Can a meso-type dinuclear complex be chiral?: Dinuclear β-diketonato Ru(III) complexes

Article abstract of DOI:10.1039/c1dt11133g

Dinuclear Ru(III) complexes, [Ru(III)(acac)₂(dabe)Ru(III)(acac) ₂] (acacH = acetylacetone; dabeH₂ = 1, 2-diacetyl-1,2-dibenzoylethane) and [Ru(III)(acac)₂(tbet)Ru(III) (acac)₂] (tbetH₂ = 1,1,2,2-tetrabenzoylethane) were synthesized by reacting [Ru(acac)₂(CH₃CN) ₂]PF₆ with dabeH₂ and tbetH₂ respectively, in toluene. The X-ray structural analysis of a meso-type dinuclear Ru(III) complex, ΔΛ-[Ru(III)(acac)₂(dabe)Ru(III)(acac) ₂], showed that the bridging part became chiral due to the orthogonal twisting of two non-symmetrical β-diketonato moieties. To confirm this conclusion, the complex was resolved chromatographically to provide a pair of optical antipodes. Such chirality in the bridging part was not generated for [Ru(III)(acac)₂(tbet)Ru(III)(acac)₂], because the β-diketonato moieties in tbet^2- are symmetrical.

Full text of DOI:10.1039/c1dt11133g

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PAPER
Can a meso-type dinuclear complex be chiral?: dinuclear b-diketonato Ru(III)
complexes†
Hisako Sato,*a Ryoichi Takase,b Yukie Moric and Akihiko Yamagishi*b
Received 16th June 2011, Accepted 27th July 2011
DOI: 10.1039/c1dt11133g
Dinuclear Ru(III) complexes, [Ru(III)(acac)2(dabe)Ru(III)(acac)2] (acacH = acetylacetone; dabeH2 = 1,
2-diacetyl-1,2-dibenzoylethane) and [Ru(III)(acac)2(tbet)Ru(III)(acac)2] (tbetH2 = 1,1,2,2-tetrabenzoyl-
ethane) were synthesized by reacting [Ru(acac)2(CH3CN)2]PF6 with dabeH2 and tbetH2 respectively, in
toluene. The X-ray structural analysis of a meso-type dinuclear Ru(III) complex, DK–[Ru(III)(acac)2-
(dabe)Ru(III)(acac)2], showed that the bridging part became chiral due to the orthogonal twisting of
two non-symmetrical b-diketonato moieties. To confirm this conclusion, the complex was resolved
chromatographically to provide a pair of optical antipodes. Such chirality in the bridging part was not
generated for [Ru(III)(acac)2(tbet)Ru(III)(acac)2], because the b-diketonato moieties in tbet2- are
symmetrical.
bination thereof causes the diastereomers, which exhibit different
spectroscopic properties. Now the question comes: Can chirality
be induced in the bridging part? And if so, does the chirality affect
any properties (e.g. dichroic properties and molecular recognition
behaviour) of the multi-nuclear complexes? Recently dinuclear
Ru(III) complexes, [Ru(III)(acac)2(taet)Ru(III)(acac)2] (taetH2 =
1,1,2,2-tetraacetylethane), were synthesized by Hashimoto et al.
and their stereochemical effects on electronic and electrochemical
Introduction
The chemistry of multi-nuclear metal complexes plays a significant
role in metal-ion directed self-assembly processes.1–3 Multi-nuclear
complexes with various shapes (e.g. boxes and helicates) can be
constructed and spatial arrangement of metal centres therein can
be controlled on the basis of well-defined geometries of coordina-
tion spheres. In these architectures, N-coordination entities such as
properties were studied.6 We extended this work to the syntheses
of trinuclear and tetranuclear Ru(III) complexes bridged by the
same taet ligands.8,9 In these complexes, tris-b-diketonato parts
have DK-chirality, while the bridging parts are achiral since the
orthogonal twisting of symmetrical b-diketonato moieties causes
D2d symmetry.
