Chiral Self-Sorting Process with Ditopic Ligands: Alternate or Block

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Article Cite This: ACS Omega 2019, 4, 2676−2683

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Chiral Self-Sorting Process with Ditopic Ligands: Alternate or Block Metallopolymer Assembly as a Function of the Metal Ion Maya Marinova, Maria Torres-Werle,́ Greǵ ory Taupier, Aline Maisse-François, Thierry Achard, Alex Boeglin, Kokou Dodzi “Honorat” Dorkenoo, and Steṕ hane Bellemin-Laponnaz* Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), CNRS-Université de Strasbourg, 23 rue du Loess BP 43, F-67034 Strasbourg, France

ACS Omega 2019.4:2676-2683. Downloaded from pubs.acs.org by 5.188.217.89 on 02/12/19. For personal use only.

S Supporting Information *

ABSTRACT: We report an extensive study on the coordination behavior of chiral ditopic bridging ligands, which lead to metallosupramolecular polymers in the presence of Zn(II) and Cu(II) in solution. With the help of UV−vis and circular dichroism spectroscopies, we show that the metallopolymer sequence can be controlled by chirality and by the choice of the metal ion. Although the formation of a block metallopolymer proceeds through the assembly of homoleptic complexes, an alternate metallopolymer may be obtained only when heteroleptic complexes are formed. This demonstrates how the prevalent coordination geometries at metal centers may be used to control the sequences of the metallopolymers.

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INTRODUCTION Metallo-supramolecular polymers are highly attractive materials, especially compared with their covalent counterparts.1,2 They have been investigated in a broad variety of applications and some are now emerging in various fields including nanoelectronics, catalysis, controlled release, information storage, or sensing.3,4 Most of these polymers are usually obtained by a simple self-assembly process between a metal ion and a monomer that has ligand units placed at both ends. Typical ligands of choice are multidentate ligands such as bipyridine, terpyridine, or 2,6-bis-(1′-methylbenzimidazolyl)pyridine, and a broad range of metal ions have been used (Fe2+, Zn2+, La2+, Co2+, etc.).2 Variations in the conformation and length of the organic core employed to connect the ligands along with choices of the metal salt are being actively pursued currently to confer new properties to the resulting metallopolymers. Nevertheless, this methodology relies upon interactions between a single binding pattern and a metal ion and, as a consequence, suffers from a lack of diversity. The development of new materials with emerging properties implies the ability to generate complex molecular systems in a controlled manner. With the aforementioned metallopolymers, we may introduce interesting additional levels of complexity by introducing several monomers giving rise to a dynamic combinatorial library.5,6 However, the spontaneous formation of metallosupramolecular copolymers makes it a challenging task to develop some form of control over the assembly process.7 For example, combining two different homoditopic monomers in the presence of a metal ion leads to a random distribution within the polymer chain (Figure 1, top).8 Consequently, the successful preparation of supramolecular alternate copolymers © 2019 American Chemical Society

containing metals may be obtained only by strategies that use two types of supramolecular interactions that do not communicate with each other as described by Schubert and collaborators (i.e., combining metal coordination and hydrogen bonding for example).9 Construction of separate block supramolecular polymer chains may also be achieved by using two different noncovalent interactions. Here, we show that the metallopolymer sequence can be controlled by chirality and the choice of the metal ion.10 Whereas the formation of a block metallopolymer may be obtained through the formation of homoleptic complexes, an alternate metallopolymer may be obtained only when heteroleptic complexes are formed. More precisely, we show that the heteroformation and homoformation of ML2-type complexes may be controlled by chirality-directed selfassembly using neutral chiral C2-symmetric bisoxazoline ligands. In particular, we show how the prevalent coordination geometries at metal centers control the sequences of the metallopolymers. We thus report the successful utilization of this approach for the selective formation of metallocopolymers as a function of the metal salt used (Cu2+ or Zn2+).

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RESULTS AND DISCUSSION Ligand Synthesis. Expanding on our previous work on the metal−bisox-based self-assembly, where we showed that neutral chiral bisoxazolines are versatile motifs for the construction of alternating coordination polymers with zinc ions,11 we recently designed a chiral ditopic ligand with a π-

Received: December 12, 2018 Accepted: January 22, 2019 Published: February 5, 2019 2676

DOI: 10.1021/acsomega.8b03484 ACS Omega 2019, 4, 2676−2683

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Figure 1. Illustration of supramolecular assemblies with two ditopic ligands and a metal ion with (top) random distribution and (bottom) controlled polymerization: alternate metallo-copolymer or block metallopolymer.

