1,2,4-Triazole-derived carbene complexes of gold: Characterization, solid-state aggregation and ligand disproportionation

Article abstract of DOI:10.1039/c4dt03201b

Ligand redistribution reactions are well documented for silver(i) N-heterocyclic carbene (NHC) complexes of the type [AgX(NHC)] (X = halido ligand), but only two reports have been described in the literature for gold analogues of the general formula [AuX(NHC)]. In both cases, the NHCs in question were exceptionally strong donors. To probe the dependence of ligand redistribution processes on NHC donor strength, a model study was conducted using a weakly donating 1,2,4-triazolin-5-ylidene (tazy) ligand and different halido coligands. For [AuX(tazy)] (X = Cl, Br, OAc, tazy = 4-benzyl-1-methyl-1,2,4-triazolin-5-ylidene), no ligand redistribution was found, while a reversible disproportionation between [AuI(tazy)] in solution and [Au(tazy)₂][AuI₂] in the solid state was observed and studied by means of X-ray crystallography, NMR and UV-Vis spectroscopy, as well as DFT calculations.

Full text of DOI:10.1039/c4dt03201b

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1,2,4-Triazole-derived carbene complexes of gold:
characterization, solid-state aggregation and
ligand disproportionation†
Cite this: DOI: 10.1039/c4dt03201b
Shuai Guo, Jan Christopher Bernhammer and Han Vinh Huynh*
Ligand redistribution reactions are well documented for silver(I) N-heterocyclic carbene (NHC) complexes
of the type [AgX(NHC)] (X = halido ligand), but only two reports have been described in the literature for
gold analogues of the general formula [AuX(NHC)]. In both cases, the NHCs in question were exception-
ally strong donors. To probe the dependence of ligand redistribution processes on NHC donor strength, a
model study was conducted using a weakly donating 1,2,4-triazolin-5-ylidene (tazy) ligand and different
halido coligands. For [AuX(tazy)] (X = Cl, Br, OAc, tazy = 4-benzyl-1-methyl-1,2,4-triazolin-5-ylidene), no
ligand redistribution was found, while a reversible disproportionation between [AuI(tazy)] in solution and
[Au(tazy)2][AuI2] in the solid state was observed and studied by means of X-ray crystallography, NMR and
UV-Vis spectroscopy, as well as DFT calculations.
Received 17th October 2014,
Accepted 20th January 2015
DOI: 10.1039/c4dt03201b
www.rsc.org/dalton
of Au–Ccarbene bonds, is probably less favourable due to the
higher BDE of the Au–NHC bond.
Introduction
Coinage metal complexes incorporating N-heterocyclic carb-
To the best of our knowledge, there are only two examples
enes (NHCs) have received a great deal of interest1 due to in this regard.8 In 2008, Bertrand and co-workers reported an
their intriguing coordination chemistry as well as their appli- ionic complex [Au(CAAC)2][AuCl2] (A′),8a which can be gener-
cations in catalysis,2 luminescence,3 and biological science.4 ated via the autoionization of the neutral complex [AuCl-
In this context, much attention has been paid to Ag(I) mono- (CAAC)] (A) in solution (I, CAAC = 2-(2,6-diisopropylphenyl)-
NHC complexes of the type [AgX(NHC)] (X = halido ligand), 3,3-dimethyl-2-azaspiro[4.5]decan-1-ylidene, Scheme 1). In
particularly due to their ability to serve as carbene transfer 2012, we published the 2nd example for ligand disproportiona-
agents.5 The structural diversity of silver NHC complexes in tion, involving an indazole-derived carbene complex [AuI-
this category has proved to be fairly broad. A large number of (Indy)] (B, Indy = 6,7,8,9-tetrahydropyridazino[1,2-a]indazolin-
neutral (e.g. [AgX(NHC)], [AgX(NHC)]2) and ionic structures
(e.g. [Ag(NHC)2][AgX2]) have been reported. Additionally, fluxio-
nal behaviour has been observed for different Ag–NHC species
in solution, indicating rapid ligand disproportionations.6
In contrast, only scattered examples of ligand disproportio-
nations of complexes with the general formula [AuX(NHC)]
have been reported, although such complexes have been exten-
sively employed in homogenous catalysis.2 A DFT study by
Frenking et al. has revealed that the bond dissociation energy
(BDE) of group 11 metal–NHC bonds follows the trend Au >
Cu > Ag.7 The disproportionation process of complexes [AuX-
(NHC)], obviously involving the breakage and reconstruction
Department of Chemistry, National University of Singapore, 3 Science Drive 3,
117543 Singapore, Singapore. E-mail: chmhhv@nus.edu.sg; Fax: +65 6779 1691;
Tel: +65 6516 2670
†Electronic supplementary information (ESI) available: Selected crystallographic
data; Computational details. CCDC 1029440–1029445. For ESI and crystallo-
Scheme 1 Previous reports on ligand disproportionations of complexes
graphic data in CIF or other electronic format see DOI: 10.1039/c4dt03201b
of the type [AuX(NHC)] (X = halido ligand).
