Two Coordination Polymers Involving Triangular and Linear Trinuclear Co(II) Clusters Created Via In situ Ligand Synthesis Lu-Fang Ma,†,‡ Yao-Yu Wang,*,† Li-Ya Wang,*,‡ Dan-Hua Lu,‡ Stuart. R. Batten,*,§ and Jian-Ge Wang‡
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 5 2036–2038
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, Department of Chemistry, Northwest UniVersity, Xi’an 710069, P. R. China, College of Chemistry and Chemical Engineering, Luoyang Normal UniVersity, Luoyang 471022, P. R. China, and School of Chemistry, Monash UniVersity, Victoria 3800, Australia ReceiVed October 6, 2008; ReVised Manuscript ReceiVed March 5, 2009
ABSTRACT: Two novel 3D metal-organic frameworks with cobalt(II) clusters as nodes and mixed bridging ligands as links, {[Co3(OH)(tbip)2(Htbip)(dps)(dpds)0.5]}n (1) and {[Co2 (H2O)(tbip)2(dps)] · CH3OH · 2 H2O}2n (2) (dps ) 4,4′-dipyridylsulfide, dpds ) 4,4′dipyridyldisulfide, H2tbip ) 5-tert-butyl isophthalic acid), were obtained from the same reaction mixture but tuned by different hydrothermal temperatures: at 120 °C, product 1 is a 3D MOF based on the novel triangular trinuclear cluster node, whereas at 160 °C, product 2 is a 3D MOF constructed from both a linear trinuclear cluster node and a mononuclear CoII node. The starting dpds reagent was partly converted into dps ligand in 1 and wholly transferred into dps in 2 via new in situ cleavage of both S-S and S-C bonds and temperature-dependent in situ ligand rearrangement of dpds. The design and synthesis of new metal-organic frameworks has been shown to be a promising field, not only because of their potential applications as functional solid materials in host-guest chemistry, ion exchange, and catalysis but also because of their intriguing variety of architectural features and fascinating new topologies.1-3 To build these molecular architectures, researchers often employ bridging polycarboxylate and bipyridyl ligands to construct coordination polymers because of their versatile coordination modes and high structural stability.4 In contrast to the large and increasing amount of work on the synthesis with ligands containing O or N donors, there have been fewer reports of studies based on organothiolate ligands. Two kinds of organothiolate ligands, thiolatopyridine and thiolatocarboxylate, have been explored to construct structures with luminescent or magnetic properties.5,6 4,4′-Dipyridyldisulfide (dpds) has recently attracted attention not only because of its twisted conformation, with a C-S-S-C torsion angle of ca. 90° and axial chirality that potentially generate M and P enantiomers in chiral crystal engineering, but also the easy cleavage of the S-S bond.7,8 As reported, the dpds ligand in most of the compounds tends to adopt the pyridyl form that coordinates with metal atoms through the nitrogen donor without S-S bond cleavage under room temperature.9 However, in some cases, dpds could be converted into other related compounds.10 Recently, in a report by Tong et al., where reactions of the dpds ligand with CuI were conducted under solvothermal conditions, the dpds reagent was unprecedentedly converted into two isomeric ligands, 4,4′-dipyridylsulfide (dps) and 1-(4-pyridyl)4-thiopyridine (ptp) at 120 and 160 °C in CH3CN solution, respectively.11 Carla Aragoni et al. also reported the reactions between dpds and dithiophosphato NiII complexes yielding three different bridging ligands from in situ chemical rearrangement of the starting dpds reagent (dpds, dps, and dipyridyltrisulfide) in CH3OH-CH2Cl2 or CH3CH2OH-CH2Cl2 mixed solution.12 Pyridine-4-thiolate was also created from a dpds precursor through reductive cleavage of the disulfide bond under solvothermal conditions in CH3OH-CH3CN mixed solution.13 Here we carried out the solvothermal reactions of Co(OAc)2 · 4H2O with dpds and H2tbip at different temperatures in CH3OH-H2O solution where * To whom correspondence should be addressed. E-mail:
[email protected] (Y.-Y.W.);
[email protected] (L.-Y.W.);
[email protected] (S.R.B.). † Northwest University. ‡ Luoyang Normal University. § Monash University.
