Observing Initial Steps in Gold-Catalyzed Alkyne Transformations by Utilizing Bodipy-Tagged Phosphine-Gold Complexes

Article abstract of DOI:10.1002/chem.201600264

The Pd-catalyzed reactions of 3-chloro-bodipy with R₂PH (R=Ph, Cy) provide nonfluorescent bodipy-phosphines 3-PR₂-bodipy 3 a (R=Ph) and 3 b (R=Cy; quantum yield Φ<0.001). Metal complexes such as [AgCl(3 b)] and [AuCl(3 b)] were prepared and shown to display much higher fluorescence (Φ=0.073 and 0.096). In the gold complexes, the level of fluorescence was found to be qualitatively correlated with the electron density at gold. Consequently, the fluorescence brightness of [AuCl(3 b)] increases when the chloro ligand is replaced by a weakly coordinating anion, whereas upon formation of the electron-rich complex [Au(SR)(3 b)] the fluorescence is almost quenched. Related reactions of [AuCl(3 b)] with [Ag]ONf)] (Nf= nonaflate) and phenyl acetylenes enable the tracking of initial steps in gold-catalyzed reactions by using fluorescence spectroscopy. Treatment of [AuCl(3 b)] with [Ag(ONf)] gave the respective [Au(ONf)(3 b)] only when employing more than 2.5 equivalents of silver salt. The reaction of the "cationic" gold complex with phenyl acetylenes leads to the formation of the respective dinuclear cationic [{(3 b)Au}₂(CCPh)]⁺ and an increase in the level of fluorescence. The rate of the reaction of [Au(ONf)(3 b)] with PhCCH depends on the amount of silver salt in the reaction mixture; a large excess of silver salt accelerates this transformation. In situ fluorescence spectroscopy thus provides valuable information on the association of gold complexes with acetylenes.

Full text of DOI:10.1002/chem.201600264

DOI: 10.1002/chem.201600264
Full Paper
&
Gold Catalysis
Observing Initial Steps in Gold-Catalyzed Alkyne Transformations
by Utilizing Bodipy-Tagged Phosphine–Gold Complexes
[a]
Roman Vasiuta and Herbert Plenio*
Abstract: The Pd-catalyzed reactions of 3-chloro-bodipy
phenyl acetylenes enable the tracking of initial steps in
gold-catalyzed reactions by using fluorescence spectroscopy.
Treatment of [AuCl(3b)] with [Ag(ONf)] gave the respective
[Au(ONf)(3b)] only when employing more than 2.5 equiva-
lents of silver salt. The reaction of the “cationic” gold com-
with R PH (R=Ph, Cy) provide nonfluorescent bodipy–phos-
2
phines 3-PR –bodipy 3a (R=Ph) and 3b (R=Cy; quantum
2
yield F<0.001). Metal complexes such as [AgCl(3b)] and
[
AuCl(3b)] were prepared and shown to display much
higher fluorescence (F=0.073 and 0.096). In the gold com-
plexes, the level of fluorescence was found to be qualitative-
ly correlated with the electron density at gold. Consequently,
the fluorescence brightness of [AuCl(3b)] increases when
the chloro ligand is replaced by a weakly coordinating
anion, whereas upon formation of the electron-rich complex
plex with phenyl acetylenes leads to the formation of the re-
+
spective dinuclear cationic [{(3b)Au} (CCPh)] and an in-
2
crease in the level of fluorescence. The rate of the reaction
of [Au(ONf)(3b)] with PhCCH depends on the amount of
silver salt in the reaction mixture; a large excess of silver salt
accelerates this transformation. In situ fluorescence spectros-
copy thus provides valuable information on the association
of gold complexes with acetylenes.
[
Au(SR)(3b)] the fluorescence is almost quenched. Related
reactions of [AuCl(3b)] with [Ag]ONf)] (Nf= nonaflate) and
Introduction
ered (signal on/off) depending on the presence or absence of
quencher. A transition metal and a ligand tagged with a fluo-
rescent dye associate to form a fluorescent or a nonfluorescent
complex, the brightness of which is reversed by a reaction.
