A Ni^II complex supported by an iminophosphorane ONP ligand: synthesis and catalytic C=C and C=O bond hydrosilylation

Abstract

An original ONP iminophosphorane ligand was synthesised and coordinated to [NiX₂(DME)] (X = Cl, Br). The corresponding complexes (2X, X = Cl, Br) were isolated and characterised among others by multinuclear NMR spectroscopy and X-ray crystallography. The collected data suggest that different geometries coexist in solution at room temperature. 2Cl proved to be an efficient hydrosilylation catalyst able to perform at a loading of 1 mol% in the presence of one equivalent of SiH₂Ph₂ and 1 mol% of ^tBuOK, with the reduction of C=C and C=O bonds in high yield in 1 h for most substrates. Moreover, the selective conversion of the C=O bond to a silylether linkage was observed for nine enones. Therefore, 2Cl presents a rather unique catalytic behaviour compared to previously described Ni catalysts. Both experimental and theoretical investigations regarding the mechanism suggest the involvement of a Ni–H complex. The computed mechanism presents a highest-lying transition state at only 19.0 kcal mol⁻¹ and shows that the reaction is driven by favorable thermodynamics.

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Introduction

Nitrogen-based ligands encompass a wide diversity of structures in which the coordinating nitrogen atom can be sp³- (amines or amides), sp²- (imines or N-heterocycles), or sp-hybridised (cyanide). They have found applications in various areas from medicinal chemistry to materials science.¹ In this large variety, iminophosphoranes or phosphinimines (PR₃NR′) represent a rather discrete class of ligands.² In such derivatives, the nitrogen atom bears two lone pairs, which are stabilised via hyperconjugation by the vicinal electropositive phosphorus atom. As a result, it can be described by two different Lewis structures: the ylene form (Fig. 1, left), largely used in the literature, which reflects the strong interaction between the P and N atoms, and the ylidic form (Fig. 1, right), which highlights the strong electron density at the N atom explaining its σ and π electron donating ability. The latter can be modulated via the nitrogen substituent.³

Fig. 1 Lewis structures of iminophosphoranes.

Even if they were first reported more than a century ago,⁴ iminophosphoranes remain underexploited in organometallic catalysis.^2b,3b,5

Nevertheless, they may be of particular interest in catalytic reduction processes due to the high hydridic character of the involved M–H species.⁶ Thirty years ago, Cavell and co-workers⁷ reported the catalytic hydrogenation of olefins with Rh^I complexes supported by mixed phosphine-iminophosphorane ligands. Rh^I and Ir^I catalysts involving other iminophosphorane-based ligands and even chiral ones have been described for this reaction.⁸ Moreover, ruthenium and rhodium iminophosphorane complexes have been shown to catalyse the transfer hydrogenation of ketones,⁹ in which the solvent (an alcohol) serves as an alternative H-donor. However, there are still very few examples of iminophosphorane catalysts involving Earth-abundant metals for these reactions. Fe^II iminophosphorane complexes were used successfully for the transfer hydrogenation of ketones 15 years ago¹⁰ and more recently for catalytic hydrodefluorination¹¹ and acetophenone hydrosilylation.¹² To the best of our knowledge, the only examples of iminophosphorane Earth-abundant metal complexes used for the catalytic hydrosilylation of olefins are a series of NNN cobalt complexes, which are moderately active.¹³ Surprisingly, there are no examples of iminophosphorane Ni catalysts for this transformation, while Ni^II complexes were established as powerful hydrosilylation catalysts.¹⁴ In addition, the catalytic ability of iminophosphorane nickel(II) complexes was demonstrated in cross-couplings,¹⁵ ethylene dimerisation reactions,¹⁶ and, more recently, in reductive couplings^3b (Scheme 1).

Scheme 1 Catalytic applications of iminophosphorane-supported Ni^II complexes.

This prompted us to investigate whether polydentate iminophosphorane Ni^II complexes could serve as hydrosilylation catalysts. We decided to prepare a tridentate L₂X ligand that should afford stable and diamagnetic Ni^II complexes. Given our past experience with phosphine-iminophosphorane^16c and iminophosphorane-phenoxy Ni catalysts,^16d we focused on an ONP ligand associating these three groups. In this article, we present the synthesis of this original ONP ligand, its coordination to Ni^II, and the catalytic performance of this ONP–Ni complex in the hydrosilylation of olefins, enones, and ketones. DFT calculations regarding the mechanism of the catalytic hydrosilylation are also presented.

Results and discussion

Synthesis and characterisation of the ligand and Ni complexes

To synthesise the targeted ONP ligand combining phosphine, iminophosphorane and phenoxy groups, we chose to use the bis(tert-butyl)-substituted phenoxyphosphine (Scheme 2), which we previously employed to prepare phosphasalen ligands.¹⁷ The tert-butyl groups indeed improved the solubility of the complexes. In order to produce the aminophosphonium by the modified Kirsanov procedure, we had to synthesise 2-diphenylphosphinoaniline. To do so, we relied on the 3-step procedure described by Stelzer and coworkers involving the regioselective lithiation of a Boc-protected aniline (Scheme S1),¹⁸ but we had to modify the final deprotection step (see the SI for more information). The aniline obtained after a basic workup is a viscous oil, which makes it difficult to weigh it accurately; therefore, we chose to isolate it as its ammonium salt. The latter was subsequently engaged in the modified Kirsanov reaction, where the phosphine was first oxidised with one equivalent of bromine in CH₂Cl₂, followed by the addition of one equivalent of DABCO (diazabicycloctane), which was used as a sacrificial base to generate in situ the 2-diphenylphosphinoaniline and then trap the equivalent of HBr eliminated while forming the P–NH bond.

Scheme 2 Synthesis of proligand 1.

The proligand 1 was obtained as a mixture of bromide and chloride salts. Exchanging all the bromide anions for chlorides by stirring with excess chloride salts as well as by washing the dichloromethane solution of 1 with saturated aqueous NaCl solution failed to completely exchange the anions. However, as the coordination step with Ni halide salts also produces one salt equivalent, we decided to remove the salt after coordination. The nature of the anions in 1 could be determined by mass spectrometry, and their proportion was assessed by X-ray diffraction.

The X-ray structure of 1 is shown Fig. 2. It presents a zigzag structure. P1 and O1 belong to the plane defined by the phenoxy ring, and N1 and P2 to that of the disubstituted phenyl rings. The angle between these planes is 67.9°. The P1–N1 bond length of 1.651(3) Å is typical of aminophosphonium derivatives.¹⁹ The ³¹P resonances were observed at −15.6 and 38.0 ppm in CDCl₃ for the phosphine and aminophosphonium groups, respectively. The ligand was generated in situ before coordination (Scheme 3). To the best of our knowledge, this is the first example of an iminophosphorane ONP ligand with a central PN functionality. The addition of ⁿBuLi (2 equiv.) to a yellow solution of 1 in THF induced a colour change to orange. The completion of the reaction was ascertained by in situ³¹P{¹H} NMR spectroscopy, showing two singlets at 14.6 and −16.5 ppm. After the addition of the nickel precursor, either [NiCl₂(DME)] or [NiBr₂(DME)], the solution darkened. The solvent was evaporated, and the residue was washed with pentane and diethyl ether. The obtained red solids were then extracted into toluene to remove the lithium salts and afford complexes 2Cl and 2Br in 76% and 68% yields, respectively, after drying.

