Interconversion and reactivity of manganese silyl, silylene, and silene complexes

Abstract

Manganese disilyl hydride complexes [(dmpe)₂MnH(SiH₂R)₂] (4^Ph: R = Ph, 4^Bu: R = ⁿBu) reacted with ethylene to form silene hydride complexes [(dmpe)₂MnH(RHSi=CHMe)] (6^Ph,H: R = Ph, 6^Bu,H: R = ⁿBu). Compounds 6^R,H reacted with a second equivalent of ethylene to generate [(dmpe)₂MnH(REtSi=CHMe)] (6^Ph,Et: R = Ph, 6^Bu,Et: R = ⁿBu), resulting from apparent ethylene insertion into the silene Si–H bond. Furthermore, in the absence of ethylene, silene complex 6^Bu,H slowly isomerized to the silylene hydride complex [(dmpe)₂MnH(=SiEtⁿBu)] (3^Bu,Et). Reactions of 4^R with ethylene likely proceed via low-coordinate silyl {[(dmpe)₂Mn(SiH₂R)] (2^Ph: R = Ph, 2^Bu: R = ⁿBu)} or silylene hydride {[(dmpe)₂MnH(=SiHR)] (3^Ph,H: R = Ph, 3^Bu,H: R = ⁿBu)} intermediates accessed from 4^R by H₃SiR elimination. DFT calculations and high temperature NMR spectra support the accessibility of these intermediates, and reactions of 4^R with isonitriles or N-heterocyclic carbenes yielded the silyl isonitrile complexes [(dmpe)₂Mn(SiH₂R)(CNR′)] (7a–d: R = Ph or ⁿBu; R′ = o-xylyl or ^tBu), and NHC-stabilized silylene hydride complexes [(dmpe)₂MnH{=SiHR(NHC)}] (8a–d: R = Ph or ⁿBu; NHC = 1,3-diisopropylimidazolin-2-ylidene or 1,3,4,5-tetramethyl-4-imidazolin-2-ylidene), respectively, all of which were crystallographically characterized. Silyl, silylene and silene complexes in this work were accessed via reactions of [(dmpe)₂MnH(C₂H₄)] (1) with hydrosilanes, in some cases followed by ethylene. Therefore, ethylene (C₂H₄ and C₂D₄) hydrosilylation was investigated using [(dmpe)₂MnH(C₂H₄)] (1) as a pre-catalyst, resulting in stepwise conversion of primary to secondary to tertiary hydrosilanes. Various catalytically active manganese-containing species were observed during catalysis, including silylene and silene complexes, and a catalytic cycle is proposed.

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Introduction

Silylenes (:SiR₂)^1,2 and silenes (R₂C=SiR₂),^2–4 heavy analogues of carbenes and alkenes, are highly reactive species, and in the absence of extremely bulky or π-donor substituents,⁵ transition metal coordination is required for stabilization.^3,6–8 However, complexes bearing unstabilized silylene ligands are involved in various catalytic processes involving silanes, including dehydrocoupling, substituent redistribution, hydrosilylation, and the Direct process for silane chlorination.⁷ Similarly, silene complexes have in several instances been hypothesized to play an important role in catalysis, typically on the basis of indirect observations. For example, they are thought to be active species in polycarbosilane synthesis from dichloromethylsilanes and sodium in the presence of [Cp₂ZrCl₂],⁹ dehydrogenative coupling of HSiMe₃ by [(Me₃P)₃RuH₃(SiMe₃)],¹⁰ transfer dehydrogenative coupling of HSiEt₃ catalysed by [(p-cymene)RuH₂(SiEt₃)₂] or [Cp*RhH₂(SiEt₃)₂],¹¹ and trialkylsilane (e.g. HSiMe₃) perdeuteration catalysed by [(Me₃P)₄OsH(SiMe₃)]¹² or [(C₆Me₆)RuH₂(SiMe₃)₂]¹³ in C₆D₆. Furthermore, silene complexes were recently proposed as off-cycle species in sila-heterocycle synthesis by intramolecular silylation of primary C–H bonds,¹⁴ and free silenes play a key role in hot wire CVD of SiC using alkylsilanes.¹⁵

Early examples of isolable transition metal complexes bearing a terminal silylene ligand featured Lewis base coordination to silicon,¹⁶ and base-free terminal silylene complexes were not isolated until 1990.¹⁷ Since then, a range of such complexes have been reported; almost exclusively mid- and late-transition metal complexes,¹⁸ which are electrophilic at silicon. By contrast, silylene complexes with an SiH substituent remain relatively rare; the first example, [(Et₃P)₃IrH₂{=SiH(C₆H₃-Mes₂-2,6)}][B(C₆F₅)₄], was reported in 2002,¹⁹ and in the same year, Tilley et al. suggested [{PhB(CH₂PPh₂)₃}IrH₂{=SiH(Trip)}] as an intermediate in the synthesis of [{PhB(CH₂PPh₂)₃}IrH₂{=Si(C₈H₁₅)(Trip)}].²⁰ Two years later, the Tobita²¹ and Tilley²² groups independently reported the first structurally characterized examples, [(C₅Me₄Et)(OC)₂WH(=SiH{C(SiMe₃)₃})] and [Cp*(dmpe)MoH(=SiHPh)], respectively. Base-free L_xM=SiHR complexes have only been isolated for groups 6, 8 and 9,^19–26 and group 7 examples are notably absent. Extensive studies by the Tilley and Tobita groups have demonstrated that hydrogen substituents on the sp² Si centers permit these silylene complexes to demonstrate unusual reactivity, including alkene insertion into the silylene Si–H bond in cationic complexes, and conversion to silylyne (M≡SiR) complexes.^20,23,25

A small number of transition metal silene complexes have also been isolated, with 2^nd and 3^rd row transition metal examples (bearing sterically and electronically unstabilized silene ligands)²⁷ limited to complexes of Ir, Ru, and W (Fig. 1).^28–30 Furthermore, outside of our recent report of manganese silene complexes (vide infra), first row transition metal complexes bearing unstabilized silene ligands have not been isolated.

Fig. 1 Second and third row transition metal complexes bearing sterically and electronically unstabilized silene ligands.

We recently communicated the synthesis (Scheme 1) of the first unstabilized terminal silylene complexes of a group 7 metal, [(dmpe)₂MnH(=SiR₂)] (3^Ph2: R = Ph, 3^Et2: R = Et), by the reaction of [(dmpe)₂MnH(C₂H₄)] (1)³¹ with secondary hydrosilanes (H₂SiR₂).³² In the solid state, the silylene and hydride ligands are cis (diphenyl analogue 3^Ph2) or trans (diethyl analogue 3^Et2) disposed, in the former case with an Si–H interligand interaction. Silylene hydride complexes with interligand Si–H interactions were first reported in 2004 by the Tobita²¹ ([(C₅Me₄Et)(OC)₂WH(=SiH{C(SiMe₃)₃})]) and Tilley²² ([Cp*(dmpe)MoH(=SiEt₂)])³³ groups, and since that time, W, Fe, Ru, and Ni examples have been reported.^25,26,34 Uniquely, the cis and trans isomers of 3^Ph2 exist in equilibrium with one another in solution (Scheme 1).

Scheme 1 Reactions of [(dmpe)₂MnH(C₂H₄)] (1) with primary and secondary hydrosilanes to generate silylene-hydride (3^R2) and disilyl hydride (4^R) complexes respectively, reactions of the latter two complexes with H₂ to generate isostructural silyl dihydride complexes (5^R2 or 5^R respectively), and reaction of silylene hydride complexes 3^R2 with ethylene to generate silene hydride complexes (6^R2).^32,35,36 Only one isomer is shown for 2, 5, and A.

In contrast to the reactions of 1 with secondary hydrosilanes, reactions with primary hydrosilanes (H₃SiR) yielded disilyl hydride complexes [(dmpe)₂MnH(SiH₂R)₂] (4^Ph: R = Ph, 4^Bu: R = ⁿBu; Scheme 1).³⁵ The syntheses of both silylene hydride complexes 3^R2 and disilyl hydride complexes 4^R from 1 were proposed to proceed via a 5-coordinate silyl intermediate [(dmpe)₂Mn(SiHRR′)] (2^Ph2: R = R′ = Ph; 2^Et2: R = R′ = Et; 2^Ph: R = Ph, R′ = H; 2^Bu: R = ⁿBu, R′ = H), which undergoes α-hydride elimination to generate 3^R2, or oxidative addition of a second equivalent of hydrosilane to afford 4^R (Scheme 1).

Reactions of the silylene hydride complexes with ethylene generated the silene hydride complexes cis-[(dmpe)₂MnH(R₂Si=CHMe)] {R = Ph (6^Ph2) or Et (6^Et2); Scheme 1}. This type of silylene to silene transformation is unprecedented, although Tilley et al. have reported conversion of [Cp*(Me₃P)Ir(Me)(=SiMe₂)]⁺ to the silene hydride isomer, [Cp*(Me₃P)IrH(Me₂Si=CH₂)]⁺.²⁸ The intermediacy of a trimethylsilyl iridium complex in this reaction was supported by trapping reactions with CO and ethylene. Additionally, the iridium silene hydride cation reacted with pyridine to afford [Cp*(Me₃P)Ir(Me){=SiMe₂(py)}]⁺, highlighting the reversibility of the silylene–silene transformation (Scheme 2).²⁸ Furthermore, many of the known silene complexes were synthesized by installation of a CH₂SiR₂H (R = Me or Ph) or SiR₂Me group, followed by β-hydride elimination.^14,28,29 These classes of reaction are combined in Scheme 2, highlighting the potential to interconvert between silylene, silyl, silene, and alkyl (CH₂SiHR₂) complexes.

Scheme 2 Reported reactions capable of converting between silylene, silyl, silene and alkyl isomers. Electron pushing arrows are shown for the forward direction.

Both silylene hydride complexes 3^R2 and disilyl hydride complexes 4^R have been shown to react with H₂ (Scheme 1) to generate silyl dihydride complexes [(dmpe)₂MnH₂(SiHRR′)] (5^Ph2: R = R′ = Ph; 5^Et2: R = R′ = Et; 5^Ph: R = Ph, R′ = H; 5^Bu: R = ⁿBu, R′ = H), suggesting the accessibility of a common low-coordinate silyl intermediate, [(dmpe)₂Mn(SiHRR′)] (2).^32,36 Therefore, disilyl hydride complexes 4^R could potentially react as sources of manganese silylene hydride complexes with an SiH substituent, and exposure of 4^R to ethylene may provide a route to silene complexes bearing an SiH substituent.

