Single-Crystal-to-Single-Crystal Structural Transformation in a Three

May 5, 2010 - §Department of Chemistry, Jadavpur University, Jadavpur, Kolkata, 700 032, India. Received October 27, 2008; Revised Manuscript Receive...
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DOI: 10.1021/cg8012014

Single-Crystal-to-Single-Crystal Structural Transformation in a Three-Dimensional Bimetallic (4f-3d) Supramolecular Porous Framework

2010, Vol. 10 2483–2489

K. L. Gurunatha,† Golam Mostafa,‡ Debajyoti Ghoshal,§ and Tapas Kumar Maji*,† †

Molecular Materials Laboratory, Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore, 560 064, India, ‡Department of Physics, and § Department of Chemistry, Jadavpur University, Jadavpur, Kolkata, 700 032, India Received October 27, 2008; Revised Manuscript Received April 17, 2010

ABSTRACT: A three-dimensional (3D) bimetallic (4f-3d) supramolecular framework, {[Nd(pyno)2(H2O)4][Fe(CN)6] 3 H2O}n (1) (pyno = pyridine-N-oxide), has been synthesized and structurally characterized. Structure determination reveals that onedimensional (1D) cyano-bridged chains of NdIII and FeIII form an alternative basket-like arrangement and are connected by (O-H 3 3 3 N) H-bonding interactions through coordinated water molecules and pendent CN groups forming a two-dimensional (2D) sheet-like structure. 2D sheets are further connected by another set of similar (O-H 3 3 3 N) interactions resulting in a 3D supramolecular framework with 1D water-filled channels. Controlled heating of the as-synthesized crystal at 85 C causes a color change from yellow to a light brown compound {[Nd(pyno)2(H2O)4][Fe(CN)6]}n (10 ) and structure determination shows significant contraction of the overall framework with selective removal of the guest water molecules. 10 exhibits a similar framework topology as 1. The dehydrated crystal (10 ) regenerates the virgin as-synthesized framework (1a) with structural expansion upon exposure to the water vapor. Adsorption studies reveal that 10 can selectively uptake H2O but not MeOH, EtOH, and CO2 consistent with the channel size. Removal of the guest as well as coordinated water molecules gives another phase {[Nd(pyno)2][Fe(CN)6]}n (2), which shows sorption of H2O and MeOH vapors but not CO2. Temperature-dependent magnetic measurement (300-2 K) suggests the as-synthesized framework is antiferromagnetically coupled at low temperature.

Introduction Three-dimensional (3D) supramolecular porous coordination frameworks constructed by the assembly of one- or two-dimensional (1D or 2D) coordination networks by noncovalent interactions have attracted much attention as a promising materials for selective adsorption, separation, recognition, and catalysis realized by their structural flexibility and dynamicity.1 Porous supramolecular assemblies with novel spin carriers may exhibit interesting magnetic properties as well as porous functionality, that is, dual functionality in a single framework system.2 Guest responsive magnetic modulations have recently been demonstrated.3 Multidimensional bimetallic assembly of transition metal ions and cyanometallate “metalloligand” anions with interesting magnetic and photomagnetic properties are well documented in the literature.4 In contrast, the lanthanide ions (LnIII) have not been well explored as nodes for the construction of a multidimensional coordination framework using cyanometallate anions.5,6 However, the large and anisotropic magnetic moment of LnIII shows versatile magnetic properties, including enhancement of the coercive field. Moreover, this bimetallic assembly of 3d-4f ions may display optical properties related to the LnIII ion included in magnetic molecular media7 and may find application in biological sciences. On the other hand, guest-induced single-crystal-to-singlecrystal structural transformation in the porous coordination framework results in shrinking or expansion in the overall framework.8 This type of structural flexibility in the porous coordination framework has been realized by the unusual

sorption and reaction properties, including guest-induced fitting, selective sorption, and separation of small molecules.9 1D/2D network materials can provide such structural dynamism by interframework translation caused by external stimuli, such as pressure and temperature.10 Such structural flexibility would be advantageous for the recognition of a particular substrate similar to metalloproteins.11 Here, we report the synthesis, reversible single-crystal-tosingle-crystal structural transformation of a 3D supramolecular framework, {[Nd(pyno)2(H2O)4][Fe(CN)6] 3 H2O}n (1), (pyno = pyridine-N-oxide) constructed by the H-bonding and π-π interactions between 1D coordination chains. Controlled heating of 1 furnished two different phases {[Nd(pyno)2(H2O)4][Fe(CN)6]}n (10 ) and {[Nd(pyno)2][Fe(CN)6]}n (2), and both the phases uptake H2O molecules but completely oppose CO2 gas. A variable temperature magnetic study of 1 indicates the overall weak antiferromagnetic nature of the compound. Experimental Section

