Photochemical dehalogenation mediated by macrocyclic nickel(II) complexes

Article abstract of DOI:10.1016/j.inoche.2011.03.024

A new photochemical dehalogenation system mediated by macrocyclic nickel(II) complexes was developed. Using this system, which consists of a macrocyclic nickel(II) complex catalyst, triethanolamine (TEOA) as a sacrificial reductant, and the ruthenium complex [Ru(bpy)₃](ClO ₄)₂ as a photosensitizer, catalytic debromination of 1-bromo-4-t-butylbenzene in acetonitrile was effectively carried out. The catalytic capabilities of various macrocyclic nickel(II) complexes in debromination reactions were also elucidated.

Full text of DOI:10.1016/j.inoche.2011.03.024

Inorganic Chemistry Communications 14 (2011) 902–905
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Photochemical dehalogenation mediated by macrocyclic nickel(II) complexes
⁎
Katsura Mochizuki , Mana Suzuki
International Graduate School of Arts and Sciences, Yokohama City University, Yokohama 236-0027, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A new photochemical dehalogenation system mediated by macrocyclic nickel(II) complexes was developed.
Using this system, which consists of a macrocyclic nickel(II) complex catalyst, triethanolamine (TEOA) as a
Received 8 February 2011
Accepted 8 March 2011
Available online 21 March 2011
sacrificial reductant, and the ruthenium complex [Ru(bpy)3](ClO4)2 as
a photosensitizer, catalytic
debromination of 1-bromo-4-t-butylbenzene in acetonitrile was effectively carried out. The catalytic
capabilities of various macrocyclic nickel(II) complexes in debromination reactions were also elucidated.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Photochemical dehalogenation
Nickel(II) complexes
Macrocyclic ligands
Catalytic reactions
Debromination
Introduction
the reactions under various conditions are shown in Table 1. The
debromination reaction was quantitative (100% conversion) when an
Tetraaza macrocyclic metal complexes are known to act as effective
catalysts in the oxidation and reduction of organic substrates because
of their high resistance to decomposition in relatively high or low
valence states and the potential for coordination of substrates at the
apical positions with respect to the macrocyclic plane. It has been
reported that some macrocyclic metal complexes catalyze reductive
dehalogenation [1] of alkyl [2,3] and aryl halides [4–6]. These reductive
dehalogenation systems for aryl halides are comprised of a macrocyclic
nickel(II) complex as a catalyst, NaBH4 as a sacrificial reductant, and an
organic halide substrate [4–6]. During the dehalogenation reaction, an
intermediate nickel(I) complex, which can be produced from
reduction by NaBH4, is thought to play an important role as an active
species [7–10]. In this study, we applied photochemically produced
macrocyclic nickel(I) complexes to dehalogenation reactions and
developed a new photochemical debromination system. Our photo-
chemical system, comprising a macrocyclic nickel(II) complex catalyst,
the ruthenium complex [Ru(bpy)3]2+ as a photosensitizer, and
triethanolamine (TEOA) as a sacrificial reductant, was effective in
the catalytic debromination of 1-bromo-4-t-butylbenzene in acetoni-
trile. Various reaction conditions were investigated, as well as the
catalytic capabilities of various nickel(II) macrocyclic complexes.
acetonitrile solution containing the nickel(II) complex 1a (catalyst:
0.8 mM), the ruthenium(II) complex (photosensitizer: 4 mM),
triethanolamine (TEOA) (sacrificial reductant: 0.4 M), and 1-bromo-
4-t-butylbenzene (substrate: 40 mM) was irradiated with visible light
for 3 h (entry 1) at room temperature (ca. 25 °C). In the absence of
either the nickel(II) complex 1a (entry 2), [Ru(bpy)3](ClO4)2 (entry
3), or TEOA (entry 4), debromination did not proceed to any large
extent, which showed that the presence of each of these components
was required for the reaction. When the reaction mixture containing
the three components was kept in the dark for 3 h (i.e., without
photo-irradiation) at room temperature (entry 5) and at 60 °C (entry
6), almost no debromination occurred, which proved that photo-
irradiation, rather than heating, is required for the reaction to take
place. Thus, it was found that three components—the nickel(II)
complex, the ruthenium(II) complex, and TEOA—and light irradiation
are all essential for this debromination reaction.
The debromination of 1-bromo-4-t-butylbenzene in this reaction
was confirmed by gas chromatography-mass spectroscopy (GC-MS)
analysis. The GC-MS spectrum of the reaction solution after photo-
irradiation showed a peak at m/z 134, which was ascribable to the
debromination product, [t-butylbenzene]+. The bromine liberated
from 1-bromo-4-t-butylbenzene was converted to bromide anions;
this was confirmed by the formation of AgBr precipitate upon addition
of AgNO3 to an aqueous solution made from the reaction solution. As
the reaction proceeded, a white precipitate appeared in suspension;
this was identified as (CH2CH2OH)3NHBr by comparison of its IR and
1H NMR spectra with those of an authentic sample prepared from the
reaction between TEOA and HBr.
Results and discussion
The photochemical system we designed was successful in the
catalytic debromination of 1-bromo-4-t-butylbenzene; the results of
Fig. 1 shows the dependence of the debromination reaction upon
photo-irradiation time. As the irradiation time was prolonged, the
⁎
Corresponding author. Tel./fax: +81 45 787 2186.
E-mail address: katsura@yokohama-cu.ac.jp (K. Mochizuki).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.03.024

