Homoleptic cis- and trans-palladium(II) bis(guanidinato) complexes derived from N-aryl-N′,N″-di(pyridin-2-yl)- and N-aryl-N′,N″-bis(6-methylpyridin-2-yl)guanidines: Catalysts for Heck-Mizoroki coupling reactions

Article abstract of DOI:10.1016/j.jorganchem.2019.04.009

N-Aryl-N′,N″-di(pyridin-2-yl)- and N-aryl-N′,N″-bis(6-methylpyridin-2-yl)guanidines (1–4 and 5–7) were isolated in 75%–81% yields. Reactions of Pd(OAc)₂ with guanidines 2–7 carried out separately in toluene at 60 °C for 3 h afforded 8–13 respectively in 69%–80% yields. Compounds 1–13 were characterized by elemental analyses, HR-MS, IR and NMR (¹H and ¹³C) spectroscopy. Molecular structures of guanidines 1, 4, 5 and 6 and those of 8–13 were determined by single crystal X-ray diffraction. The Pd(II) atom in 8–10 revealed trans geometry while that in 11–13 revealed cis geometry. DFT calculations were carried out on model compounds 9a (trans) and its hypothetical cis isomer, 9b and 12a (cis) and its hypothetical trans isomer 12b which indicated a very small energy difference between the 9a/9b pair (1.28 kcal/mol) whereas a large energy difference was observed between the 12a/12b pair (26.38 kcal/mol) in CH₂Cl₂. The catalytic utility of 9 in Heck-Mizoroki coupling reactions involving styrene and methyl acrylate and aryl bromides/aryl chlorides in the presence of NaOAc and excess of tetrabutylammonium bromide (TBAB) at 120 °C was explored. Both activated and de-activated aryl bromides and aryl chlorides were coupled with styrene and in addition, the aryl chlorides were coupled with methyl acrylate in the presence of 9 to afford the respective coupling products in 68% ? >99% yields. Neat reaction carried out with 9 and TBAB under the optimized condition released the colloidal Pd black as verified by EDAX, PXRD and SEM techniques thereby implying the heterogeneous nature of catalysis.

Full text of DOI:10.1016/j.jorganchem.2019.04.009

Accepted Manuscript
Homoleptic cis- and trans-palladium(II) bis(guanidinato) complexes derived from
N-aryl-N′,N″-di(pyridin-2-yl)- and N-aryl-N′,N″-bis(6-methylpyridin-2-yl)guanidines:
Catalysts for Heck-Mizoroki coupling reactions
Vishwesh Mishra, Jisha Mary Thomas, Sivasankar Chinnappan, Natesan Thirupathi
PII:
S0022-328X(18)31034-9
DOI:
https://doi.org/10.1016/j.jorganchem.2019.04.009
JOM 20759
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 21 December 2018
Revised Date: 20 March 2019
Accepted Date: 10 April 2019
Please cite this article as: V. Mishra, J.M. Thomas, S. Chinnappan, N. Thirupathi, Homoleptic cis-
and trans-palladium(II) bis(guanidinato) complexes derived from N-aryl-N′,N″-di(pyridin-2-yl)- and N-
aryl-N′,N″-bis(6-methylpyridin-2-yl)guanidines: Catalysts for Heck-Mizoroki coupling reactions, Journal of
Organometallic Chemistry (2019), doi: https://doi.org/10.1016/j.jorganchem.2019.04.009.
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ACCEPTED MANUSCRIPT
The graphical abstract pictogram
N
N(H)Ar
N
N(H)Ar
N
N
N
N
N
N
N
N
N
N
N
H
R
Me
Me
Pd(OAc) , 60°C
Me
Me
2
Pd(OAc) , 60°C
2
Pd
N
Ar
Pd
N
N
H
N
Toluene
R = Me
Toluene
R = H
N
N
N
N
N(H)Ar
Ar(H)N
R
12 (cis)
Ar = 4-ClC H4
9 (trans)
6
(
R = H (3) & Me (6))
The graphical abstract synopsis
The reaction of less hindered guanidine 3 with Pd(OAc) in toluene at 60 °C afforded 9 while
2
the analogous reaction with more hindered guanidine 6 afforded 12. Energetics of 9 and 12
and their geometric counterparts have been studied by DFT calculations. The utility of 9 as
catalyst in Heck-Mizoroki coupling reactions was explored.

ACCEPTED MANUSCRIPT
Homoleptic cis- and trans-palladium(II) bis(guanidinato)
complexes derived from N-aryl-N′,N″-di(pyridin-2-yl)- and
N-aryl-N′,N″-bis(6-methylpyridin-2-yl)guanidines: Catalysts
for Heck-Mizoroki coupling reactions
a
b
b
Vishwesh Mishra, Jisha Mary Thomas, Sivasankar Chinnappan and
a
Natesan Thirupathi*
a
Department of Chemistry, University of Delhi, Delhi 110 007, India
b
Catalysis and Energy Laboratory, Department of Chemistry, Pondicherry University (A
Central University), Puducherry 605 014, India
Email address (N.T.): thirupathi_n@yahoo.com, tnat@chemistry.du.ac.in
1

ACCEPTED MANUSCRIPT
Abstract N-Aryl-N′,N″-di(pyridin-2-yl)- and N-aryl-N′,N″-bis(6-methylpyridin-2-
yl)guanidines (1−4 and 5−7) were isolated in 75%−81% yields. Reactions of
Pd(OAc) with guanidines 2–7 carried out separately in toluene at 60 ºC for 3 h
2
afforded 8–13 respectively in 69%−80% yields. Compounds 1–13 were characterized
1
13
by elemental analyses, HR-MS, IR and NMR ( H and C) spectroscopy. Molecular
structures of guanidines 1, 4, 5 and 6 and those of 8–13 were determined by single
crystal X-ray diffraction. The Pd(II) atom in 8–10 revealed trans geometry while that
in 11–13 revealed cis geometry. DFT calculations were carried out on model
compounds 9a (trans) and its hypothetical cis isomer, 9b and 12a (cis) and its
hypothetical trans isomer 12b which indicated a very small energy difference
between the 9a/9b pair (1.28 kcal/mol) whereas a major energy difference was
observed between the 12a/12b pair (26.38 kcal/mol) in CH Cl . The catalytic utility
2
2
of 9 in Heck-Mizoroki coupling reactions involving styrene and methyl acrylate and
aryl bromides/aryl chlorides in the presence of NaOAc and excess of
tetrabutylammonium bromide (TBAB) at 120 °C was explored. Both activated and
de-activated aryl bromides and aryl chlorides were coupled with styrene and in
addition, the aryl chlorides were coupled with methyl acrylate in the presence of 9 to
afford the respective coupling products in 68% − >99% yields. Neat reaction carried
out with 9 and TBAB under the optimized condition released the colloidal Pd black as
verified by EDAX, PXRD and SEM techniques thereby implying the heterogeneous
nature of catalysis.
Keywords: Palladium, guanidine, homoleptic palladium(II) amido complex, Heck-
Mizoroki coupling, geometrical isomers.
2

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1
. Introduction
Transition metal complexes of sym N,N ′,N″-trisubstituted guanidines,
RNH) C=NR (sym = symmetrical; R = alkyl and aryl) are well known in the
(
2
literature due to their intriguing structural aspects and reactivity pattern [1]. In these
complexes, guanidines are present in neutral, monoanionic (i.e., guanidinate(1–)) and
dianionic (i.e., guanidinate(2–)) forms and exhibit chelating and bridging coordination
modes.
Palladium(II)
complexes
of
sym
N,N ′,N″-triphenylguanidine,
[
(PhNH) C=NPh] are known wherein the guanidine is present in neutral and
2
monoanionic
forms
[2,3].
Sym
N,N ′,N″-tris(2-
hydroxybenzylidene)triaminoguanidine, LH Cl upon reaction with (Et N) (PdCl ) in
6
4
2
4
MeCN in the presence of Et NOH afforded (Et N) (LPd Cl )⋅MeCN wherein the
4
4
2
3
3
anion was shown to possess a chiral propeller-like conformation in solid state [4].
Palladium(II) complexes of N-substituted guanidines as well as six-membered
palladacyclic complexes derived from guanidines were used as effective phosphine
free catalysts in Heck and Suzuki cross coupling reactions [5−7]. Trans-
i
[
PdCl {( PrNH) C=NAr} ] were shown to be intermediates in guanylation reactions
2
2
2
of 4-anisidine, 2-fluoro-4-iodoaniline and 3,4-(methylenedioxy)aniline by N,N ′-
diisopropylcarbodiimide catalyzed by Pd(II) source [8].
N-Substituted amidines and formamidines with additional N-donor sites and
their metal complexes are known in the literature [1c] while metal complexes of N-
substituted guanidines with additional N-donor sites remain unknown presently. We
prepared N-substituted unsymmetrical guanidines 1–7 in order to understand whether
this family of guanidines will undergo C−H or N−H activation upon treatment with
Pd(II) source. From this endeavor, interestingly, two types of geometrical isomers
3

