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Imine Nitrogen Bridged Binuclear Nickel Complexes via N−H Bond Activation: Synthesis, Characterization, Unexpected C,N-Coupling Reaction, and Their Catalytic Application in Hydrosilylation of Aldehydes Lin Wang, Hongjian Sun, and Xiaoyan Li* School of Chemistry and Chemical Engineering, Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, Shandong University, Shanda Nanlu 27, 250199 Jinan, People’s Republic of China S Supporting Information *

ABSTRACT: The reactions of NiMe2(PMe3)3 with 2,6-difluoroarylimines were explored. As a result, a series of binuclear nickel complexes (5−8, 11) were synthesized. Meanwhile, from the reactions of NiMe2(PMe3)3 with [2-CH3C6H4-C(NH)-2,6-F2C6H3] (9) and [2,6-(CH3)2C6H3-C(NH)-2,6-F2C6H3] (10), two unexpected C,Ncoupling products (12 and 13) were obtained. It is believed that these coupling reactions underwent activation of the N−H and C−F bonds. The binuclear nickel complexes showed excellent catalytic activity in the hydrosilylation of aldehydes. The mechanism of the reaction was studied through stoichiometric reactions, and the double-(η2-Si−H)− NiII intermediate was detected by in situ 1H NMR spectroscopy, which may be the key point in the catalytic cycle.

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INTRODUCTION The synthesis and property exploration of binuclear metal complexes has been a research priority for several years due to their special catalytic activities.1 Oxidation of alcohols,2 phenols,3,and sulfides,4 olefin epoxidation,5 and polymerization of olefin6 are the common processes catalyzed by bimetallic complexes. In the organism, there are various metalloproteins containing the binuclear metal complexes playing important roles in the transport of oxygen, catalysis, and photoconduction.7 On the basis of this, bimetallic complexes have been widely utilized in biomimetic catalysis,8 enzymatic reactions,9 and photocatalysis.10 In addition, C,C-coupling reactions could also be catalyzed by bimetallic complexes, such as binuclear palladium and rhodium complexes.11 Even so, the types of reactions catalyzed by bimetallic complexes are still limited. The activation of N−H bonds has been widely used in building N-bridged binuclear metal complexes: Using bimetallic complexes bearing oxo-bridging ligands as precursors,12 via the reactions of diaryl or dialkyl metal complexes with ammonia or amines13 and with the support of alkyllithium reagents,14 is a common strategy to obtain binuclear metal complexes. Moreover, C,N-coupling reaction could be realized through the N−H bond activation. Palladium, rhodium, copper, and nickel complexes performed well in cleaving the N−H bonds and prompting the C,N-coupling reactions.15 However, there were few precedents reported to obtain the bimetallic complexes and realize the C,N-coupling reaction via the activation of N(sp2)−H bonds. © XXXX American Chemical Society

In this work, a series of imine nitrogen bridged binuclear nickel complexes were synthesized via the selective activation of N−H bonds from the reactions of NiMe2(PMe3)3 with fluoroarylimines. These binuclear nickel complexes performed well in the catalytic hydrosilylation of aromatic aldehydes, aliphatic aldehydes, and α,β-unsaturated aldehydes. In addition, two unexpected C,N-coupling products were obtained from the reactions of NiMe2(PMe3)3 with (2,6-difluorophenyl)(2methylphenyl)methanimine and (2,6-difluorophenyl)(2,6dimethylphenyl)methanimine. The reaction mechanism was discussed in this paper.

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RESULTS AND DISCUSSION Reaction of NiMe2(PMe3)3 with (2,6-Difluorophenyl)(aryl)methanimine. Transition-metal complexes, especially nickel complexes, have often been utilized in the activation of N−H bonds according to the literature.16 In view of this, NiMe2(PMe3)3 was used to react with fluoroarylimines. As a result, a series of imine nitrogen bridged binuclear nickel complexes were obtained via the activation of N−H bonds (Scheme 1). A mixture of NiMe2(PMe3)3 with compound 1 was stirred at 20 °C in THF for 2 days. During this period, the solution turned from pale yellow to red. Complex 5 crystallized from the diethyl ether solution as red crystals. In the IR spectrum, the typical ν(CN−H) signal disappeared and the signals of ν(CN) and ρ1(PCH3) were found at 1612 and 939 Received: August 26, 2015

