Crystalline Anions Based on Classical N‐Heterocyclic Carbenes

Abstract

Herein, the first stable anions K[SIPr^Bp] (4 a‐K) and K[IPr^Bp] (4 b‐K) (SIPr^Bp=BpC{N(Dipp)CH₂}₂, IPr^Bp=BpC{N(Dipp)CH}₂; Bp=4‐PhC₆H₄; Dipp=2,6‐iPr₂C₆H₃) derived from classical N‐heterocyclic carbenes (NHCs) (i.e. SIPr and IPr) have been isolated as violet crystalline solids. 4 a‐K and 4 b‐K are prepared by KC₈ reduction of the neutral radicals [SIPr^Bp] (3 a) and [IPr^Bp] (3 b), respectively. The radicals 3 a and 3 b as well as [Me‐IPr^Bp] 3 c (Me‐IPr^Bp=BpC{N(Dipp)CMe}₂) are accessible as crystalline solids on treatment of the respective 1,3‐imidazoli(ni)um bromides (SIPr^Bp)Br (2 a), (IPr^Bp)Br (2 b), and (Me−IPr^Bp)Br (2 c) with KC₈. The cyclic voltammograms of 2 a–2 c exhibit two one‐electron reversible redox processes in −0.5 to −2.5 V region that correspond to the radicals 3 a–3 c and the anions (4 a–4 c)⁻. Computational calculations suggest a closed‐shell singlet ground state for (4 a–4 c)⁻ with the singlet‐triplet energy gap of 17–24 kcal mol⁻¹.

One‐electron reduction of the C2‐biphenylated 1,3‐imidazolinium cation (2 a ⁺) affords the radical 3 a, which undergoes further 1e‐reduction to yield the anion 4 a ⁻. In the solid‐state, 3 a is monomeric while 4 a‐K has a hexameric tubular structure. In addition to 3 a and 4 a‐K, synthesis, structures, and reactivity of stable radicals and anions based on unsaturated N‐heterocyclic carbene frameworks have been presented.

Introduction

Organic molecules exhibiting readily accessible multiple redox‐states are highly sought‐after species in materials science on account of their applications in data ^[1] and energy ^[2] storage as well as in logic operations suitable for quantum information science. ^[3] In this context, stable organic radicals and diradicals are of a particular significance as molecular building‐blocks for optoelectronic ^[4] and other energy related materials. ^[5] This is because of their open‐shell electronic structures, giving rise to interesting electronic, magnetic, and optical properties. ^[6] Radicals are well‐known reactive intermediates in organic chemistry, and in general highly reactive. ^[7] The first stable radical, the so‐called trityl radical Ph₃C, was isolated by Gomberg in 1900. ^[8] Currently, a variety of stable radicals are known, but most of them are based on a redox‐active (N, O, and S in particular) heteroatom. ^[9] Thus, the number of stable carbon‐centered radicals remained rather limited.[ ⁹ , ¹⁰ ]

Over the past years, stable singlet carbenes (i.e. N‐heterocyclic carbenes, NHCs; cyclic alkyl amino carbenes, cAACs) have been intensively explored for deriving stable organic as well as main‐group radicals. ^[11] Classical NHCs II (Scheme 1) are usually prepared by the deprotonation of 1,3‐imidazoli(ni)um salts (I‐H), which may formally be regarded as a two‐electron reduction process. One electron‐reduction products of I‐H would be radicals, e. g., III‐H. Already in 2004, Clyburne et al. probed the synthetic viability of an NHC‐radical III‐H (R=Mes=2,4,6‐Me₃C₆H₃) by the reduction of I‐H with potassium in boiling THF. III‐H was found to be unstable and decomposed into the free NHC (II) and, presumably, hydrogen. ^[12] This finding indicates the inappropriateness of hydrogen at the C2‐position of NHCs in accessing stable radicals. In 2015, we reported the direct C2‐arylation of NHCs by means of palladium catalysis to access C2‐arylated 1,3‐imidazol(ni)um salts IV‐Ar, ^[13] which proved to be appropriate precursors for the synthesis of C5‐protonated mesoionic carbenes (iMICs) V and related metal complexes. ^[14] Unlike I‐H, the cyclovoltammograms (CVs) of IV‐Ar show a reversible one‐electron (1e) reduction process that corresponds to the neutral radical species. In 2018, we reported the crystalline radicals VI and VII by KC₈ reduction of IV‐Ar. ^[15] Of note, C2‐arylated NHC‐radicals were previously proposed as intermediates in the n‐type doping of organic thin film transistors. ^[16] The stability of VI and VII is mainly attributed to the delocalization of the unpaired electron over the C2‐aryl substituent. Later, the radical VIII based on Bertrand's tetraarylated 1,3‐imidazole was also reported. ^[17] With a suitable reagent, radicals VI and VII can be easily oxidized back to the corresponding precursors IV‐Ar, which is consistent with their CVs. In principle, 1e‐reduction of VI and VII would lead to the corresponding anions (VI/VII)⁻, introducing an additional redox state to the systems. However, the absence of a related redox wave in the CVs of IV‐Ar rules out this feasibility. Indeed, no reaction was observed on treatment of VI and VII with KC₈. Unlike VII, radicals based on unsaturated NHCs such as VI showed limited stability in solutions and could not be characterized by single crystal X‐ray diffraction (sc‐XRD). ^[15]

