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Shuttling Motion in a Host−Guest Complex Triggered by Spiropyran to Merocyanine Reversible Chemical Transformation Denhy Hernández-Melo, Ruy Cervantes, and Jorge Tiburcio* Departamento de Química, Centro de Investigación y de Estudios Avanzados (Cinvestav), Ciudad de México 07360, México S Supporting Information *
ABSTRACT: A stimulus-responsive guest-containing spiropyran and viologen unit assembles with a 24-membered crown ether into a stable host−guest complex displaying a partially threaded geometry. Acid addition induces guest transformation to a merocyanine species activating a second recognition site, suitable for the formation of a pseudorotaxane. The simultaneous presence of two recognition sites produces a small-amplitude macrocycle shuttling motion, from the viologen to the merocyanine moiety. Base addition returns the guest to its spiropyran form, and concurrently the translation motion stops.
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This complex is held together by a series of noncovalent interactions (N+···O ion-dipole, C−H···O hydrogen bonding, and π-stacking) between the stimuli-generated PEI recognition site and the crown ether macrocycle.7 On the other hand, viologen derivatives are commonly used as binding motifs in host−guest adducts. Specifically, it has been shown that dimethyl-viologen, also known as paraquat, forms 1:1 and 1:2 supramolecular complexes with DB24C8 in a partially threaded conformation.8 Each methyl-viologen motif (MV) binds to one crown ether through a series of hydrogen bonds, ion-dipole, and π-stacking interactions. Herein, we propose the incorporation of a spiropyran fragment into the structure of a guest containing a MV recognition motif as the initial binding site for DB24C8 macrocycle. In this initial state, the crown ether would adopt a partially threaded C-type conformation (Scheme 1, left). Chemical interconversion of spiropyran to its protonated planar merocyanine would turn on a second binding station, PEI, with an associated co-conformational rearrangement to a fully threaded geometry (Scheme 1, right). These two coconformational states would coexist in equilibrium until base is added, turning PEI station of f and regenerating the initial state. The process could be repeated using acid/base stimulus as the energy source. It is important noting that both, spiropyran and merocyanine moieties, would perform as stopper groups. The spiropyran-based compound [SP-1][CF3SO3]2 was obtained as an orange solid by an alkylation reaction of a viologen precursor [SP-Bipy]Br7 with methyl iodide in acetonitrile, followed by an ion exchange procedure with concentrated aqueous sodium trifluoromethanesulfonate. This
he ability to precisely control motions at molecular level is an essential requirement for the development of artificial molecular machines.1 In this regard, host−guest complexes have been widely explored due to their dynamic nature and ability to respond to external stimuli.2 Particularly, shuttling motion has been achieved in bistable rotaxane-like systems activated by electrochemical,3 optical,4 or chemical5 inputs. Despite of the well-established binding motifs utilized for the assembly of rotaxane complexes, there is an ongoing interest in emerging novel and diverse binding modes. Upon stimulation, these new motifs must undergo significant electronic and structural changes, biasing the population between two nondegenerate energetic states; application of an opposite input must reverse the induced changes. In this context, spiropyrans have been widely studied because they can be reversibly transformed to its corresponding merocyanine species by UV-light irradiation or pH variation, resulting in substantial electronic and structural changes.6 Spiropyran is a neutral moiety with two perpendicular heterocyclic units, its transformation to a merocyanine occurs by breaking the Cspiro−O bond allowing an open-ring process to provide a fully delocalized zwitterionic species (phenolate O−, indolium N+), in the case of photoisomerization, or a protonated cationic species (indolium N+) for the pH-induced transformation. Recently, we reported that after applying an optical or chemical stimulus to a compound containing a spiropyran fragment and a benzyl-substituted viologen, an interconversion to the merocyanine species occurs, and a new recognition site for 24-crown-8 macrocycles is obtained, the pyridiniumethylene-indolium motif (PEI). When the transformation is performed in the presence of dibenzo-24-crown-8 ether (DB24C8), a stable [2]pseudorotaxane complex is generated. © 2017 American Chemical Society
Received: March 2, 2017 Published: March 22, 2017 4484
DOI: 10.1021/acs.joc.7b00509 J. Org. Chem. 2017, 82, 4484−4488
