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Cite This: J. Am. Chem. Soc. 2018, 140, 3683−3689

Ligand-to-Ligand Interactions That Direct Formation of D2‑Symmetrical Alternating Circular Helicate Tan Yan Bing,† Tsuyoshi Kawai,*,† and Junpei Yuasa*,‡,§ †

Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan § Precursory Research for Embryonic Science and Technology (PRESTO), Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ‡

S Supporting Information *

ABSTRACT: This work demonstrates that ligand-to-ligand interactions between achiral bis-β-diketonate (BTP) and chiral bis(4-phenyl-2-oxazolinyl)pyridine [(R)- or (S)-Ph-Pybox] are successfully directed to the fabrication of a D2-symmetrical alternating circular helicate with the general formula [(R)- or (S)Ph-Pybox]4(LnIII)4(BTP)6. The lanthanide(III) LnIII assemblies (LnIII4-RRRR and LnIII4-SSSS) have a nanometer-size squarelike grid (interatomic distances > 10 Å). X-ray structure analysis revealed that the circular helicate contains two double helicate LnIII2L2 units, where both show (M)-helicity for LnIII4-RRRR and (P)-helicity for LnIII4-SSSS, where π−π stacking interaction is present between the side arm of (R)-Ph-Pybox (Ph1) and the adjacent BTP ligand around the EuIII metal center (dππ = 3.636 Å: the diketonate plane···Ph1 distance). The newly obtained circular lanthanide(III) helicate exists as single and homochiral diastereomers in solution (LnIII4-RRRR and LnIII4-SSSS), exhibiting circularly dichroism (CD) and circularly polarized luminescence (CPL). Conversely, the circular helicate favors the heterochiral arrangement (i.e., LnIII4-RRRR/LnIII4-SSSS).

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INTRODUCTION Coordination-driven self-assembly with geometrically intriguing chiral nanostructures (e.g., circular helicates) is promising in view of creating new chiral materials with peculiar chiroptical properties.1 In particular, potential applications of chiral selfassembled lanthanide (LnIII)2−7 structures have attracted great interest because of the unique magnetic dipole transition nature of 4f electrons, affording large circular polarization in luminescence.8−11 Recently, intensive efforts for developing LnIII-based molecular magnetism have been successfully directed to exploiting multinuclear LnIII complexes.12 These systems typically arrange the LnIII ions at short interatomic distances with small rigid linker molecules (e.g., μ-oxo linker), thus providing dense and compact self-assembled structures (Scheme 1a). Large-scale self-assembly should contain effective ligands with at least two well-separated coordination sites,13 which increases the distance between the metal ions (Scheme 1b). In such a case, however, there are inherent difficulties due to the ligand conformational flexibility as well as the low stereochemical preferences of f-block metals. Thus, enantiopure LnIII assembly strategies for nanoscale structures still represent a considerable challenge. We report herein that ligand-to-ligand interactions are successfully directed to the fabrication of D2-symmetrical alternating circular helicates possessing nanometer-size squarelike grids (interatomic distances > 10 Å). The self-assembly © 2018 American Chemical Society

strategy is summarized in Scheme 1c, where a 1,3-bis(4,4,4trifluoro-1,3-dioxobutyl)phenyl ligand (BTP) with two βdiketonate coordination sites was employed as a bridging ligand. Doubly negatively charged bisdiketonate ligands lead to a neutral monodimensional dinuclear triple helix LnIII2L3 (stoichiometry S = 2/3 = 0.67, global complexity GC = 2 + 3 = 5).14,15 Each LnIII ion coordinates with six β-diketonate oxygen atoms, which does not satisfy the coordination number of LnIII (n ≥ 8). These LnIII high coordination numbers would make it difficult to control over the self-assembly of LnIII. However, our results reveal that versatile coordination numbers of LnIII provide the opportunity for sequential coordination with chiral coligands (R)- and (S)Ph-Pybox,16 addressing a generation of novel expanded circular lanthanide(III) helicates (LnIII4L6) with a larger global complexity (GC = 4 + 6 = 10) through the ligand-to-ligand interactions.17−19 To the best of our knowledge, the circular lanthanide(III) helicate was first successfully isolated by Senegas et al.20 This contribution theorized the effect of global complexity (GC) on the structure of the LnIII self-assembly, where the isolated LnIII self-assembly (LnIII3L3) with the global complexity GC = 3 + 3 = 6 was the D3-symmetrical trimetallic circular helicate.20 Our strategy is adaptable to various LnIII and rareearth metals (EuIII, YbIII, SmIII, and YIII), expanding the library of Received: November 30, 2017 Published: February 12, 2018 3683

