Coordination chemistry of gold catalysts in solution: A detailed NMR study

Article abstract of DOI:10.1002/chem.201201215

Coordination chemistry of gold catalysts bearing eight different ligands [L=PPh₃, JohnPhos (L2), Xphos (L3), DTBP, IMes, IPr, dppf, S-tolBINAP (L8)] has been studied by NMR spectroscopy in solution at room temperature. Cationic or neutral mononuclear complexes LAuX (L=L2, L3, IMes, IPr; X=charged or neutral ligand) underwent simple ligand exchange without giving any higher coordinate complexes. For L2AuX the following ligand strength series was determined: MeOHahex-3-yne -aMe ₂S<2,6-lutidine<4-picoline3CO₂ ^-≈DMAP3-≈Cl ^-. Some heteroligand complexes DTBPAuX exist in solution in equilibrium with the corresponding symmetrical species. Binuclear complexes dppf(AuOTf)₂ and S-tolBINAP(AuOTf)₂ showed different behavior in exchange reactions with ligands depending on the ligand strength. Thus, PPh₃ causes abstraction of one gold atom to give mononuclear complexes LLAuPPh₃⁺ and (Ph₃P) _nAu⁺, but other N and S ligands give ordinary dicationic species LL(AuNu)₂²⁺. In reactions with different bases, LAu⁺ provided new oxonium ions whose chemistry was also studied: (DTBPAu)₃O⁺, (L2Au)₂OH⁺, (L2Au) ₃O⁺, (L3Au)₂OH⁺, and (IMesAu) ₂OH⁺. Ultimately, formation of gold hydroxide LAuOH (L=L2, L3, IMes) was studied. Ligand- or base-assisted interconversions between (L2Au)₂OH⁺, (L2Au)₃O⁺, and L2AuOH are described. Reactions of dppf(AuOTf)₂ and S-tolBINAP(AuOTf) ₂ with bases provided more interesting oxonium ions, whose molecular composition was found to be [dppf(Au)₂]₃O₂ ²⁺, L8(Au)₂OH⁺, and [L8(Au)₂] ₃O₂²⁺, but their exact structure was not established. Several reactions between different oxonium species were conducted to observe mixed heteroligand oxonium species. Reaction of L2AuNCMe⁺ with S^2- was studied; several new complexes with sulfide are described. For many reversible reactions the corresponding equilibrium constants were determined. Copyright

Full text of DOI:10.1002/chem.201201215

FULL PAPER
DOI: 10.1002/chem.201201215
Coordination Chemistry of Gold Catalysts in Solution: A Detailed
NMR Study
Alexander Zhdanko,[a] Markus Strçbele,[b] and Martin E. Maier*[a]
Abstract: Coordination chemistry of
gold catalysts bearing eight different li-
gands [L=PPh3, JohnPhos (L2), Xphos
(L3), DTBP, IMes, IPr, dppf, S-tolBI-
NAP (L8)] has been studied by NMR
spectroscopy in solution at room tem-
perature. Cationic or neutral mononu-
clear complexes LAuX (L=L2, L3,
IMes, IPr; X=charged or neutral
ligand) underwent simple ligand ex-
change without giving any higher coor-
dinate complexes. For L2AuX the fol-
lowing ligand strength series was deter-
mined: MeOH !hex-3-yne <MeCNꢀ
OTfꢁ !Me2S<2,6-lutidine<4-pico-
line<CF3CO2ꢁ ꢀDMAP<TMTU<
plexes dppf
A
G
LAuOH (L=L2, L3, IMes) was stud-
ied. Ligand- or base-assisted intercon-
exchange reactions with ligands de-
pending on the ligand strength. Thus,
PPh3 causes abstraction of one gold
atom to give mononuclear complexes
versions
(L2Au)3O+, and L2AuOH are descri-
bed. Reactions of dppf(AuOTf)2 and S-
tolBINAP(AuOTf)2 with bases provid-
between
(L2Au)2OH+,
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
+
LLAuPPh3 and (Ph3P)nAu+, but other
ed more interesting oxonium ions,
whose molecular composition was
N and S ligands give ordinary dication-
2+
ic species LL
found
to
be
[dppf(Au)2]3O2 ,
with different bases, LAu+ provided
new oxonium ions whose chemistry
was also studied: (DTBPAu)3O+,
L8(Au)2OH+, and [L8(Au)2]3O22+, but
their exact structure was not establish-
ed. Several reactions between different
oxonium species were conducted to ob-
(L2Au)2OH+,
(L3Au)2OH+, and (IMesAu)2OH+. Ul-
serve mixed heteroACTHNGUTRENlUNG iACHTUNGTRENNgUNG and oxonium spe-
timately, formation of gold hydroxide
cies. Reaction of L2AuNCMe+ with
S2ꢁ was studied; several new complexes
with sulfide are described. For many
reversible reactions the corresponding
equilibrium constants were determined.
PPh3 <OHꢁ ꢀClꢁ. Some hetero
ACHUTGTNRENNUGliACTHNGUTREgNNUG and
Keywords: gold · ligand effects ·
ligand exchange · NMR spectrosco-
py · oxonium ions
complexes DTBPAuX exist in solution
in equilibrium with the corresponding
symmetrical species. Binuclear com-
Introduction
zation of the gold intermediates. Often, instead of experi-
mental elucidation of mechanisms, computational investiga-
tions are conducted to support the mechanism.[3]
Gold catalysis has emerged as an extensive area of research
that continues to evolve very rapidly today.[1] However,
while the arsenal of synthetic methods available to an or-
ganic chemist benefited enormously from gold catalysis,
knowledge about the mechanism of the corresponding trans-
formations remains more limited, and this area has just
begun to emerge as a major topic in gold catalysis.[2] Indeed,
most papers dealing with gold-catalyzed reactions confine
themselves to “proposed” mechanisms, without going into
deep experimental mechanistic investigation and characteri-
Although general coordination chemistry of gold as a sub-
ject of inorganic chemistry has been investigated with a
broad range of ligands, the coordination chemistry of gold
compounds that are used nowadays as catalysts in organic
transformations is largely unexplored.[4] This actually is not
too surprising, since many of them appeared after 2000.[5]
Here we present our pure coordination-chemistry investiga-
tions, which were conducted in parallel with our mechanistic
studies.
The present paper describes reactions of gold complexes
of the types LAuX and LAuNu+, where L is a strong, base
or parent ligand, respectively, X is a negatively charged
group in neutral complexes and Nu is a neutral weaker
ligand in cationic complexes.[6] Abbreviations, widely adopt-
ed in the text, are clear from the starting materials depicted
in Figure 1. Almost all reactions were conducted in deuter-
ated solvents (CDCl3, C6D6, [D8]THF (used pure or as a
15 vol% mixture with simple THF), CD2Cl2, MeOD) and
[a] Dipl.-Chem. A. Zhdanko, Prof. Dr. M. E. Maier
Institut fꢀr Organische Chemie
Universitꢁt Tꢀbingen
Auf der Morgenstelle 18, 72076 Tꢀbingen (Germany)
Fax : (+49)7071-295137
E-mail: martin.e.maier@uni-tuebingen.de
[b] Dr. M. Strçbele
Institut fꢀr Anorganische Chemie
Universitꢁt Tꢀbingen
Auf der Morgenstelle 18, 72076 Tꢀbingen (Germany)
Fax : (+49)7071-295702
1
were directly monitored by H and 31P NMR spectroscopy at
room temperature. Most of the reactions were conducted as
NMR titrations, which allowed us to get the necessary infor-
mation from changes observed in the spectra. We classify all
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201215.
Chem. Eur. J. 2012, 00, 0 – 0
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
1
&
ÞÞ
These are not the final page numbers!

Figure 1. Structures of gold complexes together with ligand abbreviations
used in the text.
homoACHTUNGTRENNUNGliACHTUNGTRENNUNG
gand exchanges LAuNu+/Nu as “fast” if Nu gives a
single set of broadened signals, and as “slow” if bound and
free Nu give clear or broadened separate signals. Complexes
1 and 4–8 were prepared by reaction of the corresponding
chlorides with silver triflate.
Results and Discussion
Figure 2. Equations (1)–(23) for the reactions of gold(I) complexes with
nucleophiles; Lut=2,6-lutidine, Pic=4-picoline, DMAP=4-dimethylami-
nopyridine, TMTU=tetramethylthiourea.