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DOI: 10.1021/acs.organomet.5b00734 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Reaction of NiMe2(PMe3)3 with (2,6Difluorophenyl)(aryl)methanimine

cm−1, respectively. In the 1H NMR spectrum of 5, the resonances of H3C−Ni and P(CH3)3 were recorded at −0.432 and 0.870 ppm, respectively, with a relative integral ratio of 1:3. The coupling constant 3JP,H was 9 Hz. In the 31P NMR spectrum of 5, there was only one singlet, appearing at −8.42 ppm. The signals of Ar-F appeared at −110.89 and −110.79 ppm in the 19F NMR spectrum of 5 due to the steric effect of the imine ligand. The reactions of NiMe 2(PMe3 )3 with other imines, compounds 2−4, were also explored. Although the aryl group was substituted by either an EDG or EWG, the activation of N−H bonds could still be realized and binuclear nickel complexes 6−8 were obtained. The IR and NMR spectroscopic data of these three complexes were similar to those of complex 5. Red crystals of complexes 6 and 8 suitable for single-crystal X-ray diffraction were obtained from their diethyl ether solutions at 10 °C. The molecular structure (Figure 1) of complex 6 shows the distorted-square-planar coordination geometry around each nickel center due to the influence of the bulky imine ligand: The sum of the four bond angles around the nickel center is 358.7° (N1−Ni1−N1A, 76.9°; N1A−Ni1− C1, 95.2°; C1−Ni1−P1, 91.5°; P1−Ni1−N1, 95.1°). For the same reason, the CN double bond is not coplanar with the two nickel atoms and the sum of the three bond angles around N1 is 355.5° (Ni1A−N1−Ni1, 84.5°; Ni1−N1−C12, 137.1°; C12−N1−Ni1A, 133.9°). The N1−Ni1A bond length is 1.897 Å, which is similar to the bond length of N1−Ni1 (1.914 Å. The molecular structure of complex 8 is comparable with that of complex 6 in the structural data and configuration information. In the molecular structure of 8, the sums of the bond angles around the nickel and N1 atoms are 359.15 and 355.62°, respectively. Reactions of NiMe2(PMe3)3 with Compounds 9 and 10. In addition to the binuclear nickel complexes obtained via N−H bond activation, an unexpected C,N-coupling product, compound 12, was obtained from the reaction of NiMe2(PMe3)3 with compound 9 in a low yield (Scheme 2). The binuclear nickel complex 11 was characterized by IR and NMR spectroscopy and X-ray diffraction analysis (Figure 1). Meanwhile, MS, IR, and NMR spectroscopy were used to characterize compound 12. The MS spectrum of 12 indicates that the molecular weight of 12 is 442. The IR spectrum of 12 shows a strong ν(N−H) signal at 3386 cm−1. In the 1H NMR spectrum of 12, two types of Ar-CH3 signals appear at 2.06 and 2.49 ppm with a relative integral radio of 1:1. In the 19F NMR spectrum of 12, there are two singlets at −106.55 and −109.69 ppm with a relative integral radio of 2:1. These data are consistent with the structure of compound 12.

Figure 1. Molecular structures of 6 (top), 8 (middle), and 11 (bottom). The hydrogen atoms are omitted for clarity, and thermal ellipsoids are drawn at the 50% probability level. For selected bond distances (Å) and angles (deg) see the Supporting Information.

A similar result was obtained from the reaction of NiMe2(PMe3)3 with compound 10. This reaction gave rise to the C,N-coupling product 13 in a higher yield, 35% (Scheme 3). The 1H, 19F, and 13C NMR spectra of 13 confirm the structure of compound 13. In the 1H spectrum of 13, there are three types of aryl-CH3 signals at 2.33, 2.03, and 1.69 ppm with a relative integral ratio of 2:1:1. As for 12, the 19F NMR spectrum of 13 shows two singlets at −108.35 and −111.41 ppm in a relative integral ratio of 2:1. In accord with the literature,17 a possible mechanism was proposed (Scheme 4). The activation of an N−H bond should B


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Organometallics Scheme 2. Unexpected C,N-Coupling Reaction of NiMe2(PMe3)3 with 9

This mechanism can also be used to explain why the yields of 12 and 13 are different. In comparison to 9, there are two omethyl groups in 10. This makes the electronic effect and steric hindrance more significant. Therefore, intermediate A prefers to activate the C−F bond of another imine rather than to form a binuclear nickel complex. Thus, the yield of 13 is much higher than that of 12. Catalytic Hydrosilylation Reactions of Aldehydes Catalyzed by Binuclear Nickel Complexes. The nickel complexes were reported to be applied in the catalytic hydrosilylation of alkenes, alkynes,19 aldehydes, ketones,20 imines,21 and even CO2.22 However, for binuclear nickel complexes, there were few reports involving their application in this reaction type.23 Therefore, the catalytic activities of complexes 5−8 and 11 for hydrosilylation of aldehydes were tested (Table 1). At first, we chose p-fluorobenzaldehyde as the substrate, an equal amount of H2SiPh2 as the hydrogen source, and 1 mol % complex 11 as catalyst. The reaction was completed in 2 h at 70 °C with a conversion of 99% (entry 1). If the other conditions remained unchanged, the loading of the catalyst could be

