Electrochemically Determined, and Structurally Justified Thermo-chemistry of H-atom Transfer on Ti-Oxo Nodes of the Colloidal Met-al–Organic Framework, Ti-MIL-125

Titanium dioxide (TiO2) has long been employed as (photo)electrodes for reactions relevant to energy storage and re-newable energy synthesis. Proton-coupled electron transfer (PCET) reactions with equimolar amounts of protons and electrons at the TiO2 surface or within the bulk structure lie at the center of these reactions. Because a proton and an electron are thermochemically equivalent to an H-atom, these reactions are essentially H-atom transfer reactions. Ther-modynamics of H-atom transfer has a complex dependence on the synthetic protocol and chemical history of the elec-trode, the reaction medium, and many others; together, these complications preclude the understanding of the H-atom transfer thermochemistry with atomic-level structural knowledge. Herein, we report our success in employing open-circuit potential (EOCP) measurements to quantitatively determine the H-atom transfer thermochemistry at structurally well-defined Ti-oxo clusters within a colloidally stabilized metal–organic framework (MOFs), Ti-MIL-125. The free energy to transfer H-atom, Ti3+O–H bond dissociation free energy (BDFE), was measured to be 68 ± 2 kcal mol-1. To the best of our understanding, this is the first report on using EOCP measurements on any MOFs. The proton topology, the structural change upon the redox reaction, and BDFE values were further quantitatively corroborated using computational simula-tions. Furthermore, comparisons of the EOCP-derived BDFEs of Ti-MIL-125 to similar parameters in the literature suggest that EOCP should be the preferred method for quantitatively accurate BDFE calculations. The reported success in employ-ing EOCP for nanosized Ti-MIL-125 should lay the ground for thermochemical measurements of other colloidal systems, which are otherwise challenging. Implications of these measurements on Ti-MIL-125 as an H-atom acceptor in chemical reactions and comparisons with other MOFs/metal oxides are discussed.