Figure 2. Synthesis of DiVinylBox ligands through Knoevenagel condensation.

conjugated spacer unit (named DiVinylBox, Figure 2).12 This ligand has been successfully used for constructing advanced chiral materials with nickel, and the resulting chiral metallosupramolecular films have been shown to give rise to high levels of optically active sum-frequency generation signals. However, since the ligands act as chromophores in this system, they also constitute good probes to study the metallosupramolecular assembly by means of UV−vis and circular dichroism (CD) spectroscopies. These ligands were easily obtained from a double Knoevenagel condensation using terephthalaldehyde and 2 equiv of bisoxazoline in the presence of piperidinium acetate (Figure 2).13,14 The two enantiomers using i-Pr substituents were obtained in 70% yield using conventional heating or in 60% under microwave conditions.

As mentioned previously, suitable crystals for X-ray diffraction studies were obtained with the (S) enantiomer.12 The analysis confirmed the formation of the monomer as shown on Figure 3. In the solid state, the molecule shows a conjugated π-system that includes two imine moieties. The two remaining oxazoline rings lie perpendicular to the plane of the conjugated system due to steric interaction with two hydrogen atoms of the phenyl ring. This conformation is also the one predicted by geometry optimization for an isolated molecule at the density functional theory (DFT) level of accuracy (see Supporting Information (SI) for details). As a consequence of the long conjugation path, time-dependent (TD) DFT predicts a fully allowed dipolar transition at 365 nm. Although showing a distinct yellowish hue compatible with an absorption in the 2677


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Figure 3. Molecular structure of (S)-DiVinylBox. Selected bond lengths (Å) and angles (°): C(7)−C(8), 1.342(4); C(9)−N(1), 1.265(3); C(15)−N(2), 1.252(4); C(21)−C(22), 1.343(4); C(23)−N(3), 1.262(3); C(29)−N(4), 1.272(3); N(2)−C(15)−C(8)−C(7), 87.32(3); C(15)−C(8)−C(7)−C(1), −1.80(3); N(1)−C(9)−C(8)−C(7), −171.26(4); N(3)−C(23)−C(22)−C(21), 81.20(3); C(4)−C(21)−C(22)− C(29), −176.09(4); N(4)−C(29)−C(22)−C(21), 175.30(3).

violet range in the polycrystalline form, the compound exhibits a broadband absorbance centered at 328 nm in solution in methanol, indicating that the conjugation path is most likely weakened by large-amplitude twisting motions of the ligand’s backbone involving the vinyl and imine groups. In addition, methanol being a protic solvent, the polarizable continuum model (PCM) used in the TD-DFT calculations may perform poorly for uncoordinated imine nitrogen atoms. Self-Assembly and Solution Studies of the Assemblies with Copper Ions. The formation of a metallopolymer in solution can be achieved by the simple addition of 1 equiv of the appropriate metal ion salt to a solution of the ditopic monomer and the process is easily monitored by UV spectroscopy.15,16 The self-assembly reaction with copper ions was thus studied by UV titration using the DiVinylBox ligand and Cu(BF4)2 as the metal precursor. Upon addition of Cu2+ to a solution of enantiopure DiVinylBox monomers, the main π−π* absorption peak undergoes a blue shift from 328 to 292 nm, leaving a shoulder in its place extending out from 325 to 410 nm. The absorption spectra also reveal a very weak and broad band centered around 500 nm (Figure 4), which we attribute to a ligand-to-metal charge transfer (LMCT) transition of the metal complex (see SI). Plotting the intensity of the band at 328 nm as a function of increasing Cu2+/ligand ratio (inset of Figure 4) reveals a decrease of the intensity until a 1:1 ratio is reached, strongly suggesting the formation of a homochiral metallopolymer. Titration measurements performed on a racemic mixture of ligands gave results that were nearly identical to those obtained with enantiopure monomers (see SI), consistent with the most likely formation of block homochiral metallopolymers. Additionally, the results from UV−visible titrations are corroborated by titrations monitored by circular dichroism (CD) spectroscopy as shown in Figure 5. Addition of Cu2+ to the (S)-DiVinylBox monomer leads to the formation of the corresponding chiral metallopolymer and results in a strong CD signal at 302 nm, which gradually increases until the exact 1:1 metal/ligand ratio is reached. Further addition of copper salt to the solution does not result in any change, thus demonstrating that the metallopolymer does not depolymerize in the presence of excess metal ions. Furthermore, the

