Efficient Catalysis of Transfer Hydrogenation of Ketones and Oxidation

Article pubs.acs.org/Organometallics

Efficient Catalysis of Transfer Hydrogenation of Ketones and Oxidation of Alcohols with Newly Designed Half-Sandwich Rhodium(III) and Iridium(III) Complexes of Half-Pincer Chalcogenated Pyridines Om Prakash, Pradhumn Singh, G. Mukherjee, and Ajai K Singh* Department of Chemistry, Indian Institute of Technology Delhi, New Delhi 110016, India S Supporting Information *

ABSTRACT: The in situ generated ArE¯ (E = S or Se) reacts with (2chloromethyl)pyridine in N2 atmosphere, resulting in half-pincer (N, S/Se) ligands (2-arylchalcogenomethyl)pyridine (L1−L3; aryl = Ph/2-pyridyl for S, Ph for Se). Half-sandwich complexes [(η5-Cp*)Rh(L)Cl][PF6] (1, 2), [(η5-Cp*)Rh(L2)CH3CN][PF6]2 (3), and [(η5-Cp*)Ir(L)Cl][PF6] (4− 6), where L = L1−L3, have been synthesized by reacting L with [(η5Cp*)RhCl(μ-Cl)]2 and [(η5-Cp*)IrCl(μ-Cl)]2 respectively. The ligands and complexes have been characterized by IR, HR-MS, and 1H, 13C{1H}, and 77Se{1H} NMR spectra. The single-crystal structures of all the complexes (1−6) have been determined with X-ray crystallography [Rh−S, Rh−Se, Ir−S, Ir−Se bond distances: 2.383(2)/2.353(2), 2.4867(9), 2.343(2)/2.341(3), 2.453(1) Å, respectively]. Each metal has pseudo-octahedral half-sandwich “piano-stool” geometry. The PF6− ions in crystals are packed between half-sandwich complex molecules, resulting in various H···F secondary interactions. Complexes 1−6 have been explored for catalysis of oxidation of secondary alcohols with N-methylmorpholine-N-oxide and transfer hydrogenation reaction of ketones with 2propanol and found very efficient, as revealed by TON values, which are up to 9.6 × 103 and 9.8 × 103 respectively for the two catalytic reactions. The formation of the M−H bond has been noticed in the intermediates of both the catalytic reactions. DFT calculations indicate a reactivity order for the complexes of Rh > Ir, which is also found for the two catalytic reactions experimentally. The calculated (DFT) and observed bond distances and angles of 1−6 are reasonably close.

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INTRODUCTION η -Cyclopentadienyl (Cp) and η5-pentamethylcyclopentadienyl (Cp*) groups are efficient ancillary ligands in organometallic complexes. There are reports on the synthesis, structure, and reactivity of a number of Cp or Cp* transition-metal complexes in the literature.1 Rhodium(III) and iridium(III) are two important metal ions that give with Cp/Cp* and other coligands species of interesting reactivity. Although Cp and Cp* ligands stabilize metal centers by tridentate coordination in a facial fashion, it is rather difficult to modify the electronic and steric properties of these ligands. Therefore, other ligands are used to tune properties of transition metals attached to Cp or Cp*. The complexes of the species η5-CpRh(III)/Ir(III) and η5-Cp*Rh(III)/Ir(III) with a variety of coligands have been found to show very promising catalytic activity for various organic transformations. The η5-Cp*Ir(III) complexes bearing N-heterocyclic carbene coligands are effective catalysts for Oppenauer-type oxidation of alcohols.2,3 Multialkylation of aqueous ammonia with alcohols has been catalyzed by watersoluble η5-Cp*Ir-ammine complexes.4 A variety of (N, N) and (N, O) ligands are able to tune the catalytic activity of η5Cp*Rh(III)/Ir(III) for efficient asymmetric transfer hydrogenation,5−11 even in water. There are not many examples of species having η5-CpRh(III)/Ir(III) and η5-Cp*Rh(III)/Ir(III)

units ligated with chalcogen ligands, particularly those containing selenium and tellurium. Half-sandwich iridium(III) and rhodium(III) complexes with organochalcogen (S, Se) ligands based on N-methylimidazole have been synthesized and used as catalysts12 for norbornene polymerization. [(η5Cp*)Ir(CO)(ETol)2] (E = Se, S; Tol = p-tolyl) has been synthesized and reacted with [Pt(PPh3)3] to form tri- or dinuclear mixed-metal complexes with bridging chalcogenido or chalcogenolato ligands.13,14 The coordination of chalcogenfunctionalized carboranes15−18 with η5-Cp*Rh(III)/Ir(III) has been investigated. The synthesis and structural aspects of complexes of chalcogenated phosphines with η5-Cp*Rh(III) have also been investigated.19,20 There is only one report to our knowledge in which selenoethers and telluroethers have been reacted with η5-Cp*Rh(III)/Ir(III).21 Considering the fact that selenoethers have been found recently to be important building blocks for efficient catalysts for C−C coupling22−24 [(Se, C, Se) and (Se, N, Se) pincer and (Se, N, C−) ligands], transfer hydrogenation, and oxidation of alcohols25,26 [(E, N, S) ligand E = S, Se or Te], and species η5-Cp/Cp*Rh(III)/Ir(III) have been used to design catalysts for asymmetric transfer

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© 2012 American Chemical Society

