Nickel-Mediated Stepwise Transformation of CO to Acetaldehyde and

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Nickel-Mediated Stepwise Transformation of CO to Acetaldehyde and Ethanol Ailing Zhang,† Sakthi Raje,‡ Jianguo Liu,† Xiaoyan Li,† Raja Angamuthu,‡ Chen-Ho Tung,† and Wenguang Wang*,† †

School of Chemistry and Chemical Engineering, Shandong University, 27 South Shanda Road, Jinan 250100, China Laboratory of Inorganic Synthesis and Bioinspired Catalysis (LISBIC), Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India

‡

S Supporting Information *

ABSTRACT: The insertion of CO into the Ni−C bond of synthetic Ni(II)−CH3 cationic complex ([1-CH3]+) affords a nickel−acetyl complex ([1-COCH3]+). Reduction of resultant [1-COCH3]+ by borohydrides produces CH3CHO, CH3CH2OH, and an Ni(0) compound ([1]0), which can react with CH3I to regenerate [1CH3]+. By conducting deuterium labeling experiments, we have demonstrated that CH 3 CHO is the primary product from CH3CH2OH in such CO transformation reactions. In the reduction of [1-COCH3]+, the formation of CH3CHO competes with the loss of CH4, which leads to a Ni(0)−CO compound ([1-CO]0) as a minor product. Our results establish fundamental steps in the exploration of nickel-mediated CO transformation to valuable chemicals.

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(SDmp)] (dadtEt = CH2(CH2N(Et)C2H4S¯)2; Dmp = 2,6dimesitylphenyl) has been observed to undergo carbonylation yielding thioacetate ester CH3COSDmp and a [Ni(II)Ni(0)] species, [Ni(dadtEt)Ni(COD)].13 Monomeric Ni(0)-CO or Ni(I)-CO complexes supported by anionic ligands can be methylated to yield Ni-COCH3 species.14 The complexes K2[(LtBu)Ni(CO)]2 (LtBu = [HC(C(tBu)NC6H3(iPr)2)2]¯)15 and [(PNP)Ni(CO)] (PNP = N[2-PiPr2-4-Me-C6H3]2¯)16 react with CH3I to afford neutral complexes I and II, respectively (Chart 1). In contrast, the binding of CO to Ni(II) center in cationic complexes is usually weak and reversible.17 In such cases, transmetalation of a methyl group from organometallic Grignard reagents such as CH3MgX to Ni(II) precursors is the most commonly used method for the synthesis of nickel−methyl complexes.2f,18 Several nickel− acetyl complexes (III,19 IV,20 and V21) and nickel−acyl intermediate VI22 are reported to have been formed by the insertion of CO into the Ni−C bonds of nickel−methyl or nickel−alkyl complexes. The production of thioacetate has been achieved by treatment of [Ni(II)-COCH3] complexes with thiols or thiolates (Chart 1). However, organonickel species tend to degenerate to elemental nickel and the uncoordinated protonated ligand. The reactivity of the synthetic [Ni-COCH3] complexes is particularly interesting because it not only is relevant to the biological CO transformation in thioester synthesis but also provides knowledge that can be used in exploration and

INTRODUCTION Transition-metal-mediated CO transformation into valuable chemicals is of fundamental importance and is interesting because it utilizes CO as a C1 source to produce organic carbonyl substrates or to construct C−C bonds.1,2 In addition, it is relevant to many industrial catalytic reactions such as the Monsanto process for acetic acid synthesis by catalytic carbonylation of methanol,1g hydroformylation,3 and water− gas shift reactions.4 In a natural CO dehydrogenase(CODH)/ acetyl-CoA synthase (ACS) complex, CO2 is reduced by CODH to CO, which combines with a methyl group donated by the corrinoid iron−sulfur protein (CFeSP) and coenzyme A (CoA) to generate acetyl-CoA.5 ACS is composed of a dinickel dithiolate cluster, Ni(II)Ni(II), in a ground state. The proximal nickel center formed by Fe4S4 cluster allows for its reduction to Ni(I) for CO binding or to Ni(0) for methylation. The intermediates of Ni(I)-CO or Ni(II)-CH3 postulated could result in two alternative mechanistic routes for the C−C bond formation (eqs 1 and 2).6,7 Ni(I)‐CO + CH3+ + e− → Ni(II)‐COCH3