Herein, we report the reactions of disilyl hydride complexes 4^R with ethylene to generate the first examples of silene complexes with a hydrogen substituent on silicon. Their unique reactivity is also described, including (a) silene hydride to silylene hydride isomerization, and (b) reaction with a second equivalent of ethylene to convert the SiH substituent to an SiEt group. The reactions of 4^R with ethylene likely proceed via a low-coordinate silyl or silylene hydride intermediate, and DFT calculations, high temperature NMR spectroscopy, and trapping studies are described, providing insight into the accessibility of these intermediates.

All of the silyl, silylene and silene complexes in this work are accessed via reactions of [(dmpe)₂MnH(C₂H₄)] (1) with hydrosilanes, in some cases followed by ethylene. Therefore, ethylene (C₂H₄ and C₂D₄) hydrosilylation was investigated using [(dmpe)₂MnH(C₂H₄)] (1) in combination with primary and secondary hydrosilanes, and a catalytic cycle is proposed (based on the metal species and hydrosilane products observed throughout the course of the reactions). Alkene hydrosilylation is an industrially important transition metal-catalysed process for alkylsilane production,^37–39 and the most common olefin hydrosilylation catalyst used in industry is Karstedt's catalyst, [Pt₂(O{SiMe₂(CH=CH₂)}₂)₃].³⁸ However, the development of catalytic systems based on first row transition metals such as manganese is of interest due to high abundance, low cost, reduced toxicity, and improved environmental compatibility.⁴⁰ In this regard, manganese mediated hydrosilylation of polar unsaturated bonds has been well studied,⁴¹ but only a handful of manganese catalysts have been reported for alkene hydrosilylation.^32,42

The typical mechanism for alkene hydrosilylation (Chalk–Harrod mechanism) involves oxidative addition of a hydrosilane to generate a silyl hydride complex, followed by alkene coordination, C–H bond-forming 1,2-insertion, and finally Si–C bond-forming reductive elimination. However, in some cases alkene coordination is followed by C–Si bond-forming 1,2-insertion and then C–H bond-forming reductive elimination (modified Chalk–Harrod mechanism). Furthermore, catalytic cycles which proceed via a monosilyl complex rather than a silyl hydride complex have been reported, including hydrosilylation reactions utilizing a cationic palladium(II) or cobalt(III) alkyl pre-catalyst.^38,43

Results and discussion

Reactions of disilyl hydride complexes 4^R with ethylene

The disilyl hydride complexes [(dmpe)₂MnH(SiH₂R)₂] (4^Ph: R = Ph, 4^Bu: R = ⁿBu) reacted with ethylene at room temperature to afford the silene hydride complexes [(dmpe)₂MnH(RHSi=CHMe)] (6^Ph,H: R = Ph, 6^Bu,H: R = ⁿBu). This reaction mirrors the reactions of silylene hydride complexes 3^R2 with ethylene (vide supra: Scheme 1).³² Moreover, complexes 6^R,H reacted with a second equivalent of ethylene to form silene hydride complexes with two hydrocarbyl groups on Si, [(dmpe)₂MnH(REtSi=CHMe)] (6^Ph,Et: R = Ph, 6^Bu,Et: R = ⁿBu); the products of apparent ethylene insertion into the Si–H bond (Scheme 3 and Fig. 2). This silene SiH to SiR conversion reaction is unprecedented. Complexes 6^R,Et also reacted further with ethylene to generate [(dmpe)₂MnH(C₂H₄)] (1),³¹ potentially by substitution of the silene ligand which undergoes subsequent decomposition to unidentified products.

Scheme 3 Reactions of disilyl hydride complexes [(dmpe)₂MnH(SiH₂R)₂] (4^Ph: R = Ph, 4^Bu: R = ⁿBu) with one, two, or three equivalents of ethylene to afford SiH-containing silene hydride complexes [(dmpe)₂MnH(RHSi=CHMe)] (6^Ph,H: R = Ph, 6^Bu,H: R = ⁿBu), silene hydride complexes with two hydrocarbyl groups on Si [(dmpe)₂MnH(REtSi=CHMe)] (6^Ph,Et: R = Ph, 6^Bu,Et: R = ⁿBu), and ethylene hydride complex [(dmpe)₂MnH(C₂H₄)] (1), respectively. Only one isomer is shown for each silene hydride complex.

Fig. 2 ¹H NMR spectra (298 K, C₆D₆, 600 MHz) for the reaction of [(dmpe)₂MnH(SiH₂Ph)₂] (4^Ph) with ethylene over time (initial, n_C2H4 ≈ n_silane).⁴⁴ The x-axis corresponds to the bottom spectrum, and for clarity, each spectrum above that is shifted by 0.15 ppm to lower frequency. The inset shows the relative concentration of hydride-containing species versus time; reactant [(dmpe)₂MnH(SiH₂Ph)₂] (4^Ph; light blue ✴), silene hydride [(dmpe)₂MnH(PhHSi=CHMe)] (6^Ph,H; purple ×), silene hydride [(dmpe)₂MnH(PhEtSi=CHMe)] (6^Ph,Et; red ■), and [(dmpe)₂MnH(C₂H₄)] (1; dark blue ♦).

A range of byproducts were observed in the syntheses of silene hydride complexes, including primary, secondary, and tertiary hydrosilanes {H_(3–n)SiEt_nR (n = 0, 1, 2; R = Ph, ⁿBu)};⁴⁵ the latter two are formed by stepwise manganese-catalysed hydrosilylation reactions between the primary hydrosilane byproduct and excess ethylene (vide infra). For R = ⁿBu, silene SiH to SiEt conversion did not proceed until all of the primary hydrosilane byproduct had been consumed, so conversion of 4^Bu to 6^Bu,H, and then to 6^Bu,Et, proceeded in a stepwise fashion. By contrast, for R = Ph, silene SiH to SiEt conversion commenced as soon as 6^Ph,H was available (Fig. 2).

Compounds 6^Bu,H and 6^Ph,Et were isolated as a red oil and a brown solid, respectively, in >95% purity. By contrast, 6^Ph,H and 6^Bu,Et were characterized in situ by NMR spectroscopy (Table 1). Compounds 6^Ph,H and 6^Bu,Et were not isolated due to the formation of mixtures of products (e.g.6^Ph,H accompanied by 6^Ph,Et and 1), combined with instability in solution over a period of days at room temperature.

Table 1 Selected ¹H, ¹³C, ²⁹Si and ³¹P NMR chemical shifts (ppm) and coupling constants (Hz) for silene hydride complexes [(dmpe)₂MnH(RR′Si=CHMe)] (6^Ph2: R = R′ = Ph; 6^Et2: R = R′ = Et; 6^Ph,H: R = Ph, R′ = H; 6^Bu,H: R = ⁿBu, R′ = H; 6^Ph,Et: R = Ph, R′ = Et; 6^Bu,Et: R = ⁿBu, R′ = Et); in C₆D₆ (6^R2 and 6^Ph,Et) or d₈-toluene (6^R,H and 6^Bu,Et). Unless otherwise noted, values are from NMR spectra at 298 K. For 6^R,H, NMR environments are reported for both observed isomers. Chemical shifts for 6^Ph2 and 6^Et2 are from our prior communication³²

	6^Ph,H	6^Bu,H	6^Ph,Et	6^Bu,Et	6^Ph2	6^Et2
a Due to the minor isomer of 6^Bu,H. b Due to the major isomer of 6^Bu,H. c Both isomers have identical chemical shifts. d Measured at 213 K {because this environment was not located by ^29Si{^1H} or 2D ^1H–^29Si (HSQC or HMBC) NMR spectroscopy at 298 K}. e Coupling between the Si=C̲ and Si=CH̲CH3 environments. f ^1 J C,H could only be resolved for one isomer.
MnH̲	−14.5, −14.7	−14.9,^a −15.0^b	−14.9	−15.3	−14.6	−15.3
SiH̲	4.5^c	3.7^c	—	—	—	—
Si=CH̲CH3	0.1, 0.2	−0.1,^a −0.2^b	0.2	−0.1	0.4	0.0
Si=CHCH̲3	1.9^c	1.8,^a 1.7^b	1.9	1.7	2.1	1.8
Si=C̲	−21.0, −21.2	−19.3,^a −20.8^b	−21.7	−19.3	−22.9	−19.4
^29Si	−7.1,^d −17.4^d	−8.9,^a −17.0^b	0.7	−6.5	−1.5	−3.0
^31P	63.3–85.5	65.8–79.1	65.7–79.2	65.5–79.3	62.7–78.3	65.5–79.3
^1 J C,H	139^f	138,^a 139^b	137	138	136	137

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In solution (in the absence of ethylene or free hydrosilanes), SiH-containing silene hydride complex 6^Bu,H underwent isomerization to the silylene hydride complex trans-[(dmpe)₂MnH(=SiEtⁿBu)] (trans-3^Bu,Et; Scheme 4), with 20% conversion after 2 days at room temperature in C₆D₆.⁴⁶ NMR spectra of trans-3^Bu,Et feature an MnH¹H NMR peak at −10.48 ppm (a quintet with ²J_H,P of 51 Hz), two sharp singlets in the ³¹P{¹H} NMR spectrum at 80.35 and 80.50 ppm, and a high-frequency peak in the ²⁹Si{¹H} NMR spectrum at 364 ppm. These data are consistent with a high-symmetry base-free silylene complex, and are nearly identical to the NMR data for trans-[(dmpe)₂MnH(=SiEt₂)] (trans-3^Et2).⁴⁷ Isomerization was accompanied by formation of small amounts (∼10% relative to 3^Bu,Et) of an unidentified manganese hydride complex (with a quintet ¹H NMR peak at −9.06 ppm; ²J_H,P = 47 Hz) and the silene hydride complex [(dmpe)₂MnH(ⁿBuEtSi=CHMe)] (6^Bu,Et).

Scheme 4 Solution decomposition of [(dmpe)₂MnH(ⁿBuHSi=CHMe)] (6^Bu,H) to form silylene complex trans-[(dmpe)₂MnH(=SiEtⁿBu)] (trans-3^Bu,Et) as the major product.

