Microwave and Conventional Hydro(solvo)thermal Syntheses of Three

Jul 29, 2014 - Fax: (+86)591-83793727. Synopsis. Three Co(II) coordination polymers containing two genuine supramolecular isomers have been synthesize...
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Microwave and Conventional Hydro(solvo)thermal Syntheses of Three Co(II) Coordination Polymers: Supramolecular Isomerism and Structural Transformations Accompanied by Tunable Magnetic Properties Guo-Dong Zou,†,‡ Zhang-Zhen He,† Chong-Bin Tian,†,‡ Liu-Jiang Zhou,†,‡ Mei-Ling Feng,† Xu-Dong Zhang,† and Xiao-Ying Huang*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *

ABSTRACT: Two genuine supramolecular isomers [Co(1,4-NDC)(5,5′-dmbpy)]n (1α and 1β) and compound {[Co(1,4-NDC)(5,5′dmbpy)(H2O)2]2}n (2) (1,4-NDCH2 = 1,4-naphthalenedicarboxylic acid, 5,5′-dmbpy = 5,5′-dimethyl-2,2′-dipyridyl) have been synthesized under conventional hydro(solvo)thermal reactions. The highly crystalline single-phases of 1β and 2 with a well-defined morphology could also be quickly prepared through microwave-assisted syntheses within 5 min. Compared with 2, which possesses a one-dimensional (1D) singlechain-like structure, 1α and 1β feature the structures of 1D ladder-like chain and two-dimensional (2D) layer, respectively, constructed from the ring-like [Co2(OOCR)2] dimeric units interlinked by the 1,4-NDC ligands. The drastic structural transformation from 1α to 2, as well as the interconversion between 1β and 2, was observed, which involved the reversible destruction and reconstruction of the Co2(OOCR)2 unit during the transformation and was associated with remarkable changes in magnetic properties. Remarkably, microwave heating as an external driving force was adopted to achieve structure transformation from 2 to 1β within 5 min. This work demonstrates that the microwave technique can be used as a quick and efficient synthetic method to investigate the structural conversion between compounds and further bring about property modulation.

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solvent molecule exchange or loss,6 or redox reagents.7 However, for some structural transformations, all of the external stimuli mentioned above may be ineffectual. In addition, the present task facing crystal engineers is to develop faster and economical routes, not only for a fundamental understanding of the synthesis but also for viable applications in industry. Microwave (MW)-assisted synthesis has been widely used in organic synthesis and in the preparation of nanoporous inorganic materials for several decades, which is characterized by short reaction times, the control of crystal morphology, phase selectivity, high yields of products, and better reproducibility.8 However, its application in the syntheses of CPs has been scarcely reported to date.9 Furthermore, the use of MW heating to achieve structural transformation is rarely investigated.10

INTRODUCTION

The structural transformations of coordination polymers (CPs), especially those involving changes in structural dimensionality with coordination bond cleavage and formation, are attracting increasing interest not only for gaining insights into the correlation between structure and property but also for their potential applications in molecular devices such as switches and sensors.1 The application of structural transformation to generate dynamic magnetic systems is of special interest in molecular materials research, but it is still difficult to establish a rational route for constructing such convertible structures, especially those producing reversibility. Thus far, only limited structural transformations between CPs with different structural dimensionality have been documented to exhibit drastic changes in magnetic properties.2 Moreover, the study on most of the structural conversions reported so far is generally based only on the measurements of the initial and final states, without further support from theory calculation.3 Currently, most structural transformations are triggered by external stimuli such as conventional electric heating,2b,4 light,5 © 2014 American Chemical Society

