Synthesis and structures of urea-coordinated dinickel(II) complexes with binucleating ligand 1,3-bis(N-(2-(dimethylamino)ethyl)-N-methylamino)propan-2-ol (HL1) and its analogs

Article abstract of DOI:10.1016/j.ica.2013.01.036

New urea-coordinated dinickel(II) complexes with binucleating ligands - HL¹, which contains dimethylaminoethyl arms, and its analogs HL ², which features bis(methoxyethyl) arms, and HL³, which contains one dimethylaminoethyl arm and one bis(methoxyethyl) arm - all of which contain a 2-hydroxytrimethylene bridge between the two chelating sites, were synthesized. The structures of [Ni₂L¹(μ-CH ₃COO)(CH₃COO)(urea)(EtOH)]BPh₄·EtOH (2), [Ni₂L¹(μ-Cl)₂(urea)(CH₃CN)] BPh₄·H₂O (3), [Ni₂L²(H ₂O)₂(urea)Cl](BPh₄)₂·H ₂O (5), and [Ni₂L³(μ-CH₃COO) ₂(urea)]BPh₄ (7) were revealed by X-ray crystallography, as well as those of their parent dinickel(II) complexes, [Ni₂L ¹(μ-CH₃COO)(CH₃COO)(H₂O) ₂]BPh₄ (1), [Ni₂L²(μ-CH ₃COO)₂]BPh₄·CH₃CN (4), and [Ni₂L³(μ-CH₃COO)(CH₃COO)(EtOH)] BPh₄ (6). One urea molecule was coordinated to one dinuclear nickel(II) ion, forming intramolecular hydrogen bonds with a μ-acetato bridge (2, 7), a μ-chloro bridge (3), and coordinated chloro and aqua ligands (5). The formation of 7 from the reaction of 6 with urea was associated with a change in coordination mode from a non-bridging acetato ligand to a μ-acetato ligand. Magnetic measurements revealed a weak antiferromagnetic interaction between the two nickel(II) ions in 1 (J = -16.7 cm^-1) and 2 (J = -15.0 cm^-1).

Full text of DOI:10.1016/j.ica.2013.01.036

Inorganica Chimica Acta 400 (2013) 151–158
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Inorganica Chimica Acta
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Synthesis and structures of urea-coordinated dinickel(II) complexes with
binucleating ligand 1,3-bis(N-(2-(dimethylamino)ethyl)-N-methylamino)
propan-2-ol (HL1) and its analogs
⇑
Katsura Mochizuki , Junpei Takahashi, Yukiko Ishima, Tomomi Shindo
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
New urea-coordinated dinickel(II) complexes with binucleating ligands—HL1, which contains dimethyl-
aminoethyl arms, and its analogs HL2, which features bis(methoxyethyl) arms, and HL3, which contains
one dimethylaminoethyl arm and one bis(methoxyethyl) arm—all of which contain a 2-hydroxytrimeth-
Article history:
Received 16 November 2012
Received in revised form 12 January 2013
Accepted 23 January 2013
ylene bridge between the two chelating sites, were synthesized. The structures of [Ni2L1(
l-CH3COO)
Available online 13 February 2013
(CH3COO)(urea)(EtOH)]BPh4ꢀEtOH (2), [Ni2L1(
l
-Cl)2(urea)(CH3CN)]BPh4ꢀH2O (3), [Ni2L2(H2O)2(urea)
Cl](BPh4)2 ꢀH2O (5), and [Ni2L3(
l-CH3COO)2(urea)]BPh4 (7) were revealed by X-ray crystallography, as
Keywords:
well as those of their parent dinickel(II) complexes, [Ni2L1(
l
-CH3COO)(CH3COO)(H2O)2]BPh4 (1), [Ni2L2
-CH3COO)(CH3COO)(EtOH)]BPh4 (6). One urea molecule was
-acetato
Dinickel complexes
Binucleating ligands
Synthesis
(
l
-CH3COO)2]BPh4ꢀCH3CN (4), and [Ni2L3(
l
coordinated to one dinuclear nickel(II) ion, forming intramolecular hydrogen bonds with a
l
bridge (2, 7), a
l-chloro bridge (3), and coordinated chloro and aqua ligands (5). The formation of 7 from
Urea
the reaction of 6 with urea was associated with a change in coordination mode from a non-bridging ace-
tato ligand to a l-acetato ligand. Magnetic measurements revealed a weak antiferromagnetic interaction
between the two nickel(II) ions in 1 (J = ꢁ16.7 cmꢁ1) and 2 (J = ꢁ15.0 cmꢁ1).