Fig. 2 Ortep plot of 1 with thermal ellipsoids (drawn at the 50% probability level). The anion, the H atoms and one CHCl₃ molecule are omitted for clarity. Selected bond lengths [Å] and angles [°]: C7–O1 1.382(4), C8–C7 1.398(5), C8–P1 1.798(4), P1–N1 1.651(3), N1–C1 1.453(4), C1–C2 1.398(5), C2–P2 1.835(4), C1–C2–P2 118.8(3), C2–C1–N1 119.2(3), N1–P1–C8 116.12(16).

Scheme 3 Synthesis of nickel complexes 2Cl and 2Br.

These complexes were characterised by multinuclear NMR spectroscopy, X-ray diffraction, and HR-mass spectrometry.

Surprisingly, for both complexes, only one ³¹P resonance was observed at 27 ppm at room temperature, while two inequivalent phosphorus nuclei are present in these molecules. To get further information, we recorded VT-NMR spectra in CD₂Cl₂ from −80 to 25 °C. At lower temperature, two resonances were observed at 27.4 and 22.9 ppm for 2Cl. These values are very close to those reported by Wang and co-workers²⁰ for a diamagnetic Ni^II complex supported by a tridentate NNP L₂X ligand featuring an iminophosphorane, an amide, and a diphenylphosphine (28.7 and 20.0 ppm). This points therefore towards the coordination of both phosphorus groups (as confirmed by X-ray analysis vide infra). However, the observed behaviour does not correspond to a classical dynamic phenomenon where two different resonances corresponding to the same nucleus in different environments do change and finally merge upon warming. We did not observe any changes in the chemical shifts with the temperature; only the appearance of a second peak was noted upon lowering the temperature. We also expanded the window of acquisition (−2000 to 2000 ppm) in order to track a shifted paramagnetic resonance at room temperature, but none was observed. We were interested in determining to which P-group this behaviour corresponds to and attempted to assign the ³¹P resonances. A ³¹P/¹H 2D experiment was performed: the ³¹P resonance observed at room temperature correlated with one proton of the N-substituted phenyl and not with those of the phenoxy ring. This allowed assigning this resonance to the P^III nucleus and the shielded one to the iminophosphorane (Fig. S8 and S13).† The coexistence of different geometries of these complexes in solution, among which some could correspond to paramagnetic compounds, may be an explanation for the behaviour observed. Indeed, the magnetic moments of 2Cl and 2Br were measured in solution, thanks to the Evans method.²¹ Values of 1.52 and 1.12 μ_B for 2Cl and 2Br, respectively, were obtained at room temperature. They are not unprecedented for tetracoordinated Ni^II complexes²² and suggest the coexistence of distorted tetrahedral and square-planar geometries in solution at room temperature, with the former hindering the observation of the P^V resonance. Given the square planar geometry observed in the solid state (vide infra), it would be reasonable to propose that the temperature influences the geometry of the complex, with the square planar geometry being dominant at low temperature and the tetrahedral one at higher temperatures. If so, the paramagnetic character would increase upon heating inducing the loss of ³¹P resonances and an increase of the magnetic moment. This assumption was tested with 2Cl. Its ³¹P NMR spectrum was recorded at temperatures between 25 and 60 °C in CDCl₃ (Fig. S7), and the disappearance of the ³¹P resonance was observed around 50 °C. Cooling back to room temperature restored the initial spectrum, suggesting that the phenomenon is reversible. Moreover, the magnetic moment of 2Cl measured at −80 °C is almost null, while it reaches 2.89 μ_B at 50 °C. These data agree with a change of the geometry of the complex with the temperature. At low temperature, it is square planar (as in the solid state vide infra) and acquires a more pronounced tetrahedral character at temperatures above 50 °C. This is reminiscent of the paramagnetic temperature-dependent behaviour of tetraphosphineferrocenyl nickel complexes reported by Hierso and coworkers for which the phosphine resonance was only observed at low temperature.²³ This was also explained by a change in the geometry of the metal.

Single crystals could be grown for both 2Cl and 2Br. The structure obtained for 2Cl is shown in Fig. 3. The structure of 2Br, which was of lower quality, is presented in Fig. S1. Both structures are very similar with Ni in a square planar geometry. In 2Cl, the nickel atom lies at 0.017 Å from the plane defined by Cl1, O1, and N1. The PN bond measures 1.634(7) Å, which is slightly longer than that reported for the previously mentioned square-planar NNP-supported nickel chloride complex (1.591(4) Å).²⁰ Other coordination bond lengths (Ni–Cl, Ni–N, Ni–P) are very similar to those in the latter.

Fig. 3 Ortep plot of 2Cl with thermal ellipsoids (drawn at the 50% probability level). The H atoms and one Et₂O molecule were omitted for clarity. Selected bond lengths [Å] and angles [°]: Ni1–O1 1.876(6), Ni1–N1 1.935(8), Ni1–P2 2.114(3), Ni1–Cl1 2.189(3), P1–N1 1.634(7), O1–Ni1–N1 96.8(3), N1–Ni1–P1 87.0(2), P2–Ni1–Cl1 86.12(10), O1–Ni1–Cl1 90.03(19).

Catalysis

Nickel complexes are commonly employed to catalyse the hydrosilylation of alkenes; nevertheless, examples of iminophosphorane-supported catalysts for this reaction are limited to a poorly active cobalt catalyst family.¹³ With the idea that the reactivity of the M–H active species may be boosted by the electron-donating ability of the iminophosphorane, we evaluated the catalytic behaviour of 2Cl in the hydrosilylation of 1-octene (Table 1). The reaction was first conducted in diethyl ether at room temperature with one equivalent of diphenylsilane. The conversion and yield were determined by ¹H NMR spectroscopy using trimethoxybenzene as an internal reference. Almost no conversion was observed with 1 mol% catalyst (entry 1), but the use of ^tBuOK (1 mol%) as an additive had a very positive impact (entry 2). The linear silane formed exclusively in 97% NMR yield. We confirmed that no reaction occurred in the absence of the nickel complex, while under the same conditions, [NiCl₂(DME)] afforded only 6% conversion (entries 3 and 4), underlying the importance of the ligand. When 2Br was used in place of 2Cl, the reaction was slightly less efficient (91% conversion, entry 5). Therefore, we continued the optimisation with 2Cl as the catalyst and tried to decrease the quantity used; nevertheless, conversion and yield decreased with the catalytic amount (entries 6–8). Thus, a 1 mol% catalyst loading was used for the rest of the study. We also evaluated other silicon derivatives and employed triethylsilane, and the cheaper triethoxysilane and polymethylhydrosiloxane (PMHS) (Table 1, entries 9–11). The conversion was a little lower, and the NMR analysis was not so convenient; therefore, we continued with diphenylsilane. We also tested sodium methoxide and a hydride donor (KHBEt₃) as additives. The former led to low conversion and yield, while the latter gave comparable results (entries 12 and 13). We decided to continue to use ^tBuOK and changed the solvent and the concentration. Generally, a lower concentration decreases the efficiency of the reaction (entry 14 vs. 2 and 15 vs. 16). Moreover, substituting diethyl ether with acetonitrile was detrimental to the conversion, while using THF gave excellent results at both 2 M and 1 M concentrations. It therefore offers an alternative, in particular when higher temperatures are required for the reaction. Therefore, the conditions of entry 2, which with low additive and catalyst loadings led to an excellent yield in only 1 h, were further used to examine the scope of the reaction (Scheme 4).

