Synthesis of NHC complexes by oxidative addition of 2-chloro-N- methylbenzimidazole

Article abstract of DOI:10.1021/ja110634h

The oxidative addition of 2-chloro-N-methylbenzimdazole to complexes of type [M(PPh₃)₄] yields after N-protonation compounds with NH,NMe-substituted NHC ligands. For M = Pd complex compound trans-[3]BF ₄ was obtained, while the oxidative addition for M = Pt yielded a mixture of cis-[4]BF₄ (major) and trans-[4]BF₄ (minor).

Full text of DOI:10.1021/ja110634h

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pubs.acs.org/JACS
Synthesis of NHC Complexes by Oxidative Addition of
2-Chloro-N-methylbenzimidazole
Tim K€osterke, Tania Pape, and F. Ekkehardt Hahn*
Institut f€ur Anorganische und Analytische Chemie, Westf€alische Wilhelms-Universit€at M€unster,
Corrensstrasse 30, D-48149 M€unster, Germany
S Supporting Information
b
stepwise leading to the “classical” NHC complexes of type A,
and this metal template-controlled reaction has been used, for
example, to incorporate NHC donor functions into macrocyclic
ligands.10
ABSTRACT: The oxidative addition of 2-chloro-N-methyl-
benzimdazole to complexes of type [M(PPh3)4] yields after
N-protonation compounds with NH,NMe-substituted NHC
ligands. For M = Pd complex compound trans-[3]BF4 was
obtained, while the oxidative addition for M = Pt yielded a
mixture of cis-[4]BF4 (major) and trans-[4]BF4 (minor).
Monoalkylation of the NHC ligand in complexes of type B
leads to complexes C9,11 where the remaining NH function of the
NHC ligand has been shown to act as a recognition unit via form-
ation of intermolecular hydrogen bonds.11 Recently, complexes
of type C have also been obtained, initially serendipitiously, by
tautomerization of N-coordinated azoles12 or by the oxidative
addition of the C2-H bond of donor-functionalized azoles
to selected metal centers,13 a reaction occasionally followed by
a reductive elimination and shift of the M-H hydride to the azole
N atom.14 In addition, the planned synthesis of “classical” NHC
complexes of type A followed by removal of one N-substituent
has also been reported to lead to complexes of type C.15
We have now found that not only azolium cations but also
neutral 2-chloro-substituted benzimidazoles oxidatively add to
transition metal complexes. Subsequent protonation yields com-
plexes of type C (Scheme 1). Here we describe the oxidative
addition of 2-chloro-N-methylbenzimidazole 1 to [M(PPh3)4]
(M = Pd, Pt) followed by protonation with NH4BF4 to yield the
palladium complex trans-[3]BF4 and the platinum complexes
cis-[4]BF4 and trans-[4]BF4 (Scheme 2).
he chemistry of N-heterocyclic carbenes (NHC) has been
T
studied intensely beginning with the isolation of the first
stable derivative by Arduengo et al. in 1991.1 Today, a large number
of different NHCs are known,2 and these have found numerous
applications in different fields such as organocatalysis3 and as
ligands for the preparation of metallosupramolecular4 and cata-
lytically active metal complexes.5 NHC complexes are generally
prepared from the free NHC ligands or their olefinic dimers,2,6 via
the Ag2O method developed by Lin et al.7 and, most commonly, by
in situ deprotonation of azolium cations followed by coordina-
tion of the NHC ligand to a metal center.1 The oxidative add-
ition of the C2-X (X = H, R, halogen) bond of various azolium
cations to selected transition metals has also been reported.8
The methods listed above lead to complexes of type A bearing
“classical” NR,NR-functionalized NHC ligands (Scheme 1).
In our search for NHC complexes which allow modifications
at the coordinated NHC ligand, we developed the template-
controlled cyclization of β-functionalized isocyanides, leading to
complexes of type B (Scheme 1) bearing NH,NH-functionalized
“protic” NHC ligands.9 While the free NH,NH-substituted
NHCs are not stable, the coordinated ligands can be alkylated
Ligand precursor 1 (Scheme 2) was prepared by the reaction
of commercially available 2-chlorobenzimidazole with iodomethane.
1
Formation of 1 was confirmed by H and 13C{1H} NMR spec-
troscopy and microanalytical data (see Supporting Information,
[SI]). The 1H NMR spectrum exhibits a resonance at δ = 3.77
ppm for the protons of the N-methyl group, and the 13C{1H}
NMR spectrum shows the characteristic resonances for the me-
thyl group at δ = 30.5 ppm and for the C2 carbon atom at δ =
140.9 ppm.
Reaction of 1 equiv of 1 with 1 equiv of [Pd(PPh3)4] in the
presence of an excess of NH4BF4 in toluene yielded palladium
complex trans-[3]BF4 bearing an NH,NMe-substituted NHC
ligand (Scheme 2). The reaction proceeds via oxidative addition
of the C2-Cl bond of the benzimidazole 1 to the palladium(0)
center, giving the intermediate [2] followed by protonation of
the unsubstituted nitrogen atom.16 In principle, the oxidative
addition leading to both the cis and the trans intermediate [2] is
possible. In previous studies with NHC/PPh3 complexes of
palladium(II) and platinum(II) it has been noticed that the
NHC ligand normally avoids the trans position to the PPh3
Scheme 1. Synthesis of NHC Complexes A-C
Received: December 2, 2010
Published: January 27, 2011
r
2011 American Chemical Society
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|

