Hg(II) and Pd(II) complexes with a new selenoether bridged biscarbene ligand: Efficient mono- and bis-arylation of methyl acrylate with a pincer biscarbene Pd(II) precatalyst

Article abstract of DOI:10.1039/c6dt04505g

Two equivalents of 1-benzyl-3-bromoethylbenzimidazolium bromide couple with Na₂Se to produce the first selenoether bridged bis-benzimidazolium salt (LH₂)Br₂. The nitrate (LH₂)(NO₃)₂ and tetrafluoroborate (LH₂)(BF₄)₂ salts were also synthesized from (LH₂)Br₂. The reaction of Hg(OAc)₂ with (LH₂)Br₂ gave the first pseudo pincer carbene mercury complex, [Hg(L-κ²C)][HgBr₄] (C1). Different complexes of Pd(ii) with selenoether bridged carbene were obtained using (LH₂)Br₂ and (LH₂)(NO₃)₂. Syntheses of these complexes were dependent on the counter anion and the temperature. Thus, the pincer type ionic complex [PdBr(L-κ³CSeC)]Br (C2) was isolated at 80 °C and the pseudo pincer type neutral complex cis-[PdBr₂(L-κ²C)] (C3) was isolated at room temperature from (LH₂)Br₂ and Pd(OAc)₂ in DMSO. The nitrate precursor (LH₂)(NO₃)₂ on palladation with Pd(OAc)₂ afforded [Pd(L-κ⁴C_BzCSeC)]NO₃ (C4) showing an unprecedented intramolecular metallation at the ortho position of the benzyl wingtip of the benzimidazole moiety. The ligand salts and metal complexes have been characterized using HRMS, heteronuclear NMR and IR spectroscopy. Single crystal X-ray structures of the ligand salts (LH₂)Br₂ and (LH₂)(BF₄)₂ and complexes C1-C4 have also been elucidated. Complex C2 showed good activity for C-C coupling in the mono-Heck reaction of methyl acrylate and arylbromides. Interestingly, the less common bis-arylation was also observed with deactivated arylbromides as the result of double-Heck coupling.

Full text of DOI:10.1039/c6dt04505g

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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Hg(II) and Pd(II) complexes with a new selenoether bridged
biscarbene ligand: Efficient mono- and bis-arylation of methyl
acrylate with a pincer biscarbene Pd(II) precatalyst
Received 00th January 20xx,
Accepted 00th January 20xx
Rishu,a Billa Prashanth,a Deependra Bawari,a Ushnish Mandal,b Aditya Verma,a Angshuman Roy
Choudhury,a and Sanjay Singha*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Two equivalents of 1-benzyl-3-bromoethylbenzimidazolium bromide couple with Na2Se to give the first selenoether
bridged bis-benzimidazolium salt (LH2)Br2. The nitrate (LH2)(NO3)2 and tetrafluoroborate (LH2)(BF4)2 salts were also
synthesized from (LH2)Br2. The reaction of Hg(OAc)2 with (LH2)Br2 gave the first pseudo pincer carbene mercury complex,
[Hg(L-κ2C)][HgBr4] (C1). Different complexes of Pd(II) with selenoether bridged carbene were obtained using (LH2)Br2 and
(LH2)(NO3)2. Syntheses of these complexes were dependent on the counter anion and the temperature. Thus, the pincer
type ionic complex, [PdBr(L-κ3CSeC)]Br (C2) was isolated at 80 °C and the pseudo pincer type neutral complex, cis-[PdBr2(L-
κ2C)] (C3) was isolated at room temperature from (LH2)Br2 and Pd(OAc)2 in DMSO. The nitrate precursor (LH2)(NO3)2 on
palladation with Pd(OAc)2 afforded [Pd(L-κ4CBzCSeC)]NO3 (C4) showing an unprecedented intramolecular metallation at the
ortho position of the benzyl wingtip of the benzimidazole moiety. The ligand salts and metal complexes have been
characterized using HRMS, heteronuclear NMR and IR spectroscopy. Single crystal X-ray structures of the ligand salts,
(LH2)Br2 and (LH2)(BF4)2 and complexes C1-C4 have also been elucidated. Complex C2 showed good activity for the C-C
coupling in mono-Heck reaction of methyl acrylate and arylbromides. Interestingly, less common, bis-arylation was also
observed with deactivated arylbromides as the result of double-Heck coupling.
framework to give rise to bis- or tripodal ligands (structures A-
Introduction
D
, Chart 1). In this direction some flexible bis- and tris-
imidazolium salts that are carbon- (A, C),6 boron- ( ),7
nitrogen-anchored (
)8 and rigid phenyl substituted ( )9 salts
B
N-heterocyclic carbenes (NHCs) and their metal complexes1
have gained tremendous attention due to their applications in
homogeneous catalysis including C-C cross-coupling reactions,2
C
D
have been designed (Chart 1). Usually in such cases the pivotal
centre does not interact with the appended metal ion.
olefin
metathesis,3
hydrosilylation,4
CO/ethylene
copolymerization,5 etc. Therefore, robust metal ligand
interactions are desired in such complexes to ensure high
turnover. Fine tuning of the donor characteristics of carbene
centre can be done by adjusting the steric and electronic
properties of the substituents on NHCs. Modification of the
ligand topology into multidentate ligands is yet another smart
and convenient tool to increase the metal ligand interaction.
There are two simple ways of doing it: the first approach
involves anchoring of two or three monodentate NHCs to
either
a pivotal centre (CH2/BH/CH/N etc.) or a rigid
a.Department of Chemical Sciences, Indian Institute of Science Education and
Research Mohali, Knowledge City, Sector 81, SAS Nagar, Mohali 140306, Punjab,
India
Email: sanjaysingh@iisermohali.ac.in.
b.Department of Chemical Sciences, Indian Institute of Science Education and
Research Kolkata, Mohanpur – 741246, India.
Electronic Supplementary Information (ESI) available: [multinuclear NMR spectra
Chart 1. Popular bis- and tris-N-heterocyclic carbene precursors.
of carbene salts, metal complexes C1-C4, and products of catalysis]. See
DOI: 10.1039/x0xx00000x
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Chart 3. Macrocyclic poly NHCs as pincer and pseudo-pincer NHC ligands.
with methyl acrylate. The mono- and the double-Heck
reactions proceed smoothly with low catalyst loadings (0.7-1.4
mol%). Optimizations of catalytic reactions to favor bis-
arylated products have also been demonstrated.
Results and discussion
Synthesis of selenoether bridged biscarbene ligand salts
The synthesis of selenoether bridged benzannulated bis-
carbene ligand salts involved the following sequence of
reactions: the reaction of 1-benzyl benzimidazole23 with excess
of 1,2-dibromoethane under reflux afforded 1-benzyl-3-
bromoethylbenzimidazolium bromide.11b In the next step,
nucleophilic substitution of bromide with selenide ion was
performed to couple two equivalents of 1-benzyl-3-
bromoethylbenzimidazolium bromide that yielded the
Chart 2. Important donor-functionalised, pincer and pseudo-pincer NHC ligands.
The second approach relies on connecting flexible side arm(s),
on the imidazole nitrogens, with a donor atom suitably placed
in it (structure E
, Chart 2).10 Important variation of this type
are the pincer ligands with one or two NHCs appended on a
flexible or rigid framework respectively, in structures F11 and
G12 (Chart 2). The category of pincer ligands include CNC (bis-
NHC aminopincer ligands)11a CSC (thioether bridged bis-
NHCs),11b COC (ether bridged bis-NHCs)11c (Chart 2). Contrary
to the pincer NHC complexes, the bis-NHCs bridged by a
heteroatom or a framework that does not coordinate with the
metal centre in its complexes are termed as pseudo pincer bis-
NHC ligands. These pseudo pincer/bidentate ligands include CC
pseudo pincer ligands13 i.e., bis-carbenes with (CH2)n spacers of
various lengths (H), CC pseudo pincer ligands14 (I) and cyclic
CC15a-c pseudo pincer ligands (J) (Chart 2). Some macrocyclic
poly NHCs are also known and bind as tetradentate CNC
pincer15d (K) and CC pseudo pincer15e ligand systems (L) (Chart
3).
selenoether bridged bis-benzimidazolium dibromide (
(Scheme 1). In the 1H NMR spectrum of
LH2)Br2
(
L
H2)Br2 the
imidazolium protons appeared at 10.33 ppm. A triplet for -
CH2Se- at 3.28 ppm was observed which is slightly upfield
shifted due to the replacement of bromide of the starting
material (3.83 ppm) with selenium in (LH2)Br2. Other triplet
due to -NCH2- appeared at 4.90 ppm whereas, the singlet due
to -CH2Ph was observed at 5.86 ppm. The 13C{1H} NMR
spectrum of (LH2)Br2 revealed the benzimidazolium carbon at
143.1 ppm and the signals at 50.3, 47.3 and 22.1 ppm were
due to -CH2Ph, -NCH2-, and -CH2Se-, respectively. The 77Se NMR
spectrum of (
L
H2)Br2 showed a signal at 120.5 ppm. The high
H2)Br2 showed a signal
resolution mass spectrometry (ESI+) of (
L
A few chalcogenated SCS17 and SeCSe18 ligand systems of
pincer type have been designed. In contrast, only the thioether
at m/z = 631.1055 that can be assigned to the mono cation [M-
Br]+ with correct isotopic pattern due to the presence of
selenium in (
established via single crystal X-ray diffraction and confirmed
the assigned formulation of ( H2)Br2.
