Synthesis of {15-benzyloxy-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17) ,13,15-triene}nickel(II) perchlorate and its analogs, and their catalytic behavior in reductive debromination of 1-bromo-4-tert-butylbenzene

Article abstract of DOI:10.1016/j.ica.2010.05.051

New nickel(II) complexes with macrocyclic ligands bearing benzyloxy [(5), (9)], 2-methylbenzyloxy (7), 3-methylbenzyloxy (8), and hydroxy (6) groups on the pyridine ring have been synthesized. Structures of the hydroxy substituted macrocyclic ligand (L-OH

Full text of DOI:10.1016/j.ica.2010.05.051

Inorganica Chimica Acta 363 (2010) 3151–3157
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Inorganica Chimica Acta
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Synthesis of {15-benzyloxy-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),
13,15-triene}nickel(II) perchlorate and its analogs, and their catalytic
behavior in reductive debromination of 1-bromo-4-tert-butylbenzene
a
a
a
b
Katsura Mochizuki a, , Tomoko Sugita , Satoka Kuroiwa , Fumi Yamada , Kiyoharu Hayano ,
*
Yasunobu Ohgami a, Mana Suzuki a, Syouhei Kato a
a International Graduate School of Arts and Sciences, Yokohama City University, Yokohama 236-0027, Japan
b Hokkaido University of Education Sapporo, Sapporo 002-8502, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
New nickel(II) complexes with macrocyclic ligands bearing benzyloxy [(5), (9)], 2-methylbenzyloxy (7),
3-methylbenzyloxy (8), and hydroxy (6) groups on the pyridine ring have been synthesized. Structures of
the hydroxy substituted macrocyclic ligand (L-OHÁ3HClÁH2O), and the benzyloxy substituted ligand (L-
OBnÁ3HCl) and its nickel(II) complex (5), as well as an analogous Ni(II) complex (8), have been revealed
by X-ray crystallography. Their catalytic capabilities in the reductive debromination of 1-bromo-4-tert-
butylbenzene have been elucidated, which has revealed that the pyridine ring can be a suitable position
for the introduction of functional groups while maintaining the catalytic capabilities of the nickel(II)
complexes.
Received 19 March 2010
Received in revised form 16 May 2010
Accepted 23 May 2010
Available online 1 June 2010
Keywords:
Nickel complexes
Macrocyclic ligands
Synthesis
Ó 2010 Elsevier B.V. All rights reserved.
Debromination
1. Introduction
idine ring attached to the skeleton. The introduction of some
substituents on the skeletal nitrogen atoms and a number of N-
Tetraaza macrocyclic metal complexes have been well known to
act as catalysts in oxidation and reduction of organic substrates,
because of their high resistance to decomposition to relatively
higher or lower valence states and the potential 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
[4,5]. Macrocyclic Ni(II) complexes containing a pyridine ring in
the macrocyclic skeleton have been known to act as catalysts in
the reductive debromination of bromoarenes (benzene, naphtha-
lene, and biphenyl derivatives) [4]. In order to design new catalysts
for reductive dehalogenation, we plan to introduce another chem-
ical functional group to the nickel(II) complex, e.g., 1, which is a
pyridine-containing macrocycle. For example, connecting a func-
tional group that is a photosensitizer to macrocyclic complexes is
expected to facilitate photo-driven catalysis. When introducing a
new functional substituent, it is important to know at what posi-
tion the substituent does not get reduced while maintaining cata-
lytic capability for debromination reactions. The nickel(II) complex
1 has at least two candidates for substituent-introduction posi-
tions: the nitrogen atoms of the macrocyclic skeleton and the pyr-
functionalized pyridine-containing macrocycles have been re-
ported [6–15], though only a few macrocyclic complexes with sub-
stituents on the pyridine ring have been synthesized [16–19]. In
this study, we have synthesized nickel(II) complexes with new
macrocyclic ligands bearing benzyloxy [(5), (9)], 2-methylbenzyl-
oxy (7), 3-methylbenzyloxy (8), and hydroxy (6) groups on the pyr-
idine ring of the parent complex (1) (Scheme 1). Structures of the
hydroxy substituted macrocyclic ligand (L-OHÁ3HClÁH2O), and the
benzyloxy substituted ligand (L-OBnÁ3HCl) and its nickel(II) com-
plex (5), as well as an analogous Ni(II) complex (8), have been re-
vealed by X-ray crystallography, The catalytic properties of our
synthetic nickel(II) complexes in the reductive debromination of
1-bromo-4-tert-butylbenzene are compared with those of some
other nickel(II) complexes with N-substituted macrocyclic ligands.
