Trinuclear Cu(I)–NHC as NHC transfer agent: Synthesis of mono- and di-nuclear palladium and mercury complexes

Article abstract of DOI:10.1016/j.poly.2019.114123

Mono and dinuclear palladium–NHC complexes of 1,3-bis(2-pyridyl)imidazolin-2-ylidene (py₂im) ligand, such as [Pd(py₂im)₂](PF₆)₂ (2) [Pd₂Cl₂(py₂im)₂](PF

Full text of DOI:10.1016/j.poly.2019.114123

Polyhedron 173 (2019) 114123
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Trinuclear Cu(I)–NHC as NHC transfer agent: Synthesis of mono- and
di-nuclear palladium and mercury complexes
Biswajit Santra
Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Mono and dinuclear palladium–NHC complexes of 1,3-bis(2-pyridyl)imidazolin-2-ylidene (py2im) ligand,
such as [Pd(py2im)2](PF6)2 (2) [Pd2Cl2(py2im)2](PF6)2 (3), and [PdCl2(py2im)] (4) were synthesized by car-
bene transfer method using trinuclear Cu(I)–NHC complex, [Cu3(py2im)3](PF6)3 (1). Also addition of PPh3
to the solution of 3 or in the reaction medium of 1 with PdCl2 resulted in a mononuclear bisphosphine
complex [PdCl(PPh3)2(py2im)](PF6) (5) which can also be prepared using trans-[PdCl2(PPh3)2] (6) and
[Cu(PPh3)2(py2im)](PF6) (7) as precursors. In addition a mono nuclear complex [Hg(py2im)2](PF6)2 (8)
was synthesized through carbene transfer from 1.
Received 7 June 2019
Accepted 17 August 2019
Available online 26 August 2019
Keywords:
Trinuclear Cu–NHC
Transmetallation
Trans influence
Pd–NHC
Ó 2019 Elsevier Ltd. All rights reserved.
Double transmetallation
1. Introduction
tion needs to be performed in the absence of light (b) presence of
silver ions in the product and (c) unsuccessful carbene transfer in
Since the isolation of N-heterocyclic carbene (NHC) by Ardu-
engo an impetuous effort has been given for the synthesis of metal
complexes containing NHCs [1]. Among them the late transition
metal NHCs complexes have been widely studied due to their
potential applications in catalysis [2–17]. In particular the C–C
bond coupling reactions are of interest and they are carried out
using NHC stabilized palladium dihalide as catalyst. As it is
observed that mixed donor ligands in palladium complexes, such
as phosphine and pyridine groups, have a tremendous influence
on their catalytic activity, it has been extended to palladium com-
plexes containing functionalized NHC ligands [18–23,67]. Indeed
the presence of additional tethering in NHCs, with aromatic hete-
rocycles or pendent amino/ether/thioether moiety as hemilabile
units, show strong influence in the catalytic activity, especially in
C–C bond coupling reactions [24–35].
In general, NHC stabilized metal complexes are prepared by (a)
direct reaction of isolated or in situ generated NHC with metal pre-
cursors, (b) deprotonation of imidazolium salt with basic metal
precursors, and (c) carbene transfer method. Out of these, the car-
bene transfer route using silver–NHC complexes is preferred. This
is due to its facile carbene transfer to other metals even in the pres-
ence of air or moisture and functional group tolerance. Thus a large
number of metal–NHCs, containing functional or non-functional
substitutions on the nitrogen centers, have been prepared. How-
ever, there were a few drawbacks in this method such as, (a) reac-
some cases [36–41]. Recently, an interesting reverse carbene-
transfer from nickel to silver has been described by Huynh and
co-workers [42]. Therefore, gold- or copper–NHC complexes were
chosen as carbene transfer agents alternatively [43–46]. Among
them the transfer of NHCs from copper would be preferred, from
an economical point of view, because of the availability and the
cost effectiveness of copper precursors.
Albrecht and coworkers reported the first example of NHC
transfer from copper(I)–NHC to ruthenium(II) complexes [44].
Later, Furst and Cazin synthesized the palladium and gold com-
plexes of non-functionalized NHCs by carbene transfer route from
copper(I) complexes [45]. Although a number of palladium com-
plexes 1,3-bis(2-pyridyl)imidazolin-2-ylidene containing function-
alized NHCs were successfully synthesized using the
corresponding silver carbene complexes, this method could not
be applied for the preparation of mononuclear palladium dihalide
of (py2im). Therefore, it is of our interest to synthesize the palla-
dium complexes of pyridyl functionalized NHCs, here with py2im,
starting from the corresponding copper complex.
