Lewis acid-assisted reduction of nitrite to nitric and nitrous oxides via the elusive nitrite radical dianion

Abstract

Reduction of nitrite anions (NO₂⁻) to nitric oxide (NO), nitrous oxide (N₂O) and ultimately dinitrogen (N₂) takes place in a variety of environments, including in the soil as part of the biogeochemical nitrogen cycle and in acidified nuclear waste. Nitrite reduction typically takes place within the coordination sphere of a redox-active transition metal. Here we show that Lewis acid coordination can substantially modify the reduction potential of this polyoxoanion to allow for its reduction under non-aqueous conditions (−0.74 V versus NHE). Detailed characterization confirms the formation of the borane-capped radical nitrite dianion (NO₂²⁻), which features a N(II) oxidation state. Protonation of the nitrite dianion results in the facile loss of nitric oxide (NO), whereas its reaction with NO results in disproportionation to nitrous oxide (N₂O) and nitrite (NO₂⁻). This system connects three redox levels in the global nitrogen cycle and provides fundamental insights into the conversion of NO₂⁻ to NO.

Oxyanions are a class of water pollutants of growing concern due to their high solubility in water, their potential toxicity and their negative environmental impacts^1,2. Nature has evolved several biochemical pathways capable of facile, selective reduction of oxyanions. The constituent enzymes in these pathways offer inspiration for the design of catalysts purposed towards the remediation of oxyanion pollution³. For example, the nitrite anion (NO₂⁻) serves as a terminal electron acceptor during denitrification, where it is progressively reduced to nitric oxide (NO), nitrous oxide (N₂O) and ultimately to dinitrogen (N₂). The initial, selective reduction of NO₂⁻ to NO is mediated by metalloenzymes including cytochrome c oxidase and nitrite reductases that leverage coordination of NO₂⁻ to Fe or Cu centres^4–7. These redox-active metals transfer an electron to NO₂⁻, facilitated both by the Lewis acidity of the metal centres and accompanied by binding of a proton to a nitrite O-atom in nitrite reductase^8,9. Either of these binding events may lower the reduction potential of NO₂⁻, rendering electron transfer more favourable (Fig. 1a,c).

Fig. 1 |. The role of protons and Lewis acids in nitrite reduction.

a, The standard reduction potential of nitrite to NO at pH 7 involves two protons. b, Rapid capture of β-particles (electrons) by nitrite from radioactive decay to form the nitrite dianion NO₂²⁻ and subsequent proton-induced release of NO. c, Biological nitrite reduction to NO is facilitated by metal–nitrite coordination, hydrogen-bonding networks and proton transfer. d, Lewis acid coordination enables the isolation of nitrite dianion NO₂²⁻ (this work).

Reduction of nitrogen oxyanions also occurs in high-level radioactive waste stored in nitrate- and nitrite-rich aqueous solutions¹⁰. Solvated electrons generated radiolytically undergo rapid capture by both nitrate and nitrite to form the dianions NO₃²⁻ and NO₂²⁻ at nearly diffusion-controlled rates of 9.7 × 10⁹ s⁻¹ and 3.5 × 10⁹ s⁻¹, respectively (Fig. 1b)¹⁰. Facile reaction of NO₃²⁻ and NO₂²⁻ with water to produce NO₂ and NO at rates of 5.5 × 10⁴ s⁻¹ and 4.3 × 10⁴ s⁻¹, respectively, renders these dianions extremely short-lived species in aqueous solution, with lifetimes on the order of 20 μs (ref. ¹⁰).

To decouple the effects of Lewis acid coordination and proton transfer to an oxyanion during reduction¹¹, we explored the effects of coordinating NO₂⁻ to a redox-innocent Lewis acid. We have previously shown that the coordination of the biological nitric oxide reservoir RSNOs to the Lewis acid B(C₆F₅)₃ in RSNO–B(C₆F₅)₃ adducts markedly raises the reduction potential for RSNOs (approximately −1.1 to −1.3 V versus normal hydrogen electrode (NHE)), which enables generation of the [RSNO–B(C₆F₅)₃]^•− radical anion at biologically relevant potentials (+0.1 versus NHE)¹². Using a strong, yet chemically stable capping Lewis acid for each of the two O atoms of nitrite, this strategy enables the facile reduction of NO₂⁻ to its radical dianion NO₂²⁻ without attendant proton transfer. This approach effectively decouples electron transfer and protonation¹¹ to separately show the effect of Lewis acid activation on the reduction potential of the nitrite anion and on the proton-induced release of NO from the reduced nitrite dianion NO₂²⁻. Moreover, the use of capping Lewis acids enables the characterization and reactivity study of this nitrogen oxyanion that connects NO₂⁻, NO and N₂O representing three redox levels in the global nitrogen reduction cycle.

