Iridium-mediated N-H and methyl C-H bond activations in N-(2′6′- dimethylphenyl)pyrrole-2-aldimine. Synthesis, characterization and catalytic applications

Article abstract of DOI:10.1016/j.jorganchem.2013.07.066

Reaction of N-(2′,6′-dimethylphenyl)pyrrole-2-aldimine (L-Me₂) with I r(PPh₃)₃Cl in refluxing toluene affords two organometallic complexes (1 and 2), where the imine-ligand (L-Me₂) is coordinated to the metal center, via N-H and methyl C-H activations, as a di-anionic tridentate NNC-donor, along with two triphenylphosphines. In 1 the sixth coordination site is occupied by a hydride, while in 2 by a chloride. In both cases the hydride or chloride is trans to the coordinated imine-nitrogen, and the two triphenylphosphines are mutually trans. Similar reaction of N-(2′-methylphenyl)pyrrole-2-aldimine (L-Me) with Ir(PPh₃)₃Cl affords 3, where the imine-ligand is coordinated to the metal center as a mono-anionic bidentate NN-donor, along with two triphenylphosphines, a hydride and a chloride. Structures of 1, 2 and 3 have been determined by X-ray crystallography. DFT analyses have been carried out to understand the formation of the complexes. All the complexes show characteristic ¹H NMR signals and, intense transitions in the visible and ultraviolet regions. Cyclic voltammetry on all three complexes shows two irreversible oxidations within 0.89-1.34 V vs. SCE and a reduction within -1.31 to -1.40 V vs. SCE. Complexes 1, 2 and 3 have been found to efficiently catalyze the Oppenauer oxidation of alcohols.

Full text of DOI:10.1016/j.jorganchem.2013.07.066

Journal of Organometallic Chemistry 751 (2014) 760e768
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
0
0
Iridium-mediated NeH and methyl CeH bond activations in N-(2 ,6 -
dimethylphenyl)pyrrole-2-aldimine. Synthesis, characterization and
catalytic applications
a
b
a,
*
Piyali Paul , Michael G. Richmond , Samaresh Bhattacharya
a
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India
Department of Chemistry, University of North-Texas, Denton, TX 76203, USA
b
a r t i c l e i n f o
a b s t r a c t
0 0
2 3 3
Reaction of N-(2 ,6 -dimethylphenyl)pyrrole-2-aldimine (L-Me ) with Ir(PPh ) Cl in refluxing toluene
affords two organometallic complexes (1 and 2), where the imine-ligand (L-Me ) is coordinated to the
2
Article history:
Received 29 May 2013
Received in revised form
metal center, via NeH and methyl CeH activations, as a di-anionic tridentate NNC-donor, along with two
triphenylphosphines. In 1 the sixth coordination site is occupied by a hydride, while in 2 by a chloride. In
both cases the hydride or chloride is trans to the coordinated imine-nitrogen, and the two triphenyl-
24 July 2013
Accepted 25 July 2013
0
phosphines are mutually trans. Similar reaction of N-(2 -methylphenyl)pyrrole-2-aldimine (L-Me) with
Keywords:
N-(aryl)pyrrole-2-aldimines
Iridium
NeH and methyl CeH bond activations
Oppenauer oxidation
3 3
Ir(PPh ) Cl affords 3, where the imine-ligand is coordinated to the metal center as a mono-anionic
bidentate NN-donor, along with two triphenylphosphines, a hydride and a chloride. Structures of 1, 2
and 3 have been determined by X-ray crystallography. DFT analyses have been carried out to understand
the formation of the complexes. All the complexes show characteristic 1H NMR signals and, intense
transitions in the visible and ultraviolet regions. Cyclic voltammetry on all three complexes shows two
irreversible oxidations within 0.89e1.34 V vs. SCE and a reduction within ꢀ1.31 to ꢀ1.40 V vs. SCE.
Complexes 1, 2 and 3 have been found to efficiently catalyze the Oppenauer oxidation of alcohols.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
hydrido-complex of iridium(III) (A-R) as the kinetically controlled
product, which is unstable and transforms into the thermody-
Transition-metal-mediated CeH bond activation of organic
substrates continues to command persistent attention, primarily as
it is a key step in the construction of new organic molecules [1e8].
The essential prerequisites for such transition metal catalysts are
the effective mediation of CeH bond activation and ready avail-
ability of a vacant or labile coordination site(s) at the metal center.
