Substituent-dependent structures and catalysis of benzimidazole-tethered N-heterocyclic carbene complexes of Ag(i), Ni(ii) and Pd(ii)

Article abstract of DOI:10.1039/c000722f

Homoleptic cationic benzimidazole-imidazolin-2-ylidene N-heterocyclic carbene (NHC = L) complexes of Ni^II and Pd^II have been prepared directly from the ligand precursor in salt form [H.L]Cl and from the transmetallation route via Ag^I. The N-tether of the imidazolinylidene ring imposes a significant influence on the nuclearity of the intermediate Ag(i)-NHC complexes and the geometric isomer outcome of the d⁸ products. Use of a benzyl-substituted NHC gives [Ag₄(L ^Bn)₂Cl₄], 2a (from [HL^Bn]Cl, 1a, and Ag₂O) (Bn = benzyl), which shows an alignment of four silver atoms bridged by the difunctional C-N ligands and chlorides. Its transmetallation with NiCl₂(PPh₃)₄ and PdCl₂(MeCN) ₂ results in double-metal salts 2[M(L^Bn)₂] ²⁺[Ag₄Cl₈]^4- (M = Ni (3a) and Pd (4a)). The nuclearity of the Ag₄ aggregate is maintained in the transmetallation process. Their Ag-free forms [M(L^Bn) ₂]Cl₂ (M = Ni (5) and Pd (6)) were prepared by direct deprotonation of 1a with M(OAc)₂. The two carbenic carbon donor are cis- to each other in both 3a and 4a, thus imposing the weaker σ-benzimidazole nitrogen donor to be trans to them. A sterically demanding mesityl pendant however gives the dinuclear dissymmetic [Ag₂(L ^Mes)₂Cl₂], 2b (Mes = mesityl) that shows a 12-membered metallomacrocyclic ring with a 2-coordinated [Ag^I(NHC) ₂] and 4-coordinated [Ag^I(Imd)₂Cl₂] (Imd = imidazole). Transmetallation of the latter, or direct metallation from [HL^Mes]Cl, 1b, gives [M(L^Mes)₂]Cl₂ (M = Ni (3b) and Pd (4b)) with the two carbonic carbon trans to each other. The catalytic potential of 3b and 4b, which are more effective than 5 and 6, has been demonstrated by their high activities in Ni-catalyzed Kumada at room temperature and Pd-catalyzed Heck couplings of aryl and/or heteroaryl halides, respectively.

Full text of DOI:10.1039/c000722f

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PAPER
www.rsc.org/dalton | Dalton Transactions
Substituent-dependent structures and catalysis of benzimidazole-tethered
N-heterocyclic carbene complexes of Ag(I), Ni(II) and Pd(II)† ‡
Fuwei Li,a,b Jian Jin Hu,a Lip Lin Koha and T. S. Andy Hor*a
Received 12th January 2010, Accepted 29th March 2010
First published as an Advance Article on the web 4th May 2010
DOI: 10.1039/c000722f
Homoleptic cationic benzimidazole-imidazolin-2-ylidene N-heterocyclic carbene (NHC = L)
complexes of NiII and PdII have been prepared directly from the ligand precursor in salt form [H.L]Cl
and from the transmetallation route via AgI. The N-tether of the imidazolinylidene ring imposes a
significant influence on the nuclearity of the intermediate Ag(I)-NHC complexes and the geometric
isomer outcome of the d8 products. Use of a benzyl-substituted NHC gives [Ag4(LBn)2Cl4], 2a (from
[HLBn]Cl, 1a, and Ag2O) (Bn = benzyl), which shows an alignment of four silver atoms bridged by the
difunctional C–N ligands and chlorides. Its transmetallation with NiCl2(PPh3)4 and PdCl2(MeCN)2
results in double-metal salts 2[M(LBn)2]2+[Ag4Cl8]4- (M = Ni (3a) and Pd (4a)). The nuclearity of the Ag4
aggregate is maintained in the transmetallation process. Their Ag-free forms [M(LBn)2]Cl2 (M = Ni (5)
and Pd (6)) were prepared by direct deprotonation of 1a with M(OAc)2. The two carbenic carbon donor
are cis- to each other in both 3a and 4a, thus imposing the weaker s-benzimidazole nitrogen donor to
be trans to them. A sterically demanding mesityl pendant however gives the dinuclear dissymmetic
[Ag2(LMes)2Cl2], 2b (Mes = mesityl) that shows a 12-membered metallomacrocyclic ring with a
2-coordinated [AgI(NHC)2] and 4-coordinated [AgI(Imd)2Cl2] (Imd = imidazole). Transmetallation of
the latter, or direct metallation from [HLMes]Cl, 1b, gives [M(LMes)2]Cl2 (M = Ni (3b) and Pd (4b)) with
the two carbonic carbon trans to each other. The catalytic potential of 3b and 4b, which are more
effective than 5 and 6, has been demonstrated by their high activities in Ni-catalyzed Kumada at room
temperature and Pd-catalyzed Heck couplings of aryl and/or heteroaryl halides, respectively.
with high TON (~11 750).10a Other benzimidazole hybrid ligands
Introduction
have also emerged accordingly.10b,10c In view of the very unusual
N-Heterocyclic carbenes (NHCs) have attracted considerable
structural motif found in the intermediate Ag3 complex and its
synthetic utility for other d8 carbene-hybrid complexes,10a,11 we
have investigated the coordination of other related ligands and
found that the structural assemblies of both the transmetallation
intermediates and final products are dependent on the ligand
substituents. These variations however do not appear to adversely
affect the efficiency of the transmetallation step. The synthesis
of two other ligands with bulkier N-substituents (Fig. 1, LBn 1a
and LMes 1b) (Bn = benzyl; Mes = mesityl) and their complexation
with Ag(I) (2a and 2b), Ni(II) (3a, 3b and 5) and Pd(II) (4a,
4b and 6) as well as the use of 3b and 4b as catalysts for the
Ni-catalyzed Kumada and Pd-catalyzed Heck couplings of aryl
and/or heteroaryl halides are herein reported.
interest for their diverse applications in coordination chemistry
and homogeneous catalysis.1 In recent times, much attention has
been turned to the design of carbene hybrid ligands, namely, NHCs
that are functionalized with other donor functions.2 It provides a
ready method to tune the electronic and steric characters of the
resulting complexes, as well as to raise the ligand hemilability,
both of which are essential considerations in catalytic designs.
Nitrogen donors of different dissociability such as amine,3 imine,4
oxazoline,5 pyridine,6 pyrimidine,7 quinoline8 and phenanthroline9
are among the successful examples that can be hybridized with
pure NHC. We have recently reported the use of benzimidazole-
functionalized imidazolium NHC ligand (Fig. 1, LMe) to support
the trinuclear Ag3Cl2(m-Cl)(m-LMe)2 and its use as a transmetalla-
tion agent to give PdCl2(h-LMe), which is a Suzuki-active catalyst
aDepartment of Chemistry, National University of Singapore, 3, Science
Drive 3, Singapore, 117543, Singapore. E-mail: andyhor@nus.edu.sg;
Fax: (65) 6873 1324; Tel: (65) 6874 2663
bState Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou
Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou,
73000, China
† CCDC reference numbers 760767–760776. For crystallographic data
for 1a–4a, 1b–4b, 5 and 6 in CIF or other electronic format see DOI:
10.1039/c000722f
‡ This paper is dedicated to the memory of Professor Robert Bau (1944–
2008).
Fig. 1 A structural representation of benzimidazole-functionalized NHC
ligands.