Received: April 16, 2014 Revised: July 7, 2014 Published: July 29, 2014 4430

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cooled to room temperature. Red sheet crystals were obtained, washed with ethanol, and dried in air (manually selected, yield = 0.060 g, 26.24% based on Co). Elemental analysis (%): calcd for C24H18CoO4N2, C 63.03, H 3.97, N 6.13; found, C 62.66, H 4.29, N 6.44. Synthesis and Characterization of 1β. MW-Assisted Hydrothermal Synthesis. A mixture of Co(Ac)2·4H2O (0.1245 g, 0.5 mmol), 1,4-NDCH2 (0.1081 g, 0.5 mmol), and 5,5′-dmbpy (0.0921 g, 0.5 mmol) in water (5 mL) was placed in a Biotage MW vial (Vmax = 20 mL). The MW absorption level of water was set to high. The reaction mixture was heated with continuous magnetic stirring at different temperatures (150−160 °C) and was maintained at the selected temperature for a predetermined time in the range of 5−30 min (Table S1, Supporting Information). After the solution had cooled, red powder was collected by filtration, washed with ethanol, and dried in air. Elemental analysis (%): calcd for C24H18CoO4N2, C 63.03, H 3.97, N 6.13; found (160 °C, 30 min), C 63.35, H 4.16, N 6.47. Conventional Solvothermal Synthesis. Compound 1β was prepared under conventional heating by a procedure similar to that of 1α, except for the replacement of solvent water by an equal volume of DMF. Large quantities of dark-red block-like crystals (manually selected, yield = 0.10 g, 43.73% based on Co) were obtained. Elemental analysis (%): calcd for C24H18CoO4N2, C 63.03, H 3.97, N 6.13; found, C 62.93, H 4.32, N 6.49. Synthesis and Characterization of 2. MW-Assisted Hydrothermal Synthesis. Compound 2 was obtained by a procedure similar to that of 1β under MW heating, but at a lower reaction temperature (130−150 °C) (Table S1, Supporting Information). After the solution had cooled, pink powder was collected by filtration, washed with ethanol, and dried in air. Elemental analysis (%): calcd for C48H44Co2O12N4, C 58.43, H 4.49, N 5.68; found MW synthesis (140 °C, 30 min), C 58.54, H 4.67, N 6.02. Conventional Hydrothermal Synthesis. Compound 2 was prepared by a procedure similar to that of 1α under conventional hydrothermal method except that the reaction temperature was decreased to 140 °C. Orange prismatic crystals (manually selected, yield = 0.02 g, 8.11% based on Co) were obtained. Elemental analysis (%): calcd for C48H44Co2O12N4, C 58.43, H 4.49, N 5.68; found C 58.06, H 4.70, N 5.92. X-ray Crystallography. Single crystals of 1α, 1β, and 2 obtained by the hydro(solvo)thermal method were chosen for single-crystal Xray diffraction analyses. The X-ray diffraction data for 1α and 1β were collected on an Oxford Xcalibur Eos CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature. The X-ray diffraction data for 2 were collected on a SuperNova CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 100(2) K. The structures were solved by direct methods and refined by full-matrix least-squares on F2 by using the program SHELX-97.15 Anisotropic thermal factors were assigned to all the non-hydrogen atoms. Generally, C-bound H atoms were placed geometrically and refined as riding; whereas H atoms on water molecules in 2 were located from the D−F maps, assigned isotropic thermal parameters, and allowed to ride on their respective parent atoms before the final cycle of least-squares refinement. Further crystallographic details for the three compounds are summarized in Table 1. CCDCs 966979 (1α), 966980 (1β), and 966981 (2) contain the supplementary crystallographic data for this paper. The data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html. Computational Details. To determine the total energy of each compound, we have carried out first principle density functional theory calculations with Vienna ab initio simulation package (VASP).16 The generalized gradient approximation PW91 (GGA-PW91)17 was chosen as the exchange-correlation functional, and a plane wave basis with ultrasoft pseudopotentials (US-PP)18 was used. A planewave cutoff energy of 400 eV and a threshold of 10−5 eV were set for the self-consistent-field convergence of the total electronic energy. H 1s1, C 2s22p2, N 2s22p3, O 2s22p4, and Co 3d74s2 were treated as valence electrons. The Monkhorst−Pack scheme of k-point sampling was used for integration over the first Brillouin zone.19 For the isolated H2O molecules, we also built a cubic box with the lattice length of 10

As coined by Zaworotko, supramolecular isomerism, which refers to the existence of more than one type of network superstructure for the same chemical composition,11 is an essential element in crystal engineering of CPs and can provide us a useful and unique perspective to understand structure− property relationships. Although many CPs exhibit supramolecular isomerism, a substantial number of them are based on the coexistence of different guest molecules, which are better categorized as pseudopolymorphism.12 In contrast, genuine supramolecular isomerism, that is, a fixed stoichiometry for all components, is extremely rare even for the simplest system,13 but it may provide an invaluable opportunity for the study of the factors influencing self-assembly and crystal growth. It has been well demonstrated that the generation of supramolecular isomers depends on the subtle variation of assembly environments, such as solvent, reaction temperature and time, concentration effect, pH, and additive agents.14 In this work, we report two genuine supramolecular isomers [Co(1,4-NDC)(5,5′-dmbpy)]n (1α and 1β) and another compound {[Co(1,4-NDC)(5,5′-dmbpy)(H2O)2]2}n (2) generated from assemblies of Co(Ac)2, 1,4-NDCH2, and 5,5′dmbpy (1,4-NDCH2 = 1,4-naphthalenedicarboxylic acid, 5,5′dmbpy = 5,5′-dimethyl-2,2′-dipyridyl) under conventional hydro(solvo)thermal conditions. Employing a MW-assisted hydrothermal route, we could also efficiently synthesize highly crystalline single-phases 1β and 2 in high yields within 5 min, by simply tuning the reaction temperature. Attractively, compound 1α featuring a one-dimensional (1D) ladder-like chain was able to undergo a spontaneous structural transformation to result in a distinct 1D single-chain structure of 2 under stirring in water at room temperature, via breaking the [Co2(OOCR)2] unit. Furthermore, the MW-assisted heating method was adopted to achieve the reversible structural transformation between the compound 2 and layer structure of 1β, which involved the destruction and reconstruction of the [Co2(OOCR)2] dimeric unit. Remarkable changes of magnetic properties were also observed among them. To understand the transformation processes well among them, the experimental results were complemented by a theoretical study.

