NHC‐Terphenyl Radicals and Anions: Tuning Stability and Redox Properties via Substituent Patterning

Abstract

Herein, we report the influence of C2‐terphenyl substitution patterns (i.e., p‐terphenyl versus m‐terphenyl) on the redox behavior and stability of the corresponding radicals and anions derived from N‐heterocyclic carbenes (NHCs). Three well‐known NHCs; SIPr (1a), IPr (1b), and Me‐IPr (1c) (SIPr = C{N(Dipp)CH₂}₂, IPr = C{N(Dipp)CH}₂; Me‐IPr = C{N(Dipp)CCH₃}₂; Dipp = 2,6‐iPr₂C₆H₃); were functionalized at the C2 position using 4‐bromo‐p‐terphenyl (p‐TerBr) and 5′‐bromo‐m‐terphenyl (m‐TerBr) under nickel catalysis, yielding the corresponding cations [(NHC)p‐Ter]Br (2a–c) and [(NHC)m‐Ter]Br (3a–c), respectively. Cyclic voltammetry (CV) measurements of 2a–c reveal two distinct reversible redox events, while 3a–c exhibit one reversible and one irreversible or quasi‐reversible wave. Reduction of 2a–c and 3a–c with KC₈ readily affords stable radicals [(NHC)p‐Ter]^● (4a–c) and [(NHC‐m‐Ter]^● (5a–c), isolated as crystalline solids. Further reduction of 4a–c produces diamagnetic anions [(NHC)p‐Ter]K (6a–c‐K), consistent with the electrochemical data. In contrast, 5b and 5c are unreactive toward KC₈ under similar conditions, while 5a (derived from the more electrophilic NHC 1a) can be reduced to the corresponding anion [(SIPr)m‐Ter]K (7a‐K). Selected compounds have been characterized by spectroscopic techniques and single‐crystal X‐ray diffraction, with computational studies supporting the experimental findings. The results highlight how the NHC and the C2‐terphenyl substituent influence the properties and stability of the resulting species.

Two series of C2‐terphenylated derivatives of three N‐heterocyclic carbenes (NHCs) have been reported. The para‐terphenylated species exhibit three stable (reversible) redox states (e.g., [2b]⁺, [2b]^● and [2b]⁻), while for the meta‐terphenylated derivatives only cationic and neutral states (i.e., [3b]⁺ and [3b]^●) are reversible.

Introduction

Organic compounds capable of maneuver between multiple redox states are of growing importance in modern materials science, with promising applications in data^{[ 1} , ² , ^{3 ]} and energy storage,^{[ 4} , ⁵ , ⁶ , ⁷ , ⁸ , ⁹ , ^{10 ]} quantum information technologies,^{[ 11} , ^{12 ]} and biomedical fields.^{[ 13} , ^{14 ]} Among these, stable organic radicals serve as key molecular building blocks for optoelectronic^{[ 15} , ¹⁶ , ¹⁷ , ¹⁸ , ¹⁹ , ²⁰ , ²¹ , ²² , ²³ , ²⁴ , ²⁵ , ²⁶ , ^{27 ]} and energy‐related materials,^{[ 28} , ^{29 ]} owing to their open‐shell electronic structures, which impart valuable electronic, magnetic, and optical properties.^{[ 30} , ³¹ , ^{32 ]} Traditionally, radicals in organic chemistry have been viewed as highly reactive intermediates.^{[ 33 ]} The first stable organic radical, the so‐called trityl radical Ph₃C^●, was isolated by Gomberg in 1900.^{[ 34 ]} Since then, numerous radicals have been synthesized—most featuring halogenated substituents on the carbon center or redox‐active heteroatoms such as nitrogen, oxygen, or sulfur.^{[ 35} , ³⁶ , ^{37 ]} However, truly carbon‐centered stable radicals remained rare.^{[ 38} , ³⁹ , ^{40 ]} Recent studies nonetheless emphasized that achieving stability in organic radicals requires carefully designed molecules that provide both steric protection and/or spin delocalization.^{[ 38} , ³⁹ , ^{40 ]} This insight led to the development of thermally stable carbon‐centered radicals based on polycyclic aromatic hydrocarbons^{[ 40} , ⁴¹ , ⁴² , ⁴³ , ^{44 ]} and N‐heterocyclic carbenes (NHCs),^{[ 45} , ⁴⁶ , ⁴⁷ , ⁴⁸ , ⁴⁹ , ^{50 ]} many of which were successfully isolated in the late 2010s. Among stable singlet carbenes,^{[ 51} , ⁵² , ⁵³ , ^{54 ]} the use of cyclic alkyl amino carbenes (cAACs)^{[ 55} , ⁵⁶ , ⁵⁷ , ^{58 ]} with remarkable electrophilic characteristics in the stabilization of radicals and related open‐shell species is noteworthy.^{[ 55} , ⁵⁶ , ⁵⁷ , ^{58 ]}

