On/off porosity switching and post-assembly modifications of Cu₄L₄ metal–organic polyhedra

Abstract

Synthetic porous materials composed of metal-organic polyhedra (MOPs) have found application in topical areas such as gas storage, separation and catalysis. Control over their physical properties (e.g. porosity) has typically been achieved through ligand design or judicious choice of metal ions. Here, we demonstrate pore-size control and on/off porosity in Cu 4 L 4 MOPs by exploiting their structural non-rigidity. We report an aldehyde-functionalised MOP (1) that can be isolated in five distinct solvatomorphs, each exhibiting different structural flexibility. When soaked in MeOH, two of these solvatomorphs undergo a rapid transformation to a thermodynamically favoured phase, whilst in acetone they template the crystallisation of an entirely new crystal packing. We support these findings by single and powder X-ray diffraction and rationalise the observed phase transformations by lattice energy calculations. Of the five solvatomorphs, three can be obtained as solvent-exchanged pseudo-polymorphs with distinct porosities in their activated form (SA BET ¼ 35-455 m 2 g À1 ). Further control over the crystal packing of MOPs is achieved through covalent post-assembly modifications, which promote the crystallisation of isoreticular 2-D sheet-like structures.

Metal‐Organic Frameworks and Metal‐Organic Cages – A Perspective

“…Structural transformations of MOCs can be induced by many factors, as recently reviewed. [102] Structures can adapt as a result of changes in solvent, [103] concentration, [104] temperature, [105] light, [106] and guest binding. [68] The postassembly modification of MOCs via new covalent bond formation is a particularly active area.…”

Assembly and Covalent Cross-Linking of an Amine-Functionalised Metal-Organic Cage

“…Owing to solubility of 1 in DMF and DMA, we explored whether other crystallisation conditions can lead to a different crystal-packing through solvatomorphism, ( Bloch et al, 2020 ; Deegan et al, 2021 ). Indeed, slow vapour diffusion of diisopropyl ether (DiPE) into a DMA solution of the cage yielded crystals of 2 after 4 days.…”

“…In our previous work, we found that post-assembly imine condensations can be carried out on Cu 4 L 4 MOCs using a Sc(OTf) 3 catalyst under mild conditions. ( Bloch et al, 2020 ; Matsumoto et al, 2017 ). Thus, we employed the same conditions for the covalent cross-linking of 1 .…”

“…In this context, a particularly accessible class of MOCs, based on di-carboxylate ligands and M 2 paddlewheel nodes (M = Cu 2+ , Rh 2+ Mo 2+ and Cr 2+ ), has received significant scientific attention. Recent studies have demonstrated several impressive properties for MOCs based on M 2 paddlewheel nodes, including tunable porosity ( Gosselin et al, 2020 ), solvatomorphism ( Craig et al, 2018 ; Bloch et al, 2020 ), hydrolytic stability ( Lal et al, 2019a ) ( Mollick et al, 2019 ), and transport capabilities ( Grancha et al, 2019 ).…”

“…Advances in this area have been made possible by fundamental studies into the compatibility of M 2 paddlewheel-based MOCs with various surface functionalities. These include coordinatively competitive groups such as hydroxyl ( Niu et al, 2015 ), carboxylate ( Albalad et al, 2019 ), carbonyl ( Bloch et al, 2020 ), and amine substituents ( Albalad et al, 2019 ; Schneider et al, 2020 ; Taggart et al, 2020 ) which can be installed onto the MOC exterior through ligand design or post-assembly modifications ( Zeng et al, 2020 ). Of these functionalities, amine moieties offer considerable synthetic versatility for post-assembly reactions, owing to their tunable basicity and nucleophilicity.…”