Structure–activity comparison in palladium–N–heterocyclic carbene (NHC) catalyzed arene C[sbnd]H activation- functionalization

Article abstract of DOI:10.1016/j.molcata.2016.06.027

A simple and efficient C[sbnd]H activation catalyst was identified through a model structure-activity screening applied to a noncooperative, nonsymmetric bimetallic palladium(II)-N-heterocyclic carbene complex. Mechanistic studies based on kinetics and DOSY NMR spectroscopy provided the origin of the higher efficiency of the identified catalyst.

Full text of DOI:10.1016/j.molcata.2016.06.027

Accepted Manuscript
Title: Structure–Activity Comparison in
Palladium–N-Heterocyclic Carbene (NHC) Catalyzed Arene
C<ce:glyph name="sbnd"/>H Activation-Functionalization
Author: Moumita Mondal Joyanta Choudhury
PII:
S1381-1169(16)30249-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.06.027
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Journal of Molecular Catalysis A: Chemical
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Accepted date:
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23-6-2016
27-6-2016
Please cite this article as: Moumita Mondal, Joyanta Choudhury, Structure–Activity
Comparison in Palladium–N-Heterocyclic Carbene (NHC) Catalyzed Arene
CH Activation-Functionalization, Journal of Molecular Catalysis A: Chemical
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Structure–Activity Comparison in Palladium–N-Heterocyclic
Carbene (NHC) Catalyzed Arene C–H Activation-Functionalization
Moumita Mondal, Joyanta Choudhury*
Organometallics & Smart Materials Laboratory, Department of Chemistry, Indian Institute of
Science Education and Research Bhopal, Bhauri, Bhopal By-pass Road, Bhopal 462 066 (India)
Corresponding author: Joyanta Choudhury (joyanta@iiserb.ac.in). Address: Organometallics &
Smart Materials Laboratory, Department of Chemistry, Indian Institute of Science Education and
Research Bhopal, Bhauri, Bhopal By-pass Road, Bhopal 462 066 (India)
Dedication: This work is dedicated to Professor Georgiy B. Shul’pin on the occasion of his 70th
birthday.

Graphical Abstract
Highlights



A monodentate mono-NHC palladium(II) complex was developed as efficient arene C-H
activation-functionalization catalyst
The monodentate mono-NHC catalyst was more efficient than a similar bidentate bis-NHC
palladium(II) complex
Kinetics and other mechanistic analyses provided the plausible reason for the observed
higher activity of the catalyst
Abstract
A simple and efficient C-H activation catalyst was identified through a model structure-activity
screening applied to a noncooperative, nonsymmetric bimetallic palladium(II)-N-heterocyclic
carbene complex. Mechanistic studies based on kinetics and DOSY NMR spectroscopy provided
the origin of the higher efficiency of the identified catalyst.
Keywords: Arene C-H activation / N-Heterocyclic carbene (NHC) / Palladium / Acetoxylation /
Mechanistic study