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Results and discussion
Synthesis of new benzimidazole-functionalized imidazolium salts
(1a and 1b)
The imidazolium salts 1a and 1b were prepared from the
quarternization of N-substituted imidazoles by 2-chloromethyl-
◦
1
1-ethylbenzimidazole in DMSO at 120 C (Scheme 1). H NMR
spectra of 1a and 1b show characteristic downfield resonances of
the NCHN protons at 9.59 and 9.92 ppm (in d6-DMSO). The
13C NMR spectra display characteristic downfield resonances at
148.15 and 147.71 ppm for the NCN carbons. The positive mode
of ESI-MS give base peaks at m/z = 317 and 345 which correspond
to the cations of 1a and 1b respectively. The molecular structures
of 1a and 1b have been unambiguously identified by single-crystal
X-ray diffraction (Fig. 2, depository numbers: CCDC 760767 (1a)
and CCDC 760768 (1b)†), which indeed revealed the expected
benzimidazole-functionalized imidazolium cation associated with
Cl- anion.
Scheme 2 Synthesis of NHC complexes of AgI (2a), NiII (3a and 5) and
PdII (4a and 6) from the precursor 1a.
Scheme 1 Synthesis of ligand precursor [HL]+Cl-.
Scheme 3 Synthesis of NHC complexes of AgI (2b), NiII (3b) and PdII
(4b) from 1b.
protons on the bridgehead carbon (C10/10A, Fig. 3, depository
number: CCDC 760769 (2a)†) appear as singlets (5.75 and
1
5.86 ppm respectively) in the H-NMR spectra, which probably
indicate unhindered rotation about carbon linker in solution.
The positive mode of ESI-MS spectra give no further structural
information except confirmation of the presence of [Ag(L)2]+ for
both 2a (m/z 741) and 2b (m/z 797).
Fig. 2 Molecular structures of benzimidazole-functionalized imida-
zolium salts [HLBn]Cl (1a) and [HLMes]Cl (1b) with 30% thermal ellipsoids
and labeling scheme; hydrogen atoms are omitted for clarity.
Unlike the known Ag3Cl2(m-Cl)(m-LMe)2, the X-ray structure of
2a shows an open alignment of four silver atoms with the carbene-
benzimidazole hetero-donating ligand bridging the two external
silver atoms, supplemented by alternate singly and doubly bridging
chlorides (Scheme 2 and Fig. 3). This assembly could be a result
of dimerisation of two dinuclear [ClAg(m-LBn)AgCl] moieties as a
means to gain stability. No formal AgI–AgI bonding is envisaged
Synthesis of AgI complexes (2a and 2b)
Reactions of 1a and 1b in CH2Cl2 at r.t. with Ag2O yield
colorless complexes 2a [Ag4(m-Cl)4(LBn)2] and 2b [Ag2Cl2(LMes)2],
respectively (Scheme 2 and 3). Their 1H-NMR spectra in d6-
DMSO reveal the complete disappearance of the resonances for
the acidic 2H-imidazolium protons, supporting the formation of
the silver-carbene moieties. Their 13C NMR spectra also display
significant downfield shifted resonances (180.39 and 181.66 ppm)
for the NCN carbon, thus pointing to NHC complex formation.
Similar to the reported trinuclear Ag3Cl2(m-Cl)(m-LMe)2,10a the two
˚
although Ag1–Ag2 (2.988(2) A) is within bonding distance and
˚
internal Ag(1)–Ag(1A) is confined to a close contact of 3.338(2) A.
The stark contrast between Ag3Cl2(m-Cl)(m-LMe
) and [Ag4(m-
2
Cl)4(LBn)2] demonstrates the structural sensitivity of these Ag(I),
which are intermediates in the transmetallation process, on the
N-substituent of the imidazole ring. The geometric, coordination
and nuclearity flexibilities of Ag(I) enables it to adjust and adapt to
different electronic and steric demands of different hybrid carbene
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ring. The second Ag(I) is tetrahedrally bound to the N-donating-
benzimidazoles and two terminal chlorides. The Ag–N bonds
˚
(Ag1–N1 2.359(5), Ag1–N5 2.328(5) A) are longest among this
˚
series of Ag-NHC complexes (Ag–N (2.242(2) A) in Ag3Cl2(m-
Cl)(m-LMe)2 and (2.314(9) A) in [Ag4(m-Cl)4(L )2]), probably due
Bn
˚
to the saturation of Ag(I) here. The Ag1 ◊ ◊ ◊ Ag2 non-bonding
˚
separation of 3.739(3) A is significantly larger than the combined
˚
van der Waals radii (3.20 A) and at the large end of the
˚
Ag ◊ ◊ ◊ Ag distances (3.232(1)–3.852(1) A) found in other Ag(I)
metallomacrocycles.11f
Synthesis of NiII and PdII from transmetallation of AgI-NHC
complexes
Fig. 3 ORTEP view of [Ag4Cl4(LBn)2] (2a) with 30% thermal el-
lipsoids and labeling scheme. Selected bond lengths (A) and angles
The significance of silver NHC carbene complexes is evident in
its common use as a transmetallation agent in the preparation of
NHC complexes of other late transition metals.11,12 Reaction of
2a with NiCl2(PPh3)2 in CH2Cl2 at r.t. gives [Ni(LBn)2]2[Ag4Cl8]
(Scheme 2, 3a), which is a result of phosphine replacement at
NiII and chloride migration to Ag(I). Its formation is inferred
˚
(◦): Ag1 ◊ ◊ ◊ Ag2 2.988(2), Ag1 ◊ ◊ ◊ Ag1A 3.338(2), Ag1–N1 2.314(9),
Ag1–Cl1 2.664(3), Ag1–Cl1A 2.616(3), Ag1–Cl2 2.805(4), Ag2–C11
2.068(1), Ag2–Cl2 2.337(3), Ag1A–Cl1 2.616(3), N1–Ag1–Ag2 73.4(3),
N1–Ag1–Ag1A 126.1(2), N1–Ag1–Cl1 107.1(2), N1–Ag1–Cl2 109.5(3),
N1–Ag1–Cl1A 117.0(3), C11–Ag2–Ag1 115.7(4), C11–Ag2–Cl2 176.3(4),
Ag2–Ag1–Ag1A 145.6(6), Ag2–Cl2–Ag1 70.4(9), Cl2–Ag1–Ag2 47.5(7),
Cl2–Ag1–Ag1A 124.4(8), Cl2–Ag2–Ag1 62.2(9), Ag1–Cl1–Ag1A 78.4(9),
Cl1–Ag1–Cl2 116.1(1), Cl1–Ag1–Ag2 99.7(8), Cl1–Ag1–Ag1A 50.2(7),
Cl1A–Ag1–Cl1 101.6(9), Cl1A–Ag1–Cl2 105.8(1), Cl1A–Ag1–Ag2
151.8(8), Cl1A–Ag1–Ag1A 51.4(7).
1
from the disappearance of the H-NMR resonance of the acidic
imidazolium C2–H at 9.59 ppm. The bidentate chelate at NiII shows
two doublets for the methylene linkage at 6.32 and 6.23 ppm. Its
13C-NMR spectrum shows the carbenic carbon at 159.94 ppm.
The positive mode ESI-MS shows two base peaks of [Ni(LBn)2]2+
at m/z = 345 and 690 with isotopic peak difference of 0.5 and 1.0
mass units respectively.
ligands, thus explaining the synthetic utility of the transmetallation
methodology in NHC carbene syntheses.
