Competing Allosteric Mechanisms for Coordination-Directed

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Competing Allosteric Mechanisms for Coordination-Directed Conformational Changes of Chiral Stacking Structures with Aromatic Rings Kohei Nonomura† and Junpei Yuasa*,† †

Department of Applied Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8061, Japan

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S Supporting Information *

ABSTRACT: This work revealed that significant asymmetric nonlinear effects can be found in a coordination-directed conformational alteration through competing allosteric mechanisms. Toward this aim, we have prepared new chiral bridging ligands [(S,S)- and (R,R)Im2An] containing an anthracene ring as a spacer with two ethynyllinked chiral imidazole groups at the 9,10-positions. The (S,S)- and (R,R)-Im2An ligands (L) spontaneously form the assemblies with Zn2+ ions (M) in solution phase, giving L4M2-type assemblies with a general formula [(S,S)- or (R,R)-Im2An]4(Zn2+)2. NMR studies revealed that the [(S,S)-Im2An]4(Zn2+)2 assembly has an anthracene dimer structure with a parallel-displaced geometry, leading to relatively small circular dichroism (CD) signals, as expected for nonchiral objects. Conversely, subsequent addition of chiral coligands [(R)- or (S)-Ph-box] to [(S,S)-Im2An]4(Zn2+)2 afforded an alternative Zn2+ assembly with general formula [(R)- or (S)-Phbox]2[(S,S)-Im2An]2(Zn2+)2, where the chiral coligands expel two of the (S,S)-Im2An ligands that were singly bound to the Zn2+ ions in the original [(S,S)-Im2An]4(Zn2+)2 assembly. This ligand-exchange reaction causes conformational alteration from a parallel-displaced structure to a twisted stacking between the anthracene rings inside the Zn2+ assembly, which results in a significant enhancement of CD signals due to excitonic interactions of the chiral anthracene dimer. Dissymmetry factor (gCD) for CD due to chiral stacking structures shows a significant inverse sigmoidal response to the enantiomeric excess of the chiral coligands. The observed nonlinear phenomena are results of the two conflicting mechanisms, homochiral cooperative association (homochiral self-sorting) to form CD-active assemblies [(S)- or (R)-Ph-box]2[(S,S)-Im2An]2(Zn2+)2 versus heterochiral cooperative dissociation of [(S,S)-Im2An]4(Zn2+)2 by sequestering of Zn2+ inside the assembly through formation of a heterochiral 2:1 Zn2+ complex ([(R)-Ph-box][(S)-Ph-box]Zn2+). The presented mechanisms provide a new strategy for generating switch-like OFF/ON states in chiral systems.

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INTRODUCTION Stacking interactions are some of the most important noncovalent interactions, which yield π-electron systems of aromatic molecules with intriguing electronic and photophysical properties.1−6 Typical of noncovalent interactions, interactions of aromatic stacks are weak and are formed reversibly in a dynamic equilibrium, with some tendency to give the energetically favored stacking arrangements.7 Conversely, metal−ligand interactions sometimes trigger energetically unfavorable stacking interactions.8−17 Scheme 1 represents the coordination-directed stacking approach referred to as a “metal ion clip”.11 In the presented methodology, we introduced two chiral imidazole groups at the 9,10-positions of anthracene using ethyne spacers to afford chiral bridging ligands (S,S)- and (R,R)-Im2An.18 Rotational degrees of freedom around the ethyne spacers increased the conformational possibilities of the bridging ligands, which provided perpendicular−perpendicular and perpendicular− planar coordination modes at the two imidazole groups (Scheme 1a and b, respectively). From these geometries, one © XXXX American Chemical Society

may expect two different stacking modes between the anthracene rings when they are bridged by two metal ions with a tetrahedral coordination preference (e.g., Zn2+): a parallel-displaced structure from the perpendicular−perpendicular conformation and the twisted stacking structure from the perpendicular−planar conformations (Scheme 1a and b, respectively). The latter is intriguing, as it allows for intense chiroptical activity, such as circular dichroism (CD) and circularly polarized luminescence (CPL), owing to excitonic interactions inside the twisted dimer.19−25 In the present study, we have introduced the chiral substituents into the bridging ligands for the purpose of monitoring the stacking modes between the anthracene rings by CD signals. This work shows that successive binding of bidentate chiral coligands [(S)- and (R)-Ph-box]26 to the coordinatively unsaturated assemblies causes a conformational change of the anthracene stacking structures from a parallel-displaced geometry to twisted Received: March 8, 2019

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DOI: 10.1021/acs.inorgchem.9b00665 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

acts to sequester the metal ions inside the assembly (heterochiral cooperative dissociation). The present work demonstrates that such a competing allosteric mechanism (Scheme 2b) affords highly nonlinear inverse sigmoidal responses of the chiroptical activity to the enatiomeric excess of the chiral coligands, which successfully generates switch-like chiroptical OFF/ON states.

Scheme 1

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RESULTS AND DISCUSSION Zinc-Driven π-Stacked Assembly with Anthracene Ligands. Chiral bridging ligands with an anthracene spacer [(S,S)- and (R,R)-Im2An] were synthesized by Sonogashira coupling of two chiral imidazole derivatives with 9,10di(ethynyl)anthracene (see Supporting Information for details). Upon addition of 0−1.5 equiv of zinc triflate [Zn(OSO2CF3)2] to an acetonitrile (MeCN) solution of (S,S)-Im2An (2.0 × 10−5 M), UV/vis spectral changes of (S,S)Im2An were observed with clear isosbestic points at 489, 399, 317, and 260 nm (Figure 1a, red−blue line). The isosbestic

stacking modes exhibiting intense chiroptical activity. Another important aspect to be envisaged from such coordinationdriven aromatic helicity is allosteric cooperativity in chiral systems, which has been a topic of active research.27−34 The simplest case is that homo- or heterochiral association of chiral ligands (LR and LS) to aromatic dimer systems with dual binding sites (M−(Ar)2−M) results in a nonlinear response to the enatiomeric excess of the chiral ligands (Scheme 2a). Scheme 2

Figure 1. (a) UV−vis absorption and (b) emission spectra of (S,S)Im2An (2.0 × 10−5 M) in the presence of Zn2+ [0 (red line)−1.5 × 10−5 M (blue line)] in acetonitrile at 298 K. Excitation wavelength: λex = 400 nm. Arrows indicate the direction of the spectral changes. Insets: Plots of (a) absorbance at λ = 450 nm and (b) emission intensity at 496 nm vs [Zn2+]/[(S,S)-Im2An]0, where [(S,S)-Im2An]0 denotes the initial concentration of (S,S)-Im2An (2.0 × 10−5 M). (b) Visible fluorescence change of (S,S)-Im2An upon addition of Zn2+ in acetonitrile.