1
. Introduction
In the field of N-heterocyclic carbene (NHC) chemistry, one of the major focuses has been in the
design of new carbenic architectures to offer diverse stereoelectronic properties for fulfilling a
plethora of desired functions. Among the various versatile NHC ligand frameworks, bis-, tris- and
poly-NHC motifs offer a great deal of coordination multifariousness and modularity—an attribute
that benefits the development of bi-/polymetallic catalysis [1]. In line with this, Cowie et al.
explored unsymmetrical dicarbenes toward heterodimetallic coordination suitable for tandem
catalysis [2], while Peris et al. investigated symmetrical bis- and tris-NHCs for homo-
bimetallic/trimetallic catalysis [1a,c,h,i,3]. Orthogonal reactivity and compatibility of each metal
centers are required for operating tandem catalysis. On the other hand, cooperative effect among
the individual metal centers plays an important role in bi-/polymetallic catalysis which exhibits
enhanced activity. However, Peris et al. demonstrated that bi-/polymetallic NHC complexes may
[
4a] or may not [4b,c] exhibit catalytic cooperative effect depending on the (electronic)
communication among the metal centers. Our group has been motivated for developing robust and
efficient C-H activation-functionalization catalysts based on NHC ligand framework [5]. Recently,
we reported chelating bis-NHC monopalladium(II) complexes for efficient arene C–H
acetoxylation catalysis with PhI(OAc) as oxidant [6a]. Inspired by the longstanding challenge of
2
developing more active non-directed arene C-H activation catalysts [6b-h], and the success of
polymetallic systems in a variety of catalytic reactions [1a,c,2,3,4], a new tris-NHC bound
dipalladium(II) catalyst 1 with nonsymmetrically coordinated Pd(II) centers was designed (Fig. 1)
and employed by us for anticipated synergistic benefit during oxidative arene C-H acetoxylation
catalysis with PhI(OAc) as oxidant. Delightedly, the resultant catalytic activity was observed to
2
be higher than that of previously reported chelating bis-NHC monopalladium(II) complexes for
the same reaction [6a]. However, no “cooperativity” but only an “additive” effect was observed in
the overall catalytic activity. As a part of the assay process with an aim to deconvolute the
“additive” effect exhibited by this bimetallic Pd(II) catalyst 1, two model monometallic Pd(II)
partners 2 and 3 ( Scheme 1) were synthesized and screened. Interestingly, the relatively simpler,
mono-NHC palladium(II) model 3 appeared to be a highly efficient and robust catalyst for the
reaction. This finding is significant in the context of a limited number of efficient candidates
available in the library of non-directed arene C-H acetoxylation catalysts reported so far [6b,g,h,7].

2
. Experimental
2
.1. General methods and materials
1
13
1
H , C{ H}, DOSY NMR spectra were recorded on Bruker AVANCE III 400 MHz, 500 MHz
and 700 MHz NMR spectrometers. Chemical shifts (δ) are expressed in ppm using the residual
1
proton resonance of the solvent as an internal standard (CHCl
3
: δ = 7.26 ppm for H spectra, 77.36
1
3
1
1
13
1
ppm for C{ H} spectra; DMSO: δ = 2.50 ppm for H spectra, 39.5 ppm for C{ H} spectra;
1
13
1
CD CN: δ = 2.13 ppm for H spectra, 118.2 ppm for C{ H} spectra; Toluene: δ = 7.000 ppm for
3
1
13
1
H spectra, 137.9 ppm for C{ H} spectra). All coupling constants (J) are expressed in hertz (Hz)
1
1
and only given for H- H couplings unless mentioned otherwise. The following abbreviations were
used to indicate multiplicity: s (singlet), d (doublet), t (triplet), q (quartet), dd (doublet of doublet)
and m (multiplet). ESI mass spectroscopy was performed on a Bruker microTOF QII spectrometer.
Dry solvents and reagents were obtained from commercial suppliers and used without further
purification. Deuterated solvents were purchased from Aldrich. Pd(OAc) was purchased from
2
Johnson Matthey and used as received without further purification. L1 and L2 was prepared by
following reported literature procedure [6a,8].
2
.2. Synthesis of complexes
Complex 1. Complex 1 was synthesized directly from L1 (50 mg, 0.071 mmol) by refluxing it in
acetonitrile with 2 equivalents of Pd(OAc) (31.8 mg, 0.142 mmol) in presence of KI (20.6 mg,
.124 mmol) for 16 h . After column chromatography, pure complex 1 was obtained in 29% yield.
Complex 1 can also be prepared from complex 2 by the following procedure. A mixture of 2 (58
mg, 0.071 mmol) and Pd(OAc) (15.9 mg, 0.071 mmol) and KI (20.6 mg, 0.124 mmol) in
2
0
2
acetonitrile, was refluxed for 9 h under inert atmosphere. After cooling to ambient temperature,
mixture was filtered through a pad of celite and solvent was removed under vaccum. The crude
solid was purified by column chromatography. Compound 1 was eluted with 8:2
chloroform/acetonitrile solutions. Removal of solvents afforded compound 1 as brown orange
1
solid in 41% yield. H NMR (700 MHz, CD
3
CN): δ = 8.02 (d, J = 8.6Hz, 2H), 7.66 (d, J = 1.8Hz,
2
H), 7.56 (s, 1H), 7.54 (d, J = 8.5Hz, 2H), 7.33 (d, J = 1.8Hz, 1H), 7.32 (d, J = 1.8Hz, 1H), 7.16
1
3
(
d, J = 1.9Hz, 2H), 3.94 (s, 6H), 3.93 (s, 3H); C NMR (175 MHz, CD
3
CN): δ = 165.2, 144.8,
1
40.9, 138.7, 129.0, 127.0, 126.7, 125.8, 124.7, 124.0, 123.1, 66.3, 41.0, 40.3; HRMS (ESI) m/z =

