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Synthesis, characterization and catalytic activity of gold complexes with pyridine-based selone ligands
Inorganica Chimica Acta 450 (2016) 315–320
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Research paper
Synthesis, characterization and catalytic activity of gold complexes with pyridine-based selone ligands Hai-Ning Zhang, Wei-Guo Jia ⇑, Qiu-Tong Xu, Chang-Chun Ji College of Chemistry and Materials Science, Center for Nano Science and Technology, The Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecular-Based Materials, Anhui Normal University, Wuhu 241000, China
a r t i c l e
i n f o
Article history: Received 12 April 2016 Received in revised form 26 May 2016 Accepted 10 June 2016 Available online 11 June 2016 Keywords: Selone Gold Complex Catalyst Reduction
a b s t r a c t Three neutral pyridine-based selone compounds, 2,6-bis(1-methylimidazole-2-selone)pyridine (Bmsp), 2,6-bis(1-ethylimidazole-2-selone)pyridine (Besp) and 2,6-bis(1-isopropylimidazole- 2-selone)pyridine (Bpsp) have been synthesized and characterized. Reactions of HAuCl4 with pyridine-based selone ligands result in the formation of the complexes [Au(L)Cl2]+[AuCl2] (L = Bmsp (1); L = Besp (2) and L = Bpsp (3)), respectively. All compounds have been characterized by elemental analysis, NMR IR spectra and electrospray ionization mass spectroscopic (ESI-MS). The molecular structure of 2 has been determined by X-ray crystallography. Moreover, the gold complexes are efficiently catalyzed nitroarenes reduction to aromatic amines in the presence of sodium tetrahydroborate reducing agent in water. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Functionalized anilines are important structural motifs that are found in a variety of agrochemicals, dyes, pharmaceuticals, and pigments and other industrially useful products [1–6]. Therefore, a variety of protocols for synthesizing functionalized anilines have been developed [7–19]. The desirable method for the synthesis of functionalized anilines is the reduction of nitroarenes using transition metal catalysts in the presence of sodium tetrahydroborate (NaBH4) [20]. A number of homogeneous and heterogeneous catalysts have also been reported as viable catalysts for this hydrogenation. Heterogeneous gold catalysts have become an important topic since the 1980s [21], since 2000 homogeneous gold catalysis has become a highly active field [22,23]. Furthermore, occasionally reactions typically catalyzed by homogeneous catalysts have also been reported to be catalyzed by heterogeneous catalysts [24], and occasionally reaction typically catalyzed by heterogeneous catalysts have also been reported to be catalyzed by homogeneous gold(III) complexes [25]. Gold-based catalysts supported by metal oxides (such as TiO2, Fe2O3, Fe3O4, SiO2 and Al2O3) [26–32], carbon nanotube [33], polymer [34,35], graphene oxide [36], and gold complexes [37–40] are all good candidate for the hydrogenation of nitroarenes due to their high catalytic activities.
⇑ Corresponding author. E-mail address: [email protected] (W.-G. Jia). http://dx.doi.org/10.1016/j.ica.2016.06.023 0020-1693/Ó 2016 Elsevier B.V. All rights reserved.
Imidazoline-2-chalcogenone ligands NHC = E (NHC = N-heterocyclic carbene, E = S, Se) are good r-donors and weak p-acceptors which can be easily modified using appropriate available NHC precursors to attain the desired steric-bulk or electronic properties [41–43]. Reports about transition metal complexes with imidazoline-2-chalcogenone ligands have been reported within past few years [44–49]. And the catalytic activities of these complexes are comparable with the most efficient metal-NHC complexes for organic transformation [50–52]. However, the transition metal complexes with selone ligands are reported rarely due to difficulty in synthesis, handing and malodorous nature of the organoselenium ligands [46]. Continuing our research interests in imidazoline-2-chalcogenone metal complexes for their rich chemistry [53–56], we try to prepare the gold complexes with pyridine-based [57,58] selone ligands and hope to exploit their catalytic applications in hydrogenation of nitroarenes. Herein, we report the synthesis and characterization of three gold complexes with pyridine-based selone compounds: [Au(L)Cl2]+[AuCl2] (L = Bmsp (1), Bmsp = 2,6-bis(1-methylimidazole-2-selone)pyridine; L = Besp (2), Besp = 2,6-bis(1-ethylimidazole-2-selone)pyridine and L = Bpsp (3), Bpsp = 2,6-bis (1-isopropylimidazole-2-selone)pyridine), and further examine their catalytic ability in direct nitroarenes reduction to aromatic amines in the presence of NaBH4 reducing agent in water. Our preliminary results indicate that gold complexes show promising catalytic activities in nitroarenes reduction reaction. The elucidation of solid state structure of the gold complex 2 was also obtained through single crystal X-ray diffraction.
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2. Experimental 2.1. Materials and physical measurements Commercial reagents were analytical grade and used as received from Aladdin and Energy chemical. All manipulations were carried out under nitrogen using standard schlenk and vacuum-line techniques. All solvents were purified and degassed by standard procedures. The starting materials 2,6-bis(1-methylimidazolium) pyridine dibromide, 2,6-bis(1-ethylimidazolium)pyridine dibromide and 2,6-bis(1-isopropylimidazolium)pyridine dibromide were synthesized according to the procedures described in the literature [55]. All catalytic reactions were monitored by TLC using 0.25 mm silica gel plates with UV indicator (60F-254). 1H and 13C NMR were recorded on a 300 MHz or 500 MHz NMR spectrometer at room temperature. IR spectra were recorded on a Niclolet AVATAR-360IR spectrometer. Element analyses were performed on an Elementar III vario EI Analyzer. Mass spectra were obtained with MicroTof (Bruker Daltonics, Bremen, Germany) spectrometers. 