitable applications in magnetism, luminescence materials, molecular adsorption, and bimetallic catalysis, etc.1 The azido ligand is a suitable bridging ligand that shows several coordination bridging modes. It is especially easy to predict the magnetic nature of the coupling depending on its linkage modes and structural parameters, sometimes leading to spin-canting, spinflop, and metamagnetism, so it is a good candidate for the design of magnetic coordination polymers.2 Usually, the azide bridge transmits mainly ferromagnetic (FM) interactions for the end-on (EO) bridging mode and antiferromagnetic (AFM) interactions for the end-to-end (EE) mode.3 So far, a variety of metal-azido complexes have been reported, however, most of them are only based on 3d metal atoms. Recently, we have seen a lot of Co/Ni/ Cu/Mn azido complexes induced by the second coligands, which hold the 0D to 3D structure and interesting magnetic behavior.4–7 By contrast, because of the somewhat difficult synthesis, only limited instances of 3d-4f metal-azide coordination complexes have been documented, although it is believed that the exploration of 3d-4f metal-azide coordination complexes will bring intriguing topologies and interesting magnetic behavior.8 Thus, there is still a major challenge in this theme. As an ongoing work in the design and preparation of 3d-4f MOFs,9 herein, we report a series of heterometallic Co(II)-Ln(III) metal-azido complexes, 1-5. The 3D matrix of them is the common R-Po net, but its 6-connected MBB, viz. [Co2Ln(µ3-OH)(CO2)5(N3)]2, is rather rare. And the succedent magnetic measurement suggests the ferromagnetic interactions between them. Moreover, through this work, to some extent, we also demonstrated that the Hnic ligand is a good organic spacer to construct 3d-4f metalazide coordination complexes. The hydrothermal reaction of Ln2O3, CoCl2 · 6H2O, NaN3, nicotinic acid, and H2O in a molar ratio 1:2:2:5:2000 at 190 °C led to the formation of the deep red air-stable complexes 1-5. The IR spectrum shows the characteristic peak of azide at 2075 cm-110 and the typical antisymmetric (1611 cm-1) and symmetric (1566 and 1400 cm-1) stretching bands of carboxylate groups of nic ligands.11 Additional experiments suggest that other cobalt slats such as Co(NO3)2, CoSO4, Co(ClO4)2 can be used as cobalt source to replace CoCl2 to synthesize these compounds. Furthermore, this reaction is not so sensitive to the reaction temperature, as the compounds can be obtained at 170-200 °C. The single-crystal X-ray diffraction analysis reveals that polymers 1-5 are isostructural and display the triclinic P1j space group. * Corresponding author. E-mail:
[email protected].
Figure 1. View of the [Co2Ln(µ3-OH)(CO2)5(N3)]2 MBB.
Herein, only the structure of 1 is discussed representatively. The asymmetric unit of 1 contains one Tb(III) ion, two unique Co(II) (Co1 and Co2) ions, one azide ligand, and five different nicotinate anions. As shown in Figure 1, both Co1 and Co2 show sixcoordinated distorted octahedral geometry, but such geometry for Co1 is completed by two nic N atoms (N4,N5), three nic O atoms (O4,O5,O8) and one hydroxy O(O9), whereas for Co2, it is ligated by two nic N atoms (N6,N7), two nic O atoms (O7,O11), one hydroxy O(O9), and one azide N(N1) atom. The Tb1 ion is surrounded by five nic O atoms, one azide N, and one hydroxy O atom, to create the monocapped trigonal-prism geometry. The bond lengths and angles are list in Table 1 (see the Supporting Information) and all of them are in the normal range.9 The nic liangds adopt µ3:η1:η1:η1 and µ2:η1:η1:η0 coordinated modes, whereas azide ligands give the µ1,1-coordinated fashion. The bondvalence calculation suggests the bond-valence parameter 2.16 for Co1 and 2.20 for Co2.12 As illustrated in Figure 1, the O9 atom from the OH- ion acts as the tridentate ligand in the µ3-fashion to bridge one Tb and two
10.1021/cg800431b CCC: $40.75 2008 American Chemical Society Published on Web 08/22/2008
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Crystal Growth & Design, Vol. 8, No. 10, 2008 3509
Figure 2. View of the connectivity of [Co2Ln(µ3-OH)(CO2)5(N3)]2 MBB.
