Neil R. Champness

Inorganic crystal engineering using self-assembly of tailored building-blocks

Abstract The use of transition metal complexes of bridging bidentate ligands to construct predictable, multi-dimensional infinite networks is an area of chemistry which has received ever-increasing attention over recent years. This article will review the advances that have been made in this field of research and will illustrate how ligand design and the properties of the transition metal and counter-anion can be used to control network geometry and thus crystal structure. The range of network topologies and structural motifs that have been constructed thus far will be outlined with particular emphasis upon how specific arrays can be prepared via rational design of molecular building-blocks. The unusual phenomenon of interpenetration, or polycatenation, will be discussed and methods to achieve control over this effect will be highlighted.

Supramolecular design of one-dimensional coordination polymers based on silver(I) complexes of aromatic nitrogen-donor ligands

Abstract This review discusses the design and structure of coordination polymers derived from Ag(I) with N-donor ligands and their role in the investigation of weak non-covalent interactions in the solid state. These forces include arene–arene, metal–anion, metal–arene and metal–metal interactions. The main purpose of this review is to classify and discuss the supramolecular forces which define the overall observed structure (topology, geometry and packing arrangement) of coordination polymers by comparison of a series of structurally related compounds when one parameter of the multi-component system (choice of anion, ligand or solvent) is varied. Design criteria for one-dimensional polymers are given and discussed with respect to the fundamental importance of these compounds for understanding and further development of supramolecular synthetic strategies.

Controlling molecular deposition and layer structure with supramolecular surface assemblies

Selective non-covalent interactions have been widely exploited in solution-based chemistry to direct the assembly of molecules into nanometre-sized functional structures such as capsules, switches and prototype machines. More recently, the concepts of supramolecular organization have also been applied to two-dimensional assemblies on surfaces stabilized by hydrogen bonding, dipolar coupling or metal co-ordination. Structures realized to date include isolated rows, clusters and extended networks, as well as more complex multi-component arrangements. Another approach to controlling surface structures uses adsorbed molecular monolayers to create preferential binding sites that accommodate individual target molecules. Here we combine these approaches, by using hydrogen bonding to guide the assembly of two types of molecules into a two-dimensional open honeycomb network that then controls and templates new surface phases formed by subsequently deposited fullerene molecules. We find that the open network acts as a two-dimensional array of large pores of sufficient capacity to accommodate several large guest molecules, with the network itself also serving as a template for the formation of a fullerene layer.

High Capacity Hydrogen Adsorption in Cu(II) Tetracarboxylate Framework Materials: The Role of Pore Size, Ligand Functionalization, and Exposed Metal Sites

A series of isostructural metal-organic framework polymers of composition [Cu2(L)(H2O)2] (L= tetracarboxylate ligands), denoted NOTT-nnn, has been synthesized and characterized. Single crystal X-ray structures confirm the complexes to contain binuclear Cu(II) paddlewheel nodes each bridged by four carboxylate centers to give a NbO-type network of 64.82 topology. These complexes are activated by solvent exchange with acetone coupled to heating cycles under vacuum to afford the desolvated porous materials NOTT-100 to NOTT-109. These incorporate a vacant coordination site at each Cu(II) center and have large pore volumes that contribute to the observed high H2 adsorption. Indeed, NOTT-103 at 77 K and 60 bar shows a very high total H2 adsorption of 77.8 mg g(-)- equivalent to 7.78 wt% [wt% = (weight of adsorbed H2)/(weight of host material)] or 7.22 wt% [wt% = 100(weight of adsorbed H2)/(weight of host material + weight of adsorbed H2)]. Neutron powder diffraction studies on NOTT-101 reveal three adsorption sites for this material: at the exposed Cu(II) coordination site, at the pocket formed by three {Cu2} paddle wheels, and at the cusp of three phenyl rings. Systematic virial analysis of the H2 isotherms suggests that the H2 binding energies at these sites are very similar and the differences are smaller than 1.0 kJ mol-1, although the adsorption enthalpies for H2 at the exposed Cu(II) site are significantly affected by pore metrics. Introducing methyl groups or using kinked ligands to create smaller pores can enhance the isosteric heat of adsorption and improve H2 adsorption. However, although increasing the overlap of potential energy fields of pore walls increases the heat of H2 adsorption at low pressure, it may be detrimental to the overall adsorption capacity by reducing the pore volume.