derivatized porphyrins and polypyridyl ligands are widely used for
homo- and hetero-metallic connections. In contrast, less attention
is paid to O-coordinating b-diketonato bridges in spite of the fact
that tris- or bis-b-diketonato complexes of transition metals have
unique electronic and magnetic properties.3–10
We have studied chirality effects on the multi-nuclear complexes
consisting of b-diketonato moieties.7–9 In these multi-nuclear
complexes, each tris-chelated centre has DK chirality, and a com-
In this paper, we report the chiral induction of a bridg-
ing part by use of a non-symmetrical b-diketonato ligand,
for which the twisting of two b-diketonato planes causes C2
symmetry. As the precursor of such a ligand, 1,2-diacetyl-1,2-
dibenzoylethane (dabeH2), was used (Chart 1(a)). The meso-type
and racemic diastereomers of dinuclear complex, DK- and DD-
(or KK)-[Ru(III)(acac)2(dabe)Ru(III)(acac)2], were obtained and
aGraduated School of Science and Engineering, Ehime University, Mat-
suyama, Ehime, 790-8577, Japan. E-mail: sato.hisako.my@ehime-u.ac.jp;
Fax: +81-89-927-9599; Tel: +81-89-927-9599
bDepartment of Chemistry, Toho University, Funabashi, Chiba, 274-8510,
Japan
1
cDepartment of Chemistry, Ochanomizu University, Bunkyo-ku, Tokyo, 112-
8610, Japan
their stereochemical properties were investigated by H NMR,
mass spectrometry, X-ray crystallography and electronic and
vibrational circular dichroism spectra. These results are compared
with the dinuclear complex with a symmetrical bridging ligand,
1,1,2,2-tetrabenzoylethane (tbetH2) (Chart 1(b)).
† Electronic supplementary information (ESI) available: The re-
sults of chromatographic resolution of [Ru(III)(acac)2(S- or R-
dabe)Ru(III)(acac)2)] and [Ru(III)(acac)2(tbet)Ru(III)(acac)2)]; mass
spectra and 1H NMR of [Ru(acac)2(CH3(CN)2]PF6, dabeH2 and
tbetH2; 1H NMR data of the isomers of [Ru(III)(acac)2(S- or R-
dabe)Ru(III)(acac)2] and [Ru(III)(acac)2(tbet)Ru(III)(acac)2]; chiral chro-
matographic resolution of [Ru(III)(acac)2(S- or R-dabe)Ru(III)(acac)2]
and [Ru(III)(acac)2(tbet)Ru(III)(acac)2]; CD, UV and VCD of DD-
and KK-[Ru(III)(acac)2(tbet)Ru(III)(acac)2]; UV-vis spectra of DK-
[Ru(III)(acac)2(S- or R-dabe))Ru(III)(acac)2] and DFT calculated results
of DK-[Ru(III)(acac)2(S-dabe)Ru(III)(acac)2]). CCDC reference number
819545. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1dt11133g
Results and discussion
[Ru(III)(acac)2(dabe)Ru(III)(acac)2]
[Ru(III)(acac)2(CH3CN)2]PF6 and dabeH2 were reacted in toluene
(Experimental section). When the reactant was eluted on a HPLC
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If the rotation of the b-diketonato planes around the
Ca–Ca¢ bond is prohibited, the enantiomers of F1, DK-
[Ru(III)(acac)2(S-dabe)Ru(III)(acac)2] and DK-[Ru(III)(acac)2(R-
dabe)Ru(III)(acac)2], are stable against racemization in a solution.
In order to examine the possibility of such interlocking of the
two halves of the molecule, it was attempted to optically resolve
a racemic mixture of meso-type dimers chromatographically. A
methanol solution of F1 was mounted on a chiral column packed
with D-[Ru(II)(phen)3]2+/synthetic hectorite13 and eluted with
1 : 1(v/v) methanol/chloroform mixture. As a result, two peaks
with the same area were obtained (see ESI†). As shown in Fig. 2(a),
the more and less retained fractions gave mirror-imaged electronic
circular dichroism (ECD) spectra. Chromatographic resolution
was also obtained for racemic-type dimers, F2 and F3 (see ESI†).
The comparison of their ECD spectra (Fig. 2(b) and (c)) with
that of [Ru(acac)3] allowed us to determine the configuration of
the Ru(III) centres. In the case of F2, the more and less retained
fractions were DD-[Ru(III)(acac)2(S- or R-dabe)Ru(III)(acac)2]
and KK-[Ru(III)(acac)2(R- or S-dabe)Ru(III)(acac)2], respec-
tively. In the case of F3, the more and less retained frac-
tions were KK-[Ru(III)(acac)2(S- or R-dabe)Ru(III)(acac)2] and
DD-[Ru(III)(acac)2(R- or S-dabe)Ru(III)(acac)2], respectively. It
should be noted that F1 gave much weaker ECD peaks than F2
and F3. This was reasonable, since the chirality due to the DK
isomerism of the two tris-b-diketonato parts was cancelled out,
leaving the chirality of the bridging part alone for F1.
