Highly active, homogeneous catalysis by polyoxometalate-assisted N-heterocyclic carbene gold(I) complexes for hydration of diphenylacetylene

Article abstract of DOI:10.1016/j.mcat.2019.02.014

Monomeric, N-heterocyclic (NHC) carbene/carboxylato/gold(I) complex [Au(RS-pyrrld)(IPr)] (RS-Hpyrrld = RS-2-pyrrolidone-5-carboxylic acid; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) (1, 0.01 mmol) in a mixed solvent of 1,4-dioxane: water (4: 1 v/v) showed high activity in the catalysis of the hydration of diphenylacetylene to deoxybenzoin in the presence of protonic acid forms of Keggin polyoxometalates (POMs). The reaction proceeded homogeneously. The catalytic activities were superior to that of the previously reported reaction using the PPh₃ derivative of gold(I) complex, [Au(RS-pyrrld)(PPh₃)] (3). An important difference is that the reaction of (3) + H-PW12 proceeded in a heterogeneous/suspension system, because of the poor solubility of (3). The present catalytic systems were experimentally stable: re-addition of the substrate (1.5 mmol) to the reaction systems of (1) + H-PW12, (1) + H-SiW12, and (1) + H-AlW12, after the first 2-h reaction, revealed no loss of activity. The effect of the NHC ligand was also examined using the [Au(RS-pyrrld)(NHC)] complexes (NHC = IMes (6), BIPr (7), IF³ (8) and I^tBu (9) (see Abbreviations and Scheme 1). We also isolated a novel compound, [Au(H₂O)(IPr)]₃[α-PW₁₂O₄₀] (2), in 42% yield from a 1: 3 M ratio mixture of (1) in EtOH and H-PW12 in water. Compound (2) showed almost the same catalytic activity and stability as the mixed system of (1) + H-PW12, suggesting that (2) is the actual catalyst precursor. Complexes (1), (2), and (6)-(9) were characterized by elemental analysis, IR, TG/DTA, and (¹H and ¹³C{¹H}) NMR, in addition to X-ray crystallography and ³¹P{¹H} NMR for (2). The role of POM in the reactions, i.e., the formation of ion-pair species, was elucidated based on a study of the effects of strong protonic acids such as HBF₄, HPF₆ and CF₃SO₃H, instead of POMs, on (1). Quantum-mechanical (QM) calculations indicated that both the catalytically unsaturated species [LAu]⁺ and the important intermediate [LAu(C₂Ph₂)]⁺ in the catalytic cycle are energetically more stable for L = IPr than for L = PPh₃, in good agreement with the experimental finding that (1) has much greater catalytic activity and stability than (3).

Full text of DOI:10.1016/j.mcat.2019.02.014

MolecularCatalysisxxx(xxxx)xxx–xxx
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Highly active, homogeneous catalysis by polyoxometalate-assisted N-
heterocyclic carbene gold(I) complexes for hydration of diphenylacetylene
Kenji Nomiya⁎, Yuichi Murara, Yuta Iwasaki, Hidekazu Arai, Takuya Yoshida,
Noriko Chikaraishi Kasuga, Toshiaki Matsubara⁎
Department of Chemistry, Faculty of Science, Kanagawa University, Hiratsuka, Kanagawa 259-1293, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords:
Polyoxometalate
N-Heterocyclic carbene ligand
Gold(I) complex
Homogeneous catalysis
Hydration of diphenylacetylene
Monomeric, N-heterocyclic (NHC) carbene/carboxylato/gold(I) complex [Au(RS-pyrrld)(IPr)] (RS-Hpyrrld =
RS-2-pyrrolidone-5-carboxylic acid; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) (1, 0.01 mmol) in a
mixed solvent of 1,4-dioxane : water (4 : 1 v/v) showed high activity in the catalysis of the hydration of di-
phenylacetylene to deoxybenzoin in the presence of protonic acid forms of Keggin polyoxometalates (POMs).
The reaction proceeded homogeneously. The catalytic activities were superior to that of the previously reported
reaction using the PPh3 derivative of gold(I) complex, [Au(RS-pyrrld)(PPh3)] (3). An important difference is that
the reaction of (3) + H-PW12 proceeded in a heterogeneous/suspension system, because of the poor solubility
of (3). The present catalytic systems were experimentally stable: re-addition of the substrate (1.5 mmol) to the
reaction systems of (1) + H-PW12, (1) + H-SiW12, and (1) + H-AlW12, after the first 2-h reaction, revealed
no loss of activity. The effect of the NHC ligand was also examined using the [Au(RS-pyrrld)(NHC)] complexes
(NHC = IMes (6), BIPr (7), IF3 (8) and ItBu (9) (see Abbreviations and Scheme 1). We also isolated a novel
compound, [Au(H2O)(IPr)]3[α-PW12O40] (2), in 42% yield from a 1 : 3 M ratio mixture of (1) in EtOH and H-
PW12 in water. Compound (2) showed almost the same catalytic activity and stability as the mixed system of (1)
+ H-PW12, suggesting that (2) is the actual catalyst precursor. Complexes (1), (2), and (6)-(9) were char-
acterized by elemental analysis, IR, TG/DTA, and (1H and 13C{1H}) NMR, in addition to X-ray crystallography
and 31P{1H} NMR for (2). The role of POM in the reactions, i.e., the formation of ion-pair species, was elucidated
based on a study of the effects of strong protonic acids such as HBF4, HPF6 and CF3SO3H, instead of POMs, on
(1). Quantum-mechanical (QM) calculations indicated that both the catalytically unsaturated species [LAu]+
and the important intermediate [LAu(C2Ph2)]+ in the catalytic cycle are energetically more stable for L = IPr
than for L = PPh3, in good agreement with the experimental finding that (1) has much greater catalytic activity
and stability than (3).
1. Introduction
POM, e.g., H3[α-PW12O40]·7H2O, or its sodium salt, e.g., Na3[α-
PW12O40]·9H2O, react to form various phosphanegold(I) cluster coun-
Polyoxometalates (POMs) are anionic, molecular metal-oxygen
bonding clusters that resemble discrete fragments of solid metal oxides
and mimic soluble metal oxides. Their interesting properties have led to
a range of applications, especially in catalysis, medicine, biology,
electrochromism, magnetism, and materials science [1–12].
Recently, it has been reported that a monomeric phosphanegold(I)
carboxylate, [Au(RS-pyrrld)(PPh3)] (3) (RS-Hpyrrld = RS-2-pyrroli-
done-5-carboxylic acid) [13], and the protonic acid form of Keggin
tercations of the POM anions by elimination of the carboxylato ligand
[14–20]. Formation of such phosphanegold(I) cluster cations is strongly
dependent upon the solvent system used. For example, when (3) was
dissolved in CH2Cl2, and the Keggin POM was dissolved in the solvent
system of EtOH/water, the tetrakis{phosphanegold(I)} cation-con-
taining compound, or [{Au(PPh3)}4(μ4-O)]3[α-PW12O40]2·4EtOH (4)
[15], was formed; in contrast, when the Keggin POM was dissolved in
CH3CN/water, the monomeric phosphanegold(I) cation-containing
Abbreviations: IPr, 1,3-Bis(2,6-diisopropylphenylimidazol-2-ylidene; IMes, 1,3-Bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; BIPr, 1,3-Bis(2,6-diisopropylphenyl)
benzimidazol-2-ylidene; IF3, 1,3-Bis(2,4,6-trifluorophenyl)imidazol-2-ylidene; ItBu, 1,3-Di-tert-butylimidazol-2-ylidene; acenaphthaneIPr, 7,9-Bis[2,6-bis(1-methy-
lethyl)phenyl]-6b,7,9,9a-tetrahydro- (6bR,9aS)-rel-8H- acenaphth[1,2-d]imidazol-8-ylidene; R2-imy, 1-Diphenylmethyl-3-methylimidazolin-2-ylidene
⁎ Corresponding authors.