9
26.6898 [(M)-I-CH
3
CN]+ (calcd for C19
: C, 22.98; H, 2.17; N, 8.93. Found: C, 22.72; H, 2.37; N, 8.82%.
H N
20 6
Pd I
+: m/z = 926.6961). Anal. Calc. for
2
3
C
21
H
23
N
7
I
4
Pd , CH CN, CHCl
2
3
3
Complex 2. A mixture of ligand precursor L1 (168 mg, 0.235 mmol) and Pd(OAc)
2
(52.5 mg,
0
.235 mmol) in 60 mL acetonitrile, was refluxed for 16 h under inert atmosphere. After cooling to
ambient temperature, reaction mixture was filtered through celite bed. After filtration acetonitrile
was removed in vacuum and yellow residue was washed several times with ether and dried in
1
vacuum to give 173 mg pure complex 2 as yellow powder (Yield 90%). H NMR (400 MHz,
CD
3
CN): δ = 9.85 (s, 1H), 8.35 (s, 1H), 8.03 (s, 1H), 7.96 (d, J = 1.5Hz, 2H), 7.35 (s, 1H), 7.85
1
3
(
d, J = 8.6Hz, 2H), 7.66 (d, J = 8.4Hz, 2H), 7.52 (d, J = 1.5Hz, 2H), 3.93 (s, 3H), 3.90 (s, 6H); C
NMR (100 MHz, DMSO-d
6
): δ = 162.6, 138.4, 136.2, 134.9, 128.8, 124.7, 123.5, 122.4, 121.1,
+
PdI
+: m/z
1
20.5, 113.4 73.6, 40.6, 36.2; HRMS (ESI) m/z = 692.8988 [(M)-I] (calcd for C19
H
21
N
6
2
=
692.8954); Anal. Calc. for C19
H
21
N I Pd: C, 27.81; H, 2.58; N, 10.24. Found: C, 28.26; H, 2.729;
6
3
N, 10.00%.
Complex 3. A mixture of 3-methyl-1-phenyl-1H-imidazolium iodide, L2 (85.8 mg, 0.3 mmol),
Pd(OAc) (67.2 mg, 0.3 mmol) and KI (74.7 mg, 0.45 mmol) in 3.5 mL pyridine, was stirred at
2
room temperature for 48 h in Schlenk tube under argon. Reaction mixture was diluted to 18 mL
with dichloromethane and was filtered through celite. Solvent was evaporated under vacuum.
Crude solid thus obtained was loaded to a silica gel column. Elution with 1:1 hexane/ethyl acetate
1
afforded 115 mg pure compound 3 as orange powder (Yield 64%). H NMR (400 MHz, CDCl
3
):
δ = 8.82 (dd, J = 6.3, 1.4Hz, 2H), 8.06-7.98 (m, 2H), 7.66 (tt, J = 7.7, 1.5Hz, 1H), 7.56 (dd, J =
1
2
1
0.4, 4.8Hz, 2H), 7.48 (t, J = 7.4Hz, 1H), 7.26-7.21(m, 2H), 7.18 (d, J = 2.0Hz, 1H), 7.10 (d, J =
13
.0Hz, 1H), 4.09 (s, 3H); C NMR (100 MHz, CDCl
3
): δ = 153.7, 147.8, 139.8, 137.6, 129.1,
+
28.8, 126.4, 124.5, 123.8, 123.3, 39.8; HRMS (ESI) m/z = 469.9321 [(M)-I] (calcd for
+
C
15
H
15
N
3
PdI : m/z = 469.9346). ); Anal. Calc. for C15
H
15
N I Pd: C, 30.15; H, 2.53; N, 7.03.
3
2
Found: C, 30.29; H, 2.31; N, 7.10%.
.3. Catalytic oxidative acetoxylation of arenes with complexes 1, 2 and 3
2
Catalytic acetoxylation of different arenes were performed separately using complex 1, complex
orcomplex 3 as catalyst. Diacetoxyiodobenzene (40.4 mg, 0.125 mmol), catalyst (2.5 mol% w.r.t
Pd) and arenes (1.25 mmol) were placed in a sealed tube which was equipped with a magnetic bar.
2

