Highly selective CO2 vs. N2 adsorption in the cavity of a molecular coordination cage

J. Wright, A. Metherell, W. Cullen, J. Piper, R. Dawson, M. Ward, Chem. Comm., 2017, 53, 4398.

Porous solid-state materials are attractive for gas adsorption purposes, with several classes of porous material gaining increasing attention in recent years. These include metal-organic frameworks (MOFs), covalent organic frameworks (COFs) and microporous organic polymers (MOPs).

In the case of MOFs and MOPs, impressive gas uptake capacities have been reported, and extremely highly porous materials described. However, higher uptake capacity in porous materials can come at the expense of selectivity between small gaseous molecular guests. Adsorbents which are selective for the desired adsorbate are desirable, but not necessarily at the expense of uptake capacity.

A collaboration between the Ward and Dawson groups has recently been published in the journal Chemical Communications, in which the gas adsorption behaviour of a pair of molecular coordination cages has been investigated; both cages demonstrated a high selectivity for CO2 uptake over N2 in the solid state, with good absolute CO2 uptake.

The cystal structure left and graph showing higher selectivity of CO2. The Ward group have previously described the self-assembly of a series of cubic [M8L12]16+ coordination cages; twelve bridging ligands and eight metal ions come together in solution to form a single assembly. These [M8L12]16+ cages have recently been shown to display interesting host-guest chemistry by virtue of the intrinsic cage cavity. In organic solvent, hydrogen-bond accepting guests (e.g. carbonyls) bind in a region of the cage which offers a convergent pocket of CH2 groups, which have a H-bond donating strength comparable to phenol. Importantly, the cavity (and binding pocket) persists both in solution and the solid state.

An investigation into the gas sorption capability of these materials demonstrated a high selectivity for CO2 uptake over N2 in the solid state, which was ascribed to the presence of the same H-bond donor sites on the cage interior surface that facilitate guest binding in solution. A crystal structure of CS2 within the cage cavity was used as a structural model, with the H-bond accepting CS2 molecule binding at the H-bond donor sites (Figure 1).

Significantly, the balance between absolute CO2 uptake, and CO2/N2 uptake selectivity, is amongst the best known in any kind of porous material.

Further Information.

The full article by James Wright, Alex Metherell, Will Cullen, Jerico Piper, Robert Dawson and Mike Ward can be found here: Highly selective CO2 vs N2 adsorption in the cavity of a molecular coordination cage