Complexation of Chiral Zinc-Porphyrin Tweezer with Achiral Diamines

Publication Date (Web): March 10, 2017 ... Structural insights of the host–guest complexes have been obtained spectroscopically along with molecular...
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Complexation of Chiral Zinc-Porphyrin Tweezer with Achiral Diamines: Induction and Two-Step Inversion of Interporphyrin Helicity Monitored by ECD Bapan Saha,† Sk Asif Ikbal,† Ana G. Petrovic,‡,§ Nina Berova,*,‡ and Sankar Prasad Rath*,† †

Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India Department of Chemistry, Columbia University, 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 ‡

S Supporting Information *

ABSTRACT: We report here the synthesis of a new chiral Zn(II) bisporphyrin tweezer in which two achiral Zn(II) porphyrin moieties are covalently linked by (1R,2R)-diphenylethylenediamine, which produces a strong chiral field around the porphyrin moieties. The chiral tweezer exhibits not only intensity modulation in UV−vis and CD exciton couplets but also a dramatic change, namely, the inversion in the sign of the interporphyrin helicity upon binding of achiral diamines of varying lengths. The stoichiometry-controlled formation of a 1:1 sandwich complex followed by a 1:2 open complex was realized with smaller achiral diamines (n: 2−5) at their low and high concentration regions, respectively, leading to two-step inversion of chirality. With longer achiral diamines (n: 6−8), however, only 1:1 sandwich complexes are formed with no change of sign in the CD couplet. As compared to a 1:2 open complex, a 1:1 sandwich complex shows an enhanced CD response as two porphyrin units come closer in space. Structural insights of the host−guest complexes have been obtained spectroscopically along with molecular mechanics minimizations with the newly implemented OPLS-3 force field followed by geometry optimization using density functional theory of the most stable conformer. The amide bridge in the Zn(II) bisporphyrin has a low rotational barrier, which provides conformational flexibility to change interporphyrin helicity between 1:1 and 1:2 binding depending on the size of the achiral guests in order to minimize host−guest steric interactions.

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INTRODUCTION Supramolecular chirogenesis is a modern interdisciplinary field of science dealing mostly with asymmetry information transfer upon noncovalent interactions that are widely seen in many natural (DNA double helix, the secondary α-helix structure of proteins, heme proteins, etc.) and artificial systems. There is a growing interest in the control of chirality at the molecular level, which would be immediately applicable in the field of asymmetric catalysis for organic synthesis, the development of sensors to determine the absolute configuration, and many other aspects, such as intermolecular interactions.1−7 Probing molecular chirality is a continuously evolving area of research and of great importance to chemistry, biochemistry, and many related disciplines. However, detection of chirality by electronic circular dichroism spectroscopy (ECD) can be hampered by the prerequisite for chiral species to contain UV−vis chromophores. It is not surprising therefore that © 2017 American Chemical Society

porphyrins and metalloporphyrins with their photoelectronic and geometric attributes have recently attracted much attention for applications as versatile chirality induction reporter groups in structural studies by ECD. The use of porphyrins as molecular probes however poses various challenges arising from undesired aggregation, steric, and solvent effects. To overcome some of these common drawbacks, various research groups have focused on the design of more efficient chemical/chiroptical protocols. As a result of such studies, recent developments lead to the employment of a microscale, supramolecular approach known as the tweezer methodology. In this case an achiral dimeric metalloporphyrin host (tweezer) upon complexation with chiral bidentate guests undergoes a process of chirogenesis and transformation into a Received: November 6, 2016 Published: March 10, 2017 3849

DOI: 10.1021/acs.inorgchem.6b02686 Inorg. Chem. 2017, 56, 3849−3860

Article

Inorganic Chemistry Scheme 1. Synthetic Outline of the Chiral Zn(II) Bisporphyrin (1) Host

sensor for chirality of the bidentate guest.6−12 Specifically, when the bidentate coordination to the metal and the formation of a 1:1 host/guest complex proceeds under stereocontrol, two porphyrins of the host adopt a preferred chiral twist, which in most cases leads to a very intense exciton coupled CD in the Soret band region suited for spectroscopic detection. Since the preferred interporphyrin helicity, along with other factors, is predominantly governed by the absolute configuration (AC) of the guest, it becomes clear that the tweezer approach represents a powerful and sensitive chiroptical tool for investigating molecular chirality of chiral substrates capable of interacting with metalloporphyrin dimers (tweezers) in a stereodifferentiating manner. Over the years, the application of tweezer methodology6−12 toward reliable determination of AC of chiral guests has undergone several developments, the latest being integration of X-ray structure and molecular modeling investigations of the host−guest complex in order to justify the sign of the observed CD couplet. As such, the methodology has resulted in the successful AC assignment of a wide variety of natural products and chiral synthetic compounds including chiral amines, alcohols, and carboxylic acids.6−12 While in most of the published investigations the bis-porphyrin tweezer host is achiral and the guest is chiral, there are rare cases where the tweezer is chiral, while the guest is achiral.13 These studies have demonstrated intensity modulation in the CD response of the free tweezer vs host−guest complex. In the current study, the (R,R)-diphenylethylenediamine host represents, to the best of our knowledge, the first case where the chiral tweezer exhibits not only intensity modulation but also a dramatic change, namely, the inversion in the sign of the interporphyrin helicity

