Syntheses, characterization, solution behavior and catalytic activity of trans-[(guanidine)2PdX2] (X?=?Cl and OC(O)R; R?=?Me, Ph and tBu) in Heck–Mizoroki coupling reactions involving chloroarenes/methyl acrylate

Article abstract of DOI:10.1016/j.poly.2018.05.022

Trans-[{(ArNH)₂C[dbnd]NAr}₂PdX₂] (Ar = 2,5-Me₂C₆H₃; X = Cl (1) and OC(O)R; R = Me (2), Ph (3), and ^tBu (4)) were isolated in high yields and characterized by elemental analysis, IR, and NMR (¹H and ¹³C) spectroscopy. The molecular structures of the aforementioned complexes were determined by single crystal X-ray diffraction which revealed trans syn anti-anti (1·CHCl₃), trans anti anti-syn (2·CHCl₃ and 3·C₇H₈) and trans anti anti-anti (4) stereochemistry in solid state. Complexes 2·CHCl₃, 3·C₇H₈ and 4 contain a pair of R¹₁(8) rings stabilized by an intramolecular N–H?O hydrogen bond between the guanidine ligand and the carboxylate moiety. The influence of shape and size of the anion upon the stereochemistry of the complexes in the solid state and in solution are discussed. VT ¹H NMR spectroscopic studies carried out on samples of 1 and 3 revealed the presence of a mixture of two rotamers in solution which arise due to the restricted [dbnd]C–N(H)Ar single bond rotation of the guanidine ligand in both complexes. Complexes 1–4 were shown to be active catalysts even when used in 0.001 mol% in Heck–Mizoroki coupling reactions involving chlorobenzene and methyl acrylate. The scope of 1 in Heck–Mizoroki coupling reactions involving methyl acrylate and nine distinct chloroarenes were explored at 0.01 mol% which afforded the coupling products in 81?96% yields.

Full text of DOI:10.1016/j.poly.2018.05.022

Polyhedron 151 (2018) 313–322
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Syntheses, characterization, solution behavior and catalytic activity of
t
trans-[(guanidine) PdX ] (X = Cl and OC(O)R; R = Me, Ph and Bu) in
2
2
Heck–Mizoroki coupling reactions involving chloroarenes/methyl
acrylate
a
a
b
a,
⇑
Palani Elumalai , Rishabh Ujjval , Munirathinam Nethaji , Natesan Thirupathi
a
Department of Chemistry, University of Delhi, Delhi 110 007, India
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bengaluru 560 012, India
b
a r t i c l e i n f o
a b s t r a c t
t
Article history:
2 2 2 2 6 3
Trans-[{(ArNH) C@NAr} PdX ] (Ar = 2,5-Me C H ; X = Cl (1) and OC(O)R; R = Me (2), Ph (3), and Bu (4))
Received 8 August 2017
Accepted 8 May 2018
Available online 30 May 2018
1
13
were isolated in high yields and characterized by elemental analysis, IR, and NMR ( H and C) spec-
troscopy. The molecular structures of the aforementioned complexes were determined by single crystal
X-ray diffraction which revealed trans syn anti-anti (1ꢁCHCl
3
), trans anti anti-syn (2ꢁCHCl
, 3ꢁC and 4 contain a pair of
(8) rings stabilized by an intramolecular N–Hꢁ ꢁ ꢁO hydrogen bond between the guanidine ligand and the
3
and 3ꢁC
7 8
H ) and
trans anti anti-anti (4) stereochemistry in solid state. Complexes 2ꢁCHCl
3
7 8
H
Keywords:
1
R
1
Palladium
carboxylate moiety. The influence of shape and size of the anion upon the stereochemistry of the complexes
in the solid state and in solution are discussed. VT 1H NMR spectroscopic studies carried out on samples of 1
and 3 revealed the presence of a mixture of two rotamers in solution which arise due to the restricted @CAN
(H)Ar single bond rotation of the guanidine ligand in both complexes. Complexes 1–4 were shown to be
active catalysts even when used in 0.001 mol% in Heck–Mizoroki coupling reactions involving chloroben-
zene and methyl acrylate. The scope of 1 in Heck–Mizoroki coupling reactions involving methyl acrylate
and nine distinct chloroarenes were explored at 0.01 mol% which afforded the coupling products in
0
00
N,N ,N -triarylguanidines
Heck–Mizoroki coupling
NꢀHꢁ ꢁ ꢁO hydrogen bond
Conformers
81ꢀ96% yields.
Ó 2018 Elsevier Ltd. All rights reserved.
1
. Introduction
Guanidines are popular for their role as non-ionic bases, super
dianionic forms [1–3,16]. Only a handful of Pd(II) complexes of sym
N,N’,N -triphenylguanidine, (PhNH) C@NPh are known wherein
2
0
0
the guanidine acts as neutral, monoanionic chelating/bridging N-
donor ligand, and as monoanionic chelating C,N donor ligand
[17–19]. Robson and co-workers reported the crystal structure of
bases and as N-donor ligands for a variety of metal ions. Transition
metal complexes of guanidines are well known for their intriguing
structural aspects, reactivity pattern and their utility as homoge-
neous catalysts and as MOCVD and ALD precursors [1–7]. Pd(II)
complexes of neutral monodentate [8–12], and multidentate
guanidines wherein the AN@C(NRR )
2ꢀ
3
[LPd Cl
3
]
5
(LH = tris(2-hydroxybenzylidene)triaminoguanidine)
which exhibits chiral propeller-like structure wherein the three
Pd(II) atoms are assembled via the converging imino and amido
0
2
units are linked by organic
N atoms, and phenolato oxygen atoms [20].