Similarly, compared to other transition metal–NHC complexes
less attention has been given for the synthesis of Hg–NHC com-
plexes. The first mercury–NHC complex was synthesized by Wan-
zlick and Schönherr in 1968 [47–52]. Since then a number of
mercury(II)–NHC complexes have been synthesized by direct
metallation using imidazolium salts and Hg-precursors. Recently,
mercury–NHCs have been used as transmetallating agents for the
synthesis of other transition metal–NHC complexes [48,49]. It is
E-mail address: bisa.chem.iitk@gmail.com
https://doi.org/10.1016/j.poly.2019.114123
0277-5387/Ó 2019 Elsevier Ltd. All rights reserved.

2
B. Santra / Polyhedron 173 (2019) 114123
noteworthy that the carbene ligand transfer from group 11 to
group 12 metals is not known so far.
were solved by the direct method using the program SHELXS-97 [59–
60] and refined by full-matrix least squares on F2, minimizing the
We have prepared a known trinuclear copper(I) complex (1)
containing the hemilabile ligand py2im through a simple and cost
effective method starting from copper(II) acetate monohydrate and
py2imHꢀPF6 [53–54]. The copper–NHC complex 1 possesses three
units of py2im, where the carbene carbon atom is bridged between
two copper ions in a constrained environment. Thus it may be
easier to transfer py2im from copper centre to other metal ions.
With this assumption the reactions of 1 with palladium precursors
and mercury salts were carried out by varying the stoichiometry,
solvents and by using additional coordinating reagents like PPh3.
Herein, we report the synthesis of palladium (2–5) and mercury
(8) complexes, obtained through carbene transfer route starting
from 1, and the structural characterizations of 3–5. In addition
we also report various synthetic routes to obtain [PdCl(PPh3)2(py2-
im)](PF6) (5) using trans-[PdCl2(PPh3)2] (6) and [Cu(PPh3)2(py2im)]
(PF6) (7) as precursors.
function
R
w(|Fo| ꢁ |Fc|)2, where Fo and Fc, are the observed and cal-
culated structure amplitudes, and the weight w = 1. All non-hydro-
gen atoms were refined with anisotropic displacement parameters
in all the structure. The hydrogen atom positions or thermal
parameters were not refined but were included in the structure
factor calculations.
2.4. Synthesis
2.4.1. Synthesis of [Pd(py2im)2](PF6)2 (2)
Method (A). A mixture of [Cu3(py2im)3](PF6)3 (0.13 g, 0.1 mmol)
and Pd(OAc)2 (0.02 g, 0.1 mmol) in CH3OH (12 mL) was stirred at
room temperature for 24 h. Light green precipitate was obtained
which was filtered and dried under vacuum. This solid was washed
by ethanol followed by diethyl ether. The solid was dissolved in
CH3CN. Colorless crystalline solid of 2 was obtained by slow evap-
oration at room temperature. Yield: 0.07 g (87% based on Pd
(OAc)2). Method (B): A similar procedure was used except that a
methanolic solution (20 mL) of 1 (0.39 g, 0.3 mmol) was added
PdCl2 (0.05 g, 0.3 mmol) as solid. The solution mixture was stirred
at room temperature for 24 h. Artichoke green precipitate was
obtained which was filtered. The volatiles were removed from
the filtrate under vacuum and the residue was washed with diethyl
ether. The product was dissolved in CH3CN/CH3OH (2:1) and kept
for slow evaporation at room temperature. Colorless crystals of 2
were obtained within two days. Yield of 2: 0.11 g (44% based on
PdCl2). 2: Mp: 226–228 °C, 1H NMR (CD3CN, 500 MHz): d 8.47 (d,
4H, 3J = 4.10 Hz, CHpy), 8.04 (m, 4H, CHpy), 7.82 (d, 4H,
3J = 7.75 Hz, CHpy), 7.70 (s, 4H, CHim), 7.52 (m, 4H, CHpy). 13C
NMR (CD3CN, 125 MHz): d 155.3 (Pd-C), 150.4, 150.2, 141.8,
125.9, 122.9, 117.2. IR (KBr pellet, cmꢁ1) 3159(m), 2372(m), 1618
(s), 1451(vs), 1334(m), 1289(m), 1155(w), 1101(w), 1000(w), 959
2. Experimental
2.1. Material and methods
All the carbene transfer reactions were performed using dry sol-
vent under nitrogen atmosphere. Dry CH3OH was prepared by
refluxing single distilled methanol over Mg/I2 mixture in methanol
and followed by distillation. Acetonitrile was dried over CaH2 and
distilled under nitrogen atmosphere [56,57]. Cu(OAc)2·H2O, Hg
(OAc)2, HgCl2 and PPh3 were procured from sdfine-chem limited
(SDFCL). KPF6, 2-chloropyridine, 2-bromopyridine, N-methylimi-
dazole, PdCl2, and Pd(OAc)2 were obtained from Sigma Aldrich.