This principle has been put to good use to clarify mechanistic
issues concerning the postulated release–return mechanism in
Understanding catalytic processes is of essential importance
[1]
for enhancing catalysts and catalytic transformations. Acquir-
ing such an understanding critically relies on spectroscopic
techniques, which are able to probe the status of the species
[2]
[7]
under study with sufficient precision and sensitivity. In this re-
olefin metathesis (Scheme 1a). Mirkin and co-workers report-
[
3]
spect, fluorescence spectroscopy could be a useful method,
ed a Pt complex in which the bodipy-centered fluorescence is
reversibly modulated by an added aminophosphine ligand L
since the high sensitivity of detection perfectly suits the moni-
toring of catalytic species, which (almost by definition) are
present in very small concentrations during catalytic reactions.
Ligand-exchange reactions constitute elementary steps in
the chemistry of metal complexes and are crucial in catalytic
transformations. It would thus be highly enlightening to syn-
thesize a fluorescent reporter ligand for catalytically active
metals, the fluorescence properties of which are modulated by
such elementary reactions. Only a few examples related to cat-
[
8]
(Scheme 1b). This ligand L releases a nitrogen donor from
the coordination sphere of platinum, which then quenches the
fluorophore. We recently realized that changes in the fluores-
cence signal of bodipy-tagged N-heterocyclic carbene metal
complexes also occur as a consequence of changes in the co-
ordination sphere of a metal, despite the fact that the spatial
relationship of fluorophore and transition metal remains un-
changed. In this case, the substitution of a 1,5-cyclooctadiene
(cod) ligand in an Ir complex by two CO molecules converts an
initially dark complex into a brightly fluorescent complex
[4]
alysis in single-molecule studies and in ensemble measure-
[5]
ments in solution have been realized, despite the fact that re-
lated approaches are very popular in photoluminescent sens-
[
9]
(Scheme 1c). This modulated fluorescence intensity was at-
tributed to changes in the electron density at the transition
metal and indicates that the photoinduced electron transfer
(PET) mechanism appears to be primarily responsible for fluo-
[6]
ing.
The detection processes in such complexes reported to date
have been relatively simple, in that only two states are consid-
rescence quenching. The oxidative addition of H to a fluoro-
2
[
a] R. Vasiuta, Prof. H. Plenio
Organometallic Chemistry, TU Darmstadt, Alarich-Weiss-Str. 12, 64287
Darmstadt (Germany)
phore-tagged Crabtree-type complex is another example of
a fluorogenic reaction (Scheme 1d) that is useful for the detec-
[10]
tion of H2.
E-mail: plenio@tu-darmstadt.de
For this study, it was our target to synthesize metal com-
plexes of bodipy-tagged phosphines, in which one phosphorus
donor is directly connected to the bodipy core. The resulting
Supporting information for this article and ORCID from one of the authors
are available on the WWW under
http://dx.doi.org/10.1002/chem.201600264.
Chem. Eur. J. 2016, 22, 6353 – 6360
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fluorescence quantum yields, very broad emission
band, and instability). Therefore, another synthetic
approach leading to different bodipy–phosphines
was required.
The reaction of 2,2’-(p-tolylmethylene)bis(1H-pyr-
[
17]
role) with N-chlorosuccinimide followed by chlora-
nil and BF ·Et O provides 3-chloro-bodipy 2 in a three
3
2
step, one-pot procedure involving two chromato-
graphic purifications in an acceptable overall yield of
1
9% (Scheme 2). An experimental procedure and
spectroscopic data for this compound were reported
[
18]
only very recently. Our procedure reported herein
is based on the synthesis of the closely related 3,5-di-
[
19]
chloro-bodipy. In the latter compound, the chlorine
atoms are highly activated towards nucleophilic sub-
[
20]
stitution and Pd-catalyzed cross coupling reactions
and can be replaced with various nucleophiles
(
amines, alcohols, thiols, etc.) in good yields employ-
[
21]
ing mild reaction conditions.
Consequently, reac-
tions of 2 with Ph PH or Cy PH and base (Et N) pro-
2
2
3
vide the respective bodipy–phosphines 3a and 3b,
but better yields (72–76%) were obtained, when the
same reactions were carried out in the presence of
Scheme 1. Examples of switched fluorescence (on/off) in transition metal complexes with
an appended fluorophore. Mes=2,4,6-trimethylphenyl, bdp=bodipy, SIMes=1,3-
bis(2,4,6-trimethyl)phenyl-4,5-dihydroimidazol-2-ylidene.
