Unusual nucleophilic reactivity of a dithiolene-based N-heterocyclic silane

Abstract

While the dithiolene-based N-heterocyclic silane (4) reacts with two equivalents of BX₃ (X = Br, I) to give zwitterionic Lewis adducts 5 and 8, respectively, the parallel reaction of 4 with BCl₃ results in 10, a dithiolene-substituted N-heterocyclic silane, via the Si–S bond cleavage. Unlike 5, the labile 8 may be readily converted to 9via BI₃-mediated cleavage of the Si–N bond. The formation of 5 and 8 confirms that 4 uniquely possesses dual nucleophilic sites: (a) the terminal sulphur atom of the dithiolene moiety; and (b) the backbone carbon of the N-heterocyclic silane unit.

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Silylenes, the silicon analogues of carbenes, have evolved from transient reaction intermediates^1,2 to versatile ligands impacting transition metal coordination chemistry, catalysis, small molecule activation, and the stabilization of novel low-oxidation state main group species.^3–23 A variety of four-, five- and six-membered N-heterocyclic silylenes (NHSis) have been reported^3,8,10,24,25 since the first such molecule was synthesized by the West group three decades ago.²⁶ N-Heterocyclic silylenes have demonstrated considerably different reactivity toward boron halides than their carbon analogues, N-heterocyclic carbenes (NHCs). N-Heterocyclic carbenes usually form stable Lewis adducts with boron halides.^27–29 Although stable Lewis adducts have been isolated (Scheme 1a),^30,31 reactions between N-heterocyclic silylenes and boron halides often proceed beyond this stage. Braunschweig et al. reported that Xyl-substituted NHSi (A in Scheme 1b, Xyl = 2,6-dimethylphenyl) may react with organoborane halides to give the corresponding oxidative addition products (B), which were subsequently converted to Cvia ring expansion.³² Subsequently, a series of oxidative additions of the B–X (X = halide) bonds of boron halides at the silylene centers and silylene ring expansion reactions have been reported.^33–36 When the amidinate-supported four-membered silylene is combined with organoborane halides (Scheme 1c), migration of the amidinate ligand from the silicon atom to the boron atom was reported by Roesky et al.³⁰ In addition, Cui et al. recently reported that reaction of the five-membered NHSi (D) with BBr₃ produced the N-heterocyclic boryl-substituted silicon bromide Evia silicon–boron exchange reaction (Scheme 1c).³⁷ Notably, the literature does not reveal any reports of boron halide-mediated backbone activation of N-heterocyclic silylene rings.

Scheme 1 Typical reactions of N-heterocyclic silylenes with boron halides (Xyl = 2,6-dimethylphenyl, Dipp = 2,6-diisopropylphenyl).

Recently this laboratory investigated the silylene^38,39 (1, in Scheme 2a)-mediated sulphur–sulphur bond cleavage of an imidazole-based dithione dimer (3),⁴⁰ affording a dithiolene-based N-heterocyclic silane (4, Scheme 2a).⁴¹ Herein, we report the dual nucleophilic reactivity of the carbon backbone of the N-heterocyclic silyl framework and the terminal sulphur atom of the dithiolene unit in 4 with BX₃ (X = Br, I)—resulting in the formation of zwitterionic Lewis adducts 5 and 8, respectively. This discovery is a unique example of Lewis acid-induced charge separation of a five-membered N-heterocyclic silyl ring.

Scheme 2 Synthesis of 2, 5–10 (R = 2,6-diisopropylphenyl) and canonical forms of 5 and 8.