0
00
[
13], phosphoryl and carbophosphazenyl [14] and 1,1 -fer-
We have shown that sym N,N’,N -triarylguanidines, (ArNH)
2
-
rocenediyl based [15] units are well known. Palladium(II) com-
plexes of monodentate guanidines have been shown as active
catalysts in Heck and Suzuki coupling reactions [10,11].
C@NAr that possess Me/OMe substituents in the aryl rings undergo
cyclometalation with Pd(OC(O)R) and cis-[(Me SO) PtCl ] in the
presence of external base, NaOAc under facile reaction condition
2
2
2
2
00
2
Sym N,N’,N -trisubstituted guanidines, (RHN)
2
C@NR (Sym =
to afford six-membered cyclopalladated guanidines, [
j
(C,N)Pd
and six-membered cycloplatinated guanidines, cis-
S(O))Cl] [21,22]. We herein report syntheses and
C@NAr} PdX ] (Ar = 2,5-Me
; X = Cl (1), OC(O)R; R = Me (2), Ph (3) and Bu (4)) with a view
aimed at understanding how the shape/size of the anion influences
Symmetrical; R = alkyl or aryl) are an interesting sub-class of guani-
dines, capable of ligating the metal ions in their neutral, mono- and
(
l
-OC(O)R)]
2
(C,N)Pt(Me
2
2
[j
characterization of trans-[{(ArNH)
2
2
2
2 6
C -
t
H
3
⇑
Corresponding author.
E-mail address: thirupathi_n@yahoo.com (N. Thirupathi).
https://doi.org/10.1016/j.poly.2018.05.022
277-5387/Ó 2018 Elsevier Ltd. All rights reserved.
0

3
14
P. Elumalai et al. / Polyhedron 151 (2018) 313–322
the structural aspects of these complexes in solid state and in solu-
tion. The catalytic utility of 1 in Heck–Mizoroki coupling reactions
of nine chloroarenes with methyl acrylate at 0.01 mol% are
reported. The results of our endeavor are presented in the follow-
ing sections.
2
. Results and discussion
2.1. Syntheses
2 2 2
The reaction of trans-[(PhCN) PdCl ] with (ArNH) C@NAr (Ar =
2
,5-xylyl
2
,5-Me C H
2 6 3
; LH
2
) in 1:2 mole ratio in toluene under reflux
condition for 12 h afforded 1 in 82% yield as illustrated in Scheme 1.
The aforementioned reaction carried out separately with Pd(OC(O)
t
R)
2
(R = Me, Ph and Bu) as Pd(II) source afforded 2–4 in ꢂ95%
Scheme 1. Preparation of complexes 1–4.
yields. Complexes 1–4 were crystallized from toluene/CHCl
ture at RT over a period of several days to afford 1ꢁCHCl , 2ꢁCHCl
and 4 respectively as orange red crystals suitable for single
3
mix-
3
3
,
3ꢁC
7 8
H
crystal X-ray diffraction (SCXRD).
arylguanidines [27]. Thus, eight isomers namely, (i) trans syn syn-
syn, (ii) trans syn syn-anti, (iii) trans syn anti-syn, (iv) trans syn
anti-anti, (v) trans anti syn-syn, (vi) trans anti syn-anti, (vii) trans anti
2 2
Complexes of the type trans-[L PdCl ] (L = N-donor ligands con-
taining the ˜C@NA unit) are known as model complexes to better
understand factors that favor/disfavor intramolecular cyclopalla-
anti-syn and (viii) trans anti anti-anti are possible for 1ꢁCHCl ,
3
with LH2
,5-xylyl
in 1:1 mole
dation [23]. The reaction of Pd(OAc)
2
2
2ꢁCHCl , 3ꢁC H and 4 as shown in Chart 2. The ˜C@NA unit of the
3
7
8
ratio in toluene under reflux condition for 6 h afforded 2 in 45%
yield instead of a putative six-membered cyclopalladated guani-
dine, 5 (see Chart 1). It is to be noted that a closely related sym
guanidine ligands are located in the same direction in 1ꢁCHCl while
3
in the remaining complexes, these two units are located in the oppo-
site direction. Accordingly, 1ꢁCHCl revealed trans syn anti-anti con-
3
0
0
N,N’,N -tri(2-tolyl)guanidine, (ArNH)
2
C@NAr (Ar = 2-Me
in toluene under reflux con-
dition afforded either six-membered cyclopalladated guanidine, 6
or 1:2 adduct, trans-[{(ArNH) C@NAr} Pd(OAc) ] (7) depending
upon Pd:guanidine ratio [21b,24].
C
2 6
4
H ;
formation while 2ꢁCHCl and 3ꢁC H revealed trans anti anti-syn
3
7
8
2
-tolyl
LH
2
) upon reaction with Pd(OAc)
2
conformation and 4 revealed trans anti anti-anti conformation. The
conformational difference between 1ꢁCHCl and the remaining
3
2
2
2
three complexes arises due to the difference in the shape of the
anion while the conformational difference between 2ꢁCHCl and
3
It has been shown that 1:1 adducts such as 8 and 9 are interme-
diates in stoichiometric and catalytic CAH activation mediated by
Pd [21a,25,26]. Probably, the formation of the intermediate 8 is
3ꢁC H on the one hand and 4 on the other arises due to the differ-
7
8
ence in steric bulk of the R group of the carboxylate moiety.
