Mechanism for the reduction of the mixed-valent (MnMnIV)-Mn-III[2-OHsalpn](2)(+) complex by tertiary amines

The mixed-valent dimanganese(III/IV) complex (MnMnIV)-Mn-III(2-OHsalpn)(2)( +), 1, is cleanly reduced in acetonitrile by aliphatic tertiary amines to g ive the dimanganese(III) product Mn-2(III)(2-OHsalpn)(2), 2. Thorough chara cterization of the organic reaction products shows that tributylamine is co nverted to dibutylformamide and propionaldehyde. Kinetic studies and radica l trapping experiments suggest that this occurs via initial single-electron transfer from the amine to 1 coupled with C-H-alpha proton transfer from t he oxidized amine. EPR spectroscopy and base inhibition studies indicate th at coordination of the amine to 1 is a critical step prior to the electron transfer step. Rate data and its dependence on the amine indicate that the ability of the amine to reduce 1 is correlated to its basicity rather than to its reduction potential. Weakly basic amines were unable to reduce 1 irr espective of their reduction potential. This was inferred to indicate that proton transfer from the amine radical cation is also important in the redu ction of 1 by tertiary amines. Comparison of the activation energy with rea ction thermodynamics indicates that proton transfer and electron transfer m ust be concerted to explain the rapidity of the reaction. The fate of the a mine radical is dependent on the presence of oxygen, and labeling studies s how that oxygen in the organic products arises from dioxygen, although inco rporation from trace water was also observed. These data indicate that inhi bition of the hydrolytic quenching of the amine radical in an aprotic solve nt results in a different fate for the amine radical when compared to amine oxidation reactions in aqueous solution. The proposed mechanism gives new insight into the ability of amines with high reduction potential to reduce metal ions of lower potential. In particular, these data are consistent wit h the ability of small amines and certain amine-containing buffers to inhib it manganese-dependent oxygen evolution in photosynthesis, which arises in some cases as a result of manganese reduction and its concomitant loss from the PS II reaction center.