Phenyl substitution reactions of 3-bromo-2,4-pentanedionato complexes

Article abstract of DOI:10.1016/S0020-1693(01)00531-X

Phenyl substitution reactions of 3-bromo-2,4-pentanedionate metal (M = Cr and Co) complexes catalyzed by palladium (Suzuki coupling reaction) were carried out. Variously substituted products were identified by standard spectroscopic methods. In order to c

Full text of DOI:10.1016/S0020-1693(01)00531-X

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Inorganica Chimica Acta 321 (2001) 149–152
Phenyl substitution reactions of 3-bromo-2,4-pentanedionato
complexes
Yoshiharu Nakano *, Akiko Baba, Tsuneo Adachi, Takatoshi Noguchi
Faculty of Science, Ibaraki Uni6ersity, Bunkyo 2-1-1, Ibaraki 310-8512, Japan
Received 27 March 2001; accepted 15 June 2001
Abstract
Phenyl substitution reactions of 3-bromo-2,4-pentanedionate metal (M=Cr and Co) complexes catalyzed by palladium (Suzuki
coupling reaction) were carried out. Variously substituted products were identified by standard spectroscopic methods. In order
to confirm the products, crystal structures of the several products were determined by the X-ray diffraction method. © 2001
Elsevier Science B.V. All rights reserved.
Keywords: Crystal structures; Suzuki reaction; Catalysis; Chromium complexes; Cobalt complexes; Acetylacetonate complexes
1. Introduction
of metal 3-bromo-2,4-pentanedionate with phenyl-
boronic acid as shown in Fig. 1. In this paper, we
describe the results of Suzuki coupling reactions on
M(acac)3 (M=Cr, Co).
About 40 years ago, there was a heated discussion in
the field of coordination chemistry that metal acetylace-
tonates (M(acac)n, n=2 or 3) might have an aromatic
character on the basis of large stability constants [1]. In
fact, many acetylacetonate complexes have remarkable
stabilities and show various properties similar to aro-
matic compounds. Collman, Nyholm and Kluiber per-
formed many electrophilic reactions on the g-position
of M(acac)3 (M=Cr, Co, Rh) [2]. Although the eager
hope for demonstrating the aromatic character of
acetylacetonate rings has subsided recently, the work
on the aromaticity of Collman and others attracted and
encouraged us to investigate similar substitution reac-
tions. Here, we report aryl substitution reactions
(Suzuki coupling reaction [3]) of metal g-bromoacety-
lacetonates. The Suzuki coupling reaction is one of the
most important reactions due to its mild conditions and
good results over a wide range of aryl halides and aryl
oxysulfonates. Arylboronates for example give biaryls
which can be prepared by Ullmann reaction in low
yields only. The Suzuki reaction conditions are so mild
that almost all functional groups are usually tolerated.
We attempted a Pd(0) catalyzed cross coupling reaction
2. Experimental
2.1. Materials and instruments
All reagents were obtained from chemical companies
and used without purification. Bromination of Cr(a-
cac)3 and Co(acac)3 were performed with NBS [4].
1
Using JEOL GSX-400, H NMR and 13C NMR were
obtained. Infrared spectra (400–4000 cm−1) and mass
spectra were measured with a JASCO FT/IR-300E and
JEOL DX-300, respectively. Since metal complexes are
generally not so stable in severe conditions, mass spec-
tral data of metal complexes under usual measurement
conditions are useful only in limited cases. Most of the
* Corresponding author.
E-mail address: nyoshi@mito.ipc.ibaraki.ac.jp (Y. Nakano).
Fig. 1. Cross coupling reactions of M(g-X-acac)3.
0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(01)00531-X