1
mL of freshly prepared acetic acid/acetic anhydride (9:1 v/v) mixed-solvent was added to the
mixture. Sealed tube was capped tightly and reaction mixture was stirred at 95 °C for 24 h. It was
then cooled to room temperature and the yield was calculated by GCMS analysis using PhCl
(added after reaction) as an internal standard. Following are the yield (%) and corresponding ratios
of o, m and p- isomers. In case of acetophenone, o-acetoxylated product was formed exclusively.
For evaluating time course of toluene acetoxylation with the complexes 1, 2, and 3 the following
procedure was followed. Catalytic acetoxylation of toluene were performed separately using
complex 1 (0.2 mol%), complex 2 (0.2 mol%)or complex 3 (0.2 mol%) as catalyst. Catalyst,
PhI(OAc) (40.4 mg, 0.125 mmol) and toluene (1.25 mmol) were placed in a sealed tube which
2
was equipped with a magnetic bar. 1 mL of freshly prepared acetic acid/acetic anhydride (9:1, v/v)
mixed-solvent was added to the mixture. The reaction mixture was stirred at 95 °C. It was then
cooled to room temperature and was analyzed at every 6 h interval by GCMS for a total of 24 h.
3
. Results and discussion
The complexes were synthesized by heating the corresponding ligands (L1 or L2) and Pd(OAc)
2
in acetonitrile (for 1 and 2) or in pyridine (for 3) as shown in Scheme 1. The coordination
environment of the synthesized assembled dipalladium(II) complex 1 with the tris-NHC ligand
was kept distinct at each metal center. Not surprisingly, the coordination environment around a
metal center plays a vital role in any catalysis. For every specific catalytic reaction, the significance
of metal-coordination environment is different and generally it is closely related to the inherent
mechanistic steps involved in the particular catalytic cycle. Thus, in 1, a chelating bis-NHC
scaffold was imposed to one Pd center while the other Pd was coordinated to a tethered mono-
NHC ligand. Strong σ-donating ability of NHC ligands was believed to impart robust Pd-NHC
bond as required for efficient turnover during oxidative catalysis involving Pd /PdIV cycle.
Molecular structure of 1 confirmed binding of two Pd centers in different coordination mode
within the same ligand backbone (Fig. 2). The six-membered palladaacycle with the chelating bis-
II
II
NHC preferred a boat conformation exhibiting Ccarbene–Pd –Ccarbene bite angle of 83.9(5)°, and
II
Pd –Ccarbene bond distances of 1.991(14) Å and 1.986(14) Å. The other Pd in 1 was bound by one
II
NHC in monodentate coordination mode with a relatively shorter Pd –Ccarbene bond length of
1
.928(14) Å, and two trans iodides. The two Pd metals in 1 were 8.005 Å apart through space.