The ligand 1,3-bis(2-pyridyl) imidazolium hexaflurophosphate
(py2imH∙PF6) was synthesized according to literature procedure
[54,55,61,69]. Trans-[PdCl2(PPh3)2] (6) was synthesized according
to the literature procedure using PdCl2 and PPh3 in DMF [58]. Syn-
thesis of compound 7 has been described in SI.
(w), 839(vs), 736(s), 556(s). ESI-MS: calculated for [C26H20N8Pd]2+
,
275.0423, Found: 275.0375. Anal. Calc. for C26H20F12N8P2Pd: C,
37.14; H, 2.40; N, 13.33, Found: C, 37.12; H, 2.34; N, 13.06.
2.2. Physical measurements
2.4.2. Synthesis of [Pd2Cl2(py2im)2](PF6)2 (3)
A mixture of 1 (0.39 g, 0.3 mmol) and PdCl2 (0.16 g, 0.9 mmol)
in CH3OH (60 mL) was stirred at room temperature for 24 h. A
grayish turbid solution was obtained. The solvent was evaporated
under vacuum to obtain the artichoke green colored residue. The
residue was dissolved in CH3CN and CH3OH (1:1) mixture and kept
for crystallization. Pale yellow colored hexagonal shaped single
crystals suitable for X-ray studies were obtained. Yield: 0.30 g
(66% based on PdCl2). 3: Mp: 222–224 °C. 1H NMR (DMSO-d6,
500 MHz): d 8.86 (d, 1H, 3J = 5.5 Hz, CHpy), 8.84 (d, 1H, 3J = 1.9 Hz,
CHim), 8.79 (d, 1H, 3J = 5.3 Hz, CHpy), 8.47 (m, 1H, CHpy), 8.38 (d,
1H, 3J = 8.4 Hz, CHpy), 8.26 (m, 1H, CHpy), 8.21 (d, 1H, 3J = 2.3 Hz,
CHim), 7.92 (d, 1H, 3J = 8 Hz, CHpy), 7.64 (m, 1H, CHpy), 7.48 (m,
1H, CHpy); 13C NMR (DMSO-d6, 125 MHz): d 154.5 (Pd-C), 153.5,
151.3, 150.7, 148.9, 144.6, 143.6, 127.8, 126.1, 117.9, 113.7,
124.5, 124.3. FT-IR (KBr pellet, cmꢁ1) 3129(w), 2924(w), 1609(s),
1574(w), 1484(s), 1454(s), 1367(w), 1324(m), 1282(w), 1145(w),
1094(w), 1038(w), 957(w), 841(vs), 781(s), 735(s), 557(s). ESI-
MS: calcd m/z for [C26H20Cl2F6N8PPd2]+ 872.8941. Found:
872.8950. Anal. Calc. for C26H20Cl2F12N8P2Pd2: C, 30.67; H, 1.98;
N, 11.01. Found: C, 30.52; H, 1.97; N, 10.60.
Melting point was measured in a glass capillary under air using
a JSGW melting point apparatus. IR spectra were recorded as KBr
pellets on a Bruker Vector 22 FT-IR spectrometer operating at
4000–400 cmꢁ1
. Elemental analyses of the compounds were
obtained from thermoquest CE instruments CHNS-O, EA/1110
model and Perkin Elmer Series-II 2400 Elemental Analyzers.
ESI-MS) spectra were recorded on a Waters Q-TOF Premier mass
spectrometer.1H and 13C NMR spectra were recorded on a JEOL
JNM-LA500FT and JEOL ECS-400 instrument using DMSO-d6 and
CD3CN with TMS as internal standard and all the 31P NMR was
recorded in JEOL ECS-400 instrument using DMSO-d6.
2.3. X-ray crystallographic study
Single crystals suitable for X-ray crystallographic analyses were
obtained by the slow evaporation of solvent from the reaction solu-
tion. Single crystal X-ray data were collected using graphite-
monochromated Mo Ka radiation (k = 0.71073 Å) on a Bruker
SMART APEX CCD diffractometer at 140 K. The linear absorption
coefficients, scattering factors for the atoms and the anomalous
dispersion corrections were taken from the International Tables
for X-ray Crystallography. The program SMART was used for collect-
ing frames of data, indexing reflections, and determining lattice
parameters. The data integration and reduction were processed
with SAINT software. An empirical absorption correction was applied
to the collected reflections with SADABS using XPREP. All the structures
2.4.3. Synthesis of [PdCl2(py2im)] (4)
A mixture of 1 (0.39 g, 0.3 mmol) and PdCl2 (0.16 g, 0.9 mmol)
in CH3OH or CH3CN (100 mL) was stirred at room temperature
for 24 h. Greenish turbid solution was obtained. The solution was
filtered. The residue was dissolved in CH3CN/CH3OH (6:1) mixture
(100 mL) and the volume was reduced under vacuum to half of the

B. Santra / Polyhedron 173 (2019) 114123
3
solution. During this process, a yellow block shaped crystalline
solid of 4 was obtained. Yield of 4: 0.06 g, (50%). The latter yellow
colored solid was filtered and the filtrate was kept for crystalliza-
tion to afford pale yellow colored hexagonal shaped crystals of 3.
