Philip A. Chater

A Water‐Stable Porphyrin‐Based Metal–Organic Framework Active for Visible‐Light Photocatalysis

Metal–organic frameworks (MOFs) permit the combination of high internal surface area with chemical and physical functionality conferred by the molecular linker. Porphyrins are versatile functional molecules in catalysis, light harvesting, and molecular sensing. Porphyrins have been used as building blocks for MOFs, affording catalysts, light harvesting and selective sorption in liquid and gas phases. MOFs based on Alcarboxylate coordination chemistry are amongst the most thermally and chemically stable of such systems reported to date. Here we report a waterstable porous porphyrin MOF with a BET surface area of 1400 m g 1 which performs visible-lightdriven hydrogen generation from water. The freebase porphyrin can be metalated within the rigid host structure. The reaction of AlCl3·6 H2O with the free-base meso-tetra(4-carboxyl-phenyl) porphyrin H2TCPP (Figure 1b) in water under hydrothermal conditions at 180 8C followed by washing with dimethyl formamide (DMF) to remove unreacted ligand leads to the formation of the microcrystalline porous red compound H2TCPP[AlOH]2(DMF3(H2O)2) 1 (referred to as Al-PMOF, experimental details are given in section 1.1 in the Supporting Information). The linker consists of four benzoate groups around the central porphyrin core. The analyzed composition reveals that no aluminium is coordinated within the porphyrin ring, consistent with the need to use reactive trialkylaluminium reagents for metalation of the porphyrin in solution. The reaction temperature is required to solubilize the porphyrin linker. The crystal structure of 1 was solved and refined from synchrotron powder Xray diffraction collected at 100 K. Indexing and Pawley refinement revealed an orthorhombic cell (a = 31.978(3) , b = 6.5812(4) , c = 16.862(2) , V= 3548.7(6) ) consistent with the C222, Cmm2, and Cmmm space groups. Each of these candidate space groups was evaluated by simulated annealing using a semi-rigid body to describe the TCPP unit (Figure S1 in the Supporting Information) with eight refined parameters describing distances and angles within the porphyrin. The best results were obtained for the benzoic acid group perpendicular to the central porphyrin ring, which can be best described in Cmmm symmetry, and zero occupancy for Al at the center of the porphyrin. This model was used in the final Rietveld analysis (Figure 1a). Fourier mapping revealed a single guest atom in the channels attributed to oxygen from water, which was included in the final refinement (Figure S2 in the Supporting Information). Each porphyrin linker in 1 is coordinated to eight aluminium centers (Figure 1c–e) through the four carboxylate groups which each bridge two aluminium units. There is Figure 1. a) Final Rietveld refinement of 1 (100 K) showing observed (gray crosses), calculated (line a), and difference (line b) plots (Q = 2p/d). Bragg peak positions are indicated. b) TCPP porphyrinic linker in 1. c–e) Crystal structure of 1 viewed down [001], [100], and [010] directions, respectively.

Correlated defect nanoregions in a metal–organic framework

Throughout much of condensed matter science, correlated disorder is key to material function. While structural and compositional defects are known to exist within a variety of metal–organic frameworks, the prevailing understanding is that these defects are only ever included in a random manner. Here we show—using a combination of diffuse scattering, electron microscopy, anomalous X-ray scattering, and pair distribution function measurements—that correlations between defects can in fact be introduced and controlled within a hafnium terephthalate metal–organic framework. The nanoscale defect structures that emerge are an analogue of correlated Schottky vacancies in rocksalt-structured transition metal monoxides and have implications for storage, transport, optical and mechanical responses. Our results suggest how the diffraction behaviour of some metal–organic frameworks might be reinterpreted, and establish a strategy of exploiting correlated nanoscale disorder as a targetable and desirable motif in metal–organic framework design.

Side-chain control of porosity closure in single- and multiple-peptide-based porous materials by cooperative folding

Porous materials are attractive for separation and catalysis—these applications rely on selective interactions between host materials and guests. In metal–organic frameworks (MOFs), these interactions can be controlled through a flexible structural response to the presence of guests. Here we report a MOF that consists of glycyl–serine dipeptides coordinated to metal centres, and has a structure that evolves from a solvated porous state to a desolvated non-porous state as a result of ordered cooperative, displacive and conformational changes of the peptide. This behaviour is driven by hydrogen bonding that involves the side-chain hydroxyl groups of the serine. A similar cooperative closure (reminiscent of the folding of proteins) is also displayed with multipeptide solid solutions. For these, the combination of different sequences of amino acids controls the frameworks response to the presence of guests in a nonlinear way. This functional control can be compared to the effect of single-point mutations in proteins, in which exchange of single amino acids can radically alter structure and function. A family of dipeptide-based metal–organic frameworks has been shown to respond to the presence of guests in a cooperative manner controlled by one amino acid residue. When the linker features a serine residue, guest removal enables the formation of hydrogen bonds between the residues side-chains, causing a conformational change that closes the MOFs porous domain.

Synthesis and crystal structure of Li4BH4(NH2)3

The solid solution, (LiNH2)x(LiBH4)(1-x), formed through the reaction of the two potential hydrogen storage materials, LiNH2 and LiBH4, is dominated by a compound that has an ideal stoichiometry of Li4BN3H10 and forms a body-centred cubic structure with a lattice constant of ca. 10.66 A.