K. Mochizuki, M. Suzuki / Inorganic Chemistry Communications 14 (2011) 902–905
903
Table 1
Photochemical debromination of 1-bromo-4-t-butylbenzene under various conditionsa.
Entry
Complexb
Photosensitizer
Sacrificial reductant
Conversionc/%
1
2
3
1a
None
1a
1a
1a
[Ru(bpy)3](ClO4)2
[Ru(bpy)3](ClO4)2
None
[Ru(bpy)3](ClO4)2
[Ru(bpy)3](ClO4)2
[Ru(bpy)3](ClO4)2
TEOA
TEOA
TEOA
None
TEOA
TEOA
100
2
1
2
3
4
5d
6e
1a
1
a
Ni complex: 0.8 mM, photosensitizer: 4 mM, 1-bromo-4-t-butylbenzene: 40 mM,
triethanolamine(TEOA): 0.4 M, acetonitrile, room temperature (ca. 25 °C), irradiated
for 3 h by visible light.
b
ClO−4 salt.
GC conversion.
The reaction was carried out in the dark.
The reaction was carried out at 60 °C in the dark.
c
d
e
conversion increased to ca. 100% in 3 h and then remained at a high
value up to 6 h. The debromination reaction was completed within 3 h
under the experimental conditions used. Fig. 2 shows the change in
conversion rate against the concentration of the photosensitizer [Ru
(bpy)3](ClO4)2, with the other reaction conditions being kept
constant. The conversion increased as the concentration of [Ru
(bpy)3](ClO4)2 increased, reaching a plateau (ca. 100%) at a
photosensitizer concentration of 4 mM ([1a]:[[Ru(bpy)3](ClO4)2]=
1:5). The dependence of the conversion on the concentration of 1a—
[1a]:[[Ru(bpy)3](ClO4)2]=1:1, 1-bromo-4-t-butylbenzene=40 mM,
and TEOA=0.4 M—is shown in Fig. S1. The yield increased as [1a]
increased, reaching saturation point at [1a] ([[Ru(bpy)3](ClO4)2])=
1.6 mM. The dependence of the conversion on the proportion of
ethanol (protic solvent) in the acetonitrile–ethanol mixed solvent was
also investigated (Fig. S2). The conversion yield was highest in neat
Scheme 1. Structural formulae of nickel(II) complexes.
acetonitrile solution and decreased as the ethanol content increased.
At an ethanol content of 60%, the yield was down to ca. 50%. Based on
these results, the optimum experimental conditions were concluded
to be as follows: [Ni complex]=0.8 mM, [[Ru(bpy)3](ClO4)2]=4 mM,
[1-bromo-4-t-butylbenzene]=40 mM, [triethanolamine (TEOA)]=
0.4 M, acetonitrile as solvent, and 3 h visible light irradiation. These
conditions were used in subsequent experiments.
The catalytic capabilities of various macrocyclic nickel(II) com-
plexes were investigated (see Scheme 1 and Table 2). For analogous Ni
(II) complexes containing a pyridine moiety, the introduction of a
benzyloxy group to the 4-position of the pyridine ring of 1b did not
affect their catalytic capabilities (entries 1 (1a) and 7 (1b)). The
introduction of one methyl group to the nitrogen atom at the 7-
position of the macrocyclic skeleton of 1b or 2a decreased the
conversion yield only slightly (entries 8 (1c) and 11 (2b)), whereas
the introduction of methyl groups to the three nitrogen atoms at the
3-, 7-, and 11-positions of 1b largely diminished its catalytic
capability, with the conversion decreasing from 97% to 18% (entry 9
(1d)). The introduction of methyl groups to the carbon atoms at the 2-
and 12-positions of 1b did not affect the conversion (entry 12 (2c)).
The introduction of unsaturated C=N bonds to the macrocyclic
Fig. 1. Dependence of debromination reaction on photo-irradiation time for an
acetonitrile solution containing 1a (0.8 mM), [Ru(bpy)3](ClO4)2 (4 mM), 1-bromo-4-t-
butylbenzene (40 mM), and TEOA (0.4 M).
Table 2
Photochemical debromination of 1-bromo-4-t-butylbenzene using various Ni(II)
complexesa.
Entry
Complexb
Conversionc/%
Entry
Complexb
Conversionc/%
1
7
8
9
10
11
12
13
1a
1b
1c
1d
2a
2b
2c
2d
100
97
82
18
89
86
100
77
14
15
16
17
18
19
20
21
3a
3b
3c
3d
3e
4a
4b
4c
100
97
93
98
99
10
7
12
a
Ni complex: 0.8 mM, photosensitizer: 4 mM, 1-bromo-4-t-butylbenzene: 40 mM,
triethanolamine(TEOA): 0.4 M, acetonitrile, room temperature (ca. 25 °C), irradiated
for 3 h by visible light.
b
Fig. 2. Conversion vs. concentration of [Ru(bpy)3](ClO4)2. 1a: 0.8 mM, 1-bromo-4-t-
butylbenzene: 40 mM, and TEOA: 0.4 M.
ClO−4 salt.
GC conversion.
c

