Tiara-like octanuclear palladium(II) and platinum(II) thiolates and their inclusion complexes with dihalo- or iodoalkanes

Article abstract of DOI:10.1021/ic403050c

A tiara-like octanuclear palladium thiolate complex, [Pd(μ-SCH ₂CO₂Me)₂]₈, that has a toroidal structure was synthesized via reactions of either PdCl₂ with methyl thioglycolate/N,N-diisopropylethylamine (DIEA) (conventional method) or [PdCl₂(MeCN)₂] with m-C₆H₄(CMe ₂SCH₂CO₂Me)₂ (alternative method). In the latter method, the tiara-like complex formed via the corresponding SCS-pincer complex and/or 1:1 PdCl₂ and ligand complexes. With respect to the platinum analogues, the alternative method efficiently produced the tiara-like octanuclear complex [Pt(μ-SCH₂CO ₂Me)₂]₈ in high purity. Small molecules, such as CH₂Cl₂, ClCH₂CH₂Cl, CH ₂Br₂, and CH₃I, were accommodated in the inner voids of the tiara rings to form 1:1 inclusion complexes. These complexes are stabilized not only by weak CH...X hydrogen bonds (X = Cl or Br) between the methylene protons of four or eight axially positioned methoxycarbonylmethyl groups on the tiara rings and the halogen atoms of the guest molecules but also by weak coordination of the halogen atoms to the transition-metal atoms.

Full text of DOI:10.1021/ic403050c

Article
pubs.acs.org/IC
Tiara-like Octanuclear Palladium(II) and Platinum(II) Thiolates and
Their Inclusion Complexes with Dihalo- or Iodoalkanes
Yukari Yamashina, Yasutaka Kataoka, and Yasuyuki Ura*
Department of Chemistry, Faculty of Science, Nara Women’s University, Kitauoyanishi-machi, Nara 630-8506, Japan
S
* Supporting Information
ABSTRACT: A tiara-like octanuclear palladium thiolate
complex, [Pd(μ-SCH2CO2Me)2]8, that has a toroidal structure
was synthesized via reactions of either PdCl2 with methyl
thioglycolate/N,N-diisopropylethylamine (DIEA) (conven-
tional method) or [PdCl2 (MeCN)2 ] with m-
C6H4(CMe2SCH2CO2Me)2 (alternative method). In the latter
method, the tiara-like complex formed via the corresponding
SCS-pincer complex and/or 1:1 PdCl2 and ligand complexes.
With respect to the platinum analogues, the alternative method
efficiently produced the tiara-like octanuclear complex [Pt(μ-
SCH2CO2Me)2]8 in high purity. Small molecules, such as
CH2Cl2, ClCH2CH2Cl, CH2Br2, and CH3I, were accommodated in the inner voids of the tiara rings to form 1:1 inclusion
complexes. These complexes are stabilized not only by weak CH···X hydrogen bonds (X = Cl or Br) between the methylene
protons of four or eight axially positioned methoxycarbonylmethyl groups on the tiara rings and the halogen atoms of the guest
molecules but also by weak coordination of the halogen atoms to the transition-metal atoms.
In most cases, tiara-like nickel and palladium complexes are
synthesized through the reactions of group 10 transition-metal
halides with thiols in the presence or absence of a base. We
successfully synthesized an octanuclear palladium complex
using this conventional method. Moreover, we also developed
an alternative synthetic method using m-C6H4(CMe2SR)2 as
the thiolate ligand source. The latter method was found to be
advantageous for the synthesis of a platinum complex. In this
paper, we report not only the synthesis, using these two
methods, of novel tiara-like octanuclear palladium and platinum
complexes but also the inclusion complexes with small organic
molecules. Decanuclear and undecanuclear nickel complexes
have been reported to include solvent molecules, such as
benzene,20 toluene,15 and tetrahydrofuran (THF),21 in their
inner voids. In contrast, no tiara-like palladium inclusion
complexes have been reported thus far.
INTRODUCTION
■
Metal thiolates are an important class of compounds because of
their structural diversity in coordination chemistry,1−3
relevance to cofactors of metalloproteins in biological
systems,4,5 and importance in cluster and surface science6,7
and catalysis.8 Thiolate-bridged polynuclear group 10 tran-
sition-metal complexes with chain structures that have the
general formula of [M(μ-SR)2]n (M = Ni, Pd) have been
investigated in detail because they provide efficient catalysis of
atom-economical organic reactions such as regioselective
additions of thiols and disulfides across alkynes.8−13 Tiara-like
complexes, which are also polynuclear group 10 transition-
metal thiolates and are characterized by toroidal architectures,
have been studied extensively, both in terms of their intriguing
structural features and with respect to the preparation of
monodisperse metal sulfide nanoparticles,14 nonlinear optical
materials,15 photoactive water-reducing catalysts,16,17 and host−
guest chemistry.15,18−21 Tiara-like nickel complexes have
received considerable attention, resulting in the preparation
of complexes with a variety of ring sizes that have the general
formula of [Ni(μ-SR)2]n (n = 4−6, 8−12).15,17,18,20−35 In
contrast, although several tiara-like hexanuclear palladium
complexes have been reported,9,14,19,36−43 only one octanuclear
complex, namely, [Pd(μ-SnPr)2]8, is known and was obtained as
a mixture with hexanuclear [Pd(μ-SnPr)2]6.19 For platinum,
infinite thiolates and selenolates that have the general formula
of [Pt(μ-ER)2]∞ (E = S, Se) have been synthesized using a
solvothermal method;37 however, to the best of our knowledge,
no tiara-like complexes have been reported thus far.
RESULTS AND DISCUSSION
■
Synthesis and Structure of Tiara-like Octanuclear
Palladium Complex 1. PdCl2 was reacted with 2 equiv each
of methyl thioglycolate and N,N-diisopropylethylamine (DIEA)
in nPrOH at room temperature. After 4 h, a tiara-like
octanuclear complex, that is, [Pd(μ-SCH2CO2Me)2]8 (1), was
obtained in high isolated yield (90%) (Scheme 1); a high
concentration of palladium (0.25 M) is essential to achieve a
high yield because a lower concentration (0.04 M) resulted in
lower yield (67%). The molecular structure of complex 1 was
Received: December 12, 2013
Published: March 24, 2014
© 2014 American Chemical Society
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(average: 88.4(16)°), and the dihedral angles between the PdS4
planes are within 127.75(6)−138.41(6)° (average: 135(4)°).
