A Stable N‐Heterocyclic Silylene with a 1,1′‐Ferrocenediyl Backbone

Abstract

The N‐heterocyclic silylene [{Fe(η⁵‐C₅H₄‐NDipp)₂}Si] (1DippSi, Dipp=2,6‐diisopropylphenyl) shows an excellent combination of pronounced thermal stability and high reactivity towards small molecules. It reacts readily with CO₂ and N₂O, respectively affording (1DippSiO₂)₂C and (1DippSiO)₂ as follow‐up products of the silanone 1DippSiO. Its reactions with H₂O, NH₃, and FcPH₂ (Fc=ferrocenyl) furnish the respective oxidative addition products 1DippSi(H)X (X=OH, NH₂, PHFc). Its reaction with H₃BNH₃ unexpectedly results in B−H, instead of N−H, bond activation, affording 1DippSi(H)(BH₂NH₃). DFT results suggest that dramatically different mechanisms are operative for these H−X insertions.

A stable cyclic diaminosilylene featuring a ferrocene‐based backbone is reported. The dicoordinate Si^II atom is part of a six‐membered FeC₂N₂Si ring and shows a comparatively large bond angle of ca. 107°. This N‐heterocyclic silylene can activate strong bonds (C=O, N=O, B−H, N−H, P−H, O−H) under ambient conditions.

The N‐heterocyclic silylene (NHSi) A ^[1] (Figure 1) is a heavier NHC ^[2] analogue and represents the first stable compound containing divalent and dicoordinate silicon. ^[3] Backbone‐saturated congeners are significantly more reactive. For example, whereas 1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene (IPr) is inert towards PH₃, the backbone‐saturated congener SIPr readily inserts into a P−H bond, ^[4] and silylene B undergoes self‐insertion into an Si−N bond during its tetramerisation. ^[5] The first isolable dialkylsilylene C ^[6] exhibits a more pronounced ambiphilicity than diaminosilylenes and rearranges to a Si^IV compound. ^[7]

Figure 1.

Silylenes A–F (Ad=1‐adamantyl, Dipp=2,6‐diisopropylphenyl, TMS=trimethylsilyl).

The rapid development of carbene chemistry has led to acyclic diaminocarbenes (ADACs), ^[8] ring‐expanded NHCs (reNHCs) with ring sizes >5 ^[9] and cyclic (alkyl)(amino)carbenes (CAACs), ^[10] which are all closely related to standard NHCs, but exhibit a more pronounced ambiphilicity, and hence higher reactivity. ^[11] While more than a dozen silicon analogues of standard NHCs have been isolated,[ ³ , ¹² ] only a single example each has been reported for stable silicon analogues of CAACs, ^[13] ADACs ^[14] and reNHCs, ^[15] viz. silylenes D–F (Figure 1). The ambivalent reactivity of reNHSi F was rationalised by a significant contribution of N‐ylidic canonical structures summarised as F′. We here report on the reNHSi [{Fe(η⁵‐C₅H₄‐NDipp)₂}Si] (1DippSi), which contains a six‐membered FeC₂N₂Si ring. 1DippSi is an analogue of our stable ferrocene‐based NHCs, whose ambiphilicity allowed for small‐molecule activation reactions unprecedented for diaminocarbenes.[ ¹⁶ , ¹⁷ ]

