Reductive coupling of benzyl bromide catalyzed by a novel dicobalt complex having two salen units

Article abstract of DOI:10.1039/b010022f

A novel dicobalt complex[Co^II₂L] has been synthesized, where (L)^4- is a dinucleating ligand derived by the 2 : 2 : 1 condensation of ethylenediamine, salicylaldehyde, and 5,5′-methylenebis(salicylaldehyde), and it has two N₂O₂ metal-binding sites which are linked to each other with a methylene spacer. This complex was characterized by UV-VIS, IR, and NMR as well as mass spectroscopy. Its redox behavior was investigated in dmf using cyclic voltammetry in comparison with that for the corresponding mononuclear complex [Co(salen)]. The redox couples to Co^III-Co^II and Co^II-Co^I for [Co^II₂L] were observed at +0.09 and - 1.20 V vs. Ag-AgCl, respectively. These potentials are quite similar to those for [Co(salen)]. The electro-generated [Co^I_2- reacts with alkyl halide at each metal center to give an organocobalt complex. A further one-electron reduction of the compound yields an unstable intermediate that undergoes rapid decomposition by cleavage of the cobalt-carbon bond. The electrolysis of benzyl bromide at - 1.40 V vs. Ag-AgCl in the presence of the dicobalt complex yields bibenzyl as the major product. On the other hand, when [Co(salen)] was used as a catalyst, toluene was obtained as the major product. The difference in the product distribution is due to the structural properties of the catalysts.

Full text of DOI:10.1039/b010022f

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Reductive coupling of benzyl bromide catalyzed by a novel dicobalt
complex having two salen units†
Hisashi Shimakoshi, Wataru Ninomiya and Yoshio Hisaeda*
Department of Chemistry and Biochemistry, Graduate School of Engineering,
Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan.
E-mail: yhisatcm@mbox.nc.kyushu-u.ac.jp
Received 15th December 2000, Accepted 11th May 2001
First published as an Advance Article on the web 15th June 2001
A novel dicobalt complex [CoII L] has been synthesized, where (L)4Ϫ is a dinucleating ligand derived by the 2 : 2 : 1
2
condensation of ethylenediamine, salicylaldehyde, and 5,5Ј-methylenebis(salicylaldehyde), and it has two N2O2
metal-binding sites which are linked to each other with a methylene spacer. This complex was characterized by
UV-VIS, IR, and NMR as well as mass spectroscopy. Its redox behavior was investigated in dmf using cyclic
voltammetry in comparison with that for the corresponding mononuclear complex [Co(salen)]. The redox couples
to CoIII–CoII and CoII–CoI for [CoII L] were observed at ϩ0.09 and Ϫ1.20 V vs. Ag–AgCl, respectively. These
2
potentials are quite similar to those for [Co(salen)]. The electro-generated [CoI2L]2Ϫ reacts with alkyl halide at each
metal center to give an organocobalt complex. A further one-electron reduction of the compound yields an unstable
intermediate that undergoes rapid decomposition by cleavage of the cobalt–carbon bond. The electrolysis of benzyl
bromide at Ϫ1.40 V vs. Ag–AgCl in the presence of the dicobalt complex yields bibenzyl as the major product.
On the other hand, when [Co(salen)] was used as a catalyst, toluene was obtained as the major product. The
difference in the product distribution is due to the structural properties of the catalysts.
In this paper we report the synthesis, characterization, and
Introduction
redox behavior of a novel dicobalt complex, and the electro-
reductive coupling reaction of benzyl bromide which has been
shown to be catalyzed by the dicobalt complex.
