Rhodium(III) NCN pincer complexes catalyzed transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.tetlet.2009.09.152

Air-stable monomeric rhodium(III) NCN pincer complexes were synthesized via direct C-H bond activation of 1,3-bis(2-pyridyloxy)benzene, 3,5-bis(2-pyridyloxy)toluene and 3,5-bis(2-pyridyloxy)anisole with RhCl₃·3H₂O in ethanol under reflux. The synthesized complexes were characterized by elemental analysis and ¹H NMR. One of the complexes was structurally characterized by X-ray analysis. An investigation into the catalytic activity of the complex 1a as catalyst for transfer hydrogenation of ketones to alcohols at 82 °C in the presence of iPrOH/KOH was undertaken with the conversions up to 99%.

Full text of DOI:10.1016/j.tetlet.2009.09.152

Tetrahedron Letters 50 (2009) 7014–7017
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Rhodium(III) NCN pincer complexes catalyzed transfer hydrogenation
of ketones
b
Mathiyazhagan Ulaganatha Raja a, Rengan Ramesh a, , Kyo Han Ahn
*
a School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, Tamil Nadu, India
b Department of Chemistry, POSTECH, San 31 Hyoja-dong, Pohang 790-784, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Air-stable monomeric rhodium(III) NCN pincer complexes were synthesized via direct C–H bond activa-
tion of 1,3-bis(2-pyridyloxy)benzene, 3,5-bis(2-pyridyloxy)toluene and 3,5-bis(2-pyridyloxy)anisole
with RhCl3Á3H2O in ethanol under reflux. The synthesized complexes were characterized by elemental
analysis and 1H NMR. One of the complexes was structurally characterized by X-ray analysis. An inves-
tigation into the catalytic activity of the complex 1a as catalyst for transfer hydrogenation of ketones to
alcohols at 82 °C in the presence of iPrOH/KOH was undertaken with the conversions up to 99%.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 26 August 2009
Revised 24 September 2009
Accepted 28 September 2009
Available online 1 October 2009
Keywords:
NCN donors
Rhodium(III) NCN pincer complexes
Characterization
Catalytic transfer hydrogenation
The cyclometalation reaction or intramolecular C–H bond acti-
vation, which is a major achievement of organometallic chemistry,
provides access to metallacyclic derivatives of the transition metal
including pincer metal complexes.1–7 Direct cyclometalation is one
of the attractive methods for the formation of a new M–C bond,
since it does not require prefunctionalization of the pincer ligand
in order to achieve regioselective metalation.8 Direct cyclometala-
tion of NCN pincer ligands is less common, and the topic was re-
viewed in 1998.9 Pincer-based metal complexes possess a unique
balance of stability versus reactivity which can be controlled by
systematic ligand modifications and/or variation of the metal
centre, allowing enhancement of metal complex stability, reactiv-
ity and reaction selectivity.10–14
Transfer hydrogenation of unsaturated compounds like ketones
and imines using iPrOH/KOH as hydrogen source is a widely inves-
tigated reaction that is promoted by many transition metal
complexes.15 Ru, Rh and Ir bearing nitrogen or phosphorus con-
taining ligands are most active, which allow the easy formation
of catalytically active hydride species.16–18 Particularly, ruthe-
nium-based pincer complexes have proven to be highly efficient
catalysts for the transfer hydrogenation of ketones.19–25 Albrecht
and co-workers reported the use of rhodium(III) complexes con-
taining carbenes as active catalysts for transfer hydrogenation of
ketones in the presence of iPrOH/KOH.26,27 Rhodium(III) complexes
with a tripodal bis(imidazolylidene) ligand was effectively used as
a catalyst for transfer hydrogenation of ketones.28 Further, asym-
metric transfer hydrogenation of ketones was successfully carried
out with a rhodium(III) arene complex.29 In contrast, the rho-
dium(III) complexes containing NCN pincer ligand as catalyst in
transfer hydrogenation of ketones has not been reported.
In recent years, we have reported the synthesis and catalytic
applications of a number of ruthenium complexes.30–33 In view of
the promising results obtained using ruthenium complexes, we
focused our interest on rhodium pincer complexes for investiga-
tion. Herein, we report the synthesis of new air-stable rhodium(III)
NCN pincer complexes and their catalytic application towards
transfer hydrogenation of ketones.
1,3-Bis(2-pyridyloxy)benzene (HL1), 3,5-bis(2-pyridyloxy)tolu-
ene (HL2) and 3,5-bis(2-pyridyloxy)anisole (HL3)34,35 readily reacts
with RhCl3Á3H2O in refluxing ethanol in equimolar ratio for 4 h to
give the rhodium pincer complexes (Scheme 1). The C–H activation
at the phenyl ring of the NCN ligand led to the formation of the
rhodium NCN pincer complexes composed of two six-membered
metallacycles in 71–82% isolated yield. It has been observed that
the pincer ligands replace two water molecules and one chloride
ion from the starting precursor and the oxidation state of rhodium
remains unchanged during course of cyclometalation. All the com-
plexes are found to be air stable and are soluble in common organic
solvents such as methanol, ethanol and acetonitrile. Elemental
analysis (C, H and N) of the synthesized complexes were consistent
with the composition proposed for all the complexes.
The rhodium pincer complexes were characterized by 1H NMR
by using DMSO-d6 solvent. In 1H NMR, all the aromatic protons
in the complexes appeared as a multiplet in the region of
6.9–8.7 ppm. The central –CH proton of the phenyl ring in NCN
* Corresponding author. Tel.: +91 431 2407053; fax: +91 431 2407045.
E-mail address: ramesh_bdu@yahoo.com (R. Ramesh).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.09.152

