Singlet carbenes as mimics for transition metals: synthesis of an air stable organic mixed valence compound [M₂(C₂)⁺˙; M = cyclic(alkyl)(amino)carbene]

Abstract

A cyclic C-bromo-iminium bromide reacts with lithium trimethylsilyl acetylide to afford the title compound 2⁺˙ in 40% yield. In contrast to other organic mixed valence compounds, this radical cation can be stored in air. Oxidation and reduction of 2⁺˙ afford the corresponding dication 2⁺⁺ and neutral cumulene 2, respectively, in good yields.

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During the last few years, it has been shown that due to the presence of a lone pair and an accessible vacant orbital, singlet carbenes could mimic the chemical behaviour of transition metal centers.¹ Indeed, stable and rather electrophilic carbenes such as cyclic(alkyl)(amino)carbenes (CAACs)² and diamidocarbenes³ can activate small molecules (H₂, CO and NH₃),⁴ and enthalpically strong bonds such as Si–H, P–H, B–H, P–P, and even C–H bonds.⁵ Moreover, it has been found that these carbenes can stabilize a variety of main-group compounds featuring unusual bonding situations,⁶ as well as paramagnetic species.^7,8 Since the pioneering work by Creutz and Taube,⁹ bimetallic mixed-valence systems have attracted much attention due to their potential applications in electronic conductors and non-linear optical materials, among others.¹⁰ Although organic mixed-valence compounds have been known for decades,¹¹ their applications have been hampered due to their sensitivity to oxygen and their lack of thermal stability.¹² Here we report that the peculiar electronic properties of CAACs¹³ allow for the preparation of a thermally and air stable purely organic mixed-valence derivative.¹⁴

Cyclic C-bromo-iminium bromide 1 readily reacts at −78 °C with a freshly prepared THF solution of lithium trimethylsilyl acetylide in THF (Scheme 1). Within a few minutes at room temperature the pale yellow suspension darkens to a deep red colour. After removal of all volatiles and washing of the crude mixture with diethyl ether, compound 2⁺˙ was extracted with dichloromethane and recovered as a red solid (40% yield).

Scheme 1 Synthesis of radical cation 2⁺˙ (Dipp: 2,6-diisopropylphenyl).

The paramagnetic nature of 2⁺˙ was clearly demonstrated by the X-band EPR spectrum shown in Fig. 1 (bottom left). Red single crystals were obtained by slow diffusion of diethyl ether into a THF solution of 2⁺˙ in air, and were subjected to an X-ray diffraction study. Compound 2⁺˙ crystallizes in a monoclinic P21/n space group with an inversion center in the middle of the central CC bond. The 5-membered rings are coplanar in a trans configuration. The NC1 [1.338(3) Å], C1C2 [1.376(3) Å] and C2C2′ [1.230(5) Å] bond distances are in agreement with the presence of a delocalized unpaired electron across the NC1C2C2′C1′N′ fragment (Table 1). This delocalization is substantiated by DFT calculations [B3LYP/6-311 g(d,p)], the spin density being distributed over the N (44%), C₁ (24%) and C₂ (31%) centres (Fig. 1, top right inset). Very interestingly, the radical cation 2⁺˙ is thermally very stable [m.p. > 250 °C(dec.)], and even moisture and air stable for several months in the solid state.

Fig. 1 UV-Vis spectrum (5.5 × 10⁻⁴ mol L⁻¹ in methylene chloride) (top), spin density map [hydrogen atoms omitted for clarity, B3LYP/6-311 g(d,p)] (top right inset), X-Band EPR spectrum (bottom left), and cyclic voltammogram (nBu₄NPF₆ 0.1 M, 100 mV S⁻¹, versus Fc/Fc⁺) (bottom right) of 2⁺˙.

Table 1 Solid-state structure and selected bond lengths (Å) and angles (°) of compounds 2⁺⁺, 2⁺˙, and 2 (H atoms and anions are omitted for clarity)

	NC1	C1C2	C2C2′	NC1C2	C1C2C1′	NC1C1′N′
2^++	1.289(2)	1.438(2)	1.195(4)	123.63(16)	171.04(11)	45.59
2^+˙	1.338(3)	1.376(3)	1.230(5)	123.74(19)	175.5(3)	180
2	1.3904(15)	1.3332(17)	1.266(2)	124.45(11)	177.73(16)	180

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The cyclic voltammogram of 2⁺˙ shows two reversible redox processes at E_1/2 = +0.05 V and −0.82 V (versus Fc⁺/Fc) (Fig. 1, bottom right). The large coupling of about 0.87 V is another indication of the delocalization of the unpaired electron in the extended π-system.

The UV-Vis spectrum of a methylene chloride solution of the radical cation 2⁺˙ shows strong absorption bands at 382, 408 and 540 nm, along with weak bands at 464 and 482 nm (Fig. 1, top).

Encouraged by these results, we engaged in the synthesis of the oxidized and reduced derivatives of 2⁺˙, namely the dication 2⁺⁺ and cumulene 2 (Scheme 2). An excess of bromine was added at room temperature to a chloroform solution of the radical cation 2⁺˙. An immediate yellow precipitate appeared, and after filtration and washing with diethyl ether, 2⁺⁺ was isolated as a pale yellow, air stable, solid in 53% yield. Similarly to radical cation 2⁺˙, the dication 2⁺⁺ proved to be highly thermally stable [m.p. = 200 °C (dec.)]. The diamagnetic nature of 2⁺⁺ allows for its characterization by NMR spectroscopy. In the ¹³C NMR spectrum, the four quaternary carbons C1, C1′, and C2, C2′ give signals at +178.8 and +96.7 ppm, respectively, in the usual range of iminium and alkyne carbons.

Scheme 2 Oxidation and reduction of radical cation 2⁺˙.

Treatment of a THF solution of 2⁺˙ with an excess of zinc powder, in the absence of air, led to a rapid fading of the red colour. The solvent was removed under vacuum, and the residue was extracted with diethyl ether. After evaporation of the solvent, 2 was isolated as a yellow solid in 56% yield. Although 2 is highly air sensitive, it is stable under an inert atmosphere for weeks both in the solid state and in solution. The cumulenic carbons give signals at δ = 140.8 and 127.9 ppm, in the range observed for other cumulenes.¹⁵

Single crystals of 2⁺⁺ and 2 were subjected to an X-ray diffraction study, which allows comparing the bond lengths along the series 2⁺⁺, 2⁺˙, and 2 (Table 1). As expected, addition of one and two electrons into the extended π-system results in lengthening of the N–C1 and C2–C2′ bonds, with a concomitant shortening of the C1–C2 bond. Also noteworthy is the coplanarity of the two five-membered rings in 2⁺˙ and 2, whereas they are twisted by 46° in 2⁺⁺.

Compounds 2⁺⁺, 2⁺˙, and 2 can be viewed as a C₂⁺⁺, C₂⁺˙ and C₂ fragment, respectively, stabilized by two CAACs.¹⁶ The straightforward synthesis of 2⁺˙, and its surprising air stability, pave the way for the preparation of a variety of organic mixed valence systems featuring electrophilic carbenes as stabilizing groups for different radical cation spacers.

Thanks are due to the National Science Foundation (CHE-1316956), and to the China Scholarship Council for a graduate fellowship (L.L.).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedures and analytical data; CIF files for single crystal X-ray structural analysis. CCDC 991467–991469. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4qo00072b

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