Figure 4. Titration of a (S)-DiVinylBox ligand concentration of 2.5 × 10−5 M in MeOH with Cu(BF4)2 followed by UV−vis spectroscopy. The spectra corresponding to 0 and 1 equiv of Cu2+ added are displayed as black and red traces, respectively. The black arrow indicates the trend in the spectral feature upon addition of the salt. The insets show the change in absorbance at 328 nm as a function of added Cu2+.

observed CD signals are of the opposite signs when the other enantiomer is used to assemble the (R) homochiral metallopolymer as shown in the Supporting Information (SI). Although the CD spectra display an asymmetric band shape whose maxima do not coincide exactly with the position of the absorption maximum upon chelation of the metal ions, they must be attributed to the main ligand-centered π−π* transition when making allowances for the torsional flexibility of the ligand. The optical activity therefore arises from the chiral conformation of the conjugated backbone that is being stabilized upon the formation of the metallopolymer. A modeling by DFT geometry optimizations performed on a single bimetallic complex consisting of a DiVinylBox ligand, 2678


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the differences between the position and shape of the absorption band vs those of the CD spectral band as oscillator strengths and rotary strengths depend inversely on the amount of twisting of the conjugated backbone. Self-Assembly and Solution Studies of the Assemblies with Zinc Ions. Titrations carried out under identical conditions, but employing Zn(BF4)2 as the ion source, yielded different results (Figure 7). Upon addition of Zn2+ aliquots to a solution of the enantiopure (S)-DiVinylBox monomer, the absorption band at 328 nm was again shifted to 292 nm leaving a shoulder sloping off from the position of the maximum of the original band to where it vanished, i.e., just below 380 nm. No detectable absorption has been observed at longer wavelengths, which is consistent with the fact that no electronic transitions involving metal d-orbitals are possible at energies below that of the first intraligand singlet excitation. The shoulder should therefore be attributed to free or partly free ligand molecules, having temporarily only one of its oxazolines coordinated to a zinc ion at one end for example. The same titration using the enantiopure (S)-DiVinylBox monomer gave basically the same absorption profile. However, a different behavior was observed while plotting the intensity of the band at 328 nm over an increasing Zn2+/ligand ratio. With the racemic monomer, the plot clearly shows a decrease of the intensity until a 1:1 ratio is reached, indicating the formation of the metallopolymer. Further addition of zinc cation did not result in any absorbance variation, thus demonstrating that the metallopolymer was stable and did not undergo depolymerization in the presence of excess of metal salt. In addition, an isobestic point was observed only between 0 and 0.5 equiv of zinc, suggesting that the heteroleptic species was first formed owing to the higher binding constant for the formation of ML2-type species. In contrast, the plot with the enantiopure monomer shows a decrease of the intensity until a 2:1 Zn2+/ligand is reached. In addition, circular dichroism titration gave a weak positive signal at 304 nm with a maximum reached at a 2:1 Zn2+/ligand ratio (see SI). These data are consistent with the formation of a dinuclear complex rather than a metallopolymer. Altogether, these results show that under these conditions the formation of the metallopolymer occurs only in the presence of the two enantiomers of the monomer in support of their heterochiral complementarity in the presence of zinc. Racemic metallopolymers were also synthesized in bulk and were analyzed by classical methods. IR spectra indicate that the oxazoline moieties are coordinated to the metal center (coordination shift of the ν(CN) band of 20 cm−1). In the 1H NMR spectra of the metallopolymer, the broader peaks are consistent with the polymer formation. The dynamic nature of

Figure 5. CD spectral changes in the titration of (S)-DiVinylBox solution (MeOH, c = 2.5 × 10−5 M) with Cu(BF4)2. The black arrow indicates the change in the main spectral feature upon addition of the salt. The inset shows the increase in CD intensities at 302 nm.

