Polyoxotitanate Molecular Cage Featuring Four Types of Ethylenediamines: Formation Mechanism Insight from Host-Guest Interaction and Crystallographic Study.

Titanium-oxide or polyoxotitanate clusters are a new type of inorganic host materials that can encapsulate inorganic molecules or ions. We report herein a (NH4)4(enH2)[Ti18O27(PhCOO)24(en)9] molecular cage (Ti18) that encapsulates an entire organic ethylenediamine (en) ion. A thorough investigation has revealed the extraordinary versatility of en. Besides being a guest cation, it also functions as chelating and bridging ligand. It balances the charge of the negative Ti18 cage and facilitates the deprotonation of benzoic acid at the early stage of the reaction as well. DFT calculation and a derivative of Ti18 with open sites at its equatorial position shed further light on the formation mechanism. Ti18 strongly absorbs visible light as a result of en coordination, and it exhibits superior photocatalytic activity compared to anatase TiO2.