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Synthesis and Characterization of Gold(I) and Gold(III)
Complexes Derived from Benzimidazolin-2-ylidene
Ligands
Ekkehardt Hahn, Mareike Jahnke, Tania Pape
To cite this version:
Ekkehardt Hahn, Mareike Jahnke, Tania Pape. Synthesis and Characterization of Gold(I) and
Gold(III) Complexes Derived from Benzimidazolin-2-ylidene Ligands. Journal of Inorganic and Gen-
eral Chemistry / Zeitschrift für anorganische und allgemeine Chemie, Wiley-VCH Verlag, 2010, 636
(13-14), pp.2309. �10.1002/zaac.201000234�. �hal-00599880�
Synthesis and Characterization of Gold(I) and Gold(III)
Complexes Derived from Benzimidazolin-2-ylidene Ligands
Journal:
Zeitschrift für Anorganische und Allgemeine Chemie
Manuscript ID:
zaac.201000234.R1
Wiley - Manuscript type:
Article
Date Submitted by the
Author:
01-Jul-2010
Complete List of Authors:
Hahn, Ekkehardt; Westfaelische Wilhelms-Universitaet Muenster,
Institut fuer Anorganische Chemie
Jahnke, Mareike; Westfaelische Wilhelms-Universitaet Muenster,
Institut fuer Anorganische Chemie
Pape, Tania; Westfaelische Wilhelms-Universitaet Muenster, Institut
fuer Anorganische Chemie
Keywords:
N-Heterocyclic Carbene, Gold, Oxidation, Molecular Structure
Wiley-VCH
ZAAC
1
ARTICLE
DOI: 10.1002/zaac.200((will be filled in by the editorial staff))
Synthesis and Characterization of Gold(I) and Gold(III) Complexes Derived from
Benzimidazolin-2-ylidene Ligands
Mareike C. Jahnke,
[a]
Tania Pape,
[a]
and F. Ekkehardt Hahn*
,[a]
In memory of Prof. Dr. Herbert Schumann
Keywords: N-Heterocyclic Carbene; Gold; Oxidation, Molecular Structures
Substitution of the chloro ligand in (N,N´-dialkylbenzimidazolin-2-
ylidene)gold(I) chlorides
(alkyl = methyl, ethyl, propyl, butyl)
succeeded by treatment of the gold chloride complexes with
lithium
bromide in acetone leading to the gold complexes of the
type
[AuBr(L)] 1–4. Furthermore, the Au–C
carbene
bond
in complexes
1−4 is inert towards a change of the oxidation state of the metal
center. Complexes 1−4 have been oxidized chemically with
elemental bromine leading to the gold(III) carbene complexes of
the type [AuBr
3
(L)] 5−8. The molecular structures of the gold(I)
complexes 1, 2 and 4 exhibiting a linear topology are presented
together with the two molecular structures of the square-planar
gold(III) complexes 6 and 8.
Introduction
Today, N-heterocyclic carbenes (NHCs) are well
established ligands in organometallic chemistry [1]. This
development rests on the easy access to differently
substituted NHCs making them a good alternative to
phosphines in the design of new organometallic catalysts [1,
2]. The facile access to NHCs is complemented by the
superior σ-donor properties of the carbene ligands compared
to the most basic phosphines [3] leading to a remarkable
stability of the carbene complexes against air, moisture, heat
and oxidizing conditions [1]. Recently, poly-NHC ligands
have also been used as building blocks for
metallosupramolecular structures. Linear di- [4] as well as
tri- or tetra-NHC ligands [5] have been employed for the
generation of molecular rectangles or cylindrical molecules
which provide cavities suitable as host for guest molecules
during a catalytic reaction [6].
A large portion of all known NHC complexes contain a
group 9 or 10 transition metal [2a, 7] or ruthenium [8] as the
metal center. In addition, many silver carbene complexes [9]
have been prepared after Wang and Lin [10] reported in
1998 the very useful carbene transfer reaction utilizing
silver NHC complexes. Today, this carbene transfer reaction
has developed into a versatile and well established
procedure for the synthesis of transition metal NHC
complexes [9].