Single crystals of ( H2)Br2 suitable for X-ray diffraction were
grown from MeOH solution by slow evaporation. Compound
H2)Br2 crystallizes in the triclinic crystal system with Pī space
LH2)Br2. Solid state structure of (LH2)Br2 was
bridged CSC pincer carbenes are known (structure F, Chart 2)
and no report is available on the selenoether bridged CSeC
pincer or pseudo-pincer carbenes till date. Such CSeC type of
pincer framework can be considered appropriate for soft
metals like copper,21 and palladium.22
L
L
(
L
The results presented herein, to the best of our knowledge
are the first synthesis of selenoether bridged CSeC pincer
group (Fig. 1, Table 1). The C(16)–Se(1) (1.949(3) Å) and C(17)–
ligand salts ((
metal complexes ([Hg(
(C2) cis-[PdBr2(
-κ2C)] (C3) and [Pd(
L
H2)Br2, (
L
H2)(NO3)2 and (
-κ2C)][HgBr4] (C1), [PdBr(
-κ4CBzCSeC)]NO3 (C4)). The
L
H2)(BF4)2) and their
L
L
-κ3CSeC)]Br
L
L
present results also highlight the key differences in the
bonding properties of the new selenoether bridged CSeC
pincer system compared to the thioether CSC analog.11b In the
present work, we also demonstrate the ability of [PdBr(L-
κ3CSeC)]Br (C2) pincer complex to catalyze the mono- and
relatively less common double-Heck reaction of aryl bromides
Scheme 1. Synthesis of selenoether bridged bis-benzimidazolium dibromide (LH2)Br2.
2 | J. Name., 2012, 00, 1-3
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of C1 the peak due to carbene carbon was observed at 181.4
ppm (as compared to 143.1 ppm for ( H2)Br2 and the signals at
L
51.3, 49.1 and 27.7 ppm were attributed to -NCH2Ph, -NCH2-,
and -CH2Se-, respectively. The 77Se NMR spectrum showed a
signal at 106.3 ppm comparable to that of the free ligand
precursors (LH2)Br2, (LH2)(NO3)2, and (LH2)(BF4)2 which is
indicative of no Hg···Se interaction. The high resolution mass
spectrum (ESI+) of C1 showed the base peak at m/z = 375.0654
attributed to the mononuclear carbene-mercury adduct as [M-
2H-HgBr4]2+. The ESI– spectrum of C1 showed a signal at m/z =
440.7290 due to [HgBr3]− anion.
The unambiguous composition of C1 was confirmed by
single crystal X-ray diffraction method. Crystals suitable for X-
ray analysis of C1 were grown by diffusion of Et2O vapor in its
DMSO solution giving colorless needles. Complex C1
crystallizes in the trigonal system with R3 space group. The
solid state structure of complex C1 (Fig. 2, Table 1) showed
Fig. 1 Solid state structure of the cation of (LH2)Br2, counter ions Br– have been deleted.
Ellipsoids are set at the 50% probability level. Hydrogen atoms have been omitted for
clarity. Selected bond [Å] and bond angles [deg]: C(17)–Se(1) 1.960(4), C(16)–Se(1)
1.949(3), C(18)–N(3) 1.460(4), C(15)–N(2) 1.486(5), C(1)–N(1) 1.488(5), C(26)–N(4)
1.472(4), C(16)–Se(1)–C(17) 100.85(15), C(15)–C(16)–Se(1) 113.2(3), C(18)–C(17)–Se(1)
113.6(2), N(2)–C(15)–C(16) 109.6(3), N(3)–C(18)–C(17) 112.3(3).
[HgBr4]2- as the counter anion and the cationic part as [ ·Hg]2+
L
as anticipated from the HRMS and NMR data. No meaningful
Hg···Se interaction was seen confirming the observation from
Se(1) (1.960(4) Å) bond distances are normal for C–Se
distances.24 The C(15)–N(2) (1.486(5) Å) and C(1)–N(1)
(1.488(5) Å) distances compare well with that for the sulfur
analog (L’H2)Br2.11b The central part of (
LH2)Br2 contains Se in a
bent shape with the C(16)–Se(1)–C(17) angle of 100.85(15)°
that is comparable to the corresponding angle in sulfur analog
with C–S–C bond angle of 99.6(3)°.11b
Exchange of anion with stoichiometric amount of AgNO3
and excess of HBF4 respectively, afforded its nitrate
1
(
L
H2)(NO3)2 and terafluoroborate salts (
13C{1H} NMR features and chemical shift values of (
and ( H2)(BF4)2 were very similar to that of their precursor
H2)Br2 and do not require elaborate discussion. Solid state
structure of ( H2)(BF4)2 was determined and is consistent with
L
H2)(BF4)2. The H and
L
H2)(NO3)2
L
(L
L
the spectroscopic data (see ESI).
Fig. 2 Crystal structure of pseudo-pincer complex [Hg(L-κ2C)][HgBr4]·DMSO (C1·DMSO).
Ellipsoids are set at the 50% probability level. Hydrogen atoms and DMSO solvent of
crystallization have been omitted for clarity. Selected bond lengths [Å] and bond angles
[deg]: C(1)–Hg(1) 2.107(12), C(19)–Hg(1) 2.127(14), C(16)–Se(1) 1.992(13), C(17)–Se(1)
1.953(16), C(1)–N(1) 1.330(18), C(1)–N(2) 1.313(17), C(19)–N(3) 1.33(2), C(19)–N(4)
1.290(18), C(1)−Hg(1)–C(19) 165.7(6), C(17)−Se(1)–C(16) 98.1(6), C(18)−C(17)–Se(1)
113.4(10), C(15)–C(16)–Se(1) 117.0(10), N(3)–C(19)–N(4) 108.9(13), N(1)–C(1)–N(2)
109.9(11).
Synthesis of Hg(II) complex with selenoether bridged biscarbene
ligand.
The transmetallation of silver-NHC complexes with other metal
salts is a known procedure to isolate metal-NHC complexes.25
Similarly, the ligand transfer from mercury to other transition
metals or main group elements has also been established as a
synthetic method.26 A series of di- and tri-valent lanthanoid
complexes and various Sn(IV) complexes have been prepared
by redox transmetallation using metal(0) sources and
mercury(II) complexes.27 Thus, the synthesis of a mercury(II)
complex with selenoether bridged bis-carbene precursor was
the 77Se NMR for this complex, the distance between Hg(1)
and Se(3) was found to be 3.095(1) Å. Complex C1 is the first
example of
a pseudo pincer selenoether bridged Hg(II)
complex. A molecule of DMSO per formula unit of C1 was
found as solvent of crystallization in the crystal lattice of C1
with its O atom at a distance of 2.807(2) Å from Hg(1) centre.
undertaken and a reaction of (LH2)Br2 was carried out with an
equivalent of mercuric acetate in methanol that afforded the
−Hg(1)
The C(1)
–C(19) bond angle (165.7(6)°) in C1 is
complex, [Hg(L (C1). Complex C1 was fully
-κ2C)][HgBr4]
comparable to 158.70(5)° seen in a similar ether functionalized
bis-NHC mercury(II) complex.28 The C(1)–Hg(1) and C(19)-Hg(1)
bond length of 2.107(12) and 2.127(14) Å respectively, are
consistent with the value reported for the ether functionalized
bis-NHC mercury(II) complex (2.109(15) and 2.091(13) Å).28
The anionic unit, [HgBr4]2- of C1, showed distorted tetrahedral
arrangement around Hg centre. The average Hg–Br bond
length in this anion is 2.604 Å and the Br–Hg–Br angles lie in
the range 98.07(4)–90.95(5)°.
characterized by 1H, 13C{1H} and 77Se NMR spectroscopy, IR
spectroscopy and high resolution mass spectrometry.