Results and discussion

Capture of the nitrite dianion, synthesis and characterization.

Addition of 1 equiv. B(C₆F₅)₃ to [Cp*₂Co][NO₂] (1) in fluorobenzene leads to the formation of [Cp*₂Co][(C₆F₅)₃B–ONO] (2) in near quantitative yield (Fig. 2a). The X-ray structure of unsymmetrically capped 2 shows B(C₆F₅)₃ bound to one O-atom of nitrite with markedly different N–O distances of 1.337(10) (capped O atom) and 1.200(10) Å (free O atom), respectively (Supplementary Fig. 30). Treating [Cp*₂Co][(C₆F₅)₃B–ONO] (2) with a second equivalent of B(C₆F₅)₃ in fluorobenzene forms the doubly activated nitrite anion in [Cp*₂Co][(C₆F₅)₃B–ONO–B(C₆F₅)₃] (3) (Fig. 2b). The X-ray structure of 3 shows more symmetric NO distances (1.261(2), 1.225(2) Å) that result upon coordination of a Lewis acid to each O atom in nitrite. Lewis acid coordination also results in a contraction of the nitrite O–N–O bond angle to 112.2(7)° and 109.3(17)° in mono- and di-capped species 2 and 3, respectively, compared to the free nitrite anion in 1 (120.6(6)°; Supplementary Figs. 29–31).

Fig. 2 |. Synthesis and structures of Lewis acid-capped nitrite mono- and dianions.

a, Synthesis of Lewis acid-capped nitrite anions [Cp*₂Co][(C₆F₅)₃B–ONO] (2), [Cp*₂Co][(C₆F₅)₃B–ONO–B(C₆F₅)₃] (3) and [Cp*₂Co]₂[(C₆F₅)₃B–ONO–B(C₆F₅)₃] (4). b, X-ray structures of the nitrite monoanion [Cp*₂Co] [(C₆F₅)₃B–ONO–B(C₆F₅)₃] (3) and dianion [Cp*₂Co]₂[(C₆F₅)₃B–ONO–B(C₆F₅)₃] (4) showing N–O distances (Å) and O–N–O angles (°) in the ONO moiety. Each anion is charge-balanced by the Cp*₂Co⁺ cation. Atom colours: N, blue; O, red; B, pink; C, grey; F, yellow. LA, Lewis acid.

Cyclic voltammetry studies show the effect of the Lewis acid coordination on the reduction potential of free nitrite. Under aqueous conditions, the reduction potential for free nitrite is highly proton-dependent, ranging from +0.98 V and −0.48 versus NHE at pH 0.0 and 14.0, respectively^13,14. In fluorobenzene with [PPN] [BAr^F₄] (Ar^F = 3,5-(CF₃)₂C₆H₃) as electrolyte, however, neither [Cp*₂Co][NO₂] (1) nor [Cp*₂Co][(C₆F₅)₃B–ONO] (2) exhibits any reduction wave for nitrite or monocapped nitrite anion (Supplementary Fig. 26). Rather, only a wave for Cp*₂Co⁺/Cp*₂Co couple is evident which appears at −1.35 V in 2. In marked contrast, the cyclic voltammogram of [Cp*₂Co][(C₆F₅)₃B–ONO–B(C₆F₅)₃] (3) shows a quasi-reversible wave centred at −0.74 V versus NHE that corresponds to the [(C₆F₅)₃B–ONO–B(C₆F₅)₃]⁻/[(C₆F₅)₃B–ONO–B(C₆F₅)₃]²⁻ couple (Supplementary Fig. 27). The addition of a second Lewis acid to the nitrite anion induces a profound electronic effect that facilitates nitrite reduction.

