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Concomitant Crystallization of Genuine Supramolecular Isomeric Rhombus Grid and Ribbon Jing-Yun Wu,*,† Cheng-Chu Hsiao,† and Ming-Hsi Chiang‡ †

Department of Applied Chemistry, National Chi Nan University, Nantou 545, Taiwan Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan

‡

S Supporting Information *

ABSTRACT: Two genuine Co(II) supramolecular isomers, a two-dimensional (2-D) rhombus grid 1 and a one-dimensional (1-D) ribbon 2, that have the same metal fragment and ligand conformations, were crystallized from the same reaction bath under hydro(solvo)thermal conditions. The formation of supramolecular isomers in this system is dominated by the bridging orientation of InMe-4-py ligands, which is mainly influenced by reaction temperature but also weakly swayed by pH value, reaction time, and counteranion. The major rhombus grid 1 is the thermodynamically favored product, and the minor ribbon 2 is the kinetically favored product under controlled conditions, as supported by their relative abundances in functions of temperature and time. Both polymeric networks of supramolecular isomers 1 and 2 display a high thermal stability over 350 °C. Magnetic studies of 1 and 2 indicate that the Co(II) centers in the 2-D and 1-D networks are essentially magnetically insulated. The magnetic behavior demonstrates depopulation of higher energy Kramers doublets to the ground state, which results from a spin−orbit contribution, of the highspin Co(II) center in Oh configuration upon a decrease of temperature.

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INTRODUCTION Along with the explosive increase in solid crystalline compounds, metal−ligand directed finite and infinite supramolecular coordination structures that show the existence of more than one type of network structure for a given set of components have been obtained. Such interesting supramolecular phenomena are termed supramolecular isomerism, which was first coined by Zaworotko1 and subsequently in an extensive review.2 So far, supramolecular isomerism is an important subject in the field of inorganic supramolecular chemistry and crystal engineering, especially in the realm of coordination polymers.1−15 Generally, the occurrence of supramolecular isomerism arises from the small free energy difference among the possible crystalline forms and/or polymorphs and the responsibility of kinetic factors for the crystal growth, hinting that supramolecular isomer couples may result from the competitions of kinetic and thermodynamic controlled products.6 Therefore, the formation of supramolecular isomers depends on the subtle variation of assembly environments, such as solvent,7 reaction temperature,6d,8 additive agent,6b,9 concentration effect10 and pH.11 In this connection, the study of supramolecular isomers may allow one to obtain a better understanding of the factors influencing the formation of novel crystalline materials, giving considerable insight into polymorphism and crystal growth.12,13 At the heart of the concept of supramolecular isomerism, genuine supramolecular isomers should possess identical chemical compositions for both the coordination framework and the whole crystal.6a According to the strict definition, many © 2014 American Chemical Society

so far reported supramolecular isomers should be categorized as a group of solvent-induced (guest-induced) polymorphs and pseudopolymorphs rather than genuine supramolecular isomers,1,3−14 since they have showed crystal structures that contain a different stoichiometry of solvent (guest) molecules to satisfy crystal packing, which produce different chemical compositions. Up to now, there are only sporadic examples exhibiting genuine supramolecular isomerism, and most of them have been obtained from different synthesis conditions and/or strategies.6b,7f,h,8a,g,9b,e,10,11c,15 Concomitant crystallization of couples of genuine supramolecular isomers from one single crystallization reaction is still uncommon.8a,g,9f,10a,15h As part of our ongoing efforts directed toward the design and synthesis of functional crystalline materials,16−18 we report herein on two infinite coordination architectures of different polymorphs and network topologies (a rhombus grid and a ribbon) obtained from the assembly of Co(NO3)2·6H2O and 4(2-pyridylmethyl)-1,3-dioxoisoindoline-5-carboxylic acid (HInMe-4-py). These two cobalt(II) compounds having exactly the same chemical components are important genuine supramolecular isomers because they are crystallized in the same reaction bath under hydro(solvo)thermal conditions. In addition, the influences of several factors including not only temperature and pH value but also reaction time and Received: March 18, 2014 Revised: July 25, 2014 Published: August 19, 2014 4321

dx.doi.org/10.1021/cg500380v | Cryst. Growth Des. 2014, 14, 4321−4328

Crystal Growth & Design

Article

counteranion toward the formation of the two cobalt(II) supramolecular isomers have also been investigated.