*To whom correspondence should be addressed. E-mail: tmaji@jncasr. ac.in. Tel. þ91 80 2208 2826. Fax: þ91 80 2208 2766.

Materials. All the reagents and solvents employed were commercially available and used as supplied without further purification. Nd(NO3)3 3 6H2O, K3[Fe(CN)6], and pyridine-N-oxide were obtained from Aldrich Chemical Co. Physical Measurements. The elemental analyses were carried out using a Perkin-Elmer 2400 CHN analyzer. IR spectrum of the compound was recorded on a Bruker IFS 66v/S spectrophotometer using the KBr pellets in the region 4000-400 cm-1. Thermogravimetric analysis (TGA) was carried out on METTLER TOLEDO TGA 850 instrument in the temperature range of 25-300 C under nitrogen atmosphere (flow rate of 50 mL/min) at a heating rate of 3 C/min. Inductively coupled plasma-optical emission spectral (ICP-OES) analysis was carried out using a Thermoelectron 6500 ICP-OE spectrometer. 0.014 mg (0.02 mmol) of compound 1 was

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dissoluted in 25 mL of H2O with conc. HNO3 (100 μL) in a 100 mL volumetric flask. The solution was made up to the mark using double-distilled water and subjected to ICP-OES analysis. X-ray powder diffraction (PXRD) patterns in different states of the samples were recorded on a Bruker D8 Discover instrument using Cu-KR radiation. Measurement of Adsorption. The adsorption isotherms of CO2 (195 K) and solvents in the vapor state (like H2O (298 K), MeOH (293 K), and EtOH (298 K)) for 10 and 2 were measured by using QUANTACHROME AUTOSORB-1C analyzer and BELSORPaqua 3 volumetric adsorption equipment from BEL, Japan, respectively. In the sample chamber (∼17.5 mL) maintained at T ( 0.03 K was placed the adsorbent sample 1 (∼100-150 mg), which had been prepared at 353 K (for 10 ) and 413 K (for 2) under reduced pressure (0.1 Pa) for about 5 h prior to measurement of the isotherms. The adsorbate was placed into the sample tube, and then the change of the pressure was monitored and the degree of adsorption was determined by the decrease of the pressure at the equilibrium state. All operations were computer-controlled and automatic. Synthesis of {[Nd(pyno)2(H2O)4][Fe(CN)6] 3 H2O}n (1). An aqueous solution (15 mL) of K3[Fe(CN)6] (0.329 g, 1 mmol) was slowly added to an aqueous solution (10 mL) of Nd(NO3)3 3 6H2O (0.438 g, 1 mmol) and the reaction mixture was stirred for 30 min. Then an ethanolic solution (25 mL) of pyridine-N-oxide (pyno) (0.190 g, 2 mmol) was slowly added to the above solution and the whole yellow color reaction mixture was stirred for 1 h. Then solution was filtered and the filtrate was kept in an open atmosphere for slow evaporation and a slight yellowish precipitate was discarded. After one week, light yellow color shiny crystals were separated and washed with EtOH/water mixture and dried in air. Yield: 80%. Anal. Calcd for NdFeC16H20N8O7: C, 30.16; H, 3.14; N, 17.59. Found: C, 29.88; H, 3.29; N, 17.92. IR (KBr, cm-1): ν(O-H), 31353630; ν(NO), 1382; ν(CdC), 1625 and ν(CN), 2121. Both EDAX (Figure S2, Supporting Information) and ICP-OES analyses of 1 indicated the ratio of Fe/Nd to be 1:1. Preparation of {[Nd(pyno)2(H2O)4][Fe(CN)6]}n (10 ) and {[Nd(pyno)2(H2O)4][Fe(CN)6](H2O)}n (1a). Compound 1 was placed in a glass sample cell and heated at 80 C for 5 h under reduced pressure that yields 10 . Anal. Calcd for NdFeC16H18N8O6: C, 31.04; H, 2.91; N, 18.10. Found: C, 30.76; H, 3.11; N, 18.29. IR (KBr, cm-1): ν(O-H), 3205-3580; ν(NO), 1380; ν(CdC), 1622 and ν(CN), 2125. The dehydrated compound 10 was placed for three days in a small sample vial which was placed in a bigger sample vial containing water. Anal. Calcd for NdFeC16H20N8O7: C, 30.16; H, 3.14; N, 17.59. Found: C, 30.58; H, 3.01; N, 17.22. IR (KBr, cm-1): ν(O-H), 3235-3630; ν(NO), 1380; ν(CdC), 1625 and ν(CN), 2120. Preparation of {[Nd(pyno)2][Fe(CN)6]}n (2). Compound 1 was placed in a glass sample cell and heated at 140 C for 5 h under reduced pressure that yields 2. Anal. Calcd for NdFeC16H10N8O2: C, 35.13; H, 1.82; N, 20.49. Found: C, 34.82; H, 2.01; N, 19.97. IR (KBr, cm-1): ν(NO), 1384; ν(CdC), 1624 and ν(CN), 2122. Single-Crystal X-ray Diffraction. A suitable yellow color single crystal of compound 1 was mounted on a thin glass fiber with commercially available super glue. X-ray single crystal structural data were collected on a Bruker Smart-CCD diffractometer equipped with a normal focus, 2.4 kW sealed tube X-ray source with graphite monochromated Mo-KR radiation (λ = 0.71073 A˚) operating at 50 kV and 30 mA. The program SAINT was used for integration of diffraction profiles and absorption correction was made with SADABS program. The structure was solved by SIR-92 and refined by a full matrix least-squares method using SHELXL. For all the compounds, the non-hydrogen atoms were refined anisotropically. All hydrogen atoms were located by Fourier analysis and refined isotropically. A suitable single crystal of 1 was heated at around 85 C in N2 atmosphere and became 10 , which was covered with the paraffin oil and mounted on a thin glass fiber with commercially available super glue. Structure was solved using a similar method as for 1. During refinement of 10 , it was found that the crystal is twinned. The use of ROTAX12 shows 180 rotations are possible about (1 0 0) and (-1 0 2) reciprocal lattice directions leading to a three component twin. The corresponding nonmerohedral twin matrices are (1 0 0.995/0

