Dynamic scaffold of chiral binaphthol derivatives with the alkynylplatinum(II) terpyridine moiety

Abstract

Platinum(II)-containing complexes with inherently chiral binaphthol derivatives display a versatile scaffold between random coils and single-turn helical strands, in which the conformational transition is controlled by the Pt···Pt and π−π interactions of alkynylplatinum(II) terpyridine moiety upon solvent and temperature modulation. The bisignate Cotton effect in the circular dichroism spectra is indicative of the cooperative transformation from random coil state to a compact single-turn M- or P- helix. More importantly, as revealed by the appearance of new UV-vis absorption and emission bands during conformational change, the self-assembly of the platinum(II)-containing complex into a helical structure is assisted by the metal···metal and π−π interactions of the alkynylplatinum(II) terpyridine moieties. The folded structure with stabilization via metal···metal and π−π interactions has been supported by density functional theory calculations, which provide insights into the folded geometry of these kind of metallo-foldamers.

The photophysical and spectroscopic properties of the d⁸ platinum(II) polypyridine system have been extensively studied (1−36) and are pioneered by the works of various groups, such as those of Miskowski and colleagues (1–3), Gray and colleagues (4–6), Lippard and colleagues (7–11), Che and colleagues (12–15), Vogler and colleagues (16), and McMillin and colleagues (17). Such square-planar platinum(II) complexes have been shown to exhibit interesting and intriguing photophysical properties associated with a tendency to form metal···metal and/or π−π stacking interactions (1–17). Most of the studies on these platinum(II) polypyridine systems related to the metal···metal interactions are mainly associated with their rich solid-state polymorphism and are confined to that in the study of solid-state spectroscopy and electronic structures (4, 8, 14). Corresponding studies in solution are relatively unexplored (8).

In view of the relatively small association constants for dimerization or oligomerization found for these compounds in solution, high solubility would be an important prerequisite for them in the preparation of solutions for the study of aggregation in solution. However, poor solubility of the platinum(II) polypyridine system is commonly encountered in solution and limits an extensive study of its aggregation properties in solution.

As an extension of our previous work on transition metal alkynyl complexes (37–45), efforts have been made to prepare the first luminescent alkynylplatinum(II) terpyridine system, [Pt(tpy)(C≡CR)]⁺ (20), with improved solubility through the incorporation of strong σ-donating and solubilizing alkynyl ligands into the platinum(II) metal center. Apart from the rich photophysical and luminescence properties, as well as interesting solid-state polymorphism, such complexes exhibit notable color changes and luminescence enhancements as a result of Pt···Pt and π−π stacking interactions that occur from solvent composition change (21), polyelectrolyte addition (22, 23), and gel formation (24). Such studies have benefited from the early works on the understanding of the influence of the metal···metal interaction on the spectroscopic properties of the d⁸ platinum(II) system (1−36,46−54) and a recent study on the energy landscape of the ligand-bridged dinuclear iridium compound, which demonstrates the versatility of metal···metal interactions in governing the conformational change of the geometry (55). Apart from using metal···metal and π−π stacking interactions in stabilizing supramolecular nanostructures (28), hairpin conformation (25), and organogel formation (24), our group recently demonstrated the utilization of Pt···Pt and π−π interactions in controlling the helix-coil transition in meta-phenylene ethynylene (mPE) oligomers and in stabilizing reversibly the construction of a single-turn helix (26). The use of metallophilic interactions as the driving force has opened up a new strategy for the construction of helical strands of metallo-foldamers that are of immense interest in supramolecular architectures.

