A Planar π-Conjugated Naphthyridine-Based N-Heterocyclic Carbene

Article pubs.acs.org/Organometallics

A Planar π‑Conjugated Naphthyridine-Based N‑Heterocyclic Carbene Ligand and Its Derived Transition-Metal Complexes Xiaolong Liu,† Shanfei Pan,† Junshi Wu,‡ Yapei Wang,‡ and Wanzhi Chen*,†,§ †

Department of Chemistry, Zhejiang University, Xixi Campus, Hangzhou 310028, People’s Republic of China Department of Chemistry, Renmin University of China, Beijing 100872, People’s Republic of China § State Key Laboratory of Elemento-organic Chemistry, Nankai University, Tianjin 300071, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: HL·(PF6) (L = 2,4-dimethyl-8-phenyl[1,2,4]triazolo[4,3-a][1,8]naphthyridin-9-ylidene) was synthesized from the nucleophilic reaction of 7-chloro-2,4-dimethyl-1,8naphthyridine with phenylhydrazine and subsequent acidification, anion exchange, and condensation with triethyl orthoformate. Its silver, copper, cobalt, and nickel complexes [Ag2(L)2(CH3CN)2](PF6)2 (1), [CuL(CH3CN)2](PF6) (2), [CuL(phen)](PF 6 ) (3), [CuL(dppe)](PF 6 ) (4), [Co(L)2(CH3CN)2](PF6)2 (5), and [Ni(L)3](PF6)2 (6) have been synthesized and fully characterized by NMR, elemental analysis, and X-ray diffraction analysis. The copper complex 2 exhibits excellent catalytic activity in the Cu(I)-catalyzed azide−alkyne cycloaddition reaction of 2,2,6,6-tetramethylpiperidinyl-1-oxytethered alkynes in an air atmosphere at 50 °C.

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INTRODUCTION 1,10-Phenanthroline (A in Figure 1) and its derivatives have been widely used as bidentate ligands in coordination and

A great number of transition-metal complexes with NHCs have been synthesized10 and showed excellent catalytic activities in various reactions, including ruthenium-catalyzed olefin metathesis11 and hydrogenation,12 palladium-catalyzed C−C13 and C−heteroatom couplings,14 and copper-promoted 3 + 2 cycloaddition reactions.15 Recently, a pyridazine-based NHC ligand (B) analogous to 1,10-phenanthroline and its transition-metal complexes have been reported.16 The ligand showed coordination behavior different from that of 1,10phenanthroline in its corresponding coinage-metal complexes. Most recently, Monkowius and co-workers reported Ag and Au complexes of 8-(2,6-diisopropylphenyl)-8H-imidazo[1,5-a][1,8]naphthyridin-9-ylidene (C), which combined an imine and an NHC function as a new C,N ligand.17 We have been interested in the synthesis and catalysis of transition-metal complexes of pyridine,18 pyrazole,19 naphthyridine,20 pyridazine,21 and phenanthroline-functionalized NHCs.22 As an extension, herein we present the synthesis and characterization of a naphthyridine-based triazolium salt (D) and its Ag(I) and Cu(I) complexes, in which the ligand acts in a bidentate fashion similarly to 1,10-phenanthroline. Additionally, the coppercatalyzed 3 + 2 cycloaddition reaction is also described.

Figure 1. 1,10-Phenanthroline and its NHC analogues.

organometallic chemistry.1 A large number of metal complexes of 1,10-phenanthroline show interesting photophysical, 2 electrochemical,3 and structural properties.4 1,10-Phenanthroline has also been well established as a powerful ligand in transition-metal-catalyzed C−heteroatom coupling reactions5 and C−C bond formation reactions such as fluoroalkylation,6 direct arylation of heterocycles,7 and decarboxylation reactions.8 However, the performance tuning of metal−phenanthroline complexes through modification of the 1,10-phenanthroline ligand is restricted, since the introduction of substituents is difficult.2a,3b,9 We speculate that replacement of one nitrogen donor by an NHC carbon donor would alter the electronic and steric behavior but still maintain its role in stabilizing transition-metal complexes. The introduction of an NHC to the planar π-conjugated scaffold would offer the opportunity to study the coordination and catalytic properties of new C,N-bidentate ligands. © XXXX American Chemical Society

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EXPERIMENTAL SECTION

All the chemicals were obtained from commercial suppliers and used without further purification. 7-Chloro-2,4-dimethyl-1,8-naphthyriReceived: October 22, 2012

