Single electron reduction of NHC–CO₂–borane compounds

Abstract

The carbon dioxide radical anion [CO₂˙⁻] is a highly reactive species of fundamental and synthetic interest. However, the direct one-electron reduction of CO₂ to generate [CO₂˙⁻] occurs at very negative reduction potentials, which is often a limiting factor for applications. Here, we show that NHC–CO₂–BR₃ species – generated from the Frustrated Lewis Pair (FLP)-type activation of CO₂ by N-heterocyclic carbenes (NHCs) and boranes (BR₃) – undergo single electron reduction at a less negative potential than free CO₂. A net gain of more than one volt was notably measured with a CAAC–CO₂–B(C₆F₅)₃ adduct, which was chemically reduced to afford [CAAC–CO₂–B(C₆F₅)₃˙⁻]. This room temperature stable radical anion was characterized by EPR spectroscopy and by single-crystal X-ray diffraction analysis. Of particular interest, DFT calculations showed that, thanks to the electron withdrawing properties of the Lewis acid, significant unpaired spin density is localised on the carbon atom of the CO₂ moiety. Finally, these species were shown to exhibit analogous reactivity to the carbon dioxide radical anion [CO₂˙⁻] toward DMPO. This work demonstrates the advantage provided by FLP systems in the generation and stabilization of [CO₂˙⁻]-like species.

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Introduction

The carbon dioxide radical anion [CO₂˙⁻] is a very unstable molecule that was characterized from trapping experiments with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) by EPR,¹ using pulse radiolysis with time-resolved IR detection in acetonitrile,² by IR (hydrated form,³ on surfaces⁴ or in Ne matrix⁵), and by mass spectrometry in the gas phase.⁶ As a classical distonic anion, the charge and radical sites of [CO₂˙⁻] are separated.⁷ Notably, the radical is located on a σ-type orbital centred on the carbon atom.⁶ This highly reactive molecule was exploited in diverse synthetic approaches to produce carboxylic acid-containing molecules through C–C bond formation,⁸ or as a powerful single electron reductant in challenging reductive processes.^8a,r,9

The use of alkali formate compounds [MHCO₂] has recently emerged as an efficient method to generate [CO₂˙⁻] by Hydrogen Atom Transfer (HAT) (Chart 1a).^8o–t As an alternative, the direct one-electron reduction of CO₂ has been employed to generate [CO₂˙⁻] in few synthetic transformations by photo- or electroreduction.^8b–n However, such reduction is energy demanding since it requires very negative reduction potentials (e.g. E_1/2 = −2.65 V in DMF, −2.70 V in CH₃CN and −2.53 V in aqueous solution vs. Fc^+/0),^8b,10 which limits practical applications (Chart 1a). In the context of using CO₂ as a sustainable source of carbon,¹¹ it is therefore appealing to seek CO₂ activation modes that would enable an easier one-electron reduction process to access [CO₂˙⁻]. With this in mind, we decided to investigate the single electron reduction of FLP (Frustrated Lewis Pair)-type activated CO₂ molecules, corresponding to a bifunctional activation of the molecule by Lewis basic and Lewis acidic entities. The ambiphilic nature of carbon dioxide makes it an ideal candidate for FLP-type activation.¹² Quickly after the pioneering disclosure of the FLP concept on H₂ activation, the FLP activation was explored for CO₂.^12a,c,d This process was then exploited for the trapping and reactivity of CO₂. However, while the FLP concept was recently extended to single electron transfer between the frustrated donor and acceptor counterparts,¹³ the one-electron-reduction of FLP-activated CO₂ has seldom been investigated. To the best of our knowledge, only the (t-Bu)₃P–CO₂–B(C₆F₅)₃ adduct was subjected to one-electron reduction by Heiden et al., in a broader investigation on the role of main group elements in CO₂ reduction.¹⁰ They studied the reduction of the (t-Bu)₃P–CO₂–B(C₆F₅)₃ adduct by means of DFT, electro- and chemical-reduction (Chart 1b). The DFT study indicated that the monoreduced species is a minimum on the energy surface with an unpaired spin density of 16% on the carbon of the CO₂ unit. However, the electroreduction of (t-Bu)₃P–CO₂–B(C₆F₅)₃ gave rise to irreversible waves at rather low potential of −2.56 V and −2.61 V vs. Fc^+/0 in THF and acetonitrile, respectively while the chemical reduction led to an ill-defined mixture of products.