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namely 8–10 (trans) and 11–13 (cis) were isolated via N−H activation of guanidines
and these complexes were fully characterized. Molecular structures of four guanidines
(1, 4, 5 and 6) and those of complexes 8–13 were determined by single crystal X-ray
diffraction (SCXRD). The utility of 9 as a catalyst in Heck-Mizoroki coupling
reaction (H-MCR) involving styrene and methyl acrylate and aryl bromides and aryl
chlorides as coupling partners were explored.
2
. Results and discussion
2
.1 Syntheses Reactions of N ′,N″-di(pyridin-2-yl)thiourea and N ′,N″-bis(6-methyl
pyridin-2-yl)thiourea with variously substituted anlines in the presence of 70% aqs.
KOH in nitrobenzene following the procedure previously reported by us on the
preparation of sym N,N ′,N″-triarylguanidines [9] afforded N-aryl-N′,N″-di(pyridin-2-
yl)guanidines, 1–4 and N-aryl-N′,N″-bis(6-methylpyridin-2-yl)guanidines, 5–7 in
7
5%−81% yields as illustrated in Scheme 1. Guanidine 2 is known in the literature
[
10] but it was prepared from the reaction of N-(2-pyridyl)-N ′-(4-tolyl)carbodiimide
with 2-aminopyridine in 18% yield which is far inferior than our yield of 76%
obtained through the route illustrated in Scheme 1.
Scheme 1
Reactions of Pd(OAc) with guanidines, 2–7 carried out separately in toluene
2
at 60 °C for 3 h afforded 8–13 respectively in 69%−80% yields as illustrated in
Schemes 2 and 3. A plausible mechanism of the formation of 8–13 is shown in
Scheme 4. The reaction of Pd(OAc) with the guanidine could afford a transient
2
intermediate A due to chelate effect and this intermediate upon amine-imine
tautomerisation could afford another intermediate B. Subsequently, the intermediate B
upon N−H activation via the loss of AcOH could afford either a dinuclear
4

ACCEPTED MANUSCRIPT
intermediate C or a mononuclear intermediate D depending upon the steric
encumbrance of the R group in the coordinated pyridyl or methyl pyridyl units of the
guanidine. It is believed that sterically less hindered coordinated 2-pyridyl ring in the
intermediate C stabilizes the dinuclear core, [Pd(µ2-OAc)] while sterically more
2
hindered 6-methyl-2-pyridyl ring in the intermediate D stabilizes the mononuclear
2
core, [Pd(
κ
-OAc)]. Compounds of both types C and D are known in the literature
and a few of these complexes are crystallographically characterized [11]. Our
attempts to secure NMR experimental support for the aforementioned hypothesis
were thwarted as reactions of Pd(OAc) with 2 and 3 carried out separately in toluene
2
even at ambient temperature lead to the precipitate formation in each case and these
precipitates were subsequently isolated and identified as 8 and 9, respectively.
Schemes 2−4
2
.2 X-ray Crystallography Molecular structures of guanidines 1 and 4−6 were
determined by SCXRD and are shown in Figs. 1 and 2. In principle, sym N,N′,N′′-
triarylguanidines can exhibit four different conformations namely syn-syn, syn-anti,
anti-syn and anti-anti [9]. From the dispositions of the aryl and the pyridyl
substituents of amino N atoms with reference to the >C=N− unit in 1 and 4−6, syn-
anti conformation is evident in solid-state. In this conformer in all guanidines, there
are two intramolecular N–H⋅⋅⋅N hydrogen bonds involving pyridyl N atom and H
atom of the amino moiety and further in guanidines 1 and 4, one intramolecular C–
H⋅⋅⋅N hydrogen bond involving an imino N atom and o-hydrogen atom of the aryl ring
is observed (see Fig. 1). The aforementioned intramolecular E–H⋅⋅⋅N (E = N and C)
hydrogen bonds places all non-hydrogen atoms of pyridyl and aryl rings in a plane
which almost coincides with the plane of the CN unit of the guanidine. The angle
3
between the mean plane of the CN unit and the mean plane of the aryl and pyridyl
3
5

ACCEPTED MANUSCRIPT
rings attached to the N atom of the CN unit respectively are 3.8(2)°, 6.9(1)° and
3
4
.5(1)° (1), 3.2(5)°, 5.1(6)° and 5.4(6)° (4), 1.5(1)°, 9.6(1)° and 4.9(1)° (5) and
.8(4)°, 4.2(4)° and 5.4(4)° (6). Both the amino N atoms and the carbon atom of the
1
CN unit in all guanidines are planar (Σ ≈ 360°).
3
Figs. 1 and 2
A somewhat planar disc shaped molecule of 1 are stacked one over the other
in anti-parallel fashion continuously while a similar disc shaped molecules of 4 and 6
are stacked one over the other in parallel fashion continuously and this growth leads
to the formation of a one-dimensional column (see the Supplementary data). The two
one-dimensional columns are further linked through a weak intermolecular hydrogen
bonds such as C–H···N, C–H···X (X = Cl and Br) and N–H···Br and the columns are
stacked at an angle of 66.39° (1), 78.83° (4) and 73.89° (6) with each other. In the
case of 5, no such stacking arrangement is observed in the crystal lattice.
Molecular structures of 8−13 were determined by SCXRD and are shown in
Figs. 3 and 4. Complexes 8−10 revealed trans geometry around the Pd(II) atom while
complexes 11−13 revealed cis geometry. The angle between two mean planes
constituted by N1Pd1N5 and N6Pd1N10 in 8 is 0.5(1)° and the corresponding angle
in 9 and 10 are 0.0(1)° and 0.0(2)° respectively indicating a very minimal or no
distortion around the Pd(II) atom in these complexes. However, the angle between
two mean planes constituted by N1Pd1N5 and N6Pd1N10 in 11 is 10.2(1)° and the
corresponding angle in 12 and 13 are 10.4(1)° and 12.2(1)° respectively indicating a
significant distortion around the Pd(II) atom in these complexes.
Figs. 3 and 4
6

ACCEPTED MANUSCRIPT
Key structural parameters of 8−13 are listed in Table 1. The two six-
membered rings linked by the Pd(II) atom in these complexes revealed a pseudo boat
conformation. The Pd(II) atom deviates from the basal plane of the pseudo boat more
than the imine N atom as reflected from the greater values of x and x′ than those of y
and y′ and the greater values of θ1 and θ1′ than those of θ2 and θ2′. Further, the
magnitude of distortion around the Pd(II) atom is greater in 11−13 than in 8−10. This
difference arises due to both steric and electronic factors. The repulsive interaction
between Me groups in two cis positioned coordinated 6-methyl 2-pyridyl substituent
of the chelate rings in 11−13 causes more geometrical distortion around the Pd(II)
atom. Further, both the amido N atoms in 8−10 and 11−13 can act as σ and π-donors
to the Pd(II) atom. It has been shown for four-membered lithium amidinate complexes
that the Li atom moves towards the plane of a charge delocalized diazally anion,
–
{
(Me Si)NC(R)N(SiMe )} when the N atom in this anion acts as a σ-donor while the
3
3
Li atom moves above the plane of the anion when the N atom acts as a π-donor [12].
The CN carbon atom and the exocyclic amino N atom in 8−13 are planar while the
3
amido N atom slightly deviates from planarity. Thus, the amido N atom in 8−13 acts
as σ and π-donors and additionally, there is a greater geometrical distortion around
the Pd(II) atom in 11−13 due to steric factor. Homoleptic palladium(II) amido
complexes with a trans geometry around the Pd(II) atom is more prevalent than the
analogous complexes with a cis geometry [13,14].
Table 1
Interestingly, an intramolecular π⋅⋅⋅π stacking interaction was observed
between 6-methyl-2-pyridyl rings attached to N1 and N6 in 11−13 (π⋅⋅⋅π = 3.589 Å
(11), 3.579 Å (12) and 3.599 Å (13)) and two intermolecular π⋅⋅⋅π stacking
7

