Chloride Encapsulation by a Tripodal Tris(4-pyridylurea) Ligand and

Jan 22, 2014 - Anion coordinated capsules and pseudocapsules of tripodal amide, urea and thiourea scaffolds. Sandeep Kumar Dey , Arghya Basu , Romen ...
4 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/crystal

Chloride Encapsulation by a Tripodal Tris(4-pyridylurea) Ligand and Effects of Countercations on the Secondary Coordination Sphere Rui Zhang,† Yanxia Zhao,† Jiamin Wang,‡ Liguo Ji,† Xiao-Juan Yang,† and Biao Wu*,†,§ †

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an 710069, China ‡ State Key Laboratory for Oxo Synthesis & Selective Oxidation, Lanzhou Institute of Chemical Physics, CAS, Lanzhou 730000, China § State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China S Supporting Information *

ABSTRACT: A series of anion complexes of the 4-pyridyl-functionalized tripodal tris(urea) receptor (L) have been synthesized. Ligand L forms the 1:1 anion complex [Cl⊂L]− with various metal chloride salts, MClx (M = Na, K, Mg, Ca, Mn, Co, x = 1 or 2). When M = Na, K, Mg, and Ca, the metal ions are not coordinated by the pyridyl groups of L but are involved in second-sphere coordination to form three-dimensional structures. However, in the complex of Co2+, the transition metal ions are directly coordinated by the pyridyl groups. Interestingly, the Mn2+ ion forms two complexes with both of the above two types of structure. In all complexes, one chloride ion is “half” encapsulated in the cleft of one ligand by N−H···Cl hydrogen bonds to form the [Cl⊂L] units, which are further linked via intermolecular interactions into three-dimensional structures. Moreover, the fluoride and carbonate complexes of L have also been obtained. The solution anion binding properties of L have been studied by 1H NMR spectroscopy and electrospray ionization mass spectrometry.

■

ligands. In addition, Das et al.9i discussed the binding of halides and oxoanions by a protonated tris(amide) receptor, and Ghosh et al.9b reported a pentafluorophenyl-substituted tripodal amine receptor that encapsulates a Cl− or Br− ion within the cleft upon protonation of the secondary amines. Furthermore, the encapsulation of Cl− ion by a tris(amide) analogue was also reported.9j Very recently, Gale et al.11g reported the effects of fluorination of tris(thio)urea receptors on the transmembrane transport of chloride through a lipid bilayer. Our recent research has focused on anion coordination by urea-based receptors.12 The 3-pyridyl-substituted tripodal tris(urea) (L3‑py) has been synthesized, which can selectively encapsulate sulfate ion by one or two receptor molecules.11b,12a,b Furthermore, we have modified the tripodal tris(urea) with the redox-active ferrocenyl groups as electrochemical reporting units (LFc),12c−e as well as with quinolinyl groups (LQn)12f or Ru(bipy)3 moieties12g for the fluorescent signaling purposes. It is noticeable that, although the terminal 3-pyridyl groups in the ligand L3‑py (or the 4-pyridyl analogue L)12a,h are potentially metal coordination sites, they do not coordinate to any transition metal (Mn2+, Zn2+, Cd2+, Co2+) or Mg2+ ions in the capsular sulfate complexes; instead, the metal ions exist as

INTRODUCTION Since Park and Simmons discovered in 1968 that katapinands can encapsulate a chloride ion,1a which was structurally confirmed in 1975,1b the supramolecular chemistry of anions has attracted much attention because of the relevance of anions in chemistry, biology, and environmental events.2 The chloride ion plays pivotal roles in biological processes, for example, in signal transduction or transport of organic solutes through the cell membrane. A number of cyclic and acyclic hosts, such as triazolophanes or aryl-triazole systems, macrobicyclic and macrotricyclic amines, octaaminocryptands, and oligoureas, have been reported to show very high binding affinities for halide species.1a,3−7 The tris(2-aminoethyl)amine moiety was first introduced into the framework of anion hosts by Reinhoudt and coworkers.8 This tripodal structure has proven to be a promising scaffold for anion binding due to its favorable conformation for multiple hydrogen bonds, and many tren-based receptors with a variety of anion-binding sites such as amine, amide, urea, and thiourea groups, etc. have been synthesized.9−12 Such receptors not only can complementarily encapsulate oxoanions (sulfate, carbonate, and phosphate) but also display excellent binding abilities to the spherical halide ions. For instance, Beer et al.9a reported that a tripodal tris(amido-benzo-15-crown-5) ligand can cooperatively bind chloride, iodide, or perrhenate anions, while Fabbrizzi et al.10a studied the incipient and definitive proton transfer of urea-fluoride interaction with tripodal © 2014 American Chemical Society

