Marc A. Little

Separation of rare gases and chiral molecules by selective binding in porous organic cages

The separation of molecules with similar size and shape is an important technological challenge. For example, rare gases can pose either an economic opportunity or an environmental hazard and there is a need to separate these spherical molecules selectively at low concentrations in air. Likewise, chiral molecules are important building blocks for pharmaceuticals, but chiral enantiomers, by definition, have identical size and shape, and their separation can be challenging. Here we show that a porous organic cage molecule has unprecedented performance in the solid state for the separation of rare gases, such as krypton and xenon. The selectivity arises from a precise size match between the rare gas and the organic cage cavity, as predicted by molecular simulations. Breakthrough experiments demonstrate real practical potential for the separation of krypton, xenon and radon from air at concentrations of only a few parts per million. We also demonstrate selective binding of chiral organic molecules such as 1-phenylethanol, suggesting applications in enantioselective separation.

Functional materials discovery using energy–structure–function maps

Molecular crystals cannot be designed in the same manner as macroscopic objects, because they do not assemble according to simple, intuitive rules. Their structures result from the balance of many weak interactions, rather than from the strong and predictable bonding patterns found in metal–organic frameworks and covalent organic frameworks. Hence, design strategies that assume a topology or other structural blueprint will often fail. Here we combine computational crystal structure prediction and property prediction to build energy–structure–function maps that describe the possible structures and properties that are available to a candidate molecule. Using these maps, we identify a highly porous solid, which has the lowest density reported for a molecular crystal so far. Both the structure of the crystal and its physical properties, such as methane storage capacity and guest-molecule selectivity, are predicted using the molecular structure as the only input. More generally, energy–structure–function maps could be used to guide the experimental discovery of materials with any target function that can be calculated from predicted crystal structures, such as electronic structure or mechanical properties.

Porous Organic Cages for Sulfur Hexafluoride Separation

A series of porous organic cages is examined for the selective adsorption of sulfur hexafluoride (SF6) over nitrogen. Despite lacking any metal sites, a porous cage, CC3, shows the highest SF6/N2 selectivity reported for any material at ambient temperature and pressure, which translates to real separations in a gas breakthrough column. The SF6 uptake of these materials is considerably higher than would be expected from the static pore structures. The location of SF6 within these materials is elucidated by X-ray crystallography, and it is shown that cooperative diffusion and structural rearrangements in these molecular crystals can rationalize their superior SF6/N2 selectivity.

Acid- and Base-Stable Porous Organic Cages: Shape Persistence and pH Stability via Post-synthetic “Tying” of a Flexible Amine Cage

Imine cage molecules can be reduced to amines to improve their chemical stability, but this introduces molecular flexibility. Hence, amine cages tend not to exhibit permanent solid-state porosity. We report a synthetic strategy to achieve shape persistence in amine cages by tying the cage vertices with carbonyls such as formaldehyde. Shape persistence is predicted by conformer stability calculations, providing a design basis for the strategy. The tied cages show enhanced porosity and unprecedented chemical stability toward acidic and basic conditions (pH 1.7-12.3), where many other porous crystalline solids would fail.

Controlling the Crystallization of Porous Organic Cages: Molecular Analogs of Isoreticular Frameworks Using Shape-Specific Directing Solvents

Small structural changes in organic molecules can have a large influence on solid-state crystal packing, and this often thwarts attempts to produce isostructural series of crystalline solids. For metal-organic frameworks and covalent organic frameworks, this has been addressed by using strong, directional intermolecular bonding to create families of isoreticular solids. Here, we show that an organic directing solvent, 1,4-dioxane, has a dominant effect on the lattice energy for a series of organic cage molecules. Inclusion of dioxane directs the crystal packing for these cages away from their lowest-energy polymorphs to form isostructural, 3-dimensional diamondoid pore channels. This is a unique function of the size, chemical function, and geometry of 1,4-dioxane, and hence, a noncovalent auxiliary interaction assumes the role of directional coordination bonding or covalent bonding in extended crystalline frameworks. For a new cage, CC13, a dual, interpenetrating pore structure is formed that doubles the gas uptake and the surface area in the resulting dioxane-directed crystals.

