Transformation networks of metal-organic cages controlled by chemical stimuli

Abstract

The flexibility of biomolecules enables them to adapt and transform as a result of signals received from the external environment, expressing different functions in different contexts. In similar fashion, coordination cages can undergo stimuli-triggered transformations owing to the dynamic nature of the metal-ligand bonds that hold them together. Different types of stimuli can trigger dynamic reconfiguration of these metal-organic assemblies, to switch on or off desired functionalities. Such adaptable systems are of interest for applications in switchable catalysis, selective molecular recognition or as transformable materials. This review highlights recent advances in the transformation of cages using chemical stimuli, providing a catalogue of reported strategies to transform cages and thus allow the creation of new architectures. Firstly we focus on strategies for transformation through the introduction of new cage components, which trigger reconstitution of the initial set of components. Secondly we summarize conversions triggered by external stimuli such as guests, concentration, solvent or pH, highlighting the adaptation processes that coordination cages can undergo. Finally, systems capable of responding to multiple stimuli are described. Such systems constitute composite chemical networks with the potential for more complex behaviour. We aim to offer new perspectives on how to design transformation networks, in order to shed light on signal-driven transformation processes that lead to the preparation of new functional metal-organic architectures.

Fig. 1. Examples of cage transformations triggered by ligand exchange. (a) A network of interconverting Pd^II₂L₄ cages driven by the binding hierarchy of the ligands to the Pd^II centres. (b) Formation of cage 12 from the double-layered ‘pregnant molecular nanoball’ cage 10. Adapted from ref. with permission from American Chemical Society, copyright 2021. (c) Transformation between Mukherjee's cages 13–15, attributed to enthalpic factors. (d) Chiral memory observed upon exchange of the stereochemically fixed ancillary ligand 21 with the more labile 22 to transform cage 19 to 20.

Fig. 2. (a) Heteroleptic cage 25 assembled via dimerization of two equivalents of cage 23 upon addition of ligand 24. (b) Transformation of cage 26 to homoleptic 27 and heteroleptic 28via ligand displacement involving a more electron-rich ligand.

Fig. 3. Network of interconverting structures 32–39, with transformations driven by electronic effects and relief of steric hindrance.

Fig. 4. Examples of cage transformation triggered by subcomponent exchange. (a) Exchange of bulky 4-methoxybenzylamine by methylamine leads to the transformation of cage 46 into cage 47. (b) An enantiopure ΔΔΔΔ cage 51 is formed by exchange of a chiral amine by achiral tren through a stereochemically retentive pathway. (c) Aldehyde exchange transforms high-spin 53 into low-spin 54, with spin-state switching a result of the release of steric crowding around the iron(ii) metal centres.

Fig. 5. Examples of metal-ion induced cage-to-cage transformations. (a) Addition of a Pd^II salt to cage 58 promoted coordination of its free pyridyl arms to the Pd^II centres, thus forming stellated cuboctahedron 59 with enclosed faces. (b) Addition of Rh^III transforms macrocycle 60 into cage 61. (c) Transmetallation allows the formation of a series of Ln^III₈L₆ (Ln^III = Pr^III, Nd^III or Eu^III) cubes, 63 and Yb^III₈L₆ cube 64, which could not be formed via direct metal–ligand assembly. (d) Two Co^II₅L₂ cages 67 were formed from Cu^I₁₀L₄ cage 66via displacement of the Cu^I ions and 2-formyl-6-methylpyridine by Co^II ions and 2-formylphenanthroline. (e) The replacement of tripalladium (Tr₂Pd^II₃) by triplatinum (Tr₂Pt^II₃) clusters drove the conversion of 69 to the intermediate (Tr₂Pd^II₃)(Tr₂Pt^II₃)L₃70 and final triple helicate (Tr₂Pt^II₃)₂L₃ cage 71.

Fig. 6. Heteroleptic cages 76, 79 and 80 formed via cage fusion of the corresponding homoleptic cages and subsequent integrative self-sorting of ligands. Thermodynamically stable heteroleptic 76 is favoured when cages 79 and 80 are mixed in the presence of catalytic Cl⁻, due to complementarity between the ligand binding angles. Adapted from ref. with permission from John Wiley and Sons, copyright 2021.