Scheme 3. Unexpected C,N-Coupling Reaction of NiMe2(PMe3)3 with 10

Scheme 4. Possible Mechanism for the C,N-Coupling Reaction of 10

Table 1. Hydrosilylation of p-FC6H4CHO Catalyzed by Binuclear Nickel Complexesa

be the first step. Due to the steric hindrance of 10, intermediate A was formed instead of the binuclear nickel complex. The intermediate B, a nickel(IV) species, could be produced through the ligand replacement of a PMe3 by 10 and C−F bond activation. As the electron-donating groups, the methyl groups on the aryl made the nickel center electron-rich and, therefore, the C−F bond activation of another imine could be realized via oxidative addition.17d−g Intermediate B was so unstable that the reductive elimination would happen immediately in the presence of PMe3. Therefore, the C,Ncoupling product 13 was obtained with nickel(II) fluoride ([Ni−F]) as a byproduct. In order to find this byproduct, in situ 19F NMR spectroscopy was used to monitor the reaction process. Because of the low yield, there was only a weak signal at −300.0 ppm, which could be regarded as the typical signal of a Ni−F bond.18 However, when we tried to obtain the intermediate via shortening the reaction time and lowering the temperature, we failed due to the instability of the intermediate.

entry

cat.

silane

amt of cat. (%)

T (°C)

t (h)

conversion (%)b

1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16

11 11 11 11 11 11 11 11 11 11 11 5 6 7 8 5

H2SiPh2 H2SiPh2 H2SiPh2 H2SiPh2 HSi(OEt)3 HSiPh3 HSiMePh2 HSiEt3 H2SiPh2 H2SiPh2 H2SiPh2 H2SiPh2 H2SiPh2 H2SiPh2 H2SiPh2 H2SiPh2

1 0.6 0.5 0.5 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 2

70 70 70 70 70 70 70 70 50 45 35 45 45 45 45 60

2 2 2 4 4 4 4 4 3 3 7 3.5 3.5 3.5 3.5 12

99 99 70 99 65 52 38 25 99 99 99 60 53 55 56 99

a

Reaction conditions: (1) p-FC6H4CHO (1 mmol); silane (1 mmol), and binuclear nickel complex in 1 mL of THF at T °C for t h; (2) 3 mL of CH3OH and 5 mL of 10% NaOH added to the mixture at 60 °C for 24 h. bDetermined by GC, with n-dodecane as the standard. C


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Organometallics

Table 2. Hydrosilylation of Aldehydes Catalyzed by 11a

reduced to 0.6% (entries 2−4). Different silanes were tested as hydrogen sources, and H2SiPh2 was found to be the best (entries 4−8). In the presence of 0.6 mol % of complex 11, the reaction could be completed under milder conditions (entries 9 and 10). The conversion of aldehyde could still be up to 99% at 35 °C, although a longer reaction time was needed (entry 11). In addition to complex 11, complexes 5−8 were also able to catalyze the hydrosilylation reaction of aldehyde (entries 12− 16). However, 11 showed the best activity among them. On the basis of the above facts, the optimized catalytic conditions for the hydrosilylation of aldehydes were found to be 0.6 mol % catalyst loading of 11 and an equimolar amount of H2SiPh2 as the hydrogen source at a temperature of 45 °C. Under the optimized conditions, the catalytic hydrosilylations of aromatic aldehydes with different substituted groups, aliphatic and α,β-unsaturated aldehydes, were explored (Table 2). For most aromatic aldehydes, their corresponding alcohols could be obtained in good yields in a short time (Table 2, entries 1, 2, 4, 5, 7, and 8). An EDG on the aryl (entry 3) and steric hindrance (entry 6) may influence the interaction between substrate and catalyst thus, the reaction needed a longer time to be completed. For aliphatic aldehydes, the loading of the catalyst should be increased to 1 mol % and the reaction mixture should be heated to 55 °C due to their low activity (entries 11 and 12). The selective hydrosilylation reaction of α,β-unsaturated aldehydes was also explored (entries 13−16). In the presence of 1 mol % of complex 11, the formyl group was selectively hydrosilylated with retention of the CC double bond at 60 °C after 7 h. After workup, the corresponding cinnamyl alcohols were obtained in good yields. In comparison to most mononuclear nickel catalysts, complex 11 is more stable and easily prepared. In addition, this catalytic system is only suitable for the hydrosilylation of aldehydes. Ketone, ester, and amide could not be reduced catalytically to the corresponding products with this system. The catalytic activities of these catalysts are selective. In order to understand the catalytic transformation, the stoichiometric reaction of complex 11 with H2SiPh2 was explored. Two components reacted in C6D6 at 45 °C, just as for the catalytic conditions. After 24 h, the reaction was monitored by in situ IR and 1H NMR spectroscopy (Scheme 5a). The 1H NMR spectrum shows a signal at −6.52 ppm as a quadruplet of doublets which is coupled with methyl protons (q, 3JH,H = 21 Hz) and with one phosphorus atom (d, 2JH,P = 9 Hz). According to our early work and the related literature24 this signal was regarded as the resonance of the H atom of the coordinated Si−H bond linked to the nickel center. The absorption at 1976 cm−1 in the IR spectrum can be identified as the typical ν(Ni−H) signal. Under similar reaction conditions, the 1H NMR spectrum of the stoichiometric reaction of complex 7 with H2SiPh2 shows the existence of the [Ni-CH3] group at −2.26 ppm and the relative integral ratio of this signal to that of (η2-Si−H)−Ni at −5.88 ppm (q, 3JH,H = 21 Hz) is about 3:2 (Scheme 5b). These facts confirm the formation of the double-(η2-Si−H)−NiII intermediate C during the reaction process (Scheme 6). In addition, after aldehyde was added to the mixture, the signal of (η 2 -Si−H)−Ni disappeared immediately. Moreover, there was no obvious change during the reaction of nickel complex 7 or 11 with aldehyde under the same conditions. On the basis of these facts, a possible mechanism could be proposed (Scheme 6).

