Molecular recognition of planar and non-planar aromatic hydrocarbons through multipoint Ag–π bonding in a dinuclear metallo-macrocycle

Abstract

Exploration of a novel structural motif of host–guest interactions is one of the most fundamental topics to develop macrocycle-based host–guest/supramolecular systems. Herein, we present an unprecedented mode of inclusion of aromatic hydrocarbons into a macrocyclic cavity via multipoint Ag–π bonding as a driving force. A dinuclear Ag^I-macrocycle encapsulated one molecule of anthracene, a typical planar aromatic hydrocarbon, in solution and in the solid state. Single-crystal X-ray diffraction analysis of the host–guest inclusion complex revealed the binding of anthracene via multipoint Ag–π bonding to both Ag^I ions arranged within the open-ended nano-cavity of the dinuclear Ag^I-macrocycle. Notably, this binding motif based on Ag–π bonding was also applied to the inclusion of triptycene, a non-planar aromatic hydrocarbon with a steric tripodal structure, to evaluate the rotational motion of the molecular paddle-wheel in the Ag^I-macrocycle.

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Introduction

Since the discovery of crown ether by Pedersen, a great number of excellent examples of functional macrocycles have been reported which can bind guest molecules or ions within their inner spaces.¹ Guest binding abilities of macrocycles play vital roles in providing various implications to advanced functions in host–guest/supramolecular systems, such as molecular recognition,² activation,³ transportation⁴ and fabrication of specific supramolecular architectures (i.e. rotaxanes and catenanes).⁵ In general, a variety of non-covalent interactions including hydrogen bonding, coulombic interactions, van der Waals forces and solvophobic effects have been utilised as driving forces for guest uptake of macrocycles.⁶ Besides, metal coordination capable of forming stable, reversible and directional bonds is another binding mode between host and guest compounds, and therefore macrocycles with coordinatively labile metal centres in the cavity have received a lot of attention in recent years in the field of supramolecular chemistry.⁷

Aromatic hydrocarbons have been widely investigated as one of the most important classes of guest molecules for macrocycles owing to their ubiquitous structure and electronic properties related to π-conjugation.⁸ In the past few decades, a variety of macrocyclic receptors for aromatic molecules have been reported which can realise recognition,⁸ separation^8c and regulation^8d of their properties via host–guest interactions. However, in spite of the diversity of macrocyclic structures, molecular recognition of non-substituted, structurally simple aromatic hydrocarbons is still challenging because such hydrocarbons without polar functionalities are difficult to uptake by forming distinct chemical bonds (i.e. hydrogen bonds and coordination bonds) with a receptor, and alternatively vast-area contacts through a few weak interactions such as π–π, CH–π and van der Waals interactions are required to encapsulate a pristine aromatic hydrocarbon in a host framework (Fig. 1a). Such vast-area contacts generally need shape complementarity in the host–guest inclusion structure. Therefore, there are limitations in the diversity of host–guest structures, which make it difficult to rationally design a platform for the molecular recognition of non-planar aromatic hydrocarbons with a three-dimensional structure.

Fig. 1 (a) Schematic representation of the binding mode of a non-substituted aromatic hydrocarbon within a macrocyclic host via vast-area contacts. (b) The chemical structure of a dinuclear Ag^I-macrocycle [Ag₂L1X₂](SbF₆)₂ and the schematic representation of the binding modes of planar and non-planar aromatic hydrocarbons via multipoint Ag–π bonding.

Herein, we present an unprecedented mode of inclusion of non-substituted aromatic hydrocarbons into a metallo-macrocycle utilising M–π bonding as a driving force. M–π bonding is a kind of non-Werner type coordination bonding mainly originating from the interactions among d orbitals of metals and π orbitals of aromatic hydrocarbons.^9–11 In general, M–π bonding has higher directionality and site-specificity than the above-mentioned conventional intermolecular interactions, and the resulting M–π complexes have the potential to exhibit unique functions based on the chemical and physical properties of the metal centres. Therefore, metallo-macrocycles possessing a nano-space with multiple metal centres for M–π bonding are expected to provide a novel structural motif to recognise aromatic guest molecules, in which the inclusion structure and the guest selectivity can be affected by the type, number, and arrangement of the inner metal ions.¹² Based on this concept, we have recently developed a dinuclear Ag^I-macrocycle [Ag₂L1X₂](SbF₆)₂ (L1 = macrocyclic ligand and X = Et₂O or H₂O) which possesses an anthracene-based rhombic structure with a nano-space with two Ag^I centres (Fig. 1b). In this system, the Ag^I-macrocycle can accommodate a ditopic aromatic molecule such as cyclophanes and ferrocene utilising multiple Ag–π bonds as a main driving force.^12a,c Herein we report that the Ag^I-macrocycle effectively recognised an anthracene molecule, a planar aromatic hydrocarbon, in the open-ended cavity through multiple Ag–π bonds with both ends of anthracene (Fig. 1b). Moreover, we found that this guest-binding motif based on Ag–π bonding does not require vast-area contacts between the host and guest, which achieved the binding of a non-planar aromatic hydrocarbon, triptycene, in a similar manner.