In 2004, Clyburne et al., attempted synthesis of the radical I‐H (Figure 1) by reducing the corresponding 1,3‐imidazolium salt [(IMes)H]Cl with potassium in boiling THF.^{[ 59} , ^{60 ]} I‐H was found to be unstable and decomposed to form the free NHC (IMes). This outcome was in agreement with the cyclic voltammogram (CV) of [(IMes)H]Cl, which exhibits an irreversible reduction at a high negative potential (E _pc = −2.38 V versus SCE, E _pc = cathodic peak potential).^{[ 59 ]} In contrast, the CVs of the related C2‐arylated 1,3‐imidazoli(ni)um salts [(NHC)Ar]X, which can be obtained by the direct C2‐arylation of NHCs (I) under Ni or Pd catalysis,^{[ 61} , ^{62 ]} show one reversible redox process at a lower negative potential (> −2 V versus Ag/Ag⁺) related to the [(NHC)Ar]⁺/[(NHC)Ar]^● redox couple.^{[ 63 ]} Consistently, the radicals [(NHC)Ar]^● are isolable as stable crystalline solids. Moreover, the reduction potential (E _1/2) of [(NHC)Ar]⁺ can be readily adjusted by varying the NHC and/or the C2‐aryl group. Also, the applications of such NHC‐based radicals in optoelectronic materials have been started to evolve.^{[ 64} , ⁶⁵ , ⁶⁶ , ⁶⁷ , ^{68 ]}

Figure 1.

Schematic illustration of [(NHC)H]X, [(IMes)H]^●, and NHC (I). NHC‐derived stable radicals [(NHC)Ar]^●, anions [(NHC)Ar]⁻, radical cations [(NHC)₂(C₆H₄) _n ]^●+, and diradical(oid)s [(NHC)₂(C₆H₄) _n ]^●●.

Expectedly, related dicationic compounds [(NHC)₂(C₆H₄) _n ](X)₂ with a phenylene (n = 1),^{[ 69} , ^{70 ]} biphenylene (n = 2),^{[ 69} , ⁷⁰ , ⁷¹ , ^{72 ]} or terphenylene (n = 3)^{[ 73 ]} spacer show additional stable redox states and can be reduced to stable radical cations [(NHC)₂(C₆H₄) _n ]^●+ and diradical(oid)s [(NHC)₂(C₆H₄) _n ]. The latter (see the diradical form) can also be considered as NHC‐derivatives of para‐quinodimethane (p‐QDM) (see the quinoidal form).^{[ 74 ]} The stability of these radical and diradical species is largely attributed to the delocalization of the unpaired electron over the C2‐aryl or the π‐spacer moiety (C₆H₄) _n . Remarkably, a further extension of the C2‐aryl group at the para position by an aryl substituent such as biphenyl (Bp) in [(NHC)Bp]X results in the introduction of a new redox state as evident by their CVs, which show two reversible redox processes. The first corresponds to the [(NHC)Bp]⁺/[(NHC)Bp]^● and the second to the [(NHC)Bp]^●/[(NHC)Bp]⁻. In fact, both radicals [(NHC)Bp]^● and anions [(NHC)Bp]⁻ are isolable stable compounds.^{[ 75 ]} Thus, the larger size of the C2‐substituent (i.e., Bp) provides more room for the delocalization of the unpaired electron and can also accommodate an extra electron, giving rise to anions. We therefore prompted to introduce an additional phenyl group to the C2‐aryl substituent in the [(NHC)Ar] framework and explore the structure and properties of related NHC‐derivatives.

Herein, we present synthesis of two classes of terphenylated systems namely [(NHC)p‐Ter]Br (2a, 2b, 2c) and [(NHC)m‐Ter]Br (3a, 3b, 3c) with three distinct NHCs namely SIPr (1a), IPr (1b), and Me‐IPr (1c) (SIPr = C{N(Dipp)CH₂}₂, IPr = C{N(Dipp)CH}₂; Me‐IPr = C{N(Dipp)CCH₃}₂; Dipp = 2,6‐iPr₂C₆H₃) and report their structures, electrochemistry, and the corresponding radicals and anions.

