IrIIIâ1,2,3-Triazole-Based

Article pubs.acs.org/Organometallics

Catalyst Activation with Cp*RhIII/IrIII−1,2,3-Triazole-Based Organochalcogen Ligand Complexes: Transfer Hydrogenation via Loss of Cp* and N‑Methylmorpholine N‑Oxide Based vs OppenauerType Oxidation Fariha Saleem, Gyandshwar Kumar Rao, Arun Kumar, Goutam Mukherjee, and Ajai K. Singh* Department of Chemistry, Indian Institute of Technology Delhi, New Delhi 110016, India S Supporting Information *

ABSTRACT: The reactions of 1-benzyl-4-((phenylthio/phenylseleno)methyl)1H-1,2,3-triazole (L1/L2) and 4-phenyl-1-((phenylthio/phenylseleno)methyl)1H-1,2,3-triazole (L3/L4), synthesized using the click reaction, with [(η5Cp*)RhCl(μ-Cl)]2 and [(η5-Cp*)IrCl(μ-Cl)]2 at room temperature followed by treatment with NH4PF6 result in complexes of the type [[(η5-Cp*)M(L)Cl] (1− 8). Their HR-MS and 1H, 13C{1H}, and 77Se{1H} NMR spectra have been found characteristic. The single-crystal structures of 2, 3, and 6 have been established by X-ray crystallography. There is a pseudo-octahedral “piano-stool” disposition of donor atoms around Rh/Ir. In 1, 2, 5, and 6 N(3) of the triazole skeleton coordinates with Rh/Ir, whereas in the other four complexes the nitrogen involved is N(2). These complexes have been explored as catalysts for Nmethylmorpholine N-oxide (NMO) based and Oppenauer-type oxidation of alcohols and transfer hydrogenation (TH) of carbonyl compounds with 2propanol. Oppenauer type oxidation is somewhat slower than that based on NMO. The homogeneous nature of TH is supported by a poisoning test. The catalytic processes are more efficient with Rh complexes than the corresponding Ir complexes. The complexes having N(2) coordinated with the metal have shown better activity than those in which N(3) is involved in ligation. The reactivity with respect to ligands is in the order Se > S. In TH the species formed with loss of Cp* appears to be involved in catalysis with Rh as well as Ir complexes. Such a loss is noticed in the case of Rh for the first time. Generally results of DFT calculations are consistent with the experimental results.

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INTRODUCTION Oxidation of alcohols1 to the corresponding carbonyl compounds and transfer hydrogenation of aldehydes/ketones2 to alcohols are important for chemical transformations. Such transition-metal-catalyzed processes are tolerant to other functionalities. N-Methylmorpholine N-oxide (NMO), tertbutyl hydroperoxide (t-BuOOH), sodium oxychloride (NaOCl), sodium periodate (NaIO4), oxygen, and hydrogen peroxide are commonly used oxidants.1b−n In Oppenauer-type dehydrogenative oxidation of alcohols,3 generally acetone is used as a solvent and oxidizing agent. It is attractive from an environmental4 as well as an economical point of view and has become a reliable synthetic protocol beyond conventional oxidation reactions.5 A variety of transition-metal complexes bearing differently crafted ligands have been studied to catalyze the oxidation reactions6,7 of alcohols. Efficient catalysts for Oppenauer-type oxidation are complexes of Ru(II),8 especially heterobimetallic Rh(I)−Ru(II) species.8g Some N-heterocyclic carbene (NHC) based Cp*IrIII and Cp*RhIII complexes have been reported to be suitable for Oppenauer-type oxidation of alcohols.9 The transfer hydrogenation (TH) of carbonyl compounds using 2-propanol as source of hydrogen eliminates the use of inflammable hydrogen gas10 and therefore is as an © 2014 American Chemical Society

attractive alternative to hydrogenation. This reaction has been generally catalyzed with ruthenium species, including halfsandwich11 complexes. Rh(III) and Ir(III) complexes12 of various ligands have also been explored for this purpose, but their number is less than those of Ru. Recently, the promise of metal complexes of organochalcogen ligands has been demonstrated in the catalysis of some chemical transformations.13 The efficiency of these complexes has been attributed partially to the appropriate electron-donating ability of chalcogen atoms. In the recent past catalyst design based on chalcogenated 1,2,3-triazole has been reported.14 Its half-sandwich Ru complexes have been explored for TH with 2-propanol. The 1,2,3-triazole unit present in the ligand can act as a N donor through two types of nitrogen atoms: viz., N(2) and N(3). Ir(III) and Rh(III) complexes of 1,2,3-triazole-based ligands have been explored for their luminescent properties and used in cellular bioimaging. Some of them have also been used as catalysts for organic transformations such as hydroformylation of 1-octene and intramolecular hydroamination.15 The Rh and Ir complexes, Received: March 13, 2014 Published: May 1, 2014 2341