Isomerization of a silene hydride complex to a silylene hydride complex is, to our knowledge, unprecedented. However, this isomerization is related to Tilley and Bergman's report of an equilibrium between the silylene alkyl complex [Cp*(Me₃P)Ir(Me)(=SiMe₂)]⁺ and the silene hydride isomer, [Cp*(Me₃P)IrH(Me₂Si=CH₂)]⁺, which relies upon reversible α-Me and β-H elimination from a trimethylsilyl intermediate.²⁸

For silene hydride complexes 6^R,H (those with a hydride substituent on Si), two sets of NMR signals were observed due to a pair of isomers present in solution with a 1 : 1 (6^Ph,H; Fig. 2) or 1.9 : 1 (6^Bu,H) ratio, whereas only a single set of NMR signals (indicative of a single isomer) was observed for 6^R,Et (silene hydride complexes with two hydrocarbyl substituents on Si). NMR spectra of the silene hydride complexes feature (for each isomer) four ³¹P NMR signals, a single ²⁹Si NMR environment (at −17.4 to 0.7 ppm), a low frequency ¹³C NMR signal for the Si=C̲ environment (at −19.3 to −21.7 ppm), and a silene ¹J_C,H coupling constant (137–139 Hz) intermediate between those typical for sp² and sp³ hybridized carbon atoms; Table 1. Additionally, the MnH̲ signal was located at −14.5 to −15.3 ppm in the ¹H NMR spectra of 6^R,H and 6^R,Et, and the SiH̲, Si=CH̲(CH₃) and Si=CH(CH̲₃) signals were observed at 3.7 to 4.5 ppm, −0.2 to 0.2 ppm, and 1.7 to 1.9 ppm, respectively. These data are very similar to those for [(dmpe)₂MnH(R₂Si=CHMe)] (6^Ph2: R = Ph, 6^Et2: R = Et; pertinent NMR data is included in Table 1), which have been spectroscopically, and (for 6^Ph2) crystallographically, characterized.³² To the best of our knowledge, 6^R,H are the first spectroscopically observed⁴⁸ examples of transition metal silene complexes with a hydrogen substituent on silicon.

Despite numerous attempts, we were unable to obtain X-ray quality crystals of 6^R,H or 6^R,Et. Therefore, we turned to DFT calculations in order to gain further insight into the structures of these complexes (ADF, gas-phase, all-electron, PBE, D3-BJ, TZ2P, ZORA). For all four complexes, energy minima were located for four cis silene hydride isomers⁴⁹ with E or Z silene stereochemistry, and differing in the orientation of the silene methyl substituent (RR′Si=CHMe) relative to the two dmpe ligands, as shown in Fig. 3 (see Fig. 4 for the lowest energy isomer of 6^Bu,H). In all cases, isomers (i) and (ii) are within a few kJ mol⁻¹ of one another, and are 13–22 kJ mol⁻¹ lower in energy than isomers iii and iv, consistent with observation of just 2 isomers in the solution NMR spectra of 6^R,H.⁵⁰ By contrast, the apparent formation of a single isomer of compounds 6^R,Et suggests that these reactions proceed under kinetic control.

Fig. 3 Calculated isomers (i–iv) of silene hydride complexes [(dmpe)₂MnH(RR′Si=CHMe)] (6^Ph,H: R = Ph, R′ = H; 6^Bu,H: R = ⁿBu, R′ = H; 6^Ph,Et: R = Ph, R′ = Et; 6^Bu,Et: R = ⁿBu, R′ = Et) featuring Si–H interligand interactions.

Fig. 4 Calculated structure (ball and stick diagram) for the lowest energy isomer of silene hydride complex [(dmpe)₂Mn(ⁿBuHSi=CHMe)] (6^Bu,H). All hydrogen atoms have been omitted for clarity except those on Mn or the Si=C unit. The inset shows a top-down view of the Mn silene hydride core, with selected bond distances.

In the calculated structures of silene hydride isomers (i) and (ii) (for bond metrics, see Table S4†), the Si=C bond distances of 1.80–1.81 Å fall within the range for previously reported transition metal silene complexes (1.78(2)–1.838(11) Å),⁵¹ and correspond to Mayer bond orders ranging from 0.96 to 1.10 (cf. 0.70–0.91 for Si–C single bonds in the same complexes). Also, as in previously reported 6^R2,³² significant interligand interactions exist between silicon and the hydride. Computationally, this is illustrated by short Si–H_Mn distances (1.64–1.66 Å), with substantial Mayer bond orders (0.45–0.49),⁵² and is also reflected by a large negative ²⁹Si–¹H_Mn coupling constant of −80 Hz (measured using ²⁹Si_edited 2D ¹H–¹H COSY NMR spectroscopy)³⁶ for the major isomer of 6^Bu,H (cf. −30 to −31 Hz for 4^R and >0 in classical silyl hydride complexes).^36,53 Short Mn–Si distances (2.35–2.42 Å) with Mayer bond orders of 0.49–0.53, and Mn–H distances of 1.64–1.66 Å with Mayer bond orders of 0.52–0.56, combined with the short Si=C distance (vide supra), support the identification 6^R,H and 6^R,Et as cis silene hydride complexes, as opposed to 5-coordinate alkyl complexes with a strong β-Si–H–Mn interaction.

DFT calculations on low-coordinate silyl and silylene hydride intermediates derived from 4^R

The reactions of the disilyl hydride complexes [(dmpe)₂MnH(SiH₂R)₂] (4^Ph: R = Ph, 4^Bu: R = ⁿBu) with C₂H₄ (vide supra) likely proceed via either (a) 5-coordinate mono-silyl intermediates, [(dmpe)₂Mn(SiH₂R)] (2^Ph: R = Ph, 2^Bu: R = ⁿBu), or (b) silylene hydride intermediates, [(dmpe)₂MnH(=SiHR)] (3^Ph,H: R = Ph, 3^Bu,H: R = ⁿBu), formed by sequential hydrosilane reductive elimination and α-hydride elimination from disilyl hydride complexes 4^R (Scheme 6; vide infra). Therefore, DFT calculations (ADF, gas-phase, all-electron, PBE, D3-BJ, TZ2P, ZORA) were carried out to assess the thermodynamic accessibility of such intermediates (Fig. 5 and Table 2).

Fig. 5 DFT calculated Gibbs free energies at 298.15 K (ΔG^{298.15 K}; kJ mol⁻¹) to access reactive intermediates (and the H₃SiR byproduct) from disilyl hydride complexes [(dmpe)₂MnH(SiH₂R)₂] (4^Ph: R = Ph, blue dotted lines; 4^Bu: R = ⁿBu, red dashed lines). Calculated intermediates (left to right) are: (i) an isomer of [(dmpe)₂Mn(SiH₂R)] with an equatorial dmpe arrangement (trans-2^Ph: R = Ph, trans-2^Bu: R = ⁿBu), (ii) an isomer of [(dmpe)₂Mn(SiH₂R)] with a disphenoidal dmpe arrangement and a hydrocarbyl substituent on silicon oriented towards the vacant coordination site {rotamer 1 of cis-2^R: R = Ph (cis-2^Ph), ⁿBu (cis-2^Bu)}, (iii) an isomer of [(dmpe)₂Mn(SiH₂R)] with a disphenoidal dmpe arrangement and an SiH substituent oriented towards the vacant coordination site {rotamer 2 of cis-2^R: R = Ph (cis-2^Ph); a minimum was not located for R = ⁿBu}, (iv) an isomer of [(dmpe)₂MnH(=SiHR)] with interacting cis-disposed silylene and hydride ligands (cis-3^Ph,H: R = Ph, cis-3^Bu,H: R = ⁿBu), and (v) trans-[(dmpe)₂MnH(=SiHR)] (trans-3^Ph,H: R = Ph, trans-3^Bu,H: R = ⁿBu). Geometry optimized cores of the phenyl analogues of reactive intermediates are depicted below each energy level, showing Mn in red, Si in pink, C in dark grey, and H in light grey, accompanied by stick bonds to the phosphorus donor atoms.

Table 2 Thermodynamic parameters calculated by DFT for the formation of intermediates in Fig. 5 from disilyl hydride complexes [(dmpe)₂MnH(SiH₂R)₂] {4^Ph (R = Ph)/4^Bu (R = ⁿBu)}; ΔE (calculated before ZPE correction), ΔH, ΔG^{298.15 K}, ΔG^{335 K} (kJ mol⁻¹ at 298.15 K or, for ΔG^{335 K}, 335 K), and ΔS (J mol⁻¹ K⁻¹ at 298.15 K)^a

	trans-2^R	cis-2^R rotamer 1	cis-2^R rotamer 2	cis-3^R,H	trans-3^R,H
a n.o. = not observed (i.e. energy minimum not located).
ΔE	146/156	131/145	165/n.o.	110/115	115/122
ΔH	135/150	123/138	152/n.o.	117/124	100/111
ΔS	242/265	197/232	225/n.o.	234/216	199/234
ΔG^298.15 K	63/71	64/69	85/n.o.	47/60	41/41
ΔG^335 K	54/62	57/60	76/n.o.	39/52	34/33

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In the case of low-coordinate silyl species, energy minima were located for structures in which the silyl group is either cis (cis-2^R) or trans (trans-2^R) to the vacant coordination site generated by hydrosilane reductive elimination. At 298 K, ΔG for the formation of these monosilyl compounds and free hydrosilane from 4^R is very similar (63–71 kJ mol⁻¹).

In the global minima for the cis isomers (rotamer 1 of cis-2^R), the hydrocarbyl substituent on silicon engages in a γ-agostic interaction with manganese (via an ortho-CH̲ bond in cis-2^Ph or a CH₂CH̲₂CH₂CH₃ bond in cis-2^Bu), with Mn–H_γ distances of 1.91–1.93 Å. The Mn–H_γ–C_γ angles in this rotamer of cis-2^R are 115.9° and 131.4°, respectively, and the presence of a γ-agostic interaction is further supported by Mayer bond orders of 0.22–0.24 between Mn and H_γ, and 0.13–0.15 between Mn and C_γ.

For the phenyl analogue 2^Ph, a higher-energy cis isomer was also located, corresponding to a rotamer where one of the two hydrogen substituents on silicon is now oriented in the direction of the vacant coordination site (rotamer 2 of cis-2^Ph; Fig. 5 and Table 2). Relative to rotamer 1, this structure features an acute Mn–Si–H_Si angle of 101° (cf. 119°), an Mn–H_Si Mayer bond order of 0.06 (cf. <0.05), a marginally elongated Si–H_Si distance of 1.53 Å (cf. 1.51 Å), and a marginally lower Si–H_Si Mayer bond order of 0.80 (cf. 0.85), together suggestive of a weak α-Si–H–Mn interaction. Rotamer 2 of 2^R is presumably involved in silylene hydride formation via α-hydride elimination, and indeed, all attempts to locate an analogous energy minimum for the ⁿBu analogue structure led instead to a silylene hydride structure (cis-3^Bu,H; vide infra).