The known complex 10 revealed trans syn syn-anti/anti-anti
conformations as two halves of the molecule are related by neither
inversion symmetry nor two fold rotation symmetry (see Chart 3)
2
-tolyl
more feasible with LH
type of intermediate for LH
2
upon reaction with Pd(OAc)
2
while this
2
,5-xylyl
2
in the absence of a suitable ligand
orientation toward the acetate moiety for cyclopalladation quickly
undergoes a symmetrical bridge-splitting reaction with the second
[18]. The ˜C@NA units of the guanidine in 1ꢁCHCl are syn to each
3
other while these units in the known complexes 11–14 [12] are
2
,5-xylyl
equiv of LH
2
to form a stable 1:2 adduct, 2. It is to be noted that
anti to each other due to less sterically demanding nature of the
guanidine ligands in the former complex (see Chart 4). The Pd(II)
both LH2
and despite this similarity, the non-reactive nature of the latter
guanidine toward Pd(OAc) is believed to arise from the presence
-tolyl
and LH
2,5-xylyl
revealed anti-anti conformation [22,27]
2
2
atom in 1ꢁCHCl revealed a significantly distorted square planar
3
2
geometry while that in 2–4 revealed undistorted square planar
of a Me group in 5th position of the 2,5-xylyl ring in one of the
amino moieties. However, the Me substituent in 5th position of
geometry. The dihedral angle between two planes constituted by
‘‘ClPdNimine” unit in 1ꢁCHCl is 9.45(11)° while that in 2ꢁCHCl ,
3
3
0
0
the 2,5-xylyl ring in ArC(H)@NAr (Ar = 2,5-Me
,4,6-Me ) upon treatment with Pd(OAc) facilitates cyclopal-
ladation and leads to the formation of a five-membered cyclopalla-
C
2 6
H
3
and Ar =
3ꢁC H and 4 is close to 0.0°. The presence of two sterically
7
8
2
3 6
C H
2
2
demanding guanidine ligands on the same side in 1ꢁCHCl could
3
be the reason for greater geometrical distortion around the Pd(II)
2
dated imine of the type [
j
(C,N)Pd(
l
-OAc)]
2
[28].
atom. The CN unit of the guanidine moiety lies approximately per-
3
pendicular to the square plane around the Pd(II) atom (Dihedral
angles = 78.61(7), and 73.58(6)° (1ꢁCHCl
3
), 84.10(7)° (2ꢁCHCl
3
),
2
.2. Molecular structures
8
7 8
3.69(8)° (3ꢁC H ) and 85.98(7)° (4)).
The degree of n–
amino N atom with the ˜C@NA
p
conjugation involving the lone pair of the
Complex 1ꢁCHCl
3
revealed no crystallographic symmetry while
and 4 revealed an inversion symmetry (see Figs. 1
⁄
p
orbital of the guanidine ligand
2
ꢁCHCl
3
, 3ꢁC
7 8
H
0
can be estimated from the values of
22,30]. The
between the ˜C@NA double bond and the @CAN(H)Ar single bond
of the CN unit placed away from the Pd(II) atom while the
value is the difference in the bond distance between the ˜C@NA
double bond and the @CAN(H)Ar single bond of the CN unit
placed toward the Pd(II) atom. The value reflects elongation of
D
CN
,
D
CN and
q
parameters
and 2). The Pd(II) atom in these complexes is surrounded by the
imine nitrogen atom of two guanidines placed in mutually trans
[
DCN value is the difference in the bond distance
position as are two chlorides (1ꢁCHCl
monodentate carboxylate moieties (2ꢁCHCl
(N-acylamidines) PdCl ] were shown to exhibit interesting struc-
tural aspects due to many isomeric possibilities with different con-
formations and configurations [29]. Trans configured 1ꢁCHCl
and 4 can further exhibit isomerism due to two pos-
3
) or the oxygen atom of two
CN
D ’
3
, 3ꢁC and 4). Trans-
7 8
H
3
[
2
2
3
q
3
,
the ˜C@NA double bond upon coordination relative to the con-
2
3 7 8
ꢁCHCl , 3ꢁC H
comitant shortening of the average @CꢀN(H)Ar single bond [30].
sible orientations of the ˜C@NA unit of the guanidine ligands with
respect to each other. Additionally, the xylyl ring of two amino moi-
eties (NHAr) can orient parallel and anti parallel to the ˜C@NA unit
The
a maximum n–
indicate an impediment to the n–p
DCN and DCN’ values of 0.00 and the q value of 1.00 indicate
p
conjugation while deviation from these values
0
0
conjugation [22,30].
of the guanidine ligand as discussed previously for sym N,N’,N -tri-

P. Elumalai et al. / Polyhedron 151 (2018) 313–322
315
2
Chart 1. Possible structures of proposed/known six-membered cyclopalladated guanidine and 1:1 adducts of Pd(OAc) with guanidine.
Fig. 1. ORTEP representations of 1ꢁCHCl
3
and 2ꢁCHCl
3
at the 50% probability level. Solvent molecules have been omitted for clarity and only hydrogen atom of the amino
) Pd1–Cl1 = 2.3082(6), Pd1–Cl2 = 2.3232(6), Pd1–N1 = 2.024(2), Pd1–N4 = 2.035(2), N1–C1 =
.304(3), N2–C1 = 1.373(3), N3–C1 = 1.353(3), N4–C26 = 1.298(3), N5–C26 = 1.380(3), N6–C26 = 1.357(3); N1–Pd1–N4 = 177.59(7), N1–Pd1–Cl1 = 89.93(6), N1–Pd1–Cl2 =
8.62(6), Cl1–Pd1–N4 = 90.56(6), Cl2–Pd1–N4 = 90.53(6), Cl1–Pd1–Cl2 = 170.74(2); (2ꢁCHCl ) Pd1–O1 = 2.011(1), Pd1–N1 = 2.033(1), N1–C1 = 1.311(2), N2–C1 = 1.358(2),
moiety is shown for clarity. Selected bond parameters (Å and deg): (1ꢁCHCl
3
1
8
3
N3–C1 = 1.354(2), N1–Pd1–N1/O1–Pd1–O1 = 180.000(1), N1–Pd1–O1 = 89.05(6).
