Complexation of Chiral Zinc(II) Porphyrin Tweezer with Achiral

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

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Complexation of Chiral Zinc(II) Porphyrin Tweezer with Achiral Aliphatic Diamines Revisited: Molecular Dynamics, Electronic CD, and 1H NMR Analysis Bapan Saha,†,# Ana G. Petrovic,‡,§,# Avinash Dhamija,† Nina Berova,*,‡ and Sankar Prasad Rath*,† †

Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, India Columbia University, Department of Chemistry, 3000 Broadway, New York, New York 10027, United States § Department of Life Sciences, New York Institute of Technology, 1855 Broadway, New York, New York 10023, United States Downloaded via NOTTINGHAM TRENT UNIV on August 15, 2019 at 06:58:02 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: We have reported here the complexation and chiroptical behavior of the host−guest complexes using a new chiral Zn(II) bisporphyrin tweezer host and a series of achiral aliphatic diamine guests varying the chain length. (1R,2R)-Cyclohexanediamine covalently links two Zn(II) porphyrin moieties, which thereby produces a strong chiral field around the bisporphyrin tweezer framework. The chiral tweezer upon complexation with achiral guest exhibited large changes in the UV−vis spectra and CD exciton couplets due to a sudden change in the porphyrin disposition, which is controlled by the host−guest stoichiometry as well as the chain lengths of the diamine guest. Addition of smaller diamines (n: 2−5) to the host resulted in the formation of 1:1 sandwich and 1:2 open complexes, respectively, at the low and high guest concentration, which eventually display two-step inversions of the CD couplet. With longer diamines (n: 6−8), however, only 1:1 sandwich complexes are formed with retention of the CD sign. Similar observations were also reported by us recently using another chiral bisporphyrin tweezer having (1R,2R)-diphenylethylenediamine as the spacer. In an effort to obtain deeper insights into the sudden changes of interporphyrin disposition just by changing the length of the achiral diamines, we have extended a series of computational studies and correlated closely with the results obtained from the experiment. While the previously published study has relied on commonly applied Monte Carlo (MC) sampling of the potential energy surface in addition to being guided by porphyrin effective transition moment approximation, the present study uses a considerably more robust molecular modeling protocol, namely Molecular Dynamics (MD) simulations followed by full ab initio geometry optimization and TD-DFT CD prediction. The experimental data corroborate with the results obtained from the theoretical conformational analysis. The latter are also supported by experimental 1H NMR data empowered by the porphyrin ring-current effect. The NMR spectral patterns of pyrrolic protons of the free host and the 1:1 sandwich complexes appear very diagnostic and reflect the changes in the mutual porphyrin disposition on moving from the free host to the complexed ones with short and long diamines. Overall, the experimental NMR data underscore the sensitivity of pyrrolic protons chemical shifts to subtle alterations of the geometrical features, and as such, they come in agreement with the theoretically derived models.

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INTRODUCTION

at the visible region, one of the important prerequisites for an efficient chirogenic performance. The tweezer methodology has been extensively used for the utilization of porphyrins as molecular probes.6−13 In this case, an achiral metallobisporphyrin host and a chiral bidentate guest forms a host−guest supramolecular complex, thereby generating a stereospecific twist between the two chromophores due to the transfer of chirality from the chiral guest to the achiral host. This results in the generation of a bisignate Electronic Circular Dichroism (ECD) response (so-called exciton couplet) in the porphyrin spectral region around ca. 400 nm with the sign being controlled by the through space interaction between the chromophores. Since the absolute configuration (AC) of the guest, along with other factors,

Supramolecular chirogenesis is one of the most important interdisciplinary fields due to its relevance in many natural (such as DNA double helix, the secondary α-helix structure of proteins, heme proteins, etc.) and artificial systems. It mostly deals with the processes such as asymmetry transfer, induction, modulation, and amplification that are solely governed by the various noncovalent interactions.1−5 Nowadays researchers are highly interested in the control of chirality at the molecular level, for the direct implementation of enantiomer resolution, enantiomeric excess (ee) analysis, molecular recognition and asymmetric catalysis, and many other aspects, such as intermolecular interactions.1−6 Porphyrins have been considered as one of the most efficient chromophores for probing molecular chirality because of their unique photoelectronic and geometrical attributes.6−11 They display an intense Soret band © XXXX American Chemical Society

Received: April 2, 2019

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DOI: 10.1021/acs.inorgchem.9b00962 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

but rather a circular oscillator, where the 10−20 transition dipole cannot be ignored. Therefore, clarifying the reasons for the unexpected CD response of tweezer 1 upon complexation with achiral diamines of different size sounds intriguing and justifies more studies for rationalizing the impact of conformational factors on the chiroptical signature and other spectroscopic properties as diagnostic structural tools.

predominantly governs the preferred interporphyrin helicity, the tweezer methodology is a quite powerful and sensitive chiroptical tool for investigation of molecular chirality of the chiral substrates. In most of the studies, the metallobisporphyrin tweezer host has been achiral and the guests are chiral. Only a handful of examples are available where a metallobisporphyrin tweezer is chiral, while the guest is achiral.14a−f An interesting case of chiral discrimination by chiral diamines based on an inherently chiral bisnaphthalene bridged porphyrin dimer was recently reported. It highlighted the binding mode dependent molecular recognition and sensing by 1 H NMR.14g In 2017, we reported in this journal about the chiroptical properties of a large series of host−guest complexes formed between the chiral (1R,2R)-diphenylethylenediamine bridged zinc(II) bisporphyrin tweezer host 1 and achiral aliphatic diamines of varying lengths as guest.10 The chiral tweezer upon binding of achiral diamines of varying lengths revealed a dramatic change in the chiroptical response associated with the intermolecular porphyrin−porphyrin helicity in the complexes. The sign of the CD exciton couplet has shown first inversion from negative to positive and again inversion to a negative sign upon stepwise complexation with the shorter (n = 2−5) diamines. However, no change in the sign of CD couplet has been observed with the longer (n = 6−8) diamines. Obviously, the length of the achiral diamine guest dictates the nature of the interporphyrin helicity and thereby the sign of the CD couplets.10 In fact, the addition of just one −CH2− group in the achiral diamine (between n = 5 and 6) leads to such a conspicuous change! New extensive investigation became desirable in order to understand what causes such dramatic changes in the ECD signature upon moving from shorter to longer diamines. We also have extended the investigation further using another chiral tweezer in which (1R,2R)diphenylethylenediamine spacer is replaced by (1R,2R)cyclohexanediamine. On the basis of many recent reports, it is now accepted that the sign of the ECD couplet is very sensitive to the mutual orientation of the porphyrin macrocycles attached to the stereogenic centers.15,16a The concept of porphyrin effective electric transition dipole moment that mainly takes into account the optical contributions in the 5−15 direction was introduced as a practical approach in 1996.15b The latter played a driving force in extending the wide application of the intense ECD exciton couplet observed within the porphyrin’s UV−vis Soret band region for stereochemical analysis.15−17 We have considered this concept also in the ECD analysis upon complexation of the (1R,2R)-diphenylethylenediamine bridged zinc(II) bisporphyrin with achiral diamines.10 Surprisingly, in some cases, depending on the size of diamine guests, the sign of the CD couplet for the complexes remained unchanged and similar to that of the host, while in other cases it got reversed. Upon careful re-examination, it was found that MM/MC search when taking into account only 5−15 effective transition dipole lead to some discrepancies with the experimental data.10 Even though the effective dipole moment concept was successfully applied in the past on other types of tweezer hosts with more flexible linker,17 it fails in the case of 1, where the short ethylenediamine linker may have restricted porphyrin’s libration.16a The present work supports the previous study16a that when conformational flexibility related to metalloporphyrin coordination comes into play, the porphyrin chromophore can no longer be considered as linear,

