Heterobimetallic carbene complexes by a single-step site-selective metalation of a tricarbene ligand

Article abstract of DOI:10.1021/ja401546h

The unsymmetrical tris(imidazolium) salt H₃-1(Br)₃, featuring a 1,2,4-substitution pattern of the central phenyl ring, after triple imidazolium C2 deprotonation reacts in a one-pot reaction with Pd(OAc) ₂ and [M(Cp*)(Cl)₂]₂ (M = Rh ^III, Ir^III) to yield heterobimetallic complexes [3] (M = Rh) and [4] (M = Ir), in which the Pd^II ion is chelated by two ortho N-heterocyclic carbene (NHC) donors while the third NHC donor coordinates to the M^III center, which orthometalates the central phenyl ring.

Full text of DOI:10.1021/ja401546h

Communication
pubs.acs.org/JACS
Heterobimetallic Carbene Complexes by a Single-Step Site-Selective
Metalation of a Tricarbene Ligand
Ramananda Maity, Hannah Koppetz, Alexander Hepp, and F. Ekkehardt Hahn*
Institut fur Anorganische und Analytische Chemie and NRW Graduate School of Chemistry, Westfalische Wilhelms-Universitat
̈
̈
̈
Munster, Corrensstrasse 30, D-48149 Munster, Germany
̈
̈
S
* Supporting Information
donors in a chelating fashion and a second metal center
coordinated by only one NHC donor. The metalation pattern
of this tris(NHC) ligand depended on the sequence of addition
of the metal sources, underlining the stepwise nature of the
metalation process.12
We became interested in the heterobimetallic metalation of
polycarbenes in a one-pot procedure. We envisaged that an
unsymmetrical tricarbene ligand could be metalated simulta-
neously with two different metals by utilizing the differences in
their coordination chemistries. The 1,2,4-tris(imidazolium)-
substituted NHC precursor H3-1(Br)3 reacts simultaneously
with Pd(OAc)2 and [M(Cp*)(Cl)2]2 (M = Rh, Ir) to give
complexes [3] (M = Rh) and [4] (M = Ir) in a one-pot
synthesis (Scheme 1).
ABSTRACT: The unsymmetrical tris(imidazolium) salt
H3-1(Br)3, featuring a 1,2,4-substitution pattern of the
central phenyl ring, after triple imidazolium C2 deproto-
nation reacts in a one-pot reaction with Pd(OAc)2 and
[M(Cp*)(Cl)2]2 (M = RhIII, IrIII) to yield heterobimetallic
complexes [3] (M = Rh) and [4] (M = Ir), in which the
PdII ion is chelated by two ortho N-heterocyclic carbene
(NHC) donors while the third NHC donor coordinates to
the MIII center, which orthometalates the central phenyl
ring.
-Heterocyclic carbenes (NHCs) have emerged as a very
Nuseful class of ligands in organometallic chemistry over
the last few decades.1 NHCs have been used extensively as
spectator ligands in catalytically active metal complexes,2 in
organocatalysis,3 and in biologically active compounds.4
Applications for NHCs may be further enhanced by
incorporation of the NHC donor groups into poly-NHC
ligands.5 Very recently, poly-NHC ligands have also been used
for the generation of two- or three-dimensional metal-
losupramolecular assemblies.6
Scheme 1. Synthesis of the Monometalated Complex [2]Br
and the Heterobimetallic Complexes [3] and [4]
Since the emergence of the first poly-NHC ligands, the
preparation of polymetallic and heterobimetallic carbene
complexes with applications in cooperative or tandem catalysis
has become a focus of interest. The first attempts were reported
in 2000, when a 1,2,4-triazolidin-3,5-diylidene was doubly
metalated with AgI, leading to a one-dimensional polymeric
chain.7 Later, Arduengo et al.8 described a cyclopentadienyl-
fused imidazolium cation that was employed to prepare a
ruthenocene derivative by reaction with [Ru(Cp*)(MeCN)3]-
(CF3SO3). Subsequent oxidative addition of Pd0 to the
imidazolium C2−C1 bond gave the heterobimetallic complex
featuring an η5,η5-coordinated RuII and an NHC-coordinated
PdII. Doubly metalated benzobiscarbenes bearing two identical
metals have been described9 in addition to several dinuclear
complexes derived from flexible bis(NHC) ligands,10a some of
which could be metalated stepwise with two different
metals.10b,c Probably the most promising approaches to
heterobimetallic NHC complexes were described by Peris and
co-workers, who attached two different complex fragments to a
1,2,4-triazolyldiylidene in a stepwise manner.11 In addition, they
prepared a Y-shaped tris(NHC) ligand derived from a 4,5-
bis(NHC)-substituted imidazolin-2-ylidene. This ligand could
be metalated in a stepwise fashion, leading to heterobimetallic
complexes featuring one metal coordinated by two NHC
The design of the ligand precursor H3-1(Br)3 was based on
previous observations. It is known that 1,2-bis(NHC)-
substituted benzenes react with PdII or PtII to form cis-
dicarbene chelate complexes of type A (Figure 1).6h
Alternatively, 1,3,5-tris(NHC)-substituted benzenes react with
[M(Cp*)(Cl)2]2 (M = Rh, Ir) to yield complexes in which the
metal center bears only one NHC ligand and in addition
orthometalates the central phenyl group. Even trinuclear
Received: February 12, 2013
Published: March 22, 2013
© 2013 American Chemical Society
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Communication
match well with previously reported chemical shifts for related
15
13,14,16
PdII and IrIII
complexes.