Fig. 2. ORTEP representations of 3ꢁC
7
H
8
and 4 at the 50% probability level. Solvent molecule has been omitted for clarity and only hydrogen atom of the amino moiety is
) Pd(1)–O(1) = 2.016(2), Pd(1)–N(1) = 2.031(2), N(1)–C(1) = 1.312(3), N(2)–C(1) = 1.363(3), N(3)–C(1) = 1.348
3); O(1)–Pd(1)–O(1) = 180.0(1), O(1)–Pd(1)–N(1) = 91.02(8)/88.98(8), N(1)–Pd(1)–N(1) = 180.0(2); (4) Pd(1)–O(1) = 2.019(1), Pd(1)–N(1) = 2.026(1), N(1)–C(1) = 1.312(2), N
2)–C(1) = 1.340(2), N(3)–C(1) = 1.354(2), O1–Pd1–O1/N1–Pd1–N1 = 180.0, O1–Pd1–N1 = 87.35(6)/92.65(6).
shown for clarity. Selected bond parameters (Å and deg): (3ꢁC
7 8
H
(
(

3
16
P. Elumalai et al. / Polyhedron 151 (2018) 313–322
Chart 2. Eight possible configurational and conformational isomers of 1–4.
0
0
The values of
D
CN
,
D
CN and
q
(
D
CN
/D
CN = 0.069(3)/0.049(3),
[9], which is absent in 1 due to the presence of sterically more
2
,5-xylyl
0
(
(
.059(3)/0.082(3) Å (1ꢁCHCl
3)/0.036(3) Å (3ꢁC ), 0.042(2)/0.028(2) Å (4);
1ꢁCHCl ), 0.97 (2ꢁCHCl
3
), 0.047(2)/0.043(2) (2ꢁCHCl
3
), 0.051
demanding LH
The hydrogen bond interactions associated with the ligands
around the Pd(II) atom in 2ꢁCHCl are illustrated in Fig. 3. The car-
acts as a bifur-
2
in the latter complex.
7
H
8
q
= 0.96 and 0.95
3
3
), 0.97 (3ꢁC
7
H
8
), and 0.97 (4)) in conjunc-
3
P
tion with planarity of the amino nitrogen atoms ( N = 360°)
bonyl oxygen atom, O2 of the acetate in 2ꢁCHCl
3
clearly indicated greater n–
p
conjugation in complexes than in
CN = 0.098(3) Å/(0.097(3) Å;
cated hydrogen bond acceptor by simultaneously forming
intramolecular N–Hꢁ ꢁ ꢁO hydrogen bond with one of the N(H)Ar
moieties of the guanidine ligand and an intermolecular C–Hꢁ ꢁ ꢁO
2
,5-xylyl
0
uncoordinated LH
2
(
D
CN
/D
q
=
P
0
.93; N = 349.8° and 353.9° [22]).
Complex 1 associates with CHCl
3
through the CAHꢁ ꢁ ꢁCl hydro-
3
hydrogen bond with a CHCl . This results in the formation of a pair
1
7 8 1
H (8)
and 4 also contain a pair of R1
gen bond (H51ꢁ ꢁ ꢁCl2 = 2.556(2) Å, C51ꢁ ꢁ ꢁCl2 = 3.463(4) Å, C51–
of R
rings formed between one of the N(H)Ar moieties of the guanidine
ligand and the carboxylate moiety as analogously found in 2ꢁCHCl
(H3ꢁ ꢁ ꢁO2 = 2.008(2) Å, N3ꢁ ꢁ ꢁO2 = 2.842(3) Å, N3–H3ꢁ ꢁ ꢁO2 = 163.221
(2)° (3∙C ) and H2ꢁ ꢁ ꢁO2 = 1.948(2) Å; N2ꢁ ꢁ ꢁO2 = 2.784(2) Å; N2–
1
(8) rings. Complexes 3ꢁC
H51ꢁ ꢁ ꢁCl2 = 157.39(5)°). Trans-[(hPPH)
2 2
PdCl ] (hPPH = 1,3,4,6,7,8-
hexahydro-2H-pyrimido[1,2-a] pyrimidine) illustrated in Chart 5
was shown to be stabilized by a pair of intramolecular N–Hꢁ ꢁ ꢁCl
3
hydrogen bonds through the formation of a pair of R1
1
(6) rings
7 8
H

P. Elumalai et al. / Polyhedron 151 (2018) 313–322
317
and these isomers were shown to interconvert via hindered rota-
tion of two hydrazones about the PdAN bonds [32], (ii) any one
isomer namely pseudo syn or pseudo anti wherein one hydrazone
is different from the other due to non-bonded Pdꢁ ꢁ ꢁH (H is part
1 2
of either R or R whichever is closer to Pd) interaction leading to
a concerted oscillation of two hydrazones about the PdAN bonds
33]. In the second example, it was proposed that the coordinated
[
N of one hydrazone makes an angle with Pd(II) atom which is dif-
ferent from the angle made by the coordinated N atom of the other
hydrazone so that Pd(II) atom would be pseudo five-coordinated at
any given moment. Trans-{[Me(EtO)C@NCH
also shown to exist as a mixture of syn and anti isomers in solution
23b]. This complex was shown to undergo a more rapid @CAO
single bond rotation as predicted from variation in chemical shift
of CH O protons.
2 2 2
Ph] PdCl ] (15) was
[
2
0
00
Chart 3. Structurally characterised 1:2 adduct of N,N ,N -triphenylguanidine with
PdCl
In order to understand the nature of solution species of 1–4, as
representative examples, 1 and 3 were subjected to variable tem-
2
.
