Modular Cavities: Induced Fit of Polar and Apolar Guests into Halogen

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Modular Cavities: Induced Fit of Polar and Apolar Guests into Halogen-Based Receptors Kentaro Aoki,† Kazuya Otsubo,*,† Garry S. Hanan,‡ Kunihisa Sugimoto,§ and Hiroshi Kitagawa*,†,⊥,∥ †

Division of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan Département de Chimie, Université de Montréal, 2900 Edouard-Montpetit, Montréal, Quebec H3T 1J4, Canada § Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan ⊥ Institute for Integrated Cell-Material Sciences, Kyoto University, Yoshida-Ushinomiya-cho, Sakyo-ku, Kyoto 606-8501, Japan ∥ INAMORI Frontier Research Center, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-3095, Japan ‡

S Supporting Information *

receptor for ion pairs by including the guest ion pairs was predicted by density functional theory (DFT) calculation. However, its detailed molecular structure still remains unclear, and there is no direct evidence of interaction between the triangular unit and guest molecules. Herein, we report the synthesis and X-ray structural characterization of the neutral triangular macrocyclic compound [PdCl2(4,7-phen)]3·(DMF)3· Et2O (1; DMF = N,N′-dimethylformamide; Et2O = diethyl ether) and novel isostructural analogue [PdBr2(4,7-phen)]3· (DMF)3·Et2O (2). Moreover, it was revealed that a triangular unit works as a halogen-based receptor for polar (DMF) and apolar (Et2O) solvents in the solid state. Figure 1a shows the synthetic scheme of the triangular macrocyclic compounds 1 and 2. The triangular macrocyclic compounds were synthesized according to a previous report.11 PdX2(CH3CN)2 (72.0 mg, 0.277 mmol, for X = Cl and 96.6 mg, 0.277 mmol, for X = Br) and 4,7-phen (50.1 mg, 0.277 mmol) were dissolved in N,N′-dimethylacetamide (DMAc; 20 mL) separately. A palladium precursor was added dropwise to the solution of 4,7-phen over a period of 30 min under vigorous stirring. The solution was stirred for another 1 h and then left to stand overnight. After that, insoluble polymeric precipitate was removed by filtration, and the filtrate was poured into excess Et2O (150 mL) to obtain the crude product as a pale-yellow powder. The crude product was collected by filtration and washed extensively with Et2O. After drying under a vacuum, target triangular macrocyclic compounds 1 and 2 were obtained (yellow powder, 48 mg, and 38% yield for 1 and brown powder, 40 mg, and 32% yield for 2). The obtained compounds were characterized from solutionstate 1H NMR and elemental analyses. Parts b and c of Figure 1 show the 1H NMR spectra of 1 and 2 in DMAc measured just after the addition of a palladium precursor to the 4,7-phen solution (see the experimental procedure). For 1H NMR measurement, CD3CN was added as a tracer with the ratio of 4:1. The detailed peak assignments are as follows: δ 11.48 (s, 6H, Hb), 9.802 (d, 6H, J = 7.6 Hz, Hd), 9.726 (d, 6H, J = 4.8 Hz, Hc), 8.178 (dd, 6H, J = 5.0 and 3.3 Hz, Ha) for 1 and δ 11.28 (s, 6H, Hb), 9.713 (d, 6H, J = 5.2 Hz, Hd) 9.697 (d, 6H, J = 3.5 Hz, Hc),

ABSTRACT: Neutral triangular macrocyclic compounds, [PdX2(4,7-phen)]3·(DMF)3·Et2O (X = Cl, Br; 4,7-phen = 4,7-phenanthroline; DMF = N,N′-dimethylformamide; Et2O = diethyl ether), were synthesized, and their molecular structures were characterized. Solution-state 1 H NMR results suggested the formation of metal−ligand bonds, and single-crystal X-ray crystallography revealed clear triangular structures. A detailed examination of the structures indicated the formation of two kinds of cavities in the solid state, where a triangular unit works as a halogen-based receptor for polar and apolar solvents through weak hydrogen-bonding and dipole−dipole interaction.