E-mail addresses: nomiyk01@jindai.jp (K. Nomiya), matsubara@kanagawa-u.ac.jp (T. Matsubara).
https://doi.org/10.1016/j.mcat.2019.02.014
Received 12 December 2018; Received in revised form 30 January 2019; Accepted 15 February 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:KenjiNomiya,etal.,MolecularCatalysis,https://doi.org/10.1016/j.mcat.2019.02.014

K. Nomiya, et al.
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compound, or [Au(CH3CN)(PPh3)]3[α-PMo12O40
]
(5) [21], was
quantum mechanical (QM) calculations showed that the electronic and
steric effects of the ligand IPr are both significant for the catalytic ac-
tivity and stability of the complex (1).
We propose that the activity of the catalyst precursor [Au(IPr)
(H2O)]+ and the catalytically active species [Au(IPr)(alkyne)]+ for
alkyne hydration in the system of (1) and H-PW12 can be understood in
terms of formation of ion-pair species with the POM.
formed. Also, while studying the chemistry of (3) in the presence of
POM, we very recently observed the formation of the unusual μ5-N
ammonium(2+) cationic species, {[Au(PPh3)]5(μ5-N)}2+, by reaction
of (3) in MeOH/CH2Cl2 containing aqueous NH3 with H3[α-
PW12O40]·13H2O in MeOH [22]. Formation of such a species can be
attributed to successive proton substitution of the coordinated NH3 in
the initially formed [Au(NH3)(PPh3)] by [Au(PPh3)]+ generated
through elimination of RS-pyrrld ligand in the presence of protonic
acids of POMs. Similar formation of several μ4-O and/or μ3-O phos-
phanegold(I) cluster cations by successive substitution of protons of
coordinated water molecule in the initially formed [Au(PPh3)(H2O)]+
by [Au(PPh3)]+ in the presence of POMs has been reported [14–20].
Several phosphanegold(I) complexes have been reported to be ef-
fective as catalysts for organic synthesis in homogeneous systems by
Schmidbaur’s group [23], Hashmi’s group [24–26], Toste’s group
[27–29], and so on. One of the most straightforward ways of synthe-
sizing compounds with carbon–oxygen bonds is the hydration of al-
kynes [30–32].
2. Experimental
2.1. Materials
The following reagents were used as received: H[AuCl4]·4H2O, di-
phenylacetylene, deoxybenzoin, phenylacetylene, acetophenone,
acetone, EtOH, hexane, MeOH, toluene, Et2O, CHCl3, CH2Cl2, 1,4-di-
oxane, tetrahydrofuran, tetrahydrothiophene, 3-chloropyridine, paraf-
ormaldehyde, formic acid, triethylorthoformate, 40% glyoxal aqueous
solution, 60% HPF6 aqueous solution, 98% CF3SO3H aqueous solution,
42% HBF4 aqueous solution, Na2SO4, Na2CO3, NaHCO3, K2CO3, NaPF6,
CF3SO3Na, NaBF4 (Wako), 1-phenyl-1-butyne, butyrophenone, 1-
phenyl-2-butanone, 4 M HCl in dioxane, DL-pyroglutamic acid, 2,4,6-
trimethylaniline, tert-butylamine, 2,4,6-trifluoroaniline, 2,6-diisopro-
pylaniline, Pd(dba)2, tri-tert-butylphosphonium tetrafluoroborate, tert-
BuONa, 1,2-dibromobenzene, trimethylsilyl chloride (TCI), and D2O,
CD2Cl2, CDCl3 and DMSO-d6 (Isotec). Polymeric silver(I) carboxylate,
Regarding POM-based phosphanegold(I) catalysts, Blanc’s group
showed that proton-containing hybrid complexes [Au(CH3CN)
(PPh3)]xH4-x[SiW12O40
] (x = 1–4) exhibited good activity and se-
lectivity as heterogeneous catalysts for the conversion of enyne acetate
to cyclopentenone and the rearrangement of propargylic gem-diester to
(E)-3-oxycarbonyl enone derivatives [33]. In addition, the related
proton-containing hybrid complexes [Au(CH3CN)(PR3)][POM-H]
∞{[Ag(RS-pyrrld)]2} (RS-Hpyrrld
= RS-2-pyrrolidone-5-carboxylic
(R = Ph, Me; POM-H
= H3SiW12O40, H2PM12O40 (M = Mo, W),
acid), was prepared according to the literature [52] and characterized
by CHN elemental analysis, TG/DTA, IR, 1H and 13C{1H} NMR in D2O.
[AuCl(THT)] was prepared by reaction of H[AuCl4]·4H2O and tetra-
hydrothiophene (THT) according to the literature [53,54]. NHC ligands
such as IPr, IMes, BIPr, IF3, and ItBu (see Abbreviations of NHC ligands
and Scheme 1) were introduced into the [AuCl(NHC)] complexes by
reaction of H[AuCl4] and the NHC precursors in the form of imidazo-
lium chlorides (Data in Brief). The precursor imidazolium chlorides,
H5P2W18O62 also exhibited bifunctional catalytic properties for
)
tandem aza-Prins cyclization/enol ether hydrolysis [34]. Our group has
reported diphenylacetylene hydration catalyzed by acid-free inter-
cluster compounds of phosphanegold(I) cluster species with Keggin
POMs, i.e., (4) and [{{Au(PPh3)}4(μ4-O)} {{Au(PPh3)}3(μ3-O)}][α-
PW12O40]·EtOH, as well as (5), under the following conditions: catalyst
loading (0.67 mol%), diphenylacetylene (1.5 mmol), 1,4-dioxane/water
(4/1 v/v) mixed solvent, and reaction temperature 80 °C [21]. These
complexes efficiently catalyzed diphenylacetylene hydration in het-
erogeneous systems. The presence of POMs was indispensable, but their
role was not clear.
On the other hand, Nolan’s group have studied the hydration of
terminal and internal alkynes using NHC-gold(I) complexes (NHC = N-
heterocyclic carbene) [35–45]. Corma et al. recently observed a clear
induction time in the gold(I)–carbene-catalyzed hydration of dipheny-
lacetylene, and they suggested that the gold(I)-carbene decomposes
under these reaction conditions, forming catalytically active 3–5 atom
gold–alkyne clusters after an induction period [46,47]. Carbene ligands
and bulky ligands have provided highly active gold catalysts [48,49].
In the metal complexes, the degree of back-donation, i.e, the σ-
donor and π-acceptor properties of the NHC ligand, is dependent upon
the metal [50,51] and is comparable with that of phosphane ligand.
Most NHC-Au(I) complexes are water-soluble, and their catalytic reac-
tions can be examined in homogeneous systems.
In this work, we prepared several [Au(RS-pyrrld)(NHC) complexes
(NHC = IPr (1), IMes (6), BIPr (7), IF3 (8), ItBu (9) in order to compare
their chemistry with that of (3) in the presence of POMs. The catalytic
activities and stability of (1) in the presence of H3[α-PW12O40]·9H2O
(H-PW12) were remarkably superior to those of the system of (3) + H-
PW12. The contributions of the IPr and PPh3 ligands, the effect of
substituents of the NHC ligands, and the effect of addition of a strong
protonic acid HX, such as HBF4, HPF6 and CF3SO3H, and H-PW12 were
clarified by running the catalytic reactions under various conditions.
Separately, we obtained the crystalline complex [Au(H2O)(IPr)]3[α-
PW12O40]·2Et2O (2) by reaction of (1) in EtOH with H-PW12 in water,
and found that its activity for homogeneous catalysis of diphenylace-
tylene hydration was almost the same as that of the mixed system of (1)
and H-PW12. The molecular structure of (2) with seven Et2O molecules
of solvation was determined by X-ray crystallography. Furthermore,
Scheme 1. NHC ligands.