Chart 1 Structures of (a) 1,2-diacetyl-1,2-dibenzoylethane (dabeH2),
(b) 1,1,2,2-tetrabenzoylethane (tbetH2) and (c) DK-[Ru(III)(acac)2-
(dabe)Ru(III)(acac)2]
column packed with silica gel, three peaks (denoted as F1, F2 and
F3, respectively) were separated (see ESI†). All of the peaks gave
the molecular weight of a dimeric species (m/z = 919 (obs.); 918.9
1
(calc.)). From the H NMR spectra of these fractions in CDCl3,
the two tris-b-diketonato moieties were inequivalent for F1, while
they were equivalent for F2 and F3 (see ESI†). The results were
rationalized by assuming that the bridging part (dabe2-) was chiral
(denoted as S-dabe2- and R-dabe2- tentatively), resulting in the
diastereometic relation with the octahedral Ru(III) moieties. Based
on this assumption, F1 was assigned to be DK-[Ru(III)(acac)2(S-
or R-dabe)Ru(III)(acac)2] (meso-type), while F2 and F3 were
assigned to be either DD- (or KK-)[Ru(III)(acac)2(S-dabe or R-
dabe)Ru(III)(acac)2] or DD- (or KK-)[Ru(III)(acac)2(R-dabe or S-
dabe)Ru(III)(acac)2] (racemic type) (Chart 1(c)).
The molecular structure was determined for the racemic mixture
of the meso-type dimer by X-ray crystallographic analysis. The
results are shown in Fig. 1. It was unequivocally confirmed that the
two tris(b-diketonato)Ru(III) moieties had DK-antipodal struc-
tures. Notably the two non-symmetrical b-diketonato moieties
in dabe2- were orthogonally twisted in the Ca–Ca¢ bond. Thus
the bridging part has axial chirality according to the sign of the
twisting angle. It should be noted that the chirality in the bridging
part is different from that in the keto-form of (R,R)- or (S,S)-
dabeH2.11,12
Fig. 2 The circular dichroism spectra of the resolved enantiomers of
fractions (1), (2), and (3) in methanol. (a), (b) and (c) are F1 (meso-type),
F2 (racemic-type) and F3 (racemic-type), respectively. Black and red lines
are the less and more retained enantiomers, respectively.
The vibrational circular dichroism (VCD) spectroscopy, which
is an extension into the infrared and near-infrared regions of
the circular dichroism spectroscopy, provides a powerful tool to
identify a chiral part in a molecule in a solution.14–17 Fig. 3(a),
(b) and (c) show the VCD spectra of F1, F2 and F3, respectively.
These diastereomers gave mirror-imaged VCD spectra between the
Fig.
1
The molecular structure of DK-[Ru(III)(acac)2(dabe)-
Ru(III)(acac)2] in the crystal. H-atoms are omitted for clarity.
748 | Dalton Trans., 2012, 41, 747–751
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NMR spectra of these fractions in CDCl3, four acac ligands were
all equivalent, giving only two kinds of methyl groups for both F1
and F2 (see ESI†). Thus the bridging part was achiral and the two
tris-b-diketonato moieties were equivalent. When eluted on the
chiral column, F1 gave a single band with no indication of optical
resolution, while F2 gave two peaks with an equal area (see ESI†).
Thus F1 and F2 were concluded to be achiral meso-type dimer,
DK-[Ru(III)(acac)2(tbet)Ru(III)(acac)2], and chiral racemic-type
dimer, DD- (or KK-)[Ru(III)(acac)2(tbet)Ru(III)(acac)2], re-
spectively. The bridging part was achiral due to the or-
thogonal twisting of the symmetrical b-diketonato moieties.
These situations were the same as previously observed for
[Ru(III)(acac)2(taet)Ru(III)(acac)2].6 As for the racemic-type
dimer, the less and more retaining isomers were assigned to be DD-
and KK-enantiomers, respectively, when their ECD spectra were
compared with that of [Ru(acac)3] (see ESI†). The VCD spectra
of these resolved enantiomers are given in the ESI†. They were
similar to those of racemic [Ru(III)(acac)2(dabe)Ru(III)(acac)2].
It is interesting to note that the elution order of racemic
[Ru(III)(acac)2(tbet)Ru(III)(acac)2] on the chiral column was
similar to that of racemic [Ru(III)(acac)2(dabe)Ru(III)(acac)2] in
F3 and opposite to that in F2. These situations might reflect the
subtle orientational difference of phenyl groups at the bridging
part among these molecules.