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Table 1. Crystallographic Data for Compounds 1 and 2

EXPERIMENTAL SECTION

empirical formula Mw crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z T (K) λ (Å) Dcalc (g cm−3) F000 μ (mm−1) θmin, θmax (deg) refl collected unique refl (Rint) obs refl (I > 2σ(I)) parameters R1a (I > 2σ(I)) wR2b (I > 2σ(I)) R1a (all data) wR2b (all data) GOF on F2 Δρmax, Δρmin (e·Å−3)

Materials and Instruments. Chemical reagents were purchased commercially and were used as received without further purification. 4(2-Pyridylmethyl)-1,3-dioxoisoindoline-5-carboxylic acid (HInMe-4py) ligand was prepared according to a literature method.18 Thermogravimetric (TG) analyses were performed under nitrogen with a Thermo Cahn VersaTherm HS TG analyzer. X-ray powder diffraction (XRPD) measurements were recorded on a Siemens D5000 diffractometer with a graphite monochromatized Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 30 mA, with a step size of 0.02° in θ and a scan speed of 1 s per step size. Infrared (IR) spectra were recorded on a PerkinElmer Frontier Fourier transform infrared spectrometer using attenuated total reflection (ATR) technique; abbreviations used for the IR bands are s = strong, m = medium, w = weak, b = broad. Elemental analyses (C, H, N) were performed on an Elementar Vario EL III analytical instrument. The data of the temperature dependence of the magnetic susceptibility of the compounds 1 and 2 were recorded on a SQUID magnetometer (SQUID-VSM, Quantum Design) under an external magnetic field of 1000 Oe in the temperature range of 2−300 K. The data of the field dependence of the magnetization were collected at 1.8 K. The magnetic data were corrected with ligands’ diamagnetism by the tabulated Pascal’s constants. Preparation of [Co(InMe-4-py)2(H2O)2] (1 and 2). Co(NO3)2· 6H2O (29.1 mg, 0.10 mmol), HInMe-4-py (28.3 mg, 0.10 mmol), THF (8 mL), and H2O (2 mL) were conducted in an acid digestion bomb at 80 °C for 72 h and then cooled to room temperature, resulting in the formation of pink-colored rodlike crystals 1 (13.5 mg, yield 42%) and pink-colored blocklike crystals 2 (0.66 mg, yield 2%). For 1, Anal. Calcd for C30H22CoN4O10: C, 54.81; H, 3.37; N, 8.52%. Found: C, 54.80; H, 3.43; N, 8.44%. IR (ATR, cm−1): 3406b, 2975w, 2935w, 1772m, 1714s, 1622m, 1551m, 1428m, 1377s, 1303m, 1192w, 1096m, 1068m, 1024w, 955m, 873w, 786m, 741m, 703m. For 2, Anal. Calcd for C30H22CoN4O10: C, 54.81; H, 3.37; N, 8.52%. Found: C, 54.66; H, 3.74; N, 8.49%. IR (ATR, cm−1): 3471w, 3384w, 3059w, 2957w, 1775m, 1712s, 1614m, 1548m, 1425m, 1373s, 1195w, 1096m, 1067w, 1021w, 957m, 807w, 789m, 738m, 701m. Crystal Structure Determination. Data collections for 1 and 2 were performed at 296(2) K on a Bruker Smart 1000 CCD-based diffractometer equipped with a graphite monochromated Mo Kα radiation (λ = 0.71073 Å). Starting models for structure refinement were found using direct methods (SHELXS-9719), and the structural data were refined by full-matrix least-squares methods on F2 using the WINGX20 and SHELX-9719 program packages. All non-hydrogen atoms were found from the different Fourier maps and refined anisotropically. Generally, carbon-bound hydrogen atoms were placed by geometrical calculation and refined as riding mode. Oxygen-bound hydrogen atoms were first located on difference Fourier maps and then fixed at calculated positions and included in the final refinement. Isotropic displacement parameters of all hydrogen atoms were derived from the parent atoms. Experimental details for X-ray data collection and the refinements are summarized in Table 1.

