Syntheses and catalytic application of hydrido iron(II) complexes with [P,S]-chelating ligands in hydrosilylation of aldehydes and ketones

Abstract

Four hydrido iron(II) complexes (1–4) with [P,S]-chelating ligands were synthesized by the reactions of (2-diphenylphosphanyl)thiophenols, C₆H₃(1-SH) (2-PPh₂) (4-R₁) (6-R₂), abbreviated as (P^SH), with Fe(PMe₃)₄. (1: R₁ = R₂ = H; 2: R₁ = H, R₂ = SiMe₃; 3: R₁ = CH₃, R₂ = H; 4: R₁ = SiMe₃, R₂ = H). Among them, complexes 2–4 are new and were completely characterized by spectroscopic methods. The molecular structures of complexes 2, 3, and 4 were confirmed by X-ray single crystal diffraction. The catalytic properties of hydrido iron(II) complexes 1–4 were explored in the hydrosilylation of aldehydes and ketones. They showed a good activity in catalytic hydrosilylation of aldehydes and ketones by using (EtO)₃SiH as a hydrogen source under mild conditions.

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Introduction

Hydrido metal complexes are of great importance in the areas of synthetic chemistry, coordination chemistry and homogeneous catalysis. In general, they are postulated as key intermediates in a variety of catalytic reactions, such as hydrogenation, transfer hydrogenation and hydrosilylation. Their applications are not only in the laboratory as a reactant or catalyst but also in the commercial production of chemicals, perfumes, pharmaceuticals and so on.¹ Noble metals like Ir², Re³, Ru⁴, Mo⁵ and Au⁶ have been deeply reported for more than 20 years in the area of catalysis because their compounds have good catalytic activities. In contrast, the inexpensive metals had been rarely reported. Iron as an inexpensive transition metal has shown many significant advantages for its cheap, environment-friendly, biological importance, and abundant in the nature.⁷ Recently the iron-catalyzed reactions of organic carbonyl compounds have appeared more frequently in organic synthesis. The first example of iron-catalyzed direct hydrogen addition reactions of carbonyl compounds had been shown by Casey under mild catalytic conditions of room temperature, low hydrogen pressure, and low catalyst loading in 2007.^8,9 Chirik synthesized a series of bis(imino)pyridine iron catalysts with excellent catalytic activities in the hydrogenation and hydrosilylation of unsaturated compounds, such as carbonyls, alkenes and alkynes and the selectivity of these iron catalysts under certain moderate conditions performed perfectly.¹⁰ Guan and his co-workers synthesized a iron compound in the hydrogenation of carbonyls.¹¹ Morris and his co-workers stepped towards the goal of both atom economic and environment-friendly to explore a new iron complex for the asymmetric hydrogenation of prochiral ketones.¹² Meanwhile Milstein found some iron (PNP) pincer complexes with good catalytic effects (TONs up to 1980) at complete conversions of ketones.¹³ In 2014, Schneider and Jones reported well-defined iron catalysts for the acceptorless reversible dehydrogenation-hydrogenation of alcohols and ketones at ambient temperature under mild H₂ pressure.¹⁴ Compared to hydrogenation, transfer hydrogenation provided a safety and convenient method to reduce carbonyl compounds using alcohols (e.g. i-PrOH) and base (e.g. t-BuOK).¹⁵ In 2006, Beller exploited the transfer hydrogenation of ketones with the iron porphyrins generated in situ as homogeneous catalysts at first time.¹⁶ Additionally, hydrosilylation had been widely reported recently in catalytic reactions by Chirik¹⁰ and Guan.¹⁷ Nishiyama focused mainly on in situ catalyst system and reported achiral and chiral iron complexes with phebox ligands.¹⁸ Complex 1 was synthesized by Klein through the reaction of (2-diphenylphosphanyl)thiophenol with Fe(PMe₃)₄.¹⁹ Regretfully, the catalytic property of complex 1 has not been discussed. Recently, our group has found that the hydrido cobalt(III) complex²⁰ supported by trimethylphosphine ligands and the hexa-coordinate octahedral hydrido iron(II) complex²¹ had an effective catalytic activity in hydrosilylation of carbonyl compounds. In 2014, we obtained the compound ((Ph₂P(C₆H₄))₂CH)Fe(H) (PMe₃)₂ through C–H activation of the PCP ligand with Fe(PMe₃)₄ and the hydrido iron complex displayed perfect character in the catalytic hydrosilylation of aldehydes and ketones.²²