a

Reaction conditions unless stated otherwise: (1) aldehyde (1 mmol), H2SiPh2 (1 mmol), and complex 11 (0.006 mmol) in 1 mL of THF at 45 °C for t h; (2) 3 mL of CH3OH and 5 mL of 10% NaOH added to the mixture at 60 °C for 24 h. bIsolated yield. cThe temperature of the reaction was 55 °C. d11 (0.01 mmol); 55 °C. e11 (0.01 mmol); 60 °C. f Determined by GC, with n-dodecane as the standard.

At first, the binuclear nickel complex disintegrated in the presence of silane to form the mononuclear double-(η2-Si− H)−NiII intermediate C. With the synergy of aldehyde, cleavage of the Si−H bond and the insertion of the CO double bond into the Ni−H bond occurred. As a result, intermediate D was generated. Intermediate D was unstable. With the assistance of another H2SiPh2, the reductive elimination at intermediate D took place with the formation of siloxane. Meanwhile, the nickel−silane intermediate C was regenerated in the catalytic cycle. D


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Organometallics

Scheme 5. Hydrogen Signals of (η2-Si−H)−NiII in the in Situ 1H NMR Spectra of the Reaction between 11 and H2SiPh2 (a) and That between 7 and H2SiPh2 (b)

reference at room temperature. Meanwhile, 13C NMR spectra (100 MHz) were recorded on a Bruker Avance 400 spectrometer. 13C and 31 P NMR resonances were obtained with broad-band proton decoupling. Elemental analyses were carried out on an Elementar Vario ELIII instrument. Melting points were measured in capillaries sealed under nitrogen and are uncorrected. MS were measured on an Agilent Technologies 6510 QTOF LC/MS instrument. X-ray Crystal Structure Determinations. Single-crystal X-ray diffraction data of the complexes were collected on a Bruker SMART Apex II CCD diffractometer equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). During collection of the intensity data, no significant decay was observed. The intensities were corrected for Lorentz−polarization effects and empirical absorption with the SADABS program. The structures were resolved by direct or Patterson methods with the SHELXS-97 program and were refined on F2 with SHELXTL. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included in calculated positions and were refined using a riding model. A summary of crystal data, data collection parameters, and structure refinement details is given in the Supporting Information. CCDC 975064 (6), 1400364 (8), and 975063 (11) contain supplementary crystallographic data for this paper. Copies of the data can be obtained free of charge on application to the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax, (+44)1223-336-033; e-mail, deposit@ ccdc.cam.ac.uk). Synthesis of Complexes. Synthesis of 5. NiMe2(PMe3)3 (0.50 g, 1.58 mmol) dissolved in 20 mL of THF was added to a 30 mL of THF solution of (2,6-difluorophenyl)phenylmethanimine (1; 0.34 g, 1.58 mmol) at 20 °C. Then the mixture was stirred at the same temperature for 48 h. During this period the mixture turned from orange to red. After removal of the solvent at reduced pressure the solid residue was extracted with diethyl ether (30 mL × 2). Red crystals of 5 were obtained from the diethyl ether solution at −20 °C. Yield: 0.36 g, 63%. Mp: 178 °C dec. Anal. Calcd for C34H40F4N2Ni2P2·C4H10O (804.20 g mol−1): C, 56.62; H, 6.25; N, 3.47. Found: C,56.72; H,5.28; N, 3.64. IR (Nujol, cm−1): 1612 ν(CN), 1583, 1563 ν(CC), 940 ρ1(PCH3). 1H NMR (300 MHz, C6D6, 298 K, ppm): δ −0.43 (d, 3 JP,H = 8.4 Hz, 6H, Ni−CH3), 0.87 (d, 3JH,P = 8.7 Hz, 18H, P(CH3)3), 6.51−9.01 (m, 16H, Ar-H). 31P NMR (121.5 MHz, C6D6, 298 K, ppm): δ −8.42 (s, P, P(CH3)3). 19F NMR (282.4 MHz, C6D6, 298 K, ppm): δ −110.8 (s, 2F, Ar-F), −110.9 (s, 2F, Ar-F). 13C NMR (100 MHz, C6D6, 298 K, ppm): δ −14.1 (d, 2JP,C = 32.0 Hz, H3C-Ni), 13.8 (d, 1JP,C = 26.0 Hz, (CH3)3P), 29.9 (Ar-CH3), 110.1 (d, 2JF,C = 22.0 Hz, CAr), 111.6 (CAr), 128.4 (CAr), 131.0 (CAr), 142.7 (d, 3JF,C = 9.0 Hz, CAr), 155.6 (CAr), 160.2 (dd, 1JF,C = 249.0 Hz, 3JF,C = 10.0 Hz, FCAr), 160.8 (dd, 1JF,C = 240.0 Hz, 3JF,C = 7.0 Hz, F-CAr). Synthesis of 6. NiMe2(PMe3)3 (0.50 g, 1.58 mmol) dissolved in 20 mL of THF was added to 30 mL of a THF solution of (2,6difluorophenyl)(4-methylphenyl)methanimine (2; 0.36 g, 1.58 mmol) at 20 °C. Then the mixture was stirred at the same temperature for 48 h. During this period the mixture turned from orange to red. After removal of the solvent at reduced pressure the solid residue was extracted with diethyl ether (30 mL × 2). Red crystals of 6 were obtained from the diethyl ether solution at −20 °C. Yield: 0.35 g, 59%. Mp: 186 °C dec. Anal. Calcd for C36H44F4N2Ni2P2·2C4H10O (906.31