1

2

C30H22CoN4O10 657.45 monoclinic P21/c 6.6287(3) 24.2017(12) 8.4264(5) 90 92.110(2) 90 1350.90(12) 2 296(2) 0.71073 1.616 674 0.707 1.68, 26.49 10430 2792 (0.0350) 2318 205 0.0491 0.0907 0.0640 0.0948 1.171 0.355, −0.271

C30H22CoN4O10 657.45 triclinic P1̅ 10.519(3) 12.038(3) 13.100(3) 95.687(5) 111.148(5) 112.522(6) 1373.9(6) 2 296(2) 0.71073 1.589 674 0.695 1.73, 26.50 19550 5597 (0.0663) 4438 409 0.0488 0.1253 0.0618 0.1314 1.082 0.838, −0.702

R1 = Σ||Fo| − |Fc||/Σ|Fo|. bwR2 = {Σ[w(Fo2 − Fc2)2]/Σ[w(Fo2)2]}1/2. w = 1/[σ2(Fo2) + (0.0174P)2 + 2.2377P] where P = (Fo2 + 2Fc2)/3 for 1; w = 1/[σ2(Fo2) + (0.0675P)2 + 0.0699P] where P = (Fo2 + 2Fc2)/3 for 2. a

Scheme 1. Synthesis and Photographs of Crystal of Supramolecular Isomers 1 and 2

absorptions of the trimellitic imide carbonyl groups were observed at 1772 and 1714 cm−1 for 1 and at 1775 and 1712 cm−1 for 2, which are slightly blue-shifted relative to those of the free HInMe-4-py ligand (1770 and 1703 cm−1).18 Further, the broad bands centered at 3406 cm−1 for 1 and 3471 and 3384 cm−1 for 2 indicate the presence of hydroxyl groups from the coordinated water molecules. Crystal Structure of [Co(InMe-4-py)2(H2O)2] (1). X-ray crystal structure of 1 reveals that the Co(II) center lies on a center of inversion and displays an octahedral {CoN2O4} coordination geometry defined by two pyridine nitrogens and two carboxylate oxygens of four different InMe-4-py ligands and two aqua ligands in an all trans manner (Figure 1a). Pertinent hydrogen bonds form between the aqua ligands and the noncoordinated carboxylate oxygens of InMe-4-py ligands (O−H···O, 2.646(3) Å, Table 2). The InMe-4-py ligand using its carboxylate and pyridine donating groups bridges two

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RESULTS AND DISCUSSION Synthesis. Compounds 1 and 2 as pink rod- and blocklike crystals, respectively, were simultaneously obtained from the hydro(solvo)thermal reactions of Co(NO 3) 2·6H2 O and HInMe-4-py at 80 °C for 72 h in the same reaction vessel (Scheme 1). Their molecular structures were determined by single-crystal X-ray structure analyses and further characterized by elemental analyses, infrared (IR) spectra, and thermogravimetric (TG) analyses, and the bulk phase purity of the compounds were confirmed by X-ray powder diffraction (XRPD) patterns (Figures S1 and S2, Supporting Information). In the solid IR spectra of 1 and 2 (Figure S3), the characteristic 4322