two metal ions, and four methanol molecules to complete the coordination spheres (see SI) revealed that the two neighboring oxazolines need to adopt a nearly coplanar conformation to chelate the metal ions. This however leads to the phenyl ring being twisted out of the overall plane of the ligand because of steric hindrance involving two of its hydrogen atoms (Figure 6). Together, the two alkene moieties and the phenyl between them adopt either a slightly twisted chair conformation or one of the two comparably skewed boat conformations. In the chair geometry, the conjugation path of the π electrons, extending from one pair of imines to the other, nearly conforms to having an inversion center in the middle of the phenyl ring and the TD-DFT calculations assign it a weak rotary strength only. The two boat configurations, by contrast, give the conjugation path helical geometries, which give rise to large optical rotary strengths, of opposite signs, in TD-DFT results. Both configurations have C2 symmetry and the DFT optimizations show that they have almost identical total energies and would thus be equally likely barring solvation and entropic effects (see SI). Although we may expect the actual metallopolymer to behave as a dynamic chain whose links constantly shift between conformations, the fact that we detect a strong positive CD in solution indicates that the more open of the two boat conformations dominates in the large-scale polymer structure. Thermal motions of the solvent and the ligand in the dominant conformation lead to a heterogeneity, which explains

Figure 6. Conformational change induced by metal coordination with the DiVinylBox ligand. Left: rotation of the phenyl ring out of the plane leading to the chair conformation (weak CD activity); right: alkenes positioned in the more open of the two boat configurations, which leads to the helical structure responsible for the positive circular dichroism signal (R = iPr). 2679


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form extended strands, an aspect that will be addressed experimentally using monotopic ligands below. Complexation Studies of the Chiral Monotopic Bisoxazoline Ligand as a Model. The homo- and heterochiral complementarities of a chiral C2-symmetric ligand in an ML2-type complex may be demonstrated by evaluating their binding association constants.18 For this purpose, we synthesized the two enantiomers of the monotopic bisoxazoline ligand 2,2′-(2-phenylethene-1,1-diyl)bis(4-isopropyl-oxazoline) VinylBox (Scheme 1) and then conducted titration Scheme 1. Molecular Structure of the Monotopic Bisoxazoline Ligand VinylBox (S,S)′

experiments by UV−visible spectroscopy.19 Addition of the monotopic ligand (in its enantiopure or racemic form) to a solution of Zn(BF4)2 or Cu(BF4)2 caused changes in the spectrum, which were analyzed as a function of the amount of ligand added. The data were fitted to an ML or ML2 binding model to obtain the log β values.20 With copper salt, the fitting of the experimental data to an ML2 model showed the overall stability log β to be 8.9 ± 0.6 in both cases (enantiopure or racemic form of the ligand). The same feature was observed using zinc salt and the racemic form of the ligand (log β = 8.7 ± 0.7). However, the titration data for the addition of the enantiopure ligand to Zn(BF4)2 were fitted to a simple ML binding model with log β = 3.3 ± 0.4, thus suggesting that the homochiral zinc complex could not be formed under these conditions. Hetero- and Homocomplementarity of Bis(oxazoline) Ligands. Thus, on the basis of chiral molecular recognition processes, the ditopic ligands present an adaptive characteristic as a function of the metal salt in the medium. Harnessing the local environments around the metal ion is key to control the chain sequence in the metallopolymer. Thus, the successful construction of an alternate metallo-copolymer is only possible by dynamic heteroleptic coordination at a metal center, which should contain freely exchangeable ligands under thermodynamic control. Reliable heteroaggregation at the mononuclear level can be set up using either topological or steric constraints and the principle of maximal site occupancy.21 Oxazolinebased ligands such as bisoxazolines ligands have attracted much attention because they have been successfully used in a wide range of enantioselective catalytic reactions. With a C2symmetric bisoxazoline ligand, the orientation as well as the steric demand of the substituents at the chiral center in the 4 position of the heterocyclic ring are the critical factors that determine the formation of the homoleptic species ML2. The combination of two chiral enantiopure C2-symmetric bisox ligands in a tetrahedral geometry offers two possible complexes: heterochiral (RR + SS) or homochiral (SS + SS or RR + RR) (Figure 8).22 As already observed, the heterochiral complex, which is the least sterically hindered, is the more stable since the orientation of the substituents is complementary. A square planar geometry with two bisox

Figure 7. Titration of a DiVinylBox ligand concentration of 2.5 × 10−5 M in MeOH with Zn(BF4)2 followed by UV−vis spectroscopy. Top: racemic mixture; bottom: (S) enantiomer. The spectra corresponding to 0 and 1 equiv (top) or 2 equiv (bottom) of Zn2+ added are displayed as black and red traces, respectively. Black arrows indicate the direction of the trends in the spectral features upon addition of the salt. The insets show the change in absorbance at 328 nm as a function of added Zn2+.

