Electrochemical Reduction of N2O with a Molecular Copper Catalyst

Abstract

Deoxygenation of nitrous oxide (N₂O) has significant environmental implications, as it is not only a potent greenhouse gas but is also the main substance responsible for the depletion of ozone in the stratosphere. This has spurred significant interest in molecular complexes that mediate N₂O deoxygenation. Natural N₂O reduction occurs via a Cu cofactor, but there is a notable dearth of synthetic molecular Cu catalysts for this process. In this work, we report a selective molecular Cu catalyst for the electrochemical reduction of N₂O to N₂ using H₂O as the proton source. Cyclic voltammograms show that increasing the H₂O concentration facilitates the deoxygenation of N₂O, and control experiments with a Zn(II) analogue verify an essential role for Cu. Theory and spectroscopy support metal-ligand cooperative catalysis between Cu(I) and a reduced tetraimidazolyl-substituted radical pyridine ligand (MeIm₄P₂Py = 2,6-(bis(bis-2-N-methylimidazolyl)phosphino)pyridine), which can be observed by Electron Paramagnetic Resonance (EPR) spectroscopy. Comparison with biological processes suggests a common theme of supporting electron transfer moieties in enabling Cu-mediated N₂O reduction.

Figure 1

(a) Proposed binding of N₂O in the 4Cu^I:S active site of N₂O reductase and (b) electrochemical N₂O reduction catalysis in this work.

Scheme 1. Complex Synthesis

Figure 2

Solid-state structure of 1-Cu. Ellipsoids are shown at 50% probability, and hydrogen atoms have been omitted for clarity. C is shown in gray, N is shown in blue, O is shown in red, F is shown in green, P is shown in orange, S is shown in yellow, and Cu is shown in brown. Selected bond lengths (Å) and angles (°) are as follows: Cu–N_Im 2.048(2), 2.013(2), 2.028(2), and 1.997(2); Co–N_Py 2.432(2); Co–O 2.487(2); O–Cu–N_Py 170.81(7); N5–Cu–N7 87.35(8); and N7–Cu–N3 92.67(8).

Figure 3

Cyclic voltammograms (scan rate 100 mV s^–1) recorded using a 3 mm diameter glassy carbon working electrode in MeCN and 0.1 M NⁿBu₄PF₆ supporting electrolyte for (a) 1-Cu under 1 atm N₂ (black), N₂O (red), and N₂O in the presence of 10 μL (0.1 M) of water (blue) and (b) 1-Cu under 1 atm of N₂O with increasing amounts of water: 10 (0.11 M), 20 (0.22 M), 30 (0.33 M), 40 (0.44 M), 50 (0.55 M), and 100 μL (1.11 M). (c) Controlled potential electrolysis at −2.3 V (vs Ag⁺/Ag) of a MeCN solution of 0.1 M NⁿBu₄PF₆ supporting electrolyte with H₂O (100 mM) under 1 atm N₂O using a RVC working electrode. Charge passed as a function of time in the presence of catalyst (1-Cu, 1 mM) is shown in blue, and that in the absence of catalyst is shown in black.

Scheme 2. Reduction of 1-Cu by 2e^– using Excess Na/Hg

Figure 4

(A) EPR spectrum of the Na/Hg reduction of 1-Cu. (B) Spin density plot of the 2e^– reduced model complex (isovalue of 0.002). EPR conditions are as follows: microwave frequency of 9.6304 GHz, microwave power of 0.2 mW, and modulation of 0.03 mT/100 kHz.