Syntheses, crystal structures, reactivity, and photochemistry of gold(iii) bromides bearing N-heterocyclic carbenes

Article abstract of DOI:10.1039/c1dt11175b

Gold(i) complexes bearing N-heterocyclic carbenes (NHC) of the type (NHC)AuBr (3a/3b) [NHC = 1-methyl-3-benzylimidazol-2-ylidene (= MeBnIm), and 1,3-dibenzylimidazol-2-ylidene (= Bn₂Im)] are prepared by transmetallation reactions of (tht)AuBr (tht = tetrahydrothiophene) and (NHC)AgBr (2a/2b). The homoleptic, ionic complexes [(NHC)₂Au]Br (6a/6b) are synthesized by the reaction with free carbene. Successive oxidation of 3a/3b and 6a/6b with bromine gave the respective (NHC)AuBr₃ (4a/4b) and [(NHC)₂AuBr₂]Br (7a/7b) in good overall yields as yellow powders. All complexes were characterized by NMR spectroscopy, mass spectrometry, elemental analysis and single crystal X-ray diffraction. Reactions of the Au(iii) complexes towards anionic ligands like carboxylates, phenolates and thiophenolates were investigated and result in a complete or partial reduction to a Au(i) complex. Irradiation of the Au(iii) complexes with UV light yield the Au(i) congeners in a clean photo-reaction.

Full text of DOI:10.1039/c1dt11175b

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PAPER
Syntheses, crystal structures, reactivity, and photochemistry of gold(III)
bromides bearing N-heterocyclic carbenes†
Christa Hirtenlehner,a Charlotte Krims,a Johanna Ho¨lbling,a Manuela List,b Manfred Zabel,c Michel Fleck,d
Raphael J. F. Berger,e Wolfgang Schoefbergera and Uwe Monkowius*a
Received 21st June 2011, Accepted 20th July 2011
DOI: 10.1039/c1dt11175b
Gold(I) complexes bearing N-heterocyclic carbenes (NHC) of the type (NHC)AuBr (3a/3b) [NHC =
1-methyl-3-benzylimidazol-2-ylidene (= MeBnIm), and 1,3-dibenzylimidazol-2-ylidene (= Bn2Im)] are
prepared by transmetallation reactions of (tht)AuBr (tht = tetrahydrothiophene) and (NHC)AgBr
(2a/2b). The homoleptic, ionic complexes [(NHC)2Au]Br (6a/6b) are synthesized by the reaction with
free carbene. Successive oxidation of 3a/3b and 6a/6b with bromine gave the respective (NHC)AuBr3
(4a/4b) and [(NHC)2AuBr2]Br (7a/7b) in good overall yields as yellow powders. All complexes were
characterized by NMR spectroscopy, mass spectrometry, elemental analysis and single crystal X-ray
diffraction. Reactions of the Au(III) complexes towards anionic ligands like carboxylates, phenolates
and thiophenolates were investigated and result in a complete or partial reduction to a Au(I) complex.
Irradiation of the Au(III) complexes with UV light yield the Au(I) congeners in a clean photo-reaction.
complex or the free carbene. Reports about Au(III) complexes are
Introduction
less frequent.6,8–12 As the direct synthesis starting from an Au(III)
Numerous publications dealing with the synthesis, solid state
precursor (e.g. KAuCl4) only works with special functionality on
the NHC ligand,12 they are mostly synthesized by the oxidation
of the corresponding Au(I) complexes with halogens (Cl2, Br2,
I2). The majority of NHC–Au(III) complexes are of the type
(NHC)AuX3 or [(NHC)2AuX2]X. The exchange of the halides
by other anionic (or neutral) ligands seems to be critical and
leads frequently to the reduction to Au(I) or decomposition. Some
of the rare examples are the recently published carbene bearing
complexes of the type (NHC)Au(R)X2 (R = C6H5, C6F5, Me; X =
Cl, I).6,11
structures and applications of silver(I), gold(I), and gold(III)
complexes bearing N-heterocyclic carbenes (NHC) have been
published in the last decade and have been summarised in
several review articles.1 In particular, the interest in NHC–Ag(I)
complexes has grown significantly since Wang and Lin proved
their capability as versatile carbene transfer agents and a plethora
of compounds have been reported.1–4 In the case of NHC–
Au complexes intensive research activities were stimulated by
their pharmaceutical potential as anticancer, antiarthritic, and
antibacterial agents as well as their catalytic activities for unique
C–C, C–O, and C–N bond-forming reactions.5–8 Nevertheless, the
chemistry of gold is still dominated by the oxidation state +1
and the linearly coordinate complexes of the type NHC–Au(I)–X
or [(NHC)2Au]X, which are usually synthesized by the reaction
of R2SAuX (R2S = Me2S, tht, X = halide) with a NHC–Ag
Light and temperature sensitivity is a general feature of
coinage metal complexes. The decomposition of such complexes
often leads to the metal, sometimes in colloidal form. For
(organometallic) gold(III) complexes, the reductive elimination
involving carbon–carbon or element–element bond formations is
well documented.13 Albeit known for a very long time, studies
about photochemical transformations of gold(I) and especially of
gold(III) compounds are still scarce.14 The thoroughly investigated
tetrachloroaurate anion [AuCl4]- undergoes photoreduction in
the presence of oxidizable substrates (e.g. alcohols) via [AuCl2]-
to eventually form elemental gold.15 If reducible substances like
alkyl halides or O2/Cl- are present, [AuCl2]- can be photo-
aInstitut fu¨r Anorganische Chemie, Johannes Kepler Universita¨t Linz,
Altenbergerstr. 69, A-4040, Linz, Austria. E-mail: uwe.monkowius@jku.at
bInstitut fu¨r Chemische Technologie Organischer Stoffe, Johannes Kepler
Universita¨t Linz, Altenbergerstr. 69, 4040, Linz, Austria
cZentrale Analytik der Universita¨t Regensburg, Ro¨ntgenstrukturanalyse,
Universita¨tsstr. 31, 93053, Regensburg, Germany
oxidized to AuCl4 .16 Analogous photo-reactivity has been found
-
dGeozentrum – Universita¨t Wien, Institut fu¨r Mineralogie und Kristallogra-
phie, Althanstr. 9, 1090, Wien, Austria
for LAuX3 (L = phosphane, X = Br, Cl).17 For clean photo-
reductive elimination of X2 from a LAuX3 complex according
to eqn (1), a halogen trap (alkene, alcohol) is required in solution,
whereas in the solid state due to the volatility of X2 no trap is
needed. Remarkably, no reduction to elemental gold took place
under these conditions.
eMaterialwissenschaften und Physik, Abteilung Materialchemie, Paris-
Lodron Universita¨t Salzburg, Hellbrunner Str. 34, 5020, Salzburg, Austria
† Electronic supplementary information (ESI) available: Figures, spectra,
and tables giving additional analytical and computational details. CCDC
reference numbers 830155–830166, 830315. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1dt11175b
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Scheme 1
−X
imidazolium bromides and LiN(SiMe3)2 in DCM at rt under inert
conditions. The oxidation of 6a/b yields the Au(III) complexes
[(NHC)2AuBr2]Br 7a/b as yellow powders. All compounds are
perfectly stable in solution and solid state at ambient conditions.