Chemical reduction of [Cp*₂Co][(C₆F₅)₃B–ONO–B(C₆F₅)₃] (3) with Cp*₂Co in fluorobenzene effects a colour change from yellow to grey (Fig. 2a). Crystallization of the grey product confirms the formation of the borane-capped nitrite radical dianion [Cp*₂Co]₂[(C₆F₅)₃B–ONO–B(C₆F₅)₃] (4) (Fig. 2b). Although Lewis acid-capped mono- and dianions 3 and 4 possess similar overall structures, the bond lengths and angles in nitrite dianion 4 are greatly changed in comparison with nitrite 3. In nitrite dianion 4, the N–O bonds are markedly elongated (1.385(7), 1.373(7) Å) as compared to monoanion 3 (1.225(2), 1.261(2) Å) as a consequence of the extra electron present in the nitrite dianion 4. Furthermore, the nitrite O atoms in dianion 4 are more tightly bound to the capping boranes (B–O distances: 1.488(9), 1.490(9) Å), consistent with its greater negative charge in comparison to doubly capped nitrite anion 3 (B–O distances: 1.603(3), 1.628(3) Å). Moreover, the O–N–O bond angle in dianion 4 (103.5(5)°) is contracted relative to monoanion 3 (109.3(17)°). Infrared (IR) spectroscopy reports a markedly lower-energy N–O stretching frequency for dianion 4 (1,010 cm⁻¹) in comparison with monoanion 3 (1,265 cm⁻¹), consistent with a weakening of the N–O bonds of nitrite upon one-electron reduction.

Electronic structure of the nitrite dianion core.

Qualitative molecular orbital analysis for NO₂^2−/1− predicts that the lowest unoccupied molecular orbital of 3/singly occupied molecular orbital (SOMO) of 4 should be an N-localized π* combination between N 2p_x and O 2p_x orbitals (Supplementary Fig. 38). This is confirmed by analysis of electron paramagnetic resonance (EPR) spectra. The continuous-wave (c.w.) X-band EPR spectrum obtained at room temperature for 4 (Fig. 3a) shows the expected isotropic three-line pattern arising from ¹⁴N hyperfine coupling, with the isotropic component of the hyperfine coupling (HFC) A_iso(¹⁴N) = 44.0 MHz. Accordingly, the ¹⁵N isotopologue exhibits a two-line pattern with A_iso(¹⁵N) = 62.0 MHz (Fig. 3b). The isotropic g-value (g_iso) value of 2.005 obtained from both measurements is consistent with a radical composed solely of light atoms. Frozen-solution spectra recorded at both the X- (Fig. 3c) and Q-band (Fig. 3d) (the latter obtained by field-swept echo detection) show substantial anisotropy of the ¹⁴N hyperfine values, with the best simulation of the X-band data giving A_x = 122 MHz, A_y = 4 MHz and A_z = 6 MHz. Assignment of the larger HFC as A_x is supported by the corresponding Q-band data. The g values obtained from simulating the Q-band data of 4 are g_x = 1.991, g_y = 1.996 and g_z = 1.996. For completeness, frozen-solution spectra and simulations for the ¹⁵N isotopologue are provided in the Supplementary Information.

Fig. 3 |. Probing the electronic structure of the nitrite dianion by ePR and X-ray absorption spectroscopy, as well as DFT calculations.

a, 298-K solution c.w. X-band (9.3886 GHz) EPR spectrum of ¹⁴N–4 (9.7249 GHz). b, Solution c.w. X-band EPR spectrum of ¹⁵N–4. c, 96-K frozen-solution c.w. X-band (9.2710 GHz) EPR of ¹⁴N–4. d, 10-K field-swept echo-detected Q-band (33.9544 GHz) EPR of ¹⁴N–4. dχ″/dH is the derivative of the imaginary component of the molecular magnetic susceptibility (χ″) with respect to the external static magnetic field H. Experimental data are shown in black and simulations in red. The simulated spectra are in very good agreement with the experimental results. e, Smoothed partial fluorescence yield-detected N K-edge XAS of 3 (black) and 4 (red). Raw data are in grey. f, B3LYP/def₂⁻TZVP(-f) calculated spin density plots (isovalue 0.002 a.u., maroon) of the dianion in 4 and geometry-optimized NO₂²⁻. Atom colours: N, blue; O, red; B, pink; C, grey; F, yellow.

For an electron localized to a MO with substantial N 2p_x character, the A values [A_x, A_y, A_z] are expected to be [A_iso + T_x, A_iso + T_y, A_iso + T_z], where T is the anisotropic value for ¹⁴N. Using values tabulated by Morton and Preston¹⁵, the value of aiso corresponding to an electron localized to N 2s is 1,811 MHz, whereas an electron localized to N 2p_x should have anisotropic [T_x, T_y, T_z] = [4/5, −2/5, −2/5] × 138.8 MHz = [111.04, −55.52, −55.52] MHz. Using the well-resolved A_x value to obtain T_x, the unpaired spin in 4 resides in a MO with 2.5% N 2s and 69% N 2p_x character.

