Cationic gold(I) π-complexes of terminal alkynes and their conversion to dinuclear σ,π-acetylide complexes

Article abstract of DOI:10.1021/om200840g

Treatment of a suspension of (IPr)AuCl [IPr = 1,3-bis(2,6- diisopropylphenyl)imidazol-2-ylidene] and AgSbF₆ with terminal arylacetylenes led to the formation of thermally unstable gold π-alkyne complexes of the form [(IPr)Au(η ²-HC≡ CAr)]+SbF ₆ ^- in ≥86 ± 5% yield, which were characterized by spectroscopy without isolation. Warming these complexes to 0 °C led to C(sp)-H bond cleavage and formation of dinuclear gold(I) σ,π-acetylide complexes of the form {[(IPr)Au]₂(η 1,η ²-C≡ CAr)}+SbF₆ ^-, three of which were isolated in 99% yield and one of which was characterized by X-ray crystallography. ₁H NMR analysis of the conversion of gold π-arylacetylene complexes to σ,π-acetylide complexes established protonation of free arylacetylene, indicating the generation of a strong Bronsted acid under reaction conditions.

Full text of DOI:10.1021/om200840g

Article
pubs.acs.org/Organometallics
Cationic Gold(I) π-Complexes of Terminal Alkynes and Their
Conversion to Dinuclear σ,π-Acetylide Complexes
Timothy J. Brown and Ross A. Widenhoefer*
Department of Chemistry, French Family Science Center, Duke University, Durham, North Carolina 27708-0346, United States
S
* Supporting Information
ABSTRACT: Treatment of a suspension of (IPr)AuCl [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] and AgSbF6
with terminal arylacetylenes led to the formation of thermally unstable gold π-alkyne complexes of the form [(IPr)Au(η 2-HC
CAr)]+SbF6− in ≥86 ± 5% yield, which were characterized by spectroscopy without isolation. Warming these complexes to 0 °C
led to C(sp)−H bond cleavage and formation of dinuclear gold(I) σ,π-acetylide complexes of the form {[(IPr)Au]2(η1,η2-C
CAr)}+SbF6 , three of which were isolated in 99% yield and one of which was characterized by X-ray crystallography. H NMR
analysis of the conversion of gold π-arylacetylene complexes to σ,π-acetylide complexes established protonation of free
arylacetylene, indicating the generation of a strong Brønsted acid under reaction conditions.
−
1
transformations represent only a small subset of the gold(I)-
catalyzed transformations that employ terminal alkynes, and
the potential involvement of gold acetylide intermediates in
this larger collection of transformations remains largely
unexplored.8 Noteworthy in this regard is the combined
experimental and computational analysis of the cycloaddition of
1,5-allenynes that invoked dual activation of the terminal CC
moiety through formation of a dinuclear gold σ,π-acetylide
species.11
Despite the widespread employment of terminal alkynes in
gold(I)-catalyzed transformations and the potential involve-
ment of gold acetylide complexes in these transformations, little
experimental information is available regarding the conversion
of gold π-(terminal alkyne) complexes to gold σ-acetylide
complexes. This deficiency is due in large part to the scarcity of
well-defined gold π-(terminal alkyne) complexes, which contrasts
with the large number of known gold σ-acetylide complexes,12
including those containing N-heterocyclic carbene ligands,13
and the growing number of gold(I) π-(internal alkyne) com-
plexes.14,15 In a singular and particularly relevant example,
Bertrand has described the isolation of a gold(I) π-phenyl-
acetylene complex containing a cyclic alkyl amino carbene
(CAAC) ligand and the conversion of this complex to the
corresponding σ-acetylide complex upon treatment with diethyl-
amine.16 Here we report the in situ generation and spectroscopic
characterization of thermally unstable gold π-arylacetylene
INTRODUCTION
■
The formation of coinage metal acetylide complexes from ter-
minal alkynes under basic conditions is a process central to a
number of important synthetic procedures.1 Notable among
these are the palladium/copper-catalyzed coupling of terminal
alkynes with aryl or alkenyl halides (Sonogashira reaction)2 and
the copper-catalyzed 1,3-dipolar cycloaddition of azides and
terminal alkynes.3 A diverse range of transformations likewise
exploit the reactivity of silver acetylide complexes generated
in situ from terminal alkynes.4 The generally accepted mech-
anism for conversion of a terminal alkyne to a copper or silver
acetylide involves initial formation of a transient metal π-alkyne
complex, which activates the acetylenic proton toward deproto-
nation by external base.1−5 DFT analysis of the copper-catalyzed
1,3-dipolar cycloaddition of azides and terminal alkynes suggests
that the pKa of an alkynyl proton decreases by 10 pKa units upon
coordination to Cu(I).6
Gold(I) complexes are particularly active catalysts for the
functionalization and cycloaddition of alkynes,7,8 reactivity that
is presumably initiated by π-complexation of the alkyne to the
electrophilic gold(I) center, which renders the alkyne sus-
ceptible toward nucleophilic attack. The depletion of π-electron
density from the alkyne upon coordination to the electrophilic
gold(I) center should also enhance the acidity of the acetylenic
proton, facilitating acetylide formation. Indeed, there is a grow-
ing body of gold-catalyzed transformations of terminal alkynes
in which the involvement of gold(I) acetylides is implicit,9
notably the coupling of terminal alkynes with secondary amines
and aldehydes to form propargylic amines.10 However, these
Received: September 6, 2011
Published: October 19, 2011
© 2011 American Chemical Society
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complexes containing an N-heterocyclic carbene ligand and the
facile conversion of these complexes to dinuclear gold η1,η2-
acetylide complexes in the absence of external base.