Scheme 4 Scope of the catalytic hydrosilylation of olefins with 2Cl. NMR yields using trimethoxybenzene as the internal reference are given with the isolated yields specified in brackets. ^a Reaction conducted with the olefin (1 mmol) in the presence of 2Cl (0.01 mmol, 1 mol%), trimethoxybenzene (0.07 mmol) as reference, SiH₂Ph₂ (1 mmol), and ^tBuOK (0.01 mmol, 1 mol%) for 1 h at room temperature in Et₂O (2M) (method A). ^b Same conditions as method A but at 60 °C in THF (2M) (method B). ^c Reaction time extended to 48 h. ^d The product could not be isolated. ^e Reaction conducted for 24 h at 60 °C in THF with 5 mol% of 2Cl. ^f NMR and isolated yields correspond to the mixture of linear and branched products (l/b). ^g Reaction conducted with 2-octene.

Table 1 Optimisation of the hydrosilylation of 1-octene^a

	Catalyst (1 mol%)	Si–H	Solv. (x M)	Additive (1 mol%)	Conv.^b (%)	Yield^c (%)
a Reactions were conducted with 1-octene (1 mmol) and SiH2Ph2 (1 mmol) in diethyl ether at room temperature with 1 mol% catalyst and 1 mol% additive. b Determined by NMR using the integration of the singlet at 4.92 ppm for SiH2Ph2 relative to the CH aromatic resonances of the reference at 6.10 ppm. c Determined by NMR using the triplet resonance at 4.86 ppm (C8H15SiHPh2). d 0.8 mol%. e 0.5 mol%. f 0.2 mol%. g Yield could not be determined because of signal overlap from the reagent and the product.
1	2Cl	SiH2Ph2	Et2O (2)	—	1	1
2	2Cl	SiH 2 Ph 2	Et 2 O (2)	^t BuOK	100	97
3	—	SiH2Ph2	Et2O (2)	—	0	0
4	[NiCl2(DME)]	SiH2Ph2	Et2O (2)	^tBuOK	6	6
5	2Br	SiH2Ph2	Et2O (2)	^tBuOK	91	91
6	2Cl	SiH2Ph2	Et2O (2)	^tBuOK^d	66	63
7	2Cl	SiH2Ph2	Et2O (2)	^tBuOK^e	45	44
8	2Cl	SiH2Ph2	Et2O (2)	^tBuOK^f	17	16
9	2Cl	Et3SiH	Et2O (2)	^tBuOK	79
10	2Cl	HSi(OEt)3	Et2O (2)	^tBuOK	88
11	2Cl	PMHS	Et2O (2)	^tBuOK	100
12	2Cl	SiH2Ph2	Et2O (2)	NaOMe	45	20
13	2Cl	SiH2Ph2	Et2O (2)	KHBEt3	97	97
14	2Cl	SiH2Ph2	Et2O (1)	^tBuOK	94	94
15	2Cl	SiH2Ph2	MeCN (1)	^tBuOK	72	69
16	2Cl	SiH2Ph2	MeCN (2)	^tBuOK	83	83
17	2Cl	SiH2Ph2	THF (1)	^tBuOK	99	98
18	2Cl	SiH2Ph2	THF (2)	^tBuOK	99	99

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Different aliphatic terminal olefins formed the linear silane (4a–c) with high efficiencies after 1 h. In the case of modest yields at room temperature (method A, Scheme S2), a large improvement was observed when heating at 60 °C for 1 h using THF as the solvent (method B). Heteroatoms are relatively well tolerated, as alkylsilanes 4d–f were obtained in average to high yields. Olefins incorporating a cyclohexene or a phenyl group were efficiently transformed, but again some required heating (3g–3l). Notably, electron-poor or electron-rich groups were equally well tolerated (4k and 4l). In contrast, for styrene derivatives (3m–3q), the efficiency and the selectivity of the reaction depend on the nature of the substituent on the aromatic ring. The yields are high from 3m and 3n, leading mainly to the linear isomer. From 3o, bearing a more electron-donating methoxy substituent, the yield remains high (94%), but both isomers were formed in similar proportions. From 3p, with a chloro-substituent, the change in regioselectivity is amplified, with the branched isomer 4p′ being twice as abundant as 4p. In the presence of a more electron-withdrawing group (3q), the yield drops, but the branched isomer 4q′ becomes largely dominant. Obtaining mixtures of regioisomers from styrene derivatives is not unprecedented in Ni-catalysed hydrosilylation, and the preference for the linear²⁴ or branched²⁵ product depends on the catalyst. In reports describing the transformation of variously substituted styrenes,^25c,d,26 no such change in regioselectivity was observed with the substituent. Note that α-olefins incorporating a hydroxy group or an aromatic allyl ether were not converted into the silylether. For some of them, competitive experiments showed that they inhibited the catalysis (Schemes S3 and S4). We then turned our attention to disubstituted olefins. They were in general poorly reactive even under forcing conditions (Scheme S3). However, 2-octene was converted to 4a in an excellent 91% yield when using 5 mol% catalyst within one day at 60 °C. Under the same conditions, α-methylstyrene led to 4r in 93% NMR yield, while 4s could be isolated in 36% yield.

Moreover, strained internal olefins 3t and 3u led to good yields under forcing conditions (60 °C with 5 mol% catalyst for, respectively, 24 and 48 h). 2Cl was efficient at catalysing the hydrosilylation of a large range of olefins. In the literature, Ni hydrosilylation catalysts for olefin substrates are reported more often than for carbonyl ones.^14b,c In addition, Hu and coworkers demonstrated the selective conversion of the C=C double bond within keto- and formyl-containing olefins.²⁷ This stimulated the investigation of the catalytic behaviour of complex 2Cl with such substrates (Scheme 5).

Scheme 5 Scope of the catalytic hydrosilylation of enones/enals with 2Cl. NMR yields using trimethoxybenzene as an internal reference are given with the isolated yields specified in brackets. ^a Reaction conducted with the keto-olefin (1 mmol) in the presence of 2Cl (0.01 mmol, 1 mol%), trimethoxybenzene (0.07 mmol) as reference, SiH₂Ph₂ (1 mmol), and ^tBuOK (0.01 mmol, 1 mol%) within 1 h at room temperature in Et₂O (2 M) (method A). ^b Same conditions but within 1 h at 60 °C in THF (2 M) (method B). ^c No isolated yield could be determined.

