Stable nickel complexes of a siliconoid/silylene hybrid ligand: competent hydrosilylation catalysts for terminal olefins

Abstract

The use of abundant, first row transition metals instead of platinum group metals is a contemporary goal in homogenous catalysis research. Electron-rich silylenes are increasingly established as viable alternatives to carbene-based ligand systems. Here we report that – depending on the first row transition metal fragment employed – a silylene hybridized with a Si₇-siliconoid (Tip₅Si₇NHSi 1) either coordinates to the metal centre via the silylene side-arm exclusively (Fe(CO)₄ complex) and thus retains an unperturbed metalloid core or coordinates in a chelating manner involving the donation by unsubstituted vertices (Ni(cod)-fragment, cod = 1,5-cyclooctadiene). The siliconoid/silylene ligand 1 is bonded more strongly than cod so that the latter is readily replaced by PPh₃ or CO ligands. The cod derivative is catalytically active in the hydrosilylation of terminal olefins with the secondary silane Ph₂SiH₂.

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Introduction

Transition metal complexes are still the benchmark in homogenous catalysis. Especially the precious late 4d and 5d transition metals show outstanding catalytic activity in the value-added transformations of small molecules such as CO, CO₂, NH₃ or H₂.^1,2 The toxicity, low abundance and high price of these metals moved 3d transition metals into focus over the last years.³ Their lower propensity for 2e⁻ redox steps, however, requires the use of non-innocent ligands and hence metal–ligand cooperativity in many cases.^4,5 More recently, the incorporation of heavier elements of the p-block into the ligand system has proven to induce chemical non-innocence.^6–10 In particular, low-valent motifs such as silylenes, germylenes and stannylenes have been employed as ligands towards base metals in order to improve their performance in homogenous catalysis.^11–19

Even with chemically innocent coordination environment, nickel is an outstanding catalyst in many homogenous transformations, for instance, in alkene hydrogenation and cross-coupling reactions.^20,21 The nickel complexes with amidinato-stabilized silicon(II) I ²² and with dialkylsilylene ligand II (Fig. 1)²³ paved the way for a broad variety of stable Ni(0) complexes with low-valent group 14 ligands.^11,24–28 A mixed NHC/NHSi-stabilized Ni complex III reported by Driess et al. activates dihydrogen and thus reductively transforms HBcat into a Ni^II borylene complex.²⁴ Subsequently, various bis(silylenes) with different bridging backbones such as IV,²⁵ V ²⁶ or VI ²⁷ were employed as bidentate ligands for nickel to result in the activation of H₂, CO, or NH₃ and other small molecules.^11–19,28

Fig. 1 Selected examples of low-valent group 14 ligands with nickel (a, b and c) ( = BH, = Ge, = Ge-Hyp (Hyp = Si(SiMe₃)₃), = Si, Dip = 2,6-diisopropylphenyl, 1,3-cod = 1,3-cyclooctadienyl, cod = 1,5-cyclooctadienyl, NHC = N,N′-diisopropyl-3,4-dimethylimidazol-2-ylidene).

Group 14-based cluster systems^29–31 have recently come to the fore as potentially redox-active ligands, but examples of their coordination to nickel are scarce and studies on catalytic activity even rarer. The Goicoechea group reported on the capping of a Ni(II) centre onto the dianionic Ge₉ Zintl cluster VII ³² (Fig. 1). The corresponding rhodium derivative can be converted to the heterodimetallic Ni₂-Rh-Ge₉ cluster VIII.³³ Although the Rh-Ge₉-cluster is an active catalyst in the hydrogenation of cyclooctadiene (cod), in case of VII no information on catalytic activity is available. The only silicon-cluster based nickel complex thus far was isolated by Korber et al. as the eightfold negative charged, dimeric Si₉–(Ni(CO)₂)₂–Si₉ cluster IX ³⁴ with no reported catalytic activity. Siliconoids (unsaturated silicon clusters)^{29,30,35–40} have been employed as ligands to various transition metals.^36,37 The incorporation of an amidinato silicon(II) side arm proved a useful strategy to enable coordination to electroneutral transition metal fragments, for instance to Fe(CO)₄ with Si₆ siliconoid/silylene hybrid ligand X,³⁶ yet nickel complexes of siliconoids remain elusive.

In view of the catalytic activity of an iridium complex derived from X in alkene isomerization,³⁷ we had previously attempted to coordinate it to a nickel carbonyl fragment (in situ-generated from Ni(cod)₂ under CO atmosphere) but isolated nickel-free XI instead (Scheme 1) as the product of complete CO cleavage. DFT calculations regarding the mechanism of CO splitting, however, suggested cooperativity between the siliconoid core and the nickel centre in experimentally undetected intermediates.³⁸ Plausibly, the lack of space at the Si₆ core of X prevents the observation, let alone isolation of any involved nickel complex.