2.2. Synthesis of Bmsp In a 25 mL Schlenk tube were placed with 2,6-bis(1-methylimidazolium)pyridine dibromide (201.5 mg, 0.5 mmol), Se (94.8 mg, 1.2 mmol), K2CO3 (165.9 mg, 1.2 mmol) and 6.0 mL methanol as solvent. The mixture was allowed to reflux for 12 h after which the methanol was removed with a rotary evaporator. The remaining solid was shaken with 3 10 mL CH2Cl2 which was then filtered and rotary evaporated. The product was recrystallized from CH2Cl2/MeOH to give white solid. Yield: (258 mg 65%) (based on 2,6-bis(1-methylimidazolium)pyridine dibromide). Anal. Calcd. for C13H13N5Se2 (397.20): C, 39.10; H, 3.28; N, 17.77. Found: C, 39.15; H, 3.30; N, 17.58. 1H NMR (300 MHz, CDCl3): d 8.90 (d, J = 8 Hz, pyridine, 2H), 8.05 (t, pyridine, 1H), 7.54 (d, J = 3 Hz, imidazole, 2H), 6.92 (d, J = 3 Hz, imidazole, 2H), 4.52 (s, 2CH3, 6H). 13C NMR (500 MHz, CDCl3): d 156.72, 149.40, 140.15, 120.95, 119.18, 118.97, 37.86 ppm. IR (KBr, cm 1): 3358 (w), 3281 (w), 3188 (w), 3165 (w), 3130 (w), 3099 (w), 1666 (w), 1624 (vs), 1571 (m), 1471 (vs), 1448 (s), 1395 (m), 1369 (m), 1290 (m), 1236 (m), 1121 (w), 1074 (w), 985 (w), 874 (w), 797 (m), 671 (w), 555 (w). 2.3. Synthesis of Besp The synthesis procedure was similar to the ligand Bmsp to afford the white solid Besp, using 2,6-bis(1-ethylimidazolium)pyridine dibromide (217 mg, 0.5 mmol), Se (94.8 mg, 1.2 mmol), K2CO3 (165.9 mg, 1.2 mmol). Yield: (213 mg 50%) (based on 2,6bis(1-ethylimidazolium)pyridine dibromide). Anal. Calcd. for C15H17N5Se2 (426.98): C, 42.16; H, 4.01; N, 16.40. Found: C, 42.20; H, 4.05; N, 16.36. 1H NMR (300 MHz, CDCl3): d 8.86 (d, J = 8 Hz, pyridine, 2H), 8.12 (t, pyridine, 1H), 7.57 (d, J = 3 Hz, imidazole, 2H), 6.98 (d, J = 3 Hz, imidazole, 2H), 4.26 (m, 2CH2 4H), 1.46 (t, 2CH3, 6H). 13C NMR (500 MHz, CDCl3): d 155.73, 149.39, 139.91, 119.60, 119.35, 119.15, 45.34, 14.60 ppm. IR (KBr, cm 1): 3177 (w), 3140 (w), 3097 (w), 2965 (w), 2864 (w), 1647 (w), 1605 (s), 1576 (m), 1533 (w), 1464 (s), 1408 (s), 1392 (s), 1354 (w), 1303 (m), 1281 (s), 1258 (s), 1223 (s), 1155 (m), 1134 (m), 1115 (m), 1076 (w), 1040 (w), 995 (w), 985 (w), 966 (w), 949 (m), 903 (w), 802 (s), 800 (s), 785 (m), 773 (m), 735 (w),716 (m), 688 (w), 655 (m), 599 (w), 509 (w). 2.4. Synthesis of Bpsp The synthesis procedure was similar to the ligand Bmsp to afford the white solid Bpsp, using 2,6-bis(isopropyllimidazolium)
pyridine dibromide (220.4 mg, 0.5 mmol), Se (94.8 mg, 1.2 mmol), K2CO3 (165.9 mg, 1.2 mmol). Yield: (245 mg 55%) (based on 2,6-bis (isopropyllimidazolium)pyridine dibromide). Anal. Calcd. for C17H21N5Se2 (455.01): C, 44.83; H, 4.65; N, 15.39. Found: C, 44.88; H, 4.62; N, 15.36. 1H NMR (300 MHz, CDCl3): d 8.82 (d, J = 8 Hz, pyridine, 2H), 8.04 (t, pyridine, 1H), 7.58 (d, J = 3 Hz, imidazole, 2H), 7.02 (d, J = 3 Hz, imidazole, 2H), 5.39 (m, 2CH, 2H), 1.45 (d, J = 6 Hz, 4CH3, 12H). 13C NMR (500 MHz, CDCl3): d 155.30, 149.39, 139.72, 119.96, 119.93, 119.79, 119.7, 51.36, 22.28, 22.25 ppm. IR (KBr, cm 1): 3175 (w), 3132 (w), 3094 (w), 2970 (w), 2866 (w), 1666 (w), 1598 (m), 1572 (m), 1456 (s), 1416 (m), 1400 (m), 1385 (w), 1368 (w), 1337 (m), 1327 (w), 1277 (m), 1225 (s), 1153 (w), 1134 (w), 1113 (m), 1047 (w), 1018 (w), 991 (w), 883 (w), 835 (w), 804 (m), 781 (m), 718 (m), 685 (w), 664 (m), 584 (w), 554 (w), 500 (w). 2.5. Synthesis of complex [Au(Bmsp)Cl2][AuCl2] (1) In a 25 mL Schlenk tube were placed with Bmsp (59.55 mg, 0.15 mmol), HAuCl4 (132 mg, 0.32 mmol), 5 mL methanol as solvent. The mixture was stirred at room temperature overnight and then the solvent was removed with a rotary evaporator; the resulting solid was washed with methanol and diethyl ether, and then dried in vacuo. The product was recrystallized from MeCN/ CH2Cl2 to give brown deep-red powder. Yield: (67.5 mg, 45%) (based on Bmsp ligand). Anal. Calcd. for C13H13Au2Cl4N5Se2 (932.94): C, 16.74; H, 1.40; N, 7.51. Found: C, 16.77; H, 1.16; N, 7.50. 1H NMR (300 MHz, CD3OD): d 8.52 (t, pyridine, 1H), 8.17 (d, J = 2 Hz, pyridine, 2H), 7.95 (d, J = 6 Hz, imidazole, 2H), 7.70 (d, J = 2 Hz, imidazole, 2H), 4.25 (s, 6H). 13C NMR (500 MHz, CD3OD): d 148.06, 143.98, 138.89, 125.57, 124.02, 121.55, 38.5. ppm. ESI-MS (positive ions) for [Au(Bmsp)]3+: m/z 595.9160 (calcd for [Au (Bmsp)]3+ 595.9167). IR (KBr, cm 1): 3421(w), 2943(w), 1816(w), 1672(m), 1627(s), 1562(m), 1508(w), 1476(s), 1440(m), 1408(w), 1379(w), 1300(w), 1238(m), 1138(w), 986(w), 878(w), 802(w), 734(w), 551(w), 501(w). 2.6. Synthesis of complex [Au(Besp)Cl2][AuCl2] (2) Prepared by the same procedure as described above for 1, using Besp (49.65 mg, 0.15 mmol) and HAuCl4 (132 mg, 0.32 mmol). Yield: (77.3 mg, 50%) (based on Besp ligand). Anal. Calc. for C15H17Au2Cl4N5Se2 (961.00): C, 18.75; H, 1.78; N, 7.29. Found: C, 18.72; H, 1.71; N, 7.22. 1H NMR (300 MHz, CD3OD) d 8.51 (t, pyridine,1H), 8.20 (d, J = 2 Hz, imidazole, 2H), 8.08 (d, J = 2 Hz, imidazole,2H), 7.97 (d, J = 2 Hz, pyridine, 2H), 4.70–4.72 (m, 4H, CH2), 1.60 (t, 2CH3, 6H). 13C NMR (500 MHz, CD3OD) d 148.08, 146.59, 143.99, 124.64, 123.90, 122.09, 47.19, 13.95. ESI-MS (positive ions) for [Au(Besp)]3+: m/z 623.9461 (calcd for [Au(Besp)]3+ 623.9480). IR (KBr, cm 1): 3395 (w), 3138 (w), 3070 (w), 2976 (w), 2929 (w), 2868 (w), 1732 (w), 1627 (w), 1597 (w), 1554 (w), 1458 (s), 1431 (m), 1398 (w), 1352 (w), 1301 (w), 1267 (m), 1221 (m), 1134 (w), 1088 (w), 1043 (w), 995 (w), 954 (w), 814 (w), 775 (w), 735 (w), 679 (w), 544 (w), 501 (w). 2.7. Synthesis of complex [Au(Bpsp)Cl2][AuCl2] (3) Prepared by the same procedure as described above for 1, using Bpsp (68.0 mg, 0.15 mmol) and HAuCl4 (132 mg, 0.32 mmol). Yield: (87.3 mg, 55%) (based on Bpsp ligand). Anal. Calc. for C17H21Au2Cl4N5Se2 (989.05): C, 20.64; H, 2.14; N, 7.08. Found: C, 20.70; H, 2.18; N, 7.15. 1H NMR (300 MHz, CD3OD) d 8.53 (t, pyridine, 1H), 8.25 (d, J = 2 Hz, imidazole, 2H), 8.12 (d, J = 2 Hz, imidazole, 2H), 7.99 (d, pyridine, 2H), 5.48 (m, CH, 2H), 1.57 (d, CH3, 6H), 1.28 (d, CH3, 6H). 13C NMR (500 MHz, CD3OD) d 148.08, 144.47, 137.10, 124.93, 121.99, 121.42, 54.40, 21.64, 21.00. ESI-MS