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Scheme 2. Synthesis of NHC Complexes by Oxidative
Addition
Figure 1. Molecular structure of trans-[3]þ in trans-[3]BF4 CH2Cl2
3
(hydrogen atoms except the one bound to N2, the anion and the
solvent molecule have been omitted for clarity). Selected bond dis-
tances (Å) and angles (deg): Pd-Cl1 2.3374(7), Pd-P1 2.3173(8),
Pd-P2 2.3264(8), Pd-C1 1.979(3), N1-C1 1.346(4), N2-C1
1.342(4); Cl1-Pd-P1 87.79(3), Cl1-Pd-P2 90.39(3), Cl1-Pd-C1
178.88(8), P1-Pd-P2 176.79(3), P1-Pd-C1 91.22(8), P2-Pd-C1
90.59(8).
solution of trans-[3]BF4 at ambient temperature. An X-ray
diffaction analysis confirmed the composition and coordina-
tion geometry of trans-[3]BF4 (Figure 1, see also SI). The bond
lengths for the Pd-Ccarbene bond (1.979(3) Å) and the Pd-P
bonds (Pd-P1 2.3173(8) Å, Pd-P2 2.3264(8) Å) fall into the
range previously reported for similar palladium NHC complexes.8e
The plane of the carbene ligand is oriented perpendicular to the
nearly square-planar palladium coordination sphere (Figure 1).
The platinum complexes trans-[4]BF4 and cis-[4]BF4 were
prepared as described for the palladium complex trans-[3]BF4
from [Pt(PPh3)4] as starting material (Scheme 1). Surprisingly
and in contrast to the palladium complex, both the cis (95%) and
trans (5%) platinum complexes were detected in the product
mixture. This coexistence of both complexes was confirmed by
13C{1H} and 31P{1H} NMR spectroscopy. The 13C NMR spec-
trum of the product mixture shows two signals for the carbene
carbon atom. The resonance at δ = 170.7 ppm appears as a
doublet of doublets (dd, 2JCP(cis)=10.3 Hz, 2JCP(trans)=142.4 Hz)
and can therefore be assigned to cis-[4]BF4, while the carbene
carbon resonance for complex trans-[4]BF4 was observed as a
broad singlet at δ = 166.9 ppm. The 31P{1H} NMR spectrum
shows a singlet at δ=18.2 ppm (Pt satellites, 1JPtP=2524 Hz) for
the two chemically equivalent phosphorus atoms of complex
trans-[4]BF4 and two doublets at δ=15.7 ppm (d, 2JPP= 19.2 Hz,
Pt satellites 1JPtP =2223 Hz, PPh3 trans to NHC) and δ = 11.1
ppm (d, 2JPP = 19.2 Hz, Pt satellites 1JPtP =3702 Hz, PPh3 cis to
NHC). Similarly to trans-[3]BF4, a downfield resonance at δ =
12.98 ppm was observed for the N-H proton in the 1H NMR
spectrum (in CDCl3/DMSO-d6).
ligand and that the cis-[M(NHC)(PPh3)L2] complexs are the
thermodynamically most stable reaction products.17 In accord
with these results, the oxidative addition of 1 to Pd0 leads ulti-
mately to complex trans-[3]BF4 which was isolated in 73% yield
as colorless solid. Complex trans-[3]BF4 is unstable in solution
but is quite stable toward air and moisture in the solid state.
The important intermediate [2] could not be isolated. The
conceivable oxidative addition of the benzimidazolium derivative
of 1, obtained by N-protonation of 1 with the excess of NH4BF4
present, appears unlikely as NH4BF4 is not acidic enough to pro-
tonate free N-alkylbenzimidazoles. A derivative of 1, 2-chloro-
N-picolylbenzimidazole, also reacts with [Pt(PPh3)4] in the
absence of NH4BF4 under oxidative addition. The resulting
complex [5] is a dinuclear species obtained via coordination of
the free ring imine nitrogen atom to a second platinum(II) center
(see SI). This reaction confirms the ability of 2-chloro-N-
alkylbenzimidazoles to react under oxidative addition in the
absence of proton acids and corroborates the reaction mechan-
ism proposed in Scheme 2.
1
Formation of complex trans-[3]BF4 was confirmed by H,
1
13C{1H}, and 31P{1H} NMR spectroscopy. The H spectrum
showed a dependence of the chemical shift for the N-H proton
