Effects of central metal ions on vibrational circular dichroism spectra of tris-(β-diketonato)metal(III) complexes

Article abstract of DOI:10.1021/ic070300i

Vibrational circular dichroism (VCD) spectra of a series of [M(III)(acac)₃] (acac = acetylacetonato; M = Cr, Co, Ru, Rh, Ir, and Al) and [M(III)(acac)₂(dbm)] (dbm = dibenzoylmethanato; M = Cr, Co, and Ru) have been investigated experimentally and/or theoretically in order to see the effect of the central metal ion on the vibrational dynamics of ligands. The optical antipodes give the mirror-imaged spectra in the region of 1700-1000 cm^-1. The remarkable effect of the central metal ion is observed experimentally on the VCD peaks due to C-O stretches (1500-1300 cm-1) for both [M(III)(acac)₃] and [M(III)(acac)₂(dbm)]. In the case of Δ-[M(III)(acac)₃], for example, the order of frequency of two C-O stretches (E and A₂ symmetries) is dependent on the kind of a central metal ion as follows: E (-) > A₂ (+) for M = Co, Rh, and Ir, while A₂ (+) > E (-) for M = Cr and Ru. In the case of Δ-[M(III)(acac)₂-(dbm)], the order of frequency of three C-O stretches (A, B, and B symmetries) is as follows: A (-) > B (+) > B (+) for Co(III), B (+) > A (-) > B (-) for Cr(III), and A (-) > B (+) > B (-) for Ru(III). These results imply that the energy levels of C-O stretches are delicately affected by the kind of central metal ion. Since such detailed information is not obtained from the IR spectra alone, the VCD spectrum can probe the effect of the central metal ion on interligand cooperative vibration modes.

Full text of DOI:10.1021/ic070300i

Inorg. Chem. 2007, 46, 6755−6766
Effects of Central Metal Ions on Vibrational Circular Dichroism Spectra
of Tris-( -diketonato)metal(III) Complexes
â
Hisako Sato,† Tohru Taniguchi,‡ Atsufumi Nakahashi,‡ Kenji Monde,‡ and Akihiko Yamagishi*,§
Department of Earth and Planetary Science, Graduate School of Science, The UniVersity of Tokyo,
Tokyo 113-0033, Japan, PRESTO, Japan Science and Technology Agency, Japan, Laboratory of
AdVanced Chemical Biology, Graduate School of AdVanced Life Science, Hokkaido UniVersity,
Sapporo 001-0021, Japan, Department of Chemistry, Faculty of Science, Ochanomizu UniVersity,
Tokyo 112-8610, Japan, and CREST, Japan Science and Technology Agency, Japan
Received February 14, 2007
Vibrational circular dichroism (VCD) spectra of a series of [M(III)(acac)3] (acac
Ru, Rh, Ir, and Al) and [M(III)(acac)2(dbm)] (dbm dibenzoylmethanato; M Cr, Co, and Ru) have been investigated
experimentally and/or theoretically in order to see the effect of the central metal ion on the vibrational dynamics of
ligands. The optical antipodes give the mirror-imaged spectra in the region of 1700
1000 cm-1. The remarkable
effect of the central metal ion is observed experimentally on the VCD peaks due to C O stretches (1500 1300
cm-1) for both [M(III)(acac)3] and [M(III)(acac)2(dbm)]. In the case of
-[M(III)(acac)3], for example, the order of
frequency of two C O stretches (E and A2 symmetries) is dependent on the kind of a central metal ion as follows:
E ( ) > A2 ) for M Co, Rh, and Ir, while A2 ) > E ( ) for M Cr and Ru. In the case of -[M(III)(acac)2-
(dbm)], the order of frequency of three C O stretches (A, B, and B symmetries) is as follows: A ( ) > B ( ) > B
) for Co(III), B ( ) > A (-) > B (-) for Cr(III), and A (-) > B ( ) > B ( ) for Ru(III). These results imply that the
energy levels of C O stretches are delicately affected by the kind of central metal ion. Since such detailed information
) acetylacetonato; M ) Cr, Co,
)
)
−
−
−
∆
−
−
(+
)
(+
−
)
∆
−
−
+
(
+
+
+
−
−
is not obtained from the IR spectra alone, the VCD spectrum can probe the effect of the central metal ion on
interligand cooperative vibration modes.
Introduction
metal complexes lies in understanding the cooperative effects
of the dynamical motions among vicinal ligands and the
degree of participation of the central metal ion in the effects.
Knowledge of this kind has been obtained by various
Chirality of metal complexes has been attracting continu-
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istry.1 In the practical area, chiral octahedral metal complexes
are employed extensively as catalysts, sensors, and chiral
source of induction of helix and so on.2-8 Their functions
as a chiral source are manifested through the discrimination
of molecular chirality, the transfer of chirality from one
species to another, and the amplification of chirality to a
supramolecular scale. A key to useful application of chiral
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* To whom correspondence should be addressed at the Department of
Chemistry, Faculty of Science, Ochanomizu University, Bunkyo-ku, Ootsuka
2-1-1, Tokyo 112-8610, Japan. Tel: +81-3-5978-5575. Fax: +81-3-5978-
5575. E-mail: yamagishi.akihiko@ocha.ac.jp.
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‡ Hokkaido University.
§ Ochanomizu University and CREST, Japan Science and Technology
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10.1021/ic070300i CCC: $37.00
Published on Web 07/11/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 16, 2007 6755

Sato et al.
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6756 Inorganic Chemistry, Vol. 46, No. 16, 2007