2. Experimental
2.1. Synthesis of ligands and nickel(II) complexes
The nickel(II) complexes, [NiL](ClO4)2 (1) [6], [Ni(LMe)](ClO4)2
(2) [7], [Ni(LMe3)](ClO4)2 (3) [8], and [Ni(L-Bn2)](ClO4)2 (4) [6],
were prepared by reported methods. 4-(Benzyloxy)pyridine-2,6-
dicarboxaldehyde was synthesized by a modified method reported
by Froidevaux et al. [20].
* Corresponding author. Tel.: +81 45 787 2186; fax: +81 45 787 2413.
E-mail address: katsura@yokohama-cu.ac.jp (K. Mochizuki).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.05.051

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K. Mochizuki et al. / Inorganica Chimica Acta 363 (2010) 3151–3157
Scheme 1. Structures of macrocyclic ligands.
2.1.1. 15-Benzyloxy-3,7,11,17-tetraaza-bicyclo[11.3.1]heptadeca-
1(17),13,15-triene (L-OBn)
J = 5.3 Hz, 4H). 13C NMR (75 MHz, CDCl3): d 28.9(t), 46.2(t), 46.9(t),
54.9(t), 70.0(t), 107.8(d), 127.8(d), 128.6(d), 129.0(d), 135.9(s),
To an ethanol/water (1:1, v/v) solution (40 mL) containing 4-
(benzyloxy)pyridine-2,6-dicarboxaldehyde (1.50 g, 6.22 mmol)
and Cu(NO3)2Á3H2O (1.52 g, 6.29 mmol) was added dropwise an
ethanol solution (10 mL) of N,N-bis(3-aminopropyl)amine (0.82 g,
6.25 mmol). After the mixture was heated at 60 °C for 6 h, the dark
violet solution was evaporated to dryness. The resulting residue
was dissolved in 100 mL methanol/water (1:1, v/v) solution, which
was cooled to 5 °C, followed by addition of NaBH4 (5.0 g,
132 mmol). The mixture was heated to 60 °C and was stirred for
2 h after further addition of NaBH4 (5.0 g, 132 mmol). The violet
solution was acidified to pH 1 by addition of conc. HCl, stirred
for 2 h, and neutralized by addition of NaOH. The resulting solution
was evaporated to ca. 50 mL and cooled in an ice bath to form
white precipitates of boric acid, which were filtered off. NaCl was
also removed by precipitation by addition of ethanol to the solu-
tion and subsequent filtration. After the ethanolic solution was
evaporated to dryness, the resultant residue was dissolved in a
minimum amount of water. To the violet solution, addition of Na-
ClO4 yielded precipitates of the perchlorate salt of the copper com-
plex with L-OBn, which was filtered and used for the isolation of
free ligand.
161.3(s), 165.5(s). HR-FAB-MS: Found: m/z = 341.2342. Calcd for
12
C
20
1H2914N416O1: m/z = 341.2341 [M+H]+.
The hydrochloride salt, L-OBnÁ3HCl, was obtained from conver-
sion of the free ligand, L-OBn, using hydrochloric acid and a crystal
suitable for X-ray analysis was obtained by recrystallization from
ethanol/ether (vapor diffusion method).
2.1.2. 15-Hydroxy-3,7,11,17-tetraaza-bicyclo[11.3.1]heptadeca-
1(17),13,15-triene (L-OH)
L-OBnÁ3HCl (0.89 g, 1.98 mmol) was hydrogenated in an aque-
ous solution containing palladium oxide (0.24 g, 1.96 mmol) as a
catalyst under 1 atm hydrogen at room temperature for 24 h. After
debenzylation was complete, the catalyst was filtered and the
product was isolated by evaporation of the solution (yield:
0.63 g, 84%). Crystals (L-OHÁ3HClÁH2O) suitable for X-ray structural
analysis were obtained by recrystallization from 1 mol cmÀ3 HCl/
ethanol. 1H NMR (300 MHz, D2O): d 7.05 (s, 2H), 4.50 (s, 4H),
3.38 (t, J = 6.3 Hz, 4H), 3.26 (t, J = 7.4 Hz, 4H), 2.35 (quintet,
J = 6.8 Hz, 4H) 13C NMR (75 MHz, D2O): d 21.2(t), 42.5(t), 43.7(t),
49.8(t), 113.0(d), 150.9(s), 166.4(s). HR-FAB-MS: Found: m/
12
z = 251.1875. Calcd for
C
13
1H2314N416O1: m/z = 251.1872 [M+H]+.