Scheme 1 Reported reaction of Si₆ siliconoid/silylene hybrid with CO in the presence of Ni(cod)₂ ( = Si, R = 2,4,6-triisopropylphenyl).³⁸

We therefore anticipated that an expansion to a Si₇ core could increase the available space for metal coordination. Indeed, we now report the isolation and full characterization of siliconoid-transition metal complexes employing a core-expanded Si₇ cluster with pending amidinato-silylene as a ligand.⁴⁰ While the Fe(CO)₄ fragment is only coordinated by the silylene sidearm, the reaction with Ni(cod)₂ results in a product with distinct interactions of the Si₇ core with the nickel centre. Straightforward ligand exchange of cod yields the corresponding PPh₃ and CO complexes. Competent catalytic activity in hydrosilylation of terminal alkenes is demonstrated. During benchmark reactions with Ni(cod)₂, we unexpectedly found that this simple Ni(0) precursor exhibits remarkable activity in these alkene hydrosilylations as well, which once more illustrates the importance of appropriate control experiments in homogenous catalysis.

Results and discussion

In order to quantify our assumption regarding the reduced steric demand of the Si₇ system, we initially investigated the complexation of the Si₇ siliconoid/silylene hybrid 1 ⁴⁰ to an Fe(CO)₄ fragment, aiming for a direct comparison with the Si₆ system X-Fe(CO)₄.³⁶ Reaction of 1 with one equivalent of Fe₂(CO)₉ at 60 °C (aluminium heating block temperature) in benzene overnight indeed leads to quantitative conversion. The desired iron tetracarbonyl complex 2 crystallizes in 46% yield from pentane (Fig. 2, top). Notably, the corresponding products could not be obtained with the corresponding germylene or stannylene-Si₇-hybrids.⁴⁰

Fig. 2 Top: Reactivity of Tip₅Si₇NHSi 1 towards Fe₂(CO)₉ ( = Si, R = 2,4,6-triisopropylphenyl). Bottom: Molecular structure of iron tetracarbonyl siliconoid/silylene hybrid 2 in the solid state. Hydrogen atoms are omitted for clarity. Thermal ellipsoids at 50% probability. Selected bond lengths [Å] and angles [°]: Fe–Si8 2.253(6), Si3–Si8 2.391(8), Si1–Si2 2.618(8), Si1–Si3 2.315(8), Si1–Si5 2.332(8), Si1–Si7 2.323(7), Si2–Si3 2.366(7), Si2–Si5 2.346(8), Si2–Si7 2.356(8), Si3–Si42.428(8), Si4–Si5 2.388(7), Si5–Si6 2.381(7); Fe1–Si8–Si3 129.0(3), Si4–Si5–Si6 171.0(3), Si8–Fe1–C4 171.7(9).

The molecular structure of 2 in the solid state as determined by single crystal X-ray diffraction (Fig. 2, bottom) reveals a silylene-coordinated Fe1 centre with a Fe–Si8 bond length of 2.253(6) Å, considerably shortened compared to X-Fe(CO)₄ (2.384 Å)³⁷ but well within the range of reported sterically unencumbered Si–Fe(CO)₄ species (2.196–2.405 Å).^41–44 The short Fe–Si bond length is a first indication for the validity of our working hypothesis that the expanded cluster core may ease the steric strain in the envisaged complexes. For a more quantitative description, we calculated the percent buried volume %V_bur ^45,46 of 2 vs. that of X-Fe(CO)₄ using the SambVca open-source application.⁴⁷ As expected, X shows a somewhat larger buried volume of 64.9%V_bur in comparison with 1 (55.3%V_bur) at the iron centre of the Fe(CO)₄ fragment.³⁶ The assumption of a better accessibility of 1 compared to X is thus confirmed in principle; both values are at the upper end of the range found for typical bulky ligands. For context, common ligands for transition metals exhibit %V_bur values of around 30–60%.⁴⁸ Sterically more demanding ligands exceed these values: the carbazole ligand by Hinz with 74.8%,⁴⁹ the triazene by Masuda (80.1%)⁵⁰ or a tetradentate combination of N,N and P,P ligands by Constable and Housecroft (88.0%).⁵¹

As in X-Fe(CO)₄,³⁶ the Si₇ system 2 shows no interactions between the cluster core and the iron moiety. The Si1–Si2 distance is at 2.618(8) Å almost identical with that in the free ligand 1 (2.620 Å).⁴⁰ The CO ligands complete the trigonal–bipyramidal coordination sphere of the iron centre with C–O bond lengths (1.144–1.152 Å) and Fe1–C distances (1.769–1.794 Å) in the range of reported Fe(CO)₄ complexes.^41–44,52 The most notable distortion from the cluster bonding situation in the free ligand 1 is the Si4–Si5–Si6 seesaw angle of 171.0(3)° in 2, which is slightly smaller than in 1 (174.8°).⁴⁰

The ²⁹Si NMR spectrum of 2 shows the familiar wide signal distribution of siliconoids with eight signals in the range of 172.7 to −209.0 ppm. The signal at 172.7 ppm, assigned to Si3 based on ¹H/²⁹Si 2D NMR spectra, is significantly upfield-shifted compared to the free ligand 1 (211.7 ppm).⁴⁰ In contrast, the signal of the silylene moiety (Si8) is strongly downfield-shifted from 24.1 ppm (1)⁴⁰ to 100.5 ppm (2), which is very similar to the reported shift differences between X and its iron complex.³⁶ Owing to Berry pseudorotation, the ¹³C NMR spectrum of 2 reveals only one single signal for the four characteristic CO ligands at high field at 216.79 ppm.⁵³