Table 2. Equilibrium Constants for Displacement of NCArF
[ N C A r F
=
N  C - 3 , 5 - C 6 H 3 ( C F 3 ) 2 ] f r o m
[(IPr)Au(NCArF)]+SbF6 (2) with para-Substituted
Arylacetylenes in CD2Cl2 at −60 °C
−
RESULTS AND DISCUSSION
■
Synthesis of Gold π-Complexes of Terminal Alkynes.
Treatment of a suspension consisting of a 1:1 mixture of the
gold(I) N-heterocyclic carbene complex (IPr)AuCl [IPr = 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene] and AgSbF6 with
phenylacetylene (1.0 equiv) in CD2Cl2 at −78 °C for 10 min
led to formation of the cationic gold π-alkyne complex
Ar
Keq
4-C6H4OMe
4-C6H4Me
C6H5
1.25 ± 0.07
0.52 ± 0.03
0.21 ± 0.02
[(IPr)Au(η2-HCCC6H5)]+SbF6 (1a) in 92 ± 5% yield by
−
1H NMR analysis. Complex 1a was thermally unstable (see
below) and was characterized in solution by H and 13C NMR
1
4-C6H4Br
4-C6H4CF3
0.065 ± 0.006
0.025 ± 0.003
spectroscopy at −60 °C. Coordination of phenylacetylene to
gold was established by the downfield shift of the acetylenic
proton of 1a (δ 4.56) relative to that of free phenylacetylene
alkynes to the 12-electron gold fragment (IPr)Au+, we likewise
determined equilibrium constants for the displacement of
NCArF from 2 with para-substituted arylacetylenes HCC-4-
C6H4X (X = OMe, Me, Br, CF3).17,18 The relative binding
affinities of the arylacetylenes decreased with decreasing electron
density by a factor of ∼50 (Table 2), and a plot of log(KX/KH)
versus the Hammett σ-parameter displayed a slope of ρ = −2.1 ±
0.3 (Figure 1). While the magnitude of the binding affinities of
1
(δ 3.06) in the H NMR spectrum and by the difference in the
13C NMR shifts of the acetylenic carbon atoms of 1a [δ 72.7 (d),
93.5 (s)] relative to free [δ 77.3 (d), 83.7 (s)] phenylacetylene.
1
The JCC coupling constant of the phenylacetylene ligand of
the 13C-isotopomer [(IPr)Au(η 2-H13CCC6H5)]+SbF6
−
(1a-13C1) (1JCC = 133.8 Hz) was diminished significantly
relative to free phenylacetylene (1JCC = 175.9 Hz). This
behavior contrasts the minimal diminution (∼6 Hz) of the 1JCC
of the isobutylene ligand of [(IPr)Au(η2-H213CCMe2)]+SbF6−
relative to free isobutylene17 and points to a more significant
π-back-bonding component to the gold π-alkyne bond than to
the gold π-alkene bond. In addition to 1a, thermally unstable
gold π-alkyne complexes [(IPr)Au(η2-HCCAr)]+SbF6− [Ar =
4-C6H4Me (1b), 4-C6H4CF3 (1c), 4-C6H4OMe (1d), and
4-C6H4Br (1e)] were generated and characterized in situ by
NMR at −60 °C (Table 1).
Table 1. Synthesis of Gold π-Complexes of Arylacetylenes
Figure 1. Plot of log KX/KH versus the Hammett σ-parameter for the
equilibrium displacement of HCArF from 2 with arylacetylenes in
CD2Cl2 at −60 °C (ρ = −2.1 ± 0.2).