As with the 5-coordinate silyl species (vide supra), multiple energy minima (Fig. 5 and Table 2) were located for silylene hydride structures [(dmpe)₂MnH(=SiHR)] (3^Ph,H: R = Ph, 3^Bu,H: R = ⁿBu). The two lowest energy structures are (a) a cis silylene hydride isomer with a significant interaction between silicon and the neighbouring hydride ligand (the Si⋯H_Mn distances are 1.68 Å, with Mayer bond orders of 0.52; cf. 0.84–0.85 for the terminal Si–H bonds), and (b) a trans silylene hydride isomer. These isomers are isostructural to the X-ray crystal structures of cis-[(dmpe)₂MnH(=SiPh₂)] (3^Ph2) and trans-[(dmpe)₂MnH(=SiEt₂)] (3^Et2), respectively.³² Calculated ΔG values to access 3^R,H from 4^R range from 41 kJ mol⁻¹ (trans isomers) to 47–60 kJ mol⁻¹ (cis isomers) at 298.15 K, decreasing to 33–34 kJ mol⁻¹ (trans isomers) and 39–52 kJ mol⁻¹ (cis isomers) at 335 K, highlighting their thermodynamic accessibly.

In silylene hydride complexes 3^R,H, Mn–Si double bond character is apparent from relatively short Mn–Si distances (2.16–2.20 Å), Mn–Si Mayer bond orders ranging from 1.17 (cis-3^R,H) to 1.54–1.57 (trans-3^R,H), and a planar or near-planar environment about Si (∑(R–Si–R) > 356°); Table S2.† These parameters are comparable to those previously observed and/or calculated for the two isomers of [(dmpe)₂MnH(=SiR₂)] (3^R2; R = Et or Ph).³²

High temperature NMR spectra of 4^R: in situ generation of trans-silylene hydride (trans-3^R,H) species

At 335 K, ¹H NMR spectra of the disilyl hydride complexes [(dmpe)₂MnH(SiH₂R)₂] (4^Ph: R = Ph, 4^Bu: R = ⁿBu) revealed the formation of a small amount of a new manganese complex and free hydrosilane (H₃SiR; R = Ph or ⁿBu). This process is reversible, and clean spectra of 4^R were observed upon cooling back to room temperature. The new manganese complex exhibits a high frequency (9.83 or 9.53 ppm) and a low frequency (−9.01 or −9.60 ppm) ¹H NMR signal. The former is in the range observed for the terminal =SiRH̲ (R = hydrocarbyl substituent) environment in diamagnetic silylene complexes of Mo, W, Fe, Ru, Os, and Ir, (6.34–12.1 ppm),^{19,20,22–26} while the latter is consistent with a metal hydride environment. The low frequency hydride signal is a quintet (²J_¹H,³¹P = 54 or 51 Hz) consistent with a hydride ligand apical to a plane of four equivalent phosphine donors. Taken together, these data suggest that the new complex observed at elevated temperature is trans-[(dmpe)₂MnH(=SiHR)] (trans-3^Ph,H: R = Ph, trans-3^Bu,H: R = ⁿBu); the most thermodynamically accessible silyl or silylene species in Fig. 5.

Characterization of trans-3^R,H (R = Ph or ⁿBu) by ²⁹Si NMR spectroscopy was not successful since the new species were formed at very low concentrations (∼4% and ∼2% relative to 4^Ph or 4^Bu, respectively). However, EXSY NMR spectroscopy at 335 K indicates exchange between the two diastereotopic SiH̲ protons in 4^Ph or 4^Bu, the free hydrosilane SiH̲ peak, the high frequency trans-3^R,H silylene SiH̲ environment, and the MnH̲ signals from both 4^R and trans-3^R,H (shown in Fig. 6 for R = ⁿBu).⁵⁴ This is consistent with an equilibrium in which 4^R eliminates free H₃SiR to form trans-3^R,H (vide infra).

Fig. 6 1D NOESY/EXSY NMR spectrum of a solution of [(dmpe)₂MnH(SiH₂ⁿBu)₂] (4^Bu) at 335 K with excitation at the SiH̲ signal of 4^Bu, showing chemical exchange between the SiH̲ and MnH̲ environments of [(dmpe)₂MnH(SiH₂ⁿBu)₂] (4^Bu) and trans-[(dmpe)₂MnH(=SiHⁿBu)] (trans-3^Bu,H), and the SiH̲ environment of free H̲₃SiⁿBu. Positive (EXSY) peaks are indicative of chemical exchange and negative (NOESY) peaks are indicative of through-space coupling (500 MHz, C₆D₆).

Trapping experiments with isonitriles and N-heterocyclic carbenes

To provide experimental corroboration for the accessibility of 5-coordinate silyl [(dmpe)₂Mn(SiH₂R)] (2^Ph: R = Ph, 2^Bu: R = ⁿBu) and silylene hydride [(dmpe)₂MnH(=SiHR)] (3^Ph,H: R = Ph, 3^Bu,H: R = ⁿBu) species from 4^R, reactions with neutral donor ligands were carried out, with a view towards coordination to manganese in 2^R or silicon in 3^R,H (Scheme 5).

Scheme 5 Trapping of putative silyl (2^R) and silylene hydride (3^R,H) intermediates: synthesis of silyl isonitrile complexes [(dmpe)₂Mn(SiH₂R)(CNR′)] (7a: R = Ph, R′ = o-xylyl; 7b: R = ⁿBu, R′ = o-xylyl; 7c: R = Ph, R′ = ^tBu; 7d: R = ⁿBu, R′ = ^tBu) and NHC-stabilized silylene hydride complexes [(dmpe)₂MnH{=SiHR(NHC)}] (8a: NHC = ^iPrNHC, R = Ph; 8b: NHC = ^iPrNHC, R = ⁿBu; 8c: NHC = ^MeNHC, R = Ph; 8d: NHC = ^MeNHC, R = ⁿBu).

Addition of o-xylylN≡C or ^tBuN≡C to solutions of 4^R resulted in hydrosilane elimination, and isolation of yellow or orange silyl isonitrile complexes [(dmpe)₂Mn(SiH₂R)(CNR′)] {R′ = o-xylyl, R = Ph (7a) or ⁿBu (7b); R′ = ^tBu, R = Ph (7c) or ⁿBu (7d)}, effectively trapping silyl complexes 2^R (Scheme 5). In solution, all four reactions initially led to mixtures of two complexes identified by NMR spectroscopy as cis (85–97%) and trans (3–15%) isomers of 7a–d. Slow isomerization was observed between the cis and trans isomers of 7a–d in solution, and unexpectedly, these isomerization reactions proceeded in the direction of the trans isomers at elevated temperature (resulting in an increase in the proportion of trans isomer to 44–74% after heating solutions containing exclusively the cis isomer at 65–80° for 4–21 days), and in the opposite direction upon leaving the same solutions at room temperature for 3 weeks (e.g. leaving cis/trans mixtures of 7a,b containing 44–48% trans isomer at room temperature resulted in solutions containing 99% cis isomer after 3 weeks).

X-ray quality crystals were obtained for the four silyl isonitrile complexes 7a–d, in each case as the cis isomer (Fig. 7). All four structures are octahedral with Mn–Si distances of 2.3552(5)–2.3618(5) Å and Mn–C distances of 1.805(4)–1.847(3) Å. The isonitrile ligands show elongated C_Mn–N distances of 1.176(4)–1.225(9) Å and non-linear C–N–C angles of 159.2(8)–167.5(1)°, indicative of appreciable π-backbonding.

Fig. 7 X-ray crystal structures of (top left) cis-[(dmpe)₂Mn(SiH₂Ph)(CNxylyl)] (cis-7a), (top right) cis-[(dmpe)₂Mn(SiH₂ⁿBu)(CNxylyl)] (cis-7b), (bottom left) cis-[(dmpe)₂Mn(SiH₂Ph)(CN^tBu)] (cis-7c), and (bottom right) cis-[(dmpe)₂Mn(SiH₂ⁿBu)(CN^tBu)] (cis-7d), with ellipsoids drawn at 50% probability. Most hydrogen atoms have been omitted for clarity. cis-7c and cis-7d crystallized with two independent and essentially isostructural molecules in the unit cell, only one of which is shown. All structures exhibit some disorder, and only the dominant conformation is shown.

In contrast to reactions with isonitriles, reactions of disilyl hydride complexes 4^R with 1,3-diisopropylimidazolin-2-ylidene (^iPrNHC) or 1,3,4,5-tetramethyl-4-imidazolin-2-ylidene (^MeNHC) afforded the base-stabilized silylene hydride complexes [(dmpe)₂MnH{=SiHR(NHC)}] {NHC = ^iPrNHC, R = Ph (8a) or ⁿBu (8b); NHC = ^MeNHC, R = Ph (8c) or ⁿBu (8d)}, trapping the proposed silylene hydride species 3^R,H (Scheme 5). Compounds 8b–d were isolated as analytically pure red powders, whereas 8a evaded purification.

A variety of NHC-stabilized silylene complexes have been reported for V, Cr, W, Fe, Co, Rh, and Ni,⁵⁵ and relative to base-free silylene complexes, they feature longer metal–silicon bond distances, pyramidalization at silicon, and lower frequency ²⁹Si NMR chemical shifts (typically 25–100 ppm,⁵⁶ compared with >200 ppm for base-stabilized silylene complexes).⁸

Room temperature NMR spectra of ^iPrNHC adducts 8a,b revealed two sets of broad NMR signals in the process of coalescence/decalescence, due to a pair of rapidly interconverting isomers. Cooling the solutions afforded two sets of well resolved NMR signals corresponding to compounds with a disphenoidal arrangement of the phosphorus donor atoms, each with a single SiH̲ signal (5.1 to 6.4 ppm), a single MnH̲ resonance (−12.3 to −12.6 ppm), a single ²⁹Si NMR environment (22.2 to 29.6 ppm), and four unique ³¹P NMR environments (65.6–81.9 ppm). These data are indicative of NHC-coordinated cis silylene hydride complexes existing as a pair of interconverting diastereomers (due to chirality at Si and Mn).

In contrast, NMR spectra of the ^MeNHC silylene hydride complexes (8c,d) revealed the same two rapidly interconverting cis diastereomers plus a third isomer which afforded a sharp set of ¹H and ³¹P NMR signals at room temperature. This third isomer corresponds to an NHC-coordinated trans silylene hydride complex, as evidenced by a single MnH̲ (−14.9 or −15.0 ppm) signal with a quintet coupling pattern (²J_H,P = 48–49 Hz) and two sharp signals in the ³¹P{¹H} NMR spectra (78.7–80.6 ppm) due to diastereotopic phosphorus atoms. The ¹H NMR SiH̲ and ²⁹Si NMR chemical shifts in these trans isomers (4.9–5.8 ppm and 22.4 ppm, respectively)⁵⁷ are similar to those in the cis isomers.