DOI: 10.1021/jacs.7b12663 J. Am. Chem. Soc. 2018, 140, 3683−3689

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methanol (Scheme 1c).21 After removing the solvent by evaporation, the resulting yellow−white powder was successfully crystallized by slow diffusion of diethyl ether into a solution of the sample in toluene (for the crystallographic data, refer to Tables S1 and S2 in Supporting Information). X-ray crystallography revealed the circular lanthanide(III) helicates with the general formula [Ph-Pybox]4(LnIII)4(BTP)6 (Figure 1a and Figure S1), where each four metal ions possess the nonacoordination geometry with the same chirality (Figure 1b,

Scheme 1

circular lanthanide(III) helicates LnIII4-RRRR and LnIII4-SSSS wi th the g eneral formula [ ( R) - an d (S )-P h -Pybox]4(LnIII)4(BTP)6. The obtained circular helicates are kinetically inert and thus can be isolated and identified by X-ray structure analysis in comparison with NMR spectroscopy. The NMR analysis suggests D2-symmetry for the circular lanthanide(III) helicates (LnIII4L6) with the global complexity GC = 10. Additionally, the present study reveals that the circular lanthanide(III) helicate favors the heterochiral arrangement (Scheme 2). The heterochiral aggregate (EuIII4-RRRR/EuIII4Scheme 2

SSSS) was successfully analyzed by X-ray crystallography, demonstrating that the two enantiomers are packed in an alternating fashion into the crystal form.

Figure 1. (a) X-ray crystal structures of [(R)-Ph-Pybox]4(EuIII)4(BTP)6 (Left, CCDC 1570102) and [(S)-Ph-Pybox]4(EuIII)4(BTP)6 (Right, CCDC 1570115). Solvent molecules and hydrogen atoms are omitted for clarity. (b, Left) M4L6 core structure obtained from the crystal structure of [(R)-Ph-Pybox]4(EuIII)4(BTP)6; (Right) coordination geometries around Eu1 and Eu2. (c) Distance and angle between the EuIII ions. (d) Space-filling view of [(R)-Ph-Pybox]4(EuIII)4(BTP)6. Arrows indicate π−π stacking interactions between the side arm of (R)Ph-Pybox and the β-diketonate plane. (e) Coordination arrangement around Eu1, where arrows denote coordination direction.

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RESULTS AND DISCUSSION X-ray Crystal Structures of D2-Symmetrical Alternating Circular Helicate. The circular lanthanide(III) and rare-earth metal (EuIII, YbIII, SmIII, and YIII) helicates were synthesized by reacting dinuclear tris(BTP) LnIII complexes [(LnIII)2(BTP)3]14 with (R)-Ph-Pybox or (S)-Ph-Pybox in 1:2 stoichiometry in 3684