dx.doi.org/10.1021/om500266p | Organometallics 2014, 33, 2341−2351

Organometallics

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pellets. The C, H, and N analyses were carried out with a C, H, and N analyzer. Single-crystal structure data were collected using Mo Kα (0.71073 Å) radiation at 298(2) K. The software SADABS was used for absorption correction (if needed) and SHELXTL for space group and structure determination and refinements. Hydrogen atoms were included in idealized positions with isotropic thermal parameters set at 1.2 times that of the carbon atom to which they were attached in all cases. The least-squares refinement cycles on F2 were performed until the model converged. High-resolution mass spectral (HR-MS) measurements were performed with electron spray ionization (10 eV, 180 °C source temperature, and sodium formate as calibrant), with the sample being taken in CH3CN. The melting points determined in an open capillary are reported as such. All DFT calculations were carried out at the Supercomputing Facility for Bioinformatics and Computational Biology, Department of Chemistry, IIT Delhi, using Gaussian 09 software. The B3LYP functional implemented in Gaussian 09 software was used for calculations. For Ir and Rh the LANL2DZ (having ECP for core electrons) basis set and for the rest of the elements the 6-311G** basis set were used. Coordinates obtained from single-crystal X-ray diffraction studies were directly used for geometry optimization. In the absence of a single-crystal structure (Xray diffraction based) of the input complex, its structure was modeled by Gaussview software by replacing the atomic center to the atom of interest. The frequency calculations at the end of the geometry optimization were carried out for all eight complexes, to characterize the stationary points as minima. Molecular orbital calculations were carried out with optimized coordinates using the same level of theory and basis set. Chemcraft software (http://www.chemcraftprog.com) was used for visualization of molecular orbitals. Chemicals and Reagents. The ligands L1−L4 were synthesized as reported earlier.14 [(η5-Cp*)RhCl(μ-Cl)]219 and [(η5-Cp*)IrCl(μCl)]220 were prepared according to literature methods. Ammonium hexafluorophosphate procured from Sigma-Aldrich (USA) was used as received. The common reagents, alcohols, ketones, and bases used in catalytic reactions were procured from local sources. All of the solvents were dried and distilled before use by standard procedures.21 Synthesis of Complexes 1−4. A solution of L1 (0.056 g, 0.2 mmol)/L2 (0.065 g, 0.2 mmol)/L3 (0.053 g, 0.2 mmol)/L4 (0.063 g, 0.2 mmol) in CH3OH (5 mL) was mixed with a solution of [(η5Cp*)RhCl(μ-Cl)]2 (0.050 g, 0.1 mmol) prepared in CH3OH (5 mL), 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 mL) with a rotary evaporator. It was mixed with solid NH4PF6 (0.032 g, 0.2 mmol), and the resulting orange microcrystalline solid was filtered, washed with 5 mL of ice-cold CH3OH, and dried in vacuo. Single crystals of complexes 2 and 3 were obtained by diffusion of diethyl ether into their solutions (1 mL) made in a mixture (1/4) of CH3OH and CH3CN. 1: Yield: 0.114 g (82%). Mp: 160 °C dec. Anal. Found: C, 44.59; H, 4.30; N, 6.02. Calcd for [C26H30N3ClRhS][PF6]: C, 44.62; H, 4.32; N, 6.00. 1H NMR (CD3CN, 25 °C, TMS; δ (ppm)): 1.51 (s, 15H, H of CpMe5), 4.10−4.12 (m, 1H, H5), 4.27 (d, 1H, 3JH−H = 15.9 Hz, H5), 5.71 (s, 2H, H8), 7.22 (s, 2H, H10), 7.40−7.56 (m, 8H, H1−H3, H11, H12), 7.96 (s, 1H, H7). 13C{1H} NMR (CD3CN, 25 °C, TMS; δ (ppm)): 8.1 (C of Me(Cp*)), 36.0 (C5), 55.3 (C8), 98.9 (C of Cp*), 123.1 (C12), 128.4 (C3), 128.8 (C2), 128.4 (C1), 128.9 (C10), 129.6 (C11), 130.5 (C7), 131.3 (C4), 133.8 (C9), 147.1 (C6). IR (cm−1): 490 (b), 557 (s), 750 (m; νC−H (aromatic)), 838 (s; νP−F), 1025 (m), 1090 (b), 1160 (b), 1260 (b), 1398 (b), 1452 (m; νC−C (aromatic)), 1574 (b; νNN), 2360 (m), 2942 (m; νC−H (aliphatic)), 3164 (m; νC−H (aromatic)). HR-MS (CH3CN): [M]+ m/z 554.0899; calcd value for C26H30ClN3RhS 554.0899 (δ 0.0). 2: Yield: 0.119 g (78%). Mp: 170 °C dec. Anal. Found: C, 41.79; H, 4.03; N, 5.60. Calcd for [C26H30N3ClRhS][PF6]: C, 41.81; H, 4.05; N, 5.63. 1H NMR (CD3CN, 25 °C, TMS; δ (ppm)): 1.52 (s, 15H, H of CpMe5), 3.97 (d, 1H, 3JH−H = 13.2 Hz, H5), 4.18 (d, 1H, 3JH−H = 17.7 Hz, H5), 5.70 (d, 2H, 3JH−H = 2.7 Hz, H8), 7.17 (d, 2H, 3JH−H = 7.8 Hz, H10), 7.36−7.52 (m, 8H, H1−H3, H11, H12), 7.96 (s, 1H, H7). 13 C{1H} NMR (CD3CN, 25 °C, TMS; δ (ppm)): 8.1 (C of Me(Cp*)), 28.6 (C5), 55.3 (C8), 98.6 (C of Cp*), 123.4 (C12), 126.7

particularly half-sandwich-type of organochalcogen ligands, have been hardly explored16 as catalysts. To the best of our knowledge, Rh(III)/Ir(III) complexes of 1,2,3-triazole-based organochalcogen ligands have not been reported so far. There have been limited reports on the syntheses and applications of other metal complexes17 of ligands which contain both the 1,2,3-triazole unit and a chalcogenoether moiety. Therefore, Rh(III)/Ir(III) complexes have been synthesized using ligands L1/L2 and L3/L4 (Scheme 1). A comparative study of their Scheme 1. Synthesis of Ligands L1−L4 and Complexes 1−8

catalytic potential for NMO-based oxidation of alcohols and Oppenauer-type oxidation (alcohols) with acetone has been carried out. The present half-sandwich complexes of Rh(III)/ Ir(III) have been found to be promising for both oxidations. The efficiency of Oppenauer-type oxidation is comparable to that of NMO-based oxidation. Transfer hydrogenation of carbonyl compounds with i-PrOH catalyzed with these Rh/Ir complexes has also been found to be efficient. In the case of both Rh(III) and Ir(III) complexes Cp* appears to be lost in the catalysis and the resulting species is also active catalytically. Two such examples are known for Ir(III),18 but none is known for Rh(III), and the present example is the first one where loss of Cp* has been noticed. All of these results are given in the present paper. The complexes of Se-containing ligands are more catalytically active than the corresponding sulfur analogues. Similarly, Rh complexes have been found to show higher activity than Ir complex. The coordination of N(2) of 1,2,3-triazole gives a complex of activity higher than that in which there is involvement of N(3) in the coordination. This may be due to the higher stability of latter complexes in comparison to the former complexes.14 DFT calculations have also been carried out and found to support the experimental results.