Results and Discussions

The desired 1,3‐imidazoli(ni)um bromides [(SIPr)p‐Ter]Br (2a), [(IPr)p‐Ter]Br (2b), [(Me‐IPr)p‐Ter]Br (2c), [(SIPr)m‐Ter]Br (3a), [(IPr)m‐Ter]Br (3b), and [(Me‐IPr)m‐Ter]Br (3c) (Scheme 1a) were prepared by the direct C2‐arylation of NHCs (i.e., SIPr (1a) = C{N(Dipp)CH₂}₂, IPr (1b) = C{N(Dipp)CH}₂, Me‐IPr (1c) = C{N(Dipp)CCH₃}₂; Dipp = 2,6‐iPr₂C₆H₃) with an appropriate aryl bromide (4‐Bromo‐p‐terphenyl = p‐TerBr or 5′‐Bromo‐m‐terphenyl = m‐TerBr) using 3 mol% of Ni(cod)₂ (cod = cyclooctadiene).^{[ 61 ]} Compounds 2a–c and 3a–c are colorless air‐stable crystalline solids. The ¹H and ¹³C{¹H} NMR spectra of 2a–c and 3a–c exhibit expected signals (see, Figures S5–S22), which are in line with those of known C2‐arylated 1,3‐imidazol(ni)um salts.^{[ 61} , ⁶² , ⁶³ , ^{75 ]} Single crystal X‐ray diffraction (sc‐XRD) analyses of 2a and 3a (Figure 2) show the expected atom connectivity (see below).^{[ 76 ]}

Scheme 1.

a) Synthesis of C2‐p‐terphenylated (2a, 2b, and 2c) and C2‐m‐terphenylated (3a, 3b, and 3c) 1,3‐imidazoli(ni)um salts from classical NHCs 1a, 1b, and 1c. b) Cyclic voltammograms (CVs) of 2a, 2b, 2c and 3a, 3b, 3c (in CH₃CN, 0.1 M nBu₄NPF₆, 100 mV s⁻¹, versus Ag/Ag⁺).

Figure 2.

Solid‐state molecular structures of 2a, 3a, 4a, and 5a. Hydrogen atoms (and bromide anions for 2a and 3a) are omitted and Dipp groups are shown as wireframes for clarity. Thermal ellipsoids are set at 50% probability.

The CVs of 2a–c (Scheme 1b) show two reversible redox processes, which are similar to those of [(NHC)Bp]Br.^{[ 75 ]} The first redox event at E _1/2 = −1.35 (for 2a), −1.60 (for 2b), and −1.70 V (for 2c) (versus Ag/Ag⁺) is in line with the electrophilic characteristics of NHCs (1a > 1b ≥ 1c). A similar trend for the second reversible redox wave is also observed for 2a–c [E _1/2 = −1.85 (for 2a), −2.00 (for 2b), −2.10 V (for 2c)]. The E _1/2 values for the first and second redox events for 2a–c are comparable to those of [(NHC)Bp]Br (NHC = SIPr: −1.31, −1.92 V; IPr: −1.51, −2.02 V, Me‐IPr: −1.59, −2.09 V).^{[ 75 ]} Like 2a–c, the CVs of 3a, 3b, and 3c (Scheme 1b) also show one reversible redox event at E _1/2 = −1.40, −1.60, and −1.75 V (versus Ag/Ag⁺), respectively. However, the second reduction event at E _pc = −2.20, −2.30, and −2.35 V (E _pc = cathodic peak potential) for 3a, 3b, and 3c, respectively, is irreversible. The outcomes of CVs studies suggest that one‐electron reduction of 2a–c and 3a–c to the corresponding neutral radicals should be feasible for all, but their further reductions to the corresponding anions seem more likely for the p‐terphenyl derivatives, i.e., [(NHC)p‐Ter]⁻.

Treatments of 2a, 2b, and 2c with one equivalent of potassium graphite (KC₈) yield the radicals, [(SIPr)p‐Ter]^● (4a) (green), [(IPr)p‐Ter]^● (4b) (green), and [(Me‐IPr)p‐Ter]^● (4c) (yellow‐green) as crystalline solids, respectively (Scheme 2). Similarly, reactions of 3a, 3b, and 3c with one equivalent of KC₈ afford crystalline radicals [(SIPr)m‐Ter]^● (5a) (green), [(IPr)m‐Ter]^● (5b) (green), and [(Me‐IPr)m‐Ter]^● (5c) (yellow‐green), respectively. Compounds 4a–c and 5a–c are stable at room temperature under an inert gas atmosphere and are NMR silent. The radicals 4a–c and 5a–c have been characterized by UV–vis and EPR spectroscopy as well as by sc‐XRD (Figure 2).

Scheme 2.

KC₈ reductions of 2a–c and 3a–c to the radicals 4a–c and 5a–c, respectively.