Ar′
cmpd
yield
arylacetylenes to (IPr)Au+ are similar to vinyl arenes, the
electronic dependence of the binding affinity of arylacetylenes is
somewhat diminished relative to vinyl arenes (ρ = −2.4),17
suggesting a greater π-back-bonding contribution to the Au−
(π-alkyne) bond relative to the Au−(π-alkene) bond. Never-
theless, the large negative ρ value points to a gold−alkyne
interaction dominated by σ-donation from the alkyne CC
bond to gold.19
Ph
1a
1b
1c
1d
1e
92 ± 5%
97 ± 5%
99 ± 5%
86 ± 5%
91 ± 5%
4-C6H4Me
4-C6H4CF3
4-C6H4OMe
4-C6H4Br
Binding Affinity of Arylacetylenes to (IPr)Au+. Arylace-
tylenes bind weakly to the 12-electron gold fragment (IPr)Au+
and were readily displaced by weak two-electron σ-donors such
as triflate anion and 3,5-bis(trifluoromethyl)benzonitrile
(NCArF). In the case of the former, reaction of an equimolar
mixture of (IPr)Au(OTf) and phenylacetylene at −60 °C in
CD2Cl2 generated a 3:1 equilibrium mixture of 1a and
(IPr)Au(OTf). In the latter case, treatment of [(IPr)Au-
(NCArF)]+SbF6− (2) with phenylacetylene (1 equiv) at −60 °C
in CD2Cl2 generated a 4.6:1.0 equilibrium mixture of 2 and 1a,
which corresponds to an equilibrium constant for displacement
of NCArF from 2 with phenylacetylene of Keq = [1a][NCArF]/
[2][phenylacetylene] = 0.21 ± 0.02 (Table 2). To evaluate the
effect of alkyne electron density on the binding affinity of
Formation of Dinuclear Gold σ,π-Acetylide Com-
plexes. Addition of 2,6-di-tert-butylpyridine (DTBP; 1 equiv)
to a solution of 1a (51 mM) at −60 °C led to immediate (<10
min) formation of the dinuclear gold(I) σ,π-acetylide complex
{[(IPr)Au]2(η1,η2-CCC6H5)}+SbF6 (3a) in 91 ± 5% (1H
−
NMR) yield with concomitant release of phenylacetylene in
48% yield (based on 1a; Scheme 1). Conversion of the π-alkyne
ligand of 1a to the σ,π-acetylide ligand of 3a was evidenced by
1
the disappearance of the acetylenic proton at δ 4.56 in the H
NMR spectrum and by the significant downfield shifts of the
acetylenic carbon atoms of 3a [(δ 124.5 (d), 113.0 (s)] relative
to 1a [δ 72.7 (d), 93.5 (s)] in the 13C NMR spectrum. More
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Scheme 1
surprising, however, was that warming a CD2Cl2 solution of
phenylacetylene complex 1a at 0 °C for 1 h in the absence of
base also led to formation 3a in 99 ± 5% (1H NMR) yield along
with phenylacetylene in 41% yield (based on 1a) (Scheme 1). In
a preparative-scale experiment, treatment of a methylene
chloride suspension of (IPr)AuCl and AgSbF6 with phenyl-
acetylene (1 equiv) at room temperature for 10 min followed
by filtration and crystallization gave 3a in 99% yield as air and
thermally stable yellow crystals, which were further charac-
terized by MALDI-MS, combustion analysis, and X-ray
crystallography (see below). These solid-state analyses firmly
established the presence of two (IPr)Au groups per phenyl-
acetylide moiety.
Figure 2. ORTEP diagram of 3a·3CH2Cl2. Ellipsoids are shown at the
50% probability level with solvent, counterion, and hydrogen atoms
omitted for clarity. Selected bond lengths (Å) and angles (deg) for
3a·3CH2Cl2: C1−C2 = 1.214(10), C2−C3 = 1.450(10), Au1−C1 =
2.193(7), Au1−C2 = 2.234(7), Au2−C1 = 1.997(8), Au1−C9 =
1.991(8), Au2−C9a = 2.001(8), Au1−Au2 = 3.624, C1−C2−C3 =
165.3(8), Au2−C1−C2 = 164.4(7), C1−Au2−C9a = 172.0(3), C9−
Au1−C2C3(centroid) = 171.6.
containing trialkyl phosphine ligands have recently been
structurally characterized by Russel15 and Finze.22
Although the (IPr)Au fragments of complexes 3 reside in
nonequivalent environments, the room-temperature 1H and
13C NMR spectra of σ,π-acetylide complexes 3a−c displayed
resonances corresponding to a single type of (IPr)Au group
that neither broadened nor resolved at −60 °C. These
observations point to facile σ,π-interconversion of the two
chemically inequivalent (IPr)Au groups on the NMR time
scale. The minimal process required for this interconversion is
through a symmetric gem-bis(gold) intermediate or transition
state I in which both gold atoms are bound symmetrically to
the terminal acetylenic carbon atom, perhaps involving a three-
center, two-electron bonding interaction with a gold−gold
bond (Scheme 2). The proposed intermediate/transition state I
Gold π-arylacetylene complexes 1b and 1c displayed
reactivity similar to 1a, and warming solutions of complexes
1b and 1c to 0 °C in both cases led to formation of the
corresponding dinuclear gold(I) σ,π-acetylide complexes
{[(IPr)Au]2(η1,η2-CC-4-C4H4R)}+SbF6 [R = Me (3b),
−
CF3 (3c)] as the exclusive gold-containing species. Complexes
3b and 3c were subsequently isolated in 99% yield from the
corresponding preparative-scale reactions of (IPr)AuCl and
AgSbF6 with p-tolylacetylene and p-trifluoromethylphenylace-
tylene, respectively. In comparison, warming CD2Cl2 solutions
of gold π-arylacetylene complexes 1d and 1e to 0 °C led to
apparent formation of the corresponding gold σ,π-acetylide
1
complexes 3d and 3e as judged by H NMR and MALDI-MS
Scheme 2
analysis, but these transformations led to comcomitant decom-
position and significant darkening of the solutions, which
precluded isolation of the pure acetylide complexes.