FT-IR spectroscopy reveals four CO stretching vibrations (Ṽ_CO,exp = 2026, 1953, 1923, 1907 cm⁻¹), in line with the absence of symmetry in 2. The carbonyl stretching frequency at Ṽ_CO,exp = 2026 cm⁻¹ is in accordance with that of the silylene complex of Fe(CO)₄ (Ṽ_CO,exp = 2026 cm⁻¹) reported by Jana and Roesky,⁴¹ suggesting a similar ligand-to-metal σ-donation but a slightly higher donation than in the Si₆-analogue (Ṽ_CO,exp = 2022 cm⁻¹). Theoretical support is provided by calculations at the B3LYP/def2-TZVPP^54–57 level of theory with stretching frequencies at Ṽ_CO,calc = 2009 and 1901 cm⁻¹. The longest wavelength UV-Vis absorption of 2 occurs at λ_max,exp = 525 nm (ε = 380 M⁻¹ cm⁻¹) with a bathochromic shift compared to that of the Si₆-analogue (λ_max,exp = 470 nm, ε = 1890 M⁻¹ cm⁻¹)³⁶ but a marginally hypsochromic shift compared to the starting material 1 (λ_max,exp = 532 nm, ε = 704 M⁻¹ cm⁻¹).⁴⁰ The experimental values for 2 are in good agreement with the TD-DFT results (λ_max,calc = 528 nm; see SI for details).

For the envisaged coordination to a nickel centre, we opted for bis(cyclooctadiene) nickel(0) Ni(cod)₂ as precursor, given that Ni(CO)₄ is highly volatile and toxic (and not commercially available). Addition of 1 eq. of solid Ni(cod)₂ to a solution of 1 in toluene at ambient temperature caused a colour change of the mixture from brown to blackberry red. After work-up (see SI), the desired Ni-siliconoid complex 3a was isolated as dark red, block-shaped crystals in 46% yield (Scheme 2). The residual cod ligand in 3a can be readily replaced by adding 1 eq. of solid PPh₃ to the reaction mixture of 1 and Ni(cod)₂ affording the triphenylphosphine-derivative 3b in 49% yield.

Scheme 2 Synthesis of nickel siliconoid/silylene complexes 3a and 3b ( = Si, R = 2,4,6-triisopropylphenyl).

Single crystals were obtained from a concentrated pentane solution of 3a,b at −26 °C. X-ray diffraction analysis confirmed the solid-state structure of 3a in the triclinic space group P1̄ and 3b in the monoclinic space group I2/a (Fig. 3). In contrast to the previously discussed iron complex 2, an interaction of the transition metal atom with the Si1 and Si3 atom is observed for 3a/3b. The overall decreased congestion at the unsubstituted silicon vertices given their increased number in the Si₇/silylene scaffold 1 compared to the Si₆-hybrid X not only allows stable nickel coordination in the first place but also gives rise to significant interactions with the cluster core. In 3a,b, the Ni1 atom is not only coordinated by the central Si8 atom of the silylene unit (Si3–Si8 3a: 2.183(8) Å/3b: 2.212(5) Å), as in the case for iron in 2, but also by the Si1 and Si3 atoms. With bond distances of 2.302(5) Å (3a)/2.216(8) Å (3b) the Si1–Ni1 bond lengths are well within the common range (2.075–2.499 Å),^{19,22–28,34,58–61} while the distances between Si3 and Ni1 are with 2.428(5) Å (3a)/2.379(8) Å (3b) slightly elongated. In 3a, a longer Si1–Ni1 bond accompanied by a shorter Si1–Si3 bond (2.429(6) Å) is observed while the reduced donor strength of PPh₃ leads to a contraction of the Si1–Ni1 bond and thus a slight elongation of the Si1–Si3 bond in 3b (3a: 2.429(6) Å; 3b: 2.468(1) Å). These structural parameters reflect the stronger σ-donor character of the cod ligand compared to PPh₃, while also highlighting the influence of the appreciable π-acceptor ability of PPh₃ on the electronic structure at the nickel centre. In agreement with lowered electron density in the Si₇ scaffold due to tighter bonding of the Ni1 centre in 3b compared to 3a, the distance between the Si1 and Si2 atom of 2.782 (1) Å in 3b exceeds the distances between the unsubstituted vertices of so far reported propellane-like siliconoids (2.551 to 2.728 Å), while for 3a it is with 2.674(6) Å in the reported range.^{29,30,34–40} The concomitant upfield-shift of the Si2 atom in the ²⁹Si NMR spectrum of 3b (3a: −340.7, 3b: −287.6 ppm) is in line with the above mentioned shift of cluster electron density towards the perimeter of the Si2 atom. The seesaw Si4–Si5–Si6 angle in 3a of 171.74(2)° is similar to that in 2 (171.0(3)°) and hence, slightly smaller than for 3b (173.10(4)°) and the previously reported Si₇ cluster scaffolds (Si₇Tip₅Cp*: 173.8°, Si₇Tip₅Li: 174.0°, Tip₅Si₇NHSi 1: 174.8°).^39,40 The geometry around the Ni centre in 3a is approximately tetrahedral with the coordinating silicon vertices (Si1 and Si8) and the two geometrical centres of the η²-coordinated C=C double bonds of the cod ligand, which is comparable to the case of NHSi(Cl)–Ni(CO)₃ I.²² In stark contrast, the substitution of the cod ligand by PPh₃ leads to a trigonal planar coordination environment around Ni1 in 3b (largest deviation from Si1–Si8–Ni1–P1 plane for Ni1 = −0.018 Å), similar to the situation in V.²⁶