Metal Ion Effects on VCD Spectra of Tris-Chelated Complexes
Motivated by these works, the present work has studied
the VCD spectra of a series of tris(acetylacetonato)metal-
(III) and bis(acetylacetonato)(dibenzoylmethanato)metal(III).
The reason for choosing these molecules is that the infrared
spectra of tris(acetylacetonato)metal(III) complexes have
been thoroughly investigated theoretically.45-47 The main
intense bands are successfully assigned in conjunction with
density functional theory (DFT) calculations. According to
our preliminary VCD measurements of chiral tris(acetylac-
etonato)metal(III) complexes,43 it has been found that the
order of energy levels is opposite, for example, for the
stretching vibrations of C-O bonds (A2 and E symmetries)
between Co(III) and Cr(III) complexes. These results suggest
the significant effects of the d-electron configuration of a
central metal ion on vibration transitions. For examining the
validity of observed effects of the central metal ion for wider
classes of metal complexes, this work attempted a systematic
VCD study on two kinds of tris-chelated complexes with a
series of different metal ions. As a result, it has been revealed
that the configuration of d-electrons in a central metal ion
has a dramatic effect on the VCD spectral shapes. With the
help of theoretical calculations, it has been concluded that
the modes representing the interligand correlative vibrations
of C-O stretches are most significantly affected by the type
of central metal ion.
between vicinal ligands.27 For the Ru(III) complex alone,
the appreciable broadening of a peak is observed in this
frequency region.
The situations are contrasted with the bands due to C-O
stretches at 1500-1300 cm-1. Notably, the positions of the
positive and negative peaks are dependent on the type of
metal ion. In the case of Co(III), Rh(III), and Ir (III) (denoted
by group 1), the ∆ enantiomer exhibits negative and positive
peaks from higher to lower frequency. As expected, the Λ
enantiomer of the same complex exhibits the opposite feature.
To the contrary, in the case of Cr(III) and Ru(III) (denoted
by group 2), the ∆ enantiomer exhibits positive and negative
peaks from higher to lower frequency. In other words, the
bisignate nature of the peak is opposite in complexes of
groups 1 and 2. In the case of [Ru(III)(acac)3], the separation
between the coupled peaks is so large (ca. 40 cm-1) that the
peak at lower frequency arises from the shoulder at 1341
cm-1 in the IR spectra. As an additional feature, in the case
of the Ru(III) complex, the broad peak is observed at 1300-
1100 cm-1, which is ascribed to the participation of the d-d
transition in this region.29 Table 1 summarizes the experi-
mental VCD of C-O stretching in ∆-[M(III)(acac)3].
The calculated IR and VCD spectra of the ∆ enantiomers
of [M(III)(acac)3] with M ) Co, Rh, Ir, Cr, and Ru) are
shown in Figure 2 panels a-e, respectively. The calculation
of IR spectra well reproduces the observed spectra except
for the shift of each peak position toward higher frequency
(ca. 70 cm-1). In previously reported calculations at the same
level [DFT(B3LYP/6-31G(d)], the correct frequency was
obtained by introducing a scale factor of 0.92 for C-O
stretching.45
Results
VCD Spectra of [M(III)(acac)3]. Figure 1 panels a-e
show the observed VCD (upper) and IR (lower) spectra for
[M(III)(acac)3] with M ) Co, Rh, Ir, Cr, and Ru in CDCl3,
respectively. All five complexes give similar IR spectra in
the whole spectral range (1700-1000 cm-1). Main intense
bands were assigned previously in detail in the case of M )
Co and Cr.45 Both complexes are reported to undergo no
Jahn-Teller distortion. In the IR spectra, the intense band
around 1570 cm-1 is assigned mainly to C-O stretches. The
intense band around 1520 cm-1 is assigned mainly to C-C
stretches. The band around 1430 cm-1 involves multiple
vibrations arising from CH3 bending bands. The intense band
around 1380 cm-1 is mainly assigned to C-O stretches.45
For the Ru(III) complex alone, a small shoulder observed at
1340 cm-1 is assigned to C-O stretching.
The calculated VCD spectra in the range of 1700-1500
cm-1 are nearly identical among the investigated five
complexes. A calculated bisignate band around 1580 cm-1,
assigned to C-C stretches, corresponds to the bisignate band
around 1520 cm-1 in the observed spectra. The single peak
around 1630 cm-1, which is assigned to one of the doubly
degenerate E modes of C-O stretching, corresponds to the
peak around 1570 cm-1 in the observed spectra. Thus it is
confirmed both experimentally and theoretically that the
electronic properties of the central metal ion little affect the
spectral features of these bands.
It is interesting to see how the theory predicts the effect
of a central metal ion on the bisignate bands observed at
1500-1300 cm-1 due to C-O stretches. In the case of
∆-[Co(III)(acac)3], the calculated peaks consist of negative
(doubly degenerate E) and positive (A2) composites at 1473
and 1468 cm-1, respectively. These two peaks correspond
to the negative and positive ones at 1396 and 1381 cm-1 in
the experimental spectra, respectively. Moreover, the calcula-
tion predicts correctly the lower intensity of the negative peak
than that of the positive one. The same features are predicted
for ∆-[Rh(III)(acac)3] and ∆-[Ir(III)(acac)3], confirming the
agreement between the observed and calculated spectra.
In the case of ∆-[Cr(III)(acac)3], the calculated peaks are
composed of positive (A2) and negative (doubly degenerate
E) peaks at 1464 and 1461 cm-1, respectively. These two
peaks correspond to the positive and negative ones at 1391
In VCD measurements, the antipodes give mirror-imaged
spectra for all investigated cases. When the spectra are
compared among the complexes, there is no remarkable
difference in the range of 1700-1500 cm-1, which corre-
sponds to the C-C and C-O stretching vibrations. The band
around 1520 cm-1, which is mainly assigned to the C-C
stretching, exhibits a positive-negative bisignate shape from
higher to lower frequency for the ∆ isomers or vice versa
for the Λ isomers. Previously the splitting of the band was
interpreted in terms of the coupled oscillator mechanism
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2001, 105, 238. (b) Diaz-Acosta, I.; Baker, J.; Cordes, W.; Hinton, J.
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(46) (a) Tayyari, S. F.; Raissi, H.; Ahmadabadi, Z. Spectrochim. Acta, Part
A 2002, 58, 2669. (b) Tayyari, S. F.; Milani-nejad, F. Spectrochim.
Acta, Part A 2000, 56, 2679.
(47) Thornton, D. A. Coord. Chem. ReV. 1990, 104, 173.
Inorganic Chemistry, Vol. 46, No. 16, 2007 6757

