Chiral Discrimination of Diamines by a Binaphthalene-Bridged

Jun 24, 2017 - A pair of 1,1′-binaphthalene-bridged bisporphyrins, (R)- and (S)-H1, were designed to examine their chiral discrimination abilities t...
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Chiral Discrimination of Diamines by a Binaphthalene-Bridged Porphyrin Dimer Wenxin Lu, Huifang Yang, Xinyao Li, Chiming Wang, Xiaopeng Zhan, Dongdong Qi, Yongzhong Bian,* and Jianzhuang Jiang* Beijing Key Laboratory for Science and Application of Functional Molecular and Crystalline Materials, Department of Chemistry, University of Science and Technology Beijing, Beijing 100083, China S Supporting Information *

ABSTRACT: A pair of 1,1′-binaphthalene-bridged bisporphyrins, (R)- and (S)-H1, were designed to examine their chiral discrimination abilities toward a range of model diamines by using UV−vis absorption, CD, and 1H NMR spectroscopy with the assistance of DFT molecular modeling. The spectroscopic titrations revealed that (R)-/(S)-H1 could encapsulate (R)-/(S)-DACH and (R)-/(S)-PPDA in the chiral bisporphyrin cavities, leading to the selective formation of sandwich-type 1:1 complexes via dual Zn−N coordination interactions. In particular, the chiral recognition energy (ΔΔG°) toward (R)-/(S)-DACH was evaluated to be −4.02 kJ mol−1. The binding processes afforded sensitive CD spectral changes in response to the stereostructure of chiral diamines. Remarkable enantiodiscrimination effects were also detected in the NMR titrations of (R)-/(S)-H1, in which the nonequivalent chemical shift (ΔΔδ) can reach up to 0.57 ppm for (R)-/(S)DACH. However, due to the large steric effect, another chiral diamine ((R)-/(S)-DPEA) could not be sandwiched in the chiral bisporphyrin cavity; therefore, (R)-/(S)-DPEA could hardly be discriminated by (R)-/(S)-H1. The present results demonstrate a chiral bisporphyrin host with integrated CD and NMR chiral sensing functions and also highlight the binding-mode-dependent character of its enantiodiscrimination performance for different chiral guests.

1. INTRODUCTION Chiral discrimination originates from the different physical and chemical properties of diastereomeric supramolecular systems, which is generally involved in the supramolecular interactions of chiral species. The related effects hold very important functions in biological systems1,2 and have many practical applications in absolute configuration (AC) assignment,3 enantiomer resolution,4 enantiomeric excess (ee) analysis,5 molecular recognition,6,7 asymmetric catalysis,8 and materials science.9,10 Consequently, accurate and efficient chiral discrimination systems are highly desirable. On the other hand, porphyrin derivatives have been intensely studied in the fields of supramolecular chemistry11 and stereochemistry.12 Due to their unique electronic and photonic activities in response to structural variation and external stimuli, porphyrins are highly involved in stereoselective molecular recognition systems as chiral optical probes.13−15 In particular, covalently linked porphyrin dimers16−18 possess preorganized cavities for the binding of guest molecules via noncovalent interactions, and the chiral optical signals are very sensitive to the guest-induced allosteric effects. For example, upon binding of a chiral guest molecule to an achiral porphyrin dimer, the induced bisignate circular dichroism (CD) signals can be correlated to the stereostructure of the chiral guest by the exciton coupled circular dichroism (ECCD) protocol.19 As a © 2017 American Chemical Society

result, achiral porphyrin dimers can serve as chiral sensing systems to determine the absolute stereochemistry of chiral guest molecules.20−22 However, chiral discrimination systems require the receptors (hosts) themselves to be chiral. In this regard, chiral porphyrin dimers were constructed by incorporating intrinsically chiral porphyrins23,24 or, alternatively, by connecting two porphyrin monomers with one25−32 or two33,34 chiral spacers. Enantioselective binding of D- and L-amino acid esters was observed by using relatively rigid chiral bisporphyrin tweezers with Tröger’s base24 or binaphthyl29 spacers. The enantioselective extraction of chiral fullerene C7623 and optical resolution of P- and Mhelical peptides33 were even achieved by the confined asymmetric cavities of cyclic porphyrin dimers. In contrast, for a relatively flexible chiral bisporphyrin tweezer with a isophthalic spacer,27 very weak enantioselectivities toward chiral diamine derivatives were detected; nevertheless, remarkable sensitivities as an NMR chiral shift reagent (CSR) were revealed due to the induced-fit binding of diamine guests. It is likely that a rigid or confined host is favorable to obtain a high enantioselectivity, while a flexible host is advantageous to afford high binding affinities to a wide variety of chiral substrates by an Received: April 13, 2017 Published: June 24, 2017 8223

DOI: 10.1021/acs.inorgchem.7b00920 Inorg. Chem. 2017, 56, 8223−8231

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chiral discrimination, thus providing a new step forward in the development of an efficient and versatile chiral discrimination system.