Simple ligand exchange chemistry of gold catalysts: It has
long been known that Ph3PAu + (with weakly or noncoordi-
nating anions) reacts with PPh3 to provide mixtures of
higher coordinate complexes (Ph3P)nAu+ (n=2–4). Howev-
er, individual components of the equilibria [Figure 2,
Eqs. (2), (3)] could not be observed by NMR spectroscopy
at room temperature because of fast exchange. The system
simply gives one broadened phosphorus resonance averaged
for all species.[7] We reproduced this situation and describe it
with detailed spectra in the Supporting Information. Since
many of Ph3PAu-containing complexes had been previously
described, we largely focused on new species derived from
other ligands. The following known species were considered
of relevance for this study: Ph3PAuNu+ (Nu=Me2S,[8] 2,6-lu-
tidine,[9] 2,6-di-tert-butylpyridine, NMe3,[10] PPh3), Ph3PAuN-
MeCN gave a single broad resonance, indicating fast ligand
exchange in the L2AuNCMe+/MeCN system. After the
equivalence point, MeCN became completely free, giving
1
the normal sharp signal at 2.00 ppm in the H NMR spec-
trum. For example, in the reaction of 2 with PPh3 the corre-
+
sponding hetero
G
G
sole product, giving two doublets in the 31P NMR spectrum
ꢁ
due to P P coupling. As a characteristic feature of the
1H NMR spectrum, the protons at the 3’,5’- and 4’-positions
of the biphenyl ring are significantly shifted upfield, presum-
ably due to the influence of the inductive currents in the tri-
phenylphosphine rings situated in close proximity. Beyond
the equivalence point gradual substitution of the L2 ligand
started to occur. Experimentally we found also that this
process can be better described as Equation (10) in Figure 2,
with an equilibrium constant of approximately 0.15 Lmolꢁ1.
This demonstrates much higher preference of L2 versus the
PPh3 ligand for gold. Even 17.5 equiv PPh3 were able to sub-
ACHTUNGTRENNUNG
ꢁ
(2) with different ligands: MeOH, hex-3-yne, Me2S, 2,6-luti-
dine, 4-picoline, DMAP, TMTU, PPh3 and Cy3P in CDCl3.
All ligands except MeOH and hex-3-yne stoichiometrically
substituted acetonitrile from 2 to give 1:1 products whose
ꢁ
composition was established as L2AuNu +SbF6 [Figure 2,
+
Eq. (4)], and in all these cases, before the equivalence point,
stitute only one-half of L2. Formation of stable L2AuPPh3
&
2
&
www.chemeurj.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Coordination Chemistry of Gold Catalysts in Solution
FULL PAPER
hetero
li
E
Since gold catalysts are often prepared and used as tri-
flates or trifluoroacetates, it was of interest to compare their
binding abilities. For this purpose we prepared L2AuOTf
and L2AuOTFA and found that OTfꢁ as a ligand is bound
to gold slightly more strongly than MeCN [Figure 2,
Eq. (13)], but more weakly than Me2S. The OTFA salt was
found to be quite strong, comparable to the complex with
DMAP. This finding directly correlates with efficacy of cat-
alysis: while triflates still provide fast reactions, the trifluor-
oacetates are in fact very sluggish catalysts. However, al-
though binding of a catalyst to a substrate is important, it is
not the only factor determining the efficacy of gold catalysis.
Similar studies using Me2S, 2,6-lutidine, 4-picoline,
DMAP, TMTU, and PPh3 in CDCl3 were conducted with
L3AuNTf2 (3), which displayed exactly the same properties
HRMS analysis, while the presence of higher coordinate
L2AuACHTUNGTRENNUNG(PPh3)2 could not be confirmed. The absence of
L2Au(PPh3)2 was also evident from the NMR spectra.
ACHTUNGTRENNUNG
Thus, if any appreciable amount thereof was formed in solu-
tion, it would either give new signals or shift the
L2AuPPh3 signals due to the dynamic equilibrium that
would be established. However, even in the presence of
such a large excess of PPh3, the resonances of L2AuPPh3
broadened only slightly and appeared at constant chemical
shift, which also points to a “slow” mode of exchange in the
L2AuPPh3+/PPh3 system. The significant kinetic stability of
the L2AuPPh3 complex is in sharp contrast to the behavior
of the corresponding triphenylphosphine analogue
(Ph3P)2Au+ and must be a consequence of increased steric
hindrance of the L2 ligand. If liberated, the free L2 ligand
also gave a sharp constant resonance, while an excess of
PPh3 gave a broadened resonance due to fast exchange with
the partially liberated gold in the classical (Ph3P)nAu +/Ph3P
dynamic system [Figure 2, Eqs. (2) and (3)].
[Figure 2, Eq. (5)]. The corresponding heteroACTHUNTGRNENUGliACHUTNGTERNNUGgand complex
+
L3AuPPh3 was formed as a sole product with triphenyl-
phosphine, and beyond the equivalence point gradual substi-
tution of the L3 ligand started to occur. As in the case of
L2, L3 proved to be a much better ligand for gold than
PPh3, comparable to L2, but the equilibrium constant could
not be found due to signal overlaps. With Nu=Me2S, Lut,
Pic, DMAP, and TMTU the corresponding complexes
L3AuNu + were formed, which revealed fast L3AuNu +/Nu
ligand exchange for Nu=Me2S, DMAP, and TMTU and
slow ligand exchange for Nu=Lut and Pic. Ligand exchange
in L3AuNu +/Nu was always slower than in L2AuNu +/Nu,
as was evident from the line shapes, and this can be regard-
ed as a consequence of increased steric hindrance of the L3
ligand in comparison to L2.
Similarly, in the reaction with Cy3P, formation of the cor-
responding L2AuPCy3 occurred, which in the presence of
excess Cy3P readily underwent substitution of L2 from gold
to give (Cy3P)2Au+ [Eq. (24)].
Other L2AuNu + complexes with Nu=Me2S, Lut, Pic,
DMAP, and TMTU where readily obtained by reaction of 2
with the corresponding ligand; they all were completely
stable in the presence of excess of ligand, giving neither
higher coordinate complexes nor liberating the original
phosphine ligand. For Nu=Me2S, Lut, Pic, DMAP, and
TMTU, fast ligand exchange between L2AuNu + and Nu
was observed, but for Nu=Lut, remarkably, both free and
bound forms gave separate clear resonances indicating a
“slow” mode of ligand exchange, which is probably a conse-
quence of increased steric bulk of this ligand.
Reactions of 2 with MeOH and hex-3-yne in CDCl3 did
not reveal stoichiometric substitution of MeCN, that is, the
resulting complexes would be weaker than the starting one.
We determined that MeOH is a weaker ligand than hex-3-
yne, and both are weaker than MeCN [Figure 2, Eqs. (11)
and (12)]. The complex with methanol is so weak that only
in pure MeOD solution (ꢀ7000 equiv) does it exist as pre-
dominant form over acetonitrile. Complexes of 2 with sever-
al alkynes and alkenes were recently studied by Widenhoe-
fer et al., and hex-3-yne exhibited the strongest binding
among a range of several alkynes and alkenes.[13] Therefore,
it can be taken as a general rule for gold catalysis that bind-
Then we studied reactions of DTBPAuOTf (4) having a
phosphite ligand, which revealed several unexpected differ-
ences from the chemistry described above. First, 4 was
found to give a heteroACTHNUGRTENNUGliACHUTGTNRENNUG
gand complex DTBPAuPPh3+, which
does not exist in individual state but undergoes reversible
ligand metathesis to (Ph3P)2Au+ and (DTBP)2Au+, so that
all three species are simultaneously observed in solution.
The equilibrium constant was estimated to be Keq ꢀ0.06
[Figure 2, Eq. (19)]. On further addition of PPh3, DTBP is
completely displaced from gold to give (Ph3P)2Au+ as a sole
product. During this process higher coordinate species
+
+
(DTBP)2AuPPh3 and DTBPAu
ACHTUNGTREN(NNUG PPh3)2 were not detected.
The homoli
A
gand complex (DTBP)2Au + was generated in
the reaction between 4 and DTBP and revealed slow ligand
exchange in the presence of excess DTBP. Here again, no
higher coordinate (DTBP)nAu + (n>2) complexes were de-
tected. One of the phosphite ligands in (DTBP)2Au+ is
bound rather weakly: it is already substituted by 2,6-lutidine
with an equilibrium constant Keq ꢀ4 [Figure 2, Eq. (18)]. We
believe that such weak binding is a consequence of both
high steric hindrance and weak donor ability of the DTBP
ligand. Rather in contrast with the previous chemistry, com-
plex 4 reacts with Me2S and TMTU to give no clear picture
ing of a cationic gold catalyst to a C=C or C C bond of a
substrate would be generally weaker than with acetonitrile
by a factor of 10–100. This provides a ready explanation
why acetonitrile is often not a preferable solvent for a
gold(I)-catalyzed reaction: obviously, it would seriously
compete with an alkyne substrate for the gold center, slow-
ing down the overall process.[14]
of what is really happening in solution. In the case of Me2S
+
a
complex dynamic mixture of DTBPAuSMe2
,
(DTBP)2Au+, and (Me2S)n=1,2Au+ is formed due to ligand
exchange [Figure 2, Eq. (8)]. In contrast, lutidine reacted
Chem. Eur. J. 2012, 00, 0 – 0
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
3
&
ÞÞ
These are not the final page numbers!