The distances between two adjacent sulfur atoms in the two S8
rings are within the range of 3.336(6)−3.573(6) Å (average:
3.48(5) Å). The 16 methoxycarbonylmethyl arms are
alternately located at axial and equatorial positions (i.e., the
arms extend nearly perpendicularly and horizontally from the
tiara ring), and some are significantly disordered; in particular,
the axial arm on the S1 atom alternates between bending
inward to occupy the inner void and extending out of the
toroidal structure (∼1:1). A similar inward curvature of an arm
was also observed in [Ni(μ-SCH2CO2Et)2]8.18 The axial arm
on the S2 atom, which is located trans to S1 on the other rim of
the tiara, closes the lid from the bottom. The distances between
the diagonally positioned palladium atoms are Pd1···Pd5 =
8.7191(9) Å, Pd2···Pd6 = 8.2331(9) Å, Pd3···Pd7 = 8.2307(8)
Å, and Pd4···Pd8 = 8.5895(8) Å, resulting in an ellipsoidal
architecture of the tiara ring (average: 8.44 Å). These distances
are considerably longer than those of [Ni(μ-SCH2CO2Et)2]8
(7.59−8.34 Å, average: 7.95 Å).18
Scheme 1. Formation of Tiara-like Octanuclear Palladium
Complex 1
determined by X-ray crystallography (Figure 1, recrystallized
from CHCl3/hexane). The eight palladium atoms lie on a
nearly planar surface and form an octagonal structure with the
distances between two adjacent metal atoms ranging from
3.1398(9) to 3.2745(10) Å; the average Pd···Pd distance is
3.23(4) Å, which is slightly longer than the metal−metal
distances in [Pd(μ-SnPr)2]8 (3.17 Å)19 and an octanuclear
nickel analog, [Ni(μ-SCH2CO2Et)2]8 (3.05 Å).18 Each
palladium atom is coordinated by four μ2-bridged thiolate
ligands and has an almost square planar geometry. The Pd−S
bond distances are within 2.305(2)−2.367(4) Å (average:
2.32(2) Å). The horizontal and vertical S−Pd−S bond angles
vary between 93.50(14) and 99.21(8)° (average: 97.3(14)°)
and 81.65(8) and 85.35(14)° (average: 83.0(9)°), respectively.
The Pd−S−Pd bond angles are within 85.20(8)−91.70(17)°
The above-mentioned geometrical parameters for 1 were
compared with those of the previously reported hexanuclear
39
palladium complexes [Pd(μ-SCH2CO2Me)2]6 and [Pd(μ-
SnHex)2]6.9 The average distance between the two adjacent
palladium atoms in the hexanuclear complexes is 3.11 Å, which
is approximately 0.1 Å shorter than that in 1. Although the
Figure 1. Molecular structure of complex 1 with an inwardly bent arm (left) and outwardly stretched arm (right) on S1 atom. (upper) Top view.
(lower) Side view. Hydrogen atoms are omitted for clarity. Pd: light green, S: orange, C: gray, O: red.
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1
Figure 2. Variable-temperature H NMR spectra (400 MHz) of 1 in (left) CDCl3, −55−20 °C and (right) CD2Cl2, −85−20 °C.
averages of the horizontal and vertical S−Pd−S bond angles are
similar for these three complexes (97.8(5)° and 82.4(5)° for
[Pd(μ-SCH2CO2Me)2]6; 98.0(5)° and 81.9(5)° for [Pd(μ-
SnHex)2]6), the average Pd−S−Pd bond angle for the
hexanuclear complexes is 84.5°, which is approximately 4°
smaller than that found for 1. The dihedral angles between the
PdS4 planes are also small: 113.47(3)−124.36(4)° for [Pd(μ-
SCH2CO2Me)2]6 and 115.19(6)−126.53(7)° for [Pd(μ-
SnHex)2]6. The distances between the diagonally positioned
palladium atoms in [Pd(μ-SnHex)2]6 are within 6.053−6.461 Å.
The average distance (6.23 Å) is approximately 2.2 Å shorter
than that in 1.
appears at 3.97 ppm is assigned to the methoxy groups in the
axial arms, and that at 3.77 ppm is attributed to the methoxy
groups in the equatorial arms. Because carbonyl carbons (170.4
and 169.8 ppm) have correlations with the axial methoxy
protons (3.97 ppm) and methylene protons (3.34 ppm), as well
as the equatorial methoxy protons (3.77 ppm) and other
methylene protons (3.23 ppm) in the heteronuclear multiple-
bond correlation (HMBC) spectrum measured in CDCl3,
respectively, the singlet at 3.34 ppm is assigned to the
methylene protons in the axial arms, and the other, at 3.23
ppm, is assigned to the methylene protons in the equatorial
arms.
The matrix-assisted laser desorption ionization time-of-flight
mass spectrometry (MALDI-TOF MS) spectrum of 1 revealed
a parent peak at m/z 2556.3 (with Na+, calcd 2556.2) and a
small fragment peak at m/z 2240.4 assignable to [Pd(μ-
SCH2CO2Me)2]7 (with Na+, calcd 2238.3) (Figures S1−S3 in
Alternative Synthesis of 1. Although the conventional
method was successful, complex 1 could also be prepared using
dimethyl [1,3-phenylenebis(1-methylethylidenethio)]diacetate
(2) as a thiolate-ligand source in the absence of a base.
[PdCl2(MeCN)2] was reacted with 1 equiv of 2 in MeOH (0.3
M) at 70 °C (bath temperature). After 10 h, complex 1 was
obtained in high yield (Scheme 2).
1
Supporting Information). The H NMR spectrum of 1 (in
CDCl3) features two methylene signals at 3.34 and 3.23 ppm
and two methoxy signals at 3.97 and 3.77 ppm; these two sets
of signals are attributed to the axial and equatorial
methoxycarbonylmethyl arms, and therefore, the molecular
symmetry of 1 in solution is pseudo-D4d. Such separated NMR
signals of axial and equatorial arms have also been observed in
hexanuclear palladium complexes.9,14,19,41,42 In contrast with
[Pd(μ-SnPr)2]8, which exists in equilibrium with the corre-
sponding hexanuclear complex [Pd(μ-SnPr)2]6,19 1 was stable
in solution (CD3CN) even at 70 °C, and no hexanuclear or
Scheme 2. Alternative Synthesis of 1
1
other polynuclear complexes were observed at all. The H
NMR spectra of 1 in CDCl3 and CD2Cl2 obtained through low-
temperature measurements are shown in Figure 2. In CDCl3,
one of the two methoxy signals appears at 3.97 ppm as a slightly
broad singlet, which is broadened further than the other signal
at 3.77 ppm, upon decreasing the temperature. On the other
hand, in CD2Cl2, both methoxy signals broaden simultaneously.
