Reductive C-C Coupling from Molecular Au(I) Hydrocarbyl Complexes: A Mechanistic Study

Article abstract of DOI:10.1021/jacs.0c11296

Organometallic gold complexes are used in a range of catalytic reactions, and they often serve as catalyst precursors that mediate C-C bond formation. In this study, we investigate C-C coupling to form ethane from various phosphine-ligated gem-digold(I) methyl complexes including [Au2(μ-CH3)(PMe2Ar′)2][NTf2], [Au2(μ-CH3)(XPhos)2][NTf2], and [Au2(μ-CH3)(tBuXPhos)2][NTf2] {Ar′ = C6H3-2,6-(C6H3-2,6-Me)2, C6H3-2,6-(C6H2-2,4,6-Me)2, C6H3-2,6-(C6H3-2,6-iPr)2, or C6H3-2,6-(C6H2-2,4,6-iPr)2; XPhos = 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl; tBuXPhos = 2-di-tert-butylphosphino-2′,4′,6′-triisopropylbiphenyl; NTf2 = bis(trifluoromethyl sulfonylimide)}. The gem-digold methyl complexes are synthesized through reaction between Au(CH3)L and Au(L)(NTf2) {L = phosphines listed above}. For [Au2(μ-CH3)(XPhos)2][NTf2] and [Au2(μ-CH3)(tBuXPhos)2][NTf2], solid-state X-ray structures have been elucidated. The rate of ethane formation from [Au2(μ-CH3)(PMe2Ar′)2][NTf2] increases as the steric bulk of the phosphine substituent Ar′ decreases. Monitoring the rate of ethane elimination reactions by multinuclear NMR spectroscopy provides evidence for a second-order dependence on the gem-digold methyl complexes. Using experimental and computational evidence, it is proposed that the mechanism of C-C coupling likely involves (1) cleavage of [Au2(μ-CH3)(PMe2Ar′)2][NTf2] to form Au(PR2Ar′)(NTf2) and Au(CH3)(PMe2Ar′), (2) phosphine migration from a second equivalent of [Au2(μ-CH3)(PMe2Ar′)2][NTf2] aided by binding of the Lewis acidic [Au(PMe2Ar′)]+, formed in step 1, to produce [Au2(CH3)(PMe2Ar′)][NTf2] and [Au2(PMe2Ar′)]+, and (3) recombination of [Au2(CH3)(PMe2Ar′)][NTf2] and Au(CH3)(PMe2Ar′) to eliminate ethane.

Full text of DOI:10.1021/jacs.0c11296

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Article
Reductive C−C Coupling from Molecular Au(I) Hydrocarbyl
Complexes: A Mechanistic Study
§
§
́
Juan Miranda-Pizarro, Zhongwen Luo, Juan J. Moreno, Diane A. Dickie, Jesus Campos,*
and T. Brent Gunnoe*
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ABSTRACT: Organometallic gold complexes are used in a range of
catalytic reactions, and they often serve as catalyst precursors that
mediate C−C bond formation. In this study, we investigate C−C
coupling to form ethane from various phosphine-ligated gem-digold(I)
methyl complexes including [Au2(μ-CH3)(PMe2Ar′)2][NTf2],
[ A u ( μ - C H ) ( X P h o s ) ] [ N T f ] ,
a n d
2
3
2
2
[Au2(μ-CH3)(tBuXPhos)2][NTf2] {Ar′ = C6H3-2,6-(C6H3-2,6-Me)2,
C6H3-2,6-(C6H2-2,4,6-Me)2, C6H3-2,6-(C6H3-2,6-iPr)2, or C6H3-2,6-
(C6H2-2,4,6-iPr)2; XPhos = 2-dicyclohexylphosphino-2′,4′,6′-triisopro-
pylbiphenyl; tBuXPhos = 2-di-tert-butylphosphino-2′,4′,6′-triisopropyl-
biphenyl; NTf2 = bis(trifluoromethyl sulfonylimide)}. The gem-digold
methyl complexes are synthesized through reaction between Au(CH3)L and Au(L)(NTf2) {L = phosphines listed above}. For
[Au2(μ-CH3)(XPhos)2][NTf2] and [Au2(μ-CH3)(tBuXPhos)2][NTf2], solid-state X-ray structures have been elucidated. The rate of
ethane formation from [Au2(μ-CH3)(PMe2Ar′)2][NTf2] increases as the steric bulk of the phosphine substituent Ar′ decreases.
Monitoring the rate of ethane elimination reactions by multinuclear NMR spectroscopy provides evidence for a second-order
dependence on the gem-digold methyl complexes. Using experimental and computational evidence, it is proposed that the
mechanism of C−C coupling likely involves (1) cleavage of [Au2(μ-CH3)(PMe2Ar′)2][NTf2] to form Au(PR2Ar′)(NTf2) and
Au(CH3)(PMe2Ar′), (2) phosphine migration from a second equivalent of [Au2(μ-CH3)(PMe2Ar′)2][NTf2] aided by binding of the
Lewis acidic [Au(PMe2Ar′)]+, formed in step 1, to produce [Au2(CH3)(PMe2Ar′)][NTf2] and [Au2(PMe2Ar′)]+, and (3)
recombination of [Au2(CH3)(PMe2Ar′)][NTf2] and Au(CH3)(PMe2Ar′) to eliminate ethane.
triisopropylbiphenyl; NTf2 = bis(trifluoromethyl sulfonyl)-
imide}.
INTRODUCTION
■
Organometallic gold precatalysts have been applied to a range
of catalytic organic syntheses.1−9 Among the Au-catalyzed
processes, many involve C−C bond forming reactions as a key
step. Thus, the mechanisms of Au-mediated C−C bond
formation have been of substantial interest.10−20 Also, Au-
catalyzed partial oxidation of methane in oleum to form
methylbisulfate has been reported.21,22 Demonstration, sepa-
rately, of Au-mediated methane C−H activation23,24 and of the
ability of molecular Au complexes to mediate C−C bond
forming reactions14−22 sparked our interest in ethane
elimination since combined methane C−H activation and
ethane reductive elimination provides a strategy for the
oxidative conversion of methane to ethane.25,26 Herein, we
disclose a mechanistic study of ethane elimination from
phosphine-ligated gem-digold27 methyl complexes with the
general formula [Au2(μ-CH3)(PMe2Ar′)2][NTf2], [Au2(μ-
CH3)(XPhos)2][NTf2], and [Au2(μ-CH3)(tBuXPhos)2]-
[NTf2] {Ar = C6H3-2,6-(C6H3-2,6-Me)2, C6H3-2,6-(C6H3-
2,4,6-Me)2, C6H3-2,6-(C6H3-2,6-iPr)2, or C6H3-2,6-(C6H3-
2,4,6-iPr)2; XPhos = 2-dicyclohexylphosphino-2′,4′,6′-triiso-
Proposed mechanisms for Au-mediated C−C bond for-
mation include reductive elimination from AuIII intermediates
(Scheme 1).28 For example, reductive elimination from
n
(R)2Au(X)(L) (L = phosphine; R = Me, Et, or Pr; X =
anionic ligand such as Cl, OTf, NO3, O2CCF3, or another alkyl
ligand) was investigated by Kochi and co-workers, and
reductive eliminations from [(Me)2Au(L)2]+ complexes have
been reported.18,29,30 The proposed mechanism involves initial
phosphine dissociation followed by C−C reductive elimination
from the three-coordinate R2AuIIIX intermediate (Scheme 1a).
When R = Me, isotopic labeling studies with (Me)2AuX(L)
and (CD3)2Au(X)(L) (L = phosphine) indicate kinetically
competitive intermolecular transfer of Me between two Au
Received: October 27, 2020
Published: February 5, 2021
t
propylbiphenyl; BuXPhos = 2-di-tert-butylphosphino-2′,4′,6′-
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Germane to these proposed binuclear Au precursors to C
C elimination, several gem-digold intermediates have been
reported, including [Au2(σ,π-CHCHC3H5)(PPh3)2]-
[NTf2],44 [Au2(μ-Ph)L2][NTf2]45 (L = PPh3 or NHC), and
[Au2(μ-R)(PMe2ArDipp2)2][NTf2] (R = CH3, CHCH2, C
CH, ArDipp2C6H3-2,6-(C6H3-2,6-iPr)2).46 The thermal stabil-
ities of phosphine-ligated gem-digold hydrocarbyl complexes
have been reported to depend on the steric properties of the
ancillary ligands.46 Other related examples, including [Au2(μ-
vinylcypr)(PPh3)2][NTf2]14,15,44 and [Au2(μ-vinylcypr)(PPh3)2]-
[NTf2], readily decompose to the corresponding diene,
[Au(PPh3)2][NTf2], and colloidal gold byproducts. Nonethe-
less, a mechanistic understanding of these CC coupling
processes and, in general, of CC formation from AuI
complexes is lacking.
Scheme 1. Proposed Pathways of C−C Bond Coupling
Reactions Mediated by Molecular Gold Complexes17,18,31,36
Herein, we explore the formation of ethane from one of the
simplest possible gold-based systems, namely, Au(CH3)-
(PPh3). To enable reliable mechanistic investigations, we
extended our preliminary observations on triphenylphosphine-
ligated systems to bulkier terphenyl and biaryl phosphines that
provide kinetic stabilization of key digold intermediates. In
particular, we have focused on C−C coupling reactions from
gem-digold methyl complexes with a general formula [Au2(μ-
CH3)(PMe2Ar′)2][NTf2] and [Au2(μ-CH3)(XPhos)2][NTf2]
(Figure 1). We studied the impact of the phosphine ligand on
the stability of digold complexes, especially the influence on
ethane elimination.
centers, but these alkyl transfers appear to occur only in
nonpolar solvents.23 Further, the putative binuclear Au
intermediates responsible for alkyl transfer were not directly
implicated in the C−C coupling reactions. From the starting
complexes (Me)2Au(X)(L) (L = phosphine), it was proposed
that larger phosphines facilitate ethane reductive elimination.30
Alternatively, Kochi has proposed that ethane formation could
result from digold alkyl intermediates, but to our knowledge,
such reactions were not directly observed.31,32 Other examples
of ethane formation through bimolecular reductive elimination
from M−CH3 species include NiII,33 CuI,34 and RuII.35
The formation of C−C bonds from (NHC)AuI−R (NHC =
N-heterocyclic carbene; R = Ph, Me, or p-tolyl) occurs upon
addition of electrophiles (R′X), such as PhI, MeI, and MeOTf,
to form R−R′ as well as homocoupled products R−R and R′−
R′ (Scheme 1b).36−38 The proposed mechanism involves
formal trans oxidative addition of the electrophile (R′X) to
form an (NHC)AuIII(R′)(X)(R) intermediate, followed by
competitive (a) C−C reductive elimination to form R′−R and
(b) intermolecular transfer of RX from a Au(III) intermediate
to (NHC)AuI−R to yield (NHC)AuIII(R)2(X) followed by
C−C reductive elimination to give the homocoupled product
R−R. Related to these processes, the addition of F+-donors to
Au(I) hydrocarbyl compounds also promotes C−C coupling
reactions.39−42
Mixed-valent gold hydrocarbyl complexes have also been
proposed as intermediates responsible for the CC bond
formation.17 For example, Toste and co-workers reported a fast
biaryl C−C bond reductive elimination from a mixed-valent
bimetallic AuI/AuIII complex [ClAu]PNP[AuCl(C6H4-4-F)2]
(PNP = Ph2PN(CH3)PPh2) (Scheme 1c).17 In this study,
the Au(I) complex [Au(C6H4-4-F)]PNP[Au(C6H4-4-F)] is
oxidized with PhICl2 to generate a symmetric bimetallic Au(II)
species, [ClAu(C6H4-4-F)]PNP[Au(C6H4-4-F)Cl]. The latter
isomerizes to a mixed-valent AuI/AuIII complex, [ClAu]PNP-
[AuCl(C6H4-4-F)2], which undergoes reductive elimination to
form a biaryl product. Similarly, O’Hair and co-workers
reported a concerted redox couple mechanism from a reaction
between allylic halides (CH2 = CHCH2X, X = Cl, Br, and I)
and a gem-digold(I) compound, [(dppm)2Au2Ph]+ (dppm =
bis(diphenylphosphino)methane, (Ph2P)2CH2).43 It is hy-
pothesized that the reductive coupling occurs from a AuI/
AuIII complex, [ClAuI](dppm)[AuIII(CH2CHCH2)(Ph)].
