Cooperativity between coordinative unsaturated Fe(iii) and aryl-π electrons in MIL-100(Fe) for adsorption of small molecules

Abstract

MIL-100(Fe), one of the water-stable metal–organic frameworks, has potential applications in water purification, sensing, drug storage, and in vivo drug delivery. Molecular adsorption is an important process in these applications, thus necessitating further research to gain a more profound understanding of adsorption mechanisms. Potential explanations for this phenomenon have been proposed, including the formation of coordination bonds to the coordinatively unsaturated site (CUS) on Fe³⁺, electrostatic interactions with the CUS, or π–π interaction on the aryl group of a 1,3,5-benzenetricarboxylic acid (BTC), which is a constituent ligand of MIL-100(Fe). In this study, we investigated the adsorption behavior of 40 molecules on MIL-100(Fe) using MIL-100(Fe) as a stationary phase in liquid chromatography. The relationship between log k (k is a retention factor) and log P_ow shows that, unlike the commonly used reversed-phase and normal-phase LC systems, there was a difference in the retention trend between “aromatics with polar groups” and “alkanols and hydrocarbons”. Aromatics with polar groups were strongly retained on the MIL-100(Fe), suggesting that the interaction with CUS and the π–π interaction play a cooperative role. This study offers the first insights into the structural features of molecules affected by the cooperative effect. The findings suggest that MIL-100(Fe) may have the potential to act as a multimodal adsorbent, where the CUS could act as a polar hydrophilic site and the aryl-π electrons as a hydrophobic site simultaneously.