A compound with a cobalt–carbon bond is a useful reagent for
forming radical species. The cobalt–carbon bond can be
cleaved homolytically in a simple fashion by photolysis,
electrolysis, or thermolysis to form CoII and the corresponding
radical species as shown in Scheme 1.1,2 The application of the
Experimental
General analyses and measurements
Elemental analyses were obtained from the Service Centre
of Elementary Analysis of Organic Compounds at Kyushu
1
University. The H, 13C and 2-D NMR (COSY) spectra (in
CDCl3) were recorded on a Bruker AMX 500 spectrometer and
the chemical shifts (in ppm) are referenced to SiMe4 as the
internal standard. The ESR spectrum was obtained on a JEOL
JES-FE1G X-band spectrometer equipped with an Advantest
TR-5213 microwave counter and an Echo Electronics EFM-200
NMR field meter. The FAB mass spectra were recorded on a
JEOL JMS-HX110A, IR spectra on a JASCO IR-810 spectro-
photometer using KBr disks and UV-VIS absorption spectra
in chloroform or thf on a Hitachi U-3300 spectrophotometer
at room temperature. The complexes of CoII and CoI were
measured under anaerobic conditions to prevent auto-
oxidation. The cobalt() complex was prepared in situ from the
cobalt() complex by NaBH4/PdCl2 reduction under anaerobic
conditions.20 Cyclic voltammetry (CV) and differential pulse
voltammetry (DPV) were performed using a BAS CV 50W elec-
trochemical analyzer. A three-electrode cell equipped with a 1.6
mm diameter platinum wire and coil as the working electrode
and counter electrode was used.9 An Ag–AgCl (3.0 M NaCl)
electrode served as the reference. Non-aqueous solutions
containing the cobalt complex (5.0 × 10Ϫ4 M) and tetra-n-
butylammonium perchlorate (1.0 × 10Ϫ1 M) were deaerated
prior to each measurement, and the inside of the cell was
maintained under an argon atmosphere throughout. All the
measurements were carried out at room temperature. The scan
rate was varied over the range from 10 to 500 mV sϪ1. The E1/2
value of Fc–Fcϩ was 0.54 V vs. Ag–AgCl with this set-up.
Scheme 1
alkylated complex to organic synthesis is quite interesting from
the viewpoint of a radical forming reagent.3–5 For example, we
have been dealing with hydrophobic vitamin B12 derivatives
which have ester groups in place of the peripheral amide
moieties of the naturally occurring vitamin B12,6 and succeeded
in performing various organic reactions such as the 1,2 migra-
tion of functional groups, asymmetric reactions, and ring
expansion reactions using a hydrophobic vitamin B12 derivative
as a catalyst.7–19 Though many studies have used a compound
with a cobalt–carbon bond, there is no example of a dicobalt
complex with cobalt–carbon bonds employed as a catalyst.
When we used a dicobalt complex it was possible to activate
two molecules of the substrate at the same time to form two
active species. Therefore, we designed a novel dicobalt complex
having two [Co(salen)] units capable of forming two cobalt–
carbon bonds at each metal center. The cobalt salen complex is
widely known to form a stable cobalt–carbon bond.20,21
† Electronic supplementary information (ESI) available: 1H NMR
spectrum of ligand L and assignments; UV-VIS spectra of [CoII2L] and
[CoI2L]; ESR spectrum for the spin trapping. See http://www.rsc.org/
suppdata/dt/b0/b010022f/
DOI: 10.1039/b010022f
J. Chem. Soc., Dalton Trans., 2001, 1971–1974
This journal is © The Royal Society of Chemistry 2001
1971

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Scheme 2
Electrolysis of benzyl bromide
6.93 (2 H, d, Ph), 6.98 (2 H, s, Ph), 7.08 (2 H, d-d, Ph), 7.23 (2
H, d, Ph), 7.30 (2 H, t, Ph), 8.29 (2 H, s, N᎐CH), 8.35 (2 H, s,
᎐
Electrolysis was carried out in a two-compartment cell
equipped with platinum electrodes at room temperature under
an argon atmosphere during irradiation with a 500 W tungsten
lamp at a distance of 50 cm.9 The applied potential between the
working and reference electrodes during the electrolysis was
N᎐CH), 13.06 (2 H, s, OH) and 13.23 (2 H, s, OH). 13C NMR
᎐
(125 MHz, CDCl3, SiMe4): δ 39.7 (C6H3CH2C6H3), 59.8
(NCH2CH2N), 116.9, 117.0, 118.4, 118.6, 118.7, 131.3, 131.4,
131.5, 132.4, 133.0 (Ph), 159.4, 161.0 (N᎐CH) and 166.5 (PhO).
