Silver- and Acid-Free Catalysis by Polyoxometalate-Assisted Phosphanegold(I) Species for Hydration of Diphenylacetylene

Article abstract of DOI:10.1021/acs.organomet.6b00114

A DMSO-soluble intercluster compound consisting of a tetra{phosphanegold(I)}oxonium cation and an α-Keggin polyoxometalate (POM) anion, [{Au(PPh₃)}₄(μ₄-O)]₃[α-PW₁₂O₄₀]₂ (1), was found to be an effective precatalyst for the silver- and acid-free catalysis of diphenylacetylene hydration (0.67 mol % catalyst; conversions 36.1%, 55.2%, and 93.7% after 4, 6, and 24 h reactions, respectively). The reaction proceeded in the suspended system in 6 mL of 1,4-dioxane/water (4:1) at 80 °C because of the low solubility of 1. Similar POM-based phosphanegold(I) compounds [{{Au(PPh₃)}₄(μ₄-O)}{{Au(PPh₃)}₃(μ₃-O)}][α-PW₁₂O₄₀]·EtOH (5), which is composed of a heptakis{triphenylphosphanegold(I)}dioxonium cation and an α-Keggin POM anion, and [Au(CH₃CN)(PPh₃)]₃[α-PMo₁₂O₄₀] (6), which consists of an acid-free monomeric phosphanegold(I) acetonitrile cation and an α-Keggin molybdo-POM anion, also exhibited acid-free catalysis for the hydration of diphenylacetylene. An induction period was observed in the catalysis by 5. On the other hand, their component species, or phosphanegold(I) species without the POM anion, such as [{Au(PPh₃)}₄(μ₄-O)](BF₄)₂ (2) and [{Au(PPh₃)}₃(μ₃-O)]BF₄ (3), and the monomeric phosphanegold(I) complex [Au(RS-pyrrld)(PPh₃)] (4) (RS-Hpyrrld = RS-2-pyrrolidone-5-carboxylic acid), the last of which has been used as a precursor for the preparation of 1, 5, and 6, showed poor activities in this reaction (0.67 mol % catalysts; conversions 1.8%, 1.7%, and 0.5% after 24 h reactions, respectively). However, upon adding the free-acid form of Keggin POM, i.e., H₃[α-PW₁₂O₄₀]·7H₂O (H-POM: 0.67 mol %), 2-4 exhibited remarkably enhanced activities (conversion 97.6% each after 24 h reactions). In contrast, the activities were not enhanced after adding either the sodium salt of the Keggin POM, Na₃[α-PW₁₂O₄₀]·8H₂O (Na-POM; 0.67 mol %), or a Br?nsted acid 10% HBF₄ aqueous solution (0.67 mol %). Both H-POM and Na-POM themselves exhibited no activity. Catalysis by the phosphanegold(I) species for diphenylacetylene hydration was influenced significantly under the free-acid form or sodium salt of the Keggin POM. Acid-free catalytic hydration by 1 of other alkynes, such as phenylacetylene and 1-phenyl-1-butyne, was also examined.

Full text of DOI:10.1021/acs.organomet.6b00114

Article
pubs.acs.org/Organometallics
Silver- and Acid-Free Catalysis by Polyoxometalate-Assisted
Phosphanegold(I) Species for Hydration of Diphenylacetylene
Hidekazu Arai,† Takuya Yoshida,†,‡,§ Eri Nagashima,† Akifumi Hatayama,† Shogo Horie,†
Satoshi Matsunaga,† and Kenji Nomiya*,†
†Department of Chemistry, Faculty of Science, Kanagawa University, Hiratsuka, Kanagawa 259-1293, Japan
‡Research Center for Gold Chemistry and §Department of Applied Chemistry, Tokyo Metropolitan University, Minami-osawa 1−1,
Hachioji, Tokyo 192-0397, Japan
S
* Supporting Information
ABSTRACT: A DMSO-soluble intercluster compound consisting of
a tetra{phosphanegold(I)}oxonium cation and an α-Keggin poly-
oxometalate (POM) anion, [{Au(PPh3)}4(μ4-O)]3[α-PW12O40]2 (1),
was found to be an effective precatalyst for the silver- and acid-free
catalysis of diphenylacetylene hydration (0.67 mol % catalyst;
conversions 36.1%, 55.2%, and 93.7% after 4, 6, and 24 h reactions,
respectively). The reaction proceeded in the suspended system in 6
mL of 1,4-dioxane/water (4:1) at 80 °C because of the low solubility
of 1. Similar POM-based phosphanegold(I) compounds [{{Au-
(PPh3)}4(μ4-O)}{{Au(PPh3)}3(μ3-O)}][α-PW12O40]·EtOH (5),
which is composed of a heptakis{triphenylphosphanegold(I)}-
dioxonium cation and an α-Keggin POM anion, and [Au(CH3CN)-
(PPh3)]3[α-PMo12O40] (6), which consists of an acid-free monomeric
phosphanegold(I) acetonitrile cation and an α-Keggin molybdo-POM anion, also exhibited acid-free catalysis for the hydration of
diphenylacetylene. An induction period was observed in the catalysis by 5. On the other hand, their component species, or
phosphanegold(I) species without the POM anion, such as [{Au(PPh3)}4(μ4-O)](BF4)2 (2) and [{Au(PPh3)}3(μ3-O)]BF4 (3),
and the monomeric phosphanegold(I) complex [Au(RS-pyrrld)(PPh3)] (4) (RS-Hpyrrld = RS-2-pyrrolidone-5-carboxylic acid),
the last of which has been used as a precursor for the preparation of 1, 5, and 6, showed poor activities in this reaction (0.67 mol
% catalysts; conversions 1.8%, 1.7%, and 0.5% after 24 h reactions, respectively). However, upon adding the free-acid form of
Keggin POM, i.e., H3[α-PW12O40]·7H2O (H-POM: 0.67 mol %), 2−4 exhibited remarkably enhanced activities (conversion
97.6% each after 24 h reactions). In contrast, the activities were not enhanced after adding either the sodium salt of the Keggin
POM, Na3[α-PW12O40]·8H2O (Na-POM; 0.67 mol %), or a Brønsted acid 10% HBF4 aqueous solution (0.67 mol %). Both H-
POM and Na-POM themselves exhibited no activity. Catalysis by the phosphanegold(I) species for diphenylacetylene hydration
was influenced significantly under the free-acid form or sodium salt of the Keggin POM. Acid-free catalytic hydration by 1 of
other alkynes, such as phenylacetylene and 1-phenyl-1-butyne, was also examined.