[22]
catalytic amounts of Pd(OAc)2.
Additional phos-
phine for the activation of the palladium catalyst was
not required and we assume that the secondary
phosphine should be dark, due to PET quenching by the phos-
phorus lone pair. However, coordination of this lone pair to
a transition metal should restore fluorescence when using
metals that do not open other quenching pathways (for exam-
ple, heavy atom effect or through minor changes in the elec-
[
11]
tronic structure).
Such a partially quenched fluorescence
signal can be sensitive to subtle changes in the coordination
sphere of the metal and translate such changes into an in-
crease or a decrease in the bodipy fluorescence.
Results and Discussion
Synthesis of bodipy-tagged phosphines
Only a few examples of bodipy-tagged phosphines have been
Scheme 2. Synthesis of bodipy–phosphines. Reaction conditions: a) Ph
CH Cl
, 15 min; b) N-chlorosuccinimide, À788C, THF, followed by chloranil,
followed by BF ·Et O, NEt ; c) R PH (R=Ph, Cy), Pd(OAc) , Et N, toluene.
2
PH,
[
12]
reported to date. Higham and co-workers recently reported
the synthesis of several bodipy–phosphines, as well as metal
complexes with coinage metals ions such as Cu, Ag, Au and
2
2
3
2
3
2
2
3
[
13]
with Re.
Bodio and co-workers also reported such phos-
[14]
phines and the derived Ru, Os and Au complexes, whereas
phosphine used as a reagent forms a sufficiently active cross-
[15]
[23]
Lim and Blum reported a related Pd complex. Most of the
previously reported metal complexes are highly fluorescent,
because the distance between the phosphorus (or the transi-
tion metal) quencher and the bodipy is rather large. Our strat-
egy is different, in that the phosphorus donor and the bodipy
core are to be connected directly, to initially generate fluores-
cence-quenched bodipy–phosphines.
coupling catalyst with palladium.
The bodipy–phosphines
3a and 3b are slightly sensitive towards oxidation and in
a few reactions under ambient atmosphere, small amounts of
phosphine oxide 4a were formed and separated by chroma-
tography.
Metal complexes of bodipy–phosphines
We first tested fluorescence properties of the known 8-Ph P-
2
[
16]
bodipy (Scheme 1). However, we learnt that this ligand and
the respective gold complex show unfavorable properties (low
The new metal complexes of bodipy–phosphines (Scheme 3),
like [AuCl(1)] and [MX(3b)] (M=Cu, Ag, Au) were synthesized
Chem. Eur. J. 2016, 22, 6353 – 6360
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+
+
+
spectra of all Cu , Ag , and Au complexes could be record-
ed. However, based on the optical data, which are the same as
those of phosphine 3a, [CuI(3a)] appears to dissociate at the
À6
low concentrations used to record such spectra (c=1.0”10
À1
molL ). Complex [PdCl (3b) ] was synthesized from
2
2
[
27]
[
PdCl (MeCN) ] and 3b in 88% yield.
2 2
NMR spectra
The bodipy–phosphines are characterized by the presence of
31
several NMR active nuclei and, in particular, the P NMR spec-
31
tra display interesting features. In the P spectra of 3a and
4
31
19
3
b, large J( P– F) coupling constants are observed (3a: J=
[
28]
4
4
5.5 Hz; 3b: J=52.4 Hz; Figure 1). The same corresponding
J( P– F) values are much smaller in the respective phos-
31
19
phine–metal complexes; for example, in the gold complex
(
(
[AuCl(3a)], J is 15.4 Hz, and it is even smaller in [AuCl(3b)]
J<2 Hz; Figure 1). Our explanation for this observation is
based on the Fermi contact term mechanism, which accounts
19
31
for the coupling between F and P and the efficiency of
which increases with increasing s-orbital participation in the
[
29]
bonds connecting the two nuclei.
