A Combined Experimental and Theoretical Study on the Circular

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A Combined Experimental and Theoretical Study on Circular Dichroisms of Staggered and Eclipsed Forms of Dimethoxy[2.2]-, [3.2]-, and [3.3]Pyridinophanes and Their Protonated Forms Akinori Shimizu, Yoshihisa Inoue, and Tadashi Mori J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b08623 • Publication Date (Web): 11 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017

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The Journal of Physical Chemistry

A Combined Experimental and Theoretical Study on Circular Dichroisms of Staggered and Eclipsed Forms of Dimethoxy[2.2]-, [3.2]-, and [3.3]Pyridinophanes and Their Protonated Forms

Akinori Shimizu, Yoshihisa Inoue, and Tadashi Mori* † Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka, 565-0871, Japan * E-mail: [email protected]

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ABSTRACT.

The

circular

dichroisms

(CDs)

of

dimethoxy[2.2]-,

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[3.2]-,

and

[3.3]pyridinophanes and their protonated forms were investigated experimentally and theoretically. Characteristic multisignate Cotton effects (CEs), typical for planar chiral cyclophane derivatives, were observed. The CD spectral pattern was quite comparable for the staggered forms of [2.2]-, [3.2]-, and [3.3]cyclophanes, but significantly differed for the eclipsed forms. More interestingly, the pattern resembled, but the CE signs were practically opposite between staggered and eclipsed [2.2]pyridinophanes. Upon protonation, the signs of most CEs were inverted in both forms of cyclophanes, owing to the reversal of diploe moment in the pyridine against the pyridinium moiety. Such a change in CD spectrum upon protonation was not apparent in [3.2]pyridinophane and the CD spectral behavior was more complex in [3.3]pyridinophanes. The variation of CD caused by protonation/deprotonation process was temperature-dependent and hence utilized as a thermal sensor. The protonated forms of the homologous pyridinophanes with different tether lengths in staggered and eclipsed forms served as a model system for systematically studying the cation-π interaction and its effects on chiroptical properties. A steady increase of electronic interaction became apparent for the smaller-sized cyclophanes from the increased excitation energy and electronic coupling element of the charge-transfer (CT) band, while the observed CE at the CT band was a more complex function of the original transition dipole of donor/acceptor pair and linker atoms, as well as the strength of electronic interaction.


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Introduction Pyridinophanes have been widely exploited as scaffolds for advanced materials utilizing the pyridine unit as a metal coordination or hydrogen bonding site for constructing supramolecular assemblies and architectures.1,2,3 Possessing the inherent planar chirality, heterocyclophanes and substituted cyclophanes have been the subject of intensive experimental and theoretical studies and also employed as key components in various applications.4,5,6 We have recently shown that (unsubstituted) [2.2]- and [3.3]pyridinophanes exhibit strong coupled Cotton effects (CEs) in their circular dichroism (CD) spectra and the CE sings are completely inverted upon protonation to the pyridine unit. The latter phenomenon was rationally explained by the reversal of dipole moment in the pyridinium versus pyridine moiety, as the electronic transitions are composed of the excitonic states while the structural change upon protonation is marginal. 7 Another interesting feature of the protonated pyridinophanes is the large anisotropy (g) factor in the order of 10-2 observed for the CT band. In this study, we experimentally and theoretically investigated the CDs of dimethoxysubstituted [2.2]-, [3.2]-, and [3.3]pyridinophanes in their staggered and eclipsed forms. Particularly, the impacts of protonation on their structures and CD spectra were comprehensively investigated and the results were compared with those of parent pyridinophanes (Scheme 1). The transannular interaction in a series of cyclophanes with different tether lengths (i.e., [m.n]cyclophanes where m ≠ n) has been investigated, 8 but the chiroptical properties of [m.n]cyclophanes have rarely been examined.9,10 Such studies will provide deeper insights into the sensing mechanisms operative in flexible macrocyclic hosts with pyridine unit(s). 11 ,12 ,13 Furthermore, the protonated pyridinophanes possessing rigid framework will enable us to



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systematically evaluate the cation-π interaction between the benzene and pyridinium units and its effects on the chiroptical properties.

Scheme 1. Planar chiral dimethoxypyridinophanes investigated in this study and the parent pyridinophanes. Only the (Sp)-enantiomers (with respect to the pyridine ring), excepting 1s, are shown and the second Rp/Sp descriptor for eclipsed and staggered forms in substituted cyclophanes are omitted for clarity.

