Catalyst activation with CpRhIII/IrIII-1,2,3- triazole-based organochalcogen ligand complexes: Transfer hydrogenation via loss of cp* and N -methylmorpholine N -oxide based Vs oppenauer-type oxidation

Article abstract of DOI:10.1021/om500266p

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 [(n ⁵-Cp)RhCl(U-Cl)]₂ and [(n⁵-Cp)IrCl(U-Cl)] ₂ at room temperature followed by treatment with NH ₄PF₆ result in complexes of the type [[(n ⁵-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 N-methylmorpholine N-oxide (NMO) based and Oppenauer-type oxidation of alcohols and transfer hydrogenation (TH) of carbonyl compounds with 2-propanol. 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.

Full text of DOI:10.1021/om500266p

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 Oppenauer-
Type 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 [(η5-
Cp*)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 N-
methylmorpholine N-oxide (NMO) based and Oppenauer-type oxidation of
alcohols and transfer hydrogenation (TH) of carbonyl compounds with 2-
propanol. 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.
attractive alternative to hydrogenation. This reaction has been
generally catalyzed with ruthenium species, including half-
sandwich11 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.
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), tert-
butyl 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
Recently, the promise of metal complexes of organo-
chalcogen 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
© 2014 American Chemical Society
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Article
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 (X-
ray 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.
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
Chemicals and Reagents. The ligands L1−L4 were synthesized
as reported earlier.14 [(η5-Cp*)RhCl(μ-Cl)]2 and [(η5-Cp*)IrCl(μ-
19
20
Cl)]2 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 [(η5-
Cp*)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.
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.
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,
1
6.00. H NMR (CD3CN, 25 °C, TMS; δ (ppm)): 1.51 (s, 15H, H of
3
CpMe5), 4.10−4.12 (m, 1H, H5), 4.27 (d, 1H, JH−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).
EXPERIMENTAL SECTION
■
1
Physical Measurements. The H, 13C{1H}, and 77Se{1H} NMR
2: Yield: 0.119 g (78%). Mp: 170 °C dec. Anal. Found: C, 41.79; H,
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 anthraqui-
none, 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
4.03; N, 5.60. Calcd for [C26H30N3ClRhS][PF6]: C, 41.81; H, 4.05; N,
1
5.63. H 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
3
3
Hz, H5), 5.70 (d, 2H, JH−H = 2.7 Hz, H8), 7.17 (d, 2H, JH−H = 7.8
Hz, H10), 7.36−7.52 (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*)), 28.6 (C5), 55.3 (C8), 98.6 (C of Cp*), 123.4 (C12), 126.7
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(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).
(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).
3: Yield: 0.116 g (85%). Mp: 185 °C dec. Anal. Found: C, 43.76; H,
7: Yield: 0.136 g (88%). Mp: 185 °C dec. Anal. Found: C, 38.70; H,
4.12; N, 6.11. Calcd for [C25H28N3ClRhS][PF6]: C, 43.78; H, 4.11; N,
3.62; N, 5.39. Calcd for [C25H28N3ClRhS][PF6]: C, 38.73; H, 3.64; N,
1
1
6.13. H NMR (CD3CN, 25 °C, TMS; δ (ppm)): 1.62 (s, 15H, H of
5.42. H NMR (CD3CN, 25 °C, TMS; δ (ppm)): 1.58 (s, 15H, H of
3
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,
CpMe5), 5.66−5.80 (m, 2H, H5), 7.20 (s, 2H, JH−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).
3
H10−H11), 7.97 (d, 2H, JH−H = 6.9 Hz, H3), 8.68 (s, 1H, H6).
13C{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,
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,
1
5.73. H NMR (CD3CN, 25 °C, TMS; δ (ppm)): 1.66 (s, 15H, H of
1
5.11. H NMR (CD3CN, 25 °C, TMS; δ (ppm)): 1.66 (s, 15H, H of
3
CpMe5), 5.58 (s, 2H, H5), 7.15 (d, 2H, JH−H = 7.5 Hz, H9), 7.42−
CpMe5), 5.52 (d, 1H, 3JH−H = 9.0 Hz, H5), 5.73 (d, 1H, 3JH−H = 12 Hz,
7.56 (m, 6H, H1−H2, H10−H11), 7.96 (d, 2H, H3), 8.54 (s, 1H, H6).