Scheme 1.

Classical NHC precursors (I‐H), NHCs (II), a putative NHC‐radical (III‐H). iMIC‐precursors (IV‐Ar), iMICs (V), and stable radicals (VI, VII, VIII). Stable radicals (IX) and anions (IX)⁻ derived from C2‐biphenylated 1,3‐imidazoli(ni)um cations (IX)⁺.

To further examine the impact of the C2‐substituent on the stability and properties of radicals, ^[18] we sought to prepare new salts featuring a larger aryl substituent at the C2‐position of NHCs. Herein, we describe the synthesis of crystalline radicals IX as well as anions (IX)⁻ by the sequential 1e‐reduction of the corresponding C2‐biphenylated 1,3‐imidazoli(ni)um cations (IX)⁺. Notably, the use of a biphenyl substituent at the C2‐position of NHCs not only lowers the reduction potential of (IX) ⁺ (cf. IV‐Ar) to give the radicals IX but also brings in an additional stable redox state (i.e. the anions). The isolation of the anions (IX) ⁻ is unprecedented in the NHC chemistry.

Results and Discussion

The desired 1,3‐imidazoli(ni)um bromides (SIPr^Bp)Br (2 a), (IPr^Bp)Br (2 b), ^[14d] and (Me‐IPr^Bp)Br (2 c) (SIPr^Bp=BpC{N(Dipp)CH₂}₂, IPr^Bp=BpC{N(Dipp)CH}₂, Me‐IPr=BpC{N(Dipp)CMe}₂; Bp=4‐PhC₆H₄; Dipp=2,6‐iPr₂C₆H₃) were prepared by the direct C2‐arylation of the corresponding NHCs (SIPr (1 a), IPr (1 b), and Me‐IPr (1 c)) with 4‐bromobiphenyl (BpBr) using nickel catalysis as originally developed by this laboratory (Figure 1a). ^[14b] 2 a, 2 b, and 2 c are colorless air‐stable solids and exhibit expected ¹H and ¹³C NMR signals. The ¹H NMR spectra of 2 a–2 c each exhibit two doublets and one septet for the isopropyl groups. The 1,3‐imidazolinium back‐bone protons (CH ₂) of 2a appear as a singlet at 4.58 ppm in the ¹H NMR spectrum. The ¹³C{¹H} NMR spectrum of 2 a exhibits resonances for the Dipp and Bp groups, which can be appropriately assigned to the carbon nuclei based on ¹H^–13C heteronuclear multiple quantum coherence (HMQC) analyses. sc‐XRD analyses ^[19] of 2 a–2 c (Figure 1b) reveal the expected atom connectivity. The N1−C1−N2 bond angles (2 a 111.9(1), 2 b 107.2(2), 2 c 106.7(1)°) and the N1−C1/N2−C1 bond lengths (2 a 1.323(2)/1.333(2), 2 b 1.35(2)/1.349(2), 2 c 1.352(2)/1.353(2) Å) are consistent with the related 1,3‐imidazoli(ni)um cations. ^[13]

Figure 1.