Typically, such species contain an active MeX (X ¼ hydride or
halide) fragment that serves to facilitate the approach of the
organic substrate to the metal center. This molecular interaction
precedes the desired activation of an otherwise inert CeH bond,
followed by the release of the functionalized substrate. We have
been studying the iridium-mediated CeH bond activation of
organic ligands of diverse types, and the present work originates
from our interest in this area [9e13]. We have recently reported
namically preferred isomer (B-R) on heating (Scheme 1) [13]. Upon
modification of the pendent aryl fragment in such ligands, viz.
naphthyl replacing phenyl, in order to bring an aryl CeH fragment
into close proximity to the IreCl fragment of species A-R, we have
observed secondary activation of the pendent aryl moiety occurs
and the formation of an organoiridium complex via elimination of
HCl [13]. The facile nature of these aryl CeH activations has led us to
explore whether alkyl CeH bond activation, which is much more
difficult compared to aryl CeH activation, can be induced in similar
aldimine-based iridium complexes. For this study, we have chosen
0
0
a slightly modified N-(aryl)pyrrole-2-aldimine, viz. N-(2 ,6 -dime-
thylphenyl)pyrrole-2-aldimine (L-Me ), in which both the ortho
2
positions of the phenyl ring are substituted by methyl groups. The
primary objective of this present study was to see whether the
selected ligand can undergo dual NeH and methyl CeH activations
and display an NNC-mode of binding (I). Herein we report our re-
3 3
that the iridium(I) center in Ir(PPh ) Cl can successfully activate the
NeH bond in N-(aryl)pyrrole-2-aldimines (L-R), generating a
0 0
3 3
) Cl with N-(2 ,6 -dimethylphenyl)
sults on the reaction of Ir(PPh
pyrrole-2-aldimine (L-Me ), and a related ligand, which include the
2
formation of new complexes and, their crystal structures, DFT
modeling, and catalytic oxidation properties.
*
Corresponding author. Tel./fax: þ91 33 24146223.
E-mail address: samaresh_b@hotmail.com (S. Bhattacharya).
0
022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.07.066

P. Paul et al. / Journal of Organometallic Chemistry 751 (2014) 760e768
761
PPh
3
N
N
H
III
Ir
H
Cl
PPh
3
N
N
H
I
Ir (PPh ) Cl
R
3
3
H
A-R
PPh
3
R
N
N
Cl
H
III
Ir
L-R
H
PPh
3
R
B-R
Scheme 1
N
N
N
N
H
Ir
H
H
H C
3
CH3
H C
3
C
H
H
L-Me2
I
2
. Results and discussion
.1. Synthesis and characterization
3 3
The chosen L-Me ligand reacts with Ir(PPh ) Cl in refluxing
2
Fig. 1. (a) Molecular structure of 1 (hydrogen atoms in the organic ligands are omitted
2
for clarity), and (b) view of the equatorial plane.
toluene to yield two yellow complexes 1 and 2, which have been
separated by thin-layer chromatography over silica gel using ben-
zene as the eluant. The yield of each product is good, with 1 and 2
isolated in 33 and 38% yields, respectively. Characterization in-
dicates the presence of an imine-ligand, a hydride and two tri-
phenylphosphines in the coordination sphere of 1, and presence of
a chloride, in addition to an imine-ligand and two triphenylphos-
phines in 2. The disposition of the ligands around iridium and the
activation of an aryl methyl in the isolated products were ascer-
tained by X-ray crystallography. The structures of 1 and 2 are shown
in Figs. 1 and 2 respectively, and selected bond distances and angles
NNC atoms of the heterocyclic platform, these three ligands display
an overall mer stereochemical disposition, a feature in agreement
1
with the H NMR spectrum (vide infra). The molecular structure of 2
(Fig. 2) is similar to that of 1, except that a chloride is coordinated to
iridium instead of a hydride. In both 1 and 2, the bond distances and
angles around the metal center, as well as those within the chelated
heterocyclic ligand, are all found to be quite usual [13e20]. It is
relevant to mention here that such metal-mediated methyl CeH
bond activation is, compared to aryl CeH activation, a less common
phenomenon [21,22].
0
0
are listed in Table 1. The structure of 1 confirms that the N-(2 ,6 -
dimethylphenyl)pyrrole-2-aldimine (L-Me ) has, indeed, under-
2
gone the targeted methyl CeH activation, and coordinated to
iridium in the expected tridentate NNC-mode (I). Two triphenyl-
phosphines and a hydride are also coordinated to the metal center.