Consistent with the D-to-E conversion (Scheme 1c),³⁷ NHSi (1) reacts with BBr₃ to give 2 (Scheme 2a). Compound 2 may also be prepared via reaction of the 2-alkoxysilane-1,3,2-diazaborole with BBr₃.⁴² In contrast, room-temperature reaction of 4 with BBr₃ (in a 1 : 2 molar ratio) in toluene gave 5 (81% yield) (Scheme 2b).⁴³ X-ray quality yellow crystals of 5 were obtained via recrystallization in toluene. While 4 shows one singlet olefin proton resonance at 5.68 ppm,⁴¹ the backbone protons of the N-heterocyclic silyl framework in 5 exhibit two resonances in the ¹H NMR spectrum:⁴³ a broad singlet at 5.61 ppm (for HC:BBr₃) and a singlet at 6.05 ppm (for N=CH). The singlet (−6.36 ppm) and doublet (−10.97 ppm, ²J_BH = 7.8 Hz) ¹¹B NMR resonances of 5 correspond to the BBr₃ units bound to the sulphur and carbon atoms, respectively.⁴³ The 4-to-5 conversion results in the downfield shift of the ²⁹Si NMR resonance from −4.67 ppm (for 4, in C₆D₆)⁴¹ to 8.12 ppm (for 5, in toluene-d₈).⁴³ Compound 5 may be converted back to 4 in THF. Compound 5, in the presence of BBr₃, readily decomposes at room temperature, giving an acyclic doubly borylated (E)-N,N′-diaminoethene (6) and other uncharacterized products. Further reaction of 5 with BBr₃ (in a 1 : 2 ratio) in toluene at an elevated temperature (100 °C) gave a ca. 1 : 1 mixture of 6 and a dithiolene-based bromoborane complex (7) according to the ¹H NMR data (Scheme 2b).⁴³ The mixture of 6 and 7 may also be obtained via the 1 : 4 reaction of 4 with BBr₃ in toluene at 100 °C (Scheme 2b). Due to the similar solubilities, crystals of 6 (square blocks) and 7 (long rods) were manually separated for NMR measurements. The ¹¹B NMR resonance of 6 (28.60 ppm) compares well to that of an aminodichloroborane analogue (13) (32.60 ppm), Cl₂B=N(Aryl)–CH=CH–N(Aryl)=BCl₂ (Aryl = 2,6-Me₂C₆H₃).⁴⁴ The ¹¹B NMR spectrum⁴³ of 7 shows a singlet at −6.36 ppm and a broad singlet at 51.15 ppm, which correspond to the four-coordinated boron (in the SBBr₃ moiety) and three-coordinated boron (in the five-membered C₂S₂B ring), respectively. While the mechanistic details of the formation of 6 and 7 from reaction of 5 with BBr₃ remain unclear, this transformation may plausibly involve consecutive insertions of the BBr₃ species into the Si–N bonds in 5, accompanied by the migration of one bromide from the boron atom to the silicon atom, rendering to 6 and an intermediate 11 (i.e., the dithiolene-based SiBr₂ analogue of 7). The BBr₃ residing at the backbone carbon in 5 could be released during this process and subsequently react with the intermediate 11 to yield 7 and SiBr₄ (as a by-product) via silicon–boron exchange. However, our repeated attempts to isolate intermediate 11 were unsuccessful.

As a comparison, we also investigated the parallel reactions of 4 with BX₃ (X = Cl, I). The 1 : 2 reaction of 4 with BI₃ in toluene over 2h resulted in the isolation of 8 (the analogue of 5) (Scheme 2c). In contrast to 5, compound 8 may be readily converted to 9via the BI₃-mediated silicon–nitrogen bond cleavage (Scheme 2c). While 9 can be isolated as pale-yellow crystalline powder in 72% yield, attempts to obtain pure 8 was unsuccessful due to its high lability. Isolation of 9 further supports our proposed mechanism for the BBr₃-mediated decomposition of 5 (Scheme 2b). The formation of 8 has been confirmed by both single crystal X-ray diffraction technique (Fig. 1) and NMR studies. The singlet ¹H NMR resonances of 8 [5.58 ppm (HC:BI₃) and 6.06 ppm (N=CH)] compare well to those for 5 [5.61 ppm (HC:BBr₃) and 6.05 ppm (N=CH), respectively]. The singlet (−82.90 ppm, C=SBI₃) and doublet (−70.43 ppm, ²J_BH = 7.0 Hz, C(H)BI₃) ¹¹B NMR resonances of 8 are shifted highfield compared to those for 5 (−6.36 ppm, C=SBBr₃ and −10.97 ppm, ²J_BH = 7.8 Hz, C(H)BBr₃). Due to the high lability of 8 (which was converted to 9 during the ²⁹Si NMR measurement), we only observed the ²⁹Si NMR resonance for 9 (−18.80 ppm). The singlet ¹¹B NMR resonances (at 6.02 ppm and −82.85 ppm) of 9 correspond to the three-coordinate NBI₂ and four-coordinate C=SBI₃ units, respectively.

Fig. 1 Molecular structures of 5–10. Thermal ellipsoids represent 30% probability. All hydrogen atoms (except H(28) and H(29) in 5 and 8) have been omitted for clarity.