This difference can be attributed to the fact that CDCl3 cannot
be accommodated in the inner void of 1 but CD2Cl2 can (vide
infra). Instead of the solvent molecule, one of the axial
methoxycarbonylmethyl groups can occupy the inner void in
CDCl3, as already shown in Figure 1. The selective peak
broadening observed in the variable-temperature NMR spectra
in CDCl3 results from the dynamic behavior of the axial arms
moving into and out of the tiara ring. Therefore, the singlet that
To investigate the reaction mechanism, [PdCl2(MeCN)2]
was reacted with 2 in CHCl3 (0.04 M) at room temperature.
After 1 h, SCS-pincer complex 3 formed selectively with 83%
isolated yield; 1 was not formed at this stage (Scheme 3). After
isolation, 3 was heated in CD3CN (0.3 M) at 90 °C (bath
temperature) in the presence of 1 equiv of HCl, resulting in the
formation of 1 in moderate yield along with a small amount of
1,3-diisopropenylbenzene (4). These reactions clearly demon-
strate that tiara-like complex 1 can form via SCS-pincer
complex 3. In contrast, when dimethyl[1,3-phenylenebis-
(methylenethio)]diacetate (5), which does not contain methyl
groups at the benzylic positions, was used instead of 2, higher
temperatures and longer reaction times were required to
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Found: C, 34.11; H, 3.56.).45 Thus, the steric hindrance derived
from the four methyl groups at the benzylic positions in 2
facilitates the aromatic C(sp2)−H bond activation by placing
the palladium atom proximate to the C(sp2)−H bond at the 2-
position. Even at 90 °C, 1 did not form from the mixtures of
either [PdCl2(MeCN)2] and 5 or 6 and HCl (Scheme 4).
A possible mechanism for the formation of complex 1 from
[PdCl2(MeCN)2] and 2 is shown in Scheme 5. After the
Scheme 3. Formation of Tiara-like Complex 1 via SCS-
Pincer Complex 3
Scheme 5. Possible Mechanism for the Formation of 1 from
[PdCl2(MeCN)2] and 2
generate SCS-pincer complex 6 (Scheme 4, Figure 3).44,45
When the same reaction was performed at room temperature, it
Scheme 4. Formation and Reactivity of SCS-Pincer Complex
6
generation of SCS-pincer complex 3 via aromatic C(sp2)−H
bond activation, one of the thiolate groups is eliminated to
provide a tertiary carbocation at the benzylic position with
concurrent dissociation of a chloride ligand from palladium.
This step rationalizes the difference in the reactivity of 3 and 6
because the generation of a carbocation intermediate from 6 is
less favorable. Subsequent deprotonation affords a carbon−
carbon double bond. The protonolysis of the aromatic C(sp2)−
Pd bond by HCl followed by E1 elimination similar to that
described above occurs again to give 4 and a mononuclear
palladium thiolate, [Pd(SCH2CO2Me)2], which then aggregates
to construct a stable tiara-like octanuclear architecture.
Although the above mechanism explains the formation of 1,
the bidentate coordination of 2 to PdCl2 without aromatic
C(sp2)−H bond activation45 and the subsequent stepwise
elimination of the thiolate groups, similar to the above
mechanism, can proceed concurrently to form 1.
For examining whether a small amount of the hexanuclear
complex [Pd(μ-SCH2CO2Me)2]6 is formed through the
conventional method (Scheme 1) or the alternative method
(Scheme 2), the crude reaction mixtures were analyzed by
MALDI-TOF MS. No peak assignable to [Pd(μ-
SCH2CO2Me)2]6 (with Na+, calcd 1921.4) was detected.
Both these methods afforded 1 selectively; therefore, the
predominant formation of the octanuclear complex over a
hexanuclear complex seems to result not from the reaction
conditions but from the structure of the substituents on the
sulfur atoms, that is, the methoxycarbonylmethyl groups, which
can be regarded as a template. In the case of nickel, the
aforementioned [Ni(μ-SCH2CO2Et)2]8 is the sole example of
an octanuclear tiara-like complex, reported by Dance et al.18
They mentioned that the size of the tiara ring can be
determined by the van der Waals volume of the ethoxycarbo-
nylmethyl group, which, similar to that of 1, also bends inward
to occupy the inner void in the crystal structure. Dahl et al.
proposed that macrocyclic [Ni(μ-SPh)2]11 was constructed
Figure 3. Molecular structure of 6. Thermal ellipsoids are shown at a
50% probability level. Hydrogen atoms are omitted for clarity. Selected
bond distances (Å), bond angles (deg), and dihedral angles (deg):
Pd1−C1 = 1.980(2), Pd1−S1 = 2.2967(7), Pd1−S2 = 2.3151(7),
Pd1−Cl1 = 2.3961(7), C1−Pd1−S1 = 83.11(6), C1−Pd1−S2 =
84.46(6), S1−Pd1−Cl1 = 96.27(3), S2−Pd1−Cl1 = 96.12(2), Pd1−
S1−C2 = 99.24(7), Pd1−S1−C4 = 105.61(7), C2−S1−C4 =
104.64(10), Pd1−S2−C3 = 97.67(7), Pd1−S2−C5 = 102.18(7),
C3−S2−C5 = 100.81(11), C1−Pd1−S1−C2 = 25.12(10), C1−Pd1−
S2−C3 = 24.22(10), C1−Pd1−S1−C4 = 133.26(10), C1−Pd1−S2−
C5 = 78.64(10).
produced an insoluble solid in nearly quantitative yield; the
product appeared to be a monomer and/or oligomers of a 1:1
complex of PdCl2 and 5, according to the results of elemental
analyses (Anal. Calcd for C14H18Cl2O4PdS2: C, 34.19; H, 3.69.
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around a THF molecule, which operated as a template during
the growth of a ribbon-like nickel thiolate oligomer.21 On the
other hand, [Pd(μ-SCH2CO2Me)2]6, which is a hexanuclear
analogue of 1, was prepared from PdCl42− and thioglycolic acid
in MeOH.39 In this reaction, we presume that the formation of
the hexanuclear tiara ring with carboxylic acids as substituents
occurred first, and ester condensation between the carboxylic
acid moieties and MeOH followed.