The triphenylphosphines in 1 exhibit a mutually trans arrangement,
and with the hydride, which resides in the plane defined by the
Products 1 and 2 are presumed to derive from divergent path-
ways which are controlled by the orientation of the ligands at the
iridium, following the initial coordination of the pyrrole-aldimine
ligand, via NeH bond activation, to the metal. Scheme 2 outlines
the likely sequence of events involved, which are suggested to arise

762
P. Paul et al. / Journal of Organometallic Chemistry 751 (2014) 760e768
Table 1
ꢀ
ꢁ
Selected bond lengths (A) and bond angels ( ) for 1, 2 and 3.
1
ꢀ
Bond lengths (A)
Ir(1)eH(101)
Ir(1)eN(1)
Ir(1)eN(2)
Ir(1)eC(13)
Ir(1)eP(1)
Ir(1)eP(2)
1.100(5)
2.154(9)
2.097(10)
2.106(12)
2.301(3)
2.292(3)
N(1)eC(4)
C(4)eC(5)
C(5)eN(2)
N(2)eC(6)
C(6)eC(11)
C(11)eC(13)
1.386(18)
1.405(19)
1.261(16)
1.450(15)
1.340(17)
1.524(17)
ꢁ
Bond angles ( )
P(1)eIr(1)eP(2)
N(2)eIr(1)eH(101)
N(1)eIr(1)eC(13)
170.84(11)
168.0(18)
159.6(4)
N(1)eIr(1)eN(2)
N(2)eIr(1)eC(13)
77.90(4)
82.0(2)
2
ꢀ
Bond lengths (A)
Ir(1)eCl(1)
Ir(1)eN(1)
Ir(1)eN(2)
Ir(1)eC(13)
Ir(1)eP(1)
Ir(1)eP(2)
2.4047(11)
2.179(4)
2.031(4)
2.098(4)
2.3619(12)
2.3536(11)
N(1)eC(4)
C(4)eC(5)
C(5)eN(2)
N(2)eC(6)
C(6)eC(11)
C(11)eC(13)
1.387(7)
1.376(7)
1.317(6)
1.414(7)
1.390(7)
1.490(7)
ꢁ
Bond angles ( )
P(1)eIr(1)eP(2)
N(2)eIr(1)eCl(1)
N(1)eIr(1)eC(13)
179.37(4)
179.56(11)
161.08(14)
N(1)eIr(1)eN(2)
N(2)eIr(1)eC(13)
79.54(15)
81.54(15)
3
ꢀ
Bond lengths (A)
Ir(1)eH(101)
Ir(1)eCl(1)
Ir(1)eN(1)
Ir(1)eN(2)
Ir(1)eP(1)
Ir(1)eP(2)
1.460(4)
2.393(11)
2.033(3)
2.219(3)
2.313(11)
2.345(11)
N(1)eC(4)
C(4)eC(5)
C(5)eN(2)
1.374(5)
1.403(5)
1.305(5)
ꢁ
Bond angles ( )
P(1)eIr(1)eP(2)
N(1)eIr(1)eCl(1)
N(2)eIr(1)eH(101)
169.6(4)
176.6(10)
173.3(16)
N(1)eIr(1)eN(2)
77.4(12)
The facile activation of one of the methyl substituents that ac-
companies the formation of 1 and 2 prompted us to explore the
generality of this reaction with a pyrrole-aldimine having only one
0
aryl methyl group. Thus, we have prepared N-(2 -methylphenyl)
pyrrole-2-aldimine (L-Me) and have investigated its reactivity with
Ir(PPh ) Cl under identical conditions. Given the structurally
3 3
different ortho sites associated at the aryl ring, two types of coor-
dination modes are, in principle, possible for this ligand, (i) NNC-
mode (II) via methyl CeH activation or (ii) simple NN-mode (III)
0
without any methyl CeH activation. Treatment of N-(2 -methyl-
3 3
phenyl)pyrrole-2-aldimine (L-Me) with Ir(PPh ) Cl in refluxing
toluene furnished a single product (3), which was subsequently
1
isolated in 56% yield by thin-layer chromatography. The H NMR
spectrum of 3 shows the presence of a high-field triplet and singlet
resonances at ꢀ13.53 ppm and 1.85 ppm for the hydride and intact
methyl group, respectively, with the remaining hydrogens on the
heterocycle and triphenylphosphines appearing from 6.29 to
7.97 ppm. For an unambiguous characterization of this complex,
with particular reference to coordination mode of the imine-ligand
at the iridium center, its crystal structure has also been determined.