When 1 was tested as a catalyst (2.5 mol%) for non-directed C-H acetoxylation of toluene using
PhI(OAc) as oxidant, it resulted in 64% yield of acetoxytoluene in 24 h of reaction carried out at
5 °C (Fig. 3, A). Some more arenes were used to prove its efficacy which was found to be
2
9
promising. However, because of the 8 Å distance between two Pd centers in 1, its catalytic activity
seemed not to be due to any cooperativity. This argument prompted us to deconvolute the overall
activity of 1. Thus disassembly of 1 into its monometallic models was sought, which led to the
synthesis of 2 and 3 (Scheme 1). Molecular structures of 2 and 3 were confirmed to possess a
similar coordination environment around the Pd center along with similar geometric parameters as
was present in 1 (Fig. 2). The complexes 2 and 3 were then subjected to catalytic activity test for
acetoxylation of the aromatic compounds as above under same conditions. The results were shown
in Fig. 3A. It is noteworthy that acetoxylation of all the arenes was not accompanied by any side
reaction and maintenance of inert atmosphere was not required for these reactions, which enabled
us to compare the performance of all the catalysts quite easily. As is evident from Fig. 3A, the
simple mono-NHC complex 3 was proved to be much superior catalyst than 2. The yields of all
the arenes (except bromobenzene) were 60-70% with the use of catalyst 3 which was noteworthy
in the context of all the known Pd-based acetoxylation catalysts reported so far [6]. The remarkable
difference in the initial rates of the reaction with complex 2 and 3 evaluated by applying a very
low catalyst loading (0.2 mol%) was also evident from Fig. 3B. As the complex 3 was found to be
superior catalyst, it was applied for a few more oxidative arene C-H acetoxylation and directed
halogenation catalysis as shown in Table 1.
The origin of higher activity of 3 than 2 was investigated via mechanistic kinetics study. Kinetic
order on each component of complex 2 and 3-catalyzed acetoxylation of toluene was assessed by
initial rate method (Fig. S20-S31, supplementary content). However, surprisingly, the analysis
provided the overall rate expression as rate = kobs[Pd][toluene] for both 2- and 3-catalyzed
II
reactions (Scheme 2A). The oxidation of Pd center by PhI(OAc)
2
could be excluded to be rate-
was zero in both the cases. Involvement of
toluene in the rate expressions suggested its presence in the rds which was again verified by kinetic
isotope effect studies (Scheme 2B). Thus, catalytic reactions of toluene and toluene-d with
PhI(OAc) under identical conditions generated the k /k value of 4.01±0.14 and 3.47±0.24 in case
determining as the order dependency on PhI(OAc)
2
8
2
H
D
of 2 and 3 as catalyst, respectively. These primary kinetic isotope effect values indicated that the
breaking of arene C-H bond was involved in the rate limiting step [6a,b,h,9]. In situ ESI-MS