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EXPERIMENTAL SECTION

Materials and Physical Measurements. All chemicals were commercially purchased and used without further purification. MW syntheses were carried out in a Biotage Initiator MW synthesizer (power range 0−400 W at 2.45 GHz). Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku Miniflex II diffractometer using Cu Kα radiation. C, H, and N analyses were performed on a German Elementary Vario EL III instrument. Fourier transform infrared (FT-IR) spectra were recorded on a Magna 750 FT-IR spectrometer in the 4000−500 cm−1 region using KBr pellets. Photoluminescence spectra were recorded on a PerkinElmer LS 55 luminescence spectrometer with a R928 red-sensitive photomultiplier without correction. Thermogravimetric analyses (TGA) and differential scanning calorimetric (DSC) experiments were carried out with a NETZSCH STA 449F3 unit at a heating rate of 5 °C/min under a nitrogen atmosphere. Magnetic measurements were carried out on the powdered samples (from single crystals obtained by conventional hydro(solvo)thermal methods) of 1α, 1β, and 2 with a Quantum Design MPMS-XL SQUID magnetometer. The morphologies of compounds 1β and 2 were obtained using a scanning electron microscope (JSM-6700F). Synthesis and Characterization of 1α. A mixture of Co(Ac)2· 4H2O (0.1245 g, 0.5 mmol), 1,4-NDCH2 (0.1081 g, 0.5 mmol), and 5,5′-dmbpy (0.0921 g, 0.5 mmol) in water (10 mL) was placed in a 28 mL Teflon-lined stainless-steel autoclave at 160 °C for 5 days and then 4431

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Table 1. Crystallographic Data for 1α, 1β, and 2 empirical formula formula mass cryst syst space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z T, K F(000) ρcalcd, g cm−3 μ, mm−1 meas reflns indep reflns no. params Rint R1,a wR2b (I > 2σ(I)) R1,a wR2b (all data) GOF a

1α

1β

2

C24H18CoO4N2

C24H18CoO4N2

C48H44Co2O12N4

457.33 triclinic P1̅ 9.5968(7) 10.6725(9) 11.6254(7) 73.962(7) 69.363(6) 65.849(8) 1004.52(13) 2 295(2) 470 1.512 0.889 8484 4102 282 0.0246 0.0340, 0.0773

457.33 orthorhombic Pbca 12.5060(4) 17.1232(6) 19.2757(6) 90.00 90.00 90.00 4127.8(2) 8 295(2) 1880 1.472 0.866 11463 4342 282 0.0289 0.0327, 0.0758

986.73 triclinic P1̅ 8.8615(4) 13.2176(6) 19.1476(10) 78.192(4)) 81.697(4) 80.696(4) 2151.94(18) 2 100(2) 1020 1.523 0.843 21429 9099 623 0.0332 0.0349, 0.0847

0.0431, 0.0838

0.0466, 0.0845

0.0454, 0.0923

1.007

1.007

1.005

Scheme 1. Solvent-Directed Assemblies of 1α and 1β at 160 °C for 5 days by Conventional Heatinga

a

DMA = N,N-dimethylacetamide.

(2) when H2O was used as a solvent, the lower and higher reaction temperatures afford compounds 2 and 1α, respectively (Scheme 2). The above results clearly indicate that compound

R1 = ∑||F0|−|Fc||/∑|F0|. bwR2 = [∑w(F02−Fc2)2/∑w(F02)2]1/2.

Scheme 2. Temperature-Directed Assemblies of 1α, 1β, and 2 at Different Reaction Temperatures for 5 days by Conventional Heating

Å. A 5 × 5 × 5 Monkhorst−Pack mesh of k-points and an energy cutoff of 400 eV were employed. The phonon contribution to the free energy is not considered, because it is very time-consuming to calculate this term due to relatively large unit cells used in our study.

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RESULTS AND DISCUSSION In this work, compounds 1α, 1β, and 2 were synthesized by the conventional hydro(solvo)thermal reactions of Co(Ac)2, 1,4NDCH2, and 5,5′-dmbpy in a 1:1:1 molar ratio under different reaction conditions at first. Compounds 1α and 1β were prepared at the same temperature of 160 °C but in different solvents (H2O for 1α; N,N-dimethylformamide (DMF) for 1β), while 2 was formed by a procedure similar to that for 1α at lower temperature of 140 °C. To our knowledge, the CP constructed by the mixed ligands of 1,4-NDCH2 and 5,5′dmbpy has not been previously reported. Effects of the Solvent, Reaction Temperature, and Time during the Coordination Assembling Process. Conventional Hydro(solvo)thermal Synthesis. Our initial experiments revealed that the reaction temperature and solvent were critical to the formations of supramolecular isomers 1α and 1β and 2 under conventional hydro(solvo)thermal conditions, which inspired us to carry out further studies on the assembly system of Co(Ac)2, 1,4-NDC, and 5,5′-dmbpy. As shown in Scheme 1, compound 1β could always be obtained by replacing H2O or DMF with various other solvents when the reaction temperature was maintained at 160 °C for 5 days. In particular, 1β could also be obtained through the solid-solid reaction process without solvent in an evacuated sealed glass tube. We also carried out the reactions over various temperature in H2O or DMF solvent system: (1) when DMF was used as a solvent, only compound 1β could be formed at an interval of 10 °C within the temperature scope 130−190 °C;