points suggest the quantitative conversion of (S,S)-Im2An to a single Zn2+-assembly (Scheme 3). Assembly formation with Zn2+ is accompanied by significant emission quenching of (S,S)-Im2An (Figure 1b, inset).36 The emission spectrum shape is characteristic of the anthracene monomer fluorescence (Figure 1b, red line),18 hence (S,S)-Im2An molecules exist as monomers in the absence of Zn2+. The fluorescence quenching may be mainly due to formation of an anthracene dimer with parallel-displaced geometry through assembly with Zn2+ (vide infra), which perhaps provides a pathway to a nonradiative process.37 Absorbance at λ = 450 nm and fluorescence intensity at λem = 496 nm were plotted against the molar ratio ([Zn2+]/[(S,S)-Im2An]0) to obtain the binding stoichiometry between (S,S)-Im2An and Zn2+ (Figure 1, insets), where [(S,S)-Im2An]0 denotes the initial concentration of the bridging ligand (2.0 × 10−5 M). A clear break can be found

However, such a simple mechanism normally generates only a weak sigmoidal or inverse sigmoidal nonlinear response. We propose here a new perspective for allosteric cooperativity in chiral systems, competing allosteric mechanisms (Scheme 2b).35 In the present mechanisms, the initial assembly (M− (Ar)2−M) favors homochiral association with the chiral ligands (LR and LS) to afford optically active assemblies, LR− M−(Ar)2−M−LR and LS−M−(Ar)2−M−LS. Conversely, the metal ions (M) intrinsically favor formation of a heterochiral 2:1 complex (LR−M−LS), where a combination of LR and LS B


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Inorganic Chemistry Scheme 3

at [Zn2+]/[(S,S)-Im2An]0 = 0.50 in both titration curves (Figure 1, inset), a clear indication of a 2:1 binding stoichiometry between (S,S)-Im2An and Zn2+. The crude analysis suggests the Zn 2+ -assembly to be [(S,S)Im2An]2n(Zn2+)n (n = 1, 2, 3) with stoichiometry S = n/2n = 0.5 and global complexity G = n + 2n = 3n. NMR analysis was then conducted to quantify the global complexity (G = 3n) of the Zn2+-assembly (vide infra). Addition of 0.5 equiv of Zn2+ to a CD3CN solution of (S,S)Im2An (2.0 × 10−3 M) resulted in a significant signal alteration of the 1H NMR spectrum of (S,S)-Im2An (Figure 2b and c) and appearance of significantly upfield-shifted triplet signals at 5.43 and 5.10 ppm. These two protons (a and b) show a strong COSY correlation (Figure 2c), meaning that they are vicinal aromatic protons in the anthracene ring. This considerable upfield shift highlights the shielding effects of the anthracene rings in the Zn2+ assembly, suggesting a coordination-oriented stacking (dimer) structure of the anthracene rings. Due to the nature of the assembly, the NMR signals for the aromatic protons are rather complicated, with 14 resonances spread over a range of 5.0−8.5 ppm (Figure 2c). Conversely, the methoxy protons (−OMe) of the pendant alkyl chains of the imidazole side arms show a singlet NMR signal at 3.28 ppm (Figure 2f), which is a much simpler probe for addressing the symmetry of the resulting Zn2+-driven self-assembly. The signal at 3.28 ppm splits into four singlet peaks (3.28, 3.27, 3.13, and 3.08 ppm) upon addition of 0.5 equiv of Zn2+ (Figure 2f vs e), with the two peaks (α and β) are essentially overlapping. The four singlet peaks are a clear indication that the imidazole moieties exist in four different coordination environments (α, β, γ, and σ) for the resulting Zn2+ assembly. The simplest L2M-type assembly (i.e., [(S,S)-Im2An]2(Zn2+)) with the smallest global complexity G = 2 + 1 = 3 appears inconsistent with this observation, as one would expect two singlet peaks for [(S,S)-Im2An]2(Zn2+). Consequently, we assumed a L4M2-type assembly ([(S,S)-Im2An]4(Zn2+)2) with a global complexity G = 4 + 2 = 6 and obtained its energyminimized structure by molecular mechanics (Figure 2a). The model structure contains the two (S,S)-Im2An ligands (A and B) that are connected to both Zn2+ ions, as well as the other two (S,S)-Im2An ligands (C and D) that coordinate to the Zn2+ ions in monodentate binding modes. A C2 point group can be assigned to the structure of [(S,S)-Im2An]4(Zn2+)2; hence, the (S,S)-Im2An ligands “A” and “C” are identical to those of “B” and “D”, respectively (Figure 2a). When in the complex, each ligand loses its original ligand symmetry (C2);

Figure 2. (a) Energy-minimized structure of [(S,S)-Im2An]4(Zn2+)2. (b, f) 1H NMR spectra of (S,S)-Im2An (2.0 × 10−3 M) in CD3CN at 298 K in the region of (b) 4.5−9.0 and (f) 3.0−3.4 ppm. (c, e) 1H NMR and 1H,1H-COSY NMR spectra of (S,S)-Im2An (2.0 × 10−3 M) in the presence of Zn2+ (1.0 × 10−3 M) in CD3CN at 298 K in the region of (c) 4.5−9.0 and (e) 3.0−3.4 ppm. (d) Proposed 1H NMR assignment of [(S,S)-Im2An]4(Zn2+)2 in the methyl proton region.

therefore, one can find the imidazole moieties in four different coordination environments for [(S,S)-Im2An]4(Zn2+)2 (Figure 2d), in agreement with the four singlet peaks observed by NMR (Figure 2e). Additionally, the [(S,S)-Im2An]4(Zn2+)2 model structure shows that the two anthracene protons at the 2,3 positions (Ha and Hb) are shielded by the anthracene rings in the parallel-displaced geometry (Figure 2a), corresponding C


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Inorganic Chemistry

signals were observed in the 1Bb transition of anthracene around 260 nm (Figure 4a). The Zn2+ assemblies, [(S,S)- and

to the significantly upfield-shifted protons identified by NMR (Figure 2c). No second diastereomer (such as the two helicities of P- and M-[(S,S)-Im2An]4(Zn2+)2) was detected by NMR, suggesting that no helicity is present in [(S,S)Im2An]4(Zn2+)2, in agreement with the parallel-displaced geometry. The detailed NMR assignment is given in the Experimental Section (see Supporting Information). If the Zn 2+ -assembly is assumed to be [(S,S)Im2An]4(Zn2+)2 with the global complexity G = 4 + 2 = 6, one may logically expect detection of {((S,S)Im2An)4(Zn)2(OSO2CF3)2}2+ in a positive ESI mass spectrum of the Zn2+-assembly, as the Zn2+ ions contain possible coordination sites for OSO2CF3− anions (Figure 3, inset).