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etallosupramolecular chemistry has been one of the most intriguing topics in the field of materials science for decades,1 and self-assembly is known to create novel multifunctional materials by the combination of functional subunits.2 Among self-assembled materials, macrocycles are promising motifs not only for molecular recognition and guest inclusion but also for the construction of extended superstructures.3 Since the discovery of the square-shaped macrocyclic compound [Pd(en)(4,4′-bpy)]4(NO3)8 (en = ethylenediamine; 4,4′-bpy = 4,4′bipyridine) via self-assembly,4 a variety of square and triangular macrocyclic compounds utilizing square-planar metal ions (Pd2+ or Pt2+) and rigid organic linkers [e.g., pyrazine, 4,4′-bpy, 1,4bis(4-pyridyl)benzene, 4,4′-azopyridine, and uracil] have been reported.5 However, special attention has been paid for the synthesis of triangular macrocyclic compounds because they are often in equilibrium among other sizes of macrocycles.6 To construct a discrete triangular macrocyclic compound, many strategies such as distortion of the organic ligands by steric factors,7 control of the coordination geometry around the metal center,8 and UV-light irradiation9 have been investigated. One promising strategy is to utilize a 60° coordination angle, in such ligands as 4,7-phenanthroline (4,7-phen).10 With this strategy, Hasenknopf et al. reported the synthesis of a neutral triangular complex, [PdCl2(4,7-phen)]3.11 The realization of this triangular unit was expected by 1H NMR and mass spectroscopic observations, and its potential to work as a halogen-based © XXXX American Chemical Society

Received: April 18, 2018

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DOI: 10.1021/acs.inorgchem.8b01069 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 2. Molecular structure of 2 at 100 K. The ellipsoids are drawn with a 50% probability (orange, palladium; brown, bromine; light blue, nitrogen; black, carbon). Solvent molecules and hydrogen atoms are omitted for clarity.

Figure 1. (a) Synthetic scheme for triangular macrocyclic compounds 1 and 2. 1H NMR spectra for 1 (b) and 2 (c) measured just after the addition of a palladium precursor in a DMAc solution of 4,7-phen. Black and red lines correspond to spectra of the reagent 4,7-phen and product, respectively.

8.112 (dd, 6H, J = 5.1 and 3.4 Hz, Ha) for 2. The peak assignments were also supported by the 2D 1H COSY (correlation spectroscopy) NMR results (Figure S1). As is clearly seen, the peaks assignable to 4,7-phen are observed as a single component. In addition, compared to the spectrum of the 4,7-phen ligand, large peak shifts are observed after the reaction, implying the formation of metal−ligand bonds. These results suggest that the reaction of 4,7-phen and a palladium precursor reaches equilibrium within 30 min. Because of the poor solubility after trituration from Et2O, the triangular macrocyclic compounds were not soluble in typical organic solvents. However, the powders of 1 and 2 could be dissolved in DMF in several hours (about 10 mg/20 mL). After this solution was allowed to stand for a few days at room temperature, hexagonal platelet crystals suitable for single-crystal X-ray study were obtained. Elemental analyses of 1 and 2 showed reasonable agreement with the calculated value of the 1:1 ratio between palladium and 4,7-phen. The crystal sizes of 1 and 2 were 0.28 × 0.09 × 0.06 and 0.12 × 0.08 × 0.05 mm3, respectively. For compound 2, because of its small crystal size, synchrotron radiation was used for the X-ray crystal study. Figure 2 shows the molecular structure of 2 at 100 K. Compound 2 crystallizes in a trigonal R3̅ space group. As is clearly seen, 2 has a triangular structure, where 4,7-phen ligands and PdBr2 units are located at the vertex and side of the triangular unit, respectively. The side of the triangular unit is approximately 1.2 nm. The stacking pattern in 2 along the c axis is shown in Figure 3a. The triangular unit has a slightly bent structure, and each Br−Pd−Br part in the triangular unit is tilted 18.6° from the c axis. In addition, solvent molecules (DMF and Et2O) are located in the void space of the crystal (Figure S4, vide infra). The molecular structure and stacking pattern of 1 are quite similar to those of 2, except for the tilting angle of the Cl−Pd−Cl part (17.1° from the c axis; Figures S3 and S5).