Received: February 29, 2012 Published: April 3, 2012 3379

dx.doi.org/10.1021/om300169p | Organometallics 2012, 31, 3379−3388

Organometallics

Article

optimized geometries to characterize the stationary points as minima. The molecular orbital plots were made using the Chemcraft program package (http://www.chemcraftprog.com). Synthesis of L1. Sodium hydroxide (0.224 g, 6 mmol) dissolved in 5 cm3 of water was added dropwise to thiophenol (0.5 mL, ∼5 mmol) taken in 50 cm3 of ethanol and refluxed for 0.5 h under N2 atmosphere. (2-Chloromethyl)pyridine hydrochloride (0.82 g, 5 mmol) dissolved in 20 cm3 of ethanol was added dropwise, and refluxing of the mixture continued further for 3 h. After cooling to room temperature the mixture was poured into 100 cm3 of distilled water, neutralized with dilute sodium hydroxide, and extracted with 100 cm3 of chloroform. The extract was washed with water (3 × 50 cm3) and dried over anhydrous sodium sulfate. Its solvent was evaporated off under reduced pressure on a rotary evaporator, resulting in pale yellow oil, L1. Yield: 0.95 g, 95%. 1H NMR (CDCl3, 25 °C vs Me4Si) δ (ppm): 4.26 (s, 2H, H5), 7.17−7.60 (m, 8H, H1−3, H7−9), 8.52 (d, 3JHH = 4.5 Hz, 1H, H10). 13C{1H} NMR (CDCl3, 25 °C vs Me4Si) δ (ppm): 40.4 (C5), 122.0 (C7), 122.9 (C9), 126.2 (C3), 128.8 (C2), 129.5 (C1), 135.8 (C4), 136.5 (C8), 149.3 (C10), 157.6 (C6). HR-MS (CH3CN) [M + H]+ (m/z) = 202.0685; calcd value for C12H12NS = 202.0685 (δ 0.00 ppm). IR (KBr, cm−1): 3056 (m; νC−H (aromatic)), 2924 (s; νC−H (aliphatic)), 1585 (m; νCN), 1474 (m; νCC (aromatic)), 748 (m; νC−H (aromatic)). Synthesis of L2 and L3. Diphenyl diselenide/dipyridyl disulfide (2.0 mmol) dissolved in 30 cm3 of ethanol was treated with a solution (made in 5% NaOH) of NaBH4 (0.149 g, 4.0 mmol) (added dropwise) under N2 atmosphere until it become colorless due to the formation of PhSeNa/PySNa. (2-Chloromethyl)pyridine hydrochloride (0.6560 g, 4.0 mmol) dissolved in 5 cm3 of ethanol was mixed with this colorless solution with constant stirring. The mixture was stirred further for 3−4 h and poured into 100 cm3 of ice-cold distilled water and extracted into CHCl3 (5 × 40 cm3). The extract was washed with water (3 × 50 cm3) and dried over anhydrous sodium sulfate. Its solvent was evaporated off under reduced pressure on a rotary evaporator, resulting in a pale yellow oil, L2/L3, respectively. L2: Yield: 0.92 g, 93%. 1H NMR (CDCl3, 25 °C vs Me4Si) δ (ppm): 4.23 (s, 2H, H5), 7.08−7.61 (m, 8H, H1−3, H7−9), 8.51 (d, 3JHH = 5.7 Hz, 1H, H10). 13C{1H} NMR (CDCl3, 25 °C vs Me4Si) δ (ppm): 33.7 (C5), 121 (C7), 122.9 (C9), 127.2 (C1), 128.9 (C2), 129.7 (C4), 133.4 (C3), 136.2 (C8), 149.2 (C10), 158.5 (C6). 77Se{1H} NMR (CDCl3, 25 °C vs Me2Se) δ (ppm): 365.7. HR-MS (CH3CN) [M + Na]+ (m/z) = 271.9949; calcd value for C12H12NSe = 271.9949 (δ 0.0 ppm). IR (KBr, cm−1): 3056 (m; νC−H (aromatic)), 2925 (s; νC−H (aliphatic)), 1585 (m; νCN (aromatic)), 1474 (m; νCC (aromatic)), 739 (m; νC−H (aromatic)). L3: Yield: 0.74 g, 92%. 1H NMR (CDCl3, 25 °C vs Me4Si) δ (ppm): 4.58 (s, 2H, H6), 6.96−7.62 (m, 6H, H2−4, H8−10), 8.43−8.45 (m, 1H, H1), 8.54−8.56 (m, 1H, H11). 13C{1H} NMR (CDCl3, 25 °C vs Me4Si) δ (ppm): 35.8 (C6), 119.5 (C4), 121.9 (C2), 121.9 (C8), 123.1 (C10), 135.9 (C3), 136.5 (C9), 149.1 (C11), 149.2 (C1), 157.4 (C7), 158.1 (C5). HR-MS (CH3CN) [M + H]+ (m/z) = 203.0637; calcd value for C11H11N2S = 203.0640 (δ −1.1 ppm). IR (KBr, cm−1): 3048 (m; νC−H (aromatic), 2924 (s; νC−H (aliphatic)), 1579 (m; νCN), 1475 (m; νCC (aromatic)), 746 (m; νC−H (aromatic)). Synthesis of Complexes 1 and 2. The solid [(η5-Cp*)RhCl(μCl)]2 (0.05 g, 0.1 mmol) and L1 or L3 (0.2 mmol) dissolved in CH3OH (15 cm3) were mixed, and the mixture was stirred for 8 h at room temperature. The resulting orange solution was filtered, and the volume of the filtrate was reduced (∼7 cm3) with a rotary evaporator. It was mixed with solid NH4PF6 (0.032 g, 0.2 mmol), and the resulting orange-colored microcrystalline solid was filtered, washed with 10 cm3 of ice-cold CH3OH, and dried in vacuo. Single crystals of 1 or 2 were obtained by diffusion of diethyl ether into its solution (1 cm3) made in a mixture (1:4) of CH3OH and CH3CN. 1: Yield: 0.076 g, 78%. Anal. Calcd for C22H26ClNRhS·[PF6]: C, 42.63; H, 4.23; N, 2.26. Found: C, 42.59; H, 4.19; N, 2.25. Mp: 206.0 °C. 1H NMR (CD3CN, 25 °C vs Me4Si) δ (ppm): 1.74 (s, 15H, H of Me5Cp), 4.58−4.68 (m, 2H, H5), 7.01−8.08 (m, 8H, H1−3, H7−9), 8.60 (d, 3JHH = 4.8 Hz, 1H, H10). 13C{1H} NMR (CD3CN, 25 °C vs Me4Si) δ (ppm): 8.0 (C of Me5Cp), 45.4 (C5), 99.2 (C of Cp), 123.7 (C7),