upon binding of achiral diamines of varying lengths. Specifically, the sign of the CD exciton couplet measured for the tweezer has changed from negative to positive and again to negative upon stepwise complexation with the shorter (n = 2− 5) diamines. In sharp contrast, no change in the sign of the CD couplet is observed with longer (n = 6−8) achiral diamines. Interestingly, these are rare examples where the length of the achiral guests has sheer control over the sign of the interporphyrin helicity of the bisporphyrin-based host−guest complexes. The helicity and sign of the CD response are dictated by the relative conformation/orientation of chromophoric porphyrin planes attached to stereogenic centers.14 Stoichiometry-controlled supramolecular chirality induction and one-step inversion have already been reported in the literature.10 It was demonstrated using an achiral Znbisporphyrin receptor with chiral diamine guests. Here, we report the synthesis of a new chiral Zn(II) bisporphyrin in which two achiral Zn(II) porphyrin moieties are covalently linked by a remote chiral auxiliary, (1R,2R)-diphenylethylenediamine (DPEA), which produces a strong chiral field around the porphyrin moieties. The optically active diamine spacer provides a bias of twist nature between two Zn(II) porphyrin chromophores depending on their mutual orientation (angle and distance) characteristic for each structure (free conjugate or host−guest complex). In the present work, we demonstrate a two-step chirality inversion in a molecular assembly consisting of a chiral bisporphyrin host and an achiral diamine guest that is controlled solely by the stoichiometry of the complex. Schemes 1−3 show the complexes reported here and their abbreviations. 3850

DOI: 10.1021/acs.inorgchem.6b02686 Inorg. Chem. 2017, 56, 3849−3860

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Inorganic Chemistry Scheme 2. Complexation of the Smaller Diamine Guests with 1

Scheme 3. Complexation of the Larger Diamine Guests with 1

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RESULTS AND DISCUSSION

We describe in this paper the synthesis of (1R,2R)diphenylethylenediamine bridged zinc(II) bisporphyrin tweezer 1 (Scheme 1) and report its application as a chiral molecular and chiroptical probe for monitoring the complexation process with achiral amines. The sensitive ECD response of tweezer 1

Figure 1. UV−visible spectral changes of 1 in chloroform upon addition of (A) 1,5-diaminopentane, L5, and (B) 1,8-diaminooctane, L8, as the host−guest molar ratio changes from 1:0 to 1:10000 at 295 K.

exhibited an intense negative exciton couplet in the porphyrin Soret region, in full agreement with the theoretically predicted pattern of intramolecularly interacting porphyrins when their effective electric transition dipoles 5−15/5′−15′ form a negative chiral twist (see Figure S1). This is the same as a negatively twisted orientation between two C*−NH bonds in 1R,2R-tweezer 1 in its most stable conformation, 1A (see Figure 6 and the SI).15 Undoubtedly the very intense CD amplitude (ACD) of −574 cm−1 M−1 (−302 cm−1 M−1 at 424 nm and 272 cm−1 M−1 at 416 nm) is quite significant. It is expected that even when complexation or other effects significantly affect the ACD intensity/sign of 1, the accuracy of experimental CD measurements will still remain high enough. UV−vis. Interaction of the achiral diamine L with 1 has been monitored by UV−visible spectroscopy. The addition of 1,5diaminopentane (L5) to the chloroform solution of 1 at room temperature results in a red shift (Figure 1A) of Soret (419 to 427 nm) and Q-bands (549 to 565 nm, 587 to 605 nm) as the low host−guest molar ratio changes from 1:0.1 to 1:10 due to formation of the 1:1 sandwich complex, 1·L5, which has been isolated and characterized. The sandwich complex is proved to be stable up to the addition of 250 equiv of the guest ligand. Further addition of L5 causes dramatic changes in the UV− visible spectral pattern due to the formation of 1:2 open complex 1·(L5)2, which was characterized by the red shifts of

Figure 2. CD and UV−visible (in CHCl3 at 295 K) spectral change of 1 (2 × 10−6 M) upon addition of 1,5-diaminopentane (L5) as the host−guest molar ratio changes from (A) 1:0 to 1:10 and (B) 1:250 to 1:12200. (C) CD and UV−vis (in CHCl3 at 295 K) spectra of 1 (blue), 1·L5 (red), and 1·(L5)2 (green).

Soret (to 429 nm) and Q-bands (to 567 and 606 nm). Similar spectral changes were also observed with other smaller diamines, L2−L4 (Scheme 2), which form both the 1·L and 1·(L)2 at lower and higher guest concentrations, respectively. As the length of the diamine was increased from L2 to L5, more and more red shifts of the Soret band were observed, while the shift was largest with L5. However, upon addition of longer diamines (L6−L8) (Scheme 3), a difference in the UV−visible spectra has been observed (Figures 1 and S2). For example, the addition of 1,8diaminooctane (L8) to the chloroform solution of 1 at room 3852