The orthometalation of the central aryl ring of the ligand in
both [3] and [4] was confirmed by 2D correlation NMR
spectroscopy. For the rhodium complexes syn/anti-[3], the
resonances for the orthometalated carbon atoms were observed
at 160.6 ppm (1JRh,C = 36.9 Hz) for the syn isomer and 162.4
ppm (1JRh,C = 35.4 Hz) for the anti isomer. Two resonances at
higher field were found for syn/anti-[4] at 144.1 and 146.5
Figure 1. Coordination modes of 1,2- and 1,3,5-substituted poly-NHC
ligands.
1
ppm, respectively. For syn/anti-[3] the JRh,C coupling clearly
indicates the metalation of the central aryl ring of the ligand.
The formation of complexes [3] and [4] was also confirmed by
high-resolution electrospray ionization (ESI) mass spectrome-
try (positive ion mode), which showed peaks at m/z 903.9876
for [[3] − Br + CH3CN]+ (calcd for [[3] − Br + CH3CN]+,
903.9874) and m/z 994.0436 for [[4] − Br + CH3CN]+ (calcd
for [[4] − Br + CH3CN]+, 994.0437).
The connectivity and overall geometry of complex syn-[4]
was established by X-ray crystallography. Suitable crystals of
syn-[4]·CHCl3·3CH3CN were obtained by slow evaporation of
the solvents from a concentrated acetonitrile/chloroform
solution of the complex at ambient temperature. The structure
analysis confirmed the formation of the heterobimetallic
complex and showed that two o-NHC donors are bonded to
the PdII center in a chelating fashion while the IrIII center is
coordinated by only one NHC donor and orthometalates the
central phenyl ring of the ligand (Figure 2).
complexes such as B featuring a tris-orthometalated central
phenylene group (Figure 1) have been prepared.13 Combining
the 1,2- and 1,4-substitution patterns in one ligand led to the
unsymmetrical 1,2,4-tris(imidazolium) NHC precursor H3-
1(Br)3 which was designed to react simultaneously with
Pd(OAc)2 and [M(Cp*)(Cl)2]2 in a one-pot reaction with
regioselective metalation of the two different coordination sites.
The tris(imidazolium) salt H3-1(Br)3 (Scheme 1) was
prepared by heating a mixture of 1,2,4-tribromobenzene and
imidazole in the presence of the base K2CO3 and CuSO4
followed by alkylation of the free imine functions with ethyl
1
bromide [see the Supporting Information (SI)]. The H NMR
spectrum of H3-1(Br)3 shows three distinctly different
resonances for the chemically nonequivalent imidazolium
C2−H protons at 10.41, 10.07, and 9.91 ppm, with the
resonance for the imidazolium moiety located at the 4-position
of the aromatic ring at the lowest field.
The reaction of H3-1(Br)3 with 1 equiv of Pd(OAc)2 and 0.5
equiv of [M(Cp*)(Cl)2]2 (M = Rh, Ir) in a one-pot reaction in
the presence of sodium acetate selectively gave the hetero-
bimetallic complexes [3] and [4], respectively, in yields of 40−
53% (Scheme 1; also see the SI). No regioisomeric complexes
were detected. The Pd(II) ion is selectively coordinated by the
chelating 1,2-bis(NHC) moiety, whereas the MIII center is
coordinated by a single NHC donor and orthometalates the
phenyl ring at the sterically least encumbered ortho position
next to this NHC donor.
Two geometric isomers of both complexes [3] and [4] were
identified by NMR spectroscopy. Their formation is caused by
the two possible orientations of the halogenato ligands relative
to each other (syn or anti). The isomers were separated
(syn:anti ratio ≈ 33:67) by column chromatography using a 4:1
(v/v) dichloromethane/acetone solvent mixture (see the SI).
