Carbene Transfer Reactivities of Nickel(II)–N-Heterocyclic Carbene

Article pubs.acs.org/Organometallics

Carbene Transfer Reactivities of Nickel(II)−N-Heterocyclic Carbene Complexes and Their Applications in the Synthesis of Metal−NHC Complexes Bo Liu,† Xiaolong Liu,† Chao Chen,† Congyan Chen,† and Wanzhi Chen*,†,‡ †

Department of Chemistry, Zhejiang University, Xixi Campus, Hangzhou 310028, People’s Republic of China State Key Laboratory of Elemento-organic Chemistry, Nankai University, Tianjin, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: We describe a new synthetic procedure for transition-metal N-heterocyclic carbene complexes (NHCs). A number of Pd(II), Pt(II), Co(III), and Ru(II) complexes containing functionalized NHCs have been obtained in good to excellent yields using NiII−NHC complexes as carbene transfer reagents. NiII−NHC complexes are easily prepared from the direct reactions of the corresponding imidazolium salts with commercially available Raney nickel powder. The byproduct of transmetalation reactions, NiCl2, can be easily removed by simple filtration. This study offers a new, economical, and practical synthetic method for metal−NHC complexes.

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INTRODUCTION Due to the unique properties of N-heterocyclic carbenes (NHCs), transition-metal complexes of NHCs are playing an increasingly important role in the fields of materials, medicine, and especially in catalysis.1 Since the first application of NHCs in homogeneous catalysis was reported by Herrmann et al.,2 numerous and ever-increasing uses of NHCs in catalysis have been described. To a large extent, extensive use, especially in industrial applications, of these metal−NHC catalysts depends on the ease of their preparation.3 The known synthetic routes to metal−NHC complexes can be summarized in Scheme 1.

The poor tolerance to base-sensitive N substituents significantly restricts its application. A convenient route involves the reactions of imidazolium salts with basic metal acetates, alkoxides, amides, etc. (method B5). The limitation of method B comes from the moisture sensitivity of these metal precursors and their availability. The in situ deprotonation reaction of an imidazolium salt with a transition-metal precursor in the presence of a base (K2CO3, NEt3, etc.) is also often used (method C).6 Metal−NHC complexes may also be obtained by thermal decomposition of carbene adducts of alcohol,7 chloroform,8 pentafluorobenzene,8,9 CO2,10 CS2,10b cyanide,11 and phosphenium12 in the presence of a suitable metal precursor (method D). Method D suffers from the availability of the carbene precursors or the use of strong bases and toxic reagents. Oxidative addition of the C2−X bond of imidazolium salts (X = H) or 2-haloimidazolium salts (X = Cl, I) with moisture-sensitive zerovalent metal complexes is also applicable (method E).13 In comparison with the aforementioned methods, NHC transfer from one metal center to another represents a practical approach with the obvious advantage of broad tolerance toward imidazolium salts bearing various N substituents (method F).14 The readily available AgI−NHC complex is the most commonly used NHC transfer reagent. A great number of metal−NHC complexes have been obtained via transmetalation from AgI−NHCs.15 However, AgI−NHC complexes are somewhat light-sensitive, and silver occasionally induces degradation of the imidazolium precursors.16 Other metal−NHC complexes involving W, Mn, Cu, Au, and Ni have been scarcely employed as carbene sources.17 We have shown that metal−NHCs can be prepared from imidazolium salts and metal powders in air (method G).18

Scheme 1. Synthetic Routes to Metal−NHC Complexes

Generally, metal−NHCs can be prepared from nucleophilic reactions of free carbenes and the appropriate metal precursors (method A).4 However, the generation of a free carbene intermediate often requires a strong base and handling unstable carbene. © 2011 American Chemical Society

Received: September 20, 2011 Published: December 29, 2011 282

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Organometallics

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Table 1. Preparation of NiII−NHC Complexes

Alternatively, they can be electrochemically obtained using imidazolium salts as the carbene sources and the metal plate as the sacrificial anode (method H).19 These two routes are suitable for first-row transition metals. As a continuation of our organometallic chemistry of NHCs,18−20 we now report a new, practical, and convenient synthetic procedure for PdII−, PtII−, CoIII−, and RuII−NHC complexes using readily available NiII− NHC complexes as the carbene sources.

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RESULTS AND DISCUSSION Preparation of NiII−NHC Complexes. The transmetalation reaction is commonly used to prepare NiII−NHC complexes.21 A more convenient route is the reaction of an imidazolium salt with commercially available nickel powder in air,18 shown in Scheme 2. Scheme 2. Synthesis of NiII−NHC Complexes