According to the CVs of 2a–c (Scheme 1b), further 1e‐reductions of 4a–c or direct two‐electron reductions of 2a–c to the corresponding anions seem viable. Indeed, treatments of 2a–c (or 4a–c) with two equivalents (or one equivalent) of KC₈ yields the anions K[(NHC)p‐Ter] (6a–c‐K) (Scheme 3a). Compounds 6a–c‐K are extremely reactive and slowly decay in THF to partially form the corresponding radicals, making their characterization by NMR spectroscopy difficult. This is not surprising considering their strong tendency to be oxidized [cf. E _1/2 = −1.85 (for 2a), −2.00 (for 2b), −2.10 V (for 2c)]. This problem can be overcome by adding a small pinch of KC₈ (∼ 2 mg for 0.5 mL THF‐d ₈ solution) into a NMR sample of 6a–c–K. Compounds 6a–c–K are diamagnetic and show well‐resolved NMR signals (Figures S23–S28). The ¹H NMR spectra of 6a‐K and 6c‐K show expected signals for the NHC and p‐terphenyl moieties, which are high‐field shifted relative to those of 2a and 2c. In particular, the high‐field shifting of ¹H NMR signals for the p‐terphenyl is more distinct and relates to NHC‐based p‐QDM derivatives.^{[ 69 ]} Compound 6a‐K is rather stable and can be isolated as a blue crystalline solid. The stability of 6a–c‐K, like radicals 4a–c as well as previously reported anions K[(NHC)Bp],^{[ 75 ]} can be attributed to the delocalization of the electron lone pair over the C2‐p‐Ter group (see selected resonance structures A–F, Scheme 3b). DFT calculations further support this analysis (see below).

Scheme 3.

a) KC₈ reductions of 2a–c or 4a–c to the anions 6a–c‐K, respectively. b) Selected resonance forms (A–F) of the anion [6a]⁻. c) KC₈ reductions of 3a or 5a to the anion 7a‐K (with selected resonance forms A and B).

The CVs of 3a, 3b, and 3c (Scheme 1b) feature a second irreversible reduction at a rather higher negative potential, E _pc = −2.20, −2.30, and − 2.35 V, respectively, relative to that of 2a–c. Consistently, no reaction between a THF‐d ₈ solution 5b or 5c with KC₈ was observed at room temperature as evident by no change in green color as well as in the ¹H NMR spectrum. However, a green THF‐d ₈ solution of 5a immediately turned deep blue on addition of KC₈, suggesting the formation of the anion K[(SIPr)m‐Ter] (7a‐K). The ¹H and ¹³C{¹H} NMR spectra measured for the same sample reveal diamagnetic feature of 7a‐K, which compare well with those of 6a‐K and 6c‐K. It is worth noting that the ¹H NMR signals for the C₆H₅ protons of the m‐terphenyl group of 7a‐K (6.92–6.96 ppm) appear in the aromatic region and are nearly comparable to those of 3a (6.91–7.34 ppm in CDCl₃). This indicates that the electron lone pair in 7a‐K delocalizes only over the central C₆H₃‐ring (cf. resonance structure A and B in Scheme 3c) and both m‐C₆H₅ rings remain unchanged. This is further corroborated by DFT calculations (see below). The lower reduction potentials (2a, 3a) and greater stability of the SIPr‐derivatives (4a, 5a and 6a‐K, 7a‐K) than those of IPr‐ and Me‐IPr‐compounds (respectively, 2b,c/3b,c; 4b,c/5b,c and 6b,c‐K, 7b,c‐K) may be rationalized considering superior π‐acceptor property of SIPr with respect to that of IPr and Me‐IPr.^{[ 77} , ⁷⁸ , ^{79 ]} Thus, the extent of π‐delocalization and hence the properties of derived molecules can be controlled by varying the nature of NHCs.^{[ 80 ]} Also, the formation of 4b,c and 5b,c (featuring 7π electron C₃N₂‐ring) from 2b,c and 3b,c (containing 6π electron planar C₃N₂‐ring), respectively, occurs at the expense of Hückel's aromaticity. This is not the case with SIPr‐derived compounds 4a, 5a, 6a‐K, and 7a‐K as they all have a non‐planar C₃N₂‐ring (i.e., non‐aromatic).

Suitable single crystals of 4a–c and 5a for sc‐XRD were obtained by storing a saturated n‐hexane solution of each at room temperature. The solid‐state molecular structures of 4a and 5a as well as their precursors 2a and 3a are shown in Figure 2 (see the Supporting Information for other species, Figures S58–S68). A comparative overview of the selected bond lengths and bond angles for 2a/4a and 3a/5a is given in Table 1. Like previously reported radicals [(NHC)Ar],^{[ 63} , ^{75 ]} the C1–C4 (4a: 1.406(1); 5a: 1.397(2) Å), and C1–N1/N2 (4a: 1.402(1)/1.393(1); 5a: 1.399(2)/1.382(2) Å) bond lengths are respectively smaller and longer, while the N1─C1─N2 (4a: 108.2(1); 5a: 108.6(1) °) bond angles are smaller for 4a or 5a with respect to those of 2a or 3a (Table 1). Moreover, the bond length alteration (BLA) for the C─C bonds of the terphenyl rings in 4a and 5a (0.05–0.08 Å) is larger than that of 2a and 3a (0.01 Å), respectively.

Table 1.

