Novel Mn(II)-Based Metal–Organic Frameworks Isolated in Ionic

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Novel Mn(II)-Based Metal−Organic Frameworks Isolated in Ionic Liquids Ling Xu,†,‡ Young-Uk Kwon,*,‡ Baltazar de Castro,† and Luís Cunha-Silva*,† †

REQUIMTE & Department of Chemistry and Bichemistry, Faculty of Sciences, University of Porto, 4169-007 Porto, Portugal Department of Chemistry, BK-21 School of Chemical Materials Science, SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon, 440-176 Korea

‡

S Supporting Information *

ABSTRACT: An unprecedented series of Mn2+-based metal−organic framework (MOF) materials prepared and isolated in ionic liquids (ILs) is reported. Ionothermal reactions of Mn(OAc)2 with H3btc (benzene-1,3,5-tricarboxylic acid) in two groups of [rmi]X (rmi = 1-alkyl-3-methylimidazolium; r = ethyl or propyl, X = Cl, Br, or I) ILs produced three slightly different 3D MOFs formulated as [rmi][Mn(btc)] [r = ethyl (1), propyl (2), and (3)], whose architectures can be envisaged as (3,6)connected pyr topological nets. Compounds 1−3 are the preferred products when the metal center is half filled d-shelled Mn with the cations of ILs being [emi]+ or [pmi]+. The comparison of the ionothermal synthesized M-btc systems suggests a significant combinatorial influence of metal-direction and ILs’ cationic template, contrasting with subtle effect of ILs’ halides on the MOF structures.

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reports.10,14 Another effect that has had much less attention in the structures of MOFs isolated by ionothermal synthesis is the electronic configurations of metal centers. Zn2+ and Cd2+ both with d10-configuration are not influenced by the crystal field effect of the ligands, while the incompletely filled d-shell of Ni2+ (d8-configuration) has a mechanism to influence the geometry around the Ni-centers. In the case of Ni2+ centers, the metal atoms tend to form similar 3D architectures based on trinuclear Ni3 units,13 while in the Zn2+ and Cd2+, mononuclear based frameworks of type [rmi][M(btc)] are the dominant structures.12,14 Therefore, we are extremely interested in the investigation of the structural features of Mn-btc system, with half filled d-electron spherical distribution of Mn(II), which is immune to the crystal field effect in combination with ILs. To the best of our knowledge, such aspects are practically unexplored under ionothermal conditions. According to the general roles of crystal-engineering and selfassembly, the metal centers, the organic spacers, and the reaction pathways exert a pivotal influence on the diversity of

INTRODUCTION Ionothermal synthesis based on the use of ionic liquids (ILs) as solvents has received remarkable attention as a novel sustainable method to prepare new materials with unprecedented structures, such as zeolite, zeo-type compounds, and metal−organic frameworks (MOFs), because of the outstanding properties of the ILs: nonflammability, negligible vapor pressure, high polarity, and so forth.1−5 In the last years, enormous efforts have been devoted in the investigation of the role of ILs in the design and preparation of novel MOFs. Usually, ILs behave as solvents, structure-directional template, or charge-compensating groups in the reaction systems, thus the influence of both the cationic and anionic parts of ILs on the MOF structures have been studied in detail. More commonly, the cationic parts act as structural templates being located in the cavities of skeleton frameworks.6−11 Our previous systematic work concerning the preparation of MOFs based in the M-btc system (M = Zn, Cd, and Ni; H3btc = benzene-1,3,5tricarboxylic acid) in distinct ILs, reported an influence of anionic parts due to their hydrophilicity/hydrophobicity or nucleophilicity/basicity behavior combined with cationic template effect in the structure of the MOF materials,12,13 which has been also mentioned fragmentarily in other © XXXX American Chemical Society