Gurunatha et al. -1 0/0 0 -1) and (0.997 0 0.999/0 -1 0/0.005 0 -0.997), which are the matrices for the transformation of the reflection indices of the three domains. In order to refine the structure as a nonmerohedral twin using the HKLF 5 option in SHELXL-97, the reflection file was modified using the program TWINROTMAT implemented in PLATON. For every overlapped reflection, the reflection of the second and third twin domains calculated by the twin law has to be inserted before the prime reflection. The batch number of the reflection generated by a second and third domain has to be set to -2 and -3, respectively. The negative sign informs the program that contributions from other twin components follow. The intensity and the standard deviation of the second reflection are set to the same value as for the prime reflection. By this procedure, equivalent reflections were not merged. In order to refine the ratio of the three twin components, the refinement is performed adding a BASF instruction. The refined BASF parameters were 0.13 and 0.09 leading to three components twin ratios being 0.78: 0.13: 0.09. The final model leads to a reasonable structure with very good reliability factors. All the hydrogen atoms were fixed by HFIX and placed in ideal positions. The dehydrated single crystals (10 ) were exposed to the water vapor for three days and then single crystal data of the rehydrated crystal (1a) was collected and structure was solved by the similar procedure of 1. Here some of the hydrogen atoms were located by Fourier analysis and remaining hydrogen atoms were fixed by HFIX and placed in ideal positions. The coordinates, anisotropic displacement parameters, and torsion angles for all three compounds are submitted as Supporting Information in CIF format. All calculations were carried out using SHELXL 97,13 SHELXS 97,14 PLATON,15 and WinGX system, Ver 1.70.01.16 All crystallographic and structure refinement data of the compounds are summarized in Table 1. Selected bond distances, angles, and hydrogen bonding parameters for compounds 1, 10 , and 1a are given in Tables 2-7. Structural Description of {[Nd(pyno)2(H2O)4][Fe(CN)6] 3 H2O}n (1). Compound 1 crystallizes in monoclinic C2/c space group and structure determination reveals that the 1D bimetallic coordination chain of NdIII is connected by the cyanometallate [Fe(CN)6]3anion (Figure 1a). In the 1D chain each octacoordinated Nd1 atom ligated to the four water molecules (O1, O1_a, O2 and O2_a; a= -x, y, 3/2 - z), two oxygen atoms (O3, O3_a) of pyno ligand, and two nitrogen atoms (N1, N1_a) of [Fe(CN)6]3-. Each [Fe(CN)6]3acts as a bridging bidentate ligand and connected in trans fashion to the two Nd1 atoms and the other four cyanide groups remain as a pendent arm. The Nd1-O bond distances are in the range of 2.3408(18)-2.4842(18) A˚ and the Nd1-N bond distance is Table 1. Crystallographic and Structural Refinement Parameters for 1, 10 , and 1a empirical formula Mr crystal system space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z T (K) Fcalc (g cm-3) μ (mm-1) F(000) θmax (deg) λ (Mo KR) (A˚) total data unique data, Rint data [I > 2σ(I)] Ra Rwb S ΔF max/min/e A˚-3

