Self-Organization of K +-Crown Ether Derivatives into Double-Columnar Arrays Controlled by Supramolecular Isomers of Hydrogen-Bonded Anionic Biimidazolate Ni Complexes

Anionic tris (biimidazolate) nickelate (II) ([Ni(Hbim)3]−), which is a hydrogen-bonding (H-bonding) molecular building block, undergoes self-organization into honeycomb-sheet superstructures connected by complementary intermolecular H-bonds. The crystal obtained from the stacking of these sheets is assembled into channel frameworks, approximately 2 nm wide, that clathrate two cationic K+-crown ether derivatives organised into one-dimensional (1D) double-columnar arrays. In this study, we have shown that all five cationic guest-included crystals form nanochannel structures that clathrate the 1-D double-columnar arrays of one of the four types of K+-crown ether derivatives, one of which induces a polymorph. This is accomplished by adaptably fitting two types of anionic [Ni(Hbim)3]− host arrays. One is aΔΛ–ΔΛ–ΔΛ · · · network with H-bonded linkages alternating between the two different optical isomers of the Δ and Λ types with flexible H-bonded [Ni(Hbim)3]−. The other is a ΔΔΔ–ΛΛΛ · · · network of a racemate with 1-D H-bonded arrays of the same optical isomer for each type. Thus, [Ni(Hbim)3]− can assemble large cations such as K+ crown-ether derivatives into double-columnar arrays by highly recognizing flexible H-bonding arrangements with two host networks of ΔΛ–ΔΛ–ΔΛ · · · and ΔΔΔ–ΛΛΛ · · · .


Results and Discussion
2.1.Syntheses.The self-organization of crystals 1-5 was performed under the same conditions as those of the one-pot procedures in MeOH with Ni 2+ ions, H 2 bim, potassium nbutoxide, and the relevant crown ether derivative to obtain five types of blue or violet crystal, respectively.In preparation with cryptand, two polymorphs of the violet and blue crystals 4 and 5 were grown from the same batch solution of MeOH.All crystal structures of crystals 1-5 were identified by X-ray crystallographic analysis.Each was confirmed to consist of a cation-stuffed channel framework that included one of the four types of K + -crown ether complexes.On the other hand, preparations based on [K-(18-crown-6)] + with a nonsubstituted simple crown ether and [K-DBZ(18-crown-6)] + (potassium dibenzo-18-crown-6 complex) with a large steric hindrance of phenyl groups have no known crystal structure at present.This suggests that each one of the two cationic complexes cannot be accommodated in the hexagonal cavity as a unit of the channel framework in a crystal.

Crystal Structures.
Figure 3 shows the structures of each hexagonal cavity as repeating units of stuffed channel frameworks that clathrate two K + -crown ether complexes for each of the crystals 1-5.Each of the hexagonal cavities is constructed from alternating linkages through H-bonds between the Δ and Λ optical isomers of six [Ni(Hbim) 3 ] − molecules and contains two K + -crown ether complexes within it owing to the charge balance.As shown in Figure 3(a), it is interesting that only the cis-syn-cis structural isomer of [K-DCH(18crown6)] + is included in a hexagonal cavity, despite the use of a commercial preparation with a mixture of structural isomers of the crown ethers including the cis-anticis, cis-syn-trans, cis-anti-trans, trans-anti-trans, and transsyn-trans forms.The two K + -crown ether complexes are held to face each other in a saddle-shaped formation within the hexagonal cavity (distance between two K + ions in the crown ethers: 7.698(3) Å).The hydrophilic ether oxygens are aligned toward the outer wall of the cavity, whereas the two hydrophobic cyclohexyl groups are oriented toward the centre.Furthermore, stacking along the c-axis with two K +crown complexes leads to the formation of a small channel space, which includes the solvent molecules (methanol and water).As a result, the channel consists of both a small inner channel due to the stacking of two K + -crown complexes and a large outer channel due to the hexagonal cavities (