904
K. Mochizuki, M. Suzuki / Inorganic Chemistry Communications 14 (2011) 902–905
Scheme 2. Dehalogenation reactions.
skeleton of 2c slightly decreased the conversion without systematic
change (entries 10 (2a) and 13 (2d)).
irreversible cathodic wave, different from the original, around −1.5 V.
Because 1-bromo-4-t-butylbenzene was not reduced at −1.5 V in the
absence of the nickel(II) complex, the appearance of this irreversible
wave was probably due to the reaction of the reduced Ni(I) complex
with 1-bromo-4-t-butylbenzene, which suggests that the Ni(I)
complex is the active species in the debromination reaction. The
addition of 1-bromo-4-t-butylbenzene to a solution of [Ru(bpy)3]2+
did not affect its cyclic voltammogram.
It was found that nickel(II) complexes with 14-membered N4
macrocycles but without pyridine rings also showed catalytic
capabilities. The nickel(II) complex with cyclam 3a, which had no
substituents in its macrocyclic skeleton, gave a high conversion (entry
14). Although the introduction of methyl groups to the macrocyclic
skeleton affected the conversion only to a small extent (entries 15
(3b), 16 (3c), 17 (3d), and 18 (3e)), the introduction of unsaturated
bonds to 3e and 3b largely resulted in decreased yields (entries 19
(4a), 20 (4b: isomer of 4a), and 21 (4c)), in contrast to the above-
mentioned pyridine-containing macrocyclic nickel(II) complexes.
The photochemical reaction system under discussion was applied
to the dehalogenation of various other substrates (Scheme 2 and
Table 3). The yields for the debromination of 2-bromo-m-xylene (A)
to m-xylene (E) were 95% and 99% for 1a and 3a, respectively (entries
22 and 26), indicating that the presence of methyl groups near the
bromine atom does not affect the catalytic debromination reaction.
The dechlorination of 2-chloro-m-xylene (B) did not proceed to any
appreciable extent: the yields were very low, at 1% and 3% for 1a and
3a, respectively (entries 23 and 27). The debromination of 2,5-
dibromo-p-xylene (C) afforded a mixture of 2-bromo-p-xylene (F)
and p-xylene (H); however, the yield ratios of these products were
different for 1a (F: 39% and H: 61%, entry 24) and 3a (F: 62% and H:
38%, entry 28). Interestingly, in the dechlorination of 2,5-dichloro-p-
xylene (D), mono-dechlorination, resulting in the production of 2-
chloro-p-xylene (G), was observed (1a: 7%, entry 25, and 3a: 10%,
entry 29), although products of complete dechlorination were not
obtained using any of the Ni(II) complexes.
Although various mechanisms for the reductive dehalogenation of
aryl halides have been discussed [7–10], there has not yet been a clear
conclusion. For this photochemical dehalogenation system we
propose a tentative mechanism with two possible pathways based
on previously reported dehalogenations [7–10]. As shown in
Scheme 3, [RuII(bpy)3]2+ is initially exited to [RuII(bpy)3]2+* by
irradiation with visible light. [RuII(bpy)3]2+* then reacts with
triethanolamine (the sacrificial reductant) to become [RuI(bpy)3]+