M. E. Maier et al.
cleanly to give DTBPAuLut+ as a single product undergoing
fast DTBPAuLut+/Lut exchange [Figure 2, Eq. (9)].
might be described as a chelate or fast jumping of gold be-
tween the two phosphorus atoms.
Next, IMesAuOTf (5) was treated with PPh3 to give the
expected IMesAuPPh3+, which beyond the equivalence
point displayed fast exchange with PPh3. No other com-
pounds were formed, that is, IMesAuPPh3+/PPh3 continues
to be a simple binary system with PPh3 exchange as the only
process, which is not surprising, since N-heterocyclic carbene
(NHC) cannot be easily substituted by a phosphine. With
other ligands Nu=Lut, Pic the corresponding complexes
IMesAuNu+ were formed, and both revealed slow IMesAu-
Nu+/Nu ligand exchange [Figure 2, Eq. (6)]. Another similar
NHC carbene complex, namely, IPrAuOTf (6), displayed ex-
actly the same chemistry as 5. Ligand exchange in the IMe-
sAuNu+/Nu system was found to be fast only for Nu=Me2S
and slow for other cases [Nu=Lut, Pic, DMAP, TMTU,
PPh3; Figure 2, Eq. (7)].
In contrast to PPh3, reactions with weaker N and S ligands
revealed formation of the corresponding dicationic com-
2+
plexes dppf
(AuTMTU)22+, dppf
N
(AuLut)2
,
,
2+
2+
2+
L8ACTHNUGTRENUN(NG AuPic)2 , L8ACHTUNRTGE(NGNUN AuDMAP)2 , and L8CAHUTGTNRENU(NG AuTMTU)2
without any sign of gold abstraction [Figure 2, Eq. (23)]. In-
terestingly, the complexes with TMTU were slightly reactive
in the presence of excess TMTU, and this suggests a further
reversible unidentified process but with the equilibrium
shifted to the left.
As a result of this study, the following ligand strength
series was established, with some key Keq values indicated
(Scheme 1). Conveniently, the ligands before OTfꢁ can be
Further, we studied ligand-exchange chemistry of two bi-
nuclear complexes of gold. For example, when dppfACHTNUTRGNEUNG(AuCl)2
was titrated with PPh3, the system displayed dynamic behav-
ior with single sets of broad signals for dppf and PPh3, but
at some point sharp signals for the dppf unit were observed,
together with broad signals for PPh3 (in both 1H and
31P NMR spectra), indicating formation of a single dppf
complex. This can be either dppfAuCl or [dppfAuPPh3]+
Clꢁ. Complex dppfAuCl was previously described and its
spectra did not match those observed by as, so we suppose
[dppfAuPPh3]+Clꢁ could be formed in our case. The com-
plex [dppfAuPPh3]+ was previously described as the per-
chlorate salt and its spectra matched those observed by us
pretty well.[15]
Scheme 1. Ligand strength series for cationic gold complexes with some
key Keq values.
regarded as weak, from Me2S till TMTU as moderately
strong and beyond PPh3 very strong. The range of binding
affinities from MeOH till DMAP spans approximately ten
orders of magnitude. During further research we were able
to determine indirectly the equilibrium constant for DMAP/
TMTU exchange at Ph3PAu +, which appears to be 10 in
favor of TMTU. Unfortunately, it was not possible to quan-
titatively determine the difference in binding affinities for
TMTU/PPh3, but we suppose this may reach several (2–4)
orders of magnitude.
When dppfACHTUNGTRENNUNG(AuOTf)2 (7) was titrated with PPh3, the early-
stage spectra were complicated, indicating formation of a
multicomponent mixture. However already at three equiva-
lents of PPh3, sharp signals for the dppf unit arose, together
1
with broad signals for PPh3 (in both H and 31P NMR spec-
tra), indicating formation of a single dppf complex. Similar
properties were displayed by another binuclear complex,
namely, L8ACHTUNGTRENNUNG(AuOTf)2 (8). The observed behavior was clear
from the NMR spectra and additionally supported by ESI
MS. The observations were consistent with complete ab-
Reaction of gold catalysts with bases: formation of LAuOH
and oxonium species and their transformations: It has long
been known that Ph3PAu + reacts with base and water to
give triaurated oxonium salts (Ph3PAu)3O+ (9) stable in sol-
ution and in the solid state.[12] Indeed, when proton sponge
straction of one gold atom from the initial complex and for-
+
mation of mononuclear complexes LLAuPPh3
and
+
(Ph3P)nAu+ [Figure 2, Eq. (22)]. In case of dppfAuPPh3
the spectra matched those reported in the literature.[15] All
sharp signals (in both 1H and 31P NMR spectra) were as-
signed to the LL unit of LLAuPPh3+, and broad, time-aver-
aged signals were assigned to a PPh3 unit which experiences
fast exchange between all the species. This also causes the
phosphorus resonance of LL to appear as a singlet rather
than a doublet. Formation of free LL was not observed with
an excess of PPh3, that is, while abstraction of one gold
cation from the initial dicationic complex was easy, abstrac-
tion of the second gold ion is rather difficult. This might be
explained by a cooperative action of both phosphorus atoms
rather than the nature of each of them. Such an interaction
(PrSp) was added to
a freshly prepared solution of
Ph3PAuOTf (1) in CDCl3, a fast and quantitative reaction
with traces of water (naturally contained in CDCl3) occurred
to give (Ph3PAu)3O+ as the sole product [Scheme 2,
Eq. (25)]. Before the equivalence point, 31P NMR showed a
single broadened resonance indicating fast exchange in the
Ph3PAuOTf/ACTHNUTRGNEUNG
(Ph3PAu)3O+ system. In sharp contrast, 2 react-
ed with proton sponge to give diaurated oxonium ion
(L2Au)2OH+ (10) as a single product together with the
proton sponge salt (PrSpH+) [Scheme 2, Eq. (26)]. Its
proton spectrum exhibits
a
characteristic triplet at
ꢁ0.42 ppm (3JH-P =1.7 Hz) corresponding to a single OH
proton. The stoichiometry of the whole reaction was deter-
&
4
&
www.chemeurj.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Coordination Chemistry of Gold Catalysts in Solution
FULL PAPER
The cation (L2Au)2OH+ (10) can also be generated with
other bases. For example, even 2,6-di-tert-butylpyridine
reacts with 2 to give 10, but the reaction is reversible: two
equivalents of the base are able to transform only half of
the starting complex. Solid LiOH, and even simple water ex-
traction, trigger quantitative formation of 10, but when a
solution of 2 in CDCl3 is treated with 10% aqueous KOH,
fast reaction occurs to give L2AuOH 15 as sole product
[Scheme 3, Eq. (28)]. Its proton spectrum exhibits a charac-
teristic broad singlet at ꢁ0.73 to ꢁ0.76 ppm corresponding
to the OH proton. The exact chemical shift of the proton is
slightly affected by the presence of water, which forms
strong hydrogen bonds and causes exchange of the protons,
so that no coupling to phosphorus is observable.[17] Indeed,
in an extra-dry solution of 15 in C6D6 (above solid KOH)
the OH proton could be observed as
a doublet at
ꢁ0.20 ppm (3JH-P =4.8 Hz), and the doublet was also nicely
observed in the NMR spectrum of 15 in C6D6 layered above
saturated aqueous KOH solution.
Complex L2AuOH (15) is an example of a gold hydrox-
ide, a new class of gold complexes. Currently, only a single
Scheme 2. Reactions of gold complexes with proton sponge.
mined by integration, which showed that 1 mol of 10 is gen-
erated together with 1 mol of proton sponge salt. Observa-
tion of a single resonance in the 31P NMR and a triplet for
OH suggests a symmetrical structure for this cation with two
ꢁ
equal Au O interactions, possibly stabilized by a single au-
rophilic interaction. Other diaurated oxonium ions
(LAu)2OH+ were obtained also with ligands L=L3 (11),
IMes (12), IPr (13) as sole products of reactions of 3, 5, and
6
with proton sponge according to Equation (26a) in
Scheme 2. However, DTBPAuOTf (4) reacted with proton
sponge to give triaurated oxonium ion (DTBPAu)3O+ (14)
[Scheme 2, Eq. (27)]. Proof of the molecular composition of
oxonium ions obtained by these reactions comes not only
from the presence or absence of OH protons, but simply
from reaction stoichiometry, easily established by integra-
tion of relevant peaks in their spectra. (IPrAu)2OH+ (13)
was previously described by Nolan et al. in 2010 and until
now was the only known example of diaurated oxonium
ions.[16] According to Nolan et al., the X-ray structure of 13
contained Au···Au interactions somewhat longer than it
would be expected for aurophilic interactions. Therefore, it
is doubtful whether they contribute to the stability of this
type of structures in our case.
Scheme 3. Formation of gold hydroxide and its transformations.
Chem. Eur. J. 2012, 00, 0 – 0
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
5
&
ÞÞ
These are not the final page numbers!