analysis of the reaction of 2 or 3 with PhI(OAc) suggested the formation of acetate-bound Pd(II)
2
intermediates such as II and VI (Scheme 2C, 2D) in the reaction mixture. These acetate
intermediates could be formed via oxidation of Pd(II) to Pd(IV) followed by reductive elimination
of I
2
as supported by qualitative iodine-detection experiment (Fig. S39-S40, supplementary
content) [6a].
II
It has been reported in literature that “Pd -OAc“ species may exist in dimeric, trimeric or other
higher order aggregates in solution [6b,10]. Based on this knowledge, a diffusion-ordered
spectroscopy (DOSY) NMR experiment was undertaken to assess the catalyst resting state in case
of both 2 and 3-catalyzed reactions [6b], with an aim to complement the mechanistic kinetics
information which was alone insufficient to explain the difference in the observed reactivity with
2
and 3. Thus, the reaction of 2 or 3 with PhI(OAc)
2
was performed in CD
3
CN and toluene-d in
8
situ in an NMR tube in the presence of some inert reference compounds as molecular weight
standards and DOSY spectra were recorded at 298 K. The results of DOSY NMR analysis were
found to be significant and suggestive. Within the limits of approximation of these experiments,
II
the molecular weight of the generated “Pd -OAc“ species present in the reaction mixture
(essentially the resting states of the catalysts 2 and 3) was calculated from the plot of log(D) versus
log(MW) (D = diffusion coefficient, MW = molecular weight) as shown in Fig. 4. The calculated
values of the molecular weight suggested that in case of 2, the resting state was likely to be a
monomeric species II, but in case of 3, the resting state was most likely a dimeric species VII or
VIII (both could be in equilibrium) and unlikely to be a monomeric species VI (Fig. 4). A dimeric
resting state and a reaction order of 1 on Pd, in case of 3-catalyzed reaction, actually indicated that
the rate- determining transition state also involved a bimetallic Pd species [6b,10b,c,11]. On the
other hand, for 2-catalyzed reaction a monomeric resting state and first order rate dependency on
Pd signified a monometallic rate-determining transition state [6b,11]. Based on all these
experimental data, the most likely catalytic cycles were proposed for reaction with 2 and 3 (Scheme
3
). As per the proposal, the striking difference was in the rate-determining C–H activation step,
wherein catalyst 3 exhibited a bimetallic step. The kinetic advantage of bimetallic reaction over
monometallic one due to redox cooperativity between two metal centers reducing the activation
barrier, can thus explain the more favored and efficient catalysis with 3 than 2. Ritter et al. reported
the argument of kinetic advantage in bimetallic reductive elimination as well as oxidative addition
steps over monometallic versions [10b,c]. The favorable acetate-assisted monometallic to

bimetallic equilibrium for catalyst 3 and the less-favored one for catalyst 2 (as suggested from
DOSY data) might be due to favorable π-π stacking interaction arising from a five-membered
planar metallacycle in case of 3 as opposed to a six-membered boat-like puckered metallacycle in
case of 2 (Fig. S48, supplementary content) [12]. However, it should be mentioned that this
justification is speculative only; it requires further analysis. While species IV can undergo
monometallic reductive elimination to furnish the product ArOAc, X is likely to exhibit heterolytic
IV
II
cleavage of Pd–Pd bond to form a Pd –Pd species XI according to theoretical and experimental
results reported in literature [13], followed by similar reductive elimination of ArOAc.
4
. Conclusion
In conclusion, this study featured a structure-activity screening based on the disassembly of a non-
cooperative bimetallic Pd-NHC complex into its monometallic model counterparts. Upon full
structural characterization of all the complexes followed by comparative catalytic activity studies,
eventually it led to the identification of a simple monometallic Pd-NHC-catalyst which was found
to be highly efficient in arene C-H activation/functionalization.
Mechanistic navigation with the help of initial rate kinetics, kinetic isotope effects, ESI-MS and
DOSY NMR studies suggested that the involvement of Pd–Pd bonded bimetallic intermediate in
the rate-determining C–H activation step was the key for the enhanced reactivity of the newly
identified catalyst.The kinetic advantage of bimetallic rate-determining step over monometallic
one was in line with previous studies on similar oxidative addition and reductive elimination
reactions. The presented work is expected to enhance our knowledge-base on the important role
of Pd–Pd bimetallic intermediate in the catalytic cycle for designing active monometallic and/or
bimetallic Pd complexes for analogous C-H activation catalysis, in general, with similar chelating
and non-chelating common organic substrates.
Acknowledgements
J.C. sincerely thanks DST and IISER Bhopal for generous financial support. M.M. thanks IISER
Bhopal for fellowship. The authors thank Mr. Rajbeer Singh for help in DOSY experiments.