Using the above procedure, [Ni(L1)3](PF6)2 (1) was obtained in ca. 40% yield from [HL1](PF6) (L1 = 3-methyl1-(pyridin-2-yl)imidazolylidene) and an excess of nickel powder (200 mesh) in acetonitrile at 70 °C for 2 days. Freshly prepared Raney nickel powder is more reactive, and the yield could be increased to 60%. Similarly, 2−7 were obtained in good to excellent yields (Table 1). Complexes 2 and 6 could even be obtained in nearly quantitative yields based on the amount of imidazolium salts (entries 2 and 6). These reactions took place without removal of moisture and oxygen, and the excess nickel powder was removed via simple filtration. Compounds 1−4 and 7 have been obtained via other preparative procedures.22 The structures of 5 and 6 were determined by X-ray diffraction analysis (see the Supporting Information). Carbene Transfer from Ni(II) to Pd(II). With well-defined NiII−NHC complexes in hand, we studied the possibility of transmetalation from nickel(II) to palladium(II). The transmetalation reactions between 1−7 and Pd(COD)Cl2 were carried out in acetonitrile at 70 °C, giving 8−14 in good to excellent yields (Table 2). The initiation of the reaction was easily monitored by the observation of green NiCl2 precipitate, which could be simply removed after the reaction. Using this procedure, reaction of 1 with 1 equiv of Pd(COD)Cl2 did not give its isostructural palladium complex, but [Pd(L1)2(CH3CN)](PF6)2 (8) was obtained in 63% yield. Nickel complexes 2, 5, and 6 reacted with 2 equiv of Pd(COD)Cl2, affording 9, 12, and 13 in more than 90% yields. Complex 10 was obtained in 73% yield from 3 and Pd(COD)Cl2. Complex 10 is isostructural with 3, and the cis configuration of 10 was confirmed by NOE measurements. Similarly, 11 and 14 were obtained in 69% and 82% yields from 4 and 7, respectively. [Pd(η3-C3H5)Cl]2 could also be used as the palladium source. For example, 13 was obtained in 50% yield when [Pd(η3C3H5)Cl]2 was used. Complexes 8−11 and 14 have already been prepared utilizing other synthetic procedures.22b,d,23 The structures of 12 and 13 were further determined by X-ray diffraction studies (see the Supporting Information). Carbene Transfer from Ni(II) to Pt(II), Co(III), and Ru(II). The present procedure was also used to prepare Pt, Co, and Ru complexes, and the results are given in Table 3.

a

Reaction conditions: imidazolium salt 0.20 mmol, Raney nickel 2 mmol, CH3CN 3 mL, 70 °C, 2 days. bIsolated yields are calculated on the basis of the imidazolium salts used.

Complex 2 reacted with 2 equiv of Pt(COD)Cl2 in acetonitrile at 70 °C, affording 15 in 90% yield. Treatment of 3 with 1 equiv of Pt(COD)Cl2 afforded a mixture of cis and trans isomers 16 in 63% yield. In its 1H NMR spectrum, two sets of resonance signals were observed due to two isomers. The cis and trans isomers could be distinguished by NOE measurements. When 5 was treated with 2 equiv of Pt(COD)Cl2, 17 was isolated in 91% yield as the sole product. Similarly, the PtII−NHC complex 18 could be obtained by treatment of 6 with 2 equiv of Pt(COD)Cl2 in 94% yield. 1H NMR spectrum of 18 showed one set of resonance signals of the corresponding ligand. The 13C NMR spectrum of 18 exhibited a resonance signal at 147.0 ppm ascribed to the carbenic carbon atom.20c,24 The molecular structure of 18 is given in the Supporting Information. Treatment of 1 with 1 equiv of anhydrous CoCl2 in acetonitrile at 70 °C afforded Co(III) complex 19 as a yellow solid in 31% yield. The structure of 19 was determined by elemental analysis. Unfortunately, a Co−C resonance signal in 13 C NMR spectrum of 19 was not observed. This phenomenon has also been found in our previous study and others’ work.25 Complex 20 was obtained as a yellow solid via treatment of 283

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Table 2. Synthesis of PdII−NHC Complexesa

Reaction conditions: Ni−NHC 0.05 mmol, Pd(COD)Cl2 0.05 mmol for 8, 10, 11, and 14 or 0.10 mmol for 9, 12, and 13, CH3CN 2 mL, 70 °C, 24 h. b[Pd(η3-C3H5)Cl]2 was used instead of Pd(COD)Cl2.

a

Table 3. Carbene Transfer from Ni(II) to Pt(II), Co(III), and Ru(II)a

Reaction conditions: Ni−NHC 0.05 mmol, MLnClm 0.05 mmol for 16 and 19−22 or 0.10 mmol for 15, 17, and 18, CH3CN 2 mL, 70 °C, 24 h, MLnClm = Pt(COD)Cl2, CoCl2, [Ru(p-cycmene)Cl2]2. Yields quoted are isolated yields.

a

6 with 1 equiv of CoCl2 in 56% yield. The 13C resonance of carbenic carbon in 20 appears at 182.2 ppm, similar to case for the known CoIII−NHC complexes.25a,26 Reactions of 3 and 4 with [Ru(p-cymene)Cl2]2 in acetonitrile at 70 °C led to the isolation of 21 and 22 in yields of 94% and 71%, respectively.

Complex 21 could also be obtained via in situ transmetalation of the corresponding AgI−NHC with 1/2 equiv of [Ru(pcymene)Cl2]2 in acetonitrile. The structures of 20 and 21 were finally determined by X-ray diffraction (see the Supporting Information). 284

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Carbene Transfer from Ni(II) to Ni(II). The present procedure is also useful for the carbene transfer from one nickel to another. As shown in Scheme 3, reactions of 2 or 5 with

Scheme 5. Carbene Transfer Process

Scheme 3. Carbene Transfer from Ni(II) to Ni(II) using imidazolium salts and Raney nickel powder in excellent yields. NiII−NHCs are very stable to air, moisture, and light and thus are suitable carbene sources. The carbene transfer reactions from Ni(II) to other metal centers take place under mild conditions in air and do not require any expensive reagents and handling of air-sensitive compounds. Moreover, the reactions also tell us that the fast association and dissociation of Ni2+ ion and free NHC is possible and the Ni−C bond is kinetically active, although we have not obtained any spectroscopic evidence at present. Further research aiming at studying the kinetics of the carbene transfer reactions as well as utilizing these metal−NHC complexes in catalytic organic reactions are currently in progress.