M. E. Maier et al.
example is known in the literature. In 2010 Nolan described
IPrAuOH and studied the chemistry of this compound,
which exhibited strongly basic properties.[2e,18] In complete
accordance with expectations, L2AuOH appeared to be a
strong base as well. It readily reacted with dimethyl malo-
chanical stimulation. Thus, when a benzene solution of the
starting gold complex was ground in a small mortar with
solid KOH followed by simple filtration through Celite, a
solution of about 95% pure product was obtained
[Eq. (39)]. We were even more surprised when we found
that this reaction occurs within 5 min at room temperature
upon shaking a benzene solution of L2AuCl with concen-
trated aqueous KOH. It appears that OHꢁ can displace Clꢁ
from gold even in solution, which was not previously recog-
nized. In the same way, IPrAuOH and IMesAuOH were
prepared as well. However the corresponding chlorides re-
acted more sluggishly than L2AuCl. Notably, neither of
these methods was successful for the preparation of
Ph3PAuOH, whose existence has so far not been confirmed.
nate to give L2AuCHACHTNUGTRNEUGN(CO2Me)2 [Scheme3, Eq. (32)]. In
CDCl3 solution L2AuOH slowly reacts with the solvent
(over many hours at room temperature), but the reaction
can be strongly accelerated by PPh3 [Scheme3, Eqs. (33a)
and (33b)]. Thus, when a catalytic amount of PPh3 (ꢀ1–
5%) was added to a solution of L2AuOH in CDCl3 and the
solution concentrated in vacuo, the residue already con-
tained only the products of the reaction. Normally, a mix-
ture of L2AuCl and L2AuCCl3 (16) is formed. The chloride
ion in L2AuCl must come from CDCl3 [Scheme3, Eq. (37)].
We found that the ratio of the products can be controlled by
temperature. Thus, when evaporation of the reaction mix-
ture was conducted below room temperature, 16 could be
obtained as a main product (ꢀ95%), only slightly contami-
nated with L2AuCl, while formation of L2AuCl is favored
at higher temperature. Interestingly, 16 has exactly the same
phosphorus chemical shift as 10 and L2AuDMAP+
(57.43 ppm) in CDCl3 solution, but the proton spectra differ
sufficiently to allow differentiation. Unfortunately, a 13C
spectrum did not prove the presence of the CCl3 residue.
However the structure was unambiguously established by X-
ray analysis (Scheme 3). The catalytic action of PPh3 in this
reaction is explained by its enhancing dissociation of
L2AuOH, to provide OHꢁ, which is very reactive in organic
solution due to lack of solvation and should deprotonate
chloroform more readily [Scheme3, Eq. (34) and (35)]. Con-
trol experiments in C6D6 revealed that PPh3 is unable to sto-
ichiometrically displace OHꢁ from gold (even in excess) and
the reaction is thus highly reversible [Scheme3, Eq. (34)].
Another test performed on 15 with TMTU indicated no sign
of any exchange even if TMTU was present in excess.
Rather, prolonged warming of the reaction mixture caused
hydrolysis of TMTU to give (Me2N)2CO and some com-
plexes with sulfur. These results might seem surprising,
given the general low oxophilicity of gold, but on the other
hand OHꢁ is a highly nucleophilic ion whose elimination
from a covalent compound like LAuOH is disfavored in the
absence of good OHꢁ acceptors. Obviously, the OH of gold
hydroxide has strong desire to interact with more suitable
electrophiles, (e.g., with a proton). In other words, LAuOH
can be viewed as soluble covalent organic analogue of
KOH.
We further studied the chemistry of (L2Au)2OH+ (10)
and L2AuOH (15). When a solution of 10, generated by re-
action of 2 with PrSp, was treated with another nucleophile
(Nu), quantitative formation of L2AuNu + occurred
[Eq. (40)]. Accordingly, PrSp is completely restored from
the salt. This transformation is triggered already by Me2S,
which demonstrates high reactivity of 10 in the presence of
a proton donor. In the absence of a proton donor, however,
other pathways are followed (see below).
With regard to chemoselectivity, an interesting subject
would be the reaction with simple amines, as they can act
both as bases and ligands. To check this possibility, we con-
ducted NMR titration of 2 with triethylamine. An interest-
ing situation occurred before the equivalence point:
L2AuNEt3+, 10, and the remaining 2 were all simultaneous-
ly observed, together with the corresponding amount of
Et3NH+. Closer to the equivalence point, the amount of
+
(L2Au)2OH+ (10) and 2 decreased, while L2AuNEt3 was
formed as the major product. This situation can be described
by Equations (41) and (42). Traces of (L2Au)2OH+ were
still detectable in the presence of 1.5 equivalents of Et3N.
This experiment demonstrated that initially some competi-
tion took place, but with increasing amount of Et3N it react-
ed preferentially as a ligand and not a base.[20] When TMTU
was added to this solution, complete substitution of Et3N oc-
curred to give the L2AuTMTU+ complex. In contrast, Lut,
Pic, and DMAP reacted with 2 exclusively as ligands and
not as bases, regardless of the molar ratios, as was described
above [Figure2, Eq. (4)].
Since gold hydroxides are promising catalyst precursors
activated by protonation and useful reagents for the synthe-
sis of various organogold compounds,[19] we turned our at-
tention towards their improved synthesis. Thus, the previ-
ously described example IPrAuOH was synthesized by heat-
ing a solution of IPrAuCl in THF/toluene with solid KOH
for 24 h, which is a rather long and harsh procedure but nev-
ertheless gave the product in high yield. We were pleased to
find that the analogous reaction of L2AuCl with solid KOH
occurred in less than 1 min at room temperature under me-
&
6
&
www.chemeurj.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Coordination Chemistry of Gold Catalysts in Solution
FULL PAPER
Then we titrated a freshly prepared (Ph3PAu)3O+ (9)/
mole of gold increases for the following syntheses:
(LAu)2OH+ <(LAu)3O+ <LAuOH. Hence, we concluded
that the new species intermediately arising and disappearing
on the way from (L2Au)2OH+ (10) to L2AuOH (15) must
be (L2Au)3O+ (17). We automatically concluded that 10
and 15 react with each other to give (L2Au)3O+ (17) ac-
cording to Equation (45) in Scheme 4. This process was
found to be reversible, because 17 was often observed in
mixture simultaneously with both 10 and 15 and not just
with one of them or pure. Accordingly, brief appearance of
17 during the synthesis of L2AuOH by Equation (28) in
Scheme3 corresponds to incomplete reaction when a mix-
ture of 10 and 15 arises [Scheme3, Eqs. (29)–(31)]. Howev-
er, only in THF was (L2Au)3O+ observed free of 10 and 15.
It remains unclear if this is due to better stability of
(L2Au)3O+ in this solvent or some kinetic circumstances.
Note that (L2Au)3O+ is not an intermediate on the way
from 10 to 15; it is rather a temporary side product. The
same is valid when 15 is transformed into 10 by addition of
2. Formation of (L2Au)3O+ was rather unexpected given
the increased steric bulk of the building blocks, but this ion
is stable enough to be detected by ESI MS.
PrSpH+ mixture with triethylamine and found that gradual
+
formation of Ph3PAuNEt3 occurred, but the equilibrium
now lay on the left: approximately nine equivalents of Et3N
were able to transform only about 60% of 9 into the trie-
thylamine complex [Eq. (43)]. During this experiment both
+
9 and Ph3PAuNEt3 were observed clearly by 31P NMR.
When TMTU was added to the final solution, one broad
phosphorus resonance was observed for all the species. This
suggests that Et3N was completely substituted from gold,
but then Ph3PAuTMTU+ got involved in a fast exchange
process, together with (Ph3P)2Au+, which was present in sol-
ution as impurity from the beginning but was not involved
in any fast exchange before TMTU was added. Also, this ex-
periment allowed the stability of the two species (LAu)3O+
and (LAu)2OH+ to be compared. Obviously (LAu)3O+
should be more stable because it is stabilized by three auro-
philic interactions, while (LAu)2OH+ has only one, albeit
doubtfully. The situation may change with the size of the
ligand, and we postulate that this accounts for the relative
instability of (LAu)3O+ when a bulky phosphine ligand is
present; there is simply not enough space around the
oxygen atom for three big fragments, and only formation of
less aurated (LAu)2OH+ is possible.
Compound (L2Au)2OH+ (10) can be viewed as a complex
of L2Au+ with L2AuOH as ligand, and thus Equation (44)
in Scheme 4 can be viewed as simple ligand exchange. To
get an idea how strong L2AuOH is as a ligand, we conduct-
ed reactions with weaker nucleophiles in place of PPh3 and
found that this reaction reversibly occurs already with Me2S,
and with lutidine the equilibrium is much shifted to the
right.