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Inorganic Chemistry

formation of the 1:1 sandwich complex 1·L8, which has been isolated and characterized. However, the shifts in the Soret band were larger with longer diamines L6 to L8. Moreover, the sandwich complex is highly stable in these cases, and the 1:2 open complex formation has not been observed upon further addition of the guest. The host−guest 1:1 stoichiometry of the sandwich complex formed in solution at the ligand’s low concentration region was further supported by Job’s continuous variation plot (Figures S3 and S4). It has been found that optimum formation of a 1:1 host−guest complex takes place at an equimolar concentration of the bisporphyrin host and guest (i.e., 0.5 mole fraction). ESI-MS. ESI-mass spectroscopy has revealed peaks at m/z 1719.5310, which is assigned for [(1·L5)+H]+ (Figure S5) and m/z 1820.6487 for [1·(L5)2]+ (Figure S6) for 1:1 and 1:2 host−guest complexes, respectively. The molecular peak intensity of [(1·L5)+H]+ is ∼6 times higher (Figure S7) than that of [1·(L5)2]+, which supports the much higher stability of the sandwich complex over the 1:2 open complex. The ESImass spectrum of 1·L8 also reveals a peak at m/z 1760.5714, which has been assigned for [1·L8]+ (Figure S8). In all these cases, the isotopic distribution patterns of the experimental mass have also correlated with the calculated patterns nicely, which confirms the formation of these complexes. In spite of several attempts, we were unable to get any single crystals for the complexes suitable for structure determinations. However, structural insights have been obtained spectroscopically along with molecular mechanics (MM) minimizations with the newly implemented OPLS-3 force field16 followed by geometry optimization using density functional theory (DFT) of the most stable conformer. 1 H NMR. 1H NMR spectral studies were carried out at 295 K in CDCl3. These also play a significant role in establishing the presence of 1:1 sandwich complexes in solution.7,9 Figure S9 displays the relevant spectra coming from the reaction between 1 and L5, as a representative example. The protons of L5 after complexation with 1 show large upfield shiftsCH2 signals at −2.32, −2.62, −2.72; NH2 signals at −4.96 ppmas the L5 ligand immerges within the ring current of two porphyrin rings in the 1:1 sandwich complex. This structure also reveals the downfield shift of the DPEA protons such as CH, NH, and PhH as the two porphyrins come closer, leaving the spacer DPEA protons outside the shielding zone. Full assignment of the proton resonances was obtained from the 2D 1H−1H COSY (Figure S10). Interaction between 1 and L8 was also examined by 1H NMR spectroscopy (Figure S11). The protons of L8 after complexation with 1 show large upfield shifts: CH2 signals at −0.06, −0.47, −1.26 ppm and NH2 signals are too broad, as the L8 ligand gets encapsulated within the bisporphyrin cavity as a result of sandwich formation. Full assignment of the proton resonances of the complex was obtained from the 2D 1H−1H COSY (Figure S12). Electronic Circular Dichroism. Interaction of the bisporphyrin host 1 with the guest diamine L in chloroform was also monitored by CD spectroscopy. Figures 2−5 and S13−S24 and Table 1 summarize the experimental spectral parameters for the complexes. As mentioned earlier, the chiral Zn(II) twist bisporphyrin 1 shows a negative bisignate CD amplitude (ACD) of −574 cm−1 M−1 in the porphyrin Soret band region due to anticlockwise chiral twist between two effective electric transition moments, 5−15/5′−15′, in the Soret region. Likewise, in the UV−visible spectral titration with L, two types of spectral changes are observed in the CD spectra

Figure 3. Variation of the CD amplitude of 1 additions of 1,5diaminopentane (L5) in which the red line and green lines show the 1:1 sandwich and 1:2 open complexation processes, respectively. Inset shows the expanded regions of the CD amplitude dependence of L5 at low ligand molar excess region.

Figure 4. CD and UV−vis spectra of 1 (black) (2 × 10−6 M) in chloroform at 295 K upon addition of guests L2 (blue), L3 (brown), L4 (green), and L5 (red), respectively, at a low ligand concentration for 1:1 complexation.

Figure 5. CD and UV−vis spectra of 1 (black) (2 × 10−6 M) in chloroform at 295 K upon addition of guests L6 (green), L7 (brown), and L8 (pink), respectively, for 1:1 complexation.

temperature results in the red shift (Figure 1B) of Soret (419 to 428 nm) and Q-bands (549 to 566 nm, 587 to 606 nm) due to 3853

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Figure 6. Three representative conformations of 1, each exhibiting an antiperiplanar H1−H2 conformation, as obtained based on MM/OPLS-3 minimization. The geometry of the most stable 1A has been refined via DFT optimization at the B3LYP/6-31G(d) basis set level on C, H, N, O and with the LANL2DZ ECP basis set on Zn, in an implicit solvent environment (iefpcm, solvent = dichloromethane). While in syn−syn 1A the collinearity of the 5−15 effective transition moment and C*−N bond is preserved, this relation is lost in the less stable anti−anti conformation 1C. The same trend is seen in the conformations of representative molecular models 1·L5 and 1·L8 (vide inf ra).

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

1:1 sandwich complex CD data λ (nm) [cm−1 M−1] FCa

guest ligand L2 L3 L4 L5 L6 L7 L8

434 433 432 432 433 433 433

[151] [227] [348] [394] [−212] [−257] [−333]

SCa 423 423 424 425 427 426 426

[−121] [−166] [−257] [−272] [197] [227] [288]

Aobsb,c 272 393 605 666 −409 −484 −621

binding constant K1 (M−1)d 1.7 2.1 3.5 5.1 6.2 7.6 9.5

× × × × × × ×

105 105 105 105 105 105 105

FCa 433 433 433 435 na na na

[−257] [−227] [−212] [−197]

SCa 426 426 426 429

[242] [212] [181] [166]

Aobsb,c −499 −439 −393 −363

binding constant K2 (M−1)d 5.7 4.6 2.4 1.3

× × × ×

103 103 103 103

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]. dCalculated from CD spectral measurement.