The identity of 3a was unambiguously verified in an X-ray
single-crystal diffraction analysis (Fig. 5, depository number:
The structural variation of these Ag(I) complexes is further
exemplified in the identification of 2b which crystallizes with the
¯
triclinic space group P1. The bulky mesityl substituent exerts a
strong influence on the molecular structure of its AgI complex,
giving a dissymmetric bridged structure of 2- and 4-coordinated
Ag(I) spheres (Scheme 3 and Fig. 4, depository number: CCDC
760770 (2b)†). The former comprises a binary NHC moiety with
˚
two NHC ligands (Ag2–C11 2.090(6) and Ag2–C33 2.087(6) A)
attached to a Ag(I) that is significantly distorted from linear ((C11–
Ag2–C33) 166.4(3)◦). The distortion could be a result of the
constraint imposed by the twelve-membered metallomacrocyclic
Fig. 5 ORTEP view of one of the two [Ni(LBn)2]2+ cation (up) and
its counteranion [Ag4Cl8]4- (down) of [Ni(LBn)2]2[Ag4Cl8] (3a) with 30%
thermal ellipsoids and labeling scheme. Hydroge◦n atoms are omitted
Fig. 4 ORTEP view of [Ag2(LMes)2Cl2] (2b) with 30% thermal ellipsoids
for clarity. Selected bond lengths (A) and angles ( ): Ni1–C11 1.879(2),
˚
and labeling scheme. Hydroge◦n atoms are omitted for clarity. Selected
Ni1–C31 1.884(1), Ni–N1 1.922(1), Ni–N5 1.914(1), C11–Ni1–C31 94.4
(6), C11–Ni1–N5 178.6(6), C31–Ni1–N5 86.6(6), C11–Ni1–N1 88.2(6),
C31–Ni1–N1 177.3(6), N1–Ni1–N5 90.8(5); Ag1–Cl1 2.61(4), Ag1–Cl2
2.61(6), Ag1–Cl3 2.53(5), Ag1–Cl4 2.69(5), Ag2–Cl1 2.463(5), Ag1–Ag2A
3.26(3), Ag2–Ag2A 2.83(5).
˚
bond lengths (A) and angles ( ): Ag2–C11 2.092(6), Ag2–C33 2.088(6),
Ag1–N1 2.359(5), Ag1–N5 2.330(5), Ag1–Cl1 2.574(2), Ag1–Cl2 2.590(2),
Ag1 ◊ ◊ ◊ Ag2 3.739(3), C11–Ag2–C33 166.2(3), N1–Ag1–N5 109.2(2),
Cl1–Ag1–Cl2 117.6(7).
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CCDC 760771 (3a)†). It reveals a hydrated ionic dual-metal
complex formulated as [Ni(LBn)2]24+[Ag4Cl8]4-·2H2O. The NiII is
completely stripped of its original ligands which are replaced by
two C,N-chelate on a square planar sphere (deviation of Ni from
˚
N1N5C31C11 coordination plane is 0 A). As expected, the two
cis-directing carbon donors avoid a mutually-trans configuration.
˚
Both Ni–C (1.879(2) and 1.884(1) A) and Ni–N lengths (1.922(1)
˚
and 1.914(1) A) are comparable with related analogues with
pyridyl donors.13 A similar but in situ method was used by Jin
et al. in the preparation of 3-alkyl-1-picolylimidazolin-2-ylidene
Ni(II) through Ag2O,13b which yields the chloride salt instead of the
[Ag4Cl8]4- complex form. We are not aware of any crystallographic
report of this aggregate anion but many iodide aggregates [AgxIy]n-
are known.14 Its congeneric but structurally different [Ag4I8]4- has
also been reported in the preparation of pyrimidine-functionalized
Ni-NHC complex by Chen et al.6d A closer analogue is found
in [Ag4I8]4- which occurs as a counter-ion in the synthesis of
homoleptic crown Ag-NHC complexes.14a This Ag4 aggregate can
also be viewed as zig-zag silver chain with negligible external
˚
Ag ◊ ◊ ◊ Ag interaction (Ag(1) ◊ ◊ ◊ Ag(2A) 3.26(3) A) but strong
argentophilic contacts for the internal metals (Ag(2) ◊ ◊ ◊ Ag(2A)
˚
2.83(5) A).
Fig. 6 ORTEP view of one of the two [Pd(LBn)2]2+ cation (up) and
its counteranion [Ag4Cl8]4- (down) of [Pd(LBn)2]2[Ag4Cl8] (4a) with 30%
thermal ellipsoids and labeling scheme. Hydroge◦n atoms are omitted
This convenient transmetallation method to give homoleptic
benzimidazole-hybridized carbene cations can also be applied to
other d8 complexes. This is demonstrated by a similar transfer to
PdCl2(CH3CN)2 to give [Pd(LBn)2]2[Ag4Cl8] 4a (Scheme 2). Similar
to 3a, the disappearance of C2–H in the 1H-NMR spectrum
(9.59 ppm) and the doublet at 6.09–5.44 ppm are indicative of
successful formation of the carbene-nitrogen chelate. Unlike 3a,
however, 4a is only sparingly soluble in many deuterated solvents,
thus hampering 13C NMR analysis. The positive mode of ESI-
MS is dominated by isotopic patterns centered at m/z = 369
and 738 corresponding to [Pd(LBn)2]2+ with peak differences of
0.5 and 1.0 mass units respectively. Its X-ray diffraction analysis
shows an isostructural complex as 3a, viz. a square-planar PdII
˚
for clarity. Selected bond lengths (A) and angles ( ): Pd1–C11 1.986(5),
Pd1–C31 1.974 (4), Pd1–N1 2.066(4), Pd1–N5 2.054(4), C11–Pd1–C31
96.7(2), C11–Pd1–N1 86.0(2), C31–Pd1–N1 177.3(2), C11–Pd1–N5
178.4(2), C31–Pd1–N5 84.9 (2), N1–Pd1–N5 92.5(2); Ag1–Ag2A 3.325(2),
Ag2–Ag2A 2.838(2).
NMR spectral diagnosis comes from the disappearance of C2–H
at 9.92 ppm in d6-DMSO and the presence of doublets at 6.28
and 6.17 ppm for the methylene linkage. The downfield carbenic
carbon resonance at 176.45 ppm is also characteristic. Its ESI-MS
spectrum gives the molecular peaks of [Ni(LMes)2]2+ at m/z 373 and
746, and [NiCl(LMes)2]+ at 781.
X-Ray single-crystal diffraction of 3b (Fig. 7, depository
number: CCDC 760772 (3b)†) reveals a similar homoleptic
cationic structure of 3a except that, as anticipated, the two
˚
center (deviation of Pd atom from N1N5C31C11 plane is 0.01 A)
enclaved by two bidentate benzimidazolyl-imidazolilydenes and
the C- and N- donors opposite to each other (Fig. 6, depository
number: CCDC 760773 (4a)†). This is similar to the reported pyri-
dine functionalized imidazolium or benzimidazolium Pd-NHC
complexes,15,6b but different from the earlier reported PdCl2LMe
which is a neutral mononuclear complex. The Pd–C (1.986(5) and
˚
˚
1.974(4) A) and Pd–N bond lengths (2.066(4) and 2.054(4) A)
are normal when compared to many of the known Pd-NHC
complexes. Like 3a, 4a is also balanced by [Ag4Cl8]4-, suggesting
similar mode of formation for these congeneric complexes. Facile
formation of [AgCl2]- from AgCl and its tetramerisation to give the
stable [Ag4Cl8]4- would offer a ready driving force for the chloride
abstraction from PdCl2L that leads to the observed [PdL2]2+.
Although the cis-disposition of the two carbene donors help
to impose the weaker benzimidazole ligand at the trans-position,
it brings into proximity the two substituents on the N-atom of
the NHC ring. To examine if we can use a greater steric effect to
overcome the thermodynamically favored product, we have carried
out similar experiments by using [Ag2Cl2(LMes)2] 2b which bears
a sterically more demanding mesityl group. The NiII product viz.
[Ni(LMes)2]Cl2 3b is isolated as a greenish yellow powder which is
highly soluble in common solvents such as CH2Cl2, MeOH, DMF
Fig. 7 ORTEP view of [Ni(LMes)2][Cl]2 (3b) with 30% thermal ellipsoids
and labeling scheme. Hydrogen atoms are omitted for clarity. Selected
◦
˚
bond lengths (A) and angles ( ): Ni1–N1 1.891(7), Ni1–C11 1.909(8),
N1–Ni1–N1A 179.997(1), N1–Ni1–C11 86.3(3), N1–Ni1–C11A 93.7(3),
C11–Ni1–C11A 179.997(1). Symmetry code: A, -x, -y, -z.