Gold NHC complexes have only recently attracted interest,
although the preparation of such complexes has been
reported as early as 1998 [10]. This development supports
the recent resurgence of interest in gold catalyzed reactions
[11] which in turn has initiated a search for new gold NHC
complexes [12]. Today gold NHC complexes are used as
pharmaceuticals [13], in photophysical devices [14], and as
catalysts for selected transformations in homogeneous
catalysis [15].
The inertness of the Au−C
NHC
bond during substitution
reactions at linear [AuX(NHC)] complexes was noted by
Wang and Lin as early as 1999 [16]. Ligand X in complexes
of the type [AuX(NHC)] can be substituted for halides [16,
17], nitrogen- [18, 19] and phosphorous-donors [4a, 19] or
even hydroxide [20]. It has also been shown, that the
Au−C
carbene
bond is not affected if gold(I) NHC complexes
bearing unsaturated imidazolin-2-ylidene or saturated
imidazolidin-2-ylidene ligands are oxidized with bromine
[21] or chlorine [22] to give gold(III) complexes of the type
[AuX
3
(L)] (L = imidazolin-2-ylidene or imidazolidin-2-
ylidene; X = Br, Cl).
Most known gold NHC complexes bear the ubiquitous
unsaturated imidazolin-2-ylidenes as carbene ligands, while
there are only a few examples for gold complexes bearing
benzimidazolin-2-ylidenes [10, 16, 20a-b, 23, 24]. We have
been interested in the synthesis and properties of transition
metal complexes with benzimidazolin-2-ylidenes for several
years [25]. Recently we reported the synthesis and some
characteristic properties of gold(I) complexes of the type
[AuCl(L)] bearing N,N’-dipropyl- or N,N’-dibutyl-
substituted benzimidazolin-2-ylidene ligands [26].
Here we describe the synthesis of gold(I) complexes with
N,N’-dialkylated benzimidazolin-2-ylidene ligands and their
oxidation with elemental bromine to the gold(III) complexes.
The molecular structures of linear gold(I) NHC complexes
will be discussed together with the molecular structures of
square-planar gold(III) complexes.
____________
* Prof. Dr. F. Ekkehardt Hahn
Fax: 0049-251-833-3110
E-Mail: fehahn@uni-muenster.de
[a] Institut für Anorganische und Analytische Chemie
Westfälische Wilhelms-Universität Münster
Corrensstraße 28/30
48149 Münster, Germany
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Results and Discussion
Gold(I) complexes of the type [AuCl(NHC)] (Scheme 1)
were prepared as described [16, 26]. The oxidation of these
gold(I) chloride complexes with bromine would lead to a
mixture of complexes with bromo and chloro ligands. We
therefore performed a halide exchange by treatment of the
gold(I) chloride complexes with a tenfold excess of lithium
bromide in acetone (Scheme 1) following the procedure
described by Nolan et al.
[21]. The gold(I) bromide
complexes 1–4 were obtained in good yields (86–98%) after
purification. Although the synthesis of complexes 1 and 2
has already been reported [16], we have listed them here,
since we are providing additional analytical data.
Scheme 1. Synthesis of the gold(I) bromide complexes 1–4 by
substitution of the chloro ligand.
The
13
C NMR spectra of gold complexes 1–4 exhibit the
resonance for the carbene carbon atom in the narrow range
of
δ
180.5–181.2. Compared to the resonances of the
carbene carbon atoms of the parent gold(I) chloride
complexes (δ 178–179) [26] a slight downfield shift (∆δ 2–
3) was observed upon the halide exchange, while all other
resonances in the
1
H and
13
C NMR spectra remain nearly
unchanged. Crystals of 1, 2 and 4, which were suitable for
X-ray diffraction studies, have been obtained by slow
diffusion of diethyl ether into dichloromethane solutions of
the individual complexes. The molecular structures of
complexes 1, 2 and 4 are depicted in Figures 1 and 2 and
selected bond parameters are listed in Table 1.
Figure 1. Molecular structure of the gold(I) complexes 1 (top) and
drawing of the dinuclear species formed by Au···Au interactions
(bottom). Ellipsoids are drawn at the 50 % probability level.