At 335 K, the two cis diastereomers of 8a–d gave rise to a single set of averaged signals, with the MnH̲ peak at −12.5 to −12.7 ppm (quintets for cis-8a,c,d with ²J_H,P = 32–34 Hz, while the NMR signal for 8b remained a broad singlet in the process of coalescence), accompanied by (in solutions of 8c,d only) a set of sharp signals for the trans isomer. For 8c,d, EXSY NMR spectroscopy at 335 K showed cross peaks between the MnH̲ and SiH̲¹H NMR signals due to both the cis and trans isomers (i.e. chemical exchange between all four environments). This equilibrium between cis- and trans-8c,d mirrors that previously reported between the cis and trans isomers of base-free [(dmpe)₂MnH(=SiPh₂)] (3^Ph2).³²

Possible mechanisms for ambient temperature exchange between the cis diastereomers of 8a–d are (a) phosphine donor dissociation, isomerization of the 5-coordinate product, and phosphine re-coordination, or (b) NHC dissociation to generate cis-[(dmpe)₂MnH(=SiHR)] (cis-3^Ph,H: R = Ph, cis-3^Bu,H: R = ⁿBu), followed by re-coordination to the opposite face of the silylene ligand.⁵⁸ The latter mechanism would imply that 8a–d, like disilyl hydride complexes 4^R, could react as sources of either base-free silylene hydride complexes 3^R,H, or 5-coordinate silyl complexes 2^R. The accessibility of this pathway is implied by the reactions of 8d with ^tBuNC, and 8b with ethylene, which afforded [(dmpe)₂Mn(SiH₂ⁿBu)(CN^tBu)] (7d) and [(dmpe)₂MnH(ⁿBuHSi=CHMe)] (6^Bu,H),⁵⁹ respectively; these are the same complexes formed in reactions of these reagents with 4^Bu. Furthermore, the accessibility of 2^R provides a mechanism for the observed exchange between the SiH̲ and MnH̲ environments in the EXSY NMR spectra of 8c,d at 335 K (vide supra).

X-ray quality crystals were obtained for complexes 8a–d by recrystallization from concentrated hexanes solutions (8a,b), toluene layered with pentane (8c), or a dilute hexanes solution (8d) at −30 °C. The solid state structures of 8a–c (Fig. 8; top row and bottom left) feature a cis arrangement of the hydride and base-stabilized silylene ligands, corresponding to one of the two cis diastereomers observed in solution.⁶⁰ By contrast, 8d crystallized as the trans isomer (Fig. 8; bottom right). In all four structures {complementary DFT calculations modelled 8b,d with an ⁿBu group in place of the Et group; [(dmpe)₂MnH{=SiHEt(NHC)}] where NHC = ^iPrNHC (8b*) or ^MeNHC (8d*)}, NHC coordination to silicon resulted in elongated Mn–Si distances (2.255(1)–2.299(1) Å; calcd 2.26–2.30 Å for both isomers of 8a,b*,c,d*), and correspondingly weaker Mayer bond orders of 1.03–1.08 (Table S6†), relative to base-free silylene complexes 3^R,H (2.16–2.20 Å and 1.17–1.57, respectively). Unlike base-free analogues (vide supra), cis-8a–d display only negligible interligand Si–H interactions (with Mayer bond orders ≤0.13). Additionally, substantial pyramidalization at silicon was observed for both isomers of 8a–d, where the sum of the angles around silicon (for non-NHC substituents) ranged from 322(3) to 342(2)° (calcd 336.1–341.5°; Table S6,†cf. >356° in 3^R,H). Nevertheless, the Mn–Si distances are significantly shorter than those in related silyl complexes 7a–d (the Mn–Si distances in 7a–d range from 2.3552(5)–2.3618(5) Å {calcd 2.35–2.36 Å (cis) and 2.41–2.42 Å (trans), with Mayer bond orders of 0.89–0.93}),⁶¹ indicative of residual Mn–Si multiple bond character in 8a–d.

Fig. 8 X-ray crystal structures of (top left) cis-[(dmpe)₂MnH{=SiHPh(^iPrNHC)}] (cis-8a), (top right) cis-[(dmpe)₂MnH{=SiHⁿBu(^iPrNHC)}] (cis-8b), (bottom left) cis-[(dmpe)₂MnH{=SiHPh(^MeNHC)}] (cis-8c), and (bottom right) trans-[(dmpe)₂MnH{=SiHⁿBu(^MeNHC)}] (trans-8d) with ellipsoids drawn at 50% probability. Most hydrogen atoms have been omitted for clarity. In the case of 8a–b, the structures exhibit some disorder, and only the dominant conformation is shown.

Pathways for reactions of 4^R with ethylene

Previously, we reported the reactions of the silylene hydride complexes, [(dmpe)₂MnH(=SiR₂)] (3^Ph2: R = Ph, 3^Et2: R = Et), with ethylene to form the silene hydride complexes [(dmpe)₂MnH(R₂Si=CHMe)] (6^Ph2: R = Ph, 6^Et2: R = Et).³² Given that disilyl hydride complexes 4^R exist in equilibrium with analogous low-coordinate silyl and silylene hydride complexes (vide supra), it is likely that the reactions of 4^R with ethylene proceed via a parallel mechanism, as illustrated in Scheme 6. The initial steps in this scheme involve either (a) ethylene coordination to a silylene hydride intermediate (3^R,H) followed by 2 + 2 cycloaddition (to form B^R) and subsequent Si–H bond-forming reductive elimination, or (b) coordination of ethylene to a low coordinate silyl intermediate (2^R), forming C^R, followed by 1,2-insertion. Both of these pathways generate primary alkyl complex D^R, which can provide access to 6^R,H by sequential β-hydride elimination (to form E^R), 1,2-insertion to generate secondary alkyl complex F^R, and a second β-hydride elimination involving the hydrogen substituent on silicon. Consistent with this mechanism, the reactions of [(dmpe)₂MnH(SiH₂ⁿBu)₂] (4^Bu) or [(dmpe)₂Mn{=SiHⁿBu(^iPrNHC)}] (8b) with d₄-ethylene yielded [(dmpe)₂MnH(ⁿBuHSi=CDCD₃)] as the only observed isotopomer of 6^Bu,H.

Scheme 6 Proposed pathways for reactions of disilyl hydride complexes [(dmpe)₂MnH(SiH₂R)₂] (4^Ph: R = Ph, 4^Bu: R = ⁿBu) with ethylene to form silene hydride complexes [(dmpe)₂MnH(RHSi=CHMe)] (6^Ph,H; R = Ph, 6^Bu,H: R = ⁿBu). [Mn] = Mn(dmpe)₂. Only one isomer of 3^R,H is shown. Boxes indicate complexes which have been isolated or spectroscopically observed.

After conversion of 4^R to 6^R,H, reaction with a second equivalent of ethylene resulted in conversion of a silene SiH group in 6^R,H to an SiEt group, yielding 6^R,Et (vide supra). This reactivity likely involves the experimentally observed isomerization of 6^R,H to silylenes 3^R,Et (vide supra), presumably via a low-coordinate silyl intermediate (2^R,Et in Scheme 7) formed from 6^R,H by C–H bond-forming 1,2-insertion. Conversion to 6^R,Et can then take place via previously discussed pathways (Scheme 6) involving reactions of the silylene or low-coordinate silyl species with ethylene to afford intermediates B^R,Et and C^R,Et, respectively (B^R,Et and C^R,Et are analogues of B^R and C^R in Scheme 6, but with an ethyl group in place of one hydrogen atom on silicon).⁶²

Scheme 7 Initial steps in the pathway proposed for reactions of silene hydride complexes [(dmpe)₂MnH(RHSi=CHMe)] (6^Ph,H; R = Ph, 6^Bu,H: R = ⁿBu) with ethylene to afford [(dmpe)₂MnH(REtSi=CHMe)] (6^Ph,Et; R = Ph, 6^Bu,Et: R = ⁿBu). [Mn] = Mn(dmpe)₂. Intermediates B^R,Et and C^R,Et are analogous to intermediates B^R and C^R in Scheme 6, but with an ethyl group in place of one hydrogen atom on silicon. Only one isomer of 3 is shown. Boxes indicate complexes which have been isolated or spectroscopically observed.

Deuterium labelling studies were employed to provide experimental support for these mechanistic proposals. Specifically, [(dmpe)₂MnH(ⁿBuHSi=CDCD₃)] (d₄_6^Bu,H) isomerized to exclusively form trans-[(dmpe)₂MnH{=SiⁿBu(CHDCD₃)}] (trans-d₄_3^Bu,Et), and the reaction of [(dmpe)₂MnH(ⁿBuHSi=CHMe)] (6^Bu,H) with d₄-ethylene yielded [(dmpe)₂MnH(ⁿBuEtSi=CDCD₃)] (d₄_6^Bu,Et). Additionally, we have previously reported that [(dmpe)₂MnH(=SiEt₂)] (3^Et2) exclusively forms [(dmpe)₂MnH(Et₂Si=CDCD₃)] (d₄_6^Et2) upon exposure to d₄-ethylene.

Catalytic ethylene hydrosilylation

Our group previously reported that [(dmpe)₂MnH(Et₂Si=CHMe)] (6^Et2) catalyses ethylene hydrosilylation by diethylsilane.³² Upon monitoring the progress of this reaction by NMR spectroscopy, the silyl dihydride complex, [(dmpe)₂MnH₂(SiHEt₂)] (5^Et2), silylene hydride complex [(dmpe)₂MnH(=SiEt₂)] (3^Et2),⁶³ and ethylene hydride complex [(dmpe)₂MnH(C₂H₄)] (1), were all observed in solution, in addition to silene hydride complex 6^Et2. We were therefore motivated to investigate [(dmpe)₂MnH(C₂H₄)] (1) as a pre-catalyst for ethylene hydrosilylation (using primary and secondary hydrosilanes), given that it is a precursor to disilyl hydride, silylene hydride and silene hydride complexes in the presence of hydrosilanes and/or ethylene.