Received: September 8, 2013 Revised: January 10, 2014 Published: January 22, 2014 544

dx.doi.org/10.1021/cg401345z | Cryst. Growth Des. 2014, 14, 544−551

Crystal Growth & Design

Article

the hydrated [M(H2O)6]2+ form and interact with the ligand in the second coordination sphere.11b,12a Similarly, in the sulfate complexes of L3‑py with the alkali metal Li+, Na+, and K+ ions, these metal ions also interact with the pyridine N groups through the hydrated [Li2(H2O)2]2+, [Na(H2O)4]2+, and [K2(H2O)2]2+ units, respectively.11c,f In the current work, we report the anion binding properties of the 4-pyridyl-substituted tripodal tris(urea) L (Scheme 1).

environment of the metal cations, either the alkali and alkaline earth metals or transition metals, is significantly different, thus leading to interesting aggregations of the cationic units and anionic capsules as discussed below. It should be noted that the manganese complexes 5A and 5B were isolated from the same tube, but they have significantly different unit cell parameters and structures (vide infra). The negative-ion mode electrospray ionization mass spectrometry (ESI-MS) spectra of these complexes (Figure S1, Supporting Information) display anionic peaks at m/z 541.2 for [L + Cl]−, which further prove the 1:1 binding of chloride ion in solution. In addition, we have also studied the coordination of L with other anions, and the complex with another halide F−, K(H2O)3[F⊂L] (7), as well as two complexes of carbonate, M2(H2O)4[CO3⊂L2] (M = Na, 8; K, 9), were isolated. The fluoride complex 7 is isomorphous to complexes 1 and 2 (space group Pa3̅) (Table S3, Supporting Information). The complexes with CO32− ion (8 and 9) show the typical capsular structure in which two inversion-symmetric L molecules encapsulate a carbonate anion, as in the case of the sulfate encapsulation.12h Crystal Structure of Complexes M(H2O)3[Cl⊂L] (M = Na, 1; K, 2). The complexes 1 and 2 comprise of anionic [Cl⊂L]22− and cationic [M2(H2O)6]2+ subunits interconnected by hydrogen bonds into a three-dimensional framework. In the anionic “half capsule” [Cl⊂L]− (Figure 1a), there are six hydrogen bonds between the encapsulated Cl− ion and urea NH protons.13 Notably, two [Cl⊂L] units dimerize in a staggered manner to form a “full capsule” containing two ligands and two anions (Figure 1b). Correspondingly, the alkali metal ions (Na+, K+) are also paired up by six bridging water molecules to form the [M2(H2O)6] dimer. Each alkali metal ion is further

Scheme 1. Structure of Ligand L

The receptor readily forms the 1:1 anion complex with a series of metal chloride salts MClx (M = Na, K, Mg, Ca, Mn, Co; x = 1 or 2), in which the chloride ion is encapsulated in the tripodal cleft of the ligand. Interestingly, in the manganese and cobalt complexes, the 4-pyridyl groups of L directly coordinate to the metal ion, forming the octahedral [M(N)6]2+ metal complexes besides the anionic [Cl⊂L] “half capsule”, which is significantly different from the 3-pyridyl substituted receptor (L3‑py). Moreover, the fluoride complex shows a similar 1:1 structure, while the carbonate ion (CO32−) is encapsulated in the 2:1 (host to guest) fashion as in the case of the sulfate ion. The anion binding of L in solution was also studied by 1H NMR spectroscopy.