Received: November 24, 2012 Revised: January 18, 2013

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Figure 1. Schematic representation of (a) the reactions investigated; (b) the crystal structures packing arrangements, with the ILs cations represented in yellow and the anionic 3D frameworks drawn in orange, pink and blue for 1, 2, and 3, respectively; and (c) the (3,6)-connected pyr topological network, with the binuclear [Mn2(COO)2] subunits considered as six-connected nodes and residual [Ph-(COO)2] subunit as threeconnected nodes. max-RC diffractometer with Cu-Kα radiation (λ = 1.5406 Ǻ , 30 kV and 40 mA) with a scan speed of 2 deg·min−1 and a step size of 0.02° in 2θ. Synthesis of MOF Materials. [emi][Mn(btc)] (1). Mn(OAc)2·4H2O (1.50 mmol, 0.3677 g) and H3btc (0.50 mmol, 0.1053 g) were placed in a 25 mL Teflon-lined stainless-steel autoclave and mixed with 1.005 g of [emi]Cl. The mixtures were kept inside the furnace at 180 °C for 3 days and then naturally cooled to room temperature. The pale red crystals of 1 suitable for single-crystal XRD analysis were collected after soak clearing with acetone. Selected IR data (in KBr, cm−1): 3460(s), 3172(w), 3101(w), 2982(w), 2932(w), 2362(w), 2341(w), 1628(s), 1560(m), 1539(m), 1509(m), 1458(m), 1368(m), 1163(m), 1312(w), 1299(w), 1112(w), 923(w), 756(m), 716(m), 695(w), 668(w). An identical experimental procedure using 2.00 mmol, 0.4987 g of Mn(OAc)2·4H2O and 0.50 mmol, 0.1055 g of H3btc in 1.006 g of [emi]Cl also originated the same material 1. Furthermore, the product 1 was also obtained with a similar ionothermal processes using 1.50 or 2.00 mmol of Mn(OAc)2·4H2O and 0.50 mmol, ∼0.105 g of H3btc in ∼1.000 g of [emi]X (X = Br and I). [pmi][Mn(btc)] (2). An identical precedure to that described for 1 was used; a series of experiments using 1.50 or 2.00 mmol of Mn(OAc)2·4H2O and 0.50 mmol of H3btc mixed with 1.0 mL of [pmi]Cl or [pmi]Br produced the compound 2 as a crystalline material. Selected IR data (in KBr, cm−1) for 2: 3418(s), 3233(m), 3153(m), 3104(m), 2968(w), 2919(w), 2380(w), 1624(s), 1576(m), 1428(w), 1375(m), 1255(w), 1166(m), 1093(m), 926(w), 764(w), 707(w), 616(m). [pmi][Mn(btc)] (3). Similar to the experimental procedure previously described for 1 and 2, using 1.5 mmol, 0.3675 g of Mn(OAc)2·4H2O and 0.50 mmol, 0.1057 g of H3btc mixed with 1.0 mL [pmi]I the compound 3 was isolated as crystalline material. Selected IR data (in KBr, cm−1) for 3: 3843(m), 3433(s), 3156(w),

the framework structures.15−17 Following our scientific interest in the preparation of multifuntional MOF materials,18−24 in particular using ionothermal synthesis,12−14 we investigated a novel series of Mn(II)-based MOFs isolated in ILs. Herein, the organic ligand/spacer (H3btc), the metal source [Mn(OAc)2] and the reaction conditions are maintained unchanged to explore the possible cooperation effects of Mn2+ and ILs on the structure of Mn-btc based MOFs. Mn(OAc)2 reacted with H3btc in two groups of ILs: [emi]X and [pmi]X (emi = 1-ethyl3-methylimidazolium, pmi = 1-propyl-3-methylimidazolium, and X = Cl−, Br−, I−) (Figure 1a), generating similar porous 3D MOFs [rmi][Mn(btc)] (rmi = [emi]+ for 1, [pmi]+ for 2 and 3) based on the same [Mn2(COO)2] and [Ph-(COO)2] building units with the corresponding cations of ILs located inside the cavities.