[Au(L)(PPh3)]+ (L = solvent), during the carboxylate
elimination of a monomeric phosphanegold(I) carboxylate,
INTRODUCTION
■
Polyoxometalates (POMs) are anionic, molecular metal−
oxygen bonding clusters that resemble the discrete fragments
of solid metal oxides and mimic soluble metal oxides. These
properties have led to a range of applications in different fields,
[Au(RS-pyrrld)(PPh3)] (4) (RS-Hpyrrld = RS-2-pyrrolidone-5-
carboxylic acid),6 in the presence of the free-acid form of the
Keggin POM, H3[α-PW12O40]·7H2O (H-POM). In addition,
we reported the formation of a heptakis{triphenyl-
especially in catalysis, medicine, biology, electrochromism,
phosphanegold(I)}dioxonium cluster cation, [{{Au(PPh3)}-
magnetism, and material science.1 A combination of POMs
4(μ4-O)}{{Au(PPh3)}3(μ3-O)}]3+,5b by the reaction of [Au-
with cluster cations or macrocations has resulted in the
formation of various interesting intercluster compounds from
(RS-pyrrld)(PPh3)] with the sodium salt of the Keggin POM,
i.e., Na3[α-PW12O40]·9H2O (Na-POM). These reactions were
strongly dependent on the acidity, bulkiness, and high charge
the viewpoints of ionic crystals, crystal growth, crystal
engineering, structure, sorption properties, etc.2−4
density of the POM. Such reactions were also affected by the
Recently, we reported the formation of a novel intercluster
compound consisting of a tetrakis{triphenylphosphanegold-
substituents on the phosphane ligands.5c,e,f For example, novel
intercluster compounds [{(Au{P(p-RPh)3})2(μ-OH)}2]3[α-
(I)}oxonium cluster cation and the Keggin POM anion, i.e.,
[{Au(PPh3)}4(μ4-O)]3[α-PW12O40]2 (1).5a This compound
resulted from the POM-mediated clusterization of in situ-
Received: February 11, 2016
generated monomeric phosphanegold(I) units, [Au(PPh3)]+ or
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00114
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
PM12O40]2·nEtOH (R = Me, M = W; R = Me, M = Mo; R = F,
M = Mo) have been synthesized at room temperature5c by
reactions between [Au(RS-pyrrld){P(p-RPh)3}] (R = Me, F) in
CH2Cl2 and H3[α-PM12O40]·nH2O (M = W, n = 7; M = Mo, n
= 14) in mixed EtOH/H2O solvents. Novel intercluster
compounds, i.e., [{Au(PPh3)}4(μ4-O)][α-XW12O40 {Au-
(PPh3)}3]·3EtOH (X = Al, B), composed of the tetrakis-
{triphenylphosphanegold(I)}oxonium cluster cation and a
saturated Keggin POM, together with three monomeric
phosphanegold(I) units coordinated to the OW2 oxygen
atoms of the edge-shared WO6 octahedra of the POM, were
also reported.5d
Several phosphanegold(I) complexes have been reported to
be effective homogeneous catalysts for organic synthesis.7 One
of the most straightforward ways of synthesizing compounds
with carbon−oxygen bonds is the hydration of unsaturated
organic compounds. In particular, the synthesis of carbonyl
compounds by the addition of water to alkynes is not only
environmentally benign but also economically attractive.8a For
alkyne hydration, active catalytic systems composed of a highly
toxic mercury salt and either a Lewis or Brønsted acid have
traditionally been used. Therefore, the development of less
toxic methods has attracted considerable interest.
entire process is the addition of a second nucleophile molecule
to the formed enol ether to yield the final ketone product.
Regarding POM-based gold(I) catalysts, Blanc et al. reported
that proton-containing hybrid complexes [Au(CH3CN)-
(PPh3)]xH4−x[SiW12O40] (x = 1−4; B1−B4) exhibited effective
catalytic activity and selectivity as heterogeneous catalysts for
the conversion of enyne acetate to cyclopentenone and the
rearrangement of propargylic gem-diester to (E)-3-oxycarbonyl
enone derivatives.11a In addition, the related proton-containing
hybrid complexes [Au(CH3CN)(PR3)][POM-H] (R = Ph, Me;
POM-H = H3SiW12O40, H2PM12O40 (M = Mo, W),
H5P2W18O62) also exhibited bifunctional catalytic properties
for the tandem aza-Prins cyclization/enol ether hydrolysis.11b
In this work, we have examined diphenylacetylene hydration
catalyzed by acid-free intercluster compounds of
phosphanegold(I) cluster species with Keggin POMs, i.e.,
[{Au(PPh3)}4(μ4-O)]3[α-PW12O40]2·4EtOH (1) and [{{Au-
(PPh3)}4(μ4-O)}{{Au(PPh3)}3(μ3-O)}][α-PW12O40]·EtOH
(5), as well as [Au(CH3CN)(PPh3)]3[α-PMo12O40] (6),
consisting of an acid-free monomeric phosphanegold(I) cation
and α-Keggin molybdo-POM anion, under the following
conditions: catalysts (0.67 mol %), diphenylacetylene (1.5
mmol), toluene (4.7 mmol) as an internal standard, 1,4-
dioxane/water (4:1 v/v) mixed solvent, reaction temperature
80 °C. Complexes 1, 5, and 6 were the effective precatalysts of
the silver- and acid-free catalysis for diphenylacetylene
hydration. In addition, as control experiments, alkyne hydration
catalysis by two phosphanegold(I) cluster species without a
POM anion, ([{Au(PPh3)}4(μ4-O)](BF4)2 (2) and [{Au-
(PPh3)}3(μ3-O)]BF4 (3)), as well as by the monomeric
phosphanegold(I) complex [Au(RS-pyrrld)(PPh3)] (4) (all
0.67 mol %) was examined in the presence or absence of
additives (0.67 mol % each), such as H3[α-PW12O40]·7H2O
(H-POM), Na3[α-PW12O40]·8H2O (Na-POM), or aqueous
HBF4 solution (Table 1). This paper reports silver- and acid-
free catalysis using in situ-generated POM-assisted
phosphanegold(I) species from 1 and related compounds for
diphenylacetylene hydration.
In 2002, Tanaka et al. reported alkyne hydration, from 1-
octyne to 2-octanone, in MeOH at 70 °C catalyzed by an
organogold complex, [(PPh3)AuMe], that was activated by a
strong acid, such as H2SO4, CF3SO3H, or H3[PW12O40].8b In
2004, Schmidbaur’s group reported that (phosphane)gold(I)
carboxylates and sulfonates are highly active and reusable
catalysts for the hydration of 3-hexyne by the addition of
water.8a
With regard to gold catalysis, the “silver effect”9a,b and
“counterion effect”9c−e need to be considered. For example, it
has been claimed that gold-catalyzed alkyne hydration cannot
be catalyzed only by [Au]+ and that the presence of silver is
required for the reaction to proceed.9a However, the role of the
counterion, such as OTf, SbF6, and BF4, or the “counterion
effect”, rather than the role of the silver salt has been pointed
out.9c−e In 2009, Corma et al. reported the first incidence of
acid- and silver-free gold(I)-catalyzed alkyne hydration using
gold(I) complexes with phosphane ligands combined with a
soft noncoordinating anion, e.g., [Au(L)]NTf2 (L = SPhos,
PPh3, PtBu3; NTf2 = bis(trifluoromethanesulfonyl)imidate), at
room temperature.8c In 2011, however, Nolan et al. revealed
the hydration of terminal and internal alkynes by NHC-gold(I)
precatalysts (NHC = N-heterocyclic carbene), e.g., [(IPr)-
AuOH] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-yli-
dene), which can generate the active catalytic species
[(IPr)Au]+ by protonolysis in the presence of a Brønsted
acid, such as HSbF6, HNTf2, or HBF4.8d They recently reported
a diaurated species, [{Au(IPr)}2(μ-OH)]BF4, as a silver- and
acid-free catalyst for alkyne hydration, nitrile hydration, and
some organic transformations with relatively low catalyst
loadings (0.5 mol %).9b On the other hand, Corma et al.
recently observed a clear induction time in the gold(I)−
carbene-catalyzed hydration of diphenylacetylene and suggested
that the gold(I)−carbene decomposes under these reaction
conditions. Moreover, the latter could be the catalytically active
gold species because 3−5-atom gold−alkyne clusters were
formed after an induction period.8e
EXPERIMENTAL SECTION
■
Materials. The following reactants were used as received:
diphenylacetylene, deoxybenzoin, phenylacetylene, acetophenone,
1,4-dioxane (Wako), 1-phenyl-1-butyne, butyrophenone, 1-phenyl-2-
butanone (TCI), [{Au(PPh3)}3(μ3-O)]BF4 (3) (Aldrich), and CD2Cl2
and DMSO-d6 (Isotec). Keggin POMs, such as H3[α-PW12O40]·7H2O
(H-POM), H3[α-PMo12O40]·14H2O, Na3[α-PW12O40]·8H2O (Na-
POM), H4[α-SiW12O40]·17H2O, H5[α-BW12O40]·15H2O, H5[α-
AlW12O40]·11H2O, and H6[α-ZnW12O40]·9H2O, were prepared
using the procedures reported elsewhere.5d,12 The compounds were
identified by Fourier transform infrared (FTIR) spectroscopy,
thermogravimetric and differential thermal analysis (TG/DTA), and
solution (27Al, 31P) nuclear magnetic resonance (NMR) spectroscopy.
