Mechanism of Electrocatalytic N ₂ O Reduction to N ₂ by a Rhenium Bipyridyl Carbonyl Complex

The rhenium(I) complex, [Re(bpy)(CO)₃Cl] serves as an effective electrocatalyst for N₂O reduction to N₂ in acetonitrile. Density functional theory (DFT) calculations reveal a dinuclear mechanism initiated by two successive one-electron reductions of the precatalyst, yielding active species [<b>3</b>]^- (formally Re^1-) with concomitant Cl^- dissociation. Species [<b>3</b>]^- attacks N₂O via energetically favored <i>η</i>¹-N terminal coordination to form [<b>3-N</b>_<b>2</b><b>O</b>_{<b>(end-N)</b>}]^-, which subsequently couples with monoreduced species [2^•] (formally Re⁰) to generate dinuclear intermediate[<b>Int3</b>^•]^-(formally <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow><mml:mi>Re</mml:mi><mml:mo>⁡</mml:mo></mml:mrow><mml:mrow><mml:mi>I</mml:mi></mml:mrow></mml:msup><mml:mtext>-</mml:mtext><mml:mover><mml:mrow><mml:mi>N</mml:mi></mml:mrow><mml:mrow><mml:mover><mml:mrow><mml:mo>∥</mml:mo></mml:mrow><mml:mrow><mml:mi>N</mml:mi><mml:mi>O</mml:mi></mml:mrow></mml:mover></mml:mrow></mml:mover><mml:mtext>-</mml:mtext><mml:msup><mml:mrow><mml:mi>Re</mml:mi><mml:mo>⁡</mml:mo></mml:mrow><mml:mrow><mml:mn>0</mml:mn></mml:mrow></mml:msup></mml:math>). The catalytic cycle proceeds through a key three-membered ring transition state, forming [Int4^•]^- (formally Re^I-N₂O-Re⁰) that decomposes to release N₂ and yield oxide species [5]^- (formally Re^I-O) while regenerating [2^•]. Species [<b>5</b>]^- undergoes a proton-coupled electron transfer (PCET) reduction to form the hydroxyl intermediate [<b>2-OH</b>^•]^<b>-</b> (formally Re⁰-OH), which is further protonated to generate [<b>2-OH^₂</b>^•] (formally Re⁰-OH₂). The dissociation of water (H₂O) subsequently regenerates [<b>2</b>^•], allowing the cycle to continue. COPASI simulations demonstrated that the dinuclear mechanism is kinetically more favorable than the mononuclear mechanism. This study provides novel insight into the role of N₂O coordination modes in modulating the reaction barrier, showing that the Re-catalyst preferentially binds N₂O at an electron-transfer active center to initiate the reaction. These findings offer guidance for the future design of catalysts for N₂O conversion.