Electrochemically Determined\nand Structurally Justified\nThermochemistry of H atom Transfer on Ti-Oxo Nodes of the Colloidal\nMetal–Organic Framework Ti-MIL-125

Titanium dioxide (TiO<sub>2</sub>) has long been employed\nas a\n(photo)electrode for reactions relevant to energy storage and renewable\nenergy synthesis. Proton-coupled electron transfer (PCET) reactions\nwith equimolar amounts of protons and electrons at the TiO<sub>2</sub> surface or within the bulk structure lie at the center of these\nreactions. Because a proton and an electron are thermochemically equivalent\nto an H atom, these reactions are essentially H atom transfer reactions.\nThermodynamics of H atom transfer has a complex dependence on the\nsynthetic protocol and chemical history of the electrode, the reaction\nmedium, and many others; together, these complications preclude the\nunderstanding of the H atom transfer thermochemistry with atomic-level\nstructural knowledge. Herein, we report our success in employing open-circuit\npotential (<i>E</i><sub>OCP</sub>) measurements to <i>quantitatively</i> determine the H atom transfer thermochemistry\nat structurally well-defined Ti-oxo clusters within a colloidally\nstabilized metal–organic framework (MOF), Ti-MIL-125. The free\nenergy to transfer H atom, Ti<sup>3+</sup>O–H bond dissociation\nfree energy (BDFE), was measured to be 68(2) kcal mol<sup>–1</sup>. To the best of our understanding, this is the first report on using <i>E</i><sub>OCP</sub> measurements to quantify thermochemistry\non any MOFs. The proton topology, the structural change upon the redox\nreaction, and BDFE values were further <i>quantitatively</i> corroborated using computational simulations. Furthermore, comparisons\nof the <i>E</i><sub>OCP</sub>-derived BDFEs of Ti-MIL-125\nto similar parameters in the literature suggest that <i>E</i><sub>OCP</sub> should be the preferred method for quantitatively <i>accurate</i> BDFE calculations. The reported success in employing <i>E</i><sub>OCP</sub> for nanosized Ti-MIL-125 should lay the\nground for thermochemical measurements of other colloidal systems,\nwhich are otherwise challenging. Implications of these measurements\non Ti-MIL-125 as an H atom acceptor in chemical reactions and comparisons\nwith other MOFs/metal oxides are discussed.