Chiral supramolecular chemistry has also been of great interest over the last decade, with its profound potential for enantioselective applications (56–80), such as chiral separation (64, 65), molecular discrimination (66–69), asymmetric synthesis (70–72), liquid crystals (73–75), nonlinear optics (76, 77), and so forth. One representative class of chiral moiety is the 1,1-binaphthyls, which have extensive applications in molecular recognition (81–84), asymmetric catalysis (85), and materials (86). There are numerous examples of using 1,1-binaphthyl derivatives to achieve polygons (87) or helicates (88, 89), in which the conformational preference is limited due to a strong metal−ligand coordination. In contrast, interest in foldamers with versatile scaffolds has increased significantly because of their well-defined yet dynamic conformations that can be highly responsive to external stimuli (90–93). The introduction of inherently chiral binaphthol segments to construct a foldamer is very rare (90, 93). These examples use π−π stacking interactions (90) and hydrogen bonding (93) to transform the scaffold of binaphthol into helical strands in solution. Therefore, in contrast to those binaphthol-based helicates with irreversible conformational change using strong metal−ligand coordination, we herein report a series of metallo-foldamers using metal···metal interactions to control structural changes from random coil to helical strand upon solvent and temperature modulation. The present study demonstrates the ability of noncovalent metal···metal interactions in assisting the conformational change of a binaphthol-based complex in a metallo-foldamer system, with the modulation of electronic absorption, emission, and circular dichroism spectroscopic properties.

Results and Discussion

Our design is based on the incorporation of alkynylplatinum(II) terpyridine units into the single-turn backbone of a binaphthol derivative. The preparation of the binaphthol derivatives involves first the alkylation of 6,6′-dibromo-1,1′-bi-2-naphthol with ⁿBuBr, followed by the palladium-catalyzed coupling reaction with the corresponding mPE repeating units, H-(C≡C-1,3-C₆H₄)_n-C≡C-SiⁱPr₃ (n = 1, 2, and 3), to afford the bis-coupled products. The detailed synthetic route is given in Scheme S1. Complexes 1−3 as shown in Fig. 1 were synthesized by reacting the corresponding binaphthol derivatives with [(^tBu₃tpy)PtCl](OTf) in a molar ratio of 1:2 in degassed dimethylformamide (DMF) containing NEt₃ with a catalytic amount of CuI. The enantiomers in the (S)-form were prepared for all of the complexes for comparison, whereas only 2 was synthesized in both enantiomeric forms. All of the complexes have been characterized by ¹H and ¹³C NMR, IR spectroscopy, fast atom bombardment (FAB) mass spectrometry, and elemental analyses.

Fig. 1.

Structures of complexes 1−3.

Complex 1 gives a pale yellow solution in CH₂Cl₂, and the electronic absorption spectrum shows intense intraligand (IL) [π→π*] transitions of the terpyridine and alkynyl ligands at about 303−330 nm, together with low-energy absorptions at 430−470 nm, which are assigned as metal-to-ligand charge transfer (MLCT) [dπ(Pt)→π*(tpy)] transitions mixed with alkynyl-to-terpyridine ligand-to-ligand charge transfer (LLCT) [π(C≡CR)→π*(tpy)] character. When the solvent is changed from CH₂Cl₂ to CH₃CN, a negative solvatochromism of the MLCT absorption band (26) is observed, with a blue shift to 408−441 nm (Fig. S1A). The circular dichroism (CD) spectra of (S)-1 in CH₂Cl₂ and CH₃CN also show similar behavior (Fig. S1B), indicating that the ellipticity originates from the chromophore of the inherently chiral binaphthol backbone in both CH₂Cl₂ and CH₃CN (90). In sharp contrast to the observation of 1, complex 2, which is bis-coupled with two mPE units on the backbone (n = 2), shows a significant decrease in the intraligand [π→π*(C≡CR)] absorption band at 278−340 nm, indicative of a conformational change of the binaphthol backbone from the transoid to the cisoid form upon increasing the CH₃CN content in CH₂Cl₂, and the emergence of a low-energy absorption tail at 510 nm starting from 40% CH₃CN onward (Fig. S2). With reference to our previous works (21–27), the appearance of the absorption tail at longer wavelengths is ascribed to the formation of metal···metal interactions, and the absorption tail is tentatively assigned as the metal-metal-to-ligand charge transfer (MMLCT) transition. Because the absorption tail at 510 nm in CH₃CN is found to obey Beer’s law within the concentration range of 10⁻⁶–10⁻⁴ M, it establishes the MMLCT transition as being one that is of an intramolecular association in nature. Concomitant with the change in the UV-vis absorption spectra, (S)-2 demonstrates a remarkable Cotton effect, with the growth of a new band at 307 nm and a diminution of the band at 290 nm in the CD spectra upon increasing the CH₃CN content in CH₂Cl₂ (Fig. 2, Upper). This bisignate Cotton effect in the CD spectra at high CH₃CN contents arises from the alteration of the chiral environment for the helical backbone resulting in the twist sense bias of the helix, indicative of the formation of a single-turn P-helix, which could not have been observed in the pure organic counterpart for such a short chain with sole π−π stacking interactions (90). Furthermore, the planarization of the terpyridine moieties brought about by coordination to the Pt(II) metal center would facilitate the formation of the single-turn metallo-helix via Pt···Pt and π−π stacking interactions, which is distinct from those classically observed in the pure organic counterpart with sole π−π stacking interactions.