Sato et al.
Figure 1. Observed IR (lower) and VCD (upper) spectra of [M(III)(acac)3] [where M ) (a) Co, (b) Rh, (c) Ir, (d) Cr, and (e) Ru] in CDCl3. Solid and
dotted curves correspond to ∆- and Λ-enantiomers, respectively.
Table 1. Observed Main C-O Vibrations in VCD Spectra of
∆-[M(III)(acac)3]
molecule has C2 symmetry. The assignment of each vibration
mode is performed, however, under the approximation of
D3 symmetry. The theory predicts positive (A2) and negative
(E) peaks at 1457 and 1402 cm-1 for [Ru(III)(acac)3],
respectively. The negative peak is located at a position remote
from the positive one, so that it corresponds to the small
peak at 1402 cm-1 near the large peak in the IR spectrum.
These features roughly agree with the experimental observa-
tions. The results have confirmed the validity of the present
magnetic field perturbation (MFP) theory in the case of open-
shell systems. To summarize the above results, the effect of
metal ions on the bisignate band of C-O stretches at 1500-
1300 cm-1 is rationalized in terms of the difference in the
frequency, cm-1
∆-[Co(III)
(acac)3]
∆-[Rh(III)
(acac)3]
∆-[Ir(III)
(acac)3]
∆-[Cr(III)
(acac)3]
∆-[Ru(III)
(acac)3]
1381 (+)
1396 (-)
1383 (+)
1401 (-)
1389 (+)
1407 (-)
1374 (-)
1391 (+)
1298 (+)
1335 (-)
1377 (+)
1561 (-) broad
1575 (-)
1567 (-)
1551 (-)
1576 (-)
and 1374 cm-1 in the experimental spectra, respectively. The
calculation also predicts correctly that these peaks show
nearly the same intensity. In the case of [Ru(III)(acac)3],
calculation is performed with the assumption that the
6758 Inorganic Chemistry, Vol. 46, No. 16, 2007

Metal Ion Effects on VCD Spectra of Tris-Chelated Complexes
Figure 2. Calculated IR (lower) and VCD (upper) spectra of ∆-[M(III)(acac)3] [where M ) (a) Co, (b) Rh, (c) Ir, (d) Cr, (e) Ru, and (f) Al]. Although
[Ru(III)(acac)3] is calculated for C2 symmetry, the assignment is done under the approximation of D3 symmetry. The other five complexes are assumed to
belong to D3 symmetry. No correction is made by introducing a scale factor for these spectra.28,45
frequency order of two composite peaks (A2 and E). The
only aspect that the theory fails in predicting is the broad
peak at 1300-1100 cm-1 in the case of the Ru(III) complex.
[Al(III)(acac)3] with no d-electron was calculated as shown
in Figure 2f. In the case of ∆-[Al(III)(acac)3], the calculated
peaks are composed of positive (A2) and negative (doubly
degenerate E) peaks at 1492 and 1475 cm-1, respectively.
The single positive peak at 1656 cm-1, which is one of the
doubly degenerate E modes, is assigned as C-O stretching.
More detailed results on the experimental and calculated
frequencies of [M(III)(acac)3] are presented in the Supporting
Information. The assignment of each peak has been done
according to the calculated results.
VCD Spectra of [M(III)(acac)2(dbm)]. Figure 3 panels
a-c show, respectively, the electronic circular dichroism
(ECD) spectra of methanol solutions of enantiomeric [M(III)-
(acac)2(dbm)] (M ) Co, Cr, and Ru) (Chart 1b). The absolute
configuration of these complexes is determined as indicated
in the figures by comparing the observed spectra with those
of corresponding [M(III)(acac)3] in the d-d transition region
(400-600 nm).49
Figure 4 panels a-c show the observed IR (lower) and
VCD (upper) spectra of the CDCl3 solutions of [M(III)-
(acac)2(dbm)] (M ) Co, Cr, and Ru), respectively. [M(III)-
(48) Okamoto, Y.; Yashima, E.; Hatada, K. J. Chem. Soc., Chem. Commun.
1984, 1051.
Inorganic Chemistry, Vol. 46, No. 16, 2007 6759