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Y. Nakano et al. / Inorganica Chimica Acta 321 (2001) 149–152
Table 1
Reaction products and yields
washed with hexane and recrystallized from a mixture
of hexane and benzene. All the components were eluted
with benzene in the case of the cobalt complex.
M=Cr
M=Co
R1 R2 R3 Yield (%)
R1 R2 R3 Yield (%)
2.3. Characterization of chromium and cobalt
complexes
a
b
c
Ph Br Br
Ph Ph Br
23 a
3
a
b
c
Br Br
Ph Br
H
H
H
H
7
4
Cr-a (yield=23%): m.p. 170 °C. Anal. Found: C,
43.2; H, 3.8. Calc. for C21H23Br2CrO6 (FW=583.22):
C, 43.3; H, 4.0%. IR (KBr) w (cm−1); 441 (w), 468 (s),
610 (m), 640 (m), 707 (s), 768 (s), 922 (s), 985 (m), 1025
(s), 1335 (s), 1421 (s), 1460 (s), 1556 (s), 3019 (w). MS:
m/e 583, 585, 587, 406.
Ph Br
Ph Ph
H
H
7 a
Br
Ph
H
H
25
d
0.8
d
7 a
a The crystal structure of the product was determined by X-ray
diffraction.
Cr-b (yield=3%): m.p. 219 °C. Anal. Found: C, 56.0;
H, 4.9. Calc. for C27H28BrCrO6 (FW=580.42): C, 55.9;
H, 4.9%. IR (KBr); w (cm−1), 469 (m), 615 (w), 708
(m), 781 (w), 930 (m), 1341 (s), 1423 (s), 1448 (s), 1561
(s), 3023 (w). MS: m/e 581, 579, 402.
Cr-c (Fig. 2) (yield=7%): m.p. 107 °C. Anal. Found:
C, 56.0; H, 4.90. Calc. for C21H24BrCrO6 (FW=
504.11): C, 55.9; H, 4.9%. IR (KBr); w (cm−1), 465 (m),
613 (w), 707 (w), 921 (w), 1021 (m), 1274 (m), 1339 (s),
1424 (s), 1560 (s) cm−1. MS: m/e 503,505, 326.
Cr-d (yield=0.8%): m.p. 198 °C. Anal. Found: C,
64.6; H, 5.8. Calc. for C27H29CrO6 (FW=501.53): C,
64.7; H, 5.4%. IR (KBr); w (cm−1), 483 (w), 632 (w),
930 (w), 1013 (w), 1367 (m), 1441 (m), 1525 (m), 1573
(s). MS: m/e 501, 402, 326, 151.
Cr-e (yield=1.4%): C15H20BrCrO6 (428.23): m.p. 193
°C. IR (KBr); w (cm−1), 460 (s), 596 (m), 624 (m), 708
(m), 772 (m), 929 (m), 1016 (s), 1279 (s), 1383 (s), 1425
(s), 1520 (s), 1573 (s). MS: m/e 429, 427, 326, 250, 151.
Co-a (yield=7%): m.p. 147–149 °C. Anal. Found: C,
34.8; H, 3.6. Calc. for C15H19Br2CoO6 (FW=514.04):
Fig. 2. ORTEP drawing of Cr-c showing atom labeling scheme. Car-
bon atoms, C(22)ꢀC(24) are half of the solvated benzene.
chromium and cobalt complexes in this report are
stable enough to show molecular ions. Elemental analy-
ses were performed on a Yanaco MT-5.
1
C, 35.1; H, 3.7%. H NMR (CDCl3): l 5.55 (H, s), 2.53
(6H, s), 2.49 (6H, s), and 2.21 (6H, s). 13C NMR
(CDCl3): l 189.80, 188.40, 96.70, 94.70, 29.74, 29.61,
25.98. IR (KBr); w (cm−1), 474 (s), 648 (m), 697 (w),
771 (w), 930 (w), 986 (m), 1028 (s), 1338 (s), 1362 (s),
1419 (s), 1460 (w), 1519 (s), 1551 (vs), 1637 (w), 3435 (s,
br). MS: m/e 515, 513, 511, 334, 319, 158.
2.2. Phenyl substitution reactions
A mixture of M(3-Br-acac)3 (10 mmol), phenyl-
boronic acid (33 mmol), finely powdered solid base
(NaOH or Ba(OH)2·6H2O) (33 mmol), Pd(PPh3)4 (0.3
mmol), and benzene (50 ml) was refluxed for 24 h under
argon. The reaction mixture was filtered once and
washed with water, then dried with Na2SO4. The sol-
vent was evaporated and the residue was subjected to
chromatography. In the case of the chromium complex,
biphenyl and many components of chromium com-
plexes were discernible on TLC. In the case of
chromium, five components were isolated as listed in
Table 1 from Cr-a to Cr-e. Biphenyl and Cr-a compo-
nents were eluted at the same time with a mixture of
benzene and hexane (5:2), and the other components
were eluted with a mixture of benzene and ethyl acetate
(7:1). Since Cr-b was contaminated with several organic
species after chromatography, the solid of Cr-b was
Co-b (yield=4%): m.p. 125–128 °C. Anal. Found: C,
49.6; H, 4.7. Calc. for C21H24BrCoO6 (FW=511.25):
1
C, 49.3; H, 4.7%. H NMR (CDCl3): l 7.45–7.29 (5H),
5.57 (H, s), 2.56 (3H, s), 2.51 (3H, s), 2.23 (6H, s), 1.98
(3H, s), and 1.94 (3H, s). 13C NMR (CDCl3): l 189.88,
189.49, 188.58, 188.39, 188.28, 188.09, 139.95, 131.92,
128.80, 128.34, 127.05, 112.00, 97.35, 95.00, 77.37,
77.05, 76.72, 29.74, 26.93, 26.77, 26.20, 26.06. IR
(KBr); w (cm−1), 471 (s), 645 (m), 708 (m), 927 (w), 982
(w), 1025 (s), 1342 (vs), 1365 (s), 1421 (s), 1520 (s), 1556
(vs), 1628 (sh), 2912 (w), 3435 (m, br). MS: m/e 513,
511, 509, 333, 242, 158.
Co-c (yield=25%): m.p. 149–151 °C. Anal. Found:
C, 41.6; H, 4.4. Calc. for C15H20BrCoO6 (FW=
1
435.16): C, 41.4; H, 4.6%. H NMR (CDCl3): l 5.54