The free ligand, L-OBn, was obtained by the removal of copper
from the above perchlorate complex using NaCN and further
extraction with dichloromethane (yield: 1.59 g, 75%). 1H NMR
(300 MHz, CDCl3): d 7.40–7.31 (m), 6.62 (s, 2H), 5.07 (s, 2H), 3.79
(s, 4H), 2.70 (t, J = 5.3 Hz, 4H), 2.58 (t, J = 5.4 Hz, 4H), 1.73 (quintet,
2.1.3. 4-(2-Methylbenzyloxy)pyridine-2,6-dicarboxaldehyde
This was synthesized from the oxidation of 4-(2-methylbenzyl-
oxy)pyridine-2,6-dimethanol, which was prepared by a similar
method for 4-(benzyloxy)pyridine-2,6-dimethanol (yield: 89%).

K. Mochizuki et al. / Inorganica Chimica Acta 363 (2010) 3151–3157
3153
13C NMR (75 MHz, CDCl3): d 19.0(q), 69.9(t), 111.9(d), 126.2(d),
128.8(d), 129.1(d), 131.0(d), 132.8(s), 137.0(s), 155.0(s), 167.0(s),
192.1(s). FAB mass: m/z 256 ([M+H]+). IR (KBr): 1710.7 cmÀ1
slow evaporation of an aqueous solution of the complex containing
NaClO4 (yield: 17%). FAB mass: m/z 307 ([MÀ2ClO4ÀH]+). Anal.
Calc. for C13H22Cl2N4O9Ni: C, 30.74; H, 4.37; N, 11.03. Found: C,
30.40; H, 4.23; N, 10.92%.
(mC@O).
2.1.4. 15-(2-Methylbenzyloxy)-3,7,11,17-tetraaza-
2.1.10. [Ni(L-OBn-o-Me)](ClO4)2 (7)
bicyclo[11.3.1]heptadeca-1(17),13,15-triene (L-OBn-o-Me)
This complex was synthesized in a similar manner as 5 using L-
OBn-o-Me. Red plate crystals were obtained by recrystallization
from nitromethane/ethyl acetate (yield: 56%). FAB mass: m/z 511
([MÀClO4]+). Anal. Calc. for C21H30Cl2N4O9Ni: C, 41.21; H, 4.94; N,
9.15. Found: C, 41.04; H, 4.86; N, 9.10%.
The ligand L-OBn-o-Me was synthesized by a similar method as
that for L-OBn using 4-(2-methylbenzyloxy)pyridine-2,6-dicarbox-
aldehyde instead of 4-(benzyloxy)pyridine-2,6-dicarboxaldehyde
(yield: 82%). 1H NMR (300 MHz, CDCl3): d 7.38–7.23 (m), 6.64 (s,
2H), 5.06 (s, 2H), 3.81 (s, 4H), 2.74 (t, J = 5.6 Hz, 4H), 2.60 (t,
J = 5.7 Hz, 4H), 2.37 (s, 3H), 1.74 (quintet, J = 5.6 Hz, 4H). 13C NMR
2.1.11. [Ni(L-OBn-m-Me)](ClO4)2 (8)
(75 MHz, CDCl3):
68.1(t), 107.1(d), 125.5(d), 128.3(2d), 129.8(d), 133.0(s), 136.0(s),
d
18.9(q), 22.5(t), 46.8(t), 47.1(t), 53.0(t),
This complex was synthesized in a similar manner as 5 using L-
OBn-m-Me. Crystals suitable for X-ray structural analysis were ob-
tained by recrystallization from nitromethane/ethyl acetate (vapor
diffusion method) (yield: 61%). FAB mass: m/z 511 ([MÀClO4]+).
Anal. Calc. for C21H30Cl2N4O9Ni: C, 41.21; H, 4.94; N, 9.15. Found:
C, 40.85; H, 4.79; N, 9.10%.
159.8(s), 165.2(s). HR-FAB-MS: Found: m/z = 355.2500. Calcd for
12
C
21
1H3114N416O1: m/z = 355.2498 [M+H]+.
2.1.5. 4-(3-Methylbenzyloxy)pyridine-2,6-dicarboxaldehyde
This was synthesized from the oxidation of 4-(3-methylbenzyl-
oxy)pyridine-2,6-dimethanol, which was prepared by a similar
method as 4-(benzyloxy)pyridine-2,6-dimethanol (yield: 79%).