In the 1H NMR spectrum of complex [Hg( -κ2C)][HgBr4] (C1)
L
the signal for imidazolium hydrogens was not observed
indicating the formation of carbene-Hg bonds. The -CH2Ph
hydrogens appeared as a singlet at 6.25 ppm and -NCH2- and -
CH2Se- hydrogens appeared as two distinct broad triplets at
5.21 and 3.47 ppm, respectively. In the 13C{1H} NMR spectrum
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trend has also been well supported by the 77Se NMR chemical
shift values of the complexes prepared by Singh et al.19 The
high resolution mass spectrometry measurement showed the
base peak at m/z = 736.9833 attributed to [M-Br]+ consistent
with the isotopic pattern expected for selenium containing
Pincer and pseudo-pincer palladium(II) complexes with
selenoether bridged biscarbene ligand.
As shown in Scheme 2 palladation of (
reaction conditions gives different products (complexes C2 and
C3). Moreover, use of a different salt, ( H2)(NO3)2 gave another
new product (complex C4). Palladation of ( H2)Br2 by its
LH2)Br2 under different
L
CSeC pincer palladium complex C2
.
L
Solid state structure of C2 was determined by single crystal
X-ray diffraction technique (Fig. 3, Table 1). Crystals of C2 were
grown by diffusion of Et2O vapor into a DMF solution. Complex
C2 crystallized in orthorhombic system with Pbcn space group.
reaction with an equivalent amount of Pd(OAc)2 in DMSO
(Scheme 2) at 80 °C leads to the formation of the pincer
complex [PdBr(L
-κ3CSeC)]Br (C2) where the backbone Se
coordinates with Pd(II). This is in contrast to the observation of
11b
Huynh et al. with sulfur analog (L’H2)Br2 where palladation
The crystal structure of C2 showed
a square planar
arrangement around Pd(II) centre where Se of the selenoether
moiety is occupying one of the coordination site. Other two
sites of this square planar arrangement are occupied by
carbene carbons and the fourth site is taken up by a bromide
group. The geometry around Se atom is pyramidal with C(16)–
Se(1)–C(17), C(16)–Se(1)–Pd(1), and C(17)–Se(1)–Pd(1) angles
of 97.7(2), 101.12(15), and 103.39(15)°, respectively. Palladium
is coordinated to carbene carbons in an almost linear fashion
with C(14)–Pd(1)–C(19) bond angle of 175.69(19)° compared
to highly bent C(1)–Pd(1)–C(19) angle (94.8(3)°) of complex C3
(vide infra). The Pd(1)−Se(1) (2.4048(6) Å) distance in C2 is
consistent with the reported values for selenium containing
NHC based Pd(II) complexes with Pd–Se distance in the range
2.396-2.391 Å.20 The C(14)–Pd(1) (2.021(5) Å) and C(19)–Pd(1)
(2.042(5) Å) bond distance for C2 are slightly longer than the
observed range for Ccarbene-Pd (1.945-1.957 Å).20 The Pd(1)-
Br(1) distance 2.4396(6) Å in C2 is consistent with the
corresponding value observed for cis-[PdBr2(L’-κ2C)] (2.434(1)
and (2.479(2) Å).11b
using Pd(OAc)2 leads to the pseudo-pincer bis-carbene Pd(II)
complex, cis-[PdBr2(L
’-κ2C)] in which the thioether function
remains pendant. The Se–Pd coordination in C2 can be
attributed to the preference of softer Se for Pd as compared to
S. This Se–Pd coordination is also facilitated by carrying out the
reaction at 80 °C which probably helps one of the bromide ion
to act as the counter anion and the second Br to take up the
fourth coordination site in a square planar arrangement
1
around Pd in [PdBr(L-κ3CSeC)]Br (C2). In the H NMR spectrum
of [PdBr(L LH2)Br2,
-κ3CSeC)]Br (C2), contrary to the precursor (
Scheme 2. Synthesis of CSeC pincer type palladium complexes C2 and C4 and pseudo
pincer complex C3.
the three types of methylene groups (-NCH2-, -CH2Se-, and -
CH2Ph) showed diastereotopic nature as
a result of
metallation. The signal for each methylene group appeared as
a pair of broad signals, thus giving rise to six different signals.
The protons of −CH2Ph showed broad signals at 6.33 and 5.87
ppm whereas the −NCH2− protons were seen as broad signals
at 5.53 and 3.78 ppm. Similarly, two broad signals for −CH2Se−
appeared at 5.06 and 3.48 ppm, the latter signal overlapped
with the residual water in DMSO-d6. Recording the NMR
spectra in CDCl3/DMSO-d6 mixture to resolve this signal was
not helpful. The signal corresponding to the imidazolium
(NCHN) protons was absent in the 1H NMR spectrum of C2. The
13C{1H} NMR spectrum of C2 showed the expected downfield
shift of the carbene carbon signal at 177.7 ppm in comparison
Fig. 3 Solid state structure of CSeC pincer complex [PdBr(L-κ3CSeC)]Br (C2). Ellipsoids
are set at the 50% probability level. Hydrogen atoms have been omitted for clarity. The
Br− counter ion has also been deleted. Selected bond lengths [Å] and bond angles [deg]:
Pd(1)–Se(1) 2.4048(6), Pd(1)–Br(1) 2.4396(6), C(14)–Pd(1) 2.021(5), C(19)–Pd(1)
2.042(5), C(17)–Se(1) 1.950(5), C(16)–Se(1) 1.985(5), C(14)–Pd(1)–C(19) 175.69(19),
C(16)–Se(1)–C(17) 97.7(2), C(16)–Se(1)–Pd(1) 101.12(15), C(17)–Se(1)–Pd(1)
103.39(15), C(14)–Pd(1)–Se(1) 83.59(13), C(14)–Pd(1)–Br(1) 88.39(13), C(19)–Pd(1)–
Se(1) 92.85(14), C(19)–Pd(1)–Br(1) 95.05(13), Se(1)–Pd(1)–Br(1) 171.55(2).
The reaction of (
L
H2)Br2 with Pd(OAc)2 in DMSO at room
-κ2C)](C3
(Scheme 2). The H and 13C{1H} NMR spectral features and the
chemical shift values for complex, cis-[PdBr2(
-κ2C)] (C3) were
very similar to that of [PdBr(
-κ3CSeC)]Br (C2) and do not
temperature afforded the complex, cis-[PdBr2(
L
)
to that in (LH2)Br2 (143.1 ppm) confirming palladation of
carbene carbons. The 77Se NMR spectrum showed a downfield
1
L
shifted singlet at 300.3 ppm for complex C2 as compared to
the 77Se signal for (
LH2)Br2 (120.5 ppm). This expected
L
require elaborate discussion. Similar to complex C2, the rigidity
in the ligand backbone in C3 due to metallation was reflected
deshielding is a result of Se coordination to Pd(II) and this
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in diastereotopic nature of methylene groups (-NCH2-, -CH2Se-, 99.6(3)°) and C(1)−Pd(1)−C(19) (bond angle of 94.8(3)°) form
and -CH2Ph). The 77Se NMR spectrum for C3 showed a signal at the central part of the crown.
66.6 ppm that is upfield shifted relative to 300.3 ppm for As observed above, Se coordination to palladium in C2
selenium ligated complex C2. The 77Se NMR of crude C3 did occurred even with the Br– counter anion which is in contrast
not show the presence of complex C2. This data indicates that to that observed for the thioether analog where the sulfur
complexes C2 and C3 are not formed in the same reaction and coordination to palladium required the replacement of Br–
–
they actually are different products formed under different with weakly coordinating NO3 anion.11b Nevertheless, it was
conditions. Further, heating a DMSO solution of C3 at 80 °C worthwhile to explore the role of weakly coordinating nitrate
cleanly converts it into C2
spectrometry measurement showed the base peak at m/z = complexes. Thus, the nitrate salt of the ligand i.e., (
736.9882 and was attributed to [M-Br]+ consistent with the was reacted with Pd(OAc)2 that afforded an unprecedented
isotopic pattern expected for selenium containing palladium product [Pd(
-κ4CBzCSeC)]NO3 (C4). The aromatic region in the
complex C3
1H NMR spectrum of this product showed two different
.