Also, we studied conversions of (L2Au)2OH+ (10) and
L2AuOH in both directions in three different solvents
(CDCl3, CD2Cl2, [D8]THF). When PPh3 was added to a solu-
tion of (L2Au)2OH+, formation of new species was ob-
served, which eventually were converted to L2AuOH when
more PPh3 was added [overall Eq. (44) in Scheme 4]. The
same species were detected during the synthesis of
L2AuOH, when a solution of 2 in CDCl3 was shaken briefly
with 10% aqueous KOH. On further brief shaking,
L2AuOH was cleanly formed, according to Equation (28) in
Scheme3. From these observations, we concluded that these
new species adopt the intermediate position between
(L2Au)2OH+ and L2AuOH during the reaction with hy-
droxide ions. From general Equations (46)–(48) in Scheme 4
it becomes clear that the quantity of hydroxide ions per
Thereafter,
the
analogous
interconversion
of
(L3Au)2OH+ (11) and L3AuOH (18) was studied
[Eq. (49)], but in contrast to the previous case it appears to
be a mechanistically “clean” transformation giving no inter-
mediate species. In no situation was the corresponding triau-
rated oxonium ion (L3Au)3O+ observed [Eq. (50)]. Possibly,
it cannot exist at all given the increased steric hindrance in
comparison to (L2Au)3O+.
Finally, we studied reaction of 2 with proton sponge in
methanol [Eq. (51)]. In contrast to the previous solvents, no
clear situation was observed. Possibly, an equilibrium mix-
ture of several oxonium and methoxonium species was
formed. However, when solid KOH was added to this mix-
ture, L2AuOMe (19) was generated cleanly. Its identity was
established by reaction with PPh3, giving clean substitution
and no other products. Obviously, proton sponge was not
strong enough to generate this methoxide.
Scheme 4. Reaction of (L2Au)2OH+ with PPh3.
Chem. Eur. J. 2012, 00, 0 – 0
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
7
&
ÞÞ
These are not the final page numbers!

M. E. Maier et al.
So far no oxonium species have been described for binu-
clear gold complexes. Therefore, in line with the previous
chemistry, we studied reactions of 7 and 8 with proton
sponge. As above, by integration it was established that 7
reacts with proton sponge to give triaurated oxonium ions
for which the simplest molecular composition was establish-
So far we described homoleptic aurated oxonium species
and it is of interest to check whether some mixed aurated
oxonium complexes, a previously unknown type of com-
pounds, can be observed. For this purpose we studied the re-
action of (L3Au)2OH+ (11) with 2 in the presence of excess
proton sponge and found that 11 and 10 exist in equilibrium
2+
ed to be (dppfAu2)3O2 (20) according to Equation (52) in
with the mixed species (L2Au)ACTHGNUTERNNUG
(L3Au)OH+ (23), and the
Scheme 5, while 8 gave a kind of diaurated oxonium ion, for
equilibrium constant was determined to be 0.87 [Scheme 6,
Scheme 6. Formation of mixed oxonium species.
1
Eq. (56)], .[21] In the H spectrum 23 exhibits a triplet for the
OH proton with a chemical shift (ꢁ0.36 ppm) between the
corresponding values for the homoleptic species. In the
31P NMR spectrum 23 exhibits two singlets in a 1:1 ratio.
Further, we conducted titration of (Ph3PAu)3O+ (9) with 2
in the presence of excess proton sponge. In the early stage,
formation of 10 did not occur at all. Rather, formation of a
single product (Ph3PAu)2ACHTNUTRGENUNG
(L2Au)O+ (24) occurred quantita-
Scheme 5. Reactions of binuclear gold complexes with bases.
tively until almost all 9 had reacted [Scheme 6, Eq. (57)]. In-
terestingly, in the 31P NMR spectrum this compound exhibits
no P-P coupling and gives two singlets in 2:1 ratio. On fur-
ther addition of 2, the simultaneous formation of two com-
which the simplest molecular composition was established
as L8(Au)2OH+ 21, [Scheme 5, Eq. (53)]. In the 1H NMR
spectrum 21 exhibits a characteristic, slightly broad singlet
at +5.36 ppm, corresponding to a single OH proton (in rela-
tion to 2Au), while in the 31P NMR spectrum it shows a
single sharp resonance at 22.85 ppm. In comparison with
other such species the position of the OH signal is abnor-
mal. On addition of solid LiOH to a solution of 21 a new
species is formed, which was tentatively assigned the formu-
pounds (10 as major and (Ph3PAu)
product) indicates a reversible process [Scheme 6, Eq. (58)].
Obviously, formation of (Ph3PAu)2A
(L2Au)O + (24) with
(L2Au)2O+ 25 as minor
ACHTUNGTRENNUNG
CTHUNGTRENNUNG
three possible aurophilic interactions is rather favored over
formation of 10 in the presence of 9, but this does not hold
true for (Ph3PAu)ACTHNUTRGNEUNG
(L2Au)2O+ (25). This directly reflects the
2+
la [L8(Au)2]3O2 (22). This can be directly obtained by re-
stability of these species, which decreases as they become
more crowded. Thus, in the case of (L3Au)2OH+ (11) for-
mation of 26 is already highly reversible [Scheme 6,
1
action of 8 with solid LiOH, its H NMR spectrum exhibits
no protons assignable to OH, and its 31P NMR spectrum ex-
hibits a single sharp resonance at 21.35 ppm. However,
exact structures were not determined for either of these spe-
cies. Based on the information on structures of known oxo-
nium species, and from the fact that 20, 21, and 22 all exhibit
a single phosphorus resonance, symmetric structures are
proposed (Scheme 5).
Eq. (59)], while formation of (Ph3PAu)ACHTNUGRTNEUNG
(L3Au)2O+ was not
observed at all. Notably, phosphorus chemical shifts of the
ligand residues in these mixed oxonium species are very
close (within 0.3 ppm) to those observed in (Ph3PAu)3O+
and (L2Au)3O+, which demonstrates similarities between all
of these compounds. Assuming that the equilibrium
&
8
&
www.chemeurj.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Coordination Chemistry of Gold Catalysts in Solution
FULL PAPER
[Scheme 6, Eq. (58)] is pH-dependent, we attempted to shift
it to the right with KOH, a stronger base. However, the
equilibrium did not change significantly: both 24 and 25
were still observed; however, 10 completely transformed
into L2AuOH. We suppose that reaction of 10 with KOH to
give 15 [Scheme 3, Eq. (30)] simply happened much faster
than complex rearrangement by Equation (58) in Scheme 6.
Finally, we titrated L8(Au)2OH+ (21) with 9 in the pres-
ence of proton sponge and found that two new products
were formed simultaneously, [Scheme 7, Eqs. (60), (61)].
Scheme 8. General mechanism of formation and transformation of oxoni-
um species and gold hydroxide.
Scheme 7. Formation of mixed oxonium species bearing bidentate li-
gands.
concentration is provided from the beginning, direct substi-
tution at gold might take place as ordinary ligand exchange.
Further, III reacts with LAu+ to finally give oxonium ion V
via tetracoordinate oxonium ion IV or, alternatively, III
reacts with OHꢁ to give two equivalents of LAuOH. This
process can be viewed as a simple ligand exchange at gold,
if one considers III as a complex of LAu+ with LAuOH as a
ligand. Correspondingly, it is easy to trace the process back
when acid is added to LAuOH or V. The intermediacy of
tetracoordinate oxonium species like II and IV is postulated,
but gains support from the ability of V to form isolable tet-
racoordinate oxonium ion VI, which has been known since
1995.[22] Obviously, despite the cationic character of III and
V, the oxygen atom still can act as a nucleophile, providing
its fourth electronic pair to a proton or gold. The relative
nucleophilic nature of oxygen in these species becomes
clearer if one considers electronic density distribution within
a molecule. Indeed, according to the concept of hard and
soft Lewis acids and bases, there should be relatively weak
interaction between Au and O, with little positive charge
transfer from gold to oxygen. It can be argued that the
oxygen atom in V is more electron rich than that in III and
far more electron rich than that in H3O+, despite the fact
that gold is more electronegative than hydrogen. This hy-
However, one of them completely transformed into another
species when more 9 was added. The molecular composition
of this intermediate species was established as
1
{(L8(Au)2)2OH}
N
it exhibits OH and Me signals in the ratio 1:12:12. The final
product did not contain any OH signals, and its simplest mo-
lecular composition was established as {L8(Au)2}-
ACHTUNGTRENNUNG
(Ph3PAu)O+ (28) which can be a monomer or dimer. The
exact structures of these ions were not determined; possibly,
symmetric structures are formed.
The interconversion between different kinds of oxonium
ions and gold hydroxide can be explained by a general
mechanism outlined in Scheme 8, consisting of principally
all reversible steps, so that position of the real system at
equilibrium primarily depends on the molar ratio and
nature of reactants. Accordingly, reaction between LAu+
and a base might begin with formation of a hydrate I, which
would undergo a second addition of LAu+ to give tetracoor-
dinate oxonium ion II, which would readily expel one
proton to give intermediate (LAu)2OH+ (III). If high OHꢁ
Chem. Eur. J. 2012, 00, 0 – 0
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
9
&
ÞÞ
These are not the final page numbers!