A

dx.doi.org/10.1021/om3009876 | Organometallics XXXX, XXX, XXX−XXX

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dine,23 Co(PPh3 )2 Cl2,24 Ni(PPh 3)2 Cl2 ,25 and 4-(propargoxy)TEMPO26 were prepared according to the known procedures. Elemental analyses were performed on a Flash EA1112 instrument. 1 H and 13C NMR spectra were recorded on a Bruker Avance-400 (400 MHz) spectrometer. Chemical shifts (δ) are expressed in ppm downfield to TMS at δ 0 ppm, and coupling constants (J) are expressed in Hz. Synthesis of 2,4-Dimethyl-7-(2-phenylhydrazinyl)-1,8-naphthyridine. A solution of 7-chloro-2,4-dimethyl-1,8-naphthyridine (1.93 g, 10.0 mmol) and phenylhydrazine (1.62 g, 15.0 mmol) in 35 mL of ethanol was refluxed for 5 h. The solvent was then removed under vacuum, and the residue was purified by flash chromatography to give the desired product as an orange-red solid. Yield: 1.95 g, 74%. Anal. Calcd for C16H16N4: C, 72.70; H, 6.10; N, 21.20. Found: C, 73.03; H, 5.88; N, 20.92. 1H NMR (400 MHz, DMSO-d6): δ 8.31 (br, NH, 1H), 8.21 (d, J = 8.4 Hz, CHCH, 1H), 7.15 (m, 2Hphenyl + 2Hnaphthyridine, 4H), 6.81 (d, J = 8.4 Hz, phenyl, 2H), 6.73 (t, J = 7.2 Hz, phenyl, 1H), 2.59 (s, CH3, 3H), 2.58 (s, CH3, 3H). 13C NMR (100 MHz, DMSO-d6): δ 157.3, 152.0, 150.4, 148.7, 134.4, 128.9, 123.2, 121.6, 119.5, 118.7, 115.6, 112.1, 22.0, 18.0. Synthesis of HL·(PF6). In a solution of 2,4-dimethyl-7-(2phenylhydrazinyl)-1,8-naphthyridine (1.32 g, 5.0 mmol) in 20 mL of CH2Cl2 was bubbled dry HCl over 30 min. After the solvent was removed under vacuum, a dark red solid was isolated. Then the red solid was dissolved in 200 mL of water. The solution was filtered to remove the insoluble substances. Subsequent addition of an aqueous solution of NH4PF6 (1.64 g, 10.0 mmol) to the filtrate afforded a red precipitate, which was collected by filtration and dried. The resulting red solid (1.5 g, 3.7 mmol) was dissolved in 20 mL of chlorobenzene, and triethoxymethane (1.48 g 10.0 mmol) was added to the red solution. The solution was stirred for 8 h at 120 °C to afford an orange precipitate, which was collected by filtration and washed with Et2O (10 mL × 3). Yield: 1.41 g, 67%. Anal. Calcd for C17H15F6N4P: C, 48.58; H, 3.60; N, 13.33. Found: C, 48.23; H, 3.18; N, 13.69. 1H NMR (400 MHz, DMSO-d6): δ 12.14 (s, NCHN, 1H), 8.49 (d, J = 10.0 Hz, CHCH, 1H), 8.29 (d, J = 7.6 Hz, phenyl, 2H), 8.12 (d, J = 10.0 Hz, CHCH, 1H), 7.18 (m, 3Hphenyl + 1Hnaphthyridine), 2.79 (s, CH3, 3H), 2.78 (s, CH3, 3H). 13C NMR (100 MHz, DMSO-d6): δ 160.9, 149.2, 147.6, 140.3, 135.4, 134.4, 131.9, 131.2, 130.3, 126.6, 121.6, 116.1, 111.9, 24.2, 18.3. Synthesis of [Ag2(L)2(CH3CN)2](PF6)2 (1). A solution of HL·(PF6) (84 mg, 0.2 mmol) in 5 mL of CH3CN was treated with Ag2O (24 mg, 0.1 mmol) at 50 °C. After 0.5 h, Ag2O disappeared completely. The resulting mixture was filtered through a plug of Celite and concentrated to ca. 2 mL. Addition of Et2O (20 mL) to the filtrate afforded a light yellow precipitate, which was collected and washed with Et2O. Yield: 98 mg, 86%. Anal. Calcd for C19H17AgF6N5P: C, 40.16; H, 3.02; N, 12.33. Found: C, 40.03; H, 2.94; N, 12.18. 1H NMR (400 MHz, DMSO-d6): δ 8.28 (m, phenyl + CHCH, 3H), 7.93 (d, J = 10.0 Hz, CHCH, 1H), 7.59 (m, phenyl, 3H), 7.49 (s, naphthyridine, 1H), 2.70 (s, CH3, 3H), 2.10 (s, CH3, 3H), 2.08 (s, CH3, 3H). 13C NMR (100 MHz, DMSO-d6): δ 174.8 (Ag−C), 158.8, 148.8, 148.1, 142.4, 139.6, 138.7, 129.6, 126.3, 125.1, 122.7, 118.1, 115.3, 112.4, 23.7, 18.3, 1.1. Synthesis of [CuL(CH3CN)2](PF6) (2). A solution of HL·(PF6) (84 mg, 0.2 mmol) in 5 mL of CH3CN was treated with Ag2O (24 mg, 0.1 mmol) at 50 °C. After 0.5 h, Ag2O disappeared completely, and excess copper powder (64 mg, 1.0 mmol) was added to the yellow solution. After it was stirred for 12 h at 50 °C, the solution was filtered to remove unreacted copper powder and silver. The filtrate was then concentrated to ca. 2 mL. Addition of Et2O (20 mL) to the filtrate afforded a yellow precipitate, which was collected and washed with Et2O. Yield: 88 mg, 78%. Anal. Calcd for C21H20CuF6N6P: C, 44.65; H, 3.57; N, 14.88. Found: C, 44.32; H, 3.35; N, 14.54. 1H NMR (400 MHz, DMSO-d6): δ 8.34 (d, J = 7.6 Hz, phenyl, 2H), 8.23 (d, J = 9.2 Hz, CHCH, 1H), 7.88 (d, J = 8.0 Hz, CHCH, 1H), 7.62−7.52 (m, 3Hphenyl + 1Hnaphthyridine), 2.73 (s, CH3, 3H), 2.52 (s, CH3, 3H) 2.09 (s, CH3, 6H). 13C NMR (100 MHz, DMSO-d6): δ 171.4 (Cu−C), 159.1, 149.3, 147.2, 143.1, 139.7, 130.0, 129.9, 129.0, 125.2, 121.5, 118.4, 115.5, 112.8, 24.0, 18.4, 1.5.