Results and discussion

The binding of anthracene (Ant) to a dinuclear Ag^I-macrocycle [Ag₂L1X₂](SbF₆)₂ was first investigated by ¹H NMR titration experiments at 220 or 300 K and mass spectrometry. Upon the addition of Ant to a solution of [Ag₂L1X₂](SbF₆)₂ (0.07 mM) in CDCl₃, the ¹H NMR signals of [Ag₂L1X₂](SbF₆)₂ at 220 K were gradually replaced by a new set of signals with an increased amount of Ant (Fig. 2a–c). In the presence of more than 1.0 eq. of Ant, the original signals were almost completely replaced by a new set of signals, and the signals of free Ant (A_out, B_out and C_out) appeared at around 7.5–8.6 ppm, suggesting the formation of a 1 : 1 host–guest inclusion complex, Ant⊂[Ag₂L1](SbF₆)₂ (Fig. 2c–e). The new signals observed around the higher magnetic field region (Ant_in around 6.0–6.2 ppm) can be assigned to the included Ant, which are significantly up field shifted (|Δδ| = ca. 1.4–2.4 ppm) from their original positions due to the shielding effect from the anthracene walls of [Ag₂L1]²⁺.¹³

Fig. 2 Partial ¹H NMR spectra of [Ag₂L1X₂](SbF₆)₂ (0.07 mM) in the presence of (a) 0.0, (b) 0.5, (c) 1.0, (d) 1.5 and (e) 2.0 eq. of anthracene (Ant) (500 MHz, CDCl₃, 220 K). Ant_in represents the signals of the included Ant.

Notably, at a temperature as high as 300 K, as the intermolecular exchange between bound and free Ant molecules is faster than the timescale of ¹H NMR spectroscopy, ¹H NMR titration experiments at 300 K resulted in the sequential shift of the signals of [Ag₂L1X₂](SbF₆)₂ due to host–guest interactions (Fig. S1 and S2†). From the curve-fitting of the ¹H NMR data at 300 K based on the formation of a 1 : 1 host–guest complex, the binding constant between Ant and the Ag^I-macrocycle (K_a(Ant) = [Ant⊂[Ag₂L1]²⁺]/([[Ag₂L1X₂]²⁺][Ant]) M⁻¹) was determined to be (3.0 ± 0.4) × 10⁴ M⁻¹ in CDCl₃ at 300 K (Fig. S3†), which is comparable to those of already reported examples of macrocyclic receptors for aromatic hydrocarbons.^8c,f,14 The formation of a 1 : 1 host–guest complex was also supported by ESI-TOF mass measurements (m/z = 903.72 as Ant⊂[Ag₂L1]²⁺, Fig. S6†).

The structure and binding mode of the resulting complex were finally determined by single-crystal X-ray analysis (Fig. 3). By slow n-pentane vapour diffusion into a mixture of L1, AgSbF₆ (4.2 eq.) and Ant (10 eq.) in CH₂Cl₂ in the dark at room temperature, yellow block crystals suitable for single-crystal X-ray analysis were obtained. In the resulting crystal structure, one molecule of Ant was included in the cavity of [Ag₂L1]²⁺via η²-type Ag–π bonding at both terminal edges of the elongated π-surface in an anti-manner (Fig. 3a). These Ag^I ions were in a distorted square pyramidal five-coordinate geometry with two N-atoms of phenanthroline, two C-atoms of the included Ant, and one Cl-atom of a coordinating solvent: CH₂Cl₂ (Ag–N1 2.280(6) Å; Ag–N2 2.356(5) Å; Ag–C55 2.454(8) Å; Ag–C56 2.449(8) Å; Ag–Cl3 2.984(2) Å, Fig. 3b). These observations suggest that multipoint Ag–π bonding between Ant and Ag^I ions, which are precisely arranged on the inner surface of the nano-space, works as an effective driving force to bind Ant. Multipoint CH–π interactions between the H-atoms of the included Ant and the π-surface of anthracene-walls of the macrocycle may also contribute to stabilise the resulting complex as well. Notably, Ant⊂[Ag₂L1(CH₂Cl₂)₂]²⁺ possesses a C_i-symmetrical structure in the crystal due to the inclination of the included Ant. On the other hand, the ¹H NMR spectrum of Ant⊂[Ag₂L1](SbF₆)₂ showed a simple set of ¹H NMR signals (Fig. 2b–e) suggesting a more symmetrical (D_2h) structure with successive fluxional movement of Ant in the nano-cavity via intramolecular Ag–π exchange.