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3-ylidene).8b The latter was found to be solvent-dependant
and led to the formation of two ionic species B′ and B″ (II,
Scheme 1).
Notably, in both cases (I/II) non-classical carbenes (CAAC
and Indy) were employed, which are known to be stronger
σ-donors compared to classical NHCs.9 In order to gain better
insights into the factors affecting such processes, complexes of
a 1,2,4-triazolin-5-ylidene, as one of the most weakly donating
classical NHCs,9b were prepared with the objective to study, if
the NHC’s donating ability plays a crucial role in ligand
disproportionations.
Scheme 3 Syntheses of Au(I) bromido and iodido complexes 3 and 4.
The solubilities of complexes 3 and 4 are similar to that of
precursor 2. The 1H NMR spectra of 3 and 4 in CDCl3 resemble
that of complex 2 and thus are non-indicative for these halido
substitution reactions. However, in their 13C NMR spectra, the
carbene signals of 3 and 4 are found to be highly sensitive to
Results and discussion
Syntheses of Au(I) triazolin-5-ylidene complexes
the trans-standing halido ligands. Complexes 2–4 show a
13
gradual downfield shift of the
C
carbene
signal in the order
The carbene precursor salt tazy·HCl (1) (Scheme 2, tazy =
4-benzyl-1-methyl-1,2,4-triazolin-5-ylidene) was readily prepared
by the alkylation of 1-methyl-1,2,4-triazole, with benzyl chlor-
ide acting both as alkylating agent and solvent. The sub-
sequent auration via silver carbene transfer to [AuCl(SC4H8)]
afforded the desired Au(I) chlorido mono-carbene complex
[AuCl(tazy)] (2) in a good yield of 78%. Initial attempts to use
the same metallation route with the bromide salt tazy·HBr
gave a mixture of chlorido and bromido complexes due to
halido scrambling, which was corroborated by X-ray diffraction
analyses (vide infra).
174.8 ppm (Cl) < 178.3 ppm (Br) < 184.8 ppm (I), which corre-
lates to the increasing net donating ability of the trans halido
ligands. This is in line with the earlier studies from Baker
et al.11 and our group,9b,c,12 which revealed that an increased
Lewis acidity of the metal center will result in an upfield
13
C
carbene
shift. The formation of complexes 2 and 3 was also
supported by their ESI-MS spectra, where dominant peaks at m/z
543 corresponding to the monocationic species [Au(tazy)2]+
were observed.
The identity of complexes 2 and 3 was confirmed by X-ray
diffraction analysis on single crystals obtained by slow evapor-
ation of their concentrated solutions in CH2Cl2–n-hexane. The
molecular structures are depicted in Fig. 1. The Au–Ccarbene
distance of 3 is found to be longer than that of complex 2,
which is due to the stronger trans influence of the bromido
versus the chlorido ligand. As anticipated, the two benzyl sub-
stituents bend away from the coordination spheres to reduce
steric repulsion. In both cases of 2 and 3, two mono-carbene
molecules were found to dimerize in an antiparallel eclipsed
manner through aurophilic interactions clearly indicated by
the inter-gold distances of 3.389 Å (2) and 3.305 Å (3), respect-
Complex 2 was isolated as white solid, which is readily
soluble in common organic solvents, with exception of diethyl
1
ether, n-hexane and toluene. In the H NMR spectrum of 2 in
CDCl3, the benzylic protons and N-methyl group give singlets
at 5.37 and 4.01 ppm, respectively, which show upfield shifts
(Δδ = 0.40, 0.10 ppm) compared to those in precursor salt 1.
The formation of 2 was further corroborated by the disappear-
ance of the resonance at 11.61 ppm attributed to the acidic
proton on C5 in 1, and the presence of a carbene signal at
174.8 ppm in the 13C NMR spectrum. This carbene resonance
is found to be comparable to those of previously reported Au(I)
mono-(1,2,4-triazolin-5-ylidene) analogues.10 The ESI mass
spectrum of 2 displays a base peak at m/z 402 arising from
[M − Cl + CH3OH]+.
The chlorido complex 2 can serve as the precursor for
further ligand replacement reactions. Treating 2 with excess
LiBr in acetone gave bromido complex [AuBr(tazy)] (3) and
Finkelstein reaction with NaI led to the formation of iodido
analogue [AuI(tazy)] (4) in good yields of 81% and 88%,
respectively (Scheme 3).