ACCEPTED MANUSCRIPT
interactions were observed, between the pair of 6-methyl-2-pyridyl rings attached to
the N1 atom and its centrosymmetric neighbor and another such pair attached to the
N6 atom (π⋅⋅⋅π = 3.670 Å/3.668 Å (11), 3.684 Å/3.657 Å (12) and 4.010 Å/3.625 Å
(13)). These intra- and intermolecular π⋅⋅⋅π stacking interactions are well within the
defined boundary of 3.8 Å [15] except in 13. As a representative example, these
stacking interactions are illustrated for 11 in Fig. 5. These π⋅⋅⋅π stacking interactions
are believed to be partly responsible for stabilization of cis configuration of the Pd(II)
atom in 11−13. Neither intramolecular nor intermolecular π⋅⋅⋅π stacking interactions
such as those mentioned above were observed in the crystal lattice of 8−10.
Fig. 5
2
.3 DFT Calculations DFT calculations were performed on model compounds of 9
namely, 9a (trans) and 9b (cis) and model compounds of 12 namely, 12a (cis) and
2b (trans). All the four complexes were successfully optimized without any
1
imaginary frequency and their optimized geometries are shown in Figs. 6 and 7. The
energies of 9a, 9b, 12a and 12b in gas phase are listed in Table 2. A solvent
correction for CH Cl was performed using the polarized continuum model (PCM) on
2
2
all the four complexes, the energies of which are also listed in Table 2. Selected bond
parameters of 9a, 9b, 12a and 12b and the hydrogen bonding interactions in these
complexes are listed in the Supplementary data.
Figs. 6 and 7
Table 2
Complex 9a was found to be more stable than 9b by a value of 1.28 kcal/mol,
whereas complex 12a was found to be more stable than 12b by a value of 26.38
kcal/mol in CH Cl . It was noted that after optimization the geometry around the
2
2
Pd(II) atom in 12b changed drastically as seen from the bond angles around the Pd(II)
8

ACCEPTED MANUSCRIPT
atom (see the Supplementary data). Cis- and trans-NPdN angles between two chelate
rings in 12b are 96.79/96.77° and 143.83/157.05° instead of 90° and 180°,
respectively. Thus, the optimized geometry contained a significant tetrahedral
contribution for the Pd(II) atom in 12b.
The optimized geometries of 9a and 12a are in complete agreement with the
observed geometries of 9 and 12. The parameters x, y, θ1 and θ2 defined in Table 1
matched well with the corresponding values derived from DFT studies (see the
Supplementary data). The values of x, y, θ1, θ1′, θ2 and θ2′ parameters for 9b are
similar to the corresponding values obtained for 9 and 9a. Thus, we may conclude
that 9 can possess either cis or trans geometries on the basis of energy and steric
considerations but it prefers trans geometry in solid state due to packing forces. The
Pd(II) atom in 12b revealed a tetrahedrally distorted square planar environment as
evident from the angle between two NPdN mean planes being 43.16°, quite larger
than the ideal angle of 0.0°. Imposing a square planar geometry on the Pd(II) atom in
1
2b would result in greater instability to the complex. Tetrahedrally distorted square
2+
planar geometry around the Pd(II) atom in [PdN ] macrocyclic complex is reported
4
in the literature [16]. Thus, it can be summarized that the formation of trans
complexes 11−13 can be realized only when the environment around the Pd(II) atom
is tetrahedrally distorted. Thus, steric factor predominantly dictates the cis
configuration in 11−13.
2
.4 Heck-Mizoroki coupling reactions H-MCR is one of the important C−C bond
forming reactions [17]. Various Pd sources have been used as catalysts in H-MCR
among which phosphine free palladium catalysts are the most attractive one from the
commercial and toxicity points of view [5−7,14,17−19]. The Pd(II) amido complexes
were shown to be one of the phosphine free catalysts used in H-MCR and this class of
9

ACCEPTED MANUSCRIPT
compounds are attractive as they are cheaper, stable to air and moisture and exhibit
high activity [14,17c,18a]. Continuing our interests in utilizing Pd(II) complexes as
phosphine free catalysts in cross coupling reactions [6,7], herein we report the utility
of 8−13 and in particular the utility of 9 as catalyst in H-MCR involving styrene and
aryl bromides or aryl chlorides and methyl acrylate and aryl chlorides as coupling
partners.
Complexes 8−13 were screened for H-MCR involving styrene and 4-
bromoanisole in the presence of NaOAc as the base, 0.1% catalyst and tetra-n-butyl
ammonium bromide (TBAB) as a co-catalyst in the absence of solvent as illustrated in
Scheme 5. The screening experiments based on two runs indicated the formation of
the coupling product, E-1-(4′-methoxyphenyl)-2-phenylethene in 80%–98% yields as
listed in Table 3. From the screening study and from the fact that 9 melts sharply at
2
59.47 °C without any decomposition while 12 decomposes at 308.27 °C, the former
complex was chosen as the catalyst of choice for H-MCR involving other coupling
partners (see DSC thermograms of 9 and 12 in the Supplementary data).
Scheme 5 and Table 3
Ionic liquids have been used as a medium to carry out H-MCR [19]. TBAB
melts at 105 °C and provides an ionic liquid medium. Moreover, its low cost and
solubility in water simplifies the work up procedure [19]. A variety of bases have
been used in Pd(II) catalyzed C–C bond forming reactions. In this study, eight bases
were screened for the H-MCR illustrated in Scheme 5 and yields of the coupling
product obtained with each base are listed in Table 4. The yields of coupling product
varied from 90%–98% calculated based on the average of two runs. Though NaOAc
and NaHCO afforded the coupling product in comparable yields, the former base was
3
chosen for further catalytic studies. The progress of the reaction illustrated in Scheme
1
0

ACCEPTED MANUSCRIPT
5
was monitored by TLC after every hour from the onset of the reaction and the
reaction was complete only after 12 h.
Table 4
H-MCR illustrated in Scheme 5 was carried out at 110 °C and 120 °C for 12 h.
The progress of the reaction was monitored by TLC, which indicated the presence of
both reactants and products when carried out at 110 °C while TLC of the reaction
mixture carried out at 120 °C indicated complete consumption of 4-bromoanisole. A
number of activated, deactivated and sterically hindered aryl bromides were coupled
with styrene under optimized condition and from these reactions coupling products
were isolated in ≥98% yields (Scheme 6 and Table 5).
Scheme 6 and Table 5
The recyclability of the catalyst was studied up to five runs with styrene and
4
-bromoanisole as coupling partner under the optimized condition by extracting the
coupling product each time with diethyl ether and adding the coupling partners and
base to the reaction mixture after each cycle. The yields of the coupling product in
five consecutive cycles are 98%, 94%, 89%, 83% and 76%, respectively. The
formation of Pd in various forms such as Pd colloids, Pd nanoparticles and anionic Pd
complexes in H-MCR are well documented [18c,20]. The fact that we were able to
reuse the residue after removal of the coupling product for further catalysis suggest
that 9 releases colloidal Pd in situ and the latter catalyzes the reaction. The
heterogeneous mixture consisting of 9 and TBAB were heated at 120 °C for 12 h to
afford a dark reaction mixture which was subsequently diluted with chloroform and
centrifuged to afford Pd black at the bottom. The Pd black was characterized by
Energy Dispersive X-ray analysis (EDAX), powder X-ray diffraction (PXRD) and
Scanning Electron Microscopy (SEM) (see Figs. 8−10). The PXRD pattern of Pd
1
1