Synthesis and Structure of Tiara-like Octanuclear
Platinum Complex 7. The synthesis of tiara-like platinum
complexes was first examined using PtCl2/methyl thioglyco-
late/DIEA similar to the reaction conditions described in
Scheme 1 (in MeOH, 0.4 M, 80 °C, 10 h). A tiara-like
octanuclear complex, [Pt(μ-SCH2CO2Me)2]8 (7), was formed
in moderate yield (∼60%); however, byproducts, which may
contain tiara-like complexes with different-sized rings as
speculated from the 1H NMR analysis, were formed
concurrently, and the isolation of 7 from the mixture was
unsuccessful. MALDI-TOF MS analysis of the crude reaction
mixture showed small peaks assignable to [Pt(μ-
SCH2CO2Me)2]9 at m/z 3673.3 (with Na+, calcd 3671.2)
and [Pt(μ-SCH2CO2Me)2]10 at m/z 4075.4 (with Na+, calcd
4076.5) in addition to a parent peak assignable to 7 at m/z
3267.7 (with Na+, calcd 3264.7); no peaks corresponding to
tiara-like complexes with smaller rings were detected. In
contrast, when PtCl2 was reacted with 1 equiv of 2 in MeCN (1
M) at 90 °C (bath temperature), 7 was obtained in high
isolated yield with high purity (Scheme 6). The molecular
Scheme 6. Formation of Tiara-like Octanuclear Platinum
Complex 7
Figure 4. Molecular structure of 7. (upper) Top view. (lower) Side
view. A solvent molecule (CHCl3) outside of the tiara ring and
hydrogen atoms are omitted for clarity. Pt: pink, S: orange, C: gray, O:
red.
Pt6 = 8.7539(5) Å, Pt3···Pt7 = 8.3559(5) Å, and Pt4···Pt8 =
8.4346(5) Å, resulting in an ellipsoidal architecture; the average
distance (8.59 Å) is longer than that in 1 by ∼0.15 Å.
structure of 7 was also determined by X-ray crystallography
(Figure 4, recrystallized from CHCl3/octane). In contrast to 1,
the inside of the ring was void, and a solvent molecule (CHCl3)
was located outside the ring. The range of the distances
between two adjacent platinum atoms is 3.2409(4)−3.3345(4)
Å (average: 3.29(3) Å); these distances are slightly longer than
those between two adjacent palladium atoms in 1. The Pt−S
bond distances range within 2.298(4)−2.356(6) Å (average:
2.32(1) Å) and are almost same as the Pd−S bond distances in
1. The horizontal and vertical S−Pt−S bond angles vary within
94.88(19)−101.82(15)° (average: 98.4(16)°) and 79.42(14)−
85.3(2)° (average: 81.8(12)°), respectively, while the Pt−S−Pt
bond angles vary within 88.2(2)−92.33(7)° (average:
90.3(12)°). According to the comparison of the S−M−S and
M−S−M bond angles in 1 and 7 (M = Pd and Pt, respectively),
the tiara ring of 7 is more horizontally elongated than that of 1
as a result of the changes in these angles. The dihedral angles
between the PtS4 planes range within 129.93(13)−142.25(8)°
(average: 135(4)°). The distances between two adjacent sulfur
atoms in the two S8 rings are within the range of 3.409(8)−
3.615(6) Å (average: 3.51(4) Å) and are also slightly longer
than those in 1. The distances between the diagonally
positioned platinum atoms are Pt1···Pt5 = 8.8011(5) Å, Pt2···
The MALDI-TOF MS spectrum of isolated 7 reveals a
parent peak at m/z 3265.7 as well as peaks at m/z 3191.7 and
3118.6 (Figures S4−S7 in Supporting Information), which are
assigned to 7 with one or two methoxycarbonylmethyl
group(s) missing, respectively (with Na+, calcd 3190.7 and
1
3117.7). The H NMR spectrum of 7 (in CDCl3) reveals two
methylene signals at 3.53 and 3.42 ppm and two methoxy
signals at 3.96 and 3.76 ppm, which appeared in the same
fashion as those in 1. The variable-temperature measurements
of 7 in CDCl3 and CD2Cl2 indicated that the dynamic behavior
of the axial arms was also similar to that observed in 1 (Figure
5), whereas no inward bend of the arms was observed in the
crystal structure of 7. Compared with 1, the axial methoxy
signal at 3.96 ppm was more broadened at low temperatures in
CDCl3. The HMBC spectrum of 7 measured in CDCl3 also
revealed correlations between carbonyl carbons (169.9 and
170.1 ppm), the axial methoxy protons (3.96 ppm), and
methylene protons (3.53 ppm), as well as the equatorial
methoxy protons (3.76 ppm) and other methylene protons
(3.42 ppm); therefore, the singlet at 3.53 ppm is assignable to
the methylene protons in the axial arms, and the other singlet at
3.42 ppm is assignable to the methylene protons in the
equatorial arms. These observations indicate that the
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1
Figure 5. Variable-temperature H NMR spectra (400 MHz) of 7 in (left) CDCl3, −55−20 °C and (right) CD2Cl2, −85−20 °C.
methoxycarbonylmethyl groups should operate as a template
during the formation of 7 in a manner similar to that of 1.
A more dilute reaction (0.04 M) resulted in the formation of
SCS-pincer complex 8 (15%) in addition to 7 (57%). After
isolation, 8 was heated to 90 °C in CD3CN (0.4 M) in the
presence of 1 equiv of HCl; however, 7 did not form, thereby
this reaction differs from the corresponding reaction involving
palladium (Scheme 7). These results indicate that the
Scheme 7. Reaction of SCS-Pincer Complex 8 with HCl
Figure 6. Molecular structures of 1·CH2Cl2, 1·ClCH2CH2Cl, 1·
CH2Br2, and 1·CH3I. (upper) Top views. (lower) Side views.
Methoxycarbonylmethyl groups and solvent molecules outside of the
tiara rings are omitted for clarity. The guest molecules are depicted in a
space-filling model. Pd: light green, S: orange, Cl: green, Br: brown, I:
purple, C: gray, H: white.
formation of 7 does not proceed via SCS-pincer complex 8
but rather through monomeric and/or oligomeric 1:1
complexes of PtCl2 and 2. The plausible formation mechanism
of 7 from the 1:1 complexes is analogous to the mechanism
described for the palladium complex above.