Yield of 3: 0.06 g, (20%) 4: 1H NMR (DMSO-d6, 500 MHz): d 9.15
(br), 8.55 (br), 8.38 (br), 8.23 (br), 7.94 (br), 7.69 (br), 7.59 (br).
ESI-MS: calcd m/z for [C13H10ClN4Pd]+ 364.9626. Found:
364.9628. Anal. Calc. for C13H10Cl2N4Pd: C, 39.08; H, 2.52; N,
14.02. Found: C, 38.95; H, 2.54; N, 13.63.
C49H40ClF6N4P3Pd: C, 56.94; H, 3.90; N, 5.42. Found: C, 55.17; H,
3.90; N, 5.16.
2.4.5. Synthesis of [Hg(py2im)2](PF6)2 (8)
Method (A): A mixture of 1 (0.13 g, 0.1 mmol) and Hg(OAc)2
(0.1 g, 0.3 mmol) was added CH3OH (15 mL) and stirred at room
temperature for 13 h. Gray colored turbid solution was obtained.
The solution was filtered and the residue was washed by DCM
and CH3OH. The colorless powder of 9 was obtained. Yield:
0.12 g (85%). Method B: A mixture of 1 (0.13 g, 0.1 mmol) and HgCl2
(0.08 g, 0.3 mmol) was taken in CH3OH (10 mL) and stirred at room
temperature for 8 h. Colorless precipitate was obtained. The solu-
tion was filtered and the residue was washed by DCM. The color-
less powder of 8 was obtained. Yield: 0.11 g (81%). 1H NMR
(CD3CN, 500 MHz): d 8.35 (s, 1H, CHim), 8.13 (td, 1H, 3J = 8 Hz,
4J = 1.7 Hz , CHim), 7.98 (d, 1H, 3J = 4 Hz, CHpy), 7.94 (d, 1H,
2.4.4. Synthesis of trans-[PdCl(PPh3)2(py2im)](PF6) (5)
Method (A): A mixture of 3 (0.05 g, 0.05 mmol) and PPh3 (0.05 g,
0.2 mmol) was taken in CH3CN (20 mL) and stirred for 7 h. Pale yel-
low colored precipitate of 5 was obtained which was filtered. Yel-
low colored crystals of 5 were obtained either from the filtrate at
5 °C or CH3CN/DCM (1:2) mixture at room temperature. Collective
yield is 0.08 g (77%). Method (B): To a methanolic solution (30 mL)
of 1 (0.13 g, 0.1 mmol), PdCl2 (0.05 g, 0.3 mmol) was added as solid
and stirred for 10 min. To this solution PPh3 (0.16 g, 0.6 mmol) was
added as solid. The reaction mixture was stirred for additional
10 h. Artichoke green colored precipitate was obtained and the
solution was filtered. The residue was washed by CH3OH and crys-
tallized in CH3CN. Yield: 0.26 g (88%). Method (C): A mixture of 1
(0.13 g, 0.1 mmol) and trans-[PdCl2(PPh3)2] (0.21 g, 0.3 mmol)
was taken in CH3OH (30 mL) and stirred for 10 h. Pale yellow pre-
cipitate was obtained. The solution was filtered and washed by
CH3OH. Yield: 0.26 g (82%). Method (D): A mixture of 1 (0.10 g,
0.1 mmol) and PdCl2 (0.02 g, 0.1 mmol) was taken in CH3OH
(10 mL) and stirred for 10 h. Artichoke green precipitate was
obtained. The solution was filtered and the precipitate was
collected and washed by CH3OH. Crystals of 5 were obtained in
CH3CN through slow evaporation at room temperature. Yield:
0.073 g (71%). 5: Mp: 222 °C, 1H NMR (DMSO-d6, 500 MHz): d
8.48(d, 1H, 3J = 4.6 Hz, CHpy), 8.04(d, 2H, 3J = 8 Hz, CHpy), 7.89(m,
2H, CHpy), 7.64(s, 2H, CHim), 7.57(m, 2H, CHpy), 7.44 (broad, 6H,
CHph), 7.34(broad, 12H, CHph). 13C NMR (DMSO-d6, 100 MHz): d
163.8 (Pd-Ccarbene), 149.2, 148.8, 140.4, 134.2 (broad), 131.4,
130.5, 129.2, 128.9, 125.4, 123.6, 118.3, 117.2. 31P NMR
(DMSO-d6, 160 MHz): d 18.65 (s, PPh3), ꢁ143.57 (sept, PF6). FT-IR
(KBr pellet, cmꢁ1): 3059(w), 1592(w), 1574(w), 1482(w), 1471
(w), 1437(s), 1412(w), 1323(w), 1282(w), 1097(m), 996(w), 841
(vs), 782(w), 748(w), 698(m), 558(m), 518(m), 497(w). ESI-MS:
calcd m/z for [M-PF6]+ 887.1452, Found: 887.1451. Anal. Calc. for
3J = 8 Hz, CHpy), 7.44 (td, 1H, 3J = 7.2 Hz, 4J = 2.3 Hz, CHpy 13C
)
NMR (CD3CN, 125 MHz): d 170.9 (Hg-C), 148.6, 147.3, 141.2,
125.7, 122.3, 114.6. FT-IR (KBr pellet, cmꢁ1) 3183(w), 3155(w),
1600(s), 1579(m), 1477(m), 1447(s), 1347(s), 1272(s), 1167(w),
1136(w), 1052(w), 1000(w), 960(w), 892(m), 836(vs), 781(s), 729
(m), 624(w), 556(s), 403(w). Anal. Calc. for C26H20F12HgN8P2: C,
33.40; H, 2.16; N, 11.98. Found: C, 33.02; H, 2.14; N, 11.74 ESI-
MS: calcd m/z: 791.1159, Found: 791.1156 for {[M-PF6]+}.