Fig. 3 The vibrational circular dichroism spectra of CDCl3 solutions of
the dimeric species: (a), (b) and (c) are F1 (meso-type), F2 (racemic-type)
and F3 (racemic-type), respectively. Black and red lines are the less and
more retained enantiomers, respectively.
opposite enantiomers. The intensities of VCD peaks were much
lower for F1 than for F2 and F3 as already noticed in the
ECD spectra. In the case of F2 and F3, the couplet bands
around 1550 cm-1 were assigned to C–O stretching vibrations.
The signs of these couplets for the DD- and KK-dimer are in
accord with those for D- and K-[Ru(acac)3] respectively.17 In
the case of F1, the triple multiple peaks with different signs
were observed with lower intensity in the same wavenumber
region. The appearance of these peaks might come from the
stretching vibration of C–O bonds in the bridging part. The
multiple peaks in the region of 1300–1400 cm-1 were thought
to give a clue to determining the RS configuration of the
bridging part. As an attempt to determine the configuration, the
VCD spectrum of DK-[Ru(III)(acac)2(S-dabe)Ru(III)(acac)2] was
calculated (see ESI†). By comparing the observed and calculated
spectra, it was tentatively assigned that the less and more retained
enantiomers were DK-[Ru(III)(acac)2(S-dabe)Ru(III)(acac)2] and
DK-[Ru(III)(acac)2(R-dabe)Ru(III)(acac)2], respectively. More de-
tailed theoretical analyses are now in progress.
In order to estimate the barrier height for the rotation around the
C–C bond in the coordinated dabe2-, the enantiomeric meso-type
dimer, DK-[Ru(III)(acac)2(S-dabe)Ru(III)(acac)2], was dissolved
in toluene and kept at 90 ◦C. After the elapse of five days, the
solution was evaporated and the residue was analyzed chromato-
graphically on the chiral column. As a result, no racemization
of the initial enantiomer to its antipode, DK-[Ru(III)(acac)2(R-
dabe)Ru(III)(acac)2], took place after 120 h.
Experimental
Syntheses of ligands
The ligand, dabeH2, was obtained by reacting sodium 1-pheny-1,3-
butanedionate (5.93 g, 0.032 mol) with iodine (4.06 g, 0.016 mol)
in dioxane.6 The compound was identified to be meso-1,2-diacetyl-
1,2-dibenzoylethane by 1H NMR and mass spectra (see ESI†).10,11
The 1H NMR spectrum showed that dabeH2 took a keto form and
was not converted to the enol form in CDCl3 as was reported in the
literature.10–12 The ligand, tbetH2, was obtained by reacting sodium
1,3-diphenyl-1,3-propanedionate (9.83 g, 0.040 mol) with iodine
(5.13 g, 0.020 mol) in dioxane. The compound was identified by
1H NMR.
Syntheses of metal complexes
A dinuclear complex, [Ru(III)(acac)2(dabe)Ru(III)(acac)2], was
prepared by refluxing [Ru(III)(acac)2(CH3CN)2]PF6 (52.4 mg, 0.10
mmol) and dabeH2 (17.3 mg, 0.054 mmol) in dehydrated toluene
(120 mL) for 24 h. After evaporating the solvent, the residue
was eluted on a silica gel column (40 mm (i.d.) ¥ 30 cm) with
9 : 1 (v/v) benzene–acetonitrile to remove [Ru(III)(acac)2(dabeH)]
and [Ru(III)(acac)3]. The fraction collected was further eluted on
a HPLC silica gel column (4 mm (i.d.) ¥ 25 cm) (Inertsil, GL
Science Inc., (Japan)) with 9 : 1 (v/v) benzene–acetonitrile. Three
peaks (denoted as F1, F2 and F3, respectively) were separated