2

K. Nomiya, et al.
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i.e., HIPr+Cl− [55], HIMes+Cl- [55,56], HBIPr+Cl− [57], HIF3+Cl-
[58,59], and HItBu+Cl− [60,61] were prepared according to the cited
references, and characterized by CHN elemental analysis, TG/DTA, IR,
1H and 13C{1H} NMR. The carboxylatogold(I) complex with NHC li-
gand, [Au(RS-pyrrld)(IPr)] (1) was prepared by reaction of [AuCl(IPr)]
with ∞{[Ag(RS-pyrrld)]2} as described below. For other [Au(RS-pyrrld)
(NHC)] complexes (NHC = IMes (6), BIPr (7), IF3 (8), ItBu (9)), see
Data in Brief.
2.2. Instrumentation/analytical procedures
CHN elemental analyses were carried out using a Perkin-Elmer 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.
The 1H NMR (400 MHz), 31P{1H} NMR (161 MHz) and 13C{1H}
NMR (99 MHz) spectra of the samples were recorded in 5-mm-outer-
diameter tubes on a JEOL 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 processing system, respectively. The 1H and 13C{1H} NMR spectra
were referenced to an internal standard, tetramethylsilane (SiMe4). The
31P{1H} NMR spectra were referenced to an external standard, 25%
H3PO4 in H2O in a sealed capillary. The 31P{1H} NMR data with the
usual 85% H3PO4 reference were shifted + 0.544 ppm from these data.
The high-performance liquid chromatography (HPLC) apparatus
and conditions are as follows: Shimadzu LC-20AD with Shimadzu SPD-
20 A detector (wavelength 260 nm), column VP-ODS (150 mm x 4.6
mm), flow rate 0.7 mL per min, and solvent MeOH: water (30 : 17).
The complex (1), in combination with various POMs, was used as a
hydration catalyst for alkynes such as diphenylacetylene, phenylace-
tylene, and 1-phenyl-1-butyne. Keggin POMs, such as H3[α-
PW12O40]·9H2O (H-PW12), Na3[α-PW12O40]·10H2O (Na-PW12), H4[α-
SiW12O40]·10H2O (H-SiW12), H5[α-AlW12O40]·11H2O (H-AlW12), and
Wells-Dawson POM, H6[P2W18O62]·13H2O (H-P2W18), were prepared
according to the procedures reported elsewhere [18,62–65], and iden-
tified by FTIR, TG/DTA, and solution (27Al, 31P{1H}) NMR spectro-
scopy. The complexes [Au(RS-pyrrld)(PPh3)] (3), [{Au(PPh3)}4(μ4-
O)]3[α-PW12O40]2·4EtOH (4) and [Au(CH3CN)(PPh3)]3[α-PMo12O40
]
(5) were synthesized according to the literature procedures [13–16]
and identified by CHN elemental analysis, FTIR, TG/DTA, and solution
(1H, 13C{1H}, 31P{1H}) NMR spectroscopy. Structures of all of gold(I)
compounds (1)-(9), with and without POM anions, were shown in
Scheme 2.
2.3. Preparation
2.3.1. [Au(RS-pyrrld)(IPr)] (1)
In the dark, solid ∞{[Ag(RS-pyrrld)]2} (0.533 g, 1.13 mmol) was
Scheme 2. Structures of gold(I) compounds (1)-(9) in the manuscript.
3

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2.3.2. [Au(H2O)(IPr)]3[α-PW12O40]·2Et2O (2)
A clear, colorless solution of H-PW12 (0.456 g, 0.15 mmol) in 15 mL
of water was added slowly to a clear, pale yellow solution of (1)
(0.321 g, 0.45 mmol) in 35 mL of EtOH, and the mixture was stirred for
1 h. The resulting white powder was collected on a membrane filter (JV
0.1 μm), washed with water, EtOH and Et2O (20 mL each), thoroughly
dried by suction, and further dried in vacuo for 2 h.
Crystallization. The obtained powder (0.1496 g) was dissolved in
10 mL of CH2Cl2, and crystallized by a vapor diffusion method using the
clear, colorless CH2Cl2 solution and Et2O as an external solvent.
Colorless block crystals of compound (2) formed were collected on a
membrane filter (JV 0.1 μm), washed with Et2O (20 mL), dried thor-
oughly by suction and further dried in vacuo for 2 h. Yield 0.114 g
(76.1%). The crystalline samples were soluble in DMSO, CH3CN,
acetone, CH2Cl2, but insoluble in water, EtOH, hexane and Et2O.
Anal. Calcd for
C89H134N6O45Au3PW12 or [Au(H2O)(IPr)]3[α-
PW12O40]·2Et2O: C, 22.10; H, 2.79; N, 1.71. Found: C, 22.33; H, 2.49;
N, 1.85%. TG/DTA under atmospheric conditions: a weight loss of
4.70% due to desorption of three water molecules and two Et2O mo-
lecules at below 261.0 °C was observed with an endothermic peak at
221.6 °C; calcd 1.15% for three coordinated water molecules plus
3.10% for two Et2O molecules. Further, a weight loss of 23.19% due to
decomposition was observed at below 500.0 °C with an exothermic
peak at 500.0 °C. IR (KBr, cm−1): 1553 (vw), 1469 (w), 1459 (w), 1424
(w), 1386 (w), 1363 (w), 1330 (vw), 1274 (vw), 1256 (vw), 1216 (vw),
1182 (vw), 1079 (s), 978 (s), 896 (s), 820 (vs), 757 (m), 705 (w), 639
(w), 595 (w), 549 (vw), 521 (w), 511 (w), 452 (vw). 1H NMR (21.7 °C,
CD2Cl2): δH 1.09 (t, J 7.0 Hz, (CH3CH2)2O), 1.24 (d, J 6.8 Hz, H6), 1.37
(d, J 6.8 Hz, H6), 2.49 (sept, J 6.8 Hz, H5), 3.48 (q, J 7.0 Hz,
(CH3CH2)2O), 7.35 (s, H2), 7.38 (d, J 7.8 Hz, H7), 7.61 (t, J 7.8 Hz, H8).
13C{1H} NMR (22.0 °C, CD2Cl2): δC 15.40 ((CH3CH2)2O), 24.34 (C6),
24.64 (C6), 29.33 (C5), 124.79 (C2), 124.92 (C8), 131.72 (C7), 133.78
(C4), 146.05 (C3), 160.64 (C1). 31P{1H} NMR (23.1 °C, CD2Cl2): δP
-14.96 ([α-PW12O40]3-).
Fig. 1. Molecular structure of [Au(H2O)(IPr)]3[α-PW12O40]·7Et2O (2) by ball-
and-stick representation.
added to a solution of [AuCl(IPr)] (0.466 g, 0.750 mmol) in 40 mL
CHCl3, and the mixture was stirred overnight. The yellow-white sus-
pension containing AgCl and unreacted ∞{[Ag(RS-pyrrld)]2} was fil-
tered through a membrane filter (JV 0.1 μm), and the pale yellow fil-
trate was evaporated to dryness at ca 30 °C. The resulting clear yellow
oil was dissolved in 5 mL of acetone, and the solution was filtered
through a folded filter paper (Whatman #5). The filtrate was added to
400 mL of hexane. The resulting pale yellow powder was collected on a
membrane filter (JV 0.1μm), washed with hexane (30 mL x 2), dried
thoroughly by suction, and then dried in vacuo for 2 h. Yield 0.289 g
(51.2%). The pale yellow powder was soluble in acetonitrile, acetone,
EtOH, CHCl3, CH2Cl2, DMSO, but insoluble in water and hexane.