In the phosphine, the
phosphorus lone pair is partially delocalized into the electron-
deficient bodipy p-system. This leads to increased s-content in
the respective PÀC(bodipy) bond. In the gold complexes, as
well as in the other metal complexes, this lone pair is donated
to the transition metal rather than to the bodipy, leading to
a decrease in the s-content of the PÀC(bodipy) bond and con-
sequently to a decrease in the coupling constant. A related
Scheme 3. Synthesis of metal complexes of bodipy–phosphines. Reaction
conditions: a) [AuCl(Me S)], CH Cl , RT, 30 min; b) CuI, MeCN, RT, 1 h;
c) AgNO , KCl, water/THF, RT, 3 h; d) [PdCl (MeCN) ], CH Cl , RT, 12 h.
2
2
2
19
13
phenomenon concerning F– C coupling constants was ob-
served in fluorine-containing crown ether complexes, in which
the magnitude of the coupling constants turned out to be an
3
2
2
2
2
[
30]
following established procedures for complexes of Ph P with
effective gauge of the fluorine–metal interactions.
3
[
24]
[25]
[26]
107,109
31
copper (X=I),
silver (X=Cl),
and gold (X=Cl).
The
In [AgCl(3b)], a
cal value for Ag–phosphine complexes of J=617 Hz. The
signal is severely broadened (n1/2 =150 Hz) and this line broad-
Ag– P coupling is detected, with a typi-
[
31]
31
copper and the silver complexes with 3a and 3b are signifi-
cantly less stable than the respective gold complexes. Solu-
tions of the copper complexes decompose within a few hours,
whereas the silver complexes decompose when the solutions
are exposed to air. The most stable complexes are formed with
the electron-rich phosphine 3b and gold. Nonetheless, NMR
P
1
07
ening prevents the determination of the individual Ag or
109
31
Ag coupling with P. Such signal broadening is also ob-
31
19
served for [CuI(3a)] and [CuI(3b)] and potential P– F cou-
pling could not be resolved. Such exchange-broadened
31
1
Figure 1. P{ H} NMR spectra of phosphine 3b (left) and [AuCl(3b)] (right). Total range displayed for each spectrum is 300 Hz.
Chem. Eur. J. 2016, 22, 6353 – 6360 www.chemeurj.org ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6355

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3
1
P NMR signals are typical for Ag– and Cu–phosphine com-
Table 1. Optical properties of bodipy derivatives.
[
31]
plexes and reflect the lability of such species in solution.
The coordination of bodipy phosphines 3a and 3b to Au
[a]
Compound
F
Absorbance lmax [nm] Emission lmax [nm]
3
1
led to a significant deshielding of the P resonance from d=
À20.3 ppm for 3a to d=20.2 for [AuCl(3a)] and from d=
À14.7 ppm for 3a to d=47.5 for [AuCl(3b)]. The more pro-
1
2
3
3b
4
<0.001
0.093
<0.001
<0.001
0.026
0.008
0.016
0.073
0.087
529
506
531
519
508
552
516
515
515
508
510
510
540
–
519
–
a
3
1
–
nounced deshielding of the P NMR resonance in 3b (com-
pared to 3a) upon metal coordination is in line with a more ef-
ficient donation of the phosphorus lone pair in [AuCl(3b)] to
gold.
a
524
567
527
528
531
523
526
526
–
[
AuCl(1)]
[CuI(3b)]
[
[
[
[
[
[
AgCl(3b)]
AuCl(3a)]
Au(NTf
AuCl(3b)]
Au(CCPh)(3b)]
2
)(3b)]
0.11
0.096
0.070
X-ray crystal structure analysis of [AuCl(3a)]
[
b]
PdCl
2
(3b)
2
]
<0.001
Single crystals of [AuCl(3a)] were grown by slow evaporation
of a cyclohexane/CH Cl solution (Figure 2). The geometrical
[a] F values were determined in 1,2-dichloroethane versus rhodamine
01 (F=1.0 in EtOH) at RT with a concentration range of 0.25–1 mm;
[b] estimated value (see the Supporting Information).
2
2
1
features of the complex are very similar to those of the related
[
32]
[
AuCl(PPh )] complex [AuÀP 223.08(5), AuÀCl 229.15(5) pm].
3
Gold has a coordination number of two and adopts linear co-
ordination geometry. The distance from gold to the nearest
fluorine atom is 325.3(5) pm, which is too long to consider
(and emission) maxima of approximately 10 nm. Like other 3-
heteroatom-substituted bodipy compounds, such as the 3-
[30a]
a significant Au···F interaction.