Surprisingly, 2Cl catalysed the selective hydrosilylation of the keto group without touching the C=C bond. 4-Hexen-3-one (5a), when reacted under the previously described conditions (1 equivalent of diphenylsilane, 1 mol% of 2Cl as a catalyst and ^tBuOK as an additive in diethyl ether at room temperature), was fully converted within 1 h to vinylsilylether 6′a, which was isolated in 93% yield after column chromatography. The latter could result from the hydrosilylation of C=O followed by an isomerisation of the double bond or from a 1,4-addition. Similarly, 6′b was isolated in 87% yield when reacting 2-cyclohexen-1-one (5b). However, benzylideneacetone (5c) was converted to 6c, in which the position of the double bond had not changed. 6c was formed with modest conversion in diethyl ether at room temperature (42%) within 1 h (Scheme S5), while the conversion doubled in THF at 60 °C. This result contrasts with the report from Sortais and Ritleng,²⁸ showing that the same substrate led to a mixture of compounds resulting from C=O reduction and simultaneous C=O and C=C reduction upon catalysing the hydrosilylation reaction with an N-heterocyclic carbene (NHC)-supported nickel complex. The formation of mixtures of compounds was also reported by Rodríguez when reacting enones.²⁹ However, it remains unclear why the reaction of 5c occurred without a change in the double bond location. As stated, the reaction temperature influences only the efficiency of the transformation, not the nature of the formed product. Furthermore, 5-hexen-2-one 5d reacted at room temperature, and trans-2-hexenal (5e) at 60 °C, both without any changes in the double bond position. This phenomenon has only been observed for α,β-unsaturated ketones 5a and 5b. The hypothesis of a 1,4-hydrosilylation may be contradicted by the inertness of cyclohexene (Scheme S3) under the same reaction conditions. However, the presence of the keto group β to the C=C bond in 5b may polarise it and favour the reaction. To test this hypothesis, the hydrosilylation of crotonaldehyde was carried out at room temperature. A full conversion was observed, but a mixture of compounds was obtained as witnessed by ¹H NMR spectroscopy (Fig. S16). Indeed, two doublet resonances corresponding to aldehyde protons were identified. These products could result from a non-regioselective double bond hydrosilylation. 1,4-Hydrosilylation would lead to an ethyl-substituted vinylether for which a triplet and a quadruplet should be observed. A triplet may be seen at 0.93 ppm (J_H,H = 7.5 Hz), but it is superimposed with other resonances, and the corresponding quadruplet could not be found. This experiment showed that the reaction outcome from α,β-unsaturated ketones highly depends on the substrate structure. Because C=C bond hydrosilylation was observed for crotonaldehyde, we hypothesised that 6′a and 6′b result from a 1,4-hydrosilylation. At 60 °C in THF, enones 5f and 5g afforded the corresponding silylethers, which were isolated in high yields. Again, no modification of the double bond was observed, which may be explained by the larger steric hindrance at the γ carbon, precluding a 1,4-functionalisation. Therefore, the selectivity of the reaction may arise from a subtle balance between electronic and steric effects in the α,β-unsaturated carbonyls tested. Unconjugated ketones 5h–5i were also efficiently and selectively transformed into the corresponding olefinic silylethers 6h–i.

To the best of our knowledge, 2Cl represents a unique example of a nickel-based catalyst realising selectively and in high yield the hydrosilylation of a C=O bond in the presence of a C=C bond. It nicely complements the pincer nickel complex reported by Hu.²⁷ Surprisingly, when attempting to convert both the C=C and C=O groups by doubling the amount of silane used, no further conversion of 6d was observed. Nevertheless, the catalyst remains active since adding one equivalent of silane and one equivalent of 1-octene to the reaction mixture containing 6d led to efficient conversion of the latter (Scheme S6).

The catalytic ability of 2c towards carbonyl substrates was further investigated. We tested the previously established conditions for the hydrosilylation of ketones and were pleased to observe efficient conversion of a variety of substrates (Scheme 6). Generally, the corresponding alcohols were isolated after methanolysis. As for the olefin and enone substrates, the reaction was first attempted at room temperature for 1 h (method A), and in cases of moderate conversion, it was performed in THF for 1 h at 60 °C (method B). Acetophenone and most para-substituted acetophenones (7b,c,f) reacted at room temperature, while p-cyanoacetophenone 7g required heating for high conversion (37% at room temperature, see Scheme S7). This may be due to detrimental interactions of the CN group with the metal. Electron-donating or -withdrawing groups are equally well tolerated (8cvs.8f), but the position of the substituent impacted the yield (8c,d,e). The conversion of o-methoxyacetophenone was lower at 25 °C (24%, Scheme S7) compared to the other isomers, showing the influence of the steric hindrance or the coordinating ability of the vicinal O atom. Substrates exhibiting larger alkyl substituents on the keto group were also nicely converted at 60 °C, with isolated yields above 85% (8h–i). 8j bearing a long alkyl chain was efficiently formed at room temperature. 3-Methyl-1-phenyl-2-butanone (7k), exhibiting a methylene group between the phenyl and the keto groups, was less reactive than 7h, giving 71% yield after 14 h at 60 °C. High yields were also obtained starting from benzophenone (7l) and heterocyclic substrates (7m–o). The thiophene derivative could be easily transformed at room temperature, contrary to the pyridine and furan compounds, which required heating (prolonged to 2 h in the case of 7o). Moreover, cyclohexanone and 4-heptanone were efficiently transformed into the corresponding silylethers 8′p and 8′q, which were obtained in high yields. Of note, Mindiola and coworkers reported a lower yield for 8′p (70%) when using a nickel pincer dimer (2 mol%) after 2.5 h in benzene at 100 °C.³⁰ A range of aldehydes was also efficiently hydrosilylated. After methanolysis, benzyl alcohols 8r–u were obtained in high yields. Nevertheless, the reaction time depends on the substituent: 8r and 8s were obtained within 1 h, while 4 h were necessary to obtain 8u in high yield. Aliphatic aldehydes were also efficiently converted into the corresponding silylethers 8′v–8′x.

Scheme 6 Scope of the catalytic hydrosilylation of carbonyl compounds with 2Cl. NMR yields using trimethoxybenzene as an internal reference (except for 8v–x for which trimethylbenzene was employed) are given with the isolated yields specified in brackets. ^a Reaction conducted with the keto-olefin (1 mmol) in the presence of 2Cl (0.0 mmol, 1 mol%), trimethoxybenzene (0.07 mmol) as reference, SiH₂Ph₂ (1 mmol), and ^tBuOK (0.01 mmol, 1 mol%) within 1 h at room temperature in Et₂O (2 M) (method A). ^b Same conditions but 1 h in THF at 60 °C. ^c Same conditions but 14 h in THF at 60 °C. ^d Same conditions but 2 h in THF at 60 °C. ^e No isolated yield could be determined. ^f Same conditions but 4 h in THF at 60 °C.

The performance of 2Cl for the hydrosilylation of aldehydes is comparable to those of previous Ni catalysts,^14b,c while some reports of efficient aldehyde hydrosilylation at room temperature exist.³¹ However, for ketones, examples of powerful catalysts operating under mild conditions remain rare.^14b,c Guan reported only partial hydrosilylation of ketones at 70 °C for 24 h.^31a Royo, on the other hand, described the conversion of acetophenones within some hours in toluene at 100 °C, while aliphatic derivatives required one day of heating.³² With a similar NHC Ni catalyst, Sortais and Ritleng isolated the corresponding alcohols in good yields after 17 h of reaction at 25 °C and methanolysis.²⁸ More recently, lower performances were reported for comparable substrates with pincer nickel complexes at 70 °C.^29,33 The catalytic results with 2Cl for the hydrosilylation of carbonyls are among the best described for a nickel catalyst in the literature. Only the NHC-supported Ni complex reported by Ritleng and Sortais gave yields above 70%, for a comparable range of ketones using 5 mol% catalyst within 17 h at room temperature.²⁸ Notably, competitive hydrosilylation of 1-octene and acetophenone under the established catalytic conditions led to the formation of phenoxydiphenylsilane and octyldiphenylsilane in 98% and 2% yields, respectively (Fig. S17). This again underlines the preference for C=O over the C=C reaction for 2Cl.

