Effect of the metal within regioisomeric paddle-wheel-type metal-organic frameworks.

The effect of metal on the degree of flexibility upon evacuation of metal-organic frameworks (MOFs) has been revealed with positional control of the organic functionalities. Although Co-, Cu-, and Zn-based DMOFs (DMOF = DABCO MOF, DABCO = 1,4-diazabicyclo[2.2.2]octane) with ortho-ligands (2,3-NH2Cl) have frameworks that are inflexible upon evacuation, MOFs with para-ligands (2,5-NH2Cl) showed different N-2 uptake amounts after evacuation by metal exchange. Considering that the structural analyses were not fully sufficiently different to explain the drastic changes in N-2 adsorption after evacuation, quantum chemical simulation was explored. A new index (eta) was defined to quantify the regularity around the metal based on differences in the oxygen-metal-oxygen angles. Within 2,5-NH2Cl, the eta value becomes larger as the metal are varied from Co to Zn. A large eta value means that the structures around the metal center are less ordered. These results can be used to explain flexibility changes upon evacuation by altering the metal cation in this regioisomeric system.