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The Journal of Organic Chemistry
carbon atom (109.3° avg) in an orthogonal manner. The spiropyran moiety is connected to the methyl-viologen unit by a N−CH2CH2−N+ moiety which adopts an antiperiplanar configuration with a dihedral N−C−C−N angle of 170.8°. The aromatic rings of the viologen fragment are tilted 34.3°. It is well-known that a spiropyran species can be transformed to its merocyanine form by acid addition or UV-light irradiation.6 Since a low-yield photoconversion process was observed in a closely related spiropyran-viologen system,7 in this work, we only focus on the use of acidic conditions as stimulus for the featured guest. Three equiv of trifluoromethanesulfonic acid were added to an acetonitrile solution of [SP1][CF3SO3]2 in order to obtain the protonated merocyanine analogue [MEH-1][CF3SO3]3. Quantitative transformation was obtained after 12 h in the dark at 25 °C, with the expected color change from colorless to yellow. The electronic spectrum of [MEH-1][CF3SO3]3 in acetonitrile showed the characteristic band at 420 nm of the extended π-conjugation for the openring species in its protonated form. This compound was also characterized by 1H and 13C NMR spectroscopies as well as high-resolution mass spectrometry (see SI, S9−S14). In this compound, the indoline ring of the spiropyran transforms to an indolium ring with a formal positive charge located at the nitrogen atom. Interestingly, reverse transformation was achieved only 3 min after pyridine addition. The extended electronic π-conjugation in the merocyanine fragment of compound [MEH-1][CF3SO3]3 and the formal positive charge in the indolium ring cause notable changes in the chemical shifts of the 1H NMR spectrum (see SI, S9) as follows: (i) only one singlet (1.82 ppm) is observed corresponding to the two methyl groups of the indolium ring, which are now chemically equivalent due to the local planarity of the merocyanine fragment; (ii) a high-frequency shift of the two doublets (8.35 H8 and 7.36 H9 ppm) with a coupling constant of 16.3 Hz characteristic of a trans double bond; and (iii) a high-frequency shift (∼1.5 ppm) of the ethylene bridge protons next to the positive charge in the nitrogen atom of the indolium ring (5.19 H10 ppm). All resonances were thoroughly assigned with 2D NMR experiments. High-resolution mass spectrum showed a peak which corresponds to [MEH-1]3+ (calcd, m/z 169.0793; found, m/z 169.0796; error = 1.8 ppm).
Scheme 1. Partially Threaded Complex (Left) and Shuttling Motion (Right) Triggered by Chemical Interconversion of Guest [SP-1]2+ to [MEH-1]3+ in the Presence of DB24C8
compound was fully characterized by 1H and 13C NMR spectroscopies and single-crystal X-ray diffraction as well as high-resolution mass spectrometry (see SI, S2−S8). The 1H NMR spectrum of [SP-1]2+ in acetonitrile-d3 showed: (i) two singlets at low frequencies (1.19 and 1.05 ppm), for the nonequivalent methyl groups; (ii) two doublets at higher frequencies (6.87 H8 and 5.11 H9 ppm) with a coupling constant of 10.2 Hz, in agreement with a cis configuration double bond in the spiropyran fragment; (iii) two multiplets (3.73 H10 and 4.85 H11 ppm) for the ethylene bridge; and (iv) four aromatic doublets (between 8.2 and 9.0 ppm) and a singlet (4.41 He ppm) corresponding to the methyl-viologen fragment (see SI, S2). All the signals were accurately assigned by 2D NMR analysis. High-resolution mass spectrum showed a peak which corresponds to [SP-1 + CF3SO3]+ (calcd, m/z 655.1833; found, m/z 655.1832; error = 0.1 ppm). The proposed structure was confirmed by X-ray diffraction (see SI, S8). Single crystals were obtained by slow evaporation of a saturated methanol solution containing [SP-1][Br]2. The indoline and chromene units are bound together by a spiro
Figure 1. Partial 1H NMR spectra of [SP-1][CF3SO3]2 (0.022 M) (left) and [MEH-1][CF3SO3]3 (0.022 M) (right) with additions of DB24C8 (500 MHz, CD3CN, 298 K). (■) Residual chloroform. 4485
DOI: 10.1021/acs.joc.7b00509 J. Org. Chem. 2017, 82, 4484−4488
Note
The Journal of Organic Chemistry Once the spectroscopic features of both species, [SP-1]2+ and [MEH-1]3+, were fully understood, 1H NMR titrations were performed with DB24C8 macrocycle to establish the formation of supramolecular complexes in acetonitrile solution. Addition of DB24C8 (from 1−10 equiv) to the spiropyran species [SP-1]2+ yields only one set of signals in the 1H NMR spectrum and significant chemical shift changes for the protons on the viologen vicinity; these observations indicate the formation of a host−guest complex in fast exchange with uncomplexed species in the NMR time scale and the crown ether located around the MV site on the guest (Figure 1, left). The resonance corresponding to the N+-CH3 protons is shifted to higher