Figure 3. Positive ESI mass spectrum of an acetonitrile solution of (S,S)-Im2An (2.0 × 10−3 M) in the presence of Zn2+ (1.0 × 10−3 M) in acetonitrile. Inset: Isotopically resolved signals at m/z = 1217.35604 and the calculated isotopic distributions for {((S,S)Im2An)4(Zn)2(OSO2CF3)2}2+. Chemical description of {((S,S)Im2An)4(Zn)2(OSO2CF3)2}2+.

Figure 4. CD spectra of (R,R)-Im2An (red line) and (S,S)-Im2An (blue line; concentrations: 2.0 × 10−5 M) in the absence (a) and in the presence (b) of Zn2+ (1.0 × 10−5 M) in acetonitrile at 298 K. (c) Absorption spectrum of (S,S)-Im2An (2.0 × 10−5 M) in the presence of Zn2+ (1.0 × 10−5 M) in acetonitrile at 298 K. Dashed arrows show CD and absorbance peaks in the region of the anthracene π−π* transition band. Δε was calculated based on the ligand concentrations (2.0 × 10−5 M).

Indeed, an acetonitrile solution of (S,S)-Im2An containing 0.5 equiv of Zn2+ shows an intense peak at m/z = 1217.35604 (divalent peak) in the positive ESI mass spectrum (Figure 3), whose isotopic distribution agrees well with the calculated isotopic distribution of {((S,S)-Im2An)4(Zn)2(OSO2CF3)2}2+ (calcd 1217.35492; Figure 3, inset). As the ionization technique of ESI is known to give low fragmentation, the intensity of a monovalent peak at m/z = 2584.67549 corresponding to {((S,S)-Im2An)4(Zn)2(OSO2CF3)3}+ (calcd 2584.66969) is rather weak (Figure 3). It is difficult to coordinate an additional OSO2CF3− anion to the coordinatively saturated {((S,S)-Im2An)4(Zn)2(OSO2CF3)2}2+, which may explain the weak mass intensity. Drawing conclusions from the above-discussed spectroscopic investigations, [(S,S)Im2An]4(Zn2+)2 with the global complexity G = 4 + 2 = 6 is considered to be the most probable species in solution.11 Coordination-Directed Conformational Alteration of Chiral Stacking Structures. In such anthracene dimer systems, the anthracene π−π* transition around 260 nm assigned as 1Bb has been known to exhibit intense excitoncoupled CD signals when the two transition dipole moments of the individual anthracene chromophore are rotated in the twisted dimer.19−21 Hence, Cotton effects of [(S,S)- and (R,R)-Im2An]4(Zn2+)2 enable us to determine the screw sense of the aromatic stacking between the central anthracene dimer units. In the absence of Zn2+, (S,S)- and (R,R)-Im2An give almost no CD signal in the region of the π−π* transition band of the anthracene (380−520 nm; 1La), and only weak CD

(R,R)-Im2An]4(Zn2+)2 exhibited weak CD signals (|Δε| < 10 M−1 cm−1 in the anthracene absorption region) with a mirrorimage relationship (Figure 4b; see Figure S2 for titration analysis). The weak CD peaks at 268, 431, 452, and 493 nm in the 1Bb and 1La transitions of anthracene agree well with the those of characteristic absorptions of the anthracene (Figure 4b and c, dashed arrows), corresponding to non-excitoncoupled CD. The model structure of [(S,S)-Im2An]4(Zn2+)2 supports the parallel-displaced geometry for the central anthracene dimer unit (vide supra, Figure 2a), in agreement with a non-exciton-coupled CD.38 The [(S,S)-Im2An]4(Zn2+)2 assembly contains two (S,S)Im2An ligands that are bound to Zn2+ in monodentate binding modes (vide supra); therefore, one may expect a ligandexchange reaction to take place with the use of appropriate coligands (Scheme 4). Because (S)- and (R)-Ph-box can serve as bidentate ligands for Zn2+, one also expects that coligand binding to the possible coordination sites of the Zn2+ ions in [(S,S)-Im2An]4(Zn2+)2 would give [(S,S)-Im2An]2[(S)-Phbox]2(Zn2+)2 and [(S,S)-Im2An]2[(R)-Ph-box]2(Zn2+)2, following the dissociation of the two (S,S)-Im2An ligands that were originally bound to Zn2+ in monodentate binding modes (Scheme 4). Indeed, the anthracene monomer fluorescence recovered nearly half of the original fluorescence intensity (black dashed lines) by successive injections of the (S)- and (R)-Ph-box coligands into [(S,S)-Im2An]4(Zn2+)2 in acetonitrile (Figure 5a and b, respectively), which was accompanied D


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Inorganic Chemistry Scheme 4

Figure 5. Emission spectral changes observed upon addition of (a) (S)-Ph-box [0 (blue dashed line)−4.2 × 10−5 M (blue solid line)] and (b) (R)-Ph-box [0 (blue dashed line)−4.2 × 10−5 M (red solid line)] to an acetonitrile solution of (S,S)-Im2An (2.0 × 10−5 M) containing Zn2+ (1.0 × 10−5 M) at 298 K. Dashed lines show the emission spectra of (S,S)-Im2An (2.0 × 10−5 M) in the absence of Zn2+ in acetonitrile at 298 K. Excitation wavelength: λex = 400 nm. Inset: Visible emission color changes upon addition of (a) (S)-Ph-box and (b) (R)-Ph-box. Plots of emission intensity at λ = 495 nm vs r = (a) [(S)-Ph-box]/[[(S,S)-Im2An]4(Zn2+)2]0 and (b) [(R)-Ph-box]/ [[(S,S)-Im2An]4(Zn2+)2]0. (c) Emission decay curve of free (S,S)Im2An ligand (2.0 × 10−5 M), and those of (S,S)-Im2An (2.0 × 10−5 M) with Zn2+ (1.0 × 10−5 M) in the presence of (d) (R)-Ph-box (4.4 × 10−5 M) and (e) (S)-Ph-box (4.4 × 10−5 M) in acetonitrile at 298 K. Excitation wavelength: λex = 371 nm.