The high resolution mass counter ion in the selenoether bridged biscarbene Pd(II)
L
H2)(NO3)2
L
.
Crystals of complex C3 suitable for X-ray diffraction were multiplets at 7.44 ppm and 7.27 ppm, a scenario possible due
grown from CH3CN. Complex C3 crystallizes in the triclinic to different phenyl and benzimidazoyl rings present in this
system with Pī space group (Fig. 4, Table 1). As expected based complex (vide infra). The diastereotopic nature and different
on 77Se NMR data, the crystal structure confirmed the magnetic environment account for two separate doublets for
formation of pseudo pincer palladium complex cis-[PdBr2(
L
-
CH2Ph at 6.06 and 5.83 ppm. Similarly, two doublets appeared
κ2C)] where selenium is not coordinated to palladium. The Pd for –NCH2– groups at 5.50 and 3.73 ppm. For one set of
atom in C3 forms a square planar arrangement of two carbene protons of –SeCH2– a pseudo triplet was observed at 5.02 ppm
carbons and two bromide groups in a cis fashion. The C(1)–
whereas another pseudo triplet for the second set of protons
of –SeCH2– at 3.53 ppm was partially masked by the residual
water in DMSO-d6. The 13C{1H} NMR spectrum of C4 showed
two separate signals for the carbene carbons (NCN) at 176.9
and 175.4 ppm. This indicated towards two different
benzimidazole rings in the product. Additionally, different
magnetic behavior of benzylic protons from two CH2Ph
moieties (as discussed above) was a strong evidence towards
the metallation of C-H bond of the pendant benzyl group
making two benzimidazole rings chemically and magnetically
1
different. Based on the H and 13C{1H} NMR data, formulation
of this compound was proposed as the complex [Pd(
L-
κ4CBzCSeC)]NO3 C4). A signal at 299.3 ppm in the 77Se NMR
(
spectrum of C4 was comparable to that of complex C2 and
strongly suggestive of Pd···Se interaction. High resolution mass
spectrometric investigation, with the base peak at m/z =
655.0640 due to [M-NO3]+, was also in support of the
formation of [Pd(L
-κ4CBzCSeC)]NO3 (C4).
The solid state structure of C4, elucidated by single crystal
X-ray diffraction, confirmed the assigned formulation (Fig. 5,
Table 1). Compound C4 was crystallized as colourless crystals
by slow evaporation of its THF solution. The single crystal X-ray
structure confirmed the orthometallation of one of the benzyl
wingtip resulting into palladium carbon bond formation. The
nitrate counter ion was also found in the molecule. Complex
C4 crystallizes in the orthorhombic crystal system with Pna21
space group. We reason the weak coordinating ability of
nitrate counter ion and favorable Se···Pd interaction to assist in
the intramolecular C-H activation leading to orthometallation
in C4. The Pd atom in C4 lies in a square planar arrangement
(with the sum of bond angles as 359.86°) formed from two
carbene carbons, a Se atom and an ortho-carbon of the
metallated benzyl group. The carbenes are trans to each other
(with C(1)-Pd(1)-C(19) angle of 177.6(8)°), the bridging Se and
the ortho-carbon of the pendant benzyl group
Fig. 4 Crystal structure of pseudo pincer type complex cis-[PdBr2(L-κ2C)] (C3). Ellipsoids
are set at the 50% probability level. Hydrogen atoms have been omitted for clarity.
Selected bond lengths [Å] and bond angles [deg]: Pd(1)–Br(1) 2.4771(9), Pd(1)–Br(2)
2.4619(10), Pd(1)–C(1) 1.989(7), Pd(1)–C(19) 2.000(7), Se(1)–C(16) 1.956(7), Se(1)–
C(17) 1.937(7), C(16)–Se(1)–C(17) 99.6(3), Br(1)–Pd(1)–Br(2) 92.64(3), C(1)–Pd(1)–C(19)
94.8(3), C(1)–Pd(1)–Br(1) 176.00(19), C(19)–Pd(1)–Br(2) 179.0(2).
Pd(1) and C(19)–Pd(1) bond lengths of 1.989(7) and 2.000(7) Å
respectively, and Pd(1)–Br(1) and Pd(1)–Br(2) bond lengths
2.4771(9) and 2.4619(10) Å are consistent with the values
reported for cis-[PdBr2(L’-κ2C)]11b {Pd–C 1.951(7) & 1.984(6) Å;
Pd–Br 2.468(2) & 2.479(2) Å}. Similarly, the Br(1)−Pd(1)−Br(2)
bond angle (92.64(3)°) can be compared with the
corresponding angle in cis-[PdBr2(L’-κ2C)] (93.06(4)°).11b In
comparison to C2 that formed two six membered (C3NSePd)
palladacycles, complex C3 forms a 10 membered C6N2PdSe
palladacycle. Selenium in C3 is oriented away from the Pd
atom and the central 10 membered C6N2PdSe ring adopts a
crown like shape (Fig. 4). The C(16)–Se(1)–C(17) (bond angle of
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Table 1. Crystallographic data for the ligand precursor (LH2)Br2 and complexes C1∙DMSO, C2, C3 and C4.
Compound[a]
(LH2)Br2
C1∙DMSO
C2
C3
C4
Chemical formula
C32H34N4Br2OSe
C34H36N4Br4Hg2OSSe
C32H30N4Br2PdSe
C32H30N4Br2PdSe C32H30.5N5O3.75PdSe
Molar mass
Crystal system
Space group
T [K]
727.41
triclinic
1348.51
trigonal
R3
815.78
orthorhombic
Pbcn
815.78
triclinic
730.47
orthorhombic
Pna21
Pī
Pī
100.0(2)
11.4917(13)
11.5144(12)
12.3760(14)
96.350(5)
99.617(5)
99.243(5)
1577.6(3)
2
100.0(2)
19.153(3)
100.0(2)
30.8531(18)
10.9709(7)
17.1227(10)
90.0
100.0(2)
11.8235(15)
11.8672(14)
12.6937(15)
75.555(7)
69.682(7)
64.940(7)
1502.0(3)
2
100.0(2)
20.367(5)
18.976(5)
7.4600(17)
90
a [Å]
b [Å]
19.153(2)
19.153(3)
c [Å]
α [°]
116.8770(10)
116.8770(10)
β [°]
90.0
90
116.8770(10)
γ [°]
V [Å3]
90.0
90
3158.7(2)
3
5795.8(6)
8
2883.2(12)
4
Z
D(calcd.) [g·cm–3
]
1.53
2.127
1.870
1.804
1.683
μ(Mo-Kα) [mm–1
]
3.754
12.021
4.684
4.519
1.953
Index range
–13 ≤ h ≤ 13
–13 ≤ k ≤ 13
–14 ≤ l ≤ 14
11391
–24 ≤ h ≤ 24
–24 ≤ k ≤ 24
–13 ≤ l ≤ 13
34099
–36 ≤ h ≤ 36
–13 ≤ k ≤ 11
–19 ≤ l≤ 20
33763
–13 ≤ h ≤ 14
–14 ≤ k ≤ 14
–15 ≤ l ≤ 15
18842
–24 ≤ h ≤ 24
–22 ≤ k ≤ 22
–8 ≤ l ≤ 8
24569
Reflections collected
Independent reflections
Data/restraints/parameters
R1,wR2[I>2σ(I)][a]
5471
9631
5116
5504
5071
5471/0/419
0.038, 0.109
0.044, 0.113
1.026
9631/7/429
0.055, 0.117
0.059, 0.121
1.028
5116/0/361
0.042, 0.102
0.057, 0.110
1.037
5504/0/361
0.051, 0.125
0.076, 0.140
1.047
5071/385/400
0.086, 0.197
0.112, 0.199
1.049
R1, wR2 (all data)[a]
GOF
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo|. wR2 = [Σw(|Fo2| – |Fc2|)2/Σw|Fo2|2]1/2
are also transoid to each other (with Se(1)-Pd(1)-C(32) angle of Mono- and double-Heck coupling reactions catalyzed by [PdBr(L-
174.8(6)°). The C(16)-Se(1)-C(17) bond angle of 100.4(11)° is κ3CSeC)]Br (C2).
typical for bent geometry around Se. The Se(1)-Pd(1) distance
Transition-metal-catalyzed C-H bond activation and C-C bond
of 2.486(3) Å is comparable to that in complex C2. The Pd(1)–
formation reactions are important for constructing valuable
building blocks.29 In this context β,β-diaryl α,β-unsaturated
C(32) bond formed as a result of orthometallation and is
2.06(2) Å long. The carbene-Pd bond distances measure
esters occupy a unique position as valuable intermediates for
2.01(2) and 1.99(2) Å.
the synthesis of natural products and pharmaceuticals.