Chart 1 (a) Common strategies to generate [CO₂˙⁻], (b) study of the one-electron reduction of t-Bu₃P–CO₂–B(C₆F₅)₃, (c) one-electron reduction of CAAC–CO₂ species, (d) NHC–CO₂–borane species, (e) synthesis and one-electron reduction of NHC–CO₂–B(C₆F₅)₃ compounds.

In order to explore the monoelectronic reduction of CO₂ when activated by an FLP system, we turned our attention to N-heterocyclic carbenes (NHCs) as Lewis bases because of their high electronic and steric modularity and also because of their known ability to stabilise radical species.¹⁴ Applied to CO₂, Machan, Gilliard et al. have shown that CAAC–CO₂ adducts could be reduced by one electron (Chart 1c), either by cyclic voltammetry in a reversible manner at −2.08 V vs. Fc^+/0 in CH₃CN and −2.15 V vs. Fc^+/0 in THF for ^CyCAAC,¹⁵ or by alkali metals (Li, Na, K) for ^EtCAAC.¹⁶ Interestingly, the calculated unpaired spin density of [CAAC–CO₂˙⁻] highlights the importance of the CAAC moiety in the reduction process, since most of the added density is located on the carbenic carbene and barely any density on the carbon atom of the CO₂ moiety with a maximum of 8%.^15,16 One can notice, though, that the evaluation of the single electron reduction of other NHC–CO₂ adducts with more classical NHCs is lacking.¹⁷ On the other hand, very few NHCs have been used in FLP-CO₂ activation. To the best of our knowledge, only three examples were reported by Tamm et al.¹⁸ in their study concerning the ability of NHC/borane systems to trap CO₂ and N₂O whether the Lewis pair was frustrated or not^12d,18 (Chart 1d).

Herein, we report the expansion of the NHC–CO₂–borane library and the exploration of their single electron reduction leading to reversible or irreversible reduction depending on the nature of the NHC (Chart 1e). Two radical anion FLP-CO₂ systems could be characterized by EPR spectroscopy and one crystallized. The combined theoretical investigations enabled us to scrutinize the reduction potential of these species and the localisation of the unpaired spin density.

Results and discussion

We chose to use tris(pentafluorophenyl)borane (B(C₆F₅)₃) as the Lewis acid and varied the NHC Lewis base to prepare four NHC–CO₂–B(C₆F₅)₃ molecules 2a–d. We reproduced the synthesis of the adduct 2a, described by Tamm et al., featuring 1,3-di-tert-butylimidazolin-2-ylidene (It-Bu) and B(C₆F₅)₃ by reacting the ItBu-CO₂ adduct 1a with the borane.^18a Following a similar strategy, the NHC–CO₂–B(C₆F₅)₃ systems 2b–d were synthesized from the reaction between the NHC–CO₂ adducts 1b–d of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr, 1b), 1,3-bis(1-adamantyl)imidazol-2-ylidene (IAd, 1c),¹⁹ and cyclohexyl cyclic (alkyl)(amino)carbenes (^cyCAAC, 1d) and an equimolar amount of B(C₆F₅)₃ under a CO₂ atmosphere (Scheme 1). The adducts 2b–d were fully characterized by NMR and IR spectroscopy. Overall, these compounds exhibit similar signatures compared to 2a. In the ¹³C{¹H} NMR spectra, the resonances for the carbon atoms of the CO₂ moiety appear at δ = 156.0, 152.8, 161.1 and 158.0 ppm for 2a–d, respectively, which represents a de-shielding of approximatively 30 ppm compared to free CO₂. In the ¹¹B NMR spectra, the tetrahedral boron atom displays characteristic resonance at δ = −3.9, −3.4, −3.8 and −2.9 ppm for 2a–d, respectively. In the solid-state, the IR analysis indicates C=O stretching frequencies at 1714, 1717, 1715 and 1709 cm⁻¹ for 2a–d, respectively (Table 1). These values are included in the large range of FLP-CO₂ molecules (1645 < υ(C=O) < 1742 cm⁻¹).^12b As a comparison, the antisymmetric O=C=O stretching frequency of CO₂ appears at 2349 cm⁻¹.²⁰ Theoretical investigations revealed that the bands observed at slightly lower frequencies correspond to the υ(C=C) and υ(C–F) (Table 1 and ESI†). In the case of 2a for example, these bands were observed at 1644 and 1513 cm⁻¹, respectively.