Fig. 1 Molecular structures of 2 (left) and 3 (right) showing 50% prob-
ability ellipsoids. The other crystallographically independent molecule
without aurophilic interactions observed in the solid state of 3 is not
shown. Hydrogen atoms and a partial bromido ligand (19%) also found
in the structure of 2 have been omitted for clarity. Selected bond
lengths (Å) and bond angles (°) for complex 2: Au1–C1 1.970(3), Au1–Cl1
2.274(7), Au1–Au1X 3.389; C1–Au1–Cl1 179.9(3), N2–C1–N1 103.6(3),
C1–Au1–Au1X 83.48, Cl1–Au1–Au1X 96.42. 3: Au1–C1 1.989(4), Au1–
Br1 2.4057(5), Au1–Au1X 3.305; C1–Au1–Br1 176.15(10), N2–C1–N1
103.9(3), C1–Au1–Au1X 77.61, Br1–Au1–Au1X 106.23.
Scheme 2 Synthesis of Au(I) chlorido mono-carbene complex 2.
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ively.13 A computational study by Pyykkö et al. on the dimeriza-
tion of model complex [AuX(PH3)] (X = anionic ligand) con-
cluded that a “softer” anionic ligand X leads to a stronger
aurophilic bonding.14 In the present cases with NHCs as sup-
porting ligands, a shorter inter-gold separation (i.e. stronger
aurophilic interaction) was also found in complex 3, where the
bromido is “softer” compared to the chlorido coligand in the
corresponding complex 2. Notably, a deviation of the C1–Au1–
Br1 vector from linearity was observed in the case of 3, while
the Au(I) centre of complex 2 adopts an essentially linear
geometry.
Single crystals of iodido complex 4 suitable for X-ray diffrac-
tion study were obtained by slow evaporation of a concentrated
solution of 4 in CH2Cl2. In contrast, the molecular structure
was found to be the complex salt [Au(tazy)2][AuI2] (4′), indicat-
ing a ligand disproportionation of neutral complex 4. The ion
pair depicted in Fig. 2 contains bis(carbene) [Au(tazy)2]+ as the
cationic fragment and [AuI2]− as the counteranion. Notably,
both Au(I) centers adopt a perfectly linear coordination geome-
try. The ions aggregate in a staggered orientation with a C1–
Au1–Au2–I1 torsion angle of ∼81°, leading to the formation of
a polymeric array of “crossed-sticks” with an inter-gold contact
of 3.316 Å.
X-ray powder diffraction was carried out to study the bulk of
the crystalline material obtained from the CH2Cl2 solution of
4. The experimental diffraction pattern is in very good agree-
ment with the simulated one based on the molecular structure
of 4′ (Fig. 3), confirming that 4′ indeed represents the whole
solid material. Nevertheless, it should also be noted that the
neutral complex 4 may only undergo ligand disproportionation
yielding the ionic isomer 4′ upon crystallization. This dis-
proportionation behavior is similar to the one that has been inves-
tigated for an indazole-derived carbene complex [AuI(Indy)].8b
The Ccarbene signal of the tazy ligand is a diagnostic tool to
distinguish tazy-containing complexes with different co-
ligands (vide supra). Thus, to identify the prevalent species
present in the solution, Au(I) bis(carbene) complex [Au(tazy)2]-
Fig. 3 Simulated (top) and experimentally determined (bottom) X-ray
powder diffraction patterns of bulk crystalline solids obtained from a
saturated CH2Cl2 solution of complex 4.
Scheme 4 Syntheses of Au(I) acetato complex 5 and bis(NHC) complex 6.
PF6 (6) was synthesized for comparison via a two-step protocol acetato complex [Au(CH3COO)(tazy)] (5) can serve as a basic
(Scheme 4). In the first step, the chlorido ligand of complex 2 precursor, which reacted with azolium salt tazy·HPF6 and gave
was substituted by an acetato ligand with AgOAc. The obtained the desired bis(NHC) complex 6 in a good yield of 88%.
The solubilities of complexes 5 and 6 are very similar to
1
that of the precursor 2. Their H NMR spectra also resemble
that of 2 apart from the additional signals due to the acetato
group in the case of 5. Again, the formation of 5 and 6 was
better evidenced by their 13C NMR spectra, where the carbene
resonances are found to shift markedly upfield (Δδ = 6.8 ppm,
5) and downfield (Δδ = 11.4 ppm, 6), respectively.
Single crystals of 5 and 6 for X-ray diffraction studies were
obtained by slow evaporation of their concentrated CH2Cl2–n-
hexane solutions. The molecular structures are shown in
Fig. 4. In the structure of complex 5, the acetato moiety is co-
ordinated to the Au(I) center in a monodentate fashion. In con-
trast to the chlorido and bromido counterparts 2 and 3 (vide
Fig. 2 Molecular structure (left) and packing pattern (right) of 4’
showing 50% probability ellipsoids. Hydrogen atoms have been omitted
for clarity. Selected bond lengths (Å) and bond angles (°): Au1–C1 2.024
(5), Au2–I1 2.5586(4), Au1–Au2 3.3161(3); C1–Au1–C1X 180.0(3), I1–
supra), a weaker aurophilic interaction with a longer inter-gold
separation of 3.471 Å was observed. This is again due to
the increased “hardness” of acetato versus chlorido/bromido
Au2–I1X 180.000(1), N2–C1–N1 104.3(4), C1–Au1–Au2 94.81(14), C1X–
Au1–Au2 85.19(14), I1–Au2–Au1 83.545(9), I1X–Au2–Au1 96.455(9).