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RESULTS AND DISCUSSION In the present study, we have revisited the 1:1 sandwich complex formation of (1R,2R)-diphenylethylenediamine bridged zinc(II) bisporphyrin tweezer 1 (Scheme 1) with the Scheme 1. Formation of 1:1 Sandwich Complexes of the Chiral Zn(II) Bisporphyrin (R,R)-Zn(II)BP (1) Host with the Shorter and Longer Diamines

shorter and longer diamines and tried to obtain deeper insight into the unsuspected change in sign of the exciton CD couplet for the 1:1 sandwich complexes. The complexation results in either two-step inversions of interporphyrin disposition when shorter diamine guests (L2-L5) were used or retention of the interporphyrin mutual orientation of the host with longer diamine guests (L6-L8).10 In both the cases, the R,R-absolute configuration of the host remained intact. Scheme 1 shows the complexes discussed here and their abbreviations. We have chosen 1:1 sandwich complexes 1·L5 and 1·L8 as representatives of shorter and longer diamines, respectively, and detailed investigations are carried out on these complexes by using molecular modeling and NMR and CD analysis. Even though the unexpected switch in the CD sign occurs as diamine length extends from L5 to longer diamines, we choose L5 and L8 as the representative for two reasons: first, the two selected models are representative examples of one shorter and one distinctly longer diamine guests; second, the selected complexes exhibit not only opposite exciton couplet sign (positive vs negative) but also largest difference in CD amplitude based on shorter vs longer diamine. Additionally, it was of interest to investigate what would be the conformational behavior of the tweezer when complexed with the longest of the diamine family that could give the complex a higher degree of flexibility. We also have synthesized two new chiral hosts here: (1R,2R)-cyclohexanediamine bridged zinc(II) bisporphyrin tweezer 2 and its enantiomer(1S,2S)-cyclohexanediamine bridged zinc(II) bisporphyrin tweezer 2A. Scheme S1 describes the synthetic outline of the chiral tweezer 2. B


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Inorganic Chemistry Scheme 2. Complexation of the Shorter Diamine Ligands with the Chiral Tweezer 2

Scheme 3. Complexation of the Longer Diamine Ligands with the Chiral Tweezer 2

Host−Guest Complexation with Chiral Tweezer 2. The complexations of chiral tweezers 2 and 2A have been examined with the same series of achiral diamines Ln. Schemes 2 and 3 display the synthetic outline and list of all the complexes formed with tweezer 2. An intense negative CD response, Aobs of −623 cm−1 M−1, has been observed for tweezer 2 in the porphyrin Soret region. Based on the former approach in

interpreting the CD couplet, the negative exciton couplet could be associated with the negative chiral twist between the effective electric transition dipoles 5-15/5′-15′.17 Interestingly, the tweezer host 2 also shows two different types of spectral changes upon complexation with the smaller and longer diamines. C


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Inorganic Chemistry UV−Vis. Complexation of the achiral diamines Ln with the chiral tweezer 2 has been monitored by UV−visible spectroscopy. The gradual addition of 1,5-diaminopentane (L5) to a chloroform solution of 2 at room temperature results in red shifting (Figure 1A) of the Soret (419 to 427 nm) and Q-

distribution has correlated well with the calculated pattern, thereby confirming the formation of the mentioned complexes. We are, however, unable to get any single crystals for the complexes for structure determination. 1 H NMR. 1H NMR spectral studies are very useful in establishing the 1:1 stoichiometry of the host−guest complexes in solution.6,10,11 The spectral studies have been carried out in CDCl3 at 295 K. The protons of the (1R,2R)-cyclohexanediamine spacer are highly upfield shifted in the free tweezer 2 due to their exposure to the ring current of the porphyrin rings. Upon complexation with L5, the guest gets sandwiched between the two porphyrin rings in the 1:1 sandwich complex and the protons of L5 get exposed to the ring current of the porphyrin rings, thereby showing large upfield shifting: CH2 signals at −2.31, −2.64, −2.78; NH2 signals at −4.89 ppm. Downfield shifting of the CHDA protons such as CH, NH, and CH2 have also been observed as the two porphyrins come closer leaving the spacer CHDA protons outside the shielding zone (Figure S3). The 1:1 sandwich complexation of 2 and L8 has also been examined by 1H NMR spectroscopy. Upon formation of the sandwich complex 2·L8, the guest L8 gets encapsulated within the bisporphyrin cavity and the protons of L8 show large upfield shifting: CH2 signals at −0.43, −0.99, −2.01, and −2.30 ppm and a broad signal for NH2 protons at −4.62 ppm (Figure S4). Electronic Circular Dichroism. Interactions of the (1R,2R)cyclohexanediamine-bridged zinc(II) bisporphyrin tweezer 2 with the achiral guests Ln have also been monitored by CD spectroscopy in chloroform at 295 K. As already mentioned, the chiral tweezer 2 displays a negative exciton couplet with CD amplitude Aobs of −623 cm−1 M−1 in the porphyrin Soret band region. The enantiomeric host 2A also produces a similar CD couplet but with opposite sign (Figure S5). Upon complexation with the achiral diamines Ln, two types of spectral changes have been observed depending on the length of the diamines, like that in the UV−visible spectra; one is for the smaller diamines (L2-L5) and the other one is for the longer diamines (L6-L8). Upon addition of L5, as an example, the negative CD signal of 2 gradually decreases and generates a new red-shifted positive CD signal (Figure 2) at the Soret band region with an optimum value of +712 cm−1 M−1 at 8 equiv of the ligand, which was then maintained up to 320 equiv of the ligand. The total amplitude of the CD signal (Aobs) of +712 cm−1 M−1 observed for the sandwich complex 2·L5 is larger than the amplitude of −623 cm−1 M−1 observed for 2. Further addition of L 5 resulted in the second inversion of interporphyrin helicity (Figure 2B) with the positive CD signal gradually transformed into a new negative CD signal with a optimum value (Aobs) of −392 cm−1 M−1 at 11000 equiv of the ligand added, at the more red-shifted region due to the formation of 2·(L5)2. This reveals an opposite spatial orientation (anticlockwise) in the 1:2 open complex 2·(L5)2 in comparison to the sandwich complex, 2·L5 (Figure 2). Similar spectral changes have been observed for the other smaller guests (L2-L4) (Figures S6−S8). The amplitude of the CD signal also varies with the increasing length of diamines. In the case of the longer diamines (L6-L8), interestingly, completely different types of ECD spectral changes have been observed (Figure 3). The addition of L8 results in the gradual change of intensity of the negative CD signal of 2 to produce a new negative red-shifted CD signal at the Soret band region (Figure 3), showing an optimum value (Aobs) of −756 cm−1 M−1 at 7 equiv of the guest. Further addition of the guest

Figure 1. UV−visible spectral changes of 2 in chloroform at 295 K upon addition of (A) 1,5-diaminopentane (L5) and (B) 1,8diaminooctane (L8) as the host−guest molar ratio changes from 1:0 to 1:11000.

bands (549 to 565 nm, 587 to 605 nm) at the lower guest concentration due to the formation of the 1:1 sandwich complex, 2·L5, which has been isolated and characterized. The sandwich complex 2·L5 has been found stable up to the addition of 320 equiv of the guest. Further addition of L5 triggers the formation of 1:2 open complex 2·(L5)2, which is clearly reflected from the further red shifting of the Soret (to 429 nm) and Q bands (to 566 and 606 nm). The other smaller diamines, L2-L4, also show similar spectral changes upon formation of 2·Ln and 2·(Ln)2 at lower and higher guest concentrations, respectively. The extent of red shifting of the Soret band increased with increase in the length of the diamines from L2 to L5, with the shift being largest for L5. However, in case of the longer diamines (L6-L8), different UV−visible spectra have been observed (Figure 1). Addition of 1,8-diaminooctane (L8) to the chloroform solution of 2 results in the large red shifting (Figure 1B) of Soret (419 to 428 nm) and Q-bands (549 to 566 nm, 587 to 605 nm) due to the 1:1 sandwich complex formation, 2·L8, which has been isolated and characterized. The extent of red shifting of the Soret band is larger for the longer diamines (L6-L8). No further complexation has been observed upon further addition of the guest, thereby supporting the stability of the 1:1 sandwich complex and no formation of the 1:2 open complex. The UV−visible spectral changes shown by the chiral tweezer 2 are similar to those shown by the (R,R)-Zn(II)BP (1). ESI-MS. ESI-mass spectroscopy revealed peaks at m/z 1519.4037 and 1621.5183 assigned for [2+H]+ and [(2·L5) + H]+, respectively (Figures S1,S2). The experimental isotopic D


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Inorganic Chemistry

Figure 4. Variation of the CD amplitude (in CHCl3 at 295 K) of 2 upon gradual additions of (A) 1,5-diaminopentane (L5) and (B) 1,8diaminooctane (L8) ligands. The inset shows the expanded regions of the CD amplitude dependence of Ln at low ligand molar excess region. Figure 2. CD and UV−visible (in CHCl3 at 295 K) spectral change of 2 (2 × 10−6 M) upon addition of 1,5-diaminopentane (L5) as the host−guest molar ratio changes from (A) 1:0 to 1:8 and (B) 1:320 to 1:11000.