The compound can also be prepared from the reaction of HL·(PF6) (84 mg, 0.2 mmol) and copper powder (64 mg, 1.0 mmol) in 5 mL of CH3CN at 60 °C. Yield: 61 mg, 54%. Synthesis of [CuL(phen)](PF6) (3). A mixture of 2 (57 mg, 0.1 mmol) and 1,10-phenanthroline (19 mg, 0.1 mmol) in 3 mL of CH3CN was stirred for 5 h at 50 °C. The solution was filtered and concentrated to ca. 1.0 mL. Addition of Et2O (10 mL) to the filtrate afforded an orange precipitate, which was collected and washed with Et 2 O (10 mL). Yield: 58 mg, 87%. Anal. Calcd for C29H22CuF6N6P·0.5CH3CN: C, 52.71; H, 3.47; N, 13.32. Found: C, 52.85; H, 3.47; N, 13.47. 1H NMR (400 MHz, DMSO-d6): δ 9.17 (s, phen, 2H), 8.92 (d, J = 7.6 Hz, phenyl, 2H), 8.41 (d, J = 8.0 Hz, phen, 2H), 8.34 (s, phen, 2H), 8.25 (d, J = 9.6 Hz, CHCH, 1H), 8.08 (m, phen, 2H), 7.94 (d, J = 9.6 Hz, CHCH, 1H), 7.54 (t, J = 7.6 Hz, phenyl, 2H), 7.43 (t, J = 7.2 Hz, phenyl, 1H), 7.38 (s, Hnaphthyridine, 1H), 2.69 (s, CH3, 3H), 1.49 (s, CH3, 3H). 13C NMR (100 MHz, DMSO-d6): δ 171.9 (Cu−C), 158.1, 151.0, 148.8, 147.0, 143.3, 143.1, 139.7, 138.7, 129.7, 129.6, 129.0, 128.4, 127.1, 125.9, 124.6, 121.1, 115.2, 112.6, 22.1, 18.1. Synthesis of [CuL(dppe)](PF6) (4). A mixture of 2 (57 mg, 0.1 mmol) and 1,2-bis(diphenylphosphino)ethane (40 mg, 0.1 mmol) in 3 mL of CH3CN was stirred for 5 h at 50 °C. The solution was filtered and concentrated to ca. 1.0 mL. Addition of Et2O (10 mL) to the filtrate afforded a yellow precipitate, which was collected and washed with Et2 O (10 mL). Yield: 81 mg, 92%. Anal. Calcd for C43H38CuF6N4P3·2H2O: C, 56.30; H, 4.62; N, 6.11. Found: C, 56.49; H, 4.35; N, 5.79. 1H NMR (400 MHz, DMSO-d6): δ 8.32 (d, J = 10.4 Hz, CHCH, 1H), 8.01 (d, J = 9.6 Hz, CHCH, 1H), 7.90 (d, J = 8.0 Hz, phenyl, 2H), 7.49−7.22 (m, 23Hphenyl + 1Hnaphthyridine, 24H), 2.92−2.77 (m, CH2 + CH3, 7H), 1.42 (s, CH3, 3H). 13C NMR (100 MHz, DMSO-d6): δ 180.9 (t, J = 36.3 Hz, Cu−C), 158.9, 149.4, 147.5, 143.4, 139.2, 132.0, 130.4, 130.2, 129.9, 129.3, 128.9, 128.8, 124.6, 121.7, 115.9, 113.0, 24.0, 18.2, 15.1. Synthesis of [Co(L)2(CH3CN)2](PF6)2 (5). A solution of HL·(PF6) (84 mg, 0.2 mmol) in 5 mL of CH3CN was treated with Ag2O (24 mg, 0.1 mmol) at 50 °C. After 0.5 h, Ag2O disappeared completely, and Co(PPh3)2Cl2 (65 mg, 0.1 mmol) was added to the yellow solution. After it was stirred for 12 h at 50 °C, the solution was filtered. The filtrate was then concentrated to ca. 2 mL. Addition of Et2O (20 mL) to the filtrate afforded a brown precipitate, which was collected and washed with Et 2 O. Yield: 55 mg, 56%. Anal. Calcd for C38H34CoF12N10P2: C, 46.59; H, 3.50; N, 14.30. Found: C, 46.73; H, 3.59; N, 14.01. The compound is paramagnetic. Synthesis of [Ni(L)3](PF6)2 (6). A solution of HL·(PF6) (84 mg, 0.2 mmol) in 5 mL of CH3CN was treated with Ag2O (24 mg, 0.1 mmol) at 50 °C. After 0.5 h, Ag2O disappeared completely, and Ni(PPh3)2Cl2 (65 mg, 0.1 mmol) was added to the yellow solution. After it was stirred for 12 h at 50 °C, the solution was filtered. The filtrate was then concentrated to ca. 2 mL. Addition of Et2O (20 mL) to the filtrate afforded an orange-red precipitate, which was collected and washed with Et2O. Yield: 46 mg, 59%. Anal. Calcd for C51H42F12N12NiP2·2CH3CN: C, 52.69; H, 3.86; N, 15.64. Found: C, 52.47; H, 4.11; N, 15.82. 1H NMR (400 MHz, DMSO-d6): δ 8.51 (d, J = 10.0 Hz, CHCH, 1H), 8.14 (m, 2H), 8.07 (d, J = 10.0 Hz, CHCH, 1H), 7.68 (m, phenyl, 2H), 7.61 (d, J = 8.8 Hz, CHCH, 2H), 7.50 (d, J = 8.0 Hz, CHCH, 2H), 7.38 (m, 3H), 7.21 (t, J = 7.6 Hz, phenyl, 1H), 6.97 (m, 3H), 6.86 (t, J = 7.2 Hz, phenyl, 1H), 6.78−6.69 (m, phenyl, 6H), 2.84 (s, CH3, 3H), 2.74 (s, CH3, 3H), 2.73 (s, CH3, 3H), 1.89 (s, CH3, 3H), 1.84 (s, CH3, 3H), 1.82 (s, CH3, 3H). 13C NMR (100 MHz, DMSO-d6): δ 180.2 (Ni−C), 173.2 (Ni−C), 167.0 (Ni−C), 161.9, 158.9, 158.8, 152.9, 149.4, 149.0, 148.7, 148.2, 146.2, 143.6, 141.8, 141.3, 136.7, 136.4, 136.3, 133.4, 133.3, 131.7, 130.8, 130.6, 130.4, 130.0, 129.0, 128.2, 128.1, 126.2, 125.9, 124.4, 124.0, 123.9, 118.1, 116.6, 116.3, 113.3, 112.0, 111.8, 24.0, 22.8, 22.7, 18.4, 18.3, 18.2. General Procedure for the CuAAC Reaction. In a glass tube, a mixture of 2 (6 mg, 0.01 mmol), alkyne (1.0 mmol), and azide (1.1 mmol) in 3.0 mL of CH3OH was stirred in air at 50 °C. The stirring was continued at 50 °C for 2 h. The solvent was then removed under vacuum, and the residue was purified by column chromatography on B

dx.doi.org/10.1021/om3009876 | Organometallics XXXX, XXX, XXX−XXX

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Scheme 1. Synthesis of the Triazolium Salts [Ag2(L)2(CH3CN)2](PF6)2 (1), [CuL(CH3CN)2](PF6) (2), [CuL(phen)](PF6) (3), and [CuL(dppe)](PF6) (4)