(1)

Ni(II)‐CH3 + CO → Ni(II)‐COCH3

(2)

Synthetic nickel−acetyl (Ni-COCH3) models are essential to reproduce nickel-mediated CO methylation that the enzymatic system does.5d,8 In this context, monomeric,9 homo-,10 or heterobimetallic10e,11 synthetic nickel complexes have been investigated with the aim of reproducing the nickel−acetyl (NiCOCH3) complexes by methylation of CO.12 For example, the Ni−C bond in trithiolato-dinickel complex [Ni(dadtEt)Ni(Me)© XXXX American Chemical Society

Received: June 20, 2017

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DOI: 10.1021/acs.organomet.7b00472 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Chart 1. Examples of Monomeric Nickel−Acetyl Complex for the Thioacetate Synthesis

Figure 1. Structures of the dications [1]2+ (left) and [1(NCMe)2]2+ (right). Thermal ellipsoids are shown at the 50% probability level, and hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): [1]2+, Ni1−P1 2.196(1), Ni1−P2 2.203(1), Ni1−S1 2.197 (1), Ni1−S2 2.176(1), S1−Ni1−P2 156.83(5), S2−Ni1−P1 150.13(5); [1(NCMe)2]2+, Ni1−S1 2.4024(9), Ni1−P1 2.4966(8), Ni1−N 2.053(3), P1−Ni1−P1′ 177.86(4).

which features an octahedral geometry. The complex [1(NCMe)2]2+ is paramagnetic with a magnetic moment of 3.01 μB established by the Evans method. This is consistent with expectations for an uncoupled nickel(II) ion.28 [(P2S2)NiH]+ and [(P2S2)NiCH3]+. The reactivity of [1]2+ or [1(NCMe)2]2+ was illustrated by the reaction with pinacolborane (HBpin), which results in the formation of cationic nickel(II) hydride [1-H]+ (Scheme 2). The 1H NMR spectrum

development of nickel-based catalysts for manufacturing of value-added products such as CH3CHO or CH3CH2OH.23−25 In this paper, we describe the production of CH3CHO and CH3CH2OH by reduction of synthetic [Ni-COCH3]+ cationic complexes with borohydride salts (Scheme 1). Scheme 1. Reduction of Synthetic Nickel−Acetyl Cationic Complex by BH4¯ Salts

Scheme 2. Reactions of [1(NCMe)2]2+ with HBpin and CH3MgBr and Formation of a Nickel−Acetyl Complex

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RESULTS AND DISCUSSION [(P2S2)Ni]2+ and [(P2S2)Ni(NCMe)2]2+. Owing to its soft and flexible coordination properties, the phosphine-thioether ligand has been investigated intensively for group 10 metals such as Pd or Pt.26 In our previous study, we found that the tetradentate ligand (Ph2PC6H4CH2S)2(C2H4), abbreviated as P2S2, coordinated with iron(II) to form the cationic complex [(P2S2)FeH(CO)]+ folded in such a way that the hydride and CO ligand are in a cis-α disposition.27 The dicationic complex [Ni(P2S2)](BF4)2 ([1]2+) was prepared by the straightforward treatment of a solution of P2S2 in THF with a solution of Ni(BF4)2·6H2O in isopropanol. The molecular structure of [1]2+ was established by single-crystal X-ray diffraction, which showed that the Ni(II) center adopts a distorted-tetrahedral geometry instead of the square-planar geometry common in four-coordinate nickel(II) complexes (Figure 1). The angle ∠P2−Ni1−S1, 156.83(5)°, is comparable to that of 150.13(5)° observed for ∠P1−Ni1−S2. Interestingly, dissolving [1](BF4)2 in MeCN led to the formation of an uncommon 20-electron bis-acetonitrile complex [1(NCMe)2](BF4)2, the structure of