■

RESULTS AND DISCUSSION Syntheses and Crystal Structures. The receptor L was synthesized from the reaction of pyridyl isocyanate with tris(2aminoethyl) amine (molar ratio 3:1) under a nitrogen atmosphere.12h The binding of L with various chloride salts was tested, and crystals of seven chloride complexes (1−6) were obtained by slow diffusion of diethyl ether into the CH3OH/H2O solution of the ligand and an equimolar amount of NaCl, KCl, MgCl2·6H2O, CaCl2, MnCl2·4H2O, or CoCl2· 6H2O. X-ray diffraction and elemental analysis revealed the compositions of complexes 1−6 to be M(H2O)3[Cl⊂L] (M = Na, 1; K, 2), M(H2O)6[Cl⊂L]2 (M = Mg, 3; Ca, 4; Mn, 5A) and M[Cl⊂L]2, (M = Mn, 5B; Co, 6), respectively. The main structure of these anion complexes is similar, characterized by a [Cl⊂L]− “half capsule” in which a chloride ion is encapsulated by the three arms of one tripodal ligand. All of the NH groups point to the inside of the cleft and form N−H···Cl hydrogen bonds with the Cl− ion (N···Cl distances range from 3.1 to 3.6 Å and N−H···Cl angles from 143° to 167°; Table S1, Supporting Information). These compounds are only sparingly soluble in methanol and DMSO and are insoluble in other common organic solvents. Interestingly, the countercation has minimum effects on the chloride-encapsulating [Cl⊂L] moiety but greatly alters the whole structure of the complexes, which can be categorized to three isomorphous groups: 1 and 2 (Pa3̅); 3, 4, and 5A (R3̅); and 5B and 6 (P3̅). In these structures, the coordination

Figure 1. Crystal structure of 1. (a) Molecular structure of the [Cl⊂L]− anion complex showing the encapsulation of a chloride ion in L by six N−H···Cl hydrogen bonds. (b) The [Cl⊂L]22− dimer. (c) Pseudo-octahedral arrangement of six [Na2(H2O)6]2+ clusters around the [Cl⊂L] 2 2− capsule. (d) Coordination environment of [Na2(H2O)6]2+ unit showing the six bridging water molecules between two Na+ ions and the interaction of Na+ ion with urea carbonyl groups. 545

dx.doi.org/10.1021/cg401345z | Cryst. Growth Des. 2014, 14, 544−551

Crystal Growth & Design

Article

Figure 2. Crystal structure of 3. (a) Interaction of [Cl⊂L]22− with six [Mg(H2O)6]2+ cations through second-sphere coordination, (b) one [Mg(H2O)6]2+ at the center of an octahedron formed by six [Cl⊂L]22− capsules, (c) hydrogen bonding between the hydrated cation and the surrounding ligands.

Figure 3. Crystal structure of complex 6. (a) The [Cl⊂L] anionic unit and three surrounding [Co(N)6]2+ cations. (b) The coordination sphere of the transition metal cations with six pyridyl groups. (c) The Co atom sits at the center of a pseudo octahedron formed by six [Cl⊂L] units.

cavity,9b while the tripodal hexaamide receptor encapsulates Cl− ion in its cleft through three hydrogen bonds.9j However, for the protonated 3,5-dinitrobenzoyl-based tris(amide) receptor, the Cl− ion is not encapsulated in the cleft but interacts with the amide functionality of each side arm.9i Theoretical studies have also been carried out, which reveal that the p-cyanophenyl-based tripodal tris(urea) receptor can include Cl− ion inside the cavity by six N−H···Cl interactions.13 Recently, Gale et al.11g reported the tris-urea and -thiourea receptors with fluorinated aryl groups, which encapsulate Cl− ion by six N−H···Cl hydrogen bonds (3.200(2)−3.600(2) Å) similar to the complexes in the present work. These results demonstrate the ability of tripodal receptors to bind chloride ions in different environments. Crystal Structure of Complexes M(H2O)6[Cl⊂L]2, (M = Mg, 3; Ca, 4; Mn, 5A). Complexes 3, 4, and 5A with the divalent alkaline earth metal (Mg2+ and Ca2+) or transition metal (Mn2+) cations are isomorphous. Like in 1 and 2, two chloride “half capsules” are dimerized, and each chloride is bonded by three urea groups through six hydrogen bonds. This aggregation of two [Cl⊂L] units is similar to the sulfate- and phosphate-encapsulating complexes.10f−h,11b,12a,g For example, Das et al.10c reported that two tris(urea) receptor molecules can pack in a face-to-face fashion with an encapsulated sulfate(H 2 O) 3 -sulfate adduct. Ghosh et al. 10b reported the