a

cm−1 M−1 at 10 equiv of the ligand, which was then maintained up to 250 equiv of the ligand. The total amplitude of the CD couplet (Aobs) of +666 cm−1 M−1 observed for 1·L5 at 295 K is larger than a value of −574 cm−1 M−1 for 1. Remarkably, further addition of L5 resulted in the second chirality inversion (Figure 2B), during which the positive CD couplet gradually

depending on the length of the diamines; one is for the smaller guest ligands (L2−L5), and other one is for the larger ligands (L6−L8). Upon addition of L5, as a representative case, the negative CD couplet of 1 decreases to produce a new bathochromically shifted positive CD couplet (Figure 2A) at the Soret band region, showing an optimum value of +666 3854

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Table 2. Energy and Conformational Parameters Based on MM/OPLS-3 Minimization for Three Representative Conformations 1A−1C of 1, 2A−2C of 1·L5, and 3A−3C of 1·L8 conformations antiperiplanar H−H

CO/H, CO/H

Δ energy (kcal/mol)

HPh−HPh distance of the two porphyrin’s aromatic periphery (Å)

Zn−Zn distance (Å)

Hbonding

interporphyrin helicity (15−5 direction)

1A 1B 1C 2A 2B 2C 3A 3B 3C

syn, syn syn, anti anti, anti anti, anti syn, anti syn, syn syn, syn anti, anti syn, anti

0.0 3.5 17.1 0.0 13.4 53.1 0.0 4.7 7.6

2.8 2.3 5.4 7.4 3.0 2.2 2.8 5.4 6.1

7.2 7.5 13.7 11.1 5.5 9.6 12.6 13.1 12.08

no no no no no no yes no no

−5.1 11.7 19.1 24.2 11.0 −29.1 −19.3 17.1 −5.3

HySS2009 (Protonic Software, UK).18 Two sets of ECD titration data were analyzed considering a binding model (Scheme 2) with three colored stoichiometric states of Zn(II) bisporphyrin (1), 1:1 sandwich complex 1·L, and 1:2 open complex 1·(L)2, as shown in Figures S25−S28. For example, for complexation between 1 and L5, K1 and K2 were found to be 5.1 × 105 and 1.3 × 103 M−1, respectively (Figure S28). Similarly, the interactions of the other substrates (L2−L4) with 1 were also monitored (Figures S25−S27), and the observed binding constants are tabulated in Table 1. Binding constants between 1 and larger diamines L6−L8 were also determined using UV−vis and the CD spectroscopic titration method. Two sets of titration data were analyzed considering a binding model (Scheme 3) with two colored stoichiometric states of Zn(II)bisporphyrin (1), 1:1 host−guest complex 1·L as shown in Figures S29−S32. For example, the binding constant value of 9.5 × 105 M−1 has been observed with L8 for the 1:1 sandwich complex (Figure S32). The interactions of the other larger substrates (L6, L7) with 1 were also monitored (Figures S29−S31), and the binding constants are also tabulated in Table 1. From the binding constant values, it can be concluded that a longer diamine binds much strongly with 1 to form a 1:1 sandwich complex than their respective smaller ligands due to the well-matched size between the host and the guest molecule (vide inf ra) of the former.9,17c−e This has resulted in the conversion of 1:1 sandwich complex 1·L to 1:2 open complex 1·(L)2 upon addition of a large excess of smaller guests (L2−L5), while the 1:1 sandwich complex is fairly stable with longer guest (L6−L8). It is, however, interesting to see here the transition on moving from L5 to L6. As can be seen from the relative population plots (Figure S28), both the 1:1 sandwich and 1:2 open form of the complexes are present in the solution at a given concentration when smaller diamine substrates (L2−L5) are used to titrate with the Zn(II) bisporphyrin 1. However, the populations of 1:1 complex would increase at lower substrate concentration, while the addition of a large excess of achiral diamine results in an increase of 1:2 open complex. In contrast, the addition of the longer substrates (L6−L8) forms the 1:1 sandwich complex exclusively (Figures S29 and S32).

transformed into a new negative CD couplet with the optimum value of −363 cm−1 M−1 at 12200 equiv of the ligand added, in the higher red-shifted wavelength region due to the formation of 1·(L5)2. This reveals an opposite spatial orientation (anticlockwise) in the 1:2 open complex in comparison to the sandwich complex, 1·L5 (Figures 2 and 3). Similar spectral changes were also observed with other smaller guest ligands (L2−L4). The CD amplitude also varies with the increasing length of the diamines (Figure 4). As we move from L2 to L5, the CD amplitude of 1·L increases progressively, while the magnitude decreases for 1·(L)2 (Figure S19). This is because the binding constant (K1) for the 1:1 sandwich complexation is maximum with 1,5-diaminopentane (L5), while the binding constant K2 is lowest for the 1:2 open complex (1·(L5)2) (vide inf ra). The stoichiometry-controlled formation of the 1:1 sandwich complex followed by transformation into the 1:2 open complex can be confirmed further from the large variation of CD amplitude along with the two-step inversion of chirality (Figures 2 and 3) with smaller guests (L2−L5) at their low and high concentration conditions. As compared to the 1:2 open complex, the 1:1 sandwich complex shows an enhanced CD response since the two porphyrin moieties come closer in space in a more rigid structure and thereby experience a stronger exciton coupling between their corresponding electronic transitions. The NH−CO bond (amide bond) of the host bisporphyrin has a very low rotational energy barrier, so the porphyrin moiety can rotate rapidly around the amide bond.17a,bAs a result, the host molecule can easily take different conformations17c−e to accommodate different linkers. For longer diamine guests (L6−L8), however, completely different spectral changes have been observed in the ECD spectral titrations (Figure 5). For example, upon addition of L8, the negative CD couplet of 1 decreases to produce a new negative bathochromically shifted couplet at the Soret band region (Figure S24), showing an optimum value of −621 cm−1 M−1 at 8 equiv of the guest. There is no change seen in the amplitude and sign of the ECD signal upon further addition of the guest. As we move from L6 to L8, the ECD amplitude of 1·L increases progressively with a maximum value for L8 (Figure 5), in which the binding is strongest in the series (vide inf ra). Unlike in the smaller guests, the CD signs have not changed during the host−guest complexation process. Binding constants between 1 and the diamine L were determined using the ECD spectroscopic titration method. The binding constants were calculated using the HypSpec computer program (Protonic Software, UK), and species distribution plots of the complex were calculated using the program