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EXPERIMENTAL SECTION

Materials and General Procedures. All chemicals were commercially purchased and used without further purification except most ILs {[emi]Cl (98%, Aldrich)}. The two groups of ILs were prepared according the methods reported in literature:25 [emi]Br (white solid, mp = 80−82 °C, yield = 92%), [emi]I (yellow solid, mp = 80−82 °C, yield = 72%), [pmi]Cl (pale yellow oil, yield = 60%), [pmi]Br (pale yellow oil, yield = 80%), [pmi]I (pale brown oil, yield = 78%). FT-IR spectra were obtained on a Bruker Tensor27 FT-IR spectrometer using KBr pellets prepared with sample powders, and measured in the wavenumber range of 4000−400 cm−1. Thermogravimetric (TG) analyses were carried on a TA4000/SDT 2960 instrument with a heating rate of 10 °C·min−1, and under N2 flow. Powder X-ray diffraction (XRD) data were recorded on a Rigaku D/ B

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3104(w), 2970(w), 2925(w), 2396(m), 2350(m), 2317(m), 1615(w), 1582(s), 1413(s), 1353(w), 1239(w), 1166(m), 1103(m), 1027(m), 941(m), 763(w), 713(w), 662(m), 612(w). Single-Crystal XRD. Crystalline materials of [emi][Mn(btc)] 1, [pmi][Mn(btc)] 2, and [pmi][Mn(btc)] 3 were harvested and a suitable single-crystal of each compound was mounted on adequate cryoloops.26 Data were collected at −100 °C (for 1 and 3) and at 23 °C (for 2) on a Bruker CCD diffractometer (Mo Kα graphitemonochromated radiation, λ = 0.7107 Å) with the acquisition being controlled by the APEX2 software package.27 Images were processed using the SAINT+ software package,28 and the integrated data sets were corrected for absorption using the multiscan method implemented in SADABS.29 The structures of all complexes were solved by the direct methods using SHELXS-97,30,31 which allowed the direct location of most of the heaviest atoms, with the remaining non-hydrogen atoms being located from difference Fourier maps calculated from successive full-matrix least-squares refinement cycles on F2 using SHELXL-97.31,32 All non-hydrogen atoms were successfully refined using anisotropic displacement parameters. Hydrogen atoms attached to carbons were located at their geometrical positions using appropriate HFIX instructions in SHELXL (43 for the aromatic, 23 for the CH2 groups, and 33 for the terminal methyl groups) and included in subsequent refinement cycles in riding-motion approximation with isotropic thermal displacements parameters (Uiso) fixed at 1.2 or 1.5 × Ueq of the carbon atom to which they are attached. Information concerning crystallographic data collection and structure refinement details is summarized below, while the selected bond lengths and angles for the Mn(II) coordination environments of the materials 1, 2, and 3 are systematized in Tables S2, S3, and S4 (Supporting Information, respectively. Crystal data for 1: formula C15H14N2O6Mn; Mr = 373.22; T = 173(2) K; crystal system orthorhombic; space group Pbca; a = 12.4378(6) Å, b = 14.7717(8) Å, c = 16.6097(9) Å; V = 3051.7(3) Å3; Z = 8; ρcalcd = 1.625 g·cm−1; crystal size 0.27 × 0.16 × 0.15 mm3; F(000) = 1528; μ(MoKα)cm−1 = 0.901 mm−1; 2.45 ≤ θ ≤ 33.74°; 3774 independent reflections (Rint = 0.0747); GOF = 1.086; final R indices were R1 = 0.0638 [I > 2σ(I)] and R2 = 0.1726 (all data); residual electron density 1.061/−0.983 e·Å−3. Crystal data for 2: formula C16H16N2O6Mn; Mr = 387.25; T = 296(2) K; crystal system orthorhombic; space group Pbca; a = 12.6991(5) Å, b = 15.5711(6) Å, c = 16.3313(6) Å; V = 3229.3(2) Å3; Z = 8; ρcalcd = 1.593 g·cm−1; crystal size 0.63 × 0.40 × 0.25 mm3; F(000) = 1592; μ(MoKα)cm−1 = 0.854 mm−1; 2.42 ≤ θ ≤ 34.89°; 3988 independent reflections (Rint = 0.0330); GOF = 1.027; final R indices were R1 = 0.0700, [I > 2σ(I)] and R2 = 0.2012 (all data); residual electron density 0.921/−0.903 e·Å−3. Crystal data for 3: formula C16H16N2O6Mn; Mr = 387.25; T = 173(2) K; crystal system orthorhombic; space group Pbca; a = 12.6358(8) Å, b = 15.2897(9) Å, c = 16.4183(10) Å; V = 3172.0(3)Å3; Z = 8; ρcalcd = 1.622 g·cm−1; crystal size 0.20 × 0.15 × 0.11 mm3; F(000) = 1592; μ(MoKα)cm−1 = 0.870 mm−1; 2.43 ≤ θ ≤ 30.20°; 3925 independent reflections (Rint = 0.1150); GOF = 1.098; final R indices were R1 = 0.0744, [I > 2σ(I)] and R2 = 0.1835 (all data); residual electron density 1.784/−0.814 e·Å−3. CCDC: (1)-711648, (2)-711649, and (3)-711645 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data via www.ccdc.cam.ac.uk/ data_request/cif. The topological motifs of compounds 1−3 were determinated using the ADS program of the TOPOS 4.0 Professional structure− topological program package.33−35