1β is statistically found to be the more universal and favorable product, compared with its isomer 1α. However, concomitant impurities were always observed in the products, and the yields were low for the preparation of the three compounds no matter how hard we tried to adjust the reaction conditions. Thus, we attempted to develop alternative synthetic approaches toward the solution of the aforementioned problems. MW-Assisted Hydrothermal Synthesis. By the MW-assisted hydrothermal method, highly crystalline pure phases of 1β and 2 with higher yield could be synthesized very quickly from an identical starting mixture by simply altering the reaction temperature. However, compound 1α could not be prepared with water as solvent by MW heating, though numerous trials were carried out by altering the reaction temperature and time. Supramolecular isomerism is very sensitive to the synthetic conditions, due to the low formation energy differences. So it is easy to understand the above results, considering the differences in heating mode, stirring method, crystal nucleation 4432

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and growth rate, and so on between MW and conventional heating methods. From Table S1, Supporting Information, we could see clearly that both the reaction temperature and time played crucial roles during the MW syntheses. The average yields of 1β and 2 in all trials were ca. 82% and 89% (based on Co), which were nearly 2 and 11 times more, respectively, than those prepared under conventional heating. Compound 1β began to form when the temperature reached 150 °C, and the heating time must be not less than 10 min, while it only needed 5 min to prepare 1β at 160 °C. For the preparation of compound 2, a lower heating temperature was required, and once the temperature rose to 150 °C, the heating time must not exceed 10 min. Actually, PXRD patterns (Figures S5 and S6, Supporting Information) and elemental analyses of the as-synthesized products via MW heating matched well with the simulated ones of 1β and 2, respectively. Compared with conventional heating, the MW route produced highly phase-pure materials of both compounds in a much shorter time. As shown in Figure 1a,b (or Figure

Figure 1. Typical photographs for 1β (a) and 2 (c) prepared by conventional heating and SEM images for 1β (b) and 2 (d) prepared by MW heating.

1c,d), the respective morphologies of the fully crystallized 1β (or 2) prepared by the conventional and MW methods are very similar, but the crystals obtained by MW heating are smaller, micrometer (or submicrometer) size, as one would expect due to the higher number of nucleation sites. In particular, the size distribution of 2 is more homogeneous from the MW heating than from the conventional hydrothermal method, which further shows the efficiency of the synthesis by the MW approach. Crystal Structure Descriptions. Single-crystal X-ray diffraction analyses reveal that compounds 1α and 1β are genuine supramolecular isomers without any solvent molecule in the structures. Compound 1α crystallizes in the space group P1̅, and its asymmetric unit contains one crystallographically independent Co(II) ion, one NDC2−, and one 5,5′-dmbpy ligand. The NDC bridging ligand in 1α adopts a (κ2)-(κ1-κ1)-μ3 coordination mode (type a, Scheme S1, Supporting Information), which is common in the previously reported coordination networks.20 As shown in Figure 2a, Co(II) adopts a distorted octahedral geometry and is coordinated by four O atoms from three NDC ligands and two N atoms from one 5,5′-dmbpy ligand. Two centrosymmetric Co(II) ions are bridged by a pair of carboxylate (COO−) groups from two NDC ligands to afford a ring-like [Co2(OOCR)2] dimeric unit with the Co···Co

Figure 2. (a, c) Similar coordination environments of Co(II) and the [Co2(OOCR)2] dimeric units in 1α and 1β, as well as the 1D doublechain in 1α and 2D layer in 1β. (b) View of the coordination environments of the Co(II) and two adjacent 1D chains in 2 with labeling schemes. Symmetry codes: (a) 2 − x, 1 − y, 1 − z; (b) 1 + x, 1 + y, −1 + z; (c) 1 − x, −y, 1 − z.

distance of 4.7062(7) Å. Furthermore, the dimeric units are interconnected through a pair of bridging NDC ligands, giving rise to a 1D ladder-like chain of [Co(1,4-NDC)(5,5′-dmbpy)]n extending along the b axis. Notably, the naphthalene rings from the NDC ligands are parallel to each other in 1α. Intrachain face-to-face π−π interactions are observed between the naphthalene rings with the plane-to-plane distance of 3.7623(12) Å (Figure S1a, Supporting Information). Interchain π−π stacking interactions also exist between the pyridine rings of the 5,5′-dmbpy ligands to result in a three-dimensional (3D) supramolecular network (Figure S1b, Supporting Information). 4433