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MOLECULAR MODELING Considering the known (1R,2R)-configuration of tweezer 1 and assuming the antiperiplanar orientation of methine hydrogens as preferred (Figure S33), the negative twist sense is the expected relative orientation of the two amino moieties at the stereogenic centers. According to previous observations based on the exciton chirality method, the twist sign should be 3855

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Figure 7. Most stable conformation 2A of 1·L5 (left panel) and most stable conformation 3A of 1·L8 (right panel) based on MM/OPLS-3 minimization followed by geometry optimization at the B3LYP/6-31G(d) basis set level on C, H, N, O and with the LANL2DZ ECP basis set on Zn, in an implicit solvent environment (iefpcm, solvent = dichloromethane). Both 2A and 3A exhibit anti-periplanar H−H geometry. While 2A exhibits “anti CO/H1, anti CO/H2”, 3A exhibits “syn CO/H1, syn CO/H2” orientation on both sides of the tweezer.

1A−1C, 2A−2C, and 3A−3C, are listed in Table 2. The preferred conformation of the free tweezer 1 and that complexed with L8, designated as 1A in Figure 6 and 3A in Figure 7, both exhibit negative interporphyrin helicity and “syn CO/H1, syn CO/H2” orientation. This computational outcome is in agreement with the observed negative CD couplet. While “anti CO/H” orientation(s) and positive interporphyrin helicity are also identified as plausible, the associated energies based on the newly developed and comprehensively parametrized OPLS-3 force-field are appreciably higher (Table 2), rendering such conformations less preferred. The relative energies and conformational parameters of the three most stable representative conformations of 1·L5, namely, 2A−2C, are provided in Table 2. Interestingly, the original negative interporphyrin helicity of host 1 inverts in the experimental CD of 1·L5 to positive upon interaction with shorter diamine guest L5, corroborated by conformation of the most preferred conformer 2A with “anti CO/H1, anti CO/ H2” orientation (Figure 7). The molecular modeling investigation provides evidence that the conformational parameter that is correlated with the interporphyrin helicity (positive or negative) is the relative orientation between the amido CO group and methine H at the stereogenic center C*−H. The change in the sign of the exciton CD couplet is associated, therefore, with a conformational change in the CO and C*− H orientation. Since MM/OPLS-3 minimizations have demonstrated consistency between interporphyrin helicity and CD data for 1:1 complexes, we have resorted to this theory to explore the predominant geometry of the 1:2 complex between tweezer and 1,5-diaminopentane, namely, 1·(L5)2. In order to assess the

propagated from amino moieties to acyl-derivatized bisporphyrins as long as both acyl carbonyls (CO) exhibit a syn orientation with respect to methine hydrogens and the stereogenic center (C*−H). The broadly accepted generalization for acylated bisporphyrin chromophores covalently bound to NH or OH moieties has been demonstrated in the past for tweezer hosts involving derivatization with diamines, amino alcohols, and diols.6f,h,15b−d As suspected, the observed negative exciton CD response of the free host 1 as well as 1 complexed with longer diamines (i.e., 1,8-diaminooctane, L8) confirms that the negative relative orientation of porphyrins is preserved. The relative orientation between CO and C*−H is, hence, undoubtedly syn on both sides of the tweezer host (Figure 6). It is puzzling, however, why supramolecular complex of 1 with shorter diamines (i.e., 1,5-diaminopentane, L5) does not exhibit the same negative twist, as evidenced by the positive sign of the exciton CD couplet. Provided that we do not have opportunity to examine the crystal structure(s), resorting to systematic molecular modeling analysis to provide structural insight (rationalization) for the origin of CD sign change upon complexation with shorter diamines is imperative. We, hence, resorted to molecular modeling to find evidence for our suspicion that the CO at the acyl moiety does not preserve the syn orientation with the methine hydrogen (C*−H). In order to understand the difference in the sign of the chiroptical response, the iterative MM/OPLS-316 minimizations have been systematically undertaken on free tweezer 1, followed by its complexes 1·L5 and 1·L8 as representative supramolecular models with shorter and longer guests, respectively. The relative energies and conformational parameters of the three most stable representative conformations, 3856

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pocket. Molecular dynamics simulations and theoretical ECD calculations may provide a further, more definitive interpretation as part of the future consideration of 1·Ln complexes.