1

10

1a

NdFeC16H20N8O7 636.49 monoclinic C2/c (No. 15) 17.4068(4) 9.4583(2) 16.6362(6) 90 117.7970(10) 90 2422.90(12) 4 293 1.745 2.769 1256 25.7 0.71073 11049 2303, 0.022 2252 0.0160 0.0407 1.10 0.52/-0.39

NdFeC16H18N8O6 618.47 monoclinic C2/c (No. 15) 16.861(7) 9.123(4) 16.133(7) 90 118.44(3) 90 2182.1(17) 4 293 1.883 3.068 1216 27.3 0.71073 2140 2140, 0.00 1231 0.0854 0.2336 0.98 1.74/-1.59

NdFeC16H20N8O7 636.49 monoclinic C2/c (No. 15) 17.4001(16) 9.4537(8) 16.6235(13) 90 117.798(6) 90 2418.9(4) 4 293 1.748 2.773 1256 27.6 0.71073 9719 2587 1429 0.103 0.1533 1.10 1.78/-0.99

R = Σ||Fo| - |Fc||/Σ|Fo|; bRw = [Σ{w(Fo2-Fc2)2}/Σ{w(Fo2)2}]1/2.

a

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Crystal Growth & Design, Vol. 10, No. 6, 2010 Table 2. Bond Distances and Angles (A˚, ) for {[Nd(pyno)2(H2O)4][Fe(CN)6](H2O)}n (1)a

Nd1-O1 Nd1-O3 Nd1-O1_a Nd1-O3_a

2.4842(18) 2.3408(18) 2.4842(18) 2.3408(18)

O1-Nd1-O2 O1-Nd1-N1 O1-Nd1-O2_a O1-Nd1-N1_a O2-Nd1-N1 O2-Nd1-O2_a O2-Nd1-N1_a O1_a-Nd1-O3 a

Nd1-O2 Nd1-N1 Nd1-O2_a Nd1-N1_a

74.46(6) 131.97(6) 77.63(6) 128.79(7) 144.80(6) 145.74(6) 69.46(6) 140.36(6)

Table 7. Hydrogen Bonds (A˚, ) for {[Nd(pyno)2(H2O)4][Fe(CN)6](H2O)}n (1a)a 2.4693(19) 2.5845(18) 2.4693(19) 2.5845(18)

O1-Nd1-O3 O1-Nd1-O1_a O3-Nd1-N1_a O2-Nd1-O3 N1-Nd1-N1_a O2-Nd1-O3_a O3-Nd1-N1 O3-Nd1-O3_a

70.33(6) 70.18(7) 77.90(6) 95.07(6) 75.35(6) 93.88(6) 77.90(6) 149.28(5)

Symmetry code: a = -x, y, 3/2 - z; b = -1/2 - x, -1/2 - y, 1 - z. Table 3. Hydrogen Bonds (A˚,) {[Nd(pyno)2(H2O)4][Fe(CN)6](H2O)}n (1)a

O1--H1 3 O1--H1 3 O1--H2 3 O2--H3 3 O2--H4 3

3 O4 3 O4 i 3 N2ii 3 N3iii 3 N3

0.79(3) 0.79(3) 0.64(3) 0.68(3) 0.74(3)

2.10(3) 2.10(3) 2.17(3) 2.11(3) 2.59(3)

2.851(3) 2.851(3) 2.813(3) 2.790(3) 3.308(3)

161(4) 161(4) 178(3) 172(3) 163(3)

a Symmetry code: i = -x, -y, 1 - z; ii = -x, 1 þ y, 3/2 - z, iii = 1/2 þ x, -1/2 - y, 1/2 þ z.

