Unprecedented HAT rate acceleration in water by a non-heme manganese oxo complex

Nature employs water as the reaction medium for enzymatic redox transformations, taking advantage of its unique physicochemical properties to precisely regulate reaction kinetics, selectivity, and proton-coupled electron transfer. In a biomimetic endeavour, using a newly developed non-heme (Et₄N)[Mn^V(O)(Ph,Me-bTAML)] complex, we report a water-induced, enzyme-like rate acceleration in hydrogen atom transfer (HAT) reactivity with various substrates possessing BDEs of 67-78 kcal mol^-1. In acetonitrile, the reactivity is sluggish, but switching to water - particularly beyond 85% content - results in a dramatic rate enhancement, peaking in pure water with up to a 20 000-fold increase. This effect occurs without any structural changes or addition of external additives. Mechanistic insights suggest that water stabilises the minimum energy crossing point (MECP) more effectively than acetonitrile through enhanced electrostatics and hydrogen bonding in transition-state energetics. This is the first demonstration of a non-heme Mn(V)-oxo complex mimicking enzymatic rate enhancement solely via solvent modulation. The work highlights water's active role in driving selective, efficient, and green oxidation chemistry, unlocking new potential in bioinspired catalysis.