Fig. 3 Molecular structure of Ni(cod)-3a (left) and Ni(PPh₃)-siliconoid/silylene complex 3b (right) in the solid state. Hydrogen atoms are omitted for clarity. Thermal ellipsoids at 50% probability. Selected bond lengths [Å] and angles [°]: (a) Ni1–Si1 2.302(5), Ni1–Si3 2.428(5), Ni1–Si8 2.212(5), Si1–Si2 2.674(6), Si1–Si3 2.429(6), Si1–Si5 2.362(6), Si1–Si7 2.324(6), Si2–Si5 2.335(6), Si2–Si7 2.352(6), Si2–Si3 2.392(1), Si3–Si8 2.291(6), Si3–Si4 2.424(6), Si4–Si5 2.371(6), Si5–Si6 2.362(6), Si6–Si7 2.425(6); Si8–Ni1–Si1 108.6(2), Si1–Ni1–Si3 61.7(2), Si8–Ni1–Si3 59.0(2), Si4–Si5–Si6 171.7(2); (b) Ni1–P1 2.200(7), Ni1–Si1 2.216(8), Ni1–Si3 2.378(8), Ni1–Si8 2.183(8), Si1–Si2 2.782(1), Si1–Si3 2.468(1), Si1–Si5 2.366(1), Si1–Si7 2.294(1), Si2–Si5 2.336(1), Si2–Si7 2.384(1), Si2–Si3 2.400(1), Si3–Si8 2.293(1), Si3–Si4 2.422(1), Si4–Si5 2.373(9), Si5–Si62.378(1), Si6–Si7 2.409(1); Si8–Ni1–Si1 120.2(3), Si1–Ni1–Si3 64.9(3), Si8–Ni1–Si3 60.2(3), Si4–Si5–Si6 173.1(4).

Interestingly, in case of V, a substitution of the cod moiety by two PMe₃ units was shown to lead to a more symmetric tetrahedral geometry, a scenario likely prevented in case of 3b by insufficient space to accommodate two PPh₃ ligands. Although the smaller %V_bur^45,46 of 1 compared to X in the context of Fe(CO)₄ coordination indicates better accessibility as noted above, the tighter coordination under participation of the siliconoid core results in a much higher value of %V_bur = 69.9 at the nickel centre in 3b.

The ²⁹Si NMR spectra of 3a and 3b retain the typical wide distribution observed for siliconoids,^{29,30,36–40} albeit less pronounced than in 2. The range of the chemical shifts covered in 3b is with Δδ = 360.8 ppm considerably smaller in comparison to 1 but also to 3a (Table 1), (1: Δδ = 430.7;⁴⁰ 3a Δδ = 429.8 ppm). The former low-field signal of the SiTip group Si7 (1: 211.7 ppm, 2: 172.7 ppm) is upfield-shifted by no less than Δδ ≥ 122 ppm for 3a (δ = 50.0 ppm) and 3b (δ = 63.9 ppm, d, ⁴J_P,Si = 8.80 Hz). We suggest that this pronounced shielding of Si7 is a consequence of the perturbation of the cluster current, characteristic of siliconoids,^{29,30,36–40} by coordination of Ni to the Si₇ core. The basal unsubstituted vertex without nickel contact Si2 is strongly shielded in both 3a and 3b compared to the free ligand (3a: −341 ppm; 3b: −287.6 ppm (d, ³J_P,Si = 28.5 Hz); cf. 1: −219 ppm). The most downfield-shifted signals are now due to the amidinato-substituted silicon(II) centre Si1 coordinated to nickel (3a: 89.1 ppm; 3b: 73.6 ppm, cf. 1: 24.1 ppm (ref. 40)). The apical, substituent-free cluster vertex Si5 resonates at comparatively lower field (3a: −154.0 ppm; 3b: −87.3 (d, ³J_P,Si = 8.50 Hz) ppm; cf. 1: −211.6 ppm). The ³¹P{¹H} NMR spectrum of 3b shows a single signal at −26.4 ppm. The shifts are in good agreement with the GIAO calculated values for the optimized structures 3a and 3b (B3LYP-D3(BJ)/def2-TZVP level of theory^54–57,62) at TPSSh/def2-TZVP level of theory (Table 1).^62,63

Table 1 Experimental and calculated ²⁹Si NMR shifts (ppm) of siliconoid complexes 3a–c at ambient temperature in C₆D₆. Calculated shifts at the TPSSh-D3(BJ)/def2-TVP^56,57,62,63 level of theory in brackets