᎐
HRMS (FAB, m/z): calc. for C33H33N4O4, [MH]ϩ 549.2502;
maintained constant with
a
Hokuto Denko HA-501
found 549.2505.
potentiostat/galvanostat and the reaction was monitored on a
Hokuto Denko HF-201 coulomb/ampere-hour meter. Initial
concentrations: [CoII2L], 5.0 × 10Ϫ4 M; [CoII(salen)], 1.0 × 10Ϫ3
M; benzyl bromide, 5.0 × 10Ϫ3 M; dmf solvent containing
0.05 M tetra-n-butylammonium tetrafluoroborate. The GLC
analyses were carried out on a Shimadzu GC-9A apparatus
equipped with a Shimadzu C-R6A chromatopac.
Synthesis of [CoII L]
2
All procedures were carried out using a standard Schlenk
apparatus to avoid oxidation by atmospheric dioxygen. To a
solution of L (82 mg, 0.15 mmol) in 10 mL of chloroform was
dropwise added Co(OAc)2ؒ4H2O (78 mg, 0.32 mmol) in 10 mL
of methanol. A reddish brown solid was immediately precip-
itated and the mixture stirred at room temperature for 1 h. The
resulting solid was collected by filtration, washed with methanol
and chloroform and dried in vacuo to afford a reddish brown
powder (94 mg, 95%). Found: C, 58.40; H, 4.33; N, 8.14%.
C33H28Co2N4O4ؒH2O requires C, 58.25; H, 4.44; N, 8.23%. IR,
Materials
5,5Ј-Methylenebis(salicylaldehyde) was prepared by the method
reported.22 All solvents and chemicals used in the syntheses
were of reagent grade, used without further purification. For
the electrochemical studies, dmf was stirred for one day in the
presence of BaO under N2, then distilled at reduced pressure.
NBu4ClO4 was purchased from Nakarai Chemicals (special
grade) and dried at 60 ЊC under vacuum before use. NBu4BF4
was prepared based on a reported procedure.23 N-Benzylidene-
tert-butylamine N-oxide (PBN) was purchased from Aldrich
and used without further purification. [Co(salen)] was prepared
as described.24
ν/cmϪ1: 1640s (C᎐N) and 1310s (C–O) (KBr). HRMS (FAB,
᎐
m/z): calc. for C33H28Co2N4O4, [M]ϩ 662.0774; found 662.0754.
Results and discussion
Preparations of the ligand and complex
The dinucleating ligand L was synthesized by the one-pot
reaction of 5,5Ј-methylenebis(salicylaldehyde) and double the
molar quantity of ethylenediamine and salicylaldehyde as
shown in Scheme 2. Polymeric products and a monomeric one
(H2(salen)) were also produced, but they could easily be sep-
arated from the desired dinucleating ligand due to the difference
in their solubility in organic solvents. The IR analysis of L
shows an intense ν(C–N) band and broad ν(O–H) band at 1630
and 3450 cmϪ1, respectively. The structure of the ligand L was
characterized by NMR and high resolution mass spectro-
scopies (HRMS).