1
and DMSO, etc. It is also moderately stable in dry air. Its H
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mesityl substituents have moved away to avoid conflict, which
is achieved by placing the two carbene carbon at a mutually trans
configuration in a near-perfect square planar geometry (N1–Ni1–
N1A 179.999(1) and C11–Ni1–C11A 179.998(1)◦, distortion of
(1a, HLBnCl) with M(OAc)2 (M = Ni, Pd) is heated in DMSO for
12 h to afford the corresponding chloride salts of NiII (5) and PdII
(6) (Scheme 2). Replacing the bulky and highly-charged [Ag4Cl8]4-
in 3a and 4a by the more solvating Cl-, 5 and 6 give highly soluble
products in common solvents such as CH2Cl2, MeOH, DMF and
DMSO. The X-ray crystal structures confirm that the cations of
5 and 6 are isostructural to those of 3a and 4a (Fig. 9 and 10,
depository numbers: CCDC 760775 (5) and CCDC 760776 (6)†).
˚
Ni from the N1C11N1AC11A coordination plane is 0 A). Such
geometric arrangement is also found in similar NHC-phosphine
chlelates imposed by a sterically bulky aryl or naphthyl.16 It is
intriguing that, unlike 3a, 3b is formed as a chloride salt. It
suggests that, upon carbene departure, the residual Ag2 moieties
in 2b probably picks up the adventitious phosphine and hence less
reactive towards the chloride in 3b. These collectively suggests that
the N-substituent of imidazole ring could influence both anionic
and cationic structures of the product as well as that of the Ag(I)
intermediate.
The analogous Pd-NHC complex 4b of LMes is also easily
obtained from 2b and PdCl2(CH3CN)2. The X-ray crystal structure
reveals that 4b is isostructural to 3b with an ideal square pla-
nar geometry (N1–Pd1–N1A 180.0 and C11–Pd1–C11A 180.0◦,
distortion of Pd from the N1C11N1AC11A coordination plane
˚
is also 0 A) (Fig. 8, depository number: CCDC 760774 (4b)).
It is remarkably consistent that, like 3b, 4b is also balanced
by chloride instead of the Ag4 aggregate, thus supporting that
the anion of the transmetallation product is influenced by the
composition of the Ag(I) precursor. The trans-configuration of
the cation of 4b is in contrary to the reported cations of [Pd(N-
Fig. 9 ORTEP view of [Ni(LBn)2][Cl]2 (5) with 30% thermal ellipsoids
and labeling scheme. Hydrogen atoms are omitted for clarity. Selected
◦
˚
bond lengths (A) and angles ( ): Ni1–C11 1.868(2), Ni1–C31 1.876(3),
Ni–N1 1.933(2), Ni–N5 1.919(2), C11–Ni1–C31 92.4 (1), C11–Ni1–N5
179.4(1), C31–Ni1–N5 87.9(1), C11–Ni1–N1 86.7(1), C31–Ni1–N1
177.4(1), N1–Ni1–N5 93.1(9).
15
phenyl-N-(a-pyridyl)imidazolin-2-ylidene)]2[PF6]2 and [Pd(N-
alkyl-N-(a-picolyl)benzimidazolin-2-ylidene)]2X2 (alkyl = methyl,
ethyl, propyl, butyl; X = Br, BF4),6b but structurally similar to
the cation of [Pd(N-butyl-N-(8-quinolinyl)methyl)imidazolin-2-
ylidene)]2[PF6]2.8a
Fig. 10 ORTEP view of [Pd(LBn)2][Cl]2 (6) with 30% thermal ellipsoids
and labeling scheme. Hydrogen atoms are omitted for clarity. Selected
◦
˚
bond lengths (A) and angles ( ): Pd1–C11 1.978(3), Pd1–C31 1.980 (3),
Pd1–N1 2.061(3), Pd1–N5 2.070(3), C11–Pd1–C31 94.9(1), C11–Pd1–N1
85.8(1), C31–Pd1–N1 179.1(1), C11–Pd1–N5 177.3(1), C31–Pd1–N5 84.5
(1), N1–Pd1–N5 94.9(1).
Fig. 8 ORTEP view of the molecule [Pd(LMes)2]Cl2 (4b) with 30%
thermal ellipsoids and labeling scheme. Hydrogen atoms are omitted
◦
˚
for clarity. Selected bond lengths (A) and angles ( ): Pd1–N1 2.009(2),
Pd1–N1A 2.009(2), Pd1–C11 2.023(3), Pd1–C11A 2.023(3), N1–Pd1–N1A
180.0, N1–Pd1–C11 84.09(10), N1–Pd1–C11A 95.91(10), N1A–Pd1–C11
95.91(10), N1A–Pd1–C11A 84.09(10), C11–Pd1–C11A 180.0. Symmetry
code: A, -x, -y, -z.
Complexes 3b and 4b can also be prepared directly from 1b with
M(OAc)2. In general, the yields of these Ni and Pd complexes
are higher when prepared by this direct method compared to
transmetallation through silver carbenes.
Synthesis of NiII (5 and 3b) and PdII (6 and 4b) complexes from
direct deprotonation of imidazolium salt with M(OAc)2 (M = Ni
or Pd)
Pd-NHC complex catalyzed Heck coupling reaction
The N-mesityl substituted complex 4b (trans-NHCs) and N-benzyl
substituted complex 6 (cis-NHCs) were compared in their catalytic
activities towards the Heck reaction of 4-acetylphenyl bromide
with styrene. Reactions conducted in N,N-dimethyl acetamide
To obtain the silver-free form of 5 and 6, we have used the
established direct deprotonation method on the imidazolium
salt.17 A mixture of benzimidazolyl imidazolium chloride salts
◦
(DMAc) in the presence of NaOAc as the base at 130 C under
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Table 1 Mizoroki–Heck reaction of aryl bromides with styrene or 2-vinyl pyridine catalyzed by Pd-NHC complex 4ba
Entry
1
Ar–Br
Olefin
Time
24
Product
Conv. (%)c
>99
2b
3
24
24
87
96
4
5
24
24
>99
100
6
7
20
24
85
81
8
9
24
24
83
100
10d
30
X = C, N
99
11d
12d
30
30
95
96
X = C, N
13d
30
95
a Reaction conditions: aryl halide 0.5 mmol, olefin 0.75 mmol, 0.25 mol% complex 4b, DMAc 3 mL, NaOAc 1.0 mmol, temperature 130 ◦C. b Catalyst is
complex 6. c Conversion of aryl halide analyzed by GC-MS. d NaOAc 2.0 mmol.
a low catalyst load of 0.25% showed that the former gives almost
quantitative conversion of the aryl halide whereas the latter returns
with 87% (Table 1, entries 1 and 2). Complex 4b was hence chosen
as a model of this series to examine its Heck activities towards
other aryl bromides (Table 1). It gives an excellent conversion
of 96% in the coupling between 4-actyl bromobenzene and 2-
vinyl pyridine (entry 3), suggesting its potential to be applied to
the more demanding Heck reactions using heterocycle-bearing
olefins. Reactions of styrene or 2-vinyl pyridine with 4-aldehyde
bromobenzene proceed quantitatively (entries 4 and 5). Use of
bromobenzene or 1-bromonaphthalene (entries 6 and 7) also
give satisfactory outcomes. Use of electronic rich (entry 8) and
sterically hindered ortho-substituted substrates (entry 9) could also
result in 83% and 100% conversions respectively. This method
can also be applied to dual-coupling on dihaloarenes. This is
exemplified by the efficient Heck reactions between styrene or
2-vinyl pyridine with 1,4-diiodo benzene or 1,3-diiodo benzene
(entries 10–13). The di-pyridyl products would be useful ligands
for complexation and supramolecular assembly. The catalytic
activity of complex 4b is superior to the pyridine-functionalized
benzimidazolium Pd-NHC complex.6b
Ni-NHC complex catalyzed Kumada–Corriu coupling reaction
Many boronic acids, stannanes, and organozincs are obtained
from Grignard reagents or organolithium compounds, but Suzuki,
5236 | Dalton Trans., 2010, 39, 5231–5241
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Table 2 Ni-NHC complex catalyzed Kumada–Corriu cross-coupling
Stille, and Negishi coupling routes may be the preferred choices be-
cause of the higher functional group tolerance. Kumada coupling,
however, offers a more direct access to biaryls when the substrates
can tolerate the background reactivity of a Grignard reagent or
when sensitive functional groups are absent in the product. For this
reason, the Kumada coupling reaction still remains an attractive
route to bi- or ter-aryls.18 N- or S-containing heteroaryls are
relatively less used in coupling with Grignard reagent.
reactiona
Entry Aryl halide
1
Catalyst Time Product
Conv. (%)b
90
5 (0.5%) 12
The catalytic difference of 3b (trans-NHCs) and 5 (cis-NHCs)
was compared in the coupling of 3-chloro-6-methoxypyridazine
with p-Me-C6H4MgBr. Both give good conversions (≥ 90%) in
THF at r.t. under a low catalyst loading (0.5 mol%) but, like the
Pd analogue, the former is slightly better (Table 2, entries 1 and 2).