N-localization of the reduction of 3 to 4 is further supported by N K-edge (1s → 2p) XAS (Fig. 3e). The spectrum obtained for 3 has an intense pre-edge feature at 400.6 eV this is shifted to 397.8 eV in 4. The intensity of this feature is diminished in 4, consistent with occupation of a formerly empty MO. The edge position in the spectrum obtained for 3 appears at ~5 eV higher energy relative to the edge in 4. Together, these are consistent with the reduction of N. The intensity of the pre-edge feature in 4 can be related to N 2p participation in the acceptor orbital via equation (1)¹⁶:

$$D_0=\frac{{\alpha }^2{I}_{\text{S}}h}{3n},$$

where D₀ is the pre-edge peak intensity, h is the number of holes, n is the number of photoabsorbers (n and h = 1 in 4), I_s is the radial dipole integral between N 1s and N 2p, and α² is the percent N 2p character of the acceptor MO. Using the experimental D₀ of 4.2 and the 69% N 2p_x character determined by EPR, a value of 18.3 is obtained for I_s. This value is smaller than an estimate of I_s obtained by studying a library of N-ligated coordination complexes¹⁷ and reflects decontraction of the 2p orbital of the highly electron-rich N-centre in 4¹⁸.

Hybrid density functional theory (DFT) calculations using the B3LYP hybrid density functional and the scalar-relativistically recontracted ZORA-def2-TZVP(-f) basis set on all atoms further support the formulation of 4 as containing a NO₂²⁻ core. The calculated spin density occupies a b₁-symmetry O–N–O π* orbital composed 2% N 2s and 67% N 2p_x, in excellent agreement with values obtained from EPR (Fig. 3f). The residual spin density resides predominantly on O. Calculated spin Hamiltonian parameters exhibit high fidelity to values extracted from spectral simulations. Time-dependent DFT calculations of the N K-edge XAS confirm that the pre-edge feature in the spectrum obtained for 4 corresponds to a promotion from N 1s to the b₁-symmetry SOMO. Additionally, these calculations reproduce the energy shifts observed between N K features of 3 and 4 (Supplementary Fig. 40).

Given the fidelity of the calculated electronic structure to that deduced from experiment, we sought to examine the role of the capping Lewis acids on the electronic structure of the NO₂²⁻ core. Geometry optimization of the bare anion NO₂²⁻ at this level of theory affords N–O distances of 1.372 Å and an angle of 111.5°, consistent with an electron in the O–N–O π* orbital. A single-point electronic structure calculation at the stationary point shows that the NO₂²⁻ dianion is also a N-centred radical with a similar N orbital heritage (1.3% N 2s and 59% N 2p_x) as in Lewis acid-capped 4. The inductive effect of the Lewis acid bound to the nitrite dianion through the O atoms results in greater unpaired electron density on the less electronegative N atom in the O–N–O π* orbital (Fig. 3f).

Chemical reactivity of the nitrite dianion.

The nitrite radical dianion [Cp*₂Co]₂[(C₆F₅)₃B–ONO–B(C₆F₅)₃] (4) is stable in the solid state and at room temperature in fluorobenzene solution. Because the N atom of nitrite dianion 4 is at the same oxidation level as NO gas (N_ox = +2), we explored avenues to produce NO gas from 4. Heating a fluorobenzene solution of 4 at 75 °C for 48 h leads to the formation of NO gas in 65% yield (Fig. 4a). ¹⁹F NMR analysis of this reaction indicates multiple B(C₆F₅)₃ derived products, including [Cp*(η⁵-C₅Me₄CH₂–B(C₆F₅)₃)Co] (5) that has undergone deprotonation of the one Cp* methyl group followed by capture by the Lewis acid B(C₆F₅)₃ (Supplementary Fig. 35). Mimicking proton transfer in nitrite reduction, however, addition of 2 equiv. trifluoroacetic acid to dianion 4 at room temperature triggers the instant release of NO gas in 72% yield with 2 equiv. [Cp*₂Co][(C₆F₅)₃B–OC(O)CF₃] (6), isolated in 80% yield (Fig. 4a).

Fig. 4 |. Reactivity of the nitrite dianion.

a, Release of NO from nitrite dianion 4 upon heating or addition of proton sources. b, Reduction of NO to N₂O by nitrite dianion 4 generating 2 equiv. [(C₆F₅)₃B–ONO] (2). c, Reaction with isotopically labelled NO shows a net outer-sphere reduction of NO to N₂O.