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Co(II) centers, with a Co···Co separation of 14.36 Å (Figure S4). Compound 1 adopts a two-dimensional (2-D) rhombus grid of topologic (4,4)-net (Figure 1b and Figure S5), with Co(II) centers as four-connected square-planar nodes and InMe-4-py ligands as linear linkers. The Co···Co distances around the rhombic pore are 15.45 × 24.20 Å2 in diagonals. It is noted that such porous are very large to be avoid, and thus these rhombus grids are stacked offset in an ABAB fashion along the crystallographic c axis in whole crystal packing (Figure S6a), with the shortest net-to-net Co···Co distance of 6.63 Å, causing a loss of potential openings. Net-to-net hydrogen bonds form between the aqua ligands and the carboxylate groups of InMe4-py ligands (O−H···O, 2.796(3) Å) among the offset-stacked grids, which combine with the intramolecular aqua−carboxylate hydrogen bonds to give rise to a R24(8) pattern (Figure S6b). These supramolecular interactions undoubtedly provide supports for stabilizing the resultant three-dimensional (3-D) hydrogen-bonded supramolecular net that, in the topological point of view, can be described an oblique pcu net with a Schäfli symbol of 41263 (Figure 1c). Crystal Structure of [Co(InMe-4-py)2(H2O)2] (2). X-ray crystal structure of 2 reveals that the asymmetric unit contains two crystallographic distinct Co(II) centers, Co1 and Co2, and two crystallographic independent InMe-4-py ligands. The coordination geometries of Co1 and Co2 centers are both {CoN2O4} octahedral, bonded with two pyridine nitrogens and two carboxylate oxygens of four different InMe-4-py ligands and two aqua ligands in an all trans manner (Figure 2a). The two crystallographically independent InMe-4-py ligands both adopt a bismonodentate bridging mode to connect either two Co1 or two Co2 centers with the monodentate carboxylate and pyridine donating groups (Figure S7, Supporting Information), where the Co1···Co1 and Co2···Co2 separations are 13.52 and 13.10 Å, respectively. Interestingly, the two InMe-4-py ligands show differently coordination orienting carboxylate groups. The first one, termed as InMe-4-pyI hereafter, coordinates to the Co1 center by using its carboxylate oxygen atom (O4) that has a cisoid relationship to the trimellitic imide carbonyl group (C1O1) at the 1-position (Figure 2a), while the second one, InMe-4-pyII, coordinates to the Co2 center by using its carboxylate oxygen atom (O8) that shows a transoid relationship to the 1-carbonyl group (C16O6) of the trimellitic imide moiety. Compound 2 adopts a one-dimensional (1-D) ribbon structure (Figure 2b). Pertinent hydrogen bonds form between the aqua ligands and the noncoordinated carboxylate oxygen atoms of InMe-4-py ligands (O−H···O, 2.560(3) and 2.662(3) Å, Table 2), which reinforces the ribbon chain. There are two

Figure 1. (a) ORTEP plot of the local coordination environment around the Co(II) center in 1 with thermal ellipsoids at the 50% probability level. Symmetry codes: #1, −x, −y + 1, −z + 1; #2, −x + 1, y + 1/2, −z + 3/2; #3, x − 1, −y + 1/2, z − 1/2. (b) Ball-and-stick representation of the 2-D rhombus grid in 1. Carbon-bound hydrogen atoms are omitted for clarity. Color scheme: cyan, Co; red, O; blue, N; gray, C; yellow, H. (c) Schematic representation of the stacking of rhombus grids, showing the oblique pcu net. Cyan and yellow lines represent the InMe-4-py linkers and the net-to-net aqua−carboxylate O−H···O hydrogen bonds, respectively.