Figure 1. (a) Coordination environment of the triangular trinuclear cobalt units in 1. (b) View of extended cluster environment. Hydrogen bonds are shown by the striped bonds. (c) View of the bridging of each cluster to five others (hydrogen bonds omitted for clarity). (d) View of the overall 5-connected network topology; nodes represent the [Co3(µ3-OH)] clusters.
dpds can unpredictably partly converted into dps in compound 1 and predictably wholly transferred into dps in compound 2. Although detailed studies are still required to better understand the reason for the generation of various results from a dpds precursor, the results show that the solvent is an important factor for the formation of the final structures. The crystal structure of 1 exhibits a 3D network composed of trinuclear [Co3(µ3-OH)]5+ cluster nodes and dps, dpds, tbip, and Htbip bridging ligands. The dps ligand in 1 was generated from the in situ cleavage and rearrangement reactions of the dpds ligand in the presence of Co(II). To the best of our knowledge, this is the only example of dpds partly converting into dps in situ. There are three crystallographic independent cobalt atoms in 1 (Figure 1a and Figure S1a in the Supporting Information). Co1 is six-coordinated with a distorted octahedral geometry with two nitrogen atoms of
10.1021/cg801120y CCC: $40.75 2009 American Chemical Society Published on Web 03/19/2009
Communications two dps ligands, two oxygen atoms from two tbip ligands, one oxygen atom from a Htbip ligand, and one hydroxyl group. Co2 adopts a distorted octahedral [O5N] coordination geometry with two carboxylic oxygen atoms from two tbip ligands, two carboxylic oxygen atoms from two Htbip ligands, one hydroxyl group and one nitrogen atom of a dpds ligand. Different from Co1 and Co2, Co3 adopts a distorted tetrahedral [O4] coordination environment and is coordinated by three carboxylic oxygen atoms from three tbip ligands and one oxygen atom from a µ3-hydroxyl group. The Co · · · Co distances bridged by the µ3-hydroxyl group are 3.168, 3.345, and 3.528 Å, respectively. As mentioned above, there are trinuclear [Co3(µ3-OH)]5+ clusters which act as nodes in an overall 3D network. Each cluster is supported by four carboxylate groups that also bridge between cobalt atoms (Figure 1b). Two further carboxylates coordinate in monodentate fashions, however the uncoordinated oxygen of one accepts a hydrogen bond from the central µ3-OH, whereas the other monodentate carboxylate has an uncoordinated hydroxide that hydrogen bonds to an oxygen of a bridging carboxylate. Thus there are six carboxylate groups associated with each cluster, which come from both tbip and Htbip ligands. Furthermore, there are three pyridyl groups also coordinated to the cluster: two from dps ligands and one from a dpds ligand. All nine of these carboxylate and pyridyl ligands bridge to adjoining clusters; however, each cluster is connected to only five neighboring clusters (Figure 1c). This is because some pairs of clusters are connected by more than one ligand. Each cluster is connected to two others by single tbip bridges, to two more by both a tbip and a dps bridge, and to a fifth by two Htbip and one dpds bridge. The cluster thus acts as a 5-connecting node, and the overall 3D 5-connected network topology is shown in Figure 1d. It has boron nitride (or bnn) topology,14 with the Schla¨fli symbol 46.64. This net may be described as cross-linked (6,3) sheets; in 1 the tbip and Htbip/Htbip/ dpds bridges link the clusters into the (6,3) sheets, whereas the connections between the sheets are provided by the tbip/dps intercluster bridges. The effective free volume of 1 was calculated by PLATON analysis as 7.8% of the crystal volume (429.8 out of the 5520.5 Å3 unit cell volume), shown in Figure S2 in the Supporting Information. The structure of 2 contains three different cobalt atoms, two different tbip ligands, one type of dps ligand, one type of water ligand, as well as intercalated methanol and water molecules. The Co1 center is coordinated by five oxygen atoms from three different tbip ligands and one nitrogen atom from one dps ligand. Co2 adopts a distorted octahedral [O6] coordination environment and is coordinated by six carboxylic oxygen atoms from six different tbip ligands. One Co2 and two Co1 ions are bridged by six carboxylate groups to form a linear Co3(CO2)6 trimeric cluster (Figure 3a and Figure S3b in the Supporting Information). The third type of cobalt, Co3, is bound by two tbip ligands, two dps ligands, and two terminal H2O molecules. Each trimer is connected to two others via two pairs of tbip ligands that show the bridging mode shown in Scheme S1c in the Supporting Information. It is also connected to two Co3 atoms via two of the other type of tbip ligand, which show the bridging mode shown in Scheme S1d in the Supporting Information, and two other Co3 atoms via bridging dps ligands (Figure 2b). Thus each trimer is connected to two other trimers (two via pairs of tbip ligands) and four Co3 atoms (via single tbip and dps bridges). Therefore, the trimer acts as a 6-connecting node in the overall structure. The Co3 atoms, in turn, act as 4-connecting nodes, coordinating to two trimers via tbip ligands and two others via dps bridges. The overall topology, therefore, is that of a binodal 4,6-connected network with the (44.62)(44.610.8) Schla¨fli symbol (Figure 2c).The structure may also be described in terms of chains of trimers connected by the double tbip bridges, which are then interconnected by the single tbip bridges and Co3 atoms to give sheets (Figure 3a). These sheets are then pillared by the dps spacers into a noninterpenetrated 3D MOF structure (Figure 2d and Figure S3b in the Supporting
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Figure 2. (a) Coordination environment of the linear trinuclear cobalt units in 2. (b) Local connectivity in the structure of 2. Co1 (purple), Co2 (pink), Co3 (orange and labeled). Water ligands, intercalated solvent molecules and tbip tert-butyl groups are omitted for clarity. (c) Schematic representation of the overall network topology. Orange spheres are Co3 atoms, pink spheres represent trimers, red bonds represent single or double tbip bridges, and blue bonds represent dps bridging ligands. (d) A polyhedral view of the 3D structure of 2 view along [100] plane.