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2 equiv of Ni(PPh3)2Cl2 at 70 °C readily afforded 23 and 24. Complex 23 has also been obtained from the corresponding AgI−NHC and Ni(PPh3)2Cl2.22b As expected, complex 24 could also react with Pd(COD)Cl2 and Pt(COD)Cl2, giving 12 and 17, respectively. Heating the mixture of 2 or 5 in the presence of an excess of anhydrous KCl, LiCl, or nBu4NCl at 70 °C for 24 h did not afford 23 and 24, which illustrated that 23 and 24 did not result from the thermal decomposition of 2 and 5. In Situ Carbene Transfer Reactions. The metal−NHC complexes could also be prepared from imidazolium salts without isolation of the NiII−NHC complexes. For example, as shown in Scheme 4, heating the mixture of [HL5](PF6) and an excess of Raney nickel powder in acetonitrile at 80 °C for 24 h and subsequent addition of 1 equiv of M(COD)Cl2 (M = Pd, Pt) afforded 12 and 17 in more than 80% yields after a simple workup. It is generally believed that metal−carbene bonds are strong and inert. Dissociation of a NHC ligand or its substitution by an incoming ligand has been scarcely observed in Ni−NHC complexes.27,28 We noted that 1 decomposed slowly in CD3CN at 75 °C to give the corresponding imidazolium and unidentified inorganic nickel species, demonstrating the lability of the Ni−NHC bond. An equilibrium between Ni species and a free carbene in solution may exist.27a The formation of more stable M−C bonds might be the driving force for the reactions. The formation of insoluble NiCl2 in acetonitrile obviously promoted the dissociation of the Ni−C bond and the carbene transfer process (Scheme 5).

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EXPERIMENTAL SECTION

All chemicals were obtained from commercial suppliers and used without further purification. [HL1](PF6),29 [HL2](PF6),22b [HL3](PF6),30 [HL4](PF6),23c [HL5](PF6),31 and [H2L7](PF6)222d were prepared according to known procedures. The synthetic procedure of [HL6](PF6) is given in the Supporting Information. The elemental analyses were performed on a Flash EA 1112 instrument. 1H and 13C NMR spectra were recorded on a Bruker Avance-400 (400 MHz) spectrometer. Chemical shifts (δ) are expressed in ppm downfield from TMS at δ = 0 ppm, and coupling constants (J) are expressed in Hz. General Procedure for the Preparation of NiII−NHC Complexes. NiII−NHCs 1−7 were synthesized by the following route: a solution of the imidazolium salt (0.20 mmol) in 3 mL of CH3CN was treated with an excess of Raney nickel powder (120 mg). The mixture was allowed to react at 70 °C for 2 days in air. After it was cooled to room temperature, the solution was filtered through Celite. Then the filtrate was concentrated to ca. 1 mL. The compounds were obtained by adding diethyl ether to the filtrate. [Ni(L1)3](PF6)2 (1). This complex was prepared from [HL1](PF6) (61 mg, 0.20 mmol) and Raney nickel (120 mg). Yield: 33 mg (60%), red-brown powder. 1H NMR (400 MHz, DMSO-d6): δ 8.47 (s, 1H), 8.16 (br, 2H), 8.07 (s, 1H), 7.99 (m, 3H), 7.72 (br, 4H), 7.64 (s, 1H), 7.60 (s, 1H), 7.54 (s, 1H), 7.50 (s, 1H), 7.36 (s, 1H), 7.32 (s, 1H), 7.24 (s, 1H), 3.69 (s, 3H, CH3), 3.34 (s, 3H, CH3), 3.32 (s, 3H, CH3). Anal. Calcd for C27H27F12N9NiP2: C, 39.25; H, 3.29; N, 15.26. Found: C, 39.42; H, 3.28; N, 14.86. No satisfactory 13C NMR spectrum of 1 was obtained. [Ni(L2)2](PF6)2 (2).22b This complex was prepared from [HL2](PF6) (90 mg, 0.20 mmol) and Raney nickel (120 mg). Yield: 87 mg (93%), brown powder. Anal. Calcd for C40H39F12N9NiP2 ([Ni(L2)2](PF6)2·CH3CN): C, 48.31; H, 3.95; N, 12.68. Found: C, 48.59; H, 3.95; N, 12.31. The compound is paramagnetic, and thus no NMR spectrum was obtained. [Ni(L3)2](PF6)2 (3). This complex was prepared from [HL3](PF6) (64 mg, 0.20 mmol) and Raney nickel (120 mg). Yield: 48 mg (69%), pale yellow powder. 1H NMR (400 MHz, DMSO-d6): δ 8.14 (t, J = 7.2 Hz, 2H, 4-PyH), 7.99 (d, J = 5.6 Hz, 2H, 6-PyH), 7.91 (d, J = 8.0 Hz, 2H, 3-PyH), 7.70 (s, 2H, NCHCHN), 7.48 (t, J = 6.4 Hz, 2H,

CONCLUSION

We have described a convenient method for the synthesis of metal−NHC complexes using NiII−NHC complexes as the carbene sources. NiII−NHC complexes can be easily prepared

Scheme 4. In Situ Carbene Transfer from Ni(II) to Pd(II) and Pt(II)