[
21b]
amino-substituted bodipy,
b are essentially nonfluorescent with fluorescence quantum
yields of F<0.001. It is likely that the phosphorus lone pair
the bodipy–phosphines 3a and
3
[
3]
engages in PET quenching of the excited state. In case this is
true, the respective complexes of 3a and 3b with transition
metals coordinated through the phosphine lone pair can be
expected to display brighter fluorescences, unless the presence
of the metal opens new relaxation pathways. Indeed, the re-
+
+
spective metal complexes of Ag and Au are fluorescent
2
+
with Fꢀ0.1, whereas that of Pd is nonfluorescent. Spectro-
scopic reporter groups are important since the d ions Cu ,
10
+
+
+
Ag , and Au do not show d–d transitions in the UV/Vis spec-
trum. Interestingly, the phosphine oxide 4a is fluorescent (F=
0
.026), but less so than the metal complexes, which is consis-
[
34]
tent with the results of a study on fluorescent phospholes.
Monitoring ligand substitution reactions by fluorescence
spectroscopy
Figure 2. X-ray crystal structure analysis and ORTEP representation of X-ray
crystal structure of [AuCl(3a)]: Thermal ellipsoids are set at 50% probabili-
In contrast to the bright bodipy–NHC conjugates recently re-
[
35]
[
9,10]
ported,
the fluorescence in the bodipy–phosphines report-
ty (color coding: Au yellow, P orange, F greenish yellow, B pink, Cl green,
o
N blue). Important bond lengths (pm) and angles ( ): AuÀP 222.7(2), AÀCl
ed herein is largely quenched, probably due to PET-type
quenching of the fluorophore. However, once the phosphorus
lone pair is donated to a metal, an increase in the fluorescence
quantum yields is likely, unless the metal itself opens new
2
29.8(3), Au···F 325.3(5), 377.1(6); P-Au-Cl 174.88(8).
Optical properties
I
quenching pathways. Au is normally a non-quenching metal
Fluorescence quantum yields and absorbance and emission
maxima of the newly synthesized bodipy compounds are sum-
marized in Table 1. All fluorophores reported herein display
a strong UV/Vis absorbance at 506–552 nm and are character-
ized by small Stokes shift of approximately 15 nm. The small
Stokes shifts point to a similar geometry of the excited and
ion and we therefore decided to study different ligand substi-
tution reactions in the stable complex [AuCl(3b)]. This is espe-
cially interesting with a view to the prominent role that closely
[
36]
related complexes have played in homogeneous catalysis.
Gold–thiolato complexes
[33]
the ground state of the fluorophore. Replacing Cl with PPh2
or PCy leads to a long-wave shift of the absorbance maximum
The addition of an alkanethiol (1-dodecanethiol) to [AuCl(3b)]
leads to an exponential decrease in the fluorescence intensity
within approximately two minutes; upon addition of base
2
(
Dl= +13 or +25 nm); complexation of the phosphorus by
group 10 metals leads to short-wave shift in the absorbance
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alkyl thiols can be explained based on the much higher acidity
of aryl thiols. The analogous experiments (addition of 1 equiv
thiol and 1 equiv Et N to [AuCl(3b)], albeit at ca. 10000 times
3
higher concentration of gold complex) were carried out in
31
NMR tubes. Resonances detected in the P NMR spectra of the
nonfluorescent solutions show the clean transformation of
[
AuCl(3b)] (d=47.5 ppm) into [Au(SC H )(3b)] (d=49.6 ppm)
12 25
or into [Au(SPh)(3b)] (d=50.5 ppm); no other peaks are ob-
31
served in the P NMR spectra.
Gold complexes with weakly coordinating anions
Figure 3. Fluorescence intensity vs. time plot for the reaction of thiol (do-
Based on the results with Au thiolates, a decrease in the elec-
tron-density at the transition metal could lead to enhanced
brightness. We were therefore interested, whether it is possible
to study the formation of “cationic” gold complexes at catalyti-
cally relevant concentrations using fluorescence spectroscopy.