The CD spectral changes for the chiral tweezer 2 are similar to that of the (R,R)-Zn(II)BP (1) with the smaller and longer diamines. The chiral tweezer 2 also forms a 1:1 sandwich complex followed by a 1:2 open complex with the smaller diamines, whereas it forms only a 1:1 sandwich complex with the longer diamines (Schemes 2 and 3), like the chiral tweezer 1. Moreover, using the enantiomeric (1S,2S)-cyclohexanediamine-bridged zinc(II) bisporphyrin tweezer 2A, CD spectral changes with smaller and longer diamines are also similar (Figure S13) but with opposite signs of the CD couplets. It is also interesting to note here that on moving from L5 to L6, the nature of the host−guest complexes with chiral tweezer 2 (or 2A) has changed completely, which is clearly displayed in Figure 5. Figure 6 indeed compares the spectral changes for the two chiral hosts (1 and 2) upon complexation with the diamines. Interestingly, we found the amplitude for the free host 2 as well as the 1:1 sandwich complex 2·L5 is comparatively larger than 1 and 1·L5, respectively. We assume the structure of the spacers plays an important role for these differences and cumulatively contributes to the higher amplitude of the CD couplet in the case of the chiral tweezer 2 and its sandwich complex 2·L8 (Aobs = −621 of 1·L8 and −756 of 2·L8). The (R,R)-CHDA spacer is more flexible than that of (R,R)-DPEA whose phenyl groups are more involved in steric interactions. Binding constants between chiral host 2 and diamine guests were determined using ECD spectroscopic titration and were calculated using the HypSpec computer program (Protonic Software, U.K.). Distribution plots were calculated using the program HySS2009 (Protonic Software, U.K.).18 Two sets of ECD titration data were analyzed considering a binding model (Schemes 2 and 3) with three colored stoichiometric states of

Figure 3. CD and UV−visible (in CHCl3 at 295 K) spectral change of 2 (2 × 10−6 M) upon addition of 1,8-diaminooctane (L8) as the host−guest molar ratio changes from 1:0 to 1:600.

results in no change in the amplitude and sign of the ECD signal. Similar spectral changes have been observed for the other longer guests (L6-L7) (Figures S9−S10). The amplitude of 2·Ln gradually increases and shows the maximum value for L8. The variations of the CD amplitude with ligand molar excess display two-step inversion of the CD couplets for the smaller diamines while no inversion has been observed in the case of the longer diamines (Figures 4 and S11−S12). E


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Inorganic Chemistry

spectroscopic titrations (Figures S16−S17, Table 1), are very similar to the values observed from the CD spectral data. The consistent observations regarding the effect of the size of the achiral guests on the spectroscopic properties upon formation of the host−guest complexes with the chiral tweezers 1 and 2 prompted us to further investigate in more detail about the factors responsible for this dramatic change in the spectroscopic response upon increase in the length of the achiral diamine. MD Simulations and Conformational Analysis of the Free Tweezer 1 and Its Host−Guest Complexes 1·Ln. In the present paper, we extend the study of the (1R,2R)diphenylethylenediamine bridged zinc(II) bisporphyrin host (1) (Figure 7) in an effort to obtain deeper insight into the unsuspected change in sign of the exciton CD couplet upon complexation of the chiral tweezer with achiral diamines of variable lengths. While the previously published study10 has relied on commonly applied MM/MC sampling of the potential energy surface in addition to being guided by a porphyrin effective transition moment approximation16c (vide infra), the study herein uses a considerably more robust molecular modeling protocol, namely molecular dynamics (MD) simulations followed by full ab initio geometry optimization and TD-DFT CD prediction. A more rigorous MD and ab initio based conformational analysis followed by simulation of CD response on fully optimized geometries of the free tweezer 1 as well as its 1:1 sandwich complexes with 1,5- and 1,8-diamines was motivated by several factors. By performing comprehensive conformational analysis and subsequently TD-DFT level of theory, we expected to find the rational for unsuspected CD signatures of some of the complexes. Furthermore, MD simulations were expected to identify the rotatable bond(s) associated with the changes in the CD response upon complexation. As in the case of the previously published MC based conformational survey, three representative initial geometries of (R,R) Zn(II)BP (1) have been selected as related to SynSyn, Syn-Anti, and Anti-Anti dispositions of carbonyl oxygen (CO) and methine hydrogen (C*−H) on both sides of the tweezer linker (Figure 7B). Specifically, the relative orientation of the CO/C*−H moiety positions the arms from which the porphyrins extend. It is worth noting that all the initial conformations exhibited antiperiplanar arrangement of methine hydrogens at stereogenic centers (Figure 7C). As outlined in Figure 8, each of the three initial geometries in the case of free tweezer host 1, as well as the complexes with 1,5- and 1,8-diamine have been subjected to 500 ns MD simulations. In order to verify the consistency of the MD-based outcomes, we have extended the MD simulation to an additional 500 ns, thus providing data for an overall 1 ms run. MD runs were carried out via Desmond software19 within an explicit CHCl3 solvent environment (10 Å solvation box) with constant mole, pressure, and temperature (300 K, 1 atm) NPT ensemble. For each MD trajectory, 100 structures have been extracted and subsequently analyzed for their variation in the conformational parameters that include Zn···Zn distance and dihedrals related to electric transition dipole moments ETDM. Additionally, MD simulations for each of the three initial geometries were run under three different initial velocity conditions, thus resulting in a total of nine MD trajectories toward exploring the conformational space of the free tweezer 1 as well as its host−guest tweezer complexes with 1,5- and 1,8diamines. The three initial velocities were applied in the MD

Figure 5. Experimentally observed CD spectra of (A) 2 (black), 2·L5 (red), and 2·(L5)2 (green), and (B) 2 (black) and 2·L6 (red).

Figure 6. Experimentally observed CD and UV−visible (Soret band only) spectra of (A) 1 (black), 1·L5 (red), and 1·L8 (green), and (B) 2 (black), 2·L5 (red), and 2·L8 (green).

Zn(II) bisporphyrin (2), 1:1 sandwich complex 2·Ln and 1:2 open complex 2·(Ln)2. Figures S14 and S15 demonstrate such plots with L5 and L8, as a representative of shorter and longer diamines, respectively. Table 1 compares the CD spectral data and binding constants between two chiral hosts 1 and 2. As can be seen, although the binding constants are similar, the intensities of the CD couplets of the host−guest complexes are greater with chiral guest 2 keeping other factors similar. Binding constants between chiral host 2 and diamine guests have also been determined from the UV−visible spectroscopic titration, considering a binding model with the three-colored stoichiometric states for 2, 2·Ln, and 2·(Ln)2. However, the binding constant values, obtained from the UV−visible F


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Inorganic Chemistry Table 1. CD Spectral Data and Binding Constants of the Complexes in CHCl3 at 295 K 1:2 open complex CD data λ (nm) [Δε, cm−1 M−1]

1:1 sandwich complex CD data λ (nm) [Δε, cm−1 M−1] Guest ligand

FCa

Complexation with chiral L5 432 [394] L8 433 [−333] Complexation with chiral L5 432 [434] L8 433 [−418]

SCa host 1 425 [−272] 426 [288] host 2 424 [−278] 426 [338]

Aobsb,c

Binding constant K1 (M−1)d,e

FCa

SCa

Aobsb,c

Binding constant K2 (M−1)d,e

666 −621

5.1 × 105 9.5 × 105

435 [−197] na

429 [166]

−363

1.3 × 103

712 −756

5.2 × 105(5.3 × 105) 1.1 × 106(1.3 × 106)

435 [−199] na

428 [193]

−392

1.1 × 103(9.9 × 102)

FC: first Cotton effect; SC: second Cotton effect. bAobs (= Δε1 − Δε2) represents the total amplitude of the observed CD couplets. cAobs for the chiral host 1 is −574 cm−1 M−1 [FC, −302 cm−1 M−1 at 424 nm and SC, 272 cm−1 M−1 at 416 nm] and Aobs for the chiral host 2 is −623 cm−1 M−1 [FC, −324 cm−1 M−1 at 424 nm and SC, 299 cm−1 M−1 at 416 nm]. dCalculated from CD spectral measurement. eValues shown within the bracket are calculated from UV−visible spectral titration. a

