Framework-isomerism:highly augmented copper(II) paddlewheel-based MOF with unusual (3,4)-net topology

The synthesis and structure of a new, highly augmented {Cu2} ‘paddlewheel’-based metal-organic framework (MOF) that is stabilized by tritopic benzoate ligands is reported. The structure adopts an uncommon, less-symmetrical (3,4)- connected net topology and represents a rare framework isomer of the extensively studied {Cu2}-based pto and tbo analogues. The concomitant formation of three isomeric forms using {Cu}2- ‘paddlewheel’ complexes and a single triangular, tri-functional ligand is unique and stems from the rotational flexibility of the benzoate moieties. Computational analyses support the structural studies and underline that this observed net topology can give rise to an exceptionally high surface area (>3500 m2/g), despite the triple-interpenetrated nature of the compound

Consecutive single-crystal-to-single-crystal isomerization of novel octamolybdate anions within a microporous hybrid framework with robust water sorption properties

The 3D hybrid framework [{Cu(cyclam)}3(kMo8O27)]· 14H2O (1) (cyclam=1,4,8,11-tetraazacyclotetradecane) undergoes sequential single-crystal-to-singlecrystal transformations upon heating to afford two different anhydrous phases (2 a and 3a). These transitions modify the framework dimensionality and enable the isomerization of k-octamolybdate (k-Mo8) anions into λ (2 a) and μ (3 a) forms through metal migration. Hydration of 3 a involves condensation of one water molecule to the cluster to afford the γ-Mo8 isomer in 4, which dehydrates back into 3a through the 6a intermediate. In contrast, 2a reversibly hydrates to form 5, exhibiting the same Mo8 cluster as that of 1. It is remarkable that three of the Mo8 clusters (k, λ and μ) are new and that up to three different microporous phases can be isolated from 1 (2 a, 3a, and 6a). Water vapor sorption analyses show high recyclability and the highest uptake values for POM-based systems. The isotherms display an abrupt step at low humidity level desirable for humidity control devices or water harvesting in drylands.Funded by Eusko Jaurlaritza/Gobierno Vasco (EJ/GV, grants IT1722-22 and KK-2022/00045). E.R.B. thanks EJ/GV for her doctoral fellowship (PRE_2018_1_0143)

Consecutive single-crystal-to-single-crystal isomerization of novel octamolybdate anions within a microporous hybrid framework with robust water sorption properties

The 3D hybrid framework [{Cu(cyclam)}3 (κ-Mo8 O27 )] ⋅ 14H2 O (1) (cyclam=1,4,8,11-tetraazacyclotetradecane) undergoes sequential single-crystal-to-single-crystal transformations upon heating to afford two different anhydrous phases (2 a and 3 a). These transitions modify the framework dimensionality and enable the isomerization of κ-octamolybdate (κ-Mo8 ) anions into λ (2 a) and μ (3 a) forms through metal migration. Hydration of 3 a involves condensation of one water molecule to the cluster to afford the γ-Mo8 isomer in 4, which dehydrates back into 3 a through the 6 a intermediate. In contrast, 2 a reversibly hydrates to form 5, exhibiting the same Mo8 cluster as that of 1. It is remarkable that three of the Mo8 clusters (κ, λ and μ) are new and that up to three different microporous phases can be isolated from 1 (2 a, 3 a, and 6 a). Water vapor sorption analyses show high recyclability and the highest uptake values for POM-based systems. The isotherms display an abrupt step at low humidity level desirable for humidity control devices or water harvesting in drylands.Funded by Eusko Jaurlaritza/Gobierno Vasco (EJ/GV, grants IT1722-22 and KK-2022/00045). E.R.B. thanks EJ/GV for her doctoral fellowship (PRE_2018_1_0143). Technical and human support from SGIker (UPV/EHU) is gratefully acknowledged.Peer reviewe

Flexibility in Coordination Polymers: Structure, Topology, Porosity, and Addressability

