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 O-2 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.