Interestingly, the parallel reaction of 4 with BCl₃ gave 10 as colourless crystalline powder (in 19% yield) (Scheme 2d) via BCl₃-mediated cleavage of the Si–S bond in 4. Formation of the zwitterionic analogue of 5 and 8 was not observed in terms of the ¹H NMR tube experiments. 10 exhibits singlet ¹¹B NMR (53.28 ppm) and ²⁹Si NMR (−33.47 ppm) resonances, revealing the presence of three-coordinate boron atom and four-coordinate silicon atom. Compound 10 is labile in solution, which may gradually decompose to give 12, the analogue of 6, in benzene. Compound 12 can be directly synthesized via 1 : 5 reaction of 4 with BCl₃ (in 58% yield). The ¹¹B NMR resonance of 12 (32.27 ppm) compares to that of 6 (28.60 ppm).

The molecular structures of 5–10 were determined by single crystal X-ray diffraction and supported by DFT computations (5-Ph, 7, and 10-Ph models, B3LYP/6-311G** level; 8-Ph and 9-Ph models, mPW1PW91/LANL2DZ level).⁴³ The crystal unit cell contains an enantiomeric pair of 5 (with identical bonding parameters) (Fig. 1). The formation of 5 reveals that 4 can serve as a double donor ligand to bind two equivalents of BBr₃ at two nucleophilic sites: the terminal sulphur atom of the dithiolene unit and the backbone carbon of the C₂N₂Si ring in 4. Each boron atom in 5 is four-coordinate and adopting a distorted tetrahedral geometry. The backbone protons of the C₂N₂Si ring [i.e., H(28) and H(29)] were located from difference Fourier map.⁴³ With the BBr₃ coordination, the C(1)–S(1) bond is elongated from 1.6638(9) Å (as observed in 4)⁴¹ to 1.725(2) Å, which compares well to that [1.7256(18) Å] of the zwitterionic boron dithiolene complex with a terminal SR group (R = cyclohexyl) residing at the C2 carbon.⁴⁵ Accordingly, the Wiberg bond index (WBI) of the C(1)–S(1) bond in 5 (1.17) is somewhat lower than that in 4 (1.49),⁴¹ indicating its modest multiple bond character. The S–B bond in 5 [1.932(2) Å] is shorter than that in C₄H₈S·BBr₃ [1.966(13) Å].⁴⁶ The C–B bond in 5 [1.656(3) Å] is similar to that [1.660(2) Å] in [{Ph₂(S=)P}(H)(Ph₃Si)C(BH₃)][Li(THF)₃].⁴⁷ The structural features of the C₂N₂Si ring in 5 are remarkably different from those of 4.⁴¹ While the C=C double bond [1.3375(15) Å] in the C₂N₂Si ring of 4 is elongated to the C(28)–C(29) single bond in 5 [1.462(3) Å], one of the two C–N single bonds in the C₂N₂Si ring of 4 [1.414 Å, av] is concomitantly shortened to the N(4)–C(29) double bond in 5 [1.293(3) Å]. The Si(1)–N(3) bond in 5 [1.6797(18) Å (experimental value), 1.703 Å (theoretical value)] compares to the covalent Si–N single bonds in 4 [1.713 Å, av].⁴¹ The obviously elongated Si(1)–N(4) bond in 5 [1.8197(18) Å (experimental value), 1.829 Å (computed value)] is comparable with the reported dative Si–N single bonds (such as that [1.858(9) Å] in [Me₃Si(py)]⁺[I]^− 48 and those [1.8290(18) Å and 1.8617(18) Å] in a chlorosilyliumylidene complex).⁴⁹ Accordingly, the WBI of the Si(1)–N(4) bond in 5 (0.49) is considerably smaller than that (0.70) of the Si(1)–N(3) bond in 5. However, there have no obvious changes for the structural parameters of the C₂S₂Si rings in both 4 and 5. Compound 5 may be regarded as an intramolecular base-stabilized dithiolene-based silylium species. Natural bond orbital (NBO) analysis of 5-Ph model supports its zwitterionic feature (as shown in Scheme 2)—the silicon atom has a positive charge of +1.57, whereas the carbon atom [i.e., C(28)] bound to the BBr₃ unit bears a negative charge of −0.37. The electrostatic potential map of 4 (Fig. S22†) reveals the negative potential resides predominately in the region around the terminal sulphur atom of the dithiolene unit, while the region of the backbone carbon atoms of the N-heterocyclic silyl unit has a very weak negative electrostatic potential. Thus, it is somewhat surprising to observe the nucleophilic behaviour of the backbone carbon atom of the N-heterocyclic silyl ring in 4.