Table 2. Hydrogen Bonding Parameters for 1 and 2a D−H···A

D−H (Å)

H···A (Å)

D···A (Å)

D−H···A (deg)

1.81 2.01

2.646(3) 2.796(3)

157 163

1.75 2.09 1.86 2.00

2.560(3) 2.860(3) 2.662(3) 2.818(3)

158 156 154 169

1 O5−H101···O3 O5−H102···O3#1

0.88 0.81

O5−H101···O3 O5−H102···O6#1 O10−H103···O9 O10−H104···O5#2

0.86 0.82 0.86 0.83

2

a

Symmetry codes: For 1: #1 −x + 1, −y + 1, −z + 1. For 2: #1, −x + 1, −y + 2, −z; #2, −x, −y + 2, −z − 1. 4323



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Figure 2. (a) ORTEP plots of the local coordination environment around the two crystallographic distinct Co(II) centers in 2 with thermal ellipsoids at the 50% probability level. Symmetry codes: #1, −x + 2, −y, −z + 3; #2, −x + 1, −y, −z + 2; #3, x + 1, y, z + 1; #4, −x + 2, −y + 1, −z + 3; #5, x, y, z + 1; #6, −x + 2, −y + 1, −z + 2. (b) Ball-and-stick representation of the two independent 1-D ribbons in 2. Carbon-bound hydrogen atoms are omitted for clarity. Color scheme: green, Co1; pink, Co2; red, O; blue, N; gray, C; yellow, H. (c) Schematic representation of the AB-type cross-like packing of ribbons, showing a 3-D “plywood-like array”. Green and pink lines represent the Co1/InMe-4-pyI and Co2/InMe-4-pyII ribbons, respectively.

−21.1(4)°) and 2 (DA = 88.1(1)°; TA = 146.1(3)°, −34.6(4)° and DA = 87.7(1)°; TA = 172.1(2)°, −10.5(4)°), within slight differences in TA values. However, such negligible geometric differences are not sufficient in determining the formation of supramolecular isomers. Therefore, the occurrence of supramolecular isomerism of 1 and 2 must arise from more specific structural characteristics. Through careful insight into the structures of the two isomers, it can be seen that the main difference in both isomers is the bridging orientation of the four InMe-4-py ligands around the Co(II) centers, presumably due to the instinctive freely rotatable methylene (−CH2−) hinge. In 1, the four InMe-4-py ligands display a completely divergent bridging form, in which each of the ligands connects with one further metal center, to produce a gridlike layer structure (Figure 1). However, in 2 they adopt a 1,2-alternate ditopic arrangement on binding two further metal centers to result a ribbon structure (Figure 2). In other words, these bridging orientations/characteristics, which allow bond formation with further metal centers, cause different connectivity between the metal centers and thus lead to the formation of supramolecular isomers 1 and 2. Temperature and pH Effects. From the energetic pointof-view, it is reasonable to expect both structures of isomers 1 and 2 being apparently of similar energy since they are crystallized from a single crystallization under hydro(solvo)thermal conditions. It is a well-known fact that temperature is an important factor in controlling the network topology and the dimensionality of the structures.6,8 To some extent the kinetic and/or thermodynamic conformers could be controlled by changing the temperature that the former can generally be obtained at low temperatures while high temperature favors the latter.6a,d,8g Herein, to study the possible supramolecular

independent ribbons resulting from the combination of Co1 centers and the InMe-4-pyI ligands (i.e., Co1/InMe-4-pyI) and Co2 centers and the InMe-4-pyII ligands (i.e., Co2/InMe-4pyII), respectively (Figure 2b). These 1-D ribbons show a nonparallel cross-like packing fashion with two differently orienting neighboring layers of AB type, which can be described as a 3-D “plywood-like array”,21 as shown in Figure 2c. As such a packing fashion, there are two kinds of net-to-net hydrogen bonds (Figure S8). One is observed between the Co1-bound aqua (O5) ligands in one Co1/InMe-4-pyI ribbon and the trimellitic imide 1-carbonyl groups (O6) in the neighboring Co2/InMe-4-pyII ribbons, whereas another forms from the Co2-bound aqua (O10) ligands in one Co2/InMe-4-pyII ribbon to the Co1-bound aqua (O5) ligands in the neighboring Co1/ InMe-4-pyI ribbons. These supramolecular interactions undoubtedly provide supports for stabilizing the resultant 3-D hydrogen-bonded supramolecular net. Structural Comparison of 1 and 2. 2-D Rhombus grid 1 and 1-D ribbon 2 are genuine supramolecular isomers that they have different network structures, but the whole crystals or the coordination networks have exactly the same chemical composition. As described above, isomers 1 and 2 have the same metal fragment that the octahedral Co(II) center is made up of three pairs of trans-coordinated N(py), O(carboxylate), and O(water) donors (Figures 1a and 2a). Further, in addition to the (nearly) identical bis-monodentate coordination mode (Figures S4 and S6, Supporting Information), the conformation of the InMe-4-py ligand that can be described in terms of the dihedral angle (DA) defined by the plane of the pyridine ring and the plane of the 1,3-dioxoisoindoline moiety and the N(imide)−C(methylene)−Cα(py)−Cβ(py) torsion angles (TA) are very similar in 1 (DA = 82.5(1)°; TA = 159.6(3)°, 4324