Figure 1. Phosphine-ligated gem-digold methyl complexes with the
general formula [Au2(μ-CH3)(PR2Ar′)2][NTf2] investigated in this
work (Xyl = 2,6-C6H3-Me2; Mes = 2,4,6-C6H2-Me3; Dipp = 2,6-
C6H3-iPr2; Tripp = 2,4,6-C6H2-iPr3).
RESULTS AND DISCUSSION
■
Synthesis of Neutral Gold Complexes Based on
Terphenyl and Biaryl Phosphines. Gold complexes with
terphenyl phosphine (complexes 1a−1d in Scheme 2) and
with biaryl “Buchwald phosphine” ligands (1e and 1f) were
synthesized by methylation of Au(I) chloride precursors with
MeMgX (X = Cl or Br) in 60−80% isolated yields.47
Formation of the new Au−C bonds is evidenced by the
1
appearance of H NMR resonances in the range from 0.08 to
0.45 ppm with associated 13C{1H} signals at 3.4 to 8.3 ppm
(2JCP ≈ 100 Hz). Single crystals of 1a, 1e, and 1f were obtained
by slow evaporation from a mixture of pentane and diethyl
ether or pentane and dichloromethane solution from 5 to −25
°C (Figure 2). The solid-state structures of complexes 1e and
1f show a weak κ1 type interaction (localized Au···π(arene)
contact)48−50 between the Au(I) center and the ipso carbon of
the arenes (C20, 1e; C16, 1f) with bond distances of
3.1748(17) and 3.180(4) Å, respectively. The distances
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by a similar procedure to their methyl analogues and
characterized by spectroscopic techniques and single-crystal
X-ray diffraction (Figures 3 and 4). The σ Au−C bond
Scheme 2. Synthesis of Phosphine-Ligated Gold Methyl
Compounds with Terphenyl Phosphines (1a−1d) and
Buchwald Phosphines (1e and 1f)
Figure 3. ORTEP of Au(C2H5)(PMe2ArMes2) (2a) at 50% probability
(one of the two crystallographically distinct structures, the other one
being Au(C2H5)(PMe2ArXyl2), see Figure S1). Hydrogen atoms on
the phosphine ligands are omitted for clarity. Selected bond lengths
(Å): Au1−C1 = 2.079(8); C1−C2 = 1.411(15). Selected bond angles
(deg): C1−Au1−P1 = 179.6(3); Au1−C1−C2 = 115.0(7); C3−P1−
Au1 = 111.5(3); C5−P1−Au1 = 115.63(19); C4−P1−Au1 =
112.2(3).
Figure 2. ORTEPs of Au(CH3)(PMe2ArXyl2) (1a), Au(CH3)-
(XPhos)(1e), and Au(CH3)(tBuXPhos) (1f) represented at 50%
probability. (For 1f, one of the two chemically equivalent, but
crystallographically distinct, structures is shown. For the second
structure, see the Supporting Information.) Hydrogen atoms on the
phosphine ligands are omitted for clarity. Selected bond lengths (Å):
1a, Au1−C1 = 2.123(2); P1−C2 = 1.825(3); P1−C3 = 1.823(3);
P1−C4 = 1.852(2); Au1−P1 = 2.2900(7). 1e, Au1−C1 =
2.1146(14); Au1−C20 = 3.1748(17); Au1−C25 = 3.2510(18);
Au1−C21 = 3.5023(18); Au−arene (arene ring centroid) =
3.2659(10); Au1−P1 = 2.2917(4). 1f, Au1−C1 = 2.096(4); Au1−
C16 = 3.180(4); Au1−C17 = 3.551(4); Au1−C21 = 3.409(4); Au−
arene (benzene ring centroid) = 3.449(2); Au1−P1 = 2.3007(11).
Selected bond angles (deg): 1a, P1−Au1−C1 = 178.97(8); C2−P1−
Au1 = 112.8(1); C3−P1−Au1 = 111.95(10); C4−P1−Au1 =
113.14(8). 1e, P1−Au1−C1 = 179.57(4); C14−P1−Au1 =
117.53(5). 1f, C1−Au1−P1 = 172.80(12); C10−P1−Au1 =
115.25(13).
Figure 4. ORTEP of Au(C6H5)(PMe2ArXyl2) (3a) at 50% probability.
Hydrogen atoms on the phosphine ligands are omitted for clarity.
Selected bond lengths(Å): Au1−C1 = 2.089(7); Au1−P1 = 2.302(2).
Selected bond angles (deg): C1−Au1−P1 = 177.7(2); C8−P1−Au1
= 110.4(3); C7−P1−Au1 = 109.9(3); C9−P1−Au1 = 117.5(2).
distances are 2.079(8) Å (2a) and 2.087(7) (3a) Å, which are
similar to neutral Au−CH3 bond distances discussed above.
Complex 2a cocrystallizes in a 1:1 ratio with a molecule of
Au(C2H5)(PMe2ArMes2) (2b, see Figure S1), whose geometric
parameters are comparable to those of 2a. This is due to the
fact that the crystals were obtained from a phosphine exchange
experiment between 2a and free PMe2ArMes2 that was
conducted as part of our mechanistic investigations (see
below, Figure S1). The bond distance between C1 and C2 in
the Au-ethyl fragment of 2a is 1.411(15) Å, lying between the
carbon−carbon lengths of ethylene (1.34 Å) and ethane (1.54
Å). The electrophilic nature of gold may enhance the C−C
bond strength and thus shorten bond length compared to a
typical C−C single bond. The structure of complex 3a is
similar to those of compounds 1 and 2a and does not require
further discussion.
Ethane Elimination from Au(CH3)(PPh3). For the sake of
simplicity and considering the widespread utilization of PPh3-
based gold complexes, we commenced our studies by exploring
ethane elimination from Au(CH3)(PPh3). This compound is
stable at moderate temperatures as heating at 40 °C caused no
apparent alteration when monitoring by 1H and 31P{1H} NMR
spectroscopy, and no ethane formation was detected. However,
in the presence of 1 equiv of Au(PPh3)(NTf2), Au(CH3)-
between Au centers and arene ring centroids are 3.2659(10) Å
(1e) and 3.449(2) Å (1f), also indicative of intramolecular
Au···π(arene) interactions.50,51 Structure 1a does not exhibit
this type of contact, in agreement with the preferred geometry
adopted by the smaller phosphines of the terphenyl series.52
The Au−CH3 bond distances are 2.123(2) Å (1a), 2.1146(14)
Å (1e), and 2.096(4) Å (1f).
Terminal ethyl and phenyl complexes Au(C2H5)-
(PMe2ArXyl2) (2a) and Au(C6H5)(PMe2ArXyl2) (3a) were
synthesized with the aim of exploring the possibility of C−C
bond heterocoupling with different hydrocarbyl substituents
bound to gold (see below). These compounds were prepared
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(PPh3) evolves ethane immediately at room temperature with
complete consumption of Au(CH3)(PPh3) by the time of
placing the sample in the NMR probe (<10 min; Scheme 3).
for the possibility of a radical-mediated pathway. To do so, we
combined equimolar amounts of Au(CH3)(PPh3) and [Au-
(PPh3)][NTf2] in the presence of excess toluene (10 equiv) as
a radical probe. Under these conditions, the formation of
CH3• radicals should be quenched by toluene by means of
hydrogen atom abstraction from the benzylic position.56 This
process would have released methane, which was not observed
during our experiments, thus favoring the likelihood of a
nonradical route.
Scheme 3. Ethane Elimination from Au(CH3)(PPh3) in the
Presence of 1 equiv of Au(PPh3)(NTf2)
Synthesis of Cationic gem-Digold Methyl Complexes.
To probe if gem-digold methyl complexes are relevant
intermediates during C−C coupling reactions, bulky terphenyl
and Buchwald phosphines were used. Some of us have recently
demonstrated that gem-digold methyl species are kinetically
stabilized by large phosphine substituents,46 which should
facilitate kinetic investigations. Indeed, using the aforemen-
tioned bulky phosphines enabled the isolation and character-
ization of various uncommon gem-digold methyl complexes of
type [Au2(μ-CH3)(PMe2Ar′)2][NTf2] (4a−4d). These were
synthesized in high yields by mixing a 1:1 molar ratio of a
Au(I) methyl complex Au(CH3)(PMe2Ar′) and the corre-
sponding Au(I) bis(trifluoromethyl sulfonyl)imide (Scheme
4). Alternatively, the addition of 0.5 equiv of [Ph3C][B-
(C6F5)4] to neutral Au(I) methyl complexes Au(CH3)-
(PR2Ar′) leads to the same gem-digold species in comparable
yields.
The release of ethane is accompanied by clean formation of the
homoleptic bisphosphine complex [Au(PPh3)2][NTf2], along
with Au(0), as evinced by the formation of black insoluble
material. The nature of this solid was interrogated by
transmission electron microscopy (TEM) analysis (Figure 5).
Scheme 4. General Synthesis of the gem-Digold Methyl
Complexes with Terphenyl Phosphines (4a−4d) and
Buchwald Phosphine-Ligated gem-Digold Methyl
Complexes (4e and 4f)
Figure 5. Transmission electron microscopy (TEM) analysis of the
insoluble Au0 particles produced during ethane evolution in Scheme
3.
When a 1:1 molar mixture of Au(PPh3)(NTf2) and Au(CH3)-
(PPh3) was dissolved in dichloromethane at −70 °C, ethane
formation was detected immediately by 1H NMR spectroscopy
(Figure S9). Variable temperature 1H and 31P{1H} NMR
analysis from −70 to 25 °C revealed the formation of an
intermediate species characterized by a broad 1H NMR
resonance at 1.6 ppm associated with a 31P{1H} NMR
resonance at 37.5 ppm, which we attribute to the
corresponding gem-digold methyl species [Au2(μ-CH3)-
(PPh3)2][NTf2] (Figure S8).46 However, this compound is
only detectable at temperatures below −40 °C, and it rapidly
evolves to the final products above this temperature.