2
(1)
LAuX3 ⎯⎯⎯→LAuX
The photoreaction proceeds from a ligand-to-metal charge
transfer (LMCT) state. In the case of late transition metal halides,
a LMCT excitation often causes formal one-electron reduction
of the metal and release of a X∑ radical,18 which means for
Au(III) complexes formal reduction of the gold atom to the
relatively unstable Au(II) in the initial step. In subsequent thermal
reactions either reduction to Au(I) and liberation of another X∑ or
disproportion [2 Au(II) → Au(I) + Au(III)] takes place.
Characterization
All compounds yield the expected 1H NMR spectra. Upon
complex formation the signals for the C2-H imidazolium protons
vanish (see Experimental section). In the 13C NMR spectra, the C2
carbon atom is most affected by the ligation to the metal and by
the change of the oxidation state and coordination environment
(Table 1). With respect to the imidazolium salt, the C2 nucleus
of the Ag compounds 2a/b resonates considerable downfield by
around 44 ppm at ~181 ppm. There are considerable differences
between the neutral and ionic gold complexes: the carbene carbon
atom of both the neutral Au(I) and the Au(III) complex (3a/b and
4a/b) each have signals at higher field than the corresponding
ionic counterpart (6a/b and 7a/b) reflecting the higher positive
charge density on the gold atom. The highest downfield shifts for
the C2 are found for the ionic Au(I) complex, whereas the neutral
Au(III) have signals in the region of the imidazolium salt. For com-
parison, typical 13C chemical shifts of gold complexes bearing the
In this contribution we present the synthesis and structural char-
acterization of gold(III) complexes of the type (NHC)AuBr3 and
[(NHC)2AuBr2]Br [(NHC = 1-Methyl-3-benzylimidazol-2-ylidene
(= MeBnIm), and 1,3-dibenzylimidazol-2-ylidene (= Bn2Im)] and
their reactivity towards anionic ligands. In addition, their photo-
chemical reactivity is studied.
Results and discussion
Synthesis
The imidazolium bromides [MeBnImH]Br (1a) and [Bn2ImH]Br
(1b) are readily prepared by the reactions of benzylbromide with
methylimidazole or sodium imidazolide in toluene, respectively.
Reactions with an excess of Ag2O in DCM yield the Ag-carbenes
2a/b in good yields and high purity as white powders. The
stoichiometric reactions with LAuBr (L = tht, Me2S) in DCM
give the colourless Au(I) complexes 3a/b, which can be oxidized
by Br2 at -40 ◦C to the corresponding yellow (NHC)AuBr3
4a/b complexes. Oxidation with iodide under similar conditions
yields the mixed ligand complex (NHC)AuI2Br (5) as brown
powder. The homoleptic, ionic compounds [(NHC)2Au]Br 6a/b
are synthesized by the reaction of LAuBr (L = tht, Me2S) and the
free carbenes, which are in situ prepared by the reaction of the
Table 1 13C-NMR chemical shift for the C2 carbon atom in compounds
1–7 in d6-DMSO
R = Me, R¢ = Bn
R = R¢ = Bn
Imidazolium bromide
Ag
Neutral Au(I)
Ionic Au(I)
Neutral Au(III)
Ionic Au(III)
137.0
180.7
172.7
186.4
134.6
149.7/149.8
136.7
181.920
169.1
182.9
136.3
150.1
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1
non-substituted, unsaturated imidazol-2-ylidene moiety are de-
picted in Fig. 1. An analogue illustration for NHC–Ag complexes
would be less meaningful due to extensive ligand scrambling
equilibria in solution, which means that the molecular constitution
of the NHC–Ag complexes in the solid state might not be
retained in solution and a well-founded description of the silver
coordination environment would be difficult.9e For gold this
dynamic behaviour is less pronounced and it can be assumed that
the constitution is identical both in the solid state and in solution.
It was previously pointed out that the carbene carbon resonance
is upfield shifted with increasing Lewis acidity of the metal.9e,19
The higher electronegativity of the chloride ligand leads to higher
Lewis acidity of the gold(I) atom compared to the bromide
congeners. Accordingly, the C2 of the gold chlorides resonates
upfield compared to the gold bromides. In complexes of the
type [(NHC)2Au]+ the coordination of an additional NHC ligand
enhances the electron density on the gold atom, thus lowering
its Lewis acidity: the C2 signal is considerably shifted downfield.
Upon oxidation to Au(III), the acidity is further increased resulting
in a significant upfield shift of the C2 resonances. Interestingly, the
C2 resonances of the neutral Au(III) complexes are very similar to
the imidazolium salt. Albeit the proton and a –AuBr3 group are
not isolobal, it is tempting to speculate that the Au(III) atom has at
least a comparable electronic effect on the C2 atom like a proton.
C-atoms have unambiguously been assigned by H-1H-NOESY
and H-13C HSQC spectra (Fig. S2 and S3).† In DMSO-d6, the
1
separation of the signals of the methyl protons is 52 Hz and of◦the
methylene groups 40 Hz (at 200 MHz). Upon heating to 90 C,
the peaks of the methyl and methylene groups are only slightly
approaching each other (46 Hz for CH3 and 39 Hz for CH2), but
virtually no change of the integrals and signal shape could be
1
determined in the H-NMR spectra proving the high rotational
barrier about the C–Au axis. In C2D2Cl4 the chemical shifts are
more affected by increasing temperature. At 100 ◦C the signals are
broadened and the splitting is reduced from 75 to 60 Hz (CH2) and
from 80 to 64 Hz (Me). Unfortunately, the coalescence temperature
could not be reached because the compound starts to thermally
reduce to the Au(I) complex above this temperature (vide infra).
For an estimation of the rotational barrier, quantum-chemical
calculations were performed. Unlike the crystal structure, the
optimized structure does not exhibit coplanar orientation of the
imidazole ring planes but a somewhat twisted arrangement which
reduces the sterical repulsion of the substituent (Fig. S4).† All
calculated interatomic distances are in excellent agreement with
the measured parameters from the solid state. In the following
calculations one carbene ligand was kept fixed while the other
was rotated about the C2–Au axis giving a rotational barrier of
~60 kJ mol-1 (Fig. 3). Remarkably, this value is in good agreement
with the determined rotational barriers of similar Pd and Pt
compounds,21 but is inconsistent with the variable temperature
NMR experiments from this work. For a barrier of ~60 kJ mol-1 the
coalescence temperature is estimated to be around 60 ◦C applying
the standard approximation for the relation between the activation
enthalpy DG# and the coalescence temperature.22 In an attempt
to clarify this discrepancy also solvent effects were included into
the calculation using the COSMO model,23 however the calculated
Fig. 1 13C NMR shifts of the carbene carbon atom C2 of different gold
complexes bearing the (non-substituted) imidazol-2-ylidene moiety. NMR
data were taken from ref. 1b, 6, 9f and 10b].
energy difference between the approximate rotational saddle point
and the minimum conformation only slightly increased by ~4 kJ
The unsymmetrical substituted NHC Au(III) complex 7a shows
mol-1. Currently we are not able to give a satisfying explanation
of these findings.
1
two sets of signals in its H- and 13C-NMR spectra indicative of
syn-/anti-isomerism of this compound (Fig. 2 and Fig. S1).† The
ratio of the integrals of each signal set varies slightly from batch to
batch but is close to 1 : 1 for both isomers. The shifts of all H- and
Fig. 3 DFT calculations on the PBE0/TZVPP level of theory.
