A Stable Anionic Dithiolene Radical

Abstract

Sulfurization of anionic N-heterocyclic dicarbene, [:C{[N(2,6-Prⁱ₂C₆H₃)]₂CHCLi}]_n (2), with elemental sulfur (in a 1:2 ratio) in Et₂O at low temperature gives 3 by inserting two sulfur atoms into the Li–C (i.e., C2 and C4) bonds in polymeric 2. Further reaction of 3 with 2 equiv of elemental sulfur in THF affords 4^• via unexpected C–H bond activation, which represents the first anionic dithiolene radical to be structurally characterized in the solid state. Alternatively, 4^• may also be synthesized directly by reaction of 1 with sulfur (in a 1:4 ratio) in THF. Reaction of 4^• with GeCl₂·dioxane gives an anionic germanium(IV)–bis(dithiolene) complex (5). The nature of the bonding in 4^• and 5 was probed by experimental and theoretical methods.

Metal–dithiolene complexes, extensively studied since the early 1960s,^1–10 are intriguing not only due to their unique structural and bonding motifs but also for their remarkable capabilities in such disparate fields as materials science^4,10 and biological systems.^2,11 While both molybdenum and tungsten enzymes contain the dithiolene unit,^2,5 transition metal–bis-dithiolenes, possessing unique optical, conductive, and magnetic properties, have shown promise in the development of optoelectronic devices.^4,7,10

The fascinating redox chemistry demonstrated by metal–dithiolenes may largely be attributed to the non-innocent behavior of dithiolene ligands (Figure 1).³ Gray boldly proposed the likely presence of dithiolate radical anion moieties (L^•−) in transition metal–dithiolene complexes more than five decades ago.^13,14 The electronic structures of transition metal–dithiolene complexes were recently probed by sulfur K-edge X-ray absorption spectroscopy (XAS), providing the strong support for the non-innocence of dithiolene ligands.⁸ While the radical character of the ligands in transition metal–dithiolene complexes has been extensively explored,^8,15–24 free anionic dithiolene radicals are highly reactive and have only been studied computationally and by electron paramagnetic resonance (EPR).^25–28 Indeed, the electronic absorption spectrum of the prototype dithiolene radical anion (C₂H₂S₂^•−) was recently observed in a low-temperature matrix.²⁹ Consequently, the captivating chemistry of anionic dithiolene radicals remains highly relevant. Herein, we report the synthesis, structure, spectra, computations,³⁰ and reactivity of the lithium salt of anionic dithiolene radical (4^•), an R₂timdt-type ligand (R₂timdt = disubstituted imidazolidine-2,4,5-trithione).^31–36 Notably, 4^• is uniquely synthesized via a carbene-based strategy and represents the first stable anionic dithiolene radical to be structurally characterized.

Figure 1.

Three oxidation levels of a dithiolene ligand.¹²

Reaction of N-heterocyclic carbene [NHC = :C{N(2,6-Prⁱ₂C₆H₃)CH}₂] with an excess of elemental sulfur gives thione (1) (Scheme 1).³⁷ However, di- and tri- sulfurization of the imidazole ring may be achieved by sulfurization of C4-metalated N-heterocyclic carbenes. The first anionic N-heterocyclic dicarbene (NHDC, 2) was synthesized by this laboratory in 2010 via C4-lithiation of a NHC ligand [L: = :C{N(2,6-Prⁱ₂C₆H₃)CH}₂].³⁸ Reaction of 2 with two equivalents of elemental sulfur in Et₂O at low-temperature and subsequent workup in THF gives colorless disulfurized product 3 (in 73.5% yield) (Scheme 1, R = 2,6-diisopropylphenyl). Compound 3 may be purified by recrystallization in THF/hexane mixed solvent at −40 °C. Further reaction of crystalline 3 with elemental sulfur (in a 1:2 ratio) in THF at room temperature gives 4^• as a crystalline purple powder in quantitative yield, which can be employed for synthesis without further purification. Notably, 4^• may also be synthesized by directly reacting 2 with elemental sulfur in a 1:4 ratio (Scheme 1). However, the purity of the product 4^• from the 1:4 route is relative low. Radical 4^• solid is stable indefinitely under an inert atmosphere of argon. Interestingly, the transformation of 3 to 4^• involves unexpected C–H bond activation. The metal-catalyzed C–S bond formation via C–H bond functionalization has received substantial attention.³⁹ In addition, the disulfide bridge in dinuclear Ru(III) complexes has been reported to be involved in C–S bond formation.⁴⁰ Notably, elemental sulfur has been utilized in copper-mediated C–S bond forming reactions via C–H activation.⁴¹ While the mechanism of the transformation of 3 to 4^• remains unclear, our study reveals that this reaction requires two equivalents of elemental sulfur (Scheme 1). The corresponding 1:1 reaction only affords a mixture containing 4^• and unreacted 3.

