Brønsted Acids Promote Olefin Oxidations by Bioinspired Nonheme Co(PhIO)(OH) Complexes: A Role for Low-Barrier Hydrogen Bonds.

Introduction of Brønsted acids into biomimetic nonheme reactions promotes the oxidative ability of metal-oxygen complexes significantly. However, the molecular machinery of the promoted effects is missing. Herein, a comprehensive investigation of styrene oxidation by a cobalt(III)-iodosylbenzene complex, [(TQA)Co(OIPh)(OH)] (, TQA = tris(2-quinolylmethyl)amine), in the presence and absence of triflic acid (HOTf) was performed using density functional theory calculations. Results revealed for the first time that there is a low-barrier hydrogen bond (LBHB) between HOTf and the hydroxyl ligand of , which forms two valence-resonance structures [(TQA)Co(OIPh)(HO--HOTf)] () and [(TQA)Co(OIPh)(HO--OTf)] (). Due to the oxo-wall, these complexes ( and ) cannot convert to high-valent cobalt-oxyl species. Instead, styrene oxidation by these oxidants ( and ) shows novel spin-state selectivity, i.e., on the ground closed-shell singlet state, styrene is oxidized to an epoxide, whereas on the excited triplet and quintet states, an aldehyde product, phenylacetaldehyde, is formed. The preferred pathway is styrene oxidation by , which is initiated by a rate-limiting bond-formation-coupled electron transfer process with an energy barrier of 12.2 kcal mol. The nascent PhIO-styrene-radical-cation intermediate undergoes an intramolecular rearrangement to produce an aldehyde. The halogen bond between the OH/HO ligand and the iodine of PhIO modulates the activity of the cobalt-iodosylarene complexes and . These new mechanistic findings enrich our knowledge of nonheme chemistry and hypervalent iodine chemistry and will play a positive role in the rational design of new catalysts.