A trial for the formation of tiara-like palladium/platinum
mixed complexes was performed through the reaction of 1 with
7 in CD3CN at 70 °C. However, these complexes remained
intact even after 48 h, and mixed complexes were not obtained,
as confirmed by the 1H NMR and MALDI-TOF MS
measurements.
Inclusion Complexes with Dihalo- or Iodoalkanes. The
diameters of the inner voids of 1 and 7 are ∼5.1−5.2 Å, taking
into account the van der Waals radii of palladium (1.63 Å) and
platinum (1.75 Å).46 We found that the recrystallization of 1
and 7 from the mixtures of good solvents, such as CH2Cl2,
ClCH2CH2Cl, CH2Br2, and CH3I, and poor solvents, such as
hexane and Et2O, gave the corresponding inclusion complexes,
that is, 1·CH2Cl2, 1·ClCH2CH2Cl, 1·CH2Br2, 1·CH3I, 7·
CH2Cl2, 7·ClCH2CH2Cl, and, 7·CH2Br2, respectively. The
top and side views of the molecular structures of these
complexes are shown in Figures 6 and 7. The guest molecules
are disordered in these inclusion complexes, except for that in
1·CH2Cl2; only the configurations with highest occupancy are
depicted. In 1·CH2Cl2 and 1·CH2Br2, one of the two C−X
Figure 7. Molecular structures of 7·CH2Cl2, 7·ClCH2CH2Cl, and 7·
CH2Br2. (upper) Top views. (lower) Side views. Methoxycarbonyl-
methyl groups and solvent molecules outside of the tiara rings are
omitted for clarity. The guest molecules are depicted in a space-filling
model. Pt: pink, S: orange, Cl: green, Br: brown, C: gray, H: white.
bonds (X = Cl, Br) is arranged vertically, and the other C−X
bond is nearly horizontal. In contrast, in 7·CH2Cl2 and 7·
CH2Br2, the guest molecules themselves are oriented more
vertically. The ClCH2CH2Cl molecules in 1·ClCH2CH2Cl and
7·ClCH2CH2Cl are also vertically oriented. In 1·CH3I, the
CH3I molecule maintains a relatively horizontal position. The
distances between the diagonally positioned metal atoms,
average distances, and oblateness values, which is defined as 1
− (short radius/long radius), are summarized in Table 1. The
M1···M5 and M3···M7 distances correspond to the lengths of
the major and minor axes of the ellipsoidal architectures,
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Table 1. Diagonally Positioned Metal···Metal Distances (Å), Average Distances (Å), and Oblateness Values for 1, 7, and Their
Inclusion Complexes
complex
M1···M5
M2···M6
M3···M7
M4···M8
average
oblateness
1
8.7191(9)
8.7614(9)
8.5720(19)
8.7857(13)
8.7466(6)
8.8011(5)
8.7880(23)
8.7416(23)
8.7935(23)
8.2331(9)
8.5509(9)
8.5711(15)
8.4094(13)
8.4364(6)
8.7539(5)
8.6775(28)
8.7017(28)
8.5500(20)
8.2307(8)
8.0743(9)
8.3447(18)
8.0890(13)
8.1490(6)
8.3559(5)
8.3819(19)
8.3991(20)
8.4057(20)
8.5895(8)
8.4117(9)
8.3465(16)
8.5747(12)
8.5861(6)
8.4346(5)
8.5212(19)
8.5102(22)
8.6355(29)
8.44
8.45
8.46
8.46
8.48
8.59
8.59
8.59
8.60
0.056
0.078
0.027
0.079
0.068
0.051
0.046
0.039
0.044
1·CH2Cl2
1·ClCH2CH2Cl
1·CH2Br2
1·CH3I
7
7·CH2Cl2
7·ClCH2CH2Cl
7·CH2Br2
respectively. In the palladium complexes, the toroidal
architecture flexes to accommodate the guest molecules, as is
evident from the distances mentioned above (major axis: 8.57−
8.79 Å and minor axis: 8.07−8.34 Å), which is similar to the
behavior reported for [Ni(μ-StBu)(μ-mtet)]10 (mtet = 2-
methylthioethanethiolate).20 In contrast, the platinum com-
plexes did not undergo any significant change in shape in the
presence of the guest molecules (major and minor axes lengths
of 8.74−8.79 Å and 8.38−8.41 Å, respectively). The oblateness
also indicates the difference in the flexibilities of 1 and 7.
Among these complexes, the architecture of 1·ClCH2CH2Cl is
the closest to a true circle, whereas 1·CH2Cl2 and 1·CH2Br2
have the most ellipsoidal structures.
Table 2. Ranges of CH···X Distances (X = Cl, Br) (Å) and
Average Distances (Å) in 1·CH2Cl2, 1·ClCH2CH2Cl, 1·
CH2Br2, 7·CH2Cl2, 7·ClCH2CH2Cl, and 7·CH2Br2
complex
1·CH2Cl2
1·ClCH2CH2Cl
1·CH2Br2
7·CH2Cl2
7·ClCH2CH2Cl
7·CH2Br2
range of CH···X distances
average
2.50−2.99
2.69−3.01
2.47−3.18
2.55−3.19
2.70−2.96
2.86−3.18
2.79(22)
2.87(9)
2.8(4)
2.93(18)
2.83(9)
2.98(11)
and 0.3 Å, respectively; this indicates weak coordination of the
halogen atoms to the palladium atoms.
As shown in Figure 8, the weak CH···Cl hydrogen bonds
between the methylene protons of the four axially positioned
CONCLUSION
■
We synthesized a tiara-like octanuclear palladium complex via a
conventional method, which involved a thiol and base, and via
an alternative method using dimethyl [1,3-phenylenebis(1-
methylethylidenethio)]diacetate. In the latter method, the
resultant tiara-like complex formed via the corresponding
SCS−pincer complex and/or 1:1 complexes of PdCl2 and the
ligand. The alternative method is effective for the synthesis of a
highly pure tiara-like octanuclear platinum complex. Both of
these tiara-like complexes had an ellipsoidal architecture; the
platinum tiara ring was more horizontally elongated than the
palladium tiara ring because of the differences in the S−M−S
and M−S−M bond angles, whereas the M−S bond distances
were almost equivalent in the palladium and platinum
complexes. In CDCl3, these complexes showed the dynamic
behaviors in which the axial arms would move into and out of
the tiara ring, whereas no such behaviors were observed in
CD2Cl2. The inclusion of dihalo- or iodoalkanes into the tiara-
like complexes was also successful. The toroidal architecture
flexed to accommodate the guest molecules in the palladium
inclusion complexes, whereas no significant change in the shape
occurred upon the inclusion of guest molecules in the platinum
inclusion complexes. Weak CH···X hydrogen bonding (X = Cl,
Br) as well as weak coordination of the halogen atoms to
palladium atoms stabilize these inclusion structures. Other
small molecules would also be expected to be accommodated in
the inner void of the tiara rings, and some of them may have
much stronger interactions with transition-metal atoms,
resulting in a unique behavior and reactivity.