Reaction of nitrite dianion 4 with 3 equiv. NO results in a disproportionation reaction that generates N₂O and 2 equiv. [Cp*₂Co] [(C₆F₅)₃B–ONO] (2) quantified in 43% and 90% yields, respectively. Isotopic labelling experiments show that N₂O forms solely from the added NO_gas. Reaction of natural-abundance nitrite dianion 4–¹⁴N with 3 equiv. ¹⁵NO_gas results in the exclusive formation of ¹⁵N¹⁵NO, and reaction of nitrite dianion 4–¹⁵N with 3 equiv. ¹⁴NO_gas solely forms ¹⁴N¹⁴NO, as monitored by IR spectroscopy (Supplementary Fig. 25). In each case, a 1:1 mixture of monocapped nitrite anions [(C₆F₅)₃B–O¹⁴NO] (2–¹⁴N) and [(C₆F₅)₃B–O¹⁵NO] (2–¹⁵N) forms, according to ¹⁵N and ¹⁹F NMR analysis (Fig. 4c).

As an initial step, we propose net electron and Lewis acid transfer from nitrite dianion [(C₆F₅)₃B–ONO–B(C₆F₅)₃]²⁻ to NO to form [(C₆F₅)₃B–ONO]⁻ and the nitroxide Lewis acid adduct [(C₆F₅)₃B–NO]⁻ (Supplementary Figs. 39 and 40). Hybrid DFT calculations that follow the model reaction of free nitrite dianion NO₂²⁻ with NO via an inner-sphere pathway indicate favourable thermodynamics for conversion to the nitrite anion NO₂⁻ and NO⁻ (ΔG = −45.9 kcal mol⁻¹) with a minimal barrier (ΔG^‡ = 6.9 kcal mol⁻¹) via the [O₂N–NO]²⁻ intermediate (ΔG = −31.5 kcal mol⁻¹) (Supplementary Fig. 39). Reaction of the borane-capped species [(C₆F₅)₃B–ONO–B(C₆F₅)₃]²⁻ with NO to give [(C₆F₅)₃B–ONO]⁻ and [(C₆F₅)₃B–NO]⁻ is also exergonic (ΔG = −17.5 kcal mol⁻¹) (Supplementary Fig. 40). Stepwise capture of 2 equiv. NO by [(C₆F₅)₃B–NO]⁻ provides the hyponitrite radical anion [(C₆F₅)₃B–ONNO]⁻ (ΔG = −7.2 kcal mol⁻¹) followed by [(C₆F₅)₃B–ONO]⁻ and N₂O (ΔG = −57.5 kcal mol⁻¹). The overall reaction of [(C₆F₅)₃B–ONO–B(C₆F₅)₃]²⁻ with 3 equiv. NO to give the experimentally observed 2 equiv. [(C₆F₅)₃B–ONO]⁻ and N₂O (Fig. 4b) is highly exergonic (ΔG = −82.2 kcal mol⁻¹ overall) (Supplementary Fig. 40). This latter sequence follows the previously studied reaction of NO⁻ with NO to give [ONNO]⁻ and [ONNONO]⁻ intermediates en route to N₂O and NO₂⁻ (refs. ^19,20).

Conclusion

In summary, by using the Lewis acid B(C₆F₅)₃, electron transfer and proton transfer steps in nitrite reduction become decoupled from N–O bond scission, permitting isolation of the nitrite dianion NO₂²⁻ stabilized by a redox-innocent Lewis acid at each O atom. Although strong Lewis acids such as B(C₆F₅)₃ enable the capture of reactive species such as O₂²⁻ (ref. ²¹) or may combine with bulky, strong Lewis bases to form frustrated Lewis pair (FLP) adducts of N₂O (ref. ²²) and NO (ref. ²³), this Lewis acid enables the isolation of the nitrite dianion 4. This new polyoxoanion with N_ox = +2 provides fundamental insights into the conversion of NO₂⁻ to NO. Moreover, the nitrite dianion 4 is capable of further reducing NO to N₂O with formation of the nitrite monoanion 2, connecting three nitrogen oxidation states in the global nitrogen cycle (N_ox = +3, +2, +1). Use of potent, redox-innocent Lewis acids to enable the reduction of small molecules and ions without breaking bonds affords an opportunity to gain insight into the reduction of other environmentally relevant species, perhaps opening new approaches for catalysis. This is especially relevant for anions such as nitrate (NO₃⁻) and perchlorate (ClO₄⁻), for which transition metals mediate electron- and oxygen-atom transfer steps, yet bind to metal sites weakly, requiring assistance via non-covalent binding to facilitate oxyanion reduction³.

Online content

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Data availability

Crystallographic data for the structures reported in this Article have been deposited at the Cambridge Crystallographic Data Centre, under deposition numbers CCDC 2074366 (1), 2074367 (2), 207468 (3), 2074369 (4), 2074370 (5), 2074371 (6) and 2074372 (7). Copies of the data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/. All other data supporting the findings of this study are available within the Article and its Supplementary Information.