Y. Nakano et al. / Inorganica Chimica Acta 321 (2001) 149–152
151
1027 (s), 1193 (w), 1281 (m), 1362 (vs), 1423 (s), 1519
(vs), 1556 (vs), 1637 (w), 2921 (w), 3005 (w), 3435 (s,
br). MS: m/e 435,433, 334, 257, 158.
Co-d (Fig. 3) (yield=7%): m.p. 165–166 °C. Anal.
Found: C, 61.1; H, 6.2. Calc. for C21H25CoO6 (FW=
1
432.36): C, 61.2; H, 6.0%. H NMR (CDCl3): l 7.38–
7.17 (5H), 5.56 (2H, s), 2.24 (6H, s), 2.20 (6H, s), and
1.94 (6H, s). 13C NMR (CDCl3): l 189.63, 189.41,
188.33, 140.26, 131.98, 128.74, 126.90, 112.34, 97.29,
77.38, 77.05, 76.73, 26.83, 26.15. IR (KBr); w (cm−1),
474 (w), 633 (w), 707 (w), 771 (w), 930 (w), 1023 (w),
1388 (m), 1522 (s), 1571 (vs), 1637 (w), 3446 (s, br). MS:
m/e 432, 333, 242, 158.
2.4. X-ray crystallography
Crystallographic data are listed in Table 2. Since two
chloroform molecules were included in the complex of
Cr-a and the crystals tend to deteriorate probably due
to loss of part of them during the measurement, they
had to be coated with a sticking agent. Reflection
intensities of all the crystals were measured at room
temperature with Rigaku AFC7R diffractometer. In-
tensity data were corrected for absorption. The struc-
tures were solved by direct methods (SIR92) [5] and
Fig. 3. ORTEP drawing of Co-d showing atom labeling scheme.
Carbon atoms, C(22)ꢀC(24) are half of the solvated benzene.
(2H, s), 2.50 (6H, s), 2.22 (6H, s), and 2.18 (6H, s). 13C
NMR (CDCl3): l 189.72, 189.56, 188.14, 97.15, 77.37,
77.05, 76.73, 29.71, 26.12, 25.98. IR (KBr); w (cm−1),
470 (s), 637 (m), 678 (m), 766 (m), 936 (w), 984 (m),
Table 2
Crystallographic data a
Complex
Cr-a
Cr-c
Co-d
Crystallizing solvent
Formula
Formula weight
Space group
CHCl3
benzene–hexane
C24H27BrCrO5
543.47
benzene–hexane
C24H28CoO6
471.41
C
23H25Br2CrO6Cl6
821.97
P21/c (no. 14)
(
(
P1 (no. 2)
P1 (no. 2)
Unit cell dimensions
,
a (A)
12.849(7)
14.646(8)
17.458(7)
90.0
100.35(4)
90.0
3231(2)
4
1.689
12.004(2)
14.413(3)
7.490(1)
101.46(1)
92.79(1)
102.25(2)
1235.5(4)
2
11.702(2)
14.052(4)
7.478(1)
97.60(2)
99.15(1)
101.05(2)
1174.5(5)
2
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
V (A )
3
,
Z
Dcalc (g cm−3
)
1.461
21.20
1.333
7.66
v (cm−1
)
33.64
2qmax
55.0
60.0
60.0
h
k
l
0 to 16
−22 to 22
−19 to 0
0.63×0.56×0.53
7766
0 to 15
−20 to 19
−10 to 10
0.50×0.20×0.13
6172
0 to 16
−19 to 19
−10 to 10
0.53×0.50×0.25
8031
Crystal size (mm)
Total reflections
Unique reflections
7435
5851
7694
Reflections observed [I\2.0|(I)]
3527
0.057
0.070(0.04)
1.81
2477
0.058
0.071(0.036)
1.75
3898
0.041
0.041(0.019)
1.67
b
R
c
Rw (p-factor)
Goodness-of-fit
a Radiation: Mo Ka (graphite monochromated, u=0.71069).
b R=(ꢀꢀFoꢀ−ꢀFcꢀꢀ/SꢀFoꢀ).
c Rw=[(Sw(ꢀFoꢀ−ꢀFcꢀ)2/SwFo
] ]
2 1/2, w=[|c2(Fo)+p2/4)Fo2 −1, p=p-factor.

