Coordination-Driven Self-Assembly of Truncated Tetrahedra Capable of Encapsulating 1,3,5-Triphenylbenzene

Abstract

The design and synthesis of coordinative truncated tetrahedra is described. Coordination-driven self-assembly of a truncated tetrahedron was achieved using 90° organoplatinum acceptors and a hexapyridyl ligand with six-fold symmetry under mild conditions. This assembly can act as a host towards 1,3,5-triphenylbenzene. The truncated tetrahedral structures and the host-guest complex were identified using multinuclear (³¹P and ¹H) NMR spectroscopy, electrospray ionization (ESI) mass spectrometry, X-ray crystallography, and pulsed field gradient spin-echo (PGSE) NMR, along with computational simulations.

Coordination-driven self-assembly is a successful methodology for preparing three-dimensional (3-D) supramolecular structures.¹ During the past two decades, a variety of novel 3-D coordinative structures of high complexity, high symmetry, and well-defined size and shape, such as tetrahedra, cubes, double squares, cuboctahedra, adamantanoids, dodecahedra, and a sphere, have been developed.² By virtue of their robust structural backbone and well-defined 3-D cavity, 3-D coordinative supramolecules have been used in a variety of applications such as guest encapsulation, gas storage, catalysis, and drug delivery.³ With further exo/endo functionalization,⁴ 3-D supramolecules are endowed with a variety of novel properties, like 3-D dendrimers, nanoscale fluoro-droplet, and confined polymerization.⁵ Among these 3-D supramolecules, because of their unique structural features and fascinating host-guest properties, tetrahedral structures represent one of the most widely studied systems, as witnessed by the impressive results of Raymond⁶ and Fujita.⁷ However, the design of tetrahedral structures is still limited.^6–8

Homoleptic self-assembly of a hexapyridyl ligand and a 90° acceptor should form a tetrahedral structure, where the hexadentate ligand acts as faces and the organoplatinum acceptors are the connectors at the corners as shown in Scheme 1. We therefore carried out the self-assembly of the hexapyridyl donor 2⁹ with the 90° organoplatinum acceptors 1, resulting in a truncated tetrahedron in quantitative yield. The structures were characterized by multinuclear (³¹P and ¹H) NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), pulsed field gradient spin-echo (PGSE) NMR, as well as X-ray crystallography. Furthermore, it was found that truncated tetrahedron 3b is able to encapsulate 1,3,5-triphenylbenzene in an aqueous acetone solution. This host-guest complex was characterized by NMR spectroscopy and ESI-MS as well as computational simulations.

Scheme 1.

Graphical representation of the [12 + 4] self-assembly of 90° organoplatinum acceptors 1 and hexapyridyl ligand 2 into a truncated tetrahedron 3.

By mixing 90° organoplatinum acceptor 1 and hexapyridyl donor 2⁹ in a 3:1 ratio in an acetone-d₆/CD₃NO₂ (v/v 7:3) solution for 3a and an aqueous acetone solution (v/v 1:1) for 3b, self-assembly of truncated tetrahedra 3 were obtained after 16 h of heating at 80 °C. The assemblies 3 can be quantitatively isolated via ion exchange with KPF₆.

In the ³¹P{¹H} NMR spectra (Figure 1) of 3, only one singlet at 0.92 ppm (3a) and −28.2 ppm (3b) with concomitant ¹⁹⁵Pt satellites can be found, respectively, for the coordinated platinum centers. Likewise, the ¹H NMR spectra (See Supporting Information) also exhibit sharp signals for the pyridyl protons of 3 (δ_Pyα-H: 8.91 ppm and δ_Pyβ-H: 7.44 ppm for 3a; δ_Pyα-H: 8.68 ppm and δ_Pyβ-H: 7.48 ppm for 3b), with approximately 0.1 ppm (H_Py-β) and 0.2–4 ppm (H_Py-α) downfield shifts due to the loss of electron density upon coordination with the platinum centers. Signals corresponding to the phenyl protons on the donors are split into two sets of doublets, presumably caused by the difference between the exterior and interior of the cage structure. In the ESI mass spectra of 3 (see Supporting Information), peaks corresponding to [3a – 5PF₆]⁵⁺ and [3a – 6PF₆]⁶⁺ can be found at m/z = 2383.1 and m/z = 1961.9, as are those for 3b at m/z = 2197.0 [3b – 5OTf]⁵⁺ and m/z = 1806.2 [3b – 6OTf]⁶⁺. These signals are isotopically resolved and in agreement with their theoretical distributions.

Figure 1.

³¹P{¹H} NMR spectra (121.4 MHz) of truncated tetrahedra 3a in acetone-d₆/CD₃NO₂ (a) and 3b in acetone-d₆/D₂O (b).