The precursor [Au(RS-pyrrld)(PPh3)] (4) was synthesized using the
literature procedure6 and identified by CHN elemental analysis, FTIR
spectroscopy, TG/DTA, and solution (1H, 13C{1H}, 31P{1H}) NMR
spectroscopy.
The precatalysts [{Au(PPh3)}4(μ4-O)]3[α-PW12O40]2·4EtOH (1)
and [{{Au(PPh3)}4(μ4-O)}{{Au(PPh3)}3(μ3-O)}][α-PW12O40]·
EtOH (5) were prepared as crystalline samples using liquid−liquid
diffusion methods of the reaction systems consisting of [Au(RS-
pyrrld)(PPh3)] (4) in CH2Cl2 and H3[α-PW12O40]·7H2O (H-POM)
or Na3[α-PW12O40]·8H2O (Na-POM) dissolved in an EtOH/H2O
mixed solvent, respectively,5a,b and characterized by elemental analysis,
FTIR spectroscopy, TG/DTA, and 31P{1H} NMR spectroscopy.
Sicilia et al. discussed the theoretical aspects of the
[Au(PR3)]+-catalyzed hydration of diphenylacetylene.10 The
calculations suggested that the rate-determining step of the
−
[{Au(PPh3)}4(μ4-O)](BF4)2 (2) was prepared in BF4 form from 1
B
DOI: 10.1021/acs.organomet.6b00114
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
JNM-ECA 400 FT-NMR or a JEOL JNM-ECS-400 FT-NMR
spectrometer and a JEOL ECA-400 NMR or ECS-400 NMR data
Table 1. Results of the Hydration of Diphenylacetylene
Catalyzed by Phosphanegold(I) Species
a
1
processing system, respectively. The H and 13C{1H} NMR spectra
were referenced to an internal standard, tetramethylsilane (SiMe4).
The 31P{1H} NMR spectra were referenced to the external standard
25% H3PO4 in H2O in a sealed capillary. The 31P{1H} NMR data with
the usual 85% H3PO4 reference were shifted to +0.544 ppm from the
data.
conversion (%)
The high-performance liquid chromatography (HPLC) apparatus
and conditions are as follows: Shimadzu LC-20AD with Shimadzu
SPD-20A (wavelength 260 nm) detector, using a VP-ODS (150 mm ×
4.6 mm) column, flow rate 0.7 mL per min, and the solvent MeOH/
water (30:17).
after
4 h
after
6 h
after
24 h
entry
1-1(1)
[{Au(PPh3)}4(μ4-O)]3[α-PW12O40]2
(1)
36.1
55.2
93.7
1-1(2)
1-1(3)
1-1(4)
1-2
1 + HBF4(aq)
71.2
17.4
21.5
2.1
72.3
1.5
6.0
2.0
73.1
2.8
3.1
0.7
71.5
1.4
1.2
0
84.0
22.9
27.1
2.0
84.7
2.3
7.0
1.4
87.0
3.6
3.0
1.1
84.9
2.1
1.2
0
>99
54.0
68.0
1.8
Preparation of [Au(CH3CN) (PPh3)]3[PMo12O40] (6). Compound 6
was obtained by a liquid−liquid diffusion method using a CH3CN/
water (5:1) mixed solvent, instead of the EtOH/H2O (5:1) mixed
solvent used in the synthesis of 1. [Au(RS-pyrrld)(PPh3)] (70.5 mg,
0.120 mmol) was dissolved in CH2Cl2 (10 mL). A yellow clear
solution of H3[α-PMo12O40]·14H2O (41.5 mg, 20 μmol) dissolved in
6 mL of a CH3CN/H2O (5:1, v/v) mixed solvent was added slowly
along the interior wall of a round-bottomed flask containing the
colorless clear solution of [Au(RS-pyrrld)(PPh3)]. The round-
bottomed flask containing two layers, i.e., the gold(I) complex
solution in the lower layer and the POM solution in the upper layer,
was sealed and left in the dark at room temperature. After 3 days,
yellow block crystals were formed, collected using a membrane filter
(JG 0.2 μm), washed with CH3CN (20 mL × 2) and Et2O (20 mL ×
2), and dried in vacuo over a 2 h period. Yield: 30 mg (45.1%). The
crystalline samples were soluble in DMSO but insoluble in H2O,
CH3CN, and Et2O. Anal. Calcd for C60H54N3O40P4Au3Mo12 or
[Au(CH3CN)(PPh3)]3[PMo12O40]: C, 21.69; H, 1.64; N, 1.26.
Found: C, 21.80; H, 1.33; N, 1.26. TG/DTA under atmospheric
conditions: a 3.02% weight loss due to desorption of CH3CN
molecules below 141.9 °C was observed with an endothermic peak at
143.4 °C; calcd 3.57% for three coordinated CH3CN molecules in 6.
FTIR (KBr): 1637 (m), 1479 (w), 1436 (m), 1312 (vw), 1181 (vw),
1158 (vw), 1102 (w), 1064 (s), 1027 (vw), 962 (vs), 874 (s), 803 (vs),
753 (s), 713 (s), 692 (s), 597 (m), 545 (s), 500 (s) cm−1. 31P{1H}
1 + NaBF4
1 + KBF4
[{Au(PPh3)}4(μ4-O)](BF4)2 (2)
2 + H-POM
1-3
97.6
9.7
1-4
2 + Na-POM
1-5
2 + HBF4(aq)
7.2
1-6
[{Au(PPh3)}3(μ3-O)]BF4 (3)
3 + H-POM
1.7
1-7
97.6
16.3
3.4
1-8
3 + Na-POM
1-9
3 + HBF4(aq)
1-10
1-11
[Au(RS-pyrrld) (PPh3)] (4)
4 + H-POM
0.5
97.6
4.9
b
1-12
4 + Na-POM
1-13
1-14
1-15
4 + HBF4(aq)
2.2
H3[α-PW12O40]·7H2O (H-POM)
Na3[α-PW12O40]·8H2O (Na-POM)
0
0
0
0
1-16(1) [{{Au(PPh3)}4(μ4-O)}
{{Au(PPh3)}3(μ3-O)}] [α-
PW12O40]·EtOH (5)
1-16(2) 5 + HBF4(aq)
1-16(3) 5 + NaBF4
1-16(4) 5 + KBF4
2.0
4.2
89.0
14.9
0.3
24.1
0.5
87.3
8.4
2.7
3.1
19.0
91.6
1-17
[Au(CH3CN) (PPh3)]3[α-PMo12O40] 71.8
(6)
84.9
1
NMR [22.9 °C, DMSO-d6]: δ −3.32, 27.32 ppm. H NMR [22.2 °C,
DMSO-d6]: δ 2.08 (s, CH3CN), 5.76 (s, CH2Cl2), 7.56−7.70 (m, Ph)
ppm. Crystal data for 6: C60H54Au3Mo12N3O40P4; M = 3323.12;
rhombohedral, space group R3; a = 19.2205(6) Å, c = 19.179(12) Å, V
= 6134.4(5) Å3, Z = 3, Dc = 2.699 g cm−3, μ(Mo Kα) = 19.399 mm−1.