1
perature (VT) H NMR spectroscopic studies and stack plots for
CH
are illustrated in Figs. 4 and 5 respectively. At 313 K, six singlets
of comparable intensity were observed for CH protons of the
3
protons of the guanidine ligand for both of these complexes
3
guanidine ligand (Fig. 4). Upon lowering the temperature, signals
for the second species emerged at temperatures ꢃ263 K albeit in
lower population (isomer ratio ꢄ 1.00:0.25). The signal at d =
1
.98 ppm at 313 K broadens upon lowering the temperature,
moves upfield and finally stays at d = 1.56 ppm at 213 K. This
chemical shift trend parallels with that reported for CH O protons
2
of 15 [23b]. The presence of two solution species of 1 was also
apparent from two signals, one upfield and one downfield shifted,
for each species assignable to NH protons of the guanidine ligand
(
see Fig. S1 in the Supplementary data). These spectral patterns
exclude the PdAN bond rotation as a mean for the observance of
two solution species discussed in the preceding paragraph and
suggest that trans syn anti-anti isomer found in the solid state upon
3
dissolution in CDCl equilibrates with a small but detectable
amount of trans syn anti-syn isomer which is formed from the for-
mer isomer via the restricted @CꢀN(H)Ar single bond rotation of
3
the CN unit as shown in Scheme 2. Further, isomer interconver-
sion through the PdAN bond rotation in 1ꢁCHCl
3
is ruled out due
to voluminous nature of the guanidine ligand. It has been shown
0
00
Chart 4. Structurally characterised 1:2 adduct of unsymmetrical N,N ,N -trisubsti-
tuted guanidine with PdCl
5
5
that trans-[(
5 4 2 2 2 2
g -Cp)Fe(g -C H -CH@N(CH ) OH)] PdCl exists as a
2
.
mixture of two solution species, which was invoked to arise from
5
the difference in the relative orientation of the Fe(
g -Cp) moieties
in each one of the ligands [34].
1
VT H NMR study of 3 also revealed the presence of two solution
species in the 273–243 K temperature interval as one could see
twelve singlets of equal intensity ratio assignable to CH protons
3
of the guanidine ligand (see Fig. 5). Further, two pairs of signals
of NH protons of the guanidine ligands at low temperatures con-
firmed the presence of two solution species (see Figs. S2 and S3
in the Supplementary data). The presence of a pair of downfield
shifted singlets (d = 178.74 and 178.98 ppm (2) and d = 173.4 and
1
73.8 ppm (3)) assignable to OC(O) carbon of the carboxylate moi-
Chart 5. Structurally characterised 1:2 adduct of hPPH with PdCl
2
.
13
1
ety in their respective C{ H} NMR spectra further supported the
presence of two solution species for both complexes.
1
H2ꢁ ꢁ ꢁO2 = 163.991(2)°(4)). The
R
1
(6) rings formed through an
As one N(H)Ar proton in each guanidine ligand is locked up in
intramolecular NAHꢁ ꢁ ꢁO hydrogen bond, only the remaining N(H)
Ar moiety in both guanidine ligands in both 2 and 3 is free to
undergo the restricted CAN(H)Ar single bond rotation and this
would result in the formation of two species. These two species
are assigned to trans anti anti-syn, the solid state isomer and trans
anti anti-anti as illustrated in Scheme 3. The xylyl rings of the pair
of amino moieties are placed away from each other in 3 while
these rings are on the same side in 4 in the crystal lattice. This
observation indirectly supports the fact that two isomers found
in solution for 2 and 3 are formed via the restricted CAN(H)Ar sin-
intramolecular N–Hꢁ ꢁ ꢁO hydrogen bond between the secondary
amine, R NH ligand and the acetate moiety in trans-[(R NH) Pd
OAc) ] are prevalent in the literature (see Chart 6) [31]. The pair
2
2
2
(
2
1
of R
1
(8) rings found in 2ꢁCHCl
3
, 3ꢁC
7 8
H and 4 is, to the best of our
knowledge, unprecedented in the literature.
2
.3. Solution behavior
1 2 3 4 2 2
Complexes of the type trans-[{R (R )C@N-N(R )R } PdCl ] were
known to exist as (i) a mixture of syn and anti isomers in solution

3
18
P. Elumalai et al. / Polyhedron 151 (2018) 313–322
Fig. 3. Hydrogen bond interactions present in 2∙CHCl
C28ꢁ ꢁ ꢁO2 = 3.166(4), C28–H28ꢁ ꢁ ꢁO2 = 170.199(2); H3ꢁ ꢁ ꢁO2 = 2.144(2), N3ꢁ ꢁ ꢁO2 = 2.870(3), N3–H3ꢁ ꢁ ꢁO2 = 141.88(2).
3
. Only the ipso carbon of xylyl rings is shown. Selected hydrogen bond parameters (Å, and deg): H28ꢁ ꢁ ꢁO2 = 2.197(2),
Continuing our interests in exploring the utility of phosphine
free Pd catalysts in cross coupling reactions [37], we used 1–4 in
Heck–Mizoroki coupling reaction involving one equiv. of
chlorobenzene and 1.5 equiv of methyl acrylate in the presence
3
of 1.5 equiv of Et N as a base in DMF at 90 °C. The mol % of the cat-
alysts was fixed at 0.1, 0.01 and 0.001 with respect to chloroben-
zene. The yields of coupling product, trans methylcinnamate
obtained with different mol % of catalysts are listed in Table 1.
The results indicate that 1–4 are active catalysts for difficult sub-
strate such as chlorobenzene even when used at 0.001 mol%. We
chose 1 as a catalyst at 0.01 mol% to test the scope of these com-
plexes as catalysts in Heck–Mizoroki coupling reactions involving
variously substituted chloroarenes and methyl acrylate and the
yields coupling products obtained from our study is listed in
Table 2. Chloroarenes are difficult substrates in cross coupling
reactions due to inert nature of CACl bond. However, these sub-
strates are inexpensive and are thus attractive as substrates in
cross coupling reactions [38]. Surprisingly, the yields of coupling
products listed in Table 2 are high in the range of 81%ꢀ96% and
thus 1 exhibits high activity. The activity exhibited by 1 is compa-
rable or even superior than those Pd(0)/Pd(II) complexes ligated
with sophisticated ligands [39].