of [1-H]+ in CD3CN exhibits a triplet at δ − 13.96 (t, JP−H = 48.3 Hz) corresponding to the hydride resonance, and the 31P NMR spectrum exhibits a sharp singlet at δ 28.5. The crystal structure of [1-H]+ reveals a trigonal-bipyramidal geometry (Figure S8). The generation of the nickel(II) hydride [1-H]+ using a hydroborane such as HBpin has been reported for the pincer-type nickel formate complex [(tBuPCP)NiOC(O)H] with catecholborane.29 It is noteworthy that transferring hydride from typical hydroboranes to the nickel(II) dications providing a five-coordinate [Ni(II)-H]+ cationic complex has not been reported to date, and [1-H]+ was found to be stable toward CO or CO2 gas. The reaction of [[1(NCMe)2](BF4)2 with CH3MgBr in THF at 0 °C produced [1-CH3]BF4, which was isolated in 79% yield as a red solid. The 1H NMR spectrum of [1-CH3]+ in CD2Cl2 displays a triplet at δ −0.04 (t, JP−H = 10.5 Hz, 3H) B

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Organometallics corresponding to Ni(II)-CH3. This is consistent with the 1H NMR spectra of related cationic nickel(II)−methyl complexes.18e,30 Exchanging the BF4¯ counteranion for BPh4¯ allowed growth of single crystals of [1-CH3]BPh4, suitable for X-ray diffraction. Crystallographic analysis revealed that the nickel(II) center is coordinated with both the folded P2S2 and the methyl ligand with a pseudo-trigonal-bipyramidal geometry (Figure 2).

C2H5SH, but it reacted with NBu4BH4 salts producing CH3CHO and releasing CH4 (Scheme 3). Treatment of a Scheme 3. Formation of Acetaldehyde, Ethanol, and Methane from the Reaction of [1-COCH3]+ with BH4¯ Salts

CD3CN solution (1 mL) of [1-COCH3]BPh4 (18.8 mM) with 1 equiv of NBu4BH4 led to color change from orange to red within 5 min with concurrent formation of red precipitate. The red solid was isolated by filtration and identified as [(P2S2)Ni] ([1]0, 63% yield) with the phosphorus resonance signal at δ 21.4 in its 31P NMR spectrum. Alternatively, [1]0 could be prepared by the reaction of P2S2 ligand with [Ni(COD)2] in MeCN; as shown by 31P NMR spectroscopic studies, it is identical to [1]0 generated from [1-COCH3]BPh4. The filtrate from the reaction mixture of [1-COCH3]+ with NBu4BH4 was further analyzed by NMR and IR spectroscopy. The 1H NMR spectrum exhibits a characteristic quartet at δ 9.71 corresponding to the aldehyde proton of CH3CHO (q, JH−H = 2.8 HZ, Figure 4a). The 31P NMR spectrum indicated the presence of another organonickel product, distinct from that of [1]0 and exhibiting a 31P signal at δ 30. The molecular structure of this new nickel(0) compound [(P2S2)Ni(CO)], [1CO]0 (νCO = 1907 cm−1), was characterized crystallographically. [1-CO]0 is a four-coordinate neutral compound with an tetrahedral geometry (Figure 3). One of the S atoms is

Figure 2. Solid-state structures of the cations [1-CH3]+ (left) and [1COCH3]+ (right). Thermal ellipsoids are shown at the 50% probability level, and hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): [1-CH3]+, Ni1−S1 2.253(1), Ni1−S2 2.264(1), Ni1−P1 2.278(1), Ni1−P2 2.178 (1), Ni1−C1 2.042(4), P2−Ni1−S2 130.07(5), P2−Ni1−P1 129.71(4); [1-COCH3]+, Ni1−S1 2.349(1), Ni1−S2 2.278(1), Ni1−P1 2.198(1), Ni1−P2 2.228(1), Ni1−C1 1.922(4), P1−Ni1−P2 153.82(4), O1−C1−Ni1 123.0(3), O1−C1− C2 120.1(4), C2−C1−Ni1 116.9(3).

