Interconversion and reactivity of manganese silyl, silylene, and silene complexes
Interconversion and reactivity of manganese silyl, silylene, and silene complexes This work involves the reaction of a monosubstituted manganese silylene complex (LxHMn=SiHR) with ethylene to aff ord the fi rst transition metal silene complex with an SiH substituent (LxHMn(RHSi=CHMe)), unprecedented isomerization to a silylene isomer (LxHMn=SiEtR), and further reaction with ethylene to aff ord a new silene complex (LxHMn(REtSi=CHMe)). Reaction mechanisms were elucidated through reactions with C2D4, and we report the involvement of the aforementioned silene complexes in catalytic ethylene hydrosilylation. The image shows Mn atoms traversing 4 mountain peaks representing the stepwise silylene → silene → silylene → silene transformations in the research.
Introduction<!>Reactions of disilyl hydride complexes 4 R with ethylene<!>DFT calculations on low-coordinate silyl and silylene hydride intermediates derived from 4 R<!>Trapping experiments with isonitriles and N-heterocyclic carbenes<!>Pathways for reactions of 4 R with ethylene<!>Catalytic ethylene hydrosilylation<!>Summary and conclusions
<p>Silylenes (:SiR 2 ) 1,2 and silenes (R 2 C]SiR 2 ), [2][3][4] heavy analogues of carbenes and alkenes, are highly reactive species, and in the absence of extremely bulky or p-donor substituents, 5 transition metal coordination is required for stabilization. 3,[6][7][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 [Cp 2 ZrCl 2 ], 9 dehydrogenative coupling of HSiMe 3 by [(Me 3 P) 3 RuH 3 (SiMe 3 )], 10 transfer dehydrogenative coupling of HSiEt 3 catalysed by [(p-cymene)RuH 2 (-SiEt 3 ) 2 ] or [Cp*RhH 2 (SiEt 3 ) 2 ], 11 and trialkylsilane (e.g. HSiMe 3 ) perdeuteration catalysed by [(Me 3 P) 4 OsH(SiMe 3 )] 12 or [(C 6 Me 6 ) RuH 2 (SiMe 3 ) 2 ] 13 in C 6 D 6 . 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 rst example, [(Et 3 P) 3 IrH 2 {]SiH(C 6 H 3 -Mes 2 -2,6)}][B(C 6 F 5 ) 4 ], was reported in 2002, 19  [Cp*(dmpe) MoH(]SiHPh)], respectively. Base-free L x M]SiHR complexes have only been isolated for groups 6, 8 and 9, [19][20][21][22][23][24][25][26] and group 7 examples are notably absent. Extensive studies by the Tilley and Tobita groups have demonstrated that hydrogen substituents on the sp 2 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][29][30] Furthermore, outside of our recent report of manganese silene complexes (vide infra), rst row transition metal complexes bearing unstabilized silene ligands have not been isolated.</p><p>We recently communicated the synthesis (Scheme 1) of the rst unstabilized terminal silylene complexes of a group 7 metal, [(dmpe) 2 MnH(]SiR 2 )] (3 Ph2 : R ¼ Ph, 3 Et2 : R ¼ Et), by the reaction of [(dmpe) 2 MnH(C 2 H 4 )] (1) 31 with secondary hydrosilanes (H 2 SiR 2 ). 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 rst reported in 2004 by the Tobita 21 ([(C 5 Me 4 Et)(OC) 2 WH(]SiH{C(SiMe 3 ) 3 })]) and Tilley 22 ([Cp*(dmpe)MoH(]SiEt 2 )]) 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).</p><p>In contrast to the reactions of 1 with secondary hydrosilanes, reactions with primary hydrosilanes (H 3 SiR) yielded disilyl hydride complexes [(dmpe) 2 MnH(SiH 2 R) 2 ] (4 Ph : R ¼ Ph, 4 Bu : R ¼ n Bu; 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) 2 Mn(SiHRR 0 )] (2 Ph2 : R ¼ R 0 ¼ Ph; 2 Et2 : R ¼ R 0 ¼ Et; 2 Ph : R ¼ Ph, R 0 ¼ H; 2 Bu : R ¼ n Bu, R 0 ¼ H), which undergoes a-hydride elimination to generate 3 R2 , or oxidative addition of a second equivalent of hydrosilane to afford 4 R (Scheme 1).</p><p>Reactions of the silylene hydride complexes with ethylene generated the silene hydride complexes cis-[(dmpe) 2 -MnH(R 2 Si]CHMe)] {R ¼ 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*(Me 3 P)Ir(Me)(]SiMe 2 )] + to the silene hydride isomer, [Cp*(Me 3 P)IrH(Me 2 Si]CH 2 )] + . 