Selected sc‐XRD [and calculated at PBEh‐3c] bond lengths and angles for 2a, 3a, 4a, and 5a.

Bond length [Å]/ bond angle or torsion angle [°]
	2a	4a	3a	5a
C1 − N1	1.334(2) [1.326]	1.402(1) [1.387]	1.324(2) [1.324]	1.399(2) [1.389]
C1–C4	1.475(2) [1.456]	1.406(1) [1.401]	1.471(3) [1.458]	1.397(2) [1.404]
C4–C5	1.396(2) [1.395]	1.430(1) [1.424]	1.397(2) [1.389]	1.434(2) [1.417]
C5–C6	1.389(2) [1.379]	1.376(1) [1.371]	1.396(3) [1.389]	1.368(2) [1.382]
C7–C10	1.485(2) [1.468]	1.467(2) [1.462]	–	–
C13–C16	1.490(2) [1.473]	1.479(1) [1.473]	–	–
C6–C10	–	–	1.489(3) [1.474]	1.484(2) [1.476]
N1–C1–N2	111.1(1) [111.3]	108.2(1) [107.7]	111.8(2) [111.6]	108.6(1) [108.1]
N1–C1–C4–C5	37.1(3) [38.7]	14.6(1) [18.4]	37.2(3) [39.5]	18.4(2) [19.6]
C6–C7–C10–C11 a)	26.4(2)	8.4(1)	–	–
C12–C13–C16–C17 a)	26.8(2)	31.1(1)	–	–

^a) The values correspond to small (positive) angles (according to its 180° inversion).

Similar to the C1─C4 bond lengths, the C7–C10 (2a: 1.485(2); 4a: 1.467(2) Å) and C13–C16 (2a: 1.490(2); 4a: 1.479(2) Å) bond lengths are shorter in 4a than in 2a. The bond length reduction is most pronounced at C1–C4 and becomes less significant across C7–C10 and C13–C16. In contrast, the C6–C10 bond lengths in 3a and 5a (3a: 1.489(3); 5a: 1.484(2) Å) remain essentially unchanged. These observations suggest that in 4a the electrons are well delocalized across all three rings, whereas in 5a they are localized within a single ring. Consistent with the delocalization of the unpaired electron over the C2‐terphenyl ring, the N1–C1–C4–C5 torsion angle for 4a (14.6(1)°) and 5a (18.4(2)°) is smaller than that in 2a (37.1(3)°) and 3a (37.2(3)°), respectively. Thus, the C2‐aryl ring in 4a/5a becomes more planar and less twisted relative to that in 2a/3a. The central ring of the p‐Ter moiety of 4a also adopts somewhat planarity (cf. C6–C7–C10–C11: 8.4(1)°) relative to that of 2a (26.4(2)°), this is however less pronounced for the terminal C₆H₅ ring (cf. C12–C13–C16–C17 = 26.8(2)° for 2a and 31.1(1)° for 4a). These features indicate the delocalization of the unpaired electron over the respective C2‐terphenyl unit, which accounts for the remarkable thermal stability of the radicals 4a–c and 5a–c.

Remarkably, the C5–C6–C10–C11 torsion angle of 5a (39.0(2)°) is larger than that of 3a (27.1(3)°), excluding the delocalization of the unpaired electron to the m‐C₆H₅ rings. Similar trends in the structural parameters are observed for 4b and 4c. (Figures S63, S64). Attempts to obtain suitable single crystals of 6a‐K and 7a‐K were unfortunately unfruitful. The NMR data of 6a‐K and 7a‐K are fully consistent with a related anion characterized by sc‐XRD.^{[ 75 ]} The molecular structures of the anionic fragments [6a]⁻ and [7a]⁻ (i.e., excluding counter cation K⁺) have been analyzed by quantum chemical calculations (see below).

The optimized molecular structures of the selected radicals 4a (Figure S71) and 5a (Figure S72) at the PBEh‐3c level of theory are in good agreement with their solid‐state molecular structures (Figure 2) determined by sc‐XRD. The molecular structures of the anions [6a]⁻ (Figure S73) and [7a]⁻ (Figure S74) have been also optimized at the PBEh‐3c level of theory. The calculated natural population analysis (NPA) charges, Wiberg bond indices (WBIs), and/or spin densities according to NBO analyses for 4a (Table S10), 5a (Table S11), [6a]⁻ (Table S12), and [7a]⁻ (Table S13) are given in the Supporting Information. The EPR spectra of the radicals 4a and 5a (Figure 3, see the Supporting Information for 4b,c and 5b,c) measured at 298 K in toluene exhibit a characteristic EPR signal for S = ½ systems. The EPR spectra of 4a–c and 5a–c were successfully simulated using calculated coupling constants (Table S1). For 4b, 4c, and 5a, the EPR spectrum of each exhibits a featureless signal, while the EPR spectra of 4a, 5b, and 5c reveal fine structure. The hyperfine coupling constants (hfc) for 4a–c and 5a–c (Table S1) are in good agreement with those calculated for related [(NHC)Ar] radicals.^{[ 63 ]}

Figure 3.