Figure 7. (A) Chiral (1R,2R)-diphenylethylenediamine bridged zinc(II) bisporphyrin tweezer, (R,R) Zn(II)BP (1), with depiction of the key structural parameters under consideration: ETDM in 5−15 and 10−20 directions. (B) Syn vs Anti dispositions of carbonyl oxygen (CO) and methine hydrogen (C*−H) on either side of the tweezer linker. (C) Relative orientation (antiperiplanar vs synclinal) of the methine hydrogens at the stereogenic centers.

setups as a means of maximizing the thoroughness of the conformational exploration. From each of the nine 500 ns MD simulations, nine representative structures have been selected based on the lowest root-mean-square deviation value (RMSD) relative to all other conformations within the entire MD trajectory. RMSD is considered an unbiased conformational parameter, as it takes into account all atoms, rather than focusing on a more restrictive localized parameter. The representative structures were subsequently subjected to first AM1 based geometry optimization, followed by full geometry optimization using higher level of theory: Wb97xd/ 6-31G+* for C,H,N,O and LANL2DZ for Zn, implicit solvent (iefpcm= CHCl3). Subsequently, each of the fully optimized geometries were then subjected to CD calculations based on 10 electronic transitions at the level of theory: Wb97xd/6-31G +* for C,H,N,O and an effective core potential LANL2DZ for Zn, implicit solvent (iefpcm= CHCl3). It is worth noting that LANL2DZ (Los Alamos National Laboratory 2 Double-Zeta)20 is extensively tested ab initio effective core potential to model metal atoms.21 Gaussian 0922 software was employed for all ab

Figure 8. Outline of the overall molecular modeling, MD, and ab initio workflow. *In the case of free host 1 and 1·L5, the consecutive 500 ns MD was carried out. G


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Inorganic Chemistry

Table 2. (A) Percent (%) Occurrence Representing the Time Spent in Syn-Syn, Syn-Anti, and Anti-Anti Geometry during a 500 ns MD Run for Free Tweezer 1. (B) Boltzmann Population of the Nine Representative Structures Identified from 500 ns MD Runs for Free Tweezer 1

a

Initial conformations all display antiperiplanar orientation of methine hydrogens at the stereogenic centers. bRepresentative conformations are isolated based on the lowest value of the root-mean-square deviation (RMSD) of conformations within the MD trajectory. cGeometries with antiperiplanar orientation are labeled “ap”, while those with minus synclinal are labeled “− sc”.

Figure 9. Four representative conformations and corresponding predicted CD responses for free tweezer 1.

initio based predictions. The overall CD response has been

responses by Boltzmann populations. The detailed workflow is summarized in Figure S18. In each of the three case studies, specifically the free tweezer host 1 and its sandwich complexes with 1,5- and 1,8-diamines, the prevalence of time spent in a given distinct geometry (SynSyn, Syn-Anti, Anti-Anti) during the 500 ns MD run provides %

obtained by two different weighing mechanisms: (a) weighting representative ECD responses by the occurrences (prevalence) of Syn-Syn vs Syn-Anti vs Anti-Anti conformations within the nine MD trajectories, (b) weighting representative ECD H


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Inorganic Chemistry

Table 3. (A) Percent (%) Occurrence Representing Time Spent in Syn-Syn, Syn-Anti, and Anti-Anti Geometry during a 500 ns MD run for 1·L5. (B) Boltzmann Population of the Nine Representative Structures Identified from 500 ns MD Runs for 1·L5

a

Initial conformations all display antiperiplanar orientation of methine hydrogens at the stereogenic centers. bRepresentative conformations are isolated based on the lowest value of root-mean-square deviation (RMSD) of conformations within the MD trajectory. cGeometries with antiperiplanar orientation are labeled “ap”, while those with minus synclinal are labeled “− sc”.

calculations that corroborate with the experimentally observed negative CD couplet. Four representative conformations of the free tweezer 1 are displayed inFigure 9. The most stable conformation MD-1 (f ree) with Boltzmann population of 42.7% displays a negative CD couplet, which is very similar in sign and intensity with the experimentally observed CD response. The preferred geometry of the core segment of the host is Syn-Syn, while the arrangement of methine hydrogens at the stereogenic centers is anti-periplanar. The less stable MD-8 (f ree), with 29.8% population, maintains similar geometry of the chiral core yet elicits an overall lower intensity positive CD response. The differences in CD sign between MD-1 (f ree) and MD-8 (f ree) reflect the geometrical differences in mutual porphyrin orientation and in coupling modes of the 15−5/10−20 transition (Figure 7A). Even the less stable MD-7 (f ree) with a population of 1.6% displays a Syn-Syn geometry yet unfavorable and weak positive CD couplet. The steric interference associated with the close proximity of meso phenyls (∼3 Å) is likely the reason for lower stability and consequently depleted population of MD-7 ( f ree). The considerably less stable MD-2 ( f ree) exhibits Syn-Anti disposition of the CO/C*−H moiety with unfavorable synclinal H−H orientation due to the proximity of bulky substituents at the chiral core. It elicits a positive CD response, opposite in sign of that observed experimentally. In addition, it is worth pointing out that regardless of the starting geometry (Syn-Syn, Syn-Anti, Anti-Anti) as well as the three initially assigned velocities implemented in the MD algorithm, the free tweezer exhibits longer residency of conformers with a negative interporphyrin helicity as measured by the 5−15 dihedral (Figure S20). In terms of the 10−20 dihedral, it also displays a consistent trend in all nine trajectories which is seen in a larger conformational flexibility with angles spanning from around −170° to +170°. This observation is seen not only among the 900 extracted geometries from the first set of nine MD runs but also within 900 additional geometries extracted from a subsequent 500 ns MD trajectory (Figure S21). It should be noted that neither dihedral 5−15 nor 10−20 are individually diagnostic of the expected sign of the CD couplet, which is why the previously mentioned TDDFT CD prediction was necessary on the

occurrences. Furthermore, the Gibbs free energies of nine optimized representative structures have been converted to Boltzmann populations in order to identify the most stable geometries. Data with % occurrences obtained from 500 ns MD runs and Boltzmann populations of representative conformations are given in Tables 2−4, respectively, for each of the model systems under consideration. MD trajectories disclose a vast conformational diversity of all three types of complexes by host 1 within the explicit solvation box, reveal many delicate changes in the mutual orientation, and tilt between porphyrin macrocycles. Nonetheless, several basic structural parameters have been identified to undergo gradual yet distinct conformational changes trackable within the MD trajectory for each of the model systems (free tweezer host 1, sandwich complexes 1·L5 and 1·L8). As depicted in Figure 7, these parameters include (a) the relative orientation between CO and C*−H on both sides of the tweezer host (Syn-Syn vs Syn-Anti vs Anti-Anti), (b) the relative orientation of methine hydrogens at the stereogenic centers (antiperiplanar vs synclinal), (c) the dihedral angle along the 5− 15 direction, (d) the dihedral angle along 10−20, and (e) the Zn···Zn distance. While parameters (a) and (b) do not directly elicit a CD signal, they both influence mutual orientation and electronic interaction of porphyrins and, hence, indirectly affect the sign of the CD response. On the other hand, parameters (c) and (d) are related to the electric transition dipole moments (ETDM) that do contribute to the overall exciton CD couplet. Free Tweezer Host 1. The CO/C*−H orientations have been monitored for all the MD runs, as displayed for representative examples of free tweezer 1 in Figure S19. Collective examination of 500 ns MD trajectories, focusing on the CO/C*−H geometrical parameter, leads to identification of 73% of geometries as Syn-Syn, 22% as Syn-Anti, and only 5% as Anti-Anti (Table 2A). Similar to occurrences analysis, the Boltzmann distribution of the nine selected representative geometries (Figure S20) supports the predominance of Syn-Syn geometry over Syn-Anti in the case of free tweezer with total population given by 99.9% vs 0.1%, respectively. Importantly, in the case of free tweezer host 1, the conformations with average Boltzmann populations of 69% show a CD negative exciton coupled CD upon theoretical I


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Figure 10. Three representative conformations and their predicted CD responses for 1·L5.