(a) Synthesis of C2‐biphenylated 1,3‐imidazoli(ni)ium salts 2 a–2 c.(b) Solid‐state molecular structures of 2 a and 2 c, only the cationic part is shown. Hydrogen atoms are omitted and the aryl substituents are shown as wire‐frames for clarity. Thermal ellipsoids are drawn at the 50% probability. (c) Cyclovoltammograms (CVs) of 2 a–2 c (in CH₃CN/ 0.1 M nBu₄NPF₆, 100 mV s⁻¹, vs Fc⁺/Fc).

The CVs of C2‐monophenylated salts (IV‐Ar, Ar=Ph or 4‐Tol) ^[15] show only a single 1e‐reversible reduction process that corresponds to the redox couple IV‐Ar/VI or VII (Scheme 1). In strong contrast, the CVs of the C2‐biphenylated species 2 a, 2 b, and 2 c (Figure 1c) exhibit two well‐separated 1e‐reversible redox processes in the negative potential region of −0.5 to −2.5 V (vs. Fc/Fc⁺). The first reduction potential (E _1/2) for 2 a (−1.3 V), 2 b (−1.4 V), and 2 c (−1.6 V) each is consistent with the corresponding neutral mono‐radicals. They are, however, at slightly lower negative potential than those of monophenylated derivatives (SIPr^Ph)Br (−1.4 V) and (IPr^Ph)Br (−1.7 V). ^[15] Treatments of 2 a, 2 b, and 2 c with KC₈ lead to the formation of radicals 3 a (blue), 3 b (green), and 3 c (green) as stable crystalline solids, respectively (Scheme 2). The appearance of the second reversible redox wave at E _1/2=−1.9 V (2 a), −2.0 V (2 b), and −2.1 V (2 c) is remarkable and suggests the feasibility of further reduction of the radicals 3 a–3 c into the anions. ^[20] Treatment of a blue (or green) THF solution of 3 a (or 3 b/3 c) with KC₈ leads to the clean formation of the anion 4 a‐K (or 4 b‐K/4 c‐K) as a violet solid. The accessibility as well as the identity of the neutral radicals 3 a–3 c and anions (4 a–4 c)⁻ has also been corroborated by spectro‐electrochemical (SEC) studies (see the Supporting Information).

Scheme 2.

Synthesis of radicals 3 a–3 c and anions 4 a‐K–4 c‐K. Selected resonance forms A‐D of the anion (4 a)⁻.

The radicals 3 a–3 c are stable in solutions as well as in the solid‐state under an inert gas atmosphere and have been characterized by UV/Vis, EPR spectroscopy, and sc‐XRD (Figure 2). Note, unlike 3 b and 3 c, previously reported radicals VI (Scheme 1) based on an unsaturated NHC (i.e. IPr) ^[15] slowly decayed in solutions and hence could not be characterized by sc‐XRD. ^[21] Thus, the use of a biphenyl substituent at the C2‐position of NHCs enhances the stability of derived radicals by providing an additional room for the delocalization of the unpaired electron. ^[11l] The anions 4 a‐K–4 c‐K are extremely reactive and slowly decay in THF to partly form the corresponding radicals 3 a–3 c, leading to the broadening of NMR signals. Remarkably, freshly prepared THF solutions of 4 a‐K and 4 b‐K containing a small pinch of KC₈ (≈1 mg for 0.5 mL solution) exhibited well‐resolved sharp ¹H and ¹³C NMR signals, suggesting their diamagnetic property. This approach was unfortunately not successful for 4 c‐K, presumably due to its slightly higher reducing property (−2.1 V) than 4 a‐K/4 b‐K (−1.9/−2.0 V). The ¹H NMR spectra of 4 a‐K and 4 b‐K show expected signals for the biphenyl (p‐C₆ H ₄C₆ H ₅) moiety, which are high‐field shifted compared to those of 2 a and 2 b. The high‐field shifting of the Bp protons of 4 a‐K and 4 b‐K may be rationalized considering the delocalization of the electron lone‐pair over the Bp‐group as shown with the resonance structures A‐D (Scheme 2). This is further corroborated by DFT calculations (see below).

Figure 2.