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Figure 3. Crystal yields of supramolecular isomers 1 and 2 obtained from a single crystallization bath under hydro(solvo)thermal conditions at different temperatures and pH values.

dominated by the bridging orientation of InMe-4-py ligands, which is mainly influenced by reaction temperature. On the other hand, when the reactions were carried out at a reaction temperature of 80 °C via a similar synthetic procedure by using the Cl−, ClO4−, and BF4− salts of Co2+ ion to replace Co(NO3)2·6H2O as a starting reagent, a mixture of crystal isomers 1 and 2 was always obtained directly, while in the cases of OAc− and SO42− anions, no crystalline solids of desired isomeric products could be obtained. Thermal Properties. The thermal behaviors of supramolecular isomers 1 and 2 were studied by thermogravimetric (TG) analysis (Figure 4). The TG curve of 1 reveals that the

isomerism of this system, the same reaction mixtures (Co(NO3)2·6H2O, HInMe-4-py, THF, and H2O) were subjected to different synthetic routes by varying the reaction temperatures at 60, 80, and 120 °C, and pH values at 5, 6, 7, and 8. As shown in Figure 3, it is noted that the formations of rhombus grid 1 and ribbon 2 in crystalline solids are closely associated with both the synthetic temperature and pH values, even though the crystal yields of 1 and 2 under different conditions seem not so different. For example, reaction conditions in a more acidic solution (pH = 5) resulted in only a clear solution at all three reaction temperatures, i.e., no crystalline or powder products could be directly obtained after the reaction vessel was cooled to room temperature. At pH ≥ 6, rhombus grid 1 was found to be predominantly crystallized under most synthetic conditions, with the temperature at 60, 80, and 120 °C. In contrast, ribbon 2 always occurred as a minor product (manual separation, < 2%) under more rigid reaction conditions at pH = 6 and T = 80 and 120 °C. In other words, pure 1 as microcrystals can be directly isolated under slightly basic (7 ≤ pH ≤ 8) reaction conditions, and a mixture of 1 and 2 is simultaneously obtained from a single crystallization at pH = 6 when the temperature is higher than 80 °C. These observations clearly indicate that an acidic assembly environment is unfavorable to crystal growths of 1 and 2 in this system. On the other hand, as the reaction temperature is increased from 60 to 80 °C and 120 °C, the isolated crystal yield of rhombus grid 1 increases while, in comparison, that of ribbon 2 decreases at controlled pH values. The relatively high crystal yields of 1 are generally observed at pH ≈ 7 and T ≥ 80 °C. In other words, the higher the temperature, the higher the isolated crystal yields of 1. Accordingly, it is reasonable to assume that 1 is a thermodynamically favored product and 2 is a more kinetically favored product under the present experimental conditions.6a,d,8g To confirm this view, the influence of the reaction time was evaluated, with preparation of a set of reactions at pH = 6 and T = 80 °C, varying only the reaction time between 12, 24, 36, and 48. During very short periods (12 and 24 h), only a clear solution could be obtained. After longer reaction times (36 and 48 h), isomers 1 and 2 both appeared as a mixture of crystalline solids recorded in the XRPD diffractograms (Figure S9, Supporting Information). The relative abundances of 1 and 2 are time-dependent in that a relative high 2 to 1 value is observed in a shorter period of 36 h, which is decreased as evaluating reaction time to 48 h. This observation suggests that 2 is discarded to be formed and/or possibly transformed into 1 as a function of time. In other words, the formation of ribbon 2 is likely to be conditioned by kinetic control, while rhombus grid 1 is probably by thermodynamic control.6a,22 So, as mentioned above, the formation of supramolecular isomers in this system is