hydrogenation, it was thought worthwhile to study the complexes of η5-Cp*Rh(III)/Ir(III) with chalcogenated pyridine (half-pincer) ligands (L1−L3; see Chart 1) for catalyzing Chart 1

transfer hydrogenation of ketones and oxidation of secondary alcohols. The results are promising and given in the present paper. DFT studies have also been made and found to support experimental results.

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EXPERIMENTAL SECTION

Diphenyldiselenide, thiophenol, bis(2-pyridyl)disulfide, NaBH4, (2chloromethyl)pyridine hydrochloride, and ammonium hexafluorophosphate procured from Sigma-Aldrich (USA) were used as received. [(η5-Cp*)RhCl(μ-Cl)]227 and [(η5-Cp*)IrCl(μ-Cl)]228 were prepared according to literature methods. All the solvents were dried and distilled before use by standard procedures.29 Common reagents and chemicals available locally were used. The 1H, 13C{1H}, and 77Se{1H} NMR spectra were recorded on a Bruker Spectrospin DPX-300 NMR spectrometer at 300.13, 75.47, and 57.24 MHz, respectively, with chemical shifts reported in ppm relative to normal standards. Yields refer to isolated yields of compounds that have purity ≥ 95% [established by 1H NMR]. All reactions were carried out in glassware dried in an oven, under ambient conditions, except the synthesis of L1, L2, and L3. Commercial nitrogen gas was used after passing it successively through traps containing solutions of alkaline anthraquinone, sodium dithionite, alkaline pyrogallol, concentrated H2SO4, and KOH pellets. Nitrogen atmosphere if required was created using Schlenk techniques. IR spectra in the range 4000−400 cm−1 were recorded on a Nicolet Protége 460 FT-IR spectrometer as KBr pellets. The C, H, and N analyses were carried out with a Perkin-Elmer 2400 Series II C, H, and N analyzer. For single-crystal structures the data were collected with a Bruker AXS SMART Apex CCD diffractometer using Mo Kα (0.71073 Å) radiation at 298(2) K. The software SADABS30 was used for absorption correction (if needed) and SHELXTL for space group, structure determination, and refinements.31 Hydrogen atoms were included in idealized positions with isotropic thermal parameters set at 1.2 times that of the carbon atom to which they are attached in all cases. The catalytic oxidation yields were determined with a NUCON Engineers (New Delhi, India) gas chromatograph (with FID detector), model 5765, equipped with an Alltech (EcTM‑1) column of 30 m length and 0.25 mm diameter and having liquid film of 0.25 mm thickness. High-resolution mass spectral measurements were performed with electron spray ionization (10 eV, 180 °C source temperature, sodium formate as reference compound) on a Bruker MicroTOF-Q II, taking samples in CH3CN. All DFT calculations were carried out at the Department of Chemistry, Supercomputing Facility for Bioinformatics and Computational Biology, IIT Delhi, with the GAUSSIAN-0332 programs with an immediate objective of identifying the reactivity in the present series of compounds. The geometry of complexes 1 to 6 was optimized at the B3LYP33 level using a SDD basis set for metal atoms and chalcogen and 6-31G* basis sets for C, N, and H. Geometry optimizations were carried out without any symmetry restriction with X-ray coordinates of the molecule. Harmonic force constants were computed at the 3380


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Scheme 1. Synthesis of Ligands L1−L3 and Complexes 1−6