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(positive ions) for [Au(Bpsp)]3+: m/z 651.9778 (calcd for [Au (Bpsp)]3+ 651.9793). IR (KBr, cm 1): 3180 (w), 3134 (w), 3096 (w), 2976 (w), 2875 (w), 1672 (w), 1598 (s), 1573 (m), 1456 (s), 1416 (m), 1400 (m), 1385 (w), 1369 (w), 1337 (m), 1329 (w), 1311 (w), 1277 (m), 1224 (s), 1155 (w), 1136 (w), 1114 (m), 1069 (w), 1047 (w), 1022 (w), 991 (w), 889 (w), 843 (w), 804 (m), 787 (m), 718 (m), 685 (w), 664 (m), 590 (w), 559 (w), 503 (w). 2.8. General procedure for the reduction of nitroarenes to anilines using gold catalysts Gold complex (0.01 mmol, 0.02 equiv) was dissolved in water (2.0 mL), then appropriate nitroarenes (0.5 mmol, 1.0 equiv) and NaBH4 (2 mmol, 4.0 equiv) was added. Subsequently, the resulting mixture was stirred at room temperature. After completion of the reaction (monitored by TLC), the crude reaction mixture was extracted with EtOAC (3 2 mL). After solvents were removed in vacuo from combined organic extracts, the crude products loaded directly onto a column of silica gel and purified by column chromatography to give the corresponding products [59–61]. 2.9. X-ray crystallography for 2 Diffraction data of complex 2 was collected on a Bruker Smart CCD diffractometer with graphite-monochromated MoKa radiation (k = 0.71073 Å). All the data were collected at room temperature and the structure was solved by direct methods and subsequently refined on F2 by using full-matrix least-squares techniques (SHELXL) [62], and SADABS absorption corrections [63] applied to the data. The crystallographic data for complex 2 are summarized in Table 1, and selected bond lengths and angles are shown in Table 2. 3. Result and discussions Pyridine-based selone compounds: 2,6-bis(1-methylimidazole2-selone)pyridine (Bmsp), 2,6-bis(1-ethylimidazole-2-selone)pyridine (Besp) and 2,6-bis(1-isopropylimidazole- 2-selone)pyridine (Bpsp) were prepared in one-pot via the reactions of pyridine bridged imidazolium dibromide derivatives with selenium powder in the presence of K2CO3 with moderate yields (50%) (Scheme 1) [55]. All these compounds were thermally stable and inert toward
Table 1 Crystallographic data and structure refinement parameters for complex 2. 2 Empirical formula Formula weight Crystal syst., Space group a (Å) b (Å) c (Å) b (°) Volume (Å3), Z Dc (mg/m3) l (Mo-Ka) (mm 1) F(0 0 0) h range (°) Limiting indices Reflections/unique[R(int)] Completeness to h (°) Data/restraints/parameters Goodness-of-fit on F2 R1, wR2 [I > 2r(I)]a R1, wR2 (all data) Larg. diff. peak/hole (e/Å 3) a
R1 = R||Fo|
|Fc||/R|Fo|; wR2 = [Rw(|F2o|
C15H17Au2Cl4N5Se2 960.99 Monoclinic, P2(1)/c 13.0360(7) 15.1772(8) 11.7753(6) 92.105(1) 2328.2(2), 4 2.742 16.187 1744 1.563–27.499 15, 16, 19, 19, 15, 14 19587/5254 [0.0384] 27.49 (98.2%) 5254/0/253 1.018 0.0312, 0.0747 0.0411, 0.0787 2.12/ 1.82 |F2c |)2/Rw|F2o|2]1/2.
Table 2 Selected bond distances and angles for complex 2. Bond distance (Å) in 2 Au(1)-Se(1) Au(1)-Cl(1) Au(2)-Cl(3) Se(1)-C(3)
2.4391(6) 2.3496(15) 2.2491(3) 1.8882(6)
Au(1)-Se(2) Au(1)-Cl(2) Au(2)-Cl(4) Se(2)-C(13)
2.4427(6) 2.3279(17) 2.2540(2) 1.8862(5)
Bond angle (deg) in 2 Se(1)-Au(1)-Cl(1) Se(2)-Au(1)-Cl(1) Cl(1)-Au(1)-Cl(2) Cl(3)-Au(2)-Cl(4)
176.24(4) 90.94(5) 90.97(6) 177.22(9)
Se(1)-Au(1)-Cl(2) Se(2)-Au(1)-Cl(2) Se(1)-Au(1)-Se(2)
91.21(5) 175.56(5) 87.10(2)
air and moisture in the solid state, and were soluble in common organic solvents such as CH2Cl2, CHCl3 and THF. All compounds were characterized by elemental analysis, NMR IR spectra as well as electrospray ionization mass spectroscopic (ESI-MS). The 1H NMR spectrum of Bmsp show signals at d 4.52, 6.92, 7.54, 8.05 and 8.90 ppm, which can be assigned to the methyl, imidazole, and pyridyl groups, respectively. And the 13C NMR spectra show singlet at about d 156.72 ppm for C = Se group in Bmsp, which also prove the formation of the selone compound. The mononuclear mixed-valent gold complexes [Au(L)Cl2] [AuCl2] (L = Bmsp (1); L = Besp (2) and L = Bpsp (3)) were obtained by reacting HAuCl4 with three pyridine-based selone ligands in MeOH at room temperature respectively (Scheme 2). All gold complexes were confirmed by IR, NMR spectroscopy as well as elemental analyses. The gold complexes are air and moisture stable in the solid state, soluble in MeOH, MeCN, DMSO solvents but slightly soluble in chlorohydrocarbon. The 13C NMR chemical shift values for the C = Se group were comparable (d 148.06 ppm (1), 148.08 ppm (2) and 148.08 ppm (3), respectively) in comparison with free selone compounds. Positive-ion electrospray ionization mass spectroscopic (ESI-MS) studies further supported the formation of gold complex. For example, the ESI-MS spectrum for [Au(Bmsp)Cl2][AuCl2] showed peaks at m/z 595.9160, corresponding to [Au(Bmsp)]3+ (calcd for [Au(Bmsp)]3+ 595.9167). 3.1. Structural description of [Au(Besp)Cl2][AuCl2] (2) Crystal suitable for X-ray crystallography of 2 was obtained by slow diffusion of hexane into concentrated solution of the complex in acetonitrile solution. The molecular structure of 2 is solved in the monoclinic crystal system and P2(1)/c space group. As can be seen in Fig. 1, the coordination geometry of the central gold center can be best described as a nearly square-planar geometry with two Cl atoms and two Se atoms (Se(1)-Au(1)-Se(2) 87.10(2)°, Se(1)-Au (1)-Cl(1) 176.24(4)°, Se(1)-Au(1)-Cl(2) 91.21(5)°, Se(2)-Au(1)-Cl(1) 90.94(5)°, Se(2)-Au(1)-Cl(2) 175.56(5)° and Cl(1)-Au(1)-Cl(2) 90.97 (6)°). Both selenium atoms and two chlorine atoms of Bptp ligand adopt a cis disposition. Au-Clav average bond distance (2.338 Å) is within the range normally found for Au-Cl bonds in Au(III) complexes [64,65]. Bond distances around the gold ion with selenium
Scheme 1. Synthesis of pyridine-based selone ligands.
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H.-N. Zhang et al. / Inorganica Chimica Acta 450 (2016) 315–320 Table 3 Screening of gold catalysts for 4-nitroanisole reduction.a,b
NO2
NH2
Gold catalysts 2 mol% NaBH4, Water, RT OMe
Scheme 2. Synthesis of gold complexes (1–3) with pyridine-based selone ligands.
Fig. 1. Crystal structure of [(Besp)AuCl2][AuCl2], hydrogen atoms are omitted for clarity.