M. U. Raja et al. / Tetrahedron Letters 50 (2009) 7014–7017
7015
The bond angles for C(11)–Rh(1)–N(1) and C(11)–Rh(1)–N(2)
are 88.94(16)° and 89.13(17)°, respectively, which are very close
to 90°, and thus a less bond angle strain is expected around the me-
tal than the five-membered analogues.36 The two chlorine atoms
are mutually trans to each other with almost equal distance from
the metal center.
O
O
O
Cl
C-H Bond
Activation
N
N
N
Rh OH2
N
R
H
R
RhCl3.3H2O
Cl
O
Selected bond lengths (Å): C(11)–Rh(1) 1.952(4), Cl(1)–Rh(1)
2.3362(11), Cl(2)–Rh(1) 2.3329(11), N(1)–Rh(1) 2.047(4), N(2)–
Rh(1) 2.051(4), O(3)–Rh(1) 2.229(4). Selected bond angles (°):
C(11)–Rh(1)–N(1) 88.94(16), C(11)–Rh(1)–N(2) 89.13(17), N(1)–
Rh(1)–N(2) 178.00(17), C(11)–Rh(1)–O(3) 177.27(18), N(1)–Rh(1)–
O(3) 88.37(15), N(2)–Rh(1)–O(3) 93.57(16), C(11)–Rh(1)–Cl(2)
91.42(13), N(1)–Rh(1)–Cl(2) 90.73(10), N(2)–Rh(1)–Cl(2) 89.81(10),
O(3)–Rh(1)–Cl(2) 88.20(12), C(11)–Rh(1)–Cl(1) 90.76(13), N(1)–
Rh(1)–Cl(1) 89.18(10), N(2)–Rh(1)–Cl(1) 90.35(10) O(3)–Rh(1)–
Cl(1) 89.61(12), Cl(2)–Rh(1)–Cl(1) 177.81(5).
R = H
R = CH3 : HL2
R = OCH3 : HL3
: HL1
R = H
R = CH3
R = OCH3
:
:
:
1a
1b
1c
1a = 1,3-bis(2-pyridyloxy)phenyl)aquorhodium(III)
dichloride (yield = 82%); 1b = 3,5-bis(2-
pyridyloxy)tolyl)aquo rhodium(III)dichloride (yield = 75%)
and 1c = 3,5-bis(2-pyridyloxy)anisolyl)aquo rhodium(III)
dichloride (yield = 71%).
The synthesized rhodium(III) pincer complex 1a catalyzes the
hydrogenation of ketones via hydrogen transfer from iPrOH/KOH
at 82 °C (Scheme 2).37
Scheme 1.
Using complex 1a, both aliphatic and aromatic ketones were
efficiently reduced to their corresponding alcohols in 80–99% con-
versions (Table 1). The conversion of ketone to secondary alcohol
in the case of acetophenone was 99% (entry 1, 24 h), which takes
place at a faster rate than that for benzophenone (88%, entry 4).
The presence of electron-withdrawing (Cl) or electron-donating
(OCH3) substituent on acetophenone (entries 2 and 3) has signifi-
cant effect on the reduction of ketones to their corresponding alco-
hols. The maximum conversion of 4-methoxy acetophenone to
ligands (6.94 ppm for HL1, 6.66 ppm for HL2 and 6.45 ppm for HL3)
disappeared as pincer complexes were formed, together with con-
comitant chemical shift changes. A singlet appeared at 2.35 and
3.73 ppm due to the presence of methyl and methoxy protons in
the complexes 1b and 1c, respectively.
Further, the molecular structure of the pincer complex 1a was
resolved by single crystal X-ray crystallography. A chloroform/
methanol solution of complex 1a was slowly evaporated at room
temperature to afford a single crystal suitable for X-ray crystallo-
graphic study. There are two crystallographically independent
molecules present in the asymmetric unit cell of the complex 1a.
Both molecules have essentially identical coordination geometries,
but the corresponding bond lengths and bond angles are slightly
different. An ORTEP view of the complex 1a (Fig. 1) shows clearly
that the pincer ligand is coordinated to the rhodium metal via
two pyridyl nitrogens and one aryl carbon in a tridentate fashion
in addition to two Cl and one H2O groups. A distorted octahedral
geometry is observed as reflected in all the bond parameters
around rhodium.
O
OH
R'
catalyst 1a
KOH / iPrOH, 82 o
C
R
R'
R
Scheme 2.
Figure 1. ORTEP diagram of the complex 1a.