At 60 °C, addition of 7 mol% of [(dmpe)₂MnH(C₂H₄)] (1) to primary or secondary hydrosilanes (H₃SiPh, H₃SiⁿBu, H₂SiPh₂ or H₂SiEt₂) in C₆D₆ under ethylene (1.7 atm initial pressure) led to catalytic incorporation of one or two equivalents of ethylene into the Si–H bonds of the free hydrosilanes, leading to a mixture of new hydrosilanes (Table 3). The major products in reactions of secondary hydrosilanes were tertiary hydrosilanes (HSiEtPh₂ or HSiEt₃), while reactions involving primary hydrosilanes first formed secondary hydrosilanes (H₂SiEtⁿBu or H₂SiEtPh), followed by reaction with an additional equivalent of ethylene to generate the tertiary hydrosilane (HSiEt₂Ph or HSiEt₂ⁿBu) as the major product. Hydrosilylation reactions with H₃SiⁿBu, H₂SiPh₂, and H₂SiEt₂ produced fewer byproducts than those with H₃SiPh (as noted in Table 3). Additionally, hydrosilylation with H₂SiEt₂ progressed much more rapidly than that with H₂SiPh₂. By contrast, no reactivity was observed when 1 was exposed to ethylene and the tertiary hydrosilanes HSiEt₃ or HSiEtPh₂; various other hydrosilylation catalysts exhibit higher activities than 1, especially precious metal catalysts,^38,64 but the ability of 1 to selectively form tertiary but not quaternary hydrosilanes from ethylene is uncommon.⁶⁵

Table 3 Ratio of hydrosilane products (assigning the tertiary hydrosilane product a value of 100) observed by ¹H NMR spectroscopy after hydrosilylation of ethylene catalysed by [(dmpe)₂MnH(C₂H₄)] (1) pre-catalyst (7 mol%) with 1.7 atm ethylene (initially n_silane ≈ n_ethylene; for reactions with H₃SiR, the headspace was re-filled with ethylene after 1 week) at 60 °C in C₆D₆ after 50 days (H₃SiPh), 25 days (H₃SiⁿBu) or 6 days (H₂SiR₂)

Substrate	R/R′	Substrate	H2SiEtR	HSiEtRR′	HSiViRR′	Unidentified^a
a Relative amounts of unidentified SiH-containing silanes were determined assuming that they contain a single SiH proton. b At least seven unassigned SiH environments were observed. c Two unassigned SiH environments were observed.
H3SiPh	Ph/Et	0	0	100	<5	20^b
H3Si^nBu	^nBu/Et	0	<5	100	20	10^c
H2SiPh2	Ph/Ph	50	n.a.	100	<5	20
H2SiEt2	Et/Et	6	n.a.	100	11	<5

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Organic byproducts were observed during conversion of secondary to tertiary silanes, but not conversion of primary to secondary silanes. The major byproduct was a hydrosilane with a vinyl group (Vi) in place of an ethyl substituent (HSiEtViR, R = Et or ⁿBu or HSiPhViR, R = Et or Ph), accompanied by one or more unidentified SiH-containing silanes (Table 3). Vinyl silanes are commonly observed byproducts in olefin (e.g. H₂C=CHR) hydrosilylation, formed by β-hydride elimination from an M(CH₂CHRSiR₃) intermediate in the catalytic cycle,⁶⁶ and were an impetus for the initial proposal of a modified Chalk–Harrod catalytic cycle involving C–Si rather than C–H bond-forming 1,2-insertion from an alkene-coordinated silyl hydride intermediate.³⁷

During catalysis using primary and secondary hydrosilanes, a variety of manganese-containing complexes were observed by NMR spectroscopy, including disilyl hydride complexes (for reactions involving primarily hydrosilanes only), silylene hydride complexes (for reactions involving secondary silanes only),⁶³ silyl dihydride complexes, silene hydride complexes, and ethylene hydride complex 1. Furthermore, all of these classes of complex are catalytically active. For example, [(dmpe)₂MnH(Et₂Si=CHMe)] (6^Et2)³² and [(dmpe)₂MnH(=SiEt₂)] (3^Et2) are catalysts for ethylene hydrosilylation using secondary hydrosilanes, and [(dmpe)₂MnH(SiH₂ⁿBu)₂] (4^Bu) and [(dmpe)₂MnH₂(SiH₂ⁿBu)] (5^Bu) are active for ethylene hydrosilylation by H₃SiⁿBu. Reactions involving 6^Et2, 3^Et2, and 4^Bu rapidly generated distributions of Mn-containing species and hydrosilane products which are very similar to those formed when [(dmpe)₂MnH(C₂H₄)] (1) was used as the pre-catalyst. By contrast, when 5^Bu was employed, hydrosilylation proceeded at a slower rate, and even after 24 hours the dominant manganese-containing species was 5^Bu.

In order to monitor ethylene hydrosilylation reactions under conditions where ethylene concentration does not vary significantly during the course of the reaction, multiple aliquots from a stock solution of 1 and H₃SiⁿBu in C₆D₆ were placed under a large excess of ethylene in a sealed 50 mL flask (initial pressure 1.7 atm, n_C2H4 ≈ 40 × n_silane) and heated at 60 °C for various time periods prior to analysis by NMR spectroscopy (Fig. 9). Key observations were; (a) nearly complete conversion of the primary hydrosilane to secondary hydrosilane H₂SiEtⁿBu was observed before any formation of the tertiary silane (HSiEt₂ⁿBu) product or vinyl silane (HSiEtViⁿBu) byproduct, (b) during hydrosilylation by the primary hydrosilane H₃SiⁿBu, the dominant metal-containing species was the SiH-containing silene hydride [(dmpe)₂MnH(ⁿBuHSi=CHMe)] (6^Bu,H), with small amounts of the disilyl hydride [(dmpe)₂MnH(SiH₂ⁿBu)₂] (4^Bu), (c) after 13 hours, almost all H₃SiⁿBu had been consumed, (d) from 13 to 18 hours, conversion of H₂SiEtⁿBu to HSiEt₂ⁿBu proceeded rapidly with concurrent formation of the vinylsilane byproduct HSiEtViⁿBu (see below for experiments to determine the manganese species present between 13 and 18 hours), (e) after 18 hours, 1 was the dominant manganese species in solution {accompanied by small amounts of the silene hydride [(dmpe)₂MnH(ⁿBuEtSi=CEtMe)] (6^Bu,Et) and the silyl dihydride [(dmpe)₂MnH₂(SiHEtⁿBu)] (5^Bu,Et)}, and conversion of H₂SiEtⁿBu to HSiEt₂ⁿBu now proceeded more slowly, and (f) after 12 days, >99.5% of the H₂SiEtⁿBu intermediate had been consumed yielding 81% HSiEt₂ⁿBu, 16% HSiEtViⁿBu, and 3% of an unidentified SiH-containing byproduct (assuming that this species contains one SiH̲ proton), which is non-volatile at room temperature (5 mTorr); at this point, the only Mn-containing species in the reaction mixture was [(dmpe)₂MnH(C₂H₄)] (1). Relative amounts of the different hydrosilane and MnH-containing species in solution during ethylene hydrosilylation by H₃SiⁿBu are plotted as a function of time in Fig. 9.

Fig. 9 SiH̲ (left) and MnH̲ (middle) regions of the ¹H NMR spectra (298 K, 500 or 600 MHz) for the hydrosilylation of ethylene by H₃SiⁿBu using [(dmpe)₂MnH(C₂H₄)] (1) pre-catalyst (7 mol% relative to the hydrosilane) under ∼1.7 atm of ethylene (initial, n_C2H4 ≈ 40 × n_silane) in C₆D₆ and after various time intervals at 60 °C. The x-axis corresponds to the bottom ¹H NMR spectrum, and for clarity, each spectrum above that is shifted by 0.3 (SiH region) or 0.4 (MnH region) ppm to lower frequency. Right: graphs showing the ratio of (top) hydrosilanes (dark blue ♦ = H₃SiⁿBu; red ■ = H₂SiEtⁿBu; green ▲ = HSiEt₂ⁿBu; purple × = HSiViEtⁿBu; light blue ✴ = unidentified SiH-containing silane) and (bottom) MnH-containing species {dark blue ♦ = [(dmpe)₂MnH(C₂H₄)] (1); light blue ✴ = [(dmpe)₂MnH(SiH₂ⁿBu)₂] (4^Bu); green ▲ = [(dmpe)₂MnH₂(SiHEtⁿBu)] (5^Bu,Et); purple × = [(dmpe)₂MnH(ⁿBuHSi=CHMe)] (6^Bu,H); red ■ = [(dmpe)₂MnH(ⁿBuEtSi=CHMe)] (6^Bu,Et)} in these reactions, as measured by ¹H NMR spectroscopy. Reaction details can be found in the ESI pg. S15.†

Between 13 and 18 hours in Fig. 9, conversion of H₂SiEtⁿBu to HSiEt₂ⁿBu proceeded rapidly (to more than 50% conversion), and then slowed down dramatically, as the resting state of the catalyst switched to [(dmpe)₂MnH(C₂H₄)] (1). However, during secondary to tertiary hydrosilane conversion, [(dmpe)₂MnH(ⁿBuHSi=CHMe)] (6^Bu,H) cannot be regenerated, indicating that a different manganese species may have spiked in concentration between the 13 and 18 hour data points (this species is presumably responsible for continued rapid H₂SiEtⁿBu to HSiEt₂ⁿBu conversion observed during this time period). Consequently, the reaction in Fig. 9 was repeated and stopped after most but not all of the primary hydrosilane had been consumed {the resulting mixture of hydrosilanes and Mn-containing species in C₆D₆ was similar to that observed at 13 h in Fig. 9; i.e. mostly secondary hydrosilane H₂SiEtⁿBu and [(dmpe)₂MnH(ⁿBuHSi=CHMe)] (6^Bu,H), with a small amount of the primary hydrosilane, H₃SiⁿBu}. This mixture was then sealed under a near-stoichiometric (relative to the hydrosilane) amount of ethylene in an NMR tube, and monitored by NMR spectroscopy at 56 °C in 5 minute intervals (Fig. 10).

Fig. 10 Graphs showing the ratio of (top) hydrosilanes (dark blue ♦ = H₃SiⁿBu; red ■ = H₂SiEtⁿBu; green ▲ = HSiEt₂ⁿBu; purple × = HSiViEtⁿBu; light blue ✴ = unidentified SiH-containing silane) and (bottom) MnH-containing species {dark blue ♦ = [(dmpe)₂MnH(C₂H₄)] (1); red ■ = [(dmpe)₂MnH(ⁿBuEtSi=CHMe)] (6^Bu,Et); green ▲ = [(dmpe)₂MnH₂(SiHEtⁿBu)] (5^Bu,Et); purple × = [(dmpe)₂MnH(ⁿBuEtSi=CHMe)] (6^Bu,Et)} measured over time by ¹H NMR spectroscopy (in C₆D₆ at 56 °C) for the hydrosilylation of ethylene (initial, n_C2H4 ≈ n_silane) by a mixture of hydrosilanes corresponding to the 13 h mark in Fig. 9. Reaction details can be found in the ESI pg. S15.†

In Fig. 10, consumption of remaining primary hydrosilane was complete after 10 minutes, followed by rapid secondary to tertiary hydrosilane conversion and a spike in the concentration of a new silene hydride complex, [(dmpe)₂MnH(ⁿBuEtSi=CHMe)] (6^Bu,Et), while the concentrations of 6^Bu,H (the resting state of the catalyst during primary to secondary hydrosilane conversion) and [(dmpe)₂MnH(C₂H₄)] (1) (the Mn-containing species dominant after the 18 h mark) diminished and increased, respectively. Furthermore, a small amount (∼12%) of the silyl dihydride complex [(dmpe)₂MnH₂(SiHEtⁿBu)] (5^Bu,Et) grew in over this time period, and vinylsilane (HSiViEtⁿBu) production was observed to accompany the formation of 1 and 5^Bu,Et. The slowdown in the rate of catalysis between 13 and 18 hours in Fig. 9 can therefore be attributed to a change in the resting state of the catalyst, from more active silene hydride complexes 6^Bu,H and 6^Bu,Et, to 1 and 5^Bu,Et, both of which are formed via vinylsilane elimination (vide infra), and re-enter the catalytic cycle slowly (5^Bu,Et is particularly slow to enter the cycle; vide supra).