Fig. 7. Examples of cage-to-cage transformations occurring via cage fusion reported by the Clever group. (a) Reaction of homoleptic dinuclear Pd^II₂85₂ cage 81 with a mixture of Pd^II₃86₆ and Pd^II₄86₈ cages 82 and 83 led to the formation of heteroleptic pseudo-tetrahedron 84. (b) The selective formation of heteroleptic Pd^II₂91₂92₂ cage 90 is dictated by the steric hindrance of the ortho and para methyl substituents on the pyridyl rings of ligands 92 and 91, positioned inside and outside with respect to the cage cavity. (c) Unprecedented heteroleptic Pd^II₄96₄97₄ cage 95 formed from mixing Pd^II₆96₁₂ cage 93 with Pd^II₃97₆ triangular ring 94.

Fig. 8. Examples of cage-to-cage transformations occurring via cage fusion. (a) Our triple-decker cage 102 formed through fusion of two Zn^II₄L₆ tetrahedral cages 99 and 101. (b) Yan's heteroleptic cage 107 formed through the fusion of trigonal prism 104 and macrocycle 106.

Fig. 9. Chand's multi-cavity cages Pd^II₄108₂109₄, 112 and Pd^II₅109₄110₂, 115, formed through fusion of cage 113 with either cage 111 or 114, respectively.

Fig. 10. Anion driven conversion between ring 116 and cage 117. Nitrate anions act as templates, driving the formation of the smaller Pd^II₂118₄ cage.

Fig. 11. Examples of anion-driven transformations producing interlocked cages. (a) Anion templation allows a mixture of assemblies to be driven towards a single interlocked Pd^II₈L₁₆ cage 121. Alkyl chains in the X-ray structure of 121 are omitted for clarity. (b) The double-cage 123 was transformed into a highly interpenetrated architecture by stoichiometric addition of chloride, affording new species 124 with five consecutive cavities. Alkyl chains in the X-ray structure of 124 are omitted for clarity. (c) Interpenetrated metal–organic cage 127 is formed via subcomponent self-assembly and templation by ClO₄⁻ or BF₄⁻ anions. The use of labile Zn^II plays a crucial role in allowing dynamic reconfiguration of the components. Only the centrally bound BF₄⁻ anion is shown in the structure of 127.

Fig. 12. Cage-to-cage transformation through crystallisation in the presence of 2,7-naphthalene disulfonate anions. Anions do not play a templating role but act instead as bridges between the Pd^II centres in the crystal lattice.

Fig. 13. Examples of anion-driven cage-to-cage transformations. (a) Anion-induced formation of supramolecular helicate hexamer 134. At very high concentrations, helicate 133 aggregates into superstructure 134 where anions play the role of templates, holding the six building blocks together via hydrogen-bonding interactions. (b) Transformation network of cages, where all transformations are driven by anion metathesis. The induced-fit phenomenon drives reconfiguration of the assemblies towards the most stable host–guest complex 137. (c) A library of cages obtained by subcomponent self-assembly collapsed to produce uniquely species 142 following introduction of BF₄⁻.

Fig. 14. (a) Mixing tetrahedron 146 and cube 148 gives rise to the formation of triangular prism 149, templated by the anionic guest cobalticarborate (CoC₄B₁₈H₂₂⁻). (b) Self-assembly of a library of up to four diastereomeric trigonal prismatic cages 154 and guest induced reconfiguration to form a single diastereomer upon addition of the pesticide Mirex. The crystal structure of cage 154 (crystallised in the absence of a guest) where all the pyrene ligands adopt the L orientation is depicted.

Fig. 15. Examples of fullerene induced cage-to-cage transformations. (a) Mixing cages 156 and 158 results in the formation of a library of Pd^II₂L₄ assemblies, which is driven towards unique host–guest architecture 159 by the introduction of C₆₀. (b) Double cage 161 loses a Pd^II centre upon encapsulation of two fullerenes in order to optimise binding in 162. (c) Both cages 163 and 164 were transformed into 165 upon C₆₀ encapsulation, to maximise interactions between the guest molecules and the walls of the cage.