the metallopolymers precludes gel filtration chromatography/ size-exclusion chromatography studies as only free monomers are detected as already observed for this family of compounds.17 All UV−visible spectra showed that metal−ligand complexation induced a hypsochromic shift of the main absorption band of the ligand from 328 to 292 nm regardless of the metal. This shift is concurrent with the coordination of the bis(oxazoline) to the metal and indicates that the coordination induces a conformational change of the monomer. Indeed, as we have already stated with copper, the coordination of the ligand requires having both oxazoline moities in the equatorial plane of the metal and thus induces a conformational change of the vinyl−phenyl−vinyl moiety. DFT geometry optimizations of a dinuclear Zn(II) complex leads to ligand conformations (see SI) that are very similar to the ones obtained with Cu(II) ions, although the energies of formation now differ to a greater extent and even favor the more compact boat conformation. However, almost identical observations can be made with regard to the TD-DFT excitation energies (except for the obvious absence of LMCT transitions) and dipole and rotary strengths of an individual dinuclear complex. The fact that the CD spectra are only very weak and noisy for enantiopure ligands in the presence of zinc ions must be due to the lack of prevalence of a single optically active conformation of the chromophores, which we attribute to their lesser propensity to 2680


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Figure 8. Shaping coordination spheres with chiral C2-symmetric bisoxazoline derivatives: modular assemblies and their possible combinations in [M(bisox)2]n+ complexes in a tetrahedral or square planar environment, demonstrating the complementarity or noncomplementarity of the various combinations.

loaded mechanism, but the available positions for the isopropyl groups on the bisoxazolines with respect to the phenyl ring and to each other allow the polymer some flexibility between linear or coiled segments. Strikingly, the observation of optical activity reveals that solvent and entropic effects, and not direct steric hindrance, lead to a preferred helical conformation of the ligand and thus to a specific coiled form of the polymer. If it turns out to be possible to induce a preference for one or the other through changes in the environment, then this should be detectable using optical titration experiments as well.

ligands also gives two possible stereoisomers (heterochiral or homochiral); however, the least sterically hindered should be the homochiral complex. Such features should also be observed in an octahedral environment in which the two ligands arrange each other in the equatorial position.23

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SUMMARY In conclusion, using UV−visible titration, we have shown that we can take advantage of a self-sorting process through the pertinent choice of chiral ditopic ligands and metal ions. With copper(II) ions, block homochiral metallopolymers have been formed, whereas with zinc(II) ions alternate metallocopolymers have been produced. In the course of our study, we have been able to show that the specific way in which chiral DiVinylBox ligands may act as helicene-like chromophores made it possible to gain additional information about their metallopolymer-forming abilities when carrying titration experiments over from the UV−vis absorption spectroscopy to the CD domain. In particular, examining the data in the light of DFT calculations performed on simple complexes has revealed the importance of the changes in the geometry that the ligand may undergo. Not only does the deconjugation of the central phenyl ring upon chelation of a metal ion by a bisoxazoline unit favor the reversible nature of the coordination by acting as a spring-

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EXPERIMENTAL SECTION Materials and Methods. All organic syntheses were performed under an inert atmosphere of dry nitrogen by using standard vacuum line and Schlenk tube techniques. Solvents were purified and degassed by standard procedures. All reagents were used without further purification. MeOH purchased was of spectrometric grade and used without any purification. 1H and 13C NMR spectra were recorded by using a Bruker Avance 300 spectrometer using the residual solvent peak as a reference (CDCl3: δH = 7.26 ppm; δC = 77.16 ppm) at 298 K. Positive-mode electrospray ionization mass spectra (ESI-MS) were recorded on microTOF, Bruker Daltonics. UV−visible absorption spectra were recorded on a HITACHI 2681