In this paper, four hydrido iron(II) complexes (1–4) with [P,S]-chelating ligands were synthesized by the reactions of (2-diphenylphosphanyl)thiophenols with Fe(PMe₃)₄. Among them, complexes 2–4 are new and were completely characterized by spectroscopic methods. The catalytic properties of hydrido iron(II) complexes 1–4 in hydrosilylation reactions were fully investigated. It was found that the four complexes 1–4 could be used as effective catalysts for hydrosilylation reduction of aldehydes and ketones by using (EtO)₃SiH as a hydrogen source under mild conditions. Among them 1 is the best catalyst.

Results and discussion

1. Synthesis of iron hydride complexes 1–4

(2-Diphenylphosphanyl)thiophenols reacted with Fe(PMe₃)₄ to give the hydrido iron complexes (1–4) as S–H bond activation products (eqn (1)).¹⁷ Among them, complexes 2–4 are new and were completely characterized.

The typical signals for ν(Fe–H) vibration in the IR spectra of complexes 2, 3 and 4 were found at 1855 (2) 1845 (3) and 1852 cm⁻¹ (4). In the ¹H NMR spectra, the H_(Fe–H) resonance appears at −12.92 (2), −12.51 (3) and −12.67 ppm (4) respectively as dddd peaks with coupling constants ²J(PH) = 33–99 Hz. In the ³¹P NMR spectra there are four different phosphorus signals which appear at 90.5, 29.0, 14.8, 7.5 ppm for 2, at 90.5, 29.8, 14.6, 7.4 ppm for 3 and at 90.3, 29.3, 15.9, 7.3 ppm for 4 respectively. Complexes 2–4 should have the similar structures as the analogue hydrido iron(II) complexes formed through the reaction of (2-diphenylphosphanyl)phenol with Fe(PMe₃)₄.¹⁷ X-ray diffraction study confirmed the structures of complexes 2 (Fig. 1), 3 (Fig. 2) and 4 (Fig. 3). In complex 2, the iron atom is centered in a slightly distorted octahedral geometry. The Fe–H distance (1.43(2) Å) is within the range of Fe–H bonds.²³ Fe1–P3 (2.2524(4) Å) is longer than the other three Fe1–P3 distance (Fe1–P1 2.1949(4), Fe1–P2 2.2265(4) and Fe1–P4 2.1886(4) Å) because of the stronger trans-influence of the hydrido ligand. The Fe1–P4 bond with 2.1886(4) Å is the shortest Fe–P bond which is found opposite to the thiophenolato ligand. In complex 3, the iron atom is centered in a slightly distorted octahedral geometry. The Fe–H distance (1.47(3) Å) is within the range of Fe–H bonds.²³ Fe1–P1 (2.25(5) Å) is slightly longer than the other three Fe–P distance (Fe1–P2 2.19(5), Fe1–P3 2.19(5) and Fe1–P4 2.24(5) Å) because of the stronger trans-influence of the hydrido ligand. In complex 4, the iron atom is centered in a slightly distorted octahedral geometry. The Fe–H distance (1.38(2) Å) is within the range of Fe–H bonds.

Fig. 1 Molecular structure of complex 2. Most hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe1–H 1.43(2), Fe1–P1 2.19(4), Fe1–P2 2.23(4), Fe1–P3 2.25(4), Fe1–P4 2.19(4), Fe1–S1 2.32 (4); P3–Fe1–H 179.0(7), P1–Fe1–S1 85.12(1), P2–Fe1–S1 80.52(2), P4–Fe1–P1 97.19(2), P4–Fe1–P2 94.59(2).

Fig. 2 Molecular structure of complex 3. Most of the hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe1–H100 1.47(3), Fe1–P1 2.25(5), Fe1–P2 2.19(5), Fe1–P3 2.19(5), Fe1–P4 2.24(5), Fe1–S1 2.35(5); P1–Fe1–H 177.7(1), P4–Fe1–S1 80.40(2), P2–Fe1–S1 83.88(2), P4–Fe1–P3 93.64(2), P3–Fe1–P2 98.73(2).