Therefore, development of efficient catalyst systems for mono
and double-Heck coupling reaction leading to β-aryl acrylates
and β,β-diaryl acrylates respectively, would be of great utility.
A literature survey reveals that there are numerous reports for
the mono-Heck reactions leading to trans substituted β-aryl
acrylates however, bis-arylation of acrylates via the double-
Heck reaction has scarcely been investigated.16,19,20,29-43
Additionally, attempts to obtain bis-arylated acrylates are even
lesser and these systems, capable of delivering tri- and tetra-
substituted alkenes, use Pd(OAc)2 and a base under different
conditions30-37 such as use of additives, electron-transfer
mediators (ETMs),38 presence of oxygen and aerobic direct C-
Fig. 5 Crystal structure of [Pd(L-κ4CBzCSeC))]NO3 (C4). Ellipsoids are set at the 50%
H functionalization.38 The palladium NHC complexes capable of
̶
probability level. The disordered NO3 counter anion and hydrogen atoms have been
catalyzing the formation of bis-arylated acrylates are limited to
a dicarbene dipalladium complex,30,31 N-heterocycle carbene
(NHC)-ligated cyclopalladated N,N-dimethylbenzylamine,32 in-
omitted for clarity. Selected bond lengths [Å] and bond angles [deg]: Se(1)–Pd(1)
2.486(3), Pd(1)–C(32) 2.06(2), Pd(1)–C(1) 2.01(2), Pd(1)–C(19) 1.99(2), C(32)–C(27)
1.41(3), Se(1)–C(16) 1.95(3), Se(1)–C(17) 1.99(2), C(1)–Pd(1)–C(19) 177.6(8), C(17)–
Se(1)–C(16) 100.4(11), Se(1)–Pd(1)–C(32) 174.8(6), C(1)–Pd(1)–Se(1) 86.1(5), C(32)–
situ generated Pd carbene complex from PdCl2–5,6-dibutoxy-
Pd(1)–C(1) 94.5(8), C(32)–Pd(1)–C(19) 87.3(8), C(19)–Pd(1)–Se(1) 92.0(6).
N,N'-dibutylbenzimidazolium bromide–NaOAc system in
TBAB33 and Pd-benzothiazole carbene complex.34
6 | J. Name., 2012, 00, 1-3
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Based on various limitations with the existing systems we or the use of heterogeneous Pd and solid support.39 Herein,
were interested to check the ability of our catalyst to perform we present the role of selenium ligated Pd(II) complex,
the mono- and double-Heck reaction without having to resort [PdBr(L
-κ3CSeC)]Br (C2) in catalyzing the mono- and bis-
to ligand bases (amines35 or phosphines36), additives & ionic arylation of methyl acrylate. The merit of this reaction lies in
liquids (TBAB),30,31,35,37 harsh aerobic route (applicable for the fact that neither any additive nor large excess of substrates
dehydrogenative coupling reactions on saturated carbonyls)38
Table 2. Mono- and double-Heck C–C coupling of aryl bromides and methyl acrylate using complex C2 as catalyst.
[PdBr(L-k3CSeC)]Br (C2)
Ar
CO2Me
+
+
Ar Br
Ar
CO2Me
Na2CO3, DMA
140 °C
CO2Me
Ar
1.2 equiv.
1.0 equiv.
mono arylation
bis arylation
1a - 8a
1b - 8b
mono-arylation product
(% yield)
mono-arylation product
(% yield)
bis-arylation product
(% yield)
bis-arylation product
(% yield)
COMe
CO2Me
CO2Me
CO2Me
CO2Me
MeOC
MeOC
1a, (55 %)
1b, (18 %)
5a, (68 %)
5b, (22 %)
CHO
CO2Me
CO2Me
CO2Me
F
OHC
OHC
6a, (52 %)
2a, (63 %)
2b, (25 %)
CN
CO2Me
CO2Me
CO2Me
NC
NC
7a, (55 %)
3a, (45 %)
7b, (5 %)
NO2
CO2Me
CO2Me
CN
CO2Me
O2N
O2N
8b, (21 %)
8a, (62 %)
4a, (50 %)
were required to achieve bis-arylation. Moreover, in only afforded the expected trans coupled product but the
comparison to the previously reported dipalladium dicarbene unprecedented bis-arylation products (due to activation of the
complex,30,31 present complex showed good catalytic results second β-hydrogen) were also obtained with 4-bromotoluene,
with lesser catalyst loading.
The reactions of activated and deactivated aryl bromides bromobenzonitrile and 4-bromonitrobenzene (Table 2).
(1.2 equivalent) with methyl acrylate (1.0 equivalents) in the The bis-arylation observed in these reactions encouraged
4-bromobenzaldehyde,
4-bromoacetophenone,
4-
presence of 0.7 mol% of C2 and Na2CO3 as base (1.1 us to optimize the reactions with the aim to improve yield of
equivalents) in DMA at 140 °C were investigated (Table 2). In the bis-arylation products in the cases of 4-
the present scheme the catalytic behavior of complex C2 not bromobenzaldehyde and 4-bromoacetopheneone (Table 3 and
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4). For all optimizations, excess of arylbromides were used (3 that the reaction proceeds via classical Pd(II)/Pd(0) mechanism
fold with respect to methyl acrylate). It is apparent from Table as the formation of black particles (probably of metallic Pd)
3 that doubling the concentration of the catalyst C2 does not was observed at the end of the catalysis in all reactions. The
affect the yield of the bis-arylation products whereas, when selenium ligated palladacycle C2 under study acts as reservoir
the amount of base was doubled the proportion of bis- for active palladium nanoparticles. This was supported by the
arylation products marginally improved (compare Entry 1 in positive mercury drop test performed for catalysis with 4-
Table 3 & 4 and Entry 3 in Table 3 & 4). Moreover, when the bromobenzaldehyde as substrate, where no catalytic activity
concentration of the base as well as that of the catalyst was was found.44-47 This confirms that the colloidal palladium
doubled, a drastic improvement in the formation of the bis- formed in the reaction was amalgamated by mercury which is
arylation product was observed (Entry 2 and 4, Table 4). known to poison catalysts. Therefore, it is possible that the
Therefore, it can be concluded that the catalyst C2 with low present pincer CSeCPd/(II) complex act as precatalyst and is
catalyst loadings (0.7 mol%) is capable of mono and bis- responsible for the controlled release of tiny Pd nanoparticles
arylation of methyl acrylate and further, the yields of bis- making the system catalytically active.17,48 A likely reason for
arylation products can be easily optimized by varying the the bis-arylation could be a prolonged catalyst lifetime due to
concentration of the catalyst and base.
the more efficient stabilization of the catalytically active
Though a detailed investigation of the mechanistic insights species by the hemilabile selenoether moiety.
of the catalysis reactions are yet to be explored, it is observed
Table 3. Optimization for bis-arylation of methyl acrylate by doubling the catalyst concentration with a fixed amount of base.
Entry
Substrate
4-Bromobenzaldehyde
(18.0 mmol)
Alkene
C2 (mol%)
0.7
Na2CO3 (mmol)
Yield (%)a
1
2
3
4
Methyl acrylate
(6.0 mmol)
5.5
5.5
5.5
5.5
(2a):(2b) 30: 49
(2a):(2b) 33:53
(5a):(5b) 95:1
(5a):(5b) 94:2
1.4
4-Bromoacetophenone
(18.0 mmol)
Methyl acrylate
(6.0 mmol)
0.7
1.4
a Yields are calculated after the products were purified by column chromatography.