Our attempts to obtain reNHSis of the type 1RSi by reduction of corresponding Si^IV dihalides 1RSiX₂ (X=Cl, Br) or by α‐elimination of HCl from 1RSi(H)Cl were unsuccessful. ^[18] An alternative approach, which was introduced for the acyclic diaminosilylene (ADASi) E, is the reaction of [SiCl₂(IPr)] ^[19] with the corresponding lithium amide. ^[14] This Si^II precursor turned out to be the key to success. Its reaction with 1MesLi₂ in C₆D₆ afforded the silylene 1MesSi together with IPr (Scheme 1). Although too unstable for isolation, 1MesSi was sufficiently persistent at room temperature for detecting its ²⁹Si NMR signal (δ=121.5 ppm), which is significantly downfield‐shifted with respect to reNHSi F (δ=88.4 ppm) ^[15] and NHSi A (78.3 ppm). ^[1] The signal of the Si^II atom in ADASi E was observed at even lower field (δ=204.6 ppm). ^[14] Trapping of 1MesSi with (PhSe)₂ at room temperature in benzene solution afforded 1MesSi(SePh)₂; details are provided in the Supporting Information (SI). The bulkier homologue 1DippSi, obtained from [SiCl₂(IPr)] and 1DippLi₂ in toluene at room temperature, is sufficiently stable for isolation (Scheme 1). IPr and 1DippSi could not be separated by crystallisation or sublimation. It was possible to remove IPr from ADASi E by stirring a hexane solution of the mixture at room temperature under an atmosphere of CO₂, which led to the precipitation of IPr(CO₂). ^[14] This method was not successful in our case, because, in contrast to E, 1DippSi reacts swiftly with CO₂ under the same mild conditions, affording the orthocarbonate (1DippSiO₂)₂C (Scheme 2; see the SI). The primary products are most likely CO and the silanone 1DippSiO, ^[20] which subsequently undergoes a cycloaddition with CO₂ in a 2:1 ratio. When generated by reaction of 1DippSi with N₂O in benzene at room temperature, this silanone forms the expected dimer (1DippSiO)₂ (Scheme 2; see the SI). ^[21] An analogous stepwise reaction with CO₂ was first reported for decamethylsilicocene (Cp*₂Si). ^[22] Dialkylsilylene C ^[23] as well as IPr=N‐Si‐OSitBu₃ and IPr=N‐Si‐Si(SiMe₃)₃, an acyclic (imino)(siloxy)‐ and (imino)(silyl)silylene, ^[24] are the only examples containing dicoordinate Si^II in this context to date. ^[25] We found that IPr is easily removed by complexation with ZnCl₂, which is inert towards 1DippSi. In contrast to 1DippSi, [ZnCl₂(IPr)] ^[26] is insoluble in hexane.

Scheme 1.

Synthesis of 1MesSi (persistent, Mes=mesityl) and 1DippSi (stable) and reactions of the latter with H₂O, NH₃, FcPH₂ (Fc=ferrocenyl), and H₃BNH₃ under ambient conditions in benzene or toluene. Selected bond lengths [Å] and angles [°] for 1DippSi: Si1‐N1 1.7327(12), Si1‐N2 1.7344(12), N1‐Si1‐N2 106.58(6); sum of angles (Σ∡) at N1 359.9, at N2 360.0.

Scheme 2.

Reactions of 1DippSi with CO₂ and N₂O under ambient conditions in benzene, respectively affording (1DippSiO₂)₂C and (1DippSiO)₂ via the silanone 1DippSiO as assumed intermediate.

The ²⁹Si NMR signal of 1DippSi is located at δ=115.7 ppm, upfield‐shifted by 6 ppm with respect to 1MesSi. 1DippSi was structurally characterised by X‐ray diffraction (Scheme 1). The Si bond angle (106.6°) lies in between the values determined for reNHSi F (99.3°) ^[15] and ADASi E (110.9°) ^[14] and is close to that reported for a heterocyclic silylene with a six‐membered ring containing an NSi^IIBP unit. ^[27] Silylenes whose dicoordinate Si^II atom is part of a five‐membered ring exhibit more acute Si bond angles close to 90°.[ ³ , ⁶ , ¹² , ¹³ , ²⁸ ]