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Journal of the American Chemical Society Right). The LnIII4L6 core structure was obtained from the crystal structure of [(R)-Ph-Pybox]4(EuIII)4(BTP)6 (Figure 1b, Left), where four EuIII ions exist in two different forms (Eu1 and Eu2). This is regarded as the subtle static effects due to the crystal packing that contains solvent molecules (diethyl ether), leading to a reduction in symmetry from D2 to C2 (Figure S1). D2symmetric solution-phase structure is supposed to be based on 1 H NMR, as discussed later (vide infra). The four EuIII ions are bridged by the six BTP ligands, where the two BTP ligands bridge the two Eu1 ions and the two Eu2 ions, and the other BTP ligands bridge the Eu1 and Eu2 (Figure 1b). Consequently, this facilitates formation of a nanometer-size (1 nm × 1 nm) squarelike grid with the four EuIII cores (Figure 1c). The Eu1−Eu1, Eu2−Eu2, and Eu1−Eu2 distances are 11.813, 11.621, and 10.014 Å, respectively; Eu1−Eu2-Eu2 (ϕ) and Eu1−Eu1-Eu2 angles (θ) are close to right angles (90.49° and 89.40°, respectively). Importantly, X-ray crystallography (Figure 1d and e) revealed π−π stacking interaction between the side arm of (R)-Ph-Pybox (Ph1) and the adjacent BTP ligand (BTPA) around Eu2 (dππ = 3.636 Å: the diketonate plane···Ph1 distance). Conversely, the other side arm (Ph2) has no appreciable molecular interaction with the adjacent BTP ligands (BTPB and BTPC), where BTPB and BTPC exist in a helix arrangement (Figure 1e). A similar coordination arrangement can be found around Eu1 (dππ = 3.511 Å). Consequently, the ligand-to-ligand interactions direct fabrication of the circular helicate structure with a nanosized square with two helical chains: P-helix for [(S)-PhPybox] 4 (Eu III ) 4 (BTP) 6 and M-helix for [(R)-Ph-Pybox]4(EuIII)4(BTP)6 (Figure S2), showing nonsuperimposable complete mirror images of each other (Figure 1a). The 1H NMR spectrum of the EuIII self-assembly shows a total of 24 resonances spread over a wide chemical shift range (−3 to 18 ppm) according to the paramagnetic properties of EuIII core (Figure 2b). To facilitate the rationalization, we synthesized a chiral self-assembly of yttrium(III) ions (YIII), which exhibit similar coordination modes to EuIII ions but are not paramagnetic. The YIII self-assembly [(R)-Ph-Pybox]4(YIII)4(BTP)6 was successfully crystallized by use of the same procedure described above, and chiral self-assembly with the same circular helicate was confirmed by the X-ray structure analysis (Figure S1).22 Hence, the chiral self-assembly of YIII is isomorphous of the EuIII self-assembly. The 1H NMR spectrum of [(R)-PhPybox]4(YIII)4(BTP)6 also exhibits a total of 24 resonances (Figure 2c). In the region for the α-proton (5−7 ppm), three singlets are found with an integration ratio of 1:1:1 (α1−3 in Figure 2c). One of the three singlets corresponds to the αprotons of BTPA with C2-symmetry, and the other two singlets are assignable to the α-protons of BTPB and BTPC without C2symmetry (Figure 2a). BTPB and BTPC would be identical, assuming D2-symmetry for the self-assembly in solution. This assumption was confirmed by the 19F NMR spectrum, where three 19F NMR signals due to BTP ligands can be observed for the self-assembly (Figure S3). Consequently, the 1H NMR signals of [(R)-Ph-Pybox]4(YIII)4(BTP)6 and [(R)-Ph-Pybox]4(EuIII)4(BTP)6 were reasonably assigned sequentially by comparison with 1H NMR signals of free BTP and (R)-Ph-Pybox ligands (Figure 2d and e, respectively) using the correlation spectroscopy (COSY) and rotating-frame Overhauser spectroscopy (ROESY) correlations (Figures S4−S6; for a detailed discussion, see the Supporting Information). Interestingly, no (R)-Ph-second diastereomer was detected in both 1H NMR spectra (Figure 2b and c), a clear indication of a single homochiral conformation in solution. The obtained solution