Im2An]2[(S)-Ph-box]2(Zn2+)2. The ligand-exchange reaction is not stoichiometrically related to the amount of the coligands elucidated from the titration curves (Figure 5a,b, inset). More than 2 equiv of the coligands [(S)- and (R)-Ph-box] are required to complete the ligand-exchange reaction, indicating that the coordination of (R)- and (S)-Ph-box to the [(S,S)Im2An]2(Zn2+)2 unit is probably rather weak.39,40 Under the same experimental conditions as used for the fluorescence spectral titration experiments (Figure 5), CD spectral changes were recorded upon successive injections of (S)- and (R)-Ph-box coligands to monitor the change in the chiral dimer structure in the Zn2+-assembly (vide infra). The results are shown in Figure 6, where (S)- and (R)-Ph-box coligands enhance the CD signals in the 1Bb and 1La transitions of anthracene. The remarkably enhanced CD signals (Figure 6a) appear to be exciton-coupled biphasic CD, as observed in multichromophoric systems, a clear indication of a chiral stacking structure between the anthracene rings in twisted stacking modes (Scheme 4). CD spectra of [(S,S)Im2An]2[(R)-Ph-box]2(Zn2+)2 and [(S,S)-Im2An]2[(S)-Phbox]2(Zn2+)2 are nearly mirror images of each other, following the sequence “−,+” for [(S,S)-Im2An]2[(R)-Ph-box]2(Zn2+)2 and “+,−” for [(S,S)-Im2An]2[(S)-Ph-box]2(Zn2+)2 at the first and second Cotton bands in the 1Bb transition of anthracene around 260 nm (Figure 6a red and blue solid lines, respectively). Those splitting patterns correspond to the Mhelix excess for [(S,S)-Im2An]2[(R)-Ph-box]2(Zn2+)2, and the P-helix excess for [(S,S)-Im2An]2[(S)-Ph-box]2(Zn2+)2.19−21 Molecular mechanics models of [(S,S)-Im2An]2[(R)-Phbox]2(Zn2+)2 and [(S,S)-Im2An]2[(S)-Ph-box]2(Zn2+)2 are consistent with the M and P helicity, as indicated by the CD analysis (inset of Figure 6). These results suggest that the coligand-binding to the Zn2+ ions of [(S,S)-Im2An]4(Zn2+)2 causes structural alteration of the central anthracene dimer from the parallel-displaced geometry to the twisted stacking

by a visible fluorescence enhancement (Figure 5a and b, insets). The fluorescence lifetimes of the resulting fluorescent solutions (Figure 5d,e) were identical to that of the free (S,S)Im2An ligand (Figure 5c, τ = 3.1 ns), suggesting dissociation of the (S,S)-Im2An ligands through a ligand-exchange reaction, as described in Scheme 4. A minor long-lived lifetime component (τ = 15.0 ns, 1%) was present in the emission decay of the resulting fluorescent solutions (Figure 5d,e), which is thought to be a weak anthracene-dimer emission from the Zn2+ assemblies, [(S,S)-Im2An]2[(R)-Ph-box]2(Zn2+)2 and [(S,S)E


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Inorganic Chemistry

Im2An]4(Zn2+)2. The results are shown in Figure 7, where the CD and absorption spectral changes were observed on

Figure 7. (a) CD spectra of (S,S)-Im2An (2.0 × 10−5 M) with Zn2+ (1.0 × 10−5 M) in the presence of different enantiomeric excess ratios of Ph-box (total concentration: [R] + [S] = 5.0 × 10−5 M) in acetonitrile at 298 K, ([R] − [S])/([R] + [S]) = 0 (orange line), 0 to 1 (R)-Ph-box excess (red lines), 0 to −1 (S)-Ph-box excess (blue lines)]. Δε was calculated based on the ligand concentration of (S,S)Im2An (2.0 × 10−5 M). (b) Corresponding UV−vis absorption spectra. Black dashed line shows UV−vis absorption spectrum of free (S,S)-Im2An (2.0 × 10−5 M) in acetonitrile at 298 K.

changing the enantiomeric ratios of the Ph-box in the range from ([R] − [S])/([R] + [S]) = 0 (orange line) to +1 and to −1 (red and blue lines, respectively). It is noteworthy that the observed absorption spectrum under the racemic conditions (Figure 7b, orange solid line) was identical to that of the free (S,S)-Im2An ligand (Figure 7b, black dashed line). Thus, [(S,S)-Im2An]4(Zn2+)2 underwent dissociation into the free ligand under the racemic conditions (Scheme 5, dashed arrows). Then, the dissymmetry factor for CD (gCD) at the 1Bb transition of anthracene (λ = 281 nm) was determined using the equation gCD = (θ281/A281)/33000, where θ281 and A281 denote ellipse angle (in mdeg) and absorbance at 281 nm. The obtained gCD was plotted against enantiomeric excess ee = ([R] − [S])/([R] + [S]) of Ph-box, where [R] and [S] denote the concentrations of (R)- and (S)-Ph-box, respectively (Figure 8). Interestingly, the resulting gCD plot shows a highly nonlinear inverse sigmoidal relationship with a clear threshold, where the gCD values were almost zero below the threshold region, ([R] − [S])/([R] + [S]) = −0.2 ∼ +0.2 (Figure 8). The observed absorption spectra below the threshold region were almost identical to the free ligand absorption spectrum of (S,S)-Im2An (Figure 7b), which again underlines the dissociation of [(S,S)Im2An]4(Zn2+)2 into the CD-inactive free ligand [(S,S)Im2An] at the lower enantiomeric excess of Ph-box. Above the threshold region, the gCD begins to increase and decrease with increasing enantiomeric excess (Figure 8), which suggests the formation of the CD-active assemblies ([(S,S)-

Figure 6. (a) CD spectral changes observed upon addition of (S)-Phbox [0 (blue dashed line)−4.2 × 10−5 M (blue solid line)] and (R)Ph-box [0 (blue dashed line)−4.2 × 10−5 M (red solid line)] to an acetonitrile solution of (S,S)-Im2An (2.0 × 10−5 M) containing Zn2+ (1.0 × 10−5 M) at 298 K. Corresponding UV−vis absorption spectral changes observed upon addition of (b) (S)-Ph-box and (c) (R)-Phbox. Red and blue arrows indicate the direction of spectral changes upon addition of (R)-Ph-box and (S)-Ph-box, respectively. Inset: (a) Dominant helicity of the chiral stacking structures between the anthracene rings inside [(S,S)-Im2An]2[(S)-Ph-box]2(Zn2+)2 (blue) and [(S,S)-Im2An]2[(R)-Ph-box]2(Zn2+)2 (red) estimated by the exciton-coupled biphasic CD signals. Energy-minimized structures of [(S,S)-Im2An]2[(S)-Ph-box]2(Zn2+)2 and [(S,S)-Im2An]2[(R)-Phbox]2(Zn2+)2. Δε was calculated based on the ligand concentration of (S,S)-Im2An (2.0 × 10−5 M).

