Electronically Asynchronous Transition State Tuned from Remote Site for Oxygen Atom Transfer by CuII-Nitrite Complexes.

Nitrite (NO2-) reduction to nitric oxide (NO) is of paramount interest in biology. Cu-nitrite reductase reduces NO2- to NO while alternatively NO2- can be reduced to NO at copper centre via oxygen atom transfer (OAT) to electron-rich substrate like PPh3. This work demonstrates tuning of LUMO (Lowest Unoccupied Molecular Orbital) energy by remote site modification which leads to change in electrochemical property and OAT activity of CuII-NO2- involving electronically asynchronous transition state. For this purpose, we report here four CuII-NO2- complexes: [CuII(LCH2)(NO2)(ClO4)] (1), [CuII(LO)(NO2(ClO4)] (2), [CuII(LCH2Me)(NO2)(ClO4)] (3), [CuII(LOMe)(NO2)(ClO4)] (4) with similar primary coordination sphere but different substituents at remote site. In going from 1 - 4, by remote site substitution, there is systematic stabilization of LUMO energy which correlates linearly with the increased OAT to PPh3 resulting in 130 times reactivity enhancement for 4 compared to 1. This kind of significant reactivity enhancement by tuning LUMO energy from remote site is very rare. Mechanistic study involving experimental and computational study reveals asynchronous mechanism which was hitherto not reported for any OAT. The observed increase in OAT reactivity from 1 to 4 is attributed to an increase in the extent of asynchronicity in corresponding transition states controlled from remote site modification.