The Ni−C distance 2.042(4) Å is very close to 2.02(2) Å which was observed in [(NP3)Ni-CH3]+ (NP3 = N(CH2CH2PPh2)3)18e and slightly longer than 1.94(2) Å in [(NS3iPr)Ni-CH3]+ (NS3iPr = N(CH2CH2SiPr)3)19b and 1.92 (2) Å in [(CH2SiMe3)(PMe3)2Ni-CH3]BF4.30 Insertion of CO into Ni−C Bond. [1-CH3]BPh4 reacts readily with CO, affording the Ni(II)−acetyl complex [1COCH3]BPh4 in 86% yield. Exposure of a CH2Cl2 solution of [1-CH3]+ to CO gas at atmospheric pressure resulted in a rapid color change from red to orange. The IR spectrum of the reaction mixture displays a νCO band at 1663 cm−1 for Ni(II)COCH3.20 The acyl group was also identified in the 13C NMR spectrum, which features a characteristic Ni-COCH3 resonance at δ 242.6 (t, JP−C = 15 Hz),15a,31 while the Ni-CH3 resonance was observed at δ 4.1 (t, JP−C = 17.2 Hz) for [1-CH3]+. The 1H NMR spectrum displayed a distinct signal for Ni-COCH3 at δ 1.74. The structure of [1-COCH3]+ was confirmed by X-ray crystallographic analysis (Figure 2). The overall stereochemistry of [1-COCH3]+ is similar to that of [1-CH3]+, and the acyl group was unambiguously coordinated with the metal center. Carbonylation of the Ni-CH3 bond decreased the distortion of the trigonal-bipyramidal geometry, and this was reflected in the increase of ∠P2−Ni1−P1 from 129.71(4)° in [1-CH3]+ to 153.82(4)° in [1-COCH3]+. The Ni−C bond length in [1-COCH3]+ is 1.922(4) Å, which is 0.12 Å shorter than the Ni−C bond length in [1-CH3]+. The angles ∠Ni1− C1−C2 and ∠Ni1−C1−O1 in [1-COCH3]+ are 116.9(3) and 123.0(3)°, respectively, compared to ∠Ni−C−CH3 = 118(1)° and ∠Ni−C−O = 123 (1)° for III. Reduction of Nickel−Acetyl Complex by BH4− Salts. In contrast to the previously discussed Ni(II)−acetyl complexes, [1-COCH3]+ failed to react with thiols such as C6H5SH and

Figure 3. Solid-state structure of tetrahedral [1-CO]0. Selected distances (Å) and angles (deg): Ni1−S1 2.2456(8), Ni1−P1 2.1947(8), Ni1−P2 2.2005(8), Ni1−C1 1.750(2), O1−C1 1.154(2), P1−Ni1−S1 95.59(3), P1−Ni1−P2 121.06(3), P2−Ni1−S1 102.98(3), C1−Ni1−S1 117.32(7), C1−Ni1−P1 117.31(7), C1− Ni1−P2 102.46(7), O1−C1−Ni1 173.4(2).

not coordinated to the nickel(0) center, revealing the coordination versatility of P2S2. We have also found that the reaction of [1-COCH3]+ with BH4¯ salts releases CH4 (yield: 28.6 ± 4%), which was identified and quantified by GC analysis. In the cases of nickel-mediated thioester formation,2e,19 the reaction of nickel−acetyl complexes with thiols was proposed to occur via nucleophilic attack of the thiolate on the acetyl C

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Organometallics

For the reduction of [1-COCH3]+ by BH4¯ salts, the yield of [1-CO]0 was 32%, which is comparable to the yield of 28.6 ± 4% for CH4. The formation of [1-CO]0 should be along with the release of CH4. Surprisingly, the C−C bond of acetyl moiety undergoes cleavage in the reduction of [1-COCH3]+ by BH4¯ salts. Although both [1]0 and [1-CO]0 are nickel(0) species, their reactivities differ significantly. For example, [1]0 reacts with CD3I affording [1-CD3]+, from which [1-COCD3]+ is formed through CO insertion. However, there is no reaction observed between [1-CO]0 and CH3I. The inactivity of [1CO]0 is due to Ni(0) center ceding electron density to the carbonyl group through back-donation.

ligand.32 It has also been reported that exposure of (bpy)Ni(CH3)(SR) to CO atmosphere resulted in reductive elimination of CH3C(O)SR.22 In our case, the formation of two types of the carbonyl products, CH3CHO and [1-CO]0 indicate the existence of two different pathways (Scheme 4). [1Scheme 4. Nickel-Mediated Stepwise Transformation of CO to CH3CHO and C2H5OH