(see ESI†). Mass spectral measurements showed that all of these
peaks corresponded to dinuclear complexes (m/z = 919 (obs.);
918.9 (calc.)). The identification of these complexes is described
in the Results and discussion section. The chromatographic
resolution of F1 was performed by use of a HPLC column
(4 mm (i.d.) ¥ 25 cm) packed with D-[Ru(II)(phen)3]2+/synthetic
hectorite.13 This column has been developed in our laboratory,
[Ru(III)(acac)2(tbet)Ru(III)(acac)2]
[Ru(III)(acac)2(CH3CN)2]PF6 and tbetH2 were reacted in toluene
and the reaction mixture was eluted on a HPLC column packed
with silica gel. Two peaks (denoted as F1 and F2, respectively) were
separated (see ESI†). Both the peaks gave the molecular weight of
1
a dimer (m/z = 1044 (obs.); 1043.0 (calc.)). According to the H
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Table 1 Crystallographic data of DK-[Ru(III)(acac)2(dabe)Ru(III)-
showing high capability of resolving multi-nuclear b-diketonato
complexes.9 F1: DK-[Ru(III)(acac)2(dabe)Ru(III)(acac)2]. d = -43.0
(1H, CH), -34.22 (1H, CH), -17.62 (1H, CH), -14.31 (3H,
CH3), -12.70 (3H, CH3), -12.48 (1H, CH), -11.59 (3H, CH3),
-11.03 (3H, CH3), -9.70 (3H, CH3), -6.52 (3H, CH3), 0.12 (3H,
CH3), 2.29 (3H, CH3), 3.77 (3H, CH3), 4.52 (2H, aromatic), 4.67
(2H, aromatic), 5.47 (3H, CH3), 7.25 (1H, aromatic), 8.02 (2H,
aromatic), 8.28 (1H, aromatic), 8.34 (2H, aromatic). F2: DD- (or
KK) -[Ru(III)(acac)2(dabe)Ru(III)(acac)2]. d = -35.0 (2H, CH),
-16.12 (2H, CH), -15.31 (6H, CH3), -10.13 (6H, CH3), -6.90
(6H, CH3), 0.77 (6H, CH3), 4.00 (4H, aromatic), 4.70 (6H, CH3),
6.26 (4H, aromatic), 7.58 (2H, aromatic). F3: DD- (or KK) -
[Ru(III)(acac)2(dabe)Ru(III)(acac)2]. d = -40.5 (2H, CH), -13.81
(6H, CH3), -12.86 (6H, CH3), -10.00 (2H, CH), -9.31 (6H, CH3),
3.37 (6H, CH3), 4.12 (4H, d, J = 7 Hz, aromatic), 6.65 (6H, CH3),
7.89 (2H, aromatic), 8.92 (4H, aromatic).
(acac)2]
Empirical formula
Formula weight
Crystal size/mm3
Crystal system
C40H44O12Ru2
918.89
0.15 ¥ 0.05 ¥ 0.01
Triclinic
˚
a/A
9.8075(16)
13.263(2)
15.335(3)
79.540(2)
86.223(2)
82.549(2)
1943.1(5)
˚
b/A
˚
c/A
a/deg
b/deg
g /deg
3
˚
V/A
¯
Space group
P1
Z value
2
D
calc/g cm-3
F(000)
1.571
936
13098
No. of reflections
No. of observations
Parameters
5647
497
The dinuclear complex, [Ru(III)(acac)2(tbet)Ru(III)(acac)2],
was prepared by refluxing [Ru(III)(acac)2(CH3CN)2]PF6 (0.22 g,
0.42 mmol) and tbetH2 (0.17 g, 0.380 mmol) in dehydrated toluene
(70 mL) for 24 h. After evaporating the solvent, the residue was
eluted on a silica gel column (40 mm (i.d.) ¥ 30 cm) with 9 : 1
(v/v) benzene–acetonitrile to remove [Ru(III)(acac)2(tbetH)] and
[Ru(III)(acac)3]. The desired fraction was further separated into
the two peaks. Mass spectral measurements showed that both
the peaks corresponded to dinuclear complexes (m/z = 1044
(obs.); 1043.0(calc.)): F1 DK-[Ru(III)(acac)2(tbet)Ru(III)(acac)2].
d = -30.10 (4H, CH), -7.03 (12H, CH3), -3.51 (12H, CH3), 6.10
(8H, aromatic), 7.70 (4H, aromatic), 8.26 (8H, aromatic); F2: DD-
(or KK) -[Ru(III)(acac)2(tbet)Ru(III)(acac)2]. d = -27.1 (4H, CH),
-5.73 (12H, CH3), -1.56 (12H, CH3), 4.69 (8H, aromatic), 6.76
(8H, aromatic), 7.46 (4H, broad t, J = 6 Hz).