Fig. 3 Molecular structure of complex 4. Most of the hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe1–H71 1.38(2), Fe1–P1 2.20(6), Fe1–P2 2.24(6), Fe1–P3 2.24(6), Fe1–P4 2.19(6), Fe1–S1 2.34(6); P2–Fe1–H 178.4(1), P4–Fe1–S1 83.99(2), P3–Fe1–S1 80.46(2), P4–Fe1–P1 98.16 (2), P3–Fe1–P1 94.14(2).

2. Catalytic application of iron hydride complexes 1–4 in hydrosilylation of aldehydes and ketones

It has been reported that hydrido iron complexes as low-cost and environment-friendly transition metal compounds could be used as catalysts in the reduction of unsaturated species, such as in the hydrosilylation processes of aldehydes and ketones.^18–20

In order to study the catalytic activities of hydrido iron complexes 1–4, the hydrosilylation of aldehydes and ketones was studied with complexes 1–4 as catalysts. Preliminary studies of the catalytic activity of iron complex 1 were performed to explore its potential in the hydrosilylation reduction of benzaldehyde (eqn (2), Table 1). The studies on the influence of the reaction conditions were carried out with benzaldehyde as the model substrate by using (EtO)₃SiH as the hydrogen source in THF at 50 °C. The reaction did not occur in the absence of any catalyst (entry 1, Table 1). However, 1 mol% of complex 1 catalyzed the desired transformation and afforded benzyl alcohol in 48% yield (entry 2, Table 1). To our delight, a good yield (94%) was observed in the presence of 2 mol% of 1 (entry 3, Table 1). At the given catalytic conditions, the reduction reaction was completely finished within 2 h. The yield was low when reaction time was shorter than 2 h (entries 4–6, Table 1).

Table 1 Catalytic activity of hydrido iron complexes 1–4 for hydrosilylation of PhCHO^a

Entry	Complex	Loading (mol%)	Hydrogen source	Solvent	Temp. (°C)	Time (h)	TOF (h^−1)	Yield^b (%)
a Catalytic reaction conditions: PhCHO (1.0 mmol), (EtO)3SiH (1.2 mmol) and n-dodecane (internal standard) (1.0 mmol), 2 mL THF.b Determined by GC analysis.
1	1	0	(EtO)3SiH	THF	50	2	0	0
2	1	1	(EtO)3SiH	THF	50	2	13.3	48
3	1	2	(EtO)3SiH	THF	50	2	24.8	94
4	1	2	(EtO)3SiH	THF	50	0.5	37	33
5	1	2	(EtO)3SiH	THF	50	1.0	32.5	61
6	1	2	(EtO)3SiH	THF	50	1.5	27.7	77
7	1	2	Et3SiH	THF	50	2	8.8	31
8	1	2	Ph3SiH	THF	50	2	0	0
9	1	2	Me2PhSiH	THF	50	2	0	0
10	1	2	Ph2SiH2	THF	50	2	21.8	75
11	1	2	(EtO)3SiH	THF	15	2	13.3	44
12	1	2	(EtO)3SiH	THF	30	2	19	71
13	1	2	(EtO)3SiH	THF	60	2	23.3	88
14	1	2	(EtO)3SiH	Toluene	50	2	12.8	48
15	1	2	(EtO)3SiH	Ether	50	2	11.3	41
16	1	2	(EtO)3SiH	Pentane	50	2	10	33
17	1	2	(EtO)3SiH	Isopropanol	50	2	11.8	42
18	2	2	(EtO)3SiH	THF	50	2	9	33
19	2	2	(EtO)3SiH	THF	50	8	6.3	93
20	3	2	(EtO)3SiH	THF	50	2	22.3	83
21	3	2	(EtO)3SiH	THF	50	3	16.7	95
22	4	2	(EtO)3SiH	THF	50	2	18.5	67
23	4	2	(EtO)3SiH	THF	50	5	10	94

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When we changed the hydrogen source to Ph₃SiH, Et₃SiH, Me₂PhSiH and Ph₂SiH₂ the results seemed to be expected (entries 7–10, Table 1). Comparing the results at different temperatures, it was found that the yield was 94% at 50 °C (entry 3, Table 1). The yields were not ideal at the temperatures below or even above 50 °C (entries 11–13, Table 1). This template reaction was carried out in four different solvents (entries 14–17, Table 1). The results showed that THF was the best solvent among them with the yield of 94% for the catalytic reaction (entry 3, Table 1). The yields in toluene, diethyl ether, pentane and isopropanol are moderate (entries 14–17, Table 1). Among four catalysts 1–4, complex 1 seemed to be the best catalyst for the hydrosilylation of benzaldehyde (entries 3, 19, 21 and 23, Table 1) because a complete conversion needed a longer time with complexes 2, 3 or 4 as a catalyst.