structure associated with the change of the Zn2+ assemblies (Scheme 4). The induced twisted stacking geometry may be attributed to ligand−ligand repulsion or interaction between the coligands and (S,S)-Im2An inside in the assembly.41 Highly Nonlinear Effects in Competing Allosteric Mechanisms. Owing to these results, we have investigated the coordination-induced CD signal upon the addition of different enantiomeric ratios of the chiral coligands [(R)- and (S)-Phbox], where the total concentration of Ph-box was kept constant at each molar ratio, [R] + [S] = 5.0 × 10−5 M: 10 equiv with respect to the initial concentration of [(S,S)F


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Inorganic Chemistry

εL and εRR represent the molar absorption coefficient (at λ = 281 nm) of the free (S,S)-Im2An ligand and the complexed (S,S)-Im2An ligand in [(S,S)-Im2An]2[(R)-Ph-box]2(Zn2+)2, respectively. In the same way, the εL/εRR value was determined experimentally as εL/εRR = 1.6 (see details in Supporting Information S6). It should be noted that enantiomerically pure (R)- and (S)-Ph-box coligands induced almost the same absorption spectra (Figure 7b, red solid and blue dashed lines in bold, respectively). Hence, the two diastereomers complexes ([(S,S)-Im 2 An] 2 [(R)-Ph-box] 2 (Zn 2 + ) 2 and [(S,S)Im2An]2[(S)-Ph-box]2(Zn2+)2) exhibited almost the same absorption spectra (εRR = εSS). Conversely, γ denotes the ratio of the concentration of the free (S,S)-Im2An ligand in the solution to the total (initial) concentration of (S,S)-Im2An, [(S,S)-Im2An]f/[(S,S)-Im2An]0, where γ changed from γ = 0.5 at ee = −1 and +1 to γ = 1.0 at ee = 0 (vide infra). Thus, eq 1 theoretically predicts that gCD becomes zero at ee = 0 and reaches positive and negative maxima at ee = −1 and +1, respectively, which was consistent with the observed gCD plot (Figure 8). The observed nonlinear relationship between gCD and enantiomeric excess (Figure 8) can be well explained by competing allosteric mechanisms given in Scheme 5 (vide infra). In the present mechanisms, homochiral association of the Ph-box coligands to [(S,S)-Im2An]4(Zn2+)2 results in formation of the CD-active assemblies, [(S,S)-Im2An]2[(R)Ph-box]2(Zn2+)2 and [(S,S)-Im2An]2[(S)-Ph-box]2(Zn2+)2 (Scheme 5, solid arrows). Conversely, once a combination of (R)- and (S)-Ph-box acts to sequester Zn2+ inside the assembly, [(S,S)-Im2An]4(Zn2+)2 undergoes dissociation into the CD-inactive (S,S)-Im2An ligands (Scheme 5, dashed arrows). The above two processes have opposite effects on gCD, where the larger gCD (in absolute value) would be obtained with a larger contribution of the homochiral cooperative association (Scheme 5, solid arrows). Conversely, the smaller gCD (in absolute value) is expected with a larger contribution of the heterochiral cooperative dissociation (Scheme 5, dashed arrows). It is noteworthy that we assumed a heterochiral 2:1 Zn2+ complex ([(R)-Ph-box][(S)-Ph-box]Zn2+) based on the NMR titration data (Figure 9, vide infra). Upon successive addition of 0−2.0 equiv of Zn2+ to a CD3CN solution of enantiomerically pure (R)-Ph-box (4.0 × 10−3 M), the free ligand NMR proton signals gradually decreased, which was associated with the appearance of new signals corresponding to (R)-Ph-box bound to Zn2+ (Figure 9a). The NMR signals arising from free (R)-Ph-box completely disappeared at the molar ratio of 1.0 ([Zn2+]/[(R)-Ph-box]0 = 1.0, Figure 9a, red line). No further spectral change was observed above the molar ratio of 1.0 (Figure 9a), which suggested a 1:1 binding stoichiometry between enantiomerically pure Ph-box ligands and Zn2+ (i.e., [(R)-Ph-box]Zn2+ and [(S)-Ph-box]Zn2+).27 Conversely, different NMR titration patterns were observed under racemic conditions (Figure 9b). Upon successive addition of 0−1.0 equiv of Zn2+ (0−4.0 × 10−3 M) to a racemic mixture of (R)- and (S)-Ph-box (concentrations: 2.0 × 10−3 M), the free ligand NMR signals gradually decreased with a concomitant appearance of NMR signals arising from a Zn2+-complex (Figure 9b). The free ligand NMR signals disappeared completely on titration with approximately 0.5 equiv of Zn2+ (2.0 × 10−3 M, Figure 9b blue line) toward the total concentration of Ph-box ligands ([R]0 + [S]0 = 4.0 × 10−3 M), which indicated a 2:1 complex stoichiometry under the racemic conditions. The resulting

Scheme 5

Figure 8. Plot of gCD at λ = 281 nm vs ([R] − [S])/([R] + [S]) in CD spectra of (S,S)-Im2An (2.0 × 10−5 M) with Zn2+ (1.0 × 10−5 M) in the presence of different enantiomeric excess ratios of Ph-box (total concentration [R] + [S] = 5.0 × 10−5 M) in acetonitrile at 298 K. Theoretical curve (dashed line) is calculated from a combination of eq 1 and 2 using values for (K′S·K′SR)/(KS·KSS) = 2 × 10−4 M, gRR = −0.0038 and gSS = +0.0033.

Im2An]2[(R)-Ph-box]2(Zn2+)2 and [(S,S)-Im2An]2[(S)-Phbox]2(Zn2+)2) in the higher enantiomeric excess regions. The observed gCD can be expressed by eq 1 (for derivation of eq 1, see Supporting Information S6), where gRR and gSS denote gCD values (at λ = 281 nm) of [(S,S)-Im 2 An] 2 [(R)-Phbox]2(Zn2+)2 and [(S,S)-Im2An]2[(S)-Ph-box]2(Zn2+)2, respectively. The gRR and gSS values can be determined experimentally by measurement of CD and absorption spectra at ee = ([R] − [S])/([R] + [S]) gRR

gCD =

2

·(1 − γ ) ·

(1 + ee)2 1 + ee 2

(

1+γ

+ εL εRR

gss 2

·(1 − γ ) ·

)

−1

(1 − ee)2 1 + ee 2

(1)

= −1 and +1, and given as gRR = −0.0038 and gSS = +0.0033 (see details in Supporting Information S6). On the other hand, G