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EXPERIMENTAL SECTION

Physical Measurements. The 1H, 13C{1H}, and 77Se{1H} NMR spectra were recorded at 300.13, 75.47, and 57.24 MHz, respectively. The chemical shifts (ppm) are reported relative to the normal standards. Glassware was dried in an oven, under ambient conditions, before use. Before use commercial nitrogen gas was passed successively through traps containing solutions of alkaline anthraquinone, sodium dithionite, alkaline pyrogallol, concentrated H2SO4, and KOH pellets. Schlenk techniques were used to create a nitrogen atmosphere. IR spectra (4000−400 cm−1) were recorded as KBr 2342


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(C4), 128.8 (C3), 129.1 (C1), 129.2 C2), 130.0 (C11), 131.5 (C7), 131.6 (C10), 134.0 (C9), 150.5 (C6). 77Se{1H} NMR (CD3CN, 25 °C, Me2Se; δ (ppm)): 404. IR (cm−1): 466 (b), 556(s), 747 (s; νC−H (aromatic)), 837 (s; νP−F), 1036 (m), 1091 (b), 1155 (m), 1254 (m), 1391 (b), 1449 (m; νC−C (aromatic)), 1573 (b; νNN), 2362 (m), 2921 (m; νC−H (aliphatic)), 3167 (m; νC−H (aromatic)). HR-MS (CH3CN): [M]+ m/z 692.0917; calcd value for C26H30ClN3IrSe 692.0909 (δ 1.1). 7: Yield: 0.136 g (88%). Mp: 185 °C dec. Anal. Found: C, 38.70; H, 3.62; N, 5.39. Calcd for [C25H28N3ClRhS][PF6]: C, 38.73; H, 3.64; N, 5.42. 1H NMR (CD3CN, 25 °C, TMS; δ (ppm)): 1.58 (s, 15H, H of CpMe5), 5.61 (d, 1H, 3JH−H = 12.9 Hz, H5), 5.77 (d, 1H, 3JH−H = 14.1 Hz, H5), 7.16 (d, 2H, 3JH−H = 7.5 Hz, H9), 7.46−7.59 (m, 6H, H1−H2, H10−H11), 7.97 (d, 2H, 3JH−H = 6.9 Hz, H3), 8.68 (s, 1H, H6). 13 C{1H} NMR (CD3CN, 25 °C, TMS; δ (ppm)): 7.7 (C of Me(Cp*)), 57.9 (C5), 92.5 (C of Cp*),123.2 (C11), 125.6 (C10), 127.8 (C8), 128.0 (C4), 128.9 (C2), 129.5 (C1), 130.0 (C9), 130.9 (C3), 132.6 (C6), 151.1 (C7). IR (cm−1): 463 (b), 558 (s), 744 (m; νC−H (aromatic)), 845 (s; νP−F), 1027 (m), 1085 (b), 1268 (b), 1386 (b), 1482 (m; νC−C (aromatic)), 1573 (b; νNN), 2363 (m), 2942 (m; νC−H (aliphatic)), 3147 (m; νC−H (aromatic)). HR-MS (CH3CN): [M]+ m/z 630.1310; calcd value for C25H28ClN3RhS 630.1307 (δ 0.5). 8: Yield: 0.134 g (82%). Mp: 175 °C dec. Anal. Found: C, 36.49; H, 3.44; N, 5.13. Calcd for [C25H28N3ClRhS][PF6]: C, 36.52; H, 3.43; N, 5.11. 1H NMR (CD3CN, 25 °C, TMS; δ (ppm)): 1.66 (s, 15H, H of CpMe5), 5.52 (d, 1H, 3JH−H = 9.0 Hz, H5), 5.73 (d, 1H, 3JH−H = 12 Hz, H5), 7.17 (d, 2H, 3JH−H = 7.5 Hz, H9), 7.44−7.61 (m, 6H, H1−H2, H10−H11), 7.98 (d, 2H, 3JH−H = 6.9 Hz, H3), 8.70 (s, 1H, H7). 13 C{1H} NMR (CD3CN, 25 °C, TMS; δ (ppm)): 8.0 (C of Me(Cp*)), 50.2 (C5), 92.3 (C of Cp*),123.7 (C11), 125.23 (C8), 125.6 (C10), 128.1 (C4), 128.9 (C2), 129.5 (C1), 130.1 (C9), 131.6 (C3), 131.9 (C6), 151.0 (C7). 77Se{1H} NMR (CD3CN, 25 °C, Me2Se; δ (ppm)): 420. IR (cm−1): 467 (b), 557 (s), 742 (m; νC−H (aromatic)), 844 (s; νP−F), 1027 (m), 1083 (b), 1269 (b), 1383 (b), 1482 (m; νC−C (aromatic)), 1578 (b; νNN), 2361 (m), 2945 (m; νC−H (aliphatic)), 3148 (m; νC−H (aromatic)). HR-MS (CH3CN): [M]+ m/z 678.0715; calcd value for C26H30ClN3RhSe 678.0752 (δ, 5.5). Procedure for Catalytic Oxidation of Alcohols with NMO. A complex catalyst out of 1−4 (0.01 mol %) and 5−8 (0.1 mol %) was taken in 20 mL of CH2Cl2 and mixed with the alcohol (1 mmol), K2CO3 (1.2 mmol), and solid NMO (1.2 mmol). The mixture was refluxed for 3 h. Thereafter its solvent was evaporated off with a rotary evaporator. The residue having an oxidized product was extracted with 20 mL of chloroform. The solvent of the extract was evaporated off with a rotary evaporator, and the residue was purified by column chromatography on silica gel in the case of complex 1. The products were analyzed by 1H and 13C{1H} NMR spectra. Procedure for Catalytic Oppenauer-Type Oxidation of Alcohols. In 15 mL of acetone, a complex catalyst out of 1−4 (0.01 mol %) and 5−8 (0.1 mol %) was dissolved and mixed with alcohol substrate (1 mmol) and potassium tert-butoxide (1.5 mmol). The mixture was refluxed for 6 h. The reaction was followed by 1H NMR spectroscopy. On completion of the reaction the mixture was cooled to room temperature and its solvent evaporated off with a rotary evaporator. The residue having an oxidized product was extracted with 20 mL of petroleum ether (40−60 °C). The solvent from the extract was evaporated off and the oxidized product present in the residue analyzed by 1H NMR spectra. Procedure for Catalytic Transfer Hydrogenation. A solution of a carbonyl compound (1 mmol) made in 2-propanol (15 cm3), KOH (1 mmol), and a complex catalyst from 1−4 (0.001 mol %) and 5−8 (0.01 mol %) were mixed, and the mixture was heated at 80 °C for 3 h. Thereafter, 2-propanol was removed with a rotary evaporator and the resulting product extracted with diethyl ether. The solvent from the extract was evaporated off with a rotary evaporator, resulting in a residue which was purified by column chromatography on silica gel using a mixture (7:3) of hexane and ethyl acetate as eluent. The products were analyzed by 1H and 13C{1H} NMR spectra.