Crystal structures
¯
Complex 2a crystallizes in the triclinic space group P1 (Z = 2)
Fig. 2 1H NMR spectrum of 7a in DMSO-d6 (᭢- anti, ꢀ – syn isomer).
in an ionic form with [(NHC)2Ag]+ cations and [AgBr2]- anions
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the type [(NHC)2Ag][AgX2] and among theses structures, only one
does not exhibit short Ag ◊ ◊ ◊ Ag distances, presumably because of
steric reasons.27 Another frequently found structural motive is that
of the dimeric [Ag2X4]2- anion bridging two adjacent [(NHC)2Ag]+
. . .
cations by short Ag Ag contacts.28 Therefore, it is tempting to
speculate that the ionic aggregation pattern is governed synerget-
ically by both attractive closed shell d10-d10 as well as Coulombic
and higher mulipolar interactions (e.g. quadropole–quadropole
interactions which can in cases even outperform metallophilic
interactions29). Nevertheless, the energetic differences between the
different aggregation types seem to be small and the aggregation
is very sensitive to small changes in crystallization conditions and
alterations of the substitution of the NHC-ligand.
In complexes 3a and 3b (Fig. 5 + 6) the Au(I) atoms are almost
linearly coordinated by the carbene carbon and the bromide atoms
[C1–Au1–Br1 176.5(2)◦ (3a); 178.1(2)◦ (3b)]. Examples of a ligand
distribution forming the ionic species [L2Au]+[AuX2]- analogous
to 2a are very limited for gold(I) complexes.30 All interatomic
distances lie in the expected range: whereas the Au1–Br1 distances
Scheme 2
aggregated to infinite [+ –]–chains by short metal–metal contacts
of 3.223(1) A (Fig. 4). The asymmetric unit contains one half of
˚
a formula unit with a C2-axis passing through both Ag atoms
resulting in a perfect linear coordination environment for the
silver atoms in the ions [(NHC)2Ag]+ and [AgBr2]- with an
almost perpendicular orientation to each other (C1–Ag1–Ag–Br1
92.7◦). Both imidazole ring planes are coplanar—a frequently
found structural feature for the cation [(NHC)2M]+ (M = Au,
Ag). Despite the steric repulsion of the substituents in 1- and
3-position and the small energetic differences between a coplanar
and perpendicular orientation of the imidazolyl rings, the coplanar
arrangement is found more often, because p-backbonding contri-
butions are more pronounced in a coplanar than in a perpendicular
orientation.24
˚
˚
are almost identical [3a: 2.392(1) A; 3b: 2.397(1) A], the Au1–C1
˚
distance in 3a [1.979(8) A] is slightly shorter than in 3b [2.022(8)
A]. The molecules exist as monomeric units not associated via
˚
aurophilic interactions.
˚
Fig. 5 Molecular structure of 3a. Selected bond lengths (A) and angles
(deg): Au1–C1 1.979(8), Au1–Br1 2.392(1), C1–Au1–Br1 176.5(2).
Fig. 4 Molecular structure of the silver complex 2a (ORTEP; displace-
ment ellipsoids at the 50% probability level, H atoms are omitted for
˚
The Au(III) atoms in 4b and 5 exist in a distorted square-planar
environment defined by the carbene-C, and three halogene atoms
(Fig. 7 + 8). In complex 5 the bromide ligand is strictly in the trans
position. The angles between the imidazolyl ring plane and the
gold coordination plane are ~80◦. It should be noted that Au(III)
complexes bearing different halide ligands are rare and so far
only the crystal structure of the complex Ph3PAuBr2Cl has been
published.31
clarity). Selected bond lengths (A) and angles (deg): Ag1–C1 2.092(2),
Ag2–Br1 2.452(1), Ag1–Ag2 3.223(1), C1–Ag1–C1i 180◦, Br1–Ag2–Br1i
180◦.
As we point out in a recent publication,25 there are subtle effects
defining the aggregation of NHC-Ag-complexes in the solid state:
the competition between the neutral form (NHC)AgCl and the
ionic form [(NHC)2Ag][AgCl2] is caused by ligand scrambling
reactions and the ionic form is favoured by using polar solvents
during the crystallization process. In general, the majority of
halogenoargentates are oligo- or polymer anions. Isolated [AgX2]-
units (X = halogenide) are rarely found in the solid state.26
Interestingly, a CSD query revealed that the majority of crystal
structures containing an isolated [AgX2]- anion are complexes of
¯
Complex 6a crystallizes in the trigonal space group R3c
(Z = 18) as a highly symmetrical trinuclear, bromide centred
aggregate of the formula {[(NHC)2Au]3(m3-Br)}Br2 (Fig. 9). The
three [(NHC)2Au]+ cations are disposed about a threefold axis
linked by one bromine atom with an Au1–Br1 distance of 3.2647(3)
˚
A, which is well below the sum of their van-der-Waals radii
9902 | Dalton Trans., 2011, 40, 9899–9910
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˚
Fig. 6 Molecular structure of 3b. Selected bond lengths (A) and angles
(deg): Au1–C1 2.022(8), Au1–Br1 2.397(1), C1–Au1–Br1 178.1(2).
˚
Fig. 8 Molecular structure of 5. Selected bond lengths (A) and angles
(deg): Au1–C1 2.012(8), Au1–Br1 2.475(1), Au1–I1 2.614(1), Au1–I2
2.608(1), C1–Au1–Br1 178.4(2), C1–Au1–I1 88.7(2), C1–Au1–I2 81.1(2),
I1–Au1–C1–N1 82.9(7).
˚
Fig. 7 Molecular structure of 4b. Selected bond lengths (A) and angles
(deg): Au1–C1 2.031(8), Au1–Br1 2.420(1), Au1–Br2 2.442(1), Au1–Br3
2.429(1), C1–Au1–Br1 87.3(2), C1–Au1–Br2 177.1(2), C1–Au1–Br3
87.3(2), Br1–Au1–C1–N1 80.6(6).
Fig. 9 [Au3Br]2+ cation in the crystals of 6a (displacement ellipsoids at
the 30% probability level, H-atoms are omitted and the carbon atoms of
the methyl- and benzyl-substituent are drawn as small spheres for the sake
˚
(3.51 A) but substantially longer than those of non-bridging,
˚
˚
terminal bromide ligands (~2.4 A). This arrangement results in
of clarity). Selected bond lengths (A) and angles (deg): Au1–C1 2.035(4),
C1–Au1–C1i 175.0(2), Au1–Br1 3.2647(3), N1–C1–C1i–N1i 99.7(4).
the very rare T-shape coordination of the gold atom which is
known for only a few phosphane complexes of the type (R3P)2AuX
(X = Cl, Br, I, BF4).32 The net 2+ charge is balanced by two non-
interacting bromide ions. The distance between two gold atoms in
The crystals of 6b are monoclinic (P21/c) and contain isolated
[(NHC)2Au]+ cations and bromide anions (Fig. 10). The gold atom
is almost perfectly linearly coordinated by the carbene carbon
atom [C1–Au1–C18 178.4(4)◦], the imidazole ring planes are
almost coplanar [N1–C1–C18–N4 8.4(9)] and the Au–C distances
are comparable to the other Au–C bond lengths presented
here.