Anal. Calcd for C32H41N3O3Au or [Au(RS-pyrrld)(IPr)]: C, 53.93; H,
5.80; N, 5.90. Found: C, 53.54; H, 6.13; N, 5.75%. TG/DTA under at-
mospheric conditions: a weight loss of 10.04% due to decomposition at
below 300.0 °C was observed with an exothermic peak at 226.7 °C. IR
(KBr, cm−1): 1697 (vs), 1654 (vs), 1471 (s), 1419 (m), 1386 (m), 1364
(m), 1331 (m), 1257 (m), 1181 (w), 1119 (vw), 1060 (w), 936 (vw),
804 (w), 759 (m), 706 (w), 453 (vw). 1H NMR (22.6 °C, CDCl3): δH 1.23
(d, J 6.8 Hz, H6), 1.36 (d, J 6.8 Hz, H6), 1.99–2.17 (m, CH2 pyrrld),
2.55 (sept, J 6.8 Hz, H5), 3.91–3.95 (m, CH pyrrld), 5.76 (s, NH pyrrld),
7.22 (s, H2), 7.31 (d, J 8.0 Hz, H7), 7.52 (t, J 8.0 Hz, H8). 13C{1H} NMR
(22.0 °C, CDCl3): δC 24.07 (s, C6), 24.40 (s, C6), 25.26 (s, CH2CH
pyrrld), 28.87 (s, C5), 30.25 (s, CH2CO pyrrld), 57.79 (s, CH pyrrld),
123.40 (s, C2), 124.30 (s, C8), 130.78 (s, C7), 133.92 (s, C4), 145.62 (s,
C3), 167.36 (s, C1), 175.58 (s, COO pyrrld), 177.64 (s, CO).
Crystallization. The powder sample (0.200 g) was dissolved in 3 mL
of CH2Cl2, and hexane was added dropwise until the solution formed a
suspension. Then, 5 drops of CH2Cl2 were added, and the resulting
solution was slowly evaporated in the refrigerator. After 7 days, pale
yellow plate crystals deposited. The crystals were collected on a
membrane filter (JG 0.2 μm), washed with hexane (20 mL x 2), dried
thoroughly by suction and further dried in vacuo for 2 h. The resulting
pale yellow-white powder was characterized by CHN elemental ana-
lysis, TG/DTA, 1H and 13C{1H} NMR in CDCl3. Yield 0.14 g (62.6%).
The quality of the single crystals obtained here was not suitable for X-
ray crystallography.
The molecular structure with seven Et2O molecules of solvation was
determined (Fig. 1) by X-ray crystallography (CCDC no. 1864226).
Crystal data and selected bond distances (Å) and angles (deg) are pre-
sented in Tables 1 and 2 in Data in Brief.
2.4. Catalytic reaction
The catalytic reaction of diphenylacetylene hydration was per-
formed at 80 °C, to provide comparability with the previously reported
heterogeneous catalytic systems of (3), (3) + H-PW12, (3) + Na-
PW12, (4) and (5). The catalytic activities of the mixed-component
catalysts such as (1) + H-PW12 etc. and of the single-component
catalysts such as (1) etc. (Tables 1–5) were discussed by use of the yield
of product, but not the turnover numbers (TON) and the turnover fre-
quencies (TOF) [21].
The NHC gold(I) complex (1) (7.1 mg, 0.01 mmol) and H-PW12
(0.03 g, 0.01 mmol) were dissolved in 6.0 mL of a mixed solvent of 1,4-
dioxane and water (4:1), followed by addition of diphenylacetylene
(0.267 g, 1.5 mmol). The solution was flushed with N2 gas for 5 min,
and then stirred in an oil bath at approximately 80 °C for 0.5, 1, 1.5, 2,
2.5, or 3 h. The homogeneous, pale yellow clear solution (10 μL) and
toluene (5 μL, 0.047 mmol) as an internal standard were transferred to a
10 mL volumetric flask and the solution was diluted to 10 mL with
MeOH. HPLC analysis of a 5 μL aliquot of the homogeneous MeOH
solution showed that deoxybenzoin was produced exclusively as the
hydration product. As for other alkynes, hydration of phenylacetylene
to afford acetophenone and 1-phenyl-1-butyne to afford butyrophenone
and 1-phenyl-2-butanone was also examined (Table 5).
In the hydration of diphenylacetylene, H-PW12 was exchanged with
other POMs such as H-SiW12, H-AlW12, H-P2W18 and Na-PW12
(Table 1).
Also, in the hydration of diphenylacetylene, (1) was replaced with
4

K. Nomiya, et al.
MolecularCatalysisxxx(xxxx)xxx–xxx
Table 1
Results of diphenylacetylene hydration catalyzed by NHC- or phosphane-gold(I) complexes in the absence and presence of POMsa)
.
Entry
Catalysts (0.67 mol%) and additives (0.67 mol%)
Conversion (= Yield) (%)
1 h
2 h
4 h
6 h
24 h
1-1
[Au(RS-pyrrld)(IPr)] (1)
0
5.1
10.1
15.3
27.3
1-2
1-3
1-4
1-5
1-6
1-7
(1) + H3[α-PW12O40]･9H2O (H-PW12)
(1) + H4[α-SiW12O40]･10H2O (H-SiW12)
(1) + H5[α-AlW12O40]･11H2O (H-AlW12)
(1) + Na3[α-PW12O40]･10H2O (Na-PW12)
(1) + H6[α-P2W18O62]･13H2O (H-P2W18)
[Au(IPr)(H2O)]3[α-PW12O40]･2Et2O (2)
[Au(IPr)(H2O)]3[α-PW12O40]･2Et2O (2)
[Au(RS-pyrrld)(PPh3)] (3)
(3) + H-PW12
(3) + Na-PW12
[{Au(PPh3)}4(μ4-O)]3[α-PW12O40]2･4EtOH (4)
[Au(CH3CN)(PPh3)]3[α-PMo12O40] (5)
79.0
79.2
69.2
4.5
76.3
98.6
71.6
95.1
93.7
90.9
21.0
93.9
99.4
95.4
–
–
–
38.7
–
–
–
52.9
–
–
–
96.2
–
–
–
1-8b
1-9c
1-10c
1-11c
1-12c
1-13c
0.7
71.5
1.4
36.1
71.8
1.1
84.9
2.1
55.2
84.9
0.5
97.6
4.9
93.7
91.6
–
–
a) Reaction conditions : catalysts 0.01 mmol (catalyst loading 0.67 mol%), additives 0.01 mmol (loading 0.67 mol%), diphenylacetylene 1.5 mmol, N2 1 atm, 6 mL of
4 : 1 mixed solvent of 1,4-dioxane and water, temperature 80 °C. Entries 1-1 ∼ 1–8 are homogenous systems, but entries 1–9 ∼ 1–13 are heterogeneous systems. b)
catalyst loading 0.22 mol%. c) Ref. [21].
Table 2
Table 4
Effects of re-addition of diphenylacetylene after diphenylacetylene was con-
Effects of various protonic acids and their sodium salts on diphenylacetylene
hydration catalyzed by NHC-gold(I) complex (1) a).
sumed in the first reaction a).
Entry Catalysts (0.67 mol%) and additives Conversion (= Yield) (%)
(0.67 mol%)
Entry Catalysts (0.67 mol%) and additives Conversion (= Yield) (%)
(0.67 mol%)
1 h
2 h
4 h
6 h
24 h
1 h
2 h
4 h
6 h
24 h
2-1
2-2
2-3
2-4
2-5
2-6b
(1) + H-PW12
(1) + H-SiW12
(1) + H-AlW12
(1) + H-P2W18
(2)
63.6
74.8
78.1
67.1
98.5
–
86.6
95.7
92.2
88.7
99.1
–
–
–
–
–
–
–
–
–
–
–
–
–
1-1
1-2
1-5
4-1
4-2
4-3
4-4
4-5
4-6
(1)
0
5.1
10.1 15.3
27.3
–
96.2
> 99
–
(1) + H-PW12
(1) + Na-PW12
(1) + HBF4 aq
(1) + HPF6 aq
(1) + CF3SO3H
(1) +NaBF4
79.0 95.1
4.5 21.0 38.7 52.9
18.3 33.4 54.6 70.2
62.2 79.3 95.9 > 99
64.5 88.9 99.0 > 99
–
5.2
4.0
–
–
(4)
0
0
0
–
8.4
7.1
8.5
18.1 26.8
11.8 26.7
18.3 28.5
41.0
56.1
54.4
(1) + NaPF6
(1) + CF3SO3Na
a) Reaction conditions : catalysts 0.01 mmol (catalyst loading 0.67 mol%), ad-
ditives 0.01 mmol (loading 0.67 mol%), diphenylacetylene 1.5 mmol, N2 1 atm,
6 mL of 4 : 1 mixed solvent of 1,4-dioxane and water, temperature 80 °C, re-
action time after re-addition of the substrate. Entries 2-1 ∼ 2–5 are homo-
geneous systems, but entry 2–6 is a heterogeneous system. b) Ref. [21].
a) Reaction conditions : catalysts 0.01 mmol (catalyst loading 0.67 mol%), ad-
ditives 0.01 mmol (loading 0.67 mol%), diphenylacetylene 1.5 mmol, N2 1 atm,
6 mL of 4 : 1 mixed solvent of 1,4-dioxane and water, temperature 80 °C.