The X-ray structural analysis⁴³ of 6 (Fig. 1) reveals a planar Br₂B–N–C–C–N–BBr₂ framework, while the two 2,6-diisopropylphenyl substituents are nearly perpendicular to this plane. The structural parameters of the C₂N₂B₂(BBr₂)₂ core in 6 [d_C=C = 1.310(9) Å; d_N=B = 1.387(6) Å] compare well to those for 13 [d_C=C = 1.333(2) Å; d_N=B = 1.395(1) Å].⁴⁴ In the solid state⁴³ (Fig. 1), the terminal S(1) atom of 7, as that in 5, is capped by a boron tribromide species. The C₂S₂B ring in 7 is nearly planar [bend angle (η) between the BS₂ plane and the C₂S₂ plane = 1.7°]. The three-coordinate boron atom, involved in the five-membered dithiolene ring, adopts a trigonal planar geometry. In 7, the B(1)–S bonds [1.809(4) Å, av; WBI = 1.24, av] are somewhat shorter than the B(2)–S(1) bond [1.935(4) Å, av; WBI = 0.83], which should be due to π-donation of the S-lone pairs into the empty p orbital of the B(1) atom. The C–S bonds (1.737 Å, av) in the C₂S₂ unit of 7 are longer than those (1.710 Å, av) for the reported four-coordinate boron-based dithiolate complex,⁴⁵ which may be attributed to the electron donation from the sulphur atoms to the three-coordinate boron in 7.

X-ray structural analysis (Fig. 1) shows that 8 is isostructural with 5. The BI₃ bound to the backbone carbon of the N-heterocyclic silyl unit in 8 was released and subsequently cleaved one of the two Si–N bonds to give the NHSi-ring opened product 9. The solid-state structure of 9 (Fig. 1) shows that while a BI₂ species is bonded to a nitrogen atom [d_N=B = 1.393(11) Å], one iodine atom is attached to the central four-coordinate silicon atom. The Si(1)–I(4) bond distance in 9 [2.406(3) Å] is somewhat shorter than the computed value (2.492 Å). The B_sp²–I bonds (2.128 Å, av) in 9 is shorter than those B_sp³–I bonds in 8 and 9 (2.236 Å, av). NBO analysis shows that while the silicon atom in 9-Ph bears a positive charge of +1.10, the silicon atom and the carbon atom (next to BI₃) in zwitterionic 8 have a charge of +1.56 and −0.39, respectively. These results, coupled with the elongated Si(1)–N(4) bonds in 5 and 8, suggest that the canonical form A (Scheme 2) represents the predominant formulation of both 5 and 8. The X-ray structure of 10 (Fig. 1) indicates that one Si–S bond in 4 is cleaved by BCl₃via the formation of a Si–Cl bond and a B–S bond. The Si–Cl bond distance in 10 [2.0522(11) Å] is marginally shorter than the computed value (2.088 Å). The B–S bond in 10 [1.793(4) Å] compares well to those (involving the three-coordinate boron atom) in 7 [1.809(4) Å, av].

Computations of the simplified 5-Ph model⁴³ (Fig. 2) show that while the LUMO involves both C–B bonding and C–N π-anti-bonding character, the HOMO is dominated by the sulphur- and bromine-based lone pair character of the terminal SBBr₃ unit. HOMO−2 and HOMO−3 contain the S–B and C–B σ-bonding character, respectively. According to natural bond orbital (NBO) analysis, the C–B σ bond polarization is 28.2% toward boron and 71.8% toward carbon that has 28.90% s-, 71.08% p-, and 0.02% d-character.

Fig. 2 Molecular orbitals of the simplified 5-Ph model.

Conclusions

Dithiolene-based N-heterocyclic silane (4) reacts with two equivalents of BX₃ (X = Br, I) to give zwitterionic Lewis adducts 5 and 8, respectively, whereas the parallel reaction of 4 with BCl₃ gives 10via the Si–S bond cleavage. Further reaction of 5 with BBr₃ (in a 1 : 2 ratio) in toluene at an elevated temperature (100 °C) resulted in its decomposition, giving a mixture of 6 and 7. In contrast to 5, the labile zwitterion (8) may be readily converted to 9via BI₃-mediated Si–N bond cleavage. The 4-to-(5 and 8) conversions reveal that both the terminal sulphur atom of the dithiolene unit and the backbone carbon of the N-heterocyclic silyl moiety in 4 may serve as nucleophilic sites to bind BX₃ (X = Br and I) moieties. The potential broad utility of 4 as a species with dual nucleophilic sites is being investigated in this laboratory.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful to the National Science Foundation (CHE-2153978 to G. H. R. and Y. W.) and the Department of Energy (DOE-SC0015512 to H. F. S.) for support.

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Footnote

† Electronic supplementary information (ESI) available: Synthetic and computational details and structural and spectral characterization. CCDC 2294615–2294617, 2327747–2327749. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3dt03843b

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