ACCEPTED MANUSCRIPT
black exhibited five well defined characteristic Bragg’s peaks appearing at 2θ = 39.3°,
4
5.92°, 67.72°, 81.46° and 86.14° which correspond to the (111), (200), (220), (311)
and (222) crystal planes of face centered cubic structure of metallic Pd. For
comparison, the standard pattern of Pd obtained from JCPDS (Card No. 46-1043) is
also shown in Fig. 9. The SEM image illustrates that Pd black possesses a roughly
spherical morphology with an average diameter of about 300 nm (see Fig. 10).
Figs. 8−10
A comparative study of the catalytic efficacy of 9 in H-MCR of
bromobenzene or 4-bromotoluene with styrene carried out in TBAB with those Pd(II)
complexes in the related H-MCR in ionic liquid is summarized in Table 6
[
5b,19c,19e]. Considering the catalyst loading, temperature and duration of the
reaction, 9 appears to comparable or even better than those catalysts listed in Table 6.
Table 6
H-MCR carried out separately with styrene and 4-bromopyridine and 2-
bromopyridine under the optimized condition did not lead to the formation of
coupling products suggesting the catalyst deactivation. Under the optimized
condition, when Pd(OAc) was used instead of 9 as a catalyst, styrene coupled with 4-
2
bromoanisole to afford the coupling product in only 17% yield. This result illustrates
beneficial effect of the guanidinato ligand environment around the Pd(II) atom in 9 in
H-MCR.
Aryl chlorides are attractive substrates in cross coupling reactions as they are
inexpensive. Nevertheless, aryl chlorides are relatively difficult to activate as the C–
Cl bond is more inert than the C–Br bond in aryl bromides. The bond energies of C–X
bonds in aryl halides mentioned above are 96 kcal/mol (X = Cl) and 81 kcal/mol (X =
Br) [21]. When 4-chloroanisole was subjected to H-MCR with styrene under the
1
2

ACCEPTED MANUSCRIPT
optimized condition adopted for bromoarenes, the coupling product, E-1-(4′-
methoxyphenyl)-2-phenylethene was isolated only in 52% yield. However, when the
reaction was carried out in the presence of 0.5 mol% of 9, the coupling product was
isolated in 78% yield. Further, extending the reaction period up to 24 h improved the
yield of the coupling product to 89%. Thus, the optimized reaction condition for H-
MCR of aryl chlorides with styrene and methyl acrylate is illustrated in Scheme 7.
Eight aryl chlorides were coupled with styrene under the optimized condition and
from these reactions coupling products were isolated in 68% to 95% yields (see Table
7
). Both activated and deactivated aryl chlorides couple with styrene more efficiently
than chlorobenzene (entries 1−3 and 4−7). Further, aryl chlorides were coupled with
methyl acrylate under the condition illustrated in Scheme 7 and the coupling products
were isolated in 85% to 93% yields (see Table 8).
Scheme 7 & Table 7 and 8
3
. Conclusion
Guanidines 1–7 were prepared and isolated in good yields. Complexes 8–13 were
prepared from Pd(OAc) and guanidines, 2–7 through N-H activation. The new
2
1
compounds were characterized by elemental analyses, mass spectrometry, IR and H
NMR spectroscopy. Molecular structures of 1 and 4−6 were determined by SCXRD
which revealed syn-anti conformation in solid state. Molecular structures of 8−13
were also determined by SCXRD which revealed trans geometry (8−10) and cis
geometry (11−13) around the Pd(II) atom. DFT calculations carried out on the 9a/9b
and the 12a/12b pairs indicated a minimal energy difference (1.28 kcal/mol) and
maximal difference, (26.38 kcal/mol), respectively. Cis geometry of the Pd(II) atom
in 11−13 is shown to be dictated predominantly by the steric factor. Complex 9 was
shown to be an excellent catalyst for H-MCR involving styrene and aryl bromides at
1
3

ACCEPTED MANUSCRIPT
0
.1 mol% catalyst loading and as a good catalyst for H-MCR involving styrene and
methyl acrylate and aryl chlorides at 0.5 mol% catalyst loading. Neat reaction carried
out with 9 and TBAB indicates that the former acts as a reservoir of colloidal Pd
black in H-MCR as independently verified by EDAX, PXRD and SEM investigations
and thus points out heterogeneous nature of catalysis.
4
4
. Experimental section
.1 Materials and instrumentation Pd(OAc) , anilines, NaOAc, styrene, methyl
2
acrylate, aryl chlorides and aryl bromides (Sigma Aldrich), 2-aminopyridine and 2-
amino-6-methylpyridine, CS , Et N, TBAB and Et O (Spectrochem) were purchased
2
3
2
from commercial vendors and used as received. N ′,N″-Di(pyridin-2-yl)thiourea and
N ′,N″-bis(6-methyl pyridin-2-yl)thiourea were prepared following the literature
procedure [22]. The IR spectral data were obtained through ATR method from
powdered samples on an IRAffinity-1 Shimadzu FTIR spectrometer in the frequency
–
1
range 400–4000 cm . Elemental analyses were performed on an Elementar
Analysensysteme GmbH VarioEL Cube. Time of flight mass (TOF−MS) spectra were
recorded on Agilent Technologies 6530, Accurate-Mass Q-TOF LC/MS instrument
1
13
1
19
using electrospray positive ion mode. H NMR, C{ H} NMR and F NMR spectra
were recorded on a JEOL ECX 400 NMR spectrometer operating at 400, 100.5, and
3
76.5 MHz (with CF COOH as an external standard), respectively. The chemical
3
shifts are reported in ppm relative to tetramethylsilane or residual solvent signal.
Melting points were recorded on a Buchi melting point apparatus (Model: M−560)
and the reported values are uncorrected.
SCXRD data for 1, 4−6 and 8−13 were collected on Oxford Xcalibur S
diffractometer (4-circle kappa goniometer, Sapphire−3 CCD detector, omega scans,
1
4

ACCEPTED MANUSCRIPT
graphite monochromator, and a single wavelength Enhance X-ray source with MoKα
radiation) [23]. Pre-experiment, data collection, data reduction, and absorption
corrections were performed with the CrysAlisPro software suite [24]. The structures
were solved and refined using the SHELX-2017 program package [25] and SHELXL-
2
017/1 (within the WinGX program package) [26]. Non-hydrogen atoms were refined
anisotropically. C−H/N−H hydrogen atoms were placed in geometrically calculated
positions by using a riding model (except those in 1 and 4−6 which have been located
from difference Fourier map) and refined. The molecular structures were created with
Olex2 program [27]. Details of data collection, structure solution, refinement, selected
bond parameters and packing diagrams for 1, 4−6 and 8−13 are presented in
Supplementary data.
SEM and EDAX analyses were carried out on a JEOL JSM-6610LV scanning
electron microscope equipped with EDS. PXRD was carried out using a Rigaku
–1
Ultima (IV) diffractometer with Cu−Kα radiation at a scanning rate of 2° min in the
2
θ range of 30–90° (40 kV, 40 mA).
DFT calculations were performed on 9a, 9b, 12a, and 12b using the density
functional theory (B3LYP) [28−30]. Cl, N, C and H atoms were described using the
-31G* basis set [31] and the Pd atom was described using the LANL2DZ basis set
32−35]. The solvent correction for CH Cl was carried out using the PCM [36−38].
6
[
2
2
The computational analyses on 9a, 9b, 12a and 12b were carried out using the
Gaussian 09 software [39].
4
.2 [ArN=C(NHAr)(NHR)] (R = 2-C H N, Ar = 2-MeC H ; 1) N ′,N″-Di(pyridin-
5 4 6 4
2
-yl)thiourea (230 mg, 1.00 mmol), 2-toluidine (118 mg, 1.10 mmol), KOH (140 mg
as 70% aq. solution) and nitrobenzene (100 mg) were charged into a 25 mL round
bottom (RB) flask. The contents in the RB flask were gradually heated up to 110 °C
1
5

