The Effect of Sterically Active Ligand Substituents on Gas Adsorption within a Family of 3D Zn-Based Coordination Polymers.

An investigation of the adsorption properties of two structurally related, 3D coordination polymers of composition Zn(2-Mehba) and Zn(2,6-Mehba) (2-Mehba = the dianion of 2-methyl-4-hydroxybenzoic acid and 2,6-Mehba = the dianion of 2,6-dimethyl-4-hydroxybenzoic acid) is presented. A common feature of these structures are parallel channels that are able to accommodate appropriately sized guest molecules. The structures differ with respect to the steric congestion within the channels arising from methyl groups appended to the bridging ligands of the network. The host network, Zn(2-Mehba), is able to take up appreciable quantities of H (77 K) and CO and CH (298 K) in a reversible manner. In regard to the adsorption of N by Zn(2-Mehba), there appears to be an unusual temperature dependence for the uptake of the gas such that when the temperature is increased from 77 to 298 K the uptake of N increases. The relatively narrow channels of Zn(2,6-Mehba) are too small to allow the uptake of N and CH, but H molecules can be adsorbed. A pronounced step at elevated pressures in CO and NO isotherms for Zn(2,6-Mehba) is noted. Calculations indicate that rotation of phenolate rings leads to a change in the available intraframework space during CO dosing.