A catalytic cycle (Scheme 8) can be envisaged based on the reaction pathways already proposed for (a) reaction of ethylene with disilyl hydride complexes 4^R (formed in reactions of 1 with primary hydrosilanes) to afford silene hydride complexes 6^R,H, and (b) reaction of ethylene with silylene hydride complexes 3^R2 (formed in reactions of 1 with secondary hydrosilanes)³² to afford silene hydride complexes 6^R2. These reactions are identified with blue reaction arrows in Scheme 8. In the presence of free hydrosilane substrate, the catalytic cycle can be completed (green reaction arrows) by reaction of hydrosilanes with primary alkyl intermediate D (pathway ‘a’ in Scheme 8) or secondary alkyl intermediate F (pathway ‘b’ in Scheme 8); via oxidative addition followed by reductive elimination, or σ-bond metathesis.⁶⁷

Scheme 8 Proposed catalytic cycle for ethylene hydrosilylation by primary and secondary hydrosilanes. [Mn] = Mn(dmpe)₂. Only one isomer is shown for complexes 3 and 5. Boxes represent complexes observed by NMR spectroscopy during catalysis.

Alternatively, for conversion of primary to secondary hydrosilanes, the catalytic cycle in Scheme 8 can be completed by C–H bond-forming 1,2-insertion from intermediate 6^R,H (pathway ‘c’, green reaction arrow) to generate silyl and silylene hydride species 2^R,Et and 3^R,Et. Isomerization of 6^Bu,H to 3^Bu,Et has been observed in the absence of ethylene and hydrosilanes (Scheme 4), and this pathway is also thought to be involved in the reactions of [(dmpe)₂MnH(RHSi=CHMe)] (6^R,H) with ethylene to afford [(dmpe)₂MnH(REtSi=CHMe)] (6^R,Et) in which an SiH group is converted to an SiEt group (Scheme 7). If pathway ‘c’ is involved in the catalysis, the resulting [Mn]SiHREt (2^R,Et) complex must react with free H₃SiR to form [Mn]SiH₂R (2^R) and eliminate H₂SiREt (likely via an unobserved disilyl hydride intermediate analogous to 4^R), given that the observed reactivity converts primary hydrosilanes to free secondary hydrosilanes prior to the formation of significant amounts of tertiary hydrosilane products. The accessibility of this reaction pathway is highlighted by the reaction of [(dmpe)₂MnH(=SiR₂)] {R = Ph (3^Ph2) or Et (3^Et2)} with excess H₃SiⁿBu at 20 °C to afford [(dmpe)₂MnH(SiH₂ⁿBu)₂] (4^R) and free H₂SiPh₂ or H₂SiEt₂, respectively. This reaction was complete in several hours (for 3^Et2) or minutes (for 3^Ph2).

Unidentified SiH-containing byproducts {formed in larger amounts in reactions with H₂SiPh₂ and H₃SiPh (after conversion to H₂SiEtPh); Table 3} may arise from reactions of D (or less likely F) with hydrosilanes resulting in C–Si rather than C–H bond-formation to eliminate a disilylated organic product and generate manganese hydride intermediate [(dmpe)₂MnH] (A), which can re-enter the proposed catalytic cycle (vide infra) as shown in Scheme 8. This reactivity bears resemblance to that of ‘(dmpe)₂MnEt’ (an isomer of 1)³⁵ with H₂SiPh₂ to afford a 1 : 1 mixture of (a) [(dmpe)₂MnH(=SiPh₂)] (3^Ph2) and ethane, the products of C–H bond-forming oxidative addition/reductive elimination (or σ-bond metathesis) followed by α-hydride elimination, and (b) [(dmpe)₂MnH₂(SiHPh₂)] (5^Ph2) and Ph₂SiEtH, the products of C–Si bond-forming oxidative addition/reductive elimination (or σ-bond metathesis) to form [(dmpe)₂MnH] (A), followed by oxidative addition of a second equivalent of H₂SiPh₂.³²

Pathways ‘a’, ‘b’ and ‘c’ described above (green reaction arrows in Scheme 8) generate the observed disilyl hydride (4^R), silylene hydride (3) and silene hydride (6) complexes. However, they do not explain the formation of vinyl silane byproducts. These byproducts can be accessed by vinylsilane dissociation from intermediate E,⁶⁸ forming low-coordinate hydride species A, which can react with either of the available organic substrates: ethylene to form 1, or hydrosilanes to form silyl dihydride complexes (5); Scheme 8. While 1 reacts with primary or secondary hydrosilanes (but not tertiary hydrosilanes) to generate ethane and low-coordinate silyl species 2, complex 5 can slowly rejoin the catalytic cycle by H₂ elimination to afford 2. Support for this H₂ elimination process was obtained experimentally at elevated temperatures. For example, heating a solution of [(dmpe)₂MnH₂(SiH₂Ph)] (5^Ph) under D₂ at 70–80 °C overnight resulted in >90% deuterium incorporation into the MnH̲ environments, exclusively. Furthermore, reactions of 5^Bu with ^tBuNC, and [(dmpe)₂MnD₂(SiH₂Ph)] (d₂_5^Ph) with o-xylylNC, afforded [(dmpe)₂Mn(SiH₂ⁿBu)(CN^tBu)] (7d) and [(dmpe)₂Mn(SiH₂Ph)(CNXyl)] (7a), respectively, after 1 h at 75 °C.

In an effort to determine whether pathway ‘a’ (via primary alkyl intermediate D), ‘b’ (via secondary alkyl intermediate F), or ‘c’ (via a silene hydride complex with an SiH substituent; 6^R,H) in Scheme 8 is operative, catalysis was carried out using d₄-ethylene; pathway ‘a’ would generate CD₂CD₂H groups, whereas pathways ‘b’ and ‘c’ would generate CHDCD₃ groups.⁶⁹ Hydrosilylation of C₂D₄ by the secondary hydrosilane H₂SiEt₂ yielded d₄-HSiEt₃, primarily as HSiEt₂(CD₂CD₂H) (97%), with a minor amount of HSiEt₂(CDHCD₃) (3%), as determined by ¹H, ²H, and ¹³C{¹H} NMR analysis (Fig. S471†), indicating that pathway ‘a’ in Scheme 8 is dominant. By contrast, C₂D₄ hydrosilylation by H₃SiⁿBu under identical conditions yielded a solution containing 20% HSiⁿBu(CD₂CD₂H)₂, and 80% HSiⁿBu(CD₂CD₂H)(CDHCD₃). Given that H₂SiEt₂ has been shown to react almost exclusively via pathway ‘a’ (affording a CD₂CD₂H substituent on silicon), and H₂SiⁿBuEt can be expected to react analogously, this product distribution indicates that H₃SiⁿBu is converted to H₂SiⁿBuEt primarily via pathway ‘b’ and/or ‘c’ (∼77%), with a lesser contribution from pathway ‘a’ (∼23%).

DFT calculations indicate that alkyl intermediates D and F are very similar in energy (within 5 kJ mol⁻¹). Therefore, the preferential reactivity of secondary silanes towards less hindered D (pathway ‘a’) may be sterically driven. By contrast, for conversion of primary to secondary hydrosilanes, where pathway ‘b’ and/or ‘c’ is dominant, it is not obvious why pathway ‘b’ would be preferred over pathway ‘a’. Pathway ‘c’ is therefore a viable alternative, especially given that ‘c’ has been demonstrated (vide supra) in room temperature stoichiometric reactions involving silenes with a hydrogen substituent on silicon (6^R,H). Furthermore, it is notable that silenes (6^R,H) are the dominant metal-containing species during the first phase of catalysis (conversion of primary to secondary hydrosilanes).

Summary and conclusions

The disilyl hydride manganese complexes, [(dmpe)₂MnH(SiH₂R)₂] (R = Ph or ⁿBu), reversibly dissociate H₃SiR to access low-coordinate silyl ([(dmpe)₂Mn(SiH₂R)]) and silylene hydride ([(dmpe)₂MnH(=SiHR)]) complexes. The trans isomers of the silylene hydride complexes were observed in small amounts (<5% relative to the disilyl hydride) by NMR spectroscopy at 333 K, and are the first spectroscopically observed examples of group 7 L_xM=SiHR compounds. DFT calculations support the thermodynamic accessibility of cis- and trans-isomers of these low coordinate silyl and silylene species, and both sets of intermediates were trapped by coordination of isonitriles (to manganese) or N-heterocyclic carbenes (to silicon).

The reactivity of [(dmpe)₂MnH(SiH₂R)₂] (R = Ph or ⁿBu) with ethylene was investigated, affording silene hydride complexes [(dmpe)₂MnH(RHSi=CHMe)]. This reaction represents a unique method to access silene complexes (analogous to reactions of ethylene with [(dmpe)₂MnH(=SiR₂)] compounds in our previous communication),³² and the resulting silene complexes are the first transition metal examples with an SiH substituent. As such, they displayed unusual reactivity: for example, [(dmpe)₂MnH(RHSi=CHMe)] slowly converted to a more stable silylene hydride isomer, [(dmpe)₂MnH(=SiEtR)]; the first example of isomerization of a silene hydride complex to a silylene hydride complex. Furthermore, [(dmpe)₂MnH(RHSi=CHMe)] reacted with a second equivalent of ethylene to convert the SiH substituent to an SiEt substituent, which is an unprecedented transformation for a silene ligand.