It was hence used as a representative model to examine coupling
of selected aryl- and heteroaryl halides with p-Me-C6H4MgBr
at r.t. It is highly effective towards 2-chloropyridine and 2-
chloropyrimidine, giving near-quantitative conversions (entries 3
and 4). High efficiency is also observed towards bromoarenes
under 1.0 mol% catalyst loading (entries 9 and 10). Coupling
of 2-bromopyridine or 8-bromoquinoline is also satisfactory
(entries 6 and 7). This system is also effective towards ortho-
or meso-dibromobenzene (entries 11 and 12). Some limitations
are experienced when electron-rich 3-methoxyl-2-chloropyridine
(entry 5) or other heterocycles like 2-bromothiophene (entry
8) is used. These activities are comparable with the reported
imidazolium9,18h and benzimidazolium Ni-NHC complexes.18c
2
3
3b (0.5%) 12
3b (1.0%) 24
95
99
4
5
3b (0.5%) 24
3b (0.5%) 12
97
62
6
7
3b (0.5%) 12
3b (1.0%) 24
81
89
8
3b (1.0%) 24
69
Conclusions
9
3b (1.0%) 24
3b (1.0%) 24
97
96
The N-substituent on the imidazolin-2-ylidene ring has a profound
effect on the structures on the transmetallation Ag(I) intermediate
and the Ni(II) and Pd(II) products. It does not appear however to
influence the course or efficiency of the transmetallation process.
Its use in the benzimidazol-imidazolin-2-ylidene difunctional C–
N ligand enables the isolation of the intermediate Ag(I) complexes
of different nuclearities, and both geometric isomers of the
homoleptic products. Crystallographic analysis of the latter reveals
that the metal is almost completely engulfed by the ligands. This
suggests that in the catalytic activation process, some form of
ligand dissociation process would be inevitable for the metal to
be exposed to substrate approach. This is in contrary to the
classical chelating ligands and illustrates the value of using a more
labile donor such as the nitrogen-donating benzimidazole. Current
experiments are directed at the stoichiometric reactions of these
active d8 dications with key catalytic substrates in an attempt to
isolate and identify products that could shed clearer light on the
mode of action of catalytically important species.
10
11c
12c
3b (2.0%) 36
3b (2.0%) 36
92
84
a Reaction conditions: aryl halide 0.5 mmol, p-Me-C6H4MgBr 0.75 mmol,
Ni complex 0.5–2.0 mol%, THF 3 mL, r.t. b Conversion of aryl halide
analyzed by GC-MS. c p-Me-C6H4MgBr 1.5 mmol.
95XL-T spectrometer. Elemental analyses were performed by the
microanalytical laboratory in house. All Kumada-Corriu reactions
were conducted in a glovebox. GC-MS analyses were recorded on
Agilent 6890N/5973N system.
Experimental
Preparation of ligand precursors
General procedures
Synthesis of 1-(1-ethyl-benzimidazol-2-ylmethyl)-3-benzyl-
imidazolium chloride (1a). 1-Benzylimidazole (650 mg,
4.11 mmol) was added to a solution of 2-chloromethyl-1-
ethylbenzimidazole (779 mg, 4.00 mmol) in DMSO (5 mL), and
the mixture was heated at 120 ◦C for 24 h. The solution was
reduced to 2 mL under vacuum and Et2O (10 mL) was added
to precipitate the product. The resultant NHC ligand precursor
[HLBn]Cl 1a was washed with Et2O (3 ¥ 10 mL) and collected as
All operations were carried out without exclusion of air un-
less otherwise stated. 2-Chloromethyl-1-ethylbenzimidazole was
synthesized according to the reported method.10a N-substituted
imidazole,19a PdCl2(CH3CN)219b and NiCl2(PPh3)19c were prepared
by using published procedures. NMR spectra were measured on
Bruker ACF300 300 MHz and AMX500 500 MHz FT NMR
spectrometers. Mass spectra were obtained on a Finnigan Mat
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a white solid powder (1.30 g, 92%). Single crystals of 1a were
grown by slow evaporation of its CH2Cl2 solution. 1H NMR
(500 MHz, d6-DMSO): 9.59 (s, 1H, NC(H)N), 7.93 (t, J = 1.9 Hz,
2H, Ar–H), 7.65 (d, J = 8.2 Hz, 1H, Ar–H), 7.61 (d, J = 7.6 Hz,
1H, Ar–H), 7.49–7.40 (m, 5H, Ar–H), 7.32–7.29 (t, J = 7.6 Hz,
1H, Ar–H), 7.25–7.22 (t, J = 7.6 Hz, 1H, Ar–H), 5.97 (s, 2H,
CH2), 5.56 (s, 2H, CH2), 4.40–4.35 (q, 2H, CH2CH3), 1.33–1.31 (t,
J = 6.9 Hz, 3H, CH3); 13C(1H) NMR (125.77 MHz, d6-DMSO):
148.15 (s, NCN), 141.81, 137.30, 134.93, 129.00, 128.78, 128.28,
124.09, 122.76, 122.45, 121.98, 119.09, 110.46 (s, Ar–C), 52.04,
45.27 (s, CH2), 38.18 (s, CH2CH3), 14.96 (s, CH3). MS (ESI):
m/z 317 [HLBn]+. Crystal data for 1a: formula C20H21N4Cl. H2O,
colorless crystal, monoclinic, space group P21/c; a = 4.994(5),
of 2a were obtained by slow diffusion of Et2O into its CH2Cl2
solution. Crystal data for 2a: formula C40H40Cl4N8Ag4, colorless
crystal, monoclinic, space group P21/c; a = 13.040(4), b =
◦
3
˚
˚
9.580(3), c = 16.361(6) A; b = 98.899(7) ; V = 2019.3(1) A ;
Z = 2; crystal size 0.12 ¥ 0.10 ¥ 0.02 mm3; GOF = 1.194;
reflections collected: 10 445; independent reflections: 3553 [Rint
=
0.0684]; R1 = 0.0966; wR2 = 0.1883. Depository number: CCDC
760769.