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5-PyH), 7.37 (s, 2H, NCHCHN), 6.79 (d, J = 14.8 Hz, 2H, CH2), 5.81 (d, J = 15.2 Hz, 2H, CH2), 3.17 (s, 6H, CH3). 13C NMR (100 MHz, DMSO-d6): δ 157.64 (Ni−C), 155.09, 153.05, 141.42, 125.47, 125.31, 124.94, 123.58, 53.82, 36.08. Anal. Calcd for C20H22F12N6NiP2: C, 34.56; H, 3.19; N, 12.09. Found: C, 34.82; H, 3.19; N, 11.82. The cis configuration of 3 was confirmed by NOE measurement. [Ni(L4)2](PF6)2 (4).22a This complex was prepared from [HL4](PF6) (77 mg, 0.20 mmol) and Raney nickel (120 mg). Yield: 63 mg (77%), light yellow powder. 1H NMR (400 MHz, DMSO-d6): δ 8.97 (d, J = 4.8 Hz, 4H, 4,6-pyrimidinyl H), 8.32 (d, J = 5.2 Hz, 2H, 6-pyridyl H), 8.21 (t, J = 7.6 Hz, 2H, 4-pyridyl H), 8.04 (d, J = 7.6 Hz, 2H, 3-pyridyl H), 7.62−7.52 (m, 8H, NCHCHN + 5-pyridyl H + 5-pyrimidinyl H), 6.84 (d, J = 15.2 Hz, 2H, CH2), 6.04 (d, J = 14.8 Hz, 2H, CH2). Anal. Calcd for C26H22F12N10NiP2: C, 37.94; H, 2.69; N, 17.02. Found: C, 38.02; H, 2.69; N, 16.56. [Ni(L5)2](PF6)2 (5). This complex was prepared from [HL5](PF6) (80 mg, 0.20 mmol) and Raney nickel (120 mg). Yield: 72 mg (85%), yellow powder. 1H NMR (400 MHz, CD3CN): δ 8.69 (s, 2H), 8.00 (br, 2H), 7.92 (s, 2H), 7.74 (br, 4H), 7.31 (m, 6H), 7.23 (d, J = 7.6 Hz, 2H), 7.03 (s, 2H), 6.44 (d, J = 15.2 Hz, 2H, CH2), 5.49 (d, J = 14.8 Hz, 2H, CH2), 5.01 (d, J = 15.2 Hz, 2H, CH2), 4.38 (d, J = 15.6 Hz, 2H, CH2). 13C NMR (100 MHz, CD3CN): δ 158.88 (Ni-C), 154.70, 154.26, 152.25, 149.73, 140.85, 137.60, 125.33, 124.02, 123.67, 123.24, 122.57, 54.60, 54.24. Anal. Calcd for C30H28F12N8NiP2: C, 42.43; H, 3.32; N, 13.19. Found: C, 42.34; H, 3.25; N, 12.98. [Ni(L6)2](PF6)2 (6). This complex was prepared from [HL6](PF6) (122 mg, 0.20 mmol) and Raney nickel (120 mg). Yield: 117 mg (92%), yellow powder. Anal. Calcd for C54H48F12N16NiP2: C, 51.08; H, 3.81; N, 17.65. Found: C, 51.00; H, 3.91; N, 17.70. No satisfactory NMR spectra of 6 were obtained, although we repeatedly measured in different deuterated solvents. [Ni(L7)](PF6)2 (7).22d This complex was prepared from [H2L7](PF6)2 (124 mg, 0.20 mmol) and Raney nickel (120 mg). Yield: 108 mg (80%), greenish yellow powder. 1H NMR (400 MHz, DMSO-d6): δ 8.58 (d, J = 5.6 Hz, 2H, 6-PyH), 8.22 (d, J = 7.6 Hz, 2H, 4-PyH), 7.86 (m, 6H, NCHCHN + 3-PyH), 7.58 (d, J = 6.4 Hz, 2H, 5-PyH), 6.42 (s, 2H, NCH2N), 5.80 (s, 4H, NCH2C). Anal. Calcd for C19H18F12N6NiP2: C, 33.61; H, 2.67; N, 12.38. Found: C, 33.73; H, 2.71; N, 12.12. General Procedure for Carbene Transfer from Ni(II) to Pd(II), Pt(II), Co(III), Ru(II), and Ni(II). Complexes 8−24 were synthesized via the following route: a solution of NiII−NHC (0.05 mmol) in 2 mL of CH3CN was treated with the corresponding equivalent of Pd(COD)Cl2, Pt(COD)Cl2, CoCl2, [Ru(p-cycmene)Cl2]2, or Ni(PPh3)2Cl2. The mixture was allowed to react at 70 °C for 24 h. The solution was filtered through Celite to remove the precipitate of NiCl2. Then the filtrate was concentrated to ca. 1 mL. The compounds were obtained by adding diethyl ether to the filtrate. [Pd(L1)2(CH3CN)](PF6)2 (8). This complex was synthesized from 1 (41 mg, 0.05 mmol) and Pd(COD)Cl2 (14 mg, 0.05 mmol). Yield: 24 mg (63%), white powder. 1H NMR (400 MHz, DMSO-d6): δ 8.97 (d, J = 4.8 Hz, 1H, PyH), 8.58 (br, 1H, PyH), 8.56 (s, 1H, imidazolylidene H), 8.42 (t, J = 7.6 Hz, 1H, PyH), 8.40 (s, 1H, imidazolylidene H), 8.19 (d, J = 8.0 Hz, 1H, PyH), 8.18 (s, 1H, imidazolylidene H), 8.07 (t, J = 7.8 Hz, 1H, PyH), 7.88 (s, 1H, PyH), 7.73 (t, J = 6.0 Hz, 1H, PyH), 7.57 (s, 1H, imidazolylidene H), 7.52 (t, J = 6.0 Hz, 1H, PyH), 4.06 (s, 3H, CH3), 2.98 (s, 3H, CH3), 2.07 (s, 3H, CH3CN). 13C NMR (100 MHz, DMSO-d6): δ 160.29 (Pd−C), 156.92 (Pd−C), 151.43, 150.39, 149.53, 148.18, 143.99, 140.03, 125.88, 125.83, 124.71, 124.11, 122.78, 118.43, 117.92, 117.56, 112.73, 39.00, 36.51, 1.51. Anal. Calcd for C24H27F12N9P2Pd ([Pd(L1)2(CH3CN)](PF6)2·2CH3CN): C, 34.40; H, 3.25; N, 15.05. Found: C, 34.57; H, 3.32; N, 14.87. [Pd(L2)Cl](PF6) (9).22b This complex was synthesized from 2 (47 mg, 0.05 mmol) and Pd(COD)Cl2 (28 mg, 0.10 mmol). Yield: 56 mg (96%), yellow powder. 1H NMR (400 MHz, DMSO-d6): δ 9.05 (d, J = 8.8 Hz, 1H, phenanthroline H), 8.83 (d, J = 8.4 Hz, 1H, phenanthroline H), 8.54 (s, 2H, NCHCHN + phenanthroline H), 8.39 (d, J = 8.8 Hz, 1H, phenanthroline H), 8.14 (s, 2H, phenanthroline H), 8.02 (dd, J = 8.0 Hz, J = 5.2 Hz, 1H, phenanthroline H), 7.82 (d, J = 2 Hz, 1H, NCHCHN), 4.36 (t, J = 6.8 Hz, 2H, NCH2CH2CH2CH3),