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*(Me 3 P)Ir(Me){]SiMe 2 (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 CH 2 SiR 2 H (R ¼ Me or Ph) or SiR 2 Me group, followed by b-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 (CH 2 SiHR 2 ) complexes.</p><p>Both silylene hydride complexes 3 R2 and disilyl hydride complexes 4 R have been shown to react with H 2 (Scheme 1) to Fig. 1 Second and third row transition metal complexes bearing sterically and electronically unstabilized silene ligands.</p><p>Scheme 1 Reactions of [(dmpe) 2 MnH(C 2 H 4 )] (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 H 2 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. Scheme 2 Reported reactions capable of converting between silylene, silyl, silene and alkyl isomers. Electron pushing arrows are shown for the forward direction.</p><p>generate silyl dihydride complexes [(dmpe) 2 MnH 2 (SiHRR 0 )] (5 Ph2 : R ¼ R 0 ¼ Ph; 5 Et2 : R ¼ R 0 ¼ Et; 5 Ph : R ¼ Ph, R 0 ¼ H; 5 Bu : R ¼ n Bu, R 0 ¼ H), suggesting the accessibility of a common lowcoordinate silyl intermediate, [(dmpe) 2 Mn(SiHRR 0 )] (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.</p><p>Herein, we report the reactions of disilyl hydride complexes 4 R with ethylene to generate the rst 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 lowcoordinate 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.</p><p>All of the silyl, silylene and silene complexes in this work are accessed via reactions of [(dmpe) 2 MnH(C 2 H 4 )] (1) with hydrosilanes, in some cases followed by ethylene. Therefore, ethylene (C 2 H 4 and C 2 D 4 ) hydrosilylation was investigated using [(dmpe) 2 MnH(C 2 H 4 )] (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][38][39] and the most common olen hydrosilylation catalyst used in industry is Karstedt's catalyst, [Pt 2 (O{SiMe 2 (CH]CH 2 )} 2 ) 3 ]. 38 However, the development of catalytic systems based on rst 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 nally Si-C bond-forming reductive elimination. However, in some cases alkene coordination is followed by C-Si bond-forming 1,2insertion and then C-H bond-forming reductive elimination (modied 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 precatalyst. 38,43</p><!><p>The disilyl hydride complexes [(dmpe) 2 MnH(SiH 2 R) 2 ] (4 Ph : R ¼ Ph, 4 Bu : R ¼ n Bu) reacted with ethylene at room temperature to afford the silene hydride complexes [(dmpe) 2 MnH(RHSi]CHMe)] (6 Ph,H : R ¼ Ph, 6 Bu,H : R ¼ n Bu). 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) 2 MnH(REtSi]CHMe)] (6 Ph,Et : R ¼ Ph, 6 Bu,Et : R ¼ n Bu); 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) 2 MnH(C 2 H 4 )] (1), 31 potentially by substitution of the silene ligand which undergoes subsequent decomposition to unidentied products.</p><p>A range of byproducts were observed in the syntheses of silene hydride complexes, including primary, secondary, and tertiary hydrosilanes {H (3-n) SiEt n R (n ¼ 0, 1, 2; R ¼ Ph, n Bu)}; 45 the latter two are formed by stepwise manganese-catalysed hydrosilylation reactions between the primary hydrosilane byproduct and excess ethylene (vide infra). For R ¼ n Bu, 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 ¼ Ph, silene SiH to SiEt conversion commenced as soon as 6 Ph,H was available (Fig. 2).</p><p>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.</p><p>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) 2 MnH(]SiEt n Bu)] (trans-3 Bu,Et ; Scheme 4), with 20% conversion aer 2 days at room temperature in C 6 D 6 . 