Synthesis and thermal properties of salts comprising cationic bis(oxazoline)-AuIII complexes and fluorinated anions

Article abstract of DOI:10.1016/j.poly.2016.04.012

Salts comprising cationic bis(oxazoline) (box)-Au^III complexes and fluorinated anions were synthesized and their thermal properties were investigated. Cations that contain box ligands with butyl substituents (butyl-box) were used with either achiral-(R,S) ([1]⁺) or chiral-(S,S) ([2]⁺) configurations. [1][Tf₂N] (Tf₂N^- = bis(trifluoromethanesulfonyl)amide) is a solid that can also be classified as an ionic liquid (T_m = 62 °C, T_g = -11 °C). [1]X and [2]X (X = SbF₆^- and BF₄^-) exhibit melting points higher than 100 °C, where [1]X exhibits a higher melting point than [2]X. A salt with an ethyl-box ligand ([3][SbF₆]) exhibits no melting up to its decomposition temperature of 267 °C. Crystal structure determination revealed that the achiral cation in [1][BF₄] has a bent structure. The potential utility of [1][Tf₂N] for catalysis was demonstrated by the catalytic formation of an acetal from an alkyne.

Full text of DOI:10.1016/j.poly.2016.04.012

Polyhedron 113 (2016) 1–4
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and thermal properties of salts comprising cationic
III
bis(oxazoline)-Au complexes and fluorinated anions
Yuji Miura , Tomoyuki Mochida , Satoshi Motodate , Keisuke Kato b
a
a,
⇑
b
a
Department of Chemistry, Graduate School of Science, Kobe University, Rokkodai, Nada, Hyogo 657-8501, Japan
Faculty of Pharmaceutical Sciences, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274-8510, Japan
b
a r t i c l e i n f o
a b s t r a c t
Salts comprising cationic bis(oxazoline) (box)-AuIII complexes and fluorinated anions were synthesized
and their thermal properties were investigated. Cations that contain box ligands with butyl substituents
Article history:
Received 5 March 2016
Accepted 8 April 2016
Available online 16 April 2016
+
+
(
(
(
butyl-box) were used with either achiral-(R,S) ([1] ) or chiral-(S,S) ([2] ) configurations. [1][Tf
2
N]
À
Tf
2
N
= bis(trifluoromethanesulfonyl)amide) is a solid that can also be classified as an ionic liquid
À
À
4
T
m
= 62 °C, T
g
= À11 °C). [1]X and [2]X (X = SbF
6
and BF
) exhibit melting points higher than 100 °C,
]) exhibits
no melting up to its decomposition temperature of 267 °C. Crystal structure determination revealed that
the achiral cation in [1][BF ] has a bent structure. The potential utility of [1][Tf N] for catalysis was
demonstrated by the catalytic formation of an acetal from an alkyne.
Ó 2016 Elsevier Ltd. All rights reserved.
Keywords:
where [1]X exhibits a higher melting point than [2]X. A salt with an ethyl-box ligand ([3][SbF
6
Bis(oxazoline) complexes
Metal complexes
Thermal properties
Ionic liquids
4
2
Catalytic activity
À
À
1
. Introduction
related box-AuIII salt [9]. Counter anions Tf
2
N
(Tf
2
N = bis(trifluo-
À
À
4
romethanesulfonyl)amide), SbF
6
, and BF
were used in this study.
Metal complexes exhibit a variety of functions, including cat-
Although box-containing complexes with bulky substituents have
often been used for catalytic reactions [1,2], butyl substituents
were introduced into the box ligand to lower the melting points.
To investigate the effect of chirality, cations with achiral
(R,S)- and chiral (S,S)-configurations were employed, which are
alytic activities. Among the number of complex catalysts reported
to date, metal complexes with bis(oxazoline) (box) ligands are a
representative group of metal chelate catalysts [1,2]. Since box
ligands have two asymmetric carbons, the resultant complexes
have chiral and achiral isomers. Although complex catalysts are
often prepared in situ in catalytic reactions and not isolated,
X-ray structures of many box metal complexes have been
determined to understand their stereochemistry. Their thermal
properties, however, are not well understood.
We have developed a variety of ionic liquids containing func-
tional metal complexes [3,4]. Ionic liquids are salts with melting
points below 100 °C, and their unique characteristics have
attracted much attention [5,6]. Ionic liquids with catalytic ability
have been recently developed [7]; we previously synthesized ionic
liquids from half-sandwich complexes that function as catalysts
+
+
designated as [1] and [2] , respectively (Fig. 1). A salt with ethyl
III
substituents on the box ligand, [Au Cl
2 6
{(S,S)-ethyl-box}][SbF ]
([3][SbF ]), was also synthesized to investigate the effect of the
6
alkyl chain length on the complex. We attempted the synthesis
of other salts with different substituents and anions; however,
those results are not included in this study because we were
unable to obtain sufficiently pure samples.
A search of the Cambridge Structural Database (CSD) revealed
that structure determination of box-containing complexes is lim-
ited to chiral complexes. This is because of the interest in box com-
plexes as asymmetric catalysts. Therefore, the crystal structure of
[
8]. As such, we are interested in the possibility of ionic liquid for-
4
[1][BF ] was determined by X-ray crystallography to elucidate
mation from box complexes. In this study, therefore, we synthe-
the stereochemistry of the achiral complex.
III
sized
a
series of Au
complexes with a butyl-box ligand
III
(
[Au Cl (butyl-box)]X, Fig. 1) and investigated their thermal prop-
2
III
2. Results and discussion
erties. Au -containing complexes were chosen because of our
recent work on the isolation and structural characterization of a
2.1. Preparation
À
À
À
À
4
⇑
Corresponding author. Tel.: +81 78 803 5679.
E-mail address: tmochida@platinum.kobe-u.ac.jp (T. Mochida).
Complexes [1]X and [2]X (X = Tf
2
N , SbF
6
, and BF
) were syn-
], and a
thesized by the reaction of the butyl-box ligand, K[AuCl
4
http://dx.doi.org/10.1016/j.poly.2016.04.012
277-5387/Ó 2016 Elsevier Ltd. All rights reserved.
0

2
Y. Miura et al. / Polyhedron 113 (2016) 1–4
6
on the ligand [3][SbF ] did not show melting upon heating, only
decomposition at 267 °C. This result demonstrates the effective-
ness of introducing butyl substituents on the box ligand for lower-
ing the melting point.
Among the salts synthesized, [2][SbF
6
] and [3][SbF
6
] exhibit a
À1 À1
phase transition in the solid state at 73.8 °C (
D
S = 1.7 J mol
K )
À1 À1
and 69.3 °C (
D
S = 10.6 J mol
K ), respectively.
2.3. Crystal structure
The crystal structure of [1][BF
4
] containing the achiral ligand
Fig. 1. Structural formulas of salts of butyl-bis(oxazoline) (box)-AuIII complexes
was determined by X-ray crystallography. As per our knowledge,
there are no X-ray structures of achiral box ligand-containing com-
plexes reported to date. An ORTEP drawing of the cation is shown
in Fig. 2a. The complex has a square-planar coordination geometry,
whereas the ligand exhibits a bent structure (Fig. 2b). The two oxa-
zoline rings make an angle of 145°. The central six-membered ring
involving the Au ion forms a boat conformation, and the angle
between the mean plane of the ring and the coordination plane
is ꢀ20°. The butyl groups are extended outside of the coordination
plane. The cation and anion form an ion-pair-like arrangement
prepared in this study. (a) Achiral salts with the (R,S)-butyl-box ligand ([1]X) and
À
(
b) chiral salts with the (S,S)-butyl-box ligand ([2]X). The anions (X ) are shown
below the formulas.
salt of the anion in acetonitrile. [3][SbF
method. We failed to purify a pale yellow oil of [2][Tf
6
] was prepared by the same
N]; the
2
results are not included here. Other salts were obtained as pale yel-
low solids after recrystallization. These salts were soluble in
dichloromethane and polar solvents, such as alcohol; however,
they appeared to be rather unstable in polar solvents.
with a distance of 4.39 Å between the metal and anion centers.
III
The structure contrasts with that of [Au Cl
2
{(R,R)-benzyl-box}]
2.2. Thermal properties
[
6
SbF ] containing a chiral ligand, which we previously reported
[
9]. In this complex, the ligand is almost planar and the sub-
The thermal properties of these salts were investigated by dif-
ferential scanning calorimetry (DSC). DSC traces are shown in
Fig. S1 (electronic supporting information). The melting points
stituents are oriented above and below the coordination plane.
Moreover, the ion pairing is less prominent because of the more
symmetric cationic structure concomitant with symmetric
cation–anion arrangements. Indeed, chiral box complexes usually
exhibit a planar ligand conformation in the solid state [1].
Although the bent structure of the achiral cation as shown here
may partially result from the packing effects, the higher melting
points of the achiral complexes compared with those of chiral
complexes may be ascribed to a more effective ion-pair formation
resulting from the less symmetrical structure of the achiral cation.
(
T
m
g
), glass transition temperatures (T ), and relevant parameters
of the salts are listed in Table 1. In [1]X, the melting points
À
À
À
4
increases with decreasing anion size (Tf
Tf N] (T = 62.0 °C, T = À11 °C) can be regarded as an ionic liquid
but is a solid at room temperature. The rather high melting point of
1][Tf N] is likely caused by the large polarity, planar shape, and
2 6
N > SbF > BF ). [1]
[
2
m
g
[
2
heavy molecular weight of the cation, considering the thermal
À
properties of ionic liquids of pincer complexes [10]. The SbF
6
and
À
BF
4
salts exhibit melting points higher than 100 °C, and the achiral
salts exhibit higher melting points than the chiral salts. The melt-
ing point of [1][SbF ] (T = 131.5 °C) is higher than that of [2][SbF
= 115.4 °C) and that of [1][BF ] (T = 173.0 °C) is higher than
6
m
6
]
(T
m
4
m
that of [2][BF
value for [1][Tf
4
] (T
m
= 101 °C). For the melting entropies (DS), the
À1 À1
2
N] is larger (110 J mol
K ) than those for [1]
K ); this result is consistent with
À
À
À1 À1
[
X] (X = BF
4
, SbF
6
; ꢀ55 J mol
N anion.
When cooled from a melt, the salts do not crystallize but exhibit
glass transitions (T
the flexibility of the Tf
2
g
= À29 to 30 °C). The glass transition tempera-
tures were much higher than those of the free box ligands. The box
ligands are liquids and do not crystallize at low temperatures,
undergoing glass transitions at À79 °C ((R,S)-butyl-box) and
À77 °C ((S,S)-butyl-box).
All the salts bearing butyl substituents on the ligand exhibit
melting as described above, but the salt with ethyl substituents
Table 1
Melting points (T
m
), melting enthalpies (
D
H) and entropies (
D
S), and glass transition
temperatures (T
g
) of the salts synthesized in this study.
Compound
T
m
(°C)
D
H (kJ mol–1
)
D
S (J mol–1 K–1
)
g
T (°C)
[
[
[
[
[
[
1][Tf
1][SbF
1][BF
2][SbF
2
N]
62.0
35.8
22.2
25.9
22.0
105.8
54.4
57.9
55.3
À11
30
25
À29
6
]
]
]
131.5
173.0
115.4
4
]
6
a
b
2][BF
4
]
101
3][SbF
6
267 (dec.)b
4
Fig. 2. (a) ORTEP drawing of cation in [1][BF ]. Thermal ellipsoids are drawn at the
a
Elemental analysis data deviated by 0.7% from the calculated value for the salt.
Visually observed under a microscope.
50% probability level. Hydrogen atoms have been omitted for clarity. (b) Cation–
anion arrangement viewed from two directions.
b

Y. Miura et al. / Polyhedron 113 (2016) 1–4
3
2
.4. Catalytic reaction
stirring the solution in the dark for a day, a small amount of lithium
chloride was added to the solution. Then, the solution was filtered
through a short plug of celite and the solvent was removed under
reduced pressure. The residue was extracted with dichloro-
methane, washed several times with water, dried over magnesium
sulfate, and evaporated. The residue was dissolved in methanol
and washed several times with hexane, and the methanol layer
was separated and evaporated. Recrystallization of the resultant
The catalytic activities of box-AuIII complexes have been previ-
ously demonstrated [9]. To investigate the potential utility of the
ionic liquid [1][Tf N] for catalysis, the conversion of an alkyne to
an acetal, a simple reaction catalyzed by Au catalysts [11], was
2
III
examined. Heating a solution of phenylacetylene (PhC„CH) with
a
(
(
slight excess of methanol in the presence of [1][Tf
2 mol%) for 1 h at 60 °C produced 1,1-dimethoxyethylbenzene
PhCCH (OCH ) in 77% yield. No additional solvent was used in
2
N]
yellow oil from ethanol (À50 °C) produced the desired compound
1
3
3
)
2
i.e., a pale yellow powder (29 mg, yield 20%). H NMR (CDCl
3
,
the reaction. The solution was initially homogeneous, whereas
the product separated from the ionic liquid upon consumption of
the methanol. The use of 0.2 mol% catalyst gave a lower yield of
ppm, 400 MHz): d = 0.94 (t, 6H, J = 7.0 Hz), 1.23–1.40 (m, 8H),
1.81 (s, 3H), 1.92 (m, 2H), 1.98 (s, 3H), 2.10–2.12 (m, 2H),
4.85–4.97 (m, 6H). Anal. found: C, 27.43; H, 3.78; N, 4.92%; calc.
ꢀ
40%. The catalytic activity was thus demonstrated, although the
for C19
[1][SbF
the synthesis of [1][Tf
Recrystallization of the product from ethanol (À50 °C) produced
H
30AuCl
] was synthesized by the same procedure as used for
N] using silver hexafluoroantimonate.
2 6 3 6 2
F N O S : C, 27.09; H, 3.59; N, 4.99.
yield was lower than that for a reaction using Na[AuCl ] as the cat-
4
6
alyst (96%) [11]. The lower yield may be partly attributed to the
gradual decomposition of the catalyst, forming black precipitate.
This tendency likely results from the instability of the Au -con-
2
III
1
a pale yellow powder (Yield ꢀ20%). H NMR (CDCl
3
, ppm,
taining complex.
400 MHz): d = 0.94 (t, 6H, J = 7.2 Hz), 1.29–1.45 (m, 8H), 1.78 (s,
3
4
H), 1.95 (m, 2H), 1.98 (s, 3H), 2.12–2.15 (m, 2H), 4.79–4.89 (m,
H), 4.99–4.94 (m, 2H). Anal. found: C, 25.74; H, 3.81; N, 3.51%;
3
. Conclusion
calc. for C17
[1][BF ] was synthesized by the same procedure as used for the
synthesis of [1][Tf N] using silver tetrafluoroborate. Slow diffusion
of ether into a dichloromethane solution of the product gave a pale
2 6 2 2
H30AuCl F N O Sb: C, 25.58; H, 3.79; N, 3.51.
Salts of cationic AuIII complexes containing butyl-box ligands
4
2
were prepared and their thermal properties were investigated.
1][Tf N] is regarded as an ionic liquid; however, its melting point
is higher than room temperature. Once melted, [1][Tf N] maintains
[
2
1
yellow powder (yield 71%). H NMR (CDCl
d = 0.93 (t, 6H, J = 7.0 Hz), 1.29–1.43 (m, 8H), 1.81 (s, 3H), 1.94 (s,
H), 2.08–2.17 (m, 4H), 4.81–4.95 (m, 6H). Anal. found: C, 31.61;
3
, ppm, 400 MHz):
2
its liquid state at room temperature. Moreover, its rather high
melting point can be attributed to the planar shape, large polarity,
and heavy molecular weight of the cation. The melting points of
3
H, 4.92; N, 4.61%; calc. for C17
4.66; N, 4.32.
2 4 2 2
H30AuBCl F N O : C, 31.46; H,
À
À
4
SbF and BF salts are higher than 100 °C, and the salts from the
6
chiral (S,S)-butyl-box complex exhibit lower melting points than
the achiral (R,S)-butyl-box complex. The achiral complex exhibits
a bent structure in [1][BF ] in the solid state. The salts from the less
4
4.3. Preparation of [AuCl {(S,S)-butyl-box}]X ([2]X; X = SbF , BF )
2
6
4
symmetrical achiral complex likely contain stronger electrostatic
[2][SbF ] was synthesized by the same procedure as used for
6
À
interactions, yielding high melting points. The Tf
2
N
salt exhibits
the synthesis of [1][Tf N] using (S,S)-butyl-bis(oxazoline) and sil-
2
catalytic activity for the conversion of an alkyne to an acetal; how-
ever, gradual decomposition of the catalyst occurs during the reac-
tion. Indeed, a similar problem has been observed in catalytic
reactions using ionic liquids from half-sandwich catalysts [8].
The use of a more robust catalytic species may be needed to enable
recycling of the catalyst.
ver hexafluoroantimonate. Recrystallization of the resultant yellow
oil from ethanol (À50 °C) produced a pale yellow powder (yield
1
9%). H NMR (CDCl , ppm, 400 MHz): d = 0.93 (t, 6H, J = 6.8 Hz),
3
1.37–1.40 (m, 8H), 1.77 (m, 8H), 2.01 (m, 2H), 4.37–4.45 (m, 4H),
4.90 (t, 2H, J = 8.6 Hz). Anal. found: C, 25.48; H, 3.97; N, 3.59%; calc.
for C17H30AuCl F N O Sb: C, 25.58; H, 3.79; N, 3.51.