1.78 (m, 2H, NCH2CH2CH2CH3), 1.33 (m, 2H, NCH2CH2CH2CH3), 0.92 (t, J = 6.8 Hz, 3H, NCH2CH2CH2CH3). Anal. Calcd for C19H18ClF6N4PPd: C, 38.73; H, 3.08; N, 9.51. Found: C, 39.04; H, 3.10; N, 9.25. [Pd(L3)2](PF6)2 (10). This complex was synthesized from 3 (35 mg, 0.05 mmol) and Pd(COD)Cl2 (14 mg, 0.05 mmol). Yield: 27 mg (73%), white powder. 1H NMR (400 MHz, DMSO-d6): δ 8.29 (d, J = 5.2 Hz, 2H, 6-PyH), 8.24 (t, J = 7.6 Hz, 2H, 4-PyH), 7.96 (d, J = 7.6 Hz, 2H, 3-PyH), 7.76 (d, J = 1.6 Hz, 2H, NCHCHN), 7.62 (t, J = 6.4 Hz, 2H, 5-PyH), 7.49 (d, J = 1.6 Hz, 2H, NCHCHN), 6.17 (d, J = 14.8 Hz, 2H, NCH2), 5.80 (d, J = 15.6 Hz, 2H, NCH2), 3.31 (s, 6H, NCH3). 13C NMR (100 MHz, DMSO-d6): δ 156.05 (Pd−C), 154.28, 152.88, 142.00, 126.28, 125.91, 124.35, 123.64, 54.76, 37.04. Anal. Calcd for C20H22F12N6P2Pd: C, 32.34; H, 2.99; N, 11.31. Found: C, 32.29; H, 3.10; N, 11.66. [Pd(L4)2](PF6)2 (11).23c This complex was synthesized from 4 (42 mg, 0.05 mmol) and Pd(COD)Cl2 (14 mg, 0.05 mmol). Yield: 30 mg (69%), white powder. 1H NMR (400 MHz, DMSO-d6): δ 8.86 (d, J = 4.8 Hz, 4H, 4,6-pyrimidinyl H), 8.53 (d, J = 5.6 Hz, 2H, 6-pyridyl H), 8.30 (t, J = 7.6 Hz, 2H, 4-pyridyl H), 8.08 (d, J = 7.6 Hz, 2H, 3-pyridyl H), 7.72−7.56 (m, 8H, NCHCHN + 5-pyridyl H + 5-pyrimidinyl H), 6.23 (d, J = 14.4 Hz, 2H, NCH2), 6.00 (d, J = 14.8 Hz, 2H, NCH2). Anal. Calcd for C26H22F12N10P2Pd: C, 35.86; H, 2.55; N, 16.08. Found: C, 35.79; H, 2.59; N, 15.77. [Pd(L5)Cl](PF6) (12). This complex was synthesized from 5 (43 mg, 0.05 mmol) and Pd(COD)Cl2 (29 mg, 0.10 mmol). Yield: 49 mg (92%), white powder. 1H NMR (400 MHz, DMSO-d6): δ 9.43 (d, J = 5.6 Hz, 2H, 6-PyH), 8.24 (t, J = 7.6 Hz, 2H, 4-PyH), 7.89 (d, J = 7.6 Hz, 2H, 3-PyH), 7.70 (t, J = 7.2 Hz, 2H, 5-PyH), 7.64 (s, 2H, NCHCHN), 5.72 (s, 4H, NCH2). 13C NMR (100 MHz, DMSO-d6): δ 156.20 (Pd−C), 153.03, 149.19, 141.70, 127.01, 125.71, 122.13, 53.95. Anal. Calcd for C15H14ClF6N4PPd: C, 33.54; H, 2.63; N, 10.43. Found: C, 33.67; H, 2.65; N, 10.25. 12 was also obtained from 24 (49 mg, 0.10 mmol) and Pd(COD)Cl2 (29 mg, 0.10 mmol). Yield: 51 mg (95%). [Pd(L6)Cl](PF6) (13). This complex was synthesized from 6 (64 mg, 0.05 mmol) and Pd(COD)Cl2 (29 mg, 0.10 mmol). Yield: 71 mg (95%), white powder. 1H NMR (400 MHz, DMSO-d6): δ 8.63 (s, 2H, 5-triazole H), 7.93 (q, J = 2.8 Hz, 2H, 4,7-benzimidazole H), 7.55 (q, J = 2.8 Hz, 2H, 5,6-benzimidazole H), 7.41−7.38 (m, 10H, PhH), 5.89 (s, 4H, NCH2), 5.82 (s, 4H, NCH2). 13C NMR (100 MHz, DMSOd6): δ 157.85 (Pd−C), 139.94, 134.67, 132.50, 129.30, 129.13, 128.75, 125.74, 125.22, 112.21, 55.28, 40.96. Anal. Calcd for C28H25.5ClF6N8.5PPd ([Pd(L6)Cl](PF6)·0.5CH3CN): C, 43.79; H, 3.35; N, 15.50. Found: C,43.81; H, 3.46; N, 15.33. 13 was also obtained from 6 (64 mg, 0.05 mmol) and [Pd(η3C3H5)Cl]2 (18 mg, 0.05 mmol). Yield: 36 mg (50%). [Pd(L7)](PF6)2 (14).22d This complex was synthesized from 7 (34 mg, 0.05 mmol) and Pd(COD)Cl2 (14 mg, 0.05 mmol). Yield: 30 mg (82%), colorless powder. 1H NMR (400 MHz, DMSO-d6): δ 8.94 (d, J = 5.2 Hz, 2H, 6-PyH), 8.32 (t, J = 7.6 Hz, 2H, 4-PyH), 7.97 (d, J = 7.6 Hz, 2H, 3-PyH), 7.85−7.80 (m, 6H, NCHCHN + 5-PyH), 6.50 (s, 2H, NCH2N), 5.74 (s, 4H, CCH2N). Anal. Calcd for C19H18F12N6P2Pd: C, 31.40; H, 2.50; N, 11.56. Found: C, 31.64; H, 2.50; N, 11.34. [Pt(L2)Cl](PF6) (15).20c This complex was synthesized from 2 (47 mg, 0.05 mmol) and Pt(COD)Cl2 (38 mg, 0.10 mmol). Yield: 61 mg (90%), yellow powder. 1H NMR (400 MHz, DMSO-d6): δ 8.98 (d, J = 8.8 Hz, 1H, phenanthroline H), 8.88 (d, J = 7.6 Hz, 1H, phenanthroline H), 8.58 (d, J = 4.8 Hz, 1H, phenanthroline H), 8.47 (s, 1H, NCHCHN), 8.31 (d, J = 8.4 Hz, 1H, phenanthroline H), 8.10 (br, 3H, phenanthroline H), 7.85 (s, 1H, NCHCHN), 4.25 (br, 2H, NCH2CH2CH2CH3), 1.75 (m, 2H, NCH2CH2CH2CH3), 1.30 (m, 2H, NCH2CH2CH2CH3), 0.90 (t, J = 7.2 Hz, 3H, NCH2CH2CH2CH3). Anal. Calcd for C19H18ClF6N4PPt: C, 33.66; H, 2.68; N, 8.27. Found: C, 33.87; H, 2.69; N, 8.30. [Pt(L3)2](PF6)2 (16). This complex was synthesized from 3 (35 mg, 0.05 mmol) and Pt(COD)Cl2 (19 mg, 0.05 mmol). Yield: 26 mg (63%, cis/trans = 0.8/1), white powder. 1H NMR (400 MHz, DMSOd6): δ 8.78 (d, J = 5.6 Hz, 2H, 6-PyHtrans), 8.39 (d, J = 5.6 Hz, 1.6H, 286