T h o u g h t h e t r a n s i e n t n a t u r e o f
[Au2(μ-CH3)(PPh3)2][NTf2] prevented us from exploring its
role in further detail, our initial kinetic investigations using 1
equiv of the related Au(PPh3)(NO3) revealed a second-order
dependence on neutral Au(CH3)(PPh3) for ethane elimination
(Figure S6). Nonetheless, we could carry out these studies
with related methyl complexes based on biaryl and terphenyl
phosphines, as discussed in the following sections. Since we
observed the formation of gold nanoparticles during ethane
elimination, we decided to probe for a possible catalytic role of
Au nanoparticles in the C−C coupling reaction, particularly
considering their catalytic role in related processes.53−55
However, using independently prepared gold nanoparticles
(i.e., Au/TiO2 and Au/Fe2O3) as catalysts did not promote
methyl C−C coupling at a comparable rate (t1/2 ≈ 1 day at 25
°C). Thus, it seems unlikely that Au nanoparticles play a
catalytic role in the formation of ethane. In addition, we tested
Compounds 4a−4f were characterized by multinuclear
NMR spectroscopy, and their purity was confirmed by
microanalysis. Distinctive H NMR signals due to the methyl
1
group, which are slightly shifted to higher frequencies (ca. 0.5−
1.2 ppm) compared to their corresponding neutral precursors
(1a−1f), are consistent with the formation of the gem-digold
complexes. The presence of the bridging methyl ligand is
further confirmed by 13C{1H} NMR resonances shifted to
lower frequencies by approximately 5 ppm compared to the
parent compounds 1a−1f and characterized by a drastically
reduced scalar-coupling to 31P (ca. 50 Hz; cf. ∼100 Hz for 1a−
1f). The compounds [Au2(μ-CH3)(XPhos)2][NTf2] (4e) and
[(Au)2(μ-CH3)(tBuXPhos)2][NTf2] (4f) were additionally
authenticated by single-crystal X-ray diffraction (Figure 6;
Table 1). The gold methyl bond distances in 4e and 4f are
∼0.1 Å longer than in their corresponding neutral methyl
complexes 1e and 1f. A characteristic Au−arene interaction is
discernible for the two structures. While the structure of 4f
exhibits a slightly shortened Au−arene distance (3.390(3) Å
on average) than its neutral complex 1f (3.449(2) Å),
compound 4e (3.432(2) Å on average) presents an apparently
weaker Au−arene interaction than its neutral gold compound
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Scheme 5. Thermal Decomposition of Terphenyl and Biaryl
Phosphine Methyl-Bridged Digold Complexes (4a−4e) to
Gold Bisphosphine (5a−5e)
Figure 6. ORTEPs of [Au2(μ-CH3)(XPhos)2][NTf2] (4e) and
[(Au)2(μ-CH3)(tBuXPhos)2][NTf2] (4f) at 50% probability (for 4e,
only one of the three chemically equivalent, but crystallographically
distinct, structures is represented). Hydrogen atoms on the phosphine
ligands are omitted for clarity. Selected bond lengths (Å): 4e, Au1−
C1 = 2.221(5); Au2−C1 = 2.235(5); Au1−Au2 = 2.7466(4); Au1−
P1 = 2.2637(12); Au2−P2 = 2.2662(13). 4f, Au1−C1 = 2.204(9);
Au2−C1 = 2.207(8); Au1−Au2 = 2.7763(7); Au1−P1 = 2.285(2);
Au1−P2 = 2.2798(18). Selected bond angles (deg): 4e, C1−Au1−P1
= 168.41(14); C1−Au2−P2 = 172.39(13); Au1−C1−Au2 =
76.11(16); C1−Au1−Au2 = 52.17(13); C1−Au2−Au1 =
51.72(12). 4f, C1−Au1−P1 = 162.9(2); C1−Au2−P2 = 160.9(2);
Au1−C1−Au2 = 78.0(3); C1−Au1−Au2 = 51.0 (2); C1−Au2−Au1
= 50.9 (2).
shielding provided by the cyclohexyl and tert-butyl groups
directly bound to the phosphorus center in close proximity to
the gold nuclei.
Overall, these observations indicate that kinetic analysis by
1H and 31P{1H} NMR spectroscopy monitoring is facilitated
by larger phosphine ligands compared to PPh3. For instance,
heating complex 4e in dichloroethane at 90 °C enabled us to
monitor by NMR spectroscopy its evolution to [Au(XPhos)2]-
[NTf2] (5e) with concomitant release of ethane and formation
of Au(0) (Figure S5). The thermolysis of 4e follows a second-
order dependence on the digold complex with kobs = 5.2(1) ×
10−4 M−1 s−1 at 90 °C (Table 2), as previously observed for
the PPh3-based system. In the case of the more hindered
compound 4f, this reaction does not take place at 100 °C, and
intractable digold decomposition occurs at temperatures above
100 °C where the formation of methane, instead of ethane, was
observed (Figure S14). This finding indicates that C−C
coupling is likely not viable in the most sterically constrained
digold system studied herein. This seems to be consistent with
a second-order dependence on digold complex concentration
during ethane formation, which might imply the need for more
than two gold nuclei in close proximity along the reaction
coordinate (see below for additional discussion). Similar to
complex 4e, ethane elimination from terphenyl-ligated gem-
digold methyl complexes follows a second order dependence
on 4a−4d (Figure 7a; see the Supporting Information).
Kinetic studies provide rates for ethane elimination from the
more sterically hindered 4c and 4d of kobs = 4.8(3) × 10−3 and
2.0(1) × 10−2 M−1 s−1 at 50 °C, respectively. In contrast, the
rates of ethane elimination from 4a and 4b had to be analyzed
at lower temperatures (0 °C), resulting in rates of kobs = 9.8(3)
× 10−2 and 4.9(1) × 10−1 M−1 s−1 at 0 °C, respectively. The
corresponding half-life (t1/2) values associated with these
kinetic parameters at the working temperatures are approx-
imately 260 (4a, 0 °C), 800 (4b, 0 °C), 5600 (4c, 50 °C), and
13 000 (4d, 50 °C) s.
1e (3.2659(1) Å). The presence of intense aurophilic
interactions57,58 is evinced by Au···Au distances in complexes
4e and 4f of 2.7466(6) and 2.7763(7) Å, respectively. These
Au···Au distances are slightly longer than those reported for
the related 4c (2.7120(8) Å) and ∼0.1 Å shorter than the Au−
Au distance in metallic gold (2.878 Å).
Ethane Elimination from gem-Digold Methyl Com-
plexes. As anticipated, the stability of gem-digold methyl
complexes largely depends on the steric shielding provided by
the phosphine ligand. Thus, the compound [Au2(μ-CH3)-
(PMe2ArXyl2)2][NTf2] (4a) is only stable in dichloromethane
solution at −30 °C or below. Above −20 °C, 4a cleanly
converts into [Au(PMe2ArXyl2)2][NTf2] (5a), metallic gold,
and ethane (Scheme 5). Complex [Au2(μ-CH3)-
(PMe2ArMes2)2][NTf2] (4b) reacts in a similar way, whereas
bulkier phosphines provide enhanced stability. As such,
compounds [Au2(μ-CH3)(PMe2ArDipp2)2][NTf2] (4c) and
[Au2(μ-CH3)(PMe2ArTripp2)2][NTf2] (4d), in which the
methyl substituents in the lateral aryl rings of the terphenyl
moiety have been substituted by iso-propyl groups, are fairly
stable at room temperature, while complexes 4e and 4f remain
unaltered for hours even at temperatures up to 80 °C. Thus,
the investigated Buchwald phosphines confer enhanced
stability to gem-digold methyl species compared to terphen-
yl-based ligands, most likely as a result of the increased steric
Table 2 collects the corresponding activation barriers for C−
C coupling from the methyl-bridged complexes 4a−4e, which
Table 1. Summary of Selected Bond Distances of the gem-Digold Methyl Complexes
a
gem-digold methyl complexes
[Au2(μ-CH3)(PMe2ArDipp2)2][BArF]34 (4c)
Au−arene (Å)
Au−Au (Å)
Au−ipso carbon of arene (Å)
Au−CH3 (Å)
3.259(3)
3.321(3)
3.400(2)
3.465(2)
3.366(3)
3.413(3)
2.7120(8)
2.7120(8)
2.7330(4)
2.7330(4)
2.7765 (5)
2.7765 (5)
3.027(3)
3.102(3)
3.093(5)
3.185(5)
3.082(8)
3.406(8)
2.210(5)
2.227(4)
2.215(5)
2.238(5)
2.207(8)
2.204(9)
b
[Au2(μ-CH3)(XPhos)2][NTf2] (4e)
[(Au)2(μ-CH3)(tBuXPhos)2][NTf2] (4f)
a
b
Distance from Au to the centroid of the arene rings. Average over three independent molecules present in the asymmetric unit.
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Table 2. Summary of Kinetic Data for Ethane Elimination from gem-Digold Complexes 4a−4e
compound
T (°C)
k (M−1 s−1
)
ΔG⧧ (kcal/mol)
[Au2(μ-CH3)(PMe2ArXyl2)2][NTf2] (4a)
[Au2(μ-CH3)(PMe2ArMes2)2][NTf2] (4b)
[Au2(μ-CH3)(PMe2ArDipp2)2][NTf2] (4c)
[Au2(μ-CH3)(PMe2ArTipp2)2][NTf2] (4d)
[Au2(μ-CH3)(XPhos)2][NTf2] (4e)
0
0
9.8(3) × 10−2
4.9(1) × 10−2
4.8(3) × 10−3
2.0(1) × 10−3
5.2(1) × 10−4
N.A.
17.2(1)
17.6(1)
22.4(5)
22.9(4)
26.4(3)
N.A.
50
50
90
a
[Au2(μ-CH3)(tBuXPhos)2][NTf2] (4f)
100
a
Methane formation observed instead; N.A. (not available).
temperature interval from −20 to 10 °C. An Eyring analysis
provided activation parameters of ΔH⧧ = 20.5 1.3 kcal/mol
and ΔS⧧ = 11.9 4.8 e.u. (Figure 7b), which correspond to
⧧
ΔG298 = 16.9 2.7 kcal/mol.
To obtain a deeper insight into the nature of the Au species
involved in C−C coupling processes, we first considered
whether dissociation of complexes 4 into their monometallic
components,59 namely, neutral methyl compounds 1 and
triflimide species of type Au(PR2Ar′)(NTf2), might be
relevant. To check the viability of such equilibria, we explored
exchange processes of the methyl bridge in compound 4a. In a
first experiment, we examined the exchange between 1a and 4a
at variable temperatures. For experimental convenience, we
accessed an equimolar mixture of both species by adding 0.33
equiv of [Ph3C][B(C6F5)4] to 1a at −40 °C. Under these
conditions, one-third of the neutral methyl compound is
Figure 7. (a) Second-order kinetic representation for the con-
sumption of 4a at −5 °C in CD2Cl2. (b) Eyring plot for ethane
formation from gem-digold methyl [Au2(μ-CH3)(PMe2ArXyl2)2]-
[NTf2] (4a).
range from 17.2 kcal/mol at 0 °C for 4a to 26.4 kcal/mol at 90
°C for 4e. To complete these studies, we monitored the
evolution of ethane from the gem-digold complex 4a in the
Scheme 6. (a) Dynamic Me/Me Exchange Equilibrium between [Au2(μ-CH3)(PMe2ArXyl2)2][NTf2] (4a) and
Au(CH3)(PMe2ArXyl2) (1a) Species at −40°C; (b) C−C Coupling and Product Distribution in the Reaction between
Au(C2H5)(PMe2ArXyl2) (2a) and [Au2(μ-CH3)(PMe2ArXyl2)2][NTf2] (4a); and (c) C−C Coupling and Product Distribution in
the Reaction between Au(C6H5)(PMe2ArXyl2) (3a) and [Au2(μ-CH3)(PMe2ArXyl2)2][NTf2] (4a)
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transformed by methyl abstraction into a cationic gold species
that is immediately trapped by unreacted 1a to provide gem-
digold 4a. Variable temperature 1H and 31P{1H} NMR
spectroscopy analysis revealed dynamic behavior in solution
(Figure S10), which we attribute to the exchange equilibrium
depicted in Scheme 6a. It was possible to identify 4a by a
31P{1H} NMR resonance at 0.1 ppm recorded at −85 °C,
whereas a broad signal at 21.1 ppm was assigned to 1a. These
signals coalesce at approximately −40 °C, while the major
component when reaching 25 °C is the homoleptic bis-
phosphine compound 5a that accompanies ethane formation.