Figure 2. (a) Solution structure and NMR signal assignment of [(R)-PhPybox]4(YIII)4(BTP)6. 1H NMR (600 MHz) spectra of (b) [(R)-PhPybox]4(EuIII)4(BTP)6 and (c) [(R)-Ph-Pybox]4(YIII)4(BTP)6 in CDCl3. 1H NMR (400 MHz) spectra of (d) BTP and (e) (R)-PhPybox in CDCl3. Asterisk denotes the solvent NMR peak.

structure of the self-assembly, which possesses D2-symmetry exhibiting the same chirality for each metal ion, is given in Figure 2a.23 In light of these results, we investigated the supramolecular geometry of [(R)-Ph-Pybox]4(EuIII)4(BTP)6 in solution by electrospray ionization (ESI) mass spectroscopy (Figure 3). The ESI mass (positive) spectrum of [Ph-Pybox]4(EuIII)4(BTP)6 in chloroform displays two major mass peaks with mass range above m/z 3500 (Figure 3b). Two peaks at m/z = 4201.4 and 4223.4 can be unambiguously assigned to {[Ph-Pybox]4(EuIII)4(BTP)6 + H}+ and {[Ph-Pybox]4(EuIII)4(BTP)6 + Na}+, respectively, by using their calculated isotopic distribution (Figure 3b, Inset). Conversely, we found a mass peak assignable to the LnIII2L3 type complex at m/z = 2101.20, which corresponds to {[Ph-Pybox]2(EuIII)2(BTP)3 + H}+. This can be interpreted as possible thermodynamic equilibria existing in solution at low concentration and possible gas-phase dissociation processes. Alternatively, the simple dinuclear helicate should be considered as the most probable species in solution, because the 3685


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Figure 3. (a) Positive ESI mass spectrum of [Ph-Pybox]4(EuIII)4(BTP)6 in chloroform. An intense peak corresponding to {Ph-Pybox + Na}+ was found below m/z 500, which is attributed to dissociated Ph-Pybox during gas-phase dissociation processes. (b) Intensity was enhanced for better visibility of mass peaks corresponding to EuIII complexes. (Inset) Isotopically resolved signals at m/z = 2101.20, 4201.4, and 4223.4 with the calculated isotopic distributions for {[Ph-Pybox]2(EuIII)2(BTP)3 + H} + , {[Ph-Pybox] 4 (Eu I I I ) 4 (BTP) 6 + H} + , and {[Ph-Pybox]4(EuIII)4(BTP)6 + Na}+, respectively. (c) Reproducibility for detection of the signals at m/z = 4201.4 and 4223.4.

Figure 4. (a, Top) CD spectra of [(R)-Ph-Pybox]4(EuIII)4(BTP)6 (red) and [(S)-Ph-Pybox]4(EuIII)4(BTP)6 (blue) in chloroform at 298 K (concentrations = 1.7 × 10−7 M). (Bottom) Absorption spectrum of [(S)-Ph-Pybox]4(EuIII)4(BTP)6 (1.7 × 10−7 M) in chloroform at 298 K. (b, Top) CPL spectra of [(R)-Ph-Pybox]4(EuIII)4(BTP)6 (red solid line) and [(S)-Ph-Pybox]4(EuIII)4(BTP)6 (blue solid line) in chloroform at 298 K. Blue dashed line shows CPL spectrum of [(S)-PhPybox]4(EuIII)4(BTP)6 in KBr pellet. (Bottom) Emission spectra of [(S)-Ph-Pybox]4(EuIII)4(BTP)6 in chloroform (solid line) and in KBr pellet (dashed line) at 298 K. Excitation wavelength: λex = 360 nm. (b, Inset) Emission decay curves at λ = 618 nm of [(R)-PhPybox]4(EuIII)4(BTP)6 (red dots) and [(S)-Ph-Pybox]4(EuIII)4(BTP)6 (blue dots) (A) in chloroform and (B) those in KBr pellet at 298 K.