representative conformations. On a qualitative level, nonetheless, Figure S21 reveals that the direction of the 5−15 dihedral angle overall predominates in a negative sign, which corroborates with the same sign of the observed CD couplet. Notably, the large variation of the direction of the 10−20 dihedral angle is indicative of considerable porphyrin libration, as expected for the free tweezer in the absence of Zn−N coordination. Sandwich Complex 1·L5. The occurrences based analysis of the 1:1 sandwich complex 1·L5 provides evidence for some reorientation of the CO/C*−H moiety in one arm of the tweezer host 1, namely from Syn to Anti. This one-sided conformational rearrangement is not surprising considering that Zn−N coordination takes place with a guest of shorter N···N distance. Collective examination of 500 ns MD trajectories leads to identification of 41% of geometries as Syn-Syn, 55% as Syn-Anti, and only 4% as Anti-Anti (Table 3A). On the other hand, the Boltzmann distribution of the nine selected representative geometries (Figure S22) supports the predominance of Syn-Syn geometry over Syn-Anti (Table 3B). The outcomes from the two analyses, % occurrences and Boltzmann distribution based, provide the following interesting comparison: Syn-Anti representative structures with 55% occurrences represent the majority, and all display a positive CD couplet in agreement with the experimentally observed positive couplet. The Boltzmann distribution results in overall 60% Syn-Syn geometry and corresponding positive CD couplet. To the best we can tell, at unfavorable circumstances between a large Zn···Zn distance of the host and the short N···N

distance in the ligands, a Zn···N coordination may occur only upon a certain degree of geometrical distortion of the system. The geometry that dominates with 54.3% Boltzmann population is Syn-Syn MD-4 (1,5). It exhibits a positive predicted CD couplet, matching the sign of the experimentally observed one (Figure 10). The less stable MD-8 (1,5), with 22.1% population, also maintains a Syn-Syn geometry of the chiral core, yet elicits an overall negative, unfavorable CD response. The steric interference due to the 2.8 Å distance of meso porphyrins may contribute to the lower stability of this representative conformation. The specific governing parameter of the CD sign cannot be explained based on the Zn···Zn distances of MD-4 (1,5) and MD-8 (1,5), which are very similar (11.4 Å vs 11.5 Å). The considerably less stable MD-6 (1,5) also exhibits a favorable positive CD couplet, although of lower intensity than the most stable MD-4 (1,5). The difference between porphyrin macrocycle orientation and tilt that lead to positive vs negative CD couplet is subtle. From this analysis it becomes obvious that in the case of 1,5diamine, steric requirements for bidentate Zn−N coordination force the initial positioning of the porphyrin−porphyrin macrocycles to change toward closer Zn···Zn distance. While in the free tweezer Zn···Zn is ∼14.6 Å, in the most stable conformation of 1·L5 it is notably shorter, ∼11.4 Å. The steric changes of the overall complex, though very subtle, appear as highly diagnostic. They sensitively affect the porphyrin− porphyrin exciton coupling modes and hence the resulting sign of the couplet for 1·L5. J


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Table 4. (A) Percent (%) Occurrence Representing Time Spent in Syn-Syn, Syn-Anti, and Anti-Anti Geometry during 500 ns MD run for 1·L8. (B) Boltzmann Population of the Nine Representative Structures Identified from 500 ns MD Runs for 1·L8

a

Initial conformations all display antiperiplanar orientation of methine hydrogens at the stereogenic centers. bRepresentative conformations are isolated based on the lowest value of root-mean-square deviation (RMSD) of conformations within the MD trajectory. cGeometries with antiperiplanar orientation are labeled “ap”, while those with minus synclinal are labeled “− sc”.

Figure 11. Three representative conformations and their predicted CD responses for 1·L8.

As was the case with the free tweezer, the L5-tweezer complex also provides similar results by the subsequent 500 ns MD trajectory. Therefore, for a given geometry/velocity setup, conformational diversity is consistent and converged on the 1 microsecond time scale. For this reason, we have not carried out an additional 500 ns MD run on 1·L8 discussed below. In addition, analysis of 900 conformations extracted from each of the two subsequent 500 ns MD runs displays

predominance and therefore overall convergence to a positive interporphyrin helicity as measured by the 5−15 dihedral (Figure S23). As can be seen from Figure S21 vs S23, there is a clear distinction in the trend of both 5−15 and 10−20 dihedral angles over a 1 ms time-lapse in the case of free tweezer vs 1·L5 complex. The 10−20 dihedral angles, expectedly due to bidentate coordination, display a much more restrained degree of freedom manifested by smaller variation in amplitude over K


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Inorganic Chemistry

Figure 12. (A) Occurrences weighted ECD spectral response for free tweezer (1) based on data in Table 2A and the algorithm: ECD signal = 0.73*[CD(1 + 3 + 4 + 7 + 8 + 9)/6] + 0.22*[CD(2 + 5)/2] + 0.5*[CD(6)/1]. (B) Boltzmann weighted ECD response for free tweezer (1) based on relative populations provided in Table 2B. (C) Experimentally observed CD spectrum of 1 in CHCl3 at 295 K.

Figure 13. (A) Occurrences weighted ECD spectral response for sandwich complex with 1,5-diamine (1·L5) based on data in Table 3A and the algorithm: ECD signal = 0.55*[CD(1 + 2 + 3 + 5 + 6)/5] + 0.41*[CD(4 + 7 + 8 + 9)/4]. (B) Boltzmann weighted ECD response for 1·L5 based on relative populations provided in Table 3B. (C) Experimentally observed CD spectrum of 1·L5 in CHCl3 at 295 K.

positive CD response. Based on both the Boltzmann and occurrences analysis, the less prevailing Syn-Anti disposition such as exhibited by MD-3 (1,8) with the synclinal H−H orientation at the chiral core elicits a positive CD response, which is not observed experimentally. Molecular modeling demonstrates that neither the transition dipole moment in the 5−15 direction nor the disposition of the CO/C*−H group governs the sign of the CD couplet. For L8 complex, 900 conformations extracted from MD display more prevalence of negative rather than positive interporphyrin helicity (5−15/5′−15′ direction) which on a qualitative level matches the observed negative sign of the CD couplet (Figure S25). The 10−20 dihedral angles display two types of trends, one that is more restrained and the other that exhibits larger deviation in the amplitude, possibly reflecting bidentate coordination with a longer diamine. Therefore, similarly to the free tweezer, the complex 1·L8 exhibits larger deviations in value of the 10−20 dihedral angle, which point to a higher degree of libration. Weighted ECD Properties. The overall weighted ECD signatures of the free tweezer host (1) are provided in Figure 12, with the occurrence weighted CD response given under panel A, while the Boltzmann weighted CD responses are given under panel B and the experimentally observed CD couplet is given under panel C. Both occurrence and Boltzmann weighted CD signatures match the experimentally observed negative CD couplet for the free tweezer. However, the intensity of the predicted CD response is weaker that observed experimentally (Aobs ∼ 400 cm−1 M−1). Note that the intensity of the negative CD couplet exhibited by the most stable conformer MD-1 (f ree) is quantitatively closer to the experimental intensity. The magnitude of overall CD becomes attenuated due to the presence of an extensive variety of conformations, each of which exhibits a unique mutual

the entire trajectory. The dominance of a positive sign in the 5−15 dihedral angle within the entire MD trajectories of the 1· L5 complex qualitatively corroborates with the observed positive CD couplet. Tweezer Complex 1·L8. For 1·L8, collective examination of 500 ns MD trajectories leads to identification of 54% of geometries as Syn-Syn, 43% as Syn-Anti, and only 3% as AntiAnti (Table 4A). Visualization of MD trajectory discloses that the conversion from unfavorable Anti-Anti to more populated Syn-Anti geometry occurs via C*H−NH bond rotation. Boltzmann distribution of the nine selected representative geometries (Figure S24) supports the predominance of SynSyn geometry over Syn-Anti (Table 4B). Most importantly, conformations exhibiting a predicted negative CD couplet, which corroborates the observed CD in the case of tweezer complex with 1,8-diamine, are the distinct majority with ∼99.7% additive Boltzmann population. From DFT analysis, three representative conformations for a sandwich complex with 1,8-diamine (1·L8) are shown in Figure 11. Similar to the free tweezer 1, the most stable conformation MD-4 (1,8), with population of 64.4%, displays a negative CD couplet which correlates with the sign of the experimentally observed CD response. The relatively elongated conformation of the 1,8-diamine preferentially conforms the tweezer host 1 in the Syn-Syn CO/C*−H orientation with anti-periplanar H− H arrangement at stereogenic centers. The Zn···Zn distance of MD-4 (1,8) is 14.1 Å which is comparable to that of the most stable form of the free tweezer 1, MD-1 (f ree), which exhibits 14.6 Å. Clearly, 1,8-diamine fits to the tweezer host (1) so that the coordination proceeds without any constraints. The considerably less stable MD-6 (1,8), with 0.2% population, maintains similar geometry of the chiral core, but a more buckled conformation of the ligand contributes to a change in the porphyrins mutual disposition, resulting in an overall L


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Figure 14. (A) Occurrences weighted ECD spectral response for a sandwich complex with 1,8-diamine (1·L8) based on data in Table 4A and the algorithm: ECD signal = 0.54*[CD(1 + 4 + 7 + 8 + 9)/5] + 0.43*[(2 + 3 + 6)/3]. (B) Boltzmann weighted ECD response for 1·L8 based on relative populations provided in Table 4B. (C) Experimentally observed CD spectrum of 1·L8 in CHCl3 at 295 K.