Recent advances in crystal engineering

The articles published in the tenth anniversary issue of CrystEngComm are reviewed. The issue highlighted the state-of-the-art of crystal engineering and new trends and developing areas in crystal engineering. In particular, the following article emphasises developments in the areas of intermolecular interactions, notably hydrogen and halogen bonds; metal–organic frameworks or coordination polymers; polymorphism and solvates.

Structural diversity of building-blocks in coordination framework synthesis—combining M(NO3)2 junctions and bipyridyl ligands

Abstract The construction of coordination framework polymers using transition metal complexes of bridging bidentate ligands is a high profile area of chemistry that has received considerable attention over recent years. This article will review the complexity of the area of coordination frameworks which aims to use M(NO 3 ) 2 (M=Co, Ni, Cu, Zn, Cd) building-blocks as junctions in combination with bipyridyl-based ligands. The variety of network topologies and structural motifs that have been constructed thus far will be outlined illustrating the range of coordination environments that are adopted by the M(NO 3 ) 2 nodes and the influence that this has on the coordination framework topology.

OLEX: new software for visualization and analysis of extended crystal structures

We have developed new software (OLEX) for the visualization and analysis of extended crystal structures. This software has a Windows-compatible mouse-driven graphical interface which gives full control over all structural elements. OLEX provides the user with tools to construct topological networks, visualize interpenetrating or overlapping fragments, and analyse networks constructed fully or partially by exploiting short interactions. It is also easy to generate conventional ellipsoid, ball-and-stick or packing plots.

Terminology of Metal-Organic Frameworks and Coordination Polymers (IUPAC recommendations 2013)

A set of terms, definitions, and recommendations is provided for use in the classification of coordination polymers, networks, and metal–organic frameworks (MOFs). A hierarchical terminology is recommended in which the most general term is coordination polymer. Coordination networks are a subset of coordination polymers and MOFs a further subset of coordination networks. One of the criteria an MOF needs to fulfill is that it contains potential voids, but no physical measurements of porosity or other properties are demanded per se. The use of topology and topology descriptors to enhance the description of crystal structures of MOFs and 3D-coordination polymers is furthermore strongly recommended.

Cation-induced kinetic trapping and enhanced hydrogen adsorption in a modulated anionic metal–organic framework

Metal–organic frameworks (MOFs)—microporous materials constructed by bridging metal centres with organic ligands—show promise for applications in hydrogen storage, which is a key challenge in the development of the ‘hydrogen economy’. Their adsorption capacities, however, have remained insufficient for practical applications, and thus strategies to enhance hydrogen–MOF interactions are required. Here we describe an anionic MOF material built from In(iii) centres and tetracarboxylic acid ligands (H4L) in which kinetic trapping behaviour—where hydrogen is adsorbed at high pressures but not released immediately on lowering the pressure—is modulated by guest cations. With piperazinium dications in its pores, the framework exhibits hysteretic hydrogen adsorption. On exchange of these dications with lithium cations, no hysteresis is seen, but instead there is an enhanced adsorption capacity coupled to an increase in the isosteric heat of adsorption. This is rationalized by the different locations of the cations within the pores, determined with precision by X-ray crystallography. Porous metal–organic frameworks are promising for hydrogen storage applications, but adsorption capacities have remained too low for practical use. Now, the adsorption behaviour of such a framework has been modulated by exchanging cations within its pores resulting in either kinetic trapping or enhanced hydrogen affinity.

New trends in crystal engineering

The articles and highlights presented at the second CrystEngComm meeting, held in Nottingham in September 2004 are reviewed. The discussion has highlighted the current development of crystal engineering and allowed to emerge some of its future potentials. In particular, the papers described in this highlight focus on four fundamental aspects: (i) intermolecular interactions: evaluation and application to crystal design; (ii) networks: design and applications; (iii) approaches to crystal synthesis; (iv) polymorphism, solvates and chiral crystal resolution.