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Fig. 4 Molecular structures of 5 (left) and 6 (right) showing 50% prob-
ability ellipsoids. Hydrogen atoms and solvent molecules are omitted for
clarity. Selected bond length (Å) and angles (°) for complex 5: Au1–C1
1.981(4), Au1–O1 2.039(3); C1–Au1–O1 178.12(13), N2–C1–N1 104.6(3).
6: Au1–C1 2.017(4), C1–Au1–C1X 180.0(2), N2–C1–N1 103.2(3).
Fig. 6 Normalized absorption spectra of precursor salt 1 and com-
plexes 2–6.
ligands (vide supra). In contrast to 5, no aurophilic interaction
was found in complex 6, which likely results from the high
steric demand of the two tazy ligands.
dihalido aurate species [AuX2]− (X = Cl, Br, I) have been
thoroughly studied by Mason et al.15 It has been disclosed that
the [AuI2]− anion features a characteristic LMCT band at
362 nm, which is a good indicator for the presence of [AuI2]−-
containing species (such as 4′) in solution. As shown in Fig. 6,
the absorption patterns of complexes 2–6 are quite similar and
all absorption bands are within the high-energy region (λ <
300 nm). Notably, in the case of iodido complex 4, no LMCT
absorption at ca. 362 nm characteristic for diiodidoaurate(I) is
observed, which suggests the absence of the ionic species 4′ in
solution.
Comparison of diagnostic 13C NMR spectroscopic data
The carbene resonances of complexes 2–6 are illustrated in
13
Fig. 5. It was found that the
C
carbene
signal of the fixed tazy
ligand correlates well with the net donating ability of the trans-
standing ligand. The weakest donor, the acetato ligand, leads
to the most upfield shift of the tazy carbene resonance,
whereas the carbene signal of bis(carbene) complex 6 bearing
an additional strong carbene ligand in the trans position is the
most downfield. This observation is in line with previous
reports.9b,c,11,12
13
Notably, the
C
(tazy) signal of bis(NHC) complex 6
carbene
Oxidative addition
at 186.2 ppm shows a clear downfield shift compared to reso-
nance observed in complex 4 (184.8 ppm), which suggests that
the mono-carbene complex is the prevalent species in solution.
When 4′ was redissolved in CDCl3 and subjected to NMR
spectroscopic analysis, the same spectroscopic features were
observed as for the freshly prepared complex 4, which supports
a full reversibility of the ligand disproportionation process.
The identity of mono-carbene species 4 in solution can be also
indirectly verified by its further transformation. The oxidative
addition of I2 to complex 4 afforded the triiodido(mono-
carbene) complex [AuI3(tazy)] (7) in a very good yield of 93%
(Scheme 5).
Complex 7 was isolated as dark-red solid. The color orig-
inates most likely from the ligand-to-metal-charge-transfer
(LMCT) from a iodido ligand to the d8 Au(III) center.12a The for-
mation of 7 was corroborated by its ESI mass spectrum, where
a base peak at m/z 797 assignable to [M − I + tazy]+ was
observed. The 1H NMR spectrum of 7 does not show pro-
nounced differences despite the increased Lewis acidity of the
metal center upon oxidation. Nevertheless, the Ccarbene signal
is again a good indicator for the successful oxidation. This
resonance has shifted significantly upfield (Δδ = 21.8 ppm)
compared to that in Au(I) precursor 4.
UV-Vis absorption spectroscopy
To provide additional evidence that complex 4 is the prevalent
species in solution instead of 4′, UV-Vis absorption spectro-
scopic analyses were performed. The absorption spectra of
Fig. 5 13C carbene signals of tazy ligand in complexes 2–6.
Scheme 5 Synthesis of Au(III) triiodido complex 7.
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Fig. 7 Molecular structure and packing pattern of 7 showing 50% prob-
ability ellipsoids. Solvent molecules, disordered atoms are omitted for
clarity. Selected bond length (Å) and angles (°): Au1–C1 2.038(11), Au1–
I2 2.5888(10), Au1–I1 2.6183(8), Au1–I3 2.6259(7); C1–Au1–I2 177.1(3),
I1–Au1–I3 173.26(3), C1–Au1–I1 85.8(3), C1–Au1–I3 88.9(3), I2–Au1–I1
92.27(3), I2–Au1–I3 93.11(3).