13C{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 ice-
cold 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).
3
H5), 7.17 (d, 2H, JH−H = 7.5 Hz, H9), 7.44−7.61 (m, 6H, H1−H2,
3
H10−H11), 7.98 (d, 2H, JH−H = 6.9 Hz, H3), 8.70 (s, 1H, H7).
13C{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
5: Yield: 0.134 g (85%). Mp: 160 °C dec. Anal. Found: C, 39.55; H,
1
were analyzed by H and 13C{1H} NMR spectra.
3.82; N, 5.31. Calcd for [C26H30N3ClIrS][PF6]: C, 39.57; H, 3.83; N,
1
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).
5.32. H 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
3
3
Hz, H5), 5.75 (d, 2H, JH−H = 4.5 Hz, H8), 7.21 (d, 2H, JH−H = 6.9
Hz, H10), 7.43−7.55 (m, 8H, H1−H3, H11,H12), 8.08 (s, 1H, H7).
13C{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).
1
The mixture was refluxed for 6 h. The reaction was followed by H
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
1
in the residue analyzed by H 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
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,
1
5.03. H 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
3
3
Hz, H5), 5.73 (d, 2H, JH−H = 3.3 Hz, H8), 7.18 (d, 2H, JH−H = 7.2
Hz, H10), 7.37−7.56 (m, 10H, H1−H3, H11, H12), 8.07 (s, 1H, H7).
13C{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
1
products were analyzed by H and 13C{1H} NMR spectra.
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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.
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
1
by 10.6 ppm. The signals of H5 in H NMR spectra of all the
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).
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
1H and 13C{1H} NMR spectra of 1−4/5−8, the signals of the
1
η5-pentamethylcychlopentadienyl group (singlet in H NMR)
were found to be shifted to lower frequency (maximum shift
∼0.16 and 2.5 ppm, respectively) with respect to those of [(η5-
Cp*)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-((phenyl-
thio/phenylseleno)methyl)-1H-1,2,3-triazole (L1/L2) or 4-
phenyl-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
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.
−
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
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
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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-dodeca-
borane-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′-(1,2-
ethanediyl)bis(3-methylimidazole-2-thione)]Cl (2.3967(11)
Å),24 [η5-Cp*RhCl{2-(phenylthiomethyl)pyridine}]PF6
[(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 [η5-
Cp*Rh(2,6-(mesityl)2C6H3S)(bpy)][B(3,5-(CF3)2C6H3)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
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a
Table 1. Catalytic Oxidation of Alcohols using 1−8 as Catalysts
yield, %
b
c
c
c
c
c
c
c
entry no.
alcohol
1
2
3
4
5
6
7
8
1
2
3
4
5
6
benzyl alcohol
93
94
89
82
64
48
100
100
91
100
100
93
100
100
91
100
100
85
100
100
90
100
100
89
100
100
94
4-methoxybenzyl alcohol
4-methylbenzyl alcohol
4-chlorobenzyl Alcohol
1-phenylethanol
86
92
89
81
88
88
93
81
85
80
80
84
85
88
diphenylethanol
70
73
69
50
54
60
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
b
c
60 °C, time 3 h. Isolated yield. NMR yield.
Scheme 3. Oppenauer-Type Oxidation of Alcohols
Table 2. Effect of Base on Oppenauer-Type Oxidation of
a
Alcohols
b
yield , %
entry no.
base
KOH
4
8
1
2
3
2
4
40
50
90
10
32
45
95
K2CO3
t-BuOK
Cs2CO3
CH3ONa
a
Reaction conditions: catalyst 4 (0.01 mol %) and 8 (0.1 mol %),
Figure 6. Time profile of conversion in oxidation reactions.
alcohol (1 mmol), 15 mL of acetone, potassium tert-butoxide (1.5
mmol), 80 °C, time 6 h. Conversion determined by NMR.