Scheme 1 Synthesis of compounds 2a–d.

Table 1 Selected IR stretching frequencies of compounds 2a–d and CO₂

IR stretching frequencies (cm^−1)	υ(C=O)	υ(C=C)	υ(C–F)
a υ asym.
2a	1714	1644	1513
2b	1717	1643	1514
2c	1715	1645	1514
2d	1709	1624	1514
Free CO2	2736^a

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The solid-state structures were further analysed by single crystal X-ray diffraction analysis (Fig. 1). Selected structural parameters of 2b–d as well as the comparison with 2a are reported in Table 2. In all four cases, the incorporation of the CO₂ molecule in the FLP systems resulted in its bending with OCO angles comprised between 130.09(15)° for 2a and 128.78(13)° for 2b and with a significant lengthening of the carbon–oxygen bonds leading to an ester like trigonal planar geometry. The structures of 2a–d feature similar parameters, except for the dihedral angle between the plane of the CO₂ and the plane of the NHC moieties, noted D(NHC–CO₂) in Table 2. In fact, while structures 2a, 2c and 2d show torsion angles of 80.5(2), 76.7(3) and 79.2(2)°, respectively, the CO₂ moiety and the imidazole ring are almost coplanar in 2b with a dihedral angle of 7.3(2)°. These data can be correlated with the C_NHC–C_CO2 bond distances, which are longer for 2a, 2c and 2d (1.516(2), 1.52071(19), and 1.5223(3) Å, respectively) than for the adduct 2b (1.5071(19) Å). This feature is in accordance with data gathered on NHC–CO₂ adducts showing that the C–C bond distance between the carbenic carbon (C_NHC) and the CO₂ carbon (C_CO2) is larger with a greater dihedral angle. These C–C distances and torsion angles have then been correlated with the temperature of decarboxylation which is lower with higher dihedral angles and longer C–C distances.²¹

Fig. 1 Structures of compounds 2b (left), 2c (middle) and 2d (right). Thermal ellipsoids drawn at 30% probability.

Table 2 Structural parameters of NHC–CO₂–B(C₆F₅)₃ (2a–d)

Structural parameters	2a ^18a	2b	2c	2d
d(C=O)/Å	1.2024(19)	1.2074(18)	1.207(3)	1.2070(17)
d(C–O)/Å	1.2972(19)	1.2980(17)	1.298(3)	1.2977(17)
d(O–B)/Å	1.535(2)	1.5363(18)	1.540(3)	1.5492(18)
d(Ccarbenic–CO2)/Å	1.516(2)	1.5071(19)	1.523(3)	1.5247(19)
α(CO2)/°	130.09(15)	128.78(13)	129.96(19)	129.31(13)
D(NHC–CO2)/°	80.5(2)	7.3(2)	76.7(3)	79.2(2)