silica gel (ethyl acetate/petroleum ether) to give the desired product. The paramagnetic TEMPO derivatives were characterized by elemental analyses and mass spectroscopy. They were further identified by 1H and 13C spectroscopy after reduction to TEMP-OH compounds. X-ray Structural Determination. Single-crystal X-ray diffraction data were collected at 298(2) K on an Oxford Diffraction Gemini A Ultra diffractometer with Mo Kα radiation (λ = 0.71073 Å) by using an ω−2θ scan mode. Unit-cell dimensions were obtained with leastsquares refinement. Data collection and reduction were performed using the Oxford Diffraction CrysAlisPro software.27 The structures were solved by direct methods, and the non-hydrogen atoms were subjected to anisotropic refinement by full-matrix least-squares on F2 using the SHELXTXL package.28 Hydrogen atom positions for all of the structures were calculated and allowed to ride on their respective C atoms with C−H distances of 0.93−0.97 Å and Uiso(H) = (1.2− 1.5)[Ueq(C)]. Disordered solvents in the lattice for 5 could not be modeled successfully and were removed from their reflection data with SQUEEZE (solvent accessible void volume 100.0 Å3).29

exhibits a singlet at 7.49 ppm due to the 3-position proton of the naphthyridine. Its 13C NMR spectrum shows the carbenic carbon resonance signal at 174.8 ppm, which is consistent with signals for other Ag−NHC complexes ranging from 168.0 to 185.8 ppm.16a,21a,30 A single-crystal X-ray diffraction study showed that 1 is a dinuclear silver complex and the whole molecule is generated by a symmetry operation. As shown in Figure 2, the cation contains two Ag(I) ions, two L ligands, and two acetonitrile molecules. As expected, the ligand L coordinates in a bidentate fashion, as is the case for 1,10-phenanthroline. Each Ag(I) ion is coordinated by one carbenic carbon and two nitrogen atoms in a triangular conformation. Silver ions and the ligands are almost perfectly coplanar. The two coordination planes are parallel to

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RESULTS AND DISCUSSION Synthesis of the Triazolium Salts. As shown in Scheme 1, nucleophilic substitution of 7-chloro-2,4-dimethyl-1,8-naphthyridine by phenylhydrazine in refluxing ethanol afforded 2,4dimethyl-7-(2-phenylhydrazinyl)-1,8-naphthyridine in 74% yield. Its reaction with triethyl orthoformate in the presence of HPF6 gave the required triazolium salt in 67% yield. The triazolium salt was characterized by 1H and 13C NMR spectroscopy as well as elemental analysis. In its 1H NMR spectrum, a resonance signal was observed at 12.14 ppm assignable to the NCHN proton of the triazolium ring, indicating its quite acidic character. A doublet appeared at 8.29 ppm due to the ortho H of the phenyl ring, which showed an apparent downfield shift in comparison to those of typical phenyl hydrogen atoms because of the electron-withdrawing ability of the triazole ring. Synthesis and Characterization of [Ag2L2(CH3CN)2](PF6)2 (1). The deprotonation reaction of HL·(PF6) and Ag2O proceeded quickly in CH3CN at 50 °C, affording a light yellow solution from which [Ag2(L)2(CH3CN)2](PF6)2 (1; L = 2,4dimethyl-8-phenyl[1,2,4]triazolo[4,3-a][1,8]naphthyridin-9-ylidene) was isolated. The 1H NMR spectrum of 1 in DMSO-d6

Figure 2. ORTEP drawing of the cationic part of [Ag2(L)2(CH3CN)2](PF6)2 (1). Thermal ellipsoids are shown at the 30% probability level, with hydrogen atoms omitted for clarity. Selected bond distances (Å) and angles (deg): Ag(1)−C(11) = 2.107(2), Ag(1)−N(5) = 2.105(2), Ag(1)−N(1) = 2.602(2), Ag(1)− Ag(1)#1 = 3.218(1); C(11)−Ag(1)−N(5) = 169.17(9), C(11)− Ag(1)−N(1) = 72.92(9), N(5)−Ag(1)−N(1) = 116.68(8). Symmetry code: (#1) −x + 1, −y + 1, −z. C

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between the planes defined by the tricyclic scaffold and the phenyl group. The CuI−C distance is 1.934(3) Å, which is consistent with the reported examples.15c,34b,35 The Cu− Nnaphthyridine distance (2.307(2) Å) is much longer than those of Cu−Nacetonitrile (1.973(2) and 1.980(2) Å) due to the steric strain. Synthesis and Characterization of [CuL(phen)](PF6) (3) and [CuL(dppe)](PF6) (4). Acetonitrile could be easily substituted by other N and P donors.18d Treatment of 2 with 1 equiv of 1,10-phenanthroline and 1,2-bis(diphenylphosphino)ethane in acetonitrile afforded complexes 3 and 4 in good yields, respectively. In their 1H NMR spectra, all the ligands could be easily identified and the H resonances are consistent with their formations. The 13C NMR spectrum of 3 exhibits a singlet at 171.9 ppm due to the NHC carbon, basically the same as that of 2 at 171.4 ppm, whereas the carbenic carbon resonance of 4 was observed at 180.9 ppm as a triplet with JCP = 36.3 Hz due to its coupling with phosphorus atoms, showing an apparent shift downfield in comparison to those of 2 and 3. CuI−NHC complexes 2−4 are all stable toward air and moisture in the solid state and in solution. The structure of 3 is shown in Figure 4. Orange-red single crystals were grown by slow diffusion of Et2O into its