M. E. Maier et al.
pothesis gains further support from NMR spectra, which
typically show chemical shift of OH in III below 0 ppm. The
same statements can be made to account for the strongly
basic properties of LAuOH.
However, to account for reversible formation of V during
transformation of (LAu)2OH+ into LAuOH under basic
conditions, equilibria VÐIVÐIII are not feasible, due to
the absence of H+. In this case we propose alternative
mechanisms, which are also given in Scheme 8. Thus, one
might consider direct reaction of V with water, through hy-
drogen-bond activation, to give an equilibrium mixture of
III and LAuOH. From here, transformation of III to
LAuOH in the presence of OHꢁ is clear. Alternatively, one
can consider the possibility of V reacting directly with OHꢁ,
but then formation of gold oxide VIII would follow. Howev-
er, at present, such gold derivatives remain unknown and
their intermediacy is thus highly speculative. But if formed,
they would be highly basic substances and would react im-
mediately with water to give initially III and OHꢁ and final-
ly lead to LAuOH. We conclude that, depending on the
availability of H+ and OHꢁ, gold may undergo transforma-
tions in different directions through different pathways, but
the final outcome of a reaction would be dependent on the
molar ratio and nature of reactants.
Formal LAu+ exchange at oxygen between (LAu)3O+
and (L’Au)3O+ was previously observed, but nothing was
said about formation of any mixed intermediate species on
the way.[23] In our study, due to the presence of at least one
large unit, such exchanges were observed as stepwise proc-
esses for the first time.
Scheme 9. Formation and transformations of complexes with sulfur.
a C3 symmetry axis passing through the central sulfur atom
and perpendicular to the base of the Au3S pyramid.
Reaction of 2 with Li2S: complexes with sulfur and their in-
terconversion: When 2 was allowed to react with substoi-
chiometric amounts of Li2S in a biphasic CDCl3/H2O mix-
ture, (L2Au)3S+ (29) was formed as
a sole product
[Eq. (62)]. Under these reaction conditions formation of tet-
raaurated sulfonium ion (L2Au)4S2+ could be expected, and
exclusive formation of 29 evidences for inability of any
higher coordinated compound to exist due to steric rea-
sons.[24] On further addition of excess Li2S, multiple process-
es occurred. Thus, 29 predominantly transformed into hy-
drosulfide L2AuSH (30), but the reaction was also accompa-
nied by formation of several minor products [Eqs. (63) and
(64)]. Among them, free ligand L2 was identified. Libera-
tion of free L2 implies that part of the gold was converted
to Au2S, and this idea gains support from the fact that some
brown precipitate was indeed observed. The same decompo-
sition was described in the case of (Ph3PAu)2S long ago,[25]
and later the structure of this compound could be establish-
ed by X-ray analysis.[26] Accordingly, we concluded that neu-
tral sulfide (L2Au)2S would also be unstable: it either de-
composes to Au2S and L2 or is hydrolyzed back to hydrosul-
fide 30. Notably, complete precipitation of Au2S and libera-
tion of L2 could not be achieved, at least within a short
time interval. The structure of (L2Au)3S+ was established
by X-ray analysis (Scheme 9). The ligand residues are locat-
ed in such a way that the cation can be described as having
Interestingly, when a chloroform solution of 2 was ground
in a mortar together with excess of solid Li2S (even without
addition of water), clean formation of L2AuSH (30) occur-
red and no brown Au2S color or free L2 was observed,
[Scheme 9, Eq. (65)]. Based on this fact, we suppose that 30
is the only stable ultimate product of the reaction with sul-
fide (correspondingly, formation of LiOH should be invoked
1
to account for this process). In the H NMR spectrum 30 ex-
hibits a characteristic doublet at ꢁ1.44 ppm corresponding
to a single SH proton.
Transformations of L2AuSH (30) were further studied in
the following experiment. When 2 was added to a fresh solu-
tion of 30 in CDCl3, (L2Au)3S+ (29) was formed as sole
product [Scheme 9, Eq. (68)]. This reaction is accompanied
by liberation of H2S, which could be also recognized by
smell. On further addition of 2, 29, H2S, and 2 reacted to
give (L2Au)2SH+ 31 as sole product, [Scheme 9, Eq. (69)].
&
10
&
www.chemeurj.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Coordination Chemistry of Gold Catalysts in Solution
FULL PAPER
1
In the H NMR spectrum it exhibited a characteristic triplet
with complexes bearing small ligands. By observation of dis-
crete rather than continuous changes in NMR spectra, we
were able to detect and characterize intermediates and got
closer insight to mechanisms of processes involving multiple
reactive species.
at ꢁ0.41 ppm (3JH-P =5.3 Hz), corresponding to a single SH
proton. Ultimately, when more 2 was added, 29 reappeared
again, but now reversibly [Scheme 9, Eq. (70)]. These inter-
esting transformations are explained as follows. Initially,
when L2AuSH reacts with 2, (L2Au)2SH+ would be expect-
ed as a direct product [Scheme 9, Eq. (66)]. Observation of
29 at an early stage implies that (L2Au)2SH+ quantitatively
reacted with 29 to give (L2Au)3S+ and H2S [Scheme 9,
Eq. (67)], and only when no more 30 is present in solution
is further reaction possible. Next, when (L2Au)2SH+ reacted
with an excess of 2, formation of (L2Au)3S+ occurred, but
now liberation of super acid HSbF6 makes this process re-
versible. However, the equilibrium could be completely
shifted to the right on extraction with water. Such an inter-
esting appearance, disappearance, and reappearance of
(L2Au)3S+ 29 is completely understandable if one considers
material balance in the system. Thus, the initial formation of
29 happens before one equivalent of gold is added according
to Equation (68) in Scheme 9. At this moment the molar
ratio of elements in the system is Au+/S2ꢁ/H+ =3/2/2 (1S
and 2H are present as H2S and unreactive). Stoichiometric
formation of 31 happens exactly when the molar ratio is
Au+/S2ꢁ/H+ =2/1/1, as in the compound. Finally, 29 reap-
pears again when Au/S>2/1 and becomes complete provid-
ed Au+/S2ꢁ =3/1 and no acid is present.
From these two experiments it can be concluded that the
outcome of the reaction between 2 and S2ꢁ depends on the
molar ratio of reactants and availability of reactive protons.
The mechanism of these transformations would resemble
the previously described mechanism for oxonium species.
An attempt was made to synthesize the elusive neutral
(L2Au)2S by reaction of 30 and 15 in C6D6, but this led to
mixtures of products. Interestingly, liberation of free L2 was
observed, but now it was not accompanied by any brown
precipitate, which suggests that formation of colorless prod-
ucts of general formula (L2Au)2S·xAu2S could be possible.
Four new components were observed in the NMR spectra of
these mixtures; two of the compounds contained character-
istic singlets at ꢁ0.61 and ꢁ0.66 ppm (in C6D6); none of
them was identified. One of the products precipitated from
benzene. The solid was soluble in chloroform and did not
exhibit any signals below 0 ppm. The crystals, however, were
not analyzed by X-ray analysis leaving this puzzle to be
solved in the future.
Experimental Section
Synthesis of 2: An improved literature procedure was used.[27] AgSbF6
(0.213 g, 0.620 mmol) was quickly weighed in a vial. To this was added
cold MeCN (ꢀ0.3 mL) and CH2Cl2 (0.3 mL) to make a clear solution
(dissolution of AgSbF6 in MeCN is rather exothermic). To this solution
was added a solution of L2AuCl (0.329 g, 0.620 mmol) in CH2Cl2 (3 mL).
A white precipitate immediately formed. The reaction mixture was al-
lowed to stand in the dark for about 15 min (rather than overnight as is
often given in the literature) before it was passed through Celite (we rec-
ommend conducting halogen abstraction reactions by silver in CH2Cl2 as
a main co-solvent; see our remarks on synthesis of LAuOTf in the Sup-
porting Information). The reaction flask and filter cake were washed
with copious CH2Cl2. The clear colorless filtrate was evaporated in vacuo
till dryness and redissolved in a minimum amount of MeCN (ꢀ0.2 mL).
This solution was layered with benzene (ꢀ1.5 mL) to allow for slow crys-
tallization. First crystals appeared rather slowly, but then crystal growth
occurred faster and rather big crystals were obtained. The supernatant
was removed by Pasteur pipette and the crystals washed once with ben-
zene and dried in vacuo to yield 2 in about 95% yield. We note that both
commercial and self-made 2 normally contain only 0.93 equiv MeCN
rather than 1 equiv, and some 10 (ꢀ3%) as impurity or as hydrolysis
product. However, when the final product was moistened with MeCN
and dried in vacuo immediately, but not strongly at first, and then mixed
with pentane and finally dried thoroughly, it was possible to obtain the
product with 1 equiv MeCN as in the formula.