Slow diffusion of hexanes into a CH2Cl2 solution of 3a at
4 °C gave colorless crystals of 3a·3CH2Cl2 suitable for X-ray
analysis (Figure 2). The solid-state structure of 3a established a
dinuclear complex consisting of an η1,η2-phenylacetylide ligand
σ-bound to one (IPr)Au fragment (Au2−C1 = 1.997 Å) and
π-bound to a second (IPr)Au group (Au1−C1 = 2.193 Å,
Au1−C2 = 2.234 Å). Both gold atoms adopt distorted linear
conformations with C9a−Au2−C1 and C9−Au1−C1
C2(centroid) ≈ 172°. The gold acetylide moiety is bent
(Au1−C1−C2 = C1−C2−C3 = 165°) with no significant
elongation of the CC bond (C1−C2 = 1.214 Å) relative to
that of free phenylacetylene (1.2 Å). The Au1−Au2 distance of
3.624 Å falls outside the range typically attributed to a Au−Au
bonding interaction.20 A large number of homonuclear and
heteronuclear transition metal σ,π-acetylide complexes are
known,21 and similar dinuclear gold σ,π-acetylide complexes
is analogous to the ground-state structures for dinuclear gold
́
σ-alkenyl complexes first proposed by Gagne23 and Toste11 and
later confirmed structurally by Furstner.24
̈
We sought to evaluate the stability of the gold π-acetylide bond
of 3a toward two-electron donor ligands and the σ-acetylide bond
of 3a toward protodeauration. Regarding the former, treatment of
a CD2Cl2 solution of 3a (22 mM) with pyridine (1 equiv) at
25 °C for 10 min led to complete displacement of the π-bound
acetylide group to form a ∼1:1 mixture of the mononuclear
acetylide complex (IPr)Au(η 1-CCPh) (4a) and the cationic
pyridine complex [(IPr)Au(pyridine)]+SbF6 (5; Scheme 3);
−
complexes 4a and 5 were unambiguously identified by
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ammonium triflate.27 Similarly, when an equimolar solution of
(IPr)Au(OTf) and phenylacetylene was warmed from −60 to 25
°C, complete conversion of (IPr)Au(OTf) to σ,π-acetylide
complex 3a·OTf was observed within 10 min along with traces
(≤5%) of 7. The concentration of 7 continued to increase with
time, reaching a maximum relative concentration of 33% after
1 h (eq 1).27
Scheme 3
comparison to authentic samples. In comparison, treatment of a
CD2Cl2 solution of 3a (22 mM) with NCArF (17 mM) led to
formation of a 4.9:1.0:1.0:3.5 equilibrium mixture of 3a, 4a, 2,
and NCArF (−60 °C), which corresponds to an equilibrium
The similar spectroscopy of vinyl triflates 6 and 7 as
compared to A and B and the rapid formation of 6 and 7 from
reaction of A and B with tetrabutylammonium triflate suggest
that A and B are vinyl arenes bearing a labile anionic group at
the α-carbon atom. As an additional point of comparison,
treatment of phenylacetylene with magic acid (HSO3F·SbF5;
2 equiv) in CD2Cl2 at −60 °C generated a compound,
presumably 1-phenylethenyl fluorosulfonate (8),28,29 that
constant for displacement of NCArF from 2 with 4a of Keq
=
[3a][NCArF]/[4a][2] = 17.2 ± 1.2. This observation indicates
that the binding affinity of the neutral gold σ-acetylide complex
4a to the (IPr)Au+ fragment exceeds the binding affinity of free
phenylacetylene (Keq = 0.21; Table 2) by a factor of ∼80
(ΔΔG = 1.9 kcal mol−1).
The gold σ-acetylide interaction of 3a was stubbornly resis-
tant to protodeauration, and treatment of a solution of 3a
(30 mM) in CD2Cl2 with triflic acid (1 equiv) at 25 °C for 12 h
led to no detectable decomposition of 3a, as determined by 1H
NMR analysis (Scheme 3). The stability of σ,π-acetylide com-
plex 3a toward Brønsted acid stands in contrast to the rapid
protodeauration of the σ-acetylide complex 4a with HOTf
(−80 °C, 10 min).25 The pronounced attenuation of
1
displayed vinylic H NMR resonances at δ 5.77 (d, J = 4.5
Hz, 1 H) and 5.48 (dd, J = 3.5, 1.0 Hz, 1 H). Warming a
solution of 8 at 25 °C for 1 h led to complete decomposition,28
while treating a solution of 8 with tetrabutylammonium triflate
at −60 °C and then warming to room temperature for 20 min
led to 66% conversion of 8 to 7. Therefore, compounds A and
B appear even less stable and more reactive than 8, pointing
to the presence of an extremely labile anionic group at the
α-position of these compounds. Assigning complexes A and B
to an α-hexafluoroantimonate vinyl arene species is suspect,
̀
protonolysis reactivity of bis(gold) complex 3a vis-a-vis the
mono(gold) complex 4a mirrors the diminished protonolysis
reactivity of bis(gold) σ-alkenyl complexes relative to the
corresponding mononuclear gold σ-alkenyl complexes.23
−
−
however, as the SbF6 or Sb2F11 anions coordinate poorly
to carbenium ions,30 and there was no detectable fluorine
coupling in the H NMR spectra of complexes A and B.