ACCEPTED MANUSCRIPT
and maintained at the same temperature, while being stirred for 8 h. The reaction
mixture was cooled to room temperature and diluted with water (10 mL). The organic
portion was extracted with CH Cl (3 × 5 mL) and the extract dried over anhydrous
2
2
Na SO for an hour and filtered. The volatiles from the filtrate were removed under
2
4
vacuum to afford a yellow solid. The yellow solid was dispersed in n-hexane (10 mL),
stirred for 15 min and allowed to stand. The soluble portion was discarded. This
process was repeated two more times to afford a white solid. The solid was filtered
off and washed with diethyl ether (2 × 10 mL), and dried on a hot plate at 70 °C for 6
h to afford the guanidine 1. Yield: 81% (246 mg, 0.81 mmol). Mp: 131 °C. ATR−IR
–
1
1
(
cm ):
ν
(NH) 2958 (s, m), 2926 (s, m);
ν
(C=N) 1569 (s, sh). H NMR (CDCl , 400
3
MHz):
δ
2.45 (s, 3 H, CH ), 6.80 (br t, J = 5.2 Hz, 1 H, ArH), 6.89 (br t, JHH = 5.2
3 HH
Hz, 2 H, ArH), 6.98–7.04 (m, 2 H, ArH), 7.19–7.26 (m, 2 H, ArH), 7.54 (t, JHH = 7.0
Hz, 1 H, ArH), 7.63 (t, JHH = 6.8 Hz, 1 H, ArH), 8.21 (dd, JHH = 5.2; 1.6 Hz, 2 H,
1
3
1
ArH), 8.52 (br, 1 H, ArH), 11.86 and 14.04 (each s, 2 × 1 H, NH). C{ H} NMR
(
CDCl , 100.5 MHz): 19.0 (CH ), 113.3, 115.9, 116.8, 121.7, 122.7, 122.9, 126.3,
δ
3
3
1
28.6, 130.2, 137.6, 138.2, 138.4, 145.1, 145.9, 149.2, 153.6 (ArCH/ArC), 161.9
+
(
CN ). ESI Mass (HRMS) [M+H] Calc. 304.1562, Found 304.1559.
3
4
.3 [ArN=C(NHAr)(NHR)] (R = 2-C H N, Ar = 4-MeC H ; 2) Guanidine, 2 was
5 4 6 4
prepared from N ′,N″-Di(pyridin-2-yl)thiourea (230 mg, 1.00 mmol), 4-toluidine (118
mg, 1.10 mmol), KOH (140 mg as 70% aq. solution) and nitrobenzene (100 mg) by
following the procedure analogous to that outlined for 1. Yield: 76% (260 mg, 0.76
–
1
mmol). Mp: 107 °C. ATR−IR (cm ):
ν (NH) 3026 (br, w); ν (C=N) 1653 (s, m). The
1
H NMR spectrum of 2 revealed the presence of two isomers in ca 1.00:0.05 ratio as
1
estimated from the integrals of NH protons (see Fig. 11). H NMR (CDCl , 400
3
MHz):
δ 2.30 (s, 3 H, CH , Isomer 2), 2.35 (s, 3 H, CH , Isomer 1), 6.79 (t, J = 6.0
3 3 HH
1
6

ACCEPTED MANUSCRIPT
Hz, 2 × 1 H, ArH, Isomers 1 & 2), 6.89 (t, JHH = 6.0 Hz, 2 H, ArH, Isomer 1), 7.05 (d,
JHH = 8.0 Hz, 2 × 1 H, ArH, Isomers 1 & 2), 7.16 (d, JHH = 8.8 Hz, 2 × 2 H, ArH,
Isomers 1 & 2), 7.36 (t, JHH = 8.0 Hz, 2 H, ArH, Isomer 2), 7.55 (t, JHH = 7.0 Hz, 2 × 1
H, ArH, Isomers 1 & 2), 7.62 (t, JHH = 7.2 Hz, 2 × 1 H, ArH, Isomers 1 & 2), 7.73 (d,
JHH = 8.0 Hz, 2 H, ArH, Isomer 1), 7.87 (d, JHH = 7.2 Hz, 2 H, ArH, Isomer 2), 8.23
(d, JHH = 7.2 Hz, 2 H, ArH, Isomer 1), 8.37 (d, JHH = 2.8 Hz, 2 H, ArH, Isomer 2),
1
1
1.95 (s, 1 H, NH, Isomer 1), 12.07 (s, 1 H, NH, Isomer 2), 13.96 (s, 1 H, NH, Isomer
1
3
1
). The signal for second NH proton of isomer 2 was not observed. C{ H} NMR
(
CDCl , 100.5 MHz):
δ
20.9 (CH ), 113.3, 115.8, 116.8, 121.3, 121.6, 129.2, 132.2,
3
3
1
37.0, 137.5, 138.3, 145.1, 146.0, 149.0, 153.7 (ArCH/ArC), 162.0 (CN ). ESI Mass
3
+
(
HRMS) [M+H] Calc. 304.1562, Found 304.1551. Anal. Calc for C H N (Mw =
1
8
17
5
3
03.361): C, 71.27; H, 5.65; N, 23.09. Found: C, 71.18; H, 5.36; N, 22.87.
Fig. 11.
4
.4 [ArN=C(NHAr)(NHR)] (R = 2-C H N, Ar = 4-ClC H ; 3) Guanidine 3 was
5 4 6 4
prepared from N ′,N″-Di(pyridin-2-yl)thiourea (230 mg, 1.00 mmol), 4-chloroaniline
140 mg, 1.10 mmol), KOH (140 mg as 70% aq. solution) and nitrobenzene (100 mg)
by following the procedure analogous to that outlined for 1. Yield: 78% (253 mg, 0.78
(
–
1
1
mmol). Mp: 137 °C. ATR−IR (cm ):
NMR (CDCl , 400 MHz): 6.82–6.90 (m, 3 H, ArH), 7.05 (d, JHH = 7.6 Hz, 1 H,
ArH), 7.29 (d, JHH = 8.4 Hz, 2 H, ArH), 7.57 (t, JHH = 6.8 Hz, 1 H, ArH), 7.63 (t, JHH
7.2 Hz, 1 H, ArH), 7.82 (d, JHH = 8.8 Hz, 2 H, ArH), 8.23 (br, 2 H, ArH), 12.13 and
ν (NH) 2999 (br, w); ν (C=N) 1649 (s, m). H
δ
3
=
1
3
1
1
1
1
3.98 (each, 2 × 1 H, NH). C{ H} NMR (CDCl , 100.5 MHz): δ 113.4, 116.2,
3
17.0, 121.6, 122.2, 127.3, 128.6, 128.7, 137.7, 136.3, 138.4, 145.1, 146.0, 148.7,
+
53.5 (ArCH/ArC), 161.6 (CN ). ESI Mass (HRMS) [M+H] Calc. 324.1016, Found
3
1
7

ACCEPTED MANUSCRIPT
3
24.1007. Anal. Calc for C H N Cl (Mw = 323.78): C, 63.06; H, 4.36; N, 21.63.
17
14
5
Found: C, 63.37; H, 4.38; N, 22.00.
.5 [ArN=C(NHAr)(NHR)] (R = 2-C H N, Ar = 4-BrC H ; 4) Guanidine 4 was
4
5
4
6
4
prepared from N ′,N″-di(pyridin-2-yl)thiourea (230 mg, 1.00 mmol), 4-bromoaniline
189 mg, 1.10 mmol), KOH (140 mg as 70% aq. solution) and nitrobenzene (100 mg)
by following the procedure analogous to that outlined for 1. Yield: 75% (276 mg, 0.75
(
–
1
1
mmol). Mp: 171 °C. ATR−IR (cm ):
NMR (CDCl , 400 MHz): 6.81−6.87 (m, 2 H, ArH) 6.91 (t, JHH = 6.2 Hz, 1 H,
ArH), 7.06 (d, JHH = 8.4 Hz, 1 H, ArH), 7.43 (d, JHH = 9.2 Hz, 2 H, ArH), 7.58 (t, JHH
7.6 Hz, 1 H, ArH), 7.64 (t, JHH = 7.2 Hz, 1 H, ArH), 7.78 (d, JHH = 9.2 Hz, 2 H,
ν (NH) 3015 (br, w); ν (C=N) 1655 (m). H
δ
3
=
1
3
1
ArH), 8.23 (t, JHH = 5.8 Hz, 2 H, ArH), 12.13 and 13.98 (each, 2 × 1 H, NH). C{ H}
NMR (CDCl , 100.5 MHz): 113.4, 114.9, 116.2, 117.0, 121.6, 122.6, 131.5, 137.7,
38.5, 138.8, 145.1, 146.0, 148.7, 153.5 (ArCH/ArC), 161.6 (CN ). ESI Mass
δ
3
1
3
+
(
HRMS) [M+H] Calc. 368.0510, Found 368.0508. Anal. Calc for C H N Br (Mw =
17 14 5
3
68.231): C, 55.45; H, 3.83; N, 19.02. Found: C, 55.64; H, 3.74; N, 19.05.
.6 [ArN=C(NHAr)(NHR)] (R = 6-Me-2-C H N, Ar = 4-MeC H ; 5) Guanidine 5
4
5
3
6
4
was prepared from N ′,N″-bis(6-methyl pyridin-2-yl)thiourea (258 mg, 1.00 mmol), 4-
toluidine (118 mg, 1.10 mmol), KOH (140 mg as 70% aq. solution) and nitrobenzene
(100 mg) by following the procedure analogous to that outlined for 1. Yield: 77%
–
1
(
255 mg, 0.77 mmol). Mp: 135 °C. ATR−IR (cm ): ν(NH) 2918 (br, w); ν (C=N)
1
1
1
655 (m). The H NMR spectrum of 5 revealed the presence of two isomers in ca
1
.00:0.06 ratio as estimated from the integrals of NH protons (see Fig. 11). H NMR
2.34 (s, 3 H, CH , Isomer 1), 2.38 (s, 3 H, CH , Isomer 2), 2.52,
(
CDCl , 400 MHz):
δ
3
3
3
2
6
.54 (each s, 2 × 3 H, CH , Isomer 1), 2.68, 2.70 (each s, 2 × 3 H, CH , Isomer 2),
3 3
.60 (d, JHH = 7.2 Hz, 2 × 1 H, ArH, Isomers 1 & 2), 6.65 (d, JHH = 7.3 Hz, 2 × 1 H,
1
8