Table 4. Bond Distances and Angles (A˚, ) for {[Nd(pyno)2(H2O)4][Fe(CN)6](H2O)}n (10 )a Nd1-O1 Nd1-O3 Nd1-O1_a Nd1-O3_a O1-Nd1-O2 O1-Nd1-N1 O1-Nd1-O2_a O1-Nd1-N1_a O2 -Nd1-N1 O2-Nd1-O2_a O2-Nd1-N1_a O1_a-Nd1-O3 a

2.386(13) 2.255 (12) 2.386 (13) 2.255 (12) 75.0(4) 132.9(5) 78.1(5) 128.3(6) 143.1(5) 146.5(4) 70.4(5) 70.0(5)

Nd1-O2 Nd1-N1 Nd1-O2_a Nd1-N1_a O1-Nd1-O3 O1-Nd1-O1_a N1-Nd1-N1_a O2-Nd1-O3 O3_a-Nd1-N1 O2-Nd1-O3_a O3-Nd1-N1 O3-Nd1-O3_a

2.371(12) 2.427 (15) 2.371 (12) 2.427 (16) 142.3(5) 72.3(5) 72.7(5) 95.3(4) 78.8(5) 93.9(4) 75.3(5) 147.7(4)

a = 1 - x, y, 3/2 - z; b = 1/2 - x, -1/2 - y, 1 - z.

Table 5. Hydrogen Bonds (A˚, ) for{[Nd(pyno)2(H2O)4][Fe(CN)6](H2O)}n (10 )a D-H 3 3 3 A O2--H12 3 O1--H21 3 O2--H22 3

i

3 N2ii 3 N3iii 3 N2

D-H

H3 3 3A

D3 3 3A

— D-H 3 3 3 A

0.85 0.85 0.85

2.32 1.87 2.6

2.675(19) 2.70(2) 3.177(18)

105 163 127

a Symmetry code: i = 1/2 þ x, 3/2 - y, 1/2 þ z; ii = 2 - x, 1 - y, 1 - z; iii = 2 - x, y, 3/2 - z.

Table 6. Bond Distances and Angles (A˚, ) for {[Nd(pyno)2(H2O)4][Fe(CN)6](H2O)}n (1a)a Nd1-O1 Nd1-O3 Nd1-O1_a Nd1-O3_a O1-Nd1-O2 O1-Nd1-N2 O1-Nd1-O2_a O1-Nd1-N2_a O2-Nd1-N2 O2-Nd1-O2_a O2-Nd1-N2_a O1_a-Nd1-O3 a

2.325(7) 2.483(8) 2.325(7) 2.483(8) 93.4(3) 77.9(2) 95.8(3) 77.7(3) 145.4(3) 144.9(3) 69.7(3) 140.4(3)

Nd1-O2 Nd1-N2 Nd1-O2_a Nd1-N2_a O1-Nd1-O3 O1-Nd1-O1_a O2-Nd1-O3_a O2-Nd1-O3 O3-Nd1-N2 O3-Nd1-O3_a N2-Nd1-N2_a O3_a-Nd1-N2

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2.453(9) 2.580(7) 2.453(9) 2.580(7) 70.6(3) 148.9(2) 74.1(3) 77.3(3) 128.4(3) 70.0(3) 75.8(2) 132.2(3)

Symmetry code: a = 1 - x, y, 3/2 - z; b = 1/2 - x, -1/2 - y, 1 - z.