Sato et al.
Figure 3. ECD spectra of [M(III)(acac)2(dbm)] [where M ) (a) Co, (b) Cr, and (c) Ru] in methanol. Solid and dotted curves correspond to ∆- and
Λ-enantiomers, respectively.
Chart 1. Structures of (a) ∆-[M(III)(acac)3] and (b)
∆-[M(III)(acac)2(dbm)]
In the VCD spectra of [M(III)(acac)2(dbm)], peaks are low
and not definite in the 1700-1500 cm-1 range due to the
low signal-to-noise ratio. In the case of [Cr(III)(acac)2(dbm)],
the small peaks around 1580 and 1520 cm-1 are assigned to
C-O and C-C stretches, respectively. These peaks show
the same signs for the same enantiomers of the investigated
three complexes. The remarkable effect of the central metal
ion is observed in the main peaks at 1500-1300 cm-1 arising
from C-O stretches. In the case of ∆-[Co(III)(acac)2(dbm)],
the intense band at 1380 cm-1 in its IR spectrum is split
into two with one negative peak at 1396 cm-1 and one
positive peak at 1377 cm-1. To the contrary, in the case of
∆-[Cr(III)(acac)2(dbm)], the corresponding strong peak at
1380 cm-1 in the IR spectrum is split into two with one
positive peak at 1389 cm-1and one negative peak at 1373
cm-1. In the case of ∆-[Ru(III)(acac)2(dbm)], the bisignate
feature at 1500-1300 cm-1 is identical with that of ∆-[Cr-
(III)(acac)2(dbm)], or the positive peak at 1373 cm-1 and
the negative peak at 1334 cm-1. The positive peak is thought
to originate from the strong peak at 1373 cm-1 in the IR
spectrum, while the negative peak is so distant from that
peak that it originates from the weak broad band around 1340
cm-1. As was already seen for [Ru(III)(acac)3], the broad
band exists at 1300-1100 cm-1. Table 2 summarizes the
main C-O peaks in the VCD observed spectra of ∆-[M(III)-
(acac)2(dbm)].
(acac)2(dbm)] displays C2 symmetry. The IR spectra of
[Co(III)(acac)2(dbm)] and [Cr(III)(acac)2(dbm)] are nearly
identical in the entire range of 1700-1000 cm-1. Compared
with the IR spectra of the corresponding tris-chelated
complexes, the additional peak appears around 1480 cm-1.
This is assigned to the C-H ring stretching of a dbm ligand.
Another additional peak at 1543 cm-1 is assigned to the
combination of C-O and C-C stretches of a dbm ligand.
The combination of C-O and C-C stretches of a dbm ligand
is observed at 1580 cm-1. The IR spectrum of [Ru(III)(acac)2-
(dbm)] is similar to those of [Co(III)(acac)2(dbm)] and [Cr-
(III)(acac)2(dbm)] in the same range except for the overlap-
ping of composite peaks in the range of 1600-1500 cm-1
due to slight broadening.
Detailed assignment of the above peaks can be done with
the help of the calculated VCD spectra. Figure 5 panels a-c
(49) Drake, A. F.; Gould, J. M.; Mason, S. F.; Rosin, C.; Woodley, F. J.
Polyhedron 1983, 2, 537.
6760 Inorganic Chemistry, Vol. 46, No. 16, 2007

Metal Ion Effects on VCD Spectra of Tris-Chelated Complexes
Figure 4. Observed spectra of [M(III)(acac)2(dbm)] [where M ) (a) Co, (b) Cr, and (c) Ru] in CDCl3. Solid and dotted curves correspond to ∆- and
Λ-enantiomers, respectively.
Table 2. Observed Main C-O Vibrations in VCD Spectra of ∆-
In the case of ∆-[Cr(III)(acac)2(dbm)], the positive B-mode
peak at 1463 cm-1, the negative A-mode peak at 1461 cm-1,
and the negative B-mode peak at 1451 cm-1 in the calculated
spectrum correspond to the positive peak at 1389 cm-1, the
negative peak at 1373 cm-1, and the small shoulder around
1323 cm-1 in the observed spectrum, respectively. The single
negative peak that is assigned to C-O stretch (A-mode) is
predicted at 1602 cm-1. This corresponds to the small
negative peak at 1543 cm-1 in the observed spectrum.
In the case of ∆-[Ru(III)(acac)2(dbm)], the positive B-
mode peak at 1452 cm-1 , the negative B-mode peak at 1397
cm-1, and the additional small negative A-mode peak
predicted at 1465 cm-1 in the calculated spectrum correspond
to the positive peak at 1373 cm-1 and the negative peak at
1334 cm-1 in the observed spectrum, respectively. The large
separation between the positive and negative peaks in the
bisignate nature for this complex is predicted theoretically.
The single negative peak at 1626 cm-1 is assigned to C-O
stretch (B-mode), in which two acac ligands stretch out of
phase with a dbm ligand frozen. This corresponds to the
broad band at 1700-1500 cm-1 in the observed spectrum.
Detailed results on the observed and calculated spectra of
[M(III)(acac)2(dbm)] are presented in the Supporting Infor-
mation.
[M(III)(acac)2(dbm)]
frequency, cm-1
∆-[Co(III)(acac)2(dbm)] ∆-[Cr(III)(acac)2(dbm)] ∆-[Ru(III)(acac)2(dbm)]
1354 (+) small
1377 (+)
1396 (-)
1323 (-)
1373 (-)
1389 (+)
1585 (-)
1296 (+) weak
1334 (-)
1373 (+)
1546 (-) weak
show the calculated IR (upper) and VCD (lower) spectra of
∆-[M(III)(acac)2(dbm)] (M ) Co, Cr, and Ru), respectively.
The complexes are assumed to have C2 symmetry. No
correction is made by use of any scale factor. The calculated
IR spectra reproduce the experimental IR spectra in the whole
wavenumber region except for the shift of the peak positions
toward higher frequency.
The bisignate features observed around 1370-1390 cm-1
due to C-O stretches are interpreted as follows. In the case
of ∆-[Co(III)(acac)2(dbm)], the negative A-mode peak at
1473 cm-1, the positive B-mode peak at 1471 cm-1, and the
additional positive B-mode peak at 1463 cm-1 correspond
to the negative peak at 1396 cm-1, the positive peak at 1377
cm-1, and the small shoulder around 1354 cm-1 in the
observed spectrum, respectively. In the calculated spectra,
the negative peak predicted at 1607 cm-1 is assigned to C-O
stretch (A-mode), in which a dbm ligand is out of phase
with the other acac ligands in-phase. This peak is not
confirmed to exist in the observed spectra because of the
low signal-to-noise ratio at 1700-1500 cm-1.
Discussion
The IR spectra of a series of tris(acetylacetonato)metal-
(III) complexes were previously investigated in detail by both
Inorganic Chemistry, Vol. 46, No. 16, 2007 6761