(Au-Seav = 2.441 Å) are comparable to the distances observed in the gold complexes containing selone ligands [66–68]. The Au-Se distances (2.4391(6) and 2.4427(6) Å) in complex 2, which are longer than those of in Au(I) complex [AuCl(SeUr)] (Ur = NHC-derived selenoureas ligands) (2.353–2.398 Å) [67] and [Au2{SeC6H4(CH2NMe2)-2}2(l-dppf)] (2.4245(5) and 2.4250(8) Å) [68]. As shown in the Fig. 2, there are two forms of gold in 2 unit cell. One Au is coordinated by two Se atoms of ligand and two Cl atoms, the other Au is coordinated by two Cl atoms which as coordination anion. The [AuCl2] anions, arranged in a spiral, bridge two [Au (Besp)Cl2]+ cations through long axial contacts. For [Au(Besp) Cl2]+[AuCl2] (2), cations (+) and anions ( ) are arranged alternatively in the solid state. This alternate arrangement of the oppositely charge Au(III)/Au(I) ions is favorable because it allows the electrostatic interaction to be optimized in the solid state. Moreover, there is no Au(I) Au(I) contract between adjacent [AuCl2] units as both the Au(I) Au(I) distance (7.6478(4) Å) is longer than 4 Å [69]. There are two kinds of weak intermolecular CAH Cl hydrogen bonds between chlorine anion and carbon of ligand. The coordination anion chloride form two CAH Cl hydrogen bonds with adjacent molecules [C(7) Cl(4) = 3.677(2) Å; C(7)AH
OMe
Entry
Catalyst
Solvent
Time (h)
Yield (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
HAuCl4 [(bmtp)AuCl2]Cl [(betp)AuCl2]Cl [(bptp)AuCl2]Cl [(bmsp)AuCl2]AuCl2 (1) [(besp)AuCl2]AuCl2 (2) [(bpsp)AuCl2]AuCl2 (3) [(bpsp)AuCl2]AuCl2 (3) [(bpsp)AuCl2]AuCl2 (3) [(bpsp)AuCl2]AuCl2 (3) [(bpsp)AuCl2]AuCl2 (3) [(bpsp)AuCl2]AuCl2 (3) [(bpsp)AuCl2]AuCl2 (3) [(bpsp)AuCl2]AuCl2 (3) [(bpsp)AuCl2]AuCl2 (3) [(bpsp)AuCl2]AuCl2 (3) None
H2O H2O H2O H2O H2O H2O H2O EtOH DCM DMSO DMF MeCN H2Oc H2Od H2Oe H2Of H2Og
3 3 3 3 3 3 3 6 6 6 6 6 2.5 1 15 48 6
12 50 53 65 83 85 95 95 20 15 10 10 95 99 90 87 No reaction
a Reaction conditions: 0.5 mmol nitrobenzene, 2.0 mmol sodium tetrahydroborate, gold catalysts (2 mol%), water (2 mL), room temperature. b Isolated yield. c 30 °C. d 80 °C. e Complex 3, 1 mol%. f Complex 3, 0.5 mol%. g No catalysts.
(7) Cl(4) = 154° and C(11) Cl(4) = 3.5417(1) Å; C(11)AH(11) Cl(4) = 134°], which play crucial roles in the construction of 1D network structure in the solid states. 3.2. Catalytic reduction of nitroarenes The reactivity of the gold complexes for the reduction of nitroarenes was first studied using 4-nitroanisole as model substrate. As shown in Table 3, the gold catalysts with selone ligands are highly active in the 4-nitroanisole reduction, with the reaction catalyzed by complex 3 reaching the yields of 95% within 3 h, whereas for the reaction catalyzed by complex 1 and 2 the same yield were achieved after 4 h of reaction time, respectively. Moreover, the catalytic activities using gold complexes with selone ligands are better than those of gold complexes with thione ligands [38] and HAuCl4. Then, the complex 3 was chosen as the optimal catalyst to screen the various solvents (Table 3, entries 8–12). High yield of 95% of the desired products was achieved within 3 h in
Fig. 2. View of 1D polymeric chains in complex 2.
H.-N. Zhang et al. / Inorganica Chimica Acta 450 (2016) 315–320 Table 4 Screening of substrates for nitroarenes reduction catalyzed by gold complexes 1, 2 and 3.a,b Entry
Substrate
Product
Catalyst
Time (h)
Yield (%)
1
O2N
H2N
2
O2N
H2N
1 2 3 1 2 3
20 17 15 18 11 10
92 90 89 90 92 90
3
O2N
1 2 3
18 12 10
91 90 90
1 2 3
22 20 17
92 92 90
1 2 3
15 12 10
90 87 90
1 2 3
15 11 9
90 92 89
Cl
Cl
H2N I
I 4
O2N
5
O2N
H2N CN
CN H2N
OH
OH
6
319
reduced with NaBH4 to form gold particles (Fig. S6). Then the reduction action occurred via electrons transfer from the donor BH4 to the acceptor nitroarene after both adsorbed onto the gold particles surface. The hydrogen atom from the hydride attacked nitroarene to reduce it to aromatic anilines after the electron transfer to gold particles. There are still some gold complexes in solution (confirmed by ESI-MS), which centrifugal separation after the catalytic reaction. The nitroarene reduction procedure developed to access gold complex and particle to containing homogeneous and heterogeneous systems as previous reported [70]. 4. Conclusions In summary, we have synthesized and characterized three novel gold complexes with pyridine-based selone ligands. A combination of spectroscopic studies and X-ray crystallographic study confirmed the molecular structure of all gold complexes. And these gold complexes are highly catalyzed hydrogenation of nitroarenes to aromatic anilines to proceed in the presence of sodium borohydride reducing agent in water solvent. The reaction offers wide functional group compatibility, broad substrate scope and proides aromatic anilines in good to excellent yields.
O2N
H2N
O2N
H2N
1 2 3
13 11 7
93 91 95
8
O2N
H2N
1 2 3
12 11 9
87 89 90
9
O2N
H2N
1 2 3
6 6 5
93 95 95
This work was supported by the National Nature Science Foundation of China (21102004), the NSF of Anhui province (1408085MB32) and the Project-sponsored by SRF for ROCS and National Training Programs of Innovation and Entrepreneurship for Undergraduates (201410370040 and 201510370011).
10
O2N
1 2 3
4 4 3
89 90 91
Appendix A. Supplementary data
1 2 3
30 24 20
90 87 89
1 2 3
12 10 9
91 90 93
7
OMe
OMe H2N
CH3
CH3
O
11
OH
H2N
O2N
OH
CHO
12
O2N
Acknowledgments
NH2
H2N
NH2
Supplementary data (CCDC number 1470554 for compound 2) associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2016.06.023. References
a
Reaction conditions: 0.5 mmol nitrobenzene, 2.0 mmol sodium tetrahydroborate, gold catalyst (2 mol%), water (2 mL), room temperature. b Isolated yield.