each other. The two silvers are held together by a ligandunsupported Ag−Ag bond (3.218(1) Å). Short Ag−Ag contacts have often been observed in silver complexes due to argentophilicity.10c,31 However, ligand-unsupported Ag−Ag interactions with Ag−Ag distances in the range 2.80−3.30 Å are relatively less studied.32 An unsupported Ag−Ag interaction was also observed in an Ag−NHC complex due to electrostatic interactions between [Ag(NHC)2]+ and [AgBr2]−.32b The Ag− C distances are 2.107(2) Å, showing no difference from those of the known silver−NHC complexes.31b,33 For comparison, 8(2,6-diisopropylphenyl)-8H-imidazo[1,5-a][1,8]naphthyridin9-ylidene (C) with Ag+ forms [Ag(NHC)2]+, showing very weak Ag−N interactions between the naphthyridinyl nitrogen and silver ion.17 Synthesis and Characterization of [CuL(CH3CN)2](PF6) (2). Further reaction of [Ag2(L)2(CH3CN)2](PF6)2 either isolated or generated in situ with an excess of commercially available copper powder at 50 °C afforded a yellow solution of [CuL(CH3CN)2](PF6) (2). The copper complex could also be obtained by reaction of HL·(PF6) with copper powder in acetonitrile without any other reagents.18b,19c In the 1H NMR spectrum of 2, two doublets were observed at 8.23 and 7.88 ppm due to the CHCH of the naphthyridine ring, showing apparent upfield shifts. The corresponding protons of HL·(PF6) were found at 8.49 and 8.12 ppm. The 13C NMR spectrum of 2 showed a singlet at 171.4 ppm due to the carbenic carbon, which falls in the normal range of 166.7−204.5 ppm for known Cu−NHC complexes.15c,16b,18b,34 The structure of 2 was further confirmed by X-ray diffraction and is depicted in Figure 3. The Cu(I) center is tetracoordinated in a distorted-tetrahedral sphere by one 2,4dimethyl-8-phenyl[1,2,4]triazolo[4,3-a][1,8]naphthyridin-9-ylidene and two acetonitrile molecules. The ligand is almost coplanar, as indicated by the small dihedral angle (2.42°)

Figure 4. ORTEP drawing of the cationic section of [CuL(phen)](PF6) (3). Thermal ellipsoids are shown at the 30% probability level, with hydrogen atoms omitted for clarity. Selected bond distances (Å) and angles (deg): Cu(1)−C(11) = 1.892(3), Cu(1)−N(5) = 2.026(3), Cu(1)−N(6) = 2.073(3), Cu(1)−N(1) = 2.422(3); C(11)−Cu(1)−N(5) = 148.02(14), C(11)−Cu(1)−N(6) = 127.54(14), N(5)−Cu(1)−N(6) = 81.62(12), C(11)−Cu(1)−N(1) = 78.69(14), N(5)−Cu(1)−N(1) = 113.57(12), N(6)−Cu(1)−N(1) = 97.80(12).

Figure 3. ORTEP drawing of the cationic section of [CuL(CH3CN)2](PF6) (2). Thermal ellipsoids are shown at the 30% probability level, with hydrogen atoms omitted for clarity. Selected bond distances (Å) and angles (deg): Cu(1)−C(11) = 1.934(3), Cu(1)−N(6) = 1.973(2), Cu(1)−N(5) = 1.980(2), Cu(1)−N(1) = 2.307(2); C(11)−Cu(1)−N(6) = 127.39(10), C(11)−Cu(1)−N(5) = 124.26(10), N(6)−Cu(1)−N(5) = 101.31(10), C(11)−Cu(1)−N(1) = 80.81(9), N(6)−Cu(1)−N(1) = 113.43(8), N(5)−Cu(1)−N(1) = 105.42(9).

concentrated CH 3 CN solution. The cation features a distorted-tetrahedral copper(I) center that is chelated by one L ligand and one 1,10-phenanthroline. The non-hydrogen atoms of the tricyclic plane and the phenyl plane are not coplanar, and the dihedral angle between these two planes is 23.55°. The Cu−C distance is 1.892(3) Å, slightly shorter than D

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NHC complexes prepared from transmetalation reactions via silver−carbene intermediates with Co(PPh3)2Cl2 have been reported by our group.22b,37 All these reactions afforded Co(III) complexes rather than the expected Co(II) complexes, due to their spontaneous oxidation in air. Interestingly, treatment of a solution of [Ag2(L)2(CH3CN)2](PF6)2 with 0.5 equiv of Co(PPh 3 ) 2 Cl 2 in CH 3 CN gave [Co(L)2(CH3CN)2](PF6)2 (5) as a hexacoordinated Co(II) complex in 56% yield (Scheme 2). Its paramagnetic properties prevented its NMR spectroscopic characterization. EPR spectroscopy was carried out at room temperature, and a pronounced peak was observed at g = 2.244 (see the Supporting Information), characteristic of the typical low-spin state of the Co(II) ion. Its structure was determined by X-ray diffraction and elemental analysis. Treatment of the solution of [Ag2(L)2(CH3CN)2](PF6)2 with 0.5 equiv of Ni(PPh3)2Cl2 gave [Ni(L)3](PF6)2 (6) in 59% yield rather than the expected [Ni(L)2]2+ square-planar complex. Variation of the ratio of HL·(PF6) and Ni(PPh3)2Cl2 from 3:1 to 1:1 did not change the product. We assume that a bis(bidentate) square-planar complex would not be stable due to steric repulsion.20b In the 1H NMR spectrum of 6, three L ligands could be easily identified by the resonance signals at 2.84, 2.74, 2.73, 1.89, 1.84, and 1.82 ppm due to the six methyl protons of the three naphthyridine rings. The 13C NMR spectrum of 6 shows three singlets at 180.2, 173.2, and 167.0 ppm ascribed to the NHC carbons, which are consistent with the known values (156.2−184.8 ppm).18a,20b,38 Both 1H and 13 C NMR spectra illustrated that the two dangling naphthyridines do not show dissociation and association processes in solution. As shown in Figure 6, the Co(II) center is hexacoordinated in an elongated-octahedral sphere by two carbons and four nitrogen atoms. Two NHC carbons are trans to two nitrogen atoms of the acetonitrile, and two Nnaphthyridine atoms are located in the axial positions. The almost linear N(5)−Co−N(1), as indicated by its angle (178.51(6)°), is perpendicular to the equatorial plane defined by CoC2N2. The Co−C distances are 1.914(2) and 1.915(2) Å, which are apparently shorter than those of the reported examples, normally ranging from 2.03 to 2.08 Å.39 The axial Co−Nnaphthyridine distances are 2.402(2) and 2.430(2) Å, which are much longer than the Co−Nacetonitrile distances at 1.937(2) and 1.944(7) Å. The structure of 6 is shown in Figure 7. X-ray diffraction revealed that the nickel center is surrounded by three NHC ligands. One NHC carbon apparently deviates from the equatorial plane defined by NiC2N with a distance of 0.766

that of 2 (1.934(3) Å). The Cu−Nnaphthyridine distance (2.422(3) Å) is apparently longer than the average Cu−Nphen distance (2.05 Å). The solid-state structure of complex 4 was also characterized by X-ray diffraction, and the structural analysis showed that complex 4 adopts the same geometry as 3. As illustrated in Figure 5, the cation is composed of one Cu(I) ion, one L