Hydrogen atoms are omitted for clarity.
Complexes 1 and 4 reside on general positions in the unit
cell, while complex 2 resides on a twofold axis passing
through atoms Br, Au and C1. As expected, complexes 1, 2,
and 4 exhibit a linear coordination geometry. Two
molecules of 1 form a dinuclear species via intermolecular
Au···Au interactions [27] (Au···Au 3.1179(3) Å, Figure 1,
bottom). A similar aurophilic interaction (Au···Au 3.1216(4)
Å) was found for 4 but not for 2.
Figure 2. Molecular structures of the gold(I) complexes 2 (top) and
4 (bottom). Atoms Au, Br, and C1 of complex 2 lie on a
crystallographic twofold axis. Ellipsoids are drawn at the 50 %
probability level. Hydrogen atoms are omitted for clarity.
The Au–C bond lengths in 1 (1.982(3) Å), 2 (1.989(5) Å)
and 4 (1.994(3) Å) are nearly identical to the Au−C
separations in the parent monocarbene gold(I) chloride
complexes of the type [AuCl(L)] with L = benzimidazolin-
2-ylidene (Au−C 1.969(6)–2.01(3) Å) [16, 23, 26]. These
bond distances are also very close in magnitude to the
values observed for Au–C bonds found in complexes of the
type [AuBr(imidazolin-2-ylidene)] (1.975 Å) [21]. The Au–
Br bond lengths in 1, 2, and 4 also fall in the range
previously observed for complexes of the type
[AuBr(NHC)] [21]. As expected and previously observed
for the parent [AuCl(NHC)] complexes [16, 23, 26],
complexes 1, 2, and 4 exhibit an almost linear C–Au–Br
bond. The N–C
carbene
–N bond angles in the gold(I)
complexes are slightly enlarged in comparison to the free
benzimidazolin-2-ylidenes (N–C
carbene
–N ≈ 104°) [28].
Table 1. Selected bond lengths (Å) and angles (deg) in complexes
1, 2, and 4.
Compounds 1 2 4
Bond lengths
Au–C1 1.982(3) 1.989(5) 1.994(3)
Au–Br 2.4083(4) 2.3910(7) 2.4065(4)
N1–C1 1.352(4) 1.345(4) 1.343(4)
N2–C1 1.354(4) – 1.360(4)
Au···Au* 3.1179(3) – 3.1216(4)
Bond angles
C1–Au–Br 177.75(10) 180.0 179.30(9)
N1–C1–N2/
N1–C1–N1*
106.3(3) 107.5(4) 106.8(3)
N1
N2
C1
Au
Br
Au
Au*
Br
Br*
C1
N1
N2
N1
N1*
C1
Au
Br
N1
N2
C1
Au
Br
N
N
R
R
Au Cl
LiBr
acetone
N
N
R
R
Au Br
1 - 4
1: R = Me
2: R = Et
3: R = Pr
4: R = Bu
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Reaction of the gold complexes 1–4 with elemental
bromine in dichloromethane gave the orange gold(III)
complexes of the type [AuBr
3
(NHC)] 5–8 with NHC =
N,N´-dialkylated benzimidazolin-2-ylidene ligands in good
yields (82–89 %, Scheme 2). The
13
C NMR spectra of the
gold(III) complexes 5−8 show the resonances of the carbene
carbon atoms in the range of δ 146.6–149.7. These values
represent an upfield shift (∆δ 31–35) of the C
carbene
resonance upon oxidation of gold(I) to gold(III). This strong
upfield shift is surprising when compared to the downfield
shift of ∆δ 1−2 observed for the carbon atoms of the
aromatic ring upon the gold(I)→gold(III) oxidation. The
resonances observed for C
carbene
in 5−8, however, resemble
the values found for the N−CH−N resonances in the parent
benzimidazolium salts (δ 141–144) [25a−c, 25e]. The
upfield shift of the C
carbene
resonance upon the
gold(I)→gold(III) oxidation has been observed for related
[AuX
3
(NHC)] complexes bearing imidazolin-2-ylidene or
imidazolidin-2-ylidene ligands [21, 22, 29] and some
authors have tried to explain this observation with the higher
Lewis acidity of gold(III) in comparison to gold(I) [21].