This result might be caused by the substituent group (methyl or trimethylsilyl) at the phenyl ring of complexes 2–4 at the ortho- and para-position. Both the methyl and trimethylsilyl as the strong electron-donating groups make the central iron atom more electron-rich. Therefore, the polarity of the Fe–H bond becomes smaller and it is more difficult to break off the Fe–H bond. In other words, the nucleophilic ability of the hydrido hydrogen of complexes 2–4 is lower than that of complex 1.

According to the experimental results in Table 1, the optimized catalytic reaction conditions can be summarized as follows: PhCHO (1.0 mmol), (EtO)₃SiH (1.2 mmol), and 1 (0.02 mmol) in 2.0 ml of THF, 50 °C, 2 h.

After optimization of the catalytic reaction conditions, more substrates of aldehydes bearing different functional groups were selected to explore the scope of this catalytic system under the optimized reaction conditions (eqn (3), Table 2).

Table 2 Catalytic hydrosilylation of aldehydes with 1 as a catalyst^a

Entry	Substrates	TOF (h^−1)	Isolated yield (%)
a Catalytic reaction conditions: RCHO (1.0 mmol), (EtO)3SiH (1.2 mmol) and n-dodecane (internal standard) (1.0 mmol), 2 mL THF, 50 °C, 2 h.
1		20	70
2		20	71
3		10	21
4		25	91
5		25	94
6		25	86
7		25	95
8		9.5	11
9		3.5	5
10		5.3	11
11		24	93
12		24.5	94
13		23	89
14		25	90
15		23.8	83

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Notably, the high yields of the corresponding alcohols were achieved in the presence of halide substituents irrespective of their positions at the phenyl ring (entries 1–7, 11, and 12, Table 2). Because of the steric effect, the higher yields of the corresponding alcohols could be obtained for the aldehydes with halogens at the para-position (entries, 4–6, Table 2) than aldehydes substituted by halogens at the ortho-positions (entries 1–3, Table 2). Quite poor yields were obtained for the benzaldehydes with strong electron-donating groups at the para-positions (entries 9, 10, Table 2). For the dihalogeno substrates (entries 7, 11, 12, Table 2), the aldehydes could be completely converted to the corresponding products. For benzylacetaldehyde, the aliphatic aldehyde, the hydrosilylation seemed to be ineffective (entry 8, Table 2). For other aromatic aldehydes, such as 1-naphthaldehyde, furan-2-carbaldehyde and thiophene-2-carbaldehyde, excellent conversions and yields could be obtained for this catalytic system (entries 13–15, Table 2).

From Table 3, it could be concluded that for the ketones these hydrosilylation reactions were obviously difficult and the yields of the reactions were lower than those for the aldehydes in Table 2 under the same reaction conditions. Even if the reaction time was extended to 24 h, the conversions and yields are only moderate. Although the catalytic reaction was performed in toluene at 90 °C, the conversion had not been significantly improved. This result can be attributed to the steric hindrance of the alkyl group in the ketone molecule while the aldehydes have reactive hydrogen.

Table 3 Catalytic hydrosilylation of ketones with 1 as a catalyst^a

Entry	Substrates	TOF (h^−1)	Isolated yield (%)
a Catalytic reaction conditions: RCOR′ (1.0 mmol), (EtO)3SiH (1.2 mmol) and n-dodecane (internal standard) (1.0 mmol), 2 mL THF, 50 °C, 24 h.
1		14	33
2		11.5	29
3		14.3	40
4		16	42
5		13.3	47
6		9.3	21
7		17.8	66
8		16	51