induced-fit binding which defines the sensitivity and scope of a chiral optical or NMR sensor. A subtle balance between rigidity and flexibility is required for improving the enantioselectivity and chiral sensing properties, which causes the design of highperformance chiral discrimination systems to remain a challenging task, particularly for bisporphyrin systems with integrated functions of high CD and NMR sensitivities. In this direction, we recently reported a series of 1,1′-bi-2naphthol (BINOL)-bridged porphyrin tweezer hosts.35 We demonstrated that, by changing the length and flexibility of the chemical connections between the chiral auxiliary (BINOL) and the functional terminals (porphyrin moieties), tunable asymmetric bisporphyrin pockets could be obtained to accommodate rigid ditopic guests via an induced-fit binding mode. Here we report the design and synthesis of a new pair of chiral bisporphyrin hosts (R)-/(S)-H1, which feature a semirigid binaphthalene linkage and a preorganized chiral twist (Scheme 1). The chiral discrimination abilities toward 1,2-

2. RESULTS AND DISCUSSION 2.1. Design and Synthesis. Zinc(II) porphyrin units can serve as signaling functionalities and recognition sites on the basis of their optoelectronically active π system and their selective axial coordination ability. In addition, due to the diamagnetic porphyrin ring current, an axial ligand will receive a strong anisotropic shielding effect (ASE) depending on the relative distance and orientation to the porphyrin plane;36 as a result, the NMR signals of the axial ligand shift significantly upfield and become easily distinguished from background signals. 1,1′-Binaphthalene derivatives have stable C2 axial chirality and are readily available chiral auxiliaries for the research of asymmetric synthesis37 and molecular recognition.38 In this work, the 2,2′-sites of 1,1′-binaphthalene were selected to attach two porphyrin units to provide confined bisporphyrin pockets with C2 symmetry for the binding of 1,2-diamines. The rigidity and dimension of the bisporphyrin hosts were further adjusted by choosing the chemical connections between the chiral auxiliary and the functional terminals.35 The new porphyrin dimers (R)-/(S)-H1 were synthesized via Williamson’s coupling reaction of (R)- or (S)-2,2′-bis(bromomethyl)-1,1′-binaphthyl with a zinc(II) porphyrin phenolic derivative39 and adequately characterized (see the Experimental Section and the Supporting Information for details). Split Cotton effects are observed in the Soret band region with opposite signs for (R)- and (S)-H1 (Figure S3 in the Supporting Information). As expected, (R)-H1 shows negative CD couplets corresponding to an anticlockwise twist angle between the effective electric transition moments (EETMs, the C5/C15 direction) of the two porphyrin units,19 while (S)-H1 exhibits positive CD couplets corresponding to a clockwise chiral twist of the EETMs, indicating that the C2 chirality of the 1,1′-binaphthyl linker is translated to a defined chiral twist in the interporphyrin arrangement. Furthermore, the CD intensity value of (R)- and (S)-H1 (|A| ≈ 400 mol−1 L cm−1) is between those of formic acid ester and 1,2-dioxyethyl linked BINOL-bisporphyrin analogues,35 corresponding to the order of rigidity and interporphyrin distances. 2.2. UV−Vis Spectrophotometric Titration. The binding behaviors of (R)- and (S)-H1 with chiral 1,2-diamines were first examined by a UV−vis spectrophotometric titration method. For example, upon titration of (R)-H1 with (R)-DACH (0−20

Scheme 1. Structures of the Zinc(II) Bis-Porphyrin Hosts and the 1,2-Diamine Guests

diamines, such as (R)-/(S)-DACH, (R)-/(S)-PPDA, and (R)-/ (S)-DPEA, were assessed by using UV−vis absorption, CD, and 1 H NMR spectroscopy with the assistance of DFT molecular modeling. The results demonstrate that a well-designed chiral bisporphyrin host can possess high enantioselectivity together with the dual function of high CD and CSR sensitivities for

Figure 1. (a) Spectral change upon titration of (R)-H1 with (R)-DACH in CHCl3 at 298 K. (b) Changes in ΔA at 420 nm for evaluating Kassoc, where the solid line represents the nonliner least-squares fit for 1:1 complexation. Conditions: [(R)-H1] = 1.0 × 10−6 M; [(R)-DACH]/[(R)-H1] = 0−20. 8224