31,32
1
The conversion of π-alkyne complexes 1 to σ,π-acetylide
complexes 3 in the absence of any obvious proton acceptor
pointed to the generation of a strong Brønsted acid under
reaction conditions. To gain insight into the fate of this proton,
an equimolar solution of 1b and 4-ethynyltoluene (51 mM)
was monitored periodically by 1H NMR spectroscopy at
−20 °C. Early in the reaction, in addition to resonances
corresponding to 1b, 3b, and free 4-ethynyltoluene, a pair of
mutually coupled one-proton doublets at δ 5.91 and 5.46
(JHH ≈ 5.0 Hz) and a three-proton singlet at δ 2.90, assigned
to the vinylic and aryl methyl protons of a protonated
p-tolylacetylene derivative (A), were observed. The concen-
tration of this species rose concurrently with 3b to ∼7 mM and
then remained relatively constant as 3b continued to form.26
Warming a solution containing A to 25 °C led to rapid (≤1 min)
disappearance of the resonances corresponding to A without
detectable increase in resonances corresponding to p-tolylacety-
lene. Similarly, treating a solution containing A with tetrabutyl
ammonium triflate at −20 °C led to immediate (≤2 min) and
complete consumption of A to form 1-tolylethenyl trifluoro-
methanesulfonate [6; δ 5.66 (d, J = 4.0 Hz, 1 H), 5.30 (d, J =
4.0 Hz, 1 H), and 2.90 (s, 3 H)] in ∼25% yield.27 Analogous
experiments employing 1a and phenylacetylene formed traces
(≤2.5 mM) of a protonated phenylacetylene derivative (B) [δ
6.02 (d), 5.61 (d), JHH = 5.0 Hz], which decomposed upon
warming to 25 °C and which formed 1-phenylethenyl
trifluoromethanesulfonate (7) upon treatment with tetrabutyl
Likewise, assigning compounds A and B as free 1-arylvinyl
cations is inconsistent with the inequivalence of the β-protons
of A and B.33 The ambiguities in the structures of A and B
notwithstanding, formation of A and B in the conversion of 1b
to 3b and 1a to 3a, respectively, and also the formation of vinyl
triflate 7 in the conversion of (IPr)Au(OTf) and phenyl-
acetylene to 3a·OTf provides convincing evidence for the
generation of a strong Brønsted acid under reaction conditions.
The simplest mechanism that accounts for the conversion of
gold π-arylacetylene complexes 1 to dinuclear σ,π-acetylide
complexes 3 involves deprotonation of the acetylenic proton
either through a two-electron, three-centered transition state or
by C−H activation followed by deprotonation. Trapping of
the resulting monogold acetylide complex 4 with a second
molecule of 1 would then form 3 with concomitant release of
arylacetylene (Scheme 4). The facile protodeauration of 4a
with HOTf suggests that if such a pathway is operational,
conversion of 1 to 4 is endoergonic and the overall conversion
of 1 to 3 is driven by formation of the stable Au−(π-acetylide)
bond. An intriguing mechanistic alternative involves the initial
formation of a transient dicationic bis(gold) π-alkyne complex
II involving both of the orthogonal alkyne π-faces, which would
presumably lead to significant enhancement of the acidity of the
acetylenic proton, followed by deprotonation to form 3
(Scheme 4). In any event, because the conversion of 1 to 3
involves both deprotonation and π-complexation events, it is
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7.52−7.46 (m, 2 H), 7.40 (s, 2 H), 7.27 (d, J = 8 Hz, 4 H), 7.24 (t, J =
8 Hz, 1 H), 6.87 (d, J = 7.5 Hz, 2 H), 4.56 (s, 1 H), 2.35 (sept, J = 6.5
Hz, 4 H), 1.19 (d, J = 6.5 Hz, 12 H), 1.08 (d, J = 7 Hz, 12 H).
13C{1H} NMR: δ 175.2, 145.3, 133.0, 132.7, 130.8, 129.2, 124.5, 124.3,
123.0, 114.7, 93.5, 72.7, 28.5, 24.5, 23.3.
Scheme 4
Gold(I) π-arylacetylene complexes 1a-13C1 and 1b−e were
synthesized using a procedure similar to that used to generate 1a.