R1 = 0.0274, wR2 = 0.0648 (for all data). Rint = 0.0194, R1 = 0.0271,
wR2 = 0.0647, GOF = 1.033 (14 433 total reflections, 5185 unique
reflections where I > 2σ(I)). The same compound was recently
synthesized from a reaction of [Au(Me)(PPh3)] (1.0 equiv) with
H3[PMo12O40]·nH2O (1.0 equiv) in CH3CN (2.5 mL) at room
temperature, and reported together with the crystal data (CCDC
1407462) by Blanc et al.,11b even though their expected structure was
based on the proton-containing, bifunctional catalyst [Au(CH3CN)-
(PPh3)][H2PMo12O40].
Species Formed in the Catalyst System 3 + H-POM (Entries 1−7).
A colorless clear solution of H3[α-PW12O40]·7H2O (H-POM) (0.060
g, 0.02 mmol) dissolved in 6 mL of an EtOH/H2O (5:1, v/v) mixed
solvent was added slowly to a colorless clear solution of [{Au-
(PPh3)}3(μ3-O)]BF4 (3) (0.0592 g, 0.040 mmol) dissolved in 10 mL
of CH2Cl2. The vial containing two layers was sealed and left in the
dark at room temperature. After a few days, pale yellow block crystals
formed. A portion was used for X-ray crystallography, and the
remainder was collected with a membrane filter (JG 0.2 μm), washed
with EtOH (4 mL × 2) and Et2O (6 mL × 2), and dried in vacuo for 2
h. Yield of crystals: 55.6 mg (48.7%). Yellow block crystals were also
characterized by TG/DTA, X-ray crystallography, and 31P{1H} NMR
in DMSO-d6.
a
Reaction conditions: main catalysts (0.67 mol %), additives (0.67 mol
%), substrate diphenylacetylene 1.5 mmol, toluene (4.7 mmol) as an
internal standard, solvent 6 mL of solvent mixture of 1,4-dioxane and
water (4:1), temperature 80 °C. In entry 1-12, the reaction was
followed after 24 h until 1 week, but their conversions were unchanged
(conversions 9.1% after 48 h, 8.7% after 72 h, and 9.7% after 168 h),
suggesting that the Na+ ion inhibits catalysis by the
heptaphoshanegold(I) cluster compound 5 or significantly lengthens
the induction period.
b
using an anion-exchange resin (Amberlyst A-27) via a batch method,
and the [PW12O40]3−-free powder sample obtained was confirmed by
31P{1H} NMR in CD2Cl2.5a
[Au(PPh3)2]BF4 was prepared13e and characterized by elemental
analysis, TG/DTA, and 31P{1H} NMR spectroscopy. Phosphanegold-
(I) cluster compounds, [Au9(PPh3)8](NO3)3 and [Au9(PPh3)8][α-
14a
PW12O40],2a were prepared using the methodology reported in the
literature and characterized by elemental analysis, TG/DTA, FTIR
spectroscopy, 31P{1H} NMR spectroscopy, and X-ray crystallography.
Instrumentation/Analytical Procedures. CHN elemental anal-
yses were carried out using a PerkinElmer 2400 CHNS elemental
analyzer II (Kanagawa University). IR spectra were recorded on a
Jasco 4100 FT-IR spectrometer in KBr disks at room temperature.
TG/DTA was performed using a Rigaku Thermo Plus 2 series TG/
DTA TG 8120 instrument.
1
The H NMR (500.00 MHz) and 31P{1H} NMR (202.00 MHz)
TG/DTA under atmospheric conditions: a 0.61% weight loss due to
two desorbed EtOH molecules was observed below 212.3 °C; calcd
0.41% for one EtOH molecule and 0.81% for two EtOH molecules.
31P{1H} NMR [21.7 °C, DMSO-d6]: δ −14.89, 24.90 ppm. X-ray
crystallography revealed the formation of a tetrakis{phosphanegold-
spectra in a DMSO-d6 solution were recorded in 5 mm-outer-diameter
tubes on a JEOL JNM-ECP 500 FT-NMR spectrometer using a JEOL
ECP-500 NMR data processing system. The 13C{1H} NMR (99
MHz) spectra were recorded in 5 mm-outer-diameter tubes on a JEOL
C
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a
Table 2. Effect of Various Amounts of Diphenylacetylene in Catalysis by 1
deoxybenzoin formed (TON)
conversion (%) = yield
entry
after 4 h
after 6 h
after 24 h
after 48 h
521.5
after 4 h
after 6 h
after 24 h
after 48 h
69.5
b
2-1
214.4
54.2
233.9
82.7
396.5
140.6
140.3
28.6
36.1
47.0
31.2
55.2
44.9
52.9
93.7
46.8
c
2-2
d
2-3
141.1
134.7
a
Reaction conditions: catalyst [{Au(PPh3)}4(μ4-O)]3[α-PW12O40]2·4EtOH (1) 0.01 mmol, toluene (4.7 mmol) as an internal standard, solvent 6
b
mL of a mixed solvent of 1,4-dioxane and water (4:1), temperature 80 °C. Entry 2-1. Catalyst 1 (0.13 mol %), substrate diphenylacetylene 7.5
mmol, max TON = 750.0. Entry 2-2. Catalyst 1 (0.67 mol %), substrate diphenylacetylene 1.5 mmol, max TON = 150.0. Entry 2-3. Extra substrate
(1.5 mmol) was added to the solution (entry 2-2) after 24 h of reaction by 1, max TON = 300.
c
d
(I)}oxonium cation [{Au(PPh3)}4(μ4-O)]2+ as a countercation of the
(CH3CN)(PPh3)]3[α-PMo12O40] (6) as a DMSO-soluble
[α-PW12O40]3− anion. Crystal data: C216H180Au12O83P14W24; M =
11313.18; monoclinic, space group C2/c; a = 28.653(6) Å, b =
crystalline sample. The compound was characterized by CHN
elemental analysis, FTIR spectroscopy, X-ray crystallography,
19.107(4) Å, c = 32.601(7) Å, α = 90°, β = 107.31(3)°, γ = 90°, V =
and solution (1H, 31P{1H}) NMR spectroscopy. Compound 1
17040(6) Å3, Z = 2, Dc = 2.205 g cm−3, μ(Mo Kα) = 13.326 mm−1. R1
has been obtained by a similar reaction of [Au(RS-pyrrld)-
= 0.0715, wR2 = 0.1662 (for all data). Rint = 0.0548. R1 = 0.0681, wR2 =
(PPh3)] dissolved in CH2Cl2 with H3[α-PW12O40]·nH2O
dissolved in an EtOH/H2O (5:1) mixed solvent.5a Compounds
6 and 1 were formed only in a different solvent system using
the same reagents. Compound 6 did not contain a proton in
the countercation, but it showed acid-free effective catalysis for
the hydration of diphenylacetylene, which is in contrast to the
bifunctional catalysis by the proton-containing hybrid com-
plexes [Au(CH3CN)(PR3)][POM-H] (R = Ph, Me; POM-H =
H3SiW12O40, H2PM12O40 (M = Mo, W), H5P2W18O62).11b
The solution 31P{1H} NMR spectrum of 6 in DMSO-d6
revealed two sharp signals at −3.32 and 27.32 ppm due to the
α-Keggin POM [PMo12O40]3− and the PPh3 ligand in the
[Au(CH3CN)(PPh3)]+, respectively. The signal of the PPh3
ligand at 27.32 ppm was similar to the signal observed at 27.61
ppm of the [Au(RS-pyrrld)(PPh3)] precursor. The tendency
toward the 31P{1H} NMR spectroscopic resonance of the
oligomeric phosphanegold(I) clusters appeared at a higher field
than that of the monomeric phosphanegold(I) complexes. This
suggests that the monomeric structure of 6 had been
maintained in the solution.5 The 31P{1H} NMR spectrum of
6 in DMSO-d6 was comparable to the reported 31P NMR
spectrum reported for [Au(MeCN)(PPh3)][H2PMo12O40]
(−4.1 and 26.6 ppm).11b
Catalysis by the Intercluster Compound 1 and the
Effect of HBF4(aq), NaBF4, and KBF4 Addition (Entries 1-
1(1)−(4)). The intercluster compound [{Au(PPh3)}4(μ4-
O)]3[α-PW12O40]2·4EtOH (1) showed acid-free, effective
catalysis for the hydration of diphenylacetylene (0.67 mol %,
entry 1-1(1), Table 1), in which its conversion to deoxybenzoin
was 36.1% after a 4 h reaction, 55.2% after a 6 h reaction, and
93.7% after a 24 h reaction, in 6 mL of 1,4-dioxane/water (4:1)
at 80 °C. Compound 1 is soluble in DMSO, but sparingly
soluble in the reaction solvent of 1,4-dioxane/water (4:1).