Chart 6. Crystallographically characterised known 1:2 adducts of secondary
amines with Pd(OAc)
2
.
gle bond rotation of the guanidine ligand discussed previously. The
CAN(H)Ar single bond rotation of the guanidine ligand in 1 is more
restricted than in 2 and 3 due to greater steric encumbrance
around the Pd(II) atom in the former complex.
1
13
1
H and C{ H} NMR spectra of 4 revealed the presence of only
one species in solution and this species is assigned to the solid
state isomer, trans anti anti-anti. The presence of a bulky pivalate
moiety in 4 apparently does not permit the guanidine ligand to
undergo the CAN(H)Ar single bond rotation as mentioned previ-
ously for 2 and 3 to afford the more congested trans anti anti-syn
isomer.
3
. Conclusions
Complexes 1–4 were isolated in excellent yields and character-
ized by elemental analyses, IR, NMR spectroscopy. Molecular struc-
tures of four complexes were determined by SCXRD which
revealed trans syn anti-anti (1ꢁCHCl
and 3ꢁC ), trans anti anti-anti (4) conformations. Complexes 2–
4 represent the first structurally characterised palladium(II) car-
3
), trans anti anti-syn (2ꢁCHCl
3
7 8
H
2.4. Heck–Mizoroki coupling reactions
0
0
boxylate complexes of sym N,N’,N -trisubstituted guanidines. The
guanidine, LH
2
,5-xylyl
Heck–Mizoroki coupling is one of the important Pd catalysed
2
resists cyclopalladation upon reaction with
CAC bond forming reactions in cross coupling chemistry [35].
Phosphine free Pd(II) complexes especially those in which Pd(II)
atom is ligated by N-donor ligands are one of the sought after cat-
alysts in Heck–Mizoroki coupling reactions [36]. Prior to this paper,
only a few Pd(II) complexes of guanidines have been used as cata-
lysts in Heck–Mizoroki coupling reactions [11].
Pd(OAc) (Pd:guanidine = 1:1) but rather conveniently forms 1:2
2
adduct (Pd:guanidine = 1:2) and this reactivity pattern is ascribed
to the presence of Me group in 5th position of the xylyl ring in
one of the amino moieties of the ligand which makes the proton
on C(6) carbon inaccessible. The size and shape of the anions in
complexes dictate not only the conformations of the complexes

P. Elumalai et al. / Polyhedron 151 (2018) 313–322
319
Fig. 4. VT 1H NMR (400 MHz, CDCl
) stack plot for CH protons of the guanidine ligand in 1.
3 3
in solid state but also the number of solution species. Complexes 1
and 3 revealed a restricted CAN(H)Ar single bond rotation of the
guanidine ligand in solution and thus present as a mixture of
two isomers at lower temperatures as revealed by VT 1H NMR
spectroscopy. The new complexes in general and 1 in particular
were shown to be effective catalysts in Heck–Mizoroki coupling
reactions involving chloroarenes and methyl acrylate at low cata-
lyst loading.
4
. Experimental (For general experimental details see the
Supplementary data)
4
4
.1. Synthesis
.1.1. Complex 1
2
,5-xylyl
LH
2
2 2
(0.199 g, 0.536 mmol) and trans-[(PhCN) PdCl ]
(
0.100 g, 0.261 mmol) were charged into a 25 mL RB flask. To the
aforementioned flask, freshly distilled toluene (15 mL) was added
and the resulting solution stirred under reflux condition for 12 h.
Subsequently, the reaction mixture was concentrated under vac-
uum to half the volume, layered with CHCl
tion allowed to stand at RT over a period of several days to afford
ꢁCHCl as orange red crystals in 82% (0.224 g, 0.215 mmol) yield.
FT–IR (KBr, cm ):
3
and the resulting solu-
1
3
ꢀ
1
m
(NH) 3400 (m), 3243 (m);
H NMR (400 MHz, CDCl ): d 1.91 (s, 3 H, CH
CH
), 2.09, 2.15, 2.27, 2.41 (each s, 4 ꢅ 3 H, CH
NH), 6.55 (d, JHH = 7.8 Hz, 2H, ArH), 6.61–6.62 (br, 1 H, ArH), 6.68
d, JHH = 7.8 Hz, 1 H, ArH), 6.86 (d,JHH = 6.4 Hz, 2H, ArH), 6.92 (s, 1
m
3
(C@N) 1600 (vs).
), 1.98 (br, 3 H,
), 5.58 (s, 1H,
1
3
3
3
(
H, ArH), 6.99 (d, 1H, JHH = 7.8 Hz, ArH), 7.71 (s, 1H, ArH), 8.40 (s,
1
3
1
⁄
1
1
1
1
1
1
H, NH). C{ H} NMR (100.5 MHz, CDCl
3
): d 17.0, 17.1 , 17.2,
⁄
⁄
7.6 , 18.7, 20.6, 20.8, 21.0 (CH
24.9 (br, ArCH), 125.5 , 125.8 (br, ArCH), 126.5, 126.9 (br, ArCH),
28.5 , 129.1, 129.3 , 129.7, 129.9, 130.8, 131.0 (ArCH), 135.4,
35.5, 135.6 , 135.8, 135.9, 136.5, 136.6 , 137.5 , 143.5, 145.3 ,
52.1, 152.2 (ArC and ˜C@NA). The star symbol indicates the sig-
nals of the minor isomer. C{ H} NMR chemical shifts were
assigned with the aid of 2D HETCOR NMR spectroscopy (see
3
), 122.5, 123.8, 124.0 (br, ArC),
⁄
⁄
⁄
⁄
⁄
⁄
⁄
⁄
1
3
1
+
Figs. S4–S7 in the Supplementary data). MS (TOF-ES ), m/z (inten-
+
2,5-xylyl +
sity %), [ion]: 849 (20), [Mꢀ2Cl] ; 476 (80), [Pd + L
] , 372
3
(Mw:
2
,5-xylyl +
(
100), [LH
2
] . Anal. Calc. for
C
50
H
58
N
6
Cl
2
PdꢁCHCl
Fig. 5. VT 1H NMR (400 MHz, CD
920.38 + 119.38): C, 58.91; H, 5.72; N, 8.08. Found: C: 58.95, H:
5.69, N: 8.14.