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
decided in the present study to synthesize urea-coordinated
dinickel(II) complexes using these ligands. Their structures were
revealed by X-ray structure analysis, and the magnetic behavior
of dinickel(II) complexes with L1 was also investigated. In this
way we elucidated the coordination behavior of urea with the
dinickel(II) sites formed by our synthesized alkoxo-bridged binu-
cleating ligands (Scheme 2).
Since the active site of urease (from Klebsiella aerogenes [1] and
Bacillus pasteurii [2]), a metallo-enzyme that hydrolyzes urea to
ammonia and carbon dioxide, was revealed by X-ray structural
analysis, a number of dinickel(II) complexes have been synthesized
as active site models [3,4]. In order to design active site models
bearing two nickel(II) ions, binucleating ligands, which have two
chelating side arms linked by various functional groups to attract
two nickel(II) ions, have been widely used [3,4]. In this way,
urea-coordinated complexes have been synthesized from carboxy-
lato- [5], phenolato- [6–12], alkoxo- [13,14], pyrazolato- [15–20],
and phthalazine- [21] bridged dinickel(II) complexes. These urea-
coordinated complexes are important in examining mechanisms
for catalytic hydrolysis of urea, because they are thought to be
the first intermediate in the hydrolysis reaction [3,4].
2. Experimental
2.1. Synthesis of binucleating ligands
1,3-Bis(N-(2-(dimethylamino)ethyl)-N-methylamino)propan-2-
ol (HL1), 1,3-bis(bis(2-methoxyethyl)amino)propan-2-ol (HL2), and
1-(N-(2-(dimethylamino)ethyl)-N-methylamino)-3-(bis(2-methoxy-
ethyl)amino)propan-2-ol (HL3), were synthesized by a previously re-
ported method [22].
We recently reported the synthesis of binucleating ligands HL1,
HL2, and HL3, in which two chelating sites are linked by a 2-
hydroxytrimethylene bridge (Scheme 1), and their use in the prep-
aration of dizinc(II) complexes [22]. Because fewer examples of
urea-coordinated complexes have been reported for binucleating
ligands with an alkoxo-bridge than for those described above, we
2.2. Synthesis of nickel complexes
2.2.1. [Ni2L1(
l-CH3COO)(CH3COO)(H2O)2]BPh4 (1)
To an ethanol solution (5 ml) of HL1 (0.10 g, 0.38 mmol) was
added Ni(CH3COO)2ꢀ4H2O (0.19 g, 0.76 mmol), and the solution
was heated at 60 °C for 2 h. The addition of NaBPh4 (0.13 g,
⇑
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 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.01.036

152
K. Mochizuki et al. / Inorganica Chimica Acta 400 (2013) 151–158
Scheme 1. Structural formulae of ligands.
0.38 mmol) resulted in a green precipitate, which was filtered and
dried under vacuum (yield 0.16 g). Slow diffusion of ethyl acetate
vapor into a nitromethane solution of the precipitate (nitrometh-
ane/ethyl acetate vapor diffusion method) yielded green rectangu-
lar crystals. Elemental analysis of a sample of 1 which was dried
under vacuum over P2O5 at 90 °C for 12 h gave the corresponding
complex formed by the loss of one water molecule from the parent
complex. Anal. Calc. for (C41H61N4O7B1Ni2-H2O): C, 59.18; H, 7.15;
N, 6.73; C/N = 8.79. Found: C, 59.52; H, 7.23; N, 6.82%; C/N = 8.73.