(2), and [pmi][Mn(btc)] (3) (Figure 1). Meticulous searches in the literature and in the Cambridge Structural Database (CSD, version 5.33, with four updates)36,37 revealed that the Mn2+-based MOF materials synthesized through ionothermal reactions are very scarce. In fact, to the best of our knowledge, there is only one previous example reported in the literature, [emi]2[Mn3(1,3-bdc)4] (where 1,3-H2bdc stands for 1,3benzene-dicarboxylic acid).38 The MOFs were isolated as crystalline materials allowing the determination of their structures by single-crystal XRD, and revealing 3D porous structures with the ILs anions occupying the channels. Furthermore, all the materials were characterized by powder XRD, TG analysis, and FT-IR spectroscopy. The experimental powder XRD patterns for compounds 1−3 confirm their crystallinity and the comparison with those simulated from single-crystal data, indicating that all three compounds were isolated as single phases (Figure S1, Supporting Information). TG analysis of 1-3 revealed that all the MOF materials have good thermal stabilities, since they start to decompose beyond 300 °C (Supporting Information Figure S2). The notable stabilities of 1−3 confirm the robustness of porous 3D frameworks, further reinforced by the strong interactions between the frameworks and the guest cations, whose existence in the channels contributes to the thermal stability of this class of compounds. The characteristic absorption peaks of the main functional groups in FT-IR spectra for 1−3 are listed in Supporting Information Table S1. The asymmetric stretching vibrations υas(COO−) were observed in the range of 1560−1628 cm−1 and symmetric stretching vibrations υs(COO−) in 1353−1458 cm−1. The above stretching vibrations shift to lower values, compared to the carbonyl frequencies of free H3btc ligand. The difference Δ(υas(COO−)−υs(COO−)) were around 200 cm−1, characteristic for coordinated carboxylate groups.39−41 The peaks for 1− 3 from 2919 to 2982 cm−1 are attributed to C−H stretching vibrations.42 Those around 1165 cm−1 can be assigned as Nalkyl−C stretching of the imidazolium rings.43,44 Compound 1, [emi][Mn(btc)], features a porous 3D MOF, with the asymmetric unit (asu) comprising one Mn2+ center, one btc3− ligand and one [emi]+ cation. As shown in Figure 2a, Mn1 center is surrounded by five carboxylic O-atoms from four crystallographic equivalent btc3− ligands, displaying a coordination geometry that resembles a distorted square pyramid [τ = 0.394; τ = (β−α)/60, where α and β are the two biggest bond angles around Mn(II) center; τ = 0 for ideal square pyramid; and τ = 1 for ideal trigonal bipyramid]:44 O13A, O14B, O15C, and O16C are in the equatorial positions [dMn1−O = 2.107(3)− 2.477(4)Å] and O11 is in the axial position [dMn1−O1 = 2.022(4) Å] (for details of the distances and angles of the Mn1 coordination environment see Supporting Information Table S2). The MOF structure is generated from two kinds of building units: 8-membered binuclear units [Mn2(COO)2] formed by the connection of two Mn1 atoms through two bidentate oxygen atoms (O13 and O14), and 32-membered rings units [Mn4(btc)4] built by the association of four Mn1 atoms via four antiparallel btc3− ligands. The 32-membered rings share common edges to produce a 2D layer in a −ABAB− fashion along the bc plane of the unit cell, with the 8-membered rings between them (Figure 2b). The neighboring 2D layers are further bridged via monodentate oxygen atoms O11 to generate a 3D framework with channels along the c-direction (Figure 1b). The [emi]+ cations are located inside the channels and establish strong interactions with the negatively charged