(CH3CN) [M]+ (m/z) = 521.9967; calcd value for C22H26ClNRhSe = 521.9962 (δ 1.0 ppm). IR (KBr, cm−1): 3040 (m; νC−H (aromatic)), 2920 (s; νC−H (aliphatic)), 1605 (m; νCN), 1478 (m; νCC (aromatic)), 841 (s; νP−F), 765 (m; νC−H (aromatic)). Synthesis of Complexes 4−6. The solid [(η5-Cp*)IrCl(μ-Cl)]2 (0.05 g, 0.1 mmol) and L1, L2, or L3 (0.2 mmol) dissolved in CH3OH (15 cm3) were mixed, and the mixture was stirred for 10 h at room temperature. The resulting yellow solution was filtered. After reducing the volume of the filtrate to ∼7 cm3 with a rotary evaporator, it was mixed with solid NH4PF6 (0.032 g, 0.2 mmol). The resulting yellowcolored microcrystalline solid was filtered, washed with 10 cm3 of icecold CH3OH, and dried in vacuo. Single crystals of 4−6 were obtained by diffusion of diethyl ether into their solution (1 cm3) made in a mixture (1:4) of CH3OH and CH3CN: 4: Yield: 0.096 g, 85%. Anal. Calcd for C22H26ClNIrS·[PF6]: C, 37.26; H, 3.70; N, 1.98. Found: C, 37.37; H, 3.47; N, 1.90. Mp: 221.0 °C. 13C{1H} NMR (CD3CN, 25 °C vs Me4Si) δ (ppm): 1.71 (s, 15H, CpMe5), 4.40−4.46 (m, 2H, H5), 6.99−8.02 (m, 8H, H1−3, H7−9), 8.60 (d, 3JHH = 4.8 Hz, 1H, H10). 13C{1H} NMR (CD3CN, 25 °C vs Me4Si) δ (ppm): 8.0 (C of Me5Cp), 47.9 (C5), 92.6 (C of Cp), 123.5 (C7), 126.6 (C9), 129.1 (C3), 129.6 (C2), 130.0 (C1), 130.1 (C4), 140.8 (C8), 154.5 (C10), 160.5 (C6). HR-MS (CH3CN) [M]+ (m/z) = 564.1089; calcd value for C22H26ClN2IrS = 564.1094 (δ −0.9 ppm). IR (KBr, cm−1): 2967 (m; νC−H (aromatic)), 2924 (s; νC−H (aliphatic)), 1609 (m; νCN), 1479 (m; νCC (aromatic)), 845 (s; νP−F), 770 (m; νC−H (aromatic)). 5: Yield: 0.100 g, 85%. Anal. Calcd for C22H26ClNIrSe·[PF6]: C, 34.95; H, 3.47; N, 1.85. Found: C, 34.76; H, 3.34; N, 1.91. Mp: 203.0 °C. 1H NMR (CD3CN, 25 °C vs Me4Si) δ (ppm): 1.74 (s, 15H, CpMe5), 4.38−4.86 (m, 2H, H5), 7.04−8.07 (m, 8H, H1−3, H7−9), 8.69 (d, 3JHH = 5.7 Hz 1H, H10). 13C{1H} NMR (CD3CN, 25 °C vs Me4Si) δ (ppm): 7.8 (C of Me5Cp), 41.4 (C5), 91.6 (C of Cp), 124.1 (C7), 126.0 (C9), 128.9 (C1), 129.7 (C2), 129.8 (C4), 130.95 (C3), 140.0 (C8), 154.8 (C10), 161.0 (C6). 77Se{1H} NMR (CD3CN, 25 °C vs Me2Se) δ (ppm): 339.4. HR-MS (CH3CN) [M]+ (m/z) = 612.0534; calcd value for C22H26ClNIrSe = 612.0562 (δ: −4.6 ppm). IR (KBr, cm−1): 3045 (m; νC−H (aromatic)), 2922 (s; νC−H (aliphatic)), 1608 (m; νCN), 1453 (m; νCC (aromatic)), 843 (s; νP−F), 765 (m; νC−H (aromatic)). 6: Yield: 0.095 g, 85%. Anal. Calcd for C21H25ClN2IrS·[PF6]: C, 35.52; H, 3.55; N, 3.94. Found: C, 35.26; H, 3.42; N, 4.24. Mp: 213.0

125.8 (C9), 126.9 (C3), 128.7 (C2), 129.1 (C1), 130.6 (C4), 140.1 (C8), 153.7 (C10), 159.4 (C6). HR-MS (CH3CN) [M]+ (m/z) = 474.0524; calcd value for C22H26ClNRhS = 474.0541 (δ −3.6 ppm). IR (KBr, cm−1): 3068 (m; νC−H (aromatic)), 2928 (s; νC−H (aliphatic)), 1605 (m; νCN), 1477 (m; νCC (aromatic)), 837 (s; νP−F),762 (m; νC−H(aromatic)). 2: Yield: 0.078 g, 80%. Anal. Calcd for C21H25ClN2RhS·[PF6]: C, 40.63; H, 4.06; N, 4.51. Found: C, 40.64; H, 4.02; N, 4.53. Mp: 210.0 °C. 1H NMR (CD3CN, 25 °C vs Me4Si) δ (ppm): 1.77 (s, 15H, H of Me5Cp), 4.49−4.55 (m, 2H, H6), 7.19−8.04 (m, 6H, H2−4, H8−10), 8.22−8.24 (m, 1H, H1), 8.54 (d, 3JHH = 5.4 Hz, 1H, H11). 13C{1H} NMR (CD3CN, 25 °C vs Me4Si) δ (ppm): 8.1 (C of Me5Cp), 42.1 (C6), 99.3 (C of Cp), 123.2 (C4), 123.7 (C2), 125.2 (C8), 125.9 (C10), 136.4 (C3), 139.5 (C9), 147.2 (C11), 149.4 (C1), 152.8 (C7), 160.6 (C5). HR-MS (CH3CN) [M]+ (m/z) = 475.0477; calcd value for C21H25ClN2RhS = 475.0481 (δ −1.0). IR (KBr, cm−1): 3045 (m; νC−H (aromatic)), 2922 (s; νC−H (aliphatic)), 1608 (m; νCN), 1453 (m; νCC (aromatic)), 843 (s; νP−F), 765 (m; νC−H (aromatic)). Synthesis of Complex 3. The mixture of solid [(η5-Cp*)RhCl(μCl)]2 (0.05 g, 0.1 mmol) and L2 (0.2 mmol) dissolved in CH3OH (15 cm3) was stirred for 10 h at room temperature. The resulting orange solution was mixed with AgOTf (0.05 g, 0.2 mmol) dissolved in CH3CN (15 cm3), and the mixture refluxed for 6 h. The precipitated AgCl was filtered off. The yellow filtrate was mixed with solid NH4PF6 (0.064 g, 0.4 mmol), and the volume of the solution was reduced to ∼7 cm3 with a rotary evaporator. On addition of diethyl ether (7 cm3) to the concentrate, 3 was precipitated. It was filtered, washed with 10 cm3 of CH3CN−diethyl ether mixture (1:5), dried in vacuo, and recrystallized with a CH3CN−diethyl ether mixture (1:5). Single crystals of 3 were obtained by diffusion of diethyl ether into its solution (1 cm3) made in CH3CN. Yield: 0.10 g, 80%. Anal. Calcd for C24H29N2RhSe·[PF6]2: C, 35.27; H, 3.58; N, 3.43. Found: C, 35.10; H, 4.69; N, 3.39. Mp: 199.0 °C. 1H NMR (CD3CN, 25 °C vs Me4Si) δ (ppm): 1.55 (s, 15H, H of Me5Cp), 2.19 (s, 3H, CH3CN), 4.55−4.79 (m, 2H, H5), 7.23−8.05 (m, 8H, H1−3, H7−9), 8.67 (d, 3JHH = 5.4 Hz 1H, H10). 13C{1H} NMR (CD3CN, 25 °C vs Me4Si) δ (ppm): 8.5 (C of Me5Cp), 23.7 (CH3 of CH3CN), 40.1 (C5), 99.1 (C of Cp), 125.0 (C7), 125.8 (C9), 129.2 (C1), 129.8 (C2), 130.3 (C4), 131.9 (C3), 134.1 (CN of CH3CN), 140.1 (C8), 154.63 (C10), 161.5 (C6). 77 Se{1H} NMR (CD3CN, 25 °C vs Me2Se) δ (ppm): 380.8. HR-MS 3381