Figure 3. (a) Stacking pattern of 2 along the c axis. Two DMF molecules and one Et2O molecule were included in cavities A and B, respectively, and the interplane separations of cavities A and B are 8.70 and 9.82 Å, respectively. The plane was defined by the three palladium atoms in the triangular unit. (b) Detailed short contacts between the guest molecules and triangular unit.

As pointed out in previous DFT calculations, 1 and 2 have the potential to work as receptors for the ion and ion pairs forming adducts through electrostatic and ion−dipole interaction.11 As shown in Figure 3a, because of the stacking pattern along the c axis with a tilted Br−Pd−Br part, two kinds of cavities composed of two bent triangular units are formed. Two kinds of cavities, A (large) and B (small), are formed in the solid state. Two DMF molecules and one Et2O molecule are separately included in cavities A and B, respectively, where the bromine atoms are B

DOI: 10.1021/acs.inorgchem.8b01069 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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opening up for DMF and closing in for Et2O. Upon examination of these two cavities in detail, it was found that, for cavity A, the distances between the oxygen atom of DMF and Hb (H7 and H8) in 4,7-phen are 2.75−2.76 Å (Figure 3b). This implies dipole−dipole interaction between the bent triangular unit and polar DMF molecule. On the other hand, for cavity B, the distances between the terminal carbon atom of Et2O and adjacent bromine atoms are 3.19−3.47 Å, indicating weak C− H···Br hydrogen bonding.12 These interactions are examined by Hirshfeld surface analyses.13 Figure 4 shows the Hirshfeld surface

Communication

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01069. Synthesis, characterization, crystallographic data, stacking patterns of 1 and 2, and a comparison regarding the interaction between 1 and 2 (PDF) Accession Codes

CCDC 1838106−1838107 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Kazuya Otsubo: 0000-0003-4688-2822 Garry S. Hanan: 0000-0001-6671-5234 Kunihisa Sugimoto: 0000-0002-0103-8153 Hiroshi Kitagawa: 0000-0001-6955-3015 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by ACCEL from Japan Science and Technology Agency, JSPS KAKENHI Grants JP20350030, JP23245012, JP15H05479, and JP17H05366 (Coordination Asymmetry). Synchrotron XRD measurements were supported by the JASRI (Proposals 2016A1361, 2016B1438, 2017A1366, and 2017B1483). G.S.H. thanks the Natural Sciences and Engineering Research Council of Canada and the Direction des Affaires Internationales for financial support.

Figure 4. Hirshfeld surface views (a) for the cavity A side and (b) for the cavity B side in 2. The upper panels show the triangular units for each cavity side, and the lower panels show the corresponding Hirshfeld surfaces. The Hirshfeld surface was mapped with dnorm over the range of −0.10 to +2.0, where dnorm is a normalized contact distance. The red and blue parts denote the short and long contacts compared to the van der Waals contact, respectively. The short DMF−triangular unit and Et2O− triangular unit contacts are highlighted as black circles.

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of the triangular unit for the cavity A side (Figure 4a) and for the cavity B side (Figure 4b) in 2. In each case, a short contact between the guest molecules and triangular unit is clearly visualized, where the bent triangular unit works as a receptor for polar DMF and apolar Et2O molecules. As mentioned above, single crystals were obtained after the powder was dissolved in DMF. This also suggests that there is interaction between the triangular unit and DMF molecules. A comparison between 1 and 2 regarding the intermolecular interaction is discussed in the Supporting Information. In summary, neutral triangular macrocyclic compounds 1 and 2 were obtained via the self-assembly of 4,7-phen and palladium precursors. 1H NMR spectroscopy revealed that metal−ligand bonds are formed within 30 min. Their molecular structures were successfully characterized from single-crystal X-ray crystallography. X-ray structural analysis indicated that the triangular units have bent structures and stack in a modular fashion, resulting in the formation of two kinds of cavities. Moreover, the bent triangular units work as receptors for the included polar DMF and apolar Et2O molecules, where weak hydrogen-bonding and dipole−dipole interactions play an important role in the crystallization.

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DOI: 10.1021/acs.inorgchem.8b01069 Inorg. Chem. XXXX, XXX, XXX−XXX