In order to test the robustness of the catalyst regarding scaling-up, as representative examples, the hydrosilylation of 1-octene and acetophenone was performed on the gram scale (Scheme 7). The reactions were conducted neat with 0.1 mol% of 2Cl and ^tBuOK. Excellent yields were obtained in both cases, but the reaction was much faster with acetophenone with a total conversion within a few minutes (TOF, 11 040 h⁻¹). This again demonstrates that 2Cl is particularly effective for ketones.

Scheme 7 Gram-scale hydrosilylation of 1-octene and acetophenone.

Mechanistic investigations

Given the catalytic performance of 2Cl for the hydrosilylation of olefins and C=O bonds, we were interested in investigating the mechanism of the reaction. First, when the hydrosilylation of 1-octene was conducted in the presence of mercury, the expected silane was obtained in 81% yield (Fig. S18) which suggests a homogeneous catalytic process. When the reaction was carried out with SiD₂Ph₂, the deuterium atoms were localised in the formed silane on the silicon and the β carbon as expected (Fig. S19–21). As such reactions generally involve Ni–H complexes as the active species, the synthesis of 2H was attempted (Scheme 8-1). However, no compound could be isolated from the reaction of 2Cl with KHBEt₃ (1 equivalent); nevertheless, the in situ¹H NMR analysis of the reaction mixture showed a doublet at −25.0 ppm (J_P,H = 133.0 ppm) (Fig. S22). The latter became a singlet upon phosphorus decoupling. No signal was observed in the ³¹P NMR spectrum.

Scheme 8 Attempts to generate reaction intermediates.

This chemical shift falls in the range of Ni–H resonances.³⁴ The P,H coupling constant is rather large³⁵ but close to those reported for square-planar [(N,P)NNiH] complexes featuring a phosphinoamidinate and a pyridine-type ligand.³⁶ This suggests the formation of a Ni–H complex in which the hydride couples with the PPh₂ group.

The synthesis and isolation of [LNi(O^tBu)] (2(O^tBu)) by the addition of ^tBuOK to 2Cl remained unsuccessful. The NMR spectroscopy monitoring provided little information; the ³¹P resonance was lost, suggesting the formation of a paramagnetic species, and the ¹H NMR spectrum was difficult to interpret (Fig. S23). However, the addition of silane to this mixture led, within minutes, to a ¹H NMR spectrum very similar to that obtained upon the addition of the hydride donor (Scheme 8-(2) and Fig. S23). Moreover, upon mixing 2Cl with the silane and ^tBuOK in stoichiometric amounts in THF at room temperature (Scheme 8-3), a singlet at 5.75 ppm, corresponding to the silane SiHPh₂O^tBu,³⁷ was observed in the ¹H NMR spectrum as well as the hydride resonance at −25.0 ppm (doublet, J_P,H = 133.0 ppm) previously mentioned (Fig. S24 and 25). This reaction was conducted in the presence of trimethoxybenzene as a reference to quantify the efficiency of the conversion of 2Cl into 2H. As shown in Fig. S27, the conversion was almost quantitative (96%). In line with the absence of catalytic activity in the absence of an additive, no change was observed by NMR upon mixing 2Cl and the silane. These observations confirm that Ni–H can form from the alkoxide complex, as previously shown by Hu and coworkers,²⁷ probably thanks to the silane via σ-bond metathesis³⁸ (Scheme 9a). Nevertheless, activation of the silane by alkoxides was also described.³⁹ This was therefore also investigated by ¹H NMR spectroscopy (Scheme 9b and Fig. S26). The ¹H NMR spectrum drastically changed upon the addition of ^tBuOK to SiH₂Ph₂: the initial silane disappeared and various species formed. Addition to this mixture of 2Cl led to the observation of the signal characteristic of the Ni–H complex (Fig. S26). Therefore, this complex can form by either route depicted in Scheme 9.

Scheme 9 Possible pathways for the formation of 2H from 2Cl under the reaction conditions.

As variable-temperature NMR studies suggest that the geometry of the precatalyst 2Cl evolves with the temperature (vide supra), the hydrosilylation of 1-octene was attempted at 193 K (see the SI for details). At this temperature, almost no conversion was observed (8%). Although the reaction rate is known to increase with the temperature, the geometry of 2Cl at low temperature may also disfavour the catalysis.

DFT calculations

To get a deeper insight into the reaction mechanism, DFT computations were performed using the Gaussian 16⁴⁰ suite of programs (see Computational details). The values presented are ΔG₂₉₈ in kcal mol⁻¹. The putative nickel hydride complex A, propene and trimethylsilane were used as the model catalyst and substrates (Fig. 4). Note that the reaction works with Et₃SiH in a similar manner as with SiH₂Ph₂ (Table 1, entry 9), justifying the simplified model employed. The overall reaction, i.e., the anti-Markovnikov hydrosilylation of propene, is exergonic by 13.5 kcal mol⁻¹. The coordination of propene to Ni can take place without strict ligand exchange, although the Ni–P bond elongates significantly (from 2.10 to 3.0 Å). While A is virtually square planar, the ONP ligand adopts a butterfly geometry, allowing the formation of the 5-coordinate geometry of B (8.2 kcal mol⁻¹). Of note, the involvement of pentacoordinated Ni complexes is not unprecedented in hydrosilylation reactions.³³ However, the low energy level of this intermediate may be surprising, considering the structure of the employed ONP ligand. Insertion of propene into the Ni–H bond might lead to the Markovnikov complex C or the anti-Markovnikov one D. The latter is the only one that can account for the major products observed; however, TS_BD (12.9 kcal mol⁻¹) is 1.0 kcal mol⁻¹ less stable than TS_BC (11.9 kcal mol⁻¹). It turns out that the regioselectivity is dependent on the steric hindrance brought about by the alkene substituent (see lower box in Fig. 4). The 0.98 kcal mol⁻¹ difference between TS_BD and TS_BC with propene is actually lowered to 0.62 kcal mol⁻¹ with butene. Albeit small, the selectivity favors the anti-Markovnikov process with pentene (−0.28 kcal mol⁻¹). It still increases with hexene (−0.66 kcal mol⁻¹) and even more with heptene (−0.94 kcal mol⁻¹), which is consistent with the experimental findings. Intermediate D (7.2 kcal mol⁻¹) is still a 5-coordinate complex due to the coordination of the β-hydrogen. The hydrogen at Ni can be replaced by that of the silane to give intermediate E (16.8 kcal mol⁻¹). Through TS_EF (17.2 kcal mol⁻¹), E transforms into the 3-center-2-electron complex F in a virtually thermoneutral manner. This compound can be seen as a Ni^II species of the form [L₂NiX₃]⁻ with a silylium ion (MeSi⁺) linked to the Ni–H bond. The silylium ion will then play the role of an electrophile in the cleavage of the Ni–C bond viaTS_FG (19.0 kcal mol⁻¹) to furnish G (8.4 kcal mol⁻¹). As noted above, dissociation of the latter into the hydrosilylation product and A is appreciably exergonic. Thus, the highest-lying transition state is only 19.0 kcal mol⁻¹ above the reference system, and the driving force of the reaction is the favorable thermodynamics associated with it. This step was reinvestigated with SiH₂Ph₂ and hexene (Fig. S130), showing little impact on the overall barrier to reach TS_F′G′ (19.7 kcal mol⁻¹vs. 19.0 kcal mol⁻¹ for TS_FG).