(C4), 128.4 (C3), 128.8 (C1), 128.9 (C2), 129.7 (C11), 130.9 (C7), 131.4 (C10), 133.9 (C9), 147.9 (C6). 77Se{1H} NMR (CD3CN, 25 °C, Me2Se; δ (ppm)): 431. IR (cm−1): 490 (b), 557 (s), 748 (m; νC−H (aromatic)), 838 (s; νP−F), 1025 (m), 1089 (b), 1153 (b), 1253 (b), 1399 (b), 1454 (m; νC−C (aromatic)), 1572 (b; νNN), 2359 (m), 2942 (m; νC−H (aliphatic)), 3164 (m; νC−H (aromatic)). HR-MS (CH3CN): [M]+ m/z 602.0342; calcd value for C26H30ClN3RhSe 602.0342 (δ 0.1). 3: Yield: 0.116 g (85%). Mp: 185 °C dec. Anal. Found: C, 43.76; H, 4.12; N, 6.11. Calcd for [C25H28N3ClRhS][PF6]: C, 43.78; H, 4.11; N, 6.13. 1H NMR (CD3CN, 25 °C, TMS; δ (ppm)): 1.62 (s, 15H, H of CpMe5), 5.66−5.80 (m, 2H, H5), 7.20 (s, 2H, 3JH−H = 2.7 Hz, H9), 7.40−7.53 (m, 6H, H1−H2, H10−H11), 8.00−8.09 (m, 2H, H3), 8.58 (s, 1H, H6). 13C{1H} NMR (CD3CN, 25 °C, TMS; δ (ppm)): 8.1 (C of Me(Cp*)), 57.7 (C5), 100.0 (C of Cp*), 123.7 (C11), 125.6 (C10), 128.8 (C8), 128.9 (C2), 129.3 (C1), 130.1 (C9), 130.9 (C3), 132.0 (C4), 132.1 (C6), 150.0 (C7). IR (cm−1): 481 (b), 558 (s), 745 (m; νC−H (aromatic)), 842 (s; νP−F), 1020 (m), 1083 (b), 1164 (b), 1262 (b), 1376 (b), 1483 (m; νC−C (aromatic)), 1570 (b; νNN), 2358 (m), 2944 (m; νC−H (aliphatic)), 3149 (m; νC−H (aromatic)). HR-MS (CH3CN): [M]+ m/z 540.0735; calcd value for C25H28ClN3RhSe 540.0742 (δ, 1.3). 4: Yield: 0.114 g (78%). Mp: 185 °C dec. Anal. Found: C, 40.96; H, 3.80; N, 5.71. Calcd for [C25H28N3ClRhS][PF6]: C, 40.98; H, 3.85; N, 5.73. 1H NMR (CD3CN, 25 °C, TMS; δ (ppm)): 1.66 (s, 15H, H of CpMe5), 5.58 (s, 2H, H5), 7.15 (d, 2H, 3JH−H = 7.5 Hz, H9), 7.42− 7.56 (m, 6H, H1−H2, H10−H11), 7.96 (d, 2H, H3), 8.54 (s, 1H, H6). 13 C{1H} NMR (CD3CN, 25 °C, TMS; δ (ppm)): 8.2 (C of Me(Cp*)), 50.5 (C5), 99.8 (C of Cp*), 124.2 (C11), 125.0 (C8), 125.6 (C2), 128.4 (C4), 128.9 (C10), 129.3 (C1), 130.2 (C9), 131.5 (C6), 131.7 (C3), 150.9 (C7). 77Se{1H} NMR (CD3CN, 25 °C, Me2Se; δ (ppm)): 440. IR (cm−1): 466 (b), 557 (s), 742 (m; νC−H (aromatic)), 842 (s; νP−F), 1021 (m), 1082 (b), 1160 (b), 1265 (b), 1380 (b), 1482 (m; νC−C (aromatic)), 1574 (b; νNN), 2361 (m), 2952 (m; νC−H (aliphatic)), 3148 (m; νC−H (aromatic)). HR-MS (CH3CN): [M]+ m/ z 588.0171; calcd value for C25H28ClN3RhSe 588.0185 (δ, 2.4). Synthesis of Complexes 5−8. The solid [(η5-Cp*)IrCl(μ-Cl)]2 (0.05 g, 0.1 mmol) and ligand L1 (0.056 g, 0.2 mmol)/L2 (0.065 g, 0.2 mmol)/L3 (0.053 g, 0.2 mmol)/L4 (0.063 g, 0.2 mmol) were dissolved in CH3OH (15 mL), and the mixture was stirred for 8 h at room temperature. The resulting orange solution was filtered, and the volume of filtrate was reduced (∼7 mL) with a rotary evaporator. It was mixed with solid NH4PF6 (0.032 g, 0.2 mmol), and the resulting orange microcrystalline solid was filtered, washed with 10 mL of icecold CH3OH, and dried in vacuo. Single crystals of 6 were obtained by diffusion of diethyl ether into its solution (1 mL) made in a CH3OH and CH3CN mixture (1/4). 5: Yield: 0.134 g (85%). Mp: 160 °C dec. Anal. Found: C, 39.55; H, 3.82; N, 5.31. Calcd for [C26H30N3ClIrS][PF6]: C, 39.57; H, 3.83; N, 5.32. 1H NMR (CD3CN, 25 °C, TMS; δ (ppm)): 1.52 (s, 15H, H of CpMe5), 4.15 (d, 1H, 3JH−H = 16.2 Hz, H5), 4.46 (d, 1H, 3JH−H = 15.9 Hz, H5), 5.75 (d, 2H, 3JH−H = 4.5 Hz, H8), 7.21 (d, 2H, 3JH−H = 6.9 Hz, H10), 7.43−7.55 (m, 8H, H1−H3, H11,H12), 8.08 (s, 1H, H7). 13 C{1H} NMR (CD3CN, 25 °C, TMS; δ (ppm)): 7.8 (C of Me(Cp*)), 37.5 (C5), 55.7 (C8), 91.6 (C of Cp*), 123.4 (C12), 128.8 (C3), 129.1 (C1), 129.2 (C2), 129.9 (C11), 130.4 (C4),130.8 (C10), 132.1 (C7), 133.96 (C9), 149.5 (C6). IR (cm−1): 492 (b), 556 (s), 713 (s; νC−H (aromatic)), 838 (s; νP−F), 1036 (m), 1091 (b), 1153 (b), 1260 (b), 1393 (b), 1450 (m; νC−C (aromatic)), 1574 (b; νNN), 2362 (m), 2921 (m; νC−H (aliphatic)), 3171 (m; νC−H (aromatic)). HR-MS (CH3CN): [M]+ m/z 644.1486; calcd value for C26H30ClN3IrS 644.1464 (δ 3.4). 6: Yield: 0.147 g (80%). Mp: 170 °C dec. Anal. Found: C, 37.30; H, 3.59; N, 5.01. Calcd for [C26H30N3ClIrSe][PF6]: C, 37.35; H, 3.62; N, 5.03. 1H NMR (CD3CN, 25 °C, TMS; δ (ppm)): 1.56 (s, 15H, H of CpMe5), 3.99 (d, 1H, 3JH−H = 15.0 Hz, H5), 4.34 (d, 1H, 3JH−H = 15.3 Hz, H5), 5.73 (d, 2H, 3JH−H = 3.3 Hz, H8), 7.18 (d, 2H, 3JH−H = 7.2 Hz, H10), 7.37−7.56 (m, 10H, H1−H3, H11, H12), 8.07 (s, 1H, H7). 13 C{1H} NMR (CD3CN, 25 °C, TMS; δ (ppm)): 8.0 (C of Me(Cp*)), 29.55 (C5), 55.7 (C8), 91.3 (C of Cp*), 123.7 (C12), 127.1 2343