2 2 3
Cl ) stack plot for CH protons of the guanidine
ligand in 3.

3
20
P. Elumalai et al. / Polyhedron 151 (2018) 313–322
Scheme 2. Solution conformers of 1.
Scheme 3. Solution conformers of 3.
mixture of two species in about 1:1 ratio as revealed by 1H NMR
Table 1
Heck–Mizoroki coupling reaction involving chlorobenzene and methyl acrylate
catalyzed by 1–4.
1
spectroscopy. H NMR (400 MHz, CDCl
ꢅ 3 H, OC(O)CH ), 1.84 (s, 2 ꢅ 3H, CH
CH ), 2.10 (s, 3H, CH ), 2.18 (s, 3H, CH
s, 3 ꢅ 3H, CH ), 2.42 (s, 3 H, CH ), 5.44, 5.46 (each s, 2 ꢅ 1H,
NH), 6.50, 6.52 (each br, 2 H, ArH), 6.59–6.65 (m, 6 H, ArH), 6.82–
.88 (m, 6 H, ArH), 6.94, 6.96 (each s, 2 H, ArH), 7.45, 7.56 (each
3
): d 1.65, 1.72 (each s, 2
), 2.07 (each s, 3 ꢅ 3H,
), 2.28 (s, 3H, CH ), 2.33
3
3
3
3
3
3
(
3
3
6
1
3
1
s, 2 ꢅ 1 H, ArH), 9.94, 10.10 (each s, 2 ꢅ 1 H, NH). C{ H} NMR
100.5 MHz, CDCl ): d 16.93, 17.37, 17.54, 17.71, 17.82, 20.76,
0.81, 20.96, 21.06, 22.74 (CH ), 121.97, 122.55, 122.89, 123.25,
Complex
mol%
0.1
Yield (%)a
TONb
(
3
1
91
90
89
81
80
75
83
81
78
80
78
76
913
2
1
1
3
0
0
.01
.001
9036
89130
813
8049
74630
826
8107
78000
800
7820
76300
24.46, 124.80, 125.20, 125.32, 125.75, 126.06, 126.24, 127.10,
27.62, 128.17, 128.30, 129.58, 129.94, 130.56, 130.72, 134.86,
2
3
4
0.1
0
0
.01
.001
135.01, 135.75, 135.78, 136.39, 136.46, 136.54, 136.99 (br),
143.76, 143.87, 153.06, 153.27 (ArC/ArCH and ˜C@NA), 178.74,
0.1
+
1
[
78.98 (OC(O)). MS (TOF-ES ), m/z (intensity %), [ion]: 847 (71),
0
0
.01
.001
+
2,5-xylyl +
2,5-xylyl +
M–2 AcOH] ; 476 (85), [Pd + L
Anal. Calc. for C54 Pd (Mw: 967.56): C, 67.03; H, 6.67; N,
8.69. Found: C, 67.15; H, 6.75; N, 8.65.
] ; 372 (100), [LH
2
] .
64 6 4
H N O
0.1
0
0
.01
.001
a
Isolated yields of the product are based on chlorobenzene from the average of
two runs.
4
.1.3. Complex 3
2,5-xylyl
LH
2
2
(0.219 g, 0.589 mmol) and [Pd(OC(O)Ph) ] (0.100 g,
b
TON = mole of the product/mole of the catalyst.
0
.287 mmol) were charged into a 25 mL RB flask. To the aforemen-
tioned flask freshly distilled toluene (15 mL) was added and the
resulting solution stirred under reflux condition for 12 h. Subse-
quently, the reaction mixture was concentrated under vacuum to
4
.1.2. Complex 2
2
,5-xylyl
LH
2
(0.348 g, 0.937 mmol) and Pd(OAc)
2
(0.100 g, 0.445
half the volume, layered with CHCl
for several days to afford 3ꢁC as orange red crystals in 95%
(0.323 g, 0.273 mmol) yield. FT–IR (KBr, cm ):
3392 (m); (OCO) 1637 (vs); (C@N) 1584 (vs);
3
and allowed to stand at RT
mmol) were charged into a 25 mL RB flask. To the aforementioned
flask freshly distilled toluene (15 mL) was added and the resulting
solution stirred under reflux condition for 12 h. Subsequently, the
reaction mixture was concentrated under vacuum to half the vol-
7 8
H
ꢀ
1
m
(NH) 3424 (m),
(OCO) 1370
m
a
m
m
s
(s). Complex 3 exists as a mixture of two species in solution in
1
ume, layered with CHCl
ꢁCHCl as orange red crystals in 95% (0.409 g, 0.423 mmol) yield.