b
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 [η5-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(2-
arylselenomethyl)pyridine)Cl][PF6] (2.453(1) Å),16 [η5-
Cp*IrCl(μ-SeCOC6H5)(κ2-SeCOC6H4)Ir(η5-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(2-
arylthiopyridyl) pyridine)Cl][PF6] (2.099(8) Å)16 and shorter
than the values reported for Ir(III) complexes of the pyridine-
triazole ligand (2.137(5) Å)15a and 2-(1-benzyl-1H-1,2,3-
triazol-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
a
Table 3. Catalytic Oppenauer-Type Oxidation of Alcohols
yield, %
b
c
c
c
c
c
c
c
entry no.
alcohol
benzyl alcohol
1
2
3
4
5
6
7
8
1
2
3
4
5
6
95
91
85
85
90
84
97
100
90
100
100
93
100
100
88
95
90
78
80
85
87
100
100
82
100
100
95
100
100
100
95
4-methoxy benzyl alcohol
4-methylbenzyl alcohol
4-chlorobenzyl alcohol
1-phenylethanol
85
95
90
86
90
92
100
88
90
90
92
95
diphenylmethanol
86
89
90
88
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
b
c
of acetone, 80 °C, in air, time 6 h. Isolated yield. NMR yield.
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a
Table 4. Catalytic Transfer Hydrogenation of Carbonyl Compounds using Complexes 1−8 as Catalysts
yield, %
b
c
c
c
c
c
c
c
entry no.
substrate
benzaldehyde
1
2
3
4
5
6
7
8
1
2
3
4
4
5
6
91
89
92
68
68
88
82
100
87
85
70
75
90
93
100
100
100
70
100
100
100
78
100
90
100
100
100
90
88
100
100
85
100
83
4-methylbenzaldehyde
furfuraldehyde
100
85
100
92
acetophenone
4-chloroacetophenone
4-methylacetophenone
benzophenone
79
69
65
84
60
80
95
80
83
85
86
88
95
88
76
78
84
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
b
c
temperature 80 °C, time 3 h. Isolated yield. NMR yield.
Scheme 5. Proposed Mechanism of Transfer Hydrogenation
of Carbonyl Compounds
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 N-
methylmorpholine 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
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.
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Figure 8. Frontier molecular orbital diagrams of complexes 5−8.
Figure 9. Mulliken partial charges of complexes 1−8.
1H 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
1H 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 oxida-
tions, 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 oxida-
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tion is much less expensive than the NMO-based oxidation and
isolation of the product is easier in the case of the former
reaction.
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 Hydro-
genation 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.
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 electron-
donating 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 2-
propanol 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.
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-1,2,3-
triazole, 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
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 correspond-
ing 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
1
catalyzed with 2 and 8 were also monitored with H NMR
spectroscopy. After 1 h a broad singlet was noticed around δ
−9.8 and −13.1 ppm, respectively. These signals are character-
istic 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
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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).
REFERENCES
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CONCLUSIONS
■
The reactions of [(η5-Cp*)RhCl(μ-Cl)]2 and [(η5-Cp*)IrCl(μ-
Cl)]2 with 1-benzyl-4-(phenylthio/phenylseleno)methyl)-1H-
1,2,3-triazole (L1/L2) and 4-phenyl-1-(phenylthio/phenyl-
seleno) methyl)-1H-1,2,3-triazole (L3/L4) at room temper-
ature followed by treatment with NH4PF6 result in complexes
of the type [(η5-Cp*)M(L)Cl] (1−8), characterized by HR-
MS, 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.
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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.
AUTHOR INFORMATION
■
Corresponding Author
*A.K.S.: e-mail, aksingh@chemistry.iitd.ac.in, ajai57@hotmail.
com; fax, +91 11 26581102; tel, +91 11 26591379.
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Notes
The authors declare no competing financial interest.
Chem. Eur. J. 2012, 18, 11550. (f) Fu, Y.; Yang, Y.; Hugel, H. M.; Du,
̈
ACKNOWLEDGMENTS
Z.; Wang, K.; Huang, D.; Hu, Y. Org. Biomol. Chem. 2013, 11, 4429.
(g) Chorghade, R.; Battilocchio, C.; Hawkins, J. M.; Ley, S. V. Org.
Lett. 2013, 15, 5698.
■
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 Computa-
tional Biology (SCFBio), Department of Chemistry, IIT Delhi,
for providing access to computational facilities.
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