Article

Inorganic Chemistry

also KS·KSS) determine the nonlinear response of gCD to the enantiomeric excess of Ph-box. One can derive eq 2 by assuming that the KR·KRR is equal to KS·KSS (see details in Supporting Information S9), where [Phbox]0 denotes the total concentrations of (R)- and (S)-Ph-box ([R] + [S]).A theoretical curve was calculated from a combination of eqs 1 and 2 using values for (K′S·K′SR)/(KS· KSS) = 2 × 10−4 M, which successfully reproduced the highly nonlinear inverse sigmoidal relationship (dashed line in Figure 8). Conversely, there was a deviation between the theoretical curve and the experimental data points (Figure 8). This deviation may arise from the fact that there is a difference between KR·KRR and KS·KSS, as well as from contributions of minor complexes, that is, [(R)- or (S)-Ph-box]Zn2+ (1:1 complexes). The minor complexes do not essentially coexist at the lower enantiomeric excess, since the heterochiral 2:1 complex ([(R)-Ph-box][(S)-Ph-box]Zn2+) is favorably formed under racemic conditions (vide supra). Perhaps contributions of the minor complexes were not negligibly small in the higher enantiomeric excess, which resulted in a relatively large deviation between the theoretical curve and the experimental data points around ([R] − [S])/([R] + [S]) = −1 and +1 (Figure 8). By considering these minor contributions, overall, eqs 1 and 2 theoretically explained the highly nonlinear inverse sigmoidal response of gCD to enantiomeric excess. More importantly, eqs 1 and 2 theoretically predicted that a significant nonlinear response would be obtained with an increased concentration of Ph-box (in the total amount) when the [Ph-box]02/[(S,S)-Im2An]03 term became large. Under coligand excess conditions of 30 equiv with respect to the initial concentration of [(S,S)-Im2An]4(Zn2+)2, the relationship between gCD and enantiomeric excess was theoretically predicted by eqs 1 and 2 (Figure 10, dashed line), where the other parameters, gRR, gSS, and (K′S·K′SR)/(KS·KSS) remained unchanged. The theoretically predicted relationship between gCD and enatiomeric excess showed a significant inverse

Figure 9. Stacked 1H NMR spectra of (a) (R)-Ph-box (4.0 × 10−3 M) in the presence of Zn2+ (0−8.0 × 10−3 M) and (b) racemic mixture of (R)-Ph-box (2.0 × 10−3 M) and (S)-Ph-box (2.0 × 10−3 M) in the presence of Zn2+ (0−4.0 × 10−3 M) in CD3CN at 298 K.

NMR chemical shifts obtained under the racemic conditions (Figure 9b, blue line) differed significantly from those obtained with enantiomerically pure (R)-Ph-box (Figure 9a, red line).42 The different NMR titration patterns dependence on enantiomeric ratios of Ph-box is clear proof that the heterochiral 2:1 complex ([(R)-Ph-box][(S)-Ph-box]Zn2+) was favorably formed under racemic conditions as compared to the 1:1 complexes ([(R)-Ph-box]Zn2+ and [(S)-Ph-box]Zn2+). This observation also implies that the second (heterochiral) binding constant (K′RS = K′SR) is much larger than the initial binding constant (K′R = K′S) in the homochiral cooperative dissociation, that is, K′RS ≫ K′R and K′SR ≫ K′S (Scheme 5, dashed arrows). Conversely, formation of a heterochiral dimer assembly, such as [(S,S)-Im2An]2[(R)-Ph-box][(S)-Ph-box](Zn2+)2, was not considered in Scheme 5, because the observed absorption spectral changes showed a clear isosbestic point at 487 nm (Figure 7b), which indicated that only free (S,S)-Im2An and the CD-active assemblies ([(S,S)-Im 2 An] 2 [(R)-Phbox]2(Zn2+)2 and [(S,S)-Im2An]2[(S)-Ph-box]2(Zn2+)2) were present in the solution at each enantiomeric composition.43 The isosbestic point also suggested the much larger second (homochiral) association constants (KRR and KSS) than the initial association constants (KR and KS) in the homochiral cooperative association, that is, KRR ≫ KR and KSS ≫ KS (solid arrows in Scheme 5). The heterochiral cooperative dissociation (Scheme 5, dashed arrows) was dominant in the lower enantiomeric excess, owing to the favorable formation of the heterochiral 2:1 complex ([(R)-Ph-box][(S)-Ph-box]Zn2+). Conversely, the homochiral cooperative association (Scheme 5, solid arrows) became dominant in the higher enantiomeric excess because of the favorable formation of the homochiral assemblies ([(S,S)-Im2An]2[(R)- or (S)-Ph-box]2(Zn2+)2) and the less-favored formation of the heterochiral 2:1 Zn2+ complex ([(R)-Ph-box][(S)-Ph-box]Zn2+). Consequently, the magnitude balance between K′R·K′RS (= K′S·K′SR) and KR·KRR (and

Figure 10. Plot of gCD at λ = 281 nm vs ([R] − [S])/([R] + [S]) in the CD spectra of (S,S)-Im2An (2.0 × 10−5 M) with Zn2+ (1.0 × 10−5 M) in the presence of different enantiomeric excess ratios of Ph-box (total concentration [R] + [S] = 1.5 × 10−4 M) in acetonitrile at 298 K. Theoretical curve (dashed line) is calculated from a combination of eq 1 and 2 using values for (K′S·K′SR)/(KS·KSS) = 2 × 10−4 M, gRR = −0.0038 and gSS = +0.0033. H


Inorganic Chemistry

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sigmoidal relationship with a wide threshold range (Figure 10 dashed line). Based on the theoretical prediction, gCD remains zero over the wide range of enantiomeric ratios; however, gCD suddenly increases and decreases at around ([R] − [S])/([R] + [S]) = −1 and +1, respectively (Figure 10, dashed line). The experimentally obtained gCD plot (Figure 10, closed circles) agreed well with the calculated curve (for the corresponding UV and CD spectra, see Supporting Information, Figure S10). Thus, the presented competing allosteric mechanism, homochiral cooperative association (homochiral self-sorting) versus heterochiral cooperative dissociation, works well for regulating switch-like chiroptical OFF/ON states.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Junpei Yuasa: 0000-0003-1117-7904 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partly supported by JSPS KAKENHI Grant Number JP17H05386 (J.Y.) in Scientific Research on Innovative Areas “Coordination Asymmetry” and the Asahi Glass Foundation, as well as Takahashi Industrial and Economic Research Foundation. We thank the Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.

