Decisive Role of Spin-States and Spin-Coupling in Dictating Selective O2 Adsorption in Chromium(II) Metal-Organic Framework

By computing and analyzing the magnetic exchange coupling, binding energies, the partial density of states (pDOS), and adsorption isotherms for the pristine and gas bound MOFs [(Cr4(X)4Cl)3(BTT)8]3- (X=O2, N2, and H2), we unequivocally establish the role of spin-states and spin-coupling in controlling the gas selectivity. The computed geometries and gas adsorption isotherms are consistent with the earlier experiments. The O2 binding to the MOF follows an electron-transfer mechanism resulting in a Cr(III) superoxo species (O2-•) with a very strong antiferromagnetic coupling between the two centers, while N2/H2 found to only weakly perturb the metal center. Although the gas bound and unbound MOFs have S =0 ground state (GS), the nature of spin configurations and the associated magnetic exchanges are dramatically different. This study offers a hitherto unknown strategy of utilizing spin-state/spin-couplings to control gas adsorption selectivity in MOFs.