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The redox chemistry of species 2a–d was probed by cyclic voltammetry experiments at room temperature in dry dichloromethane (nBu₄PF₆, 0.1 M) to ensure good solubility. Satisfyingly, the reduction of adducts 2a–d led to clear waves depicted in Fig. 2 with reduction potentials expressed vs. Fc^+/0. The cyclic voltammograms of 2a (orange wave) and 2c (blue wave) show irreversible reduction at E^red_p = −2.30 V and −2.44 V, respectively. Although, upon reduction, the related (t-Bu)₃P–CO₂–B(C₆F₅)₃ and ^CyCAAC–CO₂ led to interesting CO₂ disproportionation transformations under certain conditions,^10,15 we did not explore further the fate of the reduced 2a and 2c. The cyclic voltammogram of 2b (brown wave) reveals a quasi-reversible reduction at E_1/2 = −2.08 V (ΔE_p = 150 mV). Finally, the quasi-reversible one-electron reduction of compound 2d (green wave) occurs at a potential of E_1/2 = −1.34 V (ΔE_p = 90 mV). The reduction potential of compound 1d was also recorded at E_1/2 = −2.13 V (ΔE_p = 132 mV), in accordance with the data reported in the literature.¹⁵ Overall, the recorded E^red data indicate that compounds 2a–d featuring an FLP-type activated molecule of CO₂ are more easily reduced by one electron than free CO₂. The most important gain is obtained with the combination of CAAC and B(C₆F₅)₃ enabling the achievement of a remarkable reduction potential E_1/2 of −1.34 V in CH₂Cl₂ compared to free CO₂ (i.e., E^red_p = −2.65 V in DMF, −2.70 V in CH₃CN and −2.53 V in aqueous solution vs. Fc^+/0).^8b,10 The less negative reduction potential observed in the case of CAAC vs. the more classical NHC probably results from the higher π-accepting ability of CAAC, which is the consequence of a more accessible LUMO orbital.²² This feature would thus indicate that the reduction event is at least partially localised on the carbenic carbon, which is confirmed by the following experimental and theoretical investigations. The beneficial role of the borane – which was expected because it decreases the electronic density from the carbene–CO₂ adduct – is quantified by a net gain of 0.79 V observed between the E_1/2 of 2d compared to that of 1d.

Fig. 2 Normalized cyclic voltammograms of NHC–CO₂–B(C₆F₅)₃ adducts 1d and 2a–d on a GC microdisk in 0.1 M [nBu₄N][PF₆]/CH₂Cl₂ media under an Ar atmosphere (5 × 10⁻³ M of adduct, rt, scan rate: 0.2 V s⁻¹).

In order to gain further insights into the role of the NHC/borane pair in the activation of CO₂, DFT calculations were undertaken using the M06-2X functional combined with Grimme's D3 correction to consider dispersion effects (see Computational details in the ESI†). The structures of 2a–d and of NHC–CO₂ adducts 1a–d were computed. In addition, the structures of 2(a–d)-BH₃, featuring BH₃ in place of B(C₆F₅)₃ were also computed as models for a less Lewis acidic borane. The calculated charges by Natural Population Analysis (NPA) on the C_CO2 reflect the electron withdrawing effect of the borane to a moderate extent (Table S9†). For example, the NPA charge of 1a is 0.75e and is 0.81e for 2a, while the NPA charge of the C_CO2 of free CO₂ is significantly more positive (1.05). Similar variations of 0.05 to 0.09e are observed in the case of the other NHC. The LUMO orbitals of the series were also analysed (Fig. 3). In the adducts 1a–d the π-type LUMOs are mostly located on the carbenic centres, which is in line with the less negative reduction potentials observed for the CAAC adducts. The addition of borane (BH₃ and B(C₆F₅)₃) leads to the delocalisation of the LUMO over the C_CO2 as well. This phenomenon is enhanced with the increased Lewis acidity of B(C₆F₅)₃vs. BH₃. The role of the dihedral angle D(NHC–CO₂) was also explored. Relaxed potential energy scans were computed for the rotation, carrying out constrained optimisations for each value of the dihedral angle (see Fig. S76†). From them, a minimum and rotational transition states were located for each structure. In each case, the minimum corresponds to the relative orientation of the NHC and CO₂ moieties observed in the crystal structures: orthogonal in the case of 2a, 2c–d, and coplanar for 2b. Computations also evidenced these preferences in the absence of borane and with BH₃, since the NHC and CO₂ moieties are also found orthogonal for 1a, 1c–d, as well as for the corresponding BH₃ adducts and coplanar for 1b and 2b-BH₃. These features suggest that the observed variations between 2 and 2-BH₃ stem mostly from the difference in the electronic properties between B(C₆F₅)₃ and BH₃ of lower Lewis acidity and not from steric differences. The barriers of rotation of 2a–d were evaluated to be accessible at room temperature since the highest is calculated at 16.5 kcal mol⁻¹. The barrier for 2b (4.6 kcal mol⁻¹) is significantly smaller compared to the one for 2a, 2c and 2d (13.5, 16.5 and 11.8 kcal mol⁻¹, respectively).

