Electronically Asynchronous Transition State Tuned from Remote Site for Oxygen Atom Transfer by Cu^II-Nitrite...

Nitrite (NO₂ ^-) reduction to nitric oxide (NO) is of paramount interest in biology. In biology, Cu-nitrite reductase reduces NO₂ ^- to NO, while alternatively NO₂ ^- can be reduced to NO at copper center via oxygen atom transfer (OAT) to electron-rich substrate like PPh₃. This work demonstrates systematic tuning of lowest unoccupied molecular orbital (LUMO) energy by remote site modification, which leads to systematic change in electrochemical property and OAT activity of Cu^II-NO₂ ^- involving electronically asynchronous transition state. For this purpose, we report here four Cu^II-NO₂ ^- complexes: [Cu^II(L_CH2)(NO₂)(ClO₄)] (1), [Cu^II(L_O)(NO₂)(ClO₄)] (2), [Cu^II(L_CH2 ^Me)(NO₂)(ClO₄)] (3), [Cu^II(L_O ^Me)(NO₂)(ClO₄)] (4) with similar primary coordination spheres but different substituents at remote sites. In going from 1 to 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 that 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, which was controlled from remote site modification.