46 NMR spectra of trans-3 Bu,Et feature an MnH 1 H NMR peak at À10.48 ppm (a quintet with 2 J H,P of 51 Hz), two sharp singlets in the 31 P{ 1 H} NMR spectrum at 80.35 and 80.50 ppm, and a high-frequency peak in the 29 Si{ 1 H} 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) 2 MnH(]SiEt 2 )] (trans-3 Et2 ). 47 Isomerization was accompanied by formation of small amounts ($10% relative to 3 Bu,Et ) of an unidentied manganese hydride complex (with a quintet 1 H NMR peak at À9.06 ppm; 2 J H,P ¼ 47 Hz) and the silene hydride complex [(dmpe) 2 MnH( n BuEtSi]CHMe)] (6 Bu,Et ).</p><p>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*(Me 3 P)Ir(Me)(]SiMe 2 )] + and the silene hydride isomer, [Cp*(Me 3 P)IrH(Me 2 Si]CH 2 )] + , which relies upon reversible a-Me and b-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 31 P NMR signals, a single 29 Si NMR environment (at À17.4 to 0.7 ppm), a low frequency 13 C NMR signal for the Si]C environment (at À19.3 to À21.7 ppm), and a silene 1 J C,H coupling constant (137-139 Hz) intermediate between those typical for sp 2 and sp 3 hybridized carbon atoms; Table 1. Additionally, the MnH signal was located at À14.5 to À15.3 ppm in the 1 H NMR spectra of 6 R,H and 6 R,Et , and the SiH, Si]CH(CH 3 ) and Si]CH(CH 3 ) signals were observed at 3.7 to 4.5 ppm, À0.2 to 0.2 ppm, and 1.7 to 1.9 ppm, respectively. These data are very similar to those for [(dmpe) 2 -MnH(R 2 Si]CHMe)] (6 Ph2 : R ¼ Ph, 6 Et2 : R ¼ Et; pertinent NMR data is included in Table 1), which have been Table 1 Selected 1 H, 13 C, 29 Si and 31 P NMR chemical shifts (ppm) and coupling constants (Hz) for silene hydride complexes [(dmpe) 2 -MnH(RR 0 Si]CHMe)] (6 Ph2 : R ¼ R 0 ¼ Ph; 6 Et2 : R ¼ R 0 ¼ Et; 6 Ph,H : R ¼ Ph, R 0 ¼ H; 6 Bu,H : R ¼ n Bu, R 0 ¼ H; 6 Ph,Et : R ¼ Ph, R 0 ¼ Et; 6 Bu,Et : R ¼ n Bu, R 0 ¼ Et); in C 6 D 6 (6 R2 and 6 Ph,Et ) or d 8 -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 communication  spectroscopically, and (for 6 Ph2 ) crystallographically, characterized. 32 To the best of our knowledge, 6 R,H are the rst spectroscopically observed 48 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 isomers 49 with E or Z silene stereochemistry, and differing in the orientation of the silene methyl substituent (RR 0 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 À1 of one another, and are 13-22 kJ mol À1 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.</p><p>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 signicant 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 re-ected by a large negative 29 Si-1 H Mn coupling constant of À80 Hz (measured using 29 Si_edited 2D 1 H-1 H COSY NMR spectroscopy) 36 for the major isomer of 6 Bu,H (cf. À30 to À31 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 identication 6 R,H and 6 R,Et as cis silene hydride complexes, as opposed to 5-coordinate alkyl complexes with a strong b-Si-H-Mn interaction.</p><!><p>The reactions of the disilyl hydride complexes [(dmpe Bu,H : R ¼ n Bu), formed by sequential hydrosilane reductive elimination and a-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).</p><p>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, DG for the formation of these monosilyl compounds and free hydrosilane from 4 R is very similar (63-71 kJ mol À1 ).</p><p>In the global minima for the cis isomers (rotamer 1 of cis-2 R ), the hydrocarbyl substituent on silicon engages in a g-agostic interaction with manganese (via an ortho-CH bond in cis-2 Ph or a CH 2 CH 2 CH 2 CH 3 bond in cis-2 Bu ), with Mn-H g distances of 1.91-1.93 Å. The Mn-H g -C g angles in this rotamer of cis-2 R are 115.9 and 131.4 , respectively, and the presence of a g-agostic interaction is further supported by Mayer bond orders of 0.22-0.24 between Mn and H g , and 0.13-0.15 between Mn and C g .