Scheme 6 Ligand redistribution process and decomposition into indi-
vidual steps.
step contributing to ΔG1. To mitigate this problem, a polariz-
able continuum model for dichloromethane, the solvent in
which the ligand redistribution was observed, was included in
the calculations.19
The optimized geometries are in good agreement with the
molecular structures obtained by X-ray diffraction, insofar the
latter are available. Bond angles and lengths are accurately
reproduced, with Au–X and Au–C bonds being overestimated
in the theoretical structures by less than 3%. The largest devi-
ations were observed for species, which exhibit strong auro-
philic interactions in the solid state. These are absent in the
theoretical structures, explaining the slight changes in bond
lengths and angles.
Regardless of the halido ligand, all reactions were found to
be endothermic (Table 1). However, it should be noted that the
energetic contributions from ion pairing and aurophilic inter-
actions have not been accounted for. Aurophilic interactions
can contribute up to 60 kJ mol−1 in additional stabilization,
depending on the exact nature of the species involved,13,20
which is sufficient for the ligand redistribution to proceed
upon crystallization.21
The calculated energy difference between 4 and 4′ is by
∼12 kJ mol−1 smaller for the iodido complexes when com-
pared to the chlorido and bromido congeners, and aurophilic
interactions are known to increase with softer coligands such
as iodido. Hence it is very plausible that aurophilic inter-
actions can offset the energy difference between 4 and 4′ in the
solid state, while allowing the system to revert back to the
mono-NHC form in solution. The larger energy difference and
the weaker aurophilic interactions prevent a similar equili-
brium for 2 and 3.
X-ray diffraction study on single crystals obtained from a
concentrated CH2Cl2–n-hexane solution of 7 confirmed the
essentially square planner geometry adopted by the Au(III)
center (Fig. 7). No adduct of complex 7 with I2 was found in
the solid state molecular structure, which was previously
observed for Au(I) phosphine analogues reported by Schmid-
baur et al.16 The molecules of 7 were found to aggregate
through an inter-iodine contact of 3.628 Å, which is within the
sum of van der Waals radii for two iodine atoms (3.80 Å).17
The two cis-standing iodido ligands (I1, I3) bend to the more
bulky tazy ligand with a I1–Au1–I3 bond angle of 173.26(3)°.
Similar bending has been reported for a number of trihalido
Au(III) carbene complexes,8b,12 which has been attributed to
the electron donation of cis-halido ligand to the formally
vacant pπ orbital at the Ccarbene atom, and the electrostatic
repulsion between the trans- and cis-halido ligands. Another
interesting feature in the structure of 7 is that the Au1–I2 bond
distance is shorter compared to those of Au1–I1 and Au1–I3
bonds. In earlier studies on complexes of the type [AuX3(NHC)]
(X = halido ligand), the more strongly donating carbene ligand
always has a greater trans influence and lengthens the Au–
Xtrans bond.12a,18 In the present case, the weakening of the Au–
Xcis bond is likely to be due to the intermolecular I–I inter-
action (vide supra).
Computational study
To gain further insight into the factors contribution or ham-
pering ligand redistribution, the process was studied in greater
detail by means of DFT calculations. The reaction between two
molecules of [AuX(tazy)] can be formally decomposed into two
bond-breaking and two bond-making steps, each of which con-
tributes to the overall Gibbs free energy of the reaction
(Scheme 6). While this decomposition most likely does not
reflect the actual mechanism of the ligand redistribution
process, it allows gauging the energetic contributions each
broken and newly formed bond makes towards the overall
Gibbs free energy.
When examining the individual energetic contributions in
detail, it becomes apparent that cleaving the Au–X bond in 2–4
Table 1 ΔGR of the redistribution and contributions of each bond,
energies in kJ mol−1
X = Cl
X = Br
X = I
157.9
204.3
−112.3
−205.3
44.5
The geometries of all species involved in this hypothetical
reaction outlined in Scheme 6 were optimised at the B3LYP/ ΔG1
215.5
230.0
−183.7
−205.3
56.8
213.5
223.0
−171.7
−205.3
59.0
ΔG2
cc-pVDZ-(PP) level of theory. Initially, no solvent model was
included in the optimization, but it became quickly apparent
ΔG3
ΔG4
that this leads to a vast overestimation of the charge-separation ΔGR
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requires considerably less energy for the iodido ligand than Bruker AMX 500 spectrometer and the chemical shifts (δ) were
for the lighter homologues (ΔG1). The more polarizable iodide internally referenced to the residual solvent signals relative to
is a better leaving group than the harder bromide and chloride [Si(CH3)4] (1H, 13C) or externally to CF3CO2H (19F) and 85%
anions. However, this energetic difference is negated by a H3PO4 (31P). ESI mass spectra were measured using a Finnigan
lower energy gain from aurate formation (ΔG3) due to the MAT LCQ spectrometer. Elemental analyses were carried out
stronger trans-influence of the iodido ligands. This stronger on a Elementar Vario Micro Cube elemental analyzer at the
trans-influence also makes the cleavage of the Au–C bond in Department of Chemistry, National University of Singapore.