The structure of 3 is shown in Fig. 4, where the coordination of the
Fig. 2. (a) Molecular structure of 2 (hydrogen atoms are omitted for clarity), and (b)
view of the equatorial plane.
from the isomeric hydrido-chlorides (A and B) generated in the
initial step. Loss of HCl from species A and hydrogen from species B
affords the final products 1 and 2, respectively. The intermediate
species were investigated by DFT analysis and their optimized
structures are shown in Fig. 3. While the free-energy difference
between the two intermediates lies in favor of species A by 5.9 kcal/
0
N-(2 -methylphenyl)pyrrole-2-aldimine (L-Me) and the presence
of an intact methyl group are confirmed. The heterocyclic ligand is
bound to iridium as a mono-anionic bidentate NN-donor (III), and
the chloride and hydride ligands reside in the plane containing the
NN-donor. Two triphenylphosphines and a chloride are also coor-
dinated to the metal center. The NN-coordinated imine-ligand is
sharing the same equatorial plane with the chloride and hydride,
where the chloride is trans to the pyrrole-nitrogen and the hydride
mol at room temperature, the temperature-corrected value for DG
at 383 K is smaller and favors A by less than 0.4 kcal/mol. In
refluxing toluene, reasonable concentrations of species A and B are
expected, and the ensuing CeH bond activation of a methyl group
in each species readily accounts for the observed cyclometalated
products 1 and 2, respectively.

P. Paul et al. / Journal of Organometallic Chemistry 751 (2014) 760e768
763
P = PPh3
P
P
N
N
N
N
N
H
isomerization
Cl
H
H
I
Ir (PPh ) Cl
III
III
3
3
Ir
Ir
-
PPh3
H
H
N
Cl
H
P
CH3
P
CH3
H C
3
CH3
H C
3
H C
3
L-Me2
A
B
-
HCl
-
H2
P
P
N
N
N
N
H
Cl
Ir
P
Ir
P
H
C
H
H
C H
H
H
H C
3
H C
3
1
2
Scheme 2. Probable steps behind the formation of 1 and 2.
Fig. 3. DFT-optimized structures of the intermediate species A and B depicted in Scheme 2.
N
N
N
N
N
N
H
Ir
C
Ir
H
H
H
CH3
H C
3
H
H
L-Me
II
III

764
P. Paul et al. / Journal of Organometallic Chemistry 751 (2014) 760e768
2.2. Spectral studies
Magnetic susceptibility measurements show that all the three
iridium complexes are diamagnetic, which corresponds to the þ3
6
1
oxidation state of iridium (low-spin d , S ¼ 0). The IR and H NMR
spectroscopic data are consistent with the observed solid state
structures of 1e3. Complexes 1, 2 and 3 are found to be readily
soluble in all common organic solvents producing yellow solutions.
Electronic spectra of 1, 2 and 3 have been recorded in dichloro-
methane solution. Each complex shows several intense absorptions
in the visible and ultraviolet regions (Table 2). The absorptions in
the ultraviolet region are attributable to usual transitions within
the ligand orbitals. In order to understand the nature of absorptions
in the visible region, DFT calculations have been performed on 1, 2
and 3. Compositions of the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO) are
given in Table 3. Contour plots of these molecular orbitals for 1 are
shown in Fig. 6 and the same for 2 and 3 are shown in Figs. S1 and
S2 (Supplementary material) respectively. In 1, 2 and 3 both the
HOMO and LUMO have a major contribution from the imine ligand
and minor contribution from iridium and triphenylphosphine. In 2
and 3, the HOMO also has some contribution from the coordinated
chloride. The lowest energy absorption in all these complexes may
therefore correspond to a ligand based transition.
2.3. Electrochemical properties
Electrochemical properties of the three iridium complexes have
been studied by cyclic voltammetry in acetonitrile solution (0.1 M
TBHP). Each complex shows two oxidative responses on the posi-
tive side of SCE and a reductive response on the negative side
(Table 2). All responses were found to be irreversible. In view of
composition of the HOMO in all these complexes, the first oxidative
response is assigned to oxidation of the coordinated imine-ligand.
The second oxidation is also believed to be ligand centered. Based
on the composition of the LUMO, the reductive response is assigned
to reduction of the coordinated imine-ligand.
2.4. Catalysis
Iridium complexes are well known to serve as efficient catalysts
in Oppenauer-type oxidation of alcohols [23e30], which is a
promising method as the non-noxious acetone is used as an
oxidant. This has led us to explore such catalytic properties in 1e3.
We first examined the oxidation of 4-methoxybenzylalcohol to 4-
methoxybenzaldehyde and, after several trials for optimization,
we have observed that the best results are obtained with 0.1 mol%
ꢁ
of catalyst, K
2
CO
3
as base, 40 C temperature and 5 h reaction time
Fig. 4. (a) Molecular structure of 3 (hydrogen atoms in the organic ligands are omitted
for clarity), and (b) view of the equatorial plane.