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RESULTS AND DISCUSSION A novel series of Mn2+-based metal−organic framework (MOF) materials were isolated by ionothermal reactions of Mn(OAc)2 with H3btc (benzene-1,3,5-tricarboxylic acid) in two groups of [rmi]X (rmi =1-alkyl-3-methylimidazolium; r = ethyl or propyl, X = Cl, Br, or I) ILs leading to three slight different 3D MOFs formulated as [emi][Mn(btc)] (1), [pmi][Mn(btc)] C

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Figure 2. (a) Coordination environment of Mn1 in compound 1, showing the labbeling scheme for all the atoms of the first coordination sphere and the asu. (b) 2D layer extended in the [100] direction of the unit cell. H-atoms were omitted for clarity reasons. Color scheme: Mn, light blue; O, red; N, blue; C, gray.

Figure 3. (a) Coordination environement of Mn1 in compound 2, with labeling scheme for all the atoms of the first coordination sphere and the asu. (b) 2D layer extended in the [100] direction of the unit cell. H-atoms were omitted for clarity reasons. Color scheme: Mn, light blue; O, red; N, blue; C, gray.

framework. The minimum distance between the centroids of [emi]+ and carboxylic group (O15−C19−O16) is 3.220 Å, indicating the existence of strong electrostatic interactions. The location of [emi]+ in the cavities is also strengthened by C5− H5A···O15 (2.764 Å, 127.9°) hydrogen bonding interactions. In the MOF 2, [pmi][Mn(btc)], Mn1 atom lies in the center of a five-coordinated distorted square pyramid (Figure 3a, details of the distances and angles involving the Mn1 coordination environment are systematized in Supporting Information Table S3), with the 2D layers also generated by two kinds of similar [Mn4(btc)4] and [Mn2(COO)2] subunits via the same arrangement observed in 1 (Figure 3b). The final 3D architecture with channels is also constructed by the Mn1− O11 bridges. The [pmi]+ cations are located in the channels along the c-direction (Figure 1b), which interacts with the negative framework from electrostatic interactions (dO13−C19−O14···Cg = 3.508 Å; Cg represents the centroid of the [pmi]+ cation) and weak C−H···O hydrogen bonds (dC1−H1A···O14 = 2.528 Å and angle C1−H1A···O14 = 119.4°). MOF materials 2 and 3 are polymorphic compounds with the molecular formula [pmi][Mn(btc)], and the asu of compound 3 contains the same components as 2. However, regarding the structural features, compound 3 reveal considerable differences of 2, such as the coordination geometry of Mn center, coordination mode of the btc3− ligand, and the

formation fashion of the 3D framework from the 2D layer. As shown in Figure 4a, Mn1 center is surrounded by six carboxylic oxygen atoms from four crystallographic equivalent btc3− ligands to display a distorted octahedral geometry with O12A, O13B, O14B, and O16C (dMn1−O = 2.122(4)− 2.462(5) Å) in the equatorial positions and O11, O15C in the axial positions (dMn1−O = 2.201(4)−2.186(6) Å) (for more details about the distances and angles around the Mn1 coordination center see the Supporting Information Table S4). The btc3− ligands adopt the bidentate and chelating modes linking Mn centers and leading to the formation of [Mn4(btc)4] and [Mn2(COO)2] building units. The interconnection of these two kinds of units with same edges by −ABAB− fashion generates 2D layers extended in the ab plane of the unit cell (Figure 4b). The adjacent 2D layers are bridged via chelating oxygen atoms O15 and O16 to generate an anionic porous 3D framework with channels along the b-direction, in which the [pmi]+ cations are located and further interacting with the network by C2−H···O16 (2.936 Å, 111.8°) hydrogen bonds (Figure 1b). Compounds 1−3 feature 3D frameworks that result of the interconnection of a 8-membered binuclear ring [Mn2(COO)2] (formed by two crystallographic equivalent Mn1 atoms interconnecting two bidentate COO− bridges from two btc3− D