Table 3
other carboxylatogold(I) complexes such as (6)-(9) (Table 3). In addi-
tion, the mixed catalyst system, i.e., (1) + H-PW12, was replaced with
the proton-free single-component catalyst (2) (Table 1).
Results of diphenylacetylene hydration catalyzed by various NHC-gold(I)
complexes in the absence and presence of H-PW12 a).
Entry Catalysts (0.67 mol%) and additives Conversion (= Yield) (%)
(0.67 mol%)
1 h
2 h
4 h
6 h
24 h
2.5. Computational method
1-1
1-2
3-1
3-2
3-3
[Au(RS-pyrrld)(IPr)] (1)
(1) + H-PW12
[Au(RS-pyrrld)(IMes)] (6)
(6) + H-PW12
0
79.0
0
5.3
75.9
5.1
95.1
0
5.8
94.4
10.1
–
0
6.1
–
15.3
–
0
8.3
–
27.3
–
4.4
8.9
–
Quantum mechanical (QM) calculations were performed for
[LAu]+, [L2Au]+ and [LAu(Sub)]+ (L = IPr, PPh3; Sub = H2O, C2Ph2)
with the hybrid density functional method at the M06-2X [66] level
using the Gaussian 09 program [67]. The basis set we used, which is
called basis set I (BSI) in this paper, is an effective core potential re-
placing core electrons up to 4f and double-ζ valence basis functions
(7s6p3d)/[3s3p2d] according to Hay and Wadt [68] augmented by the
single set of f orbitals with an exponent of 1.050 [69] for the Au atom
and 6-31+G* for the other atoms, H, C, N, O and P. All of the structures
were confirmed to be equilibrium structures by means of frequency
calculations, although [(PPh3)Au(C2Ph2)]+ still showed one imaginary
frequency of 5.6i cm−1 for PPh3 rotation. The atomic charge and the
atomic orbital population were obtained by natural bond orbital (NBO)
analysis [70]. The basis set superposition error (BSSE) included in the
[Au(RS-pyrrld)(BIPr)] (7) + H-
PW12
3-4
3-5
[Au(RS-pyrrld)(IF3)] (8) + H-
PW12
3.5
3.7
4.1
4.3
9.3
[Au(RS-pyrrld)(ItBu)] (9) + H-
PW12
34.8
44.4
60.2
68.9
73.4
a) Reaction conditions : catalysts 0.01 mmol (catalyst loading 0.67 mol%), ad-
ditives 0.01 mmol (loading 0.67 mol%), diphenylacetylene 1.5 mmol, N2 1 atm,
6 mL of 4 : 1 mixed solvent of 1,4-dioxane and water, temperature 80 °C.
5

K. Nomiya, et al.
MolecularCatalysisxxx(xxxx)xxx–xxx
Table 5
Hydration of phenylacetylene and 1-phenyl-1-butyne catalyzed by NHC- or phosphane-gold(I) complexes a).
Entry
5-1
Catalysts
Alkynes
Products
Time (h)
Yield (%)
(1) + H-PW12
0.5
1
39.6
75.0
phenylacetylene
2
> 99.9
acetophenone
acetophenone
5-2
5-3b
5-4
(2)
(4)
(2)
phenylacetylene
phenylacetylene
0.5
1
2
4
6
39.4
76,6
89.5
32.4
52.8
94.3
18.1
acetophenone
24
0.5
1-phenyl-1-butyne
butyrophenone
0.5
81.9
1- phenyl-2-butanone
butyrophenone
1-phenyl-2-butanone
5-5b
(4)
1-phenyl-1-butyne
2
2
38.8
57.0
a) Reaction conditions: catalysts 0.01 mmol (catalyst loading 0.67 mol%), additive 0.01 mmol (loading 0.67 mol%), substrates 1.50 mmol, 6 mL of 4 : 1 mixed solvent
of 1,4-dioxane and water, temperature 80 °C. b) Ref. [21].
interaction energies (INT) between [LAu]+ (L = IPr, PPh3) and sub-
strates, H2O and C2Ph2, was corrected by the counterpoise method
[71,72]. In order to analyze the components of the interaction energies,
we performed energy decomposition analysis (EDA) [73] for [Au
(Sub)]+ (Sub = H2O, C2H2) at the HF level with a reduced basis set of
double-ζ valence basis functions with polarization functions for all the
atoms using the GAMESS program [74].
2 [AuCl(THT)] + 2 HBIPr+Cl− + K2CO3 → 2 [AuCl(BIPr)] + 2 KCl
+2 THT + 2 H+ + CO3
(3)
2-
The carboxylatogold(I) complexes with NHC ligands, (1) and (6)-
(9), were prepared by reaction of [AuCl(NHC)] with [Ag(RS-pyrrld)]2
(see the text for (1) and Data in Brief for (6)-(9)).
These reactions can be written as follows.
2 [AuCl(NHC)] + [Ag(RS-pyrrld)]2 → 2 [Au(RS-pyrrld)(NHC)] + 2
AgCl
(4)
3. Results and discussion
On the other hand, the novel compound (2) was synthesized by the
reaction of (1) in EtOH and H-PW12 in water.
Complexes (1), (2) and (6)-(9) are new, whereas complexes of (3)-
(5) have been reported previously [21]. Synthesis of (1) and (2) was
described in Experimental of the present text, but that of (6)-(9) was
put into Data in Brief, because synthesis of (6)-(9) is similar to that of
(1).
[Au(RS-pyrrld)(IPr)] (1) + H3[α-PW12O40] + 3 H2O → [Au(H2O)
(IPr)]3[α-PW12O40] (2) + 3 RS-Hpyrrld
(5)
These compounds were obtained in reasonable yields (51.2% for
(1), 76.1% for (2), 50.4% for (6), 60.5% for (7), 46.2% for (8) and
71.5% for (9)), and were characterized by elemental analysis, IR, TG/
DTA, and (1H and 13C{1H}) NMR, in addition to X-ray crystallography
and 31P{1H} NMR for (2).
3.1. Synthesis and characterization of [Au(RS-pyrrld)(NHC)] (NHC = IPr
(1), IMes (6), BIPr (7), IF3 (8), ItBu(9)) and [Au(H2O)(IPr)]3[α-
PW12O40]·2Et2O (2)
The compositions and formulae of (1) with no molecules of solva-
tion and (2) with two molecules of Et2O of solvation, which were used
for the catalytic reactions, were determined based on elemental ana-
lyses and TG/DTA using thoroughly dried samples. On the other hand,
the single crystals used for X-ray crystallography were not dried. In fact,
the sample of (2) used for X-ray crystallography contained seven Et2O
molecules of solvation. Thus, the solvation was not necessarily con-
sistent between the samples used for X-ray analysis and for other ana-
lyses.