Shape Selectivity by Guest‐Driven Restructuring of a Porous Material

A flexible metal-organic framework selectively sorbs para- (pX) over meta-xylene (mX) by synergic restructuring around pX coupled with generation of unused void space upon mX loading. The nature of the structural change suggests more generally that flexible structures which are initially mismatched in terms of fit and capacity to the preferred guest are strong candidates for effective molecular separations.

Stable amorphous georgeite as a precursor to a high-activity catalyst

Copper and zinc form an important group of hydroxycarbonate minerals that include zincian malachite, aurichalcite, rosasite and the exceptionally rare and unstable—and hence little known and largely ignored—georgeite. The first three of these minerals are widely used as catalyst precursors for the industrially important methanol-synthesis and low-temperature water–gas shift (LTS) reactions, with the choice of precursor phase strongly influencing the activity of the final catalyst. The preferred phase is usually zincian malachite. This is prepared by a co-precipitation method that involves the transient formation of georgeite; with few exceptions it uses sodium carbonate as the carbonate source, but this also introduces sodium ions—a potential catalyst poison. Here we show that supercritical antisolvent (SAS) precipitation using carbon dioxide (refs 13, 14), a process that exploits the high diffusion rates and solvation power of supercritical carbon dioxide to rapidly expand and supersaturate solutions, can be used to prepare copper/zinc hydroxycarbonate precursors with low sodium content. These include stable georgeite, which we find to be a precursor to highly active methanol-synthesis and superior LTS catalysts. Our findings highlight the value of advanced synthesis methods in accessing unusual mineral phases, and show that there is room for exploring improvements to established industrial catalysts.

Hydrogen storage and ionic mobility in amide–halide systems

We report the results of a systematic study of the effect of halides on hydrogen release and uptake in lithium amide and lithium imide, respectively. The reaction of lithium amide and lithium imide with lithium or magnesium chloride, bromide and iodide resulted in a series of amide-halide and imide-halide phases, only two of which have been reported previously. On heating with LiH or MgH2, the amide-halides synthesised all released hydrogen more rapidly than lithium amide itself, accompanied by much reduced, or in some cases undetectable, release of ammonia by-product. The imide-halides produced were found to hydrogenate more rapidly than lithium imide, reforming related amide-halide phases. The work was initiated to test the hypothesis that the incorporation of halide anions might improve the lithium ion conductivity of lithium amide and help maintain high lithium ion mobility at all stages of the de/rehydrogenation process, enhancing the bulk hydrogen storage properties of the system. Preliminary ionic conductivity measurements indicated that the most conducting amide- and imide-halide phases were also the quickest to release hydrogen on heating and to hydrogenate. We conclude that ionic conductivity may be an important parameter in optimising the materials properties of this and other hydrogen storage systems.

Computationally Assisted Identification of Functional Inorganic Materials

Modules of Desire Using computational methods to design materials with specific properties has found some limited success. Dyer et al. (p. 847, published online 11 April) have devised a method, based on extended module materials assembly, that combines chemical intuition and ab initio calculations starting from fragments or modules of structure types that show the desired functionality. The method was tested by identifying materials suitable for a solid oxide fuel cell cathode. A method using extended building blocks is developed for computationally viable predictions of stable crystal structures. The design of complex inorganic materials is a challenge because of the diversity of their potential structures. We present a method for the computational identification of materials containing multiple atom types in multiple geometries by ranking candidate structures assembled from extended modules containing chemically realistic atomic environments. Many existing functional materials can be described in this way, and their properties are often determined by the chemistry and electronic structure of their constituent modules. To demonstrate the approach, we isolated the oxide Y2.24Ba2.28Ca3.48Fe7.44Cu0.56O21, with a largest unit cell dimension of over 60 angstroms and 148 atoms in the unit cell, by using a combination of this method and experimental work and show that it has the properties necessary to function as a solid oxide fuel-cell cathode.

Nanoscale structural heterogeneity in Ni-rich half-Heusler TiNiSn

The structural implications of excess Ni in the TiNiSn half-Heusler compound are examined through a combination of synchrotron x-ray and neutron scattering studies, in conjunction with first principles density functional theory calculations on supercells. Despite the phase diagram suggesting that TiNiSn is a line compound with no solid solution, for small x in TiNi1+xSn there is indeed an appearance—from careful analysis of the scattering—of some solubility, with the excess Ni occupying the interstitial tetrahedral site in the half-Heusler structure. The analysis performed here would point to the excess Ni not being statistically distributed, but rather occurring as coherent nanoclusters. First principles calculations of energetics, carried out using supercells, support a scenario of Ni interstitials clustering, rather than a statistical distribution.

Single sublattice endotaxial phase separation driven by charge frustration in a complex oxide.

Complex transition-metal oxides are important functional materials in areas such as energy and information storage. The cubic ABO3 perovskite is an archetypal example of this class, formed by the occupation of small octahedral B-sites within an AO3 network defined by larger A cations. We show that introduction of chemically mismatched octahedral cations into a cubic perovskite oxide parent phase modifies structure and composition beyond the unit cell length scale on the B sublattice alone. This affords an endotaxial nanocomposite of two cubic perovskite phases with distinct properties. These locally B-site cation-ordered and -disordered phases share a single AO3 network and have enhanced stability against the formation of a competing hexagonal structure over the single-phase parent. Synergic integration of the distinct properties of these phases by the coherent interfaces of the composite produces solid oxide fuel cell cathode performance superior to that expected from the component phases in isolation.