	3a (L = cod)	3b (L = PPh3)	3c (L = (CO)2)
Si1	−154.0 [−148.1]	−87.3 (d) [−68.5]	−160.5 [−160.9]
Si2	−340.7 [−354.9]	−287.6 (d) [−291.0]	−342.2 [−354.4]
Si3	−10.2 [−9.0]	0.4 [−6.5]	−4.0 [−7.7]
Si4	−13.3 [−18.7]	−41.4 [−45.2]	−10.1 [−11.6]
Si5	−83.1 [−81.1]	−60.0 (d) [−68.2]	−81.9 [−78.5]
Si6	39.0 [35.1]	39.7 [30.6]	44.6 [37.1]
Si7	50.0 [56.5]	63.9 [65.4]	70.9 [80.1]
Si8	89.1 [90.4]	73.5 [84.6]	96.5 [93.1]

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In case of the siliconoid/silylene hybrid ligand X with the Si₆ core, the addition of Ni(cod)₂ in a CO atmosphere had been shown to give rise to the incorporation of a completely cleaved CO moiety into the periphery of the cluster scaffold. This led to the formation of a formal Si=C double bond to the amidinato-substituted silicon(II) centre in XI (Scheme 1).³⁸ We therefore probed the behaviour of 3a,b towards carbon monoxide. Stirring of 3a,b under CO (1 bar) led to a colour change from blackberry red to blood red (Scheme 3). To avoid secondary decomposition reactions, the reaction was stopped immediately after disappearance of the last violet tint by removal of all volatiles under reduced pressure. The product 3c crystallizes from pentane in 44% yield. Due to the facile decomposition in reaction mixtures and under reduced pressure, the complete work-up needed to be carried out at slightly reduced temperatures (<10 °C) until isolation of 3c in the form of a crystalline solid, which is then stable even when redissolved.

Scheme 3 Reactivity of nickel siliconoid/silylene hybrids 3a,b towards CO ( = Si, R = 2,4,6-triisopropylphenyl).

The ¹³C NMR spectrum of isolated 3c shows two additional signals at low field, δ = 196.3 and 204.9 ppm, which are assigned to the carbonyl ligands based on their similarity with those of comparable Ni-carbonyl complexes (δ = 199.3 and 202.5 ppm for Roesky's amidinato silylene-Ni(CO)₃ I ²² and δ = 198.8 and 206.2 ppm for the mixed silylene–carbene coordinated Ni(CO)₂ complex reported by Driess⁶⁴). The ²⁹Si NMR signals of isolated 3c are similar to the Ni(cod) complex 3a: a lowfield signal at 96.6 ppm for the remote silicon atom and three highfield signals at −81.9, −160.4 and −342.0 ppm for the unsubstituted Si atoms. This supports a structurally and electronically similar geometry to that of 3a.

The longest wavelength absorption in the UV-Vis spectrum of 3a at λ_max,exp = 538 nm (ε = 8190 M⁻¹ cm⁻¹) is reproduced well by TD-DFT calculations on the optimised minimum structure of 3a. It is assigned to a combination of three transitions at λ_max,calc = 546 nm (HOMO → LUMO 52.6%), 520 nm (HOMO−1 → LUMO 72.6%) and 516 nm (HOMO → LUMO+1 65.4%) at the TPSSh/def2-TZVP level of theory^56,57,62,63 (for details see SI). The different coordination geometry of the central nickel atom in 3b is manifest in a strong red-shift of the experimental longest wavelength UV-Vis absorption of approximately Δλ = 56 nm. It is observed as a broad band at λ_max,exp = 596 nm (624 M⁻¹ cm⁻¹) tailing to about 700 nm. Notably, the extinction coefficient is one order of magnitude smaller than in case of 3a, which suggests considerable effect of the ligand exchange on the electronic structure (vide infra).

TD-DFT calculations at the TPSSh/def2-TZVP level of theory assign the observed bands to a weak transition at λ_max,calc = 740 nm (HOMO → LUMO 89.6%) and slightly stronger transitions at λ_max,calc = 572 nm (HOMO → LUMO+1 76.8%) and 555 nm (HOMO → LUMO+2 92.3%). A second absorption in the experimental UV-Vis spectrum of 3b at λ_max,exp = 519 nm (ε = 1127 M⁻¹ cm⁻¹) is assigned to the calculated transition at 472 nm for the (HOMO−4 → LUMO 31.0%). The colour change in the reaction to 3c to a lighter red becomes apparent in the blue-shifted absorption in the UV-Vis spectrum (λ_max,exp = 525 nm, ε = 9630 M⁻¹ cm⁻¹), compared to the precursors 3a and 3b (3a: λ_max,exp = 538 nm, ε = 8190 M⁻ cm⁻¹, 3b: λ_max,exp = 596 nm, ε = 624 M⁻¹ cm⁻¹), but in reasonable agreement with the calculated value at λ_max,calc = 506 nm, (HOMO → LUMO + 2 99.6%). The calculated transitions correspond to HOMO–LUMO gaps of ΔE = 3.00 eV (3c)/2.88 eV (3a)/2.39 eV (3b), which reproduces the experimentally observed trend (3c (2.37 eV) > 3a (2.31 eV) > 3b (2.08 eV)). Owing to the geometry of 3b, featuring the trigonal planar-coordinated Ni centre, DFT calculations suggest an amidinato-centred LUMO in contrast to 3a and 3c, where the LUMO is mostly localized at the unsubstituted Si vertices of the siliconoid ligand, resulting in a smaller HOMO–LUMO gap in 3b (Fig. 4). The lack of spatial coincidence of HOMO and LUMO results in the smaller extinction coefficient of 3b compared to 3a and 3c.