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U-3000 spectrophotometer. The stability constants of metal complexes were calculated by the Bindfit program.20 Circular dichroism absorption experiments were carried out with a UV−visible Jasco J-810 CD spectrometer using a 1 cm optical path quartz cell at 20.0 ± 0.1 °C. The monotopic VinylBox ligand was synthesized according to reported procedures.24 All structures have been optimized and their excited states calculated at the B3-LYP/6-31G(d,p) level of theory while applying the polarizable continuum model (PCM) for methanol with the Gaussian 09 Rev. D01 software. Computational details of DFT calculations can be found in the Supporting Information. X-ray diffraction studies were carried out by Dr. Lydia Brelot and Corinne Bailly at the Chemistry Institute X-ray Facility of the University of Strasbourg: crystal data were collected at 173 K using a Mo Kα graphite monochromated (λ = 0.71073 Å) radiation on a Nonius KappaCCD diffractometer. The structure was solved using direct methods with SHELXS97552 and refined against F2 using the SHELXL97 software.553. Non-hydrogen atoms were refined anisotropically. Hydrogen atoms were generated according to stereochemistry and refined using a riding model in SHELXL97. 1,4-Bis(2,2-bis((S)-4-isopropyl-4,5-dihydrooxazol-2yl)vinyl)benzene (S,S,S,S)-DivinylBox. 1,1-Bis[(4S)-4-isopropyl-4,5-dihydrooxazol-2-yl]methane (0.84 mmol, 200 mg), piperidinium acetate (0.17 mmol, 24.22 mg), and terephtaldehyde (0.42 mmol, 56.33 mg) were put together in a Schlenk tube. Pyridine (3 mL) was added and the mixture was heated and stirred at 65 °C for 72 h. After this time, the solvent was evaporated under vacuum. The resulting yellow-orange solid was purified by silica gel column chromatography (AcOEt/ MeOH, 95:5), yielding a white solid (0.30 mmol, 174.8 mg, 71%). Microwave-assisted conditions were as follows: 1,1bis[(4S)-4-isopropyl-4,5-dihydrooxazol-2-yl]methane (0.839 mmol, 200 mg, 2 equiv), 1,4-phthalaldehyde (0.461 mmol, 62 mg, 1.1 equiv), and freshly made piperidinium acetate (0.168 mmol, 24 mg) were suspended in dry pyridine (4 mL). The mixture was then stirred for 2 min before being subjected to microwave irradiation for 4 h at 70 °C. After cooling to room temperature, the pyridine was removed under vacuum, yielding a yellow oil, which was purified by silica-gel column chromatography (AcOEt/MeOH, 95:5) to yield a white solid (150 mg, 62%). [α]D = −154 cm3 g−1 dm−1 (c = 0.02 in MeOH, 293 K); 1H NMR (300 MHz, CDCl3) δ 7.53 (s, 2H), 7.44 (s, 4H), 4.28−4.41 (m, CH-N, 4H), 4.19−3.96 (m, CH2O, 8H), 1.74−1.89 (m, CH(CH3)2, 4H), 1.07−0.77 (m, 6 × CH3, 24H). 13C NMR (101 MHz, CDCl3) δ = 161.8 (2 × N CO), 160.4 (2 × NCO), 139.9 (2 × HCC), 135.4 (2 × CAr), 129.6 (4 × CHAr), 120.0 (2 × CCH), 73.1 (2 × NCH), 73.1 (2 × NCH), 70.6 (2 × OCH2), 70.4 (2 × OCH2), 33.1 (2 × CH(CH3)2), 32.6 (2 × CH(CH3)2), 19.2, 18.9, 18.7, and 18.4 (8 × CH3). IR (KBr): ṽ = 1674 cm−1 (s, CN); MS (ESI + ): m/z = 239.18 [C 13 H 22 N 2 O 2 + H] + , 327.19 [C20H26N2O2 + H]+, 597.34 [M + Na]+, 813.52 [M + C13H22N2O2 + H]+; elemental analysis calculated (%) for C34H46N4O4 (574.75): C 71.05, H 8.07, N 9.75; found C 70.78, H 8.11, N 9.48. (R,R,R,R)-DivinylBox [α]D = +155 cm3 g−1 dm−1 (c = 0.02 in MeOH, 293 K). UV−Vis and CD Titration Experiments. Ligands in their enantiopure or racemic form were titrated with zinc tetrafluoroborate Zn(BF4)2 or copper tetrafluoroborate Cu(BF4)2 under the following conditions: a 2.5 × 10−5 M solution of ligand in methanol was titrated with aliquots of a 2.5 × 10−4

M solution of M(BF4)2 also in methanol. The addition was carried out stepwise and the formation of metal coordination polymer was monitored by UV−vis or CD spectroscopies.

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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/acsomega.8b03484. Copies of 1H, 13C spectra of the ligands; UV−visible and CD titration experiment spectra; density functional theory (DFT) and time-dependent (TD) DFT calculations; X-ray tables (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Stéphane Bellemin-Laponnaz: 0000-0001-9462-5703 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the CNRS and the Ministère de l’Enseignement Supérieur et de la Recherche (MESR) for a PhD grant to M.T.-W. This work is a part of the Strasbourg IDEX program and we thank the NIE Labex for support (ANR-11-LABX-0058_NIE).