Fig. 3 LUMO orbitals of the neutral NHC-based adducts. Visualization (IBOview,²³ threshold = 45) with colour code: carbon (grey), nitrogen (light blue), boron (green), fluorine (dark yellow), oxygen (red).

Reduction potentials of compounds 2a–d were computed accurately with maximum deviation from the experimental values of 0.14 V. However, the calculated potential for 1d is off by 0.25 V (vide infra for the proposed explanation). We then explored the effect of the borane on the reduction potential. As for 1dvs.2d, the absence of borane in 1a–c has a significant impact with calculated potentials of −3.42, −3.00 and −3.44 V vs. Fc^+/0, respectively. These values are more negative than that of free CO₂, explaining the absence of any reduction potential values for the classical NHC–CO₂ adduct in the literature.^17a

Having calculated that the relative rotations of NHC and CO₂–B(C₆F₅)₃ moieties are accessible at room temperature, we wondered if the reduction potentials vary with the dihedral angle. Transition states [1d]^‡ and [2a–d]^‡ were found resulting from the rotation of the NHC and CO₂ moieties in 1d and 2a–d, respectively. The calculated values indicate that indeed the reduction potential is sensitive to the dihedral angle since much less negative potentials were calculated for these TS (Table 3), in line with the fact that the LUMOs are lower in energy after rotation (Fig. S76†). Nonetheless, the values calculated for [2a–d]^‡ do not correspond to the experimental values suggesting that it is indeed 2a–d that are reduced in the CV and not [2a–d]^‡. However, in the case of [1d]^‡, the calculated potential is significantly closer to the experimental value (Δ = 0.04 vs. 0.25 V), which could indicate that it is in fact the reduction of [1d]^‡ that is measured experimentally. Further exploration aiming at controlling the degree of rotation would nonetheless be necessary to confirm this hypothesis.

Table 3 Calculated and experimental (in brackets) reduction potentials vs. Fc^+/0 for compounds 1a–d, 2a–d, 1d_rot and 2a–d_rot

Compounds	a	b	c	d
a Coplanar TS. b Orthogonal TS.
1	−3.42	−3.00	−3.44	−2.38 (−2.13)
2	−2.26 (−2.30)	−2.04 (−2.08)	−2.30 (−2.44)	−1.29 (−1.34)
[1]^‡	—	—	—	−2.09^a
[2]^‡	−1.68^a	−1.84^b	−1.58^a	−0.78^a