As precursor complexes, the [AuCl(NHC)] complexes (NHC = IPr,
IMes, BIPr, IF3 and ItBu) were prepared from H[AuCl4]·4H2O, NHC li-
gand precursors (HIPr+Cl−, HIMes+Cl−) and Na2CO3, or from [AuCl
(THT)], NHC precursors (HBIPr+Cl−, HIF3 +Cl−, HItBu+Cl−) and
K2CO3, according to the cited references (see Data in Brief). They were
obtained in reasonable yields (24.3, 28.5, 70.5, 34.7, and 50.0%, re-
spectively), and were characterized by CHN elemental analysis, TG/
DTA, IR, 1H and 13C{1H} NMR in D2O.
Formation of [AuCl(IPr)] can be written as follows.
In the IR spectrum (KBr disk) of [Au(RS-pyrrld)(NHC)], very intense
C–O stretching bands of coordinated RS-pyrrld− ligand were observed
at 1697 and 1654 cm-1 for IPr, 1686 and 1652 cm-1 for IMes, 1705 and
1636 cm-1 for BIPr, 1694 and 1648 cm-1 for IF3, and 1697 and 1653 cm-
Na[Au Cl4] + THT + 2 (3-chloropyridine) + H2O → Na[AuICl2] +
Ⅲ
THT(=O) + 2 (3-chloropyridinium chloride)
(1)
Na[AuCl2] Na2CO3
+
HIPr+Cl−
+
→
[AuCl(IPr)]
+
2
for ItBu.
1
In the 13C{1H} NMR in CDCl3 of [Au(RS-pyrrld)(NHC)], carbene
carbon of NHC and carboxylato carbon and carbonyl carbon of RS-
pyrrld− were observed at 167.36, 175.58 and 177.64 ppm for (1),
165.38, 175.99 and 177.78 ppm for (6), 174.47, 175.53 and
177.54 ppm for (7), 171.38, 176.31 and 177.70 ppm for (8), and
159.66, 176.68 and 177.77 ppm for (9), respectively, while in the case
NaCl + NaHCO3
(2)
The chloridogold(I) complexes with other NHC ligands (BIPr, IF3,
ItBu)
were also generated from [AuCl(THT)]
(THT = tetrahydrothiophene), NHC precursors (HBIPr+Cl− HIF3
+Cl−, HItBu+Cl−) and K2CO3 according to the following scheme.
,
6

K. Nomiya, et al.
MolecularCatalysisxxx(xxxx)xxx–xxx
of [AuCl(NHC)], carbene carbon of NHC was found at 175.26 ppm for
IPr, 173.27 ppm for IMes, 181.68 ppm for BIPr, 178.61 ppm for IF3, and
168.03 ppm for ItBu.
of equimolar amounts of (1) and H-PW12, the amount of the catalyst
precursor, i.e., [Au(IPr)(H2O)]+, formed in the solution will be dif-
ferent from that in the reaction catalyzed by (2) containing three [Au
(IPr)(H2O)]+ cations. Indeed, when the catalyst loading of (2) was
lowered to 1/3 (0.22 mol%, Entry 1–8), the conversion was almost the
same as in Entry 1–2.
(3) The reactions of gold(I) complex (1) with IPr ligand (Entries 1-1
∼ 1–7) can be compared with those of (3), (Entries 1–9 ∼ 1–11) and its
derivatives (Entries 1–12, 1–13). Differences in the catalytic activities
of (1) and (3) without POMs (Entries 1-1 and 1–9) reflect the ease of
substitution of carboxylate. If the catalyst precursor is a species such as
[Au(L)(H2O)]+ (A, L = IPr, PPh3), the results can be attributed to the
weaker σ-donor and weaker π-acceptor properties of IPr, compared
The molecular structure of (2) was determined by single-crystal X-
ray structure analysis (Fig. 1). The complex (2) crystallized in the or-
thorhombic system with the P212121 (No. 19) space group (Data in
Brief, Table 1). In the unit cell, three [Au(H2O)(IPr)]+ cations, one [α-
3−
PW12O40
]
POM and seven Et2O molecules were assigned. The Au-C
bond distances (1.948(7), 1.936(5), 1.955(6) Å), the Au-O bond dis-
tances (2.068(5), 2.062(5), 2.071(5) Å), and the C-Au-O angles
(177.5(3), 179.2(3) and 177.5(3)o are included in Table 2 in Data in
Brief. These data show that the three [Au(H2O)(IPr)]+ are linear 2-
coordinated species. These data can be compared with those for the
related Au-NHC complexes [38,75–79]. The Au-C distances are slightly
shorter than those of the linear Au-NHC carboxylate complexes,
1.977(15), 1.978(5), 2.006(17), 1.949(4), 1.961(5) and 1.982(3) Å for
[Au(IPr)(OAc)] [38], [Au(IPr)(OPiv)] [75], [Au(IPr)(1,3-dimethox-
ybenzoate)] [76], [Au(acenaphtheneIPr)(OAc)] [77], [Au(R2-imy)
(OAc)] [78], and [Au(ItBu)(OAc)] [79], respectively (Scheme 1) as well
as that of the Au-hydroxide NHC complex, [Au(IPr)(OH)] (Au1–O1
2.078(6) Å, Au1–C1 1.935(6) Å, C1–Au1–O1 177.1(3) o) [38]. On the
other hand, the Au-O distances in Au-NHC complexes including com-
plex (2) showed little variation [38].
with PPh3, as supported by QM calculations (see later section).
−
On the other hand, in the presence of a stronger acid HX (X = BF4
,
PF6−, CF3SO3−, Entries 4-1∼4-3, Table 4), the reaction is greatly ac-
celerated and the generated complex [Au(L)(H2O)]+ (A) can serve as
the actual catalyst precursor for alkyne hydration (eq. 6).
[Au(RS-pyrrld)(L)] (1) + H2O + HX ⇄ [Au(L)(H2O)]+ (A) + X−
RS-Hpyrrld
+
(6)
(4) This tendency of the gold(I) complexes with L = IPr and PPh3 is
also observed in the reactions in the presence of H-PW12 (Entries 1–2
vs 1–10) and Na-PW12 (Entries 1–5 vs 1–11). In fact, the complex (1)
with L = IPr in the presence of H-PW12 (conversion of 95.1% after 2 h
(Entry 1–2)) showed much higher activity than the complex (3) with
L = PPh3 (97.6% after 24 h (Entry 1–10)), and the complex (1) in the
presence of Na-PW12 (96.2% after 24 h (Entry 1–5) showed much
higher activity than the complex (3) (4.9% after 24 h (Entry 1–11)).
(5) The catalytic behavior of complexes (2) and (5) is significantly
influenced by (i) the ligand (IPr vs PPh3) and (ii) the reaction system
(homogeneous vs heterogeneous), but it appears to be less affected by
The minimum distances between the oxygen atoms of the Keggin
moieties and the gold atoms of the cationic species are 5.478
(Au3i…O14 i = 1-x, -0.5+y, 0.5-z), 5.645 (Au2ii…O36 ii = -0.5+x, 0.5-
y, 1-z) and 6.118 (Au1…O24) Å, indicating that there is no van der
Waals interaction between [Au(H2O)(IPr)]+ and [α-PW12O40 3−, as is
]
also the case in two very recently reported Au-NHC complexes with
POMs [80], i.e., [Au(IPr)(MeCN)]2[H][PMo12O40] (Au-C and Au-N
bond distances 1.957(10) and 2.034(10) Å; C-Au-N angle 175.4(4)°)
and [Au(IPr)(MeCN)]2[H][PW12O40]·8MeCN (Au-C and Au-N bond
distances 1.981(15) and 2.042(13) Å; C-Au-N angle 179.3(5)°). On the
other hand, intermolecular hydrogen bonds are observed between
water molecules of [Au(H2O)(IPr)]+ and the six ether molecules of
solvation as shown in Fig. 1 in dotted lines, and Table 2 in Data in Brief.
the auxiliary, eliminating ligands (water vs CH3CN) and the poly-
3−
oxoanions [α-PM12O40
]
(M = W, Mo).