13C NMR (75 MHz, CDCl3): d 21.3(q), 70.3(t), 111.9(d), 124.9(d),
128.2(d), 128.8(d), 129.3(d), 134.7(s), 138.6(s), 154.5 (s), 166.9(s),
192.1(d). FAB mass: m/z 256 ([M+H]+). IR (KBr): 1703.0 cmÀ1
2.1.12. [Ni(LMe-OBn)](ClO4)2 (9)
This complex was synthesized in a similar manner as 5 using
LMe-OBn. Red needle crystals were obtained by recrystallization
from nitromethane/ethyl acetate (yield: 28%). FAB mass: m/z 511
([MÀClO4]+). Anal. Calc. for C21H30Cl2N4O9Ni: C, 41.21; H, 4.94; N,
9.15. Found: C, 41.13; H, 4.82; N, 9.17%.
(mC@O).
2.1.6. 15-(3-Methylbenzyloxy)-3,7,11,17-tetraaza-
2.2. Physical measurements
bicyclo[11.3.1]heptadeca-1(17),13,15-triene (L-OBn-m-Me)
The ligand L-OBn-m-Me was synthesized by a similar method as
that for L-OBn using 4-(3-methylbenzyloxy)pyridine-2,6-dicarbox-
aldehyde (yield: 80%). 1H NMR (300 MHz, CDCl3): d 7.31–7.18 (m),
6.62 (s, 2H), 5.04 (s, 2H), 3.79 (s, 4H), 2.73 (t, J = 5.6 Hz, 4H), 2.58 (t,
J = 5.6 Hz, 4H), 2.38 (s, 3H), 1.73 (quintet, J = 5.6 Hz, 4H). 13C NMR
(75 MHz, CDCl3): d 21.3(q), 70.9(t), 111.9(d), 124.9(d), 128.2(d),
Cyclic voltammograms were measured under Ar (99.9999%)
with an ALS/chi Electrochemical Analyzer (acetonitrile solutions;
potential sweep rate: 0.1 V sÀ1; Ag/Ag+ reference electrode (BAS
RE-5); Pt or glassy-carbon working electrode; Pt counter elec-
trode). Tetraethylammonium perchlorate (0.1 mol dmÀ3) was used
as a supporting electrolyte.
128.8(d), 129.3(d), 134.7(s), 138.6(s), 154.5(s), 166.9(s), 192.1(s).
1H and 13C NMR spectra were recorded with Bruker DRX 300,
JEOL FX90Q, and JEOL EPC-400 spectrometers.
12
HR-FAB-MS: Found: m/z = 355.2499. Calcd for
C
1H3114N416O1:
21
m/z = 355.2498 [M+H]+.
Vis-absorption spectra were measured with a Varian Cary 500
scan UV–Vis–NIR spectrophotometer.
2.1.7. 15-Benzyloxy-7-methyl-3,7,11,17-tetraaza-
bicyclo[11.3.1]heptadeca-1(17),13,15-triene (LMe-OBn)
2.3. Reductive debromination
The ligand LMe-OBn was synthesized by a similar method as
that for L-OBn using N,N-bis(3-aminopropyl)methylamine instead
of N,N-bis(3-aminopropyl)amine (yield: 81%). 1H NMR (300 MHz,
CDCl3): d 7.41–7.34 (m), 6.61 (s, 2H), 5.08 (s, 2H), 3.79 (s, 4H),
2.52 (t, J = 5.6 Hz, 4H), 2.36 (t, J = 5.7 Hz, 4H), 2.05 (s, 3H), 1.71
(quintet, J = 5.7 Hz, 4H). 13C NMR (75 MHz, CDCl3): d 26.9(t),
40.5(q), 46.0(t), 54.2(t), 55.9(t), 69.5(t), 105.7(d), 127.0(d),
To a three-necked 50-mL flask with an Ar inlet tube and a three-
way stopcock, the nickel(II) complex [0.04 mmol per nickel(II) ion],
NaBH4 (4 mmol), acetonitrile (or diglyme) (2.5 mL), and ethanol
(2.5 mL) were added. After the mixture was purged with Ar for
20 min, 1-bromo-4-tert-butylbenzene (2 mmol) was added. The
reaction mixture was stirred for 3 h at 40 °C. The mixture was fil-
tered and an internal standard (naphthalene) was added to the fil-
trate, which was analyzed by GC chromatography (internal
reference method using authentic samples, Shimadzu GAS chro-
matograph GC-14A with a chromatopac C-R6A, glass column
packed with Thermon-3000 5% SHINCARBON A 60/80).