˚
the trimer is 5.655 A and out of the standard range for Au ◊ ◊ ◊ Au
33
˚
bonds (<3.5 A ). Within the cation both imidazole ring planes
are approaching a perpendicular orientation [N1–C1–C1i–N1i
99.7(4)], presumably because of steric reasons. To the best of our
knowledge, this aggregation pattern is unprecedented for gold, but
the isostructural silver complex {[(NHC)2Ag]3(m3-I)}I2 (NHC = 3-
methyl-1-picolyl-imidazol-2-ylidene) was reported previously.34
In the oxidized gold complexes 7a and 7b (Fig. 11 and 12),
the gold atom exists in a square-planar coordination environment
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Fig. 12 Structure of the cation in crystals of 7b. Selected bond lengths
˚
(A) and angles (deg): Au1–C1 2.05(2), Au1–C18 2.03(2), Au1–Br1
Fig. 10 Structure of the cation in crystals of 6b. Selected bond lengths
(A) and angles (deg): Au1–C1 2.06(1), Au1–C18 2.04(1), C1–Au1–C18
2.425(2), Au1–Br2 2.417(2), C1–Au1–C18 176.6(9), C1–Au1–Br1 90.5(6),
C1–Au1–Br2 89.7(5), N1–C1–Au1–Br1 104(2), N3–C12–Au1–Br1 108(2),
N1–C1–C18–N3 35.1(8).
˚
178.4(4), N1–C1–C18–N4 8.4(9).
not affected by the oxidation state of the gold atom. Again, the
imidazole moiety is approaching coplanarity – presumably more
because of the steric repulsion between the Br atoms and the
carbene substituent than by p-backbonding effects which should
be much weaker for Au(III) than for Au(I). Only the syn-isomer of
7a could be characterized by single crystal X-ray diffraction.
Reactivity study of the gold(III) complexes
For the Au(III) complexes, the possibility to exchange bromide
anions by other anionic ligands was investigated in a reactivity
study. The scope of this screening was to find stable, isolatable
compounds with different kinds of ligands capable of binding to
Au(III) atoms. The main focus was laid on ligands with oxygen
donor atoms as they might resist the oxidative environment
of the Au(III) atom. Another motivation was to explore the
still underdeveloped area of gold–oxygen chemistry.35 Beside
the reported analysis, no excessive attempts were undertaken to
identify possibly formed side products.
Both the neutral as well as the ionic complexes were treated
with three (for 4b) or two (for 7c = [(NHC)2AuBr2]Cl) equiv-
alents of AgNO3, Ag(acetate), Ag2(oxalate), and Ag(benzoate),
or a mixture of AgBF4/Na(OPh), AgBF4/Na(OC6Cl5), and
AgBF4/Na(SPh). After filtration of the formed AgBr over celite
the solvent was removed and the residue was re-crystallized from
DCM/Et2O. In the case of 4b, the reactions led to partial or
complete decomposition to elemental gold and only intractable
oily substances were formed. The 1H-NMR spectra of the residues
Fig. 11 Structure of the cation in crystals of 7a. Selected bond lengths
(A) and angles (deg): Au1–C1 2.02(1), Au1–C12 2.04(1), Au1–Br1
2.416(1), Au1–Br2 2.433(1), C1–Au1–C12 176.7(2), C1–Au1–Br1 89.6(1),
C1–Au1–Br2 89.5(1), N1–C1–Au1–Br1 100.9(4), N3–C12–Au1–Br1
104.4(4), N1–C1–C12–N3 3.6(6).
˚
defined by two carbene carbon atoms and two bromide ligands.
Interestingly, the Au–C bond lengths in both complexes are nearly
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revealed that basically the imidazolium salt was present. Only for
AgNO3 a few crystals were isolated. A single crystal X-ray analysis
revealed that [(NHC)2Au]NO3 was generated by the reduction
and ligand scrambling (Fig. S5).† Because of the easy oxidation of
thiolates a complete reduction and the formation of (NHC)AuSPh
were observed for the reaction with thiophenolate. A similar
reduction was reported for the anion [Au(III)(SR)4]- forming the
gold(I) species [Au(I)(SR)2]- and dithiol RS-SR.13c Due to the low
quality of the crystals, the quality of the structural data is also low
and will not be discussed here, but the identity of the substance is
unambiguous. The molecular structure plot and detailed structure
data can be found in the ESI (Fig. S6).† For 7b the reactions
yield a complete or partial reduction of the Au(III) compound.
The main product is in all cases the cation [(NHC)2Au]+ which
can be detected by 1H-NMR. From these reactions a single crystal
suitable for X-ray diffraction of the chloride and tetrafluoroborate
salt of [(NHC)2Au]+ could be isolated (Fig. S7 and S8).†
Fig. 13 Irradiation of [(Bn2Im)2AuBr2]Br in methanolic solution (c = 1.2
10-5 mol L-1) with monochromatic light l = 280 nm. For comparison, the
spectrum of [(Bn2Im)2Au]Br (methanol, c = 1.0 10-5 mol L-1) is also shown.
In further experiments it was attempted to oxidize the neutral
2-
Au(I) complex 3b with benzoylperoxide or [NBu4]2S2O8 to the
corresponding Au(III) complex. No reaction and no decomposi-
◦
1
tion could be detected by H-NMR spectroscopy at rt or 80 C
proving the high stability of the NHC-gold(I) complex.
Upon irradiation with polychromatic light all gold(III) bromide
complexes underwent photo-reduction to the respective gold(I)
compounds. In no cases a decomposition and formation of
elemental gold and also no plasmon resonance of colloidal
gold at ~520 nm37 are detectable. In Fig. 13 the time course
of the absorbance changes while irradiating (l = 280 nm) a
methanolic solution of complex 7b at room temperature is shown.
The LMCT band at 316 nm is decreasing while the signal at
261 nm is increasing. The two isosbestic points at 239 and
278 nm are indicative of a clean photo-reaction.38 After 18
min the resulting spectrum shows the same spectral features as
the gold(I) compound [(Bn2Im)2Au]Br. Both the 1H-NMR as
well as the ESI-MS spectra prove the reduction to the gold(I)
complex (Fig. S17 – S19).† There are no spectral variations of
the non-irradiated solution even after several days under ambient
conditions. All other Au(III) bromide complexes are reacting
analogously (Fig. S13 – S16). The released bromine is further
reacting with the solvent and therefore not visible in the UV-Vis
spectra (Fig. S20†).39 Accordingly, the photo-reduction proceeds
faster in methanol than in DMSO but there is no photo-reaction
in water even upon elongated irradiation due to the decreasing
ease of oxidation of these solvents. The oxidation of methanol
and DMSO with bromine is astonishingly complex and leads to
various reaction products,40 i.e. no oxidation products could be
unambiguously identified by NMR or MS experiments. In similar
experiments,41,42 ethanol or 2-propanol was used as reductant and
indeed for the photo-reaction of 7b in a mixture of EtOH/d6-
DMSO (1 : 2) acetaldehyde and diethylacetal could be detected by
1H- and 13C-NMR.