Synthesis of Ag2Cl2(1-(1-ethyl-benzimidazol-2-ylmethyl)-3-
mesitylimidazolin-2-ylidene)2 (2b). Similar procedure to 2a, a
white powder of Ag2Cl2(LMes)2 2b was isolated from the CH2Cl2
solution of [HLMes]Cl 1b (570.30 mg, 1.50 mmol) and Ag2O
(693 mg, 2.30 mmol) after stirring at r.t. for 18 h with exclusion
of light. Yield 564.5 mg (77%). 1H NMR (300 MHz, d6-DMSO):
7.77 (d, J = 7.6 Hz, 1H, Ar–C), 7.65–7.59 (m, 2H, Ar–H), 7.49
(d, J = 1.8 Hz, 1H, Ar–H), 7.31–7.18 (m, 2H, Ar–H), 6.96 (s, 2H,
CH2), 5.87 (s, 2H, CH2), 4.38–4.31 (q, 2H, CH2CH3), 2.30 (s, 3H,
CH3), 1.74 (s, 6H, CH3), 1.17–1.13 (t, J = 7.0, 3H, CH2CH3);
13C(1H) NMR (75.77 MHz, CDCl3): 181.66 (s, NCN), 149.29,
142.34, 138.86, 136.04, 135.19, 134.67, 129.23, 123.67, 123.39,
123.15, 122.31, 119.69, 110.88 (s, Ar–C), 47.51 (s, CH2), 38.71
(s, CH2CH3), 20.97, 17.43, 15.37 (s, CH3). MS (ESI): m/z 797
[Ag(LMes)2]+. Anal. Calc for C44H50Cl2N8Ag2: C, 54.06; H, 5.16;
N, 11.46. Found: C, 54.07; H, 5.06; N, 11.27. Crystal data for
2b.1.5CH3CN: formula C47H52.50N9.50Ag2Cl2, colorless crystal,
◦
3
˚
˚
b = 12.846(1), c = 30.023(3) A; b = 92.145(3) ; V = 1924.6(3) A ;
Z = 4; crystal size 0.60 ¥ 0.20 ¥ 0.06 mm3; GOF = 1.052;
reflections collected: 13 565; independent reflections: 4430 [Rint
=
0.0487]; R1 = 0.0582; wR2 = 0.1285. Depository number: CCDC
760767.
Synthesis of 1-(1-ethyl-benzimidazol-2-ylmethyl)-3-mesityl-
imidazolium chloride (1b). 1-Mesitylimidazole (391.02 mg,
2.1 mmol) and 2-chloromethyl-1-ethylbenzimidazole (389.40 mg,
2.00 mmol) were heated at 120 ◦C in DMSO (5 mL) for 24 h.
[HLMes]Cl 1b was obtained as a white powder (685 mg, 90%) and
its single crystals were grown by slow evaporation of its CH2Cl2
solution. 1H NMR (300 MHz, d6-DMSO): 9.92 (s, 1H, NC(H)N),
8.22 (s, 1H, Ar–H), 8.03 (s, 1H, Ar–H), 7.65 (d, J = 7.7 Hz, 1H,
Ar–H), 7.57 (d, J = 7.7 Hz, 1H, Ar–H), 7.32–7.17 (m, 4H, Ar–H),
6.15 (s, 2H, CH2), 4.44–4.41 (q, 2H, CH2CH3), 2.34 (s, 3H, CH3),
2.10 (s, 6H, CH3), 1.39–1.34 (t, J = 6.7, 3H, CH2CH3); 13C(1H)
NMR (75.77 MHz, d6-DMSO): 147.71 (s, NCN), 141.88, 140.23,
138.97, 135.10, 134.27, 131.15, 129.24, 124.33, 123.65, 122.73,
121.93, 119.09, 110.45 (s, Ar–C), 45.54, 38.21 (s, CH2), 20.55,
16.90, 15.01 (s, CH3). MS (ESI): m/z 345 [HLMes]+. Crystal data
for 7: formula C22H25N4Cl. 2H2O, colorless crystal, monoclinic,
¯
triclinic, space group P1; a = 14.016(1), b = 19.413(2), c =
◦
3
˚
˚
19.940(2) A; b = 88.669(2) ; V = 4817.9(8) A ; Z = 4; crystal
size 0.14 ¥ 0.10 ¥ 0.02 mm3; GOF = 0.963; reflections collected:
33 829; Independent reflections: 22 017 [Rint = 0.0509]; R1
0.0722; wR2 = 0.1904. Depository number: CCDC 760770.
=
Synthesis of [Ni(1-(1-ethyl-benzimidazol-2-ylmethyl)-3-benzyl-
imidazolin-2-ylidene)2]2[Ag4Cl8] (3a). To solution of
a
Ag4Cl4(LBn)2 (121 mg, 0.1 mmol) in CH2Cl2 (20 mL) was
added NiCl2(PPh3)2 (33 mg, 0.1 mmol). The mixture was stirred
overnight at r.t. with the exclusion of light. The resultant
suspension was filtered through a short column of Celite and
the filtrate was then concentrated to ca. 3 mL. Addition of Et2O
to the filtrate afforded a white powder [Ni(LBn)2]2[Ag4Cl8] 3a.
Yield: 54.6 mg (52%). 1H NMR (500 MHz, d6-DMSO): 7.79
(s, 1H, Ar–H), 7.65 (d, J = 8.2 Hz, 1H, Ar–H), 7.34–7.29 (m,
6H, Ar–H), 7.12-7.09 (t, J = 85.3 Hz, 1H, Ar–H), 6.71–6.68
(t, J = 7.6 Hz, 1H, Ar–H), 6.35 (s, 1H, Ar–H), 6.32 (d, J =
12.6 Hz, 1H, CH2), 6.23 (d, J = 16.4 Hz, 1H, CH2), 5.03 (d, J =
15.2 Hz, 1H, CH2), 4.59-4.56 (t, J = 6.3 Hz, 2H, CH2), 4.52 (d,
J = 15.1 Hz, 1H, CH2), 1.47 (t, J = 6.6 Hz, 3H, CH3); 13C(1H)
NMR (75.47 MHz, d6-DMSO): 159.94 (s, NCN), 150.12, 138.56,
136.33, 129.28, 128.57, 127.71, 125.07, 124.50, 124.03, 123.56,
116, 57, 112. 20 (s, Ar–C), 53.17, 45.62, 40.79 (s, CH2), 15.85
(s, CH3). MS (ESI): m/z 345 and 690 [Ni(LBn)2]2+. Crystal data
for 3a.2H2O: formula C80H84Cl8N16O2Ag4Ni2, colorless crystal,
˚
space group P21/c; a = 14.227(7), b = 9.707(5), c = 16.060(8) A;
◦
3
˚
b = 96.868(1) ; V = 2202.1(2) A ; Z = 4; crystal size 0.48 ¥
0.36 ¥ 0.16 mm3; GOF = 1.028; reflections collected: 15 263;
independent reflections: 5046 [Rint = 0.0345]; R1 = 0.0492; wR2 =
0.1233. Depository number: CCDC 760768.
Preparation of complexes
Synthesis of Ag4Cl4(1-(1-ethyl-benzimidazol-2-ylmethyl)-3-
benzylimidazolin-2-ylidene)2 (2a).
A
slurry of [HLBn]Cl 1a
(500 mg, 1.42 mmol) and Ag2O (670 mg, 2.90 mmol) in CH2Cl2
(50 mL) was stirred for 18 h at r.t. with exclusion of light.
Filtration of the reaction mixture through Celite gave a colorless
solution, which was then concentrated to about 5 mL. Upon the
addition of Et2O to the crude reaction mixture, Ag4Cl4(LBn)2 2a
was obtained as a white powder. Yield: 727.8 mg (85%). 1H NMR
(300 MHz, d6-DMSO): 7.57–7.54 (m, 4H, Ar–H), 7.29–7.17
(m, 7H, Ar–H), 5.75 (s, 2H, CH2), 5.33 (s, 2H, CH2), 4.30 (q,
J = 7.2, 2H, CH2CH3), 1.10 (t, J = 7.1, 3H, CH2CH3); 13C(1H)
NMR (75.47 MHz, d6-DMSO): 180.39 (s, NCN), 149.18, 141.95,
137.06, 134.79, 128.67, 127.97, 127.61, 123.20, 122.62, 122.17,
121.82, 119.18, 110.38 (s, Ar–C), 54.39, 47.17, 38.26 (s, CH2),
14.84 (s, CH3). MS (ESI): m/z 741 [Ag(LBn)2]+. Anal. Calc for
C40H40Cl4N8Ag4: C, 39.77; H, 3.55; N, 9.28. Found: C, 39.83; H,
3.55; N, 9.33. Single crystals suitable for X-ray crystallography
¯
triclinic, space group P1; a = 11.350(1), b = 13.156(1), c =
◦
3
˚
˚
14.964(2) A; b = 82.205(3) ; V = 2036.9(4) A ; Z = 2; crystal
size 0.20 ¥ 0.16 ¥ 0.10 mm3; GOF = 1.069; reflections collected:
12 105; independent reflections: 7180 [Rint = 0.0377]; R1 = 0.0826;
wR2 = 0.1731. Depository number: CCDC 760771.
