Assembly of C-propyl-pyrogallol[4]arene with bipyridine-based spacers and solvent molecules

1 Introduction

The assembly of molecular capsules by the introduction of complementary hydrogen bonds between small molecular components has played a key role in the development of supramolecular chemistry [1], [2], [3], [4], [5]. Various frameworks based on resorcinol[4]arenes and bifunctional pillar compounds, such as bipyridine and trans-bis(4-pyridyl)-ethylene, have been shown to contain large cavities capable of encapsulating organic or inorganic guests [6], [7], [8]. The pioneering work by MacGillivray and Atwood showed that an assembly of C-methylcalix[4]resorcinarenes, pyridine or 4,4′-bipyridine, and an appropriate guest could effectively produce multi-component host-guest complexes [9]. In particular, multi-component supramolecular systems have attracted great attention in this regard, owing to their potential applications in molecule recognition, ion exchange, small molecule inclusion, catalysis, and the rational design of functional materials [10], [11], [12]. Interactions of resorcinol[4]arene with bipyridine in different solvents led to unique architectures of host structures, including discrete capsules in one-, two-, and three-dimensional arrangements [13], [14], [15], [16], [17], [18], [19]. Co-crystallization of pyrogallol[4]arene and bipyridine building moieties into supramolecular architectures, which may selectively encapsulate guest molecules, was first studied by Atwood and coworkers in 2005 [20]. The Atwood group reported the engineering of supramolecular organic frameworks composed of C-alkylpyrogallol[4]arene (alkyl=methyl, propyl) and 4,4′-bipyridine or 1,2-(4-pyridyl)acetylene spacers in 2015 [21]. Previously, we have reported two host-guest compounds FcH@(PgC4)₂·CH₃OH·3H₂O (PgC4=C-iso-butylpyrogallarene) and FcH@(PgC2)₂·3EtOH·2H₂O (PgC2=C-ethylpyrogallarene), in which ferrocene molecules are encapsulated by hydrogen-bonded pyrogallarene capsules [22]. Moreover, we have studied the co-crystallization of C-alkyl-calix[4]resorcinarenes, bis-pyridines and ferrocene and found that ferrocene molecules could also be enclosed by the deepened wave-like framework [23]. Following extensive contributions, which have been made to supramolecular chemistry using resorcinarenes and their derivatives, we have turned our attention to the hydrogen-bonded assembly of pyrogallol[4]arene with solvent molecules and bifunctional pillar molecules to form different supramolecular complexes (Scheme 1).

Scheme 1:

Synthesis of complexes 1 and 2 (PgC3=C-propyl-pyrogallarene).

2 Experimental

3 Results and discussion

PgC3 was synthesized in fairly high yield by an acid-catalyzed condensation reaction of pyrogallol with butyraldehyde [24]. The result is usually an all-cis configured (r-ccc), bowl-shaped cyclic tetramer. As shown in Scheme 1, the treatment of PgC3 with bpy in a 1:3 ratio in a warm ethanol solution, followed by recrystallization, afforded colorless block-shaped crystals of (PgC3)·3(bpy)·(EtOH) (1) in 72% yield. Variation in the PgC3:bpy ratio from 1:3 to 1:6 only afforded crystalline material of the 1:3 product 1. Complex 1 is a brick-wall framework, but unlike the common brick-wall assembly, part of the bpy molecules act as linkers to bridge the PgC3 molecules (Fig. 1A). In 1, each PgC3 molecule adopts a bowl conformation and connects with the neighboring PgC3 molecules through intermolecular O···O hydrogen bonds [O···O separations (Å): O2···O7 3.094(3) and O1···O7 2.682(3) Å] to form a column. The PgC3 columns are further connected by the bpy linker in a monodentate fashion through O−H···N hydrogen bonds [O···N separations (Å): O6···N3 2.685(3)] to form the brick wall, while there are two bpy molecules at other locations without hydrogen bonding to PgC3. This unusual brick-wall framework structure, formed by four pyrogallol[4]arene molecules and two juxtaposed bpy molecules, entraps two other bpy molecules as guests within each cavity (Fig. 1B). The remaining bpy molecule is located in the outside of the cavity, and the ethanol molecules lie on the other side connecting with PgC3 molecules through hydrogen bonding.

Fig. 1:

Molecular packing diagram of (PgC3)·3(bpy)·(EtOH) (1), showing the hydrogen-bonded network. (a) View of the bpy guests at the base of two PgC3 units; (b) View of two other bpy molecules as guests within each cavity in the molecular packing diagram.

In addition to bpy, the longer spacer molecule trans-1,4-bis-(pyridyl)ethylene (bpe) was investigated as a partner for PgC3 in an ethanol-water mixture. A novel crystalline complex of (PgC3)·(bpe)·2(EtOH)·(H₂O) (2) was obtained in 67% yield. In 2, adjacent bowl-shaped C-propylpyrogallol[4]arenes are connected into a 2D wave-like polymer structure. The wave chain propagates along the crystallographic b axis [O···O separations (Å): O3···O5 3.096(3), O3···O4 2.884(4), O12···O2 2.739(3), O1···O12 3.130(4), O2···O5 2.772(3), O11···O2 3.110(4)] as shown in Fig. 2A, and the parallel chains propagate along the crystallographic a axis [O···O separations (Å): O8−H8···O3 2.838(3)] as shown in Fig. 2B. The pyrogallol[4]arene molecules in one layer are all oriented in the same direction to the opposite layer, and these bis-layers have seven intermolecular hydrogen bonds along their upper rims. The distance from the wave crest to the wave trough is about 11.89 Å, and the adjacent wave crests have a spacing of 10.98 Å. Based on the 2D wave-like arrangement, the bpe molecules are embedded in the wave trough between two PgC3 molecules.

Fig. 2:

Molecular packing diagram of (PgC3)·(bpe)·2(EtOH)·(H₂O) (2), showing the hydrogen-bonded network. Ethanol and water molecules are omitted for clarity. (a) The wave chain propagates along the crystallographic b axis; (b) the parallel chains propagate along the crystallographic a axis.

In summary, two multi-component complexes, (PgC3)·3(bpy)·(EtOH) (1) and (PgC3)·(bpe)·2(EtOH)· (H₂O) (2), based on C-propyl-pyrogallol[4]arene and 4,4′-bipyridine or trans-1,4-bis-(pyridyl)ethylene spacer molecules, have been obtained in good yields. Complex 1 with bpy units is a 1D brick-wall framework, while complex 2 with bpe units is a 2D wave-like polymer, indicating that the length of the spacer molecules may have an influence on the hydrogen bonding pattern and thus the overall architecture of the resultant complexes. Atwood et al. have previously demonstrated that an assembly of C-propyl-pyrogallol[4]arene with bpy in acetone and acetonitrile solvents led to the isolation of a nano-container (PgC3)·4(bpy)·(C₃H₆O) with acetone molecules as the guests [20] and a 2D wave-like polymer structure (PgC3)·(bpy)·(MeCN) [21], respectively. Based on these results, it seems that both the reaction media and the nature of the spacer are crucial for the assembly of the supramolecular complexes.