Fig. 3 Crystal structure of Ant⊂[Ag₂L1(CH₂Cl₂)₂](SbF₆)₂. (a) Space filling model and (b) ORTEP view (50% probability level) of the partial structure (solvents, side-alkyloxy chains and counter anions in (a) are omitted for clarity). Ag: magenta, C: grey, C of Ant: blue, Cl: pale green, F: yellow, H: white, N: light blue and Sb: purple.

Based on these results, the binding properties of [Ag₂L1X₂](SbF₆)₂ to other aromatic hydrocarbons of different sizes were examined. We previously reported that a smaller aromatic hydrocarbon, p-xylene, formed not a 1 : 1 but a 2 : 1 complex, (p-xylene)₂⊂[Ag₂L1]²⁺, in the crystalline state.^12a On the other hand, a ¹H NMR titration experiment using p-xylene showed negligible shift in the host signals (|Δδ| < 0.04 ppm) even after adding 4 eq. of p-xylene, suggesting a lower affinity to [Ag₂L1X₂](SbF₆)₂ or a different binding mode from that of Ant⊂[Ag₂L1](SbF₆)₂ in solution (Fig. S22 and S24†). Similarly, naphthalene, which has an intermediate size between p-xylene and Ant, showed a smaller shift in the host signals (|Δδ| < 0.02 ppm) even after adding 4 eq. of naphthalene in the ¹H NMR titration experiment at 300 K (Fig. S23† and S24†). These results may reflect the insufficiency of the molecular size of p-xylene and naphthalene to simultaneously form Ag–π bonds at both Ag^I centres arranged in the macrocyclic skeleton. These results suggest the importance of the arrangement mode of Ag^I ions in the macrocycle in controlling the stability, selectivity and structure of the resulting host–guest complex.

In contrast, triptycene (Trip), which is a non-planar aromatic hydrocarbon with a steric tripodal structure, was found to be effectively included within the cavity of [Ag₂L1X₂](SbF₆)₂. The inclusion behavior of Trip was revealed by a ¹H NMR titration experiment in CDCl₃ at 220 K, in which the host signals were replaced by a new set of signals in the presence of more than 1.0 eq. of Trip to form an 1 : 1 host–guest complex Trip⊂[Ag₂L1](SbF₆)₂ (Fig. 4).¹³ The signals of the included Trip (B_in) showed a strong rotating frame Overhauser effect (ROE) correlation with the proton inside [Ag₂L1]²⁺ (H_i), which suggests the existence of Trip within the nano-cavity of the macrocycle (Fig. S18†). The formation of Trip⊂[Ag₂L1](SbF₆)₂ was also supported by ESI-TOF mass analysis (m/z = 941.21 as Trip⊂[Ag₂L1]²⁺, Fig. S20†).

Fig. 4 Partial ¹H NMR spectra (500 MHz, CDCl₃, 220 K) of [Ag₂L1X₂](SbF₆)₂ (0.11 mM) in the presence of (a) 0.0, (b) 0.5, (c) 1.0, (d) 1.5 and (e) 2.0 eq. of triptycene (Trip). Hexyl represents the signal of side-alkyloxy chains of L1.

Similar to the case of Ant, the rates of intermolecular exchange of bound and free Trip at a higher temperature (300 K) became faster than the timescale of ¹H NMR, which resulted in the observation of a dynamically averaged ¹H NMR spectrum (Fig. S9 and S10†). From the curve-fitting of the ¹H NMR data of the guest titration experiment at 300 K based on the formation of a 1 : 1 host–guest complex, the binding constant between Trip and the Ag^I-macrocycle (K_a(Trip) = [Trip⊂[Ag₂L1]²⁺]/([[Ag₂L1X₂]²⁺][Trip]) M⁻¹) was determined to be (3.1 ± 0.2) × 10⁴ M⁻¹ in CDCl₃ at 300 K (Fig. S11†), which is comparable to that of Ant (K_a(Ant) = (3.0 ± 0.4) × 10⁴ M⁻¹).¹⁴ This result suggests host–guest interactions between Trip and Ag^I-macrocycle similar to that of Ant and Ag^I-macrocycle, where multipoint Ag–π bonding simultaneously formed at both ends of the aromatic molecule works as a dominant driving force for guest binding. Molecular modeling of Trip⊂[Ag₂L1]²⁺ based on the crystal structure of the aforementioned Ant⊂[Ag₂L1](SbF₆)₂ (Fig. 3) and that of a previously reported Ag^I complex of Trip supports the multipoint binding of Trip, where Ag^I ions coordinate to the π-planes of Trip in the anti or syn form (Fig. 5a and S21†).^10d,g,h Such a binding motif of Trip suggests that the open-ended nano-cavity of [Ag₂L1X₂](SbF₆)₂ with two Ag^I ions would work as an effective receptor for not only planar but also non-planar aromatic molecules which allows multipoint Ag–π bonding.