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In addition to electrochemical studies, we investigated the chemical reduction of adducts 2a–d with one equivalent of potassium graphite (KC₈) as the chemical reductant in THF under argon. In accordance with the irreversible waves observed for 2a and 2c, no radical species could be detected upon chemical reduction. However, the reduction of 2b and 2d at 298 K led to a deep red and yellow solution, respectively, exhibiting EPR signals. As shown in Fig. 4b, five (g = 2.0026) and three (g = 2.0026) hyperfine lines were observed for [2b˙⁻] and [2d˙⁻], respectively, indicative of a radical electron coupling with the two equivalent nitrogen nuclei (a_N = 4.6 G) for [2b˙⁻] and one nitrogen nucleus (a_N = 6.1 G) for [2d˙⁻]. DFT calculation reproduced the EPR signals accurately (Fig. S66 and S67†). In accordance with the reduction potential measured at −1.34 V for 2d, [2d˙⁻] could also be generated using Co(Cp*)₂ as a milder reductant (E_red = −1.94 V vs. Fc^+/0 in CH₂Cl₂), as proved by the observation of a very similar EPR signal (Fig. S69†). These radical species are stable in solution under an inert argon atmosphere at room temperature, at least for two days as shown by EPR analyses (Fig. S66 and S67†). IR frequency involving the C=O bond was measured at 1596 cm⁻¹ in the solid state for the isolated [2d˙⁻], while only the solution IR frequency was recorded for the in situ generated [2b˙⁻]. DFT calculations indicate that these frequencies correspond to the combination of the stretching of the C=O and C–C_NHC bonds (ESI†). While attempts to obtain crystals were unsuccessful with the radical [2b˙⁻], single crystals of [2d˙⁻][K⁺].(THF)₂, suitable for X-ray diffraction, were grown from a concentrated THF/pentane solution at −35 °C. In comparison with the structural parameters of the parent neutral form 2d, the additional electron in [2d˙⁻] is responsible for important changes. The most striking difference concerns the torsion between the plane of the CO₂ moiety and the plane of the CAAC ring, which, from a dihedral angle of 79.2(2)° in 2d, reaches a minimum in [2d˙⁻] with a dihedral angle of 0.1(3)°. This phenomenon was also observed in the absence of borane by Machan, Gilliard et al. and highlights the increased resonance between the NHC and the CO₂ fragments in the reduced form. In line, important shortening of the C_NHC–C_CO2 bond [1.433(3) A°] and the O–B bond [1.486(3) A°] is witnessed in [2d˙⁻], along with lengthening of the N–C_NHC [1.374(3) Å], C=O [1.241(3) Å] and C–O [1.331(2) Å] bonds. In addition, the bending of the CO₂ moiety is enhanced reaching an angle of 122.26(18)° (vs. 129.31(13) for 2d, Fig. 4d). This bending is very similar to the bending observed in the reduction of the CAAC–CO₂ adduct with Li, Na and K leading to a bending of 122.9(2)° with Na to 123.71(9)° for Li.¹⁶ DFT calculations were conducted on [NHC–CO₂–BR₃˙⁻] and [NHC–CO₂˙⁻] adducts and indicate that the coplanar rearrangement of the NHC and CO₂ planes is a general phenomenon since it is predicted for the all series of [NHC–CO₂–BR₃˙⁻] and [NHC–CO₂˙⁻] adducts (Fig. 4c and ESI†). All cases feature separated anionic and radical sites and can thus be described as distonic anions. While as in [CO₂˙⁻] the anionic charge is mostly located on the oxygen atoms, the radical charge is however mostly located on the C_NHC in the case of [1a–d˙⁻] compounds and not on the C_CO2. Moreover, the radical resides in a π-type orbital instead of a σ-type orbital in [CO₂˙⁻]. Interestingly, the addition of a borane enables transferring of a part of the unpaired spin density to the C_CO2 in [2(a–d)-BH₃˙⁻] and [2a–d˙⁻], which is slightly closer to the [CO₂˙⁻] situation. In more detail, the localized Mulliken atomic spin density of [2b˙⁻] and [2d˙⁻] indicates a large distribution on the C_NHC (0.57 and 0.69, respectively) and on the C_CO2 (0.42 and 0.33, respectively), while lower spin density is found on the nitrogen (0.17, 0.23, respectively) and on the oxygen (0.14, 0.12, respectively) atoms. The portion of spin density found on the C_CO2 of [2b˙⁻] and [2d˙⁻] is remarkable when taking into account the absence of any calculated spin density on the C_CO2 of the mono-reduced form of the ^cyCAAC–CO₂ adduct [1d˙⁻] (Table S8†).¹⁵ It is in line with the spin density calculation of Heiden et al. on the parent [^tBu₃P–CO₂–B(C₆F₅)₃˙⁻] adduct with 16% on C_CO2, but to a larger extent.¹⁰

Fig. 4 Characterization of the radical anions [2b˙⁻] and [2d˙⁻]: (a) mono-reduction of 2b and 2d into [2b˙⁻] and [2d˙⁻], (b) experimental (black) and simulated (red) EPR spectra, (c) calculated SOMO orbitals, (d) solid-state structure of [2d˙⁻][K⁺]·(THF)₂ and selected structural parameters of [2d˙⁻].