3. Results and discussion
3.1. Carbene transfer from 1 to palladium(II) centre
Trinuclear Cu(I)–NHC complex (1) was synthesized in multi-
gram scale by simple and facile route starting from the less expen-
sive Cu(OAc)2∙H2O and py2imH∙PF6, in the presence of L-ascorbic
acid as reducing agent, in CH3OH under aerobic conditions [54].
The carbene transfer reactions were carried out using 1 and the
palladium precursors in different stoichiometric ratios.
The reaction of 1 with one equivalent of Pd(OAc)2 affords the
complex 2 quantitatively (Scheme 1). The 1H NMR spectrum in
CD3CN shows five distinguished sharp peaks for pyridine and imi-
dazolin-2-ylidene protons with the same integral values. This indi-
cates the symmetrical arrangement of py2im around the metal
center in solution. Further, the ESI-MS reveals the formation of
[Pd(py2im)2]2+ ion (m/z 275.0375) in 2. To confirm the structure
of 2, the single crystals were subjected to X-ray diffraction study
Scheme 1. Synthetic scheme of mono and binuclear Pd–NHC complexes (2–4) from 1.

4
B. Santra / Polyhedron 173 (2019) 114123
Â
Ã
2½PdClðpy2imÞꢂðPF6Þ ! Pd2Cl2ðpy2imÞ2ðPF6Þ2
ð3Þ
and it replicates the structural data of [Pd(pyimypy)2][PF6]2
reported earlier by Chen and Lin [61]. In contrast, the reaction of
1 with one equivalent of PdCl2 resulted in 2 only in a moderate
yield. The 1H NMR spectrum of the crude product from this reac-
tion showed broad peaks in the region of 7.0–9.0 ppm. In addition,
the ESI-MS indicates that the presence of additional products, such
as [PdCl(py2im)]+ (364.9686) and [PdCl(py2im)2]+ (587.0534), apart
from the major product 2.
The reactions of 1 with PdCl2 in 1:3 molar ratios in dry CH3OH
or acetonitrile were carried out (Scheme 1). The products 3 and 4
were isolated by varying the solvent of crystallization, such as
methanol and acetonitrile mixture. Resulted products were thor-
oughly characterized by spectroscopic, analytical and single crystal
X-ray diffraction methods. The 1H NMR spectra of 3 exhibit ten dis-
tinct peaks indicating unsymmetrical coordination of two pyridine
rings around the metal ion (Fig. S3). In case of 4, the 1H NMR spec-
trum recorded in DMSO-d6 shows six broad peaks (Fig. S6). In 13C
NMR spectrum, the resonances for carbene carbon atom in 3 are
observed at 154.5 ppm and 155.9 ppm respectively, which are
similar to that of known Pd–NHC complexes [62–64]. Further,
the ESI-MS shows the masses corresponding to different types of
species present such as, dinuclear {[Pd2Cl2(py2im)2](PF6)}+ (m/z
872.8950) in (3) and mononuclear [PdCl(py2im)]+ (m/z 364.9628)
(3 and 4) complexes. To understand better the structures of 3
and 4, the single crystals obtained from these reactions were sub-
jected to X-ray diffraction analysis.