This was tested by replacing the strongly coordinating chloride
in [AuCl(3b)] with weakly coordinating (donating) anions
À6
À1
decanethiol) with [AuCl(3b)] in 1,2-dichloroethane (c=1.0”10 molL
and base (Et N). Order of addition of reactants: [AuCl(3b)]+Et N then thiol
trace a) or [AuCl(3b)]+thiol then Et N (trace b).
)
3
3
(
3
(
(
Et N), the fluorescence of this complex nearly vanishes
3
[
37]
Figure 3). Upon addition of an arylthiol (thiophenol or thio-
[
40]
(
wca), with the help of silver salts. Such reactions are very
cresol) to [AuCl(3b)] the decrease in fluorescence is more pro-
nounced, leading to an almost quenched complex (the addi-
tion of base produces only a small additional decrease in the
important in gold catalysis, often constituting the crucial acti-
[
41]
vating step in numerous gold-catalyzed transformations.
Complexes such as [LAu(wca)] are generated in situ from
brightness). The addition of Et N alone to [AuCl(3b)] does not
3
[
LAuX] (X=Cl, Br, I) through silver salt metathesis involving
have a significant influence on the fluorescence intensity, the
fluorescence is only quenched once the thiol is added. This
[
42]
precipitation of AgX,
but may also be isolated and then
[
43]
used in catalysis. The nature of the anion and of the solvent,
in which such reactions are carried out, exert a very significant
influence on the formation of solvent-separated ions (vs. close
shows that the addition of thiol and Et N leads to the forma-
3
[
38]
tion of the well-established gold thiolato complexes, with
a significantly increased electron density at the gold. It is likely
that this increase in electron density enhances the PET quench-
ing of the fluorophore because the electron-rich gold is less
[
44]
ion-pairs) and consequently on catalytic activity.
To a solution of [AuCl(3b)] in 1,2-dichloroethane were added
aliquots corresponding to 0.5 equivalents of [Ag(ONf)] (ONf=
[11a]
willing to accept the phosphorus lone pair.
This hypothesis
À
C F SO ) in the same solvent (Figure 4). The fluorescence in-
4
9
3
is supported by work from Nagano and co-workers on organic
fluorophores, who established a qualitative relationship be-
tween the electron density (rather the energy of the HOMO) of
tensity of the solution increased following the addition of each
aliquot and reaches saturation after adding approximately
2.5 equivalents of silver salt, corresponding to an increase in
[39]
a quencher and the fluorescence quantum yield.
[
45,46]
fluorescence brightness of approximately 20%.
Addition of
It is not clear why the addition of alkanethiol leads to
a smaller decrease in the fluorescence intensity than with the
arylthiol, but the different acidities of alkanethiol and arylthiol
might play an important role. Two explanations are consid-
ered: a) the thiol replaces the chloro ligand forming a cationic
more silver salt (up to 20 equiv) did not lead to significant
change in the fluorescence intensity. It is likely that the fluores-
À
cence increase is due to ONf being less electron-donating
than chloride. To test this, [Au(NTf )(3b)] was synthesized and
2
+
À
complex of the type [Au(RSH)(3b)] Cl ; b) alternatively, an
equilibrium of the type [AuCl(3b)]+RSH=[Au(SR)(3b)]+HCl is
established. In order to probe hypothesis (a) the reaction of
[
AuCl(3b)] with thiophene was studied in the fluorescence
spectrometer. No change in the fluorescence was observed
upon addition of thiophene and therefore coordination of the
thiol is less likely. To verify hypothesis (b), different amounts of
thiol (50, 100, 200, 400 equiv) were added to [AuCl(3b)]. De-
pending on the amount of added thiol, a corresponding de-
crease in the fluorescence intensity was observed, with more
thiol leading to successively weaker fluorescence. The fluores-
cence is always fully quenched upon addition of base to this
reaction mixture. This observation is in line with shifting the
equilibrium [according to (b)] upon addition of more alkane-
thiol to the product side. Based on this equilibrium, the much
larger initial drop of brightness with aryl thiols as compared to
Figure 4. Fluorescence intensity vs. time plot for the addition of six aliquots
of [Ag(ONf)] (0.5 equiv; denoted by arrows) to a solution of [AuCl(3b)]
(c=1.0”10 molL ) in 1,2-dichloroethane.
