Enabling Access to Reduced Open-Metal Sites in Metal-Organic Framework Materials through Choice of Anion Identity: The Case of MIL-100(Cr)

Metal-organic framework materials (MOFs) offer the opportunity to exploit open-metal sites having well-defined coordination environments, one of the key features of which is open-metal oxidation state. MIL-100­(Cr) is a prototypical MOF in this regard, with M²⁺ sites offering the potential for breakthroughs not only in the area of selective gas adsorption but also acid and redox catalysis. We report herein, for the first time, the use of hydroxyl anions in accessing the theoretical maximum density of MIL-100 Cr²⁺ sites. We show, using five complementary characterization techniques, that the use of hydroxyl-endowed MIL-100, unlike the fluoride-containing analogue used extensively in the literature, enables creation of one Cr²⁺ site per node. The results point to anion identity being a key synthetic choice, the judicious use of which could enable control over open-metal oxidation state in MOF materials more broadly.