Figure 5. ORTEP drawing of the cationic part of [CuL(dppe)](PF6) (4). Thermal ellipsoids are shown at the 30% probability level, with hydrogen atoms omitted for clarity. Selected bond distances (Å) and angles (deg): Cu(1)−C(11) = 1.944(2), Cu(1)−P(2) = 2.270(3), Cu(1)−P(3) = 2.280(3), Cu(1)−N(1) = 2.384(2); C(11)−Cu(1)− P(2) = 137.45(8), C(11)−Cu(1)−P(3) = 128.53(7), P(2)−Cu(1)− P(3) = 90.67(2), C(11)−Cu(1)−N(1) = 79.25(9), P(2)−Cu(1)− N(1) = 103.18(5), P(3)−Cu(1)−N(1) = 111.73(5).

ligand, and one 1,2-bis(diphenylphosphino)ethane. The L ligand and 1,2-bis(diphenylphosphino)ethane chelated the Cu(I) center in a slightly distorted tetrahedral geometry. The Cu−C distance is 1.944(2) Å, somewhat longer than those of 2 and 3 (1.934(3) and 1.892(3) Å), possibly due to the steric bulkness of the diphosphine ligand. The Cu−Nnaphthyridine distance (2.384(2) Å) is comparable to the corresponding lengths (2.307(2) and 2.422(3) Å) in 2 and 3 presented above. The Cu−P distances are 2.270(3) and 2.280(3) Å, which are within the normal range of Cu−P distances of reported examples (2.15−2.38 Å).36 Synthesis and Characterization of [Co(L)2(CH3CN)2](PF6)2 (5) and [Ni(L)3](PF6)2 (6). Previously, a series of Co−

Scheme 2. Synthesis of [Co(L)2(CH3CN)2](PF6)2 (5) and [Ni(L)3](PF6)2 (6)

E

dx.doi.org/10.1021/om3009876 | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

Figure 6. ORTEP drawing of the cationic part of [Co(L)2(CH3CN)2](PF6)2 (5). Thermal ellipsoids are shown at the 30% probability level, with hydrogen atoms omitted for clarity. Selected bond distances (Å) and angles (deg): Co(1)−C(28) = 1.914(2), Co(1)−C(11) = 1.915(2), Co(1)−N(9) = 1.937(2), Co(1)−N(10) = 1.944(7), Co(1)−N(5) = 2.402(2), Co(1)−N(1) = 2.430(2); C(28)−Co(1)− C(11) = 81.54(9), C(28)−Co(1)−N(9) = 94.21(9), C(11)−Co(1)− N(9) = 170.43(8), C(28)−Co(1)−N(10) = 172.27(9), C(11)− Co(1)−N(10) = 94.81(8), N(9)−Co(1)−N(10) = 90.39(8), C(28)− Co(1)−N(5) = 77.52(8), C(11)−Co(1)−N(5) = 100.72(8), N(9)− Co(1)−N(5) = 86.62(8), N(10)−Co(1)−N(5) = 96.59(7), C(28)− Co(1)−N(1) = 102.20(8), C(11)−Co(1)−N(1) = 77.79(8), N(9)− Co(1)−N(1) = 94.86(8), N(10)−Co(1)−N(1) = 83.56(7), N(5)− Co(1)−N(1) = 178.51(6).

Figure 7. ORTEP drawing of the cationic part of [Ni(L)3](PF6)2 (6). Thermal ellipsoids are shown at the 30% probability level, with hydrogen atoms, and methyl and phenyl groups omitted for clarity. Selected bond distances (Å) and angles (deg): Ni(1)−C(28) = 1.857(3), Ni(1)−C(11) = 1.897(3), Ni(1)−C(45) = 1.907(3), Ni(1)−N(9) = 2.163(3), Ni(1)−N(1) = 2.542(3); C(28)−Ni(1)− C(11) = 87.68(13), C(28)−Ni(1)−C(45) = 90.39(13), C(11)− Ni(1)−C(45) = 175.49(13), C(28)−Ni(1)−N(9) = 156.30(13), C(11)−Ni(1)−N(9) = 101.37(11), C(45)−Ni(1)−N(9) = 81.96(12).

workers, highly efficient Cu−NHC complexes have been subsequently developed by other groups.15a,b,d,48 However, monodentate NHC ligands played a major role in these catalytic systems, and functionalized NHC ligands with additional N donors have been rarely reported.48b The CuAAC reaction has been introduced to construct the TEMPO-modified molecules as a synthetic method; however, only a few examples have been reported and high copper loadings (6−20%) were required.26,49 With the Cu(I)-NHC complexes 2−4 in hand, we examined their catalytic activities for the cycloaddition of 4-(propargoxy)TEMPO 7a and benzyl azide 8a. The results are given in Table 1. The reaction proceeded smoothly at room temperature in CH3CN in air, and the product 9a was obtained in 77% yield (Table 1, entry 1). The yield could be increased to 92% by using CH3OH as the solvent (entry 2). Employment of Et2O or H2O as solvent gave the product in poor yields, probably due to the low solubility of the copper catalyst (entries 3 and 4). In comparison to 3 and 4, 2 showed better catalytic activity, since the coordinated CH3CN molecules are easily dissociated from the metal center, and accordingly the substrates could coordinate more easily (entries 5 and 6). When the temperature was raised to 50 °C, the target product 9a was almost quantitatively obtained within 2 h (entry 7). For comparison, [Cu(CH3CN)4]PF6 is much less active under the same conditions, and only 47% of the product was obtained (entry 8). In the absence of a copper catalyst, only a trace amount of the cycloaddition product was observed (entry 9).