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REFERENCES

(1) (a) Hart, L. R.; Harries, J. L.; Greenland, B. W.; Colquhoun, H. M.; Hayes, W. Healable supramolecular polymers. Polym. Chem. 2013, 4, 4860−4870. (b) Wojtecki, R. J.; Meador, M. A.; Rowan, S. J. Using the dynamic bond to access macroscopically responsive structurally dynamic polymers. Nat. Mater. 2011, 10, 14−27. (2) Winter, A.; Schubert, U. S. Synthesis and characterization of metallo-supramolecular polymers. Chem. Soc. Rev. 2016, 45, 5311− 5357. (3) (a) Whittell, G. R.; Hager, M. D.; Schubert, U. S.; Manners, I. Functional soft materials from metallopolymers and metallosupramolecular polymers. Nat. Mater. 2011, 10, 176−188. (b) Fox, J. D.; Rowan, S. J. Supramolecular Polymerizations and Main-Chain Supramolecular Polymers. Macromolecules 2009, 42, 6823−6835. (c) Friese, V. A.; Kurth, D. G. From coordination complexes to coordination polymers through self-assembly. Curr. Opin. Colloid Interface Sci. 2009, 14, 81−93. (d) Bellemin-Laponnaz, S.; Achard, T.; Bissessar, D.; Geiger, Y.; Maisse-François, A. Synthesis and application of dynamic self-supported enantioselective catalysts. Coord. Chem. Rev. 2017, 332, 38−47. (4) Mauro, M.; Bellemin-Laponnaz, S.; Cebrian Avila, C. Metalcontaining polymers as light-emitting and light-responsive materials and beyond. Chem. - Eur. J. 2017, 23, 17626−17636. (5) Lehn, J.-M. Dynamers: Dynamic Molecular and Supramolecular Polymers. Aust. J. Chem. 2010, 63, 611−623. (6) Lehn, J.-M. Dynamers: dynamic molecular and supramolecular polymers. Prog. Polym. Sci. 2005, 30, 814−831. (7) Safont-Sempere, M. M.; Fernández, G.; Würthner, F. Self-sorting phenomena in complex supramolecular systems. Chem. Rev. 2011, 111, 5784−5814. (8) (a) Schlütter, F.; Wild, A.; Winter, A.; Hager, M. D.; Baumgaertel, A.; Friebe, C.; Schubert, U. S. Synthesis and Characterization of New Self-Assembled Metallo-Polymers Containing Electron-Withdrawing and Electron-Donating Bis(terpyridine) Zinc(II) Moieties. Macromolecules 2010, 43, 2759−2771. (b) Chen, Y.-Y.; Lin, H.-C. Metallo-homopolymer and metallo-copolymers 2682


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ethane (“Trisox”). Chem. - Eur. J. 2007, 13, 3058−3075. (b) Gade, L. H.; Bellemin-Laponnaz, S. Exploiting Threefold Symmetry in Asymmetric Catalysis: The Case of Tris(oxazolinyl)ethanes (“Trisox”). Chem. - Eur. J. 2008, 14, 4142−4152. (24) Burke, A. J.; Carreiro, E. P.; Chercheja, S.; Moura, N. M. M.; Prates Ramalho, J. P.; Rodrigues, A. I.; dos Santos, C. I. M. Cu(I) catalysed cyclopropanation of olefins: Stereoselectivity studies with Arylid-Box and Isbut-Box ligands. J. Organomet. Chem. 2007, 692, 4863−4874.