2.2.5. [Ni2L2(H2O)2(urea)Cl](BPh4)2ꢀH2O (5)
Complex 5 was prepared by the procedure described for 3, but
using HL2 instead of HL1. The crude product was recrystallized
from nitromethane/ethanol by the vapor diffusion method to give
green crystals (yield: 68%). Anal. Calc. for C64H83N4O9B1Cl1Ni2: C,
62.66; H, 6.82; N, 4.57. Found: C, 62.62; H, 6.73; N, 4.69%. IR
(KBr):
m
(urea) = 1640 cmꢁ1
.
2.2.6. [Ni2L3(
l-CH3COO)(CH3COO)(EtOH)]BPh4 (6)
To a methanol solution (4 ml) of HL3 (0.10 g, 0.34 mmol) was
added Ni(CH3COO)2ꢀ4H2O (0.17 g, 0.68 mmol), and the mixture
was heated at 60 °C for 2 h. Addition of NaBPh4 (0.12 g, 0.85 mol)
gave a green precipitate, which was filtered, washed with metha-
nol, and dried under vacuum. The product was recrystallized from
nitromethane/ethanol by vapor diffusion to afford green crystals
(yield: 56%). Anal. Calc. for C44H64N3O8B1Ni2: C, 59.30; H, 7.24; N,
2.2.2. [Ni2L1(
l
-CH3COO)(CH3COO)(urea)(EtOH)]BPh4ꢀEtOH (2)
To an ethanol solution (5 ml) of HL1 (0.10 g, 0.38 mmol) were
added Ni(CH3COO)2ꢀ4H2O (0.19 g, 0.76 mmol) and urea (0.23 g,
3.83 mmol). After the mixture was stirred at room temperature
for 2 h, addition of NaBPh4 (0.13 g, 0.38 mmol) resulted in the for-
mation of a green precipitate (0.20 g, yield: 54%), which was
recrystallized from nitromethane/ethanol by the vapor diffusion
method to yield green crystals. Anal. Calc. for C46H73N6O8B1Ni2:
C, 57.18; H, 7.61; N, 8.70. Found: C, 56.83; H, 7.38; N, 8.83%. IR
4.72. Found: C, 59.25; H, 7.28; N, 4.79%. IR (KBr):
m(l-CH3-
COO) = 1595 cmꢁ1 (CH3COO) = 1551 cmꢁ1
,
m
.
2.2.7. [Ni2L3(
To nitromethane solution (2 ml) of complex
l-CH3COO)2(urea)]BPh4 (7)
(KBr):
m
(urea) = 1668 cmꢁ1
.
a
6 (0.25 g,
0.28 mmol) was added an excess amount of urea (0.10 g,
1.67 mmol) in a solution of ethanol (0.2 ml). The mixture was
heated at 60 °C for 2 h. After the mixture was cooled to room tem-
perature, the precipitated urea was filtered off. Slow diffusion of
ethyl acetate into the filtrate yielded green crystals (yield: 48%).
Anal. Calc. for C43H62N5O8B1Ni2: C, 57.06; H, 6.90; N, 7.74. Found:
2.2.3. [Ni2L1(
l
-Cl)2(urea)(CH3CN)]BPh4ꢀH2O (3)
Complex 3 was prepared by a similar method to 2, but using
nickel(II) chloride hexahydrate instead of Ni(CH3COO)2ꢀ4H2O. The
resulting green precipitates were recrystallized from acetonitrile/
ethyl acetate by the vapor diffusion method to afford green crystals
(yield: 50%). Anal. Calc. for C40H58N7O2B1Cl2Ni2: C, 55.35; H, 6.73;
C, 57.04; H, 6.62; N, 7.72%. IR (KBr):
m , m(l-
(urea) = 1672 cmꢁ1
CH3COO) = 1600 cmꢁ1
.
