Structural and Hydrolytic Stability of Coordinatively Unsaturated Metal–Organic Frameworks M3(BTC)2 (M = Cu, Co, Mn, Ni, and Zn): A Combined DFT and Experimental Study

Metal–organic framework (MOF) materials have shown great potential in numerous practical applications such as storage, gas separation, and heterogeneous catalysis, largely due to their versatile porous structures and functional tunability. However, most MOFs still suffer from low structural stability, particularly low hydrolytic stability in a humid environment. Herein, the structural and hydrolytic stabilities of coordinatively unsaturated, paddlewheel M3(BTC)2 (M = Cu, Co, Mn, Ni, and Zn) have been studied using density functional theory (DFT)-based simulations combined with experimental measurements. Ab initio molecular dynamics (AIMD) simulations of isostructural metal-substitution analogues of M3(BTC)2 show that the structural oscillation intensity relies heavily on the metal type, where the skeleton of Ni3(BTC)2 and Zn3(BTC)2 dramatically vibrates. The presence of water molecules further induces oscillations of all coordinatively unsaturated M–O bonds at the metal node to some extent, leading to the precursor state for the initial M–O bond breaking. For the hydrolytic breakdown of M3(BTC)2 that involves adsorption, substitution, and dissociation steps, DFT calculations indicate that water substitution is more facile than water dissociation. The hydrolytic stability trend is predicted as follows Cu3(BTC)2 > Co3(BTC)2 > Mn3(BTC)2 > Ni3(BTC)2 > Zn3(BTC)2, which is consistent with our experimental observation. Finally, it is also found that the spin state of metal plays an important role in the hydrolytic stability of Co3(BTC)2, whose higher spin states cause more facile hydrolytic breakdown.