Scheme 2. Oxidation of the gold(I) complexes 1–4 with bromine to
give the gold(III) complexes 5–8.
Crystals of 6 and 8 suitable for X-ray diffraction studies
have been obtained by slow diffusion of diethyl ether into
dmf (6) or dichloromethane (8) solutions, respectively, of
the complexes. Complex 8 crystallized as 8·0.5CH
2
Cl
2
with
two formula units in the asymmetric unit. The two
molecules of 8 in the asymmetric unit exhibit essentially
identical metric parameters. Only one of these molecules is
depicted in Figure 2 together with the molecular structure of
6. Selected bond parameters are summarized in Table 2.
The gold(III) complexes 6 and 8 feature the expected
square-planar coordination geometry. The Au–C bonds in 6
(2.003(4) Å) and in the two molecules in 8·0.5CH
2
Cl
2
(2.009(4) Å) are similar in lengths to the equivalent bond
distances observed in gold(III) complexes of the type
[AuBr
3
(NHC)] (NHC = imidazolin-2-ylidene or
imidazolidin-2-ylidene) (Au−C 2.002(6)–2.052(6) Å) [21,
29]. The Au−C distances are very similar in the gold(I) (1, 2,
4) and the gold (III) complexes 6 and 8. The oxidation state
of the gold atom apparently bears no significance for the
Au–C bond lengths. Due to the trans effect of the carbene
donor the Au–Br
trans
bonds in 6 (2.4465(4) Å) and
8·0.5CH
2
Cl
2
(2.4521(5) Å and 2.4519(5) Å) are longer than
the Au–Br
cis
bonds (2.4107(5)–2.4276(4) Å). These
parameters match the equivalent values observed for related
gold(III) monocarbene complexes [21, 29]. The Au–Br
trans
bond distances in 6 and 8·0.5CH
2
Cl
2
are significantly longer
than the Au−Br distances in 1 (2.4083(4) Å), 2 (2.3910(7)
Å), and 4 (2.4065(4) Å) (Table 1).
Figure 3. Molecular structures of the gold(III) complexes 6 (top)
and 8 in 8·CH
2
Cl
2
(bottom). Ellipsoids are drawn at the 50 %
probability level and hydrogen atoms have been omitted for clarity.
Table 2. Selected bond lengths (Å) and angles (deg) for 6 and
8·0.5CH
2
Cl
2
.
Compounds 6 8·0.5CH
2
Cl
2
molecule A molecule B
Bond lengths
Au1–C1/
Au2–C21
2.003(4) 2.009(4) 2.009(4)
Au1–Br3/
Au2–Br23
2.4107(5) 2.4256(5) 2.4159(5)
Au1–Br2/
Au2–Br22
2.4276(4) 2.4109(5) 2.4224(5)
Au1–Br1/
Au2–Br21
2.4465(4) 2.4521(5) 2.4519(5)
N1–C1/
N21–C21
1.348(4) 1.344(5) 1.339(6)
N2–C1/
N22–C21
1.346(4) 1.345(5) 1.343(6)
Bond angles
C1–Au1–Br1/
C21–Au2–Br21
179.01(10) 178.62(12) 179.82(14)
Br3–Au1–Br2/
Br23–Au2–Br22
176.43(2) 176.34(2) 175.25(2)
C1–Au1–Br3/
C21–Au2–Br23
87.67(10) 88.71(12) 87.51(12)
C1–Au1–Br2/
C21–Au2–Br22
88.82(10) 87.79(12) 88.01(12)
N1–C1–N2/
N21–C21–N22
108.8(3) 108.7(4) 109.1(4)
N
N
R
R
Au Br
Br
Br
CH
2
Cl
2
, -78 °C
Br
2
N
N
R
R
Au Br
1 - 4
5 - 8
1, 5: R = Me
2, 6: R = Et
3, 7: R = Pr
4, 8: R = Bu
AuBr1
Br2
Br3
N1
N2
C1
Au
Br1
Br2
Br3
C1
N1
N2
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