Electronic absorption spectra and photochemistry
All gold(III) complexes feature a long wavelength absorption above
300 nm (Fig. 13, S9 – S11†). To assign the electronic transitions the
thoroughly investigated square-planar d8-anion AuX4- (X = Cl, Br)
can be used for comparison:36 e.g., a solution of K[AuBr4] in
MeCN features signals at 394 and a shoulder at ~460 nm, which
can be assigned to n(Br) → 5dx2-y2(Au) and p(Br) → 5dx2-y2(Au)
ligand-to-metal-charge-transfer (LMCT) states, respectively. The
high energy band at 258 nm originates from a s(Br) → 5dx2-y2(Au)
LMCT state (Fig. S12).† In other words, in square-planar Au(III)-
halide complexes the HOMO is not constituted by 5d-orbitals
of the gold atom but by low lying orbitals of the halide ligands.
These low lying ligand orbitals facilitate a LMCT from the halides
to the 5dx2-y2 orbital of the strongly oxidizing gold atom upon
light absorption. Due to coordination of a s-donating ligand in
complexes of the type LAu(III)X3 (e.g. L = phosphine, NHC;
X = Cl, Br) the 5dx2-y2 orbital is further destabilized leading to
a hypsochromic shift of the LMCT. In the analogues phosphine
complex Ph3PAuBr3 the LMCT absorption is found at 346 nm17b
compared to ~335 nm for the neutral complexes 4a/b indicative
of the higher s-donating character of the carbene ligand. An
additional hypsochromic shift to ~315 nm is caused by the
coordination of a second NHC ligand in the case of the ionic
complexes 7a/b. The high energy bands are due to p–p* transitions
of the imidazolyl or phenyl moiety and might cover further, short
wavelength LMCT transitions (Table 2).
Table 2 UV/Vis spectroscopic data of the gold complexes in methanolic solution
lmax [nm] (lg e)
R = Me, R¢ = Bn
R = R¢ = Bn
Neutral Au(I)
Ionic Au(I)
Neutral Au(III)
Ionic Au(III)
226 (4.02), 233 (4.05), 247 (4.14)
236 (4.26), 243 (4.28), 260 (4.26)
230 (4.63), 334 (3.59)
227 (4.28), 235 (4.41), 248 (4.48)
238 (4.07), 245 (4.57), 261 (4.55)
231 (4.93), 336 (3.89)
223 (4.19), 315 (3.68)
230 (sh, 4.93), 247 (sh, 4.35), 261 (4.23), 316 (3.70)
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The iodide complex 5 was not included in the photo-reactivity
study because there are strong indications of a thermal equilibrium
between Au(III) and Au(I): (a) the 1H-NMR spectrum shows
broad unresolved and/or more than one set of signals. The shifts
of one signal set are very similar to the gold(I) complex 3a
which was used as starting material (Fig. S21);† (b) the UV-Vis
spectrum in acetonitrile solution features bands at 290 and 361 nm
which are characteristic for triiodide in acetonitrile (Fig. S22†).43
Because acetonitrile is capable of stabilizing a cationic species44
like [(NHC)Au(I)(NCMe)]+ we propose partial reduction of the
gold(III) atom and dissociation of iodine and extensive ligand
scrambling reactions, which eventually leads to the formation
of a free triiodide anion. Due to its high extinction coefficient
(38800 L¥mol-1¥L-1 at 291 nm; 25500 L¥mol-1¥L-1 at 360 nm)
the triiodide anion dominates the spectrum even if the gold(III)
complex or other (poly)halide species are still present. Similar
equilibrium reactions are well documented for gold complexes
LAuI3 bearing phosphane or isonitrile ligands45 as well as for
the anion trans-[(CN)2AuI2]- which dissociate in solution into
[Au(CN)2]- und I2.36a
band around 320 nm. This LMCT exited state causes a reductive
elimination of bromine and reduction of the Au(III) to Au(I)
complex. Conversely, solvent molecules like methanol, ethanol
or dimethyl sulfoxide function as trap of the formed bromine and
are oxidized. Particularly remarkable is the high photochemical
stability of the Au(I) complexes even under elongated illumination.
Experimental
General
All reactions and manipulations of air- and/or moisture sensitive
compounds were carried out in an atmosphere of dry nitrogen
using standard Schlenk techniques. Dichloromethane was dried
and distilled over a Na/Pb alloy. All solvents and other reagents
were commercially obtained and used as received. NMR spectra
were recorded either on a Bruker Digital Avance DPX 200
(200 MHz) or on an Avance DRX 500 (500 MHz) spectrometer
and 1H and 13C shifts are reported in ppm relative to SiMe4
and were referenced internally with respect to the residual signal
of the deuterated solvent. UV-Vis spectra were recorded on a
Cary 300 Bio photometer. Single crystal structure analysis of
the compounds 3a, 3b, 4b, 6a, 6b and 7b were carried out
on a Bruker Smart X2S diffractometer whereas the structures
of 5 and 7a were measured on a STOE-IPDS device46 with
Conclusion
Gold(III) complexes bearing NHC ligands of the type
(NHC)AuBr3 and [(NHC)2AuBr2]Br are readily accessible by
oxidation of the Au(I) congener with Br2 as yellow solids.
All attempts to exchange the bromide ligands by carboxylates,
phenolates or thiolate result in the reduction of the Au(III) complex
and isolation of a Au(I) species. Likewise, the oxidation of the
Au(I) complexes with strong oxidants like benzoyl peroxide or
˚
graphite-monochromated Mo-Ka radiation (l = 0.71073 A). For
2a, a Nonius Kappa diffractometer with a CCD area detector
was used, employing the program suite COLLECT.47,48 Further
crystallographic and refinement data can be found in Table 3
and 4. The structures were solved by direct methods (SIR-97 and
SHELXS-97)49,50 and refined by full-matrix least-squares on F2
(SHELXL-97).51 The H atoms were calculated geometrically and
2-
S2O8 does not yield a Au(III) compound. The UV-Vis spectra
of the NHC-gold(III) halides feature a broad low-energy LMCT
Table 3 Crystal data, data collection and structure refinement for compounds 2a, 3a, 3b, 4b and 5