We further investigated this dynamic behavior by DFT
methods (see the Supporting Information for details).
Calculations indicate that dissociation of the dinuclear species
[Au2(μ-CH3)(PMe2ArXyl2)2][NTf2] (4a) into the correspond-
ing fragments, Au(CH3)(PMe2ArXyl2) (1a) and Au-
(PMe2ArXyl2)(NTf2), is only slightly endergonic (ΔG = +0.5
kcal/mol), in agreement with our experimental results. The
kinetic profile of ethane evolution in these equimolar mixtures
is identical, within the experimental error, to that of pure 4a.
This suggests that, even if carbon−carbon coupling takes
place from a trimetallic species involving the participation of
compounds 1, the required dissociation of gem-digold methyl
compounds 4 into compounds 1 and [Au(PR2Ar′)]+ is not
likely kinetically relevant.
in CD2Cl2 at −40 °C to allow the exchange to take place and
then cooled to −85 °C. At the latter temperature, the exchange
process is halted, and a variety of gold-containing products are
identified by 31P{1H} NMR. These include the neutral
hydrocarbyl compounds 1a and 2a and their corresponding
gem-digold species 4a and [Au2(μ-CH2CH3)(PMe2ArXyl2)2]
(6a), whose broad resonances were recorded at 21.2, 21.9, 0.1,
and 1.4 ppm, respectively. Also, sharp signals due to
Au(PMe2ArXyl2)(NTf2) and [Au(PMe2ArXyl2)2][NTf2] (5a)
were identified at −4.2 and 10.8 ppm, respectively. The latter
likely results from local solution warm-up during sample
handling. Increasing the temperature to −40 °C results in
coalescence of all prior resonances except for that of 5a, which
is clearly not involved in the exchange process. Further raising
the temperature to 25 °C leads to full consumption of gold
precursors and quantitative formation of bisphosphine
compound 5a along with the appearance of solid Au(0).
Similarly, rapid exchange between 4a and 3a is evinced by
immediate conversion of an equimolar mixture of those
compounds into 1a and [Au2(μ-C6H5)(PMe2ArXyl2)2] (7a)
(Figure S11).
Having in mind that the above dynamic behavior reveals the
presence of compounds 1, 4, and Au(PR2Ar′)(NTf2) in
solution, and also considering the fact that ethane evolution
follows a second-order dependence on bridging methyl
complexes 4, we considered three possible routes (Scheme
7). In the first, reductive coupling from two neutral gold
Substituting methyl compound 1a by its related ethyl (2a)
and phenyl (3a) derivatives showed the formation of cross-
coupling products (Scheme 6b,c). In the case of 2a, the
1
Scheme 7. Potential Routes for Ethane Evolution with
Regards to the Gold Coupling Partners
formation of propane and butane was apparent by H NMR
spectroscopy, while in the reaction between 4a and 3a the
formation of ethane, biphenyl, and toluene was detected in
comparable amounts. GC-MS analysis of solution and gas
headspace provided further evidence for cross coupling, since
variable amounts of ethane, propane, and butane were
measured from the reaction between 2a and 4a (Figure
S21). In both cases, the main homogeneous gold-containing
species when reaching room temperature is 5a.
To gather more information on the exchange between
bridging and terminal hydrocarbyl substituents present in gem-
digold and neutral compounds, we examined the reaction
depicted in Scheme 6b at variable temperatures (Figure 8). A
solid mixture of 2a and 4a in equimolar amounts was dissolved
methyl compounds of type 1 may take place, similar to prior
work by Kochi and co-workers (Scheme 7a).31,32 However, it
is important to highlight two distinctive features of our studies
that contrast with those prior reports. First, reductive coupling
from Au(CH3)(PPh3) only occurred at high temperatures
(∼100 °C), while C−C bond formation from bridging digold
complexes 4 is more facile. In fact, the C−C coupling reaction
readily proceeds at temperatures as low as −60 °C in the case
of the PPh3-based system (Figures S8 and S9). Second,
whereas a first-order dependence on gold was demonstrated
for reductive coupling from Au(CH3)(PPh3),31,32 with
phosphine dissociation toward “AuMe” as the rate-determining
step, we have determined a second-order dependence on
digold compounds 4 during ethane evolution. These
observations suggest different operating mechanisms in the
two cases, a notion that is further supported by DFT methods
based on the PMe2ArXyl2 system. In agreement with Kochi’s
findings, the computed reaction free energy for phosphine
dissociation at 1a is +32.1 kcal/mol, much higher than the
experimentally determined activation free energy for the
Figure 8. Variable temperature of exchange processes between
Au(C2H5)(PMe2ArXyl2) (2a) and [Au2(μ-CH3)(PMe2ArXyl2)2][NTf2]
(4a) monitored by 31P{1H} NMR spectroscopy.
⧧
overall process (ΔG298 = 16.9
2.7 kcal/mol, see above).
Phosphine dissociation from 4a to yield [Au2(μ-CH3)-
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Figure 9. Free energy profile for [Au(PMe2ArXyl2)]+-promoted phosphine migration and formation of masked “AuMe” from Au(CH3)(PMe2ArXyl2
(1a, left) or [Au2(μ-CH3)(PMe2ArXyl2)2]+ (4a, right) complexes; calculated at the ωB97X-D/6-31G(d,p) level.
)
(PMe2ArXyl2)][NTf2], where the metal−metal and metal−
arene interactions could stabilize the unsaturated gold center,
presented a similarly high value [+31.1 kcal/mol; Figure S20
(including a molecule of CH2Cl2 in the calculation to
compensate the unsaturation of the metal center results in
higher reaction free energies: +33.3 and +42.6 kcal/mol for
phosphine dissociation from 1a and 4a, respectively)]. As
anticipated, these data confirm a dissimilar C−C coupling
mechanism for compounds 4 compared to that exhibited by
monometallic gold-alkyl species.
+33.6 kcal/mol was found (TS2 in Figure S16), further
suggesting that this pathway is unaffordable.
Finally, we considered the possible involvement of gold
carbene (AuCH2) and hydride (AuH) species,60 potentially
formed by hydride abstraction from Au-methyl complexes.
However, the free energy cost to access these high-energy
intermediates is calculated to be at least +36.2 kcal/mol
(Figures S17 and S18), incompatible with the determined
activation parameters. To further rule out this mechanistic
route, we carried out an additional experiment with isotopically
labeled [Au(CD3)(PMe2ArXyl2)] (1a-d3; see the Supporting
Information for details). Treating an equimolar mixture of 1a
and 1a-d3 with 2 equiv of Au(PMe2ArXyl2)(NTf2) yielded an
approximate statistic mixture of CH3CH3, CH3CD3, and
CD3CD3 (Figure S12), without further observable H/D
scrambling, thus excluding the involvement of gold methyl-
idene species.
Having ruled out the most direct mechanisms involving 1a
and 4a, we decided to interrogate the participation of
compounds Au(PR2Ar′)(NTf2), especially in consideration of
the experimental results indicating that such complexes are
accessible under the reaction conditions (see above). These
compounds serve as a source of electrophilic [Au(PR2Ar′)]+
fragments61,62 and, as such, might facilitate or drive phosphine
dissociation from other Au complexes. Potential phosphine
dissociation is implicated from straightforward ligand exchange
experiments (see Figure S13), and since it was proposed as the
rate-limiting step in Kochi’s earlier system,31,32 it is conceivable
that it could also play a role for C−C coupling from
compounds 4. To examine this, we monitored ethane
evolution from 4a in the presence of 3 equiv of Au-
(PMe2ArXyl2)(NTf2), though this excess of gold-triflimide did
not have notable effects on the rate of ethane formation. This
was, however, not surprising in line with our computational
results, where the larger barrier originates after binding of
[Au(PMe2ArXyl2)]+ to 4a. Nonetheless, even if Au-
(PMe2ArXyl2)(NTf2) is required to facilitate phosphine
An alternative route consists of two gem-digold methyl
fragments 4 approaching to facilitate the C−C coupling event
(Scheme 7b). However, a third pathway to consider, in view of
the Coulombic repulsion derived from the approach of two
cationic species in route b, is the reaction between 4 and its
corresponding neutral methyl species 1 formed by dissociation
of a second molecule of 4 into their monometallic fragments
(Scheme 7c). First, we explored computationally the direct
coupling of methyl groups between two molecules of 4a (route
a) as well as between 4a and 1a (route b) by relaxed potential
energy scans. These studies indicate that those pathways are
unfeasible, both in the singlet and triplet state. We also
evaluated the possibility of accessing the hypothetical Au(III)
species [Au(CH3)2(PMe2ArXyl2)][NTf2] from the above
routes, since reductive coupling of ethane with such a complex
should be accessible.18,29,30 In fact, we found a feasible barrier
(+16.1 kcal/mol) for ethane formation from the hypothetical
Au(III) complex [Au(CH3)2(PMe2ArXyl2)][NTf2] (Figure
S15). However, [Au(CH3)2(PMe2ArXyl2)][NTf2] would be
formed alongside the digold(0) species [Au2(PMe2ArXyl2)2],
with these species being 42.1 kcal/mol higher in energy than
its precursors, rendering this pathway inaccessible under the
reaction conditions (Figure S15). Similarly, CH3 transfer36
+
from 4a to 1a presents a computed transition state at +47.0
kcal/mol (TS1 in Figure S16). In addition, we examined
reductive coupling from the hypothetical trinuclear species
+
derived from the above CH3 transfer; a transition state at
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dissociation, its presence may also affect the observed rate of
ethane evolution in an opposite manner by reducing the
concentration of Au(CH3)(PMe2ArXyl2) (1a) in solution, the
latter species also being required for C−C coupling. This is
because [Au2(μ-CH3)(PMe2ArXyl2)2][NTf2] (4a) is in
dynamic equilibrium in solution with 1a and Au(PMe2ArXyl2)-
(NTf2), as discussed above. To circumvent the influence of
added Au(PMe2ArXyl2)(NTf2) on that equilibrium, we
investigated the effect of adding 5 equiv of BPh3 as an
alternative and less disruptive Lewis acid that could facilitate
phosphine dissociation. While ethane evolution proceeded at a
rate (t1/2 = 340 s) comparable to that of pure 4a (t1/2 = 260 s),
we did observe a distinctive change in the kinetic profile. More
precisely, this experiment revealed a first-order kinetic
dependence on 4a (Figure S7), in contrast to the second-
order profile observed when the consumption of the latter was
monitored in pure form.
Scheme 8. Proposed Mechanism for the Reductive Coupling
of Ethane from gem-Digold Compounds 4
Next, we directed our efforts to examining, by computational
means, the role of Au(PMe2ArXyl2)(NTf2) on the pathways and
energetics for the formation of ethane (Figure 9). Since we
attribute a Lewis acidic role to this fragment, as supported by
our experiments with BPh3, we first studied BH3 as a simplified
Lewis acid. Thus, we examined the reaction between BH3 and
complex Au(CH3)(PMe2ArXyl2) (1a). The formation of a Au−
BH3 adduct is slightly exergonic (ΔG = −0.9 kcal/mol), from
which the transition state for the formation of a P−B bond
(TS3) lies at +16.2 kcal/mol above the independently
computed 1a and BH3, giving the product at −7.4 kcal/mol
(Figure S19). Encouraged by this result, we studied the
analogous process with cation [Au(PMe2ArXyl2)]+ instead of
BH3 as the Lewis acid.63 A transition state for that process
(TS4) was found at +29.3 kcal/mol, leading to the formation
of a species of formula [(PMe2ArXyl2)2AuAu(CH3)]+, A in
Figure 9, that lies at +18.5 kcal/mol and represents a form of
masked “AuMe” stabilized by aurophilic and metal−arene
interactions with the [Au(PMe2ArXyl2)2]+ fragment. Nonethe-
less, the large barrier renders this process inaccessible from 4a,
in agreement with the experimentally determined second-order
dependence on its concentration.