terminal binding of chiral pybox ligands in (LnIII)2(BTP)3[PhPybox]2 removes the 3-fold axis, and one logically expects a C2point group for (LnIII)2(BTP)3[Ph-Pybox]2, in agreement with the 24 signals identified by NMR (Figure 2b). Hence, we investigated the photophysical properties of circular helicate in solution phase as well as solid state (vide infra). Chiroptical Properties of D2-Symmetrical Alternating Circular Helicate. Figure 4a shows the CD spectra of [(R)- and (S)-Ph-Pybox]4(EuIII)4(BTP)6 in chloroform (red and blue lines, respectively), where they exhibit complete mirror-image CD signals. The assemblies show CD bands in the spectral range of the π−π* transition of the β-diketonate moieties at around λ = 300−370 nm (Figure 4a), suggesting that homochiral assembly formation induces Cotton effects in the absorption band of the βdiketonate ligands. The induced CD spectra show splitting Cotton effects due to excitonic coupling between the BTP ligands,24−26 suggesting the induced chiral configuration of the BTP ligands around the EuIII metal center, as can be seen in the X-ray crystal structures (Figure 1a). Similarly, [(S)-PhPybox]4(EuIII)4(BTP)6 shows the CPL signals at the 5D0 → 7 F1 and 5D0 → 7F2 transitions of EuIII, where each line splits into several crystal-field levels (Figure 4b, blue solid lines).27 This CPL signature displayed an almost complete mirror CPL profile for [(R)-Ph-Pybox]4(EuIII)4(BTP)6 in chloroform (Figure 4b, red solid line) and is identical to the solid-state (KBr pellet) CPL spectrum (Figure 4b, blue dashed line). In the same way, there is a nearly quantitative fit between the crystal-field splitting of emission spectrum in solution and that in the solid state (Figure 4b, black solid and dashed lines, respectively), showing consistent evidence that the circular EuIII helicate is identical in solution and in the solid state. On the other hand, the dissymmetry factors (glum) of the chiral self-assembly of EuIII assemblies were determined to be +0.22 for the (R)-isomer and −0.24 for the (S)-isomer at the magnetic (5D0 → 7F1) transition band at λ = 591 nm in chloroform. The glum is defined as glum = 2(IL − IR)/(IL + IR), where IL and IR refer to the intensity of left

and right circularly polarized light, respectively. The magneticdipole transition (5D0 → 7F1) satisfies the magnetic-dipole selection rule, ΔJ = 0, ±1 (except 0 ↔ 0), which often shows particularly large circular polarization.8−10,28−31 The circular EuIII helicates are luminescent both in the solution and in the solid state. By using a calibrated integrating sphere system upon excitation of the ligand absorption band, the emission quantum yield of [(R)-Ph-Pybox]4(EuIII)4(BTP)6 was determined to be Φem = 0.05 in chloroform. The emission decay of the circular EuIII helicate was found to be monoexponential both in the solution and in the solid state (τ = 0.59 ms in chloroform and 0.48 ms in the solid state, inset of Figure 4b). This again underlines the absence of second diastereomers in solution and solid state. The circular lanthanide(III) helicates [(R)- and (S)-PhPybox]4(LnIII)4(BTP)6 (LnIII4-RRRR and LnIII4-SSSS, respectively) are sufficiently rigid for the investigation of heterochiral interactions between them, where LnIII4-RRRR and LnIII4-SSSS show no appreciable ligand exchange in racemic conditions (vide infra).32 Interestingly, the mixing of EuIII4-RRRR and EuIII4-SSSS spontaneously forms a suspension in chloroform.33 Figure 5 presents the photographs of the circular EuIII helicates with different ratios between the EuIII4-RRRR and EuIII4-SSSS in chloroform solutions. Although pure EuIII4-RRRR and EuIII4SSSS samples are transparent, each racemic mixture forms a 3686


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Figure 5. Photographs of the circular EuIII helicates (initial total concentration = 4.7 × 10−4 M) with different enantiomeric excesses (ee’s) in chloroform under bright conditions (Top) and UV/light irradiation (Bottom).