The formation of heterobimetallic complexes [3] and [4]
Figure 2. Molecular structure of syn-[4] in syn-[4]·CHCl3·3CH3CN
(H atoms and solvent molecules have been omitted for clarity, 50%
probability ellipsoids). Selected bond distances (Å) and angles (deg):
Ir−Br1, 2.5301(3); Ir−C1, 1.999(3); Ir−C31, 2.049(3); Pd−Br2,
2.4756(4); Pd−Br3, 2.4836(4); Pd−C9, 1.961(3); Pd−C15, 1.963(3);
N1−C1, 1.349(4); N2−C1, 1.361(4); N3−C9, 1.359(4); N4−C9,
1.346(4); N5−C15, 1.348(4); N6−C15, 1.340(4); Br1−Ir−C1,
86.87(8); Br1−Ir−C31, 89.44(8); C1−Ir−C31, 77.22(11); Br2−
Pd−Br3, 93.615(13); Br2−Pd−C9, 91.31(9); Br2−Pd−C15,
174.07(8); Br3−Pd−C9, 174.69(9); Br3−Pd−C15, 90.83(8); C9−
Pd−C15, 84.12(12); N1−C1−N2, 104.3(2); N3−C9−N4, 105.6(2);
N5−C15−N6, 105.8(2).
1
was confirmed by H, 13C{1H}, and 2D correlation NMR
1
spectroscopy and mass spectrometry. The H NMR spectra of
syn/anti-[3] featured no imidazolium C2−H resonances. The
13C{1H} NMR spectra of syn/anti-[3] also indicated the
absence of any imidazolium groups, which were detected in H3-
1(Br)3 at 138.2, 138.1, and 135.9 ppm. The imidazolium
resonances were replaced with the characteristic carbene
1
resonances (syn isomer: 180.6 ppm, JRh,C = 54.8 Hz for Rh−
The coordination geometry at the PdII center is best
described as slightly distorted square-planar, while that at IrIII
resembles a three-legged piano stool with the NHC, Caryl, and
bromo donors functioning as the legs. In syn-[4], the bromine
atoms bound to the Pd atom point in the same direction as the
bromine atom bonded to the Ir center relative to the central
aryl ring.
The Pd−CNHC bond lengths are equidistant within
experimental error [Pd−C9, 1.961(3) Å; Pd−C15, 1.963(3)
Å] and slightly shorter than the Ir−CNHC and Ir−Caryl bond
CNHC and 159.9 and 159.8 ppm for Pd−CNHC; anti isomer:
1
183.4 ppm, JRh,C = 55.1 Hz for Rh−CNHC and 162.2 ppm for
Pd−CNHC). These values fall in the typical range reported for
13,14
15
RhIII−CNHC
and PdII−CNHC complexes. The 13C{1H}
NMR spectra of syn/anti-[4] also showed the characteristic
resonances for the carbene carbon atoms for both isomeric
complexes (syn isomer: 164.1 ppm for Ir−CNHC and 159.8 and
159.6 ppm for Pd−CNHC; anti isomer: 166.3 ppm for Ir−CNHC
and 162.1 and 162.0 ppm for Pd−CNHC). Again, these values
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Communication
lengths [Ir−C1, 1.999(3) Å; Ir−C31, 2.049(3) Å]. Interest-
ingly, the Ir−CNHC bond distance is shorter than the Ir−Caryl
separation. Similar observations have been made for an IrIII
complex bearing a benzyl-substituted NHC ligand that was also
orthometalated, leading to a six-membered chelate ring.16c
However, the Ir−CNHC and Ir−Caryl distances in that complex
[Ir−CNHC, 2.014(7) Å; Ir−Caryl, 2.068(8) Å] are distinctly
longer than the equivalent distances in syn-[4]. The six-
membered chelate ring also leads to a larger CNHC−Ir−Caryl bite
angle of 85.7(3)° compared with the equivalent bite angle of
77.22(11)° found in syn-[4]. The Ir····Pd separation in syn-[4]
measures 7.240 Å.
shorter separation of the metal centers, and corresponding
investigations are underway.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details for the synthesis of all compounds and X-
ray crystallographic data (CIF) for [4]·CHCl3·3CH3CN. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
fehahn@uni-muenster.de
■
Complexes [3] and [4] can also be synthesized following a
stepwise synthetic procedure. In this case, the monometallic
intermediate [2]Br was prepared and isolated (Scheme 1). The
reaction of tris(imidazolium) salt H3-1(Br)3 with 1 equiv of
Pd(OAc)2 in N,N-dimethylformamide (DMF) for 5 h at 95 °C
resulted in the formation of the bis(NHC) Pd(II) chelate
complex [2]Br in a good yield of 87%. We observed exclusive
PdII coordination to the two NHC donors at the 1- and 2-
position of the central aryl ring, while the remaining
imidazolium moiety did not react (Scheme 1).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors thank the Deutsche Forschungsgemeinschaft (SFB
858) for financial support. R.M. thanks the NRW Graduate
■
School of Chemistry (GSC-MS), Munster, for a predoctoral
̈
grant.
1
The formation of [2]Br was confirmed by H and 13C{1H}
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