water solvent. Lower catalyst loading (1 mol% and 0.5 mol% 3) attributed to longer reaction time (Table 3, entry 15–16). A control experiment without the gold complex catalyst showed no reaction (Table 3, entry 17). With the optimal reaction conditions in hand, we started to expand the scope and efficiency of this methodology. A series of functionalized anilines were obtained in good to excellent yields using three gold complexes with selone ligands as catalysts, respectively (Table 4). As shown in Table 4, all gold catalyst were found to be very active toward the reduction of various nitroarenes in different reaction time. Electron-withdrawing and electrondonating substituents on the aromatic backbone were able to give the desired products in high yields. In the case of 4-nitroacetophenone and 4-nitrobenzaldehyde, the –CHO and –COMe groups were reduced together with the nitro group. It was interest to get the mechanism of the nitroarene reduction using gold complexes catalysts. The gold complex peak (263 nm) became weak and the new peak (317 nm) increase after 30 min in UV–vis spectra (Fig. S5). When finished the reduction, some black precipitate were detected. Thus we have carried out the reaction of gold complex with NaBH4, which led to the formation of black precipitate. We deduce that the gold complexes have been
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Research paper
Synthesis, characterization and catalytic activity of gold complexes with pyridine-based selone ligands Hai-Ning Zhang, Wei-Guo Jia ⇑, Qiu-Tong Xu, Chang-Chun Ji College of Chemistry and Materials Science, Center for Nano Science and Technology, The Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecular-Based Materials, Anhui Normal University, Wuhu 241000, China
a r t i c l e
i n f o
Article history: Received 12 April 2016 Received in revised form 26 May 2016 Accepted 10 June 2016 Available online 11 June 2016 Keywords: Selone Gold Complex Catalyst Reduction
a b s t r a c t Three neutral pyridine-based selone compounds, 2,6-bis(1-methylimidazole-2-selone)pyridine (Bmsp), 2,6-bis(1-ethylimidazole-2-selone)pyridine (Besp) and 2,6-bis(1-isopropylimidazole- 2-selone)pyridine (Bpsp) have been synthesized and characterized. Reactions of HAuCl4 with pyridine-based selone ligands result in the formation of the complexes [Au(L)Cl2]+[AuCl2] (L = Bmsp (1); L = Besp (2) and L = Bpsp (3)), respectively. All compounds have been characterized by elemental analysis, NMR IR spectra and electrospray ionization mass spectroscopic (ESI-MS). The molecular structure of 2 has been determined by X-ray crystallography. Moreover, the gold complexes are efficiently catalyzed nitroarenes reduction to aromatic amines in the presence of sodium tetrahydroborate reducing agent in water. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Functionalized anilines are important structural motifs that are found in a variety of agrochemicals, dyes, pharmaceuticals, and pigments and other industrially useful products [1–6]. Therefore, a variety of protocols for synthesizing functionalized anilines have been developed [7–19]. The desirable method for the synthesis of functionalized anilines is the reduction of nitroarenes using transition metal catalysts in the presence of sodium tetrahydroborate (NaBH4) [20]. A number of homogeneous and heterogeneous catalysts have also been reported as viable catalysts for this hydrogenation. Heterogeneous gold catalysts have become an important topic since the 1980s [21], since 2000 homogeneous gold catalysis has become a highly active field [22,23]. Furthermore, occasionally reactions typically catalyzed by homogeneous catalysts have also been reported to be catalyzed by heterogeneous catalysts [24], and occasionally reaction typically catalyzed by heterogeneous catalysts have also been reported to be catalyzed by homogeneous gold(III) complexes [25]. Gold-based catalysts supported by metal oxides (such as TiO2, Fe2O3, Fe3O4, SiO2 and Al2O3) [26–32], carbon nanotube [33], polymer [34,35], graphene oxide [36], and gold complexes [37–40] are all good candidate for the hydrogenation of nitroarenes due to their high catalytic activities.
⇑ Corresponding author. E-mail address: [email protected] (W.-G. Jia). http://dx.doi.org/10.1016/j.ica.2016.06.023 0020-1693/Ó 2016 Elsevier B.V. All rights reserved.
Imidazoline-2-chalcogenone ligands NHC = E (NHC = N-heterocyclic carbene, E = S, Se) are good r-donors and weak p-acceptors which can be easily modified using appropriate available NHC precursors to attain the desired steric-bulk or electronic properties [41–43]. Reports about transition metal complexes with imidazoline-2-chalcogenone ligands have been reported within past few years [44–49]. And the catalytic activities of these complexes are comparable with the most efficient metal-NHC complexes for organic transformation [50–52]. However, the transition metal complexes with selone ligands are reported rarely due to difficulty in synthesis, handing and malodorous nature of the organoselenium ligands [46]. Continuing our research interests in imidazoline-2-chalcogenone metal complexes for their rich chemistry [53–56], we try to prepare the gold complexes with pyridine-based [57,58] selone ligands and hope to exploit their catalytic applications in hydrogenation of nitroarenes. Herein, we report the synthesis and characterization of three gold complexes with pyridine-based selone compounds: [Au(L)Cl2]+[AuCl2] (L = Bmsp (1), Bmsp = 2,6-bis(1-methylimidazole-2-selone)pyridine; L = Besp (2), Besp = 2,6-bis(1-ethylimidazole-2-selone)pyridine and L = Bpsp (3), Bpsp = 2,6-bis (1-isopropylimidazole-2-selone)pyridine), and further examine their catalytic ability in direct nitroarenes reduction to aromatic amines in the presence of NaBH4 reducing agent in water. Our preliminary results indicate that gold complexes show promising catalytic activities in nitroarenes reduction reaction. The elucidation of solid state structure of the gold complex 2 was also obtained through single crystal X-ray diffraction.