Similar to 1MesSi, 1DippSi undergoes an oxidative addition with (PhSe)₂ in benzene solution at room temperature to afford 1DippSi(SePh)₂ (see the SI). We next addressed the oxidative addition of strong H−X bonds of different polarities, which is of fundamental importance for chemical synthesis and catalysis. ^[29] While 1DippSi is inert towards H₂ under ambient conditions, it reacted readily with H₂O, NH₃, and FcPH₂, affording the corresponding derivatives of the type 1DippSi(H)X (Scheme 1, Figure 2; X=OH, NH₂, PHFc; see the SI). The reaction of H₂O with stable dicoordinate Si^II compounds to the corresponding hydroxysilane has been reported only for the metallasilylene [Cp*(CO)₃Cr‐Si‐SIPr]^+[30] as well as for A ^[31] and F. ^[32] The hydroxysilanes A(H)OH and F(H)OH were not observed, but their intermediacy was merely inferred from the products isolated. In contrast, the analogous NH₃ addition product F(H)NH₂ was obtained in high yield from the reaction of F with NH₃. ^[33] F is the exception to the rule that five‐ and six‐membered NHSis cannot be employed for NH₃ activation, although they are more Lewis acidic and have a smaller singlet‐triplet gap compared to the corresponding NHCs. ^[34] Apart from [Cp*(CO)₃Cr‐Si‐SIPr]⁺, ^[30] Dipp(Me₃Si)N‐Si‐Si(SiMe₃)₃ ^[35] and IPr=N‐Si‐OSitBu₃ ^[24a] we are not aware of any other stable silylene to undergo an oxidative addition of NH₃. The reaction of NH₃ with 1DippSi is remarkable because NH₃ activation is a challenging target even for transition metal complexes ^[36] —the potential of low‐valent main‐group element compounds in this context was uncovered only recently.[ ²⁵ , ³⁷ ] The reaction of 1DippSi with FcPH₂ afforded the oxidative addition product 1DippSi(H)(PHFc). In view of the ability of 1DippSi for N‐H activation, the activation of a P−H bond, which is weaker than an N−H bond by ca. 100 kJ mol⁻¹, is not unexpected; ^[38] reNHSi F is also capable of P‐H activation. ^[39] We next addressed the reaction of 1DippSi with H₃BNH₃ (Scheme 1), ^[40] expecting the formation of 1DippSiH₂, most likely by transfer of a protic and a hydridic H atom ^[41] to the divalent atom, as was observed for 1,3‐di‐tert‐butylimidazolin‐2‐ylidene ^[33] and reNHSi F. ^[42] Instead, the reaction furnished 1DippSi(H)(BH₂NH₃) (Figure 2), although the B−H bond is stronger than the N−H bond of H₃BNH₃. ^[43] First B−H bond activation reactions with Si^II compounds were reported only recently, ^[44] and the reaction of the (silyl)(vinyl)silylene ^MeIPr=CH‐Si‐Si(SiMe₃)₃ (^MeIPr=1,3‐bis(2,6‐diisopropylphenyl)‐4,5‐dimethylimidazolin‐2‐ylidene) with pinacolborane (HBPin), which affords ^MeIPr=CH‐Si(H)(Bpin)‐Si(SiMe₃)₃, is the only example involving dicoordinate Si^II. ^[45]

Figure 2.

Molecular structures of 1DippSi(H)OH (left; selected bond lengths [Å] and angles [°]: Si1‐N1 1.7197(16), Si1‐N2 1.7199(16), Si1‐O1 1.6071(16), N1‐Si1‐N2 111.49(8); Σ∡ at N1 359.9, at N2 359.4), 1DippSi(H)NH₂ (middle; selected bond lengths [Å] and angles [°]: Si1‐N1 1.691(4), Si1‐N2 1.721(3), Si1‐N3 1.730(3), N2‐Si1‐N3 111.72(14); Σ∡ at N2 358.5, at N3 358.0) and 1DippSi(H)(BH₂NH₃) (right; selected bond lengths [Å] and angles [°]: Si1‐N1 1.7571(14), Si1‐N2 1.7694(14), Si1‐B1 2.008(2), N3‐B1 1.600(3), N1‐Si‐N2 107.84(7), Si1‐B1‐N1 116.41(13); Σ∡ at N1 358.3, at N2 356.6).

We performed a DFT study on the electronic characteristics and the reactivity of 1, the full molecular model of 1DippSi. ^[46] At the PBEh‐3c level of DFT employed, the HOMO comprises the expected silylene lone pair together with significant contributions of the ferrocene moiety and the LUMO is dominated by the silylene p‐orbital, with a substantial HOMO–LUMO energy separation of ΔE _H/L=6.4 eV. The unexpectedly low computed singlet‐triplet energy difference of ΔE _S/T=0.4 eV does not correlate with this value because the lowest triplet state arises from a local excitation within the ferrocene moiety and does not involve the silylene p‐orbital (Figure 3). ^[47]

Figure 3.

Frontier molecular orbitals and triplet spin‐density distribution computed for 1 (i. e. the full molecular model of 1DippSi; orbital energies in eV, isocontour surfaces at ±0.05 a₀ ^−3/2 for orbitals and ±0.005 a₀ ⁻³ for the spin density, α spin: yellow, β spin: green; iPr groups and H atoms not shown).