from the solvent used. In a mixture of CDCl3/DMSO-d6 the N-
H resonance was detected at δ=12.84 ppm, whereas it was obs-
erved upfield-shifted at δ=10.90 ppm in CD3CN. The downfield
shift of the N-H resonance in DMSO-d6 is due to the capability
While separation of complexes cis-[4]BF4 and trans-[4]BF4 by
column chromatography was not possible, the two isomeric
complexes led to differently shaped crystals upon diffusion of
diethyl ether into a dichloromethane solution of the mixture.
of this solvent to engage in O H-N hydrogen bonds, and
Crystals of cis-[4]BF4 and trans-[4]BF4 0.5CH2Cl2 were ob-
3 3 3
3
related hydrogen bonds involving the N-H proton of NH,NR-
functionalized NHC ligands have been described.11,12g,h,13 The
13C{1H} spectrum shows the resonance for the C2 carbene
carbon atom at δ=169.7 ppm as a triplet exhibiting coupling to
the two phosphorus atoms coordinated in cis positions. The
31P{1H} NMR spectrum shows a singlet for the two chemically
equivalent phosphorus atoms at δ = 21.4 ppm confirming the
trans arrangement of these two donors.
tained as prisms or needles, respectively. X-ray diffraction stru-
cture analyses confirmed the composition and coordination geom-
etry of the two isomeric complex cations (Figure 2).
The arrangement of the four ligands around the metal center is
nearly square-planar in both cis-[4]þ and trans-[4]þ, and the car-
bene plane in both cations is oriented almost perpendicular to the
metal coordination sphere (Figure 2). There are noticeable diff-
erences in the bond lengths of the two isomeric cations. The Pt-
CNHC bond length is significantly shorter in trans-[4]þ (Pt-C1
1.972(4) Å) than in cis-[4]þ (Pt-C1 2.017(2) Å) with the
Crystals of trans-[3]BF4 CH2Cl2 have been obtained by slow
3
diffusion of diethyl ether into a saturated dichloromethane
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COMMUNICATION
studies focus on subsequent modifications at the NH,NR-func-
tionalized NHC ligand and on the use of the N-H function of
the NHC ligand for the recognition of selected substrates via
intermolecular hydrogen bonds.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details for the
b
synthesis of all compounds; X-ray crystallographic files for comp-
ounds trans-[3]BF4 CH2Cl2, cis-[4]BF4, trans-[4]BF4 0.5CH2-
Cl2, and [5] H2O 0.5Et2O. This material is available free of
3
3
3
3
Figure 2. Molecular structures of cis-[4]þ in cis-[4]BF4 (left) and
charge via the Internet at http://pubs.acs.org.
trans-[4]þ in trans-[4]BF4 0.5CH2Cl2 (right) (hydrogen atoms, ex-
3
cluding the ones bound to N2, the anion, and solvent molecules, have
been omitted for clarity). Selected bond distances (Å) and angles (deg)
for cis-[4]þ [trans-[4]þ]: Pt-Cl1 2.3434(5) [2.3483(11)], Pt-P1 2.34-
57(5) [2.3171(12)], Pt-P2 2.2452(6) [2.3123(12)], Pt-C1 2.017(2)
[1.972(4)], N1-C1 1.347(3) [1.350(6)], N2-C1 1.352(3) [1.349-
(6)]; Cl1-Pt-P1 87.10(2) [89.59(4)], Cl1-Pt-P2 175.77(2)
[87.18(4)], Cl1-Pt-C1 87.02(6) [178.78(14)], P1-Pt-P2 96.97-
(2) [175.77(4)], P1-Pt-C1 172.77(6) [91.00(13)], P2-Pt-C1
88.99(6) [92.19(13)].
’ AUTHOR INFORMATION
Corresponding Author
fehahn@uni-muenster.de
’ ACKNOWLEDGMENT
We thank the Deutsche Forschungsgemeinschaft (SFB 858,
IRTG 1444) for financial support.
Pt-CNHC separation for trans-[4]þ falling in the range observed
previously for related complexes of type trans-[PtCl(NH,NR-
NHC)(PR3)2.15b These differences can be attributed to the
differences in the trans-influence of the π-donor chloro ligand in
comparison to the σ-donor/π-acceptor phosphine ligand.
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