containing light-emitting poly(fluorene/ethynylene/(terpyridyl)zinc(II)) backbones and 1,3,4-oxadiazole (OXD) pendants. Polymer 2007, 48, 5268−5278. (c) He, T. C.; Gao, Y.; Chen, R.; Ma, L.; Rajwar, D.; Wang, Y.; Grimsdale, A. C.; Sun, H. D. Multiphoton Harvesting in an Angular Carbazole-Containing Zn(II)-Coordinated Random Copolymer Mediated by Twisted Intramolecular Charge Transfer State. Macromolecules 2014, 47, 1316−1324. (9) Mansfeld, U.; Winter, A.; Hager, M. D.; Hoogenboom, R.; Gunther, W.; Schubert, U. S. Orthogonal self-assembly of stimuliresponsive supramolecular polymers using one-step prepared heterotelechelic building blocks. Polym. Chem. 2013, 4, 113−123. (10) For selected examples of chiral metallopolymers, see: (a) Andres, P. R.; Schubert, U. S. Formation of metallo-polymers and -macrocycles by complexation of alkyl-linked di-terpyridines with iron(II) ions. Synthesis 2004, 8, 1229−1238. (b) Pal, R. R.; Higuchi, M.; Kurth, D. G. Optically Active Metallo-Supramolecular Polymers Derived from Chiral Bis-terpyridines. Org. Lett. 2009, 11, 3562−3565. (11) Torres, M.; Heinrich, B.; Miqueu, K.; Bellemin-Laponnaz, S. Chirality-driven metallo-co-polymer formation. Eur. J. Inorg. Chem. 2012, 3384−3387. (12) Taupier, G.; Torres-Werlé, M.; Boeglin, A.; Maisse-François, A.; Achard, T.; Bellemin-Laponnaz, S.; Dorkenoo, H. D. Optically active sum-frequency generation as an advanced tool for chiral metallopolymer material. Appl. Phys. Lett. 2017, 110, No. 021904. (13) Yuryev, R.; Liese, A. Synthesis of a Novel Unsymmetrical Bisoxazoline Ligand with sp2 Bridging Carbon. Synlett 2009, 16, 2589−2592. (14) Sun, Y.-J.; Li, N.; Zheng, Z.-B.; Liu, L.; Yu, Y.-B.; Qin, Z.-H.; Fu, B. Highly Enantioselective Friedel-Crafts Reaction of Indole with Alkylidenemalonates Catalyzed by Heteroarylidene Malonate-Derived Bis(oxazoline) Copper(II) Complexes. Adv. Synth. Catal. 2009, 351, 3113−3117. (15) Winter, A.; Schubert, U. S. Synthesis and characterization of metallo-supramolecular polymers. Chem. Soc. Rev. 2016, 45, 5311− 5357. (16) (a) Borré, E.; Stumbe, J.-F.; Bellemin-Laponnaz, S.; Mauro, M. Light-powered Self-healable Metallosupramolecular Soft Actuators. Angew. Chem., Int. Ed. 2016, 55, 1313−1317. (b) Borré, E.; Stumbe, J.-F.; Bellemin-Laponnaz, S.; Mauro, M. Control of the light-response in supramolecular metallopolymeric gels by tuning the coordination metal. Chem. Commun. 2017, 53, 8344−8347. (17) Hossain, M. D.; Sato, T.; Higuchi, M. A Green Copper-Based Metallo-Supramolecular Polymer: Synthesis, Structure, and Electrochromic Properties. Chem. - Asian J. 2013, 8, 76−79. (18) Hancock, R. D.; Martell, A. E. Ligand design for selective complexation of metal ions in aqueous solution. Chem. Rev. 1989, 89, 1875−1914. (19) Sun, Y.-J.; Li, N.; Zheng, Z.-B.; Liu, L.; Yu, Y.-B.; Qin, Z.-H.; Fu, B. Highly Enantioselective Friedel-Crafts Reaction of Indole with Alkylidenemalonates Catalyzed by Heteroarylidene Malonate-Derived Bis(oxazoline) Copper(II) Complexes. Adv. Synth. Catal. 2009, 351, 3113−3117. (20) (a) Thordarson, P. Determining association constants from titration experiments in supramolecular chemistry. Chem. Soc. Rev. 2011, 40, 1305−1323. (b) www.supramolecular.org. (21) Krämer, R.; Lehn, J.-M.; Marquis-Rigault, A. Self-recognition in helicate self-assembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 5394−5398. (22) Dagorne, S.; Bellemin-Laponnaz, S.; Maisse-François, A. Metal Complexes Incorporating Monoanionic Bisoxazolinate Ligands: Synthesis, Structures, Reactivity and Applications in Asymmetric Catalysis. Eur. J. Inorg. Chem. 2007, 913. (23) The non-autocomplementarity of chiral C3-symmetric ligands such as tris-oxazoline has also been investigated and demonstrated, see: (a) Gade, L. H.; Marconi, G.; Dro, C.; Ward, B. D.; Poyatos, M.; Bellemin-Laponnaz, S.; Wadepohl, H.; Sorace, L.; Poneti, G. Shaping and Enforcing Coordination Spheres: The Implications of C3 and C1 Chirality in the Coordination Chemistry of 1,1,1-Tris(oxazolinyl)2683


Chiral Self-Sorting Process with Ditopic Ligands: Alternate or Block

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