Oxidative versus basic asynchronous hydrogen atom transfer reactions of Mn( iii )-hydroxo and Mn( iii )-aqua complexes

Hydrogen atom transfer (HAT) reactions of metal–oxygen intermediates such as metal-oxo, -hydroxo and -superoxo species have so far been studied extensively. However, HAT reactions of metal-aqua complexes are yet to be studied in comparison with those of the corresponding metal-hydroxo complexes. In this study, a series of Mn( iii )-aqua complexes, [(dpaq 5R )Mn III (OH 2 )] 2+ (dpaq 5R = 2-[bis(pyridin-2-ylmethyl)]amino- N -quinolin-8-yl-acetamidate with a substituent at the 5-position; R = NO 2 , Cl, H, Me and OMe) were synthesized by protonating the corresponding Mn( iii )-hydroxo complexes, [(dpaq 5R )Mn III (OH)] + , with trifluoromethanesulfonic acid (HOTf). X-ray crystal structures of both [(dpaq 5R )Mn III (OH)] + and [(dpaq 5R )Mn III (OH 2 )] 2+ complexes showed a mononuclear Mn III center in a distorted octahedral environment. The electronic substitution effect of the dpaq ligand on the HAT reactions of [(dpaq 5R )Mn III (OH)] + and [(dpaq 5R )Mn III (OH 2 )] 2+ was then examined by determining the rate constants ( k 2 ) of the HAT reactions from 4-methoxy-2,6-di- tert -butylphenol to the Mn( iii )-hydroxo and -aqua complexes. Hammett plots indicate that HAT from 4-methoxy-2,6-di- tert -butylphenol to [(dpaq 5R )Mn III (OH)] + proceeds via an oxidative asynchronous coupled proton–electron transfer (CPET), whereas HAT from 4-methoxy-2,6-di- tert -butylphenol to [(dpaq 5R )Mn III (OH 2 )] 2+ proceeds mainly via a basic asynchronous CPET. The contribution of a stepwise electron transfer (ET)/proton transfer (PT) pathway is discussed by comparing the electronic substitution effect on the rate constants of outer-sphere electron transfer and HAT reactions. This study has provided valuable insights into the asynchronous CPET mechanisms of HAT reactions in which the oxidative vs . basic pathways are determined to exhibit reversed electronic substitution effects on HAT reactions of Mn( iii )-hydroxo and Mn( iii )-aqua complexes.