2a
3a
3b
4b
5
Formula
MW
C22H24N4Ag, AgBr2
719.99
C11H12N2AuBr
449.10
C17H16N2AuBr
525.19
0.85 ¥ 0.14 ¥ 0.12
Tetragonal
P42/n
12.961(1)
12.961(1)
19.407(2)
90
90
90
3260.2(5)
2.140
8
11.47
200
1.9 – 25.0
20582
2887
2002
190/0
multi-scan
0.04, 0.34
1.36/-0.84
0.040
C17H16N2AuBr3
685.01
C11H12N2AuBrI2
702.90
Crystal size/mm
Crystal system
Space group
0.06 ¥ 0.07 ¥ 0.09
0.96 ¥ 0.75 ¥ 0.56
Orthorhombic
P212121
6.502(1)
10.664(1)
17.425(2)
90
90
90
1204.9(3)
2.476
4
15.50
300
3.0 – 25.1
11033
2136
2011
137/0
multi-scan
0.04, 0.02
1.55/-1.81
0.032
0.39 ¥ 0.20 ¥ 0.16
0.20 ¥ 0.20¥ 0.18
Triclinic
Triclinic
Triclinic
¯
¯
¯
P1
P1
P1
˚
a/A
6.445(1)
8.650(2)
11.772(2)
69.05(3)
85.31(3)
69.63(3)
573.9(3)
2.083
8.279(1)
10.762(2)
11.112(2)
98.946(5)
92.925(4)
100.948(4)
956.8(2)
2.378
8.437(1)
8.767(1)
11.357(1)
94.81(1)
98.10(1)
105.83(1)
793.5(2)
2.942
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
rc/g cm-3
Z
1
2
2
m/mm-1
5.204
293
4.5 – 36.3
5534
5534
5038
188/0
multi-scan
0.652, 0.745
5.48/-2.48
0.043
13.95
200
2.9 – 25.0
9393
3340
2666
208/0
multi-scan
0.07, 0.21
0.99/-1.15
0.038
15.661
123
2.55 – 25.89
6071
2870
2228
155/0
analytical
0.0839, 0.1547
1.981/-0.860
0.0298
T/K
q range [◦]
Reflections collected
Unique reflections
Observed reflections [I > 2s(I)]
Parameters refined/restraint
Absorption correction
T
min, Tmax
-3
˚
sfin (max/min)/e A
R1 [I ≥ 2s(I)]
wR2
0.126
830315
0.078
830155
0.093
830156
0.090
830157
0.0764
830158
CCDC number
9906 | Dalton Trans., 2011, 40, 9899–9910
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Table 4 Crystal data, data collection and structure refinement for compounds 6a, 6b, 7a, and 7b
6a
6b
7a
7b
Formula
MW
C22H24N4AuBr
621.33
C34H32N4AuBr
773.51
0.24 ¥ 0.32¥ 0.81
Monoclinic
P21/c
15.030(1)
11.044(1)
22.672(1)
90
128.227(3)
90
2956.3(4)
1.738
4
6.358
300
2.5 – 27.0
32232
6399
4563
361/0
multi-scan
0.07, 0.15
6.76, -4.71
0.075
0.191
830160
C22H24N4AuBr3
781.14
0.26 ¥ 0.18¥ 0.08
Monoclinic
P21/c
15.816(1)
9.941(1)
16.001(2)
90
109.15(1)
90
2376.3(4)
2.183
4
11.25
123
2.45 – 25.92
21383
4358
3699
268/0
analytical
0.101, 0.407
2.27, -0.73
0.028
0.067
830161
C34H32N4AuBr3, CH2Cl2
1018.23
0.30 ¥ 0.35 ¥ 0.50
Orthorhombic
P212121
13.276(2)
14.874(2)
18.914(2)
90
90
90
3734.8(8)
1.811
4
7.321
300
1.1 – 18.5
19419
1618
1382
393/0
multi-scan
0.120,0.220
0.53, -0.48
0.031
Crystal size/mm
Crystal system
Space group
0.61 ¥ 0.60 ¥ 0.59
Trigonal
¯
R3c
˚
a/A
13.054(1)
13.054(1)
68.414(4)
90
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
90
120
10095.9(2)
1.839
18
8.352
300
2.2 – 27.0
26871
2421
3
˚
V/A
rc/g cm-3
Z
m/mm-1
T/K
q range [◦]
Reflections collected
Unique reflections
Observed reflections [I > 2s(I)]
Parameters refined/restraint
Absorption correction
1537
129/0
multi-scan
0.08, 0.08
0.50, -0.64
0.0233
0.0442
830159
T
min, Tmax
-3
˚
sfin (max/min)/e A
R1 [I ≥ 2s(I)]
wR2
0.068
830162
CCDC number
a riding model was applied during the refinement process. Mass
spectra were collected on a Finnigan LCQ DecaXPplus Ion trap
Mass spectrometer with ESI ion source. The light sources were
a 1000 W Hanovia Xe/Hg 977 B-1 lamp with a Schoeffel GM
250-1 monochromator and an Osram mercury short-arc lamp
(HBO100W/2). The following compounds were prepared accord-
ing to literature procedures: tetrahydrothiophene-gold(I)-bromide
thtAuBr and tetrahydrothiophene-gold(I)-chloride thtAuCl,52
(1,3-dibenzylimidazol-2-ylidene)silver(I) bromide, 2b.20
The calculations were performed using the Turbomole program
package,53,54 version 6.1. The potential energy scan was performed
at the DFT level of theory using the BP8655 functional Ahlrichs
def2-SV(P)56 basis sets the resolution-of-the-identity approxima-
tion (RI), as well as the multipole accelerated RI-J approximation,
and in connection with an integration grid of m4 type and a setting
of $denconv = 1.0d-07. The energy difference between the local
minimum structure and the approximate transition state with an
dihedral angle Br–Au–C–N fixed at 180◦, were recalculated with
the PBE0 functional57 and the def2-TZVPP basis58 set with relaxed
geometries with and without the COSMO23 model.
was precipitated with Et2O. Recrystallisation from DCM/Et2O
yielded the imidazolium bromide as a white powder. The analytical
data correspond to the previously reported [20]. Complementary
13C{ H}-NMR data (125.8 MHz, DMSO-d6, 30 ◦C): d = 136.7
1
(C2), 135.3 (ipso-CPh), 129.4, 129.2 (o-, m-CPh), 128.9 (p-CPh), 123.3
(C4/C5), 52.4 (CH2).
1-Methyl-3-benzylimidazol-2-ylidene-silver-bromide,
2a.
A
100 ml round bottom flask was covered with aluminium foil and
charged with the imidazolium bromide 1a (1.04 g, 4.11 mmol)
and 20 ml DCM. To the resulting solution was added dry Ag2O
(0.48 g, 2.05 mmol). After an overall stirring time of 3 h at
ambient temperature the reaction mixture was filtered over Celite.
The solvent was removed in vacuo and recrystallization from
DCM/Et2O giving the products as colourless powders. Yield:
1.02 g (80%); H-NMR (200 MHz, DMSO-d6, 30 ◦C): d = 7.48
1
(d, 3JHH = 1.64 Hz, ImH, 1H), 7.40 (d, 3JHH = 1.67 Hz, ImH, 1H),
1
7.28 (m, Ph, 5H), 5.27 (s, CH2, 2H), 3.73 (s, CH3, 3H); 13C{ H}
NMR data (125.8 MHz, DMSO-d6, 30 ◦C): d = 180.7 (C2), 137.8
(ipso-CPh), 129.2, 128.1 (o-, m-CPh), 128.2 (p-CPh), 123.7, 122.6
(C4/C5), 54.4 (CH2), 38.6 (CH3); EA: calcld for C11H12N2AgBr
(360.00): C 36.70, H 3.36, N 7.78. Found: C 36.87, H 3.52, N 7.98.
1-Methyl-3-benzylimidazolium-bromide, 1a, was prepared ac-
1
cording to a published procedure.59 Complementary 13C{ H}-
NMR data (125.8 MHz, DMSO-d6, 30 ◦C): d = 137.0 (C2), 135.5
(ipso-CPh), 129.4, 128.9 (o-, m-CPh), 129.1 (p-CPh), 124.4, 122.7
(C4/C5), 52.0 (CH2), 36.4 (CH3).