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Hg Poisoning Test. An excess of Hg (Hg:(Rh/Ir) 400:1) was placed in a reaction flask. Thereafter the transfer hydrogenation reaction of benzaldehyde (1.0 mmol) with 2-propanol (15 mL) using 1/5 (0.1 mol %) as catalyst was carried out in the flask under optimum conditions. The conversion was ∼100% after 3 h. PPh3 Poisoning Test. This test was carried in a manner similar to that of th Hg poisoning test using identical substrates except for the amount of PPh3, which was taken as 5 equiv. In 3 h of reaction ∼100% conversion was observed.

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RESULTS AND DISCUSSION The syntheses of L1−L4 (carried out with the reported method14) and their Rh(III)/Ir(III) complexes are summarized Figure 3. Structure of the cation of 6. Bond lengths (Å): Ir(1)−N(3) 2.094(10), Ir(1)−Cl(1) 2.399(4), Ir(1)−Se(1) 2.4822(15), Ir−C 2.136(12)−2.203(13); Bond angles (deg): N(3)−Ir(1)−Se(1) 81.2(3), N(3)−Ir(1)−Cl(1) 85.1(3), Cl(1)−Ir(1)−Se(1) 78.96(11).

and mass spectra of complexes 1−8 are given in the Supporting Information (Figures S2−S29). The signals in the 77Se{1H} NMR spectra of 2, 4, and 6 were found to be shifted to higher frequencies by 65.5, 8.9, and 38.4 ppm, respectively, with respect to those of the corresponding free ligands. Probably these shifts arise due to coordination of L2/L4 with the Rh/Ir center via Se. However, the signal in the 77Se{1H} NMR spectrum of 8 has been found to be shifted to lower frequency by 10.6 ppm. The signals of H5 in 1H NMR spectra of all the complexes appear at a higher frequency (up to 0.32 ppm) relative to those of free ligands except 4 (the shift is to a lower frequency by 0.14 ppm). Similarly, signals of C5 in 13C{1H} NMR spectra of these complexes have been observed at a higher frequency (up to 8.8 ppm) with respect to those of free ligands. These observations imply the coordination of ligands through S or Se with metal, probably in a half-pincer mode. In 1 H and 13C{1H} NMR spectra of 1−4/5−8, the signals of the η5-pentamethylcychlopentadienyl group (singlet in 1H NMR) were found to be shifted to lower frequency (maximum shift ∼0.16 and 2.5 ppm, respectively) with respect to those of [(η5Cp*)RhCl(μ-Cl)]2/[(η5-Cp*)IrCl(μ-Cl)]2. This may be due to substitution of Cl with S and Se, which have relatively lower electronegativity. Crystal Structures. Half-sandwich complexes of rhodium(III) (1−4) and iridium(III) (5−8) appear to be formed by chloro bridge cleavage of [(η5-Cp*)RhCl(μ-Cl)]2/[(η5-Cp*)IrCl(μ-Cl)]2 followed by reaction with 1-benzyl-4-((phenylthio/phenylseleno)methyl)-1H-1,2,3-triazole (L1/L2) or 4phenyl-1-(phenylthio/phenylseleno)methyl)-1H-1,2,3-triazole (L3/L4) at room temperature, facilitated by anion exchange with NH4PF6. Single crystals of complexes 2, 3, and 6 suitable for X-ray crystallography were grown by slow diffusion of diethyl ether into their solutions made up in 4 mL of a CH3OH/CH3CN mixture (1:4). Their crystal structures have been solved, and crystallographic and refinement data are given in the Supporting Information (Table S1). The molecular structures with ellipsoids at the 30% probability level of cations of complexes 2, 3, and 6 along with selected bond lengths and angles are given in Figures 1−3 respectively. H atoms and the PF6− anions have been omitted for clarity. All rhodium and iridium complexes appear to have almost similar molecular structures. A five-membered ring is formed on coordination of the nitrogen of triazole and E (S/Se) to the metal center in all of the complexes. More bond angles and distances are given in the Supporting Information (Table S2). In these cations there is a pseudo-octahedral half-sandwich “piano-stool” disposition

Figure 1. Structure of the cation of 2. Bond lengths (Å): N(3)−Rh(1) 2.091(4), Rh(1)−Se(1) 2.5010(7), Cl(1)−Rh(1) 2.4000(14), Rh−C 2.147(5)−2.181(5) (4); Bond angles (deg): N(3)−Rh(1)−Se(1) 82.00(11), Cl(1)−Rh(1)−Se(1) 79.39(4), N(3)−Rh(1)−Cl(1) 86.88(11).

Figure 2. Structure of the cation of 3. Bond lengths (Å): Rh(1)−N(2) 2.110(6), Rh(1)−Cl(1) 2.394(2), Rh(1)−S(1) 2.432(2), Rh−C 2.124(8)−2.163(8); Bond angles (deg): N(2)−Rh(1)−S(1) 78.74(11), N(2)−Rh(1)−Cl(1) 88.90(11), Cl(1)−Rh(1)−S(1) 81.95(4).

in Scheme 1. The complexes 1−8 are moderately soluble in common organic solvents such as CHCl3, CH2Cl2, and CH3OH and have good solubility in CH3CN. They can be stored at room temperature for several months under ambient conditions. The elemental analyses and multinuclear NMR, IR, and mass spectral data of 2, 3, and 6 are consistent with the results of single-crystal X-ray diffraction based structures given in Figures 1−3. A comparison of the spectral data of other complexes with those of 2, 3, and 6 suggests that 1−8 are isostructural. NMR Spectra. The 1H, 13C{1H}, and 77Se{1H} NMR spectra of L1−L4 were found to be consistent with literature reports, and those of their complexes 1−8 were in agreement with the molecular structures depicted in Scheme 1. All NMR 2344


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Figure 4. Noncovalent C−H···F interactions in 3.

