Given a holding material with sufficiently small and uniform pores, gaseous oxygen can be made to form regular one-dimensional chains. That gives unprecedented insight into the properties of confined gases.
Porous materials are invaluable ‘hosts’ — that is, they are useful for storing, separating and investigating various guest compounds. Typical hosts are inorganic materials called zeolites (which contain a lattice structure) or so-called granular activated carbon, which possesses a large internal surface area to accommodate guests. Just recently, a third group of materials, metal–organic frameworks known as coordination polymers, have joined this party of excellent hosts1,2. Writing in Angewandte Chemie International Edition, Takamizawa and colleagues3 report a canny use of a coordination polymer to store molecular oxygen (strictly, dioxygen, O2) as an ordered array of three molecules (a trimer) or as a regular one-dimensional chain over a wide range of temperatures.
Molecular oxygen is a particularly welcome guest molecule, for several reasons. First, its so-called ‘frontier orbitals’ include unpaired electrons, and give rise to a rich array of reduction and oxidation reactions (involving the gain and loss of electrons, respectively). These electrons, through their unpaired spins, also make O2 the smallest stable ‘paramagnetic’ molecule, meaning that it orients itself in a particular way in response to a magnetic field. Confining oxygen in an array would allow currently unknown details of the interplay of localized atomic spins and delocalized electrons to be investigated in charge-transfer and redox reactions between the oxygen and the pore surfaces.
Second, the properties of oxygen in its ordered, solid form are particularly interesting: in various three-dimensional solid phases under different conditions, it both exhibits a form of magnetism known as antiferromagnetism (in which the spins of neighbouring electrons align pointing in opposite directions) and displays metallic conductivity and superconductivity4. In lower-dimensional, confined forms of oxygen, novel magnetic and optical properties are expected5. The problem is that oxygen is a gas under ambient conditions, and solidifies only at −218.9 °C. Investigation of ordered forms of oxygen at ambient temperatures therefore requires extremely high pressures.
Coordination polymers offer just the right tool to bring about an ordered state without applying energy in the form of pressure. The key to this capability is extremely small, regular ‘ultramicropores’ of less than 0.7 nanometres in diameter that allow guest molecules to be lined up uniformly and intimately within the polymer. Conventional hosts such as activated carbons do not permit such structurally well-characterized arrangements: their larger pore sizes mean that the guest molecule always has some freedom to move, resulting in highly disordered arrays that must be modelled statistically.
Compared with conventional hosts, coordination polymers also possess unusual flexibility and their physical and chemical properties can be tuned by adding specific functional groups at the surface. Their molecules and ions can also be designed in minute detail. Complex frameworks can thus be built up, in which clusters, wires, ladders and other exotic molecular conformations can be contained. Molecules confined to these pore networks experience an unusually high, internally generated pressure6, giving rise to condensed assemblies whose properties differ from those of the bulk solid or those of assembled forms in an ambient environment.
Takamizawa et al.3 investigated oxygen trimers included along a channel constructed from a linear chain of a coordination polymer over a wide range of temperatures, from room temperature down to 10 K. They observed an evolution from a ‘softer’, more flexible assembly of oxygen trimers at higher temperatures to a rigid, three-dimensional ordered assembly at the lowest temperatures. Analysis of the geometry of the trimers showed that the internal distance between the trimer elements increased steadily with temperature, but that the distance between trimers first decreased to a minimum at around 90 K, and then increased once again. This indicated the presence of an intermediate, unstable phase at minimum trimer separation that could — according to measurements of magnetic correlations between the trimers involved — be treated as a quasi-one-dimensional chain of oxygen molecules.
Interestingly, the authors also show that this intermediate trimer structure changes its form under the influence of a magnetic field. Porous crystalline coordination polymers are often much more dynamic than is generally believed, and numerous studies have shown that they possess a structural flexibility that can lead to physical changes when an external stimulus — in most cases the chemical stimulus of a guest molecule — is applied7. In Takamizawa and colleagues' case3, it seems that the host framework itself flips back and forth between states, allowing the oxygen trimer to change its form in response to the applied magnetic field. This demonstration follows the magnetically induced rearrangement of an included oxygen dimer last year8.
These results could lead to new methods for the remote control of molecular structures. Whereas molecular devices that respond to light have been readily synthesized, similar systems that respond to magnetic fields have not, as the energy contained in magnetic interactions is generally too small to drive structural transformations. Magnetically induced phase transitions in an assembly of included guest paramagnetic molecules offer a way round this obstacle, as even a small stimulus could give rise to larger-scale structural change in both guest and host. In particular, guest molecular clusters could provide new quantum spin states that are strongly coupled to those of the polymer structure surrounding them.
Because Takamizawa and colleagues' polymer framework is so flexible, it can easily accommodate guest compounds that are not just paramagnetic, and thus responsive to magnetic fields, but also dipolar, and therefore sensitive to electric fields. These molecules range from heterodiatomic molecules such as carbon monoxide (which is dipolar) and nitrogen oxide (paramagnetic and dipolar) to larger polyatomic molecules such as sulphur dioxide (dipolar) (Fig. 1). With so many guests to choose from, it is inevitable that new topics of discussion will arise concerning ultra-micropores9. What is emerging is a new science of ‘coordination space’ that studies the role of nanospaces in determining the chemical and physical functions of guest and host frameworks.
Small gas molecules such as O2, CO, NO and SO2 have, in their separate ways, im-portant roles in gas separation and storage, catalysis and pollution control. An in-depth understanding of field-responsive properties of the hybridized gas assemblies offered by Takamizawa and colleagues could pave the way to new applications in environmental, energy and life sciences.
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Nature Chemistry (2009)