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lose single crystallinity under stirring in water and transformed to compound 2 completely after about 1 day, as confirmed by the PXRD patterns (Figure 3 and Figure S7, Supporting

Different from 1α, compound 1β crystallizes in the space group Pbca, but they have a similar asymmetric unit. The coordination environment of Co(II) ion and the binding modes of the NDC and 5,5′-dmbpy ligands in 1β are all similar to those observed in 1α (Figure 2c). Moreover, a similar ringlike [Co2(OOCR)2] dimeric unit to that in 1α also exists in 1β, with the Co···Co distance of 4.6388(5) Å slightly shorter than that in 1α. It is worthwhile to note that the two neighboring dimeric units are connected only by one NDC bridging ligand, instead of double NDC bridges in 1α, to afford a 2D sheet in the ab plane. The major difference in both isomers is the relative orientation of NDC ligands. Unlike that in 1α, only two of the three sets of the naphthalene rings in 1β are parallel to each other while the remaining one forms a dihedral angle of 50.22(4)° with each of them. The orientational changes of NDC ligands do not change the octahedral environment of the Co(II) ion but result in different connection modes of the dimeric units between 1α and 1β, which plays a key role in the formation of the isomers with different structural dimensionality. Moreover, face-to-face π−π interactions are also observed within the sheet (Figure S2a, Supporting Information). The adjacent layers further stack into a 3D supramolecular framework through the C−H···O and C−H···π hydrogen bonds (Figure S2b, Supporting Information). Compound 2 crystallizes in the space group P1̅. Its asymmetric unit consists of two crystallographically independent Co(II) ions, two NDC2− anions, two 5,5′-dmbpy ligands, and four coordinated aqua molecules. As shown in Figure 2b, both Co(II) ions are in a distorted octahedral coordination environment; the equatorial plane is composed of two N atoms from one 5,5′-dmbpy ligand and two aqua molecules adopting the cis-arrangement, while two carboxylate oxygen atoms from different NDC ligands occupy the apical positions. However, the octahedra of Co1 and Co2 are different: (1) The dihedral angles between two 2-pyridyl rings in 5,5′-dmbpy ligand chelated to Co1 and Co2 are 13.38(11)° and 4.42(13)°, respectively. (2) The angles among the two coordinated H2O and Co(II) ions are 91.83(6)° for O9−Co1−O10 and 101.29(6)° for O11−Co2−O12, respectively. (3) The angles among the two carboxylate oxygen atoms located at the apical positions and Co(II) ions are 171.94(5)° for O1−Co1−O5 and 175.92(6)° for O7−Co2−O4, respectively. From the above analyses, we can see that the six-coordinated geometry of Co1 is more distorted than that of Co2. Different from that in 1α or 1β, the NDC ligands in 2 adopt a bis-monodentate (κ1)-(κ1)-μ2 coordination mode (type b, Scheme S1, Supporting Information), linking the adjacent Co1 and Co2 ions to form an infinite 1D chain along the [111̅] direction (Figure S3a, Supporting Information). Additionally, there are abundant interchain π−π stacking and hydrogen-bond interactions in compound 2, connecting the 1D chains to afford a 3D supramolecular architecture (Figure S3b,c, Supporting Information). S t r u c t u ra l T r a n s f o r m a t i o n s . S im i la r t o t h e [Co2(OOCR)2] dimeric units in 1α and 1β, there also exist dimer-like units (denoted as pseudodimer) in 2 on the basis of the extensive hydrogen bond interactions between the coordinated water molecules and the uncoordinated carboxylate oxygen atoms on neighboring chains, as well as the shortest interchain Co···Co distances (5.9130(6) Å for Co1··· Co1 and 6.8670(6) Å for Co2···Co2) (Figure S4, Supporting Information). Inspired by the structural correlation of compounds 1α, 1β, and 2, the transformations among them were attempted. (1) For 1α to 2, single crystals of 1α would

Figure 3. Photographs showing the structural transformation from 1α to 2. Inset on the right, SEM image for 2 transformed from 1α.