Fig. 4 Computed free energy profile with Me₃SiH and various alkenes (ΔG₂₉₈, kcal mol⁻¹; some hydrogen atoms are not displayed for clarity; selected distances in Å).

Conclusions

An original ONP ligand associating a phenoxide, an iminophosphorane and a phosphine was synthesised from 2-diphenylphosphinoanilinium and a diphenylphosphinophenol derivative. The corresponding Ni^II complexes 2X (X = Cl, Br) were prepared in high yield and characterised by multinuclear NMR spectroscopy, HR-mass spectrometry, elemental analysis and X-ray crystallography. However, the ³¹P resonance observed at room temperature does not match the square planar geometry observed in the solid state. Lowering the temperature reveals the expected two resonances, but the variable temperature experiments allow us to discard a classical hemilability process. For 2Cl, the NMR experiments and the measurements of magnetic moments at different temperatures suggest the coexistence of square planar and distorted tetrahedral geometries, with the former being dominant at low temperature and the latter at temperature around 50 °C. The complexes were tested for the hydrosilylation of olefins. 2Cl proved particularly efficient, realising the selective conversion into the anti-Markovnikov silylether at 1 mol% loading in the presence of 1 mol% of ^tBuOK and one equivalent of SiH₂Ph₂ within one hour at room temperature for a variety of olefins, mostly terminal ones. For styrene derivatives, mixtures of regioisomers were obtained, with the major one depending on the substituent of the aromatic cycle. In some cases, the reaction was conducted at 60 °C, and for some disubstituted olefins, longer reaction times or a higher catalyst loading was necessary to obtain good yields. Remarkably, 2Cl selectively converted C=O preferentially to C=C in enones, a selectivity that is unprecedented to our knowledge with Ni catalysts. Reduction of carbonyl derivatives (after methanolysis of the vinylether) also provided high yields. In particular, a wide range of aromatic and aliphatic ketones was transformed. Therefore, the catalytic results (substrate scope and selectivity) of 2Cl are rather unique. Mechanistic experiments suggest the formation of a Ni–H complex (2H) under the applied catalytic conditions, and the latter was shown by DFT calculations to be able to deliver the hydrosilylation products via a 4-step mechanism. This involves a low-lying pentacoordinated olefin intermediate. The computed mechanism presents a higher activation barrier at 19 kcal mol⁻¹ and very favourable thermodynamic parameters. 2Cl is only the second iminophosphorane Earth abundant catalyst to be reported for hydrosilylation reactions. Its excellent performance should stimulate the use of Earth-abundant metal catalysts supported by iminophosphorane ligands for reductive transformations. We are currently exploring other catalytic applications for this complex and other metals for the coordination of the ligand.

Experimental part

All air- and moisture-sensitive reactions were performed under an inert atmosphere using a vacuum line, inert Schlenk techniques (N₂) and a glovebox (Ar, <0.1 ppm H₂O, <0.1 ppm O₂) with oven-dried glassware unless otherwise noted. Reagents were used as received from commercially available suppliers without further purification, unless otherwise specified. CH₂Cl₂, pentane, ether and toluene were taken from a solvent purification system (MBraun-SPS). THF was distilled and degassed using the freeze–pump–thaw technique. NMR spectra were recorded on a Bruker AC-300 SY spectrometer at 300 MHz for ¹H, 120 MHz for ³¹P and 75 MHz for ¹³C. Solvent peaks were used as internal references for ¹H and ¹³C chemical shifts (ppm). ³¹P{¹H} NMR spectra are relative to an 85% H₃PO₄ external reference. Unless otherwise mentioned, NMR spectra were recorded at 300 K. Structural assignments were made with additional information from COSY, HSQC, and HMBC experiments. The spectra were analysed with Topspin software. The following abbreviations are used: s, singlet; d, doublet; dd, doublet of doublets; t, triplet; and m, multiplet. The labeling for the proligand and complexes is given in Scheme 3.

Mass spectrometry experiments were performed on a Tims-TOF mass spectrometer (Bruker, France). An electrospray source has been used in positive and negative modes. Samples are prepared in acetonitrile with 0.1% formic acid at μM concentrations. 2 to 10 μL were introduced without separation with the Elute UHPLC module (Bruker) at a 100 μL min⁻¹ flow rate into the interface of the instrument. Capillary and end-plate voltages were set at 4.5 kV and 0.5 kV for ESI experiments. Nitrogen was used as the nebuliser and drying gas at 2 bar and 8 L min⁻¹, respectively, with a drying temperature of 220 °C for the ESI source. Tuning mix (Agilent, France) was used for calibration. The elemental compositions of all ions were determined with the instrument software, Data Analysis, and the precision of mass measurement was less than 3 ppm. Elemental analyses were carried out by the elemental analysis service of the “LCC” (Toulouse) using a PerkinElmer 2400 series II analyser. X-ray crystallography data were collected at 150 K on a Bruker Kappa APEX II diffractometer using a Mo-κ (λ = 0.71069 Å) X-ray source and a graphite monochromator. The crystal structures were solved using Shelxt⁴¹ or olex⁴² and refined using Shelxl-97 or Shelxl-2014.⁴¹ ORTEP drawings were prepared using ORTEP III⁴³ for Windows or Mercury. Details of the crystal data and structure refinements are summarised in Tables S1 and 2.