Sato et al.
Figure 5. Calculated IR (lower) and VCD (upper) spectra of ∆-[M(III)(acac)2(dbm)] [where M ) (a) Co, (b) Cr, and (c) Ru]. No correction is made by
introducing a scale factor for these spectra.28,45
experiments and ab initio calculation.45,46 The effects of
central metal ions on VCD spectra have been reported
previously in several examples.24-26,29,42,43 A complex with
a metal ion of open shell, for example, gives higher intensity
than one with a metal ion of closed shell. The facts are
rationalized in terms of the presence of low-lying excited
electronic states.29
In contrast to the above vibrations, the VCD spectra in
the region of C-O stretches (1500-1300 cm-1) of [M(III)-
(acac)3] are dramatically dependent on the type of central
metal ion as shown in Figure 1. With the help of the
theoretical analyses, the influence of a central metal ion in
this region is successfully interpreted in terms of the
difference in the frequency order of two composites (A2 and
E) of the interligand correlative vibrations. The investigated
five [M(III)(acac)3] complexes are classified into two
groups: the frequency of the C-O stretch with E symmetry
is higher than that of the C-O stretch with A2 symmetry in
the case of M ) Co (eg)6, Rh (eg)6, and Ir (eg)6 (denoted by
group 1), while the situations are reversed in the case of M
) Cr (eg)3 and Ru (eg)5 (denoted by group 2). Since the A2-
and E-mode vibrations represent the in-phase and out-of-
phase stretches of three ligands, respectively, as schematically
shown in Scheme 1a-c, the above results imply that the
energy of such an interligand correlative vibration depends
on the d-electron configuration of a central metal ion in a
symmetry-dependent way. For [Ru(III)(acac)3], broad bands
are observed around 1200 cm-1. This feature indicates the
presence of low-lying excited electronic states of 5d elec-
trons. A similar effect is not observed for [Rh(III)(acac)3]
and [Ir(III)(acac)3], because such states are absent for the
complex of a metal ion with a closed shell. Table 3
summarizes the calculated C-O stretches at 1500-1300
cm-1 of [M(III)(acac)3]. The calculated VCD spectrum of
∆-[Al(III)(acac)3] [Al(III) (eg)0] shows that the complex
For the present tris(â-diketonato)metal(III) complexes,
some of the VCD peaks are indifferent to the type of central
metal ion, while others are dependent on the central metal
ion in both intensity and location. The former category
includes the C-C stretches around 1520 cm-1 and the C-O
stretches at 1700-1500 cm-1. As for the C-C stretches, the
concerned bonds are so remote from a central metal ion that
the vibrations are not influenced by its electronic configu-
ration. As for the C-O stretches, Schemes 1 and 2 show
the representative modes of C-O stretches cooperative
among the coordinated ligands for [M(III)(acac)3] and
[M(III)(acac)2(dbm)], respectively. The above C-O stretch
at 1700-1500 cm-1 corresponds to the modes in Schemes
1d,e and 2d,e for [M(III)(acac)3] and [M(III)(acac)2(dbm)],
respectively. Since these two modes (d and e) are completely
degenerate, an isolated single peak appears in both IR and
VCD spectra. Under such situations, the difference of the
central metal ion has little effect on the spectral features of
peak intensity and position. Thus the signs of these bands
in this region can be a diagnosis of the ∆Λ configuration of
these complexes.
6762 Inorganic Chemistry, Vol. 46, No. 16, 2007

Metal Ion Effects on VCD Spectra of Tris-Chelated Complexes
Scheme 1. Schematic Drawings of A2- and E-Modes in the C-O Vibrations of ∆-[M(III)(acac)3]a
a (a) A2-mode: L1, L2, and L3 are all in-phase. (b) E-mode: L1 and L2 are in-phase, while L3 is out-of-phase. (c) E-mode: L3 is frozen, while L1 and
L2 are out-of-phase. (d) E-mode: L1 and L2 are in-phase, while L3 is out-of-phase. (e) E-mode: L1 and L2 are out-of-phase. E-mode (b, c) and E-mode
(d, e) are degenerate.
Scheme 2. Schematic Drawings of A- and B-Modes in the C-O Vibrations of ∆-[M(III)(acac)2(dbm)]a
a (a) B-mode: L1, L2, and L3 (dbm) are all in-phase. (b) B-mode: L1 and L2 are in-phase, while L3 is out-of-phase. (c) A-mode: L1 and L2 are
out-of-phase. (d) A-mode: L1 and L2 are in-phase, while L3 is out-of-phase. (e) B-mode: L1 and L2 are out-of-phase.
belongs to group 2. Optical resolution of [Al(III)(acac)3] is
reported to be performed at as low as -80 °C.48 Thus the
VCD measurement of this substitution-labile complex re-
mains a challenging work. At present, there is no simple
rule to relate the above classification of VCD features (groups
1 and 2) with the d-electron configuration of a central metal
ion.
A similar effect of a central metal ion on the VCD
spectrum in the C-O stretches (1500-1300 cm-1) is
observed for [M(III)(acac)2(dbm)]. The symmetry of a
complex is lowered from D3 to C2. Theoretically, the band
due to C-O stretches in this region is a composite of three
vibrational modes (A, B, and B) representing the correlative
vibrations among three ligands (Scheme 2a-c). In the
A-mode, the C-O bonds of two acetylacetonato ligands
stretch out-of-phase with no stretch of C-O bonds of dbm.
In one of the B-modes, all of the C-O bonds stretch in-
phase. In another B-mode, two acac ligands stretch in-phase.
By comparing the observed and calculated VCD spectra, the
following orders are deduced as to the frequencies of these
three modes: A (-) > B (+) > B (+) for ∆-[Co(III)(acac)2-
(dbm)], B (+) > A (-) > B (-) for ∆-[Cr(III)(acac)2(dbm)],
Inorganic Chemistry, Vol. 46, No. 16, 2007 6763