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In order to have a deeper understanding of the reaction mechanism, we conducted the stoichiometric reactions of complex 1 with benzaldehyde or triethoxysilane respectively under the catalytic condition (monitored by in situ NMR). To our disappointment, no reaction was observed. (see the ESI†) This is different from the mechanisms reported by Nikonov,²⁴ Tilley,²⁵ and Toste.²⁶ They proposed that the first step is the reaction of catalyst with aldehyde or silane. After that the second reactant (silane or aldehyde) attacks the intermediate to finish the catalytic cycles. It is puzzling that no signal of the dissociated trimethylphosphine was found even if we detected the equivalent reaction of these three components by ³¹P NMR (see the ESI†). This result contradicts the proposal by Guan.¹⁷

To explore the mechanism, the in situ FTIR was used to track the reaction between 1 equiv. of benzaldehyde and 20% equiv. of complex 1 (Fig. 4). It was found that an equilibrium was established at about 10 min. It is conjectured that the insertion species of the carbonyl group of benzaldehyde into the Fe–H bond of complex 1 was formed. After the addition of silane (at 27 min), the concentration of the benzaldehyde decreased significantly and the silicon ether increased remarkably. Combined with our previous work,^28,29 we believe that this catalytic reaction is the result of synergy of benzaldehyde, complex 1 and silane. More work is needed to explain the details of the mechanism.

Fig. 4 FTIR spectrum of the catalytic reaction.

Conclusion

Three new hydrido iron(II) complexes (2, 3 and 4) were synthesized by the literature method and their molecular structures were confirmed by IR, NMR, and X-ray single crystal diffraction. These four typical hydrido iron(II) complexes (1–4) displayed good catalytic activity in hydrosilylation of aromatic aldehydes and ketones under mild conditions. Among them, complex 1 has the best catalytic activity.

Experimental section

General procedures and materials

Standard vacuum techniques were used in the manipulations of volatile and air-sensitive materials. Solvents were dried by known procedures and distilled under nitrogen before use. Infrared spectra (4000–400 cm⁻¹), as obtained from Nujol mulls between KBr disks, were recorded on a Bruker ALPHA FT-IR instrument. NMR spectra were recorded using Bruker Avance 300 MHz spectrometers. GC-MS was recorded on a TRACE-DSQ instrument and GC was recorded on a Fuli 9790 instrument. X-ray crystallography was performed with a Bruker Smart 1000 diffractometer. Melting points were measured in capillaries sealed under N₂ and were uncorrected. Elemental analyses were carried out on an Elementar Vario ELIII instrument. All the aldehydes and ketones were purchased and used without further purification. Fe(PMe₃)₄ ²⁷ and complex 1 ¹⁹ were prepared according to literature procedures.

Caution! (EtO)₃SiH is flammable and highly toxic by inhalation and may cause skin irritation and blindness.

Synthesis of complex 2

A sample of Fe(PMe₃)₄ (1.15 g, 3.20 mmol) in 50 mL of diethyl ether was combined with (2-diphenylphosphanyl) (6-trimethylsilanyl)benzenethiol (0.99 g, 2.70 mmol) in 30 mL of diethyl ether at −78 °C. The reaction mixture was warmed to 25 °C and stirred for 24 h to get a red solution. Red crystals were obtained after filtering at −28 °C. Yield: 1.59 g (91%). Dec. > 163 °C. Anal. calcd for C₃₀H₅₀FeP₄SSi (650.58 g mol⁻¹): C, 55.38; H, 7.75. Found: C, 55.67; H, 8.00. IR (Nujol mull, cm⁻¹): 1855 ν(Fe–H), 1548 v(C=C), 932 ρ(PMe₃). ¹H NMR (300 MHz, C₆D₆, 298 K, ppm): 8.31–6.54 (m, 13H, Ar–H), 1.33 (s, 9H, –SiMe₃), 0.82–0.63 (m, 27H, PMe₃), −12.92 (dddd, ²J(PH) = 33 Hz, ²J(PH) = 42 Hz, ²J(PH) = 48 Hz, ²J(PH) = 97.5 Hz, 1H, Fe–H). ³¹P NMR (121.5 MHz, C₆D₆, 298 K, ppm): 90.5 (m, 1P, PPh₂), 29.0 (q, J = 37.7 Hz, 1P, PMe₃), 14.8 (dd, J = 48.6 Hz, 1P, PMe₃), 7.5 (s, 1P, PMe₃). ¹³C NMR (75 MHz, C₆D₆, 298 K, ppm): 0.00 (s, SiMe₃), 21.32 (d, ³J(PC) = 13.5 Hz, PMe₃), 24.40 (d, ³J(PC) = 16.5 Hz, PMe₃), 25.78 (d, ³J(PC) = 13.5 Hz, PMe₃), 118.88–143.04 (m, Ar–C).