Scheme 6. Possible Mechanism of the Hydrosilylation Reactions of Aldehydes Catalyzed by a Binuclear Nickel Complex

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CONCLUSION In conclusion, we explored the reactions of fluoroarylimine with NiMe2(PMe3)3 and a series of imine nitrogen bridging binuclear nickel complexes were obtained via the activation of N−H bonds. In addition, the reactions of NiMe2(PMe3)3 with (2,6-difluorophenyl)(2-methylphenyl)methanimine or (2,6difluorophenyl)(2,6-dimethylphenyl)methanimine led to unexpected C,N-coupling products. The binuclear nickel complexes showed good catalytic activity in the hydrosilylation reaction of aldehyde. From the stoichiometric reaction of the binuclear nickel complex with H2SiPh2, the formation of a double-(η2-Si−H)−NiII intermediate was detected. On the basis of this finding, a possible mechanism was proposed.

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EXPERIMENTAL SECTION

General Procedures. All operations were conducted utilizing standard Schlenk techniques under a nitrogen atmosphere. Pentane, diethyl ether, THF, toluene, and dioxane were dried by distillation from Na−benzophenone under nitrogen before use. CDCl3 and C6D6 for NMR were degassed and dehydrated. NiMe2(PMe3)3 was prepared according to the literature method.25 Fluoroimines were obtained by addition reactions of 2,6-difluorobenzonitrile and arylmagnesium bromide and subsequent quenching with methanol in diethyl ether solutions.26 Infrared spectra (4000−400 cm−1), as obtained from Nujol mulls between KBr disks, were recorded on a Bruker ALPHA FT-IR instrument. 1H, 19F, and 31P NMR spectra (300, 282.4, and 121.5 MHz, respectively) were recorded on a Bruker Avance 300 spectrometer with C6D6 or CDCl3 as the solvent without an internal E