Synthesis of the ligand (L)
To a solution of 5,5Ј-methylenebis(salicylaldehyde) (8.96 g, 35
mmol) and salicylaldehyde (8.54 g, 70 mmol) in 200 mL of
benzene, ethylenediamine (4.21 g, 70 mmol) in 50 mL of
benzene was slowly added, and the reaction mixture refluxed
for 30 min. It was cooled to room temperature and the precip-
itated polymeric product removed by filtration. The filtrate was
concentrated to dryness, chloroform added, and the insoluble
solid was filtered off again. The same procedures were repeated
three times completely to remove the polymeric product. The
filtrate was concentrated to dryness. The resulting product was
washed with hot ethanol to remove a by-product (H2(salen)),
affording the pure compound L (2.3 g, 12%). Found: C, 71.98;
H, 5.87; N, 9.85%. C33H32N4O4 requires C, 72.24; H, 5.88; N,
The dicobalt complex [CoII2L] was synthesized by the
reaction between L and Co(OAc)2ؒ4H2O in MeOH under
anaerobic conditions, and isolated as a reddish brown powder
in 95% yield. A satisfactory elemental analysis and FAB-MS
spectrum have been obtained for the complex. The electronic
spectra of the dicobalt complex with L in various oxidation
states are similar to those of the corresponding mononuclear
cobalt salen complexes.25 They suggest an analogous coordin-
ation environment and geometry to those of [Co(salen)]. The
10.21%. IR, ν/cmϪ1: 3450br (OH), 1630s (C᎐N) and 1280s (C–O)
᎐
1
(KBr). H NMR (500 MHz, CDCl3, SiMe4): δH 3.81 (2 H, s,
C6H3CH2C6H3), 3.92 (8 H, s, NCH2CH2N), 6.85 (4 H, m, Ph),
1972
J. Chem. Soc., Dalton Trans., 2001, 1971–1974

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Fig. 2 (a) Cyclic voltammogram and (b) differential pulse voltam-
mogram of [CoII2L] in the presence of benzyl bromide in dmf. Initial
concentrations: [CoII2L], 1.0 × 10Ϫ3 M; benzyl bromide, 1.0 × 10Ϫ2 M;
NBu4ClO4, 1.0 × 10Ϫ1 M. Scan rate: (a) 100, (b) 20 mV sϪ1
.
Fig. 1 Cyclic voltammograms of (a) [CoII2L] and (b) [CoII(salen)] in
dmf containing 0.1 M NBu4ClO4. Scan rate is 100 mV sϪ1
became irreversible, and shifted to a more negative potential
(refer to eqn. 1). The disappearance of the oxidation peak for
.
absorption spectrum of [CoII2L] in deaerated acetonitrile
(1)
showed the typical [CoII(salen)] bands, λmax/nm (ε/MϪ1 cmϪ1
)
350 (1.3 × 104), 408 (1.6 × 104), 493(sh) (3.1 × 103). The [CoII2L]
complex was reduced with NaBH4/PdCl2/NaOH in deaerated
thf to give a green solution containing [CoI2L]2Ϫ, which showed
a strong charge-transfer band at 703 nm (1.4 × 104 MϪ1 cmϪ1).
This is also a typical spectrum for the [CoI(salen)]Ϫ complex.25
CoI–CoII suggests that the reaction of [CoI2L]2Ϫ with benzyl
bromide gives benzyl–CoIII with a cobalt–carbon bond at
both metal centers as shown in eqn. (2). Therefore, the new
Electrochemistry of [CoII L]
2
(2)
Cyclic voltammograms of [CoII2L] and [Co(salen)] in dmf are
shown in Fig. 1. The CoII–CoI and CoIII–CoII redox couples for
[CoII2L] were observed at Ϫ1.20 and ϩ0.09 V vs. Ag–AgCl,
respectively, very similar to those of the corresponding mono-
nuclear complex [CoII(salen)]. The ratios between the anodic
and cathodic peak currents, ipa : ipc, were almost unity and
independent of the scan rate (from 10 to 500 mV sϪ1) for the
two redox couples in dmf. Plots of ip (= ipa ϩ ipc) vs. ν1/2 (ν is
the scan rate, mV sϪ1) were linear, and the separation between
the anodic and cathodic peaks varied from 100 to 80 mV for the
two redox couples, while the E1/2 values remained constant with-
in an accuracy of 5% regardless of the scan rate variation. The
present electrochemical redox reactions in dmf are consistent
with reversible one-electron transfer processes.26 Therefore, the
electron transfers occur independently at each cobalt with no
interaction between the two metal centers. In order to provide
an unambiguous assignment of the redox couples, coulometry
studies were carried out at Ϫ1.3 and ϩ0.2 V vs. Ag–AgCl. The
charge passed corresponded to two electrons at each potential.
Square-planar cobalt() complexes are strong nucleophiles
and react with various organic halides to give organocobalt
complexes.27 The addition of benzyl bromide significantly
changed the voltammetric pattern of the complex as shown
in Fig. 2(a). In the presence of an excess of benzyl bromide
a new peak appeared at a more cathodic potential for the
CoII–CoI reduction peak as shown in Fig. 2(b). This wave
peak at Ϫ1.38 V vs. Ag–AgCl is caused by the reduction of
[(CoIIICH2Ph)2L] as shown in eqn. (3).