Sato et al.
Table 3. Calculated Effects of the Central Metal Ion of [M(III)(acac)3] on VCD Spectra of C-O Vibrations
sign of split peaks of C-O
stretching, assignment
complex with D3 symmetry
∆-[Co(III)(acac)3]
config
d6
higher frequency
lower frequency
order of vibration modes in frequencya
(-) at 1473 cm-1
degenerate E-modeb
(-) at 1477 cm-1
degenerate E-modeb
(-) at 1482 cm-1
degenerate E-modeb
(+) at 1464 cm-1
A2-modec
(+) at 1468 cm-1
A2-modec
A2 (ip) < E (oop)
A2 (ip) < E (oop)
A2 (ip) < E (oop)
A2 (ip) > E (oop)
A2 (ip) > E (oop)
∆-[Rh(III)(acac)3]
∆-[Ir(III)(acac)3]
∆-[Cr(III)(acac)3]
∆-[Ru(III)(acac)3]d
d6
d6
d3
d5
(+) at 1465 cm-1
A2-modec
(+) at 1471 cm-1
A2-modec
(-) at 1462 cm-1
degenerate E-modeb
(-) at 1402 cm-1
E-modee
(+) at 1457 cm-1
A2-modec
a ip, in-phase; oop, out-of-phase. b Scheme 1b,c. c Scheme 1a. d The assignment is done with the assumption that [Ru(III)(acac)3] approximately belongs
to D3 symmetry. e Scheme 1b.
Table 4. Calculated Effects of the Central Metal Ion of ∆-[M(III)(acac)2(dbm)] on VCD Spectra of C-O Vibrations
sign of split peaks of C-O stretching, assignment
complex with C2 symmetry
∆-[Co(III)(acac)2(dbm)]
config
d6
higher frequency
middle frequency
lower frequency
order of vibration modes in frequencya
(-) at 1473 cm-1
A-modeb
(+) at 1471 cm-1
B-modec
(+) at 1463 cm-1
B-moded
A (oop) > B (oop) > B (ip)
∆-[Cr(III)(acac)2(dbm)]
∆-[Ru(III)(acac)2(dbm)]
d3
d5
(+) at 1463 cm-1
B-moded
(-) at 1461 cm-1
A-modeb
(-) at 1451 cm-1
B-modec
B (ip) > A (oop) > B (oop)
A (oop) >B (ip) > B (oop)
(-) at 1465 cm-1
A-modeb
(+) at 1452 cm-1
B-moded
(-) at 1397 cm-1
B-modec
a ip, in-phase; oop, out-of-phase. b Scheme 2c. c Scheme 2b. d Scheme 2a.
and A (-) > B (+) > B (-) for ∆-[Ru(III)(acac)2(dbm)].
Thus, in the case of these bis(chelated) complexes, too, the
order of energy levels is influenced by the central metal ion
in a symmetry-dependent way. Table 4 summarizes the
calculated C-O stretches at 1500-1300 cm-1 of [M(III)-
(acac)2(dbm)].
To summarize the above results, the VCD bands of a chiral
metal complex are influenced by the central metal under the
following conditions: (i) at least one of the atoms in a
concerned vibration is bonded with a metal ion or (ii) a
concerned band is a composite of several peaks with different
symmetries.
It is interesting to examine how the lowering of symmetry
from D3 to C2 affects the spectral properties of VCD. Table
5 summarizes the correlation of vibrational modes between
tris- and bis(acetylacetonato)metal(III) complexes. From the
comparison of interligand correlative vibrational motions
(Schemes 1 and 2), it is deduced that the A2-mode in [M(III)-
(acac)3] corresponds to the B- (in-phase) mode in [M(III)-
(acac)2(dbm)], while the doubly degenerate E-mode in
[M(III)(acac)3] splits into the B- (out-of-phase) and A- (out-
of-phase) modes in [M(III)(acac)2(dbm)]. When the VCD
spectra are compared between these two complexes for the
same type of central metal, the sign of the A2-mode is
retained for the B- (in-phase) mode for the investigated three
complexes (Co, Cr, and Ru). The sign of the E-mode is also
retained for the B- (out-of-phase) and A- (out-of-phase)
modes except for the B- (out-of-phase) mode in the case of
[Co(III)(acac)2(dbm)].
electronic configuration of the central metal ion has little
effect on the signs of both µ and m when the symmetry of
the complex is lowered from D3 to C2. It is not very clear
why the Co(III) complex alone inverts the sign of the B-
(out-of-phase) mode by replacing one of the acac ligands
with dbm. The electronic configuration of d-electrons in a
central metal ion might affect in a delicate way the sign of
the magnetic transition moment (m). As far as we know,
there has been no systematic study correlating the VCD
spectra for a series of tris(chelated) complexes in which a
ligand in the parent complex is replaced successively with a
ligand of a different kind. Such a study may be helpful to
establish a general rule for the correlations of VCD spectra
when the symmetry of a complex is lowered from D3 to C2.
In this work, magnetic field perturbation (MFP) theory
has been extended to open-shell systems such as Cr(III) and
Ru(III) complexes. There is no theoretical reason to avoid
such extension as far as the system is appropriately described
by the electronically ground state. It is, of course, even more
difficult to treat an open-shell system by the above theory,
since the spin multiplicity causes a number of energetically
close electronic states. Our application of the theory is
regarded as considering a single dominating state among
these states. The validity of such treatment may be judged
by comparing the calculated results with the observed ones.
In this sense, the fact that the calculated results well
reproduce the observed ones in both Cr(III) and Ru(III)
complexes is thought to validate the present approach. The
work provides a benchmark of VCD application for open-
shell transition metal cases. It is certain that the broad band
below 1300 cm-1 for Ru(III) complexes arises from the low-
lying excited states involving d-d transitions. It is our
fortune that the intensity of that band is much less than
The sign and intensity of a peak in the VCD spectrum are
determined by rotational strength, R, as given by R ) {Im}-
(µ‚m), where µ and m are the electric and magnetic dipole
transition moments, respectively. Thus, in most cases, the
6764 Inorganic Chemistry, Vol. 46, No. 16, 2007