(shown in Figure 10, exemplifying an unfavorable structure with steric repulsion) the initial host (1) is forced to undergo a conformational change to allow a coordination to zinc by shorter achiral diamines (n = 2−5) to take place. This certainly should have affected the porphyrin−porphyrin (P/P) mutual disposition (see later by exp. 1H NMR data). As a result, the subtle P/P geometry changes affect the exciton coupling modes between 5−15 and 10−20 electric dipole transition moments. It is critical to emphasize that the theoretical conformational and ECD analysis, although to a certain extent speculative, does provide a rational for the inversion of experimental CD of the complexes with shorter diamines. The dramatic change in CD reflects the spatial changes in the P/P mutual orientation when the zinc-porphyrin free host coordinates with the diamines of short N/N distances. Since the theoretical conformational analysis also reveals a vast array of spatial differences among the complexes in solution (explicit solvent CHCl3), an additional support and verification for P/P geometry changes upon complexation appeared very desirable. The following 1 H NMR analysis focused mainly on interpretation of chemical shifts of porphyrin pyrrolic protons aimed to provide the desired experimental evidence. NMR Analysis of Distinctive Pyrrolic Chemical Shift Pattern in the Free Tweezer 1 and Its Sandwich Complexes 1·Ln. 1H NMR chemical shifts are important source of information for determination of the structure of the complexes. The porphyrin ring plays a critical role in the assessment of the final structure obtained, since the presence of an aromatic ring causes large changes in the chemical shifts of nearby residues. This effect, the so-called “ring current”, is strongly associated with the aromatic character of a system, which influences the host−guest complex protons, even several bonds away from the metal coordination site.23 Therefore, the presence of guest molecules inside the bisporphyrin cavity can sensitively be analyzed by 1H NMR spectroscopy.11,17b Generally, a guest proton experiences an upfield shift due to the strong ring current effect when it is encapsulated within the bisporphyrin cavity. The distance dependence of such effects is quite important for determining the actual placement of a guest proton with regard to the porphyrin host. In the 1:1 sandwich complexes reported here, the amino protons being the closest to the porphyrin rings experience the most ring current effect, and thereby, they are the most upfield shifted. As the distance of the protons from the porphyrin ring increases, the exposure to the ring current decreases and the extent of upfield shifting also decreases.

orientation of porphyrin planes. The CD sign cannot be explained by the distribution of Syn-Syn vs Syn-Anti conformations. Specifically, when MD-7 (f ree) is endowed with Syn-Syn vs MD-2 ( free) displaying Syn-Anti CO/C*− H orientation, both exhibit the opposite of the experimental positive CD couplet with different magnitudes. We believe that the negative vs positive CD sign is geometrically associated with the subtle change in the orientation and tilt of porphyrin macrocycles, which influences the nature of interaction among the degenerate electronic transitions giving rise to the CD couplet. The ECD weighted signatures of 1·L5 are provided in Figure 13. It can be noted that the occurrence-based spectral signature (Figure 13A) provides a more distinct positive CD couplet, which matches with the sign of the observed CD (Figure 13C). The Boltzmann weighted CD, seen in Figure 13B, is attenuated most likely due to the presence of multiple conformations contributing to the variable signs and intensities of the CD couplet. Nonetheless, neither one of the predicted ECD weighted spectra (Figure 13) shows negative CD response, which underscores the confidence level of the newly undertaken MD and ab initio based modeling protocols. The ECD weighted signatures of the sandwich complex with 1,8-diamine (1·L8) are provided in Figure 14. It can be noted that both occurrence-based (Figure 14A) and Boltzmannbased (Figure 14B) weighted spectral signatures afford negative CD couplets, in agreement with the experimentally observed CD sign (Figure 14C). The difference in the signal intensities is again most likely due to the presence of multiple conformations. In an effort to understand the origin of CD response in the cases already discussed we performed a molecular orbitals (MOs) analysis of key electronic transitions in the most stable fully geometry optimized conformations. While the sign of exciton coupled CD in these cases cannot be directly derived from visualized MOs (Figure S26), it is evident that the two components of the porphyrin B band known also as the Soret band along the 5−15 and 10−20 directions (Figure 7) are being involved in producing the overall CD response. The current conformational and theoretical CD analysis suggests that the changes in porphyrin−porphyrin spatial disposition upon complexation lead to inversion of sign of the CD signal, which implies that it is a very geometry sensitive property (therefore structurally diagnostic). CD is so sensitive to the structural changes of the complexes, such that even the steric demands (shorter N/N distance) posed by one −CH2 group lead to the dramatic inversion of the CD signal. It is most likely that in order to minimize the steric interference M


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found for the pyrrolic protons, similar to the free tweezer 1 (Figure 16A). For detailed analysis of the different distribution patterns observed from the 1H NMR, we have considered the most stable conformations obtained from the MD simulations for the three molecules 1, 1·L5, and 1·L8. The position and the distribution pattern of the pyrrolic protons depend on two factors: (a) the ring current of the porphyrin and (b) the chemical environment. The chemical environment experienced by all the pyrrolic protons is similar, but the different exposure to the ring current of the neighboring porphyrin ring results in a differential distribution for these protons. We have used here a general pattern (shown in Figure 16) for the NMR assignment for the pyrrolic protons for all the three molecules 1, 1·L5, and 1·L8. In the case of the free host 1, the two porphyrin macrocycles are in a nearly perpendicular orientation (Figure 17) in which the Zn···Zn distance is 14.57 Å (Table 5). Based on H···H distances, we have divided the pyrrolic protons into three categories. The Hd···Hd distances are the smallest, and the Hd protons display one type of peak (Group-3) at the more upfield region. The Hc···Hc distances are smaller but comparatively higher than Hd···Hd, so they show a different type of peak for the Hc protons (Group-2). The Ha···Ha and Hb···Hb distances are comparatively higher, so they experience a lesser ring current and display a combined peak for two types of proton (Group-1) at the more downfield region. Therefore, the experimentally observed pattern and ratio of 2:1:1 for the pyrrolic protons of the free host 1 could be explained from this theoretical model. In sharp contrast, the two porphyrin rings are nearly cofacial to each other in the sandwich complex 1·L5 (Figure 18) with the Zn···Zn distance of 11.37 Å (Table 6), which is lesser compared to the free tweezer 1. Here also, we have divided the pyrrolic protons into three categories based on H···H distances. Due to the cofacial geometry, the Hc and Hd protons experience similar extent of ring current which can also be seen from similar Hc···Hc and Hd···Hd distances (Table 6), so they display one peak for the two types of protons (Group-3) at the more upfield region. The Ha···Ha and Hb···Hb

The chiral host 1 forms 1:1 sandwich complexes with the shorter and longer diamines (Scheme 1).10 However, the 1H NMR spectral patterns are slightly different, particularly for pyrrolic protons for the 1:1 sandwich complexes with the shorter and the longer diamines (Figure 15). Indeed, two different types of arrangements are seen for the pyrrolic protons of the 1:1 sandwich complexes with the smaller and longer diamines (Figure 16).

Figure 15. 1H NMR spectra in CDCl3 (at 295 K) of (A) 1·L8, (B) 1· L5, and (C) free tweezer 1. (Asterisk represents solvent or trace impurity.)