Figure 5. Noncovalent C−H···F interactions in 6.

values reported for half-sandwich complex of Rh(III) with the ligand 2-(phenylselenomethyl)pyridine (2.487(1) Å)16 and the complex [η5-Cp*RhCl{η2-(SePPh2)2N}] (2.5266(8) Å)22 but somewhat larger than the values reported for the half-sandwich complex of Rh(III) with the ligand 1,2-dicarba-closo-dodecaborane-1,2-dichalcogenolato (2.3833(5)−2.4706(6) Å).23 The Rh−S bond distance in 3 (2.432(2) Å) is greater than the values reported for the complexes [η5-Cp*RhCl(1,1′-(1,2ethanediyl)bis(3-methylimidazole-2-thione)]Cl (2.3967(11) Å), 24 [η 5 -Cp*RhCl{2-(phenylthiomethyl)pyridine}]PF 6 [(2.383(2) Å),16 and [η5-Cp*RhCl{η2-S,P-Ph2P(S)NHPPh2}]BF4 (2.404(3) Å).25 The Rh−N bond distances of the cations of 2/3 (2.091(4)/2.110(6) Å) are consistent with those of [η5Cp*Rh(2,6-(mesityl) 2 C 6 H 3 S)(bpy)][B(3,5-(CF3 ) 2 C 6 H 3 ) 4 ] (2.099(3)−2.103(3) Å).26 The Rh−C(Cp* centroid) distances

Scheme 2. Oxidation of Alcohols

of donor atoms around the Rh/Ir metal center. The centroid of the η5-Cp* ring occupies nearly the center of a triangular face of an octahedron. The nitrogen and chalcogen atoms forming a chelate ring with the metal center with a chlorine donor atom complete the coordination sphere. This results in an overall three-legged piano-stool conformation. The Rh−Se bond length of the cation of 2 (2.5010(7) Å) is consistent with the 2345


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Table 1. Catalytic Oxidation of Alcohols using 1−8 as Catalystsa yield, % entry no.

alcohol

1b

2c

3c

4c

5c

6c

7c

8c

1 2 3 4 5 6

benzyl alcohol 4-methoxybenzyl alcohol 4-methylbenzyl alcohol 4-chlorobenzyl Alcohol 1-phenylethanol diphenylethanol

93 94 89 82 64 48

100 100 91 86 81 70

100 100 93 92 85 73

100 100 91 89 80 69

100 100 85 81 80 50

100 100 90 88 84 54

100 100 89 88 85 60

100 100 94 93 88 68

a

Reaction conditions: catalysts 1−4 (0.01 mol %), catalysts 5−8 (0.1 mol %), alcohol (1 mmol), K2CO3 (1.2 mmol), solvent CH2Cl2, temperature 60 °C, time 3 h. bIsolated yield. cNMR yield.

Scheme 3. Oppenauer-Type Oxidation of Alcohols

Table 2. Effect of Base on Oppenauer-Type Oxidation of Alcoholsa yieldb, % entry no.

base

4

8

1 2 3 2 4

KOH K2CO3 t-BuOK Cs2CO3 CH3ONa

40 50 90 10

32 45 95

a

Reaction conditions: catalyst 4 (0.01 mol %) and 8 (0.1 mol %), alcohol (1 mmol), 15 mL of acetone, potassium tert-butoxide (1.5 mmol), 80 °C, time 6 h. bConversion determined by NMR.

Figure 6. Time profile of conversion in oxidation reactions.

Scheme 4. Transfer Hydrogenation of Carbonyl Compounds

of complexes 2 and 3 are in the normal range: 1.784(1)− 1.786(1) Å.25 The Ir−Se bond distance (2.4822(15) Å) in 6 is longer than the values reported for the complex [η 5 -Cp*Ir{Se2C2(CO2Me)2}] (2.3494(7) and 2.3520(7) Å)27 and is consistent with the distances reported for [(η5-Cp*)Ir(2arylselenomethyl)pyridine)Cl][PF6] (2.453(1) Å),16 [η5Cp * Ir Cl(μ -SeCO C 6 H 5 )( κ 2 -S eCOC 6 H 4 ) Ir (η 5 - Cp * )] (2.445(2)−2.495(1) Å),28 and [η5-Cp*Ir(μ3-Se)2{PtTol(PPh3)}2] (2.416(1)−2.422(1) Å).29 The Ir−N bond length of 6 (2.094(10) Å) is similar to that of [(η5-Cp*)Ir(2arylthiopyridyl) pyridine)Cl][PF6] (2.099(8) Å)16 and shorter than the values reported for Ir(III) complexes of the pyridinetriazole ligand (2.137(5) Å)15a and 2-(1-benzyl-1H-1,2,3triazol-4-yl)pyridine (2.151(2)−2.159(7) Å).30 The Ir−η5-

Cp*(centroid) distance (1.791(1) Å) of 6 is consistent with that of [(η5-Cp*)Ir(2-arylselenomethyl)pyridine)Cl][PF6] (1.781 Å) and is shorter than the reported value for the complex [(η5-Cp*)Ir(phpy)Cl] (1.863 Å).31 The PF6− group has been found to be involved in C−H···F secondary interactions in complexes 2, 3, and 6, resulting in chain like structures. The secondary interactions of complexes 3 and 6 are shown in Figures 4 and 5, respectively, while for 2

Table 3. Catalytic Oppenauer-Type Oxidation of Alcoholsa yield, % entry no.

alcohol

1b

2c

3c

4c

5c

6c

7c

8c

1 2 3 4 5 6

benzyl alcohol 4-methoxy benzyl alcohol 4-methylbenzyl alcohol 4-chlorobenzyl alcohol 1-phenylethanol diphenylmethanol

95 91 85 85 90 84

97 100 90 85 92 86

100 100 93 95 100 88

100 100 88 90 90 89

95 90 78 80 85 87

100 100 82 86 90 90

100 100 95 90 92 88

100 100 100 95 95 92

a

Reaction conditions: catalyst 1−4 (0.01 mol %) and 5−8 (0.1 mol %), 1 mmol of substrate (alcohol), potassium tert-butoxide (1.5 mmol), 15 mL of acetone, 80 °C, in air, time 6 h. bIsolated yield. cNMR yield. 2346


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Table 4. Catalytic Transfer Hydrogenation of Carbonyl Compounds using Complexes 1−8 as Catalystsa yield, % entry no.

substrate

1b

2c

3c

4c

5c

6c

7c

8c

1 2 3 4 4 5 6

benzaldehyde 4-methylbenzaldehyde furfuraldehyde acetophenone 4-chloroacetophenone 4-methylacetophenone benzophenone

91 89 92 68 68 88 82

100 87 85 70 75 90 93

100 100 100 70 79 95 95

100 100 100 78 69 80 88

100 90 100 85 65 83 76

100 100 100 90 84 85 78

88 100 100 85 60 86 84

100 83 100 92 80 88 85

a

Reaction conditions: catalyst 1−4 (0.001 mol %) and 5−8 (0.01 mol %), reactant (1 mmol), KOH (1 mmol), 15 mL of 2-propanol as solvent, bath temperature 80 °C, time 3 h. bIsolated yield. cNMR yield.