(entries 1e3 in Table 4). After successful oxidation of 4-
methoxybenzylalcohol, we tried oxidation of several other 4-R-
benzylalcohols (R ¼ CH
3 2
, H, Cl, Br, I, and NO ). With the exception of
-nitrobenzylalcohol, all other alcohols were easily oxidized (en-
4
is trans to the imine-nitrogen. The two triphenylphosphines are
mutually trans, as in the other two complexes. The metrical pa-
rameters in this structure (Table 1) are found to be comparable to
those found in similar complexes [13].
The model compound for 3 (referred as 3A) was also inves-
tigated by DFT, and the optimized structure of 3A is shown in
Fig. 5. The effect of interchanging the H/Cl groups in 3A was next
examined to confirm the thermodynamically preferred isomer
of 3. The structure of the hydrido-chloride isomer 3B was
studied by DFT, and the optimized structure is also displayed in
Fig. 5. Compound 3A is found to be the thermodynamically more
stable of the two isomers, lying 2.8 kcal/mol lower in energy
than 3B.
tries 4e18). However, slightly longer (6e8 h) reaction time was
needed for the alcohols with R ¼ H, Cl, Br and I. Oxidation of 4-
nitrobenzylalcohol required much higher (1.0 mol%) catalyst
loading and longer (12 h) reaction time, and the oxidized product
was obtained in much lower (40e57%) yield (entries 19e21).
The observed catalytic efficiency of complexes 1e3 in bringing
about Oppenauer oxidation of primary alcohols led us to explore
similar oxidation of secondary alcohols as well. Oxidation of 1-
phenylethanol was attempted first, which was observed to un-
ꢁ
dergo facile oxidation with 0.1 mol% catalyst, K
2
CO
3
as base, 40 C
temperature and 4 h reaction time to afford acetophenone in w90%
yield (entries 1e3 in Table 5). Oxidation of benzhydrol, cyclo-
hexanol and cyclopentanol was also achieved similarly to furnish

P. Paul et al. / Journal of Organometallic Chemistry 751 (2014) 760e768
765
Fig. 5. DFT optimized structures of 3A and 3B.
the corresponding ketones in good yields (entries 4e12). For the
oxidation of cyclohexanol, triethylamine needed to be used instead
Aldrich. All other chemicals and solvents were reagent grade
commercial materials and were used as received.
2 3
of K CO (entries 7e9). The iridium complexes 1, 2 and 3 are thus
found to be efficient catalysts for Oppenauer oxidation of primary,
as well as secondary, alcohols. Complexes 1 and 2 are, in general,
found to be slightly better catalysts than 3.
4
.2. Physical measurements
Microanalyses (C, H, N) were performed using a Heraeus Carlo
Erba 1108 elemental analyzer. Magnetic susceptibilities were
measured using a Sherwood MK-1 balance. IR spectra were ob-
tained on a PerkineElmer Spectrum Two spectrometer with sam-
ples prepared as KBr pellets. Electronic spectra were recorded on a
JASCO V-570 spectrophotometer. H NMR spectra were recorded in
3
CDCl solution with a Bruker Avance DPX 300 NMR spectrometer
using TMS as the internal standard. Electrochemical measurements
were made using a CH Instruments model 600A electrochemical
analyzer. A platinum disc working electrode, a platinum wire
auxiliary electrode and an aqueous saturated calomel reference
electrode (SCE) were used in the cyclic voltammetry experiments.
All electrochemical experiments were performed under a dini-
trogen atmosphere. All electrochemical data were collected at
3
. Conclusions
The pyrrole NeH bond in N-(2 ,6 -dimethylphenyl)pyrrole-2-
0
0
0
aldimine (L-Me
2
), or in N-(2 -methylphenyl)pyrrole-2-aldimine (L-
1
Me), undergoes facile activation upon interaction with the metal
center in Ir(PPh Cl to afford hydrido-iridium species. In case of L-
Me , the unavoidable proximity of methyl group to the iridium-bound
hydride orchloride results inactivation ofa methyl CeH bond yielding
organoiridium complexes. These iridium complexes can efficiently
catalyze Oppenauer oxidation of primary and secondary alcohols.
3 3
)
2
4
. Experimental
2
98 K and are uncorrected for junction potentials. GCeMS analyses
4.1. Materials
were performed using a Perkin Elmer CLARUS 680 instrument.