suspension (Figure 5). The resulting suspension was diluted (150-fold) to obtain a clear solution for CD spectral measurements of the racemic samples, with no appreciable suspension in the resulting dilute solutions. The CD profile is identical in each case (Figure 6a), but the amplitude (θ) increases almost proportionally with an increase in the initial enantiomeric excess (ee) of the circular EuIII helicate (Figure 6b). The linear relationship suggests that each racemic sample contains mostly the homochiral diastereomers (i.e., EuIII4-RRRR and EuIII4-SSSS), where heterochiral diastereomers (e.g., EuIII4-RRSS, EuIII4-RSSS, and EuIII4-RRRS) formed by ligand exchange apparently do not extensively coexist. Such ill-defined structures should possess different CD profiles with smaller amplitude than that of the homochiral diastereomers, which should result in a thresholdtype nonlinear relationship between the CD intensity and the initial enantiomeric excess. The observed linear relationship indicates the resulting precipitate as an insoluble heterochiral aggregate between the EuIII4-RRRR and EuIII4-SSSS isomers (Scheme 2).34 When starting from the racemic Ph-Pybox in the synthesis of the circular EuIII helicates, the self-assemblies crystallized as racemic mixtures, exhibiting the same chirality for each four EuIII/Ph-Pybox site (Figure 7). This empirical observation indicates that the circular EuIII helicate favors the heterochiral arrangement. Perhaps the heterochiral recognition is due to suitable crystal-packing interaction in the heterochiral aggregates (Figure S7), where the two enantiomers are packed in an alternating fashion into the crystal form.

Figure 6. (a) CD spectra of the resulting dilute solutions of EuIII4-RRRR and EuIII4-SSSS mixture with different ratios in chloroform. (b) Plot of θ at λ = 363 nm vs the initial enantiomeric excess (ee) of the circular EuIII helicate.

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SUMMARY AND CONCLUSIONS In conclusion, we have successfully demonstrated a self-assembly strategy for fabrication of nanometer-sized circular lanthanide(III) helicates by ligand-to-ligand interactions between the chiral Ph-Pybox and achiral BTP ligands through π−π stacking interactions. The ligand-to-ligand interactions with the tridentate coligands (Ph-Pybox) should thermodynamically stabilize the circular lanthanide(III) helicates (LnIII4L6) with large global complexity (GC = 4 + 6 = 10). The obtained circular lanthanide(III) helicate is homochiral (two double helicate LnIII2L2 units in the structure) and no second diastereomer coexists, thus providing circular polarization in the luminescence. In contrast, the circular EuIII helicate favors the heterochiral arrangement, where EuIII4-RRRR and EuIII4-SSSS crystallized as racemic mixtures with the same chirality for each four metal ions. The present strategy will be promising for the rational design of self-assembled LnIII showing geometrically intriguing chiral structures.

Figure 7. Molecular packing diagrams of the crystal structures of EuIII4RRRR (red) and EuIII4-SSSS (blue).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b12663. 3687


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Experimental details including 1H NMR assignment of the self-assembled LnIII, crystallographic parameters and refinement details, ORTEP views of the self-assembled LnIII, 19F NMR spectra, 1H,1H COSY NMR, 2D-ROESY NMR, DOSY NMR, molecular packing diagrams, spectra data for mixture of SmIII4-RRRR and EuIII4-SSSS, and supporting figures for the experimental section (PDF) Crystallographic data in CIF format (CCDC 1570102, CCDC 1570113, and CCDC 1570116) (ZIP)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Junpei Yuasa: 0000-0003-1117-7904 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by JST-PRESTO “Molecular technology and creation of new functions” (14530027), a Grantin-Aid for Scientific Research (C) (JP26410094) and Scientific Research (A) (JP25248019) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and also JSPS KAKENHI Grant no. JP17H05386 (Coordination Asymmetry). This manuscript was improved through the suggestions and comments of Prof. Leigh McDowell (Nara Institute of Science and Technology).

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