ACCEPTED MANUSCRIPT
ArH, Isomers 1 & 2), 6.74 (d, JHH = 7.3 Hz, 1 H, ArH, Isomer 1), 6.87 (d, JHH = 8.0
Hz, 1 H, ArH, Isomer 1), 7.04 (t, JHH = 7.4 Hz, 2 H, ArH, Isomer 2), 7.15 (d, JHH = 8.0
Hz, 2 × 2 H, ArH, Isomers 1 & 2), 7.34 (t, JHH = 8.0 Hz, 2 H, ArH, Isomer 2), 7.45 (t,
JHH = 7.7 Hz, 1 H, ArH, Isomer 1), 7.51 (t, JHH = 7.7 Hz, 1 H, ArH, Isomer 1), 7.74 (d,
JHH = 8.1 Hz, 2 H, ArH, Isomer 1), 7.87 (d, JHH = 8.4 Hz, 2 H, ArH, Isomer 2), 12.23
(s, 1 H, NH, Isomer 1), 12.35 (s, 1 H, NH, Isomer 2), 14.32 (s, 1 H, NH, Isomer 1)
1
3
(
see Fig. 11). The signal for second NH proton of isomer 2 was not observed. C
1
{
1
1
H} NMR (CDCl , 100.5 MHz): δ 21.0, 24.2 (CH ), 109.8, 115.0, 115.9, 118.3,
3 3
20.7, 129.3, 131.8, 137.4, 137.9, 138.6, 149.1, 153.2, 153.5, 155.2 (ArCH/ArC),
+
61.5 (CN ). ESI Mass (HRMS) [M+H] Calc. 332.1875, Found 332.1861. Anal. Calc
3
for C H N (Mw = 331.414): C, 72.48; H, 6.39; N, 21.13. Found: C, 72.28; H, 6.72;
2
0
21
5
N, 21.42.
.7 [ArN=C(NHAr)(NHR)] (R = 6-Me-2-C H N, Ar = 4-ClC H ; 6) Guanidine, 6
4
5
3
6
4
was prepared from N ′,N″-bis(6-methyl pyridin-2-yl)thiourea (258 mg, 1.00 mmol), 4-
chloroaniline (140 mg, 1.10 mmol), KOH (140 mg as 70% aq. solution) and
nitrobenzene (100 mg) by following the procedure analogous to that outlined for 1.
–
1
Yield: 75% (264 mg, 0.75 mmol). Mp: 176 °C. ATR−IR (cm ):
ν (NH) 2916 (br, w);
1
ν
(C=N) 1659 (m). H NMR (CDCl , 400 MHz):
δ
2.52, 2.54 (each s, 2 × 3 H, CH3)
3
6
.60 (d, JHH = 7.6 Hz, 1 H, ArH), 6.68 (d, JHH = 6.8 Hz, 1 H, ArH), 6.75 (d, JHH = 6.8
Hz, 2 H, ArH), 6.88 (d, JHH = 7.6 Hz, 1 H, ArH), 7.28 (dd, JHH = 7.0; 2.2 Hz, 2 H,
ArH), 7.47, 7.53 (each t, JHH = 7.6 Hz, 2 × 1 H, ArH), 7.82 (d, JHH = 8.4 Hz, 1 H,
1
3
1
ArH), 12.42 and 14.34 (each s, 2 × 1 H, NH). C{ H} NMR (CDCl , 100.5 MHz):
δ
3
2
1
4.2 (CH ), 109.8, 115.4, 116.1, 118.3, 121.5, 126.9, 126.6, 138.0, 138.7, 148.8,
3
+
53.1, 153.6, 155.2 (ArC/ArCH), 161.1 (CN ). ESI Mass (HRMS) [M+H] Calc.
3
1
9

ACCEPTED MANUSCRIPT
3
5
4
52.1328, Found 352.1329. Anal. Calc for C H N Cl (Mw = 351.8): C, 64.86; H,
1
9
18
5
.16; N, 19.91. Found: C, 64.78; H, 4.89; N, 19.78.
.8 [ArN=C(NHAr)(NHR)] (R = 6-Me-2-C H N, Ar = 4-FC H ; 7) Guanidine, 7
5
3
6
4
was prepared from N ′,N″-bis(6-methyl pyridine-2-yl)thiourea (258 mg, 1.00 mmol),
-fluoroaniline (122 mg, 1.10 mmol), KOH (140 mg as 70% aq. solution) and
nitrobenzene (100 mg) by following the procedure analogous to that outlined for 1.
4
–
1
Yield: 75% (238 mg, 0.75 mmol). Mp: 176 °C. ATR−IR (cm ):
ν (NH) 2918 (br, w);
1
ν
(C=N) 1657 (m). H NMR (CDCl , 400 MHz):
δ
2.52, 2.54 (each s, 2 × 3 H, CH3),
3
6
.60 (d, JHH = 7.6 Hz, 1 H, ArH), 6.67 (d, JHH = 6.8 Hz, 1 H, ArH), 6.75, 6.86 (each d,
JHH = 7.6 Hz, 2 × 1 H, ArH), 7.03 (t, JHH = 8.8 Hz, 2 H, ArH), 7.46 (t, JHH = 7.6 Hz, 1
H, ArH), 7.52 (t, JHH = 8.0 Hz, 1 H, ArH), 7.79−7.83 (m, 2 H, ArH), 12.31 and 14.34
1
3
1
(
each s, 2 × 1 H, NH). C{ H} NMR (CDCl , 100.5 MHz): δ 24.2 (CH ), 109.8,
3 3
1
1
15.2 (d, J = 22.0 Hz), 116.0, 118.3, 120.6, 121.9 (d, JCF = 7.6 Hz), 122.4, 128.8,
CF
36.1, 138.0, 138.7, 149.0, 153.2, 153.6, 155.2, 158.4 (d, JCF = 241.5 Hz)
19
(
ArCH/ArC), 161.3 (CN3). F NMR (CDCl , 376.31 MHz):
δ
−121.0. ESI Mass
3
+
(
HRMS) [M+H] Calc. 336.1624, Found 336.1615. Anal. Calc for C H N F (Mw =
1
9
18
5
3
4
35.378): C, 68.04; H, 5.41; N, 20.88. Found: C, 68.02; H, 5.65; N, 20.86.
2
.9 [Pd{
κ
(N,N)-C H N-1(N=C(NHAr)(NPy))-2} ] (Ar = 4-MeC H ; 8) Pd(OAc)
2
5
4
2
6
4
(50.0 mg, 0.223 mmol), guanidine 2 (135 mg, 0.445 mmol), and toluene (5 mL) were
taken in a 25 mL RB flask, fitted with a double surface condenser equipped with a
freshly prepared anhyd. CaCl guard tube. The mixture was stirred at 60 °C for 3 h to
2
afford 8 as yellow solid. The solid was filtered off, dissolved in CH Cl layered with
2
2,
n-hexane and stored at room temperature for several days to afford 8 as orange
crystals suitable for SCXRD. Yield: 80% (148 mg, 0.18 mmol). Mp: 276 °C.
–
1
1
ATR−IR (cm ):
ν
(NH) 2934 (br, w);
ν
(C=N) 1618 (m). H NMR (CDCl , 400
3
2
0