Iridium trichloride was purchased from Arora Matthey, Kolkata,
4
4
.3. Syntheses of complexes
India. Pyrrole-2-aldehyde was obtained from Spectrochem, Mum-
bai, India, and 2,6-dimethylaniline was procured from Lancaster,
England. Triphenylphosphine and 2-methylaniline were purchased
.3.1. Complexes 1 and 2
0 0
from S.D. Fine-Chem Limited, Mumbai, India. Ir(PPh
3
)
3
Cl was pre-
N-(2 ,6 -dimethylphenyl)pyrrole-2-aldimine (20 mg, 0.10 mmol)
0
0
pared by following a reported method [9]. N-(2 ,6 -dimethyl-
was dissolved in warm toluene (40 mL). A stream of nitrogen was
0
phenyl)pyrrole-2-aldimine (L-Me
2
)
and N-(2 -methylphenyl)
Table 3
pyrrole-2-aldimine (L-Me) were prepared by condensing pyrrole-
-aldehyde with the respective anilines in hot ethanol. Tetrabuty-
Composition of selected molecular orbitals of the complexes.
2
lammonium hexafluorophosphate (TBHP) was purchased from
Complex
Fragments
% Contribution of fragments
to
Table 2
HOMO
LUMO
Electronic spectral and cyclic voltammetric data.
a
1
Ir
PPh
3
Imine ligand
Hydride
11.1
0.8
88.1
0
4.7
2.7
92.6
0
Complex
Electronic spectral data
Cyclic voltammetric
data E/V vs. SCE
ꢀ1
ꢀ1
b
l
max, nm (
3 , M cm
)
1
430 (34,200), 412 (39,200), 392c
1.29,d 0.87,d ꢀ1.40e
1.30,d 0.89,d ꢀ1.43e
(
31,400), 268 (44,500)
2
3
Ir
PPh
3
Imine ligand
Chloride
10.9
0.6
80.7
7.8
4.4
2.3
93.3
0
2
442 (25,500), 420 (29,500), 402c
28,200), 266 (79,300)
(
3
376 (19,200), 289 (18,000)
In dichloromethane.
Solvent, acetonitrile; supporting electrolyte, TBHP; scan rate 50 mV s
Shoulder.
1.34,d 0.98,d ꢀ1.31e
Ir
PPh
Imine ligand
Chloride
Hydride
11.5
0.3
81.1
7.1
0
3.8
2.3
93.9
0
a
b
c
ꢀ1
3
.
d
e
E
E
pa value.
pc value.
0

766
P. Paul et al. / Journal of Organometallic Chemistry 751 (2014) 760e768
Table 4
a
Oppenauer oxidation of primary alcohols.
Entry
1
Substrate
Time (h) Catalyst Catalyst (mol %) Yieldb (%) TONc
5
5
5
5
1
2
3
1
0.1
0.1
0.1
0.1
93
95
81
91
930
950
810
910
2
3
4
5
6
5
5
2
3
0.1
0.1
89
73
890
730
7
8
9
6
6
6
8
1
2
3
1
0.1
0.1
0.1
0.1
77
79
64
83
770
790
640
830
1
1
1
0
1
2
8
8
2
3
0.1
0.1
78
68
780
680
1
1
1
1
1
3
4
5
6
7
8
8
1
2
3
1
2
3
1
0.1
0.1
0.1
0.1
0.1
0.1
1
86
84
71
88
82
72
57
860
840
710
880
820
720
57
8
8
8
Fig. 6. Contour plots of the HOMO and LUMO of 1.
18
8
1
9
0
12
passed through this solution for 5 min, and then Ir(PPh
3 3
) Cl (100 mg,
2
12
12
2
3
1
1
53
40
53
40
0
.10 mmol) was added to it. The mixture was refluxed for 24 h under
a nitrogen atmosphere to yield a yellowish-brown solution. The
solvent was evaporated and the solid mass, thus obtained, was
subjected to purification by thin layer chromatography on a silica
plate. With benzene as the eluant, two yellow bands separated. The
first band was extracted with acetonitrile, and evaporation of this
extract yielded 1. Similarly, the second band was also extracted with
acetonitrile, and its evaporation gave 2.
21
a
Reaction conditions: p-substituted benzyl alcohol (1.0 mmol),
2 3
K CO
(
1.7 mmol), Ir catalyst, acetone (8 mL).
b
Determined by GCeMS on the basis of residual p-substituted benzyl alcohol.
TON ¼ turnover number ((mol of product)/(mol of catalyst)).
c

P. Paul et al. / Journal of Organometallic Chemistry 751 (2014) 760e768
767
Table 5
6.13 (d, H, J ¼ 7.0); 6.27 (d, H, J ¼ 7.5); 6.49 (d, H, J ¼ 7.5); 6.54 (t, H,
J ¼ 8.3); 6.80 (d, H, J ¼ 7.9); 7.13e7.69 (m, 31H). IR: 1634, 1483, 1435,
a
Oppenauer oxidation of secondary alcohols.