Figure 8. CH···Cl hydrogen bonds in 1·CH2Cl2 (top view). Only the
tiara ring, four axial methoxycarbonylmethyl groups (two disordered),
and a CH2Cl2 molecule are shown. The numbers represent the H···Cl
atomic distances (Å). Pd: light green, S: orange, Cl: green, C: gray, O:
red, H: white.
methoxycarbonylmethyl groups and the chlorine atom of
CH2Cl2 in 1·CH2Cl2 stabilize the inclusion structure. The H···
Cl atomic distances (2.50−2.99 Å) are typical for this type of
hydrogen bonding.47,48 Weak CH···X hydrogen bonds (X = Cl,
Br) were observed not only in 1·CH2Cl2 but also in other
dihaloalkane inclusion complexes. The ranges of the corre-
sponding atomic distances and average distances are
summarized in Table 2. The shortest distances between the
halogen atoms of the guest molecules and palladium atoms in
1·CH2Cl2 (3.249(5) Å), 1·CH2Br2 (3.187(3) Å), and 1·CH3I
(3.3214(10) Å) are slightly shorter than the sum of the van der
Waals radii of palladium (1.63 Å) and the corresponding
halogen atoms (Cl: 1.75 Å, Br: 1.85 Å, I: 1.98 Å) by ∼0.1, 0.3,
EXPERIMENTAL SECTION
■
Materials and Methods. All manipulations were performed under
an argon atmosphere using standard Schlenk techniques. Dry solvents
were purchased from either Wako Chemical or Nacalai. Flash column
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chromatography was performed using silica gel SiliCycle SiliaFlash F60
(40−63 μm, 230−400 mesh).
added, and the reaction mixture was stirred at room temperature for
15 min. α,α′-Dibromo-m-xylene (335 mg, 1.27 mmol) was then added,
and the mixture was stirred at 70 °C for 10 h. After drying under
vacuum, the obtained crude compound was purified by silica gel
column chromatography (eluent: hexane/ethyl acetate = 3:1) to give 5
(392 mg, 1.25 mmol) in 98% isolated yield as a colorless oil. 1H NMR
(300 MHz, CDCl3) δ 7.31−7.21 (m, 4H, Ar-2,4,5,6-H), 3.81 (s, 4H,
ArCH2), 3.72 (s, 6H, CO2CH3), 3.08 (s, 4H, CH2CO2Me). 13C NMR
(75 MHz, CDCl3) δ 170.5 (CO2Me), 137.3 (Ar-1,3), 129.6 (Ar-5),
128.5 (Ar-2), 127.9 (Ar-4,6), 52.1 (CO2CH3), 35.9 (ArCH2), 31.8
(CH2CO2Me).
Synthesis of SCS-Pincer Palladium Complex 6. To a mixture of
[PdCl2(MeCN)2] (15 mg, 0.059 mmol) and 5 (19 mg, 0.060 mmol),
dry acetonitrile (1.5 mL) was added, and the reaction mixture was
stirred at 90 °C (bath temperature) for 20 h. The pale yellow solution
was vacuumed to dry. Reprecipitation from CHCl3/hexane gave a pale
yellow solid (15 mg, 0.033 mmol) in 57% isolated yield.
Recrystallization from CHCl3/hexane gave pale yellow block crystals.
1H NMR (300 MHz, CDCl3) δ 7.04−6.94 (m, 3H, Ar-3,4,5-H), 4.60−
3.90 (br, 8H, CH2), 3.79 (s, 6H, CO2CH3). 13C NMR (100 MHz,
CDCl3) δ 167.6 (CO2Me), 158.5 (br, Ar-1), 147.6 (br, Ar-2,6), 125.2
(Ar-4), 122.7 (Ar-3,5), 53.1 (CO2CH3), 48.5 (br, ArCH2), 39.9 (br,
CH2CO2Me). IR (νCO, KBr): 1734 cm−1. Mp 112−113 °C (dec).
Anal. Calcd for C14H17ClO4PdS2: C, 36.93; H, 3.76. Found: C, 36.58;
H, 3.99.
Physical and Analytical Measurements. NMR spectra were
recorded on either a JEOL AL-400 (400 MHz (1H), 100 MHz (13C),
85.7 MHz (195Pt)) or a Bruker AV-300N (300 MHz (1H), 75 MHz
1
(13C)) spectrometer. Chemical shift values (δ) in H and 13C NMR
spectra were expressed relative to SiMe4. 195Pt NMR spectra were
referenced using an external reference (0.3 M K2PtCl4 in D2O, δ =
−1628 ppm). Infrared (IR) measurements were carried out using a
JASCO FT/IR-6100 spectrometer. Elemental analysis was obtained
using a J-Science Lab JM-10 analyzer. MALDI-TOF MS spectra were
recorded on a Shimadzu Axima Performance spectrometer. Melting
points were measured on a Yanagimoto micro melting point apparatus.
Synthesis of [Pd(μ-SCH2CO2Me)2]8 (1). To a mixture of methyl
thioglycolate (51 μL, 0.57 mmol), DIEA (96 μL, 0.56 mmol), and n-
propanol (0.95 mL), PdCl2 (50 mg, 0.28 mmol) was added, and the
reaction mixture was stirred at room temperature for 4 h. After
filtration, the orange-yellow solid was dried under vacuum and
obtained in 90% isolated yield (81 mg, 0.032 mmol). Recrystallization
1
from CHCl3/hexane gave yellow block crystals. H NMR (400 MHz,
CDCl3) δ 3.97 (s, 24H, ax-CO2CH3), 3.77 (s, 24H, eq-CO2CH3), 3.34
(s, 16H, ax-CH2), 3.23 (s, 16H, eq-CH2). 1H NMR (400 MHz,
CD2Cl2) δ 3.75 (s, 24H, CO2CH3), 3.72 (s, 24H, CO2CH3), 3.38 (s,
16H, CH2), 3.22 (s, 16H, CH2). 13C NMR (75 MHz, CDCl3) δ 170.4
(ax-CO2Me), 169.8 (eq-CO2Me), 53.1 (ax-CO2CH3), 52.7 (eq-
CO2CH3), 35.6 (eq-CH2), 30.6 (ax-CH2). IR (νCO, KBr): 1735
cm−1. Mp 143−144 °C (dec.). Anal. Calcd for C48H80O32Pd8S16: C,
22.76; H, 3.18. Found: C, 22.65; H, 3.02.