To account for the second-order dependence on 4a, we
considered its initial dissociation into 1a and Au(PMe2ArXyl2)-
(NTf2), the latter providing 1 equiv of cation [Au-
(PMe2ArXyl2)]+ amenable to bind a second molecule of 4a.
The resulting trigonal dicationic adduct [Au3(μ-CH3)-
(PMe2ArXyl2)3]2+ (B) plus 1a are only 1.2 kcal/mol above
two molecules of 4a (Figure 9). From trimetallic adduct B, the
transition state for the formal transfer of a phosphine ligand
between gold atoms was found at +21.3 kcal/mol (TS5), close
enough to the experimentally determined value for the overall
process of ethane evolution. This transition state gives
trinuclear species C at +10.7 kcal/mol, from which dissociation
of 5a is thermodynamically accessible (ΔG = +2.5 kcal/mol).
This would render the bimetallic intermediate [Au2(CH3)-
(PMe2ArXyl2)]+ (Figure S20), which is reminiscent of the
proposed highly reactive “AuMe” fragment proposed by
Kochi.31,32 From such a reactive fragment, masked as
[Au2(CH3)(PMe2ArXyl2)]+, it is expected that the approach
of 1a would result in ethane elimination and formation of
colloidal gold, not necessarily in that order.
(NTf2), the latter functioning as a Lewis acid to favor
phosphine migration from a second molecule of 4 by forming a
trimetallic intermediate of type B.64 Following the release of
diphosphine compounds 5, the resulting masked “AuMe”
fragment reacts with 1a to liberate ethane with concomitant
formation of elemental Au, eventually leading to the formation
of Au nanoparticles. In this picture, phosphine migration from
4a constitutes the rate-limiting step of the overall process, in
analogy to the previously proposed mechanism for reductive
coupling from Au(CH3)(PPh3).31,32 In contrast, the remark-
able acceleration observed for C−C coupling in compounds 4
compared to 1 seems to be the result of stabilization of key
intermediates by the presence of aurophilic interactions
combined with the Lewis acidic character of [Au(PR2Ar′)]+
that enables phosphine migration, thus representing an
example of rate acceleration by polymetallic entities compared
to monometallic counterparts.65−67
CONCLUSIONS
■
Au-mediated C−C coupling processes have rapidly emerged as
versatile and powerful strategies for organic synthesis. Despite
numerous reports on the synthetic applicability of gold
catalysts, mechanistic understanding has evolved at a slower
pace. Previous studies have placed the Au(I)/Au(III) redox
couple at the heart of all these transformations, while
mechanistic investigations on C−C coupling processes without
the apparent advent of Au(III) species is lacking. Herein, we
have demonstrated that gem-digold methyl complexes [Au2(μ-
CH3)(PR2Ar′)2][NTf2] (4) promote the homocoupling of the
bridging methyl fragments to produce ethane at a remarkably
higher rate than from its parent neutral species Au(CH3)-
(PR2Ar′) (1). We have also demonstrated that this approach
permits the heterocoupling of the bridging methyl group with
ethyl and phenyl fragments. The stability of compounds 4
toward reductive homocoupling is highly dependent on the
steric bulk of the phosphine ligand. Whereas the system based
on PPh3 readily liberates ethane at −40 °C, those bearing
terphenyl phosphines (PMe2Ar′) exhibit considerably en-
hanced stability, which is further increased by the use of the
more hindered XPhos and tBuXPhos, the latter being unable to
mediate C−C coupling even at 90 °C. Our kinetic studies
revealed second-order dependence on gem-digold methyl
complexes 4 during ethane evolution, whereas a distinctive
change toward a first-order dependence on the latter was
ascertained in the presence of excess BPh3 as an external Lewis
acid. On the basis of our experimental studies combined with
Our combined experimental/computational approach led us
to propose the mechanistic picture for C−C coupling at gem-
digold compounds 4 depicted in Scheme 8. Compounds 4
readily dissociate in solution to form 1 and Au(PR2Ar′)-
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where appropriate. For elemental analyses, the LECO TruSpec CHN
elementary analyzer and PerkinElmer 2400 Series II analyzer were
DFT computational methods we have proposed a mechanism
that involves rapid dissociation of a molecule of [Au2(μ-
CH3)(PMe2Ar′)2][NTf2] (4) toward Au(PMe2Ar′)(NTf2)
and Au(CH3)(PMe2Ar′) (1). While Au(PMe2Ar′)(NTf2)
mediates phosphine migration from a second molecule of 4
via a trimetallic intermediate, compound 1 is proposed to react
with the resulting highly reactive and masked “AuMe”
fragment to effect the C−C coupling event, most likely by a
multinuclear gold species. These studies highlight the relevance
of multimetallic mechanisms in mediating uncommon trans-
formations, herein also boosting the rate at which the C−C
coupling transformation occurs.
utilized. GC analysis was performed using
a Shimadzu
GCMSQP2010-Plus instrument equipped with a PoraBOND-Q
capillary column (25 m, 0.25 mm i.d., 3.0 μm film thickness, Agilent
Technologies). Helium carrier gas was supplied at a head pressure of
10 psi to provide an initial flow rate of 1.4 mL/min. A 1 mL injection
with a split ratio of 1:10 was employed. GC temperature was initially
held at 40 °C for 1 min and gradually increased to 120 °C at 5 °C/
min. Full-scan mass spectra were collected from 5 to 70 m/z at a data
acquisition rate of 3.5 spectra/s. The MS transfer line was held at 250
°C, and the ion source temperature was 200 °C. Samples analyzed by
transmission electron microscopy (TEM) were prepared by dispersing
the powders in cyclohexane or hexanes (99.5%, anhydrous, Sigma-
Aldrich) and sonicating for 1 min before mounting on Cu-supported
holey carbon grids. The Au samples were imaged using an FEI Titan
80-300 operating at 300 kV. The structures of compounds 1a, 1e, 1f,
2a, 3a, 4e, 4f, and Au(tBuPhos)(NTf2) have been authenticated by X-
ray diffraction studies and their corresponding CIF files deposited in
the Cambridge Crystallographic Data Centre with nos. 2024182−
2024189.
EXPERIMENTAL SECTION
■
General Methods. Unless otherwise noted, all reactions and
manipulations were performed under a nitrogen atmosphere in a
glovebox or using standard Schlenk techniques with dried and
degassed solvents. All solvents were purified via a solvent purification
system or by common distillation techniques: Dichloromethane
(CH2Cl2) was distilled under nitrogen over CaH2. Toluene (C7H8),
benzene (C6H6), n-hexane (C6H14), and n-pentane (C5H12) were
distilled under nitrogen over sodium. Tetrahydrofuran (THF) and
diethyl ether were distilled under nitrogen over sodium/benzophe-
none. Benzene (C6D6) was dried over sodium, while CDCl3 and
CD2Cl2 were dried over molecular sieves (4 Å) and distilled under
nitrogen. Compounds PMe2Ar′,68 AuCl(THT) (THT = tetrahy-
drothiophene),69 Au(PPh3)(NTf2),70,71 Au(PPh3)(NO3),72,73 AuCl-
(XPhos),74 AuCl(tBuXPhos),75 Au(XPhos)(NTf2),76−78 Au-
(tBuXPhos)(NTf2),78 AuCl(PMe2ArXyl2),62 AuCl(PMe2ArDipp2),46
AuCl(PMe2ArTripp2),79 Au(PMe2ArXyl2)(NTf2),62 Au(PMe2ArDipp2)-
General Synthesis of Compounds 1. A suspension of the
corresponding gold chloride precursor AuCl(PR2Ar′) (0.20 mmol) in
toluene (10 mL) was cooled to −78 °C, and a solution of MeMgX (X
= Cl or Br; 2.5 equiv) in toluene was added dropwise. The mixture
was allowed to warm up slowly for 16 h. The volatiles were removed
in vacuo, and the residue was extracted with benzene for 1a−1d or
pentane for 1e−1f. Evaporation of the organic solvent led to
compounds 1a−1f as white powders in around 60−80% yields.
Isotopologue 1a-d3 was synthesized by the same procedure using
freshly prepared CD3MgI.81 Suitable crystals of compounds 1 can be
obtained by slow solvent evaporation from pentane/Et2O or pentane/
dichloromethane solutions. Spectroscopic and analytical data for
selected compounds (others can be found in the SI). Compound 1a.
Yield: 84 mg, 75%. Anal. Calcd for C25H30AuP: C, 53.8; H, 5.4.
Found: C, 53.4; H, 5.5. 1H NMR (400 MHz, CD2Cl2, 25 °C) δ: 7.53
(td, 1 H, 5JHP = 1.7 Hz, Hb), 7.23 (t, 2 H, Hd), 7.14 (d, 4 H, Hc), 7.07
(dd, 2 H, 4JHP = 2.9 Hz, Ha), 2.13 (s, 12 H, CH3(Xyl)), 1.02 (d, 6 H,
(NTf2),46 Au(PMe2ArTripp2)(NTf2),79 Au(CH3)(PMe2ArDipp2 46 (1c),
)
and [Au2(μ-CH3)(PMe2ArDipp2)2][NTf2]46 (4c) were prepared
according to previously reported procedures. Compounds 1e and 1f
were prepared according to the general method described below in
yields of around 75%, exhibiting identical spectroscopic data to those
previously reported. Au(CH3)(XPhos)78 and Au(CH3)(tBuXPhos)80
were prepared by an alternative method of the published procedures
and fully characterized. Methyl(triphenylphosphine)gold(I), chloro-
(dimethylsulfide)gold(I), silver bis(trifluoromethanesulfonyl)imide
acetonitrile adduct, chlorotriphenylphosphinegold(I), 2-dicyclohex-
ylphosphino-2′,4′,6′-triisopropylbiphenyl (XPhos), and 2-di-tert-bu-
tylphosphino-2′,4′,6′-triisopropylbiphenyl (tBuXPhos) were pur-
chased from STREM Chemicals and were used as received. Other
chemicals were purchased from Sigma-Aldrich and used as received.
3
2JHP = 7.7 Hz, PMe2), −0.08 (d, 3 H, JHP = 8.2 Hz, AuCH3). All
aromatic couplings are of ca. 7.5 Hz. 13C{1H} NMR (100 MHz,
CD2Cl2, 25 °C) δ: 147.0 (d, 2JCP = 10 Hz, C2), 142.3 (d, 4JCP = 4 Hz,
1
4
C3), 137.2 (C4), 131.8 (d, JCP = 35 Hz, C1), 131.6 (d, JCP = 3 Hz,
3
CHb), 131.5 (d, JCP = 7 Hz, CHa), 128.6 (CHd), 128.4 (CHc), 22.4
1
2
(CH3(Xyl)), 16.8 (d, JCP = 30 Hz, PMe2), 4.7 (d, JCP = 100 Hz,
AuCH3). 31P{1H} NMR (162 MHz, CD2Cl2, 25 °C) δ: 22.1. MS
(ESI) m/z: calcd for M(Na)+, 581.2; expt., 581.4. Compound 1d.
Yield: 106 mg, 70%. Anal. Calcd for C39H58AuP: C, 62.1; H, 7.7.