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Zn−N coordination complexes of ZnTPP units with monoamines.42 To evaluate the chiral discrimination ability, the association constants were compared in Table 1. Obvious differences in the binding affinity of (R)-H1 with each enantiomer of model 1,2diamines are observed, indicating remarkable enantiodiscrimination effects corresponding to the formation of diastereoisomeric complexes. The enantioselectivity (α) is up to 5.07 for host (R)-H1 with (R)-/(S)-DACH, leading to a chiral recognition energy (ΔΔG°) of −4.02 kJ mol−1 under the present conditions. However, the enantioselectivity α of (R)H1 toward (R)-/(S)-PPDA is only 1.38 with a ΔΔG° value of −0.80 kJ mol−1. The difference in α for (R)-/(S)-DACH and (R)-/(S)-PPDA can be ascribed to the different stereochemical features. Since (R)-/(S)-DACH has two asymmetric centers while (R)-/(S)-PPDA has only one, and DACH contains a relatively rigid cyclohexyl skeleton, the two NH2 groups in (R)-/(S)-DACH are more highly preorganized than those in (R)-/(S)-PPDA. In the case of 1:2 complexation for (R)-H1 with (R)-/(S)-DPEA, the enantioselectivity α is even as low as 1.27. By comparison with those of (R)-/(S)-DACH and (R)-/ (S)-PPDA, it is clear that a ditopic 1:1 binding mode is crucial for the present system to obtain a high enantioselectivity. In addition, it is interesting to note that the Kassoc values of (R)-H1 with (S)-diamines are always larger than those with (R)diamines in the present scope, thus tending to a “stereocomplementary” nature for this chiral recognition system. According to the diastereomorphic and enantiomorphic relationship, we can reasonably expect that (S)-H1 exhibits a higher binding affinity toward (R)-diamines in comparison to that of (S)-diamines, leading to an opposite enantioselectivity with essentially the same α and ΔΔG° values. 2.3. Circular Dichroism Response. The chiral discrimination abilities of (R)-/(S)-H1 toward the model 1,2-diamines were also monitored by electronic CD spectroscopy. Upon the addition of incremental amounts of (R)-DACH (0−100 equiv) to (R)-H1 (Figure 2a and Figure S9a in the Supporting Information), the original negative CD couplets at 429 and 420 nm gradually disappear, and new negative CD couplets are generated at 443 and 426 nm, which are attributed to the formation of (R)-H1⊃(R)-DACH. The CD response reached saturation when about 20 equiv of (R)-DACH was added,43 at which the intensity of the negative Cotton effect is slightly lower than that of the free host (R)-H1. In comparison, the addition of 20 equiv of (S)-DACH to (R)-H1 induced not only the same red shift of the CD signals but also a transformation to a positive Cotton ef fect with enhanced amplitude (Figure 2b and Figure S9b in the Supporting Information). This notable CD inversion and amplification phenomenon is attributed to the formation of the 1:1 complex (R)-H1⊃(S)-DACH. Binding of (R)- and (S)-PPDA to (R)-H1 also induced distinct CD spectral changes. In brief, the formation of (R)H1⊃(R)-PPDA led to a red shift (ca. 13 nm) and a remarkable decrease in the original negative CD couplets, while the formation of (R)-H1⊃(S)-PPDA resulted in slightly increased positive CD couplets with a similar red shift (Figure 2c,d). However, the CD responses associated with the formation of 1:2 complexes (R)-H1@[(R)-DPEA]2 and (R)-H1@[(S)DPEA]2 are not parallel with those of the above 1:1 complexes. After the addition of up to 200 equiv44 of (R)- or (S)-DPEA to (R)-H1 (Figures 2e,f), no CD inversion was observed and the original negative CD couplets decreased in intensity and underwent about a 5 nm red shift for both cases, while the final

equiv) at 298 K in CHCl3, the Soret absorption band decreased in intensity and underwent a bathochromic shift from 423 to 425 nm. The observation of a clear isosbestic point (428 nm) suggests the population of a sole host−guest complex (Figure 1a). When the ditopic feature of the host and the guest is taken into account, a complementary 1:1 binding mode can be proposed, in which a DACH ligand binds to two porphyrin subunits simultaneously by Zn−N coordination, forming the sandwich complex (R)-H1⊃(R)-DACH. In addition, the 2 nm red shift of the Soret absorption maxima is characteristic for the interporphyrin exciton coupling of the DACH-bonded sandwich complexes of zinc porphyrins,40 which supports a ditopic binding mode with 1:1 stoichiometry. From these results, the association constant (Kassoc) of (R)-H1⊃(R)-DACH was evaluated as 6.70 × 105 M−1 by applying a nonlinear curvefitting method to the binding isotherm41 (Figure 1b). The titration profiles of (R)-H1 with (S)-DACH, (R)-PPDA, and (S)-PPDA are similar to that of (R)-H1 with (R)-DACH; thus, the same binding mode was suggested and the Kassocs for (R)H1⊃(S)-DACH, (R)-H1⊃(R)-PPDA, and (R)-H1⊃(S)-PPDA were obtained as 3.40 × 106, 1.09 × 106 and 1.51 × 106 M−1, respectively (Table 1 and Figures S4−S6 in the Supporting Table 1. Association Constants (M−1), Enantioselectivities (α), and Chiral Recognition Energies (ΔΔG°, kJ mol−1) of (R)-H1 toward Diamine Guests in CHCl3 at 298 K host (R)-H1 (R)-H1 (R)-H1 (R)-H1 (R)-H1 (R)-H1

guest

Kassoca

(R)-DACH (S)-DACH (R)-PPDA (S)-PPDA (R)-DPEA (S)-DPEA

6.70 × 10 3.40 × 106 1.09 × 106 1.51 × 106 7.19 × 103 d 9.15 × 103 d 5

αb

ΔΔG°c

5.07

−4.02

1.38

−0.80

1.27

−0.59

a Errors estimated at ±5%. bα = Kassoc(RS)/Kassoc(RR). cΔΔG° = −RT ln α. dApparent association constant for the 1:2 open binding mode, which is the average binding ability of a single porphyrin subunit of the host to a diamine guest.