The formation of 3 and 4 indicates that the reaction might have
proceeded in a sequential manner. Earlier, it was reported that the
presence of a mixture of anions in the starting materials resulted in
anion exchanged products and this exchange depends on the ther-
modynamic stability of the final products [61]. For example, the
reaction between [PdCl2(dppf)] and AgBF4 yielded the dinuclear
palladium complex [Pd2Cl2(dppf)2](BF4)2 and AgCl [65]. Comparing
to this reaction, in the present case the products 3 and 4 could have
formed through an intermediate product [PdCl(py2im)]+ (Eqs. (1)–
(3)). Further chloride ions present in the medium is a determining
factor of product due to the favorable formation of CuCl as a major
driving force [66].
The reactivity of 3 with PPh3 was carried out. In this regard
reaction of 3 with four equivalents of PPh3 was performed which
results yellow colored complex of trans-[PdCl(PPh3)2(py2im)](PF6)
and interestingly the same product was obtained from the reaction
between 1 and PdCl2 in the presence of six equivalents of PPh3
(Scheme 2; method A and B). 1H NMR spectrum of 5 recorded in
DMSO-d6 showed five distinct peaks of protons in py2im ligand
along with two broad merged peaks for phenyl protons of PPh3.
This indicates the symmetrical arrangement of both pyridyl rings
of py2im and also suggests 1:2 compositions of py2im and PPh3
in 5. Here the preference for phosphines to coordinate cis to NHC
is due to the transphobia effect [66]. In 5 both the bulky phosphine
are cis to the NHC due to this effect. In fact a trans configuration is
only observed for bulky NHCs [67]. ESI-MS analysis of 5 shows the
presence of a peak at m/z 887.1383 which corresponds to [M-PF6]+.
Moreover, the 31P NMR exhibits a single sharp peak at 18.65 ppm
for coordinated phosphines, suggesting the magnetic equivalence
of two phosphine units, and a septet for the presence of (PF6)ꢁ
ion in 5. Elemental analysis also supports the formation of complex
5. For further confirmation multifaceted yellow colored crystals of
5 obtained in CH3CN and DCM (1:2) mixture, was subjected to sin-
gle crystal X-ray diffraction study.
The isolation of 5 led to the evaluation of other possible syn-
thetic routes for 5 starting from 1 (Scheme 2, method C and D;
see experimental section). The starting material [Cu(PPh3)2(py2-
im)](PF6) (7) for method D was obtained by the addition of six
equivalents of PPh3 to 1 in CH3OH/DCM mixture. 7 was character-
ized thoroughly by spectroscopic, spectrometric and analytical
methods. The 1H NMR spectrum of 7 shows five distinct peaks of
protons for py2im, along with the phenyl protons of PPh3 in 1:2
compositions. A peak is observed at 188.5 ppm in 13C NMR spec-
trum indicating the Cu–Ccarbene bond. Further, a single sharp peak
at 4.73 ppm in the 31P NMR spectrum suggests the symmetrical
arrangements of coordinated phosphines in 7. Moreover, the ESI-
MS [m/z 809.2026, (M-PF6)+] and the elemental analysis support
the composition of 7. In addition, the single crystal X-ray diffrac-
tion study was carried out to authenticate the structure of 7.
Reactions of [trans-PdCl2(PPh3)2] (6) with 1 (method C) or 7
with PdCl2 (method D) led to the isolation of 5 in very good yield.
It should be noted that in method C only carbene was transferred
from Cu to Pd center but in the case of method D both carbene
Â
Ã
Cu3ðpy2imÞ2 ðPF6Þ3 þ3PdCl2 ! 3½PdCl2ðpy2imÞꢂ þ3CuðPF6Þ
ð1Þ
4
1
½PdCl2ðpy2imÞꢂ þ CuðPF6Þ ! ½PdClðpy2imÞꢂðPF6Þ þ CuCl
ð2Þ
4
Scheme 2. Preparation of 5 by different synthetic routes.

B. Santra / Polyhedron 173 (2019) 114123
5
as well as PPh3 was transferred from Cu to Pd center. The latter
method describes the double transfer of ligands such as phosphine
and py2im from copper to palladium center. To the best of our
knowledge this is one of the first examples of double transmetalla-
tion reactions where both phosphine and carbene are transferred
from one metal center to the other.
diffraction studies. It reveals that 3 crystallized in monoclinic space
group P21/c. Crystallographic and structural parameters are given
in Table 1. The selected bond distances and angles are given in
the legend of Fig. 1. Molecular structure of 3 shows a dinuclear pal-
ladium complex in which both palladium ions are coordinated by a
carbene carbon atom, two pyridyl nitrogen atoms of py2im and a
chlorido ligand adopting square planar geometry. The chlorido
ligand ions occupy trans position to the carbene carbon. Interest-
ingly, each py2im ligand is involved in C^N chelation as well as
N^C^N bridging mode with the palladium ions to build an overall
‘V’ shaped structure (Fig. 1). The charge of the dicationic complex is
balanced with two (PF6)ꢁ ions.
3.2. Carbene transfer from 1 to mercury(II) ion
Finally the NHC transfer from Cu to group 12 metal ions is also
evaluated to understand the strength of py2im coordination in 1.