Figure 4. TG curves of supramolecular isomeric rhombus grid 1 (solid line) and ribbon 2 (dashed line).

coordinated water molecules are released between 54−107 °C (found 5.85%, calcd. 5.48%), and the solvent-free coordinative motif begins to decompose when the temperature reaches ca. 360 °C. The TG trace for 2 shows that the coordinated water molecules are lost between 110−168 °C (found 5.60%, calcd. 5.48%), and the coordinative framework is retained up to approximately 380 °C, followed by a decomposition of the framework. The final residual has a weight of 11.96% of the total sample that is reasonably attributed to CoO (calcd. 11.40%). It is worthy to note that releasing coordinated water molecules in 2 has occurred in a temperature more than 55 °C higher than that in 1; this may be indicative of stronger metal−ligand coordination bonds and net-to-net hydrogenbonding interactions for the coordinated water molecules within the network of 2 over that within the network of 1, as supported quantitatively in their hydrogen bonds (2 in 1 versus 3 in 2, Figures S6b and S8, Supporting Information). Further, even though the two supramolecular isomeric networks display similar thermal stabilities (360 °C versus 380 °C), the 1-D ribbon 2 completely collapses upon heating to ca. 635 °C, while, comparably, complete decomposition of the 2-D 4325



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rhombus grid 1 does not occur even after the temperature was raised to 900 °C. Magnetic Studies. Magnetic susceptibility measurements of powdered samples for rhombus grid 1 and ribbon 2 were performed in the temperature range of 2−300 K (Figure 5).

The Co(II) centers within the same 2-D network of rhombus grid 1 are separated by at least 14 Å (Figure 1b). The shortest Co···Co contact of 6.63 Å is present between grids (Figure S5, Supporting Information). On the other hand, in ribbon 2 all Co(II) centers within the 1-D array are separated with each other by 13.31 ± 0.21 Å (Figure 2b). The shortest distance between two metal sites is measured to be about 6.02 Å (Figure S10, Supporting Information), in which two Co(II) centers from neighboring chains are weakly connected via interchain hydrogen bonding through coordinated water molecules. The structural parameters suggest the presence of minimal magnetic exchange interactions in both compounds. The rapid decrease of the χMT values below 10 K could be indicative of weak intra/ inter-net antiferromagnetic interactions. Magnetic behavior of an isolated high-spin Co(II) center in Oh symmetry is strongly perturbed by unquenched orbital contribution.25 It removes degeneracy of the 4T1 state to a sextet, a quartet, and a Kramers doublet. The corresponding Hamiltonian is described as HSO = −αλ LS

where the α parameter a gauge of orbital reduction from the covalency of the metal−ligand bonds as well as the admixture of the upper 4T1g(4P1) state into the 4T1g(4F1) ground state, and the λ parameter the spin−orbit coupling constant. The Co(II) center in both compounds is coordinated by 2 N(py), 2 O(carboxylate), and 2 O(water) sites, which leads to approximately axial distortion. Under axial anisotropy, the 4 T1g ground state is split into 4Eg and 4A2g states with an energy gap defined as Δ. Therefore, the full Hamiltonian describing the spin−orbit coupling, axial distortion and Zeeman interaction is given by H = −αλ LS + Δ[Lz 2 − 1/3L(L + 1)] + βH( −α L + geS)