3.3. Stability of the catalyst systems of (1) + protonic acids of POMs (H-
PW12, H-SiW12, H-AlW12, H-P2W18), and (2)
3.2. Catalytic behavior of carboxylatogold(I) complexes with NHC and
PPh3 ligands for diphenylacetylene hydration in the absence and presence of
POMs
In the reactions of (1) + protonic acids of POMs (H-PW12, H-
SiW12, H-AlW12, H-P2W18), diphenylacetylene substrate was com-
pletely consumed after 2 h (Entries 1–2 ∼ 1–4 and 1–6, Table 1). We
also examined the effect of re-addition of diphenylacetylene (1.5 mmol)
after the first reaction (Entries 2-1 ∼ 2–4, Table 2). No loss of activity
was seen in the second reaction, suggesting that the catalyst systems are
stable and recyclable. Compound (2) also showed good stability as a
catalyst (Entry 2–5). In contrast, activity was not maintained in the
second reaction of compound (4) (Entry 2–6, Table 2). As shown in the
previous paper [21], in the [Au(Y)(PPh3)]+ species generated from (4),
the catalyst precursor (Y = solvent), or the active species (Y = alkyne)
can work only when the substrate is present. When the substrate has
almost been completely consumed, the active species readily changes to
inactive form, such as [Au(PPh3)2]+. Therefore, the reaction involving
(4) continues until the substrate has been almost totally consumed.
On the other hand, it is suggested that the inactive form such as
“[Au(IPr)2]+” cannot be formed in solution, because of the steric hin-
drance of isopropyl groups in the IPr ligand, even after the substrate is
completely consumed. The points that the inactive species [Au(L)2]+ is
easily formed when L = PPh3, whereas it is not when L = IPr, were also
supported by the QM calculations. Thus, the gold(I) complexes with
other NHC ligands were designed and prepared (Scheme 2).
Catalytic hydration of alkyne by [Au(RS-pyrrld)(L)] (L = NHC,
PPh3) with and without various polyoxometalates was examined in a
mixed solvent of 1,4-dioxane/water (4/1) at 80 °C. All of the reactions
of the gold(I) complexes with NHC ligand appeared to proceed homo-
geneously (Entries 1-1∼1–8, Table 1), whereas the reactions of the gold
(I) complexes with PPh3 ligand proceeded heterogeneously (Entries
1–9∼1–13).
(1) For the hydration of diphenylacetylene by (1), the observed
catalytic activity was modest in 1,4-dioxane/water (4/1) at 80 °C
(conversion to deoxybenzoin 27.3% after 24 h, Entry 1-1, Table 1), but
it was enhanced significantly by the addition of protonic acids of Keggin
POMs (H-PW12, H-SiW12, H-AlW12) (conversions of 90.9–95.1%
after 2 h, Entries 1–2 – 1–4) and Wells-Dawson POM (H-P2W18)
(conversion of 93.9% after 2 h, Entry 1–6). Although the addition of
sodium salt of Keggin POM (Na-PW12) was effective (conversions of
21.0% after 2 h and 96.2% after 24 h, Entry 1–5), the enhancement was
much lower than that by H-PW12. The lower yield can be attributed to
difficulty in the elimination of carboxylate in [Au(RS-pyrrld)(IPr)] by
the sodium salt, in comparison with the protonic acid form of POMs.
(2) On the other hand, a related compound (2) composed of the
monomeric [Au(IPr)(H2O)]+ cations and Keggin POM also showed ef-
fective catalysis (0.67 mol%, Entry 1–7, Table 1); the conversion was
98.6% after 1 h and 99.4% after 2 h. Thus, the catalyst precursor in the
(1) + H-PW12 system (Entry 1–2) is considered to be the same as that
in the reaction using (2). Since the catalyst system of Entry 1–2 consists
3.4. Catalysis by gold(I) complexes with various NHC ligands
In addition to the electronic effect of the NHC ligand, steric hin-
drance of the substituents of the N-bonding functional groups also
7

K. Nomiya, et al.
MolecularCatalysisxxx(xxxx)xxx–xxx
significantly influences the catalytic activities. Complex (6) with the
IMes ligand did not show any activity in the absence or presence of H-
PW12 (Entries 3-1 and 3-2, Table 3), suggesting that the inactive spe-
cies [Au(IMes)2]+ is readily formed. On the other hand, complex (1)
with the IPr ligand in the presence of H-PW12 was effective (Entry
1–2), although (1) alone showed modest activity (Entry 1-1), suggesting
that no species such as [Au(IPr)2]+ is generated in solution owing to
steric hindrance of the substituents. Although substitution of the car-
boxylate of [Au(RS-pyrrld)(NHC)]+ should be accelerated by H-PW12,
whether the resultant species [Au(NHC)]+ is changed to an active
species [Au(solvent)(NHC)]+ or to an inactive species [Au(NHC)2]+
depends strongly on the steric effect of the substituents of the N-
bonding functional groups. Thus, in the presence of H-PW12, complex
(7) with the BIPr ligand showed highly active catalysis (Entry 3-3),
while complex (8) with the IF3 ligand showed poor activity (Entry 3–4).
On the other hand, complex (9) with the ItBu ligand showed activity
upon reaction for ca 6 h, but may be deactivated or changed to an in-
active form over a longer reaction time (Entry 3–5). The results for (9)
can be compared with the data reported by Nolan’s group [36]; the
system of [AuCl(ItBu)] and AgSbF6 (2 mol%) in a mixed solvent of 1,4-
dioxane and water (2:1) at 80 °C afforded 93% conversion after 20 h.
3.6. Catalysis by (1) + H-PW12 and (2) for hydration of other alkynes
The hydration of alkyne substrates, or phenylacetylene and 1-
phenyl-1-butyne, by (1) + H-PW12 and (2), respectively, was also
examined (Table 5). The hydration of phenylacetylene by (1) + H-
PW12 and (2) resulted in the formation of acetophenone in yields
of > 99.9% and 89.5%, respectively, after 2 h at 80 °C (Entries 5-1 and
5-2). These results revealed much more effective catalysis than that by
(4) under similar conditions (conversion of 32.4% after 4 h, 94.3% after
24 h) at 80 °C (Entry 5-3). These catalytic behaviors can be understood
in terms of the differences in (i) the ligand (IPr vs PPh3) and (ii) the
reaction system (homogeneous vs heterogeneous).
The hydration of 1-phenyl-1-butyne by (2) resulted in the formation
of butyrophenone with a conversion of 18.8%, and 1-phenyl-2-buta-
none with a conversion of 81.9% after 0.5 h at 80 °C (Entry 5-4). The
combined yield of 100% was achieved after just 0.5 h, whereas the
hydration of 1-phenyl-1-butyne by (4) was almost completed in 2 h
(butyrophenone with a conversion of 38.8% and 1-phenyl-2-butanone
with a conversion of 57.0%, Entry 5-5) [21].
The hydration of 1-phenyl-1-butyne by (2) can be compared with
the reported formation of butyrophenone (15%) and 1-phenyl-2-buta-
none (50%) with the catalyst system of [Au(IPr)(OH)] (0.02 μmol) plus
aqueous HSbF6 (0.06 μmol) after 24 h at 120 °C [41]. The distribution of
the products formed by (2), i.e., butyrophenone (18.1%) and 1-phenyl-
2-butanone (81.9%), indicates that coordination of the alkyne substrate
on the catalyst precursor {[Au(IPr)(H2O)][α-PW12O40]}2− ((a) in
Scheme 3) and the subsequent nucleophilic attack of water molecule
are remarkably selective, compared with the catalysis by (4) (Entry 5-5)
[21]. Therefore, the site closer to the pH group of the unsymmetrical
alkyne coordinates to the [Au(IPr)]+ unit, and the water molecule at-
tacks the opposite location, i.e., the site closer to the Et group, leading
to the higher selectivity of (2). The reaction scheme based on the ion-
pair species with POM is shown in Scheme 2, which corresponds to the
previously reported scheme via the [Au(PPh3)(H2O)]+ species as the
ion-pair with POM [21].