Å. The axial Ni−N distances are 2.542 and 3.004 Å, assuming weak Ni−N interactions on the basis of the sum of the covalent radii of the two atoms (r(Ni) + r(N) = 3.18 Å).40 Thus, 6 can be viewed as a four-coordinate complex with weak axial Ni−N interactions. In the equatorial plane, the Ni−N distance (2.163(3) Å) is consistent with those of the reported Ni− NHC complexes, ranging from 1.85 to 2.22 Å.18a,22a,41 The average Ni−C distance (1.887 Å) is quite consistent with those of other Ni−NHC complexes.18a,20b,41,42 Catalytic Properties. The combinations of transition metals and 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) have been shown to be very efficient catalysts in alcohol oxidations.43 Especially, increasing research attention has been directed in recent years at employing copper/TEMPO/N donor systems in alcohol oxidation reactions with molecular oxygen as the terminal oxidant, due to their efficient, mild, and environmentally acceptable advantages.43b−d,44 TEMPO has often been utilized as the hydrogen transfer reagent.43b−d The Cu(I)-catalyzed azide−alkyne cycloaddition (CuAAC) reaction has attracted great attention because of its operationally simple and highly efficient character.15b,45 The 1,2,3-triazole products are capable of metal coordination.46 Thus, we might prepare TEMPO derivatives tethering a triazole as bifunctional ligands for copper-catalyzed alcohol oxidation reactions. Many copper catalysts with nitrogen ligands have been prepared for this “click” reaction.45a,c,47 Since the use of Cu−NHC complexes as catalysts for CuAAC reactions as pioneered by Nolan and coF

dx.doi.org/10.1021/om3009876 | Organometallics XXXX, XXX, XXX−XXX

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Table 1. Solvent and Catalyst Optimization Studiesa

entry

solvent

cat.

1 2 3 4 5 6 7 8 9

CH3CN CH3OH Et2O H2O CH3OH CH3OH CH3OH CH3OH CH3OH

2 2 2 2 3 4 2 [Cu(CH3CN)4]PF6 none

temp (°C) room room room room room room 50 50 50

temp temp temp temp temp temp

time (h)

yield (%)b

6 6 6 6 6 6 2 2 2

77 92 trace 11 53 68 97 47 trace

corresponding triazoles in high yields. The cycloaddition of 1,4bis(azidomethyl)benzene (8d) with 7a afforded the di-TEMPO product 9d in 88% yield (Table 2, entry 3). Cinnamyl azide (8e) could also be applied in high yield (entry 4). The cycloaddition of mesityl azide (8f) required a longer time due to steric effects (entry 5). Moreover, 2-(azidomethyl)pyridine (8g) was also successfully employed in 86% yield (entry 6). Under the same conditions, reactions of 4-(2-azidoacetoxy)TEMPO (8h) with phenylacetylene (7b) and 2-ethynylpyridine (7c) also afford the target products in almost quantitative yields (entries 7 and 8). The TEMPO derivatives are paramagnetic, and they were characterized by elemental analysis and mass spectroscopy. They were further identified by measuring 1H and 13C NMR spectra of their corresponding TEMP-OH derivatives after reduction by sodium ascorbate (see the Supporting Information). To explore the potential usefulness of the TEMPO-modified 1,2,3-triazole, we preliminarily examined the oxidation reaction of benzyl alcohol using 9g and [Cu(CH3CN)4]PF6 as the catalyst (Scheme 3). In chlorobenzene at 70 °C for 2 h, aerobic oxidation gave benzaldehyde in nearly quantitative yield.

a

Reaction conditions: 7a, 1.0 mmol; Cu, 0.01 mmol (1 mol %); 8a, 1.1 mmol; solvent, 3 mL. bisolated yield.

Under the optimized conditions, we extended the cycloaddition reaction to other TEMPO-tethered alkynes and azides. As found in Table 2, various azides reacted with 7a, giving the

Table 2. Copper-Catalyzed TEMPO-Modified Azide−Alkyne Cycloadditiona

Reaction conditions unless specified otherwise: alkynes, 1.0 mmol; 2, 0.01 mmol (1 mol %); azides, 1.1 mmol; CH3OH, 3 mL; 50 °C; 2 h. bIsolated yield. cReaction onditions: 7a, 2.2 mmol, 2, 0.02 mmol; 8d, 1.0 mmol; CH3OH, 6 mL. d8 h. eReaction conditions: alkynes, 1.1 mmol; 2, 0.01 mmol (1 mol %); 8h, 1.0 mmol. a