À6
À1
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the fluorescence intensity was found to be almost the same as
that of the in situ prepared gold nonaflate. However for both
amount of silver in the reaction mixture has a significant influ-
ence on the evolution of the fluorescence intensity. In the
presence of large amounts of [Ag(ONf)] (>10 equiv) a very fast
increase in the brightness is observed. When applying less
silver salt, the reaction with acetylene is sluggish or does not
take place. The need for a large excess of silver salt is not obvi-
ous, since our previous experiments had shown that
[Au(ONf)(3b)] is already formed with approximately 2.5 equiva-
lents of silver salt. However, it is well known that for many Au-
catalyzed reactions, a significant rate acceleration occurs in the
[
Au(wca)(3b)] complexes, the change in fluorescence intensi-
ties are rather modest compared to those seen in the other re-
actions reported herein. This may well mean that in the 1,2-di-
chloroethane solvent, cation and anion still form fairly close
ion pairs, which is reasonable with a view to the very low do-
[47,48]
nicity of 1,2-dichloroethane.
The experiments clearly show
that the reaction of gold complex with [Ag(ONf)] requires su-
perstoichiometric amounts of silver salt to form the gold nona-
flate complex. When smaller amounts of silver salt intermedi-
ate species are used, presumably Au···Cl···Au species are
[
42b,44a,54]
presence of a large excess of silver salt.
Based on these
results, we conclude that the coordination of the alkyne to the
gold and alkyne deprotonation are critical steps. Consequently,
the coordination of alkyne to excess silver salt appears to acti-
vate this substrate for the reaction with gold, leading to an
overall faster reaction.
[
49]
formed. This fits well with results reported by Echavarren
and co-workers, who showed that a significant excess of
[
Ag(OTf)] is required to produce the related [(JohnPhos)-
Au(OTf)] from [(JohnPhos)AuCl], as demonstrated by NMR
[
49]
studies.
Our studies confirm the results from Echavarren’s
The reactions of acetylenes with cationic gold complexes
can lead to several different species, including a side-on bound
group; however, in our study such information are obtained at
[50]
catalytically relevant concentrations.
complex [LAu(PhCCH)],
a
terminal acetylide complex
+
[55]
[
LAu(CCPh)], and a dinuclear species [(LAu) (CCPh)] . Based
2
on the fluorescence intensity alone, it is not possible to identi-
fy the formed species, but we were able to gather sufficient
evidence to identify the formed complex. The side-on complex
Gold–acetylene complexes
Next, we were interested to learn whether the modulation of
brightness could be used to probe the initial steps defining
gold-catalyzed reactions with acetylenes. Key species in such
is a fragile species and only a few examples of such complexes
[51a]
have been reported.
In the presence of base, this complex
[56]
is easily converted into the terminal acetylide. However, in
the reactions presented herein, the addition of an excess of
2
[51]
reactions include h -bonded acetylenes,
s-bonded acety-
[
52]
+ [52,53]
lides,
and the cationic digold species [(LAu) (CCR)] .
2
base (Et N) does not lead to significant changes in the bright-
3
Changes in the coordination sphere should influence the elec-
tron density at gold and lead to the characteristic modulation
of the fluorescence intensity.
ness. The side-on complexes with PhCCH are characterized by
[52,57]
modest formation constants,
which render their presence
À6
À1
in the 10 molL solutions of Au complex less likely. To probe
the potential formation of a side-on bound complex, the cat-
ionic gold complex was exposed to 1000 equivalents of 4-
octyne, which is known to form more stable complexes with
To a solution of [AuCl(3b)] in 1,2-dichloroethane were added
1
–30 equivalents of [Ag(ONf)]. The formation of [Au(ONf)(3b)]
is accompanied by a modest increase in the fluorescence in-
tensity (figs. 4 and 5). To the in situ-generated gold nonaflate
were added 1000 equivalents of phenyl acetylene, leading to
a very pronounced increase in the brightness (Figure 5). The
+
[57]
LAu than PhCCH. However, no change in the fluorescence
brightness was observed and based on this evidence we con-
clude that the side-on acetylene-gold complex is not formed
in such dilute solutions.