Information). These association constants are about 30−240 times greater than those of the single Zn−N coordination complexes of zinc tetraphenylporphyrin (ZnTPP) units with monoamines ((1.4−2.6) × 104 M−1),42 which is obviously due to the positive cooperativity of the intermolecular ditopic binding in the present system. However, the titration profiles of (R)-H1 with (R)-/(S)DPEA (Figures S7 and S8 in the Supporting Information) are quite different from those with (R)-/(S)-DACH and (R)-/(S)PPDA. Stepwise addition of (R)-/(S)-DPEA (0−200 equiv) to (R)-H1 led to a decrease in intensity and a more pronounced red shift of the Soret absorption maxima from 423 to 429 nm. A clear isosbestic point was also observed at 428 nm. Considering the structural characters of the semirigid bisporphyrin host and the large steric hindrance from the two phenyl groups of the DPEA guest, a 1:2 open binding mode can be proposed, in which each DPEA ligand binds to a porphyrin subunit, respectively, by Zn−N coordination, forming the 1:2 complex of (R)-H1@[DPEA]2. The 6 nm red shift of the Soret absorption maxima supports such an assignment.40 Thus, fitting the binding isotherm to a 1:2 stoichiometry returned to the apparent association constants of 7.19 × 103 and 9.15 × 103 M−1 for (R)-H1@[(R)-DPEA]2 and (R)-H1@[(S)-DPEA]2 respectively, which are reasonably close to those of the single 8225

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Figure 2. CD spectra of (R)-H1 before (black) and after (red) the addition of (a) (R)-DACH (20 equiv), (b) (S)-DACH (20 equiv), (c) (R)-PPDA (20 equiv), (d) (S)-PPDA (20 equiv), (e) (R)-DPEA (200 equiv), and (f) (S)-DPEA (200 equiv).

Figure 3. Optimized molecular structures of (S)-H1, (S)-H1⊃(S)-DACH, and (S)-H1⊃(R)-DACH by DFT methods at the B97D/6-31G(D) level. The torsion angle Φ is the spatial angle of C15−C5−C5′−C15′; the interchromophoric distance is the Zn−Zn distance in Å.

On the basis of the sensitive and diverse CD responses for the ditopic 1:1 binding of (R)-/(S)-H1 with (R)-/(S)-DACH and (R)-/(S)-PPDA, it is clear that the final CD signs are primarily decided by the chiral diamine guests. When the chiral host H1 binds a guest with different chirality, the original bisignate Cotton effects will reverse their direction accompanied by amplification and a red shift. In comparison, when the host binds a guest with the same chirality, the CD couplets

CD amplitude of (R)-H1@[(R)-DPEA]2 was obviously lower than that of (R)-H1@[(S)-DPEA]2. The CD spectral changes for (S)-H1 upon the binding of chiral 1,2-diamines were also recorded (Figure S10 and Table S1 in the Supporting Information). As expected, the CD responses of (S)-H1 are consistent with those of (R)-H1 by quoting the enantiomorphic relationship. 8226

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chiral guest molecules. The sensitive CD responses afford a convenient protocol for stereochemical assignment on the bases of the supramolecular binding induced allosteric effects and ECCD principle,19 such that a guest of different chirality from the host induces CD inversion and amplification, while a guest of the same chirality with the host leads to reduction of the CD intensity but retains the original CD signs. In comparison with the previously reported achiral bisporphyrin systems,16 one of the advantages of the present chiral bisporphyrin version is that the host itself is highly CD active, thus providing a proper calibrating signal for the final CD readout, which may be crucial for the accurate AC assignment when the final CD signals are very weak, such as the cases for (R)-H1⊃(R)-PPDA and (S)-H1⊃(S)-PPDA (Figure 2c and Figure S10d in the Supporting Information). 2.4. Chiral Sensing by 1H NMR. The prominent chiral discrimination ability of (R)-/(S)-H1 toward diamines, as indicated by the above UV−vis absorption and CD spectroscopic analysis, prompted us to test the possibility for NMR chiral shift reagents (CSRs). Consequently, the binding of (S)-H1 with each pairs of chiral diamines were further probed by 1H NMR spectroscopy. The titration spectra of (S)-H1 with (R)-DACH are shown in Figure 4 and Figure S13 in the Supporting Information.