152
Y. Nakano et al. / Inorganica Chimica Acta 321 (2001) 149–152
Table 3
tion pattern. Those strong peaks characteristic of
b-diketone complexes were underlined in Section 2.3.
Substituted chelate rings and non-substituted rings
showed strong peaks at 1550–1570 and 1515–1530
cm−1, respectively.
,
Selected bond distances (A) and angles (°)
In all the reactions, large amounts of biphenyl and
various materials reduced to non-substituted chelate
rings were isolated. The formation of biphenyl has been
explained previously [6]. Since it is formed from inter-
mediate ionic borate species, its formation was assumed
to be inevitable in our experiments and biphenyl was
isolated in varying yields (17–48%). Low yields of
phenyl-substituted products in all the reactions might
be attributable to steric hindrance at the reaction cen-
ter. In the case of iodoacetylacetonatochromium com-
plex, only a trace (0.4%) of Cr(acac)2(Phacac) was
obtained, while a large amount of Cr(acac)3 (35%) was
isolated. In the case of chloroacetylacetonatochromium
complex, none of the phenyl-substituted complex was
isolated except biphenyl which was probably formed
from phenylboronic acid.
Other electrophilic substitution reactions have to be
attempted and will be reported elsewhere.
4. Description of the structure
Crystal structures of the two chromium complexes
and the cobalt complex were determined. The crystallo-
graphic data are summarized in Table 2. In the case of
the Cr-a, one of two chloroform molecules included in
the unit cell has a disordered structure and large ther-
mal parameters. In the case of Cr-c and Co-d, both
complexes have one molecule of benzene per unit cell.
ORTEP drawings of Cr-c and Co-d were shown in Figure
2 and Figure 3, respectively. The phenyl rings intro-
duced in this reaction have an almost perpendicular
orientation against the acac ring. Selected bond dis-
tances and angles of Cr-c and Co-d are listed in Table
3. Coordination bonds (MꢀO) of chromium complexes
structure refinement were performed with full-matrix
least-squares. All the calculations were performed using
TEXSAN program system on the SGI Indy computer at
the Center for Instrumental Analysis of Ibaraki
University.
3. Results and discussion
Palladium-catalyzed cross coupling reactions of metal
(Cr(III) and Co(III)) g-haloacetylacetonates with
phenylboronic acid gave a complex mixture of vari-
ously phenyl-substituted metal complexes and reduced
products (see Table 1). After standard open column
chromatography was repeated twice, the crude products
were recrystallized from the appropriate solvents (indi-
cated in Table 2). Chromium complexes were isolated
more easily than cobalt complexes, probably due to the
inertness of chromium complexes under the hydrolytic
conditions. Chromium complexes were identified by IR,
mass spectrometry and elemental analyses, and finally
by X-ray diffraction method. In the case of cobalt
complexes, NMR spectroscopy was also useful to iden-
tify the structures. Infrared spectral features assigned to
CꢁO and CꢁC of the acac ring in the region of 1500–
1600 cm−1 were observed according to 0–3 substitu-
,
are in the range of 1.931–1.978 A and those of cobalt
,
complexes are 1.878–1.891 A. The coordination bonds
in the cobalt complex are rather short compared to
those of chromium complexes. This fact can be ascribed
to the smaller effective nuclear charge of the cobalt
atom compared to that of the chromium atom.
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