As an initiative to this direction the carbene transfer from 1 to mer-
cury(II) salts was studied. Thus the reactions of 1 with Hg(OAc)2 or
HgCl2 in methanol (Scheme 3) with 1:3 stoichiometric ratio was
carried out. The product 8 was isolated in 85% yield. 1H NMR shows
five distinct peaks indicating the symmetrical arrangement of
ligands around mercury(II) ion centre. The 13C NMR spectrum
shows a peak at 170.9 ppm which stands for carbene carbon bound
to mercury. ESI-MS analysis shows the presence of [(M-PF6)]+ ion
(m/z 791.1036) corresponding to [Hg(py2im)2](PF6)2 complex.
The structure of 8 was also confirmed by single crystal X-ray
diffraction study. However, Lin and coworkers reported this com-
plex by direct metallation using mercury acetate [68].
The Pd-N bond distances in 3 are in the range of 2.029(4)
Åꢁ2.046(4) Å. However, these Pd–N bond distances are shorter
than that of the analogous mononuclear palladium complexes
3.3. X-ray crystallography
3.3.1. Structure of 3
Pale yellow colored hexagonal shaped crystals of 3 obtained in
CH3CN/CH3OH (1:1) mixture was subjected to single crystal X-ray
Fig. 1. ORTEP diagram of 3 (50% probability), where counter anions are omitted for
clarity. Selected bond distances (Å) and angles (deg): C(20)–Pd(1), 1.972(5); C(8)–
Pd(2), 1.961(4); N(1)–Pd(1), 2.046(4); N(4)–Pd(2), 2.029(4); N(5)–Pd(1), 2.033(3); N
(8)–Pd(2), 2.038(3); Cl(1)–Pd(1), 2.345 (1); Cl(2)–Pd(2), 2.3179(12); C(20)–Pd(1)–N
(5), 79.04(16); C(20)–Pd(1)–N(1), 98.97(17); N(5)–Pd(1)–N(1), 175.52(14); C(20)–
Pd(1)–Cl(1), 174.40(13); N(5)–Pd(1)–Cl(1), 86.48(12); C(8)–Pd(2)–N(4), 79.69(17);
C(8)–Pd(2)–N(8), 97.87(16).
Scheme 3. Synthetic diagram of Hg–NHC (8).
Table 1
Crystallographic data of X-ray diffraction 3–5.
Complexes
3
4
5
CCDC No.
Formula
Fw
1024344
C26H20Cl2F12N8P2Pd2
1024346
C26H20Cl4N8Pd2
799.10
tetragonal P43212
9.3557(4)
9.3557(4)
32.501(3)
90
1024347
C50H42Cl3F6N4P3Pd
1118.54
orthorhombic P212121
14.7780(19)
16.243(2)
19.840(3)
90
1018.14
monoclinic P21/c
21.071(5)
15.185(4)
10.556(3)
90
Crystal system space group
a (Å)
b (Å)
c (Å)
a
(°)
b (°)
103.719(4)
90
3281.2(1)
4
2.080
1.461
90
90
2844.7(3)
4
1.866
90
90
4762.4(1)
4
1.560
c
(°)
V (Å3)
Z
q
l
(calc), g/cm3
(mmꢁ1
)
1.673
0.723
h (°)
1.99 to 26.00
6366/12/514
1.058
2.51 to 25.99
2801/0/181
1.113
1.86 to 26.00
9356 /0/604
1.022
Data/restraints/parameters
GOOF on F2
Final R indices [(I > 2
r
(I)]a
0.0426
0.0244
0.0434
wR2
0.0984
0.0563
0.0909
R indices (all data)
0.0539
0.0255
0.0533
wR2
0.1073
0.0569
0.0973
a
R1
=
R
||Fo| ꢁ |Fc||/
R|Fo|; wR2 = {[R R .
w(|Fo|2 |Fc|2)2/ w(|Fo|2)2]}1/2

6
B. Santra / Polyhedron 173 (2019) 114123
reported by Chen and Lin [61]. However, the average Pd–C, (1.967
(5) Å) and Pd–Cl, (2.332(1) Å) bond distances are similar to that of
the reported [Pd(CH3im(CH3-py))2Cl](PF6) complex [69]. The imi-
dazolin-2-ylidene and one of the pyridine rings of py2im are pre-
sent in the square plane. The other pyridine is twisted (dihedral
angle 74.44°) and coordinated to the second palladium ion (tor-
sional angles are C(6)–N(2)–C(5)–C(4), 53.60° and C(19)–N(7)–C
(21)–C(22), 66.45). The two twisted pyridine rings of both py2im
units are nearly parallel to each other and show
action (3.459 Å).
p–p stacking inter-
3.3.2. Structure of 4
Pale yellow colored crystals of 4 obtained in CH3CN/CH3OH
mixture was subjected to single crystal X-ray diffraction method.