will keep their original directions accompanied by reduction and red shift. On the other hand, the CD responses for the 1:2 binding events are not as sensitive as those for the 1:1 binding. Though a decrease in intensity and red shift of the CD signals can be observed, the original CD signs for (R)-/(S)-H1 cannot be altered by the binding of (R)-/(S)-DPEA. In order to rationalize the observed CD responses, the binding-induced allosteric effects were estimated by DFT molecular modeling (Figures 3 and Figures S11 and S12 and Table S2 in the Supporting Information). From the optimized molecular structures, two essential parameters regarding the relative orientation (Φ) and distance (R) of the two coupled porphyrin chromophores were acquired to correlate with the CD responses. As shown in Figure 3, the free host (S)-H1 exhibits a clockwise twist angle (+21.22°) between the EETMs of the two porphyrin units, which is consistent with the observed positive CD couplets, indicating a defined intramolecular chirality induction phenomenon in the bisporphyrin host. After the formation of stable ditopic 1:1 binding complexes with (R)- or (S)-DACH, respectively, the relative orientation and distance between the two porphyrin units changed significantly. For (S)-H1⊃(S)-DACH, the torsion angle increased to +42.49°, together with an increased Zn−Zn distance from 3.48 to 5.50 Å. However, for (S)-H1⊃(R)DACH, the relative orientation between the EETMs inverted to anticlockwise, with a torsion angle of −43.18° and a Zn−Zn distance of 5.42 Å. The relative orientations between the EETMs in the energy-minimized structures of (S)-H1⊃(S)DACH and (S)-H1⊃(R)-DACH nicely predict the experimentally observed CD signs. This is also true for the formation of ditopic 1:1 complexes with (R)-/(S)-PPDA, i.e., (S)H1⊃(S)-PPDA and (S)-H1⊃(R)-PPDA present clockwise and anticlockwise relative orientations, respectively, in the optimized structures, corresponding to the observed CD signs (Figures S10 and S11 in the Supporting Information). However, the formation of 1:2 complexes (S)-H1@[(R)DPEA]2 and (S)-H1@[(S)-DPEA]2 just disturbs the relative orientation and distance to a lesser extent and cannot change the interporphyrin clockwise twist direction of the free host (S)-H1, which is again in line with the observed CD signs (Figures S10 and S12 in the Supporting Information). It is known that, for a chiral bisporphyrin system, the CD intensity is inversely proportional to the square of interporphyrin distance and also depends on the torsion angle between the EETMs.19 However, due to the semiflexible structural feature of the present system, particularly for the free host and the 1:2 host−guest complexes which are lacking in the intermolecular ditopic binding, the origin of the observed CD amplitudes may involve the dynamic distribution of various low-energy conformers,22,40 thus leading to additional complexity for the direct correlation of the observed CD intensity with the optimized structural parameters. Nevertheless, for the ditopic 1:1 host−guest complexes, it was found that the CD intensity of a heterochiral host−guest complex is always larger than that of the corresponding homochiral host−guest complex (Table S1 in the Supporting Information), which is in accordance with that predicted by the relative Zn−Zn distances from the DFT optimized structures (Table S2 in the Supporting Information). On the basis of the above analysis, we propose that, once a ditopic 1:1 host−guest binding mode is identified, the present chiral bisporphyrin hosts (R)-/(S)-H1 can serve as chiral optical sensors for determining the absolute configuration of

Figure 4. Partial 1H NMR titration spectra of (S)-H1 (0.75 mM) with (R)-DACH (0.0−1.0 equiv, 0.25 equiv additions) at 298 K in CDCl3.

Upon the addition of 0.25 equiv of (R)-DACH, the resonance signals corresponding to the free (S)-H1 disappeared completely, indicating a fast exchange between the free and bound (S)-H1 on the NMR time scale. Further addition of (R)DACH induced a regular shift of the resonance signals for the host, corresponding to the shift of the chemical equilibrium between the free and bound host molecules. In particular, the signals of Por-β protons underwent an upfield shift and broadening. Until 1.0 equiv of the guest was added, the signals of Por-β protons became well-resolved again, which can be attributed to the dominant population of the 1:1 sandwich complex (S)-H1⊃(R)-DACH, because of the high binding 8227

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Inorganic Chemistry affinity (Kassoc ≈ 3.40 × 106) and the relatively high concentration for NMR measurements. The complexationinduced shift (CIS) value for Por-β protons is up to −0.44 (Δδ, ppm), as a result of the loss of conformational freedom induced by the ditopic guest binding, in turn leading to receiving a relatively strong shielding effect from an opposing porphyrin ring. The CIS values for the resonance signals of the guest (R)DACH are more significant than those of the host. Upon binding to (S)-H1, the signals of (R)-DACH shifted remarkably upfield, from the δ 2.2−1.0 ppm region to δ −0.3 to −7.5 ppm region, and became further resolved from each other. However, the CIS values corresponding to the bound guest (R)-DACH are almost constant in the present titration course (0.25−1.0 equiv of (R)-DACH), which are in good contrast with those of the host. This observation is reasonable for the titration of a 1:1 host−guest system when the guest is gradually added to the host. Notably for (R)-DACH, a maximum CIS value is found for the amine protons (NH2, Δδ = −8.67 ppm), due to the close proximity to zinc porphyrin subunits as indicated by the short Zn−N distance (2.323 Å) from the above DFT molecular modeling. The CIS values of other protons decrease from H1 (Δδ = −8.34 ppm) to H4 (Δδ = −2.03 ppm), following the order of the distances to the amine groups (Table 2). These Table 2. 1H NMR Chemical Shift (δ, ppm), ComplexationInduced Shift (CIS, Δδ, ppm), and Chemical Shift Nonequivalence (ΔΔδ, ppm) Values of Free and Bound (R)-/(S)-DACH in the Presence of (S)-H1 (0.75 mM) in CDCl3 at 298 K proton

free (S)DACH δ

N−H H1 H2 H3 H4 H5

1.21 2.23 1.81 1.09 1.66 1.26

a

(S)-H1⊃(R)-DACH δ (Δδ1)a −7.46 −6.11 −3.21 −5.70 −0.37 −1.73

(−8.67) (−8.34) (−5.02) (−6.79) (−2.03) (−2.99)