[(IPr)Au(η2-H13CCPh)]+SbF6− (1a-13C1): 99 ± 5% yield. 13C{1H}
1
NMR: δ 93.4 (d, JCC = 133.4 Hz), 72.8.
[(IPr)Au(η2-HCC-4-C6H4Me)]+SbF6 (1b): 97 ± 5% yield. 1H
−
NMR: δ 7.57 (t, J = 7.5 Hz, 2 H), 7.43 (s, 2 H), 7.30 (d, J = 7.5 Hz, 4
H), 7.08 (d, J = 7.5 Hz, 2 H), 6.83 (d, J = 7.5 Hz, 2 H), 4.50 (s, 1 H),
2.40 (s, 3 H), 2.35 (sept, J = 6.5 Hz, 4 H), 1.19 (d, J = 6.5 Hz, 12 H),
1.08 (d, J = 6.5 Hz, 12 H). 13C{1H} NMR: δ 175.4, 145.2, 132.9,
131.6, 131.0, 129.9, 129.0, 124.5, 124.3, 111.3, 95.0, 72.2, 28.5, 24.5,
23.4, 21.7.
difficult to estimate the increase in the acidity of an acetylenic
proton resulting from complexation to the 12-electron gold
fragment (IPr)Au+.
[(IPr)Au(η2-HCC-4-CF3C6H4)]+SbF6 (1c): 99 ± 5% yield. 1H
−
NMR: δ 7.59 (t, J = 8 Hz, 2 H), 7.53 (d, J = 7.5 Hz, 2 H), 7.46 (s, 2
H), 7.32 (d, J = 8 Hz, 4 H), 7.06 (d, J = 8 Hz, 2 H), 4.76 (s, 1 H), 2.36
CONCLUSION
(sept, J = 7 Hz, 4 H), 1.20 (d, J = 6.0 Hz, 12 H), 1.10 (d, J = 6.5 Hz,
■
1
12 H). 13C{1H} NMR: δ 174.8, 145.3, 132.9, 132.2, 126.0 (q, JCF
=
In summary, gold π-arylacetylene complexes were generated at
low temperature from reaction of mixtures of (IPr)AuCl and
AgSbF6 with terminal arylacetylenes and were characterized by
spectroscopy without isolation. Above −20 °C, these complexes
underwent C(sp)−H bond cleavage and nucleation to form
stable dinuclear gold σ,π-acetylide complexes, one of which was
characterized by X-ray crystallography. 1H NMR analysis of the
conversion of gold π-alkyne complexes to σ,π-acetylide com-
plexes provided evidence for protonation of free arylacetylene,
indicating the release of a strong Brønsted acid under reaction
conditions.
The facile formation of dinuclear gold σ,π-acetylide
complexes 3 from π-arylacetylene complexes 1 in the absence
of base suggests that gold(I) acetylide complexes may be
generated as intermediates in gold(I)-catalyzed transformations
involving terminal alkynes, although the role of supporting
ligand and alkyne structure in the conversion of gold π-alkyne
to σ,π-acetylide complexes has yet to be delineated. In much
the same way, the potential intermediacy of dinuclear gold
σ-alkenyl complexes in the gold-catalyzed additions of nucleo-
philes to allenes and alkynes is beginning to be appreciated.11,23,24
Likewise, there has been an ongoing concern that a number of
metal-based hydrofunctionalization processes that employ
electrophilic metal complexes, including cationic gold(I)
complexes, in combination with weakly basic nucleophiles
may be catalyzed by a Brønsted acid generated under reaction
conditions.34 In the same way, the generation of a strong
Brønsted acid in the conversion of π-arylacetylene complexes 1
to σ,π-acetylide complexes 3 also suggests that the potential
involvement of Brønsted acid catalysis should be given
appropriate consideration in gold-catalyzed transformations
involving terminal alkynes.
3.65 Hz), 124.7, 124.3, 118.9, 89.8, 75.0, 28.5, 24.6, 23.3. Three
carbons were unaccounted for in the 13C NMR spectrum, most likely
due to diminution of intensity resulting from long-range C−F coupling
combined with complexity in the aryl region of the spectrum.
[(IPr)Au(η2-HCC-4-C6H4OMe)]+SbF6 (1d): 86 ± 5% yield. H
NMR: δ 7.55 (t, J = 8 Hz, 2 H), 7.42 (s, 2 H), 7.30 (d, J = 8 Hz, 4 H),
6.96 (d, J = 8.5 Hz, 2 H), 6.77 (d, J = 8.5 Hz, 2 H), 4.45 (s, 1 H), 3.86
(s, 3 H), 2.33 (sept, J = 7 Hz, 4 H), 1.45−0.90 (m, 24 H). 13C{1H}
NMR: δ 175.6, 163.1, 145.2, 135.6, 132.4, 130.8, 124.4, 124.2, 114.8,
114.3, 98.1, 71.6, 55.8, 28.4, 24.5, 23.3.