4 crystallized in tetragonal chiral space group P43212. Crystallo-
graphic and structural parameters are given in Table 1. The
selected bond distances and angles are given in the legend of
Fig. 2. Molecular structure of 4 shows that palladium ion is coordi-
nated by carbene carbon and one pyridyl nitrogen atoms of py2im
in C^N chelating mode, and the two chlorido ligand ions present in
cis position to form square planar geometry (Fig. 2). Imidazolin-2-
ylidene and the coordinated pyridine rings of py2im are remained
in the same plane as that of palladium ion. The Pd–C (C(1)–Pd(1),
1.958(3) Å) and Pd–N (N(3)–Pd(1), 2.039(3)Å) bond distances are
almost similar to the reported palldium–NHC complexes [59–61].
The two Pd–Cl bond distances are 2.289(7)Å (Cl(1)–Pd(1)) and
2.343 (8)Å (Cl(2)–Pd(1)) and a longer Cl(2)–Pd(1) bond distance
is attribute to the trans influence of carbene [66,70].
Fig. 3. ORTEP diagram of 5 (50% probability). Solvent and counter anion have been
omitted for clarity. Selected bond distances (Å) and angles (deg): C(1)–Pd(1), 1.978
(4); P(1)–Pd(1), 2.3298(11); P(2)–Pd(1), 2.3421(11); Pd(1)–Cl(1), 2.3438(10); C(1)–
Pd(1)–P(1), 88.26(12); C(1)–Pd(1)–P(2), 91.84(12); P(1)–Pd(1)–P(2), 174.37(4); C
(1)–Pd(1)–Cl(1), 174.97(12); P(1)–Pd(1)–Cl(1), 90.09(4); P(2)–Pd(1)–Cl(1), 90.27(4).
4. Conclusions
3.3.3. Structure of 5
Compound 5 crystallized in orthorhombic, P212121 chiral space
group. Crystallographic and structural parameters are given in
Table 1 and selected bond distances and angles are given in the
legend of Fig. 3. As analyzed through spectroscopic methods, palla-
dium is surrounded by carbene carbon of py2im, one chlorido
ligand ion and two PPh3 molecules to form a square planar geom-
etry. In 5, carbene carbon is trans to chlorido ligand and the two
phosphines are trans to each other which are responsible for single
peak in 31P NMR. The Pd-C and Pd-Cl bond distances are compara-
ble to 3 and 4 as well as reported values in the literature [61–62].
The average Pd-P bond distance is 2.336 (11) Å.
Mononuclear (2 and 4) and dinuclear 3 palladium(II)–NHC as
well as mononuclear Hg–NHC (8) complexes were synthesized
by carbene transfer method starting from trinuclear Cu(I)–NHC
complex (1). All the other complexes (3–5) except 2 and 8 were
newly prepared using this procedure and characterized by X-ray
crystallography. 3 shows a ’V’ shaped structure. Like silver com-
plexes, the transmetallation from 1 to Pd was found to be sensitive
to the variation of the stoichiometry of the reactants and also to
the solvents. 5 was isolated as one of the most thermodynamically
stable product obtained by carbene transfer from 1 in presence of
PPh3. The complex 5 was also synthesized from various other
routes. Among them a double transmetallation of py2im and PPh3
was observed from compound 7 and this is the first time, to the
best of our knowledge such transmetallation route is reported.
Moreover we have showed for the first time that the NHC transfer
from Cu(I)–NHC (1) to mercury(II) salts to form thermodynami-
cally favored {[Hg(py2im)2](PF6)2} complex (8). Biological and cat-
alytic applications of these complexes are under progress.
Acknowledgement
BS thank the Indian Institute of Technology, Kanpur for infras-
tructure and financial support.
Appendix A. Supplementary data
CCDC 1024344, 1024346, and 1024347 contains the supplemen-
tary crystallographic data for this paper. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or
e-mail: deposit@ccdc.cam.ac.uk. Supplementary data to this article
can be found online at https://doi.org/10.1016/j.poly.2019.114123.
Fig. 2. Ortep diagram of 4 (50% probability). Selected bond distances (Å) and angles
(deg): C(1)–Pd(1), 1.958(3); N(3)–Pd(1), 2.039(3); Cl(1)–Pd(1), 2.2886(7); Cl(2)–Pd
(1), 2.3432(8); C(1)–Pd(1)–N(3), 79.99(11); C(1)–Pd(1)–Cl(1), 96.08(9); N(3)–Pd
(1)–Cl(1), 175.24(7); C(1)–Pd(1)–Cl(2),171.86(9); N(3)–Pd(1)–Cl(2), 93.27(7); Cl
(1)–Pd(1)–Cl(2), 90.86(3).

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