Matthew J. Cliffe

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.

PASCal: a principal axis strain calculator for thermal expansion and compressibility determination

This article describes a web-based tool (PASCal; principal axis strain calculator; http://pascal.chem.ox.ac.uk) designed to simplify the determination of principal coefficients of thermal expansion and compressibilities from variable-temperature and variable-pressure lattice parameter data. In a series of three case studies, PASCal is used to reanalyse previously published lattice parameter data and show that additional scientific insight is obtainable in each case. First, the two-dimensional metal–organic framework [Cu2(OH)(C8H3O7S)(H2O)]·2H2O is found to exhibit the strongest area negative thermal expansion (NTE) effect yet observed; second, the widely used explosive HMX exhibits much stronger mechanical anisotropy than had previously been anticipated, including uniaxial NTE driven by thermal changes in molecular conformation; and third, the high-pressure form of the mineral malayaite is shown to exhibit a strong negative linear compressibility effect that arises from correlated tilting of SnO6 and SiO4 coordination polyhedra.

Metal–Organic Nanosheets Formed via Defect-Mediated Transformation of a Hafnium Metal–Organic Framework

We report a hafnium-containing MOF, hcp UiO-67(Hf), which is a ligand-deficient layered analogue of the face-centered cubic fcu UiO-67(Hf). hcp UiO-67 accommodates its lower ligand:metal ratio compared to fcu UiO-67 through a new structural mechanism: the formation of a condensed “double cluster” (Hf12O8(OH)14), analogous to the condensation of coordination polyhedra in oxide frameworks. In oxide frameworks, variable stoichiometry can lead to more complex defect structures, e.g., crystallographic shear planes or modules with differing compositions, which can be the source of further chemical reactivity; likewise, the layered hcp UiO-67 can react further to reversibly form a two-dimensional metal–organic framework, hxl UiO-67. Both three-dimensional hcp UiO-67 and two-dimensional hxl UiO-67 can be delaminated to form metal–organic nanosheets. Delamination of hcp UiO-67 occurs through the cleavage of strong hafnium-carboxylate bonds and is effected under mild conditions, suggesting that defect-ordered MOFs could be a productive route to porous two-dimensional materials.

Negative area compressibility in silver(I) tricyanomethanide

The molecular framework Ag(tcm) (tcm(-) = tricyanomethanide) expands continuously in two orthogonal directions under hydrostatic compression. The first of its kind, this negative area compressibility behaviour arises from the flattening of honeycomb-like layers during rapid pressure-driven collapse of the interlayer separation.

A Breathing Zirconium Metal–Organic Framework with Reversible Loss of Crystallinity by Correlated Nanodomain Formation

The isoreticular analogue of the metal-organic framework UiO-66(Zr), synthesized with the flexible trans-1,4-cyclohexanedicarboxylic acid as linker, shows a peculiar breathing behavior by reversibly losing long-range crystalline order upon evacuation. The underlying flexibility is attributed to a concerted conformational contraction of up to two thirds of the linkers, which breaks the local lattice symmetry. X-ray scattering data are described well by a nanodomain model in which differently oriented tetragonal-type distortions propagate over about 7-10 unit cells.

Nanostructure determination from the pair distribution function: a parametric study of the INVERT approach

We present a detailed study of the mechanism by which the INVERT method (Cliffe et al 2010 Phys. Rev. Lett. 104 125501) guides structure refinement of disordered materials. We present a number of different possible implementations of the central algorithm and explore the question of algorithm weighting. Our analysis includes quantification of the relative contributions of variance and fit-to-data terms during structure refinement, which leads us to study the roles of density fluctuations and configurational jamming in the RMC fitting process. We present a parametric study of the pair distribution function solution space for C60, a-Si and a-SiO2, which serves to highlight the difficulties faced in developing a transferable weighting scheme.

Nanostructured MOFs through defect engineering

The crucial role that defects play in the chemistry of metal-organic frameworks (MOFs) is becoming increasingly clear, with recent studies revealing the existence of defects, especially ligand vacancies, in many important families of MOFs. The presence of defects has been shown to dramatically improve sorption properties [1], catalytic activity [1] and ionic conductivity [2] of MOFs. In many materials, particularly at higher defect concentrations, the interactions between defects lead to nanostructures that determine the behaviour of a material.

Complexity in supramolecular analogues of frustrated magnets at high pressure

Andrew Brian Cairns1, Matthew J. Cliffe2, Joseph A. M. Paddison3, Dominik Daisenberger4, Matthew G. Tucker5, François-Xavier Coudert6, Andrew L. Goodwin7, Mohamed Mezouar1 1ESRF The European Synchrotron, Grenoble, France, 2Department of Chemistry, University of Cambridge, Cambridge, United Kingdom, 3Department of Physics, University of Cambridge, Cambridge, United Kingdom, 4Diamond Light Source, Didcot, United Kingdom, 5Spallation Neutron Source, Oak Ridge, United States, 6CNRS & Chimie ParisTech, Paris, France, 7Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, Oxford, United Kingdom E-mail: andrew.cairns@esrf.fr

Correlated disorder and structured nanodomains in group 4 MOFs

UiO-66(Zr) was the amongst the first very stable metal-organic frameworks (MOFs) to be discovered[1], and as itis thermally stable to above 400oC and chemically stableto acid, base and boiling water it has been extremely well-studied. In addition to the exploitation of its porosity for gas and liquid separations[2][3], UiO-66 and its derivatives have been shown to be active Bronsted catalysts[4], photocatalysts[5], and sensors[6]. UiO-66 has a face-centered cubic like topology, consisting of twelve coordinate Zr6O4(OH)4 clusters, linked together by 1,4-benzenedicarboxylate (BDC) ligands. This high connectivity has been posited as anexplanation for the structural stability of the framework.This framework has also proved extremely versatile, witha wide range of other dicarboxlyate ligands also formingisoreticular MOFs. Substitution of Hf for Zr is also facile[7],[8]. The relatively simple topology and the structural stabilitygive only a partial picture of this material, and there havebeen early indications of structural complexity and dynamic behaviour, with TGA evidence suggesting that there are ligand vacancies[9], and post-synthetic transformations, such as ligand and metal exchange possible[10]. Some of the most striking evidence of structural complexity in this family is the presence ofbroad primitive superlattice reflections in samples of UiO-66, especially UiO-66(Hf)[8]. We show that these peaks are in fact consistent with diffuse scattering produced by the existence of nanodomains of correlateddefects. We have further shown that the size and concentration of these defects are amenable to chemical control through the use of modulators. These defects arean integral part of the structure of the material, and asrelatively energetic sites, may present opportunities for reactivity and catalysis. The existence of these defects and disorder could well have important implications for the properties of UiO-66, from the presence of wider pores to the effect of lowerconnectivity on the mechanical properties. These will beparticularly pertinent for calculations of the properties ofthis material. It also highlights that the idealised topologies of MOFs may not provide the complete storyfor these useful and fascinating materials.