Metal Ion Effects on VCD Spectra of Tris-Chelated Complexes
Table 5. Correlation of Vibrational Modes between Tris- and Bis(acetylacetonato)metal(III) Complexes
previously observed in Ni(II) and Co(II) complexes.29 Thus
we regard that such an effect does not affect crucially the
VCD features of C-O stretches.
gel column with a mixture of benzene-acetonitrile (10:1) as an
eluent. Yield: 0.30 g (27%). The fraction was further purified on
a reversed-phase HPLC column (2 mm i.d. × 250 mm) (Capcell
Pack, Shiseido Ind. Co., Japan) by being eluted with acetonitrile-
1
water. FAB-MS (m/z) calcd for [M]+ 480.4, found 480. H NMR
Conclusion
(400 MHz, CDCl3) δ (ppm) 7.92 (d, J ) 7.6 Hz, 4H), 7.46 (t, J )
7.6 Hz, 2H), 7.38 (t, J ) 7.6 Hz, 64H), 6.77 (s, 1H), 5.62 (s, 2H),
2.99 (s, 6H), 2.17 (s, 6H). Anal. Calcd for C25H25O6Co: C, 62.50;
H, 5.25. Found: C, 62.08; H, 5.33.
This paper reports the experimental vibrational circular
dichroism (VCD) spectra of two different series of chiral
tris(â-diketonato) complexes. With the help of theoretical
works based on the magnetic field perturbation (MFP), the
results are rationalized in terms of the effect of the central
metal ion on the energies of the correlative vibrations of
ligands. Detailed assignment of composite peaks as per-
formed in the present study is impossible by IR spectroscopy
alone. In this sense, VCD spectroscopy has enabled us to
observe directly the effect of the metal ion on determining
the vibration energy levels concerning the correlative motions
among vicinal ligands.
Synthesis of [Cr(III)(acac)2(dbm)]. [Cr(III)(acac)3 ](0.70 g; 2.0
mmol) and dbmH (0.44 g; 2.0 mmole) were mixed in solid form
and kept in an autoclave for 16 h at 160 °C. The brown crude
product was eluted from a silica gel column (30 mm i.d. × 600
mm) with acetonitrile-benzene. Yield: 0.25 g (26%). The fraction
was further purified on a reversed-phase HPLC column (2 mm i.d.
× 250 mm) (Capcell Pack, Shiseido Ind. Co., Japan) by being eluted
with acetonitrile-water. FAB-MS (m/z) calcd for [M]+ 473.46,
found 473. Anal. Calcd for C25H25O6Cr: C, 63.42; H, 5.32.
Found: C, 62.43; H, 5.42.
Synthesis of [Ru(III)(acac)2(dbm)].8a [Ru((III)(acac)2(CH3CN)2]-
ClO4 (0.18 g; 0.40 mmol), dbmH (0.09 g; 0.40 mmol), Zn powder
(0.053 g), and KHCO3 (0.059 g) were dissolved in 40 mL of ethanol
and 2 mL of water, and the solution was refluxed overnight. The
solution was cooled, filtered through Celite, and evaporated. The
residue was dissolved in 50 mL of chloroform, and the solution
was refluxed overnight again with 5-10 mL of an aqueous solution
containing 0.1-0.2 g of AgNO3 added. The solution was evaporated
to dryness and purified by HPLC (Shiseido Capcell Pak C18).
Yield: 38% of a deep red powder, mp 240 °C (decomp). FAB-MS
(m/z) calcd for [M]+ 522.5, found 523. 1H NMR (400 MHz, CDCl3)
δ (ppm) 12.82 (d, J ) 6.7 Hz, 4H), 9.96 (t, J ) 7.0 Hz, 2H), 6.58
(m, 4H), -2.46 (s, 6H), -4.87 (s, 6H), -27.58 (s, 2H), -40.97 (s,
1H). Anal. Calcd. for C25H25O6Ru‚1.5H2O: C, 54.64; H, 5.12.
Found: C, 54.14; H, 4.94.
Experimental Section
[M(III)(acac)]. [Co(III)(acac)3], [Cr(III)(acac)3], [Ru(III)(acac)3],
[Rh(III)(acac)3], and [Ir(III)(acac)3] were purchased from Kanto
Kagaku Co. Ltd. These complexes were chromatographically
resolved in methanol as described below. The concentration of an
enantiomer was determined spectrophotometrically by use of ꢀ and
∆ꢀ values reported in the literature.49
Synthesis of [Co(III)(acac)2(dbm)]. [Co(III)(acac)3] (0.80 g, 2.2
mmol) and dbmH (0.50 g, 2.2 mmol) were dissolved in dichlo-
romethane under nitrogen. BF3-CH3COOH (0.14 mmol) was added
slowly, and the mixture was stirred at room temperature for 24
h.50After the reaction, a 10% CH3COOK solution (10 mL) was
added and the mixture was extracted with ether (200 mL). The
solution was evaporated and a crude product was purified on a silica
Resolution of [M(III)(acac)3] and [M(III)(acac)2(dbm)]. Opti-
cal resolution was performed on a chiral column packed with an
(50) Nakano, Y.; Noguchi, T.; Adachi, T.; Sato, S. Inorg. Chim. Acta. 2003,
343, 202.
Inorganic Chemistry, Vol. 46, No. 16, 2007 6765

