Guest−Host Frameworks of the Anionic Metal Complex [Fe(ox)3] and Cationic Bipyridinium-Based Linkers Bonded by Charge-Assisted Hydrogen Bonds

Reported is the synthesis and characterization of guest-free and guest-included frameworks assembled from the anionic metal complex [Fe(ox)3] 3‐ and the cationic diprotonated 4,4′-bipyridinium (H2bpy) or 1,2-bis(4-pyridinium)ethylene 2+ (H2bpye) linkers, where the complex and linkers are bonded by multiple charge-assisted hydrogen bonds. In some of the compounds additional barium cations serve as bridges between the anionic metal complexes forming −{[Fe(ox)3]2[Ba]}∞− anionic infinite chains. Aromatic guest molecules of 4methoxyphenol (mp), 1,6-dimethoxynaphtalene (dmn), and 1,5-dihydroxynaphthalene (dhn) were successfully incorporated in the cavities of the frameworks. The π−π interactions between the pillars and the guests in the resulting guest-included frameworks were confirmed spectroscopically. The magnetic properties of the frameworks were measured as well. ■ INTRODUCTION The host−guest inclusion compounds are a class of flexible (or soft) frameworks capable of hosting diverse guest molecules by adjusting bonding modes (angles, distances, etc.) and/or molecular conformations in order to create appropriate cavities for the guest. At the same time, these adjustments are such that the basic framework connectivity remains largely unchanged. All this makes such compounds interesting not only from a synthetic point of view but also as potential materials for catalysis, separation, and molecular recognition. One particular class of soft frameworks are those built of cations and anions that are held together by multiple chargeassisted hydrogen bonds. The combination of electrostatic interactions and hydrogen bonds strengthens the framework and provides additional stability. The typical design of such framework is based on cationic nodes, either organic molecules or metal complexes, that are connected by anionic organic linkers such as disulfonates. It has been shown by Ward et al. that such frameworks made of guanidinium cations and different disulfonate anions can accommodate a large variety of guest molecules. Later, the guanidinium cations were “replaced” by cationic metal complexes with ligands capable of similar hydrogen bonding such as amines, ammonia, and water. The potential advantage of this replacement is eventual redox capabilities as well as valuable magnetic properties of the resulting compounds. In addition to the frameworks with cationic nodes and anionic linkers, there is also a growing number of frameworks with reversed polarity, i.e., with anionic metal complexes and cationic linkers, that exhibit the same chargeassisted hydrogen bonding. One of the problems encountered early with using metal complexes was their typically high positive charge, e.g., [ML6] , where L is a neutral ligand. This, when combined with the lower negative charge of −2 for the disulfonate linkers, resulted in a high number of linkers per cation, 1.5, and thus smaller cavities. This was resolved in two different ways. One of them used modified metal complex with some anionic ligands and lower charge such as [Co(en)2(ox)] , for example. The second approach was to include small “spectator” anions such as Cl− in [Co(NH3)6][PIPES]Cl·6H2O (PIPES = 1,4piperazinebisethanesulfonate) that do not occupy valuable space and yet effectively reduce the positive charge. Both approaches result in the need of only one dianionic sulfonate pillar per node instead of otherwise one and a half. This, in turn, leaves larger interpillar galleries in the structure. These solutions to high charge issues are, of course, applicable to reversed charge systems. Thus, we recently reported host− guest frameworks made of the monoanionic metal complex [Co(en)(ox)2] − and diprotonated bipyridinium dications, i.e., [Co(en)(ox)2]2[H2bpy]·n(guest). 