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°C. 1H NMR (CD3CN, 25 °C vs Me4Si) δ (ppm): 1.77 (s, 15 H, CpMe5), 4.39−4.82 (m, 2H, H6), 7.21−8.05 (m, 6H, H2−4, H8−10), 8.25−8.26 (m, 1H, H1), 8.46 (d, 3JHH = 5.9 Hz, 1H, H11). 13C{1H} NMR (CD3CN, 25 °C vs Me4Si) δ (ppm): 9.0 (CpMe5), 45.6 (C6), 93.6 (Cp), 124.4 (C4), 125.0 (C2), 127.0 (C8), 127.4 (C10), 137.8 (C3), 141.1 (C9), 147.6 (C11), 150.9 (C1), 154.6 (C7), 162.6 (C5). HR-MS (CH3CN) [M]+ (m/z) = 565.1042; calcd value for C21H25ClN2IrS = 565.1067 (δ −4.5 ppm). IR (KBr, cm−1): 2943 (m; νC−H (aromatic)), 2923 (s; νC−H (aliphatic)), 1611 (m; νCN), 1453 (m; νCC (aromatic)), 843 (s; νP−F), 768 (m; νC−H (aromatic)). Mass and 77Se{1H} NMR, 1H NMR, and 13C{1H} NMR spectra are given in the Supporting Information (Figures S1−S10, S18−S35). Procedure for Catalytic Transfer Hydrogenation. The ketone (1 mmol), KOH (2 cm3 of a 0.2 M solution in 2-propanol), and a complex from 1 to 6 (0.01 mol % dissolved in CH3CN) were added to 15 cm3 of 2-propanol, and the mixture was refluxed (80 °C) for 3 (1− 3) or 5 h (4−6). 2-Propanol was removed on a rotary evaporator, and the resulting semisolid was extracted with diethyl ether (3 × 20 cm3). The extract was passed through a short column (∼8 cm) of silica gel. The column was washed with ∼50 cm3 of diethyl ether. All the eluates from the column were mixed, and the solvent from the mixture was evaporated off on a rotary evaporator. The resulting residue was dissolved in 2−3 cm3 of hexane and subjected to GC. The final conversions are reported as an average of two runs of each catalytic reaction. Procedure of Catalytic Oxidation of Alcohols with NMO. A solution of a complex from 1−6 (0.01 mol %) dissolved in 20 cm3 of CH2Cl2 was mixed with alcohol substrate (1 mmol), K2CO3 (1.2 mmol), and solid NMO (1.2 mmol). The mixture was refluxed for 3 (1−3) or 4 h (4−6). Thereafter the solvent was evaporated off with a rotary evaporator. The residue having an oxidized product was extracted with 20 cm3 of petroleum ether (60−80 °C). The complexcatalyst undissolved in petroleum ether was recovered almost quantitatively for the next catalytic cycle. The oxidized product present in petroleum ether extract was analyzed by GC.

free L2. This kind of variation on coordination is not unusual in view of the fact that iridium is a 5d element. In 1H and 13C{1H} NMR spectra of 1−6 signals of protons and carbon atoms generally appear at higher frequencies relative to those of free ligands that coordinate with Rh and Ir in a half-pincer mode. The magnitude of shift to higher frequency is large for CH2(E) (E = S, Se): up to 4.24 ppm in 13C{1H) NMR, and for protons attached to them up to 0.65 ppm. These observations imply the coordination of ligands through S or Se. In the 1H and 13C{1H} NMR spectra of 1−3, the signals of η5-pentamethylcyclopentadienyl group (singlet in 1H NMR) were found shifted to lower frequency by up to 0.3 and 3.3 ppm, respectively, with respect to those of [(η5-Cp*)RhCl(μ-Cl)]2. This may be due to substitution of Cl with S and Se, which have relatively lower electronegativity. Similar observations regarding shifting of signals with respect to those of [(η5-Cp*)IrCl(μ-Cl)]2 were made in 1H and 13C{1H} NMR spectra of 4−6. Crystal Structures. The crystallographic and refinement data for 1−6 are summarized in the Supporting Information (Tables S1 and S2). L1, L2, and L3 exhibit an identical bonding mode in all complexes 1−6; that is, a five-membered ring is formed by their coordination to both the metal centers. The rhodium and iridium complexes have similar molecular structures. The ORTEP diagrams of cations of 1−6 are given in Figures 1 to 6 with selected bond lengths and angles. In these