Table 4. Optimization for bis-arylation of methyl acrylate by doubling the catalyst concentration with a fixed amount of base.
Entry
Substrate
4-Bromobenzaldehyde
(18.0 mmol)
Alkene
C2 (mol%)
0.7
Na2CO3 (mmol)
Yield (%)a
1
2
3
4
Methyl acrylate
(6.0 mmol)
11
11
11
11
(2a):(2b) 45:53
(2a):(2b) 10:85
(5a):(5b) 50:44
(5a):(5b) 8:90
1.4
4-Bromoacetophenone
(18.0 mmol)
Methyl acrylate
(6.0 mmol)
0.7
1.4
a Yields are calculated after the products were purified by column chromatography.
reactions in addition to the expected mono-arylation giving a
range of cinnamic acid methyl esters. We are currently
engaged in the synthesis of new metal complexes of the ligand
and study their reactivity. Preparation of heterosubstituted
pincer carbenes with Se in its backbone is also underway.
Conclusions
In conclusion, we have synthesized the first selenoether (CSeC)
bridged bis-benzimidazolium carbene precursors to explore
the carbene-metal bond formation with the soft donor atom
Se, present appropriately in the ligand backbone, for forming
the pincer (or pseudopincer) complexes. These precursors are
made available as their dibromide, dinitrate or
bis(tetrafluoroborate) salts. On treatment of the dibromide
Experimental
General considerations
salt with Hg(OAc)2,
a
new example of (κ2C) bridged
All the reactions were carried out using standard Schlenk
techniques, except for the palladium, and mercury
complexation reactions that can be performed under ambient
conditions. Solvents and chemicals were used as received
without any further treatment if not noted otherwise. 1-
pseudopincer carbene mercury was obtained. The dibromide
salt reacted with Pd(OAC)2 to afford different products at
different temperatures showing both the (κ3CSeC) pincer (at
80 °C) and (κ2C) pseudopincer (at RT) bonding modes. Use of
the dinitrate salt resulted in the metallation of the benzyl
ortho-carbon of the benzylbenzimidazolium moiety to form a
(CCSeC) pincer complex. Formation of this unprecedented
complex was assisted by the favorable Pd···Se interaction as
Benzylbenzimidazole
and
1-benzyl-3-
were prepared
bromoethylbenzimidazolium
bromide
according to previously reported methods.[21,11b] Sodium
selenide was prepared according to literature procedure.[49]
EtOH, DMF, DMSO, CHCl3, THF, and MeOH were dried prior to
their use. The 1H, 13C{1H}, 11B and 19F NMR spectra were
recorded on a Bruker ACF 400 spectrometer and the chemical
shifts (δ ppm) were referenced respectively, by TMS (1H, 13C)
and Et2O·BF3, CFCl3 as external references. Assignment of
–
well as weakly coordinating ability of NO3 counter ion. Heck
reaction of arylbromides with methyl acrylate occurred
smoothly when the mononuclear (κ3CSeC) pincer palladium
bromide complex was employed as precatalyst. More
importantly, this complex was also capable of bis-arylation
8 | J. Name., 2012, 00, 1-3
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signals in the 1H and 13C{1H} spectra of compounds (
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L
H2)Br2, C2 General procedure for the synthesis of the ligand precursors
and C4 were made by 1H-1H COSY and 1H-13C HSQC
measurements, for full details of these assignments see ESI.
77Se NMR spectra were recorded on JNM-ECS400
spectrometer with Me2Se as external reference. IR spectra
were recorded in nujol mull between KBr plates. High
resolution mass spectra were measured using a Waters Synapt
G2-S spectrometer.
Selenoether bridged bis-benzimidazolium dibromide (LH2)Br2:
To an in situ generated colorless solution of Na2Se (from 8.98
mmol NaBH4 and 4.49 mmol Se) in EtOH/DMF (1:1 v/v)
mixture was added 1-benzyl-3-bromoethylbenzimidazolium
bromide (1.7 g, 8.98 mmol) at once at 20 °C. The resulting
yellow suspension was then heated up to 75 °C for 48 h. The
solvents were distilled under vacuum to leave a solid residue.
The residue was then suspended in CHCl3 (70 mL) and was
stirred for 30 min. The un-dissolved solid was filtered off and
the filtrate was evaporated to dryness under vacuum to yield a
X-ray crystallographic studies
Single crystal X-ray diffraction data for ((
LH)2Br2, (LH)2(BF4)2,
white solid. Yield: (1.4 g, 93%). M.p. 172 °
C. 1H NMR (400 MHz,
C2, C3 and C4) were collected on a Bruker AXS KAPPA APEX-II
1
CCD diffractometer (Monochromatic MoKα radiation)
equipped with an Oxford cryosystem 700 plus at 100(2) K.
Data collection and unit cell refinement for the data sets were
performed using the Bruker APPEX-II suite, data reduction and
integration were performed using SAINTV 7.685A (Bruker AXS,
2009) and absorption corrections and scaling were performed
using SADABSV2008/1 (Bruker AXS, 2009). Single crystal X-ray
diffraction data for C1 (at 100.0(2) K) and the double-Heck
reaction product 2b (at 296.0(2) K) were collected using a
Rigaku XtaLAB mini diffractometer equipped with a Mercury
375M CCD detector. The data were recorded with graphite
monochromatic MoKα radiation (λ = 0.71073 Å) using scans.
The data collection and data reduction were performed using
the Crystal Clear suite.[50a] The crystal structures were solved
by using either the OLEX2[50b] or WINGX package using SHELXS-
97 and the structures were refined using SHELXL-97.[51] All
non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were fixed at geometrically calculated positions and
were refined using riding model. One of the bromide ions in
DMSO-d6): δ = H NMR (400 MHz, DMSO-d6): δ = 10.33 (s, 2H,
NCHN), 8.19 (broad d, 2H, 3JH-H = 8 Hz, Ar-H), 8.00 (broad d, 2H,
3JH-H = 8 Hz, Ar-H), 7.65-7.70 (m, 4H, Ar-H), 7.56 (broad d, 4H,
3JH-H = 8 Hz, Ar-H), 7.35-7.40 (m, 6H, Ar-H), 5.86 (s, 4H, -CH2Ph),
4.90 (t, 4H, 3JH,H = 7 Hz, -NCH2), 3.28 (t, 4H, 3JH,H = 7 Hz, -CH2Se).
13C{1H} NMR (100 MHz, DMSO-d6): δ = 143.1 (NCHN), 134.5,
131.5, 131.4, 129.4, 129.1, 128.7, 127.3, 127.1, 114.7, 114.5,
50.3 (-CH2Ph), 47.3 (-NCH2), 22.1 (-CH2Se). 77Se NMR (76.2
MHz, DMSO-d6): δ = 120.5. IR (ν/cm-1): 2924, 2854, 2720, 2373,
1576, 1506, 1458, 1376, 1168, 1083, 1026, 961, 840, 800, 656,
601, 559, 456, 426. HRMS (ESI+): m/z calcd for C32H32N4SeBr:
(631.0975) [M-Br]+; found: (631.1055).
Selenoether
bridged
bis-benzimidazolium
dinitrate
(LH2)(NO3)2: A mixture of (
L
H2)Br2 (0.71 g, 1.0 mmol) and
AgNO3 (0.34 g, 2.0 mmol) in MeOH (20 mL) was stirred at
ambient temperature for 14 h. The resulting suspension was
filtered over Celite and the solvent was removed in vacuo to
give the product as white powder. Yield: (0.63 g, 93 %). M.p.
162
8.13 (broad d, 2H, JH,H = 8 Hz, Ar-H), 7.98 (broad d, 2H, JH,H
°
C. 1H NMR (400 MHz, DMSO-d6): δ = 10.02 (s, 2H, NCHN),
(LH2)Br2 showed positional disorder among seven different
–
sites. The two BF4 ions present in the asymmetric unit of the
3
3
=
8 Hz, Ar-H), 7.64-7.71 (m, 4H, Ar-H), 7.50 (broad d, 4H, 3JH,H = 8
Hz, Ar-H), 7.36-7.42 (m, 6H, Ar-H), 5.81 (s, 4H, -CH2Ph), 4.84 (t,
4H, 3JH,H = 7 Hz, -NCH2), 3.20 (t, 4H, 3JH,H = 7 Hz, -CH2Se). 13C{1H}
NMR (100 MHz, DMSO-d6): δ = 143.1 (NCHN), 134.4, 131.5,
131.2, 129.4, 128.6, 127.3, 127.2, 114.5, 50.3 (-CH2Ph), 47.2 (-
NCH2), 22.1 (-CH2Se). 77Se NMR (76.2 MHz, DMSO-d6): δ =
120.9. IR (ν/cm-1): 2923, 2856, 2724, 2398, 1610, 1558, 1458,
1374, 1024, 822, 567, 456, 423. HRMS (ESI+): m/z calcd for
C32H32N5SeO3: (614.1673) [M-NO3]+; found: (614.1665); m/z
calcd for C32H30N4Se: (276.0897) [M-2H-2NO3]2+; found:
(276.0883).