ꢀ
1
1
384, 1308, 1188, 1119, 1092, 1028, 745, 722, 693, 542 and 521 cm .
0
.3.1.3. Complex 3. N-(2 -methylphenyl)pyrrole-2-aldimine (20
4
mg, 0.11 mmol) was dissolved in warm toluene (40 mL). A stream of
nitrogen was passed through this solution for 5 min, and then
Ir(PPh ) Cl (100 mg, 0.10 mmol) was added to it. The mixture was
3 3
refluxed for 24 h under a nitrogen atmosphere to yield a yellowish-
brown solution. The solvent was evaporated and the solid mass,
thus obtained, was subjected to purification by thin layer chro-
matography on a silica plate. With benzene as the eluant, a yellow
band separated, which was extracted with acetonitrile, and evap-
oration of this extract yielded 3. Yield: 60 mg (56%). Anal. Calc. for
Entry
1
Substrate
Time (h)
4
Catalyst
Base
Yieldb (%)
90
TONc
900
1
K
K
K
2
CO
2
CO
2
CO
3
3
3
C
48
H
42
N
2
ClP
N, 2.93. Complex 3: H NMR [31]: ꢀ13.53 (t, hydride, J ¼ 16.2); 1.85
CH
); 6.29 (t, H, J ¼ 6.2); 6.68 (d, 2H, J ¼ 7.5); 6.88 (d, 2H, J ¼ 7.6);
.10e7.70 (m, 31H); 7.88 (d, H, J ¼ 8.8); 7.97 (t, H, J ¼ 8.5). IR: 2187,
2
Ir: C, 61.56; H, 4.49; N, 2.99. Found: C, 61.59; H, 4.44;
2
3
4
4
2
3
93
84
930
840
1
(
7
3
1636, 1482, 1435, 1386, 1307, 1190, 1119, 1092, 1027, 746, 723, 694,
ꢀ1
542 and 520 cm
.
4
5
6
7
8
9
5
5
5
5
5
5
4
1
2
3
1
2
3
1
K
K
K
2
2
2
CO
CO
CO
3
3
3
89
85
78
67
61
52
77
890
850
780
670
610
520
770
4
.4. X-ray crystallography
Single crystals of 1, 2 and 3 were obtained by slow evaporation
of solvent from solutions of the respective complexes in acetoni-
trile. Selected crystal data and data collection parameters are given
in Table 6. Data were collected on a Bruker SMART CCD diffrac-
tometer using graphite monochromated MoK
a
radiation
¼ 0.71073 A). X-ray data reduction, structure solution and
refinement were done using SHELXS-97 and SHELXL-97 programs
32]. The structures were solved by the direct methods.
ꢀ
(l
[
NEt
NEt
NEt
3
3
3
4.5. Computational methodology and modeling details
The reported calculations were performed with the hybrid DFT
functional B3LYP, as implemented by the Gaussian 09 program
package [33]. This functional utilizes the Becke three-parameter
exchange functional (B3) [34], combined with the correlation
functional of Lee, Yang and Parr (LYP) [35]. The Ir atoms were
described using the LANL2DZ effective core potential (ecp) and
1
1
0
K
2
CO
3
Table 6
Crystallographic data for 1, 2 and 3.
1
2
4
4
2
3
K
K
2
CO
3
3
73
61
730
610
1
2
3
Empirical formula
Formula weight
Crystal system
Space group
C
49
H
43
N
2
P
2
Ir
C
51
H
N
45 3
2
ClP Ir
C
50 45 3 2
H N ClP Ir
977.5
Triclinic
1
2
CO
913.2
Triclinic
P1
988.7
Monoclinic
P2 /c
10.2062(3)
22.8379(8)
18.4586(6)
90
97.765(2)
90
4263.0(2)
4
1
P1
a
Reaction conditions: p-substituted benzyl alcohol (1.0 mmol),
K
2
CO
3
ꢀ
a (A)
10.0209(2)
11.8327(2)
17.2373(3)
88.562(1)
88.764(1)
77.415(1)
1993.89(6)
1
9.6410(2)
11.8637(3)
20.1526(4)
106.941(1)
95.115(1)
92.058(1)
2191.57(8)
2
(
1.5 mmol), Ir catalyst, acetone (8 mL).
ꢀ
b (A)
b
Determined by GCMS on the basis of residual secondary alcohol.