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2. Experimental 2.1. Materials and physical measurements Commercial reagents were analytical grade and used as received from Aladdin and Energy chemical. All manipulations were carried out under nitrogen using standard schlenk and vacuum-line techniques. All solvents were purified and degassed by standard procedures. The starting materials 2,6-bis(1-methylimidazolium) pyridine dibromide, 2,6-bis(1-ethylimidazolium)pyridine dibromide and 2,6-bis(1-isopropylimidazolium)pyridine dibromide were synthesized according to the procedures described in the literature [55]. All catalytic reactions were monitored by TLC using 0.25 mm silica gel plates with UV indicator (60F-254). 1H and 13C NMR were recorded on a 300 MHz or 500 MHz NMR spectrometer at room temperature. IR spectra were recorded on a Niclolet AVATAR-360IR spectrometer. Element analyses were performed on an Elementar III vario EI Analyzer. Mass spectra were obtained with MicroTof (Bruker Daltonics, Bremen, Germany) spectrometers. 2.2. Synthesis of Bmsp In a 25 mL Schlenk tube were placed with 2,6-bis(1-methylimidazolium)pyridine dibromide (201.5 mg, 0.5 mmol), Se (94.8 mg, 1.2 mmol), K2CO3 (165.9 mg, 1.2 mmol) and 6.0 mL methanol as solvent. The mixture was allowed to reflux for 12 h after which the methanol was removed with a rotary evaporator. The remaining solid was shaken with 3 10 mL CH2Cl2 which was then filtered and rotary evaporated. The product was recrystallized from CH2Cl2/MeOH to give white solid. Yield: (258 mg 65%) (based on 2,6-bis(1-methylimidazolium)pyridine dibromide). Anal. Calcd. for C13H13N5Se2 (397.20): C, 39.10; H, 3.28; N, 17.77. Found: C, 39.15; H, 3.30; N, 17.58. 1H NMR (300 MHz, CDCl3): d 8.90 (d, J = 8 Hz, pyridine, 2H), 8.05 (t, pyridine, 1H), 7.54 (d, J = 3 Hz, imidazole, 2H), 6.92 (d, J = 3 Hz, imidazole, 2H), 4.52 (s, 2CH3, 6H). 13C NMR (500 MHz, CDCl3): d 156.72, 149.40, 140.15, 120.95, 119.18, 118.97, 37.86 ppm. IR (KBr, cm 1): 3358 (w), 3281 (w), 3188 (w), 3165 (w), 3130 (w), 3099 (w), 1666 (w), 1624 (vs), 1571 (m), 1471 (vs), 1448 (s), 1395 (m), 1369 (m), 1290 (m), 1236 (m), 1121 (w), 1074 (w), 985 (w), 874 (w), 797 (m), 671 (w), 555 (w). 2.3. Synthesis of Besp The synthesis procedure was similar to the ligand Bmsp to afford the white solid Besp, using 2,6-bis(1-ethylimidazolium)pyridine dibromide (217 mg, 0.5 mmol), Se (94.8 mg, 1.2 mmol), K2CO3 (165.9 mg, 1.2 mmol). Yield: (213 mg 50%) (based on 2,6bis(1-ethylimidazolium)pyridine dibromide). Anal. Calcd. for C15H17N5Se2 (426.98): C, 42.16; H, 4.01; N, 16.40. Found: C, 42.20; H, 4.05; N, 16.36. 1H NMR (300 MHz, CDCl3): d 8.86 (d, J = 8 Hz, pyridine, 2H), 8.12 (t, pyridine, 1H), 7.57 (d, J = 3 Hz, imidazole, 2H), 6.98 (d, J = 3 Hz, imidazole, 2H), 4.26 (m, 2CH2 4H), 1.46 (t, 2CH3, 6H). 13C NMR (500 MHz, CDCl3): d 155.73, 149.39, 139.91, 119.60, 119.35, 119.15, 45.34, 14.60 ppm. IR (KBr, cm 1): 3177 (w), 3140 (w), 3097 (w), 2965 (w), 2864 (w), 1647 (w), 1605 (s), 1576 (m), 1533 (w), 1464 (s), 1408 (s), 1392 (s), 1354 (w), 1303 (m), 1281 (s), 1258 (s), 1223 (s), 1155 (m), 1134 (m), 1115 (m), 1076 (w), 1040 (w), 995 (w), 985 (w), 966 (w), 949 (m), 903 (w), 802 (s), 800 (s), 785 (m), 773 (m), 735 (w),716 (m), 688 (w), 655 (m), 599 (w), 509 (w). 2.4. Synthesis of Bpsp The synthesis procedure was similar to the ligand Bmsp to afford the white solid Bpsp, using 2,6-bis(isopropyllimidazolium)
pyridine dibromide (220.4 mg, 0.5 mmol), Se (94.8 mg, 1.2 mmol), K2CO3 (165.9 mg, 1.2 mmol). Yield: (245 mg 55%) (based on 2,6-bis (isopropyllimidazolium)pyridine dibromide). Anal. Calcd. for C17H21N5Se2 (455.01): C, 44.83; H, 4.65; N, 15.39. Found: C, 44.88; H, 4.62; N, 15.36. 1H NMR (300 MHz, CDCl3): d 8.82 (d, J = 8 Hz, pyridine, 2H), 8.04 (t, pyridine, 1H), 7.58 (d, J = 3 Hz, imidazole, 2H), 7.02 (d, J = 3 Hz, imidazole, 2H), 5.39 (m, 2CH, 2H), 1.45 (d, J = 6 Hz, 4CH3, 12H). 13C NMR (500 MHz, CDCl3): d 155.30, 149.39, 139.72, 119.96, 119.93, 119.79, 119.7, 51.36, 22.28, 22.25 ppm. IR (KBr, cm 1): 3175 (w), 3132 (w), 3094 (w), 2970 (w), 2866 (w), 1666 (w), 1598 (m), 1572 (m), 1456 (s), 1416 (m), 1400 (m), 1385 (w), 1368 (w), 1337 (m), 1327 (w), 1277 (m), 1225 (s), 1153 (w), 1134 (w), 1113 (m), 1047 (w), 1018 (w), 991 (w), 883 (w), 835 (w), 804 (m), 781 (m), 718 (m), 685 (w), 664 (m), 584 (w), 554 (w), 500 (w). 2.5. Synthesis of complex [Au(Bmsp)Cl2][AuCl2] (1) In a 25 mL Schlenk tube were placed with Bmsp (59.55 mg, 0.15 mmol), HAuCl4 (132 mg, 0.32 mmol), 5 mL methanol as solvent. The mixture was stirred at room temperature overnight and then the solvent was removed with a rotary evaporator; the resulting solid was washed with methanol and diethyl ether, and then dried in vacuo. The product was recrystallized from MeCN/ CH2Cl2 to give brown deep-red powder. Yield: (67.5 mg, 45%) (based on Bmsp ligand). Anal. Calcd. for C13H13Au2Cl4N5Se2 (932.94): C, 16.74; H, 1.40; N, 7.51. Found: C, 16.77; H, 1.16; N, 7.50. 1H NMR (300 MHz, CD3OD): d 8.52 (t, pyridine, 1H), 8.17 (d, J = 2 Hz, pyridine, 2H), 7.95 (d, J = 6 Hz, imidazole, 2H), 7.70 (d, J = 2 Hz, imidazole, 2H), 4.25 (s, 6H). 13C NMR (500 MHz, CD3OD): d 148.06, 143.98, 138.89, 125.57, 124.02, 121.55, 38.5. ppm. ESI-MS (positive ions) for [Au(Bmsp)]3+: m/z 595.9160 (calcd for [Au (Bmsp)]3+ 595.9167). IR (KBr, cm 1): 3421(w), 2943(w), 1816(w), 1672(m), 1627(s), 1562(m), 1508(w), 1476(s), 1440(m), 1408(w), 1379(w), 1300(w), 1238(m), 1138(w), 986(w), 878(w), 802(w), 734(w), 551(w), 501(w). 2.6. Synthesis of complex [Au(Besp)Cl2][AuCl2] (2) Prepared by the same procedure as described above for 1, using Besp (49.65 mg, 0.15 mmol) and HAuCl4 (132 mg, 0.32 mmol). Yield: (77.3 mg, 50%) (based on Besp ligand). Anal. Calc. for C15H17Au2Cl4N5Se2 (961.00): C, 18.75; H, 1.78; N, 7.29. Found: C, 18.72; H, 1.71; N, 7.22. 1H NMR (300 MHz, CD3OD) d 8.51 (t, pyridine,1H), 8.20 (d, J = 2 Hz, imidazole, 2H), 8.08 (d, J = 2 Hz, imidazole,2H), 7.97 (d, J = 2 Hz, pyridine, 2H), 4.70–4.72 (m, 4H, CH2), 1.60 (t, 2CH3, 6H). 13C NMR (500 MHz, CD3OD) d 148.08, 146.59, 143.99, 124.64, 123.90, 122.09, 47.19, 13.95. ESI-MS (positive ions) for [Au(Besp)]3+: m/z 623.9461 (calcd for [Au(Besp)]3+ 623.9480). IR (KBr, cm 1): 3395 (w), 3138 (w), 3070 (w), 2976 (w), 2929 (w), 2868 (w), 1732 (w), 1627 (w), 1597 (w), 1554 (w), 1458 (s), 1431 (m), 1398 (w), 1352 (w), 1301 (w), 1267 (m), 1221 (m), 1134 (w), 1088 (w), 1043 (w), 995 (w), 954 (w), 814 (w), 775 (w), 735 (w), 679 (w), 544 (w), 501 (w). 2.7. Synthesis of complex [Au(Bpsp)Cl2][AuCl2] (3) Prepared by the same procedure as described above for 1, using Bpsp (68.0 mg, 0.15 mmol) and HAuCl4 (132 mg, 0.32 mmol). Yield: (87.3 mg, 55%) (based on Bpsp ligand). Anal. Calc. for C17H21Au2Cl4N5Se2 (989.05): C, 20.64; H, 2.14; N, 7.08. Found: C, 20.70; H, 2.18; N, 7.15. 1H NMR (300 MHz, CD3OD) d 8.53 (t, pyridine, 1H), 8.25 (d, J = 2 Hz, imidazole, 2H), 8.12 (d, J = 2 Hz, imidazole, 2H), 7.99 (d, pyridine, 2H), 5.48 (m, CH, 2H), 1.57 (d, CH3, 6H), 1.28 (d, CH3, 6H). 13C NMR (500 MHz, CD3OD) d 148.08, 144.47, 137.10, 124.93, 121.99, 121.42, 54.40, 21.64, 21.00. ESI-MS