Scheme 1.

Synthesis of 1, 3, and 4^•

Although the ¹H NMR imidazole resonance of 3 (6.14 ppm) is similar to that of 2 (6.16 ppm),³⁸ single crystal X-ray structural analysis reveals that the two carbene carbons of 3 are sulfurized (Figure 2). The C(1)=S(1) bond distance in 3 [1.678(4) Å], comparing well to that of 1 [1.670(3) Å],⁴² is ca. 0.04 Å shorter than that of the C(3)–S(2) single bond in 3 [1.716(4) Å]. While the S(1) atom is terminal, the S(2) atom is bridged between the C4 carbon [i.e., C(3)] and a THF-solvated lithium cation. In addition, the Li–S bond is nearly coplanar with the imidazole ring [Li(1)–S(2)–C(3)–C(2) torsion angle = 2.1°].

Figure 2.

Molecular structure of 3. Thermal ellipsoids represent 30% probability. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): C(1)–S(1), 1.678(4); C(2)– C(3), 1.355(5); C(3)–S(2), 1.716(4); Li(1)–S(2), 2.369(8); S(1)–C(1)–N(1), 128.8(3); C(2)–C(3)–S(2), 134.7(3); Li(1)–S(2)–C(3), 108.4(2).

While the UV–vis absorption spectrum (Figure S1)³⁰ of radical 4^• (purple) in toluene shows two broad absorptions at 554 and 579 nm, the paramagnetic properties of radical 4^• are characterized by EPR spectroscopy in THF at 298 K (Figure 3a). The EPR spectrum displays a S = 1/2 quintet (g_av = 2.016) due to weak hyperfine coupling with two equivalent I = 1 ¹⁴N nuclei, A_av(¹⁴N) = 3.9 MHz. Molecular orbital calculations of the simplified [4-Ph]^• model suggest that the SOMO (Figure 3b) is primarily ligand-based, involving C–S π-antibonding and C–C π-bonding character. The total spin density (0.88) of the C₂S₂ unit in [4-Ph]^• [ρ(S₂) = ρ(S₃) = 0.31, ρ = 0.26 for the olefinic carbons] indicates that the unpaired electron is largely localized on the C₂S₂ moieties.

Figure 3.

(a) Room-temperature X-band EPR spectrum of 4^• in THF recorded at 9.78 GHz with a modulation amplitude of 0.02 mT and a microwave power of 0.1 mW. (b) SOMO of the simplified model [4-Ph]^•.

X-ray structural analysis (Figure 4) shows that a THF-solvated four-coordinate lithium cation is bound to two sulfur atoms of the dithiolene moiety in 4^•, giving a five-membered LiS₂C₂ ring. The LiS₂C₂ ring of [4-Ph]^• model is almost planar,³⁰ rather than having the lithium atom obviously puckered out of plane, as observed in 4^• [bend angle (η) between the MS₂ (M = Li) plane and the S₂C₂ plane = 14.2°]. This may be due to the steric bulk of the ligand and the packing effects.^43,44 The Li–S bond distances in 4^• [2.442(5) Å, av], similar to that of [4-Ph]^• (2.455 Å, av), is approximately 0.07 Å longer than that in monothiolate 3 [2.369(8)Å]. The Wiberg bond indices (WBIs) of the Li–S bonds in [4-Ph]^• (0.28, av), coupled with the +0.66 natural charges of [(THF)₂Li]⁺, indicate both the ionic bonding essence of the Li–S bonds and the anionic character of the [S=C{N(2,6-Prⁱ₂C₆H₃)-CS}₂^•− fragment in 4^•. While comparing well with the theoretical values for [4-Ph]^• (d_C–C = 1.426 Å, d_C–S = 1.697 Å) and for cis-C₂H₂S₂^•− (d_C–C = 1.411 Å, d_C–S = 1.693 Å),²⁹ the C(2)–C(3) bond distance [1.417(3)Å] and C–S bond distances [1.677(3) Å, av] in the C₂S₂ moieties of 4^• are in contrast to those in 3 [d_C–C = 1.355(5)Å, d_C–S = 1.716(4) Å] and in various dithiolates² such as uncomplexed dithiolate ligand [i.e., (NMe₄)₂(C₃S₅), d_C–C = 1.371(8) Å, d_C–S = 1.724(6) Å, av]⁴⁵ and silver dithiolate complex [NBu₄]₄[Ag-(mnt)]₄ (mnt = 1,2-maleonitrile-1,2-dithiolate) [d_C–C = 1.373(8) Å, d_C–S = 1.736(6) Å, av].⁴⁶ By comparison with those in dithiolates (L²⁻, Figure 1),² the elongation of the carbon–carbon bond and concomitant shortening of the carbon–sulfur bonds observed in 4^• may be attributed to the SOMO (Figure 3b) of 4^•, which has C–C π-bonding and C–S π-antibonding character. The WBI values of the C(2)–C(3) bond (1.22) and C–S bonds (1.34, av) in the C₂S₂ moieties of 4^• indicate that these bonds have a modest double bond character, which is consistent with the resonance structures of L^•− in Figure 1.