2–4 less endothermic for 4 (ΔG2), and indeed, it is energeti- UV–Vis absorption spectroscopic analyses were carried out
cally more favourable for the tazy ligand to bind to [Au(tazy)]+ with a Shimadzu 2550 UV-Vis spectrometer. The measurement
than to AuI, while this is not the case for AuCl and AuBr.
was performed using a 0.05 mM solution in CH3CN in a UV
Based on these results, it appears that the combination of a quartz cuvette. X-ray powder diffraction analyses were carried
stronger aurophilic interaction in the solid state of 4′ and the out with a Bruker D8 ADVANCE Powder X-ray diffractometer.
stronger trans-influence of the iodido ligand in 4 renders the 4-Benzyl-1-methyl-1,2,4-triazolium hexafluorophosphate was syn-
ligand redistribution energetically favorable upon crystalliza- thesized via a salt metathesis reaction of its chloride analogue
tion, while the absence of aurophilic interaction in solution and KPF6 in H2O.
explains the reversible behavior observed upon dissolving the
4-Benzyl-1-methyl-1,2,4-triazolium chloride (1). 1-Methyl-
crystalline sample. Nevertheless, it must be noted that our 1,2,4-triazole (415 mg, 5 mmol) and benzyl chloride (3 mL,
calculation can only address (i) electronic effects of the ligands 26 mmol) were mixed in a Schlenk flask. The reaction mixture
and (ii) solvent effects as two important factors contributing to was heated at 80 °C for 1 d. All the volatiles were removed
the observed ligand disproportionation upon crystallization. in vacuo, and the residue was washed with diethyl ether affording
The crystal lattice energy as the third major contributor, on the product as a hydroscopic white solid (912 mg, 4.35 mmol,
the other hand, can currently not be accounted for within the 87%). 1H NMR (500 MHz, CDCl3): δ 11.61 (s, 1 H, NC5HN),
scope of this study. Hence, this study provides a deeper insight 9.28 (s, 1 H, NC3HN), 7.60–7.58 (m, 2 H, Ar–H), 7.27–7.26 (m,
into this phenomenon, but an exact prediction of the process 3 H, Ar–H), 5.77 (s, 2 H, NCH2), 4.11 (s, 3 H, NCH3). 13C{1H}
is expectedly not possible.
NMR (125 MHz, CDCl3): δ 144.7 (NC5HN), 143.9 (NC3HN),
133.2, 130.2, 130.0, 129.9 (Ar–C), 52.2 (NCH2), 39.9 (NCH3). MS
(ESI): m/z 174 [M − Cl]+.
4-Benzyl-1-methyl-1,2,4-triazolium bromide. This com-
pound was synthesized in analogy to salt 1 but using benzyl
Conclusions
Four mono-NHC complexes of the type [AuX(tazy)] (X = Cl, Br, bromide as the alkylating agent. The product was isolated as
I, OAc) have been synthesized and fully characterized. Only the an off-white solid in a yield of 93%. 1H NMR (500 MHz,
iodido complex underwent a reversible ligand redistribution CDCl3): δ 11.32 (s, 1 H, NC5HN), 9.00 (s, 1 H, NC3HN),
upon crystallization. By careful analysis of crystallographic 7.61–7.59 (m, 2 H, Ar–H), 7.34–7.32 (m, 3 H, Ar–H), 5.79
data as well as NMR and UV-Vis spectroscopic data and com- (s, 2 H, NCH2), 4.16 (s, 3 H, NCH3). 13C{1H} NMR (125 MHz,
parison with spectra obtained from reference compound CDCl3): δ 144.3 (NC5HN), 143.7 (NC3HN), 132.8, 130.4, 130.2,
[Au(tazy)2]PF6 (6), it could be firmly established that the solid 130.0 (Ar–C), 52.5 (NCH2), 40.2 (NCH3). MS (ESI): m/z 174 [M − Br]+.
phase of the iodido complex exists exclusively as the dispropor-
Chlorido(4-benzyl-1-methyl-1,2,4-triazolin-5-ylidene)gold(I)
tionated material 4′, while in solution the neutral form 4 is (2). Triazolium salt 1 (210 mg, 1 mmol) and Ag2O (128 mg,
prevalent. Unlike previous reports of such redistributions, 0.55 mmol) were mixed in a 50 mL round bottom flask.
which occurred with unusually electron-rich NHC ligands, we CH2Cl2 (10 mL) was then added, and the reaction mixture was
used the weakly donating tazy ligand, thus ruling out a strong stirred in dark at ambient temperature for 3 h. The resulting
electron donor as requirement for such reactions. By contrast, suspension was then directly transferred into the solution of
a soft halido coligand with high trans-influence seems to be [AuCl(SC4H9)] (320 mg, 1 mmol) in CH2Cl2 (10 mL). The reac-
required for the reaction to proceed in this system, as evi- tion mixture was then stirred overnight. All insoluble material
denced by DFT calculations.