Synthesis of [Pd(1-(1-ethyl-benzimidazol-2-ylmethyl)-3-benzyl-
imidazolin-2-ylidene)2]2[Ag4Cl8] (4a). To a solution of 2a (121 mg,
5238 | Dalton Trans., 2010, 39, 5231–5241
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0.1 mmol) in 20 mL CH2Cl2 was added PdCl2(CH3CN)2 (26 mg,
Synthesis of [Ni(1-(1-ethyl-benzimidazol-2-ylmethyl)-3-benzyl-
imidazolin-2-ylidene)2][Cl]2 (5). Ni(OAc)2·4H2O (124 mg,
0.5 mmol) and [HLBn]Cl 1a (352 mg, 1.0 mmol) were dissolved
in DMSO (10 mL) and stirred at r.t. for 2 h and then 80 ◦C
for another 12 h. The solvent was reduced under vacuum to
1 mL. Addition of Et2O to the concentrated solution resulted in
a yellow precipitate, which was collected and washed with Et2O
(3 ¥ 10 mL) to give [Ni(LBn)2][Cl]2 5 as a greenish yellow powder.
0.1 mmol). Following the similar procedure to 3a, 4a was isolated
1
as a white powder. Yield: 60.5 mg (55%). H NMR (500 MHz,
d6-DMSO at 353 K): d 8.17–7.71 (m, 2H, Ar–H), 7.63–7.53 (m,
3H, Ar–H), 7.39–7.30 (m, 6H, Ar–H), 6.06 (d, J = 27.8 Hz,
1H, CH2), 5.98 (s, 2H, CH2), 5.44 (d, J = 13.3 Hz, 1H, CH2),
4.53–4.48 (m, 2H, CH2), 1.40 (t, J = 6.9 Hz, 3H, CH3); MS(ESI) :
m/z 369 and 738 [Pd(LBn)2]2+. Crystal data for 4a.2H2O: formula
1
¯
C80H84Cl8N16O2Ag4Pd2, colorless crystal, triclinic, space gro◦up P1;
Yield, 342 mg (90%). H NMR (500 MHz, MeOD): d = 7.74 (s,
˚
a = 11.284(3), b = 13.141(5), c = 15.104(4) A; b = 83.08(2) ; V =
1H, Ar–H), 7.56 (d, J = 8.2 Hz, 1H, Ar–H), 7.41–7.36 (m, 4H,
Ar–H), 7.16–7.13 (m, 3H, Ar–H), 6.73–6.70 (t, J = 7.6 Hz, 1H,
Ar–H), 6.13 (d, J = 8.2 Hz, 1H, Ar–H), 6.07 (d, J = 16.4 Hz,
1H, CH2), 5.58 (d, J = 16.4 Hz, 1H, CH2), 5.22 (d, J = 16.4 Hz,
1H, CH2), 4.60–4.58 (q, J = 7.0 Hz, 2H, CH2), 4.47 (d, J =
16.4 Hz, 1H, CH2), 1.50 (t, J = 6.6 Hz, 3H, CH3); 13C(1H) NMR
(75.47 MHz, MeOD): d = 160.54 (s, NCN), 148.83, 138.50,
136.24, 133.34, 128.98, 128.03, 126.39, 124.40, 123.78, 123.51,
115.79, 111.33 (s, Ar–C), 53.32, 39.80 (s, CH2), 14.44 (s, CH3).
MS (ESI): m/z = 345, [Ni(LBn)2]2+, 725 [NiCl(LBn)2]+. Anal. Calc
for C40H40Cl2N8Ni: C, 63.02; H, 5.29; N, 14.70. Found: C, 59.61;
H, 5.52; N, 13.89. Crystal data for 5.2CH3OH. H2O: formula
3
3
˚
2054.5(1) A ; Z = 2; crystal size 0.28 ¥ 0.16 ¥ 0.06 mm ; GOF =
1.057; reflections collected: 26 695; independent reflections: 9403
[Rint = 0.0423]; R1 = 0.0554; wR2 = 0.1285. Depository number:
CCDC 760773.
Synthesis of [Ni(1-(1-ethyl-benzimidazol-2-ylmethyl)-3-mesityl-
imidazolin-2-ylidene)2][Cl]2 (3b). Following a similar procedure
and scale in the preparation of 3a, [Ni(LMes)2][Cl]2 3b was obtained
as a yellow powder. Yield: 68.1 mg (83%). 1H NMR (300 MHz, d6-
DMSO): 7.85 (s, 1H, Ar–H), 7.70–7.68 (d, J = 7.1 Hz, 1H, Ar–H),
7.42–7.31 (m, 4H, Ar–H), 7.13 (s, 1H, Ar–H), 6.42 (s, 1H, Ar–H),
6.28 (d, J = 16.4, 1H, CH2), 6.14 (d, J = 17.4, 1H, CH2), 4.40 (m,
2H, CH2), 2.64 (s, 3H, CH3), 2.21 (s, 3H, CH3), 1.25 (s, 3H, CH2),
0.94 (t, 3H, CH3); 13C(1H) NMR (125.77 MHz, CDCl3): 176.45
(s, Ar–C), 158.80, 148.22, 147.33, 142.81, 142.74, 142.43, 142.41,
138.33, 138.28, 134.32, 134.12, 134.06, 133.11, 126.91, 121.80 (s,
Ar–C), 53. 99, 40.13, 29.98, 29.20, 25.85, 25.06 (s, CH3). MS (ESI):
m/z = 373 and 746 [Ni(LMes)2]2+, 781 [NiCl(LMes)2]+. Anal. Calc
for C44H50Cl2N8Ni: C, 64.41; H, 6.14; N, 13.66. Found: C, 64.40;
H, 6.02; N, 14.45. Crystal data for 3b.H2O.2CH3OH: formula
C47 H62N8O4NiCl2, green yellow crystal, monoclinic, space group
¯
C42H50Cl2N8O3Ni, colorless crystal, triclinic, space group P1; a =
◦
˚
12.717(6), b = 13.937(7), c = 14.023(7) A; b = 66.401(1) ; V =
3
3
˚
2052.0(2) A ; Z = 2; crystal size 0.30 ¥ 0.20 ¥ 0.10 mm ; GOF =
1.056; reflections collected: 27 158; independent reflections: 9416
[Rint = 0.0456]; R1 = 0.0526; wR2 = 0.1323. Depository number:
CCDC 760775.
Synthesis of [Pd(1-(1-ethyl-benzimidazol-2-ylmethyl)-3-benzyl-
imidazolin-2-ylidene)2][Cl]2 (6). Pd(OAc)2 (112 mg, 0.5 mmol)
and [HLBn]Cl 1a (352 mg, 1.0 mmol) were dissolved in DMSO
(10 mL). Using a similar procedure for the synthesis of 5, a
yellowish white powder of PdCl2(LBn)2 6 was obtained. Yield,
372 mg, 92%. 1H NMR (500 MHz, d6-DMSO): 8.19–8.17 (d, J =
8.2 Hz, 1H, Ar–H), 7.75–7.73 (d, J = 8.2 Hz, 1H, Ar–H), 7.64–7.59
(m, 3H, Ar–H), 7.41–7.26 (m, 6H, Ar–H), 6.11–6.08 (d, J = 14.50,
1H, CH2), 5.99 (s, 2H, CH2), 5.46–5.43 (d, J = 14.50, 1H, CH2),
4.57–4.47 (m, 2H, CH2), 1.41–1.39 (t, J = 7.25, 3H, CH3); 13C(1H)
NMR (125.77 MHz, d6-DMSO): 151.57 (s, NCN), 148.48, 139.13,
137.55, 133.30, 129.03, 128.71, 128.47, 124.67, 123.59, 123.44,
122.73, 120.59, 112.00 (s, Ar–C), 55.37, 53.06, 45.70 (s, CH2),
15.88 (s, CH3). MS (ESI): m/z = 369 and 738 [Pd(LBn)2]2+. Anal.