coordinated by three direct M−O bonds to the carbonyl groups of the ligand (Figure 1c,d). Thus, the Na+ or K+ ions are essentially nine-coordinate. Moreover, the [M2(H2O)6] dimer contacts with six pyridyl N donors in the secondary coordination sphere through 12 N···H−Ow hydrogen bonds (Figure 1d). Each [Cl⊂L]22− capsule is surrounded by six M2(H2O)6 dimers, which are arranged in a pseudo-octahedral geometry (Figure 1c). The anion−cation aggregation in 1 and 2 is different from the sulfate complexes of L3‑py and L, in which the M(H2O)62+ cation provides six chelating water molecules, forming a three-dimensional framework with octahedral connectivity.11b,12a It is also different from the clusters Na2(H2O)42+ and K2(H2O)22+ in the sulfate complexes of L3‑py. In the former case of sodium ion, the cationic Na2(H2O)42+ unit provides four water hydrogen-bond donors, and the two Na+ cations are linked by two hydrogen-bonded water dimers, whereas in the latter compound two K+ ions are bridged by two water molecules and the clusters act as pseudooctahedral 6-connecting nodes linking six pyridylurea chelating groups.11f The crystal structures of some chloride complexes of tripodal ligands have been reported recently. The triply protonated pentafluorobenzyl-substituted tris(amine) receptor can encapsulate one of the three halide counterions inside the tripodal 546