N, 11.30. Found: C, 55.65; H, 6.75; N, 11.76%. IR:
m(urea) =
1661 cmꢁ1. Elemental analysis of a sample dried under vacuum
over P2O5 at 90 °C for 12 h, showed data corresponding to a
complex formed by the loss of one acetonitrile molecule from the
parent complex. Anal. Calc. for (C40H58N7O2B1Cl2Ni2-CH3CN): C,
55.19; H, 6.70; N, 10.16. Found: C, 55.62; H, 6.70; N, 9.91%.
2.3. Physical measurements
Elemental analyses were performed by the Center for Instru-
mental Analysis at Hokkaido University. Magnetic susceptibilities
were measured using a Quantum MPMS-XL SQUID magnetometer.
Visible absorption spectra were measured using a Varian Cary 500
spectrophotometer and a JASCO V-650 spectrophotometer with an
integrating sphere. IR spectra were measured using a Shimadzu FT-
IR-8300 spectrophotometer.
2.2.4. [Ni2L2(
l
-CH3COO)2]BPh4ꢀCH3CN (4)
Complex 4 was synthesized by the reaction of HL2 with two
equivalents of Ni(CH3COO)2ꢀ4H2O in ethanol using a similar meth-
od to that used for the preparation of complex 1. However, the
reaction carried out in the presence of excess urea did not give
urea-coordinated complexes. The crude product was recrystallized
from acetonitrile/ethyl acetate by the vapor diffusion method to
give green crystals (yield: 50%). A sample dried under vacuum at
80 °C was used for elemental analysis. Anal. Calc. for C43H59N2O9B1Ni2:
C, 59.79; H, 7.93; N, 8.37. Found: C, 58.92; H, 6.80; N, 8.24%.
2.4. Crystallographic studies
X-ray crystallography of single crystals was carried out using a
Mac Science MXC3 k four-circle diffractometer (2, 3, and 5) and a
Rigaku XtaLAB-mini (1, 4, 6, and 7) diffractometer with graphite-
monochromatized Mo Ka radiation (k = 0.71073 or 0.71075 Å).
The structures were solved by the direct method (SIR92, SIR97,

K. Mochizuki et al. / Inorganica Chimica Acta 400 (2013) 151–158
153
Scheme 2. Structural formulae of complexes.
SIR2004, SIR2008 [23]), and refined on F2 by the full-matrix least-
squares method using SHELXL 97 [24]). A -scan was applied for
3. Results and discussion
u
The reaction of the binucleating ligand HL1 with Ni(CH3COO)2ꢀ
4H2O in ethanol at a molar ratio of HL1:Ni = 1:2 resulted in the for-
absorption correction [25] of 2, 3, and 5, as well as numerical
absorption correction of 1, 4, 6, and 7. All non-hydrogen atoms
were refined using anisotropic thermal parameters. Hydrogen
atoms were placed by SHELXL [24] (riding model refinement). Calcu-
lations were carried out using the MAXUS program system provided
by Mac Science (using a SiliconGraphics O2 work station) and the
CRYSTALSTRUCTURE program (ver. 4.0) provided by Rigaku (using a per-
sonal computer with WINDOWS XP OS). Structural diagrams were
drawn using ORTEP-3 for Windows [26]. Crystallographic data for
the complexes are listed in Table 1.
mation of the dinuclear nickel(II) complex [Ni2L1(
l-CH3COO)(CH3-
COO)(H2O)2]BPh4 (1), in which the deprotonated species L1 acted
as a N,N,O,N,N-pentadentate ligand and the two nickel(II) ions were
linked by a
l
-alkoxo bridge and a
l
-acetato bridge; the Niꢀ ꢀ ꢀNi dis-
tance was 3.5479(7) Å and the Ni(1)–O(20)–Ni(2) angle was
123.89(10)° (Fig. 1 and Table 2). Although HL1 is a binucleating
ligand with symmetrical chelating side arms with respect to the
central 2-hydroxytrimethylene bridge, the two six-coordinated

154
K. Mochizuki et al. / Inorganica Chimica Acta 400 (2013) 151–158
Table 1
Crystallographic data of complexes.