Calc for C42H50Cl2N8O3Pd: C, 56.54; H, 5.65; N, 12.56. Found: C,
55.86; H, 5.16; N, 12.88. Crystal data for 6.2CH3OH.H2O: formula
˚
C2/c; a = 16.880(6), b = 13.061(5), c = 22.335(8) A; b =
◦
3
˚
109.268(8) ; V = 4648.0(3) A ; Z = 4; crystal size 0.24 ¥ 0.12 ¥
0.08 mm3; GOF = 1.073; reflections collected: 13 065; independent
reflections: 4097 [Rint = 0.1254]; R1 = 0.1165; wR2 = 0.2595.
Depository number: CCDC 760772.
Synthesis of [Pd(1-(1-ethyl-benzimidazol-2-ylmethyl)-3-mesityl-
imidazolin-2-ylidene)2][Cl]2 (4b). Following the same procedure
and scale in the preparation of 4a, [Pd(LMes)2][Cl]2 4b was obtained
as a yellowish powder. Yield: 74 mg (85%). 1H NMR (300 MHz,
d6-DMSO): 7.93 (d, J = 1.3 Hz, 1H, Ar–H), 7.75 (d, J = 8.9 Hz,
1H, Ar–H), 7.49 (d, J = 1.9 Hz, 1H, Ar–H), 7.45 (t, J = 7.6 Hz,
1H, Ar–H), 7.38–7.31 (m, 2H, Ar–H), 6.92 (s, 1H, Ar–H), 6.27
(s, 1H, Ar–H), 6.18 (d, J = 16.4, 1H, CH2), 5.91 (d, J = 16.4,
1H, CH2), 4.57–4.46 (m, 2H, CH2), 2.14 (s, 6H, CH3), 1.34 (t, J =
7.6, 3H, CH3), 1.21 (s, 3H, CH3); 13C(1H) NMR (125.77 MHz,
CDCl3): 167.42 (s, NCN), 148.52, 138.71, 137.71, 133.40, 133.23,
132.99, 132.42, 128.49, 128.14, 124.93, 124.83, 124.07, 123.32,
117.50, 112.08 (s, Ar–C), 44.99, 40.00 (s, CH2), 20. 42, 18.90,
17.55, 15.73 (s, CH3). MS (ESI): m/z = 397 and 794 [Pd(LMes)2]2+.
Anal. Calc for C44H50Cl2N8Pd: C, 60.87; H, 5.80; N, 12.91. Found:
C, 60.74; H, 5.74; N, 12.75. Crystal data for 4b.4CH3OH: formula
¯
C42H50Cl2N8O3Pd, colorless crystal, triclinic, space group P1; a =
◦
˚
12.600(1), b = 13.967(1), c = 14.219(1) A; b = 72.213(2) ; V =
3
3
˚
2068.7(3) A ; Z = 2; crystal size 0.40 ¥ 0.22 ¥ 0.20 mm ; GOF =
1.061; reflections collected: 26 609; independent reflections: 9487
[Rint = 0.0266]; R1 = 0.0455; wR2 = 0.1253. Depository number:
CCDC 760776.
Complexes 3b and 4b could also be easily prepared from the
direct reaction of 1b and M(OAc)2 (M = Ni and Pd) with a yield
of 92% and 90%, respectively.
¯
C48 H64N8O4PdCl2, colorless crystal, Triclinic, space group P1; a =
Catalytic applications
◦
˚
10.672(8), b = 10.857(7), c = 11.628(8) A; b = 84.498(2) ; V =
3
3
˚
1210.2(2) A ; Z = 1; crystal size 0.20 ¥ 0.14 ¥ 0.06 mm ; GOF =
1.073; reflections collected: 8644; independent reflections: 5445
[Rint = 0.0294]; R1 = 0.0482; wR2 = 0.1127. Depository number:
CCDC 760774.
General procedure for the Heck reactions. Complex 4b or 6
(0.25 mol%), an aryl halide (0.5 mmol), styrene or 2-vinyl pyridine
(0.75 mmol), and NaOAc (1.0 or 2.0 mmol) were dissolved in
dimethylacetamide (DMAc, 3mL) in a 10 mL tube. The reaction
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Dalton Trans., 2010, 39, 5231–5241 | 5239
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mixture was stirred at 130 ◦C for a specified duration. After cooling
to r.t., the mixture was diluted with CH2Cl2 (10 mL) and washed
with water (3 ¥ 5 mL). The organic extract was dried over MgSO4,
filtered, and the resultant mixture analyzed by GC-MS.
Shih, Y. C. Chang, S. T. Lin, G. P. A. Yap, I. Chao and T. G. Ong,
Organometallics, 2009, 28, 4316; (c) M. V. Jime´nez, J. J. Pe´rez-Torrente,
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General procedure for the Kumada reactions. The reaction was
done in a glove box. A 10 mL tube was charged with an aryl
halide (0.5 mmol), a specified Ni complex and THF (3 mL). To the
solution was added a solution of p-MeC6H4MgBr (0.75 or 1.0 mL,
1.0 M in THF) at r.t. with stirring. After a specified duration,
the reaction was taken out from the glove box and quenched by
addition of water. The mixture was extracted with ethyl acetate (3 ¥
5 mL), and the combined organic phase was dried over MgSO4,
filtered, and the product mixture analyzed by GC-MS.
Organomet. Chem., 2006, 20, 322; (f) R. E. Douthwaite, J. Houghton
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Crystallographic analysis
Diffraction measurements were conducted at 100(2)–293(2) K
on a Bruker AXS APEX CCD diffractometer using Mo KR
˚
radiation (g = 0.71073 A). The data were corrected for Lorentz
and polarization effects with the SMART suite of programs and
for absorption effects with SADABS.20 Structure solutions and
refinements were performed by using the programs SHELXS-
9721a and SHELXL-97.21b The structures were solved by direct
methods to locate the heavy atoms, followed by difference maps
for the light non-hydrogen atoms. All hydrogen atoms were put at
calculated positions. All non-hydrogen atoms were generally given
anisotropic displacement parameters in the final model. In the
structure of 3a and 4a, the asymmetric unit contains one complex
cation [M(C20H20N4)2]2+ (M = Ni and Pd), half of the complex
anion [Ag4Cl8]4- and one water molecule. One of the Ag atoms
in [Ag4Cl8]4- of 4a is disordered into two parts with occupancy
75 : 25. In the structure of 2b, the asymmetric unit contains two
independent molecules of the compound Ag2Cl2(LMes)2.
7 (a) B. Liu, Q. Q. Xia and W. Z. Chen, Angew. Chem., Int. Ed., 2009, 48,
5513; (b) D. Meyer, M. A. Taige, A. Zeller, K. Hohlfeld, S. Ahrens and
T. Strassner, Organometallics, 2009, 28, 2142.
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47, 8031; (b) H. M. Peng, R. D. Webster and X. W. Li, Organometallics,
2008, 27, 4484.
Acknowledgements
9 S. J. Gu and W. Z. Chen, Organometallics, 2009, 28, 909.
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(b) U. J. Scheele, M. Georgiou, M. John, S. Dechert and F. Meyer,
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Organometallics, 2007, 26, 2742; (d) V. J. Catalano and A. O. Etogo,
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The authors are grateful to the Agency for Science, Technology
and Research (R143-000-364-305) and the Ministry of Education
(R-143-000-361-112) for their financial support. Assistance from
the technical and professional staff of the Department of Chem-
istry (NUS) is appreciated.
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