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Organometallics g mol−1): C, 58.18; H, 7.10; N, 3.08. Found: C,58.81; H, 6.87; N, 3.17. IR (Nujol, cm−1): 1632 ν(CN), 1615 ν(CN), 1588, 1573 ν(C C), 948 ρ1(PCH3). 1H NMR (300 MHz, C6D6, 298 K, ppm): δ −0.64 (d, 3JP,H = 9 Hz, 6H, Ni−CH3), 0.70 (d, 3JH,P = 9 Hz, 18H, P(CH3)3), 1.96 (s, 6H, CH3), 6.59−8.74 (m, 14H, Ar-H). 31P NMR (121.5 MHz, C6D6, 298 K, ppm): δ 5.15 (s, P, P(CH3)3). 19F NMR (282.4 MHz, C6D6, 298 K, ppm): δ −110.8 (s, 2F, Ar-F), −111.0 (s, 2F, Ar-F). 13C NMR (100 MHz, C6D6, 298 K, ppm): δ −14.2 (d, 2JP,C = 32.0 Hz, H3C-Ni), 13.8 (d, 1JP,C = 26.0 Hz, (CH3)3P), 29.9 (Ar-CH3), 110.0 (d, 2 JF,C = 23.0 Hz, CAr), 111.5 (CAr), 122.5 (CAr), 138.0 (CAr), 140.3 (d, 3 JF,C = 8.0 Hz, CAr), 155.4 (CAr), 160.3 (dd, 1JF,C = 250.0 Hz, 3JF,C = 12.0 Hz, F-CAr), 160.8 (dd, 1JF,C = 242.0 Hz, 3JF,C = 7.0 Hz, F-CAr). Synthesis of 7. NiMe2(PMe3)3 (0.50 g, 1.58 mmol) dissolved in 20 mL of THF was added to a 30 mL of a THF solution of (2,6difluorophenyl)(4-chlorophenyl)methanimine (3; 0.40 g, 1.58 mmol) at 20 °C. Then the mixture was stirred at the same temperature for 48 h. During this period the mixture turned from orange to red. After removal of the solvent at reduced pressure the solid residue was extracted with diethyl ether (30 mL × 2). Red crystals of 7 were obtained from the diethyl ether solution at −20 °C. Yield: 0.35 g, 55%. Mp: 168 °C dec. Anal. Calcd for C36H44F4N2Ni2P2 (906.31 g mol−1): C, 50.99; H, 4.78; N, 3.50. Found: C,51.36; H, 4.45; N, 3.23. IR (Nujol, cm−1): 1619 ν(CN), 1606, 1582 ν(CC), 951 ρ1(PCH3). 1 H NMR (300 MHz, C6D6, 298 K, ppm): δ −0.74 (d, 3JP,H = 9 Hz, 6H, Ni−CH3), 0.58 (d, 3JH,P = 9 Hz, 18H, P(CH3)3), 6.49−8.62 (m, 14H, Ar-H). 31P NMR (121.5 MHz, C6D6, 298 K, ppm): δ 4.81 (s, P, P(CH3)3). 19F NMR (282.4 MHz, C6D6, 298 K, ppm): δ −111.3 (s, 2F, Ar-F), −111.2 (s, 2F, Ar-F). 13C NMR (100 MHz, C6D6, 298 K, ppm): δ −13.9 (d, 2JP,C = 32.0 Hz, H3C-Ni), 13.7 (d, 1JP,C = 27.0 Hz, (CH3)3P), 110.2 (d, 2JF,C = 23.0 Hz, CAr), 111.6 (CAr), 128.8 (CAr), 134.6 (CAr), 140.7 (d, 3JF,C = 8.0 Hz, CAr), 154.4 (CAr), 160.0 (dd, 1JF,C = 249.0 Hz, 3JF,C = 8.0 Hz, F-CAr), 160.5 (dd, 1JF,C = 244.0 Hz, 3JF,C = 6.0 Hz, F-CAr). Synthesis of 8. NiMe2(PMe3)3 (0.50 g, 1.58 mmol) dissolved in 20 mL of THF was added to 30 mL of a THF solution of (2,6difluorophenyl)(thiephyl)methanimine (4; 0.35 g, 1.58 mmol) at 20 °C. Then the mixture was stirred at the same temperature for 48 h. During this period the mixture turned from orange to red. After removal of the solvent at reduced pressure the solid residue was extracted with diethyl ether (30 mL × 2). Red crystals of 8 were obtained from the diethyl ether solution at −20 °C. Yield: 0.34 g, 58%. Mp: 180 °C dec. Anal. Calcd for C30H36F4N2Ni2P2·C4H10O (816.12 g mol−1): C, 49.91; H, 5.67; N, 3.42. Found: C,50.32; H, 5.13; N, 3.17. IR (Nujol, cm−1): 1619 ν(CN), 1607, 1586 ν(CC), 950 ρ1(PCH3). 1H NMR (300 MHz, C6D6, 298 K, ppm): δ −0.66 (d, 3 JP,H = 9 Hz, 6H, Ni−CH3), 0.82 (d, 3JH,P = 9 Hz, 18H, P(CH3)3), 6.48−6.96 (m, 12H, Ar-H). 31P NMR (121.5 MHz, C6D6, 298 K, ppm): δ 4.80 (s, P, P(CH3)3). 19F NMR (282.4 MHz, C6D6, 298 K, ppm): δ −109.6 (m, 2F, Ar-F), −112.4 (s, 2F, Ar-F). 13C NMR (100 MHz, C6D6, 298 K, ppm): δ −14.3 (d, 2JP,C = 32.0 Hz, H3C-Ni), 13.8 (d, 1JP,C = 27.0 Hz, (CH3)3P), 110.0 (d, 2JF,C = 23.0 Hz, CAr), 111.6 (CAr), 125.4 (d, 2JF,C = 11.0 Hz, CAr), 126.6 (CAr), 149.1 (CAr), 150.4 (d, 3JF,C = 8.0 Hz, CAr), 160.0 (dd, 1JF,C = 249.0 Hz, 3JF,C = 8.0 Hz, FCAr), 160.5 (dd, 1JF,C = 250.0 Hz, 3JF,C = 9.0 Hz, F-CAr). Synthesis of 11. NiMe2(PMe3)3 (0.50 g, 1.58 mmol) dissolved in 20 mL of THF was added to 30 mL of a THF solution of (2,6difluorophenyl)(2-methylphenyl)methanimine (9; 0.36 g, 1.58 mmol) at 20 °C. Then the mixture was stirred at the same temperature for 48 h. During this period the mixture turned from orange to red. After removal of the solvent at reduced pressure the solid residue was extracted with diethyl ether (30 mL × 2). Red crystals of 11 were obtained from the diethyl ether solution at −20 °C. Yield: 0.25 g, 43%. Mp: 116 °C dec. Anal. Calcd for C36H34F4N2Ni2P2 (758.16 g mol−1): C, 56.89; H, 5.83; N, 3.69. Found: C, 56.75; H, 5.15; N, 3.47. IR (Nujol, cm−1): 1616 ν(CN), 1599, 1584 ν(CC), 952 ρ1(PCH3). 1 H NMR (300 MHz, C6D6, 298 K, ppm): δ −0.63 (d, 3JP,H = 6 Hz, 6H, Ni−CH3), 0.60 (d, 3JH,P = 6 Hz, 18H, P(CH3)3), 2.46 (s, 6H, CH3), 6.23−9.06 (m, 14H, Ar-H). 31P NMR (121.5 MHz, C6D6, 298