127.5(d), 128.0(d), 135.5(s), 160.8(s), 165.0(s). HR-FAB-MS: Found:
12
m/z = 355.2496. Calcd for
C
1H3114N416O1: m/z = 355.2498
21
[M+H]+.
2.1.8. [Ni(L-OBn)](ClO4)2 (5)
To a methanol solution (10 mL) of L-OBn (0.18 g, 0.54 mmol)
was added
a
methanol solution (10 mL) of Ni(ClO4)2Á6H2O
2.4. Crystallographic studies
(0.20 g, 0.54 mmol). After the mixture was heated at 60 °C for
30 min, cooling the methanol solution in an ice bath yielded orange
crystals, which were recrystallized from nitromethane/ethyl ace-
tate (vapor diffusion method) (yield: 32%). FAB mass: m/z 497
([MÀClO4]+). Anal. Calc. for C20H28Cl2N4O9Ni: C, 40.17; H, 4.72; N,
9.37. Found: C, 40.02; H, 4.70; N, 9.36%.
X-ray crystallography of single crystals was carried out on a
MAC Science MXC3k four-circle (L-OBnÁ3HCl and 5:
OHÁ3HClÁH2O and 8: scan) with graphite-monochromatized
MoK radiation (k = 0.71073 Å). The structures were solved by
x-2h scan; L-
x
a
the direct method (SIR92 and SIR97 [21]), and refined on F2 by
the full-matrix least-squares method SHELXL 97 [22]). The /-scan
was applied for absorption correction [23]. All non-hydrogen
atoms were refined using anisotropic thermal parameters (riding
model refinement). Hydrogen atoms were placed by SHELXL [22].
All calculations were carried out using a Silicon Graphics O2 work-
2.1.9. [Ni(L-OH)](ClO4)2 (6)
This complex was synthesized in a similar manner as 5 using L-
OH, which had been formed in a neutralized aqueous/methanol
solution of L-OHÁ3HClÁH2O. Red plate crystals were obtained by

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K. Mochizuki et al. / Inorganica Chimica Acta 363 (2010) 3151–3157
Table 1
Crystallographic data of ligands and complexes.
Complex
L-OBnÁ3HCl
20H31Cl3N4O1
449.85
298(2)
L-OHÁ3HClÁH2O
C13H2Cl3N4O2
377.74
298(2)
5
8
Empirical formula
Formula weight
T (K)
C
C20H28Cl2N4Ni1O9
598.07
298(2)
C21H30Cl2N4Ni1O9
904.59
298(2)
k (Å)
0.71070
0.71070
0.71070
0.71070
Crystal system
Space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/n
11.599(14)
7.181(4)
29.343(6)
105.78(4)
2352(3)
orthorhombic
Pbca
16.016(4)
11.789(5)
19.994(3)
–
3775(2)
8
monoclinic
P21
orthorhombic
Pbca
30.598(13)
19.259(11)
8.816(3)
–
5194(4)
8
14.091(3)
10.027(2)
19.153(5)
109.953(18)
2543.9(10)
4
1.562
1.029
1240
b (°)
V (Å3)
Z
4
Dcacl (Mg mÀ3
)
1.270
0.407
952
1.329
0.497
1656
1.565
1.010
2544
l
(mmÀ1
F(0 0 0)
)
Crystal size (mm)
Reflections collected
Independent reflections
Goodness-of-fit
0.80 Â 0.80 Â 0.20
0.60 Â 0.15 Â 0.15
0.85 Â 0.10 Â 0.10
1.00 Â 0.50 Â 0.15
5680
4841
6354
6680
5416 [R(int) = 0.0093]
1.011
4334 [R(int) = 0.0012]
1.016
6175 [R(int) = 0.0904]
0.976
5983 [R(int) = 0.0467]
1.013
Final R indices [I > 2
r(I)]
R1 = 0.0770, wR2 = 0.1729
0.991 and À0.591
R1 = 0.0709, wR2 = 0.1240
0.282 and À0.302
R1 = 0.0832, wR2 = 0.1718
0.469 and À0.394
R1 = 0.0869, wR2 = 0.2353
0.638 and À0.414
Largest diff. peak and hole (e Å3)
station (MaXus program system provided by MAC Science). Struc-
tural diagrams were drawn using ORTEP-3 for Windows [24]. Crys-
tallographic data for the complexes are listed in Table 1. During
the calculations, no restraints were applied for L-OBnÁ3HCl, L-
OHÁ3HClÁH2O, and 8, whereas, in the case of 5, two phenyl groups
comprised of C(15), C(16), C(17), C(18), C(19), and C(20), and C(35),
C(36), C(37), C(38), C(39), and C(40) were restrained so as to main-
tain an ideal hexagon by the SHELXL command, AFIX66 and AFIX65
(data/restraints/parameters: 6175/1/643).