,
which reduces the macrocyclic Ni(II) complex to the active Ni(I)
complex. The thus formed Ni(I) complex may react with the aryl
halide to form an unstable intermediate Ni(III)–aryl complex, which
has not yet been identified, or may cause direct formation of an aryl
halide radical. In the former case, the unstable aryl complex
immediately decomposes to a Ni(II) complex and an aryl radical Ar,
whereas in the latter case, the aryl halide radical immediately
decomposes to Ar· and a halide, X−. In either case, Ar· abstracts a
proton from triethanolamine to form Ar, the liberated X− forms the
triethanolamine adduct, and the parent Ni(II) and Ru(II) complexes
are recovered. In order to establish the mechanism with certainty,
further studies will be necessary.
Thus, a new photochemical dehalogenation system mediated by
macrocyclic nickel(II) complexes has been developed. It was found
that various macrocyclic nickel(II) complexes show catalytic capabil-
ities in the debromination of 1-bromo-4-t-butylbenzene and that
these capabilities depend on the number and the positions of methyl
groups on the macrocyclic skeletons.
Fig. 3 shows cyclic voltammograms illustrating the effect of 1-
bromo-4-t-butylbenzene on the electrochemical reduction of 3a. The
nickel(II) complex 3a showed one reversible wave, ascribable to the
NiII/NiI process, around −1.5 V. The addition of 1-bromo-4-t-butyl-
benzene to a solution of 3a resulted in the appearance of a new
Table 3
Photochemical dehalogenation of various substratesa.
Entry
Complexb
Substratec
Conversiond/%
E
F
G
H
22
23
24
25
26
27
28
29
1a
1a
1a
1a
3a
3a
3a
3a
A
B
C
D
A
B
C
D
95
1
–
–
–
–
–
–
–
39
–
–
61
0
–
7
–
99
3
–
–
–
–
–
–
62
–
–
38
0
–
10
a
Ni complex: 0.8 mM, photosensitizer: 4 mM, triethanolamine(TEOA): 0.4 M,
acetonitrile, room temperature (ca. 25 °C), irradiated for 3 h by visible light.
ClO−4 salt.
40 mM.
GC conversion.
Fig. 3. Cyclic voltammograms of 3a (2 mM) (1) and 3a (2 mM)+1-Bromo-t-
butylbenzene (20 mM) (2) in acetonitrile containing 0.1 M TEAP (electrolyte).
Potential sweep rate: 0.1 V s−1. Potentials vs. Ag/Ag+ reference electrode.
b
c
d

K. Mochizuki, M. Suzuki / Inorganic Chemistry Communications 14 (2011) 902–905
905
Scheme 3. Proposed mechanism for photochemical dehalogenation.
Experimental
Appendix A. Supplementary material
Materials
Supplementary data to this article can be found online at
doi:10.1016/j.inoche.2011.03.024.
The complexes 1a [6], 1b [11], 1c [12], 1d [13], 2a [14], 2b [15], 2c
[16], 2d [17], 3a [18], 3b [19], 3c [20], 3d [21], 4a [22], 4b [23], 4c [24],
and 4d [25] were prepared according to the previously reported
methods. Biotech-grade acetonitrile stored under argon was used,
applying syringe techniques. Argon gas (99.9999%) was used for
purging. Other reagents were purchased from Wako, Sigma-Aldrich,
and Tokyo Chemical Industry Co. and used as received unless
otherwise specified.
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C
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