Metal-Organic Frameworks (MOFs) are a research area approaching maturity and the industrial application of MOFs to a number of pressing challenges is imminent. These materials are exciting for their porosity and chemical tunability, as well as the applicability of rational principles in their design. MOFs are applied in fields as diverse as gas storage and separation, drug delivery, catalysis, and sensing. The design of MOFs is guided by a few powerful general principles that form the basis of the approach known as reticular chemistry. The reticular approach emphasises symmetric structures obtained by the use of rigid organic components. In this thesis, a variety of non-rigid behaviours are examined, such as the accommodation of torsional strain in one- and two-dimensional coordination polymer motifs, the diversity of conformations accessible to ligands due to free rotations in alkyl-chain backbones or ethynyl spacers, rotational flexibility about metal-ligand bonds in assembled MOFs, and conformational variability about p-phenylene spacers in extended ligands. In the absence of perfectly rigid organic linkers, or in frameworks that allow a degree of internal motion, new and unusual topologies are obtained due to lower-symmetry conformations. These effects are combined with chemical functionality resulting in MOFs that respond to stimuli, such as light or moisture, with changes in structure that reversibly affect porosity. In chapter 1, chemical and historical contexts for the strategies adopted and results presented in this work are described. A brief history and description of the concepts used is provided, followed by a survey of the current literature in the field and the progress made in the various applied branches of MOF chemistry. The aims of the thesis are delineated. In chapter 2 mixed-ligand one- and two-dimensional coordination polymers based on various M2+ metal ions are described. Both ligands used are tripodal. The accommodation of varying M2+ ion radii takes place in 1-4 due to the ability of the ligands used to adopt increasingly strained conformations. This effect permits the recurrent formation of the same one-dimensional coordination polymer motif, and the same packing arrangement in two dimensions. However, supramolecular packing in the third dimension, mediated by aromatic interactions between distorted ligands, varies as a result of increasing ionic radius. The chelating ligand used in 1-4 is replaced with an isomeric capping ligand, and a two-dimensional sheet motif is obtained. This mixed ligand strategy is applied to ditopic N-donor ligands used in combination with ditopic organic and inorganic charged moieties in Chapter 3. Functionalised ligands are used as pillars in 3D structures which contain accessible 1D channels in 6 and 7. Prolonged and delicate crystallisations allowed the isolation of compounds based on N-donor ligands with highly flexible alkyl-chain backbones ? 8 and 9, which were both found to be two-dimensional, rather than the three-dimensional structures shown by homologues. 8, was shown to be intrinsically porous, and showed excellent CO2 uptake characteristics and selectivity, bringing the most valuable qualities of many 3D MOFs into two dimensions. The ditopic, bridging monodentate coordination mode adopted by the aromatic 4,4?-azopyridine ligand in 10 allows it to pivot about its axis in response to its surroundings. This led to the occurrence of a sharp transition upon the adsorption of 1.5 molecules of CO2 molecules per unit cell, after which the uptake of CO2 increased dramatically. 10 was shown to be selective for CO2 over N2, and the stimulus-responsive behaviour was shown to result in the highest room temperature CO2 working capacity between 0.1 bar and 1 bar recorded till date. A reversible transition also occurs between 10 and a hydrated phase 10′, which is utilised for the instantaneous release of adsorbed CO2 from 10. In Chapter 4, an elegant synthetic strategy is described, by which neutral, ditopic, N-donor ligands of appropriate length were incorporated into frameworks with the pto topology. This may be considered a mixed ditopic+tritopic ligand strategy. 11 and 12 were built by the incorporation of the photoresponsive 4,4?-azopyridine ligand into pto scaffolds built with highly extended, flexible ligands. As a result, the photoresponsivity of the 4,4?-azopyridine ligand is expressed through static and dynamic changes in the CO2 uptake of 11 and 12. Strong responses to irradiation ? changes of 40% of the magnitude of uptake under dynamic irradiation conditions ? are observed. 11 and 12 are the first reported MOFs with photoresponsive gas uptake in which photoswitching ligands are not the sole organic component. This synthetic strategy was also used to incorporate functionalised ditopic ligands into pto scaffolds in 13-16. Chapter 5 contains a description of novel MOFs based on the highly extended bteb3- and bbc3- ligands and analyses of their structures. Conformational flexibility due to the acetylene and p-phenylene spacers allows the adoption of otherwise inaccessible dihedral angles, and lower symmetry conformations. As a result, the frameworks in 17-23 form ?non-default? networks. In 19 and 23, 4,4?-azopyridine is used as an auxiliary ligand, and novel topologies are obtained. In Chapter 6, a number of MOFs (24-28) are described which were targeted for specific attributes using the bteb3- and bbc3- ligands. Simulations are carried out to show the potential porosities of these frameworks, and some exceptional attributes are observed. Chapter 7 describes the experimental details of the work carried out. Chapter 8 concludes the thesis and offers an outline of the research questions emerging from the results presented which may be addressed in future studies

Tuning the switching pressure in square lattice coordination networks by metal cation substitution

Coordination networks that undergo guest-induced switching between “closed” nonporous and “open” porous phases are of increasing interest as the resulting stepped sorption isotherms can offer exceptional working capacities for gas storage applications. For practical utility, the gate ad/desorption pressures (Pga/Pgd) must lie between the storage (Pst) and delivery (Pde) pressures and there must be fast switching kinetics. Herein we study the effect of metal cation substitution on the switching pressure of a family of square lattice coordination networks [M(4,4’-bipyridine)2(NCS)]n (sql-1-M-NCS, M = Fe, Co and Ni) with respect to CO2 sorption. The Clausius-Clapeyron equation was used to correlate Pga/Pgd and temperature. At 298 K, Pga/Pgd values were found to vary from 31.6/26.7 bar (M = Fe) to 26.7/20.9 bar (M = Co) and 18.5/14.6 bar (M = Ni). The switching event occurs within 10 minutes as verified by dynamic CO2 sorption tests. In addition, in situ synchrotron PXRD and molecular simulations provided structural insight into the observed switching event, which we attribute to layer expansion of sql-1-M-NCS via intercalation and inclusion of CO2 molecules. This study could pave the way for rational control over Pga/Pgd in switching adsorbent layered materials and enhance their potential utility in gas storage applications

Crystal engineering of porous coordination networks to enable separation of C2 hydrocarbons

Crystal engineering, the field of chemistry that studies the design, properties, and applications of crystals, is exemplified by the emergence over the past thirty years of porous coordination networks (PCNs), including metal-organic frameworks (MOFs) and hybrid coordination networks (HCNs). PCNs have now come of age thanks to their amenability to design from first principles and how this in turn can result in new materials with task-specific features. Herein, we focus upon how control over the pore chemistry and pore size of PCNs has been leveraged to create a new generation of physisorbents for efficient purification of light hydrocarbons (LHs). The impetus for this research comes from the need to address LH purification processes based upon cryogenic separation, distillation, chemisorption or solvent extraction, each of which is energy intensive. Adsorptive separation by physisorbents (in general) and PCNs (in particular) can offer two advantages over these existing approaches: improved energy efficiency; lower plant size/cost. Unfortunately, most existing physisorbents suffer from low uptake and/or poor sorbate selectivity and are therefore unsuitable for trace separations of LHs including the high volume C2 LHs (C2Hx, x = 2, 4, 6). This situation is rapidly changing thanks to PCN sorbents that have set new performance benchmarks for several C2 separations. Herein, we review and analyse PCN sorbents with respect to the supramolecular chemistry of sorbent-sorbate binding and detail the crystal engineering approaches that have enabled the exquisite control over pore size and pore chemistry that affords highly selective binding sites. Whereas the structure-function relationships that have emerged offer important design principles, several development roadblocks remain to be overcom

Crystal engineering of porous coordination networks for C3 hydrocarbon separation

C3 hydrocarbons (HCs), especially propylene and propane, are high‐volume products of the chemical industry as they are utilized for the production of fuels, polymers, and chemical commodities. Demand for C3 HCs as chemical building blocks is increasing but obtaining them in sufficient purity (>99.95%) for polymer and chemical processes requires economically and energetically costly methods such as cryogenic distillation. Adsorptive separations using porous coordination networks (PCNs) could offer an energy‐efficient alternative to current technologies for C3 HC purification because of the lower energy footprint of sorbent separations for recycling versus alternatives such as distillation, solvent extraction, and chemical transformation. In this review, we address how the structural modularity of porous PCNs makes them amenable to crystal engineering that in turn enables control over pore size, shape, and chemistry. We detail how control over pore structure has enabled PCN sorbents to offer benchmark performance for C3 separations thanks to several distinct mechanisms, each of which is highlighted. We also discuss the major challenges and opportunities that remain to be addressed before the commercial development of PCNs as advanced sorbents for C3 separation becomes viable

Pillar modularity in fsc topology hybrid ultramicroporous materials based upon tetra(4-pyridyl)benzene

.Hybrid ultramicroporous materials (HUMs) are porous coordination networks composed of combinations of organic and inorganic linker ligands with a pore diameter of 10 for 1:99 C2H2/C2H4 and >5 for 1:1 C2H2/CO2. The approach taken, systematic variation of pillars with retention of structure, enables differences in selectivity to be attributed directly to the choice of the inorganic pillar. This study introduces fsc topology HUMs as a modular platform that is amenable to fine-tuning of structure and properties</p

The Effect of pendent groups upon flexibility in coordination networks with square lattice topology

Gas or vapor-induced phase transformations in flexible coordination networks (CNs) offer the potential to exceed the performance of their rigid counterparts for separation and storage applications. However, whereas ligand modification has been used to alter the properties of such stimulus-responsive materials, they remain understudied compared with their rigid counterparts. Here, we report that a family of Zn2+ CNs with square lattice (sql) topology, differing only through the substituents attached to a linker, exhibit variable flexibility. Structural and CO2 sorption studies on the sql networks, [Zn(5-Ria)(bphy)]n, ia = isophthalic acid, bphy = 1,2-bis(pyridin-4-yl)hydrazine, R = −CH3, −OCH3, −C(CH3)3, -N-N-Ph, and -N-N-Ph(CH3)2, 2−6, respectively, revealed that the substituent moieties influenced both structural and gas sorption properties. Whereas 2−3 exhibited rigidity, 4, 5, and 6 exhibited reversible transformation from small pore to large pore phases. Overall, the insight into the profound effect of pendent moieties of linkers upon phase transformations in this family of layered CNs should be transferable to other CN classes.</p

Water vapour induced structural flexibility in a square lattice coordination network

Herein, we introduce a new square lattice topology coordination network, sql-(1,3-bib)(ndc)-Ni, with three types of connection and detail its gas and vapour induced phase transformations. Exposure to humidity resulted in an S-shaped isotherm profile, suggesting potential utility of such materials as desiccants.</p