Experimental and Theoretical Investigation of the Mechanism of the Reduction of O 2 from Air to O 2 2- by V IV O 2+ - N , N , N -Amidate Compounds and Their Potential Use in Fuel Cells.

The two-electron reductive activation of O ₂ to O ₂ ^2- is of particular interest to the scientific community mainly due to the use of peroxides as green oxidants and in powerful fuel cells. Despite of the great importance of vanadium(IV) species to activate the two-electron reductive activation of O ₂ , the mechanism is still unclear. Reaction of V ^IV O ²⁺ species with the tridentate-planar N,N,N- carboxamide (ΗL) ligands in solution (CH ₃ OH:H ₂ O) under atmospheric O ₂ , at room temperature, resulted in the quick formation of [V ^V (═O)(η ² -O ₂ )(κ ³ -L)(H ₂ O)] and cis -[V ^V (═O) ₂ (κ ³ -L)] compounds. Oxidation of the V ^IV O ²⁺ complexes with the sterically hindered tridentate-planar N,N,N- carboxamide ligands by atmospheric O ₂ gave only cis -[V ^V (═O) ₂ (κ ³ -L)] compounds. The mechanism of formation of [V ^V (═O)(η ² -O ₂ )(κ ³ -L)(H ₂ O)] (I) and cis -[V ^V (═O) ₂ (κ ³ -L)] (II) complexes vs time, from the interaction of [V ^IV (═O)(κ ³ -L)(Η ₂ Ο) ₂ ] ⁺ with atmospheric O ₂ , was investigated with ⁵¹ V, ¹ H NMR, UV-vis, cw-X-band EPR, and ¹⁸ O ₂ labeling IR and resonance Raman spectroscopies revealing the formation of a stable intermediate ( Id ). EPR, MS, and theoretical calculations of the mechanism of the formation of I and II revealed a pathway, through a binuclear [V ^IV (═O)(κ ³ -L)(H ₂ O)(η ¹ ,η ¹ -O ₂ )V ^IV (═O)(κ ³ -L)(H ₂ O)] ²⁺ intermediate. The results from cw-EPR, ¹ H NMR spectroscopies, cyclic voltammetry, and the reactivity of the complexes [V ^IV (═O)(κ ³ -L)(Η ₂ Ο) ₂ ] ⁺ toward O ₂ reduction fit better to an intermediate with a binuclear nature. Dynamic experiments in combination with computational calculations were undertaken to fully elucidate the mechanism of the O ₂ reduction to O ₂ ^2- by [V ^IV (═O)(κ ³ -L)(Η ₂ Ο) ₂ ] ⁺ . The galvanic cell {Zn|V ^III ,V ^II || Id , [V ^IV O(κ ³ -L)(H ₂ O) ₂ ] ⁺ |O ₂ |C(s)} was manufactured, demonstrating the important applicability of this new chemistry to Zn|H ₂ O ₂ fuel cells technology generating H ₂ O ₂ in situ from the atmospheric O ₂ .