8 The charge of this complex is already reduced compared to [Co(ox)3] 3‐ in a similar way by replacing an anionic ligand with a neutral one. Continuing this work, we wanted to apply the second approach for “lowering” the charge of the homoleptic metal complexes [M(ox)3] 3‐ by introducing a small spectator cation. This was accomplished by incorporating Ba cations in the here reported frameworks made of [Fe(ox)3] 3‐ nodes and 4,4′-bipyridinium (H2bpy) or Received: February 29, 2012 Revised: April 2, 2012 Published: April 5, 2012 Article pubs.acs.org/crystal © 2012 American Chemical Society 2684 dx.doi.org/10.1021/cg300293j | Cryst. Growth Des. 2012, 12, 2684−2690 1,2-bis(4-pyridinium)ethylene (H2bpye) linkers and hosting 4-methoxyphenol (mp), 1,6-dimethoxynaphthalene (dmn), and 1,5-dihydroxynaphthalene (dhn) as guest molecules (Chart 1). ■ EXPERIMENTAL SECTION K3[Fe(ox)3]·3H2O was synthesized according to the literature. 9 4,4′Bipyridine (bpy, Sigma Aldrich), 1,2-di(4-pyridyl)ethylene (bpye, Sigma Aldrich), 4-methoxyphenol (Alfa-Aesar), 1,6-dimethoxynaphthalene (Alfa-Aesar), 1,5-dihydroxynaphthalene (TCI), and methanol (Fischer Scientific) were used as received without further purification. FT-IR spectra of the freshly prepared compounds in crystalline form were recorded on a Bruker TENSOR-27 FT-IR spectrophotometer in ATR mode in the 4000−400 cm−1 region. UV−vis spectra were recorded for solid samples on JASCO V-670 machine in reflectance mode in the 200−900 nm region. Synthesis of [H2bpy][Hbpy][Fe(ox)3]·2.5H2O (1). A solution of K3[Fe(ox)3]·3H2O (0.41 mmol) in 20 mL of water was mixed with 20 mL of an aqueous solution of 4,4′-bipyridinium dichloride (H2bpyCl2) (0.8 mmol) and the mixture was left undisturbed at room temperature to allow for slow evaporation. Light green colored block-shaped crystals of 1 were obtained in 2 days as a single phase. Yield = 85%. IR (cm−1): 1641, 1708, νCO, oxalate; 1604, 1384, νCC, 1486, νCN, 3510, νN−H, H2bpy. Anal. Calcd for C52H48N8O29Fe2: C, 45.9; H, 3.56; N, 8.24. Found: C, 44.21; H, 3.48; N, 8.02. Synthesis of [H2bpye][Hbpye][Fe(ox)3]0.88[Fe2(ox)5]0.12·2H2O(2). A solution of K3[Fe(ox)3]·3H2O (0.5 mmol) in 20 mL of water was mixed with 20 mL of aqueous solution of 1,2-di(4-pyridinium)ethylene dichloride (H2bpyeCl2) (0.8 mmol) and left undisturbed at room temperature to allow for slow evaporation. Light green blockshaped crystals of 2 were obtained in 2 days as a single phase. Yield = 80%. IR (cm−1): 1660 (s), 1706 (m), νCO, oxalate; 1625 (m), 1377 (m), νCC, 1504 (s), νCN, 3513 (m), νN−H, H2bpye. Anal. Calcd for C59.77H54N8O27.54Fe2: C, 49.97; H, 3.79; N, 7.80. Found: C, 48.77; H, 3.53; N, 7.64. Synthesis of Ba[H2bpye]2[Fe(ox)3]2·1.5(mp)·4.5H2O(3). Twenty milliliters of aqueous solutions of K3[Fe(ox)3]·3H2O (0.8 mmol) and H2bpyeCl2 (0.8 mmol) were mixed with 5 mL of methanol solutions of 4-methoxyphenol (mp) (1.5 mmol) and BaCl2 (1.0 mmol) and were then left undisturbed at room temperature to allow for slow evaporation. Dark-red, block-shaped crystals of 3 were obtained in 10 days as a single phase. Yield = 92%. IR (cm−1): 1651, 1707, νCO, oxalate; 1230, νC−O, mp; 1624, 1392, νCC, 1504, νCN, 3420, νN−H, H2bpye. Anal. Calcd for C372.8H352N32O251.2Fe16Ba8: C, 39.65; H, 3.14; N, 3.97. Found: C, 39.49; H, 3.06; N, 4.02. Synthesis of Ba[H2bpye]2[Fe(ox)3]2·(dmn)·3H2O(4). Twenty milliliters of aqueous solutions of K3[Fe(ox)3]·3H2O (0.82 mmol) and H2bpyeCl2 (0.8 mmol) were mixed with 5 mL of methanol solutions of 1,6-dimethoxynaphthalene (dmn) (1.5 mmol) and BaCl2 (1.0 mmol) and were then left undisturbed at room temperature to allow for slow evaporation. Dark-red, block-shaped crystals of 4 were obtained in 10 days. Yield = 55%. IR (cm−1): 1650, 1706, νCO, oxalate; 1221,νC−O, dmn; 1502, νCN,1396, 1624, νCC, 3454, νN−H, H2bpye. Anal. Calcd for C192H176N16O116Fe8Ba4: C, 41.48; H, 3.09; N, 4.03. Found: C, 39.76; H, 2.87; N, 4.04. Synthesis of Ba[H2bpye]2[Fe(ox)3]2·(dhn)·6H2O(5). Twenty milliliters of aqueous solutions of K3[Fe(ox)3]·3H2O (0.82 mmol) and H2bpyeCl2 (0.8 mmol) were mixed with 5 mL of methanol solutions of 1,5-dihydroxynaphthalene (dhn) (1.5 mmol) and BaCl2 (1.0 mmol) and were then left undisturbed at room temperature to allow for slow evaporation. Dark-red, block-shaped crystals of 5 were obtained in 10 days as a single phase. Yield = 90%. IR (cm−1): 1706, 1651, νCO, oxalate; 1278, νC−O, dhn; 1525, νCN, 1589, 1382, νCC, 3504, νN−H, H2bpye. Anal. Calcd for C184H176N16O128Fe8Ba4: C, 39.08; H, 3.14; N, 3.96. Found: C, 39.49; H, 3.64; N, 4.02. Synthesis of [H2bpy]3[Fe(ox)3]2·3(mp)·3H2O·MeOH (6). Twenty milliliters of aqueous solutions of K3[Fe(ox)3]·3H2O (0.42 mmol) and 4,4′-bipyridinium dichloride (0.6 mmol) were mixed with 5 mL of methanol solution of 4-methoxyphenol (1.0 mmol) and were then left undisturbed at room temperature to allow for slow evaporation. Red, block-shaped crystals of 6 were obtained in 10 days. Yield = 70%. IR (cm−1): 1660, 1702, νCO, oxalate; 1228,νC−O, mp; 1645, 1382, νCC, 1508, νCN, 3272, νN−H, H2bpy. Anal. Calcd for C127H124N12O67Fe4: C, 48.99; H, 4.01; N, 5.40. Found: C, 45.36; H, 3.01; N, 5.43. Structure Determination. Single crystal X-ray diffraction data sets were collected on a Bruker APEX-II diffractometer with a CCD area detector at 120 K (Mo Kα, λ = 0.710 73 Å). The crystals were taken from the mother liquid, dried in air, and covered with ParatoneN oil before inserting them in the cold stream. The structures were solved by direct methods and refined by full-matrix least-squares based on F using the SHELXL 97 program. All hydrogen atoms of the framework were refined as riding on the corresponding non-hydrogen atoms, while they were omitted for some disordered and solvent molecules. One of the two crystallographically independent organic linkers in 1 was refined as disordered equally among two sites in a slip fashion along with water molecules. Similarly, some of the linkers in 5 show disorder among two positions with 82 and 18% occupancies. Compound 2 exhibits a dinuclear formation [Fe2(ox)5] , where two Fe(ox)2 parts are bridged by the fifth oxalate. Such formations with bridging bis(bidentate) oxalates are well-known not only for iron but for a number of other transition metals. This dinuclear complex is refined with 12% occupancy and replaces two mononuclear [Fe(ox)3] 3‐ complexes that are positioned well apart from each other and are separated by a water molecule. This disorder causes disorder in one of the two independent linkers in the structure, which takes a minor position with the same 12% occupancy. Some of the guest molecules in the compounds are disordered as well. The 4-methoxyphenol in 3 shows some disorder, where some of the molecules are rotated at 180° and others have methoxy−hydroxy positional disorder. The 1,6dimethoxynaphthalene in 4 is disordered with 50% occupancy. In the structure 6, one guest 4-methoxyphenol molecule was disordered with a 63:37 ratio. Magnetic Measurements. The magnetic susceptibilities of pure polycrystalline samples were measured in the temperature range 10− 300 K on a Quantum Design MPMS SQUID instrument. The data were corrected for the diamagnetism of the sample holders and the constituent atoms. ■ RESULTS AND DISCUSSION The crystal structures of the guest-free frameworks 1 and 2 are topologically similar (Figure 1, Table 1). In both, the [Fe(ox)3] 3‐ anions are isolated from each other unlike the previously reported frameworks with [Co(en)(ox)2] −, which exhibit intercomplex hydrogen bonds involving the oxalate and ethylenediamine ligands. Instead, in 1 and 2 water molecules positioned between the complexes perform this role as hydrogen-bonded bridges between complexes. The structures can be described as infinite hydrogen-bonded chains of −H2bpy−Fe(ox)3−H2bpy− with each iron complex using two of its oxalates. Pairs of chains are then interconnected by water molecules that are hydrogen-bonded to the third oxalate ligands of two metal complexes from different chains thus Chart 1. Linkers and Guest Molecules Crystal Growth & Design Article dx.doi.org/10.1021/cg300293j | Cryst. Growth Des. 2012, 12, 2684−269