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SUMMARY AND CONCLUSIONS We have successfully demonstrated the competing allosteric mechanisms for coordination-directed conformational changes of chiral stacking structures between aromatic rings using chiral bridging ligands [(S,S)- and (R,R)-Im2An] containing an anthracene ring spacer. The chiral bridging ligands form L4M2type assemblies with Zn2+, which provides anthraceneanthracene stacking with a parallel-displaced geometry. The parallel-displaced geometry changes to a twisted stacking structure when the L4M2-type assembly [(S,S)-Im2An]4(Zn2+)2 converts into [(S,S)-Im2An]2[(R)-Ph-box]2(Zn2+)2 and [(S,S)Im2An]2[(S)-Ph-box]2(Zn2+)2 upon addition of chiral coligands, (R)- and (S)-Ph-box, respectively. The coordinationdirected conformational alteration of the chiral stacking structure remarkably enhances its chiroptical property, which is shown by an enhancement of the dissymmetry factor for circular dichroism (gCD). The observed gCD shows highly nonlinear inverse sigmoidal responses to enatiomeric excess of the chiral coligands owing to the competing allosteric mechanisms. Homochiral cooperative association is the dominant mechanism at the higher enatiomeric excess, where the CD-active assemblies [(S,S)-Im2An]2[(R)-Ph-box]2(Zn2+)2 and [(S,S)-Im2An]2[(S)-Ph-box]2(Zn2+)2 are favorably formed through homochiral association of the Ph-box coligands. Conversely, heterochiral cooperative dissociation becomes the dominant mechanism at the lower enatiomeric excess, where the combination of (R)- and (S)-Ph-box works to sequester the Zn2+ inside the initial [(S,S)-Im2An]4(Zn2+)2 assembly. This causes the dissociation of [(S,S)Im2An]4(Zn2+)2 into the CD-inactive (S,S)-Im2An ligand with the concomitant formation of the heterochiral 2:1 Zn2+complex ([(R)-Ph-box][(S)-Ph-box]Zn2+). The two mechanisms, homochiral cooperative association (homochiral selfsorting) and heterochiral cooperative dissociation, result in opposite effects and therefore generate switch-like chiroptical OFF/ON states. The present strategy will open up a new concept for allosteric regulation in chiral systems, ability to control aromatic stacking geometry, and chiroptical activity based on supramolecular coordination chemistry.

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Article

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00665. Experimental details, including 1H NMR assignment of the Zn2+ assembly, CD and NMR titration experiments, ESI mass spectra, ROESY NMR spectrum, and derivation of eqs 1 and 2 (PDF) I


Article

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Inorganic Chemistry 2365. (b) Kawai, T.; Kawamura, K.; Tsumatori, H.; Ishikawa, M.; Naito, M.; Fujiki, M.; Nakashima, T. Circularly Polarized Luminescence of a Fluorescent Chiral Binaphtylene-Perylenebiscarboxydiimide Dimer. ChemPhysChem 2007, 8, 1465−1468. (26) (a) Desimoni, G.; Faita, G.; Jørgensen, K. A. C2-Symmetric Chiral Bis(Oxazoline) Ligands in Asymmetric Catalysis. Chem. Rev. 2006, 106, 3561−3651. (b) Desimoni, G.; Faita, G.; Jørgensen, K. A. Update 1 of: C2-Symmetric Chiral Bis(oxazoline) Ligands in Asymmetric Catalysis. Chem. Rev. 2011, 111, PR284−437. (27) Enantiomerically pure Ph-box has been normally used in asymmetric catalytic reactions under the 1:1 conditions with Zn2+; hence, the resulting 1:1 complexes, (S)- and (R)-Ph-box-Zn2+ act as effective asymmetric catalysts, see: (a) Wu, J. H.; Radinov, R.; Porter, N. A. Enantioselective Free Radical Carbon-Carbon Bond Forming Reactions: Chiral Lewis Acid Promoted Acyclic Additions. J. Am. Chem. Soc. 1995, 117, 11029−11030. (b) Sibi, M. P.; Ji, J.; Wu, J. H.; Gürtler, S.; Porter, N. A. Chiral Lewis Acid Catalysis in Radical Reactions: Enantioselective Conjugate Radical Additions. J. Am. Chem. Soc. 1996, 118, 9200−9201. (c) Jia, Y.-X.; Zhu, S.-F.; Yang, Y.; Zhou, Q.-L. Asymmetric Friedel-Crafts Alkylations of Indoles with Nitroalkenes Catalyzed by Zn(II)-Bisoxazoline Complexes. J. Org. Chem. 2006, 71, 75−80. (28) (a) Scarso, A.; Rebek, J., Jr. In Topics in Current Chemistry: Supramolecular Chirality.; Crego-Calama, M., Reinhoudt, D. N., Eds.; Springer-Verlag: Berlin, 2006; Vol. 265, pp 1−46. (b) Girard, C.; Kagan, H. B. Nonlinear Effects in Asymmetric Synthesis and Stereoselective Reactions: Ten Years of Investigation. Angew. Chem., Int. Ed. 1998, 37, 2922−2959. (c) Shinkai, S.; Ikeda, M.; Sugasaki, A.; Takeuchi, M. Positive Allosteric Systems Designed on Dynamic Supramolecular Scaffolds: Toward Switching and Amplification of Guest Affinity and Selectivity. Acc. Chem. Res. 2001, 34, 494−503. (d) Yashima, E.; Ousaka, N.; Taura, D.; Shimomura, K.; Ikai, T.; Maeda, K. Supramolecular Helical Systems: Helical Assemblies of Small Molecules, Foldamers, and Polymers with Chiral Amplification and Their Functions. Chem. Rev. 2016, 116, 13752−13990. (29) (a) Mizutani, T.; Sakai, N.; Yagi, S.; Takagishi, T.; Kitagawa, S.; Ogoshi, H. Allosteric Chirality Amplification in Zinc Bilinone Dimer. J. Am. Chem. Soc. 2000, 122, 748−749. (b) Ikeda, T.; Hirata, O.; Takeuchi, M.; Shinkai, S. Highly Enantioselective Recognition of Dicarboxylic Acid Substrates by the Control of Nonlinear Responses. J. Am. Chem. Soc. 2006, 128, 16008−16009. (30) (a) Huang, W.-H.; Liu, S.; Zavalij, P. Y.; Isaacs, L. Nor-SecoCucurbit[10]uril Exhibits Homotropic Allosterism. J. Am. Chem. Soc. 2006, 128, 14744−14745. (b) Isaacs, L. Cucurbit[n]urils: from mechanism to structure and function. Chem. Commun. 2009, 619− 629. (31) Ruffin, H.; Boussambe, G. N. M.; Roisnel, T.; Dorcet, V.; Boitrel, B.; Gac, S. L. Tren-Capped Hexaphyrin Zinc Complexes: Interplaying Molecular Recognition, Mö bius Aromaticity, and Chirality. J. Am. Chem. Soc. 2017, 139, 13847−13857. (32) Yamasaki, Y.; Shio, H.; Amimoto, T.; Sekiya, R.; Haino, T. Majority-Rules Effect and Allostery in Molecular Recognition of Calix[4]arene-Based Triple-Stranded Metallohelicates. Chem. - Eur. J. 2018, 24, 8558−8568. (33) Kumar, M.; George, S. J. Homotropic and heterotropic allosteric regulation of supramolecular chirality. Chem. Sci. 2014, 5, 3025−3030. (34) Suzuki, Y.; Nakamura, T.; Iida, H.; Ousaka, N.; Yashima, E. Allosteric Regulation of Unidirectional Spring-like Motion of DoubleStranded Helicates. J. Am. Chem. Soc. 2016, 138, 4852−4859. (35) In the present study, we wish to use the term, competing allosteric mechanisms, as illustrating the situation in which two conflict mechanisms work in the same system (Schemes 2b and 5). For other definitions of competing allosteric mechanisms, see: Freiburger, L. A.; Baettig, O. M.; Sprules, T.; Berghuis, A. M.; Auclair, K.; Mittermaier, A. K. Competing allosteric mechanisms modulate substrate binding in a dimeric enzyme. Nat. Struct. Mol. Biol. 2011, 18, 288−294.