Therefore, the reaction appears to proceed in the suspension/
heterogeneous system. Compound 1 itself is not a catalyst, but
dissolved species generated in the dynamic process of 1, i.e., the
monomeric [Au(PPh3)]+ or [Au(L)(PPh3)]+ species (L =
solvent), will be the catalyst precursor. This was observed as an
averaged single peak in the solution 31P{1H} NMR spectrum of
1 in DMSO-d6.5a This significant feature is suggested by the
catalytic reaction by [Au(CH3CN)(PPh3)]3[α-PMo12O40] (6)
(entry 1-17). Therefore, the actual reaction will proceed
through the homogeneous system by the dissolved [Au-
(PPh3)]+ or [Au(L)(PPh3)]+ species.
0.1636 (I > 2σ(I)). GOF = 1.033.
Species Formed in the Catalyst System 3 + Na-POM (Entries 1−
8). A colorless clear solution of Na3[α-PW12O40]·8H2O (Na-POM)
(0.030 g, 0.010 mmol) dissolved in 3 mL of an EtOH/H2O (5:1, v/v)
mixed solvent was added slowly to a colorless clear solution of
[{Au(PPh3)}3(μ3-O)]BF4 (3) (0.036 g, 0.024 mmol) in 5 mL of
CH2Cl2. The vial containing two layers was sealed and left in the dark
at room temperature. After a few days, colorless plate crystals formed.
A portion was used for X-ray crystallography, and the remainder was
collected with a membrane filter (JG 0.2 μm), washed with EtOH (2
mL × 2) and Et2O (2 mL × 2), and dried in vacuo for 2 h. Yield of
crystals: 40.1 mg (64.5%).
The crystals were unsuitable for X-ray crystallography, but CHN
elemental analysis, TG/DTA, and 31P{1H} NMR spectroscopy in
DMSO-d6 identified them as an intercluster compound between the
heptakis{triphenylphosphanegold(I)}dioxonium cation and the [α-
PW12O40]3− anion. This compound was obtained previously by a
reaction of [Au(RS-pyrrld)(PPh3)] (4) in CH2Cl2 with Na3[α-
PW12O40]·8H2O (Na-POM) dissolved in an EtOH/H2O mixed
solvent.5b
Anal. Calcd for C130H117O44P8Au7W12 or [{{Au(PPh3)}4(μ4-O)}-
{{Au(PPh3)}3(μ3-O)}][α-PW12O40]·2EtOH: H, 1.90; C, 25.12.
Found: H, 1.97; C, 24.74. TG/DTA under atmospheric conditions:
a 1.20% weight loss due to two desorbed EtOH molecules was
observed below 206.2 °C; calcd 0.75% for one EtOH molecule and
1.48% for two EtOH molecules. 31P{1H} NMR [22.3 °C, DMSO-d6]:
δ −14.94, 24.35.
Catalytic Reaction. The catalytic reaction was carried out under
the conditions reported elsewhere (Table 1).8f Diphenylacetylene
(0.267 g, 1.5 mmol) and toluene (100 μL, 0.94 mmol), as an internal
standard, were added to the precatalyst [{Au(PPh3)}4(μ4-O)]3[α-
PW12O40]2·4EtOH (1) (0.115 g, 0.01 mmol) suspended in 6 mL of a
solvent mixture of 1,4-dioxoane and water (4:1). After the dark yellow
suspension was flushed with N2 gas for 5 min, it was stirred in an oil
bath at approximately 80 °C for 1, 4, 6, 18, 20, and 24 h reactions. The
heterogeneous reaction mixture in each reaction time was cooled to
room temperature. The catalyst was allowed to settle in the bottom of
the vial. The supernatant solution of 100 μL was transferred to a 10
mL volumetric flask and diluted to 10 mL with MeOH. The
homogeneous MeOH solution of 5 μL was analyzed by HPLC.
Deoxybenzoin as the hydration product was produced exclusively and
evaluated.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of 6. The reaction of
[Au(RS-pyrrld)(PPh3)] dissolved in CH2Cl2 with H3[α-
PMo12O40]·14H2O dissolved in a CH3CN/H2O (5:1, v/v)
mixed solvent resulted in the formation of the compound
consisting of the monomeric phosphanegold(I) acetonitrile
complex cation and Keggin molybdo-POM anion [Au-
Interestingly, catalysis by 1 was influenced significantly by the
addition of HBF4(aq), NaBF4, and KBF4 (entries 1-1(1)−(4)).
D
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c
Table 3. Hydration of Phenylacetylene and 1-Phenyl-1-butyne Catalyzed by 1
a
b
c
Reaction time: 24 h. Reaction time: 2 h. Reaction conditions: Catalyst 0.01 mmol (0.67 mol %), substrate 1.50 mmol, 1,4-dioxane/water = 4:1
mixed solvent 6 mL, N2 1 atm, temperature 80 °C. Max TON = 150.0.
The catalytic activity of 1 (entry 1-1(1)) was enhanced further
by the addition of HBF4(aq), which was comparable to those of
2 + H-POM (entry 1-3), 3 + H-POM (entry 1-7), 4 + H-POM
(entry 1-11), and compound 6 alone. In contrast, it was
decreased by the addition of NaBF4 (entry 1-1(3)) and KBF4
(1-1(4)). A similar tendency was observed in the catalytic
systems of 5 by the addition of HBF4(aq), NaBF4, and KBF4
(entries 1-16(1)−(4). This suggests that the generation of the
catalyst precursor [Au(PPh3)]+ or [Au(L) (PPh3)]+ (L =
solvent) from 1 and 5 is influenced significantly by the addition
of HBF4(aq), NaBF4, and KBF4. The catalytic activities
observed here strongly suggest that the catalyst precursor is
the solvent-coordinating species [Au(L)(PPh3)]+, such as the
countercation of 6 rather than the hypothetical, coordinating
unsaturated species [Au(PPh3)]+. If this is the case, the
catalytically active species will be attributed to the alkyne-
coordinating species [Au(alkyne)(PPh3)]+ that is stabilized by
the POM anion.
Under the conditions of increased amounts (7.5 mmol) of
diphenylacetylene using 1 (0.13 mol %), the conversion of
69.5% was observed after 48 h of reaction (Table 2, entry 2-1),
showing that the substrate still remains, and the hydration
reaction proceeded after 24 h. This shows that the catalyst
precursor (L = solvent) or the active species (L = alkyne) of the
[Au(L)(PPh3)]+ species stabilized by [PW12O40]3− can work
only when the substrate is present. However, after the substrate
has almost been completely consumed, the active species
readily changes to the inactive form, such as [Au(PPh3)2]+.
Therefore, the reaction continues until the substrate has almost
been totally consumed.