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CONCLUSION The present study demonstrates that the interporphyrin helicity of the host−guest complexes described here as monitored by ECD depends on the size of the achiral ligand and stoichiometry of the complex. Importantly, the complexation process can proceed either by the induction of two-step inversions of chirality (L2−L5) or by retention of the helicity of the free chiral tweezer host (L6−L8). For the first time, two-step chirality inversions in a molecular assembly consisting of a chiral bisporphyrin host and achiral diamine guest have been realized and probed by ECD measurements and molecular modeling. Our results have demonstrated that the chirality at the supramolecular level, which is one of the key issues in the chirogenic process, can be governed besides the absolute configuration at the chiral source also by slight conformational changes at more remote sites. In this respect the tweezer 1 can allow through modulations of its intense CD profile the sensitive monitoring of the conformational changes arising from its interactions with achiral or chiral substrates at the supramolecular level. The paper describes the first part of a broader study, namely, the interactions of chiral tweezer 1 with achiral diamines. The work on chiral molecular tweezer 1 for molecular recognition of chiral guests is in progress.

Figure 8. Most stable conformation of the 1:2 complex 1·(L5)2 based on MM/OPLS-3 minimization followed by geometry optimization at the B3LYP/6-31G(d) basis set level on C, H, N, O and with the LANL2DZ ECP basis set on Zn, in an implicit solvent environment (iefpcm, solvent = dichloromethane). The 1:2 complex exhibits antiperiplanar H−H geometry with “syn CO/H1, syn CO/H2” orientation on both sides of the tweezer.

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EXPERIMENTAL SECTION

Materials. (1R,2R)-(+)-1,2-Diphenylethylenediamine (99% ee (GLC), [α]20D +102, c = 1 in ethanol) 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.19 Synthesis of 1. A mixture of 100 mg (0.152 mmol) of 5-(4carboxyphenyl)-10,15,20-triphenylporphyrin and 4 mL of thionyl chloride was heated to reflux for 3.5 h. Excess thionyl chloride was removed at reduced pressure. The residue was dried under vacuum for 2 h. The acid chloride thus prepared was dissolved in 50 mL of dry dichloromethane (DCM). A 16 mg amount of (R,R)-diphenylethylenediamine (0.076 mmol) was added to it followed by 0.3 mL of dry triethylamine in a nitrogen atmosphere. The solution was stirred at 0 °C for 3 h. The resulting solution was treated 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. By passing chloroform first a yellowish dilute fraction appeared, which was rejected. Then with 1% methanol in chloroform, a second deep violet fraction was collected, and a third, purple fraction was a monosubstituted byproduct. The second fraction was dried. It was washed well with hexane and dried to get 55 mg (48%) of a dark violet solid of free base bisporphyrin. UV−vis (CHCl3) [λmax, nm (ε, M−1 cm−1)]: 418 (3.81 × 105), 516 (1.90 × 104), 551 (8.57 × 103). 590 (6.48 × 103), 647 (4.71 × 103). 1H NMR (CDCl3, 295 K): 8.72− 8.87 (m, 16H, Pyr-H); 8.20−8.37 (m, 8H, Ph-H); 8.06 (d, 4H, Ph-H); 7.76 (m, 8H, Ph-H); 7.51−7.57 (m, 18H, Ph-H); 7.40 (m, 10H, Ph-H, DPEA); 7.33 (m, 2H, N-H, amide); 5.93 (br, 2H C-H, DPEA); −2.83 (br s, 2H, N-H, Por); 2.91 (br s, 2H, N-H, Por) ppm. ESI-MS: m/z 1493.5948 ([M + H]+). Free base bisporphyrin (55 mg, 0.0368 mmol) was dissolved in CH2Cl2 (50 mL). A zinc acetate-saturated methanol solution (2 mL) was added to the solution, and the mixture was stirred for 12 h at room temperature. The resulting solution was treated with water (50 mL) and extracted with CH2Cl2 (100 mL × 2). The organic phase was evaporated under vacuum and subjected to silica gel (100−200 mesh)

presence of conformations with intermolecular H-bonding between noncoordinated amino groups, a flexible distance constraint (2.0 ± 0.5 Å) was set between the free amino groups. The MM-based analysis provided negative interporphyrin helicity of the most stable conformer of 1·(L5)2, in contrast to the positive one exhibited by the 1:1 complex of 1·L5. As it can be seen from Figure 8, the main conformation of the 1:2 complex exhibits a syn−syn orientation with the negative interporphyrin helicity, in agreement with the negative sign of the exciton CD couplet. Given the most stable conformation, the two noncoordinated amino groups are positioned outside of the tweezer inner pocket and are H-bonded. Attempts have also been made to rationalize the relative stability of conformers of the free tweezer 1 as well as those complexed with 1,5-diaminopentane (L5) and 1,8-diaminooctane (L8) based on energetic penalties attributed by steric factors and stabilizing forces such as H-bonding, as provided in Table 2 and corresponding Figures 7 and S34−S36. The MM/ OPLS-3-based rationale for chirality inversion upon complexation of tweezer 1 with short diamines L2−L5 has provided insight that the positive interporphyrin helicity in the complexes is related to conformational change from a syn− syn orientation of CO/C*−H moieties to the preferred anti−anti. We also have taken the lowest energy conformations obtained via MM/OPLS-3 for 1, 1·L5, 1·L8, and 1·(L5)2 and exposed them to geometry optimization (Figures S37−40). It was found that the structures remain similar even after ab initio geometry optimizations (Table S1), which further emphasizes the stability of these MM/OPLS3-minimized conformations. The chirality switch from negative of the free tweezer 1 to positive of 1·L2−5 most likely reflects the steric demand to accommodate a shorter guest within the tweezer binding 3857