TON ¼ turnover number ((mole of product)/(mole of catalyst)).
ꢀ
c (A)
c
ꢁ
a
b
g
( )
ꢁ
( )
ꢁ
( )
ꢀ
3
V (A )
Z
4
.3.1.1. Complex 1. Yield: 30 mg (33%). Anal. Calc. for C49
C, 64.39; H, 4.71; N, 3.07. Found: C, 64.72; H, 4.75; N, 3.03%. H NMR
H
43
1
N
2
P
2
Ir:
F (000)
915
1972
978
[
31]: ꢀ15.95 (t, hydride, J ¼ 15.0); 2.15 (CH
3 2
); 2.52 (s, CH ); 5.29 (t,
Crystal size (mm) 0.17 ꢂ 0.12 ꢂ 0.10 0.21 ꢂ 0.16 ꢂ 0.13 0.25 ꢂ 0.19 ꢂ 0.14
H, J ¼ 8.2); 5.92 (d, H, J ¼ 7.0); 6.28 (d, H, J ¼ 7.5), 6.44 (d, H, J ¼ 7.5),
T (K)
298
298
298
m
(mmꢀ1)
3.466
0.0299
0.0698
0.87
3.309
0.0335
0.1125
0.77
3.218
0.0350
0.0802
0.99
6
1
5
.59 (t, H, J ¼ 8.3), 6.85 (d, H, J ¼ 7.9); 7.13e7.67 (m, 31H). IR: 2198,
a
R1
639, 1483, 1435, 1384, 1301, 1188, 1118, 1092, 1030, 744, 721, 695,
b
wR2
GOF
ꢀ
1
42 and 519 cm
.
c
P
P
a
R
wR
1¼ jjF
o
j ꢀ j:F
c
j:j= jF
o
j.
4
C
.3.1.2. Complex 2. Yield: 36 mg (38%). Anal. Calc. for
ClP Ir: C, 62.05; H, 4.43; N, 2.95. Found: C, 62.41; H, 4.46;
N, 2.93%. H NMR [31]: 2.17 (CH ); 3.78 (s, CH
); 5.29 (t, H, J ¼ 7.9);
b
c
¼ ½ ½wðF ꢀ F Þ ꢃ= ½wðF Þ ꢃꢃ1=2.
P
2
2
2
P
2
2
2
o
c
o
P
H
49 42
N
2
2
2
2
2
GOF ¼ ½ ½wðF ꢀ F Þ ꢃ=ðM ꢀ NÞꢃ, where M is the number of reflections and N is
o
c
1
3
2
the number of parameters refined.

768
P. Paul et al. / Journal of Organometallic Chemistry 751 (2014) 760e768
basis set of Hay and Wadt [36], with the 6p orbitals replaced with
the split valence functions from Couty and Hall [37]. The P and Cl
atoms were described using the LANL2DZ(d) ecp basis set, with
Frenking’s d orbital contribution added to the P and Cl atoms
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eigenvalues obtained from the analytical Hessian. The computed
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3
[
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3377.
[
[
[
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In a typical run, an oven-dried round bottom flask was charged
with a known mole percent of catalyst, K CO (1.7 mmol), primary
2 3
or secondary alcohol (1.0 mmol) in acetone medium (8 mL). The
ꢁ
mixture was stirred at 40 C. After the specified time the reaction
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organic layer was washed with water (3 ꢂ 10 mL), and dried over
2 4
anhydrous Na SO . Solvent was removed under vacuum. The res-
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4927.
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Acknowledgments
[
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associated coupling constants (J in Hz) are given in parentheses. Overlapping
signals are marked with an asterisk
The authors thank the reviewers for their constructive com-
ments, which have been very helpful in preparing the revised
manuscript. Financial assistance received from the Department of
Science and Technology, New Delhi, India [Grant Nos. SR/S1/IC-29/
[
32] G.M. Sheldrick, SHELXS-97 and SHELXL-97, Fortran Programs for Crystal
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2
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II] is gratefully acknowledged. Piyali Paul thanks the Council of
Scientific and Industrial Research, New Delhi, for her fellowship
[33] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb,
J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson,
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K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari,
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[Grant No. 09/096(0588)/2009-EMR-I]. Financial support from the
Robert A. Welch Foundation (Grant B-1093-MGR) is greatly appre-
ciated. NSF support (CHE-0741936) of the computational facilities at
UNT is acknowledged. We also wish to thank Prof. Michael B. Hall
(
TAMU) for providing us a copy of his JIMP2 program, which was
used to prepare the geometry-optimized structures reported here.
Appendix A. Supplementary material
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