The values of α, λ, Δ are derived via matrix diagonalization techniques. The best-fit results to the experimental data are α = 1.430, λ = −127 cm−1, Δ = −396 cm−1, R (the agreement factor) = 1.1 × 10−4, which is defined as Σ(χMcalcd − χMobs)2/ Σ(χMobs)2, and α = 1.382, λ = −148 cm−1, Δ = −592 cm−1, R = 3.3 × 10−5 for 1 and 2, respectively. The fitted curves match the experimental data well in the whole temperature range. The large value of |Δ/λ| > 3 suggests significant anisotropy. The magnetic moments of 2.35 (1) and 2.24 Nβ (2) at 2 K approach the expected magnetization saturation value, 2.17 Nβ, indicating that only the ground Kramers doublets is populated. It allows disentanglement of the involvement of intra/inter-net antiferromagnetic interactions. A reduced-spin model was employed to the magnetic data below 10 K for estimation of the strength of the magnetic exchange (Figure S7, Supporting Information).26 From the results of J/kB = −0.24 and −0.07 K for 1 and 2, respectively, it is suggested that the magnetic coupling between Co(II) centers is fairly weak.

Figure 5. Plots of magnetic susceptibility (open circles) and χMT (open squares) versus temperature for (a) the rhombus grid 1 and (b) the ribbon 2. The solid line is the best fit of the experimental data to the theoretical model described in the text.

The χMT values at 300 K are 3.31 and 3.17 cm3 K mol−1 for 1 and 2, respectively. These values are substantially higher than the spin-only value for a high-spin Co(II) center (S = 3/2, 1.875 cm3 K mol−1). Instead, they are close to the value expected from additional contribution of orbital angular momentum (3.375 cm3 K mol−1). Unquenched spin−orbit coupling resulted from slightly distorted Oh geometry about the Co(II) center.23 Depopulation of high energy Kramers doublets to ground states with decrease of temperature leads to a decrease of the magnetic moment. The χMT values at 2 K reach 1.74 and 1.87 cm3 K mol−1 for 1 and 2, respectively, which are consistent with the theoretical estimation of 1.73 cm3 K mol−1 for isolated Co(II) ions with an effective spin Seff = 1/2. Fitting the magnetic data (50−300 K) of both complexes to the Curie−Weiss law yields the Curie constant C = 3.56 cm3 K mol−1, the Weiss constant θ = −23.58 K for 1, and C = 3.39 cm3 K mol−1, θ = −21.48 K for 2. The values of the Curie constant are in agreement with those reported for sixcoordinated high-spin Co(II) ions (C = 2.8−3.4 cm3 K mol−1).24

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CONCLUSION Two simple and straightforward Co(II) supramolecular isomers, a 2-D rhombus grid 1 and a 1-D ribbon 2, having exactly the same stoichiometry, metal fragment, and ligand conformations, without the presence of any guest component, were obtained from the same mother liquor under hydro(solvo)thermal conditions. Not only temperature herein has 4326



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shown to be a significant influencing factor on the concomitant crystallization of genuine supramolecular isomeric rhombus grid and ribbon, but also pH value, time of reaction, and counteranion. Through careful inspection of the two isomers, we can conclude that their structural differences mainly arise from the different bridging orientation of InMe-4-py ligands, which is mainly influenced by reaction temperature. X-ray crystal structure analysis reveals that the Co(II) centers in the 2-D (rhombus grid 1) and 1-D (ribbon 2) network structures are separated by at least 6 Å. Such long distances of the metal− metal contacts are responsible for negligible magnetic exchange couplings. The magnetic behavior of compounds 1 and 2 shows that the temperature dependence of the magnetic susceptibilities of the high-spin Oh Co(II) centers is mainly contributed from spin−orbit coupling.

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ASSOCIATED CONTENT

* Supporting Information S

Crystallographic data in CIF format for 1 and 2. X-ray powder diffractions patterns, infrared spectra, additional structural figures, and reduced-spin plots. 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: +886-49-2917956. Phone: +886-49-2910960-4918. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the National Chi Nan University and the Ministry of Science and Technology of Taiwan for financial support.

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