As a preliminary evaluation of the reactivity of the radical anionic species [2b˙⁻] and [2d˙⁻], we wished to probe their reactivity with DMPO since this is a classical trapping agent for [CO₂˙⁻], giving rise to a very characteristic EPR signal featuring six bands at g = 2.0058 (a_N = 15.8 G, a_β-H = 19.1 G) in aqueous media (Table 4, entry 1).¹ Satisfyingly, when [2b˙⁻] and [2d˙⁻] were reacted with five equivalents of DMPO, the EPR signals of [2b˙⁻] and [2d˙⁻] were replaced by a similar sextet at g = 2.0052 (a_N = 14.0 G and a_β-H = 18.6 G) and g = 2.0050 (a_N = 14.4 G and a_β-H = 20.0 G), respectively (Table 4, entries 2 and 3, Fig. S71†). A loss of signal intensity is however observed, presumably due to stability issues under these conditions. In the absence of borane, the chemically reduced compound [1d˙⁻] also reacts like [CO₂˙⁻] toward DMPO as indicated by a similar EPR signal at g = 2.0050 (a_N = 15.0 G and a_β-H = 20.1 G), (Fig. S71†). This is in line with the observation that when [1d˙⁻] was generated electrochemically under a CO₂ atmosphere by Gilliard, Machan et al., a disproportionation reaction was observed, which is a known reactivity of [CO₂˙⁻].¹⁵ At this stage, it is difficult to assess whether or not the borane moiety is maintained in the products of these spin trapping experiments. As indicated by DFT calculations, [DMPO-CO₂˙⁻] and [DMPO-CO₂-B(C₆F₅)₃˙⁻] would afford similar EPR signals (Table 4, entries 5 and 6). Further studies will be necessary to understand and exploit the reactivity of the monoreduced FLP-type activated CO₂ molecules in valuable synthetic strategies.

Table 4 Experimental and computed EPR data

Entries	[DMPO-CO2˙^−]	g	a N/G	a β-H/G
a In H2O.
1	Ref 1a^a	2.0058	15.8	19.1
2	From [2b˙^−]	2.0052	14.0	18.6
3	From [2d˙^−]	2.0050	14.4	20.0
4	From [1d˙^−]	2.0050	15.0	20.1
5	Calculated by DFT	2.0080	13.7	22.3
6	Calculated by DFT with B(C6F5)3	2.0081	13.7	21.8

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Conclusions

Three new NHC–CO₂–B(C₆F₅)₃ adducts 2b–d are reported. The one-electron reduction of these adducts and of the known adduct It–Bu–CO₂–B(C₆F₅)₃2a, shows reversible and irreversible waves depending on the nature of the NHC. This combined experimental and theoretical study highlights the beneficial role of the FLP-type activation of CO₂ with NHC/BR₃ pair for the one electron redox event. The LUMOs of the NHC–CO₂–B(C₆F₅)₃ adducts are mostly located on the C_NHC which was shown to be the main site of reduction. The adduct 2d featuring a π-accepting CAAC exhibits less negative reduction potential than adducts with more classical NHCs. DFT calculations highlighted the role of the dihedral angle between the carbene and CO₂ moieties and of the borane in the reduction potential. The withdrawing effect of the Lewis acidic borane was indeed shown to lead to a less negative reductive potential (E_1/2 = −1.34 V vs. Fc^+/0 for 2d). Furthermore, the two species exhibiting reversible waves in the CV experiment were shown to afford stable anionic radical species by chemical reduction, which could be characterised by EPR for [2b˙⁻] and [2d˙⁻] and X-ray diffraction analysis for [2d˙⁻]. In these distonic species, the radical site was shown to be located not only on the C_NHC but also on the C_CO2 with a maximum spin density on the C_CO2 of 0.42 for [2b˙⁻]. Finally, [2b˙⁻] and [2d˙⁻] were shown to exhibit a reactivity similar to [CO₂˙⁻] toward DMPO. Future studies will further explore the reactivity of the stable and unstable radical anions of type [2˙⁻], as well as the broader use of FLP-type activation of carbon dioxide in single electron reduction transformations.

Data availability

Raw data of NMR, CV, EPR and X-ray crystallography are available upon reasonable request.

Author contributions

AM conducted the theoretical and experimental work. CG conducted the experimental work. AS-S and LV conducted CV and X-ray diffraction analyses, respectively. AL, OB and SB conceptualised the project and supervised the experimental and theoretical work. All authors participated in writing, reviewing and editing the original draft.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 860322. We acknowledge the valuable support of the NMR service and EPR platform at LCC-CNRS. Calculations on this project were carried out with the use of CSUC supercomputing resources. We thank Vincent César and Nicolas Queyriaux for fruitful discussions.

References

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2306774–2306777. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3sc06325a

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