The free tweezer 1 displays a peak intensity ratio of approximately 2:1:1 (in the direction from downfield to upfield region) for the pyrrolic protons in the NMR spectra (Figure 16C). Whereas in the case of the shorter ligands, upon formation of the 1:1 sandwich complex, the ratio of the pyrrolic protons gets changed to approximately 1:1:2 as well as the distribution pattern is also different (Figure 16B). However, in the case of the longer ligands, upon formation of the 1:1 sandwich complex, the approximate ratio of 2:1:1 has been

Figure 16. Selected region of 1H NMR spectra in CDCl3 (at 295 K) of (A) 1·L8, (B) 1·L5, and (C) free tweezer 1. N


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Figure 17. Schematic representation for the assignment of the pyrrolic protons and the most stable conformer MD-1 of the free tweezer 1 obtained by MD simulation.

Table 5. Selected Structural Parameters Obtained from the Most Stable Conformer MD-1 of the Free Tweezer 1 Obtained by MD Simulation Zn···Zn Ha···Ha Hb···Hb Hc···Hc Hd···Hd

Hx,Hy

Distance (Å)

H3,H11 H4,H12 H5,H13 H6,H14 H7,H15 H8,H16 H1,H9 H2,H10

14.57 14.08 17.88 22.44 21.67 15.10 11.33 6.95 7.67

Table 6. Selected Structural Parameters Obtained from the Most Stable Conformer MD-4 of the Sandwich Complex 1· L5 Obtained by MD Simulation

Group Zn···Zn Ha···Ha

Group-1 (Ha + Hb)

Hb···Hb Group-2 (Hc)

Hc···Hc

Group-3 (Hd)

Hd···Hd

Hx,Hy

Distance (Å)


11.37 12.54 15.27 18.43 17.51 11.72 8.98 6.65 7.57

Group Group-1 (Ha) Group-2 (Hb) Group-3 (Hc + Hd)

The entire discussion has been carried out so far considering the distances between the protons, but there is also another important factor, the angular dependence involving ringcurrent shift. A proton can be moving from the shielding to a deshielding zone for the same distance just by varying the angular projection from the porphyrin ring. The ring current shift of the protons (δrc) can be calculated using the expression, δrc = fcμ(3 cos2 θ −1)/R3 where R is the distance (in Å) of the proton from the center of the aromatic ring, θ the angle of the R vector with the ring symmetry axis, μ the equivalent dipole of the aromatic ring and fc the π-electron current density of the ring.24 The angular dependences of the pyrrolic protons of 1,

distances are comparatively higher, and they display two types of peaks for the Ha and Hb protons at the more downfield region (Group-1 and Group-2). This observation made from the theoretical model of 1·L5 qualitatively explains the experimentally observed pattern and the ratio of 1:1:2 for the pyrrolic protons. In the case of sandwich complex 1·L8, two porphyrin rings are also in a nearly perpendicular orientation (Figure 19), with the Zn···Zn distance of 14.08 Å (Table 7), quite similar to that of the free tweezer 1. Therefore, a similar distribution pattern like the free tweezer 1 was observed for the pyrrolic protons in 1·L8.

Figure 18. Schematic representation for the assignment of the pyrrolic protons and the most stable conformer MD-4 of the sandwich complex 1·L5 obtained by MD simulation. O


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Inorganic Chemistry

Figure 19. Schematic representation for the assignment of the pyrrolic protons and the most stable conformer MD-4 of the sandwich complex 1·L8 obtained by MD simulation.

1:2 open complex 2·(L5)2, which is clearly reflected from the further red-shifting of the Soret and Q bands. The other smaller diamines, L2-L4, also show similar spectral changes upon formations of 2·Ln and 2·(Ln)2 at lower and higher guest concentrations, respectively. However, the additions of 1,8diaminooctane (L8) to the chloroform solution of 2 result in even larger red shifts of Soret and Q-bands due to the formation of 1:1 sandwich complex, 2·L8, with no further changes in the UV−visible spectra upon more additions of the guest. Similar is the situation with other longer diamines L6 and L7 also. Such interaction between host and guest has also been monitored by CD spectroscopy; the complexation results in either two-step inversions of interporphyrin helicity with shorter diamine guests (L2-L5) or retention of helicity of the host with longer diamine guests (L6-L8). Similar observations were also reported recently by us using another chiral bisporphyrin tweezer having (1R,2R)-diphenylethylenediamine spacer (1). Although binding constants are similar for the host−guest complexes, the intensities of their CD couplets are greater with chiral guest 2. The nature of the spacer plays an important role here for these differences; the (R,R)-CHDA spacer in 2 is indeed more flexible than that of (R,R)- DPEA in 1, whose phenyl groups are more involved in steric interactions. We have revisited here the 1:1 sandwich complex formation of (1R,2R)-diphenylethylenediamine bridged zinc(II) bisporphyrin tweezer 1 with the shorter and longer diamines and tried to obtain deeper insight into the unsuspected change in sign of the exciton CD couplet for the 1:1 sandwich complexes. Molecular dynamics (MD) simulations followed by full ab initio geometry optimization and TD-DFT CD prediction revealed a vast conformational diversity of the complexes. The predicted CD data are in good agreement with the experiment. The analysis also makes it clear that the origin of observed CD exciton couplets can be rationalized only by considering the complex nature of the Soret transition, including the impact of porphyrin ring libration. Additional experimental support of the theoretical conformational analysis and CD spectroscopic predictions was provided by the experimental 1H NMR analysis. The characteristic chemical shift pattern of porphyrin pyrrolic protons was pointed out to be highly sensitive to the subtle changes in mutual disposition between the porphyrin macrocycles. Hence, the specific demand for Zn···N (L) coordination by the diamines ligand affects the structure of the complexes and by that the porphyrin ring current.

Table 7. Selected Structural Parameters Obtained from the Most Stable Conformer MD-4 of the Sandwich Complex 1· L8 Obtained by MD Simulation Zn···Zn Ha···Ha Hb···Hb Hc···Hc Hd···Hd

Hx,Hy

Distance (Å)


14.08 18.60 20.72 20.79 18.36 10.99 8.73 9.81 12.29

Group Group-1 (Ha+ Hb)

Group-2 (Hc) Group-3 (Hd)

1·L5, and 1·L8 have also been calculated here (Tables S1−S3). The Hd protons, closest from the ring current of neighboring porphyrin, are also found to be shielded most which is indeed observed experimentally. However, as the distance increases for other types of pyrrolic protons, the angular dependence becomes negligible. The differences in experimental NMR profiles shown above underscore the diagnostic power of the porphyrin ring current, which sensitively reflects the spatial changes between two macrocycles upon metal coordination. The distribution pattern for the pyrrolic protons of the 1:1 sandwich complexes of the longer diamines (n = 6−8) is nearly identical to the free tweezer 1 due to the similar spatial orientation. In the case of shorter diamine guests (n = 2−5), however, the mutual orientation of the porphyrins significantly changes on demand of shorter distance for N/N for coordination. This demand forces the zinc(II) porphyrins to adopt an opposite spatial mutual disposition, which can be clearly seen from the resulting 1H NMR pattern for the pyrrolic protons. By that, the experimental NMR data qualitatively supports the theoretical conformational analysis.

■

CONCLUSION Chiroptical behavior of the host−guest complexes are reported here using (1R,2R)-cyclohexanediamine appended Zn(II) bisporphyrin tweezer host (2) and a series of achiral aliphatic diamine guest varying the chain lengths. The gradual addition of 1,5-diaminopentane (L5) to a chloroform solution of 2 results in a red-shift of the Soret and Q-bands at the lower guest concentration due to the formation of the 1:1 sandwich complex, 2·L5. Further addition of L5 triggers the formation of P