Surprisingly, direct oxidative addition of H₂O and NH₃ to 1 is precluded by high kinetic barriers for both substrates (35 and 42 kcal mol⁻¹, respectively), and two distinct alternative pathways were identified instead. The lowest‐energy pathway for NH₃ activation commences with the formation of adduct 2 (Scheme 3, top). Proton transfer is facilitated by a second NH₃ molecule acting as a proton shuttle and the experimentally observed product 3 is formed in a strongly exergonic step with a moderate overall barrier of 20 kcal mol⁻¹. H₂O, in turn, does not form a datively bonded adduct with 1, but directly adds across an Si−N bond via TS2 to form hydroxysilylene 4 in an exergonic step (Scheme 3, bottom). From there on out, silanone 5 is formed through a water‐assisted proton transfer; ^[48] the experimentally observed product 6 results in a strongly exergonic step after passage of a minute barrier in TS4.

Scheme 3.

Computed lowest‐energy reaction pathway for the formal H‐X oxidative addition to 1 with NH₃ (top) and H₂O (bottom, ΔG ²⁹⁸ in kcal mol⁻¹). Bonds formed or broken in transition states are dashed, unreactive H atoms are omitted and the orientation of the Dipp substituents in the transition state is indicated by showing the respective C_ipso atom only.

The quantum‐chemical evaluation discloses a concerted dehydrogenation of H₃BNH₃ by 1 as initial step along the lowest‐energy pathway for ammonia‐borane activation (Scheme 4). Alternative direct insertion of 1 into an N−H or B−H bond is precluded by high kinetic barriers (53 and 35 kcal mol⁻¹, respectively; see the SI). Whereas the resulting silane 7 forms as an unreactive side product, ^[49] H₂BNH₂ is a highly reactive species that has been thoroughly studied in the thermal and catalytic dehydrogenation of H₃BNH₃ and is known to polymerize below −150 °C. ^[50]

Scheme 4.

Computed reaction path for the formation of 7 from 1 and H₃BNH₃; ΔG ²⁹⁸ in kcal mol⁻¹.

Obviously, B‐H insertion of 1 in H₂BNH₂ competes efficiently with the polymerization of the latter, leading to the formation of 8 (Scheme 5). With a low barrier of 12 kcal mol⁻¹ 8 can dehydrogenate a second equivalent of H₃BNH₃ through intermediate 9 yielding the experimentally observed product 10 while regenerating H₂BNH₂. After initial formation of H₂BNH₂ from 1 and H₃BNH₃ the follow‐up reaction cascade involving B‐H insertion by 1 and subsequent dehydrogenation of H₃BNH₃ is kinetically favoured over the formation of 7.

Scheme 5.

Computed reaction path for the formation of 10 from 1, H₂BNH₂and H₃BNH₃ (ΔG ²⁹⁸ in kcal mol⁻¹ relative to the separated reactants, activation barriers relative to the preceding minimum).

In conclusion, we have described the synthesis and reactivity of the new stable reNHSi 1DippSi. 1DippSi reacts readily with N₂O and CO₂, which is in contrast to the inertness of F, the only other stable cyclic diaminosilylene featuring a ring‐expanded structure known to date. ^[25a] Studies on the reactivity of 1DippSi towards H‐X bonds of different strengths and polarities show parallels to previous reactivity studies on other silylenes. The reactions with NH₃ and H₂O both give the H‐X insertion products. Mechanistically, however, they differ significantly. More particularly, the lowest‐energy path of the reaction with H₂O involves the N‐Si cooperative activation of an O−H bond. For H₃BNH₃ the reaction mechanism consists of two key elementary steps, the first one being the dehydrogenation of H₃BNH₃ to H₂BNH₂, which subsequently catalyses the conversion of 1DippSi to 1DippSi(H)(BH₂NH₃) with H₃BNH₃. In contrast to H₃BNH₃, H₂BNH₂ has a vacant p‐orbital, which enables insertion of the silylene in a B−H bond in the second step. This silylborane can in turn dehydrogenate a second equivalent of H₃BNH₃ to give the final product 1DippSi(H)(BH₂NH₃).

Conflict of interest

The authors declare no conflict of interest.

Supporting information

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Supplementary

N. Weyer, M. Heinz, J. I. Schweizer, C. Bruhn, M. C. Holthausen, U. Siemeling, Angew. Chem. Int. Ed. 2021, 60, 2624.