D-H 3 3 3 A O2--H6 3 O3--H8 3 O3--H9 3 O3--H9 3 a

3 3 3 3

3 N4i 3 N3 ii 3 O4 3 O4

D-H

H3 3 3A

D3 3 3A

— D-H 3 3 3 A

0.83(14) 0.75(10) 0.96(10) 0.96(10)

1.98(12) 2.08(11) 1.90(10) 1.90(10)

2.783(12) 2.797(14) 2.846(13) 2.846(13)

163(12) 162(11) 170(9) 170(9)

Symmetry code: i = 1 - x, 1 þ y, 3/2 - z; ii = 1/2 - x, 1/2 þ y, 3/2 - z.

2.5845(18) A˚. 1D chains are connected to each other through O2-H3 (coordinated H2O) 3 3 3 N3 (pendent CN) H-bonding interactions forming a 2D interdigitated sheet in the crystallographic ab plane. The 2D sheet displays the basket shape arrangement alternatively formed by the pyno ligands and bonded CN groups of [Fe(CN)6]3- where the guest water molecules are accommodated (Figures 1b and S3). The guest water molecules (O4) are H-bonded with the coordinated water molecule O1 (O4 3 3 3 O1, 2.851(3)) (Table 3). These H-bonded 2D sheets are further linked by another set of O1-H2 (coordinated H2O) 3 3 3 N2 (pendent CN) H-bonding interactions resulting in a 3D supramolecular framework (Figures 2a and S3). Therefore, pendent CN groups, which are in cis arrangement, act as a building unit toward the 3D supramolecular framework. 3D framework contains 1D channel filled with the two water molecules along the crystallographic c-axis (Figure 2a). The size of the channel considering the van der Waals radii is about 3.7  1.8 A˚2. In the 1D coordination chain, the Nd1 3 3 3 Nd1 distance through the [Fe(CN)6]3- linker is about 10.967 A˚ and shortest Nd1 3 3 3 Nd1 separation in the 2D sheet and between the 2D sheets are 9.458 and 8.514 A˚, respectively. Fe1 3 3 3 Nd1 separation through the cyanide bridge is about 5.483 A˚. Framework Stability. TG analysis of 1 performed in the temperature range of 25-300 C is shown in Figure S4, Supporting Information. The guests as well as coordinated water molecules were released in the temperature range of 65-130 C in a stepwise fashion. The first step (65-105 C) corresponds to the three water molecules (one guest and two coordinated), and the second step (105-130 C) corresponds to another two Nd1-bound water molecules. The dehydrated framework {[Nd(pyno)2][Fe(CN)6]}n (2) is stable up to 220 C without further weight loss and then decomposes to an unidentified product. The stepwise removal of the water molecules suggests different H-bonding interactions involved in different strengths in the 3D supramolecular framework. The guest water molecule (O4) is H-bonded with coordinated water molecules (O1, O1_a), which correlate the first step concomitant release of the three water molecules at high temperature. The PXRD pattern of 1 in different states was shown in Figure 3. The PXRD pattern at 140 C, that is, after complete removal of the guest and coordinated water molecules from Nd1 atom, reveals a significant change in the intensities and shifting of the peak positions and also the appearance of some new peaks. These changes suggest severe structural rearrangement after dehydration. Therefore, removal of the water molecules ultimately leads to the dehydrated framework with the coordinated unsaturated NdIII sites, rather than the collapse of the framework (Scheme 1). It is worth mentioning that the PXRD pattern of the dehydrated compound 2 after exposing the water vapor for three days exhibits a fair resemblance in peak positions to the as-synthesized compound 1 (Figure 3). This structural change is also confirmed by the IR spectra (Figure S1, Supporting Information); however elemental analysis does not completely conform to 1 suggesting the rehydration process is not completely reversible. Single-Crystal-to-Single-Crystal Structural Transformation. High thermal stability of the framework inspired us to determine the corresponding crystal structures after removing the guest as well as coordinated water molecules from the framework by controlled heating. The as-synthesized crystals heated at 85 C for 3 h under N2 atmosphere to remove the guest water molecule and the color of the crystal changes from light yellow to brownish yellow (Scheme 1, Figure S5, Supporting Information). The brownish crystal (10 ) shows a similar crystal system and space group as for compound 1 (Table 1). Structure determination reveals a 1D coordination chain of NdIII and FeIII with the formulation [Nd(pyno)2(H2O)4][Fe(CN)6], suggesting complete removal of the guest water molecule

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Figure 1. (a) View of the 1D coordination chain of 1 with alternating basket type arrangement; (b) view of the 2D H-bonded sheet connected by the coordinated water molecules and pendent CN group from the [Fe(CN)6]3-.