K, ppm): δ 4.56 (s, P, P(CH3)3). 19F NMR (282.4 MHz, C6D6, 298 K, ppm): δ −103.2 (m, 2F, Ar-F), −109.2 (s, 2F, Ar-F). Synthesis of 12. NiMe2(PMe3)3 (0.50 g, 1.58 mmol) dissolved in 20 mL of THF was added to 30 mL of a THF solution of (2,6difluorophenyl)(2-methylphenyl)methanimine (9; 0.36 g, 1.58 mmol) at 20 °C. Then the mixture was stirred at the same temperature for 48 h. During this period the mixture turned from orange to tan. The solvent was removed at reduced pressure, and the solid residue was extracted with diethyl ether (50 mL × 2). After red crystals of 11 crystallized from this solution, the volatile materials were evaporated under vacuum. The residue was further purified by column chromatography on silica gel (petroleum ether (60−90 °C)/NEt3 10/1, v/v). After recrystallization in diethyl ether, 12 was obtained as white crystals. Yield: 0.03 g, 8%. Mp: 187 °C. QTOF LC/MS (m/z): calcd for C28H21F3N2 [M + H]+, 443.17; found, 443.1801. IR (Nujol, cm−1): 3391 ν(CNH), 1613 ν(CN), 1573 ν(CC). 1H NMR (300 MHz, C6D6, 298 K, ppm): δ 2.06 (s, 3H, CH3), 2.49 (s, 3H, CH3), 6.28−7.20 (m, 15H, Ar-H and CNH). 19F NMR (282.4 MHz, C6D6, 298 K, ppm): δ −106.6 (s, 2F, Ar-F), −109.7 (s, 1F, ArF). 13C NMR (100 MHz, C6D6, 298 K, ppm): δ 19.0 (s, CH3-Ar), 22.3 (s, CH3-Ar), 105.5 (d, 2JF,C = 23.0 Hz, CAr), 110.1 (CAr), 112.0 (d, 2JF,C = 28.0 Hz, CAr), 120.2 (t, 2JF,C = 13.0 Hz, CAr), 125.5 (CAr), 126.7 (CAr), 128.2 (CAr), 129.7 (t, 3JF,C = 11.0 Hz, CAr), 133.3 (CAr), 133.9 (d, 3JF,C = 11.0 Hz, CAr), 140.3 (CAr), 146.5 (d, 3JF,C = 6.0 Hz, CAr), 160.1 (d, 1JF,C = 253.0 Hz, F-CAr), 161.0 (d, 1JF,C = 242.0 Hz). Synthesis of 13. NiMe2(PMe3)3 (0.50 g, 1.58 mmol) dissolved in 20 mL of THF was added to a 30 mL of a THF solution of (2,6difluorophenyl)(2,6-dimethylphenyl)methanimine (10; 0.39 g, 1.58 mmol) at 20 °C. Then the mixture was stirred at the same temperature for 48 h. After removal of the solvent at reduced pressure the solid residue was extracted with diethyl ether (50 mL × 2). The extract was filtered, followed by the evaporation of the volatile materials under vacuum. The residue was further purified by column chromatography on silica gel (petroleum ether (60−90 °C)/NEt3 10/1, v/v). After recrystallization in diethyl ether, 13 was obtained as white crystals. Yield: 0.13 g, 35%. Mp: 56 °C. QTOF LC/MS: (m/z) calcd for C30H25F3N2 [M + H]+, 471.197; found, 471.2393. IR (Nujol, cm−1): 3385 ν(CNH), 1620 ν(CN), 1578 ν(CC). 1H NMR (300 MHz, C6D6, 298 K, ppm): δ 1.69 (s, 3H, CH3), 2.03 (s, 3H, CH3), 2.33 (s, 6H, CH3), 6.26−7.27 (m, 13H, Ar-H and CNH). 19F NMR (282.4 MHz, C6D6, 298 K, ppm): δ −108.4 (s, 2F, Ar-F), −111.4 (s, 1F, Ar-F). 13C NMR (100 MHz, C6D6, 298 K, ppm): δ 18.6 (s, CH3Ar), 19.3 (s, CH3-Ar), 23.8 (s, CH3-Ar), 104.9 (d, 2JF,C = 23.0 Hz, CAr), 105.5 (d, 2JF,C = 14.0 Hz, CAr), 110.2 (CAr), 112.6 (d, 2JF,C = 28.0 Hz, CAr), 121.9 (t, 2JF,C = 15.0 Hz, CAr), 127.1 (CAr), 127.5 (d, 2JF,C = 25.0 Hz, CAr), 128.9 (t, 3JF,C = 11.0 Hz, CAr), 131.1 (CAr), 133.8 (d, 3JF,C = 11.0 Hz, CAr), 134.2 (CAr), 136.1 (CAr), 139.3 (CAr), 142.3 (CAr), 146.7 (d, 3JF,C = 5.0 Hz, CAr), 159.9 (dd, 1JF,C = 249.0 Hz, 3JF,C = 8.0 Hz, FCAr), 160.3 (d, 1JF,C = 253.0 Hz), 177.4 (s, CN), 185.8 (s, CN). Representative Experimental Procedure of Catalytic Hydroslilylation of Aldehydes. A 25 mL Schlenk tube was charged with a mixture of PhCHO (1.0 mmol), H2SiPh2 (1.0 mmol), and complex 11 (0.006 mmol) in 2 mL of THF. The contents of the reaction vessel were stirred at 45 °C for 3 h. The progress of the reaction was monitored by TLC. After the mixture was cooled to room temperature, CH3OH (3 mL) and 10% NaOH (5 mL) were placed in the tube. After this mixture was stirred for 24 h at 60 °C, the product was extracted with Et2O (30 mL × 2). The combined organic phases were dried over anhydrous NaSO4 and filtered, and the solvent was evaporated under reduced pressure. The product was purified by column chromatography on silica gel (petroleum ether (60−90 °C)/ ethyl acetate 5/1, v/v) to afford PhCH2OH as a colorless liquid (0.10 g, 94%). Stoichiometric Reaction of Complex 11 with H2SiPh2. A 0.8 mL amount of a C6D6 solution of 11 (0.1 g, 0.13 mmol) and H2SiPh2 (0.048 g, 0.26 mmol) were sealed in a NMR tube. The mixture was reacted at 45 °C for 24 h. During this period the mixture turned from red to claret red. The reaction was monitored by 1H NMR spectroscopy in situ. In the 1H NMR spectrum, the signal of the Ni−H bond was found at −6.52 ppm as a quadruplet of doublets F