Synthesis of Dimethyl [1, 3-Phenylenebis(1-
methylethylidenethio)]diacetate (2). Compound 2 was prepared
by reference to a reported synthetic procedure for similar
compounds.49 To a mixture of α,α′-dihydroxy-1,3-diisopropylbenzene
(776 mg, 3.99 mmol), ZnI2 (1.28 g, 4.01 mmol), and methyl
thioglycolate (0.750 mL, 8.39 mmol), dry 1,2-dichloroethane (25 mL)
was added, and the reaction mixture was stirred at room temperature
for 1 h. The mixture was diluted with dichloromethane and washed
with water. The organic layer was then washed with 1 M NaOH and
dried over MgSO4. After filtration and removal of the solvents, the
residue was purified by silica gel column chromatography (eluent:
hexane/ethyl acetate = 3:1) to give 2 (1.42 g, 3.82 mmol) in 96%
isolated yield as a colorless oil. 1H NMR (300 MHz, CDCl3) δ 7.71 (t,
J = 1.5 Hz, 1H, Ar-2-H), 7.37 (dd, J = 6.6, 1.5 Hz, 2H, Ar-4,6-H), 7.28
(t, J = 6.9 Hz, 1H, Ar-5-H), 3.59 (s, 6H, CO2CH3), 2.98 (s, 4H, CH2),
1.73 (s, 12H, CH3). 13C NMR (75 MHz, CDCl3) δ 171.0 (CO2Me),
145.2 (Ar-1,3), 127.9 (Ar-5), 125.1 (Ar-4,6), 125.0 (Ar-2), 52.3
(CO2CH3), 48.6 (C(CH3)2), 32.1 (SCH2), 29.9 (C(CH3)2).
Alternative Synthesis of Complex 1. To a mixture of
[PdCl2(MeCN)2] (78 mg, 0.30 mmol) and 2 (112 mg, 0.30 mmol),
dry methanol (1.0 mL) was added, and the reaction mixture was
heated at 70 °C (bath temperature) for 10 h. After filtration, the
orange-yellow precipitate was dried under vacuum and obtained in
84% isolated yield (80 mg, 0.031 mmol). Recrystallization from
CHCl3/hexane gave yellow block crystals.
Synthesis of SCS-Pincer Palladium Complex 3. To a mixture of
[PdCl2(MeCN)2] (1.00 g, 3.86 mmol) and 2 (1.43 g, 3.86 mmol), dry
chloroform (100 mL) was added, and the reaction mixture was stirred
at room temperature for 1 h. The obtained crude compound was
purified by silica gel column chromatography (eluent: hexane/ethyl
acetate = 1:2) to give 3 as a pale orange-yellow solid (1.64 g, 3.20
mmol) in 83% isolated yield. 1H NMR (300 MHz, CDCl3) δ 7.11 (t, J
= 7.8 Hz, 1H, Ar-4-H), 6.80 (d, J = 7.5 Hz, 2H, Ar-3,5-H), 4.50−3.85
(br, 4H, CH2), 3.76 (s, 6H, CO2CH3), 1.81 (s, 12H, CH3). 13C NMR
(100 MHz, CDCl3) δ 167.1 (CO2Me), 154.1 (Ar-2,6), 152.7 (br, Ar-
1), 125.0 (Ar-4), 123.1 (Ar-3,5), 63.4 (C(CH3)2), 52.3 (CO2CH3),
35.7 (SCH2), 31.1 (br, CH3), 29.4 (br, CH3). IR (νCO, KBr): 1739
cm−1. Mp 119−121 °C (dec.). Anal. Calcd for C18H25ClO4PdS2: C,
42.28; H, 4.93. Found: C, 42.31; H, 4.85.
Synthesis of [Pt(μ-SCH2CO2Me)2]8 (7). To a mixture of PtCl2
(662 mg, 2.49 mmol) and 2 (922 mg, 2.49 mmol), dry acetonitrile
(1.25 mL) was added, and the reaction mixture was stirred at 90 °C
(bath temperature) for 10 h. The yellow solution was evaporated
under vacuum to dry. Recrystallization from CHCl3/heptane gave
1
yellow block crystals (950 mg, 0.293 mmol) in 94% isolated yield. H
NMR (400 MHz, CDCl3) δ 3.96 (s, 24H, ax-CO2CH3), 3.76 (s, 24H,
eq-CO2CH3), 3.53 (s, 16H, ax-CH2), 3.42 (s, 16H, eq-CH2). 1H NMR
(400 MHz, CD2Cl2) δ 3.75 (s, 24H, CO2CH3), 3.70 (s, 24H,
CO2CH3), 3.60 (s, 16H, CH2), 3.40 (s, 16H, CH2). 13C NMR (75
MHz, CDCl3) δ 170.1 (eq-CO2Me), 169.9 (ax-CO2Me), 52.9 (ax-
CO2CH3), 52.6 (eq-CO2CH3), 37.1 (eq-CH2), 30.2 (ax-CH2). 195Pt
NMR (85.7 MHz, CDCl3) δ −3247.3 (br). IR (νCO, KBr): 1733 cm−1.