(S)-H1⊃(S)-DACH δ (Δδ2)a −7.10 −5.76 −3.21 −5.13 −0.32 −1.59

(−8.31) (−7.99) (−5.02) (−6.22) (−1.98) (−2.85)

Figure 5. Selected region of 400 MHz 1H NMR spectra in the presence of (S)-H1 (0.75 mM) in CDCl3 at 298 K: (a) (R)-DACH (0.75 equiv); (b) rac-DACH (0.75 equiv); (c) (S)-DACH (0.75 equiv).

separated with a ΔΔδ value of 0.14 ppm, in spite of the long distances to the amine groups and the porphyrin centers. However, the signals of H2 and H4 could hardly be differentiated under the present conditions. This can be rationalized by the above DFT optimized structure of (S)H1⊃(R)-DACH, in which the cyclohexyl ring of DACH adopts a face-to-face orientation with respect to the porphyrin planes of (S)-H1, while H2 and H4 are located at the equatorial bond of the cyclohexyl ring; thus, they are pointed away from the anisotropic shielding area of the chiral bisporphyrin host and are relatively insensitive to the binding induced allosteric effects. The titration profiles of (S)-H1 with (R)-/(S)-PPDA are shown in Figures S16 and S17 in the Supporting Information. The typical CIS values for Por-β protons and the amine protons reach −0.30 and −8.42 (Δδ, ppm) for (S)-H1⊃(R)PPDA, and −0.26 and −8.72 (Δδ, ppm) for (S)-H1⊃(S)PPDA, respectively, which both are supportive of the formation of 1:1 sandwich complexes. The CIS values for the proton signals of the guest (R)-/(S)-PPDA also remain constant in the titration course (0.25−1.0 equiv of guest), indicating that all of the guest molecules are in the bound state. Surprisingly, although the enantioselectivity of (R)-H1 toward (R)-/(S)PPDA (α = 1.38) is quite small in comparison with that of (R)H1 toward (R)-/(S)-DACH (α = 5.07), a remarkable enantiomeric discrimination effect is also obtained for the formation of (S)-H1⊃(R)-PPDA and (S)-H1⊃(S)-PPDA in the 1H NMR spectra. All of the proton signals of (R)-/(S)PPDA can be differentiated with ΔΔδ values ranging from 0.30 to 0.09 ppm, which are mostly large enough to afford a baseline separation (Figure S18 and Table S3 in the Supporting Information). However, the titration profiles of (S)-H1 with (R)-/(S)DPEA are not in line with the above 1:1 sandwich binding

ΔΔδb 0.36 0.35 0.00 0.57 0.05 0.14

Δδ = δbound − δfree. bΔΔδ = |Δδ1 − Δδ2|.

CIS values are much larger than those of the single Zn−N coordination complex of (R)-DACH with a monomeric zinc porphyrin, while are similar to those of (R)-DACH captured within a bisporphyrin tweezer,40 thus giving strong support for the formation of the 1:1 sandwich complex (S)-H1⊃(R)DACH. The titration spectra of (S)-H1 with (S)-DACH (Figure S14 in the Supporting Information) are similar to those with (R)-DACH; the CIS values for Por-β protons and the amine protons of (S)-DACH are −0.33 and −8.31 (Δδ, ppm), respectively, also suggesting the formation of the 1:1 sandwich complex (S)-H1⊃(S)-DACH. On comparison of the 1H NMR spectra of (R)-DACH, (S)DACH, and their racemic mixture (rac-DACH) in the presence of (S)-H1, it is interesting to note that a high degree of enantiomeric discrimination is achieved for some of the proton signals (Figure 5 and Figure S15 in the Supporting Information). The chemical shift nonequivalence values reach up to 0.57, 0.36, and 0.35 (ΔΔδ, ppm) for H3, NH, and H1, respectively (Table 2), which are among the highest for the reported CSRs so far.45 The remarkable signal separations are believed to be due to the strong ASE of the chiral bisporphyrin cavity associated with the allosteric effects induced by the formation of diastereoisomers. The signals of H5 can also be 8228