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(positive ions) for [Au(Bpsp)]3+: m/z 651.9778 (calcd for [Au (Bpsp)]3+ 651.9793). IR (KBr, cm 1): 3180 (w), 3134 (w), 3096 (w), 2976 (w), 2875 (w), 1672 (w), 1598 (s), 1573 (m), 1456 (s), 1416 (m), 1400 (m), 1385 (w), 1369 (w), 1337 (m), 1329 (w), 1311 (w), 1277 (m), 1224 (s), 1155 (w), 1136 (w), 1114 (m), 1069 (w), 1047 (w), 1022 (w), 991 (w), 889 (w), 843 (w), 804 (m), 787 (m), 718 (m), 685 (w), 664 (m), 590 (w), 559 (w), 503 (w). 2.8. General procedure for the reduction of nitroarenes to anilines using gold catalysts Gold complex (0.01 mmol, 0.02 equiv) was dissolved in water (2.0 mL), then appropriate nitroarenes (0.5 mmol, 1.0 equiv) and NaBH4 (2 mmol, 4.0 equiv) was added. Subsequently, the resulting mixture was stirred at room temperature. After completion of the reaction (monitored by TLC), the crude reaction mixture was extracted with EtOAC (3 2 mL). After solvents were removed in vacuo from combined organic extracts, the crude products loaded directly onto a column of silica gel and purified by column chromatography to give the corresponding products [59–61]. 2.9. X-ray crystallography for 2 Diffraction data of complex 2 was collected on a Bruker Smart CCD diffractometer with graphite-monochromated MoKa radiation (k = 0.71073 Å). All the data were collected at room temperature and the structure was solved by direct methods and subsequently refined on F2 by using full-matrix least-squares techniques (SHELXL) [62], and SADABS absorption corrections [63] applied to the data. The crystallographic data for complex 2 are summarized in Table 1, and selected bond lengths and angles are shown in Table 2. 3. Result and discussions Pyridine-based selone compounds: 2,6-bis(1-methylimidazole2-selone)pyridine (Bmsp), 2,6-bis(1-ethylimidazole-2-selone)pyridine (Besp) and 2,6-bis(1-isopropylimidazole- 2-selone)pyridine (Bpsp) were prepared in one-pot via the reactions of pyridine bridged imidazolium dibromide derivatives with selenium powder in the presence of K2CO3 with moderate yields (50%) (Scheme 1) [55]. All these compounds were thermally stable and inert toward
Table 1 Crystallographic data and structure refinement parameters for complex 2. 2 Empirical formula Formula weight Crystal syst., Space group a (Å) b (Å) c (Å) b (°) Volume (Å3), Z Dc (mg/m3) l (Mo-Ka) (mm 1) F(0 0 0) h range (°) Limiting indices Reflections/unique[R(int)] Completeness to h (°) Data/restraints/parameters Goodness-of-fit on F2 R1, wR2 [I > 2r(I)]a R1, wR2 (all data) Larg. diff. peak/hole (e/Å 3) a
R1 = R||Fo|
|Fc||/R|Fo|; wR2 = [Rw(|F2o|
C15H17Au2Cl4N5Se2 960.99 Monoclinic, P2(1)/c 13.0360(7) 15.1772(8) 11.7753(6) 92.105(1) 2328.2(2), 4 2.742 16.187 1744 1.563–27.499 15, 16, 19, 19, 15, 14 19587/5254 [0.0384] 27.49 (98.2%) 5254/0/253 1.018 0.0312, 0.0747 0.0411, 0.0787 2.12/ 1.82 |F2c |)2/Rw|F2o|2]1/2.
Table 2 Selected bond distances and angles for complex 2. Bond distance (Å) in 2 Au(1)-Se(1) Au(1)-Cl(1) Au(2)-Cl(3) Se(1)-C(3)
2.4391(6) 2.3496(15) 2.2491(3) 1.8882(6)
Au(1)-Se(2) Au(1)-Cl(2) Au(2)-Cl(4) Se(2)-C(13)
2.4427(6) 2.3279(17) 2.2540(2) 1.8862(5)
Bond angle (deg) in 2 Se(1)-Au(1)-Cl(1) Se(2)-Au(1)-Cl(1) Cl(1)-Au(1)-Cl(2) Cl(3)-Au(2)-Cl(4)
176.24(4) 90.94(5) 90.97(6) 177.22(9)
Se(1)-Au(1)-Cl(2) Se(2)-Au(1)-Cl(2) Se(1)-Au(1)-Se(2)
91.21(5) 175.56(5) 87.10(2)
air and moisture in the solid state, and were soluble in common organic solvents such as CH2Cl2, CHCl3 and THF. All compounds were characterized by elemental analysis, NMR IR spectra as well as electrospray ionization mass spectroscopic (ESI-MS). The 1H NMR spectrum of Bmsp show signals at d 4.52, 6.92, 7.54, 8.05 and 8.90 ppm, which can be assigned to the methyl, imidazole, and pyridyl groups, respectively. And the 13C NMR spectra show singlet at about d 156.72 ppm for C = Se group in Bmsp, which also prove the formation of the selone compound. The mononuclear mixed-valent gold complexes [Au(L)Cl2] [AuCl2] (L = Bmsp (1); L = Besp (2) and L = Bpsp (3)) were obtained by reacting HAuCl4 with three pyridine-based selone ligands in MeOH at room temperature respectively (Scheme 2). All gold complexes were confirmed by IR, NMR spectroscopy as well as elemental analyses. The gold complexes are air and moisture stable in the solid state, soluble in MeOH, MeCN, DMSO solvents but slightly soluble in chlorohydrocarbon. The 13C NMR chemical shift values for the C = Se group were comparable (d 148.06 ppm (1), 148.08 ppm (2) and 148.08 ppm (3), respectively) in comparison with free selone compounds. Positive-ion electrospray ionization mass spectroscopic (ESI-MS) studies further supported the formation of gold complex. For example, the ESI-MS spectrum for [Au(Bmsp)Cl2][AuCl2] showed peaks at m/z 595.9160, corresponding to [Au(Bmsp)]3+ (calcd for [Au(Bmsp)]3+ 595.9167). 3.1. Structural description of [Au(Besp)Cl2][AuCl2] (2) Crystal suitable for X-ray crystallography of 2 was obtained by slow diffusion of hexane into concentrated solution of the complex in acetonitrile solution. The molecular structure of 2 is solved in the monoclinic crystal system and P2(1)/c space group. As can be seen in Fig. 1, the coordination geometry of the central gold center can be best described as a nearly square-planar geometry with two Cl atoms and two Se atoms (Se(1)-Au(1)-Se(2) 87.10(2)°, Se(1)-Au (1)-Cl(1) 176.24(4)°, Se(1)-Au(1)-Cl(2) 91.21(5)°, Se(2)-Au(1)-Cl(1) 90.94(5)°, Se(2)-Au(1)-Cl(2) 175.56(5)° and Cl(1)-Au(1)-Cl(2) 90.97 (6)°). Both selenium atoms and two chlorine atoms of Bptp ligand adopt a cis disposition. Au-Clav average bond distance (2.338 Å) is within the range normally found for Au-Cl bonds in Au(III) complexes [64,65]. Bond distances around the gold ion with selenium
Scheme 1. Synthesis of pyridine-based selone ligands.
318
H.-N. Zhang et al. / Inorganica Chimica Acta 450 (2016) 315–320 Table 3 Screening of gold catalysts for 4-nitroanisole reduction.a,b
NO2
NH2
Gold catalysts 2 mol% NaBH4, Water, RT OMe
Scheme 2. Synthesis of gold complexes (1–3) with pyridine-based selone ligands.
Fig. 1. Crystal structure of [(Besp)AuCl2][AuCl2], hydrogen atoms are omitted for clarity.