Under the conditions of diphenylacetylene (1.5 mmol) and 1
(0.67 mol %) (entry 2-2 = entry 1-1); the conversion was
93.7% after a 24 h reaction, suggesting that the substrate is
almost completely consumed. Extra amounts of diphenylace-
tylene (1.5 mmol) were added to the solution of entry 2-2 after
a 24 h reaction, in which the active species had already changed
to the inactive form [Au(PPh3)2]+. Time denotes the time after
the extra substrate had been added. The results are shown in
entry 2-3, in which conversions were unchanged, i.e., 47.0%
after 4 h, 44.9% after 6 h, and 46.8% after 24 h, indicating that
new deoxybenzoin has not accumulated. This means that the
active species cannot be recovered from the once formed,
inactive form and the reaction does not proceed further.
Effects of other mixed solvent systems (acetonitrile/water
(4:1), DMF/water (4:1)) on the catalytic activities of
diphenylacetylene hydration by 1 were also evaluated (Table
S1). The acetonitrile/water (4:1) system exhibited effective
activities (Table S1, entry 2), even though the activity was
found to be lower than that of the 1,4-dioxane/water (4:1)
system. The activity of the DMF/water (4:1) system (Table S1,
entry 3) was lower than those of the 1,4-dioxane/water (4:1)
and acetonitrile/water (4:1) systems.
Catalysis by 1 for Hydration of Other Alkynes. The
hydration of phenylacetylene and 1-phenyl-1-butyne as an
alkyne by 1 was also examined (Table 3). The hydration of
phenylacetylene by 1 resulted in the formation of acetophenone
with a TON of 141.5 (conversion 94.3%) after a 24 h reaction
at 80 °C (entry 3-1). These results revealed much more
effective catalysis by 1 under milder conditions compared to the
literature data (conversion 81%) using the catalyst system
NHC-gold(I) complex [Au(IPr)(OH)] (0.02 μmol) in the
presence of aqueous HSbF6 (0.06 μmol), in 2 mL of 1,4-
dioxane/water (2:1) after a 24 h reaction at 120 °C.8d
On the other hand, the hydration of 1-phenyl-1-butyne by 1
resulted in the formation of butyrophenone with a TON of
58.2 (conversion 38.8%) and 1-phenyl-2-butanone with a TON
of 85.5 (conversion 57.0%) after a 24 h reaction at 80 °C (entry
3-2). The conversion of more than 90% in the hydration of
diphenylacetylene and phenylacetylene by 1 was achieved after
a 24 h reaction, whereas the hydration of 1-phenyl-1-butyne
was complete within a 2 h reaction.
The hydration of 1-phenyl-1-butyne by 1 can be compared
with the formation of butyrophenone (15%) and 1-phenyl-2-
butanone (50%) via the catalyst system of [Au(IPr)(OH)]
(0.02 μmol) plus aqueous HSbF6 (0.06 μmol) after a 24 h
reaction at 120 °C.8d Therefore, compound 1 generates an acid-
free, effective catalyst under milder conditions.
The distribution of the products by 1, i.e., butyrophenone
(38.8%) and 1-phenyl-2-butanone (57.0%), shows that
coordination of the alkyne substrate in the [Au(L)(PPh3)]+
species/POM and the subsequent nucleophilic attack of water
molecules are comparatively selective. Therefore, the [Au-
(PPh3)]+ unit tends to coordinate to the site closer to the Ph
group of the alkyne and the water molecule attacks the opposite
site, i.e., the site closer to the Et group.
E
DOI: 10.1021/acs.organomet.6b00114
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Catalysis by Related Compounds 5 and 6 for
Diphenylacetylene Hydration. The related, intercluster
compound consisting of the heptaphosphanegold(I) dioxonium
cation and Keggin POM, [{{Au(PPh3)}4(μ4-O)}{{Au-
(PPh3)}3(μ3-O)}][α-PW12O40]·EtOH (5), also showed effec-
tive catalytic activity (0.67 mol %, entry 1-16, Table 1); that is,
its conversion to deoxybenzoin was 2.0% after a 4 h reaction,
4.2% after a 6 h reaction, and 89.0% after a 24 h reaction. Note
that an induction period was observed in the early stages of the
reaction.
Control Experiments 2: Catalysis by Phosphanegold(I)
Species Brought about in the Presence of the Free-Acid
Form of Keggin POM (H-POM). Upon the addition of the
free-acid form of α-Keggin POM, H3[α-PW12O40]·7H2O (H-
POM; 0.67 mol %), compounds 2−4 (0.67 mol %) showed
remarkably enhanced activities (conversions of 72.3% (entry 1-
3), 73.1% (entry 1-7), 71.5% (entry 1-11) after 1 h reactions;
84.7% (entry 1-3), 87.0% (entry 1-7), 84.9% (entry 1-11) after
4 h reactions; and 97.6% each (entries 1-3, 1-7, and 1-11) after
24 h reactions). That is, all phosphanegold(I) species 2−4
showed effective activities for diphenylacetylene hydration only
under the conditions assisted by the free-acid form of Keggin
POM (H-POM). The observed activities were comparable to
those of the monomeric phosphanegold(I) cation with the
POM anion (6, entry 1-17) and of the system 1 + HBF4(aq)
(entry 1-1(2)). Note that all reactions of entries 1-3, 1-7, and 1-
11 actually form the tetraphosphanegold(I) cluster cation
[{Au(PPh3)}4(μ4-O)]2+ in the solution (see Experimental
Section). Therefore, these reaction systems consist of [{Au-
(PPh3)}4(μ4-O)]2+, [α-PW12O40]3−, and acidic H+, or they are
compatible with the system of 1 plus H+ (entry 1-1(2)). Their
catalytic activities were higher than those of 1 only after 4 h and
6 h reactions (entry 1-1(1)). This suggests that the catalyst
precursor, [Au(L)(PPh3)]+/POM (L = solvent), may be
generated more easily from 1 in the presence of acidic H+.
A combination of 4 and the free-acid form of various Keggin
POMs [XW12O40]n− (X = Si, B, Al, Zn) was also used for
diphenylacetylene hydration (Table 4). All catalyst systems
The heptaphosphanegold(I) cluster cation in 5 was formed
only in the presence of POM.5 b Indeed, the
heptaphosphanegold(I) cluster was not found, when the
−
anion of 5 was exchanged with BF4 using anion-exchange
resin, but the tetraphosphanegold(I) cluster was observed in
solution due to decomposition.
On the other hand, another related compound, which was
composed of the monomeric phosphanegold(I) cation and
Keggin molybdo-POM, i.e., [Au(CH3CN) (PPh3)]3[α-
PMo12O40] (6), also showed effective catalysis (0.67 mol %,
entry 1-17, Table 1); the conversion was 71.8% after a 4 h
reaction, 84.9% after a 6 h reaction, and 91.6% after a 24 h
reaction. Compound 6 was prepared by the reaction between
[Au(RS-pyrrld)(PPh3)] and H3[α-PMo12O40]·14H2O in the
CH3CN/water solvent mixture, whereas compound 1 was
prepared using the same reagents in a different solvent system,
i.e., EtOH/water solvent mixture [see Experimental Section].
Although compound 6 also does not contain any acidic species,
it can work as an acid-free catalyst precursor.
Table 4. Hydration of Diphenylacetylene Catalyzed by
Combination of [Au(RS-pyrrld)(PPh3)] (4) + Free-Acid
Form of Various Keggin POMs
In catalytic processes by 1 and 5, the catalyst precursor will
actually be the same as 6, i.e., [Au(L)(PPh3)]+ (L = solvent),
and the active species is the L = alkyne compound derived from
it. In the catalysis by 1 and 5, the catalyst precursor will
originate from a “dynamic” process (or including fluxional or
scrambling of [Au(PPh3)]+ species in solution).5a,b
conversion (%)
main catalyst [0.67 mol %], additives
[0.67 mol %]
after
4 h
after
6 h
after
24 h
entry
Regarding the POM-based phosphanegold(I) catalysts, Blanc
et al. recently reported that the proton-containing hybrid
complexes [Au(CH3CN)(PR3)][POM-H] (R = Ph, Me; POM-
H = H3SiW12O40, H2PM12O40 (M = Mo, W), H5P2W18O62)
showed catalytic properties for the rearrangement of the enyne
esters to cyclopentenones and the cyclization of N-
propargylcarboxamides.11b They also reported bifunctional
catalytic properties for aza-Prins cyclization/enol ether hydro-
lysis.11a They also reported the X-ray structures of [Au-
(CH3CN)(PPh3)]3[PM12O40] (M = Mo, W) without acidic
protons, together with the related POM compounds, but none
of them had the expected structures.