dx.doi.org/10.1021/cg401345z | Cryst. Growth Des. 2014, 14, 544−551

Crystal Growth & Design

Article

[H2PO4]22− dimer encapsulation by the pentafluorophenylsubstituted tripodal tris(urea) via hydrogen bonding and anion···π interactions. In complexes 3−5A, the two half capsules have a separation of 11.22−11.33 Å between the two bridgehead N atoms (Table S2, Supporting Information) and a Cl···Cl separation of 3.44−3.46 Å. As in the sulfate complexes of L and the analogous ligand L3‑py,11b,e,12a,h the metal ions (Mg2+, Ca2+, Mn2+) in 3, 4, and 5A are not coordinated by the pyridine N donors of ligand L but exist as the hexahydrated [M(H2O)6]2+ units around the [Cl⊂L]22− cage. The hydrated cations are involved in secondsphere coordination with the 4-pyridyl or carbonyl groups of the ligand through O−Hw···N (2.782 Å) and O−Hw···O (2.839 Å) hydrogen bonding. Each [Cl⊂L]22− cage is located at the center of an octahedron formed by six [M(H2O)6]2+ cations (Figure 2a), whereas each [M(H2O)6]2+ cation is also surrounded by an octahedron formed by six [Cl⊂L]22− moieties (Figure 2b). Such an arrangement leads to a threedimensional NaCl-type structure, which is essentially the same as in the case of the 2:1 sulfate complexes, whose [SO4⊂L2]2− components are replaced by [Cl⊂L]22− in the chloride complexes 3−5A. It is worth noting that, unlike the sulfate complex of L3‑py, in which the full capsule was fixed by the same H2O molecule of the [M(H2O)6]2+ unit, in complexes 3−5A each capsule is linked by two different H2O molecules of the [M(H2O)6]2+ unit through a pyridyl group and a urea CO from another ligand (Figure 3c). In other words, each water molecule of the [M(H2O)6]2+ unit interacts with the urea C O and pyridyl N from two different capsules. Crystal Structure of Complexes M[Cl⊂L]2, (M = Mn, 5B; Co, 6). Notably, yellow prism crystals of another manganese chloride complex with the composition Mn(N)6[Cl⊂L]2 (5B) were isolated from the same tube where the light-yellow cubic crystals of 5A were obtained. Complex 5B and the cobalt(II) analogue 6 show isomorphous structure. Different from complexes 1−5A, the chloride ion is bound by three stronger (N−H···Cl < 3.5 Å) and three weaker (3.5 < N−H···Cl < 3.6 Å) hydrogen bonds. The most striking feature of the structure is that the transition metal ions are directly coordinated by the pyridyl groups of the ligand. Each Mn or Co atom is surrounded by six pyridyl N atoms from six different ligand molecules in an octahedral coordination environment (Figure 3) with M−N distances of 2.3048(3) Å (M = Mn, 5B) and 2.2161(2) Å (M = Co, 6), respectively. Both the [Cl⊂L] units and the [M(N)6]2+ cations are located at the C3 axes and are linked to each other by the M−N coordination bonds (Figure 3b). Viewed down the c axis, three [M(N)6]2+ units are arranged in a regular triangle (side length Mn−Mn: 13.40 Å or Co−Co: 13.40 Å), and a [Cl⊂L] unit is located at the center of the triangle (Figure 3a). On the other hand, each [M(N)6]2+ cation sits at the center of a pseudo octahedron formed by six [Cl⊂L] units (Figure 3c). Crystal Structure of Complex K(H2O)3[F⊂L] (7). The crystal structure of the fluoride complex 7 is very similar to the chloride analogues M(H2O)3[Cl⊂L] (M = Na, 1; K, 2). The fluoride ion is “half” encapsulated in the cleft of the ligand through six N−H···F hydrogen bonds (N···F, 2.85−3.17 Å; ∠N−H···F, 146−161°; Figure 4a). The potassium ions are also paired up by six bridging water molecules to form the [K2(H2O)6] dimer and interact with L through the CO and pyridine nitrogen atoms. Compared to the chloride complex 2 (with same countercation), the smaller fluoride ion leads to a more contracted capsule (the size of the

Figure 4. Comparison of the dimerized anion capsule in 7 (a, fluoride) and 2 (b, chloride). The distances are from bridgehead N to anion or between the two bridgehead N atoms.

dimerized “full capsule” in 7 is smaller than the chloride analogue 2; distance between the two bridgehead N atoms: 11.07 versus 11.25 Å; Figure 4). Similar fluoride encapsulation was reported for a tripodal tris(ferrocenylurea)12c and a pentafluorophenyl-substituted tris(urea) receptor.10d The hydrogen-bond lengths in 7 (mean N···F: 3.012 Å) are slightly longer than those in these reported compounds (2.834 and 2.814 Å, respectively) and the N−H···F angles are comparable (154°, 149° and 153°, respectively). Crystal Structure of Complexes M2(H2O)4[CO3⊂L2] (M = Na, 8; K, 9). The complexes [M2(H2O)4]·[CO3⊂L2] were obtained by slow diffusion of diethyl ether into a CH3OH/H2O solution of ligand L and Na2CO3 or K2CO3. The complexes crystallized in the triclinic P1̅ space group. In the structure, one carbonate ion is encapsulated by two ligands to form the full capsule. It should be noted that the crystal-imposed inversion symmetry caused partial distribution of the three oxygen atoms of CO32− to four positions with 0.75 occupancy each, forming a rhombus (Figure 5), which is similar to the carbonate complex of a bisurea ligand reported previously by us.12i In complex 8, the two L molecules are orientated in a face-toface fashion with a distance of 9.88 Å between the bridgehead nitrogen atoms (Table S2, Supporting Information), leading to a centrosymmetric molecular capsule. The carbonate ion is surrounded by six urea groups by multiple N−H···O hydrogen bonds. Like in the related sulfate complexes,11b,c,12h there are stronger and weaker N−H···O contacts in the range of N···O distances