ReviewBis–Calix[4]pyrroles: Preparation, structure, complexation properties and beyond
Graphical abstract
Introduction
Molecular recognition processes that serve to bring together two or more molecules or ions through non–covalent bonding are ubiquitous in nature and underlie a range of biological processes [1]. They play vital roles in information processing and have allowed for the preparation of a number of supramolecular devices that exploit changes in the electrical, optical, magnetic properties, chemical properties, etc. [2]. Not surprisingly, molecular recognition (also referred to as supramolecular and host–guest chemistry) has attracted increasing attention across the scientific community in recent decades.
Macrocyclic chemistry is one of the central topics within the cadre of host–guest chemistry. With well–defined recognition pockets and enhanced preorganization and substrate complementarity, macrocycles such as crown ethers [3], [4], cyclodextrins (CDs) [5], [6], [7], cucurbiturils [8], calixarenes and pillararenes [9], [10], [11], [12], have enjoyed wide-spread use as receptors for guest species or as building blocks for supramolecular assembly. In addition, considerable effort continues to be devoted to the development of new functional macrocycles that can complement these well-studied receptor systems in terms of their substrate selectivities or inherent binding affinities.
Calix[4]pyrrole (1) represents an “old-but-new” receptor system. The parent form dates back to 1886 when it was first synthesized by Baeyer via the acid–catalyzed condensation of pyrrole with acetone (Fig. 1) [13]. However, the utility of calix[4]pyrroles as receptors for anions and ion pair recognition was only recognized in the mid-1990s due to the pioneering work of Sessler and collaborators [14], [15]. Calix[4]pyrrole is a conformationally flexible molecule and can exist in four main conformations, namely 1,3-alternate, cone, partial cone, and 1,2-alternate [16]. The first of these conformations generally dominates in the absence of a substrate, whereas near-complete conversion to the cone form is typically seen in the presence of a strongly bound anion (e.g., fluoride, chloride, dihydrogen phosphate, etc.). In addition to anions, it was appreciated early on that calix[4]pyrroles could form complexes with small polar neutral guests; in such instances the partial cone 1,2-alternate conformations could be stabilized [17].
These inherent recognition features, coupled with the fact that calix[4]pyrroles are both simple to prepare and easy to modify synthetically, has led to a blossoming of calix[4]pyrrole-related research over the last 20 years or so. Indeed, at present a large number of calix[4]pyrrole derivatives are known. Collectively, they have seen use in ion sensing, extraction, transportation, as well as building blocks for the construction of chemical switches and logic gates [18], [19], [20]. Much of this work has been covered in recent reviews [17], [18], [19], [21], [22], [23]. Largely excluded from these prior reviews are so-called bis–calix[4]pyrrole systems, constructs that contain two parent calix[4]pyrroles subunits linked through one or more bridging “walls”. The present review is designed to summarize the chemistry and potential applications of these latter bis–calix[4]pyrrole systems.
Section snippets
Mono–walled bis–calix[4]pyrroles
In the absence of a bound anion, the parent form calix[4]pyrrole 1 exists primarily in the so-called 1,3-alternate conformation. However, at room temperature equilibration between forms is rapid in most organic media making the four pyrrolic NHs identical, as well as the eight β–pyrrolic CHs and eight meso methyl groups. Anion binding, typically mediated via pyrrole NH–anion hydrogen bonding interactions, generally serves to lock the calix[4]pyrrole into the symmetric cone conformation.
Double–walled bis–calix[4]pyrroles
Double–walled bis–calix[4]pyrroles are linked through two tethers, which serve to restrict the relative degrees of freedom. That is to say, the preorganization of bis–calix[4]pyrrole derivatives is typically higher than ostensibly similar mono–walled calix[4]pyrroles. In principle, there exist several ways to link two calix[4]pyrrole subunits. However, so far only bis–calix[4]pyrroles with double walls and high symmetry have been explored, although both covalent linkers and noncovalent bridges
Triple– and quadruple–walled bis–calix[4]pyrroles
As the number of “walls” serving to bridge two calix[4]pyrrole units increases, the degree of preorganization is expected to increase in a commensurate manner. It thus might be expected that both the inherent anion selectivity and the thermodynamics of binding for appropriately sized anionic substrates would be enhanced. However, the preparation of these systems is expected to be rather challenging. Perhaps as the result of the latter perceived limitation, to date only few examples of triple–
Conclusions and perspectives
Bis–calix[4]pyrroles may be defined as being calix[4]pyrrole derivatives that contain two calix[4]pyrrole units linked through one or more bridging “walls”. By considering the numbers of linkers involved, these “walled” bis–calix[4]pyrroles are organized into four classes: mono–walled bis–calix[4]pyrroles; double–walled bis–calix[4]pyrroles; triple–walled bis–calix[4]pyrroles; and quadruple–walled bis–calix[4]pyrroles. As a general rule, synthetic access to the targets becomes increasingly
CRediT authorship contribution statement
Zhenzhen Lai: Methodology, Writing - original draft, Software, Validation. Tian Zhao: Writing - original draft. Jonathan L. Sessler: Supervision, Writing - review & editing. Qing He: Conceptualization, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was supported by the National Natural Science Foundation of China (21901069 to Q.H.), the Special Funds for Hunan's Plan on Innovative Province Construction (2019RS1018 to Q.H.), and the Fundamental Research Funds for the Central Universities (Startup Funds to Q.H.). Further support for this work was provided by the U.S. Department of Energy, Office of Basic Energy Sciences under Award Number (DE–FG02–01ER15186 to J.L.S.) and the Robert A. Welch Foundation (F–0018 to J.L.S.).
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