Control Experiments 1: Catalysis by Phosphanegold(I)
Species without POM. For diphenylacetylene hydration, the
component species constituting 1 and 5, i.e., the tetra- and
triphosphanegold(I) cluster species [{Au(PPh3)}4(μ4-O)]-
(BF4)2 (2, entry 1-2) and [{Au(PPh3)}3(μ3-O)]BF4 (3, entry
1-6) and the POMs H3[α-PW12O40]·7H2O (H-POM, entry 1-
14) and Na3[α-PW12O40]·8H2O (Na-POM, entry 1-15),
showed poor activities (0.67 mol % each; conversions of
1.8%, 1.7%, 0%, and 0% after 24 h reactions, respectively). In
other words, the phosphanegold(I) species without POM
showed no activity. The complex precursor without POM, i.e.,
[Au(RS-pyrrld)(PPh3)] (4, entry 1-10), also showed only poor
activity (0.67 mol %; conversion of 0.5% after 24 h reaction).
Therefore, the activities of 1 and 5 suggest that the
phosphanegold(I) species exhibit catalytic activities only in
the presence of POM.
4-1
4-2
4-3
4-4
4-5
4 + H3[α-PW12O40]·7H2O
4 + H4[α-SiW12O40]·17H2O
4 + H5[α-BW12O40]·15H2O
4 + H5[α-AlW12O40]·11H2O
4 + H6[α-ZnW12O40]·9H2O
71.5
34.0
44.5
36.7
38.1
84.9
44.7
52.6
51.7
49.0
97.6
61.6
75.7
56.8
62.1
exhibited effective activities (entries 4-2−4-5), even though the
activities of X = Si, B, Al, and Zn were found to be lower than
that of X = P (entry 1-11 = entry 4-1). The origin of the
observed differences is unclear.
Control Experiments 3: Catalysis by Phosphanegold(I)
Species in the Presence of Sodium Salt of Keggin POM
(Na-POM). No enhancement of their original activities was
observed (entries 1-4, 1-8, 1-12) after adding the sodium salt of
the Keggin POM, Na3[α-PW12O40]·8H2O (Na-POM; 0.67 mol
%), to compounds 2−4.
All the reactions of 2 + Na-POM (entry 1-4), 3 + Na-POM
(entry 1-8), and 4 + Na-POM (entry 1-12) resulted in the
formation of the heptaphosphanegold(I) cluster cation in the
solution (see Experimental Section). Therefore, these reaction
systems are composed of [{{Au(PPh3)}4(μ4-O)}{{Au-
(PPh3)}3(μ3-O)}]3+, [α-PW12O40]3−, and Na+, or they are
compatible with the system of 5 plus Na+ ion (entry 1-16(2)).
Catalysis by 5 itself was achieved after a clear induction period
(entry 1-16(1)), suggesting that it takes a longer time to
generate the catalyst precursor [Au(L)(PPh3)]+. The catalytic
reaction by 5 is retarded by the presence of alkaline (Na+ and
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DOI: 10.1021/acs.organomet.6b00114
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Scheme 1
K+) ions (entries 1-16(3) and 1-16(4)), or the induction period
is lengthened remarkably, as shown in entry 1-12 [footnote b;
the reaction was followed from 24 h until 1 week, but their
conversions were unchanged (the conversions were 9.1% after
48 h, 8.7% after 72 h, and 9.7% after 168 h)]. Therefore, the
activities of the systems consisting of 5 plus Na+ ion (entry 1-
16(3)) are consistent with the results of entries 1-4, 1-8, and 1-
12. A similar tendency was found in the systems of 1 plus
NaBF4 and KBF4 (entries 1-1(3) and 1-1(4)).
Furthermore, the addition of an aqueous solution of the
Brønsted acid, 10% HBF4 (0.67 mol %), to compounds 2−4
did not have any effects on their original activities (entries 1-5,
1-9, 1-13). This shows that the free-acid form of POM, but not
the acidic proton without POM, significantly assists catalysis by
the monomeric phosphanegold(I) species [Au(L)(PPh3)]+ (L
= solvent or alkyne).
at 80 °C revealed a signal assignable to [Au(DMSO)(PPh3)]+
at 27.30 ppm, as well as signals due to the inactive
[Au(PPh3)2]+ at 43.8 ppm and [PW12O40]3− at −14.93 ppm
(Figure S2).
The insoluble materials recovered by filtering off the reaction
solution after a 24 h reaction were dissolved in DMSO-d6, and a
31P{1H} NMR spectrum of the solution was measured. In such
materials, the starting compound 1, which is observed as a
signal at 24.98 ppm, was absent, but the 31P{1H} NMR
spectrum revealed characteristic signals due to several catalyti-
cally inactive compounds, such as [PW12O40]3− at −14.85 ppm,
[Au(PPh3)2]+ at 43.68 ppm, and mixed-valence
phosphanegold(I) cluster cations, such as [Au9(PPh3)8]3+, as
signals at approximately 57−58 ppm.14a The filtrate obtained
after a 24 h reaction was evaporated to dryness, and the residue
was dissolved in DMSO-d6. The 31P{1H} NMR spectrum of the
solution showed no signal, suggesting that the reaction solution
after a 24 h reaction does not contain any materials originating
from PPh3 and [PW12O40]3−. On the other hand, the 1H NMR
spectrum of the solution revealed the signals for a CH2 group
(4.28 ppm) and the phenyl groups of deoxybenzoin as the
hydration product (7.22−8.02 ppm). Because the conversion
was 93.7% after a 24 h reaction, the signals of diphenylacetylene
would be very small in the reaction solution.