ACCEPTED MANUSCRIPT
MHz):
δ
2.34 (s, 6 H, CH ), 6.25 (dt, J = 6.6; 1.6 Hz, 2 H, ArH), 6.56 (t, JHH = 5.8
3
HH
Hz, 2 H, ArH), 6.83 (d, JHH = 8.0 Hz, 2 H, ArH), 7.10–7.23 (m, 8 H, ArH), 7.57 (d,
JHH = 8.8 Hz, 2 H, ArH), 7.65 (t, JHH = 7.7 Hz, 6 H, ArH), 8.02 (dd, JHH = 4.6; 1.8 Hz,
+
+
2
7
H, ArH), 12.61 (s, 2 H, NH). MS (TOF-ES ), m/z (intensity %), [M+H] Calc.
11.1924, Found 711.0419. Anal. Calc for C H N Pd (Mw = 711.126): C, 60.80;
3
6
32 10
H, 4.54; N, 19.70. Found: C, 60.60; H, 4.51; N, 19.91.
2
4
.10 [Pd{
κ
(N,N)-C H N-1(N=C(NHAr)(NPy))-2} ] (Ar = 4-ClC H ; 9) Complex 9
5
4
2
6
4
was prepared from Pd(OAc) (50.0 mg, 0.223 mmol) and guanidine 3 (144 mg, 0.445
2
mmol) in toluene (5 mL) by following the procedure analogous to that outlined for
complex 8. Orange crystals suitable for SCXRD were grown from
CH Cl /cyclohexane mixture over a period of one week at room temperature. Yield:
2
2
–
1
7
2% (121 mg, 0.16 mmol). Mp: 263 °C. ATR−IR (cm ): ν (NH) 2929 (br, w); ν
1
(
C=N) 1610 (m). H NMR (CDCl , 400 MHz):
δ
6.28, 6.59 (each t, JHH = 6.0 Hz, 2 ×
3
2
H, ArH), 6.85 (d, JHH = 8.4 Hz, 2 H, ArH), 7.15 (dt, JHH = 8.0 Hz; 1.8 Hz, 2 H,
ArH), 7.24 (t, JHH = 6.4 Hz, 2 H, ArH), 7.30 (d, JHH = 8.8 Hz, 4 H, ArH), 7.56 (d, JHH
=
8.4 Hz, 2 H, ArH), 7.63 (d, JHH = 6.4 Hz, 2 H, ArH), 7.76 (d, JHH = 8.4 Hz, 4 H,
+
ArH), 8.04 (d, JHH = 4.4 Hz, 2 H, ArH), 12.8 (s, 2 H, NH). ESI Mass (HRMS) [M+H]
Calc. 751.0832, Found 751.0819. Anal. Calc for C H N Cl Pd (Mw = 751.963): C,
34
26 10
2
5
4
1
0
4.31; H, 3.49; N, 18.63. Found: C, 54.00; H, 3.82; N, 18.56.
2
.11 [Pd{
κ
(N,N)-C H N-1(N=C(NHAr)(NPy))-2} ] (Ar = 4-BrC H ; 10) Complex
5
4
2
6
4
0 was prepared from Pd(OAc) (50.0 mg, 0.223 mmol) and guanidine 4 (164 mg,
2
.445 mmol) in toluene (5 mL) by following the procedure analogous to that outlined
for complex 8. Orange crystals suitable for SCXRD were grown from
CH Cl /cyclohexane mixture over a period of several days at room temperature.
2
2
–
1
Yield: 73% (138 mg, 0.16 mmol). Mp: 261 °C. ATR−IR (cm ):
ν (NH) 2922 (br,
2
1

ACCEPTED MANUSCRIPT
1
w);
ν (C=N) 1589 (m). H NMR (CDCl , 400 MHz): δ 6.28, 6.60 (each t, JHH = 5.8
3
Hz, 2 × 1 H, ArH), 6.67, 6.71 (each t, JHH = 6.2 Hz, 2 × 1 H, ArH), 6.85, 7.02 (each d,
JHH = 8.4 Hz, 2 × 1 H, ArH), 7.15 (dt, JHH = 7.9; 2.0 Hz, 1 H, ArH), 7.23 (d, JHH = 7.2
Hz, 1 H, ArH), 7.37 (d, JHH = 6.4 Hz, 1 H, ArH), 7.43 (t, JHH = 8.8 Hz, 4 H, ArH),
7
.48 (d, JHH = 6.0 Hz, 1 H, ArH), 7.51 (t, JHH = 6.6 Hz, 1 H, ArH), 7.56 (d, JHH = 8.4
Hz, 1 H, ArH), 7.61–7.64 (m, 4 H, ArH), 7.72 (d, JHH = 8.8 Hz, 2 H, ArH), 7.94 (d,
JHH = 8.4 Hz, 1 H, ArH), 8.04 (d, JHH = 5.2 Hz, 1 H, ArH), 12.26 and 12.80 (each s, 2
+
×
1 H, NH). ESI Mass (HRMS) [M+H] Calc. 838.9822, Found 838.9784. Anal. Calc
for C H N Br Pd (Mw = 840.805): C, 48.56; H, 3.12; N, 16.66. Found: C, 48.80;
34
26 10
2
H, 3.28; N, 16.99.
2
4
.12 [Pd{
κ
(N,N)-C H N-1-Me-6(N=C(NHAr)(N(6-MePy))-2} ] (Ar = 4-MeC H ;
5
3
2
6
4
1
1) Complex 11 was prepared from Pd(OAc) (50.0 mg, 0.223 mmol) and guanidine 5
2
(147 mg, 0.445 mmol) in toluene (5 mL) by following the procedure analogous to that
outlined for complex 8. Red crystals suitable for SCXRD were grown from CH Cl
2
2
/
cyclohexane mixture over a period of several days at room temperature. Yield: 79%
−1
(
135 mg, 0.18 mmol). Mp: 258 °C (decomp.). ATR−IR (cm ):
ν
(NH) 2914 (br, w);
1
ν
(C=N) 1618 (m). H NMR (CDCl , 400 MHz):
δ
2.16, 2.19, 2.33 (each s, 6 × 3 H,
3
CH ), 6.49 (d, J = 6.6 Hz, 2 H, ArH), 6.56 (d, JHH = 7.3 Hz, 2 H, ArH), 6.85 (d, JHH
3
HH
=
8.1 Hz, 2 H, ArH), 7.13 (t, JHH = 8.1 Hz, 4 H, ArH), 7.34, 7.46 (each t, JHH = 7.7
Hz, 2 × 2 H, ArH), 7.53 (d, JHH = 8.0 Hz, 4 H, ArH), 7.73 (d, JHH = 8.1 Hz, 2 H, ArH),
+
1
2.53 (s, 2 H, NH). ESI Mass (HRMS) [M+H] Calc. 767.2550, Found 767.2512.
Anal. Calc for C H N Pd (Mw = 767.233): C, 62.62; H, 5.25; N, 18.26. Found: C,
4
0
40 10
6
4
1
2.31; H, 5.24; N, 18.42.
2
.13 [Pd{
κ
(N,N)-C H N-1-Me-6(N=C(NHAr)(N(6-MePy))-2} ] (Ar = 4-ClC H ;
5
3
2
6
4
2) Complex 12 was prepared from Pd(OAc) (50.0 mg, 0.223 mmol) and guanidine 6
2
2
2