Appendix
Supplementary content
Supplementary information containing additional experimental details, kinetics details and spectra
are available.
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412; c) J. Seayad, A. M. Seayad, C. L. L. Chai, Org. Lett. 12 (2010) 1412-1415; d) A. Pradal,
P. Y. Toullec, V. Michelet, Org. Lett. 13 (2011) 6086-6089.
a) A. K. Cook, M. H. Emmert, M. S. Sanford, Org. Lett. 15 (2013) 5428-5431; b) M.
9
[
8]
2007) 4207-4210.[9]
066–3072.
A. K. Verma, J. Singh, V. K. Sankar, R. Chaudhary, R. Chandra, Tetrahedron Lett. 48
(
E. M. Simmons, J. F. Hartwig, Angew. Chem. Int. Ed. 51 (2012)
3
[
10] a) D. C. Powers, T. Ritter, Nat. Chem. 1 (2009), 302–309; b) D. C. Powers, M. A. L.
Geibel, J. E. M. N. Klein, T. Ritter, J. Am. Chem. Soc. 131 (2009) 17050–17051; c) D. C.
Powers, D. Y. Xiao, M. A. L. Geibel, T. Ritter, J. Am. Chem. Soc. 132 (2009) 14530–14536.
[
[
11] T. Yoneyama, R. H. Crabtree, J. Mol. Catal. A 108 (1996) 35-40.
12] B. E. Haines, J. F. Berry, J-Q, Yu, D. G. Musaev, ACS Catal. 6 (2016) 829-839.

[
13] a) D. C. Powers, T. Ritter, Acc. Chem. Res. 45 (2012) 840-850; b) D. C. Powers, E.
Lee, A. Ariafard, M. S. Sanford, B. F. Yates, A. J. Canty, T. Ritter, J. Am. Chem. Soc. 134
2012) 12002 –12009; c) A. Ariafard, C. J. T. Hyland, A. J. Canty, M. Sharma, N. J. Brookes,
B. F. Yates, Inorg. Chem. 49 (2010) 11249-11253.
(
H
OAc
Catalyst
PhI(OAc)2
R
R
0
AcOH/Ac O, 95 C, 24 h
2
previous work [6a]
R'
present work
N
N
N
I
Pd
N
N
N
N
Pd
I
Ph
I
I
chelating bis-NHC Pd(II)
monodentate mono-NHC Pd(II)
improvement of catalytic efficiency
Fig. 1. Development of Pd(II)-NHC catalyst for non-directed arene C-H acetoxylation
Fig. 2. ORTEP diagrams of complexes 1, 2 and 3 (30% ellipsoids probability level). Selected bond
lengths (Å) and bond angles (°), H atoms were removed for clarity: Complex 1: C22-Pd1 =
1
.928(14); Pd2-C32 = 1.991(14); Pd2-C35 = 1.986(14); Pd2-I1 = 2.6422(15); Pd2-I2 =

2
8
1
.6560(14); Pd1-I3 = 2.5919(16); Pd1-I4 = 2.6083(14); Pd1-N14 = 2.061(14); C35-Pd2-C32 =
3.9(5); I1-Pd2-I2 = 95.72(5); I3-Pd1-I4 = 172.61(6). Complex 2: C1-Pd1 = 1.995(6); Pd1-C9 =
.979(6); Pd1-I1 = 2.6236(7); Pd1-I2 = 2.6668(7); C9-Pd1-C1 = 83.2(2); I1-Pd1-I2 = 92.14(2).
Complex 3: C1-Pd1 = 1.953(3); Pd1-I1 = 2.6009(3); Pd1-N3 = 2.094(3); I1-Pd1-I2 = 170.531(12).
CCDC 1465252, 1465254, and 1465253, contains the supplementary crystallographic data for 1,
2
, and 3. These data can be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.

