Research Front on Coordination Polymers

Abstract

Coordination polymers, which are also widely known as metalorganic frameworks (MOFs), are infinite solid-state networks that can be synthesised from combinations of organic links and metals or metal oxide clusters. By careful choice of both precursor metal salt and the bridging organic ligand, materials with varying 1-D, 2-D, and 3-D topologies; tailored pore sizes, pore shapes, and internal pore chemistry; and properties can be synthesised. Research into the field of coordination polymers and MOFs appears to know no limits. A Web of Science search of both terms as part of the title of published manuscripts (Fig. 1) shows a continual and notable growth over the past 20 years; first of the term ‘coordination polymer’ and then more recently the term ‘metal-organic framework’ or ‘MOF’. This Research Front inAust. J. Chem. highlights examples of the current state of research in the field of coordination polymer or MOF chemistry and indicates briefly where some of the future challenges and opportunities in the field may lie. Australasia has strong links to the field of coordination polymers with Hoskins and Robson publishing one of the seminal papers in this area. By combining tetrahedral organic links and tetrahedral Cu metals ions, Hoskins and Robson were able to form a 3-D coordination polymer with a diamond-like cationic framework. This primary contribution outlined a strategy by which judicious choice of both metal ion and organic ligand could lead to the synthesis of coordination polymers with 1-D, 2-D, and 3-D topologies (Scheme 1). By modifying the structure metrics of a link or ligand, the pore size and chemistry could be systematically manipulated to generate desirable materials. A flurry of activity in the field has elucidated numerous structurally novel materials from a vast array of metal and ligand combinations, along with the observation of various fundamental phenomenon such as the mechanical interlocking – catenation or interpenetration – of the polymeric structures and the intrinsic physical properties of the frameworks, for example, magnetism and non-linear optics. The appreciation that such materials could be made permanently porous altered the focus of the field to the properties of coordination polymers that take advantage of the high surfaces areas and large pore volumes; to on-board gas storage initially and hence to separations, catalysis, sensing, and delivery, for example. The identification of a third class of coordination polymers, namely materials with dynamic structures, has added a further focus to the field with the ability to tie changes in the properties of a material to structural changes that occur in the framework. As noted, the robust frameworks obtained from combinations of metal ions, or metal-oxide clusters, and organic ligands possess high surfaces areas and large pore volumes that make them suitable for gas storage. Potential for onboard storage of gases such as hydrogen and methane for transportation has been investigated in considerable detail, with control over surface areas and pore volume by extension of the ligands, control of catenation, and generation of open metal sites having been shown to enhance gas uptake. More recently considerable attention has been directed towards the use of coordination polymers for gas separations, particularly for the separation of CO2 frommixtures of gases that include predominantlymethane (natural gas sweetening), hydrogen (pre-combustion CO2 capture), or nitrogen (post-combustion CO2 capture) to counter the increasing levels of CO2 in the atmosphere. [13] Two main strategies have been used to enhance the gas separation performance ofMOFs: increasing the affinity of the framework for the gas (e.g. generation of exposed metal sites, post-synthetic metallation, and functionalising pores with Lewis basic sites), or by separating on the basis of different kinetic diameters. Other separations that have been studied included olefin/alkane separations, capture of organosulfur compounds, and enantioseparations. The open, crystalline structures of coordination polymers have also led to studies of their potential as catalytic materials, including as shape-selective catalysts, framework catalysts (e.g. Lewis acid catalysis due to structural metal ions), and for heterogenising – embedding – existing homogenous catalysts. Properties of the frameworks such as luminescence, solvatochromism, or vapochromism, and localised Surface Plasmon Resonance have been utilised for sensing, while the large pore volumes have been explored for delivery of pharmaceutical