Information). By comparison in detail of the structures of 1α and 2, a reasonable conversion process is suggested as follows: as shown in Figure 2a,b, upon the attack of water molecules, the robust [Co2(OOCR)2] dimeric unit in 1α was destroyed, which involved the breakage of Co−O (COO−) and resulted in the coordination modes of NDC ligands changing from (κ2)-(κ1κ1)-μ3 to bis-monodentate (κ1)-(κ1)-μ2. Meanwhile, two water molecules coordinated to each Co(II) to fill the vacant sites to maintain the coordination geometry of Co(II) ion. Finally, the 1D ladder-like chain in 1α split into two 1D single chains in 2. (2) Conversely, however, the transformation from 2 to either of two isomers was unsuccessful following a series of trials carried out by conventional heating (see Supporting Information). Because the pure phases 1β and 2 could be obtained through microwave routes at different temperatures, we considered whether the interconversion of them could be achieved under MW heating. Indeed, we found that if compound 2 in its mother solution produced by MW heating at 140 °C for 30 min was returned to the MW synthesizer and further heated to 160 °C for another 5 min, a visible color change was observed (Figure 4a,b). Subsequent characterizations indicated that 1β was successfully generated, the size and morphology of which were similar to that of 1β obtained through the MW one-step synthetic method (Figure 4b and Figure 1b). When the obtained 1β in its mother solution was stirred at room temperature for about 5 h, 2 was regenerated (Figure 4b,c), although it showed a small color variation compared with that of 2 prepared through one step MW synthesis. Further research on the morphology of 2 transformed from 1β revealed that plate-like microcrystals were obtained instead of the submicrometer-sized particles obtained by MW one-step synthesis (Figure 4a,c), implying that the small color change may be attributed to the alteration of the morphology or size. It should be pointed out that the distances between the Co(II) centers and the uncoordinated carboxylate oxygen atoms from two adjacent chains in compound 2 are 3.9776(2) Å (Co1−O2) and 4.5062(2) Å (Co2−O8), respectively, within the distance criterion for structural transformation involving the formation of new coordination bonds (Figure 2b, right).21 After the removal of coordinated water molecules, additional carboxylate O atoms (O2 and O8) from adjacent chains participated in coordination, resulting in the formation of the [Co2(OOCR)2] dimeric unit and the 2D network of 1β finally. The above reversible transformation processes, which involved the destruction and reconstruction of the Co2(OOCR)2 unit, 4434

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Figure 4. Photographs and SEM images showing the visual color and morphology changes during the transformations among 2 (a) and 1β (b) and phase 2 transformed from 1β (c).

favorable product than its isomer 1α. Similar calculating processes had also been carried out on other structural transformations reported previously.3 Magnetic Properties. The structural transformations gave rise to remarkable changes in the magnetic behaviors of 1α, 1β, and 2. At 300 K, the χmT values for them are 2.78, 2.91, and 2.85 cm 3 ·K·mol −1 (Figure 6), respectively, which are significantly larger than the value of 1.875 cm3·K·mol−1 expected for one isolated, spin-only Co(II) with S = 3/2 and g = 2.00. This indicates the significant orbital contribution of Co(II) ion due to strong spin-orbit coupling in an octahedral environment.22 Upon cooling, the three compounds exhibit different magnetic behaviors. The χmT values of 1α and 2 monotonously decrease to attain the values of 0.43 and 1.69 cm3·K·mol−1 at 2 K, respectively; while for 1β the χmT value gradually decreases to 2.26 cm3·K·mol−1 around 11.9 K. Below 11.9 K, the χmT value rises sharply to reach a value of 2.57 cm3· K·mol−1 at 3 K and finally drops to 2.55 cm3·K·mol−1 at 2 K, indicating the onset of a weak ferromagnetic correlation. The magnetic susceptibility data of 1α above 10 K (30 K for 1β, 25 K for 2) obey the Curie−Weiss law well (Figure S13, Supporting Information), giving Curie and Weiss constants of C = 2.91 cm3·K·mol−1, θ = −14.47 K for 1α, C = 3.0 cm3·K· mol−1, θ = −9.76 K for 1β, and C = 2.97 cm3·K·mol−1, θ = −10.47 K for 2. The negative Weiss constants of the three compounds may be attributed to the complementarity effect of the exchange interaction between Co(II) ions, spin-orbital coupling, and the zero-splitting. It is well-known that the magnetic state of Co(II) ion in an octahedral ligand field usually exhibits Kramer’s doublet, which can be regarded as an effective spin Seff = 1/2.23 Taking into account the structural characters of dimer-like units in 1β and 2, an approximate fit to the magnetic susceptibility data of either compound was achieved based on a model for the binuclear system proposed by Lloret et al. based on the Lines’s theory.24 The spin Hamiltonian of this system is described as follows:

could be well confirmed by PXRD patterns (Figure S10, Supporting Information). In contrast, if the pure phase 2 with its mother solution produced by MW heating were transferred into a Teflon-lined stainless steel autoclave and heated at 160 °C for 1 day by conventional heating, most of the time a pure phase 1β was obtained, but occasionally the product was a mixture of phases 1α and 1β with nearly equal yield through several parallel trials. That is to say, conventional heating is not always a reliable method to achieve the structural conversion from 2 to 1β due to the uncertainty of transformation products. By contrast, compound 2 is able to selectively transform into the isomer 1β in less time by MW heating. In consideration of the similarities in the coordination environment of Co(II) ion and the binding modes of NDC and 5,5′-dmbpy ligands between isomers 1α and 1β, we also attempted to check for a possible interconversion between them. Although some relevant experiments were carried out, these efforts failed (see Supporting Information). Computational Discussions. In order to further understand the above dynamic transformation processes, the total energy of each compound was calculated using Vienna ab initio simulation package (VASP).16 The chemical reaction based on the unit cell of the corresponding product could be used to estimate the relative stability of the three compounds (Figure 5

A B A B H = −(25/9)Seff ·Seff − G(T , J )βH(Seff + Seff )

Figure 5. Comparison of the relative stabilities of the structures based upon each formula as a function of energy.