Mp 178−180 °C (dec). Anal. Calcd for C48H80O32Pt8S16: C, 17.78; H,
2.49. Found: C, 18.11; H, 2.55.
SCS-Pincer Platinum Complex 8. To a mixture of PtCl2 (369
mg, 1.39 mmol) and 2 (513 mg, 1.39 mmol), dry acetonitrile (36 mL)
was added, and the reaction mixture was stirred at 90 °C (bath
temperature) for 20 h. The yellow solution was evaporated under
vacuum to dry. The resulting yellow oil was dissolved in CHCl3, and
hexane was poured into the solution to remove 7. After filtration, the
filtrate was dried under vacuum, and the obtained crude compound
was purified by silica gel column chromatography (eluent: hexane/
ethyl acetate = 1:1) to give 8 (124 mg, 0.206 mmol) in 15% isolated
yield as a pale yellow solid. 1H NMR (300 MHz, CD3CN) δ 7.13 (t, J
= 7.8 Hz, 1H, Ar-4-H), 6.86 (d, J = 7.5 Hz, 2H, Ar-3,5-H), 4.08−3.73
(m, 4H, CH2), 3.67 (s, 6H, CO2CH3), 1.75 (s, 12H, CH3). 13C NMR
3
(100 MHz, CD3CN) δ 168.3 (t, JC−Pt = 23.8 Hz, CO2Me), 153.8 (t,
2JC−Pt = 28.8 Hz, Ar-2,6), 145.1 (Ar-1), 144.6 (Ar-1), 125.4 (Ar-4),
123.8 (t, 3JC−Pt = 23.2 Hz, Ar-3,5), 67.8 (t, 2JC−Pt = 18.7 Hz, C(CH3)2),
53.4 (CO2CH3), 38.6 (SCH2), 37.9 (SCH2), 32.0 (CH3), 31.3 (CH3),
29.4 (CH3), 28.6 (CH3). 195Pt NMR (85.7 MHz, CD3CN) δ −4026.5
(quintet, 3JPt−H = 42 Hz), −4028.2 (quintet, 3JPt−H = 38 Hz). IR (νCO
,
KBr): 1736 cm−1. Mp 131.5−132.5 °C (dec). Anal. Calcd for
C18H25ClO4PdS2: C, 36.03; H, 4.20. Found: C, 36.11; H, 4.11.
Crystallographic Study of Complexes 1, 6, 7, 1·CH2Cl2, 1·
ClCH2CH2Cl, 1·CH2Br2, 1·CH3I, 7·CH2Cl2, 7·ClCH2CH2Cl, and 7·
CH2Br2. Crystals suitable for X-ray diffraction measurements were
obtained by recrystallization from CHCl3/hexane for 1 (yellow block
crystals), CH2Cl2/hexane for 6 (pale yellow block crystals), CHCl3/
octane for 7 (yellow block crystals), CH2Cl2/Et2O for 1·CH2Cl2
(yellow prism crystals), ClCH2CH2Cl/hexane for 1·ClCH2CH2Cl
(yellow rod crystals), CH2Br2/hexane for 1·CH2Br2 (yellow rod
crystals), CH3I/Et2O for 1·CH3I (yellow block crystals), CH2Cl2/
hexane for 7·CH2Cl2 (yellow block crystals), ClCH2CH2Cl/Et2O for
Synthesis of Dimethyl [1,3-Phenylenebis(methylenethio)]-
diacetate (5). To a mixture of methyl thioglycolate (240 μL, 2.68
mmol) and DIEA (455 μL, 2.68 mmol), dry methanol (10 mL) was
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7·ClCH2CH2Cl (yellow block crystals), and CH2Br2/hexane for 7·
CH2Br2 (yellow cube crystals). The crystals dipped in liquid paraffin
were mounted using a cryoloop and frozen at either 153 or 123 K. The
diffraction data were collected with a Rigaku Saturn CCD detector
(Mo Kα, λ = 0.71073 Å). Crystal data and structure refinement
parameters are listed in Tables S1−S3 in Supporting Information. The
structures were solved by direct methods using either SHELXS-201350
(for 1, 1·CH2Cl2, 1·ClCH2CH2Cl, 1·CH2Br2, and 1·CH3I), SHELXS-
9751 (for 6), or SIR9752 (for 7, 7·CH2Cl2, 7·ClCH2CH2Cl, and 7·
CH2Br2) and refined by least-squares on F2, SHELXL-2013, or
SHELXL-97 (for 6).50,51,53 Non-hydrogen atoms were anisotropically
refined except for the disordered ones. Because methoxycarbonyl-
methyl groups and small organic molecules were disordered
occasionally, bond distances in these moieties were restrained to be
equal using a SADI command in SHELXL. All hydrogen atoms were
placed at calculated positions. Refinements were continued until all
shifts were smaller than one-tenth of the standard deviations of the
parameters involved. Atomic scattering factors and anomalous
dispersion terms were taken from the International Tables for X-ray
Crystallography.54
MALDI-TOF MS Analysis of Tiara-like Complexes 1 and 7.
Acetonitrile solutions of the tiara-like complexes (0.5 μL; 1, 9.9 mM
and 7, 7.9 mM) were mixed with a H2O/ethanol (1:1) solution of
trans-retinoic acid (0.5 μL, 50.0 mM) and aqueous trifluoroacetic acid
(aq TFA) (0.5 μL, 0.1%) on a MALDI plate and were analyzed.
Calibration was performed using bradykinin fragment 1−7 in 0.1% aq
TFA (10 μM), adrenocorticotropic hormone (ACTH) fragment 18−
39 (human) in 0.1% aq TFA (10 μM), and α-cyano-4-hydroxycin-
namic acid (4-CHCA).
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ASSOCIATED CONTENT
* Supporting Information
■
S
NMR spectra for new compounds, MALDI-TOF MS spectra
for 1 and 7, crystal data and structure refinement parameters for
1, 6, 7, 1·CH2Cl2, 1·ClCH2CH2Cl, 1·CH2Br2, 1·CH3I, 7·
CH2Cl2, 7·ClCH2CH2Cl, and 7·CH2Br2, and CIF files for 1, 6,
7, 1·CH2Cl2, 1·ClCH2CH2Cl, 1·CH2Br2, 1·CH3I, 7·CH2Cl2, 7·
ClCH2CH2Cl, and 7·CH2Br2. CCDC reference numbers
974025 (1), 974026 (6), 974027 (7), 974058 (1·CH2Cl2),
974059 (1·ClCH2CH2Cl), 974060 (1·CH2Br2), 976482 (1·
CH3I), 976483 (7·CH2Cl2), 976484 (7·ClCH2CH2Cl), and
976485 (7·CH2Br2). This material is available free of charge via
the Internet at http://pubs.acs.org.
Sheng, T.; Wu, X. Dalton Trans. 2012, 41, 8472.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: ura@cc.nara-wu.ac.jp.
■
Notes
(32) Mahmoudkhani, A. H.; Langer, V. Polyhedron 1999, 18, 3407.
(33) Capdevila, M.; Gonzalez-Duarte, P.; Sola, J.; Foces-Foces, C.;
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was partly supported by Grant-in-Aid for Scientific
Research on Innovative Areas (Molecular Activation Directed
toward Straightforward Synthesis) from The Ministry of
Education, Culture, Sports, Science and Technology
(MEXT), Japan. We thank Prof. Takashi Kajiwara (Nara
Women’s University) for supporting our X-ray crystallography.
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