Experimental and simulated isotropic X‐band EPR spectra of [(SIPr)p‐Ter]^● (4a) (HFC to 2 equiv. N atoms, a set of 4 equiv. H atoms, a set of 2 equiv. H atoms and a single H atom) and [(SIPr)m‐Ter]^● (5a) (in toluene, at 298 K, 9.63 GHz, 0.3162 mW).

The plots of calculated spin density distribution for 4a and 5a (Figure 4a) show that the unpaired electron is mostly localized at the C_NHC‐carbon atom (0.35 e for 4a and 0.39 e for 5a) and the ortho‐ and para‐carbon atoms of the C_NHC‐attached aryl ring (0.18–0.26 e) of the p‐Ter or m‐Ter unit. For 4a, a small part of the spin density is also found at the o‐ and p‐carbon atoms (∼ 0.06 e each) of the central C₆H₄ ring, while this is virtually lacking at the terminal aryl ring (p‐C₆H₅) of the p‐Ter unit. Thus, the extent of delocalization of the unpaired electron in 4a is comparable to that observed in [(NHC)Bp] radicals.^{[ 75 ]} Also, no spin density is found at the C₆H₅ rings of the m‐Ter unit. Nonetheless, the presence of an additional (electronegative) p‐phenyl group lowers the energy of the αHOMO (i.e., SOMO = singly occupied molecular orbital, in other notion) of 4a (−3.44 eV, Figure 4b) relative to that of [(NHC)Bp] radicals (−2.61 to −2.81 eV, Bp = p‐(C₆H₅)C₆H₄) as well as of [(NHC)Ar] (−2.40 to −2.67 eV) (Ar = mono‐phenyl substituent).^{[ 63 ]} Notably, the αHOMO of 5a (−3.43 eV a) is also stabilized to a similar extent, which may be rationalized considering the negative inductive effect (–I) of the m‐phenyl groups on the C₆H₃‐ring. The αHOMO of 4a and 5a (Figure 4b) reveals an out‐of‐phase combination of the p‐orbital at the C_NHC (or C1, see Table 1 for atom numbering) with the adjacent nitrogen lone pair orbitals, while there is an in‐phase combination between the p‐orbital of C1 and C4 atoms. This is in good agreement with the partial C1–C4 double bond character (cf. WBI for 4a = 1.30, 5a = 1.28 and for 2a = 1.07, 3a = 1.06) (Table 2). A similar orbital topology for the HOMO (highest occupied molecular orbital) of anions [6a]⁻ and [7a]⁻ can be seen (Figure 4c), which is in line with the population of the αHOMO of 4a and 5a with an additional electron to give [6a]⁻ and [7a]⁻. Consequently, the C1–C4 double bond character further increases in [6a]⁻ (WBI = 1.50) and [7a]⁻ (WBI = 1.48) as revealed by the respective WBI (Table 2). Moreover, the calculated NPA charges show the negative charge in anions largely resides on the terphenyl substituent.

Figure 4.

a) Calculated spin density plots (isosurfaces ± 0.005 a. e.) for 4a and 5a. The numbers correspond to natural spin density on the atoms. b) Plots (isosurfaces 0.05 a. u.) of the αHOMO (energy in eV) for 4a and 5a. c) Plots (isosurfaces 0.05 a. u.) of the HOMO of [6a]⁻ and [7a]⁻ [PBE0/def2‐TZVP].

Table 2.

Selected Natural charges (q) and Wiberg Bond Indices (WBIs) for 2a, 3a, 4a, and 5a.

Compound	2a	4a	3a	5a
Atom	NBO charge (q)
N1/C1	−0.36/0.58	−0.43/0.40	−0.35/0.58	−0.43/0.39
C4/C10/C16	−0.17/−0.09/−0.06	−0.19/−0.04/−0.04	−0.15	−0.17
C5	−0.16	−0.21	−0.16	−0.21
C6	−0.20	−0.20	−0.03	−0.05
C7/C13	0.02/−0.01	−0.09/−0.07	−0.15	−0.24

Bond	WBI
N1–C1	1.30	1.10	1.31	1.10
C1–C4	1.07	1.30	1.06	1.28
C4–C5	1.35	1.23	1.37	1.25
C5–C6	1.47	1.54	1.39	1.45
C6–C7	1.36	1.31	1.39	1.33
C7–C10	1.07	1.10	−	−
C13–C16	1.06	1.08	−	−