Catalytic Oxidation of Alcohols. The oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones, respectively, has an important role in organic synthesis. The oxidation protocol has to be simple to operate with good to excellent yield. The NMO giving Nmethylmorpholine and water as byproducts in the course of oxidation reactions is a suitable oxidant. Thus, oxidation in CH2Cl2 of primary and secondary alcohols under ambient conditions in the presence of 1−8 (0.1−0.01 mol %) (Scheme 2) with NMO has been studied. The conditions (temperature 60 °C; base K2CO3) standardized earlier16 were found to be suitable in the present case. Maximum conversions were reached in 3 h with 0.01 mol % loadings of 1−4 and 0.1 mol % loadings of 5−8. The percent conversions have been summarized in Table 1. The Oppenauer-type oxidation of alcohols in the presence of 0.01 mol % of 1−4 and 0.1 mol % of 5−8 (Scheme 3) using acetone as an oxidant/solvent has been studied. The reaction conditions were optimized by studying oxidation of 1-phenylethanol in the presence of 4 and 8. Potassium tert-butoxide has been found to be a better base for the reaction at 80 °C than K2CO3, KOH, cesium carbonate, or sodium methoxide, as yields were lower with them under similar reaction conditions (Table 2). The percent conversions of Oppenauer-type oxidations of alcohols have been summarized in Table 3. Both types of oxidation reactions with benzyl alcohol in the presence of 2 and 8 have been monitored with 77Se{1H} and

Scheme 5. Proposed Mechanism of Transfer Hydrogenation of Carbonyl Compounds

they are given in the Supporting Information (Figure S1). The C−H···F distances (Å) of the complexes have also been included in the Supporting Information (Table S3).

Figure 7. Frontier molecular orbital diagrams of complexes 1−4. 2347


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Figure 8. Frontier molecular orbital diagrams of complexes 5−8.

Figure 9. Mulliken partial charges of complexes 1−8. 1

H NMR. In the course of the reactions signals in the 77Se{1H} NMR spectra were found to be shifted to higher frequency, 21 and 28 ppm, respectively, relative to those of free complexes. In 1 H NMR spectra also new signals at δ −9.1 and −12.0 ppm appeared with the progress of the reactions. These signals may be ascribed to the formation of metal hydride species.9a−c Both of these observations indicate that the mechanism of oxidation involves metal alkoxide formation ,as reported earlier.9a,b To

compare NMO-based oxidations with Oppenauer-type oxidations, benzyl alcohol was subjected to both catalytic processes and percent conversions were monitored at an interval of 30 min. The results are plotted in Figure 6. In the case of NMO, oxidation was noticed after 10 min, while no significant conversion occurred even after 60 min in the Oppenauer-type process. Oxidation with NMO was complete much earlier (3 h) than the Oppenauer-type oxidation, which took 6 h for comparable conversion. However, the Oppenauer-type oxida2348


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mechanism:2b i.e., via alkoxide formation. When the TH reaction was monitored with 1H NMR, the rapid disappearance of the intense singlet at 1.56 ppm (due to bound Cp*) was observed with complexes of Rh as well as Ir. The loss of the Cp* ring becomes complete in 30 min, as no signal for bound Cp* appears thereafter. The rate of catalysis marginally increases with the loss of the Cp* ring, as in 30 min of reaction the conversion of furfuraldehyde is about 45%, whereas it becomes 100% after 1 h. Most likely the species [(iPrOH)xMClL]2+ is generated with the loss of Cp* and participates in the catalysis.18a The loss of arene and participation of the resulting species in the catalysis of TH have been recently proposed with the RuII−arene system.35 It is possible that a similar thing is happening in the present case. As the rate of catalysis does not slow down with the loss of Cp*, the role of [(iPrOH)xMClL]2+ in catalysis is most likely somewhat equal to that of the precursor complex. Therefore, for transfer hydrogenation the mechanism given in Scheme 5 appears to be the most plausible. DFT calculations support the variation in catalytic efficiencies with donor sites and metal atoms. Homogeneous vs Heterogeneous Transfer Hydrogenation Catalysis. To understand whether the transfer hydrogenation catalysts are homogeneous or heterogeneous, mercury and PPh3 poisoning tests36 have been executed with benzaldehyde substrate. The reactions in 2-propanol were carried out under the optimum conditions using complexes 1 and 5 as catalysts. No significant inhibition of conversion to products was noticed. Thus, the present catalysis appears to be homogeneous in nature. DFT Calculations. Density functional theory (DFT) calculations were performed on all complexes 1−8. Qualitative analyses of the lowest energy configurations, frontier orbitals, and charge distribution of the complexes have been carried out. The HOMOs (highest occupied molecular orbitals) of N(3)coordinated complexes are positioned primarily over the metal center and π orbital of the Cp* ring. The d orbital of the metal interacting with the π orbital of the Cp* ring and the p orbitals of N(3) of the triazole ring and the chalcogen atom constitutes their HOMO. In case of N(2)-coordinated complexes the HOMOs are composed of the π orbital of 4-phenyl-1,2,3triazole, the metal d orbital, the π orbital of the Cp* ring, and the p orbital of the chlorine atom. It has been reported that to some extent chemical the reactivity of a complex and its HOMO−LUMO energy gap may be correlated.37 The reactivity of a complex is most likely expected to be high when the energy gap is small. The differences among the HOMO−LUMO energy gaps of complexes 1−4 are smaller than those of 5−8 (Figures 7 and 8). This observation corroborates the somewhat higher reactivity of Rh complexes in comparison to those of Ir. Further, within complexes 1−4, complexes 3 and 4 are expected to be more catalytically active than 1 and 2. Similarly, complex 8 is likely to be more reactive than the other three Ir complexes. The experimentally observed variation in catalytic activities of various complexes is moderate but may be considered consistent with DFT calculations. The stability of a complex may also be rationalized in terms of strength of coordination, which is expected to be strong in the case of complexes 1, 2, 5, and 6, as the electron density at the coordinated N(3) of the triazole ring is quite high (Figure 9). When N(2) of the triazole ring is involved in coordination, the stability of the complex is expected to be somewhat less, on the basis of the charge on this N. The expected difference in the