Fig. 4 Selected frontier orbitals of nickel complexes 3a–c at the B3LYP/def2-TZVP level of theory^54–57 (contour value = 0.05).

The molecular structure of 3c in the solid state was confirmed by X-ray diffraction analysis on single crystals grown from a pentane solution at −26 °C (Fig. 5). Just as 3b, the CO complex 3c crystallizes in the monoclinic space group I2/a. The central geometry around the nickel centre (Si1–Ni1–Si8 108.6(4)°/C1–Ni1–C2 111.1(2)°) is like in 3a approximately tetrahedral. The seesaw angle Si4–Si5–Si6 (173.6(5)°) is in good agreement with 3b and the free ligand.^39,40 Ni1–Si bond distances in 3c (Ni1–Si1 2.335(9) Å, Ni1–Si3 2.501(1) Å, Ni1–Si8 2.236(1) Å) are slightly elongated compared to 3a,b (3a Ni1–Si1 2.302(5) Å, Ni1–Si3 2.428(5) Å, Ni1–Si8 2.212(5) Å; 3b Ni1–Si1 2.216(8) Å, Ni1–Si3 2.378(8) Å, Ni1–Si8 2.183(8) Å), which can be attributed to the strong π-acceptor character of CO, leading to increased competition for metal-based backdonation and an associated weakening of the Ni–Si interaction.^22,34,64,65 The Ni1–C1/C2 (Ni1–C1 1.807(4) Å, Ni1–C2 1.799(4) Å) as well as the C–O (C1–O1 1.135(4) Å, C2–O2 1.130(5) Å) bond lengths are in the expected range.^22,34,64,65

Fig. 5 Molecular structure of Ni(CO)₂-siliconoid/silylene complex 3c in the solid state. Hydrogen atoms are omitted for clarity. Thermal ellipsoids at 50% probability. Selected bond lengths [Å] and angles [°]: Ni1–Si1 2.335(9), Ni1–Si3 2.501(1), Ni1–Si8 2.236(1), Ni1–C1 1.807(4), Ni1–C2 1.799(4), C1–O1 1.135(4), C2–O2 1.130(5), Si1–Si2 2.667(1), Si1–Si3 2.444(1), Si3–Si8 2.310(1); C1–Ni1–C2 111.1(2), Ni1–C1–O1 175.4(4), Ni1–C2–O2 177.3(4), Si1–Ni1–Si8 108.6(4), Si1–Si3–Ni1 56.3(3), Si8–Si3–Ni1 55.2(3), Si1–Si3–Si8 102.6(5), Si4–Si5–Si6 173.6(5).

The FT-IR spectrum of 3c exhibits significant CO-stretching bands at 2015 and 1976 cm⁻¹. The bands in the IR spectrum of 3c are more red-shifted in comparison with other reported Si–Ni(CO)_n silylene-ligands, comparable to the NHSi–NHC–Ni(CO)₂ compound by Driess et al. (Ṽ_CO = 1952 and 1887 cm⁻¹),⁶⁴ the bis(NHSi)–Ni(CO)₂ by Radius (Ṽ_CO = 2039 and 2006 cm⁻¹)⁶⁵ and the dimeric Si₉ cluster by Korber et al. IX (Ṽ_CO 1999 and 1937 cm⁻¹).³⁴ This observation implies a similarly strong σ-donating ability of the siliconoid/silylene ligand 1 as the above mentioned low-valent silicon species, which is due to the enhanced Ni → CO π-backdonation of the electron-rich ligands.

In view of the relevance of hydrosilylation, especially in industrial scale,^66,67 precious transition metals such as platinum remain the gold standard.⁶⁸ The application of the lighter congeners, like nickel, has attracted increasing attention in recent years owing to the higher abundancy,^69–71 however most complexes only show moderate reactivity under harsh conditions.^72,73 In a preliminary test, we added a catalytic quantity of 3a to PhSiH₃ to test its activity towards primary silanes. Vigorous gas evolution indicated the formation of H₂ and indeed oligo- and polysilanes are detected in the reaction mixture as major products. Apparently dehydro-coupling is the favourable reaction pathway for sterically undemanding silanes. In contrast, the tertiary silane Et₃SiH did not show any reactivity at all towards styrene in the presence of 3a. As a compromise in terms of steric congestion, we chose the secondary silane Ph₂SiH₂, and indeed, the addition of catalytic quantities of 3a to Ph₂SiH₂ led to the desired conversion of alkene substrates with minor formation of disilanes and higher oligomers as by-products and was therefore selected for more detailed investigations regarding the scope of alkene substrates. Reaction of cyclohexene with Ph₂SiH₂ did not show any sign for the conversion to the hydrosilylation product even after 72 h, instead exhibiting signals assigned to the formation of tetraphenyldisilane and the according hydrogenation product cyclohexane. We concluded that the steric hindrance is limiting the accessibility of the catalyst and hence, only the de-hydrocoupling side-reaction followed by hydrogenation is observed in case of internal alkenes (see NMR spectra in the SI), and we thus turned our attention to terminal alkenes.