FT–IR (KBr, cm ):
vs); (C@N) 1606 (vs);
3
and allowed to stand at RT to afforded
about 1:1 ratio as revealed by H NMR spectroscopy. 1H NMR
2
3
(400 MHz, CDCl
2.12, 2.15, 2.25, 2.41 (each s, 4 ꢅ 3 H, CH
ꢅ 1H, NH), 6.44, 6.46 (each s, 2 H, ArH), 6.53, 6.55 (each s, 2H,
3
): d 1.70, 1.92, 1.95, 2.07 (each s, 8 ꢅ 3H, CH
3
),
ꢀ
1
m
(NH) 3375 (m), 3127 (br sh);
m
a
(OCO) 1625
3
), 5.37, 5.41 (each s, 2
(
m
m
s
(OCO) 1378 (m). Complex 2 exists as a

P. Elumalai et al. / Polyhedron 151 (2018) 313–322
321
Table 2
1629 (vs);
MHz, CDCl
m
(C@N) 1603 (vs);
m
3
s
(OCO) 1395 (s). 1H NMR (400
Heck–Mizoriki coupling of chloroarenes with methyl acrylate catalyzed by 1.
3
): d 1.16 (s, 9H, C(CH
)
3
3
), 1.81 (s, 3 H, CH ), 2.05 (s, 2
S.
No.
Chloroarene
Product
Yield
(%)
ꢅ 3 H, CH
3
), 2.17, 2.21, 2.25 (each s, 3 ꢅ 3 H, CH
3
), 5.37 (br, 1H,
a
NH), 6.48–6.55 (m br, 2 H, ArH), 6.57–6.65 (br, 1 H, ArH), 6.68
(br, 2 H, ArH), 6.75–6.84 (br m, 2 H, ArH), 6.90 (br, 1 H, ArH),
1
2
3
4
5
6
81
81
82
94
91
88
1
3
1
7
.28–7.40 (br, 1 H, ArH), 11.21 (s, 1 H, NH). C{ H} NMR (100.5
MHz, CDCl ): d 16.9, 17.6, 18.0, 20.8, 21.0 (CH ), 28.9 (C(CH )),
9.4 (C(CH )), 123.3, 123.4, 124.2, 125.0, 125.9, 126.4, 127.8,
128.1, 129.4, 129.7, 130.2 (br), 130.6, 130.7, 134.5 (br), 135.6,
36.3, 137.4, 144.5, 153.2 (ArC/ArCH, and ˜C@NA), 185.0 (br, OC
3
3
3
3
3
1
+
t
(
O)). MS (TOF-ES ), m/z (intensity %), [ion]: 847 (5), [M–2 BuC(O)
+
t
+
2,5-xylyl
OH] ; 817 (5), [M ꢀ BuC(O)OH, 2Me] ; 476 [Pd + L
]; 372
Pd (Mw: 1051.72):
C, 68.52; H, 7.28; N, 7.99. Found: C, 68.58; H, 7.23; N, 7.92.
2
,5-xylyl +
(100), [LH
2
76 6 4
] . Anal. Calc. for C60H N O
4.2. Heck–Mizoroki coupling reactions
A 25 mL RB flask was charged with chloroarene (1.00 mmol),
methyl acrylate (0.130 g, 1.50 mmol), triethylamine (0.152 g,
1
0
.50 mmol), and DMF (5 mL). A DMF solution of catalysts (0.1,
.01, and 0.001 mol %, Table 1; 0.01 mol %, Table 2) was added to
the aforementioned solution via the syringe. The flask was fitted
to a water condenser capped with anhydrous CaCl guard tube
2
and the contents in the flask were simultaneously stirred and
heated at 90 °C for 3 h. The reaction mixture was cooled and sub-
sequently poured into distilled water (300 mL). The product was
extracted with EtOAc (5 ꢅ 50 mL). The organic layer was dried over
anhydrous sodium sulfate and concentrated under vacuum to give
oil. The oil was purified by column chromatography on silica gel
7
96
(n-hexane/EtOAc, 10/0 ? 8/2, v/v) to afford coupling products in
yields indicated in Tables 1 and 2.
8
9
84
94
Acknowledgements
The authors acknowledge Department of Science and Technol-
ogy, New Delhi for financial support. We also acknowledge (i)
NMR Research Center, Indian Institute of Science, Bengaluru 560
0
12 and Central Drug Research Institute, Lucknow 226031 for VT
1
H NMR measurements.
a
Reaction condition is same as those given in Table 1. Yields reported for the
Appendix A. Supplementary data
products represent an average of two runs.
CCDC 1547493ꢀ1547496 contains the supplementary crystal-
lographic data for 1ꢁCHCl
3
, 2ꢁCHCl
3 7 8
, 3ꢁC H and 4. These data can
ArH), 6.58–6.91 (m, 10 H, ArH), 7.17–7.25 (m, 3 H, ArH), 7.27–7.37
be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-
33; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data asso-
ciated with this article can be found, in the online version, at
https://doi.org/10.1016/j.poly.2018.05.022.
(
2
m, 7 H, ArH), 7.90 (d, JHH = 6.6 Hz, 2H, ArH), 7.96 (d, JHH = 7.3 Hz,
13
1
H, ArH), 10.98, 11.01 (each s, 2 ꢅ 1H, NH). C{ H} NMR (100.5
): d 16.8, 17.6 (br), 17.8 (br), 20.5, 20.7, 20.8 (CH ),
23.6, 124.0, 124.6, 124.8, 125.3, 125.8, 126.0, 126.6, 127.5,
MHz, CDCl
3
3
0
1
1
1
1
27.9, 128.1, 128.4, 128.6, 129.4, 129.7, 129.8, 130.0, 130.1,
30.3, 130.4, 130.6, 135.0, 135.3, 135.6, 135.8, 135.9, 136.3,
37.4, 144.2, 153.6 (ArC/ArCH and ˜C@NA), 173.4, 173.8 (OC(O)).
+
+
MS (TOF-ES ), m/z (intensity %), [ion]: 847 (5), [M–2 PhC(O)OH] ;
References
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,5-xylyl +
2,5-xylyl +
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76 (5), [Pd + L
Pd∙C
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] ; 372 (100), [LH
2
] . Anal. Calc. for C64-
[
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