tion is much less expensive than the NMO-based oxidation and isolation of the product is easier in the case of the former reaction. For both types of oxidation processes it has been observed that the coordination with metal of N(2) and chalcogen results in a catalyst somewhat better than the complex in which the coordinating nitrogen along with chalcogen is N(3). Rh(III)based species are more efficient than their Ir(III) analogues. Generally catalytic efficiency is somewhat greater for metal complexes containing Se ligands in comparison to the corresponding S analogues. The partially higher electrondonating tendency of Se relative to S promotes the formation of hydride and thus results in better catalytic activity. However, between 3 and 4, S analogue is a somewhat better catalyst than the Se analogue. The variations in the catalytic activity have been corroborated by results of DFT calculation studies. Oppenauer-type oxidation with a Rh/Ir catalyst loading of less than 0.05 mol % has rarely been reported.3g All four Rh complexes described here are efficient catalysts even at 0.01 mol % loading. Transfer Hydrogenation of Carbonyl Compounds. Rhodium and iridium complexes, particularly half-sandwich types, have been less explored for TH than Ru species. The catalytic reactions in 2-propanol in their presence many times necessitate high temperature and an inert atmosphere.32 With the present complexes 1−8 (0.01−0.001 mol %) transfer hydrogenation of carbonyl compounds (Scheme 4) with 2propanol as hydrogen donor2a can be catalyzed at the moderate temperature of 80 °C and good conversion takes place in a short time. The TH reactions have been carried out in the presence of KOH, which was reported earlier to be the best inorganic base for such reactions.33 The percent conversions of substrates are given in Table 4. For the Rh complexes reported here, catalyst loading even at 0.001 mol % is enough for efficient catalysis. In earlier reports the level achieved12b,16 was only 0.01 mol %. The catalyst loading required for efficient catalysis in the case of the present Ir complexes is 0.01 mol %, which is significantly better than the 0.1 mol % reported earlier. Complexes having involvement of N(2) of the triazole ring in coordination with Rh/Ir are marginally efficient as catalysts in comparison to the corresponding compounds involving N(3) in coordination, as conversions are somewhat higher with the former. Moreover, the Rh(III) species appear to be better than their Ir(III) analogues in terms of efficiency. Complexes with Se ligands are somewhat more efficient as catalysts relative to the corresponding S analogues (Table 4). On monitoring of the transfer hydrogenation reactions catalyzed with 2 and 8 with 77Se{1H} NMR spectroscopy, it is observed that the signals in the spectra shift to higher frequency (20−25 ppm), indicating that probably the M−Cl 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.34 The transfer hydrogenation reactions catalyzed with 2 and 8 were also monitored with 1H NMR spectroscopy. After 1 h a broad singlet was noticed around δ −9.8 and −13.1 ppm, respectively. These signals are characteristic of hydrides and indicate the formation of an M−H bond.9a−c Thus, catalytic reactions with the present complexes probably proceed via formation of a metal hydride complex intermediate. The formation of a M−H bond suggests that transfer hydrogenation is catalyzed by a conventional 2349


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reactivity of complexes involving these two N atoms for coordination is consistent with the experimental results. Therefore, the complexes involving N(3) in coordination appear to be more stable than those involving N(2)14,38 and less reactive. Experimentally determined bond lengths/angles for complexes 2, 3, and 6 are consistent with DFT-based calculated values (Table S4 in the Supporting Information).

CONCLUSIONS The reactions of [(η5-Cp*)RhCl(μ-Cl)]2 and [(η5-Cp*)IrCl(μCl)]2 with 1-benzyl-4-(phenylthio/phenylseleno)methyl)-1H1,2,3-triazole (L1/L2) and 4-phenyl-1-(phenylthio/phenylseleno) methyl)-1H-1,2,3-triazole (L3/L4) at room temperature followed by treatment with NH4PF6 result in complexes of the type [(η5-Cp*)M(L)Cl] (1−8), characterized by HRMS, 1H, 13C{1H}, and 77Se{1H} NMR spectra, and X-ray crystallography on single crystals (2, 3, and 6 only). There is a pseudo-octahedral “piano-stool” disposition of donor atoms around Rh/Ir. In 1, 2, 5, and 6 N(3) of the triazole skeleton coordinates with Rh/Ir, whereas in the other four complexes the nitrogen involved is N(2). The catalysis with these complexes of Oppenauer-type oxidation is somewhat slower than that based on NMO. Transfer hydrogenation (TH) of ketones with 2-propanol has been catalyzed efficiently with all these complexes. The homogeneous nature of TH is supported by poisoning tests. The catalytic processes are more efficient with Rh complexes than the corresponding Ir complexes. The complexes having N(2) coordinated with the metal have shown marginally better activity than those in which N(3) is involved in ligation. The reactivity with respect to ligands is in the order Se > S. In TH the species formed with loss of Cp* appears to be involved in catalysis with Rh as well as Ir complexes. Such a loss is noticed in the case of Rh for the first time. Generally results of DFT calculations are consistent with the experimental results. ASSOCIATED CONTENT

S Supporting Information *

Tables, figures, and CIF and xyz files giving crystal and refinement data, bond lengths and angles, secondary interaction distances, and NMR (CCDC nos. 988645−988647) and mass spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*A.K.S.: e-mail, [email protected], ajai57@hotmail. com; fax, +91 11 26581102; tel, +91 11 26591379. Notes

The authors declare no competing financial interest.

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REFERENCES

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ACKNOWLEDGMENTS

A.K.S. thanks the Department of Science and Technology of India (project no. SR/S1/IC-40/2010) for financial support. F.S. thanks the University Grants Commission (UGC) for a SRF. G.K.R. and A.K. thank the CSIR of India for an RA. The authors also thank Prof. B. Jayaram (Coordinator, SCFBio) and the Supercomputing Facility for Bioinformatics and Computational Biology (SCFBio), Department of Chemistry, IIT Delhi, for providing access to computational facilities. 2350


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