between 4-(benzyloxy)pyridine-2,6-dicarboxaldehyde and N,N-
bis(3-aminopropyl)amine using copper(II) ion as a template. Anal-
ogous ligands were synthesized by this method as well. The nickel
complexes were synthesized by direct complex formation between
isolated ligands and nickel(II) ions in methanolic solutions. Signif-
icantly, the synthesis of L-OBn also led to the isolation of the ligand
L-OH, which was prepared from debenzylation of L-OBnÁ3HCl by
hydrogenation with PdO. The debenzylation process did not pro-
ceed for the free ligand L-OBn but succeeded only for the hydro-
chloride salt L-OBnÁ3HCl. The ligand with the OH group on the
pyridine ring, L-OH, enabled us to synthesize a new type of macro-
cyclic compound having Ru(bpy)3 as a photosensitizer [25] and
new bis(macrocyclic) ligands linked at the pyridine rings [26].
The crystallographic data for the ligands and two Ni(II) com-
plexes are shown in Table 1. The ligand L-OBn was crystallized as
a hydrochloride salt, L-OBnÁ3HCl, which allowed X-ray structural
3. Results and discussion
The synthetic route for the macrocyclic ligands with a substitu-
ent on the pyridine ring is shown in Scheme 2. The target ligand, L-
OBn, with a benzyloxy substituent at the 4th position of the pyri-
dine ring of L was synthesized via the 1:1 condensation reaction
Scheme 2. Synthesis of ligands.

K. Mochizuki et al. / Inorganica Chimica Acta 363 (2010) 3151–3157
3155
analysis (Fig. 1a). The macrocyclic skeleton folded along the axis
penetrating N(3) and N(11). The angle between the pyridine ring
and the mean plane consisting of N(3), N(7), and N(11) in the mac-
rocyclic skeleton was ca. 54°. Furthermore, the benzyl group was
extended with its phenyl group twisting at ca. 71° against the pyr-
idine ring of the macrocycle. The three imino nitrogens were pro-
tonated to result in a hydrogen bonding network spread through
three chloride anions1
.
The X-ray structural analysis of L-OHÁ3HClÁH2O clearly showed
the success of the debenzylation (Fig. 1b). The angle between the
pyridine ring and the mean plane consisting of N(6), N(10), and
N(14) in the macrocyclic skeleton was ca. 72°. The folding of the
macrocyclic skeleton was larger than that of the benzyl-substi-
tuted one, L-OBnÁ3HCl. In this case, the hydroxy group, one mole-
cule of water, and the three protonated nitrogens were joined to
form a hydrogen bonding network2.
The crystal of [Ni(L-OBn)](ClO4)2 obtained from recrystalliza-
tion in nitromethane/ethyl acetate contained two forms with dif-
ferent conformations around the benzyl groups (Fig. 2). One
macrocyclic complex containing Ni(2) took a nearly planar form,
where the angle between the phenyl ring of the benzyl group
and the mean plane consisting of N(5), N(6), N(7), and N(8) was
ca. 22°. In the other macrocyclic complex containing Ni(1), the
phenyl ring of the benzyl group was nearly perpendicular to the
macrocyclic N4-plane; the angle between the phenyl ring and
the mean plane consisting of N(1), N(2), N(3), and N(4) was ca.
88°. The macrocyclic skeleton took almost the same form in each
macrocyclic nickel(II) complex. Two nickel(II) ions adopted a
square planar conformation with three imino protons lying in the
same direction against the N4-plane in both macrocyclic nickel(II)
complexes. The average Ni–N distances were ca. 1.93 and 1.92 Å
for Ni(1) and Ni(2), respectively. These values are typical for low-
spin square planar macrocyclic Ni(II) complexes. Perchlorates did
not coordinate to the nickel(II) ions, though some of them likely
interacted with imines by hydrogen bonds [O(5)–N(3): 3.029;
O(4)–N(2): 3.134 Å].