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RESULTS AND DISCUSSION The syntheses of L1−L3 and their complexes have been summarized in Scheme 1. The present procedures for the synthesis of L1 and L2 are distinct from earlier known ones34−40 and give relatively higher yields, up to 95%. The important modifications are (i) ArS− or ArSe− was prepared in refluxing conditions and reacted with 2-chloromethylpyridine hydrochloride under similar conditions. Thus the reaction time for maximum yield in our case was short, and (ii) extraction of reaction product was carried out with chloroform, resulting in higher purity. Complex 3 was synthesized by substitution of Cl with CH3CN, as single crystals of the species containing a chloro group could not be grown. The ligands L1−L3 have good solubility in common organic solvents, viz., CHCl3, CH2Cl2, CH3OH, and CH3CN, while complexes 1−6 are moderately soluble in common organic solvents such as CHCl3, CH2Cl2, and CH3OH, but have good solubility in CH3CN. The pale yellow liquids L1−L3 are stable in air and moisture and can be stored at room temperature for several months. The solutions of complexes 1−6 made in CH3CN are also stable for several months under ambient conditions. NMR Spectra. The NMR spectral data for ligands L1−L3 and complexes 1−6 are consistent with their structures, which are established by single-crystal X-ray diffraction in the case of the complexes. The signal in the 77Se{1H} NMR spectrum of 3 appears shifted to a higher frequency by ∼15 ppm in comparison to that of free L2. Probably it arises due to coordination of L2 with Rh through Se. In the case of complex 5 the signal in the 77Se{1H} NMR spectrum has been observed shifted to lower frequency by ∼24 ppm with respect to that of

Figure 1. ORTEP diagram of the cation of 1 with ellipsoids at the 30% probability level. Hydrogen atoms and the PF6− anions have been omitted for clarity. Bond lengths (Å): Rh−S(1) 2.383(2), Rh−N(1) 2.133(5), Rh(1)−Cl(1) 2.407(2), Rh−C 2.154(6)−2.202(7). Bond angles (deg): S(1)−Rh(1)−N(1) 81.60(2), C(1)−S(1)−Rh(1) 112.2(2), S(1)−Rh(1)−Cl(1) 84.34(6).

cations there is a pseudo-octahedral half-sandwich “piano-stool” disposition of donor atoms around the metal center (Rh and Ir). The centroid of the η5-Cp* ring occupies nearly the center of a triangular face of an octahedron. A chelate ring with the metal center is formed through nitrogen and chalcogen atoms, and a chlorine atom completes the coordination sphere. This results in an overall three-legged piano-stool conformation. The structures of cations of complexes 1 to 3 are similar, with a five-membered chelate ring, except that in 3 in place of Cl there is a CH3CN molecule coordinated with metal (Figures 1−3). The Rh−S bond distances in 1 [2.383(2) Å] and 2 [2.353(2) Å] are consistent with each other and with the values reported for complexes [η5-Cp*RhCl(1,1′-(1,2-ethanediyl)bis(3-methylimidazole-2-thione)]Cl [2.3967(11) Å]12 and [η5Cp*RhCl{η2-S,P-Ph2P(S)NHPPh2}]BF4 [2.404(3) Å].41 The Rh−Se bond length of cation 3 [2.487(1) Å] is a little shorter than that of complex [η5-Cp*RhCl{η2-(SePPh2) 2N}]BF4 3382


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Figure 2. ORTEP diagram of the cation of 2 with ellipsoids at the 30% probability level. Hydrogen atoms and the PF6− anions have been omitted for clarity. Bond lengths (Å): Rh−S(1) 2.353(2), Rh−N(1) 2.091(4), Rh(1)−Cl(1) 2.376(2), Rh−C 2.143(5)−2.160(7). Bond angles (deg): S(1)−Rh(1)−N(1) 81.70(2), C(1)−S(1)−Rh(1) 112.70(2), S(1)−Rh(1)−Cl(1) 96.66(6).

Figure 4. ORTEP diagram of the cation of 4 with ellipsoids at the 30% probability level. Hydrogen atoms and the PF6− anions have been omitted for clarity. Bond lengths (Å): Ir−S(1) 2.343(2), Ir−N(1) 2.105(5), Ir(1)−Cl(1) 2.391(2), Ir−C 2.165(7)−2.184(6). Bond angles (deg): S(1)−Ir(1)−N(1) 81.70(2), C(1)−S(1)− Ir(1) 115.8(2), S(1)− Ir(1)−Cl(1) 96.12(6).

Figure 3. ORTEP diagram of the cation of 3 with ellipsoids at the 30% probability level. Hydrogen atoms and the PF6− anions have been omitted for clarity. Bond lengths (Å): Rh−Se(1) 2.487(1), Rh−N(1) 2.129(5), Rh−N(2) 2.074(6), Rh−C 2.156(6)−2.171(6). Bond angles (deg): Se(1)−Rh(1)−N(1) 82.80(2), C(1)−Se(1)−Rh(1) 107.1(2), Se(1)−Rh(1)−N(2) 92.40(2), N(2)−−Rh(1)−−N(1) 84.2(2).

Figure 5. ORTEP diagram of the cation of 5 with ellipsoids at the 30% probability level. Hydrogen atoms and the PF6− anions have been omitted for clarity. Bond lengths (Å): Ir−Se(1) 2.453(1), Ir−N(1) 2.106(6), Ir(1)−Cl(1) 2.388(2), Ir−C 2.174(8)−2.178(7). Bond angles (deg): Se(1)−Ir(1)−N(1) 82.40(2), C(1)−Se(1)− Ir(1) 113.4(3), Se(1)− Ir(1)−Cl(1) 95.84(6).