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membered [Mn4(btc)4] ring at four corners, leading to a 2D layer. The neighboring 2D layers are further bridged via carboxylic oxygen atom(s) to generate the same type of anionic 3D framework with the corresponding cations of ILs occupying the respective cavities (Figure 1b). Besides the template effect of the cations (the cavities of the materials with [emi]+, 1, is clearly smaller than those with [emi]+, 2 and 3; Supporting Information Figure S3), they also interact with the negative skeleton frameworks to further stabilize the structures, contributing to their high thermal stabilities which are demonstrated in the TG analysis (Supporting Information Table S2). If the binuclear [Mn2(COO)2] subunits are considered as six-connected nodes and residual Ph-(COO)2 subunit as three-connected nodes, the topological networks of 1−3 can be best described as (3,6)-connected pyr networks (Figure S4 in Supporting Information).33−35,45 This topological network was already reported for other transition metal-btc based MOFs prepared by ionothermal synthesis10,12,46,47 and other methods. 48 Although the aforementioned overall structural features of compounds 1−3 are similar, there are still some structural differences: (i) the connectivity fashions of btc3− ligand in 1 and 2 adopt the same monodentate, bidentate and chelating pattern (Supporting Information Scheme S1c), while 3 employs a bidentate and bis-chelating mode (Supporting Information Scheme S1b); (ii) the unique Mn1 centers in both compounds 1 and 2 are five-coordinated with distorted square pyramidal geometry, whereas in 3, it is sixcoordinated with the geometry resembling a distorted octahedron (Figures 1−3); (iii) in 1 and 2, the adjacent 2D layers are interconnected into 3D architectures by monodentate carboxylic oxygen atoms O11, but in 3, the neighboring layers are linked through a chelating COO− connection (O15 and O16). As consequence of the coordination described there are formation of porous 3D MOFs with the channels fully occupied by the ILs cations: [emi]+ in 1, [pmi]+ in 2 and 3. The dimension of the cavities that accommodate the [pmi]+ become slightly larger than those with [emi]+: 6.2 × 7.4 Å2 for 1, 6.3 × 7.8 Å2 for 2, and 6.3 × 8.2 Å2 for 3 (Supporting Information: Figure S3), in accordance with the order of the void volumes calculated by Platon49 after removing the contribution of respective [rmi]+: 46.3% (1), 48.7% (2) and 51.4% (3). Furthermore, the packing arrangements reveal distinct fashions

Figure 4. (a) Coordination sphere of Mn in compound 3, showing the labeling scheme for all the atoms of the first coordination sphere and the asu. (b) 2D layer extended in the [001] direction of the unit cell. H-atoms were omitted for clarity reasons. Color scheme: Mn, light blue; O, red; N, blue; C, gray.

moieties) and a 32-membered ring [Mn4(btc)4] (built up by four symmetry related Mn1 atoms alternately linking four antiparallel btc3− ligands). They assemble together through four 8-membered [Mn2(COO)2] rings sharing edges with one 32-

Figure 5. Schematic representation of (a) the reactions between M(Zn/Cd/Mn) salts and btc3− ligand; (b) the coordination sphere of Zn (top) in [pmi][Zn(btc)] and dinuclear M units (bottom); (c) the 3D framework of [pmi][Zn(btc)] (top) and other [rmi][M(btc)] compounds with the corresponding cations omitted in the cavities. E

