Abstract
It is difficult to assemble multi-component metallo-supramolecular architectures in a non-statistical fashion, which limits their development toward functional materials. Herein, we report a system of interconverting bowls and cages that are able to respond to various selective stimuli (light, ligands, anions), based on the self-assembly of a photochromic dithienylethene (DTE) ligand, La, with PdII cations. By combining the concept of “coordination sphere engineering”, relying on bulky quinoline donors, with reversible photoswitching between the ligand’s open (o-La) and closed (c-La) forms, a [Pd2(o-La)4] cage (o-C) and a [Pd2(c-La)3] bowl (c-B) were obtained, respectively. This structural rearrangement modulates the system’s guest uptake capabilities. Among three bis-sulfonate guests (G1, G2, and G3), the cage can encapsulate only the smallest (G1), while the bowl binds all of them. Bowl c-B was further used to synthesize a series of heteroleptic cages, [Pd2LA3LB], representing a motif never reported before. Additional ligands (Lc-f), with short or long arms, tune the cavity size, thus enabling or preventing guest uptake. Addition of Br–/Ag+ makes it possible to change the overall charge, again triggering guest uptake and release, as well as fourth ligand de-/recomplexation. In combination, site-selective introduction of functionality and application of external stimuli lead to an intricate system of hosts with different guest preferences. A high degree of complexity is achieved through cooperativity between only a few components.
Highlights
Natural systems are capable of fabricating supramolecular assemblies with multiple components in a precise and controlled manner
The general design of the system is based on previous work from our group on photochromic [Pd2L2n] assemblies formed from banana-shaped bis-pyridine ligands having DTE backbones.14a,21 In this work, we combine the effect of the photochromic backbones with coordination sphere engineering by installing quinolin-3-yl donor groups that introduce significant steric hindrance around the coordinated PdII cations
Heteroleptic Cages: Self-Assembly, Guest Binding, and Response to Chemical Stimuli. In bowl species such as c-B, each PdII center is coordinated by three quinoline-based ligands, while the fourth coordination site is occupied by a solvent molecule to complement the square-planar coordination sphere.[12]
Summary
Natural systems are capable of fabricating supramolecular assemblies with multiple components in a precise and controlled manner. Heteroleptic Cages: Self-Assembly, Guest Binding, and Response to Chemical Stimuli In bowl species such as c-B, each PdII center is coordinated by three quinoline-based ligands, while the fourth coordination site is occupied by a solvent molecule (here acetonitrile or H2O) to complement the square-planar coordination sphere.[12] This encouraged us to synthesize a new series of banana-shaped bis-pyridine ligands (Lc‐f, Figure 1b), having a length similar to that of La (in terms of their N···N distances). While the NMR spectrum of o-B-Ld is characterized by broad signals, stepwise titration with G1 led to a new set of sharp signals, showing at the same time guest uptake and stabilization of a more defined conformer of the host (Figure S92) This allowed a better characterization of the system with 2D NMR spectra (Figures S47 and S48), and an indirect confirmation of the o-B-Ld heteroleptic cage assembly. Upon addition of 2.0 equiv of Br− to a G1@c-B-Le pubs.acs.org/JACS
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