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Article

Inorganic Chemistry

modeling. It is interesting to note that a guest of different chirality with the host induces CD inversion and amplification, while a guest of same chirality with the host leads to reduction in the CD intensity but the original CD signs are retained. Furthermore, the 1H NMR titrations revealed that (R)-/(S)-H1 also possess remarkable enantiodiscrimination abilities toward (R)-/(S)-DACH and (R)-/(S)-PPDA via the formation of 1:1 sandwich complexes. The nonequivalent chemical shift (ΔΔδ) can reach up to 0.57 ppm due to the strong ASE from the chiral bisporphyrin cavity in conjunction with the allosteric effects induced by the formation of diastereoisomeric host−guest complexes. Accordingly, (R)-/(S)-H1 are suggested to be efficient chiral optical sensors and NMR chiral shift reagents (CSRs) for some chiral diamines. However, in the case of interaction with (R)-/(S)-DPEA, the guest molecules could not be encapsulated in the chiral bisporphyrin cavity of (R)-/(S)H1 to form stable 1:1 sandwich complexes due to the large steric hindrance effects; therefore, (R)-/(S)-DPEA could hardly be discriminated by CD and NMR spectroscopy. The observations revealed that, once a sandwich-type 1:1 binding mode is dominant, (R)-/(S)-H1 can present high enantioselectivity together with integrated functions of high CD and CSR sensitivities for chiral diamines; however, when a 1:1 or 1:2 open binding mode is prevailing, the system is insufficient for enantiomeric discrimination. We believe that the exploration of the chiral bisporphyrin hosts with other asymmetric bidentate substrates is worthy of further study.

cases. As shown in Figures S19 and S20 in the Supporting Information, upon the addition of 0.25 equiv of the guest, the resonance signals of the free host disappeared completely, indicating a fast exchange between the free and bound hosts. Further addition of the guest (0.25−1.0 equiv) induced a regular shift not only for the host signals but also for the guest signals, suggesting that the guest molecules are also in fast exchange between the free and bound states, which is different from the titrations with (R)-/(S)-DACH and (R)-/(S)-PPDA. This can be explained by the relatively low binding affinities of (R)- and (S)-DPEA with (S)-H1 (in the range of 103−104 M−1); however, the fast exchange between the free and bound guests is disfavored for the enantiomeric discrimination because it implies that the R and S guest molecules might be also in fast exchange once a racemic substrate is applied. When 1.0 equiv of (R)- or (S)-DPEA was added to (S)-H1, the CIS values of Porβ proton signals reached −0.12 (Δδ, ppm) for both cases, which are mainly contributed by the shielding effect from the phenyl rings of DPEA and are obviously smaller than those induced by 1.0 equiv of (R)-/(S)-DACH and (R)-/(S)-PPDA (in the range of −0.26 to −0.44 ppm). Meanwhile, the CIS values for the guest signals were found to be −4.51 and −4.28 (Δδ, ppm) for the amine protons of (R)- and (S)-DPEA, respectively, which are much smaller than those of (R)-/(S)DACH and (R)-/(S)-PPDA (in the range of −7.81 to −8.67 ppm) and are reasonably close to those of the single Zn−N coordination complexes of Zn porphyrin units with mono-42 or diamines.40 This is also true for the H1 signals of (R)- and (S)DPEA (Δδ = −4.99 and −4.85 ppm, respectively; Table S4 in the Supporting Information). The observations indicate that (R)-/(S)-DPEA cannot lock a stable face-to-face conformation of the two porphyrin subunits in (S)-H1 and the guest molecules only receive a shielding effect from one of the porphyrin rings, thus suggesting a 1:1 open binding mode in the present titration course (0.25−1.0 equiv of guest). However, this binding mode is insufficient to afford an enantiomeric discrimination due to the low binding affinity and the related fast guest-exchange equilibrium. The above 1H NMR titration results demonstrate the high enantiodiscrimination abilities of (R)-/(S)-H1 toward (R)-/ (S)-DACH and (R)-/(S)-PPDA via the formation of ditopic 1:1 sandwich complexes, in which the guest molecules receive strong ASE from both porphyrin subunits. However, due to the two bulky phenyl moieties of (R)-/(S)-DPEA, this guest could not be encapsulated in the chiral bisporphyrin cavity of (R)-/ (S)-H1 and, as a result, could not be discriminated under the present NMR conditions.