(1)

Hence the magnetic susceptibility of binuclear system can be written as

and Table S3, Supporting Information). The calculation results showed that the transformation from 1α to 2 (ΔE = −3.16 eV) or from 1β to 2 (ΔE = −4.56 eV) was an exothermic and enthalpy-driven process, which was consistent with the experimental result that 1α or 1β could convert into 2 spontaneously. Contrarily, compound 2 had lower energy than 1β, so an external force must be applied to bring about the transformation from 2 to 1β. Moreover, our calculations showed that 1β had a ground state energy lower than that of 1α, which agreed with the fact that 1β was a more universal and

χm = {2Nβ 2[G(T , J )]2 }/{kT[3 + exp(25J /(9kT ))]} + TIP

(2)

where Seff equals 1/2 and G(T,J) is a fictitious Landé factor, which is dependent on the spin-orbital coupling parameter (λ), orbital reduction factor (α), axial distortion factor of the octahedral field (Δ), intradimer magnetic exchange constant (J), and temperature independent paramagnetic factor (TIP). As shown in Figure 6c,d, the magnetic data were fitted well in 4435

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Figure 6. (a) The χmT vs T curve for 1α under 1000 Oe. The solid red line represents the best theoretical fit. Inset, χm vs T curves under different fields at low temperature. (b) Field dependence of the magnetization for 1α at 2 K. The insets show the magnetic hysteresis loop (upper left) and the dM/dH derivative curve of 1α at 2 K (lower right). (c) The χmT vs T curve for 1β under 1000 Oe. The solid red line represents the best theoretical fit. (d) The χmT vs T curve for 2 under 1000 Oe. The solid red line represents the best theoretical fit.

χ = χbinuclear /{1 − zj′(Nβ 2)−1[G(T , J )]−2 χbinuclear }

the whole temperature range for 1β and 2, leading to the best parameters of J = −1.60 cm−1, α = 1.07, Δ = −1008.02 cm−1, λ = −138.53 cm−1, TIP = 7.5 × 10−4, and R = 1.1 × 10−4 for 1β and J = 0.13 cm−1, α = 1.04, Δ = −601.08 cm−1, λ = −111.17 cm−1, TIP = 7.3 × 10−4, and R = 9 × 10−5 for 2, where the agreement factor R = ∑[(χmT)exp − (χmT)cal]2/∑[(χmT)exp]2. The low values of the |J/λ| quotient in 1β (0.012) and 2 (0.0012) justify the use of the above approach;24a and the values of λ, α, and Δ parameters obtained for both 1β and 2 lie within the range of those observed in other six-coordinated binuclear Co(II) compounds.25 It is noted that 1β shows an antiferromagnetic interaction between Co(II) ions in the dimeric unit due to negative J value, which is comparable to the previously reported compounds consisting of Co(II) dimers;25b,26 whereas the positive J value suggests a very weak ferromagnetic coupling between Co(II) ions in the pseudodimeric unit in 2. Also, the magnetic behaviors of both compounds can be further confirmed by the isothermal magnetization measured at 2 K (Figure S14, Supporting Information). It is noted that the experimental magnetization curve of 2 is fitted well by the Brillouin function (red line) for magnetically uncoupled Co(II) ions with S = 1/2, confirming nearly isolated Co(II) ions in 2 due to very weak interactions between Co(II) ions at low-temperature; while the magnetization curve of 1β deviates above the red line, confirming a weak ferromagnetic-like behavior at low temperature. These results are in good agreement with those obtained from the susceptibilities of 1β and 2 as shown in Figure 6. To further understand the ferromagnetic correlation of 1β, the magnetic interaction between adjacent dimeric units is assumed by the molecular field approximation (zj′) (Figure S15, Supporting Information):27

(3)

where zj′ is the parameter that describes the interactions between binuclear motifs. A fine fit gives J = −1.80 cm−1, zj′ = 0.025 cm−1, α = 1.03, Δ = −914.32 cm−1, λ = −134.44 cm−1, TIP = 8.6 × 10−4, and R = 9 × 10−5. These parameters are consistent with those calculated from the model for the binuclear system mentioned above. However, the positive value of zj′ suggests a ferromagnetic interaction between dimeric units, which may be considered as the origin of weak ferromagnetism at low-temperature. Unfortunately, the above-mentioned models, which are used to fit the magnetic data of 1β and 2 well, cannot reproduce the magnetic data of 1α very well (Figure S16, Supporting Information). To roughly describe the magnetic behaviors of 1α, we can estimate the exchange interaction between Co(II) ions by treating the ladder-like chain as the linkage of two uniform chains (viewed as the ladder leg) through the dimeric units (viewed as the rung), which finally brings about different intrachain and interchain exchange constants.28 For the intrachain interaction J, the classical 1D uniform chain model by Fisher is used.29 The spin Hamiltonian is described as follows: H = −2J ∑ Si·Si + 1