Solid‐state molecular structures of radicals 3 a, 3 b, and 3 c. Hydrogen atoms are omitted and Dipp groups are shown as wireframes. Thermal dispacement ellipsoids are drawn at the 50 % probability level.

Single crystals of the radicals 3 a, 3 b, and 3 c for sc‐XRD were obtained on storing a saturated n‐hexane solution of each at −32 °C. The solid‐state molecular structures of 3 a, 3 b, and 3 c are shown in Figure 2. As previously observed for the mono‐radicals VII (R′=H, Scheme 1), ^[15] the C1−C4 (1.402(1) Å) and C1−N1/N2 (1.388(1)/1.398(1) Å) bond lengths are longer while the N1−C1−N2 (108.1(1)°) bond angle is more acute for 3 a with respect to those of 2 a (Table 1). The C1−C4 (1.402(1) Å) and C7−C8 (1.41(2) Å) bond lengths of 3 a are shorter than 2 a (1.471(2), 1.481(2) Å, respectively). Moreover, the bond length alteration (BLA) for the C−C bonds of the Bp‐rings in 3 a (0.05–0.08 Å) is larger than that of 2 a (0.01 Å). These features indicate the delocalization of the unpaired electron over the C2‐biphenyl unit, which accounts for the remarkable thermal stability of the radicals 3 a–3 c. Similar trend in the structural parameters (C1−C2, C1−N1/N2 bond lengths, N1−C1−N2 bond angle, and BLA in Bp rings) of 3 b and 3 c is observed (see the Supporting Information for details). It is worth mentioning that the radicals VI (Scheme 1) based on an unsaturated NHC (i.e. IPr or Me‐IPr) containing a mono‐phenylated (Ar=Ph, 4‐Tol or 4‐Me₂NC₆H₄) C2‐substituent have limited thermal stability and undergo bond activation on storage of their solutions at room temperature to form oxidized products (see Scheme S1 for an example). ^[15] This may be rationalized as the formation of VI (7π electron C₃N₂‐ring) from IV‐Ar (6π‐electron planar C₃N₂‐ring) occurs at the expense of aromaticity. This is not the case with SIPr‐based radicals VII featuring a non‐planar C₃N₂‐ring. The presence of an additional phenyl ring in 3 b and 3 c provides an extra room for the delocalization of the unpaired electron and hence contributes to their stability. This is also in line with the spin‐density distribution in 3 a–3 c (see below).

Table 1.

Selected experimental [and calculated at BP86‐D3(BJ)/def2‐SVP] bond lengths and bond angles of 2 a, 3 a, and (4 a)⁻. For (4 a)⁻, the values are given only for one of the three moieties of [(4 a‐K)₃(THF)₂]₂ (Figure 3).

		C α −C i	C i −C o	C o −C m	C m−C p	C p −C i′	C i′ −C o′	C o′ −C m′	C m′ −C p′
	2 a 3 a (4 a)−	1.471(2) [1.461] 1.402(1) [1.419] 1.366(2) [1.396] {1.389}[a]	1.399(2) 1.399(2) [1.415] 1.431(1) 1.430(1) [1.438] 1.460(2) [1.457] {1.459}[a]	1.381(2) 1.388(2) [1.394] 1.368(1) 1.373(1) [1.387] 1.366(2) [1.383] {1.377}[a]	1.399(2) 1.399(2) [1.417] 1.414(1) 1.412(2) [1.425] 1.433(2) [1.434] {1.448}[a]	1.481(2) [1.479] 1.469(1) [1.472] 1.440(2) [1.454] {1.449}[a]	1.398(2) 1.402(2) [1.416] 1.406(2) 1.403(1) [1.421] 1.424(2) [1.434] {1.438}[a]	1.385(2) 1.387(3) [1.401] 1.386(1) 1.385(2) [1.400] 1.378(2) [1.397] {1.397}[a]	1.384(3) 1.387(3) [1.405] 1.388(2) 1.386(2) [1.406] 1.398(2) [1.411] {1.406}[a]

[a] within the hexamer [(4 a‐K)₃(THF)₂]₂, see Figure 3.