was filtered off through Celite, and the filtrate was dried
in vacuo affording the product as a white solid (316 mg,
0.78 mmol, 78%). H NMR (500 MHz, CDCl3): δ 7.97 (s, 1 H,
1
NCHN), 7.39–7.34 (m, 5H, Ar–H), 5.37 (s, 2 H, NCH2), 4.01 (s,
3 H, NCH3). 13C{1H} NMR (125 MHz, CDCl3): 174.8 (s, NCN),
142.5, 134.1, 130.1, 130.0, 129.0 (Ar–C), 53.4 (NCH2), 40.9
Experimental section
General considerations
Unless otherwise noted all operations were performed without (s, NCH3). Anal. Calcd for C10H11AuClN3: C, 29.61; H, 2.73;
taking precautions to exclude air and moisture. All solvents N, 10.36%. Found: C, 29.84; H, 2.69; N, 10.24%. MS (ESI): m/z
and chemicals were used as received without any further treat- 402 [M − Cl + CH3OH]+.
ment if not noted otherwise. [AuCl(SC4H8)] was synthesized
Bromido(4-benzyl-1-methyl-1,2,4-triazolin-5-ylidene)gold(I)
according to the reported procedure.22 1H, 13C, 19F and (3). Complex 2 (203 mg, 0.5 mmol) and LiBr (434 mg,
31P NMR spectra were recorded on a Bruker ACF 300 and 5 mmol) were mixed in a 50 mL round bottom flask. Acetone
Dalton Trans.
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(15 mL) was added, and the reaction mixture was stirred at (282 MHz, CDCl3): δ 3.30 (d, PF6). 31P{1H} NMR (121 MHz,
ambient temperature for 1 d. All the volatiles were removed CDCl3): δ −143.7 (m, PF6). Anal. Calcd for C20H22AuF6N6P:
in vacuo and CH2Cl2 (15 mL) was added to the residue. The C, 34.90; H, 3.22; N, 12.21. Found: C, 34.66; H, 3.09; N,
resulting suspension was filtered through Celite, and the fil- 12.16%. MS (ESI): m/z 543 [M − PF6]+.
trate was dried in vacuo. Column chromatography (SiO2)
Triiodido(4-benzyl-1-methyl-1,2,4-triazolin-5-ylidene) gold(III)
afforded the product as off-white powder (182 mg, 0.41 mmol, (7). Complex 4 (99 mg, 0.2 mmol) was dissolved in CH2Cl2
81%). 1H NMR (500 MHz, CDCl3): δ 7.93 (s, 1 H, NCHN), (10 mL) and elemental iodine (64 mg, 0.25 mmol) dissolved in
7.41–7.35 (m, 5 H, Ar–H), 5.39 (s, 2 H, NCH2), 4.04 (s, 3 H, CH2Cl2 (10 mL) was added dropwise at −30 °C. The reaction
NCH3). 13C{1H} NMR (125 MHz, CDCl3): δ 178.3 (NCN), 142.3, mixture was stirred at −30 °C for 30 min and at ambient temp-
134.0, 130.2, 130.1, 129.1 (Ar–C), 53.4 (NCH2), 40.9 (NCH3). erature for another 2 h. All the volatiles were removed. The
Anal. Calcd for C10H11AuBrN3·0.5CH2Cl2: C, 25.60; H, 2.46; residue was washed with n-hexane (5 mL × 3) and dried
N, 8.53%. Found: C, 25.64; H, 2.09; N, 8.79%. MS (ESI): m/z in vacuo affording the product as a dark-red solid (139 mg,
1
543 [M − Br + tazy]+.
0.186 mmol, 93%). H NMR (500 MHz, CDCl3): δ 7.93 (s, 1 H,
Iodido(4-benzyl-1-methyl-1,2,4-triazolin-5-ylidene)gold(I) (4). NCHN), 7.47–7.45 (m, 5 H, Ar–H), 5.34 (s, 2 H, NCH2), 4.02 (s,
Complex 2 (203 mg, 0.5 mmol) and NaI (375 mg, 2.5 mmol) 3H, NCH3). 13C{1H} NMR (75 MHz, CDCl3): δ 163.0 (NCN),
were mixed in a 50 mL round bottom flask. Acetone (15 mL) 143.7, 130.8, 130.5, 130.2 (Ar–C), 53.8 (NCH2), 41.1 (NCH3).
was added, and the reaction mixture was stirred at ambient Anal. Calcd for C10H11AuI3N3: C, 16.00; H, 1.48; N, 5.60.
temperature for 1 d. All the volatiles were removed in vacuo Found: C, 16.45; H, 1.47; N, 5.62%. MS (ESI): m/z 543 [M − 3I +
and CH2Cl2 (15 mL) was added to the residue. The resulting tazy]+, 797 [M − I + tazy]+.