We also performed fractional occupation number weighted density (FOD) calculations as an electron correlation diagnostic^{[ 81 ]} to analyze the electronic structures of 4a, 5a, [6a]⁻ and [7a]⁻ (Figure 5). FOD studies provide reliable information on the localization of “hot” (strongly correlated and chemically active) electrons in a molecule.^{[ 82} , ⁸³ , ^{84 ]} The FOD plots of 4a, 5a, [6a]⁻, and [7a]⁻ (Figure 5) nicely visualize the “hot” electrons on the C_NHC‐aryl (i.e. p‐Ter or m‐Ter) unit and nitrogen atoms. The resulting FOD number N _FOD increases from the cations ([2a]⁺ = 1.19 e and [3a]⁺ = 1.15 e), to the radicals (4a = 1.76 e and 5a = 1.77 e), and to the anions ([6a]⁻ = 2.44 e) and [7a]⁻ = 2.49 e). These N _FOD values for radicals and anions suggest an intermediate electron correlation.^{[ 71} , ^{72 ]} Based on these results, the stability of the reported radicals (or anions) may be attributed to the delocalization of the unpaired electron (or electrons lone pair) over the C_NHC‐m/p‐Ter moiety.

Figure 5.

FOD plots (isosurfaces ± 0.005 a. e. in yellow, at the FT‐PBE0/def2‐TZVP level of theory) of 4a, 5a, [6a]⁻, and. [7a]⁻.

The UV–vis spectra of 4a–c (Figures S41–S43) and 5a–c (Figures S44–S46) exhibit three main absorption bands in the 400–800 nm region. According to the TD‐DFT calculations, the absorption band at λ _max. (in nm) = 707 (4a), 801 (4b) or 780 (4c) may be assigned to the αHOMO→αLUMO transition, while the higher energy band at λ _max. (in nm) = 435 (4a), 474 (4b), or 484 (4c) corresponds to βHOMO → βLUMO and αHOMO–1 → αLUMO transitions (Table S14). Similarly, the absorption band at λ _max. (in nm) = 401 (5a), 448 (5b) or 458 (5c) is related to αHOMO → αLUMO + 1/αLUMO + 2 and βHOMO–2 → βLUMO transitions (Table S15).

The radicals 4a–c and 5a–c are stable at room temperature under an inert gas atmosphere for months. However, the anions 6a–c‐K and 7a‐K oxidize slowly to form the corresponding radicals even under nitrogen (or argon) atmosphere (∼ 5 ppm O₂). The radicals are also thermally stable in the solid‐state up to (decomposition temperature) 108 (4a), 91 (4b), 88 (4c), 99 (5a), 82 (5b), or 80 °C (5c). Thus, the SIPr‐based radicals 4a and 5a are thermally more stable than IPr‐ and Me‐IPr‐derived radicals (4b,c and 5b,c). Furthermore, the p‐Ter‐derivatives 4a–c decompose at a relatively higher temperature in the solid‐state than their corresponding m‐Ter‐analogues 5a–c. Interestingly, in solution, the decomposition trend is reversed. A green benzene solution of 4b turned completely pale yellow after 1 h at 80 °C, whereas no color change was observed for 5b under similar conditions. The color change from green to yellow (decomposition) of 5b occurred at 100 °C after 1 h.

In line with the CVs of 2a–c and 3a–c, the radicals 4a–c and 5a–c undergo 1e‐oxidation with AgOTf to quantitatively yield the corresponding cations [2a–c]⁺ and [3a–c]⁺ with a triflate counter anion (Scheme 4a). Also, two equivalents of AgOTf are required to generate [2a]⁺ and [3a]⁺ from 6a‐K and 7a‐K, while the use of one equivalent AgOTf gave 4a and 5a, respectively. Treatment of 4a with Co₂(CO)₈ also affords the diamagnetic salt [2a][Co(CO)₄], in which Co(0) is formally reduced to Co(–I). The ¹H and ¹³C NMR spectra of [2a][Co(CO)₄] exhibit characteristic signals for the [2a]⁺ moiety and has been also characterized by sc‐XRD (Figure S68).

Scheme 4.

a) Oxidations of radicals (4a–c, 5a–c) and anions (6a‐K, 7a‐K) with AgOTf or Co₂(CO)₈. b) Reactions of 4b and 5b with TEMPO to 11‐Ar.