(36) Under our experimental conditions, neither [4 + 4] photocyclodimerization nor thermally accessible [4 + 2] cyclodimerization were observed, see: (a) Tanabe, J.; Taura, D.; Ousaka, N.; Yashima, E. Chiral Template-Directed Regio-, Diastereo-, and Enantioselective Photodimerization of an Anthracene Derivative Assisted by Complementary Amidinium-Carboxylate Salt Bridge Formation. J. Am. Chem. Soc. 2017, 139, 7388−7398. (b) Kawanami, Y.; Katsumata, S.-y.; Nishijima, M.; Fukuhara, G.; Asano, K.; Suzuki, T.; Yang, C.; Nakamura, A.; Mori, T.; Inoue, Y. Supramolecular Photochirogenesis with a Higher-Order Complex: Highly Accelerated Exclusively Head-to-Head Photocyclodimerization of 2-Anthracenecarboxylic Acid via 2:2 Complexation with Prolinol. J. Am. Chem. Soc. 2016, 138, 12187−12201. (c) Ishida, Y.; Matsuoka, Y.; Kai, Y.; Yamada, K.; Nakagawa, K.; Asahi, T.; Saigo, K. Metastable Liquid Crystal as Time-Responsive Reaction Medium: Aging-Induced Dual Enantioselective Control. J. Am. Chem. Soc. 2013, 135, 6407−6410. (d) Yuasa, J.; Ogawa, T.; Kawai, T. Remarkable rate acceleration of [4 + 2] cyclodimerization of an ethynylanthracene derivative on the product crystal surfaces. Chem. Commun. 2010, 46, 3693−3695. (37) Hinoue, T.; Shigenoi, Y.; Sugino, M.; Mizobe, Y.; Hisaki, I.; Miyata, M.; Tohnai, N. Regulation of R-Stacked Anthracene Arrangement for Fluorescence Modulation of Organic Solid from Monomer to Excited Oligomer Emission. Chem. - Eur. J. 2012, 18, 4634−4643. (38) Koga, S.; Ueki, S.; Shimada, M.; Ishii, R.; Kurihara, Y.; Yamanoi, Y.; Yuasa, J.; Kawai, T.; Uchida, T.; Iwamura, M.; Nozaki, K.; Nishihara, H. Access to Chiral Silicon Centers for Application to Circularly Polarized Luminescence Materials. J. Org. Chem. 2017, 82, 6108−6117. (39) In such a case, NMR analysis is not feasible to monitor the conversion of the assemblies. The 1H NMR signals of [(S, S)Im2An]4(Zn2+)2 disappeared upon addition of 10.4 equiv of (R)-Phbox with respect to [(S, S)-Im2An]4(Zn2+)2, which is consistent with the experimental results obtained by emission titration (Figure 6). However, the 1H NMR signals due to [(S, S)-Im2An]2[(R)-Phbox]2(Zn2+)2 were mostly overlapping with those of the excess (R)Ph-box (Figure S3 in Supporting Information). The 1H NMR signals of the free (S, S)-Im2An ligands may merge into those of [(S, S)Im2An]2[(R)-Ph-box]2(Zn2+)2 due to rapid exchange on the NMR time scale. (40) In contrast to the coordinatively unsaturated [(S,S)Im2An]4(Zn2+)2, ESI mass detection of [(S,S)-Im2An]2[(R)-Phbox]2(Zn2+)2 could be problematic. There is no opportunity for OSO2CF3− to bind the coordinatively saturated [(S,S)-Im2An]2[(R)Ph-box]2(Zn2+)2, and hence one logically expects the tetravalent ion peak for {[(S, S)-Im2An]2[(R)-Ph-box]2(Zn2+)2}4+, which would be difficult to detect by ESI mass spec. No corresponding mass peak was observed in the mass range from m/z = 100−3000 (Figure S4 in Supporting Information). (41) (a) Yuasa, J.; Ohno, T.; Miyata, K.; Tsumatori, H.; Hasegawa, Y.; Kawai, T. Noncovalent Ligand-to-Ligand Interactions Alter Sense of Optical Chirality in Luminescent Tris(N-diketonate) Lanthanide(III) Complexes Containing a Chiral Bis(oxazolinyl) Pyridine Ligand. J. Am. Chem. Soc. 2011, 133, 9892−9902. (b) Bing, T. Y.; Kawai, T.; Yuasa, J. Ligand-to-Ligand Interactions That Direct Formation of D2Symmetrical Alternating Circular Helicate. J. Am. Chem. Soc. 2018, 140, 3683−3689. (c) Okayasu, Y.; Yuasa, J. Evaluation of circularly polarized luminescence in a chiral lanthanide ensemble. Mol. Syst. Des. Eng. 2018, 3, 66−72. (42) The resulting NMR chemical shift of the aliphatic protons (closed triangles) under the racemic conditions show a considerable upfield from 4.08 ppm to 3.51 ppm (Figure 9b), indicating that the aliphatic protons of the Ph-box ligand are shielded by the Ph groups of the other Ph-box ligand in the heterochiral 2:1 complex ([(R)-Phbox][(S)-Ph-box]Zn2+). This observation is consistent with the energy-minimized of the heterochiral 2:1 complex (Supporting Information, Figure S7). (43) For the energy-minimized structure of the heterochiral dimer complex, see Supporting Information, Figure S8. K


Competing Allosteric Mechanisms for Coordination-Directed

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