3.5. Effect of addition of various protonic acids HX and their sodium salts
NaX on the catalysis by complex (1), and the role of POM
Compared with (1) alone (Entry 1-1), addition of a strong protonic
acid HX such as HBF4, HPF6 or CF3SO3H to [Au(RS-pyrrld)(IPr)] (1)
resulted in greatly enhanced activity for diphenylacetylene hydration
(54.6, 95.9, 99.0% after 4 h, and 70.2, > 99, > 99% after 6 h in the
mixed solvent at 80 °C (Entries 4-1 ∼ 4-3), Table 4). These results in-
dicate that the ease of carboxylate elimination of (1) depends on the
acidity of the strong acid HX, and this affects the ease of formation of
ion-pair species {[Au(IPr)(H2O)]X} and neutral RS-Hpyrrld (Eq. 7 ra-
ther than Eq. 6).
[Au(RS-pyrrld)(IPr)] (1) + HX + H2O → {[Au(IPr)(H2O)]X} + RS-
3.7. QM calculations
−
Hpyrrld (X = BF4−, PF6−, CF3SO3
)
(7)
We performed density functional calculations in order to examine
the difference in the catalytic activity of [LAu]+ (L = IPr, PPh3)
[81–83]. Since the active species [LAu]+ is thought to be deactivated
Addition of the sodium salt NaX, i.e., NaBF4, NaPF6 or CF3SO3Na,
also resulted in enhanced activities for the same reaction (18.1, 11.8,
18.3% after 4 h, 26.8, 26.7, 28.5% after 6 h (Entries 4–4 ∼ 4–6)),
compared with (1) alone, suggesting that the ease of carboxylate
elimination of (1) and the formation of RS-Napyrrld are significantly
influenced by NaX (vs HX).
Given the results of addition of H-PW12 and Na-PW12 (Entries 1–2,
1–3), it is clear that the POM anion also plays an important role in
catalyzing the hydration of diphenylacetylene.
[Au(RS-pyrrld)(IPr)] (1) + H3[α-PW12O40] + H2O → {[Au(IPr)(H2O)]
[α-PW12O40]}2− + 2H+ + RS-Hpyrrld
(8)
It appears that not only does H-PW12 act as a strong protonic acid,
but also the POM anion with multiple negative charge contributes to
the formation and stability of ion-pair species such as {[Au(IPr)(H2O)]
[α-PW12O40]}2− in solution (eq. 8). If the completely dissociated cation
or [Au(IPr)(H2O)]+ (A) is formed as the actual catalyst precursor (eq.
6), the catalytic results of the systems of (1) + HX (Entries 4-1 ∼ 4-3)
and (1) + H-PW12 (Entry 1–2) should be the same. This is also the case
for the catalytic behavior of the systems of (1) + NaX (Entries 4-4 ∼
4–6) and (1) + Na-PW12 (Entry 1–5). Thus, it is strongly suggested
(Table 4) that ion-pair species such as {[Au(IPr)(H2O)]X} and {[Au(IPr)
(H2O)][α-PW12O40]}2− work as the actual catalyst precursors, and
their alkyne-substituted ion-pair species are the catalytically active
species.
Scheme 3. Reaction scheme.
8

K. Nomiya, et al.
MolecularCatalysisxxx(xxxx)xxx–xxx
Table 6
NBO charge (e) and AO population (e) of the Au atom in [(IPr)Au]+ and
[(PPh3)Au]+
.
+
[(IPr)Au]+
0.565
[(PPh3)Au]
0.418
NBO charge
AO population (valence)
s
px
py
pz
dxy
dxz
dyz
dx2-y2
dz2
0.620
0.019
0.014
0.018
1.998
1.980
1.965
1.998
1.829
0.738
0.002
0.002
0.004
1.999
1.975
1.975
1.999
1.889
second ligand IPr approaches the coordination distance of 2 Å, even in
the case that the two IPr ligands are orthogonal, when the steric re-
pulsion would be the smallest. On the other hand, in the case of
L = PPh3, even when the distance R between [(PPh3)Au]+ and the
second ligand PPh3 approaches the coordination distance of 2.3 Å,
[(PPh3)Au]+—PPh3 continues to have a stabilizing effect, indicating
that [(PPh3)Au]+ is easily deactivated by the formation of
Fig. 2. Plots of the relative energies of the [L2Au]+ complexes (L = IPr, PPh3)
versus the distance R at the M06-2X/BSI level. The energies are relative to the
energy of [LAu]+ and L. The following colors apply: blue, L = IPr and the
molecular planes of the two IPr are parallel; green, L = IPr and the molecular
planes of the two IPr are orthogonal; red, L = PPh3 (For interpretation of the
references to colour in this figure legend, the reader is referred to the web
version of this article).
[(PPh3)2Au]+
.
We next optimized the structures of [LAu(Sub)]+ (L = IPr, PPh3;
Sub = H2O, C2Ph2) and calculated the interaction energies (INT) of
[LAu]+ (L = IPr, PPh3) with the substrates, H2O and C2Ph2 (Fig. 3), in
order to examine the stability of the key intermediate in the catalytic
reaction. The interaction energy of [LAu]+ with C2Ph2 is larger by
10.6 kcal/mol for L = IPr than for L = PPh3, which suggests that the
formation of the intermediate [LAu(C2Ph2)]+ is easier, and so the
catalytic reaction is thermodynamically more favorable, for L = IPr
than for L = PPh3. The larger interaction in the case of L = IPr is re-
flected in the shorter Au-C(C≡C) and the longer C≡C distances for
by coordination of the second ligand, we first examined the stability of
[L2Au]+. However, we could not establish the instability of [(IPr)2Au]+
by geometry optimization due to the large steric repulsion between
[(IPr)Au]+ and IPr. We therefore followed the energy change of
[LAu]+—L upon approach of the second ligand L to the Au atom. As
displayed in Fig. 2, in the case of L = IPr, although [(IPr)Au]+—IPr is
stabilized to some extent by the approach of the second ligand L, it is
suddenly destabilized when the distance R between [(IPr)Au]+ and the
Fig. 3. Optimized structures (Å) of [LAu(Sub)]+ (L = IPr,
PPh3; Sub = H2O, C2Ph2) and the interaction energies
(INT) (kcal/mol) of [LAu]+ (L = IPr, PPh3) with the sub-
strates, H2O and C2Ph2, at the M06-2X/BSI level.
9

K. Nomiya, et al.
MolecularCatalysisxxx(xxxx)xxx–xxx
L = IPr compared to those for L = PPh3. The difference in the inter-
action energy between H2O and C2Ph2 is also larger for L = IPr than for
L = PPh3, which shows that the substitution of H2O with C2Ph2 in [LAu
(H2O)]+ to form the key intermediate [LAu(C2Ph2)]+ in the catalytic
cycle is also thermodynamically more favorable for L = IPr than for
L = PPh3. These computational results are in good agreement with the
experimental finding that the catalytic activity is larger for [(IPr)Au]+
than for [(PPh3)Au]+. Since EDA analysis showed a high ratio of
electrostatic energy (44–70%) in the interaction energy between Au+
and substrates, one reason for the larger interaction energy in the case
of L = IPr would be the fact that the positive charge of the Ag of [LAg]+
is larger for L = IPr (0.565 e) than for L = PPh3 (0.418 e), as presented
in Table 6. On the basis of the magnitude of the population of the dπ
orbital and the dσ and the s orbitals of the Ag in [LAg]+, σ donation
from the H2O oxygen to the Au in [LAu(H2O)]+ and σ donation from
the C≡C to the Au and π back-donation from the Au to the C≡C in
[LAu(C2Ph2)]+ are thought to be larger for L = IPr than for L = PPh3.
This is another reason why the interaction energy is larger for L = IPr
than for L = PPh3.
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