N

N OMe

MeO

S MeO

MeO (Sp)-1s

N

S MeO OMe

OMe (Rp)-1s

N

N

S

OMe (Sp)-3s

(Sp)-2s

(Sp)-1n N

N

N OMe

S

MeO

OMe S

MeO

(Sp)-1e

(Sp)-3e

S

S

(Sp)-3n

H+ NH+

(OMe)2 +

(Sp)-1s/e-H ~ 3s/e-H+


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Results and Discussion Structure of Dimethoxy[2.2]-, [3.2]- and [3.3]Pyridinophanes and Protonated Pyridinophanes While the conformational variation is not important in rigid [2.2]pyridinophanes, more flexible [3.2]- and [3.3]pyridinophanes connected by thiatrimethylene bridge(s) can exist as syn or anti conformation with respect to the sulfur (on tether) against the nitrogen atom (on pyridine/pyridinium).9 Thus, two conformations (syn/anti) are possible for [3.2]cyclophane 2s, while four conformers, i.e., anti-chair, anti-boat, syn-chair, and syn-boat (where the boat/chair are defined by the relative alignment of two sulfur atoms) should be considered in [3.3]cyclophanes 3s and 3e. Furthermore, additional skew conformation would be possible depending on the twist angle or the direction of nitrogen displacement (outward/inward), but such a conformation in dimethoxylated pyridinophanes was only found in the anti-chair conformation of 3e (Chart 1; also see sketches in Table S1 in the Supporting Information). The skew conformation has been reported to be of significant importance for the relevant spectral properties in some substituted cyclophanes. 14 The (chir)optical properties actually observed under a given condition should be the ones weight-averaged over all the conformers populated in the system, where the component spectra often considerably differ each other (vide infra). Structures of pyridinophanes have been discussed with various deformation parameters, 15 including the inter-plane distances (d and d’), the deformation angles (α and β), the twist angles (γ and γ’), the vertical separation (r), and the horizontal displacement (l), all of which will affect the (chir)optical properties in different degrees (Chart 2; for a detailed explanation of the definition of parameters, see ref. 7). It has been recently demonstrated that, in the case of parent pyridinophanes, the structural changes associated with protonation are less effective on the observed spectra than the electronic changes caused by protonation.7 We have also shown that



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the relative conformer population has stronger impact on the overall spectral properties than the structural deviation in each conformer.7 This conclusion is generally retained for the current dimethoxypyridinophane systems, as discussed below. In this section, however, we briefly discuss the deformation of dimethoxypyridinophane structures upon protonation with the above parameters on the basis of the calculated and crystal structures (Table 1).

Chart 1. Four conformers for dimethoxypyridinophanes


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Chart 2. Representative structural parameters for dimethoxypyridinophanes (skeletons are shown in black). d: the center-to-center distance between the mean planes defined by the non-bridged atoms of the two facing aromatic rings. d’: the averaged distance of the two sets of the facing bridgehead atoms; α: the averaged deformation angle between the mean plane and the bridgehead atom; β: the additional deformation angle of the linker atom from the mean plane; γ: the twist angle between the two axes passing through the mean planes (red lines); γ’: the twist angle between the two axes passing through the bridgehead atoms (green lines); r: the vertical separation between the mean planes; l: the horizontal displacement distance between the projected centers of the two mean planes.

X-ray Structures We succeeded in obtaining the X-ray crystal structures of enantiomerically pure staggered and eclipsed [3.3]pyridinophanes 3s and 3e and their protonated forms 3s-H+ and 3e-H+ as bromide salts at -150 °C (Figure 1). In all cases, two or four molecules were found as components, but only a single conformer was present in a unit cell. Despite considerable efforts, we were unable to obtain single crystals of [2.2]- and [3.2]pyridinophanes suitable for X-ray diffraction study. The structures thus obtained for neutral [3.3]pyridinophane crystals were shown to be the energetically most stable conformers by the theoretical calculations at the SCS-MP2/def2-



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TZVPP level with COSMO solvation model (vide infra), namely, the anti-boat and anti-chair conformers for 3s and 3e, respectively. However, the structures found in the crystals of the protonated forms were the less stable anti-boat conformers for both 3s-H+ and 3e-H+, which is ascribed to the intermolecular interactions, the coordination of bromide anion, as well as the packing requirement. Note also that the crystal of 3s-H+ contains solvent acetonitrile molecule in the unit cell. Details of the relative calculated energies of the conformers can be found in Table S1 in the Supporting Information. Judging from the packing structures, each cyclophane molecule is rather isolated and the intermolecular interactions should be negligible in the two neutral pyridinophanes. Such packing has been also found in other cyclophane crystals. 16 In contrast, the H-type stacking has been found for stronger (but neutral) donor-acceptor cyclophanes.17 Similarly, the intermolecular interaction was found significant in the staggered form of protonated pyridinophane 3s-H+, where the J-type association with the parallel alignment was observed. The J-association was favored probably due to the localized charge distribution around the nitrogen atom. Such interaction is again discouraged in the corresponding eclipsed form (3e-H+), where the cationic nitrogen is concealed by the adjacent methoxy groups. In the complex between benzene and pyridinium, the preferred inward displacement of nitrogen has been suggested by the high-level ab-initio calculations,18 which was also supported by the structures of parent pyridinophanes (1n-H+ and 3n-H+).7 However, the opposite (outward) displacement was found in the structures of dimethoxylated pyridinophanes 3s-H+ and 3e-H+, probably owing to the larger fraction of charge transfer between the dimethoxybenzene and pyridinium moieties and/or to the extended charge delocalization in the dimethoxybenzene ring. The distances d and d’ were slightly decreased (

A Combined Experimental and Theoretical Study on the Circular

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