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Organometallics

Article

6-PyHcis), 8.28 (t, J = 7.6 Hz, 3.6H, 4-PyHtrans + 4-PyHcis), 8.02 (d, J = 8.4 Hz, 1.6H, 3-PyHcis), 7.99 (d, J = 8.0 Hz, 2H, 3-PyHtrans), 7.74 (s, 1.6H, NCHCHNcis), 7.71 (s, 2H, NCHCHNtrans), 7.64 (t, J = 6.4 Hz, 1.6H, 5-PyHcis), 7.55 (t, J = 6.4 Hz, 2H, 5-PyHtrans), 7.50 (s, 1.6H, NCHCHNcis), 7.45 (s, 2H, NCHCHNtrans), 5.93 (d, J = 11.2 Hz, 1.6H, CH2 cis), 5.90 (d, J = 11.2 Hz, 2H, CH2 trans), 5.75 (d, J = 16.4 Hz, 2H, CH2 trans), 5.71 (d, J = 15.6 Hz, 1.6H, CH2 cis), 3.31 (s, 4.8H, CH3 cis), 3.17 (s, 6H, CH3 trans). 13C NMR (100 MHz, DMSO-d6): δ 164.62 (Pt−C), 156.95 (Pt−C), 155.41, 154.18, 153.17, 143.31, 142.27, 127.35, 127.28, 127.17, 126.92, 126.41, 124.10, 123.91, 123.02, 122.23, 54.63, 54.54, 36.67, 36.10. Anal. Calcd for C21H23.5F12N6.5P2Pt ([Pt(L3)2](PF6)2·0.5CH3CN): C, 29.60; H, 2.78; N,10.69. Found: C, 29.56; H, 2.84; N, 10.36. [Pt(L5)Cl](PF6) (17). This complex was synthesized from 5 (43 mg, 0.05 mmol) and Pt(COD)Cl2 (38 mg, 0.10 mmol). Yield: 57 mg (91%), white powder. 1H NMR (400 MHz, DMSO-d6): δ 9.56 (d, J = 5.6 Hz, 2H, 6-PyH), 8.28 (t, J = 7.2 Hz, 2H, 4-PyH), 7.90 (d, J = 7.2 Hz, 2H, 3-PyH), 7.70 (t, J = 6.4 Hz, 2H, 5-PyH), 7.63 (s, 2H, NCHCHN), 5.58 (s, 4H, NCH2). 13C NMR (100 MHz, DMSO-d6): δ 155.97 (Pt−C), 153.09, 141.71, 139.21, 127.49, 126.38, 121.26, 53.68. Anal. Calcd for C15H14ClF6N4PPt: C, 28.79; H, 2.25; N, 8.95. Found: C, 28.96; H, 2.21; N, 8.79. 17 was also obtained from 24 (49 mg, 0.10 mmol) and Pt(COD)Cl2 (38 mg, 0.10 mmol). Yield: 56 mg (90%). [Pt(L6)Cl](PF6) (18). This complex was synthesized from 6 (64 mg, 0.05 mmol) and Pt(COD)Cl2 (38 mg, 0.10 mmol). Yield: 78 mg (94%), white powder. 1H NMR (400 MHz, DMSO-d6): δ 8.64 (s, 2H, 5-triazole H), 7.88 (q, J = 3.2 Hz, 2H, 4,7-benzimidazole H), 7.52 (q, J = 3.2 Hz, 2H, 5,6-benzimidazole H), 7.43−7.35 (m, 10H, PhH), 5.83 (s, 4H, CH2), 5.79 (s, 4H, CH2). 13C NMR (100 MHz, DMSO-d6): δ 147.06 (Pt−C), 139.50, 134.54, 132.46, 129.32, 129.17, 128.76, 126.08, 124.96, 112.04, 55.43, 40.88. Anal. Calcd for C27H24ClF6N8PPt: C, 38.79; H, 2.89; N, 13.40. Found: C, 38.89; H, 2.95; N, 13.28. [Co(L1)3](PF6)3 (19). This complex was synthesized from 1 (42 mg, 0.05 mmol) and CoCl2 (7 mg, 0.05 mmol). Yield: 15 mg (31%), yellow powder. 