Figure 2. (a) Perspective view of the 3D supramolecular framework of 1 with 1D water-filled channels along the crystallographic c-axis; (b) view of the 3D supramolecular framework of 10 showing the empty channel; (c) view of the rehydrated framework of 10 (1a) showing virgin sample 1 regenerated. (Figure 2b). There is a significant contraction in the overall framework (Table 1) after removal of the guest water molecule and shows 3.13, 3.54, and 3.2% contraction along the a, b, and c axis, respectively, with a 9.9% decrease in cell volume. This structural contraction is reflected in channel size (3.0  1.0 A˚2) in 10 , which is smaller compared to the as-synthesized framework (Figure 2b). In [Nd(pyno)2(H2O)4][Fe(CN)6] (10 ), the Nd1-O and Nd1-N bond distances are slightly lower compared to 1 and also small changes were observed in the corresponding bond angles (Tables 2 and 4). It is worth mentioning that the single crystal breaks into several pieces with a loss of single crystallinity upon heating to high temperature (140 C) with removal of the coordinated water molecules. When the dehydrated brownish crystal 10 was exposed to water

vapor for three days, the yellow color of the crystals reappears (crystal 1a) (Figure S5, Supporting Information), and structure determination of 1a reveals that 10 returned to the virgin assynthesized compound 1 with the formulation of {[Nd(pyno)2(H2O)4][Fe(CN)6] 3 H2O}n (Figure 2c). Cell parameters of the rehydrated compound 1a are almost similar to that of 1 (Table 1), suggesting complete reversibility of the structure. The bond lengths and angles are also comparable to that of compound 1 (Tables 2 and 6). Adsorption Property. To confirm the permanent porosity and selectivity, dehydrated samples 10 and 2 were subjected to the sorption studies with gases such as CO2 (195 K) and different solvent vapors such as H2O (298 K), MeOH (293 K), and EtOH (298 K)

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(Figures 4 and 5). CO2 sorption for both 10 and 2 exhibit type-II profile suggesting only surface adsorption and nonporous to CO2 (Figure 4c (inset) and Figure 5a), which can be attributed to the large kinetic diameter (3.4 A˚)17 of CO2 compared to the pore size of the frameworks, which is contracted after dehydration. H2O (2.68 A˚)17 sorption profile for 10 shows a typical type-I curve indicating diffusion of H2O molecules in the pore surfaces (Figure 4). The final uptake amount is about 1.2 molecules per formula unit of 10 logical with the rehydrated structure, 1a. The steep uptake at low pressure regions and hysteretic sorption suggest strong interactions of the H2O molecules in the pore surfaces of 10 . However, 10 does not uptake any MeOH (kinetic diameter 3.8 A˚) and EtOH (4.3 A˚) solvent vapors which is consistent with the smaller channel aperture compared to the size of MeOH and EtOH (Figure (inset) 4a,b). On the other hand, H2O vapor sorption for 2 shows (Figure 5b) that with increasing pressure the sorption amount gradually increases and at P/P0=0.73 the amount is about 25 mL/g and then there is a steep uptake up to P/P0 = 1 with a final sorption amount of 86.2 mL/g without saturation. The desorption curve does not trace the adsorption curve showing large hysteresis and also a sudden steep uptake suggests structural transformation driven by the coordinatively unsaturated NdIII sites. The incomplete desorption suggest strong confinement of the water molecule, probably due to the coordination with the NdIII ions. On the other hand, MeOH sorption increases with increasing pressure and ended with 26 mL/g at P/P0 = 0.9 (Figure 5c). The MeOH exposed PXRD pattern of 2 (Figure 3e) exhibits some new peaks at low angles region and also with a change in intensities and peak positions suggest structural transformation, which is also realized by the hysteresis sorption phenomenon. The low angle peak suggests structural expansion for accommodating large MeOH molecules.17 The sorption amount

Figure 3. PXRD pattern of 1 in different states. (a) Simulated from X-ray single crystal data; (b) bulk as-synthesized compound; (c) at 140 C; (d) exposed to the water vapor for three days; (e) exposed to the MeOH vapor for three days.