[2.5266(8) Å]19 and somewhat larger than the values reported for the half-sandwich complex of Rh(III) with the [1,2-dicarbacloso-dodecaborane-1,2-dichalcogenolato] ligand [2.3833(5)− 2.4706(6) Å].15 The Rh−N bond distance of 1 [2.133(5) Å] is consistent with those of 2 and 3 [2.091(4) and 2.129(5) Å, respectively] and also with those of complex [η5-Cp*Rh(2,6(mesityl)2C6H3S)(bpy)][B(3,5-(CF3)2C6H3)4] [2.099(3)− 2.103(3) Å].42 The Rh−C (Cp* centroid) distances of complexes 1−3 are in the range 1.780−1.795 Å, which is normal.41 The molecular structures of cations of iridium complexes 4− 6 are shown in Figures 4−6. The Ir−S bond distance for 4 is 2.343(2) Å, consistent with that of 6 [2.341(3) Å]. Both are also within the range of 2.318(1)−2.3872(10) Å for the reported Ir−S bond lengths of [η5-Cp*Ir-(CO)(μ-STol)Pt(STol)(PPh3)],13 [η5-Cp*Ir(η2-ppy-S-p-tol)(H2O)][OTf]2,43 [η 5 -Cp*Ir(4,6-di-tert-butyl-(2-methylthiophenylimino)-obenzoquinone](PF6)·CH2Cl2,44 and [η5-Cp*Ir(nBuPPh2)({7(S)PPh2}-8-S-7,8-C2B9H10)].45 The Ir−Se distance [2.453(1) Å] in 5 is longer than the values reported for complex [η5Cp*Ir{Se2C2(CO2Me)2}] (2.3494(7) and 2.3520(7) Å).46 The Ir−Se bond distances reported for half-sandwich complexes

[η 5 -Cp*IrCl(μ-SeCOC 6 H 5 )(κ 2 -SeCOC 6 H 4 −)Ir(η 5 -Cp*)] (2.445(2)−2.495(1) Å) 47 and [η 5-Cp*Ir(μ 3-Se)2 {PtTol(PPh3)}2] [2.416(1)−2.422(1) Å]13 are consistent with those of 5. The Ir−N bond length of the cation of 4, 2.105(5) Å, is similar to that of cations of 5 and 6 [2.106(6) and 2.099(8) Å, respectively]. They are shorter than the values of the halfsandwich complex of Ir(III) with 2-(1-benzyl-1H-1,2,3-triazol4-yl)pyridine (2.151(2)−2.159(7) Å).48 The Ir−η5-Cp*(centroid) distances (Å) in 4 [1.800 Å], 5 [1.801 Å], and 6 [1.781 Å] are consistent with each other and with the reported values for complexes [(η5-Cp*)Ir(phpy)Cl] [1.863 Å].49 PF6− has been found involved in C−H···F secondary interactions in all the complexes 1−6, resulting in chains. In Figure 7 they are shown for complex 2 along with the distances. The secondary interactions for other complexes are given in the Supporting Information, Figures S13−S17. The noncovalent interactions of C−H···F distances (Å) of 1−6 have also been included in the Supporting Information, Table S6. 3383


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Scheme 2

Table 1. Complexes 3 and 5 are the most efficient catalysts among 1−6. On monitoring the transfer hydrogenation reactions catalyzed with 3 and 5 by 77Se{ 1H} NMR spectroscopy, it is observed that the signals in the spectra shift to higher frequency (18−23 ppm), indicating that probably the M−Cl (in the case of 3, Rh−NCCH3) bond is cleaved or weakened very significantly to make a coordination site on the metal center available so that formation of an intermediate having a M−H bond can take place.53 The transfer hydrogenation reactions catalyzed with 3 and 5 were monitored with 1H NMR spectra. After 1 h a broad singlet was noticed around δ −8.8 and −12.1 ppm, respectively. These signals are characteristic of hydrides and indicate the formation of a M−H bond.2 Thus catalytic reactions with the present complexes probably proceed via formation of a metal−hydride complex intermediate The catalytic efficiency varies with chalcogen ligands in the order Se > S, when other coligands are the same, which may be tailored by a stronger electron-donating tendency of Se toward the metal center, thus promoting the formation of hydride with it. The rhodium complexes show better catalytic efficiency in comparison to analogous iridium complexes.54 The formation of a M−H-containing intermediate and the absence of a NH group in the system suggest that transfer hydrogenation by 1−6 is catalyzed by a conventional mechanism. The Rh(III) species appears to be a better catalyst than Ir(III), which is supported by DFT calculation results, which suggest greater reactivity for rhodium species. Oxidation of Secondary Alcohols. A number of transition-metal systems have been used for the catalysis of oxidation of alcohols using oxygen or hydrogen peroxide as an oxidant.55 Oppenauer-type oxidation of alcohols in which acetone has been used as an oxidant is among the promising methods for the oxidation.3 We have examined the oxidation of

Figure 6. ORTEP diagram of the cation of 6 with ellipsoids at the 30% probability level. Hydrogen atoms and the PF6− anions have been omitted for clarity. Bond lengths (Å): Ir−S(1) 2.341(3), Ir−N(1) 2.099(8), Ir(1)−Cl(1) 2.379(3), Ir−C 2.110(1)−2.150(1). Bond angles (deg): S(1)−Ir(1)−N(1) 82.0(3), C(1)−S(1)− Ir(1) 114.7(4), S(1)− Ir(1)−Cl(1) 95.40(1).

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CATALYTIC STUDIES Transfer Hydrogenation of Ketones. The transfer hydrogenation reaction in which hydrogen is transferred from one organic molecule to another is of great importance in organic synthesis since one can avoid the use of molecular hydrogen.50 Transfer hydrogenation of ketones has been catalyzed by a number of ruthenium species including halfsandwich ones.25,26 However, Rh and Ir complexes, particularly half-sandwich type, are little explored. The catalytic reactions in their presence many times necessitate high temperature and inert atmosphere.51 However, transfer hydrogenation reactions of ketones (Scheme 2) can be catalyzed with present complexes 1−6 (0.01 mol %) at a moderate temperature of 80 °C. The high efficiency of the reduction of ketones to their corresponding alcohols with 2-propanol as hydrogen donor was exhibited in the presence of KOH, which was earlier reported to be the best inorganic base for such reactions.52 The most efficient conversions (up to 98%) were found in the case of acetophenone with all the catalysts, while in the case of aliphatic secondary ketones the conversions were up to 92%. The details of percent conversions and TONs are given in