The terminal acetylide complex is another potential reaction
product. However, this neutral complex should not lead to the
observed pronounced increase in the fluorescence level, but
instead to a significant decrease. To further clarify this,
[
Au(CCPh)(3b)] was synthesized and the fluorescence quantum
yield determined, which is much lower than that of [AuCl(3b)]
Table 1). For this reason, we believe the species formed
(
in our fluorescence experiments to be the dinuclear
+
[52–53]
[
{(3b)Au} (CCPh)] .
Closely related species with SIPr-type
2
N-heterocyclic carbene (NHC) ligands instead of 3b are well
[
51a]
known.
The cationic nature of this complex, is in accord
with the pronounced increase in fluorescence intensity. A reac-
tion mixture containing the dinuclear species, based on a pro-
Figure 5. Fluorescence intensity vs. time plot for the reactions of [AuCl(3b)]
[
52]
À6
À1
cedure by Widenhoefer and co-workers, was probed by NMR
spectroscopy (albeit at much higher concentration than in the
fluorescence experiment). This experiment provides clear evi-
dence for the formation of the dinuclear species. However, we
were not able to isolate this complex in its pure state due to
its instability. The phosphine ligand 3b is most likely insuffi-
(
[
c=1.0”10 molL ) in 1,2-dichloroethane with different amounts of
Ag(ONf)] (added after ca. 1.5 min: trace f: +1 equiv [Ag(ONf)]; trace e:
+
5 equiv; trace d: +10 equiv; trace c: +15 equiv; trace b: +20 equiv;
trace a: +30 equiv) to [AuCl(3b)], followed by addition of phenyl acetylene
1000 equiv added after 2.5–3.4 min). Traces a–f denote the fluorescence-
(
time changes after addition of alkyne to gold complexes activated with dif-
ferent amounts of [Ag(ONf)].
Chem. Eur. J. 2016, 22, 6353 – 6360
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6358
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
ciently bulky to render a stable dinuclear complex. Further-
more, NMR signals are observed for the known vinyl triflate,
which is the addition product of PhCCH and TfOH. Based
of the reaction of [AuCl(3b)] with [Ag(ONf)] and different ace-
tylenes. The nature of the silver additive appears to be impor-
tant both as an activator to generate the “cationic” gold com-
plex and as an activator of the alkyne. The large excess (>
15 equiv) of silver salt needed to accelerate the reaction with
the alkyne, may explain why several gold-catalyzed reactions
perform better in the presence of excess silver salt—even
though the “cationic” species is already formed with approxi-
mately 2.5 equivalents of silver salt.
on the fluorescence intensity of the in situ-formed
+
[
{(3b)Au} (CCPh)] , the fluorescence quantum yield of this
2
complex is estimated Fꢀ0.4.
Finally, we were interested to find out whether the modula-
tion of the electronic nature of the phenyl acetylene has an in-
fluence on the fluorescence intensity of the remote bodipy
unit. This was tested by using phenyl acetylenes with electron-
We believe that the observation of the modulated fluores-
cence intensity in (catalytically active) transition metal com-
plexes with suitable fluorescent tags renders a highly useful
tool for the analysis of chemical transformations.
withdrawing (R=NO ) and electron-donating groups (R=
2
NMe ) in the 4-position. The respective digold complexes
2
+
[
{(3b)Au} (CCC H -R)] were generated with the aid of Hünig’s
2
6
4
base in the cuvette and the fluorescence intensity measured
Figure 6). The fluorescence experiment proved to have excel-
(
Acknowledgements
lent sensitivity, since even the remote 4-NMe substituent led
2
to a significant decrease in the fluorescence brightness of the
peripheral bodipy unit, whereas the 4-NO2 group caused
a small increase in the (already high) brightness of the fluores-
cence emission.
This work was supported by the DFG (grant Pl 178/18-1). We
wish to thank Sabine Foro for the solution and the refinement
of the crystal structure of [AuCl(3a)]. We are grateful to
Heraeus Precious Metals GmbH & Co. KG for generous support
with gold salts.
Keywords: alkynes
·
bodipy
·
fluorescence
·
gold
·
homogeneous catalysis
[
[
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Chem. Eur. J. 2016, 22, 6353 – 6360
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Published online on March 24, 2016
Chem. Eur. J. 2016, 22, 6353 – 6360
www.chemeurj.org
6360
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim