Solvent Exchange Dynamics in M2(dobdc): An Interplay among Binding Strength, Exchange Kinetics, and Cooperativity.

Abstract

Solvent exchange is a crucial step in ensuring the complete activation of metal-organic frameworks (MOFs); however, the conditions for solvent exchange vary among MOFs, even within the isostructural variants. This study examines the factors contributing to solvent exchange by investigating the isostructural M2(dobdc) (M═Mg, Co, Zn) series. Common solvents N,N-dimethylformamide (DMF), ethanol (EtOH), and methanol (MeOH) are employed to assess the solvent exchange at coordinatively unsaturated sites (CUS) of M2(dobdc). By monitoring both solvents released from the MOF during solvent exchange and the coordination environment of metals within the MOF, a picture is constructed of exchange rates during early stages of solvent exchange as well as expulsion of the last traces of bound solvents. This differentiation is achieved by a combination of bulk monitoring of solvent phase composition and microscopic application of Raman spectroscopy on the single-crystal level. The kinetics of solvent replacement is revealed to have a substantial contribution from cooperativity; this phenomenon is observed in both the forward and reverse directions. Thermogravimetric analysis coupled with IR spectroscopy and density functional theory (DFT) calculations are employed to elucidate the relationship between solvent exchange rates and solvent binding energy. The solvent exchange rates are determined by the kinetic barriers of solvent exchange that do not follow the order of the solvent binding affinity. This work contributes to understanding the solvent exchange of MOFs by examining the interplay among the binding strength, exchange kinetics, and cooperativity. It further provides valuable insights for scrutinizing MOF activation protocols.