Dual stimuli-induced formation of a μ-hydroxido bridged [Zn9L5(μ-OH)6]12+ half-pipe

Abstract

Low-symmetry metal-organic architectures that feature unusual binding motifs are useful for exploring new modes of guest recognition. Such structures remain difficult to create using current rational design principles. One approach to constructing such architectures is to employ ligands with coordination vectors oriented to preclude the formation of simple, low nuclearity molecular assemblies upon complexation to metal ions. Here we report two new supramolecular assemblies generated from such a ligand: a simple metastable [Zn₃L₃]⁶⁺ assembly, which was observed to convert to a more complex [Zn₉L₅(μ-OH)₆]¹²⁺ twisted half-pipe architecture. Two chemically distinct stimuli-an anionic template and a base-must be applied for the conversion to occur. Perchlorate, perrhenate, trifluoromethanesulfonate and 2-naphthalenesulfonate were found to act as competent templates for the [Zn₉L₅(μ-OH)₆]¹²⁺ structure.

Scheme 1. Schematic representation of the synthesis of [Zn₃L₃]⁶⁺ assembly 2 and its conversion to a [Zn₉L₅(μ-OH)₆]¹²⁺ twisted half-pipe structure 3. Its formation requires the presence of both a base that forms the six μ-OH bridges in the structure and template anions that reside in the hydroxide lined groove of the structure. All reactions were carried out in acetonitrile at 70 °C for 12 h.

Fig. 1. X-ray crystal structure of [Zn₉L₅(μ-OH)₆](ClO₄)₁₂ twisted half-pipe 3. (a) View down the pseudo C ₂ axis of symmetry. (b) Cutaway view showing two μ-OH bridging anions coordinating to three adjacent zinc centres. (c) Cutaway showing the perchlorate anion templates inside the groove (C, grey; N, blue; Zn, yellow; O, red; Cl, green; H, white). (d) Space filling representation of 3 showing the three tightly packed ligands and helical twist. Uniform colours represent individual ligands. Disorder, non-coordinating anions and solvent are omitted for clarity.

Fig. 2. X-ray crystal structures of 3 templated by (a) perrhenate, (b) triflate and (c) 2-naphthalenesulfonate. The templating anions are shown in space-filling mode. Disorder, non-coordinating anions and solvent are omitted for clarity.

Fig. 3. View down the length of the groove in cutaway X-ray crystal structures of 3 templated by (a) triflate and (b) 2-naphthalenesulfonate. The templating anions are shown in space-filling mode.