DOI: 10.1021/acs.inorgchem.6b02686 Inorg. Chem. 2017, 56, 3849−3860

Article

Inorganic Chemistry

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. The residual 1H resonances of the solvents were used as a secondary reference. UV−visible and CD spectra were recorded on a PerkinElmer UV−visible and a JASCO J-815 spectrometer, respectively. Computational Details. Initial conformations of the free (R,R)diphenylethylenediamine tweezer host as well as host−guest complexes with achiral 1,5-diaminopentane and 1,8-diaminooctane guests (L5 and L8, respectively) have undergone MM minimizations with the newly implemented OPLS-3 force field16 within the Macro Model V-10.9 applet of Schrodinger software.20 Minimizations have been executed iteratively in the gas phase with the number of steps for each iteration set to 1000. The simulations were considered finalized once consecutive calculations resulted in energy convergence. The achiral diamine guests were “seeded” within the tweezer host under two perpendicular orientations in order to increase the number of initial input geometries that would yield local and global minima via MM/OPLS-3. During minimizations, diamine nitrogens (N) have been flexibly constrained in distance to porphyrin’s Zn with a force constant of FC = 100 and distance constraints set to 2.1 ± 0.5 Å. The most stable conformations identified via MM/OPLS-3 have undergone DFT-based full geometry optimization to achieve geometrical refinements. DFT calculations were also carried out by implementing a B3LYP hybrid functional using the Gaussian 09, revision B.01, package.21 The method applied was Becke’s three-parameter hybrid exchange functional, the nonlocal correlation provided by the Lee, Yang, and Parr expression,22 and Vosko, Wilk, and Nussair’s 1980 correlation functional (III) for local correction.23,24 Frequency calculations were carried out to ensure the existence of no imaginary frequencies in the optimized geometries. All the optimizations were carried out in an implicit solvent environment (iefpcm) for the inclusion of solvent correction using dichloromethane as solvent. Visualizations of the optimized geometries and the corresponding diagrams were made by using Chemcraft software.25