–
(LH2)(BF4)2 were disordered. The first BF4 showed
salt
positional disorder at two sites (designated as B1A & B1B) and
the second BF4– is thermally disordered at a third site (B2A and
B2B). Among the two partially occupied BF4– ions, the one with
B1 does not show thermal disorder while the other with B2
showed thermal disorder. The site occupancy factor between
–
B1A and B1B has been refined to 0.53:0.47. The second BF4
anion is refined with common positions for B2A (same as B2B)
and F5A (same as F5B) and other three fluorine atoms being
–
disordered at two different sites each. The BF4 ions were
refined with geometrical restraints with RIGU. A solvent
accessible void of volume 407 Å3 was calculated for C1 and
removed using mask in OLEX2. The S-Me bond distances in the
DMSO molecule in C1 were restrained using SADI and refined.
Selenoether
bridged
bis-benzimidazolium
bis-
tetrafluoroborate (LH2)(BF4)2: To a clear solution of (
L
H2)Br2
(1.4 g, 3.5 mmol) in MeOH (30 mL) was added HBF4 (1 mL,
48%) drop wise followed by stirring for 2 hour. This was
followed by removal of all volatiles and addition of H2O (70
mL). The aqueous solution was extracted with DCM (3 × 25 mL)
and dried over magnesium sulfate. The title compound was
–
The disordered NO3 in C4 has been refined between two
orientations with occupancies ¾ and ¼. The former position
leaves a large void in the structure occupied by a water
molecule with non-stoichiometric occupancy (assumed to be
obtained as white powder after removal of all volatiles. Yield:
1
C. H NMR (400 MHz,DMSO-d6): δ =
¾). CCDC 1485191-1485195 for (
LH2)Br2, (LH2)(BF4)2, C2, C3,
(0.8 g, 60 %). M.p. 165 °
9.96 (s, 2H, NCHN), 8.13 (broad d, 2H, JH,H = 8 Hz, Ar-H), 7.99
and C1·DMSO; 1489520 for C4; and 1485196 for 2b contain
3
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Center.
3
(broad d, 2H, JH,H = 8 Hz, Ar-H), 7.67-7.71 (m, 4H, Ar-H), 7.53
(d, 4H, 3JH,H = 8 Hz, Ar-H), 7.37-7.45 (m, 6H, Ar-H), 5.81 (s, 4H, -
CH2Ph), 4.84 (t, 4H, 3JH,H = 7 Hz, -NCH2), 3.21 (t, 4H, 3JH,H = 7 Hz,
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
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DOI: 10.1039/C6DT04505G
ARTICLE
-CH2Se). 13C{1H} NMR (100 MHz, DMSO-d6): δ = 143.0 (NCHN), mixture of (
Journal Name
L
H2)Br2 (1.0 g, 1.41 mmol) and Pd(OAc)2 (0.32 g,
134.4, 131.5, 131.2, 129.4, 129.4, 128.6, 127.3, 127.2, 114.5, 1.41 mmol) in DMSO (50 mL) was stirred at room temperature
114.4, 50.3 (-CH2Ph), 47.2 (-NCH2), 22.1 (-CH2Se). 19F NMR for overnight. The mixture was filtered over Celite and
(376.4 MHz, DMSO-d6): δ = −148.0. 11B NMR (128.4 MHz, resulting residue was washed with water. The compound was
DMSO-d6): δ = −1.2. 77Se NMR (76.2 MHz, DMSO-d6): δ = 119.5. extracted in DCM (100 mL) and the organic layer was
IR (ν/cm-1): 2953, 2724, 1563, 1455, 1376, 1288, 1261, 1157, separated, evaporated and dried under vacuum to afford the
1021, 802, 722, 621, 605, 560, 455, 422. HRMS (ESI+): m/z product as brown solid that was crystallized from CH3CN to
calcd for C32H32N4SeBF4: (639.1829) [M-BF4]+; found: afford the brown complex C3. Yield: (0.48 g, 59 %). M.p. 210
3
(639.1826).
°
Ar-H), 7.65 (d, 2H, JH,H = 8 Hz, Ar-H), 7.37-7.46 (m, 14H, Ar-H),
C. 1H NMR (400 MHz, DMSO-d6): δ = 8.03 (d, 2H, JH,H = 8 Hz,
3
Synthesis of selenoether bridged N-heterocyclic pseudo
pincer carbene Hg(II) complex [Hg(L-κ2C)][HgBr4] (C1): The salt
2
7.12-7.20 (m, 14H, Ar-H), 6.33 (d, 2H, JH,H = 16 Hz, -CHHPh),
5.84 (d, 2H, 2JH,H = 16 Hz, -CHHPh), 5.52 (broad d, 2H, 2JH,H = 16
(LH2)Br2 (1.0 g, 1.41 mmol) was dissolved in MeOH. To it was
2
Hz, -NCHH), 5.03 (ps t, 2H, JH,H = 12 Hz, -CHHSe), 3.76 (d, 2H,
added drop wise a solution of mercuric acetate (10.45 g, 1.4
mmol) in MeOH. The precipitation occurred during the
addition of the mercuric acetate solution. The white
precipitate was washed with Et2O and then with CHCl3. The
second filtrate was dried and the title compound was obtained
2JH,H = 16 Hz, -NCHH), 3.49 (ps t, 2H, JH,H = 12 Hz, -CHHSe).
2
13C{1H} NMR (100 MHz, DMSO-d6): δ = 177.6 (NCN), 136.4,
134.3, 133.8, 129.2, 129.1, 128.8, 128.3, 127.9, 127.3, 124.6,
124.4, 112.5, 112.4, 112.3, 51.6 (-CH2Ph), 49.5 (-NCH2), 34.5 (-
CH2Se). 77Se NMR (76.2 MHz, DMSO-d6): δ = 66.6. IR (ν/cm-1):
2928, 2722, 1690, 1508, 1458, 1377, 1176, 1024, 787, 738,
698, 557, 456, 430. HRMS (ESI+): m/z calcd for C32H30N4SePdBr:
(736.9858) [M-Br]+; found: (736.9882).
as a white solid. Yield: (0.62 g, 48%). M.p. 235 °
C. 1H NMR (400
MHz, DMSO-d6): δ = 8.13 (d, 2H, 3JH,H = 8 Hz, Ar-H), 7.81 (d, 2H,
3JH,H = 8 Hz Ar-H), 7.58-7.63 (m, 10H, Ar-H), 7.34 (broad d, 2H,
3JH,H = 8 Hz, Ar-H), 7.26 (broad d, 2H, 3JH,H = 8 Hz, Ar-H), 6.25 (s,
4H, -CH2Ph), 5.21 (broad t, 4H, -NCH2), 3.47 (broad t, 4H, - Synthesis of selenoether bridged N-heterocyclic pincer
CH2Se). 13C{1H} NMR (100 MHz, CDCl3): δ = 181.4 (NCN) 136.0, carbene Pd(II) complex[Pd(L-κ4CBzCSeC)]NO3 (C4): A mixture
133.5, 133.4, 129.3, 128.6, 127.7, 126.3, 126.2, 113.7, 113.4, of (LH2)(NO3)2 (0.50 g, 0.8 mmol) and Pd(OAc)2 (0.17g, 0.8
51.3 (-CH2Ph), 49.1 (-NCH2), 27.7 (-CH2Se). 77Se NMR (76.2 mmol) in DMSO (15 mL) was stirred at room temperature for
MHz, DMSO-d6): δ = 106.3. IR (ν/cm-1): 2925, 2858, 1595, 1457, 24 h. The resulting mixture was filtered over Celite. This was
1375, 1260, 1014, 757, 567. HRMS (ESI+): m/z calcd for followed by washing the residue with H2O. The residue was
C32H28N4SeHg: 375.0670 [M-2H-HgBr4]2+; found (375.0654); then dissolved in DCM (100 mL) and filtered. The filtrate was
m/z calcd for C32H30N4SeHgBr: (831.0518) [M-HgBr3]+; found: washed with water and the organic layer was separated,
(831.0509); HRMS (ESI-): m/z calcd for HgBr3: (440.7216) evaporated and dried under vacuum to afford the product as
[HgBr3]-; found: (440.7290).