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CONCLUSIONS A nickel−acetyl complex has been successfully obtained by the insertion of CO into the Ni−C bond of a nickel−methyl cationic complex. Reducing the resultant nickel−acetyl complex with borohydrides produces CH3CHO, CH3CH2OH, and a Ni(0) compound ([1]0). The formation of CH3CHO competes with the loss of CH4 which leads to Ni(0)-CO compound ([1CO]0) rather than [1]0 as a minor product. Notably, regeneration of [1-CH3]I could be achieved through the reaction of [1]0 with CH3I. The reductive elimination of acetyl iodide (CH3COI) from six-coordinate decarbonyl complex [Rh(COCH3)(CO)2I3]− regenerating [Rh(CO)2I2]− catalyst, is one of elementary steps in acetic acid synthesis.33,34 By contrast, [1-COCH3]I is too stable to undergo CH3COI reductive elimination. However, our results illustrate the important steps in the exploration of nickel-based catalysts for reductive transformation of CO to useful commodity chemicals.

COCH3]+ could undergo nucleophilic attack by the hydride ligand of BH4¯ forming a hydrido−acyl intermediate, [Ni(H)(COCH3)]. The reductive elimination of CH3CHO from the Hydrido−acyl species can generate [1]0.22 The production of CD3CHO for [1-COCD3]+ with NBu4BH4 was verified by 1H NMR and 2H NMR spectra with the signals at δ 9.72 (br) for CHO and δ 2.05 for CD3 group (Figures 4b and S28). When

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EXPERIMENTAL SECTION

Materials and Methods. All manipulations were conducted under N2 atmosphere using standard Schlenk techniques or in a glovebox, unless otherwise stated. All reagents were purchased from SigmaAldrich and used as received. (Ph2PC6H4CH2S)2(C2H4) was prepared according to our previous methods.27 Et2O, pentane, THF (dried by distillation over sodium), MeCN (dried by distillation over CaH2), and CH2Cl2 for general use were of analytical (AR) grade and stored under N2 atmosphere. CD2Cl2 and CD3CN were dried using activated molecular sieves (4 Å) and degassed with three thaw−freeze cycles. C6D6 and THF-d8 were dried over CaH2 and purified by vacuum transfer. NMR spectra were recorded on Bruker Avance 500, Avance 400, Avance 300 spectrometers in J. Young NMR tubes. 1H, 13C, and 31 P NMR chemical shifts are referenced to the proton signal of the deuterated solvent and external H3PO4, respectively. Single-crystal Xray diffraction data were collected using a Bruker SMART APEX II diffractometer with a CCD area detector (graphite monochromatic Mo Kα radiation) at 173 K. Infrared spectra were recorded on a PerkinElmer FT-IR Spectrometer Spectrum Two and reported for the νCO region only. Cyclic voltammetry was performed under N2 at room temperature using a CHI 760e electrochemical workstation (Shanghai Chen Hua Instrument Co., Ltd.) with a glassy carbon working electrode, Pt wire counter electrode, and the pseudo-reference electrode Ag wire. CH4 was identified by a Techcomp 7890 II gas chromatograph (GC) equipped with a 5 Å molecular sieve column using argon as carrier gas and a thermal conductivity detector. [(P2S2)Ni](BF4)2 ([1]2+). To a solution of Ni(BF4)2·6H2O (0.15 g, 0.44 mmol) in 5.0 mL of 2-propanol was added the solution of (Ph2PC6H4CH2S)2(C2H4) (0.28 g, 0.44 mmol) in 10.0 mL of THF over the course of 5 min. The mixture was stirred at room temperature for 2.0 h and gave red precipitates. The product was isolated by filtration and rinsed with 3.0 mL of 2-propanol followed by 2.0 mL of THF, then dried under vacuum. Yield: 0.23 g (74%). Single crystals suitable for X-ray diffraction were obtained by layering pentane into

Figure 4. Reduction of nickel−acetyl complexes by BH4¯ salts. 1H NMR spectra: (a) CH3CHO for the [1-COCH3]+/NBu4BH4 system and (b) CD3CHO for the [1-COCD3]/NBu4BH4 system in CD3CN. 2 H NMR spectra: (c) CD3CH2OH from [1-COCD3]+ with excess NBu4BH4, (d) CH3CD2OD from [1-COCH3]+ with excess NaBD4, and (e) CD3CD2OD from [1-COCD3] with excess NaBD4 in MeCN. Conditions: 2 μL of H2O was added to c, and D2O to d and e to quench the reactions prior to NMR analysis.