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indicates that 2.0 and 0.6 molecules of H2O and MeOH, respectively, were occluded per formula unit of 2. The low uptake of H2O and MeOH at the low pressure region suggesting weak adsorbateadsorbent interaction which may be realized by the smaller pore size after complete removal of the guest as well as coordinated water molecules. The adsorption data were analyzed by the DubininRadushkevich (DR) equation,18 and the values of βE0, which reflect

Figure 4. Sorption isotherm for 10 3 H2O (298 K); inset (a) MeOH (293 K); (b) EtOH; and (c) CO2 (195 K). P0 is the saturated vapor pressure of the respective adsorbate at the corresponding temperature. (Open symbol indicates adsorption and half filled symbol indicates desorption curve).

Figure 5. Sorption isotherm for 2. (a) H2O (298 K); (b) MeOH (293 K); and (c) CO2 (195 K). P0 is the saturated vapor pressure of the respective adsorbate at the corresponding temperature. (Open symbol indicates adsorption and half filled symbol indicates desorption curve).

Scheme 1. Schematic Representation of the Structural Conversion of Dehydration-Rehydration Process

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Crystal Growth & Design, Vol. 10, No. 6, 2010

Gurunatha et al.

Acknowledgment. T.K.M. thanks DST, Government of India, for financial support (Fast track proposal). G.M. and D.G. thank JNCASR for the visiting fellowship programme. Authors are thankful to Dr. C.M. Nagaraja for the valuable suggestions during the revision of the manuscript. Supporting Information Available: Figures S1-S6 and X-ray crystallographic files in CIF format for 1, 10 , and 1a. This material is available free of charge via the Internet at http://pubs.acs.org.

References

Figure 6. Plot of χMT versus T and 1/χM versus T for 1. the adsorbate-adorbent interaction, are 4.8 kJ/mol (H2O sorption for 10 ), 3.1 kJ/mol for H2O, and 2.2 kJ/mol for MeOH, suggesting that 10 is rather hydrophilic in nature compared to 2. Magnetic Property. The variable temperature susceptibility (300-2 K) data were measured by a SQUID magnetometer under the applied magnetic field of 10 kOe. The temperature dependence of the χMT versus T is shown in Figure 6 and the observed χMT value for 1 at 300 K is 2.32 cm3 mol-1 K, slightly higher than the calculated value 2.1 cm3 mol-1K for noninteracting free ions per NdIII FeIII unit.5a Upon cooling, the χMT decreases continuously from 300 to 2 K and the value at 2 K is 1.10 cm3 mol-1 K. The χM-1 versus T is nearly a straight line above 45 K, which obeys the Curie-Weiss law. The Curie and Weiss constants, C and θ, are 2.54 cm3 mol-1 K and -31.31 K, respectively, based on the equation χM=C/(T - θ). The negative Weiss constant and the nature of the χMT versus T plot apparently indicate an overall antiferromagnetic interaction, but the deviation from the Curie law may have other origins than the magnetic interactions, as suggested by Kahn.19 Particularly, for elements such as NdIII (f 3) where spin-orbit coupling is significant, the deviation from the Curie law in 1 is not necessarily for antiferromagnetic interaction only. The variable temperature susceptibility measurement was also carried out for the dehydrated solid (2) (300-10 K). The χM versus T plot for 2 (Figure S6, Supporting Information) shows a similar nature as that of 1; however, the smaller θ value cannot be explained due to the structural transformation after the removal of the coordinated water molecules, which is realized by the PXRD pattern (Figure 3c).

Conclusion We have successfully synthesized a novel bimetallic (4f-3d) 3D porous supramolecular framework of Nd(III) connected by the inorganic “metalloligand” [Fe(CN)6]3-. The channel water molecules were selectively removed by controlled heating of the framework, which is accompanied by the significant structural contraction. The virgin framework regenerates upon exposure to the water vapor with structural expansion. Complete removal of guest as well as coordinated water molecules transform to another phase which exhibit selective sorption of water and MeOH molecules and oppose to CO2, driven by the coordinatively unsaturated NdIII on the structurally compressed pore surfaces. Therefore, this type of guest-induced reversible single-crystal-to-single-crystal structural transformation with framework contraction and expansion is one of the novel examples of framework flexibility and dynamicity realized by the supramolecular interactions, and may find applications in sensors, actuators, and separation of the guest molecules.

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