(3)
When the dialkylated complex [(CoIIICH2Ph)2L] is irradiated
with visible light the cobalt–carbon bonds homolytically cleave
to form the corresponding two radical species. If the radical
parts are close to each other a coupling reaction will effectively
occur. Based on this idea, we applied this complex as a catalyst
for the reductive dimerization of the alkyl halide as shown in
eqn. (4).
(4)
J. Chem. Soc., Dalton Trans., 2001, 1971–1974
1973

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Table 1 Product analyses for the controlled-potential electrolyses of
benzyl bromide catalyzed by [CoII2L] and [CoII(salen)]a
the alkylated complex. Upon irradiation by visible light the
cobalt–carbon bonds of the alkylated complex then homo-
lytically cleave to form benzyl radicals. The benzyl radicals are
efficiently coupled before they diffuse out.
In conclusion, a novel dicobalt complex was synthesized and
characterized by various spectroscopic methods. This complex
catalyzed the dimerization of benzyl bromide under electro-
chemical conditions. This result provides a useful guideline
for designing catalysts for dimerization reactions via radical
intermediates.
Electrolysis conditions
Product ratioc
Chargeb/
F molϪ1
Conversion
(%)
Catalyst
Toluene
Bibenzyl
[CoII2L]
0.7–1.3
0.7–1.5
39–75
51–87
24–33
67–87
67–76
13–33
[CoII(salen)]
a Controlled potential electrolyses were carried out at Ϫ1.40 vs. Ag–
AgCl under an argon atmosphere with irradiation by a 500 W tungsten
lamp. Initial concentrations: CoII2L, 5.0 × 10Ϫ4; [CoII(salen)], 1.0 × 10Ϫ3
;
Acknowledgements
NBu4BF4, 5.0 × 10Ϫ2; PhCH2Br, 5.0 × 10Ϫ3 M. b Electrical charge
passed per mol of the substrate. c Products were analyzed by GC and
GC-MS.
We wish to thank Mr Hideki Horiuchi, a glassworker in our
department, for his skill in preparing the three-electrode cells.
We also thank Miss Yoko Iseki and Mr Akihiro Goto for their
experimental assistance. The present work was supported by
a Grant-in-aid for Scientific Research from the Ministry of
Education, Science, Sports and Culture of Japan.
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Fig. 3 Proposed mechanism for the electrolysis of benzyl bromide
mediated by the dicobalt complex.
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The electrolysis of benzyl bromide at Ϫ1.4 V vs. Ag–AgCl in
the presence of [CoII2L] and irradiation by visible light yielded
bibenzyl as a major product as shown in Table 1. On the
other hand, when the corresponding mononuclear complex
[CoII(salen)] was used as a catalyst, toluene was obtained as
the major product. No reaction occurred in the absence of the
catalyst under the same conditions. In order to investigate the
reaction mechanism, the electrolysis was examined by the spin-
trapping technique with PBN.28,29 The electrolysis of benzyl
bromide (5.0 × 10Ϫ3 M) was followed by ESR spectroscopy in
the presence of [CoII2L] (5.0 × 10Ϫ4 M) and PBN (3.9 × 10Ϫ1
M). The ESR signal attributable to the PBN spin adduct
(g = 2.006, AN = 12.2 G, AH = 2.4 G; 104 G = 1 T) was observed
during the electrolysis at Ϫ1.40 V vs. Ag–AgCl. Upon addition
of PBN, the formation of toluene and bibenzyl was completely
inhibited. This result indicates that the radical species are
generated as electrolysis intermediates under the present con-
ditions. A possible mechanism is shown in Fig. 3. The reaction
is initiated by the formation of a [CoI2L]2Ϫ species generated
from [CoII2L] at Ϫ1.05 V vs. Ag–AgCl. The [CoI2L]2Ϫ species,
which is a supernucleophile, reacts with benzyl bromide to yield
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1974
J. Chem. Soc., Dalton Trans., 2001, 1971–1974