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Organometallics (3JH,H = 21 Hz, 2JH,P = 9 Hz). After removal of the solvent at reduced pressure the solid residue was monitored by IR spectroscopy. In the IR spectrum, a typical ν(Ni−H) signal was found at 1975.8 cm−1. Stoichiometric Reaction of Complex 7 with H2SiPh2. A 0.8 mL portion of a C6D6 solution of 7 (0.1 g, 0.12 mmol) and H2SiPh2 (0.048 g, 0.26 mmol) were sealed in a NMR tube. The mixture was reacted at 70 °C for 48 h. During this period the mixture turned from red to claret red. The reaction was monitored by 1H NMR spectroscopy in situ. In the 1H NMR spectrum, the signal of the Ni−H bond was found at −5.91 ppm and that of Ni-CH3 was at −2.26 ppm.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00734. Confirmatory experiments to understand the mechanisms of the C,N-coupling reaction and hydrosilylation reactions of aldehydes and the NMR spectra of stoichiometric reactions, crystallographic data for 6, 8, and 11, and the original IR, 1H NMR, 31P NMR, and 13C NMR spectra of the complexes (PDF) Crystallographic data for 6, 8 and 11 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for X.L.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support by the NSF of China (No. 21372143/21172132) and support from Prof. Dr. Dieter Fenske and Dr. Olaf Fuhr (Karlsruhe Nano-Micro Facility) on the determination of the crystal structures.

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REFERENCES

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