excess of NaBH4 was used, CH3CH2OH was obtained as the major product. Furthermore, the 2H NMR spectra observed for the systems of [1-COCD3]+/NBu4BH4, [1-COCH3]+/NaBD4, and [1-COCD3]+/NaBD4 demonstrate the production of CD3CH2OH, CH3CD2OD, and CD3CD2OD (Figure 4, spectra c−e). These results suggest the pathway of reductive elimination of CH3CHO is possible. D

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Organometallics CH2Cl2 at −30 °C. 1H NMR (500 MHz, CD2Cl2): δ 7.56−7.17 (m, 28 × ArH), 4.28−3.95 (br, 4H, SCH2Ph), 3.30−2.91 (br, 4H, SCH2CH2S). 31P NMR (202 MHz, CD2Cl2): δ 32.62 (br), 17.04 (br). Anal. Calcd for C40H36NiP2S2B2F8: C, 54.90; H, 4.15. Found: C, 54.98; H, 4.50. ESI-MS: calcd for [1]2+: 350.0544; found: 350.0533. [(P2S2)Ni(MeCN)2](BF4)2 ([1(NCMe)2]2+). By dissolving 175 mg of [1](BF4)2 in 10 mL of MeCN, a yellow solution was obtained. The product was precipitated by the addition of 10 mL of Et2O. The yellow solid was collected by filtration. Yield: 0.18 g (92%). Crystals were grown via slow diffusion of Et2O into a MeCN solution. μeff = 3.01 μB. Anal. Calcd for C44H42N2NiP2S2B2F8: C, 55.21; H, 4.42; N, 2.93. Found: C, 55.31; H, 4.70; N, 2.75. ESI-MS: calcd for ([1]2+-2MeCN): 350.0544; found: 350.0545. [(P2S2)NiH]BF4 ([1-H]+). Treatment the CH2Cl2 solution of [1](BF4)2 (0.15 g, 0.21 mmol) with excess pinacolborane (HBpin, 0.1 mL) resulted in the color of solution changing from red to orange. The solvent was removed under vacuum. The residue, an orange solid, was filtered and rinsed with 2.0 mL of benzene followed by 3 × 5.0 mL of diethyl ether and dried under vacuum. Yield: 0.13 g (86%). Single crystals suitable for X-ray diffraction were obtained by layering pentane into DCM at −30 °C. 1H NMR (300 MHz, CD3CN): δ 7.78−6.82 (m, 28 × ArH), 4.02 (d, 2H, SCH2Ph) 3.15 (d, 2H, SCH2Ph), 2.72 (m, 2H, SCH2CH2S), 0.47 (m, 2H, SCH2CH2S), −13.96 (t, JP−H = 48.3 Hz, Ni-H). 31P NMR (121 MHz, CD3CN): δ 28.5 (s). Anal. Calcd for C40H37BF4NiP2S2: C, 60.87; H, 4.73. Found: C, 60.99; H, 4.92. ESI-MS: calcd for [1-H]+: 701.1165; found: 701.1156. (P2S2)Ni ([1]0). A 50 mL Schlenk flask was charged with (Ph2PC6H4CH2S)2(C2H4) (0.47 g, 0.73 mmol) and Ni(COD)2 (0.20 g, 0.73 mmol) in a nitrogen box. Addition of 30 mL of CH3CN with vigorous stirring (10 min) gave a dark red solid, which was filtered, washed twice with 2 mL of anhydrous diethyl ether, and dried under vacuum to yield 0.41 g (80%) of (P2S2)Ni. 1H NMR (500 MHz, C6D6): δ 7.94−6.78 (m, 28 × ArH), 3.24 (d, 2H, SCH2Ph), 3.06 (d, 2H, SCH2Ph), 2.20 (s, 2H, SCH2CH2S), 2.07 (s, 2H, SCH2CH2S). 