1-Methyl-3-benzylimidazol-2-ylidene-gold-bromide, 3a. To a
solution of thtAuBr (0.17 g, 0.45 mmol) in 10 mL DCM was
added 2a (0.20 g, 1.79 mmol), whereby AgBr was precipitating
immediately. After further stirring for 3 h at ambient temperature
the solvent was removed in vacuo and the remaining solid
was extracted with DCM. The product could be isolated by
precipitation with Et2O. Yield: 0.12 g (48%). 1H-NMR (200 MHz,
DMSO-d6, 30 ◦C): d = 7.49 (d, 3JHH = 1.88 Hz, ImH, 1H), 7.42 (d,
3JHH = 1.88 Hz, ImH, 1H), 7.32 (br, Ph, 5H), 5.29 (s, CH2, 2H),
1,3-Dibenzylimidazolium-bromide, 1b. Sodium imidazolide
(13.2 g, 0.15 mol) and benzylbromide (50.2 g, 0.29 mmol)
was refluxed in 250 mL toluene over night. The solvent was
removed on a rotary evaporator and the residue was extracted
with DCM/H2O. The organic phase was separated, dried with
Na2SO4 and filtered. After the reduction of the volume the product
◦
1
3.72 (s, CH3, 3H); 13C{ H}-NMR (125.8 MHz, DMSO-d6, 30 C):
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d = 172.7 (s, C2), 137.7 (s, ipso-CPh),, 129.2, 128.5, 128.0 (s, Ph),
123.6, 122.0 (s, ImC4/5), 53.9 (s, CH2), 38.0 (s, CH3); EA: calcld
for C11H12N2AuBr (449.10): C 29.42, H 2.69, N 6.24. Found: C
29.29, H 2.67, N 6.19.
10 min. To this solution thtAuBr (70 mg, 0.20 mmol) in 10
mL DCM was added and stirred for 2 h. Outside the glove
box the reaction was quenched with 10 mL water. The DCM
phase was separated, dried with Na2SO4 and the solvent was
removed in vacuo. Recrystallisation from DCM/Et2O yielded
1,3-Dibenzylimidazol-2-ylidene-gold-bromide, 3b. The com-
plex was prepared analogues to 3a: Starting materials: 2b (0.232
g, 0.531 mmol), thtAuBr (0.194 g, 0.530 mmol). Yield 0.18 g
(61%). 1H-NMR (200 MHz, DMSO-d6, 30 ◦C): d = 7.56 (s, ImH,
1
6a as a white powder. Yield 60 mg (48%). H-NMR (200 MHz,
◦
3
DMSO-d6, 30 C): d = 7.57 (d, JHH = 1.74 Hz, ImH, 2H), 7.48
3
(d, JHH = 1.62 Hz, ImH, 2H), 7.27 (m, Ph, 10H), 5.33 (s, CH2,
4H), 3.77 (s, CH3, 6H); 13C{ H}-NMR (125.8 MHz, DMSO-d6,
1
1
2H), 7.31-7.36 (m, Ph, 10H), 5.34 (s, CH2, 4H); 13C{ H}-NMR
◦
30 C): d = 186.4 (s, C2), 140.4 (s, ipso-CPh), 132.1, 131.4, 130.8
◦
(125.8 MHz, DMSO-d6, 30 C): d = 169.1 (s, C2), 136.6 (s, ipso-
(s, Ph), 127.1, 125.9 (s, ImC4/5), 56.8 (s, CH2), 40.9 (s, CH3);
EA: calcld for C22H24N4AuBr (621.33): C 42.49, H 3.89, N 9.01.
Found: C 42.47, H 3.88, N 8.98; MS (ESI, MeCN/H2O): m/z:
541.33 M+
CPh), 128.8, 128.1, 127.6 (s, Ph), 122.3 (s, ImC4/5), 53.8 (s, CH2);
EA: calcld for C17H16N2AuBr (525.20): C 38.88, H 3.07, N 5.33.
Found: C 38.83, H 3.05, N 5.34.
1-Methyl-3-benzylimidazol-2-ylidene-gold-tribromide, 4a. The
gold(III) complexes were prepared analogous to a published
procedure:9b A solution of Br2 (8 mg, 53 mmol) in DCM was
added to a solution of 3a (24 mg, 53 mmol) at -40 ◦C. The solution
is allowed to warm to room temperature and stirred for another
12 h in the dark. The solvent was removed under reduced pressure
and the residue was dissolved in a minimum of DCM. The yellow
product was precipitated with pentane, isolated by filtration and
dried i◦n vacuo. Yield: 19 mg (62%). 1H-NMR (200 MHz, DMSO-
Bis(1,3-dibenzylimidazol-2-ylidene)gold-bromide, 6b. Com-
plex 6b was prepared from 1b (0.23 g, 0.68 mmol), LiHMDS
(0.14 g, 0.86 mmol), and thtAuBr (0.13 g, 0.34 mmol) using the
method described for 6a. Yield: 0.16 g (53%). 1H-NMR (200 MHz,
DMSO-d6, 30 ◦C): d = 7.62 (s, ImH, 4H), 7.19–7.26 (m, Ph, 20H),
◦
1
5.32 (s, CH2, 8H); 13C{ H}-NMR (125.8 MHz, DMSO-d6, 30 C):
d = 182.9 (s, C2), 136.8 (s, ipso-CPh), 128.7, 128.1, 127.3 (s, Ph),
123.0 (s, ImC4/5), 53.6 (s, CH2); EA: calcld for C34H32N4AuBr
(773.53): C 52.79, H 4.17, N 7.24. Found: C 52.57, H 4.10, N 7.03;
MS (ESI, CHCl3/MeCN): m/z = 693.32 M+.
d6, 30 C): d = 7.73 (d, 3JHH = 1.93 Hz, ImH, 1H), 7.70 (d, 3JHH
=
2.08 Hz, ImH, 1H), 7.34 (m, Ph, 5H), 5.40 (s, CH2, 2H), 3.79
(s, CH3, 3H); 13C{ H}-NMR (125.8 MHz, DMSO-d6, 30 ◦C):
1
Bis(1,3-dibenzylimidazol-2-ylidene)gold-chloride, 6c. Complex
6c was prepared analogues to 6b by using thtAuCl instead of
thtAuBr. Yield: 72%. NMR data are identical within experimental
error. EA: calcld for C34H32N4AuCl (729.07): C 56.01, H 4.42, N
7.68. Found: C 56.12, H 4.29, N 7.63.
d = 134.6 (s, C2), 136.1 (s, ipso-CPh),, 128.9, 128.7, 128.2 (s, Ph),
125.0, 122.8 (s, ImC4/5), 51.9 (s, CH2), 39.0 (s, CH3); EA calcld
for C11H12N2AuBr3 (608.91): C 21.70, H 1.99, N 4.60. Found: C
21.80, H 2.08, N 4.61; MS (ESI, CHCl3/MeCN): m/z = 541.33
L2Au+.
Bis(1-methyl-3-benzylimidazol-2-ylidene)gold-tribromide,
7a.
1,3-Dibenzylimidazol-2-ylidene-gold-tribromide, 4b. The com-
plex 4b was prepared analogues to 4a: Starting materials: 3b (0.112
g, 0.213 mmol), Br2 (0.034 g, 0.212 mmol). Yield 0.071 g (49%).
Complex 7a was prepared by the procedure described for 4a. The
ionic complexes 7a and 7b are sparingly soluble in acetone and
can easily be purified by washing with acetone. Starting materials:
6a (0.20 g, 0.33 mmol), Br2 (62 mg, 0.39 mmol). Yield: 0.21 g
(81%).