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Evain, M.; Bera, N. C.; Kayanuma, M.; Sliwa, M.; Rebarz, M.; Poizat, O.; Daniel, C.; Odobel, F. Inorg. Chem. 2011, 50, 11309. (4) (a) Chandrasekhar, V.; Singh, P.; Gopal, K. Organometallics 2007, 26, 2833. (b) Murugavel, R.; Korah, R. Inorg. Chem. 2007, 46, 11048. (c) Chandrasekhar, V.; Azhakar, R.; Senapati, T.; Thilagar, P.; Ghosh, S.; Verma, S.; Boomishankar, R.; Steiner, A.; Kögerler, P. K. Dalton Trans. 2008, 1150. (d) Ricardo, C.; Matosziuk, L. M.; Evanseck, J. D.; Pintauer, T. Inorg. Chem. 2009, 48, 16. (5) (a) Gujadhur, R. K.; Bates, C. G.; Venkataraman, D. Org. Lett. 2001, 3, 4315. (b) Shafir, A.; Lichtor, P. A.; Buchwald, S. L. J. Am. Chem. Soc. 2007, 129, 3490. (c) Tye, J. W.; Weng, Z.; Johns, A. M.; Incarvito, C. D.; Hartwig, J. F. J. Am. Chem. Soc. 2008, 130, 9971. (d) Lv, X.; Bao, W. J. Org. Chem. 2009, 74, 5618. (6) (a) Chu, L.; Qing, F. L. J. Am. Chem. Soc. 2010, 132, 7262. (b) Chu, L.; Qing, F. L. Org. Lett. 2010, 12, 5060. (c) Qi, Q.; Shen, Q.; Lu, L. J. Am. Chem. Soc. 2012, 134, 6548. (7) (a) Do, H. O.; Khan, R. M. K.; Daugulis, O. J. Am. Chem. Soc. 2008, 130, 15185. (b) Shibahara, F.; Yamaguchi, E.; Murai, T. Chem. Commun. 2010, 46, 2471. (c) Cao, H.; Zhan, H.; Lin, Y.; Lin, X.; Du, Z.; Jiang, H. Org. Lett. 2012, 14, 1688. (8) (a) Goossen, L. J.; Rodriguez, N.; Melzer, B.; Linder, C.; Deng, G. J.; Levy, L. M. J. Am. Chem. Soc. 2007, 129, 4824. (b) Goossen, L. J.; Rodriguez, N.; Linder, C. J. Am. Chem. Soc. 2008, 130, 15248. (c) Goossen, L. J.; Linder, C.; Rodriguez, N.; Lange, P. P. Chem. Eur. J. 2009, 15, 9336. (9) Cheng, Y.; Han, X.; Ouyang, H.; Rao, Y. Chem. Commun. 2012, 48, 2906. (10) (a) Dı ́ez-González, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612. (b) Iglesias-Sigüenza, J.; Ros, A.; Dı ́ez, E.; Alcarazo, M.; Á lvarez, E.; Fernández, R.; Lassaletta, J. M. Dalton Trans. 2009, 7113. (c) Lin, J. C. Y.; Huang, R. T. W.; Lee, C. S.; Bhattacharyya, A.; Hwang, W. S.; Lin, I. J. B. Chem. Rev. 2009, 109, 3561. (d) Poyatos, M.; Mata, J. A.; Peris, E. Chem. Rev. 2009, 109, 3677. (e) Schuster, O.; Yang, L. R.; Raubenheimer, H. G.; Albrecht, M. Chem. Rev. 2009, 109, 3445. (f) Conrady, F. M.; Fröhlich, R.; Brinke, C. S.; Pape, T.; Hahn, F. E. J. Am. Chem. Soc. 2011, 133, 11496. (g) Naziruddin, A. R.; Hepp, A.; Pape, T.; Hahn, F. E. Organometallics 2011, 30, 5859. (h) Gonell, S.; Poyatos, M.; Mata, J. A.; Peris, E. Organometallics 2012, 31, 5606. (11) (a) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1, 953. (b) Samojłowicz, C.; Bieniek, M.; Grela, K. Chem. Rev. 2009, 109, 3708. (c) Vougioukalakis, G. C.; Grubbs, R. H. Chem. Rev. 2010, 110, 1746. (12) (a) Edwards, M. G.; Jazzar, R. F. R.; Paine, B. M.; Shermer, D. J.; Whittlesey, M. K.; Williams, J. M. J.; Edney, D. D. Chem. Commun. 2004, 90. (b) Burling, S.; Paine, B. M.; Nama, D.; Brown, V. S.; Mahon, M. F.; Prior, T. J.; Pregosin, P. S.; Whittlesey, M. K.; Williams, J. M. J. J. Am. Chem. Soc. 2007, 129, 1987. (c) Nordstrøm, L. U.; Vogt, H.; Madsen, R. J. Am. Chem. Soc. 2008, 130, 17672. (d) Prades, A.; Viciano, M.; Sanaú, M.; Peris, E. Organometallics 2008, 27, 4254. (e) Prades, A.; Peris, E.; Albrecht, M. Organometallics 2011, 30, 1162. (13) (a) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Angew. Chem., Int. Ed. 2007, 46, 2768. (b) Zhang, X.; Liu, A.; Chen, W. Org. Lett. 2008, 10, 3849. (c) Hartmann, C. E.; Nolan, S. P.; Cazin, C. S. J. Organometallics 2009, 28, 2915. (d) Lee, J. Y.; Cheng, P. Y.; Tsai, Y. H.; Lin, G. R.; Liu, S. P.; Sie, M. H.; Lee, H. M. Organometallics 2010, 29, 3901. (e) Yuan, D.; Huynh, H. V. Organometallics 2010, 29, 6020. (f) Bernhammer, J. C.; Huynh, H. V. Organometallics 2012, 31, 5121. (g) Valente, C.; Ç alimsiz, S.; Hoi, K. H.; Mallik, D.; Sayah, M.; Organ, M. G. Angew. Chem., Int. Ed. 2012, 51, 3314. (h) Yuan, D.; Huynh, H. V. Organometallics 2012, 31, 405. (14) (a) Luan, X.; Mariz, R.; Gatti, M.; Costabile, C.; Poater, A.; Cavallo, L.; Linden, A.; Dorta, R. J. Am. Chem. Soc. 2008, 130, 6848. (b) Dastgir, S.; Coleman, K. S.; Cowley, A. R.; Green, M. L. H. Organometallics 2010, 29, 4858. (c) Chartoire, A.; Frogneux, X.; Nolan, S. P. Adv. Synth. Catal. 2012, 354, 1897. (d) Meiries, S.; Chartoire, A.; Slawin, A. M. Z.; Nolan, S. P. Organometallics 2012, 31, 3402. (15) (a) Dı ́ez-González, S.; Correa, A.; Cavallo, L.; Nolan, S. P. Chem. Eur. J. 2006, 12, 7558. (b) Dı ́ez-González, S.; Nolan, S. P.

Scheme 3. Catalytic Aerobic Oxidation of Benzyl Alcohol

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CONCLUSION In summary, we have described the synthesis and structural characterization of Ag(I), Cu(I), Co(II), and Ni(II) complexes containing a planar π-conjugated naphthyridine-based Nheterocyclic carbene ligand. The bidentate ligand shows coordination behavior similar to that of 1,10-phenanthroline. Copper complex 2 is found to be highly active in the Cu(I)catalyzed azide−alkyne cycloaddition reaction for TEMPOmodified substrates under mild conditions. A preliminary study showed that the triazole anchoring TEMPO is a good ligand for copper-catalyzed aerobic oxidation of alcohols. Further work will be done to develop new bifunctional catalysts that are useful for catalytic aerobic oxidations.

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ASSOCIATED CONTENT

■

AUTHOR INFORMATION

S Supporting Information *

Text, figures, a table and CIF files giving structural parameters for 1−6, the EPR spectrum of 5, spectroscopic data of CuAAC products, and experimental details of alcohol oxidation. This material is available free of charge via the Internet at http:// pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Tel and fax: 0086-57188273314. Notes

The authors declare no competing financial interest.

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

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Organometallics

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dx.doi.org/10.1021/om3009876 | Organometallics XXXX, XXX, XXX−XXX