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Crystal Growth & Design and orientations of [rmi]+ in the cavities: in 1, [emi]+ points its −CH3 outward and the −C2H5 groups inward; while in 2 and 3, [pmi]+ shows a opposite orientation, with −CH3 inwardly and −C3H7 outwardly (Supporting Information Figure S3). In fact, these observations confirm the template-direction role of the ILs cations in the structure of [rmi][Mn(btc)] (1−3) MOFs. On the other hand, comparing the compositions and structural details of the three MOFs (1−3), we infer that despite the influence of the halides (anions) in the reaction environments possibly through their nucleophilicty and/or basicity,13 they are not included into the geometries of metal centers neither in the structure, pointing to a minor role of the ILs halides in the formation and main structural features of the isolated MOF materials. Meticulous searches in the literature and in the Cambridge Structural Database (CSD, version 5.33, with four updates)36,37 about MOF materials prepared through ionothermal synthesis (Supporting Information Table S5), revealed that the H3btc is the most extensively explored ligand. In the M-(H)btc based frameworks, the most common metal center investigated is Co2+ (accounted 8 of 26) with diverse MOF structural features. Furthermore, it can be also found that almost half of them (12 of 26) used [emi]+-containing ILs and approximately of a quarter (7 of 26) used ILs with [pmi]+. In the two kinds of the M-(H)btc systems containing [emi]+ or [pmi]+ cation is observed the compounds with the general formula [rmi][M(btc)] are the preferred products (8 of 19, 4 with [emi]+ and 4 with [pmi]+). This tendency becomes more evident when metal ions are limited in full or half full filled d-shells (d10-Zn2+, Cd2+; d5-Mn2+). The formula of all [pmi]+-containing and threequarters of the [emi]+-containing M-btc systems is [rmi][M(btc)], in which the btc3− ligands adopt the resembling μ4 coordination fashions (Supporting Information Scheme 1 and Table S6). [pmi][Zn(btc)] represents an exception from this general behavior (Figure 2). The origin of the tendency to the formation of MOF with general formula [rmi][M(btc)] systems is traceable to the metal centers with full and half full filled d-shells, which is well sustained by the combination of the influence of metal-direction and the template effect of ILs cations.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

Article

S Supporting Information *

Additional figures including powder XRD patterns, TG analysis profiles and representation of the topologies for the three MOF materials; tables with bond distances (Ǻ ) and angles (deg) of compounds 1−3, a table systematizing all the compounds synthesized by ionothermal reactions, the coordination fashions of btc3− ligand in ionothermally synthesized M-btc compounds; the CIF files. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected] (L.C.-S.); [email protected] (Y.-U.K.). Phone: +351 220402576 (L.C.-S.); +82 312907070 (Y.-U.K.). Fax: +351 220402659 (L.C.-S.); +82 312907075 (Y.U.K.). Author Contributions

The manuscript was written with contributions of all authors, and all authors approved to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Korean Priority Research Center Program (NRF-2011-0031392) and Basic Science Research Program (NRF-2011-0006268). The authors also acknowledge the Fundação para a Ciência e a Tecnologia (FCT, MEC, Portugal) for their general financial support through the strategic project Pest-C/EQB/LA0006/2011 (to Associated Laboratory REQUIMTE), the R&D project PTDC/CTM/ 100357/2008 and the postdoctoral research grant SFRH/BPD/ 73415/2010 (to L.X.).

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REFERENCES

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CONCLUDING REMARKS In summary, the ionothermal reactions of Mn-btc systems in two groups of ILs, [emi]X and [pmi]X (X = Cl, Br or I), have been investigated and explored, with the chemical composition and reaction conditions maintained unalterable. The two series of experiments produce three similar porous 3D MOF materials, generaly formulated as [emi][Mn(btc)] (1) and [pmi][Mn(btc)] (2 and 3), with the respective ILs cations occupying the cavities. These three unprecedented compounds represent a very rare case of Mn(II)-based MOFs isolated in ILs, and their crystal structures can be simplified into the same (3,6)-connected pyr topological network. The observations of all compounds synthesized in ionothermal conditions show that [rmi][M(btc)] is the preferred product when the metal center is full or half filled d-shell with the cations of ILs being [emi]+/ [pmi]+. Our work suggests a significant influence of metaldirection combined with a notable template effect of ILs’ cations in the MOF formation and structure, in contrast with a subtle effect of halides through their nucleophilicty or basicity. F

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