Synthesis of 1

A bromide solution in dichloromethane (1.27 mL, 2.47 mmol, 2.0 M) was added dropwise to a cooled (−78 °C) solution of 2-diphenylphosphine-4,6-di-tert-butylphenol 1 (963.8 mg, 2.47 mmol) in dichloromethane (20 mL). The colorless solution turned yellow and was brought back to room temperature, then stirred for 1 h. Afterwards, the solution was once again cooled to −78 °C and DABCO (277.1 mg, 2.47 mmol, 2 equiv.) was added, followed 5 min later by a solution of ortho-diphenylphosphine ammonium benzene chloride (932.7 mg, 2.47 mmol) in dichloromethane (6 mL). The yellow solution turned into a brown suspension with a white precipitate, which was warmed to room temperature then stirred for 3 h. Then, the two phases were separated by centrifugation, and an excess of potassium chloride (2 g, 27 mmol) was added to the solution. The suspension was stirred for 3 h, and then, the white precipitate was filtered off. The orange solution was concentrated under vacuum to afford a light orange solid. The solid was washed with diethyl ether (10 mL) before being dried under vacuum to give the desired product (1), as a light-yellow solid (1.40 g, 2.0 mmol, 81%). Single crystals were grown in CHCl₃/pentane layering at room temperature and afforded deep green crystals. ³¹P{¹H} NMR (CDCl₃, 121.5 MHz, 25 °C): δ −15.6 (s, P^III), 38.0 (s, PN); ¹H NMR (CDCl₃, 300 MHz, 25 °C): δ = 7.62–7.73 (m, 4H, CH_PPh2), 7.41–7.52 (m, 8H, CH_PPh2), 7.25–7.36 (m, 4H, CH_PPh2), 7.06–7.16 (m, 6H, CH_PPh2 and CH_Ar), 7.01 (t, J_H,H = 7.5 Hz, 1H, CH_Ar-b), 6.79 (m, 1H, CH_Ar), 6.73 (m, 1H, CH_Ar), 6.71 (m, 1H, CH_Ar-a), 1.48 (s, 9H, (CH₃)_tBu), 1.14 (s, 9H, (CH₃)_tBu); ¹³C{¹H} NMR (CDCl₃, 75 MHz, 25 °C): δ 157.0 (d, J_P,C = 3.0 Hz, C_OH), 144.6 (d, J_P,C = 13.5 Hz, C_Ar), 143.5 (d, J_P,C = 7.0 Hz, C_Ar), 140.8 (d, J_P,C = 21.0 Hz, C_Ar), 134.9 (d, J_P,C = 10.0 Hz, C_Ar), 134.3 (s, CH_PPh2), 134.2 (d, J_P,C = 3.0 Hz, CH_Ar), 133.9 (dd, J_P,C = 1.5 and 11.5 Hz, CH_PPh2), 133.8 (d, J_P,C = 20.0 Hz, CH_PPh2), 132.0 (s, CH_PPh2), 129.9 (s, CH_Ar), 129.2 (s, C_Ar), 129.0 (d, J_P,C = 4.0 Hz, CH_PPh2), 128.7 (d, J_P,C = 7.0 Hz, CH_PPh2), 128.5 (s, CH_Ar), 127.9 (d, J_P,C = 11.0 Hz, CH_Ar), 126.7 (m, CH_Ar), 126.4 (s, CH_Ar), 121.2 (d, J_P,C = 107.0 Hz, C_PPh2), 114.7 (d, J_P,C = 100.5 Hz, C_PPh2), 35.4 (d, J_P,C = 1.5 Hz, C_tBu), 34.6 (s, C_tBu), 31.2 (s, (CH₃)_tBu), 30.6 (s, (CH₃)_tBu). HR-MS (ESI⁺): calculated for [C₄₄H₄₆NOP₂]⁺ ([M − X]⁺) 666.3049; found 666.3036.

Synthesis of 2Cl

ⁿ BuLi (0.94 mL of a 1.6 M ether solution, 1.5 mmol) was added to a cooled (−78 °C) solution of 1 (547.5 mg, 0.75 mmol) in tetrahydrofuran (40 mL). The yellow solution turned orange upon warming to room temperature, and the deprotonation was completed after 10 min. [NiCl₂(DME)] (164.8 mg, 0.75 mmol) was added to the orange solution of the iminophosphorane, and the solution turned dark red/maroon and was stirred for 30 min. The solvent was evaporated under vacuum, and dichloromethane (40 mL) was added. A white precipitate appeared and was filtered off using a PTFE filter. The solvent was evaporated under vacuum, and then the dark red solid was washed with a 1 : 3 Et₂O : pentane mixture (20 mL). The desired complex 2Cl was extracted into toluene (80 mL). This solvent was evaporated, and 2Cl was obtained as a dark red solid after drying under vacuum (430.0 mg, 0.58 mmol, 76%). Single crystals were obtained by diffusion of diethyl ether into a THF solution. ³¹P{¹H} NMR (CD₂Cl₂, 121.5 MHz, −80 °C): δ = 27.4 (s), 23.0 (s); ¹H NMR (CDCl₃, 300 MHz, 25 °C): δ = 7.93 (m, 4H, CH_PPh2), 7.40–7.61 (m, 16H, CH_PPh2), 7.37–7.39 (m, 1H, CH_a), 6.96 (d, J_H,H = 7.5 Hz, 1H, CH), 6.73 (td, J_H,H = 1.5 and 7.5 Hz, 1H, CH), 6.53 (t, J_H,H = 7.5 Hz, 1H, CH), 6.26 (d, J_H,H = 8.5 Hz, 1H, CH), 6.21 (dd, J_H,H = 2.5 and J_P,H = 7.5 Hz, 1H, CH_b), 1.34 (s, 9H, (CH₃)_tBu), 1.07 (s, 9H, (CH₃)_tBu);¹³C{¹H} NMR (CDCl₃, 75 MHz, 25 °C): δ = 168.1 (s, C_O), 156.2 (s, C_N), 144.3 (s, C_Ar), 134.8 (s, C_Ar), 134.2 (s, CH_PPh2), 133.0 (d, J_P,C = 9.5 Hz, CH_PPh2), 132.9 (s, CH), 132.5 (d, J_P,C = 3.0 Hz, CH_PPh2), 132.1 (d, J_P,C = 10.5 Hz, C_Ar), 131.5 (s, CH), 130.9 (s, CH_PPh2), 129.7 (s, CH_b), 129.6 (d, J_P,C = 9.0 Hz, C_Ar), 129.0 (d, J_P,C = 12.5 Hz, CH_PPh2), 128.6 (s, CH_PPh2), 127.0 (d, J_P,C = 92.0 Hz, C_PPh2), 125.6 (d, J_P,C = 6.5 Hz, CH_a), 121.3 (d, J_P,C = 9.5 Hz, CH), 119.5 (s, CH), 114.0 (d, J_P,C = 126.0 Hz, C_PPh2), 35.3 (s, C_tBu), 33.8 (s, C_tBu), 31.4 (s, (CH₃)_tBu), 30.2 (s, (CH₃)_tBu). HR-MS (ESI⁺): calculated for [C₄₄H₄₄NNiOP₂]⁺ ([M − Cl]⁺) 722.2246; found 722.2244. Anal. Calcd for C₄₄H₄₄ClNNiOP₂·0.5 C₈H₈: C, 70.64; H, 5.98; N, 1.76. Found: C, 70.39; H, 6.43; N, 1.93.

Synthesis of 2Br

It was synthesised as 2Cl reacting ⁿBuLi (0.19 mL of a 1.6 M ether solution, 0.3 mmol), 1 (105.7 mg, 0.15 mmol) and [NiBr₂(DME)] (46.3 mg, 0.3 mmol) in THF (15 mL). The solid residue was washed with a 1 : 3 Et₂O : pentane mixture (8 mL). 2Br was obtained as a dark red (80.8 mg, 0.10 mmol, 68%) after extraction into toluene (30 mL) and drying under vacuum. Single crystals were obtained by diffusion of diethyl ether into a THF solution. ³¹P{¹H} NMR (CD₂Cl₂, 121.5 MHz, −80 °C): δ = 27.3 (s), 22.8 (s); ¹H NMR (CD₂Cl₂, 300 MHz, 25 °C): δ = 7.93 (dd, J_H,H = 2.0 and 7.5 Hz, 4H, CH_PPh2), 7.56–7.67 (m, 16H, CH_PPh2), 7.44–7.46 (m, 1H, CH_Ar-b), 6.89 (d, J_H,H = 7.5 Hz, 1H, CH_Ar), 6.78 (t, J_H,H = 7.5 Hz, 1H, CH_Ar), 6.56 (t, J_H,H = 7.5 Hz, 1H, CH_Ar), 6.28–6.33 (m, 2H, CH_Ar and CH_Ar-a), 1.32 (s, 9H, (CH₃)_tBu), 1.10 (s, 9H, (CH₃)_tBu); ¹³C{¹H} NMR (CDCl₃, 75 MHz, 25 °C): δ = 167.0 (s, C_O), 157.0 (s, C_N), 144.3 (s, C_Ar), 144.1 (s, C_Ar), 135.4 (d, J_P,C = 14.5 Hz, C_Ar), 134.4 (s, CH_PPh2), 133.2 (s, CH_Ar), 133.1 (d, J_P,C = 10.0 Hz, CH_PPh2), 131.7 (s, CH_Ar), 132.8 (d, J_P,C = 3.0 Hz, CH_PPh2), 131.0 (s, CH_PPh2), 129.6 (s, CH_Ar-b), 129.0 (d, J_P,C = 12.5 Hz, CH_PPh2), 128.6 (s, CH_PPh2), 126.9 (d, J_P,C = 93.0 Hz, C_PPh2), 126.1 (d, J_P,C = 7.0 Hz, C_Ar), 125.6 (d, J_P,C = 14.5 Hz, CH_Ar-a), 121.4 (d, J_P,C = 7.5 Hz, CH_Ar), 119.8 (s, CH_Ar), 115.0 (d, J_P,C = 111.0 Hz, C_PPh2), 35.0 (s, C_tBu), 33.6 (s, C_tBu), 31.1 (s, (CH₃)_tBu), 30.0 (s, (CH₃)_tBu); HR-MS (ESI⁺): calculated for [C₄₄H₄₄NNiOP₂]⁺ ([M − Br]⁺) 722.2246; found 722.2239. Anal. Calcd for C₄₄H₄₄BrNNiOP₂·1.5 C₈H₈: C, 69.52; H, 5.99; N, 1.49. Found: C, 69.44; H, 6.32; N, 1.58.

