Synthesis and O2 Reactivity of a Titanium(III) Metal-Organic Framework

Metal-organic frameworks featuring pores lined with exposed metal cations have received attention for a wide range of adsorption-related applications. While many frameworks with coordinatively unsaturated M(II) centers have been reported, there are relatively few examples of porous materials with coordinatively unsaturated M(III) centers. Here, we report the synthesis and characterization of Ti3O(OEt)(bdc)3(solv)2 (Ti-MIL-101; bdc(2-) = 1,4-benzenedicarboxylate; solv = N,N-dimethylformamide, tetrahydrofuran), the first metal-organic framework containing exclusively Ti(III) centers. Through a combination of gas adsorption, X-ray diffraction, magnetic susceptibility, and electronic and vibrational spectroscopy measurements, this high-surface-area framework is shown to contain five-coordinate Ti(III) centers upon desolvation, which irreversibly bind O2 to form titanium(IV) superoxo and peroxo species. Electronic absorption spectra suggest that the five-coordinate Ti(III) sites adopt a distorted trigonal-bipyramidal geometry that effectively shields nuclear charge and inhibits strong adsorption of nonredox-active gases.