</p><p>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 a-Si-H-Mn interaction. Rotamer 2 of 2 R is presumably involved in silylene hydride formation via a-hydride elimination, and indeed, all  attempts to locate an analogous energy minimum for the n Bu analogue structure led instead to a silylene hydride structure (cis-3 Bu,H ; vide infra).</p><p>As with the 5-coordinate silyl species (vide supra), multiple energy minima (Fig. 5 and Table 2) were located for silylene hydride structures [(dmpe) 2 MnH(]SiHR)] (3 Ph,H : R ¼ Ph, 3 Bu,H : R ¼ n Bu). The two lowest energy structures are (a) a cis silylene hydride isomer with a signicant 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) 2 MnH(]SiPh 2 )] (3 Ph2 ) and trans-[(dmpe) 2 MnH(]SiEt 2 )] (3 Et2 ), respectively. 32 Calculated DG values to access 3 R,H from 4 R range from 41 kJ mol À1 (trans isomers) to 47-60 kJ mol À1 (cis isomers) at 298.15 K, decreasing to 33-34 kJ mol À1 (trans isomers) and 39-52 kJ mol À1 (cis isomers) at 335 K, highlighting their thermodynamic accessibly.</p><p>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 ( P (R-Si-R) > 356 ); Table S2. † These parameters are comparable to those previously observed and/or calculated for the two isomers of [(dmpe) 2 MnH(]SiR 2 )] (3 R2 ; R ¼ 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, 1 H NMR spectra of the disilyl hydride complexes [(dmpe) 2 MnH(SiH 2 R) 2 ] (4 Ph : R ¼ Ph, 4 Bu : R ¼ n Bu) revealed the formation of a small amount of a new manganese complex and free hydrosilane (H 3 SiR; R ¼ Ph or n Bu). This process is reversible, and clean spectra of 4 R were observed upon cooling back to room temperature. The new manganese complex   19,20,[22][23][24][25][26] while the latter is consistent with a metal hydride environment. The low frequency hydride signal is a quintet ( 2 J 1 H, 31 P ¼ 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) 2 MnH(]SiHR)] (trans-3 Ph,H : R ¼ Ph, trans-3 Bu,H : R ¼ n Bu); the most thermodynamically accessible silyl or silylene species in Fig. 5. Characterization of trans-3 R,H (R ¼ Ph or n Bu) by 29 Si 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 SiH protons in 4 Ph or 4 Bu , the free hydrosilane SiH peak, the high frequency trans-3 R,H silylene SiH environment, and the MnH signals from both 4 R and trans-3 R,H (shown in Fig. 6 for R ¼ n Bu). 54 This is consistent with an equilibrium in which 4 R eliminates free H 3 SiR to form trans-3 R,H (vide infra).  In contrast to reactions with isonitriles, reactions of disilyl hydride complexes 4 R with 1,3-diisopropylimidazolin-2-ylidene ( iPr NHC) or 1,3,4,5-tetramethyl-4-imidazolin-2-ylidene ( Me NHC) afforded the base-stabilized silylene hydride complexes [(dmpe) 2 MnH{]SiHR(NHC)}] {NHC ¼ iPr NHC, R ¼ Ph (8a) or n Bu (8b); NHC ¼ Me NHC, R ¼ Ph (8c) or n Bu (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 purication.</p><!><p>A variety of NHC-stabilized silylene complexes have been reported for V, Cr, W, Fe, Co, Rh, and Ni, 55 and relative to basefree silylene complexes, they feature longer metal-silicon bond distances, pyramidalization at silicon, and lower frequency 29 Si NMR chemical shis (typically 25-100 ppm, 56 compared with >200 ppm for base-stabilized silylene complexes). 8 Room temperature NMR spectra of iPr NHC 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 SiH signal (5.1 to 6.4 ppm), a single MnH resonance (À12.3 to À12.6 ppm), a single 29 Si NMR environment (22.2 to 29.6 ppm), and four unique 31 P 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).