Fig. 5 Schematic representation of the plausible binding modes and molecular dynamics of Trip⊂[Ag₂L1](SbF₆)₂. (a) Plausible side cross-sectional views of Trip⊂[Ag₂L1](SbF₆)₂ where Ag^I ions coordinate to the π-planes of Trip in an anti or a syn manner. (b–d) Plausible side cross-sectional views of the rotational motion of Trip, where Trip exhibits faster intramolecular rotational motion (b and c) and a slower intermolecular guest exchange (d) than the timescale of ¹H NMR observation.

Notably, the included Trip exhibits rotational motion at 220 K, which affects the symmetry of the resulting host–guest complex, Trip⊂[Ag₂L1]²⁺ (Fig. 5b–d).¹⁵ Upon binding of Trip, the ¹H NMR signals of the protons of anthracene walls (H_g in Fig. 4a) of the macrocycle split into two separate signals (H_g1 and H_g2 in Fig. 4c), suggesting the desymmetrisation of the host skeleton from D_2h-symmetry to C_2v-symmetry (loss of the mirror plane on the cyclic skeleton of L1). On the other hand, the three aromatic panels of the included Trip (α, β and γ in Fig. 5b and c) were observed as two identical signals in the higher magnetic field region (B_in and C_in at 4.4 and 4.1 ppm in Fig. 4c), suggesting conservation of the D_3h-symmetry of the pristine tripodal structure of Trip even within the nano-cavity of the macrocycle. Considering such a respective loss and conservation of the symmetry of the host and the guest, the included Trip was supposed to be deviated from the mirror plane of the cyclic skeleton of the macrocycle L1 without fast intermolecular guest exchange to desymmetrise the host skeleton (Fig. 5d). Besides, the included Trip was supposed to exhibit fast intramolecular rotational movement around the 3-fold axis of the tripodal structure to average the magnetic environment of the three aromatic panels in the timescale of ¹H NMR observation (α, β and γ in Fig. 5b and c). Such a specific rotational motion of Trip bound to two Ag^I ions suggests that the open-ended nano-cavity of [Ag₂L1X₂](SbF₆)₂ is suitable to bind aromatic guest molecules and to maintain and regulate the degree of freedom of their molecular motion, which potentially plays a key role as functional molecular machines.

Conclusions

In this study, host–guest interactions between a dinuclear Ag^I-macrocycle [Ag₂L1X₂](SbF₆)₂ and aromatic hydrocarbons were investigated. [Ag₂L1X₂](SbF₆)₂ has an ability to strongly include not only a planar (Ant) but also a non-planar (Trip) aromatic hydrocarbon which allows multipoint Ag–π bonding at both Ag^I centres simultaneously. Such a Ag^I-based specific binding motif is quite distinct from those of conventional macrocyclic receptors for aromatic molecules which require close and vast-area contacts between the aromatic guest and the nano-cavity of the macrocycle. Such a unique binding motif utilising non-Werner type M–π coordination would provide us an important clue to the rational design of the structures, properties and metal-centred functions of the resulting host–guest complexes. This finding would help to develop a new dimension to macrocycle-based supramolecular chemistry and molecular machines based on the selectivity, reactivity and dynamics of M–π bonding within a confined space of metallo-macrocycles.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by JSPS KAKENHI grant numbers JP16H06509 (Coordination Asymmetry) to M. S. and JP16H00956 (Molecular Architectonics) to S. T. K. O. acknowledges the Advanced Leading Graduate Course for Photon Science (ALPS).

Notes and references

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13. The integral ratios of the ¹H NMR signals of the included guest molecules encapsulated in [Ag₂L1]²⁺ were evaluated to be smaller than those estimated from the structures of 1 : 1 complexes probably due to the fluxional dynamic movement of the included guest molecules within the nano-space, which caused broadening of the ¹H NMR signals..
14. The stabilities of the host–guest complexes were temperature dependent. The binding constant K_a estimated at 300 K seems to be smaller than that at 220 K, as revealed by the convergence of ¹H NMR titration experiments..
15. (a) A. Scarso, H. Onagi and J. Rebek Jr, J. Am. Chem. Soc., 2004, 126, 12728–12729; (b) T. Matsuno, M. Fujita, K. Fukunaga, S. Sato and H. Isobe, Nat. Commun., 2018, 9, 3779.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and characterization data. CCDC 1911739. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9sc02619c

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