Fig. 2.

CD spectral traces of (S)-2 (2.4 × 10⁻⁵ M) (Upper) and (R)-2 (2.3 × 10⁻⁵ M) (Lower) in CH₂Cl₂ with increasing CH₃CN content in CH₂Cl₂ at 298 K.

Moreover, it is important to note the presence of isodichroic points at 279 and 300 nm, which shows that this conformational change involves a two-state equilibrium (90). The CD spectra of (R)-2 show a similar trend, except that the CD signals are mirror images of that for (S)-2, resulting from the formation of the M-helix as shown in Fig. 2 (Lower). Therefore, it is plausible that 2 would undergo a solvophobically driven process from the random coil state to the helical strand. Such helical bias is due to the self-assembly of two platinum(II) terpyridine moieties resulting from the intramolecular metal···metal and π−π interactions at high CH₃CN contents. By monitoring the changes of A_341nm/A_290nm in the UV-vis spectra and the ellipticity at 307 nm in the CD spectra, sigmoidal curves typical of cooperative interactions are observed (Fig. 3), with helix stabilization energies ΔG(CH₃CN) of −1.71 ± 0.02 and −1.80 ± 0.02 kcal/mol obtained (Table 1) from the two independent experiments, which are in close agreement with each other.

Fig. 3.

Solvent denaturation of 2 measured by UV-vis absorbance ratios of A_{341 nm}/A_{290 nm} (■) and CD intensity at 307 nm (△).

Table 1.

Helix stabilization energy in pure CH₃CN [ΔG(CH₃CN)] for 2 and 3 obtained from the titration curves in UV-vis and CD spectra

Complex	ΔG(CH3CN)/kcal mol−1
	UV	CD
2	−1.71	−1.80
3	−0.86	−0.84

Density functional theory (DFT) calculations were performed to provide insight into the molecular structure of the folded conformation in (S)-2 (see the Computational Details). As shown in Fig. 4, the binaphthol backbone containing the couple of two mPE units (n = 2) forms a single turn of the P-helix. The two [(^tBu₃tpy)Pt] units are located outside the helical turn, with the two terpyridyl groups arranged in a staggered conformation. The Pt···Pt distance and interplanar distance of the two [Pt(^tBu₃tpy)] coordination planes are calculated to be 3.349 and 3.451 Å, respectively, indicating the presence of Pt···Pt and π−π interactions in the folded structure. The change in the free energy of the folded structure with respect to the unfolded structure (all-transoid conformation in the mPE and binaphthol backbone) as shown in Fig. S3 was computed to be −1.9 kcal/mol,* which is close to the experimental value and further supportive of the stabilization of the folded structure by metal···metal and π−π interactions.

Fig. 4.

Optimized structure of the helical conformation in (S)-2.The hydrogen atoms are omitted for clarity.