To identify optimal reaction conditions, we performed a more detailed pre-screening for the hydrosilylation of vinyltrimethylsilane with Ph₂SiH₂ in the presence of variable catalyst loadings of 3a–c (Table 2). With 0.5 mol% of catalyst 3a in C₆D₆ at ambient temperatures, we observed full conversion of vinyltrimethylsilane after two hours under formation of the anti-Markovnikov product A in 46% spectroscopic yield (entry 2). Lowering the catalyst loading to 0.05 mol%, required a longer reaction time of 8 h (A 45%, entry 3), yet the selectivity for A over by-products such as oligosilanes and presumably the twofold-substituted silylation product remains virtually unaffected. There are no indications for the formation of the Markovnikov product M in these cases. A mercury drop test revealed a slower reaction to A with an overall lower spectroscopic yield, however exhibiting a higher ratio of A compared to the twofold-substituted silylation product (23 : 6) than in the reaction without the presence of Hg (9 : 5).(entry 4), indicating that the catalyst is partially decomposed to nickel particles in the course of the reaction, which are known to be highly active but unselective in heterogenous catalysis.^74,75 A blank run without any catalyst gave no conversion at all after more than 48 h (entry 1). Employing phosphine and carbonyl derivatives 3b and 3c as catalysts resulted in incomplete conversions at 0.5 mol% and a negligible conversion at 0.05% (entries 5 to 7). Their lack of catalytic activity can be attributed to the tighter coordination of the phosphine and carbonyl ligands in 3b and 3c compared to the cod ligand in 3a, blocking the binding site for the alkene and silane substrates more effectively. Intended as another control experiment with regard to the precatalytic activity, we probed the catalysis with 0.5 mol% of Ni(cod)₂ under the same conditions as for 3a. Surprisingly, an even faster conversion of vinyltrimethylsilane (50 minutes, entry 9) was observed. With reduced catalyst loading 0.05 mol% of Ni(cod)₂ full conversion is achieved after 5.5 h, with the anti-Markovnikov species A as the major product (42.5%, entry 10). Although there are many examples reported for the hydrosilylation with Ni(cod)₂ in combination with a stabilizing ligand, e.g. phosphines or NHCs,^76,77 there are only very few reports on catalysis using Ni(cod)₂ itself on substrates such as styrene,⁷⁸ α-(trifluormethyl)styrene⁷⁹ and imines.⁸⁰ To the best of our knowledge, the Ni(cod)₂-catalysed hydrosilylation of unactivated terminal alkenes has never been reported. While 3a is thus somewhat less active than Ni(cod)₂ itself, it can still be considered a competent catalyst for the anti-Markovnikov hydrosilylation of vinyltrimethylsilane.

Table 2 Screening of the reaction conditions for the hydrosilylation catalysis of vinyltrimethylsilane (mesitylene as internal standard)

Entry	Cat.	Cat. loading [mol%]	Conversion [%]	Spectr. yield A [%]	Time [h]
a 0.6 mmol substrate, 1.1 eq. diphenylsilane.b In the presence of Hg.
1	None	0	0	0	>48
2	3a	0.5	>99	46	2^a
3	3a	0.05	>99	45	8
4	3a	0.05	>99	23	72^b
5	3b	0.5	29	24	48^a
6	3b	0.05	8	0	48
7	3c	0.5	8	0	48^a
8	3c	0.05	4	0	48
9	Ni(cod)2	0.5	>99	43	<1
10	Ni(cod)2	0.05	>99	42	5.5
11	Ni(cod)2	0.05	>99	31	5.5^b

---

After the preliminary optimization of the reaction parameters with the vinyltrimethylsilane benchmark, we sought to expand the substrate scope to other terminal alkenes. Additional electron-rich examples, namely 1-hexene, vinyl- and allyl cyclohexane, and allyltrimethylsilane were investigated but also comparatively electron-poor styrene and allylbenzene included for comparison. Reactions were carried out at ambient temperature in C₆D₆ with the catalyst load kept constant at 0.05 mol% of 3a (Table 3). As above, control experiments employing Ni(cod)₂ as the catalyst were performed under the same conditions for comparison (see SI, Table S1). While the anti-Markovnikov product A is obtained without any sign for the formation of the Markovnikov product M in the electron rich cases (entries 1 to 5), electron-poor styrene and allylbenzene unexpectedly gave rise to the branched product in an almost 1 : 1 ratio (entries 6 and 7).^68,81–83 Note that Markovnikov hydrosilylation – formerly considered an undesired outcome – has been moving into focus in recent years.⁶⁷

