Crystallographic Evidence for the Host-Guest Interaction of Metallamacromolecules (A Highlight Review)

Metallamacromolecules offer opportunities to create structural diversity and interesting properties based on their unique frameworks and host-guest chemistry. Various types of assemblies can be created by the appropriate choice of the predesigned organic ligands containing various backbones and connectivity information and metal centers. These macrocyclic hosts contain a large and tunable hydrophobic inner cavity, which can be able selectively to recognize guest molecules. The goal of this highlight review is to describe the synthetic routes for the preparation of metallamacromolecules including stepwise and self-assembly strategies as well as their molecular recognition properties. key words: Host-Guest Chemistry, Metallamacromolecules. these metal-organic hosts with the capability of binding guests strongly and selectively is difficult and still remains a challenge for chemists.7 The size and shape of the host structures could be finetuned by adjusting or modifying the ligand structure and the preferred geometry of the metal ions. The compounds of palladium(II) and platinum(II) with multidentate pyridine-based ligands were of the first reported cyclic hosts because their square planar geometry provides corners for the formation of different sizes and shapes.8 The simplicity of self-assembly has resulted in a plethora of selfassembled complexes having structures containing molecular triangles, squares, higher-order cyclic species, and closed structures with nanometersized cavities. Many impressive examples of 1258 LEE et al., Orient. J. Chem., Vol. 29(4), 1257-1266 (2013) metallasupramolecular hosts can be found in studies reported by Lehn,9 Fujita,10 Sauvage,11 Stang,12 Mirkin,13 Hupp,14 Lu,15 Jin16 and other groups.17 Numerous types of metallacyclic hosts have been constructed in a step-wise or one-step synthetic route by using metallo-corner building blocks with bidentate rigid or semirigid linkers and difunctional pyridyl linkers. The preparation of metallacyclic hosts and the investigation of their complexations have produced many insights into significant noncovalent binding mechanisms. The formation and stability of complexation depend on the magnitude and directions of intermolecular interactions in the host”guest assembly. Several examples of systems that exploit aromatic interactions (e.g., π···π, CH···π, anion···π, etc.) and hydrogen bonding interactions have been shown in the creation of host”guest assemblies. Single-crystal X-ray analysis gives information about topology and mode of interaction of the host-guest assemblies in the solid state, which is crucial for thermal stability and kinetics of formation and decomposition of such materials.18 In addition, computational studies are helpful for steering hostguest assembly into prescribed crystal architectures based on well-defined structure directing noncovalent bonding interaction.19 Therefore, the quantitative knowledge of host-guest assembly is of considerable importance for understanding the nature of relevant noncovalent interactions, including those occurring in biological systems. Herein, for the sake of brevity, we highlight some specific examples of synthetic approaches and host behavior of discrete metallacyclic compounds in the solid state. Preparation and Host-Guest Studies The use of shape-specific designed ligands such as 1,3-bis(benzimidazol-1-ylmethyl)4,6-dimethylbenzene L1, 1,3-bis(benzimidazol1-ylmethyl)-2,4,6-tr imethylbenzene L2, or 1 ,4-b is(benz imidazol -1-y lmethy l ) -2 ,3 ,5 ,6tetramethylbenzene L3 with different metal salts to form a series of metallacyclic structures [Ag2L12] (BF4)2 (1), [Ag2L22](CF3SO3)2 (2) and [CF3SO3 "⊂ Ag2L32]CF3SO3(3),[CF3SO3 ⊂Ag2L33]CF3SO3 (4), [ClO4 -⊂ Cu2L24](ClO4)3 (5) and [4H2O⊂Ni2L24Cl4]· 6H2O (6), respectively (Scheme 1). 