Chemical control of structure and guest uptake by a conformationally mobile porous material

Abstract

Metal-organic frameworks (MOFs) are crystalline synthetic porous materials formed by binding organic linkers to metal nodes: they can be either rigid 1,2 or flexible. 3 Zeolites and rigid MOFs have widespread applications in sorption, separation and catalysis that arise from their ability to control the arrangement and chemistry of guests in their pores via the shape and functionality of the internal surface defined by their chemistry and structure. 4,5 Their structures correspond to an energy landscape with a single, albeit highly functional, energy minimum. In contrast, proteins function by navigating between multiple metastable structures using bond rotations of the polypeptide, 6,7 where each structure lies in one of the minima of a conformational energy landscape and can be selected according to the chemistry of the molecules interacting with the protein. These structural changes are realised through the mechanisms of conformational selection (where a higher energy minimum characteristic of the protein is stabilised by small molecule binding), and induced fit (where a small molecule imposes a structure on the protein that is not a minimum in the absence of that molecule). 8 Here we show that rotation about covalent bonds in a peptide linker can change a flexible MOF to afford nine distinct crystal structures, revealing a conformational energy landscape characterised by multiple structural minima. The uptake of small molecule guests by the MOF can be chemically triggered by inducing peptide conformational change. This change transforms the material from a minimum on the landscape that is inactive for guest sorption to an active one. Chemical control of the conformation of a flexible organic linker offers a route to modify the pore geometry and internal surface chemistry and thus the function of open-framework materials. Flexible MOF structures 9,10 can be rearranged in the presence of guests through mechanical mechanisms such as the repositioning of a rigid linker about an inorganic unit 11-13 or the relative displacement of two rigid networks, 14 opening a range of routes to control function 15 that are not accessible to rigid frameworks with their single structural minimum (Figure 1). Similar phenomena have been observed in the host-guest chemistry of interlocked cage molecules. 16-18 Alternatively, rotations about bonds involving sp 3 carbons 19-25 allow MOF to access different structures. For example, low energy conformational changes of dipeptide Gly-X linkers produce open and closed forms of Zn(Gly-X)2 frameworks. 26,27 The greater chemical diversity and more complex conformational space of higher order oligopeptides offer MOF with multiple open structures (Figure 1). This could allow interaction with molecules in the pores to select a specific structure for a defined function from the resulting energy landscape. That structure would be accessed through the single bond rotation pathway used by proteins (Figure 1). The tripeptide glycine-glycine-L-histidine (GGH) affords a three-dimensional chiral MOF Zn...

Stepwise Evolution of Molecular Nanoaggregates Inside the Pores of a Highly Flexible Metal–Organic Framework

“…Eugenol-f1,-f2 and eugenole underwent ar emarkable change in their final position:t he first has moved closer to the amide group of the a-net (eugenol-F-bpba 3.00 )replacing DMF-II,the latter is now interacting via short C À O À HC interaction with the bpbc ligand of the a-net (eugenol-E-bdpc 3.22 ) ( Figure 4c). Concomitantly to the guests rearrangement, the framework become remarkably distorted [50] (paddlewheel SBUsd isplay bpba-centroid-bdpc angular values of 83.298 8 (c-net) and 95.368 8 (anet)), and the triple interpenetrated nets reciprocally slide apart (paddle wheel SBUsd istances increase from 7.79 to 12.08 )t ob etter accommodate the guest molecules (Figure 5).…”

Stepwise Evolution of Molecular Nanoaggregates Inside the Pores of a Highly Flexible Metal–Organic Framework

“…Eugenol‐ f1 , ‐ f2 and eugenol‐ e underwent a remarkable change in their final position: the first has moved closer to the amide group of the a‐net (eugenol‐ F –bpba 3.00 Å) replacing DMF‐ II , the latter is now interacting via short C−O−HC interaction with the bpbc ligand of the a‐net (eugenol‐ E –bdpc 3.22 Å) (Figure c). Concomitantly to the guests rearrangement, the framework become remarkably distorted (paddlewheel SBUs display bpba‐centroid‐bdpc angular values of 83.29° (c‐net) and 95.36° (a‐net)), and the triple interpenetrated nets reciprocally slide apart (paddle wheel SBUs distances increase from 7.79 to 12.08 Å) to better accommodate the guest molecules (Figure ).…”

An Allosteric Metal–Organic Framework That Exhibits Multiple Pore Configurations for the Optimization of Hydrocarbon Separation

“…Recently, unprecedented multiple energy minima (up to 9 phases) were achieved upon inclusion of solvents. This has propelled MOF chemistry into a higher level of complexity resembling that of biological systems (Scheme b), albeit with higher energy barriers (Scheme c) . Achieving multiple stable states and sufficiently high energy barriers could provide new dimensions of structural tuneability by traversing the energy landscapes and optimizing the gas‐separation properties by suppressing the structure transformation during adsorption.…”