Table 3 Catalytic hydrosilylation of terminal olefins with Ph₂SiH₂ using nickel complex 3a as a homogeneous catalyst

Entry	Substrate	Time [h]	Conversion [%]	Product	Spectr. yield [%]	TOF [h^−1]
1		0.25	>99	A	39	8000
2A		1	30	A	24	640
2B		4	79	A	40	400
2C		8	>99	A	45	250
3A		8	27	A	20	67
3B		192	83	A	45	9
3C		408	>99	A	48	5
4A		0.08	60	A	33	14 300
4B		72	97	A	37	27
4C		168	>99	A	59	12
5A		0.5	34	A	13	1350
5B		192	85	A	38	9
5C		408	>99	A	40	5
6A		2	33	A : M	9 : 16	530
6B		24	90	A : M	23 : 29	75
6C		54	>99	A : M	27 : 31	37
7A		0.5	30	A : M	18 : 13	1190
7B		24	88	A : M	33 : 32	73
7C		54	>99	A : M	38 : 34	37

---

Concerning the hydrosilylation with catalyst 3a, a general trend becomes apparent. While all cases investigated react to full conversion resulting in turn-over numbers (TONs) of approximately 2000, higher turnover frequencies (TOFs) up to 8000 h⁻¹ are observed for the open chain substrates such as 1-hexene (entry 1) and the vinyl-substituted alkenes (entries 2, 4 and 6). Spectroscopic yields of A up to 59% (entries 1, 2C and 4C) are obtained without any evidence for the formation of the Markovnikov product (M). On the other hand, the allyl-substituted substrates (entries 3 and 5) show rather slow conversions with TOFs down to 5 h⁻¹ and spectroscopic yields of A around 40–48% (entry 3C and 5C). In contrast, the electron-poor substrates styrene and allylbenzene (entry 6 and 7) produce an almost equimolar spectroscopic ratio of M to A with TOFs of around 37 h⁻¹ at full conversion. Presumably due to catalyst degradation, the TOFs are decreasing during the progress of the reactions (entries 2–7 A vs. B vs. C). Moreover, the formation of unidentifiable/volatile byproducts reduces the (spectroscopic) yield of the A/M hydrosilylation products.

To put the obtained results into perspective: Tilley et al. reported a dimeric, halogenated Ni catalyst reaching TONs of up to 970 (0.1 mol%, 17 h at 23 °C) with a selectivity ratio of 3 : 1 in favour of terminal hydrosilylation,⁸⁴ while a nickel pincer complex reported by the group of Hu on the other hand, gives even higher TONs and TOFs (up to 100 000 h⁻¹ at 0.01 mol%/83 000 h⁻¹ at 0.025 mol%).⁸⁵ Employing Ni(cod)₂ as the catalyst results in instant conversion of 1-hexene reiterating the result with vinyltrimethylsilane as a substrate (see SI, Table S1). Allylcyclohexane and allyltrimethylsilane were not fully converted in the presence of Ni(cod)₂ even after 21 days likely because of catalyst degradation (see SI, Table S1). In contrast, 3a resulted in full conversion of both substrates, albeit slowly (TOF 5 h⁻¹ each, entries 3C and 5C, Table 3).

Conclusions

In summary, we reported the preparation and full characterization of a terminal Fe(CO)₄ complex of a Si₇/silylene hybrid ligand. Unlike in the case of the corresponding Si₆/silylene system, the lower steric demand of the organic residues per silicon cluster vertex also allowed for the preparation of a series of the first stable nickel complexes of a siliconoid/silylene with 1,5-cyclooctadiene, triphenylphosphine and carbon monoxide acting as auxiliary ligands. The coordination environment of the PPh₃ derivative stands out with a trigonal planar nickel centre while the cod and CO complexes show distorted tetrahedral nickel coordination spheres. The cod-coordinated nickel complex turned out to be active in the hydrosilylation of terminal alkenes with an interesting preference for anti-Markovnikov regioselectivity in case of electron-richer substrates and unselective formation of both regiomers for electron-poor alkenes.

Author contributions

L. Giarrana performed the synthetic work and data analysis with partial support by D. Welterlich; L. Giarrana. and D. Scheschkewitz designed the study. D. Scheschkewitz acquired the funding; B. Morgenstern performed the X-ray diffraction studies. M. Zimmer performed the solid state and variable temperature NMR measurements. L. Giarrana performed the DFT calculations. L. Giarrana wrote the initial manuscript draft. L. Giarrana and D. Scheschkewitz reviewed and edited the manuscript.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

All data associated with this manuscript are available in the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6qi00025h.

CCDC 2504459 (2), 2504461 (3a), 2504467 (3b) and 2504470 (3c) contain the supplementary crystallographic data for this paper.^86a–d

Acknowledgements

Funding by the Deutsche Forschungsgemeinschaft (DFG SCHE 906/4-4) is gratefully acknowledged. Instrumentation and technical assistance for this work were provided by the Service Center X-ray Diffraction, with financial support from Saarland University and German Science Foundation (project number INST 256/506-1). The authors thank Prof. Stella Stopkowicz for access to her computational cluster.

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