Temperature/K
Final R1, Rw(I > 2s(I))
Final R1, Rw (all data)
Goodness-of-fit
90(2) K
0.0414, 0.0776
0.0709, 0.0960
1.029
data were collected at 90 K on a Bruker Smart CCD diffractometer
with graphite-monochromated Mo Ka radiation. The structure
was solved by the direct method and refined by the least-squares
on F2. The crystallographic data and refinement parameters are
listed in Table 1.†
Computational details
The IR and VCD spectra of the complex were theoretically
calculated by the use of Gaussian 09 program.18 VCD intensities
were determined by the vibrational rotational strength and
magnetic dipole moments, which were calculated by the magnetic
field perturbation (MFP) theory formulated using magnetic field
gauge-invariant atomic orbitals. The calculated intensities were
converted to Lorentzian bands with 4 cm-1 half-width at half-
height. Geometry optimization and frequency analysis were
performed at the DFT level (B3LYP functional with Stuttgart
ECP for Ru and 6-31G(d) for other atoms). Thus the peaks in the
observed spectrum were assigned on the basis of the animation of
molecular vibration with Gauss view 5.08 (Gaussian Inc.).
Spectroscopic measurements
1H NMR spectra were measured with a JNM-AL400 (JEOL,
Ltd.) in CDCl3. UV-vis spectra were recorded with a U-2810
spectrophotomer (Hitachi, Ltd., Japan). Circular dichroism (CD)
spectra were recorded with a J-720 spectropolarimeter (JASCO,
Co., Japan). VCD and IR spectra were measured with a PRESTO-
S-2007 spectrometer (JASCO, Co., Japan). The machine is the
single PEM system, at which the central wavenumber of PEM was
set at 1250 cm-1. The calibration was made by use of a quarter
wave-retardation and an analyzer. The spectra were recorded on
the instrument implemented with the shuttle system. However,
we did not use the shuttle system in this work because the
signals were large enough for the conventional measurements.
A CDCl3 solution of each complex was injected into a cell
(150 mm in optical length) with BaF2 windows. The signal was
accumulated during 10000 scans (ca. 1.5 h) for each complex. The
background correction was performed only by subtracting the
solvent spectrum. The resolution of wavenumber was 4 cm-1. The
concentration was adjusted such that the absorbance of IR spectra
below 1.0 for the optimal measurements.
Conclusion
Ru(III) dimers bridged by the non-symmetrical b-diketonato
ligand, [Ru(III)(acac)2(dabe)Ru(III)(acac)2], were synthesized and
separated into the meso- and two racemic forms, each of which
was resolved into a pair of enantiomers. Notably, the meso-form,
in which D- and K-moieties are connected by the bridging ligand,
is chiral in solid and solution states. This is the first example of
optical resolution of the enantiomers due to the axial chirality of
b-diketonato rings. The resolution was successfully achieved on
the basis of the chiral recognition by D-[Ru(phen)3]2+/synthetic
hectorite. The enantiopure meso-form showed characteristic cir-
cular dichroism, particularly in the mid IR region, even though the
chirality due to DK isomerism of the Ru(III) centers is cancelled
out. In contrast to the non-symmetical bridging ligand, a symmet-
rical bridging ligand, tbetH2, formed achiral meso-type and chiral
racemic-type dimers. The situations were the same as taetH2.
X-Ray crystal analyses
A single crystal suitable for X-ray analysis was obtained by
recrystallization from an ethanol solution of the racemic mixture
of meso-[Ru(III)(acac)2(dabe)Ru(III)(acac)2] (F1). The intensity
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9 N. Fujimoto, Y. Mori, A. Yamagishi and H. Sato, Chem. Commun.,
2010, 46, 5473; H. Sato, J. Kameda, Y. Fukuda, M. Haga and A.
Yamagishi, Chem. Lett., 2008, 37, 716.
10 J. R. Cannon, V. A. Patrick and A. H. White, Aust. J. Chem., 1986, 39,
1811.
11 The keto-form of dabeH2 has two chiral carbon atoms. The di-
astereomers, meso- and rac-dabeH2, can be discriminated based on
their 1H NMR spectra; H. Quast, M. Seefelder, S. Ivanova, M.
Heubes, E.-M. Peters and K. Peters, Eur. J. Org. Chem., 1999,
3343.
12 Quast et al. reported that the bis-enol tautomer of dabeH2 exists in a
C2-symmetric conformation, in which the two b-diketonato planes are
largely twisted, in the crystalline state. However, their sample was a
racemic crystal and no optical resolution was attempted11.
13 A. Yamagishi, J. Coord. Chem., 1987, 16, 131; J. X. He, H. Sato,
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Acknowledgements
Thanks are due to Prof. T. Kitazawa, Mr. I. Tomori and Mr. N.
Mitsuhiro (all of the Department of Chemistry, Toho University)
for help in the X-ray structural analyses.
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