Complex
1
2
3
4
5
6
7
Empirical
formula
Formula
weight
T (K)
B1C41H61N4Ni2O7
B1C46H73N6Ni2O8
B1C40H60Cl2N7Ni2O3 B1C45H62N3Ni2O9
B2C64H83Cl1N4Ni2O9 B1C44H64N3Ni2O8
B1C43H62N5Ni2O8
850.13
966.33
886.08
917.21
1226.83
891.21
905.20
298(2)
298(2)
298(2)
298(2)
298(2)
298(2)
298(2)
k (Å)
0.71075
monoclinic
P21/c
15.7424(17)
12.1464(11)
23.968(4)
0.71073
monoclinic
P21/n
17.591(8)
17.449(5)
17.256(7)
0.71075
monoclinic
P21/c
18.230(3)
15.284(3)
16.364(4)
0.71075
triclinic
0.71073
triclinic
0.71075
monoclinic
P21/c
14.169(6)
18.064(7)
18.342(8)
0.71075
monoclinic
P21/c
11.254(5)
15.027(6)
27.273(10)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
ꢀ
ꢀ
P1
P1
11.618(7)
14.000(8)
14.870(8)
76.444(15)
84.969(18)
86.123(17)
2339(3)
2
11.765(3)
14.4120(14)
18.5460(18)
95.475(8)
91.001(12)
93.904(12)
3122.1(8)
2
a
(°)
b (°)
107.058(5)
104.69(4)
101.445(16)
98.824(4)
91.374(5)
c
(°)
V (Å3)
4381.4(9)
4
1.286
0.910
1800
5124(4)
4
1.253
0.788
2064
4468.8(15)
4
1.317
1.006
1872
4639(4)
4
1.276
0.863
1896
4611(3)
4
1.304
0.871
1920
Z
Dcalc (Mg mꢁ3
)
1.302
0.860
972
1.305
0.704
1300
l
(mmꢁ1
)
F(000)
Crystal size
(mm)
0.50 ꢂ 0.20 ꢂ 0.12 0.90 ꢂ 0.80 ꢂ 0.60 0.50 ꢂ 0.40 ꢂ 0.15
0.61 ꢂ 0.46 ꢂ 0.16 0.70 ꢂ 0.50 ꢂ 0.35
0.62 ꢂ 0.48 ꢂ 0.11 0.79 ꢂ 0.50 ꢂ 0.50
Number of
reflections
8950
11712
1.015
10247
1.024
10603
1.059
14341
1.024
10648
1.099
10581
1.063
Goodness-of-fit 1.064
(GOF) on F2
Final R indices
R1 = 0.0583,
wR2 = 0.1595
R1 = 0.0586,
wR2 = 0.1586
R1 = 0.0763,
wR2 = 0.2124
R1 = 0.0531,
wR2 = 0.1502
0.840 and ꢁ0.910
R1 = 0.0387,
wR2 = 0.1049
0.617 and ꢁ0.417
R1 = 0.0476,
wR2 = 0.1197
0.32 and ꢁ0.44
R1 = 0.0599,
wR2 = 0.1615
0.810 and ꢁ0.620
[I > 2r(I)]
Largest
0.830 and ꢁ0.450 0.560 and ꢁ0.533 1.403 and ꢁ0.4123
difference in
peak and
hole (e Åꢁ3
)
site (O(52)ꢀ ꢀ ꢀO(43) 2.763(4) Å), and also intermolecular hydrogen
bonds (O(52)ꢀ ꢀ ꢀO(32)# (ꢁx + 1, y + 1/2, ꢁz + 3/2) 2.853(4) Å and
O(51)ꢀ ꢀ ꢀO(42)# (ꢁx + 1, y + 1/2, ꢁz + 3/2) 2.714(4) Å). These inter-
molecular hydrogen bonds further connected the cation parts of
1 to form a linear chain, which was similar to that found in 2
(see below).