By analogy to 2, complex 3 bis-coupled with three mPE units (n = 3) also shows the behavior of folding with the formation of metal···metal and π−π interactions upon increasing the CH₃CN content in CH₂Cl₂, as revealed from the UV-vis and CD spectra shown in Fig. S4 A and B, respectively. The helix stabilization energy was found to be less negative, with ΔG(CH₃CN) values of −0.86 ± 0.05 and −0.84 ± 0.04 kcal/mol obtained from UV-vis and CD experiments, respectively. The misalignment of the two terminal platinum(II) terpyridine units in the single turn probably accounts for the less stable helical strand with less favorable ΔG values when n = 3.

The importance of the platinum(II) terpyridine moiety in governing the conformational transition is most apparent in variable temperature UV-vis absorption and CD measurements in CH₃CN. Upon increasing the temperature from 293 to 353 K in CH₃CN, the UV-vis spectra of 2 show a disappearance of the MMLCT absorption band (Fig. S5), whereas in the CD spectra, a diminution of ellipticity at 307 nm for (S)- and (R)-2 was observed, as shown in Fig. S6 A and B, respectively. The spectral changes in the UV-vis and CD spectroscopy clearly indicate that, upon increasing the temperature, the helical bias of the helix would significantly decrease due to the destruction of the metal···metal and π−π interactions of the alkynylplatinum(II) terpyridine moiety, leading to the transformation from the folded state back to the random coil state. Therefore, it is believed that the platinum(II) terpyridine moiety plays an important role in governing the dynamic scaffold of the binaphthol derivatives between the random coil and the helical strand upon modulations by various stimuli, as depicted schematically in Fig. 5.

Fig. 5.

Schematic representation of the dynamic transformation of binaphthol derivatives between random coils and single-turn helical strands by temperature and solvent modulation.

Complexes 1−3 exhibit strong emission bands in CH₂Cl₂ at 577−582 nm upon excitation at λ > 400 nm, which are assigned as an excited state of predominantly ³MLCT/³LLCT character. Upon increasing the CH₃CN content, 2 and 3 both show a growth of a low-energy emission band at 685 nm typical of a ³MMLCT emission with the diminution of the ³MLCT/³LLCT emission of the monomeric alkynylplatinum(II) terpyridine moiety (Fig. 6 A and B). Interestingly, 2 with the optimum length for a single-turn helical strand shows a switching-off of the monomeric ³MLCT/³LLCT emission and a more red-shifted ³MMLCT emission band in CH₃CN (Fig. S7A), in accordance with the proposition that 2 would form a more stable helical strand with a tighter conformation than 3, as reflected by the more negative ΔG(CH₃CN) value of 2 than 3. In contrast, the emission maximum of 1 in CH₃CN only shows a slight blue shift relative to that observed in CH₂Cl₂ (Fig. S7B) due to the negative solvatochromism (35), without the self-assembly of the two platinum(II) terpyridine moieties to form metal···metal and π−π interactions. In the variable temperature luminescence study, upon increasing the temperature, both 2 and 3 show a decrease in ³MMLCT emission as shown in Fig. S7 C and D, respectively, indicating that these metallo-foldamers possess a high reversibility for the transformation from a helical strand back to a random coil with the reversible control of the metal···metal interactions for the folding process.

Fig. 6.

Normalized emission spectral traces of CH₃CN-CH₂Cl₂ mixtures: 0–100% for (A) rac-2 (2.4 × 10⁻⁵ M) at 616 nm with λ_exc = 435 nm and (B) rac-3 (2.0 × 10⁻⁵ M) at 606 nm with λ_exc = 420 nm upon increasing the CH₃CN fraction.