Synthesis of complex 3

A sample of Fe(PMe₃)₄ (1.15 g, 3.20 mmol) in 50 mL of diethyl ether was combined with (2-diphenylphosphanyl) (4-methyl)benzenethiol (0.83 g, 2.7 mmol) in 30 mL of diethyl ether at −78 °C. The reaction mixture was warmed to 25 °C and stirred for 24 h to get a red solution. Red crystals were obtained after filtering at −28 °C. Yield: 1.49 g (93%). Dec. > 156 °C. Anal. calcd for C₂₈H₄₄FeP₄S (596.46 g mol⁻¹): C, 56.38; H, 7.44. Found: C, 56.72; H, 7.69. IR (Nujol mull, cm⁻¹): 1845 ν(Fe–H), 1583 v(C=C), 934 ρ(PMe₃). ¹H NMR (C₆D₆, 298 K, ppm): 8.63–6.69 (m, 13H, Ar–H), 1.98 (s, 3H, CH₃), 1.60 (d, J = 6.0 Hz, 9H, PMe₃), 1.06 (dd, J = 6.0 Hz, 18H, PMe₃), −12.51 (dddd, ²J(PH) = 36 Hz, ²J(PH) = 45 Hz, ²J(PH) = 49.5 Hz, ²J(PH) = 99 Hz, 1H, Fe–H). ³¹P NMR (C₆D₆, 298 K, ppm): 90.5 (m, J = 25.5 Hz, 1P, PPh₂), 29.8 (q, J = 46.2 Hz, 1P, PMe₃), 14.6 (ddd, J = 12.1 Hz, 1P, PMe₃), 7.4 (m, 1P, PMe₃). ¹³C NMR (75 MHz, C₆D₆, 298 K, ppm): 20.50 (s, CH₃), 21.56 (d, ³J(PC) = 13.5 Hz, PMe₃), 24.40 (d, ³J(PC) = 17.25 Hz, PMe₃), 25.89 (d, ³J(PC) = 14.25 Hz, PMe₃), 128.57–143.48 (m, Ar–C).

Synthesis of complex 4

A sample of Fe(PMe₃)₄ (1.15 g, 3.20 mmol) in 50 mL of diethyl ether was combined with (2-diphenylphosphanyl) (4-trimethylsilanyl)benzenethiol (0.99 g, 2.7 mmol) in 30 mL of diethyl ether at −78 °C. The reaction mixture was warmed to 25 °C and stirred for 24 h to get a red solution. Red crystals were obtained after filtering at −28 °C. Yield: 1.56 g (89%). Dec. > 155 °C. Anal. calcd for C₃₀H₅₀FeP₄SSi (650.58 g mol⁻¹): C, 55.38; H, 7.75. Found: C, 54.89; H, 8.09. IR (Nujol mull, cm⁻¹): 1852 ν(Fe–H), 1558 v(C=C), 934 ρ(PMe₃). ¹H NMR (300 MHz, C₆D₆, 298 K, ppm): 8.48–6.85 (m, 13H, Ar–H), 1.43 (d, J = 6.0 Hz, 9H, PMe₃), 0.89 (dd, J = 6.0 Hz, 18H, PMe₃), 0.001 (s, 9H, SiMe₃), −12.67 (dddd, ²J(PH) = 33 Hz, ²J(PH) = 43.5 Hz, ²J(PH) = 48 Hz, ²J(PH) = 97.5 Hz, 1H, Fe–H). ³¹P NMR (C₆D₆, 298 K, ppm): 90.3 (m, 1P, PPh₂), 29.3 (q, J = 42.5 Hz, 1P, PMe₃), 15.9 (ddd, J = 14.6 Hz, 1P, PMe₃), 7.3 (m, 1P, PMe₃). ¹³C NMR (75 MHz, C₆D₆, 298 K, ppm): 1.06 (s, SiMe₃), 21.50 (d, ³J(PC) = 13.5 Hz, PMe₃), 24.35 (d, ³J(PC) = 16.5 Hz, PMe₃), 25.90 (d, ³J(PC) = 14.25 Hz, PMe₃), 128.57–142.85 (m, Ar–C).