(4)

Then the theoretical expression of susceptibility is presented as χchain = [Nβ 2g 2S(S + 1)/(3kT )](1 + u)/(1 − u)

(5)

where S = 3/2 and u = coth[JS(S + 1)/(kT)] − kT/[JS(S + 1)]. The interchain interaction (zj′) through the rungs of the ladder is treated by the molecular field approximation 4436

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Crystal Growth & Design χladder = χchain /[1 − 2zj′χchain /(Nβ 2g 2)]

Article

Moreover, significant differences in magnetic coupling interaction among the three CPs contribute to a deep understanding of the relationships between the structures and properties.

(6)

Using this approximate model, we simulated the susceptibility data of 1α, giving the best fit in the whole temperature range with the parameters J = 0.00025 cm−1, zj′ = −5.73 cm−1, g = 2.49, and R = 5.7 × 10−4. Because of the Co···Co distance within the dimeric unit (4.7062(7) Å) is shorter than that of intrachain (10.6725(9) Å), it is not difficult to understand that the zj′ is much higher than the J value. The low temperature magnetic susceptibilities for 1α at different fields were measured (Figure 6a, inset). The χm vs T curves below 2 T for 1α show a maximum around 6.5 K, which disappears when the applied field increased to 3 T, indicating an occurrence of field-induced metamagnetic behavior. As shown in Figure 6b, the isothermal magnetization for 1α measured at 2 K shows a sigmoidal shape, which further confirms the metamagnetic behavior. The critical field, defined as the maximum in the dM(H)/dH derivative curve, is at 3 T (the inset on the lower right of Figure 6b). Below 3 T, the M of 1α displays a linear increase, suggesting the presence of antiferromagnetic interactions. Above 3 T, it increases rapidly and reaches a saturation value of 2.34 Nβ at 8 T, which is in agreement with the expected value of 2.3 Nβ for Seff = 1/2 and g = 13/3, favoring a ferromagnetic state at high filed. In addition, this ferromagnetic state is confirmed by the observed magnetic hysteresis loop at high external field (the inset on the upper left of Figure 6b). Thus, the metamagnetism of 1α can be attributed to a magnetic transition from the antiferromagnetic state to the ferromagnetic state induced by the external field. It should be noted that 1α and 1β are genuine supramolecular isomers with nearly the same dimeric unit but they display different magnetic interactions. We speculate the possible reasons are as follows: (1) The relative orientational changes of the NDC ligands, which result in the dimensional variation from the 1D ladder-like chain for 1α to 2D sheet for 1β, may affect the orthogonality of the magnetic orbitals and are responsible for the change from antiferromagenic to ferromagnetic interaction between adjacent Co(II) ions at low temperature. (2) The supramolecular interactions, such as hydrogen bond contacts, π−π stacking interactions, and so on, may play an important role in magnetic behavior.30 Based on the structural analyses of 1α and 1β above, there are obvious differences in interchain and interlayer interactions between them, which can also result in the magnetic variation.

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ASSOCIATED CONTENT

* Supporting Information S

The series of unsuccessful trials carried out to study the possible conversions among 1α, 1β, and 2, additional magnetic data, the details of MW-assisted hydrothermal syntheses, detailed calculation results of ΔE during the transformation processes, extra figures, PXRD patterns, TGA, DSC, IR, and photoluminescence. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+86)591-83793727. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 973 program (Grant No. 2012CB821702) and the NNSF of China (Grant No. 21221001).

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REFERENCES

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CONCLUSION In summary, we have successfully synthesized three CPs under conventional hydro(solvo)thermal syntheses, including two genuine supramolecular isomers. Interestingly, highly crystalline pure phases of 1β and 2 could be quickly formed through a MW-assisted hydrothermal method. Furthermore, we have also investigated the effects of solvent, reaction temperature, and time on the formation of three CPs in detail. Remarkably, a drastic structural transformation from 1α to 2 was observed, involving the breakage of [Co2(OOCR)2] dimeric unit. More importantly, with MW heating as a driving force, the reversible structural transformation between 1β and 2 involving a dimensional change could be successfully achieved, which demonstrates that the MW technique can be used as a quick and efficient synthesis method to investigate the structural conversion between CPs. In contrast, conventional heating is not always a reliable method to achieve the conversion from 2 to 1β due to the uncertainty of the transformation products. 4437

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