Summary and Prospects

In conclusion, we demonstrated a unique class of metallo-foldamers using inherently chiral binaphthol segments to construct a single-turn helical strand. Moreover, these metal-containing binaphthol derivatives display a structural conformational transition between the random coil state and the helical strand induced by different stimuli with the modulation of metal···metal and π−π interactions. The present work demonstrates the versatility and the reversibility of the metal···metal and π−π interactions in the stabilization of dynamic metallo-foldamers in diverse structural architectures that are distinct from those classically observed with metal−ligand coordination. The solvophobically tubular cavity of the helix with the inherently chiral binaphthol segment may be effective for molecular recognition and molecular switching by monitoring the loss of stereochemical information.

Materials and Methods

General Details.

¹H NMR spectra were recorded on a Bruker AVANCE 400 or 500 (400 and 500 MHz) Fourier-transform NMR spectrometer with chemical shifts reported relative to tetramethylsilane, [(CH₃)₄Si]. Positive-ion FAB mass spectra were recorded on a Thermo Scientific DFS high-resolution magnetic sector mass spectrometer. IR spectra were obtained as a KBr disk on a Bio-Rad FTS-7 Fourier transform infrared spectrophotometer (4,000−400 cm⁻¹). Elemental analyses of the complexes were performed on a Flash EA 1112 elemental analyzer at the Institute of Chemistry, Chinese Academy of Sciences. The UV-visible spectra were obtained using a Hewlett-Packard 8452A diode array spectrophotometer. The emission spectra at room temperature were recorded on a Spex Fluorolog-3 model FL3-211 fluorescence spectrofluorometer equipped with an R2658P PMT detector. Variable temperature UV-vis absorption and emission spectra were obtained using a Varian Cary 50 UV-vis spectrophotometer and a Spex Fluorolog-3 model FL3-211 fluorescence spectrofluorometer equipped with an R2658P PMT detector, respectively. The temperature was maintained by a Varian Cary single-cell Peltier thermostat.

Synthetic Details.

The synthesis of the binaphthol derivatives involves the alkylation of 6,6′-dibromo-1,1′-bi-2-naphthol with ⁿBuBr, followed by the Sonogashira coupling reactions with the corresponding mPE repeating units [H-(C≡C-1,3-C₆H₄)_n-C≡C-SiⁱPr₃ (n = 1, 2, and 3)], to afford the bis-coupled products. After removing the triisopropylsilyl (TIPS) protecting groups with tetra-n-butylammonium fluoride, the terminal alkynes of various lengths were synthesized. Complexes 1−3 were synthesized by reacting the corresponding binaphthol alkyne derivatives with [(^tBu₃tpy)PtCl](OTf) in a molar ratio of 1:2 in degassed DMF containing NEt₃ with a catalytic amount of CuI. The details of the synthesis and characterization are given in SI Synthesis and Characterization and Scheme S1.

Computational Details.

Calculations were carried out using the Gaussian 09 software package (94). The folded and unfolded structures for (S)-2 were fully optimized in solution (CH₃CN) using the density functional theory at the M06 level of theory (95) in conjunction with the solvation model density (SMD) continuum method (96). The Cartesian coordinate of the selected structures is shown in Table S1. The M06 functional has been recommended for the study of the noncovalent interactions and transition metal thermochemistry (97). The Stuttgart effective core potentials (ECPs) and the associated basis set were applied to describe Pt (98) with two f-type polarization functions (ζ = 0.70 and 1.14) (99), whereas the 6–31G(d) basis set (100–102) was used to describe all other atoms. Vibrational frequencies were calculated at the same level of theory for the optimized structures to verify that each was a minimum on the potential surface. Single point calculations were performed on the optimized structures using a larger basis set, in which the same metal basis set was used, but Dunning’s correlation-consistent cc-pVTZ basis set (103) was used for all other atoms. The Gibbs free energies were computed at the M06/SMD(CH₃CN)/cc-pVTZ//M06/SMD(CH₃CN)/6–31g(d) level, in which thermal contributions to the free energy were obtained from the vibrational frequency calculations with the smaller basis set. A pruned (99,590) grid was used for all of the DFT calculations.