Fig. 16. Examples of cage-to-cage tranformations induced by neutral guests in aqueous solution. (a) Binding of neutral guests induced an expansion of capsule 167 in order to maximise host–guest interactions, thus transforming 167 into 168. (b) In similar fashion, the binding of three molecules of methyl(4-nitrophenyl)sulfane triggered the transformation of cage 170 into bowl-shaped 171. This process reverses when the guest molecules were extracted from the cavity. (c) The tetramerization of the trialkoxysilane guest within the cavity of 173 induced the transformation of this host into new species 174. (d) Cage 176 transformed into double cage 177 when the guest underwent a self-coupling dimerization after encapsulation.

Fig. 17. Anion-driven reconfiguration of a new class of metal–organic assemblies: polyhedral links. Component flexibility, as well as secondary π-coordination between the alkyne linkers of the ligand and the metals, allow the formation of highly entangled architectures. Strong templation effects were shown to drive the transformation between the different architectures.

Fig. 18. Concentration driven transformations. (a) Three architectures based on crown-ether ligand 188 and Zn^II interconvert depending on concentration. (b) In a similar manner, the reaction of triptycene ligand 192 and Cd^II produces three transformable species.

Fig. 19. Ward and co-workers also demonstrated that concentration and hydrophobic effects drive cage transformations among a series of Co^II_2n196_3n architectures.

Fig. 20. Concentration dependent formation of a Zn^II₈198₆ cube-like assembly 199 and unprecedented Zn^II₁₆198₁₂ structure 200 with fac Zn^II centres are shown in orange and mer Zn^II in yellow. Structure 199 converts into 200 after heating.

Fig. 21. Examples of solvent-induced transformations. (a) Reversible cage-to-macrocycle transformation induced by a switch between CHCl₃ and CH₂Cl₂. (b) Cage 204 and sandwich-like architecture 205 interconvert by switching the solvent from MeCN to NO₂Me. (c) Interconversion between ‘peanut’ cage 207 and butterfly complex 208, driven by changing between DMSO and a mixture of MeCN/H₂O.

Fig. 22. Examples of solvent-influenced transformations during crystallisation. (a) Kinetically trapped prism 212 is obtained through crystallisation of a solution of 211 or by modification of the reaction conditions. Dissolution of this assembly in water led to the recovery of tetrahedral cage 211. (b) In a similar manner, giant assembly 215 was obtained from a solution of 214 after crystallization. However, after a long period in solution, crystals of 215 were observed to transform back into trigonal prism 214. (c) Crystallization allowed the selection of specific isomers from a library of cages depending on the conditions used, and introduction of pyridine induced transformation of the cages into macrocycle 220.

Fig. 23. Acid-driven transformation between assemblies based on basicity and donor strength of the ligands of the system.

Fig. 24. Interconversion between tetrahedron 232 and bowl-shaped 233 triggered by five distinct stimuli.

Fig. 25. Stepwise self-assembly and structural transformations between heterobimetallic cages 236–239.

Fig. 26. Anion- and metal-ion directed structure interconversion pathways in a network. Adapted from ref. with permission from American Chemical Society, copyright 2021.

Fig. 27. Transformations between pseudo-icosahedron 252, helicate 251, and tetrahedron 253.

Fig. 28. Transformation pathways employing combinations of (R)-254, (S)-254, 2-formylpyridine 55, 2-formylphenanthroline 68, and Zn^II. All reactions occurred in MeCN unless otherwise indicated. L^S and L^R denote ligands derived from (S)-254 and (R)-254 respectively. Adapted with permission. Adapted from ref. with permission from American Chemical Society, copyright 2021.

Fig. 29. Self-assembly and multi-stimulus-responsive transformations between Pd₁₂262₆ cage 259 and the topologically isomeric Pd₆262₃ cages 260 and 261. The crystal structure of 259 is shown. Adapted with permission. Adapted from ref. with permission from American Chemical Society, copyright 2021.

Fig. 30. (a) Self-assembly and reversible multi-stimuli responsive transformations between monomeric cage 263 and [2]catenane 264. (b) Crystal structure of cyclic bis[2]catenane cage 268.

Fig. 31. Transformation pathways in a network starting from tetrazine-edged Fe^II₄L₆ tetrahedal cage 269, showing the major products expressed by the system following the addition of different combinations of three stimuli. The crystal structures of 269 and (PF₆⁻)₉⊂274 are shown.

Clockwise from the top left: Elie Benchimol, Jonathan R. Nitschke, Bao-Nguyen T. Nguyen, and Tanya K. Ronson