(Au-Seav = 2.441 Å) are comparable to the distances observed in the gold complexes containing selone ligands [66–68]. The Au-Se distances (2.4391(6) and 2.4427(6) Å) in complex 2, which are longer than those of in Au(I) complex [AuCl(SeUr)] (Ur = NHC-derived selenoureas ligands) (2.353–2.398 Å) [67] and [Au2{SeC6H4(CH2NMe2)-2}2(l-dppf)] (2.4245(5) and 2.4250(8) Å) [68]. As shown in the Fig. 2, there are two forms of gold in 2 unit cell. One Au is coordinated by two Se atoms of ligand and two Cl atoms, the other Au is coordinated by two Cl atoms which as coordination anion. The [AuCl2] anions, arranged in a spiral, bridge two [Au (Besp)Cl2]+ cations through long axial contacts. For [Au(Besp) Cl2]+[AuCl2] (2), cations (+) and anions ( ) are arranged alternatively in the solid state. This alternate arrangement of the oppositely charge Au(III)/Au(I) ions is favorable because it allows the electrostatic interaction to be optimized in the solid state. Moreover, there is no Au(I) Au(I) contract between adjacent [AuCl2] units as both the Au(I) Au(I) distance (7.6478(4) Å) is longer than 4 Å [69]. There are two kinds of weak intermolecular CAH Cl hydrogen bonds between chlorine anion and carbon of ligand. The coordination anion chloride form two CAH Cl hydrogen bonds with adjacent molecules [C(7) Cl(4) = 3.677(2) Å; C(7)AH
OMe
Entry
Catalyst
Solvent
Time (h)
Yield (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
HAuCl4 [(bmtp)AuCl2]Cl [(betp)AuCl2]Cl [(bptp)AuCl2]Cl [(bmsp)AuCl2]AuCl2 (1) [(besp)AuCl2]AuCl2 (2) [(bpsp)AuCl2]AuCl2 (3) [(bpsp)AuCl2]AuCl2 (3) [(bpsp)AuCl2]AuCl2 (3) [(bpsp)AuCl2]AuCl2 (3) [(bpsp)AuCl2]AuCl2 (3) [(bpsp)AuCl2]AuCl2 (3) [(bpsp)AuCl2]AuCl2 (3) [(bpsp)AuCl2]AuCl2 (3) [(bpsp)AuCl2]AuCl2 (3) [(bpsp)AuCl2]AuCl2 (3) None
H2O H2O H2O H2O H2O H2O H2O EtOH DCM DMSO DMF MeCN H2Oc H2Od H2Oe H2Of H2Og
3 3 3 3 3 3 3 6 6 6 6 6 2.5 1 15 48 6
12 50 53 65 83 85 95 95 20 15 10 10 95 99 90 87 No reaction
a Reaction conditions: 0.5 mmol nitrobenzene, 2.0 mmol sodium tetrahydroborate, gold catalysts (2 mol%), water (2 mL), room temperature. b Isolated yield. c 30 °C. d 80 °C. e Complex 3, 1 mol%. f Complex 3, 0.5 mol%. g No catalysts.
(7) Cl(4) = 154° and C(11) Cl(4) = 3.5417(1) Å; C(11)AH(11) Cl(4) = 134°], which play crucial roles in the construction of 1D network structure in the solid states. 3.2. Catalytic reduction of nitroarenes The reactivity of the gold complexes for the reduction of nitroarenes was first studied using 4-nitroanisole as model substrate. As shown in Table 3, the gold catalysts with selone ligands are highly active in the 4-nitroanisole reduction, with the reaction catalyzed by complex 3 reaching the yields of 95% within 3 h, whereas for the reaction catalyzed by complex 1 and 2 the same yield were achieved after 4 h of reaction time, respectively. Moreover, the catalytic activities using gold complexes with selone ligands are better than those of gold complexes with thione ligands [38] and HAuCl4. Then, the complex 3 was chosen as the optimal catalyst to screen the various solvents (Table 3, entries 8–12). High yield of 95% of the desired products was achieved within 3 h in
Fig. 2. View of 1D polymeric chains in complex 2.
H.-N. Zhang et al. / Inorganica Chimica Acta 450 (2016) 315–320 Table 4 Screening of substrates for nitroarenes reduction catalyzed by gold complexes 1, 2 and 3.a,b Entry
Substrate
Product
Catalyst
Time (h)
Yield (%)
1
O2N
H2N
2
O2N
H2N
1 2 3 1 2 3
20 17 15 18 11 10
92 90 89 90 92 90
3
O2N
1 2 3
18 12 10
91 90 90
1 2 3
22 20 17
92 92 90
1 2 3
15 12 10
90 87 90
1 2 3
15 11 9
90 92 89
Cl
Cl
H2N I
I 4
O2N
5
O2N
H2N CN
CN H2N
OH
OH
6
319
reduced with NaBH4 to form gold particles (Fig. S6). Then the reduction action occurred via electrons transfer from the donor BH4 to the acceptor nitroarene after both adsorbed onto the gold particles surface. The hydrogen atom from the hydride attacked nitroarene to reduce it to aromatic anilines after the electron transfer to gold particles. There are still some gold complexes in solution (confirmed by ESI-MS), which centrifugal separation after the catalytic reaction. The nitroarene reduction procedure developed to access gold complex and particle to containing homogeneous and heterogeneous systems as previous reported [70]. 4. Conclusions In summary, we have synthesized and characterized three novel gold complexes with pyridine-based selone ligands. A combination of spectroscopic studies and X-ray crystallographic study confirmed the molecular structure of all gold complexes. And these gold complexes are highly catalyzed hydrogenation of nitroarenes to aromatic anilines to proceed in the presence of sodium borohydride reducing agent in water solvent. The reaction offers wide functional group compatibility, broad substrate scope and proides aromatic anilines in good to excellent yields.
O2N
H2N
O2N
H2N
1 2 3
13 11 7
93 91 95
8
O2N
H2N
1 2 3
12 11 9
87 89 90
9
O2N
H2N
1 2 3
6 6 5
93 95 95
This work was supported by the National Nature Science Foundation of China (21102004), the NSF of Anhui province (1408085MB32) and the Project-sponsored by SRF for ROCS and National Training Programs of Innovation and Entrepreneurship for Undergraduates (201410370040 and 201510370011).
10
O2N
1 2 3
4 4 3
89 90 91
Appendix A. Supplementary data
1 2 3
30 24 20
90 87 89
1 2 3
12 10 9
91 90 93
7
OMe
OMe H2N
CH3
CH3
O
11
OH
H2N
O2N
OH
CHO
12
O2N
Acknowledgments
NH2
H2N
NH2
Supplementary data (CCDC number 1470554 for compound 2) associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2016.06.023. References
a
Reaction conditions: 0.5 mmol nitrobenzene, 2.0 mmol sodium tetrahydroborate, gold catalyst (2 mol%), water (2 mL), room temperature. b Isolated yield.
water solvent. Lower catalyst loading (1 mol% and 0.5 mol% 3) attributed to longer reaction time (Table 3, entry 15–16). A control experiment without the gold complex catalyst showed no reaction (Table 3, entry 17). With the optimal reaction conditions in hand, we started to expand the scope and efficiency of this methodology. A series of functionalized anilines were obtained in good to excellent yields using three gold complexes with selone ligands as catalysts, respectively (Table 4). As shown in Table 4, all gold catalyst were found to be very active toward the reduction of various nitroarenes in different reaction time. Electron-withdrawing and electrondonating substituents on the aromatic backbone were able to give the desired products in high yields. In the case of 4-nitroacetophenone and 4-nitrobenzaldehyde, the –CHO and –COMe groups were reduced together with the nitro group. It was interest to get the mechanism of the nitroarene reduction using gold complexes catalysts. The gold complex peak (263 nm) became weak and the new peak (317 nm) increase after 30 min in UV–vis spectra (Fig. S5). When finished the reduction, some black precipitate were detected. Thus we have carried out the reaction of gold complex with NaBH4, which led to the formation of black precipitate. We deduce that the gold complexes have been
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