31 P NMR (202 MHz, C6D6) δ 21.36 (s). Calcd for C40H36NiP2S2: C, 68.49; H, 5.17. Found: C, 68.81; H, 5.41. [(P2S2)NiCH3]BPh4 ([1-CH3]+). Method A. To a slurry of 0.45 g (0.51 mmol) of [1](BF4)2 in 50 mL of THF at −78 °C was added dropwise 0.6 mL (0.6 mmol) of a 1 M solution of CH3MgBr in THF. The dark red mixture was allowed to warm to room temperature upon stirring, and 210 mg of NaBPh4 was added. The solvent was removed under vacuum, and product was extracted into 5 mL of CH2Cl2. The extract was filtrated through Celite and diluted with 30 mL of hexane to precipitate a red powder. The resulting powder was recrystallized by layering a CH2Cl2 solution with hexane at −30 °C. Yield: 0.42 g (79%). 1H NMR (500 MHz, CD2Cl2): δ 7.56−6.81 (m, 48 × ArH), 3.65 (br, 2H, SCH2Ph), 3.35 (d, 2H, SCH2Ph), 2.53 (br, 2H, SCH2CH2S), 1.40 (br, 2H, SCH2CH2S), −0.04 (t, JP−H = 10.5 Hz, 3H, Ni−CH3). 31P NMR (121 MHz, CD2Cl2): δ 12.45 (s). 13C NMR (126 MHz, CD2Cl2): δ 4.12 (t, JP−C = 17.2 Hz, Ni-CH3). Anal. Calcd for C65H59BNiP2S2: C, 75.38; H, 5.74. Found: C, 75.68; H, 5.93. ESI-MS: calcd for [1-CH3]+: 715.1322; found: 715.1307. Method B. To a solution of (P2S2)Ni (0.27 g, 0.39 mmol) in toluene was added CH3I (26 μL, 0.42 mmol). After the reaction solution stirred for 30 min, solvent was removed under vacuum. The residue was redissolved in CH2Cl2, which was then treated with a solution of 0.16 g of NaBPh4 (0.47 mmol) in 10 mL of THF. The solvent was removed under vacuum, and product was extracted into 5 mL of CH2Cl2. The extract was filtrated through Celite. Hexane (15 mL) was layered on the top of the red solution, which gave red crystals at −30 °C overnight. Yield: 0.34 g (85%). [(P2S2)NiCD3]BPh4 ([1-CD3]+). Complex [1-CD3]BPh4 was prepared by using CD3I and (P2S2)Ni following method B for [1-CH3]BPh4. 31P NMR (121 MHz, CD2Cl2): δ 12.94 (s). 2H NMR (CH2Cl2): δ −0.09 (br, Ni−CD3). Anal. Calcd for C65H56D3BNiP2S2: C, 75.16; H, 6.02. Found: C, 75.38; H, 6.37. ESI-MS: calcd for [1-CD3]+: 718.1510; found: 718.1492. [(P2S2)Ni-COCH3]BPh4 ([1-COCH3]+). CO (1 atm) gas was bubbled through a solution of [1-CH3]BPh4 (0.25 g, 0.24 mmol) in 30 mL of CH2Cl2. The color of the solution immediately turned from red to