brown solid that was crystallized from CHCl3 to afford the
1
C. H NMR
yellow complex C4. Yield: (1.2 g, 60%). M.p. 222
3
°
(400 MHz, DMSO-d6): δ = 8.00 (d, 2H, JH,H = 8 Hz, Ar-H), 7.59
Synthesis of selenoether bridged N-heterocyclic pincer
carbene Pd(II) complex [PdBr(L-κ3CSeC)]Br (C2): A mixture of
3
(d, 2H, JH,H = 8 Hz, Ar-H), 7.44 (m, 7H, JH,H = 8 Hz, Ar-H), 7.27
3
(LH2)Br2 (2.0 g, 2.82 mmol) and Pd(OAc)2 (0.63 g, 2.82 mmol) in
2
(m, 6H, Ar-H), 6.06 (d, 2H, JH,H = 16 Hz, -CHHPh), 5.83 (d, 2H,
DMSO (50 mL) was stirred at 80 °C for overnight. The mixture
was filtered over Celite, and DMSO was distilled off under
vacuum. The resulting residue was washed with H2O (3× 200
mL) and dissolved in DCM. The un-dissolved solid was
collected by filtration followed by washing with THF. The DCM
2
2JH,H = 16 Hz, -CHHPh), 5.50 (d, 2H, JH,H = 12 Hz, NCHH), 5.02
2
(ps t, 2H, JH,H = 12 Hz, -CHHSe), 3.73 (d, 2H, JH,H = 12 Hz, -
2
2
NCHH), 3.53 (ps t, 2H, JH,H = 12 Hz, -CHHSe).‡ 13C{1H} NMR
(100 MHz, DMSO-d6): δ = 176.9 (NCN), 175.4 (NCN), 136.8,
133.9, 133.6, 129.1, 128.3, 127.9, 124.4, 112.4, 50.3 (-CH2Ph),
49.0 (-NCH2), 33.1 (-CH2Se). 77Se NMR (76.2 MHz, DMSO-d6): δ
= 299.3. IR (ν/cm-1): 2928, 2722, 2369, 1690, 1508, 1458, 1377,
1176, 1024, 787, 738, 698, 557, 456, 430. HRMS (ESI+): m/z
calcd for C32H29N4SePd: (655.0609) [M-NO3]+; found:
filtrate afforded another crop of the product. Combined yield:
1
C. H NMR (400 MHz,DMSO-d6): δ =
(0.85 g, 42%). M.p. 246
°
3
3
8.04 (d, 2H, JH,H = 8 Hz, Ar-H), 7.67 (d, 2H, JH,H = 8 Hz, Ar-H),
7.47-7.38 (m, 8H, Ar-H), 7.17-7.20 (m, 6H, Ar-H), 6.33 (broad,
2H, -CH2Ph), 5.87 (broad, 2H, -CH2Ph), 5.53 (broad, 2H, -NCH2),
5.06 (broad, 2H, -CH2Se), 3.78 (broad, 2H, -NCH2), 3.48 (broad,
2H, -CH2Se) the signal overlaps with signal of residual water in
‡
(655.0640). A signal for one hydrogen atom of –CHHSe was
overlapped with residual water in DMSO-d6.
DMSO-d6. 13C{1H} NMR (100 MHz, DMSO-d6): δ = 177.7 (NCN), General procedure for the Heck coupling reaction catalyzed
136.4, 134.3, 133.8, 129.1, 128.3, 127.9, 124.6, 124.4, 112.5, by C2: In a reaction tube, a mixture of aryl bromide (7.2 mmol,
112.4, 51.6 (-CH2Ph), 49.5 (-NCH2), 34.5 (-CH2Se). 77Se NMR 1.2 equivalent), methyl acrylate (543 μL, 6.0 mmol, 1.0
(76.2 MHz, DMSO-d6): δ = 300.3. IR (ν/cm-1): 2921, 2720, 1627, equivalent) and sodium carbonate (594 mg, 5.5 mmol, 1.1
1508, 1458, 1408, 1377, 1194, 1075, 1063, 886, 787, 757, 573, equivalent) in DMA (5 mL) was prepared. A DMA solution of
426. HRMS (ESI+): m/z calcd for C32H30N4SePdBr: (736.9858) the precatalyst C2 (0.7 mol %) was added to it via syringe. The
[M-Br]+; found (736.9833).
reaction tube was then sealed under nitrogen followed by
stirring and heating at 140 °C using J-KEMP temperature
controller for 24 h. After this the reaction mixture was
dissolved in water 50 mL and extracted with ethyl acetate
Synthesis of selenoether bridged N-heterocyclic pseudo
pincer carbene Pd(II) complex cis-[PdBr2(L-κ2C)] (C3):
A
10 | J. Name., 2012, 00, 1-3
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ARTICLE
12 (a) J. C. C. Chen and I. J. B. Lin, J. Chem. Soc., Dalton Trans.,
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Faller and R. H. Crabtree, Organometallics, 2001, 20, 5485-
5488; (c) G. T. S. Andavan, E. B. Bauer, C. S. Letko, T. K. Hollis
and F. S. Tham, J. Organomet. Chem., 2005, 690, 5938-5947;
(d) F. E. Hahn, M. C. Jahnke, V. Gomez-Benitez, D. Morales-
Morales and T. Pape, Organometallics, 2005, 24, 6458-6463;
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2007, 26, 150-154.
13 J. Gil-Rubio, V. Cámara, D. Bautista and J. Vicente, Inorg.
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14 (a) J.-W. Wang, H.-B. Song, Q.-S. Li, F.-B. Xu and Z.-Z. Zhang,
Inorg. Chim. Acta., 2005, 358, 3653-3655; (b) P. Marshall, R.
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sulfate and all volatiles were removed under vacuum to afford
the products. The obtained products were purified using
column (silica 100-200) chromatography.
Acknowledgements
We acknowledge the financial support, Grant No. SR/FT/CS-
009/2009, from Department of Science and Technology (DST),
India. Rishu thanks CSIR for a research associate fellowship.
We would like to thank Dr. S. Venkataramani for useful
scientific discussions. We thank the anonymous reviewer for
Coles, I. A. Fallisa and A. Dervisi, Dalton Trans., 2012, 41
12839-12846.
,
valuable inputs on the X-ray structures reported in the paper. 15 (a) P. J. Barnard, L. E. Wedlock, M. V. Baker, S. J. Berner-
Price, D. A. Joyce, B. W. Skelton and J. H. Steer, Angew.
Chem. Int. Ed., 2006, 45, 5966-5970; (b) S. Ramos, E. Alcalde,
G. Doddi, P. Mencarelli and L. Pérez-García, J. Org. Chem.,
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Skelton, A. H. White and C. C. Williams, J. Organomet. Chem.,
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White and C. C. Williams, Organometallics, 2002, 21, 2674-
2678; (e) W. W. H. Wong, M. S. Vickers, A. R. Cowely, R. L.
The authors thank IISER Mohali for NMR, X-ray facility and all
other infrastructural support.
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Dalton Transactions
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DOI: 10.1039/C6DT04505G
Graphical Abstract
Hg(II) and Pd(II) complexes with a new selenoether bridged biscarbene ligand: Efficient mono- and
bis-arylation of methyl acrylate with a pincer biscarbene Pd(II) precatalyst
Rishu, Billa Prashanth, Deependra Bawari, Ushnish Mandal, Aditya Verma, Angshuman Roy
Choudhury, and Sanjay Singh*
The first examples of selenoether bridged bis-bezimidazolium carbene salts have been
prepared and used further for pseudo pincer Hg(II) and Pd(II) complexes as well as pincer
Pd(II) ionic complexes, an ionic pincer Pd(II) complex showed very good activity in catalyzing
mono- and double-Heck reaction between methyl acrylate and aryl bromides.