column chromatography. By eluting with 5% ethyl acetate in chloroform the major reddish-violet fraction was collected and dried. It was washed well with hexane and dried to get 46 mg (80%) of dark red solid (1). UV−vis (CHCl3) [λmax, nm (ε, M−1 cm−1)]: 419 (6.00 × 105), 549 (3.48 × 104), 587 (6.04 × 103). 1H NMR (CDCl3, 295 K): 8.71−8.98 (m, 16H, Pyr-H); 7.96−8.19 (m, 12H, Ph-H); 7.70 (m, 8H, Ph-H); 7.25−7.35 (m, 18H, Ph-H); 7.11 (br, 10H, Ph-H, DPEA); 6.59 (br, 2H, N-H, DPEA); 3.99 (br, 2H C-H, DPEA) ppm. ESI-MS: m/z 1619.4348 ([M + 3H]+). The 1:1 sandwich complexes 1·L (L: L2−L8) were prepared by using the general procedures; details for 1·L2 as a representative case are described below. Synthesis of 1·L2. Zn(II)bisporphyrin 1 (50 mg, 0.031 mmol) was dissolved in 5 mL of distilled dichloromethane. 1,2-Ethylenediamine (L2) (2 mg, 0.033 mmol) was added to it, and the mixture was stirred for 30 min. The solution obtained was then filtered off to remove any solid residue and carefully layered with hexane in air at room temperature. On standing for 4−5 days, a green solid precipitated out, which was then isolated by filtration, washed well with hexane, and dried under vacuum. Yield: 39 mg (70%). UV−vis (CHCl3) [λmax, nm (ε, M−1 cm−1)]: 424 (6.20 × 105), 564 (2.86 × 104), 604 (1.49 × 104). 1 H NMR (CDCl3, 295 K): 8.40−8.87 (m, 16H, Pyr-H); 8.06−8.20 (m, 8H, Pyr-H); 7.92−8.04 (m, 8H, Pyr-H); 7.85 (m, 4H, Pyr-H); 7.53−7.70 (m, 18H, Pyr-H); 7.36 (m, 10H, Ph-H, DPEA); 7.32 (m, 2H, N-H, DPEA); 5.98 (br, 2H, C-H, DPEA); −4.00 (br, 4H, CH2, L2) −6.10 (br, 4H, NH2, L2) ppm. Synthesis of 1·L3. Yield: 43 mg (82%). UV−vis (CHCl3) [λmax, nm (ε, M−1 cm−1)]: 425 (6.34× 105), 564 (2.87 × 104), 604 (1.55× 104). 1H NMR (CDCl3, 295 K): 8.50−8.88 (m, 16H, Pyr-H); 8.10− 8.25 (m, 8H, Pyr-H); 7.92−8.04 (m, 8H, Pyr-H); 7.87 (m, 4H, PyrH); 7.50−7.77 (m, 18H, Pyr-H); 7.47 (m, 10H, Ph-H, DPEA); 7.29 (m, 2H, N-H, DPEA); 6.00 (br, 2H, C-H, DPEA); −2.20 (br, 2H, CH2, L3); −3.40 (br, 4H, CH2, L3);-5.70 (br, 4H, NH2, L3) ppm. Synthesis of 1·L4. Yield: 41 mg (77%). UV−vis (CHCl3) [λmax, nm (ε, M−1 cm−1)]: 426 (6.38 × 105), 565 (2.84 × 104), 605 (1.47 × 104). 1H NMR (CDCl3, 295 K): 8.53−8.76 (m, 16H, Pyr-H); 8.12− 8.20 (m, 8H, Pyr-H); 7.98−8.06 (m, 8H, Pyr-H); 7.71 (m, 4H, PyrH); 7.55−7.67 (m, 18H, Pyr-H); 7.46 (m, 10H, Ph-H, DPEA); 7.33 (m, 2H, N-H, DPEA); 6.03 (br, 2H, C-H, DPEA); −2.10 (br, 4H, CH2, L4); −2.80 (br, 4H, CH2, L4); −5.35 (br, 4H, NH2, L4) ppm. Synthesis of 1·L5. Yield: 40 mg (75%). UV−vis (CHCl3) [λmax, nm (ε, M−1 cm−1)]: 427 (6.42 × 105), 565 (2.81 × 104), 605 (1.53 × 104). 1H NMR (CDCl3, 295 K): 8.64−8.84 (m, 16H, Pyr-H); 8.12− 8.22 (m, 8H, Pyr-H); 8.00−8.06 (m, 8H, Pyr-H); 7.88 (m, 4H, PyrH); 7.50−7.75 (m, 18H, Pyr-H); 7.42 (m, 10H, Ph-H, DPEA); 7.36 (m, 2H, N-H, DPEA); 5.99 (br, 2H, C-H, DPEA); −2.32 (br, 4H, CH, L5); −2.62 (br, 2H, C-H, L5); −2.72 (br, 4H, C-H, L5); −4.96 (br, 4H, −NH2, L5) ppm. ESI-MS: m/z 1719.5310 ([1·(L5)+H]+). Synthesis of 1·L6. Yield: 44 mg (80%). UV−vis (CHCl3) [λmax, nm (ε, M−1 cm−1)]: 428 (6.46 × 105), 566 (2.82 × 104), 606 (1.6 × 104).1H NMR (CDCl3, 295 K): 8.64−8.78 (m, 16H, Pyr-H); 8.19− 8.23 (m, 8H, Pyr-H); 7.99−8.12 (m, 8H, Pyr-H); 7.62−7.70 (m, 4H, Pyr-H); 7.51−7.58 (m, 18H, Pyr-H); 7.41 (m, 10H, Ph-H, DPEA); 7.35 (m, 2H, N-H, DPEA); 5.99 (br, 2H, C-H, DPEA); −1.34 (br, 4H, C-H, L6); −1.39 (br, 4H, C-H, L6); −2.09 (br, 4H, C-H, L6) ppm. Synthesis of 1·L7. Yield: 45 mg (83%). UV−vis (CHCl3) [λmax, nm (ε, M−1 cm−1)]: 428 (6.58 × 105), 566 (2.86 × 104), 606 (1.68 × 104). 1H NMR (CDCl3, 295 K): 8.69−8.80 (m, 16H, Pyr-H); 8.21− 8.25 (m, 8H, Pyr-H); 7.97−8.14 (m, 8H, Pyr-H); 7.66−7.70 (m, 4H, Pyr-H); 7.52−7.59 (m, 18H, Pyr-H); 7.39 (m, 10H, Ph-H, DPEA); 7.34 (m, 2H, N-H, DPEA); 5.95 (br, 2H, C-H, DPEA); −0.29 (br, 6H, C-H, L7); −0.57 (br, 4H, C-H, L7); −1.20 (br, 4H, C-H, L7) ppm. Synthesis of 1·L8. Yield: 42 mg (76%). UV−vis (CHCl3) [λmax, nm (ε, M−1 cm−1)]: 428 (6.65× 105), 566 (2.89 × 104), 606 (1.72 × 104). 1H NMR (CDCl3, 295 K): 8.69−8.83 (m, 16H, Pyr-H); 8.22− 8.28 (m, 8H, Ph-H); 8.01−8.17 (m, 8H, Ph-H); 7.67−7.70 (m, 4H, Ph-H); 7.52−7.55 (m, 18H, Ph-H); 7.39 (m, 10H, Ph-H, DPEA); 7.33 (m, 2H, −NH, DPEA); 5.96 (br, 2H, −CH, DPEA); −0.06 (br, 8H, −CH2, L8); −0.47 (br, 4H, −CH2, L8); −1.26 (br, 4H, −CH2, L8) ppm. ESI-MS: m/z 1760.5714 ([1·(L8)]+).

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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.6b02686. UV−visible spectral titrations, Job’s plot, ESI-MS, 1H NMR, 1H−1H COSY, CD spectral titrations, binding constant determination, MM and DFT-optimized structures, Figures S1−S40, Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (N. Berova). Fax: (+1) 212-9328273. *E-mail: [email protected] (S. P. Rath). Tel: (+91)-512-2597251. Fax: (+91)-512-259-7436. ORCID

Sankar Prasad Rath: 0000-0002-4129-5074 Notes

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 3858

DOI: 10.1021/acs.inorgchem.6b02686 Inorg. Chem. 2017, 56, 3849−3860

Article

Inorganic Chemistry Board (SERB), India for financial support. The CARE scheme of IIT Kanpur is gratefully acknowledged for the CD facility.

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DOI: 10.1021/acs.inorgchem.6b02686 Inorg. Chem. 2017, 56, 3849−3860

Article

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DOI: 10.1021/acs.inorgchem.6b02686 Inorg. Chem. 2017, 56, 3849−3860