Fig. 3. Comparison of catalytic activity of 1, 2, and 3 in non-directed arene C-H activation reaction.

for Complex 2
2
for Complex 3
N
OAc
N
N
N
N
PdII
N
N
N
O
PdII
AcO
O
N
OAc
PdII
OAc
N
O
VI: MW = 461.8
O
N
N
N
N
VII: MW = 765.4
N
N
N
N
O
PdII
O
N
PdII
N
N
O
PdII
PdII
O
N
O
VIII: MW = 645.3
AcO
OAc
O
N
IIa: MW = 557.9
IIb: MW = 498.9
Fig. 4. DOSY NMR spectroscopic results (shown at the top) to evaluate the most probable species
shown at the bottom) generated from the reaction of 2 and 3 with PhI(OAc) via approximate
molecular weight determination method. See supplementary content for details.
(
2
I
N
N
N
I
Pd
N
N
N
N
H C
3
I
2
Pd(OAc) KI
Pd(OAc)2
2
,
CH CN, reflux, 16 h
3 I
CH CN, reflux, 16 h
3
3
N
I
N
I
N
I
N
N
N
N
N
N
N
N
N
Pd
2
Pd
1
L1
I
N
N
I
Pd(OAc) , KI
2
N
N
Pd
N
I
Pyridine
rt, 24 h
I
L2
3
Scheme 1. Synthesis of the complexes.

(A) Initial rate studies
H
OAc
[Pd]-Cat
for both catalyst 2 and 3:
PhI(OAc)2
Me
Me
rate = kobs[Pd][toluene]
0
AcOH/Ac O, 95 C
2
Scheme 2. Kinetics for order determination, isotope effect, and other control studies.

(
a) with Cat 2
(resting state)
II OAc
L
PhI(OAc)2
PhI
Pd
I2
L
OAc
(
II)(DOSY)
(ref. [6])
OAc
IV OAc
Pd
OAc
IV OAc
L
L
ArOAc (pdt)
Pd
L
I
L
rds
OAc
I(I)
OAc
OAc
(III)
IV OAc
PhI(OAc)2
PhI
L
ArH
Pd
Ar
(
kinetics)
L
II
Pd
I
L
OAc
IV)
HOAc
(
L
I
cat. 2
catalytic cycle
monometallic rds)
catalyst activation
(
(
b) with Cat 3
(
resting state)
Ac
O
Ac
O
AcO
L
II py
OAc
Ph
L
L
Pd
PdII
PdII
PdII
PdII
OAc
(
VI)
AcO
O
Ac
Ph
(DOSY)
O
Ac
L
I2
L
(
VII)
(VIII)
PhI(OAc)2
OAc
ArOAc (pdt)
AcO
L
IV py
(ref. [10b])
Pd
I (V)
PhI
I
Ac Ph
Ph
Ph
O
L
L
L
Ph
L
PhI(OAc)2
PhI
PdII
PdIV
AcO PdIII
AcO
PdIII OAc
Ar
O
I
II py
Pd
(
XI) Ac OAc
OAc
(
IX)
ArH
HOAc
L
I
rds
cat. 3
Ph
Ph
PdIII Ar
kinetics)
L
(
ref.[12])
L
AcO PdIII
AcO
catalyst activation
(
OAc
(X)
catalytic cycle
bimetallic rds)
(
Scheme 3. Plausible catalytic cycles for 2- and 3-catalyzed reactions.

Table 1 Complex 3-catalyzed arene C-H acetoxylation and directed halogenation reaction.
Ar-OAc
%Yield
Ar-X
%Yield %Yield
X= Br) (X= Cl)
(
3
9
72
81
9
5
79
77
7
0
50
44
aReaction conditions A: ArH (1.25 mmol), PhI(OAc)
AcOH/Ac O (1 mL; 9:1, v/v), 95 °C, 24 h. Reaction conditions B: ArH (1.25 mmol), NBS or NCS
0.125 mmol), Catalyst 3 (2.5 mol%), CH CN (1 mL), 95 °C, 24 h. Yields (based on the oxidant,
(0.125 mmol), Catalyst 3 (2.5 mol%),
2
2
(
3
Ox) were calculated by GCMS using chlorobenzene as internal standard added after the reaction
was over.