Figure 7. Noncovalent interaction diagram showing the C−H···F interactions in 2. Distances [Å]: C(5)−H(5)···F(2) 2.447(6), C(9)−H(9)···F(3) 2.452(6), C(17)−H(17A)···F(4) 2.490(1). 3384


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Table 1. Transfer Hydrogenation of Ketones

secondary alcohols in the presence of 1−6 (0.01 mol %) (Scheme 3) in which NMO has been used as an oxidant at 80 °C with K2CO3 in CH2Cl2. Maximum conversions were reached in 3 h with catalysts 1−3 and in 4 h with catalysts 4−6 (Scheme 3). The products were identified by GC after recovering the catalyst and doing the required workup. The percent conversion and TON results has been summarized in Table 2. On monitoring the reactions catalyzed with 3 and 5 by

Scheme 3

Table 2. Oxidation of Secondary Alcohols

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catalyzing oxidation than their S analogues containing similar coligands. Similarly Rh(III) species are somewhat better catalysts than Ir(III) species, which has been corroborated by the DFT calculations given below DFT Calculations. In an attempt to further understand the nature of bonding within these complexes and the reactivity, density functional theory (DFT) calculations (see Experimental Section for details) were performed for all six complexes 1−6. While the accuracy of calculations incorporating late transition metals is insufficient to warrant a detailed discussion of MO energy levels, an analysis of the lowest energy configurations and frontier orbitals can provide a qualitative insight into the bonding, etc., characteristics of the complexes. The HOMOs (highest occupied molecular orbitals) of all complexes are essentially identical and are positioned primarily over the metal center and Cp* ring, with some contribution toward chalcogen and Cl (if present) donors (see Figure 8 for complexes 1, 3, 4, and 5, and for complexes 2 and 6 see the Supporting Information, Figure S16). The agreement between the experimentally observed bonding parameters and the calculated one is better for M−Cl, M−N, and M−Cp* (centroid). For details see the Supporting Information (Table S4). The differences between calculated and observed M−E (E = S or Se) bond distances (0.13−0.19 Å) are not very unusual. The calculated and experimentally found bond angles are also very close (Table S4) except in a few cases, e.g., S−Rh−Cl. The absolute values for the HOMO energy levels cannot be reliably determined for complexes of heavy metals. However, relative energy levels are informative. It has been previously reported that there is a direct correlation between the HOMO−LUMO energy gap of a complex and its chemical reactivity.56−58 The larger gap reflects lower reactivity. The

Scheme 4. Proposed Mechanism of Oxidation of Alcohol

77

Se{1H} and 1H NMR spectra, as examples, it has been found that the signal in the 77Se{1H} NMR spectrum is shifted to higher frequency (20−27 ppm) in the course of the reaction. In the 1H NMR spectra new signals at δ −9.1 and −12.5 ppm appear in the case of 3 and 5, respectively. These signals may be ascribed to the formation of metal hydride species.3 Thus the oxidation of secondary alcohols appears to be catalyzed by 1−6 via a pathway suggested in Scheme 4. Like transfer hydrogenation, Se ligand containing complexes are more efficient for

Figure 8. Graphical representations of HOMOs for 1, 3, 4, and 5. 3386


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Figure 9. Partial charges in coordination spheres of 1, 3, 4, and 5.

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HOMO−LUMO energy gaps between 1 and 4, 2 and 6, and 3 and 5 sufficiently differ in energy (Table S5), indicating that reactivities of Rh complexes are greater than those of Ir complexes. This is consistent with the observed catalytic efficiency of complexes of two metals. Mulliken partial charge analyses for 1−6 show that the partial atomic charge on the Rh center is greater than that on the Ir center (Figure 9), which further supports the above conclusion. The charges on the other atoms are given in the Supporting Information (Table S5, Figure S17).

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ASSOCIATED CONTENT

* Supporting Information S

Crystal and refinement data, bond lengths and angles, secondary interaction distances and their figures, 1H NMR, 13 C{1H} NMR, 77Se{1H} NMR, and mass spectra, CIFs, and DFT calculation data [figures of HOMOs, partial charge diagrams, table of calculated bond lengths and angles, table of HOMO−LUMO energy gap and partial charges] are included. This material is available free of charge via the Internet at http://pubs.acs.org.

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CONCLUSION

AUTHOR INFORMATION

Notes

Half-sandwich “piano-stool” complexes [(η5-Cp*)Rh(L)Cl][PF6], [(η5-Cp*)Rh(L2)CH3CN][PF6]2, and [(η5-Cp*)Ir(L)Cl][PF6] (1−6) of chalcogenated pyridine ligands (L1−L3) have been synthesized and characterized by multinuclei NMR, HR-MS, and X-ray crystallography. The complexes are the first examples of half-sandwich complexes of rhodium(III)/iridium(III) with (Se/S, N) ligands explored for transfer hydrogenation of ketones and oxidation of secondary alcohols. They show promising TON values [transfer hydrogenation reactions of ketones up to 9.8 × 103, oxidation of secondary alcohols up to 9.6 × 103]. Their air and moisture stability is an additional advantage. The catalytic activities follow the orders Rh > Ir and Se > S, which are corroborated by DFT studies. The formation of the M−H bond has been noticed in the intermediates of both catalytic reactions.

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank the Department of Science and Technology, New Delhi (India), for project no. SR/S1/IC-40/2010. The partial financial assistance given by DST (India) to establish single-crystal X-ray diffraction and mass spectral facilities at IIT Delhi, New Delhi (India), under its FIST programme is gratefully acknowledged. O.P. and P.S. thank University Grants Commission (India) for the award of a Junior/Senior Research Fellowship. The authors thank Prof. B. Jayaram for computational facilities.

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REFERENCES

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