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Inorganic Chemistry

present. It was then carefully layered with hexane in air at room temperature. After standing for 4−5 days, a green solid precipitated out which was then isolated by filtration, washed well with hexane, and dried in vacuum. Yield: 39 mg (73%). UV−vis (CHCl3) [λmax, nm (ε, M−1 cm−1)]: 427 (6.54 × 105), 565 (2.86 × 104), 605 (1.58 × 104). 1H NMR (CDCl3, 295 K): 8.61−8.91 (m, 16H, Pyr-H); 8.09− 8.22 (m, 4H, Ph-H); 7.93−8.07 (m, 4H, Ph-H); 7.64−7.75 (m, 12H, Ph-H); 7.48−7.63 (m, 18H, Ph-H); 6.76 (br, 2H, N-H, CHDA); 5.35 (br, 2H C-H, CHDA); 2.72 (br, 2H C-H, CHDA); 2.44 (br, 2H C-H, CHDA); 1.76 (br, 2H C-H, CHDA); 1.41 (br, 2H C-H, CHDA); −2.31 (br, 4H, C-H, L5); −2.64 (br, 2H, C-H, L5); −2.78 (br, 4H, CH, L5); −4.89 (br, 4H, -NH2, L5) ppm. ESI-MS: m/z 1621.5183 ([(2· L5)+H]+). Synthesis of 2·L8. Yield: 43 mg (78%). UV−vis (CHCl3) [λmax, nm (ε, M−1 cm−1)]: 428 (6.68× 105), 566 (2.91 × 104), 606 (1.76 × 104). 1H NMR (CDCl3, 295 K): 8.61−8.97 (m, 16H, Pyr-H); 8.13− 8.26 (m, 4H, Ph-H); 7.95−8.12 (m, 4H, Ph-H); 7.64−7.75 (m, 12H, Ph-H); 7.42−7.61 (m, 18H, Ph-H); 6.88 (br, 2H, N-H, CHDA); 5.34 (br, 2H C-H, CHDA); 2.71 (br, 2H C-H, CHDA); 2.41 (br, 2H C-H, CHDA); 1.72 (br, 2H C-H, CHDA); 1.41 (br, 2H C-H, CHDA); −0.43 (br, 4H, C-H, L8); −0.99 (br, 4H, C-H, L8); −2.01 (br, 2H, CH, L8); −2.30 (br, 4H, C-H, L8); −4.62 (br, 4H, -NH2, L8) ppm. Instrumentation. ESI-MS spectra were recorded on a waters Micromass Quattro Microtriple quadrupole mass spectrometer. Isotopic pattern simulations were done by the software ‘‘ISOTOPE PATTERN CALCULATOR v-4.0’’ developed by Junhua Yan 2001.9. 1 H NMR spectra were recorded on a JEOL 500 MHz instrument at 295 K. The residual 1H resonances arising from the solvents were treated as a secondary reference. UV−visible and CD-spectra were recorded on PerkinElmer UV−visible and JASCO J-815 spectrometers, respectively. Computational Details. Molecular Dynamic (MD) simulation runs were carried out via Desmond sof tware19 within explicit CHCl3 solvent environment (10 Å solvation box) with constant # moles, pressure and temp (300 K, 1 atm) NPT ensemble. Two consecutive MD runs have sampled 500 ns trajectory. Gaussian 0922 software has been employed for all ab initio based predictions. Full geometry optimization was carried out via AM1, followed by Wb97xd/6-31G+* for C,H,N,O and LANL2DZ for Zn, implicit solvent (iefpcm = CHCl3) levels of theory. CD calculations based on 10 electronic transitions were carried out at Wb97xd/6-31G+* for C,H,N,O and an effective core potential LANL2DZ for Zn, implicit solvent (iefpcm = CHCl3). Visualizations of the optimized geometries, and the corresponding diagrams were made by using Maestro,19 GaussView 5,22 and Chemcraf t software.26

The current investigation demonstrates that the change in porphyrin−porphyrin spatial disposition upon complexation leads to inversion of sign of the CD couplet in the 1:1 sandwich complex having smaller chiral ligands. In order to minimize the steric interaction in the host−guest complex, the host (1 or 2) is forced to undergo a conformational change to allow a coordination to zinc by shorter achiral diamines (n = 2−5). The subtle change in the porphyrin−porphyrin (P/P) mutual disposition affects the exciton coupling modes between 5−15 and 10−20 electric dipole transition moments. Finally, the present study contributes to the currently limited pool of examples of chiral porphyrin tweezers. As such, we wanted to understand how the chain length of achiral ligand and stoichiometry of the complex can profoundly change the complex conformation and CD response. This may contribute to other cases where the chiral host (biological receptor) undergoes conformational changes upon interaction/coordination with achiral ligand (drug).

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EXPERIMENTAL SECTION

Materials. (1R,2R)-(−)-1,2-Diaminocyclohexane (98% ee (GLC)) was commercially purchased from Sigma-Aldrich and used without further purification. Other reagents and solvents were purchased from commercial sources and purified by standard procedures before use. 5-(4-Carboxyphenyl)-10,15,20-triphenylporphyrin was synthesized according to the literature procedures.25 The (1R,2R)-diphenylethylenediamine bridged zinc(II) bisporphyrin tweezer 1 and its 1:1 sandwich complexes 1·Ln (n = 2−8) with the achiral diamines were synthesized according to the procedures reported earlier.10 Synthesis of 2. A mixture of 100 mg (0.152 mmol) of 5-(4carboxyphenyl)-10,15,20-triphenylporphyrin and 4 mL of thionyl chloride was refluxed for 3.5 h. Excess thionyl chloride was removed under reduced pressure. The residue was dried under vacuum for 2 h. The acid chloride was dissolved in 50 mL of dry dichloromethane, and 9 mg of (1R,2R)-1,2-diaminocyclohexane (0.076 mmol) was added to it followed by 0.3 mL of dry triethylamine under nitrogen atmosphere. The solution was stirred at 0 °C for 3 h. The resulting solution was washed with water (100 mL) and extracted with DCM (50 mL × 3). The organic phase was evaporated and subjected to silica gel (100−200 mesh) column chromatography. The product was eluted with 1% methanol in chloroform. The product was washed well with hexane and dried to get 49 mg (46%) of the solid free base bisporphyrin. Free base bisporphyrin (49 mg, 0.0351 mmol) was dissolved in CH2Cl2 (50 mL), saturated methanol solution of zinc acetate (2 mL) was added to it, and the mixture was stirred for 12 h at room temperature. The resulting solution was washed with water (50 mL) and extracted with CH2Cl2 (100 mL × 2). The organic phase was evaporated in vacuum and subjected to silica gel (100−200 mesh) column chromatography. The major reddish violet fraction was eluted with 5% ethyl acetate in chloroform and dried. It was washed well with hexane and dried to get 45 mg (85%) of dark red solid (2). UV−vis (CHCl3) [λmax, nm (ε, M−1 cm−1)]: 419 (6.19 × 105), 549 (3.51 × 104), 587 (6.12 × 103). 1H NMR (CDCl3, 295 K): 8.55−9.09 (m, 16H, Pyr-H); 8.11−8.27 (m, 4H, Ph-H); 7.91−8.09 (m, 4H, PhH); 7.63−7.86 (m, 12H, Ph-H); 7.38−7.61 (m, 18H, Ph-H); 6.51 (br, 2H, N-H, CHDA); 3.17 (br, 2H C-H, CHDA); −0.48 (br, 2H C-H, CHDA); −1.80 (br, 2H C-H, CHDA); −2.14 (br, 2H C-H, CHDA); −2.37 (br, 2H C-H, CHDA) ppm. ESI-MS: m/z 1519.4037 ([2+H]+). The 1:1 sandwich complexes 2·Ln (L: L2-L8) were prepared by using the general procedures; details for 2·L5 as a representative case are described below. Synthesis of 2·L5. Zn(II)bisporphyrin 2 (50 mg, 0.033 mmol) was dissolved in 5 mL of distilled dichloromethane, and 1,5-pentanediamine (L5) (3 mg, 0.039 mmol) was added to it. The mixture was stirred for 30 min and then filtered off to remove any solid residue

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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.9b00962. 1 H NMR, UV−visible and CD spectral titrations, ESIMS, binding constant determination, molecular modeling, and MD and ab initio simulations studies (Scheme S1, Figures S1−S26, Tables S1−S3) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (N. Berova). Fax: (+1) 212932-8273. *E-mail: [email protected] (S. P. Rath). Tel: (+91)-512-2597251. Fax: (+91)-512-259-7436. ORCID

Sankar Prasad Rath: 0000-0002-4129-5074 Author Contributions #

B.S. and A.G.P. contributed equally to this work.

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Article

Inorganic Chemistry Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Council of Scientific and Industrial Research (CSIR), New Delhi and Science and Engineering Research Board (SERB), India, for financial support. CARE scheme of IIT Kanpur is gratefully acknowledged for the CD facility.

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DEDICATION Dedicated in honor and in memory of Professor Koji Nakanishi. REFERENCES

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Complexation of Chiral Zinc(II) Porphyrin Tweezer with Achiral

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