◦
1H-NMR (200 MHz, DMSO-d6, 30 C): d = 7.73 (s, ImH, 2H),
1
7.35 (m, Ph, 10H), 5.42 (s, CH2, 4H); 13C{ H}-NMR (125.8 MHz,
DMSO-d6, 30 ◦C): d = 136.3 (C2), 134.3 (s, ipso-CPh), 128.9, 128.7,
128.6 (s, Ph), 125.2 (s, ImC4/5), 53.7 (s, CH2); EA calcld for
C17H16N2AuBr3 (685.01): C 29.81, H 2.35, N 4.09. Found: C 29.89,
H 2.40, N 4.01; MS (ESI, CHCl3/CH3OH/NH4Ac): m/z = 684.07
M+, 693.4 L2Au+.
Anti-isomer: 1H-NMR (500 MHz, d6-DMSO, 30 ◦C): d 7.76 (d,
3
3JHH = 1.6 Hz, 2 H, CHIm), 7.75 (d, JHH = 1.6 Hz, 2 H, CHIm),
7.40 (m, 6 H, CHPh), 7.30(d, 4 H, CHPh), 5.52 (s, 4 H, -CH2), 3.68
(s, 6 H, -CH3). Syn-isomer: d 7.77 (d, 3JHH = 1.65 Hz, 2 H, CHIm),
7.68 (d, 3JHH = 1.65 Hz, 2 H, CHIm), 7.28 (m, 6 H, CHPh), 7.21 (d,
4 H, CHPh), 5.33 (s, 4 H, CH2), 3.68 (s, 6 H, CH3). Anti-isomer:
13C-NMR (125.8 MHz, d6-DMSO, 30 ◦C): d:149.7 (C2), 135.5
(ispo-CPh), 128.4 (CPh), 126.1 (CPh), 125.6 (CHIm), 124.9 (CHIm),
53.1 (CH2), 37.31 (CH3). Syn-isomer: d 149.8 (C2), 135.3 (ipso-
CPh), 128.4 (CPh), 127.7 (CPh), 125.9 (CHIm), 124.4 (CHIm), 52.9
(CH2), 37.3 (-CH3); EA: calcld for C22H24N4AuBr3 (781.14): C
33.83, H 3.10, N 7.17. Found: C 33.57, H 2.89, N 7.01, MS (ESI,
CHCl3/CH3OH): m/z = 701 M+, 542.33 L2Au+.
1-Methyl-3-benzylimidazol-2-ylidene-gold-dibromide-iodide, 5.
To a solution of 3a (0.20 g, 0.32 mmol) in 50 mL DCM was added
at -40 ◦C under stirring iodine (0.09 g, 0.32 mmol). The solution
was allowed to warm to room temperature and stirred for another
2 h. The solution was filtered over Celite and the solvent was
removed under reduced pressure. The residue was dissolved in a
minimum of DCM and precipitated with Et2O as a brown powder.
Yield: 0.20 g (89%). 1H-NMR (200 MHz, DMSO-d6, 30 ◦C): d =
7.66 (s, ImH, 2H), 7.42 (m, Ph, 5H), 5.42 (s, CH2, 2H), 3.85 (s,
CH3, 3H); EA: calcld for C11H12N2AuBrI2 (702.90): C 18.80 H
1.72 N 3.99. Found: C 18.89 H 1.81 N 4.01.
Bis(1,3-dibenzylimidazol-2-ylidene)gold-tribromide, 7b. Com-
plex 7b was prepared by the procedure described for 4a. Starting
materials: 6b (70 mg, 91 mmol), Br2 (14 mg, 90 mmol). Yield: 48 mg
(58%). 1H-NMR (200 MHz, DMSO-d6, 30 ◦C): d = 7.67 (s, ImH,
Bis(1-methyl-3-benzylimidazol-2-ylidene)gold-bromide,
6a.
1
The ionic gold complexes were prepared according to a modified
literature procedure:7f In a glove box, a solution of LiHMDS
(70 mg, 0.39 mmol) in 5 mL DCM was added to a solution
of 1a (0.10 mg, 0.40 mmol) in 5 mL DCM and stirred for
4H), 7.12–7.27 (m, Ph, 20H), 5.30 (s, CH2, 8H); 13C{ H}-NMR
◦
(125.8 MHz, DMSO-d6, 30 C): d = 150.1 (s, C2) 135.1 (s, ipso-
CPh), 129.3, 128.9, 128.1 (s, Ph), 125.8 (s, ImC4/5), 53.8 (s, CH2);
EA calcld for C34H32N4AuBr3 (933.33):C 43.75, H 3.46, N 6.00.
9908 | Dalton Trans., 2011, 40, 9899–9910
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Found: C 43.48, H 3.47, N 6.22; MS (ESI, MeCN/H2O): m/z =
B. W. Skelton and A. H. White, Dalton Trans., 2006, 3708–3715; (g) M.
V. Baker, P. J. Barnard, S. J. Berners-Price, S. K. Brayshaw, J. L. Hickey,
B. W. Skelton and A. H. White, J. Organomet. Chem., 2005, 690, 5625–
5635; (h) P. J. Barnard, M. V. Baker, S. J. Berners-Price and D. A. Day,
J. Inorg. Biochem., 2004, 98, 1642–1647; (i) J. Weaver, S. Gaillard, C.
Toye, S. Macpherson, S. P. Nolan and A. Riches, Chem.–Eur. J., 2011,
17, 6620–6624.
853.08 M+, 693.42 L2Au+.
Bis(1,3-dibenzylimidazol-2-ylidene)gold-dibromide-chloride, 7c.
Complex 7c was prepared analogues to 7b by using 6c instead
of 6b. Yield: 79%. NMR data are identical within experimental
error. EA: calcld for C34H32N4AuBr2Cl (888.88): C 45.94, H 3.63,
N 6.30. Found: C 46.12, H 3.71, N 6.53.
8 J. Lemke, A. Pinto, P. Niehoff, V. Vasylyeva and N. Metzler-Nolte,
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General procedure for the reactions of gold(III) bromides 4b and
7b with various ligands. 4b (0.05 g, 0.08 mmol) or 7c (0.05
g, 0.06 mmol)◦were suspended in 20 mL absolute acetone and
cooled to -40 C. To this mixture 3 (for 4b) or 2 (for 7c) equiv-
alents of AgNO3, Ag(acetate), Ag2(oxalate), and Ag(benzoate),
or a mixture of AgBF4/Na(OPh), AgBF4/Na(OC6Cl5), and
AgBF4/Na(SPh) were added. After the reaction mixtures were
allowed to warm to room temperature it was stirred for additional
4h. The formed AgBr/Cl was removed by filtration over Celite
and the solvent was distilled off in a vacuo. The residues were
dissolved in a minimum of DCM and crystallized by slow gas
phase diffusion of Et2O.
12 M. Boronat, A. Corma, C. Gonza´lez-Arellano, M. Iglesias and F.
Sanchez, Organometallics, 2010, 29, 134–141.
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Acknowledgements
We thank Prof. G. Kno¨r for fruitful discussions and generous
support of the experimental work.
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9910 | Dalton Trans., 2011, 40, 9899–9910
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©