ACCEPTED MANUSCRIPT
(156.6 mg, 0.445 mmol) in toluene (5 mL) by following the procedure analogous to
that outlined for complex 8. Red crystals suitable for SCXRD were grown from
CH Cl /cyclohexane mixture over a period of several days at room temperature.
2
2
−1
Yield: 69% (124 mg, 0.15 mmol). Mp: 270 °C (decomp.). ATR−IR (cm ):
ν (NH)
1
2
922 (w);
ν
(C=N) 1593 (m). The H NMR spectrum of 12 revealed the presence of
three isomers in ca 1.00:0.08:0.03 ratio as estimated from the integrals of NH protons
1
(
see Fig. 12). H NMR (CDCl , 400 MHz):
δ
2.17, 2.20 (each s, 4 × 3 H, CH , Isomer
3
3
1
), 2.41 (s, 2 × 3 H, CH , Isomer 2), 2.52, 2.54 (each s, 4 × 3 H, CH , Isomer 3), 2.84
3 3
(
s, 2 × 3 H, CH , Isomer 2), 6.14 (d, J = 7.2 Hz, 2 H, ArH, Isomer 3), 6.43 (d, JHH =
3 HH
7
.2 Hz, 2 H, ArH, Isomer 2), 6.53 (d, JHH = 7.3 Hz, 2 H, ArH, Isomer 1), 6.58 (d, JHH
7.3 Hz, 2 × 2 H, ArH, Isomers 1 & 3), 6.71 (d, JHH = 8.8 Hz, 2 H, ArH, Isomer 2),
.75 (d, JHH = 6.0 Hz, 2 H, ArH, Isomer 2), 6.86 (d, JHH = 8.0 Hz, 2 × 2 H, ArH,
=
6
Isomers 1 & 3), 7.01 (t, JHH = 7.4 Hz, 2 H, ArH, Isomer 2), 7.07 (d, JHH = 8.0 Hz, 2 H,
ArH, Isomer 2), 7.11 (d, JHH = 8.4 Hz, 2 H, ArH, Isomer 2), 7.27 (d, JHH = 8.8 Hz, 3 ×
4
7
H, ArH, Isomers 1, 2 & 3), 7.38 (t, JHH = 7.7 Hz, 3 × 2 H, ArH, Isomers 1, 2 & 3),
.44 (t, JHH = 7.7 Hz, 2 × 2 H, ArH, Isomers 1 & 3), 7.61 (d, JHH = 8.8 Hz, 2 × 4 H,
ArH, Isomers 1 & 3), 7.70 (d, JHH = 8.1 Hz, 2 × 2 H, ArH, Isomers 1 & 3), 7.82 (d,
JHH = 8.0 Hz, 2 H, ArH, Isomer 2), 12.59 (s, 2 H, NH, Isomer 2), 12.68 (s, 2 H, NH,
Isomer 1), 13.25 (s, 2 H, NH, Isomer 3). Isomers 2 and 3 could possibly arise from the
isomer 1 through two independent restricted C−N(H)Ar single bond rotations of the
+
guanidinato ligand [40]. ESI Mass (HRMS) [M+H] Calc. 807.1457, Found 807.1394.
Anal. Calc for C H N Cl Pd (Mw = 808.07): C, 56.48; H, 4.24; N, 17.33. Found:
3
8
34 10
2
C, 56.72; H, 4.42; N, 17.28.
Fig. 12
2
3

ACCEPTED MANUSCRIPT
2
4
1
.14 [Pd{
κ
(N,N)-C H N-1-Me-6(N=C(NHAr)(N(6-MePy))-2} ] (Ar = 4-FC H ;
5
3
2
6
4
3) Complex 13 was prepared from Pd(OAc) (50 mg, 0.223 mmol) and guanidine 7
2
(149.2 mg, 0.445 mmol) in toluene (5 mL) by following the procedure analogues to
that outlined for complex 8. Red crystals suitable for SCXRD were grown from
CH Cl /cyclohexane mixture over a period of several days at room temperature.
2
2
−1
Yield: 73% (126 mg, 0.16 mmol). Mp: 233 °C (decomp.). ATR−IR (cm ):
ν (NH)
1
2
922 (br, w);
ν
(C=N) 1570 (m). The H NMR spectrum of 13 revealed the presence
of two isomers in ca 1.00:0.36 ratio as estimated from the integrals of NH protons
1
(
see Fig. 12). H NMR (CDCl , 400 MHz):
δ
2.14, 2.18 (each s, 8 × 3 H, CH3,
3
Isomers 1 & 2), 6.49 (d, JHH = 6.8 Hz, 2 × 2 H, ArH, Isomers 1 & 2), 6.56 (d, JHH
=
7
.2 Hz, 2 × 2 H, ArH, Isomers 1 & 2), 6.83 (d, JHH = 8.0 Hz, 2 H, ArH, Isomer 1),
.87 (d, JHH = 8.4 Hz, 2 H, ArH, Isomer 2), 7.02 (t, JHH = 8.6 Hz, 2 × 4 H, ArH,
6
Isomers 1 & 2), 7.32−7.36 (m, 6H, ArH, 2H (Isomer 1) & 4H (Isomer 2)), 7.45 (t, JHH
7.6 Hz, 2 × 2 H, ArH, Isomers 1 & 2), 7.58−7.61 (m, 6 H, ArH, Isomer 1 (4 H) &
=
Isomer 2 (2 H)), 7.66 (d, JHH = 8.0 Hz, 2 H, ArH, Isomer 2), 7.72 (d, JHH = 8.0 Hz, 2
H, ArH, Isomer 1), 12.59 (s, 2 H, NH, Isomer 1), 12.61 (s, 2 H, NH, Isomer 2).
1
3
1
C{ H} NMR (CDCl , 100.5 MHz):
δ 23.61, 24.91 (CH ), 114.88, 115.26 (d, J =
3 CF
3
2
2.1 Hz), 116.61, 116.63 (d, J = 22.2 Hz, Isomer 2), 118.52, 121.12, 122.31, 122.53
CF
(d, JCF = 7.6 Hz), 128.78, 136.78, 137.21, 138.07, 140.75, 152.00, 152.09, 152.13,
1
55.53, 158.49 (d, J = 240.8 Hz), 159.51, 159.86 (ArCH/ArC/CN ). The isomer 2
CF
3
could possibly arise from the isomer 1 through a restricted C−N(H)Ar single bond
1
9
rotation of the guanidinato ligand. F NMR (CDCl , 376.31 MHz):
δ –121.16. ESI
3
+
Mass (HRMS) [M+H] Calc. 775.2048, Found 775.2078. Anal. Calc for
C H N F Pd (Mw = 775.16): C, 58.88; H, 4.42; N, 18.07. Found: C, 58.98; H, 4.49;
38
34 10 2
N, 18.32.
2
4

ACCEPTED MANUSCRIPT
4
.15 General procedure for H-MCR of aryl bromides with styrene To a 25 mL
RB flask was taken styrene (145.8 mg, 1.4 mmol), aryl bromide (1.0 mmol), sodium
acetate (90.2 mg, 1.1 mmol), TBAB (2.0 gm, 6.2 mol) and the catalyst (0.8 mg, 0.001
mmol). The reaction mixture was heated to 120 °C and stirred at the same temperature
for 12 h and cooled. The organic portion from the reaction mixture was extracted with
diethyl ether (2 × 15 mL). The extracts were combined and the volatiles removed on a
rotavapor. The desired coupling product was isolated on a silica gel column using
n-hexane→ethyl acetate/n-hexane (2/98, v/v) mixture as eluent.
4
.16 General procedure for H-MCR of aryl chlorides with styrene and methyl
acrylate To a 25 mL RB flask was taken styrene (145.8 mg, 1.4 mmol) or methyl
acrylate (120.5 mg, 1.4 mmol), aryl chloride (1.0 mmol), sodium acetate (90.2 mg,
1
.1 mmol), TBAB (2.0 gm, 6.2 mol) and the catalyst (4.0 mg, 0.005 mmol). The
reaction mixture was heated to 120 °C and stirred at the same temperature for 24 h
and cooled. The organic portion from the reaction mixture was extracted with diethyl
ether (2 × 15 mL). The extracts were combined and the volatiles removed on a
rotavapor. The desired coupling product was isolated on a silica gel column using
n-hexane→ethyl acetate/n-hexane (2/98, v/v) mixture as eluent.
4
.17 Neat reaction involving 9 and TBAB Complex 9 (301 mg, 0.40 mmol) and
TBAB (2.0 gm, 6.20 mol) were taken a 25 mL RB flask. The reaction mixture was
heated to 120 °C and stirred at the same temperature for 12 h and cooled.
Subsequently, the reaction mixture was diluted with chloroform (50 mL) and
centrifuged to afford a black palladium powder. The powder was washed with
chloroform (2 × 10 mL) and dried in an oven at 70 °C for an hour. Yield: 85% (36
mg, 0.34 mmol).
2
5

ACCEPTED MANUSCRIPT
Acknowledgements
We thank University of Delhi and Department of Science and Technology, New Delhi
for DU-DST research grant and one of us (V.M.) acknowledges Council of Scientific
and Industrial Research, New Delhi for a fellowship.
Supplementary data
The supplementary data contains X−ray crystallographic data, tables, selected bond
parameters of structurally characterized compounds and NMR spectra of all
compounds (PDF) and are available on the publisher’s website.
Accession Codes CCDC 1867604–1867611 and 1872496–1872497 contain the
supplementary crystallographic data (CIF files) for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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2
8