All of the silyl, silylene and silene complexes in this work were accessed via reactions of [(dmpe)₂MnH(C₂H₄)] (1) with hydrosilanes and/or ethylene. Therefore, ethylene hydrosilylation was investigated using 1 as a pre-catalyst, resulting in stepwise conversion of primary to secondary to tertiary hydrosilanes. Manganese complexes observed during catalysis include (a) disilyl hydride complexes, (b) silylene hydride complexes, (c) silene hydride complexes, (d) silyl dihydride complexes, and (e) the ethylene hydride pre-catalyst. All of these species are catalytically active (although the silyl dihydride complexes are significantly less active than the others), and a catalytic cycle is proposed on the basis of these observations, the aforementioned stoichiometric reactions, and hydrosilylation of d₄-ethylene. This catalytic cycle is unusual due to the involvement of silylene hydride and silene hydride complexes, potentially as on-cycle species.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

D. J. H. E. thanks NSERC of Canada for a Discovery Grant. We are grateful to Dr Jim Britten of the McMaster Analytical X-ray Diffraction Facility, Dr Bob Berno and Hilary Jenkins of the McMaster NMR Facility, and Dr Ignacio Vargas-Baca of McMaster University Department of Chemistry for advice and support with X-ray diffraction, NMR spectroscopy, and DFT calculations respectively. We are also grateful to Dr Yurij Mozharivskyj and Fang Yuan for collection of X-ray diffraction data for 8b,c while the department's diffractometer was out of commission. Also, we are grateful to Compute Canada for access to computational resources and Aathith Vasanthakumar for providing ^iPrNHC for the synthesis of 8a,b.

Notes and references

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43. B. Marciniec, H. Maciejewski, C. Pietraszuk and P. Pawluć, Hydrosilylation: A Comprehensive Review on Recent Advances, Springer, Netherlands, 2009.
44. Over the 4 days that this reaction was monitored by ¹H NMR spectroscopy, the ethylene concentration in solution decreased by 40%. Relative concentrations in Fig. 2 were calculated by integrating ¹H NMR data; integration relative to the residual solvent signal from C₆D₆ indicated that the total moles of manganese-containing species in the inset in Fig. 2 remained constant.
45. In the reaction of 4^Ph with ethylene, no tertiary hydrosilane was observed. However, small amounts of a vinyl hydrosilane were observed.
46. DFT calculations indicated that isomerization of SiH-containing silene hydride complexes 6^R,H to silylene hydride complexes trans-[(dmpe)₂MnH(=SiEtR)] (trans-3^R,Et) is thermodynamically favourable for R = Ph and ⁿBu; minima for the latter complexes were located 20–34 kJ mol⁻¹ lower in energy than the lowest energy silene hydride isomer. In addition, cis silylene hydride isomers were determined to be 1 (3^Ph,Et) and 9 (3^Bu,Etb) kJ mol⁻¹ less stable than the respective trans isomers.
47. NMR data for trans-[(dmpe)₂MnH(=SiEt₂)] (trans-3^Et2) includes an MnH¹H NMR peak at −10.46 ppm (quintet with ²J_H,P of 51 Hz), a single sharp singlet in the ³¹P{¹H} NMR spectrum at 80.95 ppm, and a ²⁹Si NMR chemical shift of 365 ppm; see ref. 32.
48. Iron or cobalt silene complexes with hydrogen substituents on Si have been detected in the gas phase by mass spectrometry and postulated as an intermediate in the gas-phase activation of H₃SiEt by Co cations; (a) R. Bakhtiar, C. M. Holznagel and D. B. Jacobson, J. Am. Chem. Soc., 1993, 115, 345–347; (b) D. B. Jacobson and R. Bakhtiar, J. Am. Chem. Soc., 1993, 115, 10830–10844; (c) S. Bärsch, T. Böhme, D. Schröder and H. Schwarz, Int. J. Mass Spectrom., 2000, 199, 107–125.
49. Not including enantiomers where the stereochemistry at manganese is switched from Λ to Δ.
50. The dominant isomer for 6^Bu,H was assigned as isomer i given that the SiH¹H NMR signal for this isomer exhibits a large (18 Hz) ³J_¹H,³¹P coupling to one of the phosphorus donor atoms, and in the calculated structures of isomers (i) and (ii) (for a model of 6^Bu,H where the ⁿBu group was replaced with an Et group), only isomer (i) exhibited a ¹H–³¹P coupling of comparable magnitude (23 Hz). As well, isomer i of 6^Bu,H is slightly (4 kJ mol⁻¹) lower in energy than isomer (ii).
51. C. R. Groom, I. J. Bruno, M. P. Lightfoot and S. C. Ward, Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater., 2016, 72, 171–179.
52. By comparison, terminal Si–H distances and Mayer bond orders in 6^R,H are 1.50–1.51 Å and 0.82–0.86, respectively.
53. (a) S. K. Ignatov, N. H. Rees, B. R. Tyrrell, S. R. Dubberley, A. G. Razuvaev, P. Mountford and G. I. Nikonov, Chem.–Eur. J., 2004, 10, 4991–4999; (b) P. Meixner, K. Batke, A. Fischer, D. Schmitz, G. Eickerling, M. Kalter, K. Ruhland, K. Eichele, J. E. Barquera-Lozada, N. P. M. Casati, F. Montisci, P. Macchi and W. Scherer, J. Phys. Chem. A, 2017, 121, 7219–7235.
54. In the case of 4^Ph, an additional low frequency ¹H NMR signal (a broad singlet with <2% intensity relative to the MnH̲ peak of 4^Ph) was observed at −12.1 ppm (335 K), which could potentially be from the MnH̲ environment of cis-[(dmpe)₂MnH(=SiHPh)] (cis-3^Ph,H). However, EXSY NMR spectroscopy did not show chemical exchange between this peak and those from 4^Ph or trans-3^Ph,H (potentially due to broadness and low intensity of the signal). A further low frequency ¹H NMR signal (also a broad singlet, but present in the room temperature and high temperature NMR spectra in similar intensities; ∼1.5% relative to the MnH̲ region of 4^Ph) was observed at −13.8 ppm, which could potentially be from another isomer of 4^Ph; this environment was observed by EXSY NMR spectroscopy to be in chemical exchange with the SiH̲ and MnH̲ environments of both 4^Ph and trans-3^Ph,H.
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56. A handful of NHC-stabilized silylene complexes have also been reported with ²⁹Si chemical shifts lower than 25 ppm (see ref. 55e–h), and this has been rationalized by adoption of a zwitterionic bonding motif which results in limited π-backdonation to the Si center from the metal (see ref. 55h).
57. We could not determine the ²⁹Si NMR chemical shift of trans-8c due to the low proportion of trans isomer in solution (cis : trans ratio of 14 : 1).
58. The lability of NHCs in 8a–d was also illustrated by initial generation of mixtures of reagents and products upon addition of free NHCs to 4^R; complete conversion to 8a–d required removal of the free hydrosilane byproducts. For 8b–d, this was achieved by periodically removing all solvent and hydrosilane byproducts in vacuo. By contrast, for 8a this was achieved by the reaction of the H₃SiPh byproduct with excess ^iPrNHC to form 1-phenyl-2,5-diisopropyl-3,4-dehydro-2,5-diazasilinane; this reaction has previously been reported at 100 °C, and in our hands 98% conversion was observed after 24 h at 55 °C (consistent with the reaction conditions involved in the synthesis of 8a). D. Schmidt, J. Berthel, S. Pietsch and U. Radius, Angew. Chem., Int. Ed., 2012, 51, 8881–8885.
59. Unlike the reaction of 4^Bu with ethylene, the reaction of 8b with ethylene does not generate H₃SiⁿBu as a byproduct, and under these conditions, complex 6^Bu,H reacted readily with a further equivalent of ethylene, so that both 6^Bu,H and 6^Bu,Et were formed concurrently.
60. In the structures of cis-8a,b, the dmpe ligands are disordered, and modelling this disorder allowed the structures of both diastereomers observed in solution to be elucidated.
61. 7b,d were computationally modelled with Et groups in place of ⁿBu groups; [(dmpe)₂Mn(SiH₂Et)(CNR)] (7b*: R = o-xylyl, 7d*: R = ^tBu).
62. Alternative pathways requiring initial dissociation of a phosphine donor in 6^R,H followed by ethylene coordination (with subsequent oxidative coupling or 1,2-insertion reactivity) cannot be ruled out.
63. Silylene complexes were only observed during catalysis when most of the ethylene had been consumed.
64. C. J. Kong, S. E. Gilliland III, B. R. Clark and B. F. Gupton, Chem. Commun., 2018, 54, 13343–13346.
65. For examples of ethylene hydrosilylation selective for producing secondary hydrosilanes, see ref. 23d and (a) T. I. Gountchev and T. D. Tilley, Organometallics, 1999, 18, 5661–5667. For examples of ethylene hydrosilylation selective for producing quaternary silanes, see (b) L. A. Oro, M. J. Fernandez, M. A. Esteruelas and M. S. Jimenez, J. Mol. Catal., 1986, 37, 151–156; (c) R. S. Tanke and R. H. Crabtree, Organometallics, 1991, 10, 415–418; (d) S. Lachaize, S. Sabo-Etienne, B. Donnadieu and B. Chaudret, Chem. Commun., 2003, 214–215; (e) E. A. Chernyshev, Z. V. Belyakova, S. P. Knyazev, G. N. Turkel'taub, E. V. Parshina, I. V. Serova and P. A. Storozhenko, Russ. J. Gen. Chem., 2006, 76, 225–228; (f) J. J. Adams, N. Arulsamy and D. M. Roddick, Organometallics, 2009, 28, 1148–1157; (g) S. Lachaize, L. Vendier and S. Sabo-Etienne, Dalton Trans., 2010, 39, 8492–8500.
66. M. A. Schroeder and M. S. Wrighton, J. Organomet. Chem., 1977, 128, 345–358.
67. Energy minima for alkyl intermediates D and F were found to lie 46–67 kJ mol⁻¹ higher in energy than the respective silene hydride resting states, indicating their thermodynamic accessibility from complexes observed by solution NMR spectroscopy.
68. In hydrosilylation reactions with d₄-ethylene, the vinyl byproducts contain fully deuterated vinyl groups, in keeping with the proposed pathway for their formation.
69. Hydrosilylation reactions involving C₂D₄ (with either H₃SiⁿBu or H₂SiEt₂) proceeded to completion (i.e. complete consumption of the secondary hydrosilane reagent/intermediate) after 4 days at 60 °C, while analogous reactions using a higher pressure of C₂H₄ still contained 6–7% of the secondary hydrosilane (H₂SiEtⁿBu intermediate or H₂SiEt₂ reagent), suggestive of an inverse kinetic isotope effect.

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, NMR frequencies and selected NMR spectra for complexes and hydrosilylation products, tabulated bonding parameters from X-ray structures, computational results (tables of bonding parameters, bond orders, energies, and Hirshfeld charges), visualization of calculated structures, and tables of crystal data/crystal structure refinement (PDF). Cartesian coordinates of the calculated structures (XYZ). CCDC 1946403–1946410. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9sc04513a

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