orange. By layering the resultant solution with hexane, orange microcrystals were obtained at −30 °C. Yield: 0.22 g (86%). 1H NMR (500 MHz, CD2Cl2) δ 7.64−6.86 (m, 48 × ArH), 3.82 (br, 4H, SCH2Ph), 2.07 (br, 4H, SCH2CH2S), 1.74 (s, 3H, Ni-COCH3). 31P NMR (202 MHz, CD2Cl2) δ 14.91 (s), 10.75 (s). 13C NMR (126 MHz, CD2Cl2): δ 242.63 (t, JP−C = 15.1 Hz, Ni-COCH3). Anal. Calcd for C66H59BNiOP2S2: C, 74.52; H, 5.59. Found: C, 74.77; H, 5.96. ESI-MS: calcd for [1-COCH3]+: 743.1271; found: 743.1251. FT-IR (CH2Cl2): νCO = 1663 cm−1. [(P2S2)Ni-COCD3]BPh4 ([1-COCD3]+). Complex [1-COCD3]BPh4 was prepared by using [1-CD3]BPh4 following the same procedure as that for [1-COCH3]BPh4. 31P NMR (202 MHz, CD2Cl2) δ 14.82 (s), 10.72 (s). 2H NMR (CH2Cl2): δ 1.69 (br, Ni-COCD3). 13C NMR (126 MHz, CD2Cl2): δ 242.95 (t, JP−C = 15.1 Hz, Ni-COCD3). FT-IR (CH2Cl2): νCO = 1660 cm−1. Anal. Calcd for C66H56D3BNiOP2S2: C, 74.31; H, 5.86. Found: C, 74.52; H, 6.13. ESI-MS: calcd for [1COCD3]+: 746.1459; found: 746.1437. Reduction of [1-COCH3]BPh4 by NBu4BH4 Salts. In a typical experiment, treatment of a solution of [1-COCH3]BPh4 (30 mg, 28 μmol) in CD3CN (2 mL) with 7.3 mg (28 μmol) of NBu4BH4 resulted in a color change from orange to red within 5 min and the formation of red precipitate. The red solid was isolated by filtration and identified as (P2S2)Ni by 31P NMR spectroscopic analysis. The filtrate was further analyzed by NMR and IR spectra. The production of CH3CHO was signaled by the characteristic peak of CH3CHO at δ 9.71 (q, JH−H = 2.8 HZ). The same procedure was used for analysis of CD3CHO (1H NMR: δ 9.72; 2H NMR: δ 2.05, CD3), CD3CH2OH (2H NMR: δ 1.08, CD3), CH3CD2OD (2H NMR: δ 3.52, CD2, δ 2.60, OD), and CD3CD2OD (2H NMR: δ 3.50, CD2, δ 2.60, OD, δ 1.07, CD3). Methane Identification by GC. A Pyrex tube was filled with [1COCH3]BPh4 (20 mg, 18.8 μmol) and NBu4BH4 (4.8 mg, 18.8 μmol) in the glovebox. The tube was sealed with a rubber plug and sealed with wax. Subsequently, 2 mL of CH3CN was injected into the tube. After stirring for 10 min, H2 (50 μL) was injected into the tube as an internal standard. Next, 120 μL of the gas in the headspace was sampled by a Hamilton (1750 SL) gas-tight microliter syringe and then analyzed by a Techcomp 7890 II gas chromatograph (GC), which is equipped with a 5 Å molecular sieve column using argon as carrier gas and a thermal conductivity detector. The retention time of CH4 produced was compared to that of the standard gas. The amount of CH4 was calculated according to the published methods.35 (P2S2)Ni(CO) ([1-CO]0). To a solution of [1-COCH3]BPh4 (0.19 g, 0.18 mmol) in 15 mL of MeCN was added 46 mg of NBu4BH4 (0.18 mmol). The mixture was stirred for 5 min, during which time the solution color turned from orange to red with the formation of red precipitate. The red solid, (P2S2)Ni, was isolated by filtration (Yield: 79 mg, 63%). The filtrate was dried under vacuum. The residue was extracted by anhydrous diethyl ether, and the extracts was stored at −30 °C overnight affording (P2S2)Ni(CO) as micro yellow crystals. Yield: 42 mg, 32%. 1H NMR (500 MHz, CD3CN): δ 7.59−6.86 (m, 28 × ArH), 4.09 (br, 2H, SCH2Ph), 3.73 (br, 2H, SCH2Ph), 2.32(br, 2H, SCH2CH2S), 2.18 (d, 2H, SCH2CH2S). 31P NMR (202 MHz, CD3CN): δ 30.01 (s). Anal. Calcd for C41H36NiP2S2O: C, 67.51; H, 4.97. Found: C, 67.62; H, 5.24. FT-IR (CH3CN): νCO = 1907 cm−1.

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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.7b00472. X-ray crystallography and NMR (1H, 31P) spectra (PDF) Accession Codes

CCDC 1548492−1548497 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The E

DOI: 10.1021/acs.organomet.7b00472 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Raja Angamuthu: 0000-0002-5152-0837 Chen-Ho Tung: 0000-0001-9999-9755 Wenguang Wang: 0000-0002-4108-7865 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the “1000 Youth Talents Plan”, the Natural Science Foundation of China and Shandong Province (21402107, 91427303, and ZR2014M011). We also thank Prof. Di Sun for assistance with the X-ray crystallography. S.R. acknowledges the CSIR, India, for senior research fellowship. R.A. thanks the IIT Kanpur for funding.

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