For 4 a‐K, single crystals suitable for sc‐XRD were obtained by storing a saturated n‐heptane solution of 4 a‐K at 4 °C for four days. The molecular structure of 4 a‐K has C _i symmetry featuring a hexameric cyclic structure with four THF molecules, i.e. [(4 a‐K)₃(THF)₂]₂ (Figure 3). Two potassium atoms bear a THF molecule, while one potassium coordinates with the ortho‐ and ipso‐carbons of the biphenyl substituent of two (4 a)⁻ anions. The potassium atom coordinated by THF has close contacts with the aryl‐carbon atoms of a Dipp substituent. Thus, each of the potassium atoms is formally eight‐coordinated. The C1−C4, C40−C43, and C79−C82 bond lengths (av. 1.366(2) Å) of 4 a‐K are shorter than the corresponding bonds of 2 a (1.471(2) Å) and 3 a (1.402(1) Å), indicating a considerable double bond character (see below for NBO analyses). This suggests that the negative charge in 4 a‐K is mostly located at the o,o′ and p,p′‐carbon atoms of the biphenyl group (see resonance structures A‐D, Scheme 2), which is consistent with computational calculations (see below). All three potassium atoms have close contacts with the o,o′,p,p′‐carbon atoms of the biphenyl substituents. The C−K interatomic distances range from 2.911(1) to 3.522(1) Å, which are shown as broken lines in Figure 3. As expected, the BLA for the Bp rings of 4 a‐K (0.03–0.08 Å), like in 3 a (0.05–0.08 Å), is larger than that of 2 a (0.01 Å).

Figure 3.

Solid‐state molecular structure of [(4 a‐K)₃(THF)₂]₂. Dipp groups are shown as wireframe models, H atoms and minor occupied disordered atoms have been omitted, and only the oxygen atom of THF is shown for clarity. Thermal dispacement ellipsoids are drawn at the 50 % probability level. Symmetry code used: 1−x, 1−y, −z.

The optimized structures of 3 a–3 c (Figures S35) at the BP86‐D3(BJ)/def2‐SVP as well as at the BP86‐D3(BJ)/def2‐TZVP level of theory are in good agreement with those determined by sc‐XRD (Figure 2). The calculated natural partial charges (NPAs) for 3 a–3 c (Table 2, Table S6) suggest that the Bp moiety is negatively charged by −0.31(3 a), −0.35(3 b), and −0.38 (3 c) with the first ring bearing the major part of the negative charge (see Figure S39). The EPR spectrum of 3 a (Figure 4a, see the Supporting Information for 3 b and 3 c) measured at 298 K in THF exhibits a characteristic doublet signal. The EPR spectra of 3 a–3 c were successfully simulated using calculated coupling constants (Table S12). The hyperfine coupling constants (hfc) are in good agreement with those calculated for VI and VII. ^[15] The singly occupied molecular orbitals (SOMOs) of 3 a–3 c (Figure 4b, Figure S37) reveal an out‐of‐phase combination of the p‐orbital at C _α with the adjacent nitrogen lone pair orbitals, while there is an in‐phase combination between the p‐orbital of C _α and C _i , which is in good agreement with the C _α −C _i double bond character (Wiberg Bond Index=1.27e). Strikingly, there is a strong conjugation within the biphenyl substituent. Indeed, the plots of Mulliken spin densities for 3 a–3 c (Figure 4c, Figure S38) show that the unpaired electron is mostly localized at the carbene carbon atom (27–39 %) and the para‐C atom (26–27 %) of the first benzene ring. Interestingly, ≈15 % of the radical electron are located on the C _ortho and C _para atoms of the terminal phenyl ring. This number shows the electron withdrawing effect of the para‐phenyl substituent at the C₆H₄‐ring, enhancing the overall stability of the C2‐biphenylated radicals 3 a–3 c as well as introducing the possibility of hosting an additional electron into the SOMO to give stable anions (4 a–c)⁻. Notably, the SOMOs of 3 a (−2.81 eV) and 3 b (−2.61 eV) are lower in energy than the corresponding C2‐phenylated radicals VII (−2.67 eV) and VI (−2.40 eV). ^[15] The UV/Vis spectra of 3 a–3 c (Figure 4d, Figure S19–S21) show three main absorption bands (λ_max (in nm)=314, 393, 637 (3 a); 301, 438, 684 (3 b), 308, 406, 679 (3 c)). According to TD‐DFT studies (Tables S9–S11), these absorption bands of 3 a are related to HOMO‐1→LUMO+1, HOMO→LUMO+5, and HOMO→LUMO transitions, respectively (HOMO=highest occupied molecular orbital; LUMO=lowest unoccupied molecular orbital). Based on TD‐DFT analyses (Table S12–S14), the UV/Vis absorption bands (λ_max (in nm)=294, 400, and 600 nm) in the SEC spectrum of the anion 4 a ⁻ (Figure 4d) are related to HOMO‐1→LUMO, HOMO→LUMO+5/LUMO+8, and HOMO→LUMO, respectively (see the Supporting Information for 4 b ⁻ and 4 c ⁻).