The structure of 3 was unambiguously determined by X-ray diffraction analysis using synchrotron radiation. X-ray quality crystals of 3a were obtained by slow diffusion of pentane into the acetone solution of 3a. As shown in Figure 2 and S4 in the Supporting Information, the structure has a truncated tetrahedral shape with a diameter of 3.6 nm, and bears a 1.0 nm cavity core. The shortest Pt-Pt distance is 1.1 nm. The PF₆⁻ anions are found surrounding the structure but not in the cavity, and the shortest Pt-F distance was measured to be 0.48 nm. As seen in Table S3 (see Supporting Information), the Pt atoms in the structure are coordinating with two PEt₃ and two nitrogen atoms from the pyridine moieties in a distorted square planar geometry. The mean values from the square planar Pt(II) is N-Pt-N 80.6°, P-Pt-P 98.2°, N-Pt-P 90.6°, Pt-P 2.28 Å, and Pt-N 2.09 Å, respectively.

Figure 2.

Crystal structure of truncated tetrahedron 3a (Pt: green; C: grey; N: blue; P: Orange; protons, solvent, and PF₆⁻ omitted for clarity).

PGSE NMR measurements were also used to characterize the structures in solution. Using the translational self-diffusion coefficient measured by PGSE NMR in conjunction with the Stokes-Einstein equation, the “effective size” of the overall assembly in acetone-d₆ was obtained: 3.35 ± 0.15 nm for 3a and 3.23 ± 0.21 nm for 3b, which are in good agreement with that of the crystal structure.

The host-guest properties of 3 were also studied. According to the symmetry and size of the host, 1,3,5-triphenylbenzene 4 was chosen for the investigation. The experiment was carried out by mixing 1b and 2 in a 3:1 ratio with excess triphenylbenzene 4 in an aqueous acetone solution (v/v 1:1). After 16 h of heating at 70 °C, the encapsulated complex 3b·4₃ was formed.

The singlet at −28.2 ppm with concomitant ¹⁹⁵Pt satellites in the ³¹P{¹H} NMR spectrum (see Supporting Information) and the identifiable peaks (δ = 8.76 ppm H_Py-α-3b; δ = 7.36 ppm H_Py-β-3b; δ = 6.94 ppm H_Phenyl-3b) in the ¹H NMR spectra (Figure 3b) show the formation of a truncated tetrahedron. By comparing the ¹H NMR spectra of 4 (acetone-d₆), 3b (acetone-d₆/D₂O = 1:1), and 3b·4₃ (acetone-d₆/D₂O = 1:1) in Figure 3, signals are found at 7.25 ppm (Δδ = −0.6 ppm, H_Phenyl-4), 7.06 ppm (Δδ = 0.24 ppm, H_Phenyl-3b), and 6.35 ppm (Δδ = −1.0–1.8 ppm, H_Phenyl-4), indicating that the triphenylbenzene 4 is encapsulated in the truncated tetrahedron 3b, and integration of the peaks at 8.76 ppm (H_Py-α-3b) and 6.35 ppm (H_Phenyl-4) suggest that three guest molecules are encapsulated in each cage. In the ESI mass spectra (see Supporting Information), isotopically resolved signals at m/z = 2380.8 [3b·4₃ – 5OTf]⁵⁺ and m/z = 1959.3 [3b·4₃ – 6OTf]⁶⁺ confirm the complex 3b·4₃. Elemental analysis (see Supporting Information) of the isolated complex is also consistent with the composition of 3b·4₃.

Figure 3.

Partial ¹H NMR spectra (300 MHz) of “free” 4 in acetone-d₆ (a), 3b in acetone-d₆/D₂O (b), and the encapsulated complex 3b·4₃ in acetone-d₆/D₂O (c).

While X-ray quality crystals for 3b·4₃ were not obtained, a computational simulation was used to gain insight into the structural features of the encapsulated complex 3b·4₃.¹⁰ A molecular dynamics simulation using a molecular mechanics force field (MMFF), 300K, in the gas phase was used to equilibrate each supramolecule, and the output of the simulation was then minimized to full convergence. As shown in Figure 4, in the model of 3b·4₃, three triphenylbenzene molecules 4 are stacked within the cavity of 3b. The distance between these guests is about 4.3 Å, and that between the guest and the interior of the cage is 3.6–4.3 Å.

Figure 4.

Computational model (MMFF) of the encapsulated complex 3b·4₃ (For clarity, the three guest molecules are labeled as blue, green, and orange).

In conclusion, we report the facile synthesis of a new type of 3-D truncated tetrahedra via coordination-driven self-assembly, where the highly symmetrical hexapyridyl ligand acts as the faces and 90° organoplatinum acceptors are connectors at the edges. These truncated tetrahedra show a unique 3-D nanoscale pore, and preliminary studies indicate the nano-cavity is able to encapsulate 1,3,5-triphenylbenzene.