General procedure for the catalytic hydrosilylation of alkenes 3

In the glovebox, trimethoxybenzene (11.8 mg, 0.07 mmol) and 2Cl (7.6 mg, 0.01 mmol, 1 mol%) were introduced in a 10 mL vial. Then, SiH₂Ph₂ (0.19 mL, 1 mmol, 1 equiv.) and the alkene (1 mmol, 1 equiv.) were added. Finally, the corresponding volume of solvent, either Et₂O for the reaction performed at 25 °C (method A) or THF for the reaction performed at 60 °C (method B) to reach a 2 M concentration, and ^tBuOK (1.1 mg, 0.01 mmol, 1 mol%) were added, and the vial was capped. After stirring for 1 h at the appropriate temperature, an aliquot of 10 μL was taken, quenched with 1 mL of distilled water and extracted with 2.5 mL of Et₂O then dried on MgSO₄. The solvent from the aliquot was evaporated on a rotary evaporator (5 min, 50 mbar, 40 °C) and analysed by NMR in CDCl₃. The remaining mixture was put on the rotary evaporator (30 min, 50 mbar, 40 °C), then the crude product was purified by flash chromatography (pentane/EtOAc), and the desired silyl ether was isolated.

General procedure for the catalytic hydrosilylation of enones/enals 5

In the glovebox, trimethoxybenzene (11.8 mg, 0.07 mmol) and then 2Cl (7.6 mg, 0.01 mmol, 1 mol%) were introduced in a 6 mL catalytic vial. Then, SiH₂Ph₂ (0.19 mL, 1 mmol, 1 equiv.), the enone or enal 5 (1 mmol, 1 equiv.) were also introduced. The corresponding volume of solvent; Et₂O if the reaction would be performed at 25 °C (method A) or THF if reaction would be performed at 60 °C (method A), to reach a 2 M solution, and ^tBuOK (1.1 mg, 0.01 mmol, 1 mol%) were added and the vial was capped. After stirring for 1 h at the appropriate temperature, an aliquot of 10 μL was taken, quenched with 1 mL of distilled water and extracted with 2.5 mL of Et₂O then dried on MgSO₄. The solvent from the aliquot was evaporated on a rotary evaporator (5 min, 50 mbar, 40 °C) and analysed by NMR in CDCl₃. The rest of the mixture was put on the rotary evaporator (30 min, 50 mbar, 40 °C), and then the crude product was purified by flash chromatography (pentane/EtOAc), and the desired silyl ether was isolated.

General procedure for the catalytic hydrosilylation of ketones and aldehydes 7

In the glovebox, trimethoxybenzene (11.8 mg, 0.07 mmol) and then 2Cl (7.6 mg, 0.01 mmol, 1 mol%) were introduced in a 6 mL catalytic vial. Then, SiH₂Ph₂ (0.19 mL, 1 mmol, 1 equiv.) and the ketone derivative 7 (1 mmol, 1 equiv.) were also introduced. Finally, the corresponding volume of the solvent, Et₂O if the reaction was performed at 25 °C (method A) or THF if the reaction was performed at 60 °C (method A), was added to reach a 2 M solution, then ^tBuOK (1.1 mg, 0.01 mmol, 1 mol%) was introduced, and the vial was capped. After stirring for 1 h at the appropriate temperature, an aliquot of 10 μL was taken, quenched with 1 mL of distilled water and extracted with 2.5 mL of Et₂O and then dried on MgSO₄. The solvent from the aliquot was evaporated on a rotary evaporator (5 min, 50 mbar, 40 °C) and analysed by NMR in CDCl₃. The remaining mixture was reacted with 3 mL of a saturated NaOH solution in MeOH during 2 h at room temperature. Then, water (5 mL) and Et₂O (10 mL) were added, and the aqueous layer was further extracted with Et₂O (2 × 20 mL). The combined organic layers were washed with brine (5 mL) and concentrated under vacuum. Afterwards, the crude product was purified by flash chromatography (pentane/EtOAc), and the desired alcohol 8 was isolated. In the case of 7n and 7o, after the aliquot was taken, the remaining reaction mixture was evaporated using the rotary evaporator (30 min, 0 mbar, 40 °C), and then the crude product was purified by flash chromatography (pentane/EtOAc), and the desired silyl ether was isolated.

Computational details

Optimisations were carried out with the M06 functional,⁴⁴ the LANL2DZ ECP basis set⁴⁵ for Ni and the 6-31G(d,p) basis set for the other elements. Thermal correction to the Gibbs free energy was obtained at the optimisation level. Single point energy calculations were performed at the M06-2X/def2-TZVPP level.⁴⁶

Author contributions

I. P.: formal analysis, data curation, and investigation; T. F. A. A.: investigation; S. B.: formal analysis and data curation; N. C.: formal analysis and data curation; V. G.: formal analysis and data curation; and A. A.: formal analysis, data curation, supervision, and writing of the original draft.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

Details regarding catalysis, X-ray data collection and DFT calculations, as well as NMR data of the synthesised molecules are available within the article and its supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5qi01895a.

CCDC 2452730–2452732 contain the supplementary crystallographic data for this paper.^47a–c

Acknowledgements

The authors thank Ecole Polytechnique, CNRS, and UPSaclay for financial support, and Mr Karim Hammad for his help in recording the 2D H/P NMR experiments. This work was granted access to the HPC resources of CINES under allocation 2020-A0070810977 made by GENCI to VG. The Agence Nationale de la Recherche (ANR-21-CE07-0026) is acknowledged for funding the LYMACATO project, and the RESOMAG platform for access to NMR instruments, as well as GDR Phosphore for gathering the community of P-chemists in France.

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Footnote

† The same 2D NMR experiment performed at −80 °C was not conclusive because the resolution was not sufficient to confidently assign the cross peak to one or the other of the broad ³¹P resonances.

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