</p><p>In contrast, NMR spectra of the Me NHC silylene hydride complexes (8c,d) revealed the same two rapidly interconverting cis diastereomers plus a third isomer which afforded a sharp set of 1 H and 31 P NMR signals at room temperature. This third isomer corresponds to an NHC-coordinated trans silylene hydride complex, as evidenced by a single MnH (À14.9 or À15.0 ppm) signal with a quintet coupling pattern ( 2 J H,P ¼ 48-49 Hz) and two sharp signals in the 31 P{ 1 H} NMR spectra (78.7-80.6 ppm) due to diastereotopic phosphorus atoms. The 1 H NMR SiH and 29 Si NMR chemical shis in these trans isomers (4.9-5.8 ppm and 22.4 ppm, respectively) 57 are similar to those in the cis isomers.</p><p>At 335 K, the two cis diastereomers of 8a-d gave rise to a single set of averaged signals, with the MnH peak at À12.5 to À12.7 ppm (quintets for cis-8a,c,d with 2 J H,P ¼ 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 MnH and SiH 1 H 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) 2 MnH(]SiPh 2 )] (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) 2 MnH(]SiHR)] (cis-3 Ph,H : R ¼ Ph, cis-3 Bu,H : R ¼ n Bu), 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 t BuNC, and 8b with ethylene, which afforded [(dmpe) 2 Mn(SiH 2 n Bu)(CN t Bu)] (7d) and [(dmpe) 2 -MnH( n BuHSi]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 SiH and MnH 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 À30 C. The solid state structures of 8a-c (Fig. 8; top row and bottom le) 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 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 signicantly shorter than those in related silyl complexes 7ad (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.</p><!><p>Previously, we reported the reactions of the silylene hydride complexes, [(dmpe) 2 MnH(]SiR 2 )] (3 Ph2 : R ¼ Ph, 3 Et2 : R ¼ Et), with ethylene to form the silene hydride complexes [(dmpe) 2 -MnH(R 2 Si]CHMe)] (6 Ph2 : R ¼ Ph, 6 Et2 : R ¼ 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 Aer 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 Deuterium labelling studies were employed to provide experimental support for these mechanistic proposals. Specically, [(dmpe) 2 MnH( n BuHSi]CDCD 3 )] (d 4 _6 Bu,H ) isomerized to exclusively form trans-[(dmpe) 2 MnH{]Si n Bu(CHDCD 3 )}] (trans-</p><!><p>Our group previously reported that [(dmpe) 2 MnH(Et 2 Si]CHMe)] (6 Et2 ) catalyses ethylene hydrosilylation by diethylsilane. 32 Upon monitoring the progress of this reaction by NMR spectroscopy, the silyl dihydride complex, [(dmpe) 2 MnH 2 (SiHEt 2 )] (5 Et2 ), silylene hydride complex [(dmpe) 2 MnH(]SiEt 2 )] (3 Et2 ), 63 and ethylene hydride complex [(dmpe) 2 MnH(C 2 H 4 )] (1), were all observed in solution, in addition to silene hydride complex 6 Et2 . We were therefore motivated to investigate [(dmpe) 2 MnH(C 2 H 4 )]</p><p>(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) 2 MnH(C 2 H 4 )] (1) to primary or secondary hydrosilanes (H 3 SiPh, H 3 Si n Bu, H 2 SiPh 2 or H 2 SiEt 2 ) in C 6 D 6 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 (HSiEtPh 2 or HSiEt 3 ), while reactions involving primary hydrosilanes rst formed secondary hydrosilanes (H 2 SiEt n Bu or H 2 -SiEtPh), followed by reaction with an additional equivalent of ethylene to generate the tertiary hydrosilane (HSiEt 2 Ph or HSiEt 2 n Bu) as the major product. Hydrosilylation reactions with H 3 Si n Bu, H 2 SiPh 2 , and H 2 SiEt 2 produced fewer byproducts than those with H 3 SiPh (as noted in Table 3). Additionally, hydrosilylation with H 2 SiEt 2 progressed much more rapidly than that with H 2 SiPh 2 . By contrast, no reactivity was observed when 1 was exposed to ethylene and the tertiary hydrosilanes HSiEt 3 or HSiEtPh 2 ; 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 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 ¼ Et or n Bu or HSiPhViR, R ¼ Et or Ph), accompanied by one or more