In contrast to the above mentioned [Ni(L-OBn)](ClO4)2, the crys-
tal of [Ni(L-m-MeOBn)](ClO4)2, which has a methyl group on the
phenyl ring, adopted a unique form of the complex (Fig. 3).
Although the macrocyclic skeleton was very similar to that of
[Ni(L-OBn)](ClO4)2, the benzyl group extended from the macrocy-
clic part with ca. 55° of a twisted angle between the phenyl ring
and the mean N4-plane consisting of N(3), N(7), N(11), and
N(17). The nickel(II) ion adopted a typical four-coordinate square
planar conformation, enclosed with four nitrogen donors which
were almost in the same plane (average Ni–N distance: ca. 1.91 Å).
The VIS-absorption data for the synthesized complexes are
listed in Table 2. In solid state and in nitromethane solution, all
the complex perchlorates showed only one intense absorption
band around 450–480 nm, indicating that these nickel complexes
are in a square planar coordination geometry. On the other hand,
in acetonitrile solutions, these Ni(II) complexes showed at least
two bands, one of which appeared around 450–460 nm which
was attributed to a four-coordinate square planar species and
one around 720–750 nm which was attributed to a six-coordinate
species. These results suggest that the Ni(II) complexes are in equi-
librium between a four-coordinate species and a solvent coordi-
nated six-coordinate species in acetonitrile solution. Similar
Fig. 1. Ortep drawings of the cationic portions of L-OBnÁ3HCl (a) and L-
OHÁ3HClÁH2O (b) with 50% probability thermal ellipsoids.
phenomena have been observed for analogous macrocyclic Ni(II)
complexes [26,27].
Cyclic voltammograms for the nickel(II) complexes showed two
reversible waves of which E1/2 ranged from +0.77 V to +0.9 V and
between À1.5 V and À1.59 V. These may be assignable to the
NiII/NiIII and NiI/NiII redox processes, respectively (Table 3). The
E1/2 of NiI/NiII for the N-methylated complex, 9, was À1.509, which
was slightly (ca. 80 mV) more positive than those for the other
non-methylated complexes, 5–8, as well as that for E1/2 of NiII/NiIII.
This difference in the redox behavior may play a role in differenti-
ating complex 9 from other complexes in catalytic capability for
the following debromination reactions.
Debromination of 4-bromo-tert-buthylbenzene mediated by
macrocyclic nickel(II) complexes was carried out at 40 °C for 3 h
with a reactant ratio of Ni(II):substrate:NaBH4 = 1:50:100. The
conversion yields, ranging from 14% to 63%, are shown in Table
4. The debromination proceeded only when macrocyclic nickel(II)
complexes co-existed in the reaction solution. It was found that
the introduction of methyl groups to the nitrogen atoms of the
macrocyclic skeleton reduced the catalytic activity. The conversion
yield by complex 2, which had one methyl group on the nitrogen at
the 7th position, was 26%, which was significantly lower than that
of the parent complex 1 (85%). A similar decrease in catalytic capa-
bility by the introduction of a methyl group was found for the syn-
thetic benzyloxy substituted complexes (5: 92%; 9: 10%). Further
1
The selected distances are as follows: Cl(1)–N(3): 3.24, Cl(1)–N(11): 3.20, Cl(1)–
N(11#1): 3.12, Cl(2)–N(3#2): 3.02, Cl(3)–N(7): 3.28, and Cl(3)–N(7#1): 3.03 Å
(symmetry operations: #1: (2 À x, À1/2 + y, 1.5 À z), #2: (x, 1 + y, z)).
2
The selected distances are as follows: O(1)–Cl(21#1): 2.99, Cl(19)–N(10): 3.18,
Cl(19)–N(14#2): 3.21, Cl(19)–N(6#2): 3.17, Cl(20)–N(10): 3.09, Cl(20)–O(22#3): 3.13,
Cl(21)–N(6#2): 3.10, Cl(21)–O(22#4): 3.07, and O(22)–N(14#2): 2.73 Å (symmetry
operations: #1: (1/2 + x, y, 1/2 À z), #2: (1.5 À x, À1/2 + y, z), #3: (À1/2 + x, À1/2 À y,
Àz), #4: (x, À1/2 À y, 1/2 + z)).