General procedure for catalytic hydrosilylation of aldehydes

To a 25 mL Schlenk tube containing a solution of 1 in 2 mL of THF was added an aldehyde (1.0 mmol) and (EtO)₃SiH (0.20 g, 1.2 mmol). The reaction mixture was stirred at 50–55 °C until there was no aldehyde left (monitored by TLC and GC-MS). The reaction was then quenched by MeOH (2 mL) and a 10% aqueous solution of NaOH (2 mL) with vigorous stirring at 50 °C for about 24 h. The organic product was extracted with diethyl ether (10 mL × 3), dried over anhydrous MgSO₄, and concentrated under vacuum. The alcohol product was further purified using flash column chromatography (elute with 5–10% ethyl acetate in petroleum ether). The ¹H NMR spectra of the primary alcohol products are provided in the ESI.†

General procedure for the catalytic hydrosilylation of ketones

Ketones were reduced following a similar procedure to the one used for aldehydes except that the reaction time of heat was extended to 24 h. The ¹H NMR spectra of the secondary alcohol products are provided in the ESI.†

X-ray structure determination

Intensity data were collected on a Bruker SMART diffractometer with a graphite-monochromated MoKα radiation (λ = 0.71073 Å). The structure was solved by direct methods and refined with the full-matrix least-squares method on all F² (SHELXL-97) with anisotropic approximation for non-hydrogen atoms.†

Crystallographic data of complex 2

Red block crystals, C₃₀H₅₀FeP₄SSi = 650.58 g mol⁻¹, monoclinic, P2₁/n, a = 12.4708(5) Å, b = 21.9336(7) Å, c = 12.7769(4) Å, β = 105.097(3)°, V = 3374.2(2) ų, Z = 4, T = 180.15 K, μ(MoKα) = 0.753 mm⁻¹, D_calc = 1.281 g cm⁻³, 19 120 reflections measured (3.788° ≤ 2Θ ≤ 51.254°), 6333 unique (R_int = 0.0118, R_sigma = 0.0122). R₁ (I > 2σ(I)) = 0.0240, wR₂ (all data) = 0.0662.

Crystallographic data of complex 3

Red cubic crystals, C₂₈H₄₄FeP₄SS = 592.42 g mol⁻¹, Orthorhombic, Pbca, a = 9.1740 (2) Å, b = 17.9628(3) Å, c = 36.5293(8) Å, α = 90°, β = 90°, γ = 90°, V = 6019.7 (2) ų, Z = 8, T = 150 K, μ(MoKα) = 0.711 mm⁻¹, D_calc = 1.307 g cm⁻³, 19 120 reflections measured (2.27° ≤Θ ≤ 32.65°), 6333 unique (R_int = 0.1023, R_sigma = 0.0112). R₁ (I > 2σ(I)) = 0.0498, wR₂ (all data) = 0.1463.

Crystallographic data of complex 4

Red block crystals, C₃₀H₅₀FeP₄SSi = 650.58 g mol⁻¹, Triclinic, P1̄, a = 14.5546(4) Å, b = 15.9021(4) Å, c = 18.4277(5) Å, α = 65.96°, β = 71.35°, γ = 73.81°, V = 3635.75(2) ų, Z = 4, T = 153 K, μ(MoKα) = 0.698 mm⁻¹, D_calc = 1.189 g cm⁻³, 19 120 reflections measured (1.71° ≤Θ ≤ 30.24°), 6333 unique (R_int = 0.1118, R_sigma = 0.0202). R₁ (I > 2σ(I)) = 0.0742, wR₂ (all data) = 0.0825.

Acknowledgements

We gratefully acknowledge the support by NSF China no. 21372143. We also thank the kind assistance from Prof. Dieter Fenske and Dr Olaf Fuhr (Karlsruhe Nano-Micro Facility (KNMF), KIT) for the X-ray diffraction analysis.

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Footnote

† Electronic supplementary information (ESI) available: characterization of all compounds; crystallographic information files CCDC 1040932 (2), 1062381 (3), and 1063006 (4). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra09330a

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