Conclusions
It is difficult to determine the positions and structures of guest molecules such as urea [54], zeolite [55], and MOF [56,57], included in a nanoporous framework.This is because the resultant structures have a very robust porous framework made of an inorganic polymer such as aluminosilicate, owing to the heavy disordering of the included guest molecules.However, in a cation-clathrated porous framework formed from [Ni(Hbim) 3 ] − , as we have shown here, a crystal structure is obtained under conditions in which guest molecules have already been included in the nanochannels by one-pot synthesis.This is different from typical porous crystals such as zeolite, which do not have guest molecules.Thus, it is necessary not only to isolate the crystal of unstable nanoporous frameworks in advance, but also to fix the included guest cations by adaptable fitting of self-organised porous host frameworks formed from [Ni(Hbim) 3 ] − .The crystal structure of the included guest molecules is clearly determined because adaptably fitting into the host H-bonding network prevents disordering of the guest molecules.In this study, we have compared five crystal structures induced by relatively large K + -crown ether derivatives.In contrast to host molecules that usually capture certain guest molecules in the well-known field of molecular recognition, the host arrays of [Ni(Hbim) 3 ] − suggest a new host-guest chemistry because the self-organised supramolecular isomer, which is different from H-bonded superstructures, recognises certain guest molecules in the crystal.Here, K + -crown ether derivatives have been onedimensionally arranged in the channel, the K + ion conductivity is not observed in the crystal.In the future, we hope that Li + ion-conductivity will be produced from an anionic nanochannel crystal that includes Li + -crown ether derivatives.Such a controlled crystal structure by induced fitting to [Ni(Hbim) 3 ] − must be found as new structural-chemical investigation on the guest molecules included into a porous crystal.(1).A suspension of H 2 bim (0.13 g, 1.0 mmol), DCH(18crown6) (0.12 g, 0.31 mmol), and KO t Bu (0.30 g, 2.6 mmol) was added to methanol (30 cm 3 ) and heated under reflux until the ligand dissolved.Ni(ClO) 4 •6H 2 O (0.11 g, 0.31 mmol) in methanol (20 cm 3 ) was added dropwise to the resulting solution, and the mixture was heated under reflux for 15 min.The insoluble components were removed by filtration, and the filtrate was allowed to stand at room temperature.Blue prisms were obtained from the filtrate after several days.Elemental analysis: Calcd for [Ni(Hbim) 3 ][K-DCH(18crown6)]•1.5H 2 O (C 38 H 52 N 12 O 6.5 NiK): C, 50.90%;H, 6.07%; N, 18.74%; Found: C, 50.85%;H, 5.86%; N, 18.77% (dried in vacuo 6 h at 100 • C).IR (KBr) 2937 cm −1 (ν(CH)), ∼2500 cm −1 (br, ν(NH)), 1895 cm −1 (br, 2 γ (NH)).{[Ni(Hbim) 3 ][K-MCH (18-crown-6)  C).IR (KBr) 2941 cm −1 (ν(CH)), ∼2500 cm −1 (br, ν(NH)), 1899 cm −1 (br, 2 γ (NH)).

Figure 3 :
Figure 3: Structural view of each hexagonal cavity for crystals 1-5.(a) The structure of the hexagonal cavity of 1 is constructed from nondistorted hydrogen bond connections of six [Ni(Hbim) 3 ] − molecules with alternating ΔΛ-ΔΛ-ΔΛ sequences of the Δ (red sphere) and Λ (blue sphere) optical isomers and includes two [K-DCH(18-ccrown-6)] + .The frameworks of K + -crown ether derivatives and MeOH molecules are represented by the green ball-and-stick lines.The potassium and nickel (II) ions are drawn with yellow and magenta spheres, respectively.((b) and (e)) The hexagonal cavities of 2 and 5, which are represented by H-bonded connections of the ΔΔΔ-ΛΛΛ sequence with the distorted connection of the same optical isomers, include two [K-MCH(18-crown-6)] + and two [K-cryptand] + molecules, respectively.(c) The hexagonal cavity of 3 that includes [K-MZB(18-crown-6)] + is constructed from a sequence similar to that of 1 (ΔΛ-ΔΛ-ΔΛ), although the hydrogen bonding connections are distorted.(d) The hexagonal cavity of 4 that includes [K-cryptand] + , which is also a polymorph of 5, is constructed from the ΔΛ-ΔΛ-ΔΛ sequence.

Figure 4 :Figure 5 :
Figure 4: Perspective views of the channel structures through each hexagonal cavity for crystals 1-5.Each hexagonal cavity is stacked to form the channel structures.The channel structures of all crystals are constructed from segregated stacking, that is, from different stacking columns of each of the Δ and Λ optical isomers of [Ni(Hbim) 3 ] − along the channel axis.The Δ and Λ isomers of [Ni(Hbim) 3 ] − are represented by the red and blue lines, respectively.The crown ether frameworks and MeOH molecules are shown in green.The potassium and nickel (II) ions are drawn as yellow and magenta spheres, respectively.