−
1
[(IPr)Au(η2-HCC-4-C6H4Br)]+SbF6 (1e): 91 ± 5% yield. 1H
−
NMR: δ 7.60 (t, J = 8 Hz, 2 H), 7.45 (s, 2 H), 7.43 (d, J = 8.5 Hz,
2 H), 7.32 (d, J = 8 Hz, 4 H), 6.79 (d, J = 8 Hz, 2 H), 4.66 (s, 1 H),
2.35 (sept, J = 6.5 Hz, 4 H), 1.20 (d, J = 6.5 Hz, 12 H), 1.11 (d, J = 6.5
Hz, 12 H). 13C{1H} NMR: δ 175.1, 145.2, 133.9, 132.5, 132.2, 131.1,
127.9, 124.6, 124.4, 113.8, 91.9, 74.1, 28.5, 24.6, 23.4.
Determination of Arylacetylene Binding Constants. Phenyl-
acetylene (2.10 mg, 2.4 × 10−2 mmol) was added via syringe to an
NMR tube sealed with a rubber septum that contained a CD2Cl2
solution of [(IPr)Au(NCArF)]+SbF6− (2; 25 mg, 2.4 × 10−2 mmol) at
−60 °C. The tube was shaken, placed in the probe of an NMR
spectrometer cooled at −60 °C, and allowed to equilibrate for 10 min.
The relative concentrations of 2, 1a, NCArF, and phenylacetylene were
determined by integrating the resonances corresponding to the
aromatic protons of bound [δ 8.29, 8.25 (1:2)] and free [δ 8.15, 8.14
(2:1)] NCArF and the resonances corresponding to the acetylenic
proton of bound (δ 4.56) and free (δ 3.15) phenylacetylene. From
these data, an equilibrium constant of Keq = [1a][NCArF]/
[2][phenylacetylene] = 0.21 ± 0.02 was determined.
Similar procedures were employed to determine the equilibrium
constants for the displacement of NCArF from 2 with p-tolylacetylene,
4-ethynylanisole, 1-bromo-4-ethynylbenzene, and 4-ethynyl-α,α,α-
trifluorotoluene in CD2Cl2 at −60 °C. Equilibrium data are collected
in Table 2.
Synthesis of−Digold σ,π-Acetylide Complexes. {[(IPr)Au]2(η1,η2-
CCPh)}+SbF6 (3a). A suspension of (IPr)AuCl (40 mg, 6.5 ×
10−2 mmol), AgSbF6 (23 mg, 6.5 × 10−2 mmol), and phenylacetylene
(10.2 mg, 0.100 mmol) in CH2Cl2 (2 mL) was stirred at room
temperature for 10 min. The resulting suspension was filtered through
Celite, and the filtrate concentrated to 0.5 mL. Slow diffusion of
hexanes into the concentrated CH2Cl2 solution at 4 °C for 16 h gave
3a (49 mg, 99%) as yellow crystals. 1H NMR: δ 7.54 (t, J = 8 Hz, 4 H),
7.34−7.22 (m, 13 H), 7.10 (t, J = 8 Hz, 2 H), 6.71 (dd, J = 1.5, 8.5 Hz,
2 H), 2.47 (sept, J = 6.5 Hz, 8 H), 1.19 (d, J = 7 Hz, 24 H), 1.09 (d,
J = 7 Hz, 24 H). 13C NMR: δ 182.8, 145.9, 134.0, 132.2, 131.2, 129.9,
128.8, 124.7, 124.5, 124.1, 120.9, 113.0, 29.1, 24.6, 24.1. MALDI-MS
calcd (found) for C62H77N4Au2 (M+): 1271.5 (1271.4). Anal. Calcd
(found) for C62H77N4F6Au2Sb: H, 5.15 (5.03); C, 49.38 (49.19); N,
3.72 (3.71).
EXPERIMENTAL SECTION
■
Synthesis of− Gold(I) π-Acetylene Complexes. [(IPr)Au(η2-
HCCPh)]+SbF6 (1a). Phenylacetylene (3.3 mg, 3.2 × 10−2 mmol)
and 1,3-dimethoxybenzene (1.0 μL, 7.6 μmol; internal standard) were
added via syringe to an NMR tube capped with a rubber septum that
contained a suspension of (IPr)AuCl (20 mg, 3.2 × 10−2 mmol) and
AgSbF6 (11 mg, 3.2 × 10−2 mmol) in CD2Cl2 at −78 °C. The tube
was shaken and placed in the probe of an NMR spectrometer
precooled at −60 °C. 1H NMR analysis after 10 min revealed
formation of 1a as the only (IPr)Au-containing product in 92 ± 5%
yield based on integration of the acetylenic resonance of 1a at δ 4.56
relative to the methyl resonance of 1,3-dimethoxybenzene at δ 3.8 in
the 1H NMR spectrum. 1H NMR: δ 7.55 (t, J = 7.5 Hz, 2 H),
6007
dx.doi.org/10.1021/om200840g|Organometallics 2011, 30, 6003−6009

Organometallics
Article
Gold(I) σ,π-dinuclear acetylide complexes 3b and 3c were
synthesized using a procedure similar to that used to synthesize 3a.