A Unified Treatment of the Relationship Between Ligand Substituents and Spin State in a Family of Iron(II) Complexes

Abstract The influence of ligands on the spin state of a metal ion is of central importance for bioinorganic chemistry, and the production of base‐metal catalysts for synthesis applications. Complexes derived from [Fe(bpp)2]2+ (bpp=2,6‐di{pyrazol‐1‐yl}pyridine) can be high‐spin, low‐spin, or spin‐crossover (SCO) active depending on the ligand substituents. Plots of the SCO midpoint temperature (T 1/2 ) in solution vs. the relevant Hammett parameter show that the low‐spin state of the complex is stabilized by electron‐withdrawing pyridyl (“X”) substituents, but also by electron‐donating pyrazolyl (“Y”) substituents. Moreover, when a subset of complexes with halogeno X or Y substituents is considered, the two sets of compounds instead show identical trends of a small reduction in T 1/2 for increasing substituent electronegativity. DFT calculations reproduce these disparate trends, which arise from competing influences of pyridyl and pyrazolyl ligand substituents on Fe‐L σ and π bonding.

Styrene Purification by Guest-Induced Restructuring of Pillar[6]arene

The separation of styrene (St) and ethylbenzene (EB) mixtures is important in the chemical industry. Here, we explore the St and EB adsorption selectivity of two pillar-shaped macrocyclic pillar[n]arenes (EtP5 and EtP6; n = 5 and 6). Both crystalline and amorphous EtP6 can capture St from a St-EB mixture with remarkably high selectivity. We show that EtP6 can be used to separate St from a 50:50 v/v St:EB mixture, yielding in a single adsorption cycle St with a purity of >99%. Single-crystal structures, powder X-ray diffraction patterns, and molecular simulations all suggest that this selectivity is due to a guest-induced structural change in EtP6 rather than a simple cavity/pore size effect. This restructuring means that the material “self-heals” upon each recrystallization, and St separation can be carried out over multiple cycles with no loss of performance.

Predicted crystal energy landscapes of porous organic cages

In principle, the development of computational methods for structure and property prediction offers the potential for the in silico design of functional materials. Here, we evaluate the crystal energy landscapes of a series of porous organic cages, for which small changes in chemical structure lead to completely different crystal packing arrangements and, hence, porosity. The differences in crystal packing are not intuitively obvious from the molecular structure, and hence qualitative approaches to crystal engineering have limited scope for designing new materials. We find that the crystal structures and the resulting porosity of these molecular crystals can generally be predicted in silico, such that computational screening of similar compounds should be possible. The computational predictability of organic cage crystal packing is demonstrated by the subsequent discovery, during screening of crystallisation conditions, of the lowest energy predicted structure for one of the cages.

Trapping virtual pores by crystal retro-engineering

Stable guest-free porous molecular crystals are uncommon. By contrast, organic molecular crystals with guest-occupied cavities are frequently observed, but these cavities tend to be unstable and collapse on removal of the guests—this feature has been referred to as ‘virtual porosity’. Here, we show how we have trapped the virtual porosity in an unstable low-density organic molecular crystal by introducing a second molecule that matches the size and shape of the unstable voids. We call this strategy ‘retro-engineering’ because it parallels organic retrosynthetic analysis, and it allows the metastable two-dimensional hexagonal pore structure in an organic solvate to be trapped in a binary cocrystal. Unlike the crystal with virtual porosity, the cocrystal material remains single crystalline and porous after removal of guests by heating. Organic molecular crystals with guest-occupied cavities are often observed, but the cavities tend to collapse when the guests are removed. Now, the porous domain of a crystalline solvate has been stabilized by formation of a cocrystal with a second molecule whose size and shape matches those of the unstable voids.

Ag(I) Organometallic Coordination Polymers and Capsule with Tris-Allyl Cyclotriveratrylene Derivatives

Tris-allyl-cyclotriguaiacylene combines with silver salts to give a range of crystalline network structures and one discrete complex. A number of different coordination modes are found within the complexes including η(2)-allyl, aryl, and OR groups binding to Ag(I). AgSbF(6) gives two types of three-dimensional (3-D) coordination polymer with unusual topologies, along with a discrete Ag(2)L(2) capsule dependent on reaction stoichiometry and reaction conditions. Isostructural coordination chain structures are found with AgBF(4) and AgClO(4), while use of Ag(CF(3)SO(3)) gives two-dimensional (2-D) and 3-D coordination polymers through bridging triflate anions.