Table 2.

Selected NPA charges (Q) and WBIs of 2 a, 3 a, and (4 a)⁻.

		NPA Charge (Q)
		C α	N	Bp	C i /C i′	C o /C o′	C m /C m	C p /C p′
	2 a 3 a (4 a)−	0.51 0.37 0.23	−0.33 −0.41 −0.44	0.15 −0.31 −0.80	−0.16/−0.08 −0.18/−0.05 −0.19/−0.04	−0.16/−0.19 −0.22/−0.21 −0.27/−0.24	−0.20/−0.20 −0.20/−0.21 −0.20/−0.22	0.01/−0.19 −0.08/−0.24 −0.18/−0.31

		WBI
		N−C α	C α −C i	C i −C o	C o −C m	C m −C p	C p −C i′	C i′ −C o′
	2 a 3 a (4 a)−	1.29 1.09 1.00	1.10 1.27 1.42	1.34 1.23 1.16	1.48 1.55 1.61	1.36 1.30 1.25	1.09 1.12 1.20	1.37 1.35 1.29

Figure 4.

(a) X‐Band continuous wave EPR spectrum in THF at 298 K (ν=9.63 GHz, P _mw=2 mW, Mod. Amp. 0.3 mT), (b) plot of the SOMO, and (c) Mulliken spin density distribution (isovalue 0.003 a.u.) for 3 a. (d) UV/Vis SEC (spectroelectrochemical) spectra of radical (3 a) and anion (4 a⁻ ) recorded using 2 a in 0.1 M nBu₄NPF₆ in acetonitrile (see the Supporting Information for details).

To examine the electronic structures of 4 a‐K–4 c‐K, we performed quantum‐chemical calculations at the BP86‐D3(BJ)/def2‐SVP level of theory for the anionic species (4 a–4 c)⁻ without the inclusion of the counter cation. The comparison with the computed [(4 a‐K)₃(THF)₂]₂ structure reveal similar bond lengths (Table 1). As stated above, 1e‐reduction of 3 a to give 4 a‐K leads to the elongation of the C1−N1/N2 and shortening of the C1−C4 bond lengths (Table 1). The NPA charges (Table 2) indicate that the negative charge is mostly located at the biphenyl moiety (4 a ⁻: −0.80 e, 4 b ⁻: −0.74 e, 4 c ⁻: −0.77 e). Thus, the anions (4 a–4 c)⁻ are stabilized by the delocalization of the electron lone‐pair over the biphenyl ring. Calculations predict singlet ground state of the anions (4 a–4 c)⁻ (Table 3). This is consistent with their well‐resolved NMR signals measured at room temperature (see above). We also examined the diradical character of the model species (4aM–4cM)⁻, in which the Dipp substituents of (4 a–4 c)⁻ are replaced by methyl groups, by CASSCF+NEVPT2(12,12) calculations. Our active space includes the most relevant biphenyl group. The CI vector has a contribution of 0.82 for the (222222000000) configuration and 0.03 for the (222220200000), which reveals a weak multiconfigurational nature of the anionic species (4aM–4cM)⁻ (Table S8). The diradical character calculated with these values according to Bachler et al. amounts to 6.7 %. ^[22]

Table 3.

Singlet‐triplet energy gap (ΔE _S‐T , in kcal mol⁻¹) for the anions (4 a–4 c)⁻ and their model systems (4 aM–4 cM)⁻ in which Dipp substituents were replaced by methyl groups.