7016
M. U. Raja et al. / Tetrahedron Letters 50 (2009) 7014–7017
Table 1
corresponding alcohol was achieved over a period of 24 h (entry 3)
and that for 4-chloro acetophenone was 98%. Similarly, 2-buta-
none, 2-pentanone and 3-pentanone (entries 10–12) underwent
hydrogenation to afford the corresponding alcohols in 70%, 73%
and 71% conversions, respectively, in 10 h and achieved maximum
conversions in 24 h. Interestingly, this catalyst shows excellent
activity for the conversions of six-, seven- and eight-membered
cyclic ketones (entries 5–9) to their corresponding alcohols with
88% conversion in the case of cyclopentanone and conversions
98% in case of cyclohexanone, 3-methyl cyclohexanone, cyclohep-
tanone and cyclooctanone.
In conclusion, we have demonstrated a facile and general syn-
thetic route to rhodium(III) NCN pincer complexes via direct cyclo-
metalation. The synthesized rhodium(III) pincer complexes are air
stable and composed of two six-membered metallacycles with
reduced bond angle strain around rhodium. The X-ray crystal
structure of complex 1a shows an octahedral geometry around
rhodium(III) ion with little distortion. An initial foray into the
utility of the complex 1a demonstrates its ability to form active
catalytic species for use in transfer hydrogenation of ketones.
Catalytic transfer hydrogenation of ketones using complex 1a/iPrOH/KOHa
Entry
1
Ketone
Time (h)
Conversion (%)
TON
O
10/24
94/99
188/198
O
2
3
10/24
10/24
82/98
69/80
164/196
138/160
Cl
O
Me
O
O
4
5
10/24
10/24
83/88
72/88
166/176
144/176
Acknowledgements
We gratefully acknowledge financial support from the Depart-
ment of Science and Technology (DST Ref. No. SR/S1/IC-03/2007)
New Delhi, India. M.U.R. thanks DST for the award of Junior Re-
search Fellowship (JRF). We thank Department of Chemistry, IIT-
Madras, Chennai, for providing X-ray data.
O
O
O
Supplementary data
6
10/24
95/99
190/198
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.09.152.
References and notes
7
8
10/24
10/24
93/99
89/98
186/198
178/196
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catalyst (0.01 mmol) and iPrOH (5.0 mL) was heated to 82 °C. At the desired
reaction times, aliquots were extracted from the reaction vessel, extracted
using diethyl ether, filtered through a short path of silica and the silica was
washed with diethyl ether. The combined organic filtrates were evaporated
and analyzed by GC or GC–MS.