Found: C, 62.0; H, 7.5. 1H NMR (300 MHz, CD2Cl2, 25 °C) δ: 7.42
1
All new compounds have been characterized by H NMR spectros-
copy, 31P NMR spectroscopy, 13C NMR spectroscopy, and elemental
analysis (see Figure 10). Solution NMR spectra were recorded on
Varian Inova 600 or 500 MHz or on Bruker AMX-300, DRX-400,
DRX-500, and Avance III 800 MHz spectrometers. Spectra were
referenced to external SiMe4 or using the residual proton solvent
peaks as internal standards (1H NMR experiments), or the
characteristic resonances of the solvent nuclei (13C NMR experi-
ments), while 31P was referenced to H3PO4. Spectral assignments
were made by routine one- and two-dimensional NMR experiments
5
4
(td, 1 H, JHP = 1.8 Hz, Hb), 7.15 (dd, 2 H, JHP = 3.0 Hz, Ha), 7.08
(s, 4 H, Hc), 2.94 (hept, 2 H, 3JHH = 6.9 Hz, p-iPr(CH)), 2.58 (hept, 4
H, 3JHH = 6.9 Hz, o-iPr(CH)), 1.31 (d, 12 H, 3JHH = 6.9 Hz; d, 12 H,
3JHH = 6.9 Hz, o-iPr(CH3), p-iPr(CH3)), 1.07 (d, 6 H, JHP = 7.4 Hz,
2
PMe2), 1.02 (d, 12 H, 3JHH = 6.9 Hz, o-iPr(CH3)), −0.36 (d, 3 H, 3JHP
= 8.2 Hz, AuCH3). All aromatic couplings are of ca. 7.5 Hz. 13C{1H}
NMR (125 MHz, CD2Cl2, 25 °C) δ: 149.9 (C5), 146.9 (C4), 146.6
2
4
(d, JCP = 11 Hz, C2), 137.9 (d, JCP = 5 Hz, C3), 133.7 (C1), 133.4
(d, 3JCP = 7 Hz, CHa), 129.3 (CHb), 121.8 (CHc), 35.1 (p-iPr(CH)),
31.9 (o-iPr(CH)), 26.1 (o-iPr(CH3)), 24.9 (p-iPr(CH3)), 23.4
(o-iPr(CH3)), 17.3 (d, JCP = 30 Hz, PMe2), 5.7 (d, JCP = 102 Hz,
AuCH3). 31P{1H} NMR (121 MHz, CD2Cl2, 25 °C) δ: 19.8. MS
(ESI) m/z: calcd for M(Na)+, 777.4; expt., 777.5.
1
2
General Synthesis of Compounds 4. A solid mixture of the
corresponding methyl gold precursor 1a−1f (0.0175 mmol) with 1
equiv of its parent compound [Au(PR2Ar′)][NTf2](0.0175 mmol)
was dissolved in CD2Cl2 (0.6 mL) under nitrogen at −50 °C to
rapidly yield the desired methyl-bridged complex 4a−4f in a
quantitative NMR spectroscopic yield. Characterization of the less
stable compounds 4a and 4b was carried out by multinuclear NMR
spectroscopy at low temperature without further purification.
Compounds 4c−4f were obtained as colorless microcrystalline
Figure 10. Labeling scheme used for 1H and 13C{1H} NMR
assignments.
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Article
substances by precipitation with pentane at −20 °C (4c, 4d) or 25 °C
(4e, 4f) in around 90% yields. Alternatively 4a−4f can be prepared in
comparable by treating compounds 1a−1f with 1/2 equiv of
[Ph3C][B(C6F5)4] in dichloromethane by an otherwise identical
procedure. Spectroscopic and analytical data for selected compounds
(others can be found in the SI). Compound 4a. 1H NMR (400 MHz,
CD2Cl2, −30 °C) δ: 7.61 (t, 2 H, Hb), 7.25 (t, 4 H, Hd), 7.08 (m, 12
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Jesus Campos − Instituto de Investigaciones Químicas (IIQ),
́
2
Departamento de Química Inorgánica and Centro de
Innovación en Química Avanzada (ORFEO−CINQA),
Universidad de Sevilla and Consejo Superior de
Investigaciones Científicas (CSIC), 41092 Sevilla, Spain;
orcid.org/0000-0002-5155-1262; Email: jesus.campos@
iiq.csic.es
H, Ha, Hc), 1.98 (s, 24 H, CH3(Xyl)), 1.16 (d, 12 H, JHP = 7.7 Hz,
PMe2), 0.45 (br. s, 3 H, AuCH3···Au). All aromatic couplings are of
ca. 7.5 Hz. 13C{1H} NMR (100 MHz, CD2Cl2, −30 °C) δ: 147.1 (d,
4
2JCP = 11 Hz, C2), 141.1 (d, JCP = 5 Hz, C3), 136.7 (C4), 133.3
3
(CHb), 131.8 (d, JCP = 8 Hz, CHa), 129.0 (CHd), 128.2 (CHc),
127.8 (d, 1JCP = 38 Hz, C1), 21.9 (CH3(Xyl)), 16.9 (d, 1JCP = 37 Hz,
PMe2), 0.6 (AuCH3···Au). 31P{1H} NMR (162 MHz, CD2Cl2, −20
°C) δ: 1.1. Compound 4d. Anal. Calcd for C101H113Au2BF20P2: C,
55.8; H, 5.2. Found: C, 56.1; H, 4.9. 1H NMR (400 MHz, CD2Cl2, 25
°C) δ: 7.51 (t, 2 H, Hb), 7.17 (dd, 4 H, 4JHP = 3.3 Hz, Ha), 7.05 (s, 8
H, Hc), 2.94 (hept, 4 H, 3JHH = 7.0 Hz, p-iPr(CH)), 2.41 (hept, 8 H,
3JHH = 6.7 Hz, o-iPr(CH)), 1.31 (d, 24 H, 3JHH = 7.0 Hz, p-iPr(CH3)),
T. Brent Gunnoe − Department of Chemistry, University of
Virginia, Charlottesville, Virginia 22904, United States;
orcid.org/0000-0001-5714-3887; Email: tbg7h@
virginia.edu
3
1.22 (m, 36 H,o-iPr(CH3), PMe2), 1.00 (d, 24 H, JHH = 6.6 Hz,
Authors
o-iPr(CH3)), 0.25 (s, 3 H, AuCH3···Au). All aromatic couplings are of
Juan Miranda-Pizarro − Instituto de Investigaciones Químicas
(IIQ), Departamento de Química Inorgánica and Centro de
Innovación en Química Avanzada (ORFEO−CINQA),
Universidad de Sevilla and Consejo Superior de
Investigaciones Científicas (CSIC), 41092 Sevilla, Spain;
orcid.org/0000-0002-0580-7335
Zhongwen Luo − Department of Chemistry, University of
Virginia, Charlottesville, Virginia 22904, United States;
orcid.org/0000-0002-8331-4747
Juan J. Moreno − Instituto de Investigaciones Químicas (IIQ),
Departamento de Química Inorgánica and Centro de
Innovación en Química Avanzada (ORFEO−CINQA),
Universidad de Sevilla and Consejo Superior de
Investigaciones Científicas (CSIC), 41092 Sevilla, Spain;
orcid.org/0000-0003-1809-6170
Diane A. Dickie − Department of Chemistry, University of
Virginia, Charlottesville, Virginia 22904, United States;
orcid.org/0000-0003-0939-3309
ca. 7.5 Hz. 13C{1H} NMR (100 MHz, CD2Cl2, 25 °C) δ: 151.1 (C5),
2
4
147.3 (C4), 146.8 (d, JCP = 12 Hz, C2), 137.0 (d, JCP = 6 Hz, C3),
3
1
134.0 (d, JCP = 8 Hz, CHa), 131.2 (CHb), 129.4 (d, JCP = 56 Hz,
C1), 122.2 (CHc), 35.0 (p-iPr(CH)), 32.0 (o-iPr(CH)), 25.8
(o-iPr(CH3)), 24.8 (p-iPr(CH3)), 23.5 (o-iPr(CH3)), 17.7 (d, JCP
=
1
2
1
37 Hz, PMe2), 0.5 (t, JCP = 53 Hz, JCH = 130 Hz, AuCH3···Au).
31P{1H} NMR (162 MHz, CD2Cl2, 25 °C) δ: 0.5. MS (ESI) m/z:
calcd for M+, 1493.8; expt., 1493.8. Compound 4e. Anal. Calcd for
C69H101Au2F6NO4P2S2: C, 50.5; H, 6.2; N, 0.9. Found: C, 50.3; H,
1
6.2; N, 0.9. H NMR (500 MHz, CD2Cl2, 25 °C) δ: 7.76 (m, 2 H,
Ha), 7.64 (m, 4 H, Hb), 7.22 (m, 2 H, Hc), 7.09 (s, 4 H, Hd), 3.06
(hept, 2 H, 3JHH = 7.1 Hz, o-iPr(CH), 2.37 (hept, 4 H, 3JHH = 7.0 Hz,
p-iPr(CH)), 2.15 (m, 2 H, Cy(CH2)), 1.92 (m, 8 H, Cy(CH2)), 1.85
(m, 2 H, Cy(CH)), 1.46 (m, 8 H, Cy(CH)), 1.44 (d, 12 H, 3JHH = 6.0
Hz, p-iPr(CH3)), 1.37 (m, 8 H, Cy(CH)), 1.24 (m, 8 H, Cy(CH)),
3
3
1.26 (d, 12 H, JHH = 6.0 Hz, o-iPr(CH3)), 1.03 (d, 6 H, JHH = 6.0
Hz, o-iPr(CH3), c), 0.67 (t, 3 H, JHP = 2.2 Hz, AuCH3···Au).
3
13C{1H} NMR (201 MHz, CD2Cl2, 25 °C) δ: 150.3, 147.1, 146.7 (d,
J = 14 Hz), 137.2 (d, J = 6 Hz), 134.2 (d, J = 10 Hz), 133.2, 131.1,
2
127.8 (d, J = 6 Hz), 127.5 (d, JC−P = 48 Hz), 121.3, 37.5 (d, J = 32
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c11296
Hz), 34.2, 30.8 (d, J = 4 Hz), 30.8, 30.0 (d, J = 4 Hz), 26.8 (d, J = 12
Hz), 26.7 (d, J = 14 Hz), 25.7, 24.9, 24.2, 23.0, 3.1 (t, 2JCP = 48 Hz).
31P{1H} NMR (243 MHz, CD2Cl2, 25 °C) δ: 39.5.
Author Contributions
§J.M.-P. and Z.L. contributed equally.
General Procedure to Measure Kinetic Constants. Kinetic
studies were carried using an identical procedure to that described for
the general synthesis of compounds 4, in J-Young NMR tubes under
nitrogen atmosphere, and monitoring the disappearance of the in situ
Notes
The authors declare no competing financial interest.
1
formed gem-digold methyl compounds 4 by H and 31P{1H} NMR.
ACKNOWLEDGMENTS
Each kinetic experiment was run in triplicates, and average data are
given.
■
The Gunnoe group acknowledges support from the U.S.
National Science Foundation (1800173). The Campos group
acknowledges support from the Spanish Ministry of Science
and Innovation (Project PID2019-110856GA-I00). The
Gunnoe group also acknowledges Jiahan Xie and Robert J.
Davis for the TEM analyses. The Campos group acknowledges
Carlos Navarro-Gilabert for assistance on ligand synthesis. The
two groups acknowledge Prof. Ernesto Carmona for helpful
discussions.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c11296.