Information).The channels in 2 are filled with guest methanol and water molecules. The effective free volume of 2 was calculated by PLATON analysis as 23.5% of the crystal volume (483.3 out of the 2058.5 Å3 unit cell volume). The TG curve for compound 2 shows that the first weight loss of 10.5% (calcd: 10.4%) from 30 to 280 °C is due to the loss of two free water molecules, one coordinated water molecule, and one methanol molecule per formula unit (see Figure S4 in the Supporting Information). The second process might start from 280 to 800 °C, indicating the decomposition of the coordination framework. The final residue of 21.6% is close to the calculated 18.0% based on CoO. PXRD (powder X-ray diffraction) measurements at different temperatures (see Figure S5 in the Supporting Information) show that for compound 2 after heating 280 °C for 8 h, the spectra are similar to the original one, suggesting that their crystalline nature is still maintained after removing the guest molecules. The magnetic susceptibilities, χM, of 1 and 2 were measured in the 2- 300 K temperature range, and are shown as χMT and χM versus T plots in Figure S6 in the Supporting Information. The experimental χMT values of 1 and 2 at room temperature are 7.51 and 8.24 cm3 K mol-1, respectively, which are slightly larger than the spin value expected for three uncoupled high-spin Co(II) ions (5.63 cm3 K mol-1). The χMT value of 1 steadily decreases with decreasing temperature to reach a minimum value of 7.05 cm3 K mol-1 at 35 K. Upon further cooling, the χMT values increase up to maximum of 8.06 cm3 K mol-1 at 7 K and then goes down quickly to a minimum value of 5.12 cm3 K mol-1 at 2 K. The decrease in χMT at high temperature is a typical manner of spin-orbit coupling and is mainly due to the single-ion behavior of Co(II).15 The increase in χMT between 35 and 7 K is due to the ferromagnetic coupling within the µ3-OH-bridged Co(II) ions.16 The M vs H measurement at 2 K among 0-60 KOe is given in Figure
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S7 in the Supporting Information, which further confirms the expected ferromagnetic coupling in 1. Field-cooled magnetic susceptibility at fields of 30 Oe (see Figure S8 in the Supporting Information) shows there is no apex in the plot, suggesting impossible spin-glass or superparamagnetic behavior. The χMT product of 2 continuously decreases as the temperature is lowered to 2 K, which suggests that antiferromagnetic interactions are operative in 2. The temperature dependence of the reciprocal susceptibilities (1/χM) of 2 obey the Curie-Weiss law above 50 K with θ ) -19.65 K, C ) 5.82 cm3 K mol-1, R ) 9.83 × 10-4 for 2. The moderate negative θ values indicate the presence of antiferromagnetic interactions between adjacent Co(II) ions even if a contribution from the spin-orbit coupling of Co(II) is also present.17 In summary, we have successfully synthesized two novel coordination polymers based on dpds and H2tbip with Co(II) under hydrothermal conditions. Compound 1 is a 3D framework with triangular trinuclear Co units as nodes while compound 2 forms a 3D network with linear trinuclear and mononuclear Co units as nodes. Interestingly, mixed dps and dpds ligands as well as a dps ligand from a dpds precursor through the cleavage of the S-S and S-C bond occurred in 1 and 2, respectively. Such an unprecedented in situ solvothermal organothiolate ligand synthesis provides a useful route to the construction of metalorganosulfide compounds and new materials.
Acknowledgment. This work was supported by the Natural Science Foundation of China (20771090 and 20771054) TRAPOYT and SRFDP (20050697005) and Henan tackle key problem of science and technology (072102270030 and 072102270034). Supporting Information Available: Detailed experimental procedures, some additional figures, coordination modes of H2tbip, IR spectroscopic data, elemental analysis, TGA, PXRD, patterns of magnetic properties (PDF); X-ray crystallographic data in CIF format for 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org.
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CG801120Y