3156
K. Mochizuki et al. / Inorganica Chimica Acta 363 (2010) 3151–3157
Fig. 2. Ortep drawings of the cationic portions of [Ni(L-OBn)](ClO4)2 with 50% probability thermal ellipsoids. Selected bond lengths (Å) and angles (°): N(1)–Ni(1) 1.91(3),
N(2)–Ni(1) 1.964(11), N(3)–Ni(1) 1.99(3), N(4)–Ni(1) 1.878(13), N(5)–Ni(2) 1.97(2), N(6)–Ni(2) 1.910(16), N(7)–Ni(2) 1.93(4), N(8)–Ni(2) 1.851(13), N(4)–Ni(1)–N(1)
89.0(12), N(1)–Ni(1)–N(2) 95.2(14), N(4)–Ni(1)–N(3) 78.3(13), N(2)–Ni(1)–N(3) 97.5(15), N(8)–Ni(2)–N(7) 83.4(17), N(6)–Ni(2)–N(7) 96.4(19), N(8)–Ni(2)–N(5) 83.7(12),
N(6)–Ni(2)–N(5) 96.5(15).
Fig. 3. Ortep drawing of the cationic portions of [Ni(L-m-MeOBn)](ClO4)2 with 50% probability thermal ellipsoids. Selected bond lengths (Å) and angles (°): Ni(1)–N(17)
1.841(6), Ni(1)–N(11) 1.907(8), Ni(1)–N(3) 1.922(7), Ni(1)–N(7) 1.953(7), N(17)–Ni(1)–N(11) 83.6(3), N(17)–Ni(1)–N(3) 84.2(3), N(11)–Ni(1)–N(7) 97.2(3), N(3)–Ni(1)–N(7)
94.9(3).
Table 2
Vis-absorption spectral data.
The introduction of the more bulky benzyl groups drastically de-
creased the conversion yield (4: 2%). In contrast to the introduction
kmax (nm) (e )
/molÀ1 dm3 cmÀ1
Complexa
of substituents on the nitrogen atoms, the introduction of substit-
uents to the 4th position of the pyridine ring did not greatly reduce
the catalytic activity of this type of nickel(II) complex. Conversion
yields were as follows: 5 (benzyloxy): 95%; 6 (hydroxy): 78%; 7 (2-
methylbenzyloxy): 72%; and 8 (3-methylbenzyloxy): 82%.
In conclusion, new nickel(II) complexes with macrocyclic li-
gands bearing benzyloxy [(5), (9)], 2-methylbenzyloxy (7), 3-meth-
ylbenzyloxy (8), and hydroxy (6) groups on the pyridine ring have
been synthesized. Their catalytic properties in the reductive debro-
mination of 1-bromo-4-tert-butylbenzene revealed that the pyri-
dine ring can be a suitable position for the introduction of new
Solid
Nitromethane
Acetonitrile
5
6
7
8
9
471
463
459
467
470
464(160)
462(170)
461(190)
462(178)
466(180)
725(14), 465(63), 329(shoulder)
725(14), 467(67), 330(shoulder)
748(11), 459(78), 330(shoulder)
726(15), 458(79), 330(shoulder)
727(16), 468(74), 340(shoulder)
a
[Complex]: 3.0 Â 10À3 mol dmÀ3, 25 °C.
introduction of two methyl groups to complex 2 led to complex 3,
for which the conversion yield was even further decreased (16%).

K. Mochizuki et al. / Inorganica Chimica Acta 363 (2010) 3151–3157
3157
Table 3
Appendix A. Supplementary material
Cyclic voltammetric data for Ni(II) complexes.a
Complex
Epc/V
Epa/V
E1/2/V
dEp/V
CCDC 769766, 769767, 769768 and 76769 contain the supple-
mentary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.ica.2010.05.051.
5
+0.745
À1.642
+0.747
À1.651b
+0.728
À1.656
+0.743
À1.634
+0.840
À1.549
+0.832
À1.530
+0.885
À1.526
+0.824
À1.527
+0.832
À1.539
+0.942
À1.468
+0.789
À1.586
+0.816
À1.589
+0.776
À1.592
+0.788
À1.587
+0.891
À1.509
0.087
0.112
0.138
0.125
0.096
0.129
0.084
0.095
0.102
0.081
6
7
8
9
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Table 4
Debromination of 1-bromo-4-t-butylbenzene.a
Complex
Conversion (%)b
1
2
3
4
5
6
7
8
9
85
26c
16c
2c
95
78
72
82
10
a
Ni(II) complex: 0.04 mmol, substrate:
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EtOH = 1:1 (V/V), 40 °C, 3 h.
b
GC conversion yield.
Solvent: diglyme/EtOH = 1:1 (V/V).
c
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This work was partially supported by JSP Grant-in-Aid for Sci-
entific Research (C) 20550139.