Synthesis and characterization of new compounds, X-ray
diffraction data, information on kinetic studies, variable
temperature analysis and exchange experiments, DFT
calculations, and MS and NMR spectra (PDF)
REFERENCES
■
XYZ coordinates (XYZ)
(1) Hashmi, A. S. K. Homogeneous Gold Catalysis Beyond
Assumptions and ProposalsCharacterized Intermediates. Angew.
Chem., Int. Ed. 2010, 49, 5232−5241.
(2) Henrion, G.; Chavas, T. E. J.; Le Goff, X.; Gagosz, F.
Biarylphosphonite Gold(I) Complexes as Superior Catalysts for
Oxidative Cyclization of Propynyl Arenes into Indan-2-ones. Angew.
Chem., Int. Ed. 2013, 52, 6277−6282.
Accession Codes
CCDC 2024182−2024189 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
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(3) Zeng, X.; Soleilhavoup, M.; Bertrand, G. Gold-Catalyzed
Intermolecular Markovnikov Hydroamination of Allenes with
Secondary Amines. Org. Lett. 2009, 11, 3166−3169.
(23) Lu, P.; Boorman, T. C.; Slawin, A. M. Z.; Larrosa, I. Gold(I)-
Mediated C-H Activation of Arenes. J. Am. Chem. Soc. 2010, 132,
5580−5581.
(4) Lavallo, V.; Frey, G. D.; Donnadieu, B.; Soleilhavoup, M.;
Bertrand, G. Homogeneous Catalytic Hydroamination of Alkynes and
Allenes with Ammonia. Angew. Chem., Int. Ed. 2008, 47, 5224−5228.
(5) Gaillard, S.; Bosson, J.; Ramón, R. S.; Nun, P.; Slawin, A. M. Z.;
Nolan, S. P. Development of Versatile and Silver-Free Protocols for
Gold(I) Catalysis. Chem. - Eur. J. 2010, 16, 13729−13740.
(6) Joost, M.; Amgoune, A.; Bourissou, D. Reactivity of Gold
Complexes towards Elementary Organometallic Reactions. Angew.
Chem., Int. Ed. 2015, 54, 15022−15045.
(24) Boorman, T. C.; Larrosa, I. Gold-mediated C-H bond
functionalization. Chem. Soc. Rev. 2011, 40, 1910−1925.
̈
(25) Zavyalova, U.; Holena, M.; Schlogl, R.; Baerns, M. Statistical
Analysis of Past Catalytic Data on Oxidative Methane Coupling for
New Insights into the Composition of High-Performance Catalysts.
ChemCatChem 2011, 3, 1935−1947.
(26) Farrell, B. L.; Igenegbai, V. O.; Linic, S. A Viewpoint on Direct
Methane Conversion to Ethane and Ethylene Using Oxidative
Coupling on Solid Catalysts. ACS Catal. 2016, 6, 4340−4346.
(27) Bayrakdar, T. A. C. A.; Scattolin, T.; Maa, X.; Nolan, S. P.
Dinuclear gold(i) complexes: from bonding to applications. Chem.
Soc. Rev. 2020, 49, 7044−7100.
(7) Wang, J.; Lv, S.; Chen, H.; Shi, M.; Zhang, J. Isolation and
characterization of gem-diaurated species having two C-Au σ bonds in
gold(I)-activated amidiniumation of alkynes. Dalton Trans. 2016, 45,
17091−17094.
(28) Paul, A. M.; Bent, B. E. Alkyl Coupling on Copper, Silver, and
Gold: Correlation Between the Coupling Rate and the Metal-Alkyl
Bond Strength. J. Catal. 1994, 147, 264−271.
(29) Komiya, S.; Albright, T. A.; Hoffmann, R.; Kochi, J. K.
Reductive Elimination and Isomerization of Organogold complexes.
Theoretical Studies of Trialkylgold Species as Reactive Intermediates.
J. Am. Chem. Soc. 1976, 98, 7255−7265.
̈
(8) Hashmi, A. S. K.; Lauterbach, T.; Nosel, P.; Vilhelmsen, M. H.;
Rudolph, M.; Rominger, F. Dual Gold Catalysis: sigma, pi-Propyne
Acetylide and Hydroxyl-Bridged Digold Complexes as Easy-to-
prepare and Easy-to-handle Pre-Catalysts. Chem. - Eur. J. 2013, 19,
1058−1065.
(9) Lauterbach, T.; Asiri, A. M.; Hashmi, A. S. K. Organometallic
Intermediates of Gold Catalysis. Adv. Organomet. Chem. 2014, 62,
261−297.
(10) Li, D.; Che, C.-M.; Kwong, H.-L.; Yam, V.W.-W. Photoinduced
C-C bond formation from alkyl halides catalysed by luminescent
dinuclear gold(I) and copper(I) complexes. J. Chem. Soc., Dalton
Trans. 1992, 3325−3329.
(11) Gasparrini, F.; Giovannoli, M.; Misiti, D.; Natile, G.; Palmieri,
G.; Maresca, L. Gold(III)-catalyzed one-pot synthesis of isoxazoles
from terminal alkynes and nitric acid. J. Am. Chem. Soc. 1993, 115,
4401−4402.
(12) Hashmi, A. S. K.; Schwarz, L.; Choi, J.-H.; Frost, T. M. A New
Gold-Catalyzed C-C-Bond Formation. Angew. Chem., Int. Ed. 2000,
39, 2285−2288.
(13) Hashmi, A. S. K.; Frost, T. M.; Bats, J. W. Highly Selective
Gold-Catalyzed Arene Synthesis. J. Am. Chem. Soc. 2000, 122,
11553−11554.
(30) Lawrence Kuch, P.; Stuart Tobias, R. Synthesis of cationic
dialkylgold(III) complexes: nature of the facile reductive elimination
of alkane. J. Organomet. Chem. 1976, 122, 429−446.
(31) Kochi, J. K. Electron-transfer mechanisms for organometallic
intermediates in catalytic reactions. Acc. Chem. Res. 1974, 7, 351−360.
(32) Tamaki, A.; Kochi, J. K. Formation and decomposition of alkyl-
gold(I) complexes. J. Organomet. Chem. 1973, 61, 441−450.
(33) Xu, H.; Diccianni, J. B.; Katigbak, J.; Hu, C.; Zhang, Y.; Diao,
T. Bimetallic C-C Bond-Forming Reductive Elimination from Nickel.
J. Am. Chem. Soc. 2016, 138, 4779−4786.
(34) Goj, L. A.; Blue, E. D.; Delp, S. A.; Gunnoe, T. B.; Cundari, T.
R.; Petersen, J. L. Single-Electron Oxidation of Monomeric Copper(I)
Alkyl Complexes: Evidence for Reductive Elimination through
Bimolecular Formation of Alkanes. Organometallics 2006, 25, 4097−
4104.
(35) Lail, M.; Gunnoe, T. B.; Barakat, K. A.; Cundari, T. R.
Conversions of Ruthenium(III) Alkyl Complexes to Ruthenium(II)
through Ru-Calkyl Bond Homolysis. Organometallics 2005, 24, 1301−
1305.
(14) Cheong, P. H.-Y.; Morganelli, P.; Luzung, M. R.; Houk, K. N.;
Toste, F. D. Gold-Catalyzed Cycloisomerization of 1,5-Allenynes via
Dual Activation of an Ene Reaction. J. Am. Chem. Soc. 2008, 130,
4517−4526.
(36) Johnson, M. T.; Marthinus Janse van Rensburg, J.; Axelsson,
M.; Ahlquist, M. S. G.; Wendt, O. F. Reactivity of NHC Au(I)-C σ-
bonds with electrophiles. An investigation of their possible
involvement in catalytic C-C bond formation. Chem. Sci. 2011, 2,
2373−2377.
(15) Weber, D.; Tarselli, M. A.; Gagné, M. R. Mechanistic Surprises
in the Gold(I)-Catalyzed Intramolecular Hydroarylation of Allenes.
Angew. Chem., Int. Ed. 2009, 48, 5733−5736.
(16) Levin, M. D.; Toste, F. D. Gold-Catalyzed Allylation of Aryl
Boronic Acids: Accessing Cross-Coupling Reactivity with Gold.
Angew. Chem., Int. Ed. 2014, 53, 6211−6215.
(37) Yang, Y.; Eberle, L.; Mulks, F. F.; Wunsch, J. F.; Zimmer, M.;
Rominger, F.; Rudolph, M.; Hashmi, A. S. K. Trans Influence of
Ligands on the Oxidation of Gold(I) Complexes. J. Am. Chem. Soc.
2019, 141, 17414−17420.
(17) Wolf, W. J.; Winston, M. S.; Toste, F. D. Exceptionally Fast
Carbon-Carbon Bond Reductive Elimination from Gold(III). Nat.
Chem. 2014, 6, 159−164.
̈
(38) Yang, Y.; Schießl, J.; Zallouz, S.; Goker, V.; Gross, J.; Rudolph,
(18) Komiya, S.; Kochi, J. K. Electrophilic cleavage of organogold
complexes with acids. The mechanism of the reductive elimination of
dialkyl(aniono)gold(III) species. J. Am. Chem. Soc. 1976, 98, 7599−
7607.
M.; Rominger, F.; Hashmi, A. S. K. Gold-Catalyzed sp2-sp C-C
Coupling by Alkynylation via Oxidative Addition of Bromoalkynes.
Chem. - Eur. J. 2019, 25, 9624−9628.
(39) Hashmi, A. S. K.; Ramamurthi, T. D.; Rominger, F. Synthesis,
Structure and Reactivity of Organogold Compounds of Relevance to
Homogeneous Gold Catalysis. J. Organomet. Chem. 2009, 694, 592−
597.
(40) Cui, L.; Zhang, G.; Zhang, L. Homogeneous gold-catalyzed
efficient oxidative dimerization of propargylic acetates. Bioorg. Med.
Chem. Lett. 2009, 19, 3884−3887.
(41) Zhang, G.; Cui, L.; Wang, Y.; Zhang, L. Homogeneous Gold-
Catalyzed Oxidative Carboheterofunctionalization of Alkenes. J. Am.
Chem. Soc. 2010, 132, 1474−1475.
(19) Rao, W.; Koh, M. J.; Li, D.; Hirao, H.; Chan, P. W. H. Gold-
Catalyzed Cycloisomerization of 1,6-Diyne Carbonates and Esters to
2,4a-Dihydro-1H-fluorenes. J. Am. Chem. Soc. 2013, 135, 7926−7932.
́
(20) Grirrane, A.; Garcia, H.; Corma, A.; Alvarez, E. Air-stable,
dinuclear and tetranuclear σ,π-acetylide gold(I) complexes and their
catalytic implications. Chem. - Eur. J. 2013, 19, 12239−12244.
(21) Hashiguchi, B. G.; Bischof, S. M.; Konnick, M. M.; Periana, R.
A. Designing Catalysts for Functionalization of Unactivated C-H
Bonds Based on the CH Activation Reaction. Acc. Chem. Res. 2012,
45, 885−898.
(22) Jones, C. J.; Taube, D.; Ziatdinov, V. R.; Periana, R. A.; Nielsen,
R. J.; Oxgaard, J.; Goddard, W. A., III Selective Oxidation of Methane
to Methanol Catalyzed, with C-H Activation, by Homogeneous,
Cationic Gold. Angew. Chem., Int. Ed. 2004, 43, 4626−4629.
(42) Hashmi, A. S. K.; Ramamurthi, T. D.; Todd, M. H.; Tsang, A.
S. K.; Graf, K. Gold-Catalysis: Reactions of Organogold Compounds
with Electrophiles. Aust. J. Chem. 2010, 63, 1619−1626.