Synthesis of dppfACHTNUTRGNEUNG(AuCl)2 (7): Solid dppf (0.170 g, 1 equiv) was added to a
suspension of Me2SAuCl (0.101 g, 0.343 mmol, 2.02 equiv) in CH2Cl2
(2 mL). All insoluble material soon dissolved to give an almost clear
orange solution containing trace amounts of a tiny dark precipitate. The
solution was filtered through Celite into a 10 mL flask and additional
amounts of CH2Cl2 were used to quantitatively transfer all of the materi-
al. The filtrate was concentrated in vacuo to about 0.5 mL. At this point
a major part of dppfACTHNUTRGNEUNG(AuCl)2 may already precipitate from the solution.
The filtrate was triturated with MeOH to enhance precipitation of the
product. The supernatant solution was carefully sucked out (by a Pasteur
pipette with a piece of cotton at the end) and the residue washed once
with MeOH and dried in vacuo (all in the same flask) to give the desired
complex as orange microcrystalline solid in quantitative yield (0.173 g).
Note: Me2SAuCl, dppf and dppfACHTUNTRGNEU(GN AuCl)2 are all practically insoluble in
MeOH; therefore it is not recommended to keep more significant devia-
tions from 2/1 molar ratio of the reactants.
S-TolBINAPACHTNUTRGNENG(U AuCl)2 8 was synthesized by the same method in quantita-
tive yield. It is also insoluble in MeOH.
Synthesis of (Ph3PAu)3O+ OTfꢁ (9): A solution of Ph3PAuCl (171.3 mg,
0.345 mmol) in THF (2 mL) was added to a solution of AgOTf (89.2 mg,
0.347 mmol) in THF (2 mL). AgCl immediately precipitated. The reac-
tion mixture was kept in the dark for 5 min and filtered through a pad of
Celite. To the clear colorless filtrate was added a solution of NaOH
(15 mg) in water (0.4 mL) and the emulsion was shaken briefly. The prod-
uct formed as a gray precipitate. The resulting suspension was evaporated
almost till dryness. The residue was taken up in CH2Cl2 and water. Any
remaining precipitate was filtered off and washed with portions of
CH2Cl2. The clear biphasic filtrate was collected, the water phase separat-
ed, and the organic phase dried with Na2SO4. After filtration, the clear
filtrate was transferred to a weighed round-bottomed flask. Most of the
solvent was removed in vacuo and the residue diluted with THF to cause
spontaneous precipitation of the pure product as white crystals. The su-
pernatant solution was decanted by Pasteur pipette and the precipitate
Conclusion
We have thoroughly investigated the coordination chemistry
of modern gold catalysts by NMR spectroscopy in solution.
This included simple ligand-exchange reactions at gold, as
well as formation and transformations of different oxonium
and sulfonium species. Success in this study relied on the
presence of bulky ligands in gold complexes, which allowed
the transformations to be traced by NMR clearly without
being spoiled by the dynamic situations that often happen
Chem. Eur. J. 2012, 00, 0 – 0
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
11
&
ÞÞ
These are not the final page numbers!

M. E. Maier et al.
washed with THF once, decanted, and the product dried in vacuo to
yield 0.166 g (93%) of the title compound. Note: Clearly, the synthesis
would be better performed in CH2Cl2 solution (change of the solvent
from THF to CH2Cl2 would be not necessary).
120, 2208–2211; Angew. Chem. Int. Ed. 2008, 47, 2178–2181; h) D. J.
Gorin, B. D. Sherry, F. D. Toste, Chem. Rev. 2008, 108, 3351–3378;
i) E. Jimꢃnez-NfflÇez, A. M. Echavarren, Chem. Rev. 2008, 108,
3326–3350; j) Z. Li, C. Brouwer, C. He, Chem. Rev. 2008, 108,
3239–3265; k) A. Arcadi, Chem. Rev. 2008, 108, 3266–3325; l) J.
Muzart, Tetrahedron 2008, 64, 5815–5849; m) A. S. K. Hashmi, M.
Rudolph, Chem. Soc. Rev. 2008, 37, 1766–1775; n) H. C. Shen, Tet-
rahedron 2008, 64, 7847–7870; o) H. C. Shen, Tetrahedron 2008, 64,
3885–3903; p) R. A. Widenhoefer, Chem. Eur. J. 2008, 14, 5382–
5391; q) D. J. Gorin, F. D. Toste, Nature 2007, 446, 395–403; r) A.
Fꢀrstner, P. W. Davies, Angew. Chem. 2007, 119, 3478–3519; Angew.
Chem. Int. Ed. 2007, 46, 3410–3449; s) E. Jimꢃnez-NfflÇez, A. M.
Echavarren, Chem. Commun. 2007, 333–346; t) A. S. K. Hashmi,
Chem. Rev. 2007, 107, 3180–3211.
Synthesis of gold hydroxides LAuOH (L=L2, IMes, IPr), mechano-
chemical method: A suspension of L2AuCl (33.8 mg, 43.8 mmol) and
KOH (23.5 mg, 420 mmol, 9.6 equiv) in C6D6 (0.5 mL) was ground in a
small mortar for 30–60 s (10 s is enough for smaller loadings, and less
KOH can also be used). On filtration through Celite (prewashed with
C6D6) a clear solution of 15 was obtained, which was used directly with-
out isolation. IPrAuCl reacted more slowly, but complete conversion was
reliably achieved. Surprisingly, IMesAuCl reacted even more slowly, and
complete conversion was not achieved. However, the reaction was clean,
and this suggests that prolonged reaction time should achieve complete
conversion. We suppose that solubility of the initial and final gold com-
plexes in benzene would be important. We found that L2AuCl and
L2AuOH are quite soluble, while IPrAuCl and IMesAuCl are less solu-
ble.
[2] For a recent review on intermediates in gold catalysis, see: A. S. K.
Hashmi, Angew. Chem. 2010, 122, 5360–5369; Angew. Chem. Int.
Ed. 2010, 49, 5232–5241. Recent highlight about diaurated species:
A. Gómez-Suµrez, S. P. Nolan, Angew. Chem. 2012, 124, 8278–8281;
Angew. Chem. Int. Ed. 2012, 51, 8156–8159. Most recent research
on mechanisms includes also: a) W. Wang, G. B. Hammond, B. Xu,
J. Am. Chem. Soc. 2012, 134, 5697–5705; b) Z. J. Wang, D. Benitez,
E. Tkatchouk, W. A. Goddard III, F. D. Toste, J. Am. Chem. Soc.
2010, 132, 13064–13071; c) T. J. Brown, D. Weber, M. R. Gagnꢃ,
R. A. Widenhoefer, J. Am. Chem. Soc. 2012, 134, 9134–9137;
d) A. S. K. Hashmi, I. Braun, M. Rudolph, F. Rominger, Organome-
tallics 2012, 31, 644–661; e) A. S. K. Hashmi, A. M. Schuster, S.
Gaillard, L. Cavallo, A. Poater, S. P. Nolan, Organometallics 2011,
30, 6328–6337; f) A. S. K. Hashmi, A. M. Schuster, S. Litters, F. Ro-
minger, M. Pernpointner, Chem. Eur. J. 2011, 17, 5661–5667.
[3] a) Top. Curr. Chem. Vol. 302 (Eds.: E. Soriano, J. Marco-Contelles),
Springer, Berlin, 2011, pp. 252; b) C. M. Krauter, A. S. K. Hashmi,
M. Pernpointner, ChemCatChem 2010, 2, 1226–1230; c) T. de Haro,
E. Gómez-Bengoa, R. Cribiffl, X. Huang, C. Nevado, Chem. Eur. J.
2012, 18, 6811–6824; d) R. Fang, L. Yang, Organometallics 2012, 31,
3043–3055; e) T. Fan, X. Chen, J. Sun, Z. Lin, Organometallics 2012,
31, 4221–4227; f) G. Mazzone, N. Russo, E. Sicilia, Organometallics
2012, 31, 3074–3080.
Extractive variant: A solution of LAuCl in CH2Cl2/benzene (1/5) was
shaken with concentrated (but not saturated) aqueous KOH for 5 min.
The small amount of CH2Cl2 was used to improve the solubility of
IPrAuCl and IMesAuCl. In principal, pure CH2Cl2 can be used, but the
efficacy of emulsion generation on shaking and determination of phase
separation is lower, while benzene is much better suited. No phase-trans-
fer catalysts are required. Again, reaction of IMesAuCl was found to be
rather sluggish. We recommend the mechanochemical method as the sim-
plest and most reliable for synthesis of 15, while for larger scale synthesis
of IPrAuOH some adaptation would be required due to lower solubility
in benzene. More remarks on these two procedures are given in the Sup-
porting Information.