4. EXPERIMENTAL SECTION 4.1. Synthesis of (R)-H1. Anhydrous K2CO3 (200 mg, 1.4 mmol), CuI (19 mg, 0.1 mmol), and Cs2CO3 (70 mg, 0.21 mmol) were added to a stirred solution of Zn(TTBPP) (60.3 mg, 0.07 mmol) in dry toluene (10 mL). Then a solution of (R)-2,2′-bis(bromomethyl)-1,1′binaphthyl (13.2 mg, 0.03 mmol) in dry DMF (10 mL) was added dropwise. The mixture was heated to 90 °C and reacted under nitrogen for 24 h. The mixture was poured into water and extracted with CHCl3. The organic phase was dried over anhydrous Na2SO4 and concentrated at reduced pressure. The residue was purified by silica gel column chromatography with dichloromethane/n-hexane (3/2, v/v) as the eluent. The second fraction containing the target compound was collected and evaporated. Repeated chromatography followed by recrystallization from chloroform and methanol gave pure (R)-H1 as a purple powder (12 mg, 20%). 1H NMR (CDCl3, 400 MHz, 298 K): δ 8.90 (d, 4H, J = 4.6 Hz), 8.80 (t, 8H, J = 4.4 Hz), 8.65 (d, 4H, J = 4.7 Hz), 8.23 (d, 2H, J = 8.56 Hz), 8.18 (d, 2H, J = 8.6 Hz), 8.09 (m, 6H), 8.01 (d, 4H, J = 8.4 Hz), 7.87 (d, 4H, J = 8.2 Hz), 7.74 (d, 4H, J = 8.2 Hz), 7.65 (d, 4H, J = 7.8 Hz), 7.57 (t, 2H, J = 6.9 Hz), 7.52 (d, 4H, J = 8.1 Hz), 7.40 (t, 2H, J = 7.6 Hz), 7.35 (d, 2H, J = 8.2 Hz), 7.30 (d, 4H, J = 7.8 Hz), 7.18 (d, 4H, J = 8.5 Hz), 5.31 (d, 2H, J = 11.0 Hz), 5.19 (d, 2H, J = 12.3 Hz), 1.61 (s, 18H), 1.51 (s, 6H), 1.46 (s, 30H); UV− vis (CHCl3): λmax (log ε) 423 (5.00), 550 (4.66), 588 nm (4.09). MALDI-TOF-MS: m/z calcd for C134H119N8O2Zn2 (MH+), 2004.25; found, 2004.36. Anal. Calcd for C134H118N8O2Zn2: C, 80.30; H, 5.98; N, 5.59. Found: C, 80.49; H, 5.80; N, 5.67. 4.2. Synthesis of (S)-H1. (S)-H1 was synthesized following the above method for (R)-H1, starting from (S)-2,2′-bis(bromomethyl)1,1′-binaphthyl (13.2 mg, 0.03 mmol) instead of (R)-2,2′-bis(bromomethyl)-1,1′-binaphthyl. (S)-H1 was obtained as a purple powder (13 mg, 22%). 1H NMR (CDCl3, 400 MHz, 298 K): δ = 8.90 (d, 4H, J = 4.6 Hz), 8.80 (t, 8H, J = 4.4 Hz), 8.65 (d, 4H, J = 4.7 Hz), 8.22 (d, 2H, J = 8.56 Hz), 8.19 (d, 2H, J = 8.8 Hz), 8.09 (m, 6H), 8.00 (d, 4H, J = 8.4 Hz), 7.88 (d, 4H, J = 8.2 Hz), 7.73 (d, 4H, J = 8.4 Hz), 7.64 (d, 4H, J = 7.6 Hz), 7.57 (t, 2H, J = 6.9 Hz), 7.51 (d, 4H, J = 8.1 Hz), 7.40 (t, 2H, J = 7.6 Hz), 7.36 (d, 2H, J = 8.2 Hz), 7.31 (d, 4H, J = 7.8 Hz), 7.17 (d, 4H, J = 8.4 Hz), 5.29 (d, 2H, J = 11.2 Hz), 5.17 (d, 2H, J = 12.4 Hz), 1.61 (s, 18H), 1.51 (s, 6H), 1.45 (s, 30H); Anal.

3. CONCLUSION In summary, we designed and synthesized a pair of chiral bisporphyrin hosts (R)-/(S)-H1 with the characteristics of a semirigid molecular skeleton and a preorganized interporphyrin twist. By taking advantage of an induced-fit binding to ditopic guest molecules, (R)-/(S)-H1 showed effective enantioselectivity to chiral 1,2-diamines, as indicated by the UV−vis titrations. In particular, the enantioselectivity for (R)-/(S)DACH was evaluated to be 5.07, corresponding to a chiral recognition energy (ΔΔG°) of −4.02 kJ mol−1 for the formation of diastereoisomeric sandwich-type 1:1 complexes. Sensitive CD responses were observed for (R)-/(S)-H1 in the presence of (R)-/(S)-DACH and (R)-/(S)-PPDA, which can be attributed to the allosteric effects associated with the ditopic 1:1 host−guest binding, as estimated by DFT molecular 8229

DOI: 10.1021/acs.inorgchem.7b00920 Inorg. Chem. 2017, 56, 8223−8231

Article

Inorganic Chemistry Calcd for C134H118N8O2Zn2: C, 80.30; H, 5.98; N, 5.59. Found: C, 80.52; H, 5.78; N, 5.82.

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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/acs.inorgchem.7b00920. Chemicals and instruments, spectral characterizations and titrations, and DFT molecular modeling details (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for Y.B.: [email protected]. *E-mail for J.J.: [email protected]. ORCID

Yongzhong Bian: 0000-0003-0621-3683 Jianzhuang Jiang: 0000-0002-4263-9211 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Natural Science Foundation of China (21471015, 21631003, 21290174, and 21671017), the National Ministry of Science and Technology of China (2013CB933402), Beijing Natural Science Foundation, and Fundamental Research Funds for the Central Universities is gratefully acknowledged.

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DOI: 10.1021/acs.inorgchem.7b00920 Inorg. Chem. 2017, 56, 8223−8231