Among stable radicals, nitroxyl radicals such as TEMPO (2,2,6,6‐Tetramethylpiperidinyloxyl) featuring three‐electron‐two‐center π double bond have found widespread applications in various chemical and electrochemical oxidations, polymerizations, and biological antioxidant processes.^{[ 85} , ⁸⁶ , ⁸⁷ , ⁸⁸ , ⁸⁹ , ^{90 ]} This is owing to their unique properties and reactivity.^{[ 91 ]} However, 1e‐reduction of TEMPO and related species to the corresponding anions has received rather limited attention.^{[ 92} , ⁹³ , ^{94 ]} To further probe the chemical reactivity of the radicals, we treated 4b and 5b with one equivalent of TEMPO radical, which resulted in each case a discoloration (green to light brown). The well‐resolved ¹H NMR spectrum of each sample revealed the selective formation of a diamagnetic species, i.e., 11‐Ar (Scheme 4b) instead of the expected radical‐coupling products 8‐Ar. The exact mechanism of the formation of 11‐Ar is unknown. One‐electron oxidation of 4b or 5b with TEMPO to form the salt [(NHC‐Ar)](TEMPO) (9‐Ar) is likely.^{[ 95 ]} In 9‐Ar, the deprotonation of the cation [(NHC‐Ar)]⁺ by the counter anion TEMPO⁻ in resulting the mesoionic carbene (iMIC) 10‐Ar seems plausible, which subsequently undergo proton migration via a C_sp3–H bond activation to ultimately form 11‐Ar. This type of transformation is known for iMICs.^{[ 96 ]} Notably, TEMPO is known to react with an alkali metal (M) reducing agent to afford (TEMPO)M,^{[ 97} , ^{98 ]} which show nucleophilic reactivity.^{[ 99 ]} Indeed, the deprotonation of 2b and 3b by TEMPOK (or KN(SiMe₃)₂) yields 10‐Ar, which at elevated temperature form 11‐Ar (see Figures S1–S4). For 11‐Ar, the ¹H NMR spectra show three septets for the methine protons (HCMe₂), a set of several doublets for methyl protons (HCMe ₂), and two singlets for the methyl groups (CMe ₂). In addition, the ¹H NMR spectra of 11‐Ar exhibit two distinct singlets for the backbone NCH protons, indicating that they are magnetically different. In addition to other expected signals, the ¹³C{¹H} NMR spectra of 11‐Ar show characteristic resonances for the CMe₂ unit (Me: 1.0/31.1; C: 101.3 ppm). The molecular structures of 11‐Ar (Figures S66 and S67) determined by sc‐XRD clearly show the formation of C–H activation (i.e., C–C coupling) products, which align well with their NMR data.

Conclusions

In conclusion, we have reported the synthesis and structural characterization of two series of NHC–terphenyl systems, [(NHC)p‐Ter] and [(NHC)m‐Ter], based on three distinct NHCs (SIPr, IPr, and Me‐IPr). Electrochemical studies confirm the accessibility of these systems in three redox states: cationic [(NHC)Ar]⁺, radical [(NHC)Ar]^●, and anionic [(NHC)Ar]⁻ species (Ar = p‐Ter or m‐Ter). The radicals [(NHC)p‐Ter]^● (4a–c) and [(NHC)m‐Ter]^● (5a–c) have been isolated as stable crystalline solids and fully characterized by sc‐XRD, EPR, and UV–vis spectroscopy. This is in line with the CVs of [(NHC)Ar]Br, which show a reversible redox wave for the [(NHC)Ar]⁺/[(NHC)Ar]^● couple. The second reduction step is reversible only for the p‐Ter‐derivatives, i.e., the ([(NHC)p‐Ter]^●/[(NHC)p‐Ter]⁻ redox couple, enabling the isolation of anions K[(NHC)p‐Ter] (6a–c‐K) as stable diamagnetic solids as confirmed by NMR spectroscopy. For the m‐Ter‐series, the second reduction is irreversible or quasi‐reversible. The diamagnetic anion K[(SIPr)m‐Ter] (7a‐K) is obtained by reducing radical 5a and characterized by NMR, whereas attempts to reduce the radicals [(IPr)m‐Ter]^● (5b) or [(Me‐IPr)m‐Ter]^● (5c) under similar conditions were not viable.

Reactivity studies of the radicals with TEMPO yielded C–H activation products (11‐Ar), while reactions of the radicals or anions with AgOTf quantitatively generated the corresponding cations or radicals. Overall, these findings highlight the crucial influence of the C2‐terphenyl substituent, in addition to the NHC identity, on the stability and redox behavior of NHC‐based systems. The insights gained here provide valuable information for designing new redox‐active systems with tunable and predictable properties.

Supporting Information

Experimental details, the plots of NMR, EPR, and UV–vis spectra, the details of X‐ray crystallography, and quantum chemical calculations of the reported compounds are given in the Supporting Information.

Conflict of Interests

The authors declare no conflict of interest.

Supporting information

Supporting Information

Supporting Information

Dedicated to Professor Dr. Ramaswamy Murugavel on the occasion of his 60^th birthday

Steffenfauseweh H., Vishnevskiy Y. V., Neumann B., Stammler H.‐G., de Bruin B., Ghadwal R. S., Angew. Chem. Int. Ed. 2025, 64, e202520260. 10.1002/anie.202520260

Data Availability Statement

The data that support the findings of this study are available in the Supporting Information of this article.