suspension was filtered through Celite, and the filtrate was
X-ray diffraction studies
dried in vacuo affording the product as an off-white solid
(218 mg, 0.44 mmol, 88%). H NMR (300 MHz, CDCl3): δ 7.91
1
X-ray data for 2, 3, 4, 5·H2O, 6 and 7 were collected with a
Bruker AXS SMART APEX diffractometer, using Mo-Kα radi-
ation at 100(2)K with the SMART suite of Programs.23 Data
were processed and corrected for Lorentz and polarisation
effects with SAINT,24 and for absorption effect with SADABS.25
Structural solution and refinement were carried out with the
SHELXTL suite of programs.26 The structure was solved by
direct methods to locate the heavy atoms, followed by differ-
ence maps for the light, non-hydrogen atoms. All non-hydro-
gen atoms were generally given anisotropic displacement
parameters in the final model. All H-atoms were put at calcu-
lated positions.
(s, 1 H, NCHN), 7.42–7.35 (m, 5 H, Ar–H), 5.40 (s, 2 H, NCH2),
4.05 (s, 3 H, NCH3). 13C{1H} NMR (75 MHz, CDCl3): 184.8
(NCN), 142.2, 134.1, 130.14, 130.06, 129.1 (Ar–C), 53.3 (NCH2),
40.8 (NCH3). Anal. Calcd for C10H11AuIN3: C, 24.16; H, 2.23;
N, 8.45%. Found: C, 24.28; H, 2.25; N, 8.40%. MS (ESI): m/z
543 [M − I + tazy]+.
Acetato(4-benzyl-1-methyl-1,2,4-triazolin-5-ylidene)gold(I) (5).
Complex 2 (203 mg, 0.5 mmol) and AgO2CCH3 (92 mg,
0.55 mmol) were mixed in a 50 mL round bottom flask.
CH2Cl2 (10 mL) was then added, and the reaction mixture was
stirred in dark at ambient temperature for 1 h. The resulting
suspension was filtered through Celite, and the filtrate was
dried in vacuo affording the product as a white solid (152 mg,
1
0.35 mmol, 71%). H NMR (300 MHz, CDCl3): δ 8.03 (s, 1 H,
Computational methods
NCHN), 7.35–7.32 (m, 5 H, Ar–H), 5.34 (s, 2 H, NCH2), 3.96
(s, 3 H, NCH3), 2.00 (s, 3 H, CH3COO). 13C{1H} NMR (75 MHz,
CDCl3): δ 177.9 (CH3COO), 168.0 (NCN), 142.6, 134.2, 130.0,
129.8, 129.1 (Ar–C), 53.3 (NCH2), 40.8 (NCH3), 24.3 (CH3COO).
Anal. Calcd for C12H14AuN3O2: C, 33.58; H, 3.29; N, 9.79.
Found: C, 33.25; H, 3.18; N, 9.77. MS (ESI): m/z 543
[M − CH3COO + tazy]+.
All calculations were carried out using hybrid density func-
tional theory (DFT) employing the B3LYP functional27 and the
Gaussian 09 software,28 using the IEFPCM approach to
account for solvatation in dichloromethane.19 The nature of all
stationary points was confirmed by frequency analysis, and all
geometries were found to represent minima on the potential
energy surface. For the optimisation of complex geometries
and the calculation of their Gibbs free energies, gold and
iodine were described with a cc-pVDZ-PP basis set in combi-
nation with the corresponding electronic core potential
(ECP),29 while the lighter elements were treated with the cc-
pVDZ basis set.30 The basis sets were all obtained from the
EMSL Basis Set Library.31
Bis(4-benzyl-1-methyl-1,2,4-triazolin-5-ylidene)gold(I) hexa-
fluoro-phosphate (6). Complex 5 (86 mg, 0.2 mmol) and
4-benzyl-1-methyl-1,2,4-triazolium hexafluorophosphate (64 mg,
0.2 mmol) were mixed in a 50 mL round bottom flask. CH2Cl2
(10 mL) was then added, and the reaction mixture was stirred
at ambient temperature overnight. The resulting solution was
concentrated and n-hexane was added. The precipitates were
collected and dried in vacuo affording the product as a white
1
solid (121 mg, 0.176 mmol, 88%). H NMR (500 MHz, CDCl3):
δ 8.13 (s, 2 H, NCHN), 7.35–7.33, 7.25–7.23 (m, 10 H, Ar–H),
Acknowledgements
5.34 (s, 4 H, NCH2), 3.99 (s, 3 H, NCH3). 13C{1H} NMR
(125 MHz, CDCl3): δ 186.2 (NCN), 143.7, 135.1, 130.0, 129.7, We thank the National University of Singapore and the
128.7 (Ar–C), 54.1 (NCH2), 41.2 (NCH3). 19F{1H} NMR Singapore Ministry of Education for financial support (WBS
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Prof. Lip Lin Koh for determining the X-ray molecular
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