31P{1H} NMR Characterization of the Reaction Systems
by 1, 5, and 6. (1) The reaction solution after a 6 h reaction at
80 °C using 1 (0.67 mol %, entry 1-1(1)), in which sufficient
amounts of diphenylacetylene still remained (conversion
55.2%), was evaporated to dryness. The residue was then
dissolved in CD2Cl2. The 31P{1H} NMR spectrum of a CD2Cl2
solution revealed a broad signal that was assigned to the
[Au(diphenylacetylene)(PPh3)]+ species at 36.1 ppm, in
addition to the inactive [Au(PPh3)2]+ species13 at 45.1 ppm
and [PW12O40]3− at −14.6 ppm (Figure S1a). The signal at
36.1 ppm was consistent with a 31P NMR signal at 36.1 ppm in
(2) The solution 31P{1H} NMR spectrum of 5 in DMSO-d6
revealed two sharp signals at −14.92 and 24.46 ppm. The
former peak was assigned to the phosphorus heteroatom in the
α-Keggin POM, whereas the latter peak was assigned to the
averaged signal of PPh3 groups due to fluxional motion in the
solution of the heptaphosphanegold(I) cluster, which corre-
sponds to one broad signal at 24.6 ppm in the CPMAS 31P
NMR spectrum.5b The solid-state broad single peak was
attributed to the fact that all PPh3 groups are in an
approximately equivalent state, but they are not fluxional in
the solid state. The heptaphosphanegold(I) cluster would
probably be present in solution only in the presence of POM;
that is, it cannot exist without POM.5b The solution 31P{1H}
NMR signal at 24.46 ppm shifted slightly to a higher field from
CD2Cl2 of [Au(alkyne)(PPh3)]+SbF6 reported in the liter-
−
ature.15 31P{1H} NMR spectra in CD2Cl2 of the residues
obtained from the reaction systems after hydration reactions at
80 °C of other alkynes (phenylacetylene and 1-phenyl-1-
butyne) using 1 were also measured (Figure S1b,c): after 6 h
hydration reaction ([α-PW12O40]3−, −14.7 ppm; [Au-
(phenylacetylene)(PPh3)]+, 37.2 ppm; [Au(PPh3)2]+, 45.2
ppm) and after 1 h hydration reaction ([α-PW12O40]3−;
−15.0 ppm, [Au(1-phenyl-1-butyne)(PPh3)]+; 34.4 and 36.2
ppm, [Au(PPh3)2]+; 44.9 ppm). On the other hand, the
DMSO-d6 solution of the residue collected after a 6 h reaction
G
DOI: 10.1021/acs.organomet.6b00114
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
the single peak observed at 24.84 ppm of the
tetraphosphanegold(I) cluster in solution.5a
attributed to the monomeric gold(I)−alkyne species stabilized
by POM, i.e., [Au(L)(PPh3)]+/POM (L = alkyne). Indeed, the
reaction systems without POM, such as 2−4 (entries 1-2, 1-6,
and 1-10, respectively), exhibited poor activities. In catalysis by
1, the catalyst precursor [Au(L)(PPh3)]+ (L = solvent) is
originated from the “dynamic” process (or the fluxional or
scrambling process of [Au(PPh3)]+ species in solution, as
shown in the 31P{1H} NMR spectrum of 1 in DMSO-d65a),
whereas in catalysis by 6, it gave rise to ligand exchange with
the solvent used. The acidic proton enhances the activity of 1
(entry 1-1(2)), suggesting that the generation of the catalyst
precursor is accelerated by an acidic proton. Because the
catalyst systems of 2 + H-POM (entry 1-3), 3 + H-POM (entry
1-7), and 4 + H-POM (entry 1-11) generate the same
tetraphosphanegold(I) species in solution, such catalytic
systems are actually the same as that of 1 plus a proton, the
activities of which are higher than that of 1 itself (entry 1-1(1)).
In contrast, catalysis by 5 showed an induction period (entry 1-
16(1)), suggesting that it takes a longer time to generate the
catalyst precursor [Au(L)(PPh3)]+. However, because the
catalyst systems of 2 + Na-POM (entry 1-4), 3 + Na-POM
(entry 1-8), and 4 + Na-POM (entry 1-12) form the same
heptakis{phosphanegold(I)} species in solution, their catalytic
activities are actually the same as the system of 5 plus a Na+ ion
(entry 1-16(3)). The catalytic reaction by 5 is retarded by the
presence of alkaline (Na+ and K+) ions. The induction period
may be lengthened markedly, as shown in entries 1-12
[footnote b] and 1-16(3) and -(4). Further work in this area
is currently under way. The influence of the stoichiometry of
the gold catalyst 4 versus the amount of POM (Table 4) will be
studied in our next plan. We have synthesized other types of
intercluster compounds consisting of the dimers of dinuclear
units in parallel-edge and crossed-edge arrangements of
phosphanegold(I) units and Keggin POMs and determined
their molecular structures.5c Catalytic behaviors of such POM-
based compounds will also be studied as future work.
The 31P{1H} NMR measurements in DMSO-d6 of the
insoluble materials recovered by filtering off the reaction
solution after a 24 h reaction showed that the signal of the
starting compound 5 had disappeared. However, the inactive
[Au(PPh3)2]+ species as the major signal at 43.84 ppm and the
mixed-valence phosphanegold clusters,14a such as
[Au9(PPh3)8]3+, as the minor signals at 54.45 and 58.13 ppm
appeared, in addition to the signal of [PW12O40]3− at −14.84
ppm. The 31P{1H} NMR spectrum of the reaction solution
after a 24 h reaction of 5 was similar to that of 1. The catalyst
precursor in the catalysis by 5 was also the [Au(L)(PPh3)]+
species (L = solvent), but an induction period was required for
its generation.
(3) The 31P{1H} NMR measurements in DMSO-d6 of 6
before the reaction showed a signal at 27.32 ppm. The 31P{1H}
NMR measurements in DMSO-d6 of the insoluble materials
recovered after the 24 h reaction showed that the starting
compound 6 disappeared with the concomitant appearance of
the major signal at 43.55 ppm due to [Au(PPh3)2]+ species and
many smaller signals at 56.19, 57.24, 57.85 (main peak), and
58.06 ppm due to the mixed-valence gold cluster species,14a
such as [Au9(PPh3)8]3+. In addition, the signal due to
[PMo12O40]3− at −3.32 ppm before the reaction changed to
that at −5.96 ppm after a 24 h reaction, which was attributed to
the partially reduced molybdo-POM. This shows that the
reaction contains a redox process, which will be brought about
together by the oxidation of PPh3 liberated from 6.
Reaction Scheme. The reaction scheme of diphenylacety-
lene hydration catalyzed by 1 is depicted in Scheme 1. The
generation of monomeric phosphanegold(I) species [Au(L)-
(PPh3)]+ (L = solvent; catalyst precursor) in the presence of
the POM anion from 1 initiates alkyne coordination to the
Au(I) site by ligand exchange of L (catalytically active species),
followed by the subsequent nucleophilic attack of a water
molecule on the coordinated alkyne and the deprotonation of
coordinated water, resulting in the formation of the enol form
of deoxybenzoin and a transformation to the keto form on the
Au(I) site. By subsequent protonation, the catalytic cycle would
be completed. This is consistent with the theoretical discussion
of alkyne hydration catalyzed by gold(I) species.10 In this
catalytic cycle, the catalytically active, monomeric
phosphanegold(I) species [Au(L)(PPh3)]+ (L = alkyne)
would be accompanied by the POM anion throughout the
process. The generation of the catalyst precursor [Au(L)-
(PPh3)]+ (L = solvent) from 1 would be accelerated by the
acidic proton, as indicated by the reaction 1 + aqueous HBF4
(entry 1-1(2)), but may be retarded by alkaline (Na+ or K+)
ions (entries 1-1(3) and 1-1(4)). In the catalysis by 5 (entry 1-
16(1)), the induction period would also be related to the
process of generating the catalyst precursor [Au(L)(PPh3)]+ (L
= solvent), which may be accelerated by the acidic proton
(entry 1-16(2)), but affected seriously by alkaline (Na+ and K+)
ions (entries 1-16(3) and -(4)).
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00114.
Figures S1 and S2 (31P{1H} NMR data of the residues
obtained from the reaction systems) and Table S1 (effect
of mixed solvent system on the catalytic activities)
(PDF)
AUTHOR INFORMATION
■
Corresponding Author
*Phone: 81-463-59-4111. Fax: 81-463-58-9684. E-mail:
nomiya@kanagawa-u.ac.jp (K. Nomiya).
Notes
The authors declare no competing financial interest.
CONCLUSION
■
Both compounds 1 and 6 exhibited acid-free, effective catalytic
activity for diphenylacetylene hydration (entries 1-1(1) and 1-
17, respectively). Compounds 1 and 6 were synthesized by the
same reaction process of 4 + H-POM using only different
solvent systems, EtOH/H2O vs CH3CN/H2O, respectively. In
both catalytic processes by 1 and 6, the active species was
ACKNOWLEDGMENTS
■
This study was supported by JSPS KAKENHI grant number
22550065 and by the Strategic Research Base Development
Program for Private University of the Ministry of Education,
Culture, Sports, Science and Technology of Japan.
H
DOI: 10.1021/acs.organomet.6b00114
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Article
ChemCatChem 2013, 5, 3509−3515. (f) Gaillard, S.; Bosson, J.;
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