Sato et al.
Table 6. Absorption and CD Spectra of ∆-[M(III)(acac)2(dbm)] in
specified as quartet and doublet in the open-shell system, respec-
tively. [M(III)(acac)3] was assumed to have D3 symmetry for M )
Co, Cr, Rh, Ir, and Al and to have C2 symmetry for M ) Ru.45,53
The reason that the C2 symmetry was selected for [Ru(III)(acac)3]
instead of D3 was that the energy was not converged under the
assumption of D3 symmetry. This was probably because the Jahn-
Teller distortion from D3 symmetry is not neglected for this Ru-
(III) complex. The same calculation was previously reported by
other authors.53 [M(III)(acac)2(dbm)] was assumed to have C2
symmetry. IR and VCD spectra were calculated after the geometry
optimization. The optimization was performed with tight conver-
gence, avoiding numerical errors. No correction of introducing a
scale factor, however, was made in the present calculation. VCD
intensities were determined by the vibrational rotational strength
and magnetic dipole moments, which were calculated by the
magnetic field perturbation method with gauge-invariant atomic
orbitals.14 Description of calculated modes is based on the anima-
tions of the modes and spectra to the resolution of 1 cm-1 with
Gauss view 3.09 (Gaussian Inc.).
Methanol
complex
λ, nm
ꢀ, mol-1 cm-1
∆ꢀ, mol-1 cm-1
∆-[Co(III)(acac)2(dbm)]
∆-[Cr(III)(acac)2(dbm)]
∆-[Ru(III)(acac)2(dbm)]
597
560
500
183.4
80.5
1902
-5.09
-2.68
+ 4.66
ion-exchange adduct of ∆-[Ru(phen)3]2+ (phen ) 1,10-phenan-
throline) and synthetic hectorite (Ceramosphere RU-1, Shiseido Ind.
Co.) with methanol.51 The Λ and ∆ isomers were resolved in the
baseline separation in this order. Measured ꢀ and ∆ꢀ values are
summarized for [M(III)(acac)2(dbm)] in Table 6.
Instruments. CD spectra were recorded on a JASCO J-720
spectropolarimeter. The VCD spectra were recorded at a 4 cm-1
resolution using a circular spectrometer JV-2001 (JASCO, Japan)
and Bomem/BioTools, Inc.41 For the measurements, a CDCl3
solution of a complex (ca. 0.04 M) was injected into a cell (6.2
cm2 × 100 µm) with CaF2 windows. The signal was accumulated
with 3000 scans for tris-chelated complexes and 10 000 scans for
bis-chelated complexes during about 30 min and about 2 h,
respectively. IR absorbance adjusted under 1.0 was optimal for VCD
measurements. The mole concentration of each metal complex was
0.04 M.
Calculation Details. The VCD spectra of these complexes were
theoretically calculated for their optimized geometries. Vibrational
frequencies and VCD intensities were simulated by use of Gaussian
03 at the DFT level [B3LYP functional with LANL2DZ for
transition metals and 6-31G(d) for other atoms].52 Co(III) (eg)6, Rh-
(III) (eg)6, Ir (III) (eg)6, and Al(III) were specified as singlet in the
closed-shell system, while Cr(III) (eg)3 and Ru(III) (eg)5 were
Acknowledgment. This work was financially supported
by a Grant-in-Aid for Scientific Research on Priority Areas
(417) from the Ministry of Education, Culture, Sports,
Science and Technology (MEXT) of the Japanese govern-
ment. Thanks are due to Professor Nobuaki Miura (Hokkaido
University) and Professor Haruyuki Nakano (Kyushu Uni-
versity) for theoretical support. Thanks are due to Jun
Yoshida (The University of Tokyo) for the synthesis of [Co-
(III)(acac)2(dbm)] and Mai Hashimoto (Hokkaido University)
for their support for measurement of VCD. Thanks are due
to Professor Kazunari Naka (Hiroshima University) for his
comment on the use of Gaussian 03. We also thank Kenichi
Akao and Jun Koshoubu (JASCO Corp.) for the VCD
measurements of [Ir(III)(acac)3].
(51) (a) Yamagishi, A. ReV. Coord. Chem. 1987, 16, 131. (b) Kakegawa,
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K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
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W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
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K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
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Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
Supporting Information Available: Results of ECD of [M(II-
II)(acac)3] (M ) Co, Rh, Ir, Cr, and Ru)and of chromatographic
resolution of [M(III)(acac)2(dbm)] (M ) Co, Cr, and Ru)and
assignment of the main peaks of IR and VCD spectra of [M(IIII)-
(acac)3] and [M(III)(acac)2(dbm)]. This material is available free
of charge via the Internet at http://pubs.acs.org.
IC070300I
(53) Reynolds, P. A.; Cable, J. W.; Sobolev, A. N.; Figgis, B. N. J. Chem.
Soc., Dalton Trans. 1998, 559.
6766 Inorganic Chemistry, Vol. 46, No. 16, 2007