2
6
2 2
[
4
2][BF ] was synthesized by a similar procedure but the corre-
sponding nitrate salt was first prepared using silver nitrate and
then the solid was subjected to anion exchange using sodium
tetrafluoroborate in acetonitrile. After evaporation of the solvent,
the product was dissolved in dichloromethane and washed with
water; the product was then dissolved in methanol and washed
4
. Experimental
4.1. General
Silver bis(trifluoromethanesulfonyl)amide was prepared
several times with hexane. The product was obtained as a yellow
according to the literature [12]. The box ligands were prepared
using the general procedure described for bis(oxazolines) [13]
and were purified using silica gel chromatography (eluent: hex-
1
powder (yield 28%). H NMR (CDCl
3
, ppm, 400 MHz): d = 0.91 (t,
6
2
H, J = 6.4 Hz), 1.26–1.51 (m, 8H), 1.54–1.77 (m, 8H), 1.90 (m,
H), 3.28–3.73 (m, 2H), 4.11–5.00 (m, 4H). Anal. found: C, 30.75;
ane/ethyl acetate: 5/1–10/1 + 1% triethylamine). Other reagents
were commercially available. 1H NMR spectra were measured
H, 4.76; N, 4.07%; calc. for C17H30AuBCl F N O : C, 31.46; H,
2
4
2 2
4
.66; N, 4.32. The elemental analysis results deviated by 0.71%,
using a JEOL JNM-ECL-400 spectrometer. Elemental analysis was
performed using a Yanaco CHN MT-5 recorder. DSC measurements
were conducted using a TA Instrument Q100 differential scanning
but purification by recrystallization was not possible.
calorimeter. TG analysis was conducted using a Rigaku TG8120 at
2 6 6
4.4. Preparation of [AuCl {(S,S)-ethyl-box}][SbF ] ([3][SbF ])
À1
1
K min under nitrogen atmosphere.
[
3][SbF
6
] was synthesized by the same procedure as used for [1]
À
À
6
4
BF
.2. Preparation of [AuCl
2
{(R,S)-butyl-box}]X ([1]X; X = Tf
2
N , SbF
,
[Tf N] using (S,S)-ethyl-bis(oxazoline) and silver hexafluoroanti-
2
À
4
)
monate. Recrystallization of the product from ethanol (À50 °C)
1
2 2
produced a pale yellow powder (yield 24%). H NMR (CD Cl ,
An acetonitrile solution (10 mL) of (R,S)-butyl-box (51.6 mg,
.175 mmol) and an acetonitrile solution (10 mL) of KAuCl
64.3 mg, 0.170 mmol) were mixed, to which a solution of Ag
Tf N] (106 mg, 0.273 mmol) in acetonitrile was added. After
ppm, 400 MHz): d = 0.97 (t, 6H, J = 7.4 Hz), 1.77 (m, 2H), 1.81 (s,
6H), 2.11–2.14 (m, 2H), 4.72–4.73 (m, 2H), 4.86–4.87 (m, 4H). Anal.
0
(
[
4
found: C, 21.43; H, 3.10; N, 4.19%; calc. for C13
C, 21.04; H, 2.99; N, 3.78.
2 6 2 2
H22AuCl F N O Sb:
2

4
Y. Miura et al. / Polyhedron 113 (2016) 1–4
4.5. Catalytic reaction
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2016.04.012.
2
Under a nitrogen atmosphere, [1][Tf N] (19.2 mg, 0.0228 mmol,
ca. 2 mol%) and phenylacetylene (0.125 mL, 1.32 mmol) were dis-
solved in methanol (0.092 mL, 3.62 mmol) and the solution was
stirred for 1 h at 60 °C. Then, the product was extracted with
hexane and filtered through a short plug of celite. Removal of the
solvent left 182.8 mg of a yellow liquid of 1,1-dimethoxyethylben-
zene, which contained a small amount of phenylacetylene. The
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[
[
Crystallographic parameters: C17
Z = 8, Dcalc = 1.845 g cm , monoclinic, space group C2/c, with unit
cell a = 22.688(5) Å, b = 14.477(5) Å, c = 14.231(5) Å, b = 91.569(5)°,
and V = 4672(3) Å . R
pendent reflections (Rint = 0.0434), 266 parameters. ORTEP-3 [15]
was used to produce molecular graphics.
H
30AuBCl
2
F
N
4 2
O
2
, M
W
= 649.11,
À3
3
1
(I > 2
r
(I)) = 0.0714, wR
2
= 0.1980, 4909 inde-
(
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(
(
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Acknowledgments
[
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12;
b) S. Mori, T. Mochida, Organometallics 32 (2013) 780.
We thank Y. Funasako (Kobe University) for X-ray crystallogra-
phy. This work was financially supported by JSPS KAKENHI (Grant
number 21655020 and 24350073).
1
(
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Akita, Synlett (2008) 1081.
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Appendix A. Supplementary data
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Pires, I. Villalba, Synlett (2005) 2321.
CCDC 854789 contains the supplementary crystallographic data
for [1][BF ]. These data can be obtained free of charge via http://
4
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
[14] G.M. Sheldrick, Acta Crystallogr. D64 (2008) 112.
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[