Co ions to create the Co2Tb(µ3-OH) fragment, where the angles of Co1-O9-Co2, Co1-O9-Tb1, and Co2-O9-Tb1 are 129.53, 112.74, and 99.89°, respectively, and the Co1-Co2, Co1-Tb1, and Co2-Tb1 distances are 3.812, 3.693, and 3.396 Å, respectively. Moreover, within such a fragment, Co1-Tb1 is also associated with two nic carboxyl groups, whereas Co2-Tb1 is linked by one nic carboxyl group and one azide N (N1) atom, and Co1-Co2 is only connected by one nic carboxyl group. Interestingly, to the best of our knowledge, there still is not a precedent of µ1,1-N3- connected Co-Tb structure observed in this Co2Tb(µ3-OH) fragment. The Co-N-Tb angle is 93.91°. Further, two such Co2Tb(µ3-OH) fragments separated by ca. 4.3Å (the Tb-Tb distance) connect each other by two nic carboxyl groups to furnish the [Co2La(µ3OH)(CO2)5(N3)]2 substructure. In the literature, several notable MOFs built on Cr3(µ3-OH)(CO2)6, Al3(µ3-OH)(CO2)6, Fe3(µ3OH)(CO2)3(SO4)3, and Co3(µ3-OH)(CO2)6 MBBs have been documented,13 but there is not an instance built this special 3d-4f MBB. Thus, it is believed that the present [Co2Ln(µ3-OH)(CO2)5(N3)]2 MBB is unique and unprecedented. Furthermore, these MBBs are linked together via 16 nic connectors to give the R-Po net with the 41263 topology symbol. (Figure 3) Notably, these 16 nic connectors link only to six directions: as shown in Figure 2, along some two directions, the MBBs are linked by four nic connectors, whereas along some four directions, they are associated by two nic spacers. Such phenomenon is also observed in one Cd4-based and one Cu2Gd2-based polymer that holds 12 connectors in six directions and creates the R-Po net with double edges.11,9a Comparing with them, to the best of our knowledge, the preset R-Po net owns the highest number of connectors.
Figure 3. Schematic description of R-Po net of 1.
The TGA measurement of 1 shows that at 30-320 °C, there is no weight loss until the chemical decomposition of it; this research result is consisted with the structure analysis (see Figure S1 in the Supporting Information). The phase purity of 1 is confirmed by XRD research (see Figure S2 in the Supporting Information). The magnetic properties of 1 were investigated over the temperature range 2-300 K in a field of 1000 Oe (Figure 4). At 300 K, χMT is equal to17.94 cm3 mol-1 K, which is higher than the sum of the expected value (15.56 cm3 mol-1 K, gCo ) 2.0, SCo ) 3/2, gTb ) 3/2) for one Tb(III) ion and two isolated Co(II) ions, because of the spin-orbit of the octahedral Co(II) ions.14 The plateau of χMT values at 74-300 K appears to be the result of the compromise between the spin-orbit coupling effects of the high-spin octahedral Co(II) ions with an orbital degenerate 4T1 ground term and the ferromagnetic interactions of them. It then abruptly rises to a
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Figure 4. χMT vs T plot and the inserted χM-1 vs T plot.
maximum of 9.22 cm3 mol-1 K at 22 K, indicating the onset of ferromagnetic interactions, which is further supported by the wellfitted χM-1 vs T plot that gives C ) 17.86 cm3 mol-1 K and θ ) +0.61 K. Furthermore, below this crucial temperature, χMT sharply decreases to 13.62 cm3 mol-1 K at 2 K, possibly because of ZFS (zero-field splitting) and/or inter-MBBs antiferromagnetic interactions. Within this [Co2Tb(µ3-OH)(CO2)5(N3)]2 MBB, the mainly metal-bridge-metal coupling pathway is listed below: J1/ Co1-O9-Co2, J2/Co2-O-Tb1, J3/Co1-O9-Tb1, J4/Co2-N1Tb1, zJ/intra-MBB. And the g value of Co(II) and Tb(III) is also different. Thus, at present, it is difficult to simulate it by a suitable magnetic model. In this communication, we disclosed an unique and unprecedented MBB, viz. [Co2Ln(µ3-OH)(CO2)5(N3)]2, which holds 16 connectors but only in six directions, thus resulting in the R-Po net. The succedent magnetic measurement suggests the ferromagnetic interactions between them, but at present, as discussed above, it is still difficult to simulate it by suitable magnetic model. And then we can not confirm which metal-bridge-metal coupling pathway is ferromagnetic coupling and whether the antiferromagnetic coupling is coexisting between this MBB. Thereby, the further exploration of the magnetic properties of this MBB will be our follow-up work.
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Acknowledgment. This work was supported by the Nation Natural Science Foundation of China under Project (50572040).
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Supporting Information Available: Crystallographic information in CIF format; synthesis information, tables of crystallographic data and bond lengths and angles, and TGA and XRD graphs (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
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