Method	(4 a)−	(4aM)−	(4 b) −	(4 bM)−	(4 c)−	(4 cM)−
B3LYP+D3(BJ)/def2‐TZVPP[a]	24.0	34.1	19.5	26.1	16.9	28.6
PBE0+D3(BJ)/def2‐TZVPP[a]	25.0	35.3	20.0	25.7	17.2	30.6
M06‐2X/def2‐TZVPP[a]	32.2	40.9	25.6	33.5	22.7	35.4
CASSCF(12,12)/def2‐TZVP	–	40.3	–	41.9	–	40.3
CASSCF+NEVPT2(12,12)/def2‐TZVP	–	37.1	–	34.4	–	36.2

[a] Single point on the optimized geometry at BP86+D3(BJ)/def2‐SVP.

Efforts to obtain single crystals of 4 b‐K were unsuccessful due to its slow decay in solutions to form 3 b. We attempted the synthesis of a compound featuring an organic cation by reacting 4 b‐K with (Ph₄P)Cl. The reaction however led to the formation of radical 3 b and PPh₃ instead of the expected cation exchange product 4 b‐PPh₄ (Scheme 3). This suggests strong reducing property of (4 b)⁻ that induces P−C_Ph bond cleavage of (Ph₄P)⁺ to form 3 b and PPh₃ along with the C−C coupling product biphenyl (PhPh). ^[23] The degradation of (Ph₄P)⁺ with lithium amides via free‐radical pathways is literature known. ^[24] As expected, treatment of 4 a‐K (or 4 b‐K) with 2 a (or 2 b) cleanly affords 3 a (or 3 b). The reducing property of 3 b was further explored with Ph₂PCl and Ph₂P(O)Cl, which resulted in the homocoupling products (Ph₂P)₂ (³¹P NMR=−14.9 ppm) ^[25] or (Ph₂P(O))₂ (³¹P NMR=26.6 ppm) ^[26] and the salt (2 b)Cl. Treatment of 4 a‐K with AgOTf leads to the clean formation of the radical 3 a, which further reacts with an additional AgOTf to give (2 a)OTf (see Table S1 for the titration of 4 a‐K with AgOTf).

Scheme 3.

Reaction of 4 b‐K with (Ph₄P)Cl to Ph₃P and 3 b. Reductive homocoupling of Ph₂PCl and Ph₂P(O)Cl with 3 b. Reactions of 4 a‐K and 3 a with AgOTf to 3 a and (2 a)OTf, respectively.

Conclusion

In conclusion, the accessibility of three stable redox states has been shown with C2‐biphenylated 1,3‐imidazoli(ni)um salts 2 a–2 c, which forms stable radicals 3 a–3 c and anions 4 a‐K–4 c‐K on sequential one‐electron reduction with KC₈. The use of a biphenyl substituent at the C2‐position of classical NHCs enhances the stability of the corresponding radicals (3 a–3 c) compared to mono‐phenylated (Ar=Ph or substituted monoaryl) derivatives. Remarkably, this also introduces an additional stable redox state to the systems, the anions (4 a–4 c)⁻. The isolation of crystalline anions is unprecedented in the NHC chemistry. The radicals 3 a–3 c have been characterized by EPR spectroscopy and sc‐XRD. The anions 4 a‐K and 4 b‐K have been characterized by NMR spectroscopy and the solid‐state molecular structure of 4 a‐K has been established by sc‐XRD. Reactivity studies of 3 a, 3 b and 4 a‐K, 4 b‐K towards Ph₂PCl, Ph₂P(O)Cl and (Ph₄P)Cl have been shown. The facile accessibility of 4 a‐K and 4 b‐K is expected to add new facets to the NHC as well as organometallic chemistry.

Conflict of interest

The authors declare no conflict of interest.

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Dedicated to Professor Guy Bertrand on the occasion of his 70th birthday.

Merschel A., Rottschäfer D., Neumann B., Stammler H.-G., Ringenberg M., van Gastel M., Demirer T. I., Andrada D. M., Ghadwal R. S., Angew. Chem. Int. Ed. 2023, 62, e202215244; Angew. Chem. 2023, 135, e202215244.

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.