́
́
Brouwer, C.; He, C. Chem. Rev. 2008, 108, 3239. (e) Jimenez-Nunez,
̃
E.; Echavarren, A. M. Chem. Rev. 2008, 108, 3326.
(8) (a) Jimenez-Nunez, E.; Echavarren, A. M. Chem. Rev. 2008, 108,
3326. (b) Michelet, V.; Toullec, P. Y.; Genet
Ed. 2008, 47, 4268. (c) Amijs, C. H. M.; Lop
{[(IPr)Au]2(η1,η2-CC-4-C4H4Me)}+SbF6 (3b): brown solid, 99%.
−
́
́
̃
1H NMR: δ 7.53 (t, J = 8 Hz, 4 H), 7.28 (d, J = 7.5 Hz, 8 H), 7.24 (s, 4
H), 6.91 (d, J = 7.5 Hz, 2 H), 6.61 (d, J = 8 Hz, 2 H), 2.46 (sept, J = 7
Hz, 8 H), 2.30 (s, 3 H), 1.19 (d, J = 7 Hz, 24 H), 1.09 (d, J = 7 Hz, 24
H). 13C NMR: δ 182.9, 145.9, 140.6, 134.0, 132.2, 131.2, 129.5, 129.4,
124.6, 123.0, 119.6, 117.9, 29.1, 24.6, 24.1, 21.5. MALDI-MS calcd
(found) for C63H79N4Au2 (M+): 1285.6 (1286.1). Anal. Calcd (found)
for C63H79N4F6Au2Sb: H, 5.23 (5.17); C, 49.72 (49.85); N, 3.68
(3.65).
̂
, J.-P. Angew. Chem., Int.
ez-Carrillo, V.; Raducan,
́
M.; Perez-Galen, P.; Ferrer, C.; Echavarren, A. M. J. Org. Chem. 2008,
73, 7721. (d) Jimen
́
́
́
́
ez-Nunez, E.; Echavarren, A. M. Chem. Commun.
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2007, 333.
(9) (a) de Haro, T.; Nevado, C. Synthesis 2011, 2530. (b) Boorman,
T. C.; Larrosa, I. Chem. Soc. Rev. 2011, 40, 1910.
(10) Wei, C.; Li, C.-J. J. Am. Chem. Soc. 2003, 125, 9584.
(11) Cheong, P. H.-Y.; Morganelli, P.; Luzung, M. R.; Houk, K. N.;
Toste, F. D. J. Am. Chem. Soc. 2008, 130, 4517.
{[(IPr)Au]2(η1,η2-CC-4-C4H4CF3)}+SbF6 (3c): tan solid, 99%. H
NMR: δ 7.53 (t, J = 8 Hz, 4 H), 7.36 (d, J = 8.5 Hz, 2 H), 7.34−7.22
(m, 12 H), 6.79 (d, J = 8 Hz, 2 H), 2.46 (sept, J = 6.5 Hz, 8 H), 1.20
(d, J = 7 Hz, 24 H), 1.09 (d, J = 6.5 Hz, 24 H). 13C{1H} NMR: δ
182.3, 146.0, 133.9, 132.3, 131.2, 125.7, 124.7, 124.6, 122.9, 110.4,
29.1, 24.6, 24.1. MALDI-MS calcd (found) for C63H76N4F3Au2 (M+):
1339.5 (1339.9). Anal. Calcd (found) for C63H76N4F9Au2Sb: H, 4.86
(4.76); C, 48.01 (48.06); N, 3.56 (3.41).
−
1
(12) (a) Schmidbaur, H.; Schier, A. In Comprehensive Organometallic
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ASSOCIATED CONTENT
■
S
* Supporting Information
General experimental methods, 1H NMR and MALDI-MS data
for complexes 3d and 3e, independent syntheses of 4a and 5,
X-ray crystallographic data and crystallographic information file
(cif) for 3a·3CH2Cl2, and scans of 1H and 13C NMR spectra for
complexes 1a−e. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: rwidenho@chem.duke.edu.
Commun. 1998, 1633. (j) Kohler, K.; Silverio, S. J.; Hyla-Kryspin, I.;
Gleiter, R.; Zsolnai, L.; Driess, A.; Huttner, G.; Lang, H. Organo-
̈
ACKNOWLEDGMENTS
■
metallics 1997, 16, 4970. (k) Lang, H.; Kohler, K.; Zsolnai, L. Chem.
We thank the NSF (CHE-0555425) for support of this research
and the NCBC (2008-IDG-1010) for support of the Duke Uni-
versity NMR facility. T.J.B. thanks Duke University for Zielik and
Hauser Fellowships. We thank Dr. Marina G. D. Leed (Duke
University) and Dr. Paul D. Boyle (NC State University) for
assistance with solving the X-ray crystal structure of 3a·3CH2Cl2.
̈
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(17) Brown, T. J.; Dickens, M. G.; Widenhoefer, R. A. J. Am. Chem.
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