Generation of oxonium species (LAu)2OH+
ACTHNUTRGNE(UNG 10–13); comparison of dif-
ferent reagents: Of all bases used in this study only proton sponge gener-
ated this species essentially in 100% yield according to Equation (26a) in
Scheme 2 regardless of the excess of base. In case of water extraction of
a chloroform solution of 2, reaction naturally does not reach completion,
but was at least 95% complete (trace amounts of unconverted 2 always
remained, as evidenced by the MeCN resonance not exactly at 2.00 ppm
and broadening of the OH triplet). When a chloroform solution of 2 was
extracted with dilute NaHCO3 solution, the resulting solution contained
traces of products of reaction beyond 10, that is, 15 and 17 (as evidenced
by the MeCN resonance exactly at 2.00 ppm and broadening of the OH
triplet). The use of solid inorganic bases such as LiOH or KOH gave dif-
ferent results depending on the amount of base, particle size, and reac-
tion time. Thus, on brief shaking of a chloroform solution of 2 with rela-
tively coarse LiOH powder, 10 was observed cleanly (at least 95%),
while on shaking with bigger amounts of finer powder reaction occurred
till exclusive formation of 15. Despite these ambiguities, shaking of a sol-
ution of 10 generated by reaction of 2 and PrSp according to Equa-
tion (26a) in Scheme 2 with LiOH powder was reliably used several
times to get rid of PrSpH+ while not disturbing 10.
[4] M. C. Gimeno, A. Laguna, in Comprehensive Coordination Chemis-
try II, Vol. 6, (Eds.: J. A. McCleverty, T. J. Meyer), Pergamon,
Oxford, 2003, pp. 911–1145.
[5] The true rise of homogenous gold catalysis essentially started with
the following two papers: a) A. S. K. Hashmi, L. Schwarz, J.-H.
Choi, T. M. Frost, Angew. Chem. 2000, 112, 2382–2385; Angew.
Chem. Int. Ed. 2000, 39, 2285–2288; b) A. S. K. Hashmi, T. M. Frost,
J. W. Bats, J. Am. Chem. Soc. 2000, 122, 11553–11554.
[6] With “parent ligand” we refer to heteroleptic two-coordinate gold
complexes consisting of L-Au-Nu units in which gold is bound to
two different ligands L and Nu. In their transformations the L-Au
ꢁ
unit remains integral while the Au Nu bond is disconnected. Thus,
L can be considered the non-exchangeable ligand. In most situations
L is the stronger ligand. In some rare cases it appears that the newly
incoming Nu is stronger than L (e.g., exchange of MeCN by Cy3P at
L2AuNCMe+), but Cy3P is still considered to be Nu with respect to
the parent precursor.
Acknowledgements
[7] a) C. B. Colburn, W. E. Hill, C. A. Mcauliffe, R. V. Parish, J. Chem.
Soc. Chem. Commun. 1979, 218–219; b) R. V. Parish, O. Parry,
C. A. McAuliffe, J. Chem. Soc. Dalton Trans. 1981, 2098–2104; c) R.
Colton, K. L. Harrison, Y. A. Mah, J. C. Traeger, Inorg. Chim. Acta
1995, 231, 65–71.
We thank Dr. K. Eichele (Univ. Tꢀbingen, Institut fꢀr Anorganische
Chemie) and P. Schuler (Univ. Tꢀbingen, Institut fꢀr Organische
Chemie) for allowing us to use additional NMR spectrometers and Dr.
D. Wistuba for HRMS analysis.
[8] A. Sladek, H. Schmidbaur, Z. Naturforsch., B: Chem. Sci. 1996, 51b,
1207–1209.
[9] M. Munakata, S.-G. Yan, M. Maekawa, M. Akiyama, S. Kitagawa, J.
Chem. Soc. Dalton Trans. 1997, 4257–4262.
[10] J. Vicente, M.-T. Chicote, R. Guerrero, P. G. Jones, J. Chem. Soc.
Dalton Trans. 1995, 1251–1254.
[11] G. L. Wegner, A. Jockisch, A. Schier, H. Schmidbaur, Z. Natur-
forsch., B: Chem. Sci. 2000, 55b, 347–351.
[1] For reviews on gold catalysis, see: a) A. Corma, A. Leyva-Perꢃz,
M. J. Sabater, Chem. Rev. 2011, 111, 1657–1712; b) M. Bandini,
Chem. Soc. Rev. 2011, 40, 1358–1367; c) T. C. Boorman, I. Larrosa,
Chem. Soc. Rev. 2011, 40, 1910–1925; d) A. S. K. Hashmi, M.
Bꢀhrle, Aldrichimica Acta 2010, 43, 27–33; e) N. D. Shapiro, F. D.
Toste, Synlett 2010, 675–691; f) S. Sengupta, X. Shi, ChemCatChem
2010, 2, 609–619; g) N. Bongers, N. Krause, Angew. Chem. 2008,
&
12
&
www.chemeurj.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Coordination Chemistry of Gold Catalysts in Solution
FULL PAPER
[12] A. N. Nesmeyanov, E. G. Perevalova, Y. T. Struchkov, M. Y. Antipin,
K. I. Grandberg, V. P. Dyadhenko, J. Organomet. Chem. 1980, 201,
343–349.
[19] S. Gaillard, C. S. J. Cazin, S. P. Nolan, Acc. Chem. Res. 2012, 45,
778–787; see also refs. [16] and [18].
+
[20] A similar IPrAuNEt3 complex has recently been described. Such
complexes are important in the context of dual catalysis because
they dissociate to give active gold and the base NEt3. See ref. [2d].
[21] For a related heteroleptic diaurated vinyl gold compound, see:
A. S. K. Hashmi, I. Braun, P. Nçsel, J. Schꢁdlich, M. Wieteck, M.
Rudolph, F. Rominger, Angew. Chem. 2012, 124, 4532–4536;
Angew. Chem. Int. Ed. 2012, 51, 4456–4460.
[13] a) T. J. Brown, R. A. Widenhoefer, J. Organomet. Chem. 2011, 696,
1216–1220; b) T. J. Brown, M. G. Dickens, R. A. Widenhoefer,
Chem. Commun. 2009, 6451–6453.
[14] For example, gold(I)-catalyzed hydroalkoxylation fails in MeCN: V.
Belting, N. Krause, Org. Lett. 2006, 8, 4489–4492; in contrast, some
goldACHTUNGTRENNUNG(III) reactions were performed in MeCN: G. Dyker, E. Muth,
[22] H. Schmidbaur, S. Hofreiter, M. Paul, Nature 1995, 377, 503–504.
[23] Y. Yang, V. Ramamoorthy, P. R. Sharp, Inorg. Chem. 1993, 32,
1946–1950.
A. S. K. Hashmi, L. Ding, Adv. Synth. Catal. 2003, 345, 1247–1252.
[15] M. C. Gimeno, A. Laguna, C. Sarroca, P. G. Jones, Inorg. Chem.
1993, 32, 5926–5932.
[24] F. Canales, M. C. Gimeno, P. G. Jones, A. Laguna, Angew. Chem.
1994, 106, 811–812; Angew. Chem. Int. Ed. Engl. 1994, 33, 769–770.
[25] C. Kowala, J. M. Swan, Aust. J. Chem. 1966, 19, 547–554. In contrast
to (Ph3PAu)2S and (L2Au)2S, (Et3PAu)2S was reported to be stable.
[26] C. Lensch, P. G. Jones, G. M. Sheldrick, Z. Naturforsch., B: Org.
Chem. 1982, 37B, 944–949.
[27] C. Nieto-Oberhuber, M. P. MuÇoz, S. López, E. Jimꢃnez-NfflÇez, C.
Nevado, E. Herrero-Gómez, M. Raducan, A. M. Echavarren, Chem.
Eur. J. 2006, 12, 1677–1693.
[16] a) S. Gaillard, J. Bosson, R. S. Ramón, P. Nun, A. M. Z. Slawin, S. P.
Nolan, Chem. Eur. J. 2010, 16, 13729–13740; b) R. S. Ramón, S.
Gaillard, A. Poater, L. Cavallo, A. M. Z. Slawin, S. P. Nolan, Chem.
Eur. J. 2011, 17, 1238–1246.
[17] Fast exchange and strong hydrogen bonding were previously docu-
mented for LAuOR/ROH systems: a) S. Komiya, M. Iwata, T. Sone,
A. Fukuoka, J. Chem. Soc. Chem. Commun. 1992, 1109–1110; b) T.
Sone, M. Iwata, N. Kasuga, S. Komiya, Chem. Lett. 1991, 20, 1949–
1952.
Received: April 10, 2012
Revised: July 30, 2012
Published online: && &&, 0000
[18] S. Gaillard, A. M. Z. Slawin, S. P. Nolan, Chem. Commun. 2010, 46,
2742–2744.
Chem. Eur. J. 2012, 00, 0 – 0
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
13
&
ÞÞ
These are not the final page numbers!

M. E. Maier et al.
Diving in a gold solution: A detailed
exploration of the coordination chem-
istry of eight gold catalysts by NMR
spectroscopy in solution was con-
ducted. This covered ligand-exchange
reactions, formation and transforma-
tion of different oxonium salts, and
complexes with sulfide.
Gold Complexes
A. Zhdanko, M. Strçbele,
M. E. Maier* .................. &&&& — &&&&
Coordination Chemistry of Gold Cata-
lysts in Solution: A Detailed NMR
Study
&
14
&
www.chemeurj.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!