20 X-ray diffraction analysis reveals that in proceeding from 1 to 6 the molecules display an increasingly regular shape, especially with respect to the inner cavity. The cavity in 1-3, however, is arguably not a rectangular box since all the sides are not truly face to-face parallel. The CH3CN solvent molecules and BF4 " anions in 1 are found to locate around the metallacycle with almost negligible Ag···F (3.401 Å) and Ag···N (3.021 Å) interactions. In 2, one CF3SO3 " anion is dangling on both sides of the molecule by weak Ag···O interaction. On the other hand, one CF3SO3 " anion is unambiguously located inside the rectangular cavity of 3 to generate a [CF3SO3 ⊂Ag2L32] + cation, although it is crystallographically disordered (Fig. 1). In contrast, the structure of 4 is a prismatic box in which two Ag+ ions are linked by three L3 ligands in a trigonal fashion while the structures of 5 and 6 are tetragonal prismatic cages constructed by two square planar Cu2+ or Ni2+ ions linked by four L2 ligands. Compound 4 hosts one triflate anion, which are weakly interacting with two Ag+ ions (2.543 Å). Two disordered ClO4 anions are alternately arranged inside and outside of 5, thereby linking the molecules into a one-dimensional column with axial Cu···O interactions (2.388 and 2.659 Å), producing polycages. In neutral 6, four symmetrically arranged water molecules are accommodated inside the cavity, which are well ordered and held in place by weak O-H···Cl (O···Cl, 4.555 Å) hydrogen bonds (Fig. 2). Thus, the diversity of the guest molecules endows the structures with tunable inclusion properties and also makes it ambiguous. Building on well-established methods for assembling metallosystems incorporating b-diketone ligands,21 Lindoy’s group constructed a large discrete triangular subcomponent. Treatment of Scheme 1: Synthetic route for the preparation of compounds 1-6 N N N N R L1, R = 4,6-Me2, 2,5-H2 L2, R = 2,4,6-Me3, 5-H2 L3, R = 2,3,4,6-Me4 1−3 4 5,6 = metal salts 1259 LEE et al., Orient. J. Chem., Vol. 29(4), 1257-1266 (2013) Fig. 2: ball and stick representation of the (a) [CF3SO3 ⊂Ag2L33] + cation in 4, (b) [ClO4 ⊂Cu2L34] 3+ cation in 5 and (c) [4H2O⊂Ni2L34] cage in 6 (c) (a) (b) Fig. 1: ball and stick representation of the crystal structure of (a) 2 showing two dangling CF3SO3 anions, and (b) 3 including the disordered CF3SO3 " guest (a) (b) 1260 LEE et al., Orient. J. Chem., Vol. 29(4), 1257-1266 (2013) β-diketonate ligand (L4H2 = 1,1'-(4,4'-biphenylene) bis-3,3-dimethylpentane-1,3-dione) in warm pyridine solution with cobalt(II) acetate tetrahydrate in pyridine afforded the neutral trinuclear CoIII compound, [Co3L43(py)6]· 5.55py· 0.6H2O (7, py = pyridine) (Scheme 2).22 Scheme 2: Preparation of trinuclear metallamacrocycle 7 Fig. 3: Crystal structure of 7 along with the pyridine guest occupies in the void Single-crystal structural analysis revealed that the 2-diketonate ligands in 7 were not only bridged by three Co(III) ions but also formed a neutral equilateral triangle (Fig. 50). In addition, the pyridine coligands were axially coordinated to pseudooctahedral mode of CoIII ions. In the solid state structure of 7, a triangular void space was found about 118 Å2 in which the pyridine guest molecule was disordered in this void (Fig. 3). Compound 7 was claimed as the largest neutral M3L’3 triangle so far characterized structurally.23 The characteristics associated with the recognition of molecular rectangles with respect to the planar aromatic molecules and the Ag ion was reported. The alkoxyor thiolato-bridged molecular rectangles [{(CO)3Re(m-ER)2Re(CO)3}2(m-L5)2] (8-11) (8, -OC8H17; 9, -OC12H25; 10, ER = -SC4H9; 11, ER = -SC8H17) were prepared by the reaction of Re2(CO)10 with the 4,4'-bipyridine (L5, bpy) in the presence of higher aliphatic alcohols or a mercaptan Scheme 3: Self-assembly of neutral Re(I)-based rectangles 8-11 under solvothermal reaction (Scheme 3).24 The more hydrophobic nature of rectangles containing a dodecyl group enhanced their solubility in less polar solvents compared to those carrying an octyl and a butyl groups. O O O O