Figure 4.

Molecular structure of 4^•. Thermal ellipsoids represent 30% probability. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): S(1)–C(1), 1.654(2); C(2)–C(3), 1.417(3); C(2)–S(2), 1.674(3); C(3)–S(3), 1.680(3); Li(1)–S(2), 2.437(5); Li(1)–S(3), 2.446(5); C(2)–C(3)–S(3), 129.54(19); Li(1)–S(2)–C(2), 93.08(14); S(2)–Li(1)–S(3), 93.31(16).

Although synthetic routes for dithiolene complexes have been reported,² 4^•, as a bidentate ligand with both anionic and radical character, provides a unique platform to access a variety of dithiolene compounds. To this end, we allowed 4^• to react with GeCl₂·dioxane in THF (in a ratio of 2:1). The anionic chlorogermanium-bis(dithiolene) complex (5) was isolated as a dark-blue diamagnetic crystalline solid (83.3% yield) (Scheme 2).

Scheme 2.

Synthesis of 5

X-ray structural analysis (Figure 5a) shows that the central five-coordinate germanium atom in 5 adopts an approximate square pyramidal geometry, having four coplanar basal sulfur atoms (d_S⋯S = 3.134–3.309 Å) and an apical chlorine atom. In contrast, the optimized structure (Figure 5b) of the simplified [5-H]⁻ model (in C₂ symmetry) features a distorted trigonal bipyramidal geometry around the germanium atom (S₄–Ge₁–S₈ angle = 167.2°; S₅–Ge₁–S₇ angle = 127.7°), which is comparable to that for anionic fluorogermanate [(C₇H₆S₂)₂GeF]⁻ (S_ax–Ge–S_ax = 171.1°, S_eq–Ge–S_eq = 136.2°).⁴⁷ The square pyramidal geometry about the germanium atom in 5 may well be a consequence of steric repulsion of the bulky ligands (see space-filling model of 5, Figure S2). The Ge–S bonds in 5 [2.3290(8)–2.3561(8) Å] are similar to the Ge–S_eq bonds (2.361 Å), but obviously shorter than the Ge–S_ax bonds (2.465 Å) in [5-H]⁻. It is noteworthy that, while comparing well to those of dithiolates (L²⁻, Figure 1),^2,45,46 the olefinic C–C bonds [1.347(4) Å, av] in 5 are shorter than that in 4^• [1.417(3) Å]. Meanwhile, the C–S bonds [1.719(3)–1.729(3) Å] in the C₂S₂ moieties of 5 are concomitantly elongated compared to those in 4^• [1.677(3) Å, av]. Thus, the germanium atom in 5 may be assigned an oxidation state of +4. In addition, the C₂S₂Ge rings in 5 (η = 37.3°, av) are more bent than those in [5-H]⁻ (η = 29.6°) and the C₂S₂Li ring in 4^• (η = 14.2°). The axial Ge(1)–Cl(1) bond in 5 [2.1902(9) Å] is only slightly shorter than that in [5-H]⁻ (2.265 Å).

Figure 5.

(a) Molecular structure of [5]⁻. Thermal ellipsoids represent 30% probability. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): Ge(1)–Cl(1), 2.1902(9); Ge–S, 2.3290(8)–2.3561(8); C(2)–C(3), 1.349(4); C(2)–S(2), 1.724(3); C(3)–S(3), 1.727(3); S(2)–Ge(1)–Cl(1), 103.13(4); S(2)–Ge(1)–S(6), 153.89(4); S(3)–Ge(1)–S(5), 152.66(3). (b) The optimized structure of [5-H]⁻ model in C₂ symmetry.

The anionic NHDC ligand (2) may be di- and trisulfurized to give 3 and 4^•, respectively. Compound 3 may be further transformed into 4^• via C–H bond activation. The effective transformation of 4^• to 5 suggests that 4^•, as a stable monoanionic dithiolene radical, may serve as a new platform to access a variety of unexplored dithiolene chemistry. The reactivity of both 3 and 4^• is being studied in this laboratory.