(43) Vikse, K. L.; Zavras, A.; Thomas, T. H.; Ariafard, A.; Khairallah,
G. N.; Canty, A. J.; Yates, B. F.; O’Hair, R. A. J. Prying open a
2520
https://dx.doi.org/10.1021/jacs.0c11296
J. Am. Chem. Soc. 2021, 143, 2509−2522

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
́
Reactive Site for Allylic Arylation by Phosphine-Ligated Geminally
(63) Schulz, J.; Shcherbachenko, E.; Roithova, J. Investigation of
Geminally Diaurated Arene Complexes in the Gas Phase. Organo-
metallics 2015, 34, 3979−3987.
Diaurated Aryl Complexes. Organometallics 2015, 34, 3255−3263.
(44) Seidel, G.; Lehmann, C. W.; Furstner, A. Elementary Steps in
̈
Gold Catalysis: The Significance of gem-Diauration. Angew. Chem.,
Int. Ed. 2010, 49, 8466−8470.
(64) Adams, R. D.; Luo, Z. High nuclearity clusters containing
methyl groups. Synthesis and structures of pentaosmium-gold
carbonyl cluster compounds. J. Organomet. Chem. 2016, 812, 108−
114.
́
(45) Gómez-Suarez, A.; Dupuy, S.; Slawin, A. M. Z.; Nolan, S. P.
Straightforward Synthetic Access to gem-Diaurated and Digold σ,π-
Acetylide Species. Angew. Chem., Int. Ed. 2013, 52, 938−942.
(46) Espada, M. F.; Campos, J.; López-Serrano, J.; Poveda, M. L.;
Carmona, E. Methyl-, Ethenyl-, and Ethynyl-Bridged Cationic Digold
Complexes Stabilized by Coordination to a Bulky Terphenylphos-
phine Ligand. Angew. Chem., Int. Ed. 2015, 54, 15379−15384.
(47) A convenient alternative method towards the Au-hydrocarbyl
complex based on boronic acids appeared during the preparation of
this manuscript: Tzouras, N. V.; Saab, M.; Janssens, W.;
Cauwenbergh, T.; Hecke, K. V.; Nahra, F.; Nolan, S. P. Simple
Synthetic Routes to N-Heterocyclic Carbene Gold(I)-Aryl Com-
plexes: Expanded Scope and Reactivity. Chem. - Eur. J. 2020, 26,
5541−5551.
(65) Campos, J. Bimetallic cooperation across the periodic table.
Nat. Rev. Chem. 2020, 4, 696−702.
(66) Buchwalter, P.; Rosé, J.; Braunstein, P. Multimetallic Catalysis
Based on Heterometallic Complexes and Clusters. Chem. Rev. 2015,
115, 28−126.
(67) Farley, C. M.; Uyeda, C. Organic Reactions Enabled by
Catalytically Active Metal-Metal Bonds. Trends Chem. 2019, 1, 497−
509.
́
(68) Ortega-Moreno, L.; Fernandez-Espada, M.; Moreno, J. J.;
Navarro-Gilabert, C.; Campos, J.; Conejero, S.; López-Serrano, J.;
Maya, C.; Peloso, R.; Carmona, E. Synthesis, properties, and some
rhodium, iridium, and platinum complexes of a series of bulky m-
terphenylphosphine ligands. Polyhedron 2016, 116, 170−181.
(69) Uson, R.; Laguna, A.; Laguna, M.; Briggs, D. A.; Murray, H. H.;
Fackler, J. P., Jr (Tetrahydrothiophene)Gold(I) or Gold(III)
Complexes. Inorg. Synth. 2007, 26, 85−91.
(70) Hashmi, A. S. K.; Rudolph, M.; Huck, J.; Frey, W.; Bats, J. W.;
Hamzic, M. Gold Catalysis: Switching the Pathway of the Furan-Yne
Cyclization. Angew. Chem., Int. Ed. 2009, 48, 5848−5852.
(71) Mézailles, N.; Ricard, L.; Gagosz, F. Phosphine Gold(I) Bis-
(trifluoromethanesulfonyl)imidate Complexes as New Highly Effi-
cient and Air-Stable Catalysts for the Cycloisomerization of Enynes.
Org. Lett. 2005, 7, 4133−4136.
(72) Ho, T.-L.; Fieser, M.; Fieser, L. Gold(I) Nitrate -
Triphenylphosphine. In Fieser and Fieser’s Reagents for Organic
Synthesi; Fieser, L. F., Fieser, M., Ho, T.-L., Eds.; John Wiley &
Sons, 2011. DOI: 10.1002/9780471264194.fos11741.
(73) Mueting, A. M.; Alexander, B. D.; Boyle, P. D.; Casalnuovo, A.
L; Ito, L. N.; Johnson, B. J.; Pignolet, L. H.; Leeaphon, M.; Meyer, K.
E.; Walton, R. A.; Heinekey, D. M.; Harper, T. G. P. Mixed-Metal-
Gold Phosphine Cluster Compounds. In Inorganic Syntheses; Grimes,
́
(48) Pérez-Galan, P.; Delpont, N.; Herrero-Gómez, E.; Maseras, F.;
Echavarren, A. M. Metal-Arene Interactions in Dialkylbiarylphos-
phane Complexes of Copper, Silver, and Gold. Chem. - Eur. J. 2010,
16, 5324−5332.
(49) Ni, Q.-L.; Jiang, X.-F.; Huang, T.-H.; Wang, X.-J.; Gui, L.-C.;
Yang, K.-G. Gold(I) Chloride Complexes of Polyphosphine Ligands
with Electron-Rich Arene Spacer: Gold-Arene Interactions. Organo-
metallics 2012, 31, 2343−2348.
́
(50) Caracelli, I.; Zukerman-Schpector, J.; Tiekink, E. R. T. Supra-
molecular synthons based on gold···π(arene) interactions. Gold Bull.
2013, 46, 81−89.
(51) Zuccarello, G.; Zanini, M.; Echavarren, A. M. Buchwald-Type
Ligands on Gold(I) Catalysis. Isr. J. Chem. 2020, 60, 360−372.
́
(52) Marín, M.; Moreno, J. J.; Navarro-Gilabert, C.; Alvarez, E.;
Maya, C.; Peloso, R.; Nicasio, M. C.; Carmona, E. Synthesis, Structure
and Nickel Carbonyl Complexes of Dialkylterphenyl Phosphines.
Chem. - Eur. J. 2019, 25, 260−272.
(53) Li, G.; Liu, C.; Lei, Y.; Jin, R. Au25 nanocluster-catalyzed
Ullmann-type homocoupling reaction of aryl iodides. Chem. Commun.
2012, 48, 12005−12007.
R.N., Ed.; John Wiley
9780470132609.ch63.
& Sons, 1992. DOI: 10.1002/
(54) Li, G.; Abroshan, H.; Liu, C.; Zhuo, S.; Li, Z.; Xie, Y.; Kim, H.
J.; Rosi, N. L.; Jin, R. Tailoring the Electronic and Catalytic Properties
of Au25 Nanoclusters via Ligand Engineering. ACS Nano 2016, 10,
7998−8005.
(74) Nieto-Oberhuber, C.; López, S.; Echavarren, A. M. Intra-
molecular [4 + 2] Cycloadditions of 1,3-Enynes or Arylalkynes with
Alkenes with Highly Reactive Cationic Phosphine Au(I) Complexes.
J. Am. Chem. Soc. 2005, 127, 6178−6179.
́
́
(55) Vilhanova, B.; Vaclavík, J.; Artiglia, L.; Ranocchiari, M.; Togni,
A.; van Bokhoven, J. A. Subnanometer Gold Clusters on Amino-
Functionalized Silica: An Efficient Catalyst for the Synthesis of 1,3-
Diynes by Oxidative Alkyne Coupling. ACS Catal. 2017, 7, 3414−
3418.
(75) Wang, Y.; Ji, K.; Lan, S.; Zhang, L. Rapid Access to Chroman-3-
Ones via Gold-Catalyzed Oxidation of Propargyl Aryl Ethers. Angew.
Chem., Int. Ed. 2012, 51, 1915−1918.
́
(76) Sanchez-Cantalejo, F.; Priest, J. D.; Davies, P. W. A Gold
(56) Cher, M. The Reaction of Methyl Radicals with Toluene. J.
Phys. Chem. 1964, 68, 1316−1321.
(57) Schmidbaur, H.; Schier, A. A briefing on aurophilicity. Chem.
Soc. Rev. 2008, 37, 1931−1951.
(58) Schmidbaur, H.; Schier, A. Aurophilic interactions as a subject
of current research: an up-date. Chem. Soc. Rev. 2012, 41, 370−412.
(59) Weber, D.; Jones, T. D.; Adduci, L. L.; Gagné, M. R. Strong
Electronic and Counterion Effects on Geminal Digold Formation and
Reactivity as Revealed by Gold(I)-Aryl Model Complexes. Angew.
Chem., Int. Ed. 2012, 51, 2452−2456.
(60) Adams, R. D.; Luo, Z.; Chen, M.; Rassolov, V. Multicenter
transformations of the methyl ligand in CH3Os3Au carbonyl cluster
complexes: Synthesis, characterization and DFT analyses. J. Organo-
met. Chem. 2016, 812, 95−107.
(61) Campos, J. Dihydrogen and Acetylene Activation by a Gold(I)/
Platinum(0) Transition Metal Only Frustrated Lewis Pair. J. Am.
Chem. Soc. 2017, 139, 2944−2947.
(62) Hidalgo, N.; Moreno, J. J.; Pérez-Jiménez, M.; Maya, C.; López-
Serrano, J.; Campos, J. Evidence for Genuine Bimetallic Frustrated
Lewis Pair Activation of Dihydrogen with Gold(I)/Platinum(0)
Systems. Chem. - Eur. J. 2020, 26, 5982−5993.
Carbene Manifold to Prepare Fused γ-Lactams by Oxidative
Cyclisation of Ynamides. Chem. - Eur. J. 2018, 24, 17215−17219.
(77) Zhdanko, A.; Strobele, M.; Maier, M. E. Coordination
Chemistry of Gold Catalysts in Solution: A Detailed NMR Study.
Chem. - Eur. J. 2012, 18, 14732−14744.
(78) Fehr, C.; Vuagnoux, M.; Buzas, A.; Arpagaus, J.; Sommer, H.
Gold- and Copper-Catalyzed Cycloisomerizations towards the
Synthesis of Thujopsanone-Like Compounds. Chem. - Eur. J. 2011,
17, 6214−6220.
(79) Alférez, M. G.; Moreno, J. J.; Hidalgo, N.; Campos, J.
Reversible Hydride Migration from C5Me5 to RhI Revealed by a
Cooperative Bimetallic Approach. Angew. Chem., Int. Ed. 2020, 59,
20863−20867.
(80) Han, Z.-Y.; Guo, R.; Wang, P.-S.; Chen, D.-F.; Xiao, H.; Gong,
L.-Z. Enantioselective concomitant creation of vicinal quaternary
stereogenic centers via cyclization of alkynols triggered addition of
azlactones. Tetrahedron Lett. 2011, 52, 5963−5967.
(81) Mayo, D. W.; Bellamy, L. J.; Merklin, G. T.; Hannah, R. W. The
application of resolution enhancement techniques to the study of
factors affecting group frequencies-I. Coupling of symmetric methyl
̈
2521
https://dx.doi.org/10.1021/jacs.0c11296
J. Am. Chem. Soc. 2021, 143, 2509−2522

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
deformation frequencies in o-xylene. Spectrochim. Acta 1985, 41,
355−357.
2522
https://dx.doi.org/10.1021/jacs.0c11296
J. Am. Chem. Soc. 2021, 143, 2509−2522