1H NMR (400 MHz, DMSO-d6): δ 9.44 (d, J = 5.6 Hz, 3H, 6-PyH), 8.64 (d, J = 1.2 Hz, 3H, NCHCHN), 8.43 (t, J = 8.0 Hz, 3H, 4-PyH), 8.28 (d, J = 8.4 Hz, 3H, 3-PyH), 7.69 (t, J = 6.6 Hz, 3H, 5-PyH), 7.66 (d, J = 1.6 Hz, 3H, NCHCHN), 2.80 (s, 9H, CH3). 13C NMR (100 MHz, DMSO-d6): δ 153.09, 143.30, 129.37, 123.73, 118.60, 113.36, 35.02. Anal. Calcd for C30H32.5CoF18N10O0.25P3 ([Co(L1)3](PF6)3·CH3CN·0.25Et2O): C, 34.95; H, 3.18; N, 13.59. Found: C, 35.26; H, 2.93; N, 13.53. [Co(L6)2](PF6)3 (20). This complex was synthesized from 6 (64 mg, 0.05 mmol) and CoCl2 (7 mg, 0.05 mmol). Yield: 39 mg (56%), pale yellow powder. 1H NMR (400 MHz, DMSO-d6): δ 8.53 (s, 4H, 5triazole H), 8.17 (q, J = 3.2 Hz, 4H, 4,7-benzimidazole H), 7.84 (q, J = 2.8 Hz, 4H, 5,6-benzimidazole H), 7.32 (t, J = 7.2 Hz, 4H, PhH), 7.21 (t, J = 7.2 Hz, 8H, PhH), 6.82 (d, J = 7.6 Hz, 8H, PhH), 5.39 (s, 8H, CH2), 5.31 (s, 8H, CH2). 13C NMR (100 MHz, DMSO-d6): δ 182.17 (Co−C), 142.37, 134.23, 134.18, 129.14, 129.04, 128.22, 127.24, 125.60, 112.40, 55.18, 40.17. Anal. Calcd for C54H48CoF18N16P3: C, 45.84; H, 3.42; N, 15.84. Found: C, 45.83; H, 3.40; N, 15.74. [Ru(L2)Cl(CH3CN)2](PF6) (21). This complex was synthesized from 2 (47 mg, 0.05 mmol) and [Ru(p-cycmene)Cl2]2 (31 mg, 0.05 mmol). Yield: 62 mg (94%), reddish brown powder. 1H NMR (400 MHz, DMSO-d6): δ 9.48 (d, J = 4.8 Hz, 1H, phenanthroline H), 8.76 (d, J = 8.8 Hz, 1H, phenanthroline H), 8.63 (d, J = 8.8 Hz, 1H, phenanthroline H), 8.62 (d, J = 1.2 Hz, 1H, NCHCHN), 8.43 (d, J = 8.8 Hz, 1H, phenanthroline H), 8.20 (q, J = 8.8 Hz, 2H, phenanthroline H), 8.11 (dd, J = 4.8 Hz, J = 4.8 Hz, 1H, phenanthroline H), 7.74 (d, J = 2.0 Hz, 1H, NCHCHN), 4.46 (t, J = 7.2 Hz, 2H, NCH2CH2CH2CH3), 2.85 (s, 3H, CH3CN), 2.03 (s, 3H, CH3CN), 1.96 (m, 2H, NCH2CH2CH2CH3), 1.43 (m, 2H, NCH2CH2CH2CH3), 0.98 (t, J = 7.2 Hz, 3H, NCH2CH2CH2CH3). 13C NMR (100 MHz, DMSO-d6): δ 185.34 (Ru−C), 154.85, 153.26, 147.26, 146.96, 138.29, 138.02, 130.80, 127.72, 127.16, 127.05, 126.98, 126.26, 124.48, 124.08, 119.31, 111.55, 49.78, 32,48, 19.10, 13.68, 3.75, 3.07. Anal. Calcd