unidentied SiH-containing silanes (Table 3). Vinyl silanes are commonly observed byproducts in olen (e.g. H 2 C]CHR) hydrosilylation, formed by b-hydride elimination from an M(CH 2 CHRSiR 3 ) intermediate in the catalytic cycle, 66 and were an impetus for the initial proposal of a modied Chalk-Harrod catalytic cycle involving C-Si rather than C-H bond-forming 1,2-insertion from an alkenecoordinated 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) 2 MnH(Et 2 Si]CHMe)] (6 Et2 ) 32   a Relative amounts of unidentied 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. employed, hydrosilylation proceeded at a slower rate, and even aer 24 hours the dominant manganese-containing species was 5 Bu .</p><p>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 H 3 Si n Bu in C 6 D 6 were placed under a large excess of ethylene in a sealed 50 mL ask (initial pressure 1.7 atm, n C 2 H 4 z 40 Â 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 H 2 SiEt n Bu was observed before any formation of the tertiary silane (HSiEt 2 n Bu) product or vinyl silane (HSiEtVi n Bu) byproduct, (b)   during hydrosilylation by the primary hydrosilane H Bu conversion observed during this time period). Consequently, the reaction in Fig. 9 was repeated and stopped aer most but not all of the primary hydrosilane had been consumed {the resulting mixture of hydrosilanes and Mncontaining species in C 6 D 6 was similar to that observed at 13 h in Fig. 9; i.e. mostly secondary hydrosilane H 2 SiEt n Bu and [(dmpe) 2 MnH( n BuHSi]CHMe)] (6 Bu,H ), with a small amount of the primary hydrosilane, H 3 Si n Bu}. 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). In Fig. 10, consumption of remaining primary hydrosilane was complete aer 10 minutes, followed by rapid secondary to tertiary hydrosilane conversion and a spike in the concentration of a new silene hydride complex, [(dmpe) 2 -MnH( n BuEtSi]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) 2 MnH(C 2 H 4 )] (1) (the Mn-containing species dominant aer the 18 h mark) diminished and increased, respectively. Furthermore, a small amount ($12%) of the silyl dihydride complex [(dmpe) 2 -MnH 2 (SiHEt n Bu)] (5 Bu,Et ) grew in over this time period, and vinylsilane (HSiViEt n Bu) 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).</p><p>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 iden-tied 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 s-bond metathesis. 67 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   DFT calculations indicate that alkyl intermediates D and F are very similar in energy (within 5 kJ mol À1 ). 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 rst phase of catalysis (conversion of primary to secondary hydrosilanes).  32 and the resulting silene complexes are the rst transition metal examples with an SiH substituent. As such, they displayed unusual reactivity: for example, [(dmpe) 2 -MnH(RHSi]CHMe)] slowly converted to a more stable silylene hydride isomer, [(dmpe) 2 MnH(]SiEtR)]; the rst example of isomerization of a silene hydride complex to a silylene hydride complex. Furthermore, [(dmpe) 2 MnH(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.</p><!><p>All of the silyl, silylene and silene complexes in this work were accessed via reactions of [(dmpe) 2 MnH(C 2 H 4 )] (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 (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 signicantly less active than the others), and a catalytic cycle is proposed on the basis of these observations, the aforementioned stoichiometric reactions, and hydrosilylation of d 4 -ethylene. This catalytic cycle is unusual due to the involvement of silylene hydride and silene hydride complexes, potentially as on-cycle species.</p>