frequency (Δδ = +0.24 He ppm) in accordance with the formation of hydrogen bonds with the oxygen atoms on the crown ether cavity, as it has been observed for related systems.9 In addition, resonances for the viologen protons are shifted to lower frequencies with respect to the free guest (Δδ = 0.73 Hb and 1.03 ppm of Hc), suggesting an aromatic π-stacking interaction with the catechol rings on the crown ether. These results support the idea of a host−guest complex with the crown ether in a C-type conformation capping the MV recognition site [SP-1·DB24C8]2+, (Scheme 1, left). It is worth noticing that the chemical shifts for the protons on the ethylene bridge and the merocyanine moiety do not significantly change throughout the titration experiment, discarding its participation in the complexation process. Furthermore, no evidence of transformation from the spiropyran species to the merocyanine was observed by the sole presence of the crown ether. High-resolution mass spectrum of an equimolar acetonitrile solution showed a peak corresponding to the singly charged species, reinforcing the proposed 1:1 stoichiometry (SI, S22). An association constant was derived from titration data at 25 °C by a least-squares fitting analysis of the chemical shift of the N+-CH3 protons under a 1:1 stoichiometry model resulting in 2.7 ± 0.3 × 102 M−1 (see SI, S15), a value comparable to those reported for other host−guest complexes adopting a similar spatial arrangement.8 Results obtained from a 1H NMR titration experiment of the species [MEH-1]3+, under the same experimental conditions, provided a quite different outcome (Figure 1, right). Upon addition of 1 equiv of DB24C8 to a solution of [MEH-1]3+, we observe only one set of resonances and simultaneous chemical shift variations for the viologen and merocyanine moieties in comparison with unbound guest: a high-frequency shift for the N-methyl group (Δδ = +0.11 ppm) and a low-frequency shift for the aromatic protons (Δδ = 0.32 Hb, 0.36 ppm of Hc) on the viologen fragment; also a high-frequency shift for the ethylene resonances (Δδ = +0.06 H10, + 0.05 H11 ppm) besides some aromatic and olefinic protons (Δδ = +0.09 H7 and +0.10 H8 ppm) on the merocyanine moiety. These changes are consistent with the formation of a partially threaded complex [MEH-1·DB24C8]3+ in fast exchange with a fully threaded pseudorotaxane complex [MEH-1⊂DB24C8]3+ (Scheme 1). This behavior occurs after the PEI binding station is turned on, inducing a shuttling motion of the macrocycle between the two recognition sites, MV and PEI, in a fast dynamic process at the NMR time scale. A 70/30 ratio of the partially to fully threaded complexes can be estimated by weighing the chemical shifts and mole fractions for the bound and unbound species, tracking protons from the MV and PEI stations, respectively (see Experimental Section). The bias for the partially threaded complex is in agreement with its higher
association constant regarding a related pseudorotaxane complex.7 In order to provide further support to our proposal, variabletemperature 1H NMR experiments were performed (see SI, S18). By using a solution of [MEH-1]3+ and DB24C8 in a 1:2 molar ratio, the 1H NMR spectrum showed significant changes throughout the temperature range, particularly in the ethylene protons region: (i) at high temperature (333 K), separate resonances for protons H10 and H11 are observed, due to uncomplex guest; (ii) at 273 K, only one broad signal is observed for protons H10 and H11, because of a fast exchange process between two different complex species in different geometrical arrangements; and (iii) at low temperature (233 K), three well differentiated signals are observed as a consequence of a slow exchange process in the NMR time scale between the partially and the fully threaded complexes. It is also important to mention that at this temperature, practically all resonances on the 1H NMR spectrum are split. A 2D T-ROESY experiment was also performed at 233 K to gain further information about the shuttling motion (SI, S19). A 50/50 ratio of the partially to fully threaded species was determined by integration of the ethylene resonances for both complexes at this temperature (Figure 2). A dynamic exchange constant of kex = 1.3 s−1 for the macrocycle shuttling motion between both recognition sites could be established following a reported method (SI, S20).10
Figure 2. Partial T-ROESY NMR of [MEH-1]3+ (0.022 M) with DB24C8 (0.044 M) at 233 K with tm = 0.75 s (500 MHz, CD3CN). Only EXSY-related peaks are shown.
The observed co-conformational equilibrium of the shuttling motion of the DB24C8 macrocycle between two different binding sites, MV and PEI, in the [MEH-1]3+ guest can be interrupted by a chemical stimulus. Addition of 3 equiv of pyridine-d5 rapidly transforms (