for C23H24ClF6N6PRu: C, 41.48; H, 3.63; N, 12.62. Found: C, 41.49; H, 3.63; N, 12.47. [Ru(L4)(CH3CN)3](PF6)2 (22). This complex was synthesized from 4 (42 mg, 0.05 mmol) and [Ru(p-cycmene)Cl2]2 (15 mg, 0.05 mmol). Yield: 34 mg (71%), bright yellow powder. 1H NMR (400 MHz, DMSO-d6): δ 9.17 (d, J = 8.0 Hz, 1H), 9.07 (d, J = 5.2 Hz, 1H), 9.00 (d, J = 4.4 Hz, 1H), 8.36 (s, 1H, NCHCHN), 8.06 (t, J = 8.0 Hz, 1H), 7.91 (s, 1H, NCHCHN), 7.76 (d, J = 8.4 Hz, 1H), 7.66 (t, J = 5.2 Hz, 1H), 7.59 (t, J = 5.6 Hz, 1H), 6.02 (s, 2H, NCH2), 2.20 (s, 6H, CH3CN), 2.07 (s, 3H, CH3CN). 13C NMR (100 MHz, DMSO-d6): δ 198.81 (Ru−C), 162.51, 159.38, 158.94, 156.77, 153.64, 138.13, 126.62, 124.22, 124.11, 119.03, 118.08, 53.19, 3.75, 3.40. Anal. Calcd for C19H20F12N8P2Ru: C, 30.37; H, 2.68; N, 14.91. Found: C, 30.66; H, 2.69; N, 14.95. [Ni(L2)Cl](PF6) (23).22b This complex was synthesized from 2 (47 mg, 0.05 mmol) and Ni(PPh3)2Cl2 (66 mg, 0.10 mmol). Yield: 28 mg (52%), orange powder. 1H NMR (400 MHz, acetone-d6): δ 9.04 (d, J = 8.4 Hz, 1H, phenanthroline H), 8.86 (d, J = 8.4 Hz, 1H, phenanthroline H), 8.74 (d, J = 4.8 Hz, 1H, phenanthroline H), 8.37 (d, J = 2.0 Hz, 1H, NCHCHN), 8.33 (d, J = 8.8 Hz, 1H, phenanthroline H), 8.20 (q, J = 8.8 Hz, 2H, phenanthroline H), 8.12 (dd, J = 4.8 Hz, J = 7.6 Hz, 1H, phenanthroline H), 7.72 (d, J = 2.0 Hz, 1H, NCHCHN), 4.50 (t, J = 7.2 Hz, 2H, NCH2CH2CH2CH3), 1.89 (m, 2H, NCH2CH2CH2CH3), 1.43 (m, 2H, NCH2CH2CH2CH3), 0.96 (t, J = 7.6 Hz, 3H, NCH2CH2CH2CH3). Anal. Calcd for C19H18ClF6N4NiP: C, 42.14; H, 3.35; N, 10.35. Found: C, 42.06; H, 3.30; N, 10.27. [Ni(L5)Cl](PF6) (24). This complex was synthesized from 5 (43 mg, 0.05 mmol) and Ni(PPh3)2Cl2 (66 mg, 0.10 mmol). Yield: 10 mg (20%), yellow powder. 1H NMR (400 MHz, acetone-d6): δ 8.13 (t, J = 7.6 Hz, 2H, 4-PyH), 8.11 (s, 1H, NCHCHN), 8.02 (m, 2H, PyH), 7.84−7.73 (m, 5H, PyH + NCHCHN), 5.79 (s, 4H, CH2). Anal. Calcd for C15H14ClF6N4NiP: C, 36.81; H, 2.88; N, 11.45. Found: C, 36.94; H, 2.82; N, 11.23. No satisfactory 13C NMR spectrum of 24 was obtained. General Procedure for in Situ Carbene Transfer from Ni(II) to Pd(II) and Pt(II). A solution of imidazolium salt (0.10 mmol) in 3 mL of CH3CN was treated with an excess of Raney nickel powder (100 mg). The mixture was allowed to react at 80 °C for 24 h in air. Without any additional treatment, 0.10 mmol of Pd(COD)Cl2 or Pt(COD)Cl2 was directly added to the reaction mixture. Then, the mixture was allowed to react at 80 °C overnight. After it was cooled to room temperature, the solution was filtered through Celite to remove nickel powder and the precipitate of NiCl2. The filtrate was concentrated to ca. 1 mL. The complexes were obtained by adding diethyl ether to the filtrate. 12 and 17 were obtained in 84% and 87% yields.

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S Supporting Information *

Text giving experimental details and figures, tables, and CIF files giving structural drawings and crystallographic data for 5, 6, 12, 13, 18, 20, and 21. This material is available free of charge via the Internet at http://pubs.acs.org.

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*Tel and fax: (+) 86-571-88273314. E-mail: [email protected].

ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (No. 21072170) for financial support. REFERENCES

(1) (a) Mercs, L.; Albrecht, M. Chem. Soc. Rev. 2010, 39, 1903. (b) Díez-González, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612. (c) Poyatos, M.; Mata, J. A.; Peris, E. Chem. Rev. 2009, 109, 3677. 287

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Organometallics

Article

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dx.doi.org/10.1021/om200881s | Organometallics 2012, 31, 282−288