In contrast, the formation of a complex between HL1 and
Ni(CH3COO)2ꢀ4H2O in the presence of excess urea, under reaction
conditions which were the same as those described for 1, followed
by recrystallization from nitromethane/ethanol, gave the urea-
coordinated nickel(II) complex [Ni2L1(
l-CH3COO)(CH3COO)(urea)
(EtOH)]BPh4ꢀEtOH (2) (Fig. 2a). The coordination mode of the binu-
cleating ligand L1 in 2 was similar to that found in 1. L1 was coor-
dinated as a pentadentate ligand, while the two six-coordinate
nickel(II) ions were also linked by a
l-alkoxo and a l-acetato
bridges; the Niꢀ ꢀ ꢀNi distance was 3.545(1) Å and the Ni(1)–
O(20)–Ni(2) angle was 124.53(11)°. One urea molecule was coordi-
nated to Ni(2) through the O(53) oxygen atom (Ni(2)–O(53):
2.062(3) Å) as a planar molecule. The bond lengths of C(50)–
O(53) (1.250(4) Å), C(50)–N(51) (1.329(5) Å), and C(50)–N(52)
(1.335(5) Å) were similar to those found in other urea-coordinated
nickel complexes (see Table 2) [5–21]. In addition, the N(51) nitro-
gen atom of the coordinated urea interacted with the O(33) oxygen
Fig. 1. ORTEP Drawing of the cation section of 1, [Ni2L1(
l-CH3COO)(CH3COO)(H2-
O)2]+, with 50% probability ellipsoids. Hydrogen atoms have been omitted for
clarity. Selected bond lengths (Å) and angles (°): Ni(1)–O(20) 1.997(2), Ni(1)–O(32)
2.038(3), Ni(1)–O(42) 2.166(3), Ni(1)–O(43) 2.202(3), Ni(1)–N(1) 2.128(3), Ni(1)–
N(4) 2.143(4), Ni(2)–O(20) 2.024(3), Ni(2)–O(33) 2.030(3), Ni(2)–O(51) 2.130(4),
Ni(2)–O(52) 2.144(4), Ni(2)–N(8) 2.089(3), Ni(2)–N(11) 2.136(4); O(20)–Ni(1)–
O(32) 95.92(10), O(42)–Ni(1)–O(43) 59.90(9), N(1)–Ni(1)–N(4) 83.48(12), O(20)–
Ni(2)–O(33) 94.94(10), O(51)–Ni(2)–O(52) 84.90(12), N(8)–Ni(2)–N(11) 84.55(13),
Ni(1)–O(20)–Ni(2) 123.89(10).
atom of the l-acetato bridge through an intramolecular hydrogen
bond (N(51)ꢀ ꢀ ꢀO(33): 2.831(5) Å) and formed an intermolecular
hydrogen bond with O(42)# (ꢁx + 1/2 + 1, y + 1/2, ꢁz + 1/2)
(N(51)ꢀ ꢀ ꢀO(42)#: 2.857(5) Å) of the acetato ligand coordinated to
Ni(1). These hydrogen bonds successively connected the cation
parts of 2 to form a zigzag linear chain, similar to 1 (see Fig. 2b).
In addition, one oxygen atom (O(63)) of the lattice solvent, ethanol,
was located close to N(52) of the urea molecule (O(63)ꢀ ꢀ ꢀN(52):
3.073(9) Å). Another intramolecular hydrogen bond was observed
between O(62) of the ethanol molecule coordinated to Ni(2) and
O(43) of the acetato ligand coordinated to Ni(1) (O(62)ꢀ ꢀ ꢀO(43):
nickel(II) ions had different co-ordination environments; one nick-
el(II) ion was associated with one bidentate acetato ligand and the
other nickel(II) ion with two water molecules. The two side arms
containing N(1) and N(11), respectively, adopted a syn-geometry
with respect to the plane comprised N(4), O(20), and N(8). Com-
plex 1 had an intramolecular hydrogen bond around the binuclear

K. Mochizuki et al. / Inorganica Chimica Acta 400 (2013) 151–158
155
Table 2
Structural data.
Complex
Niꢀ ꢀ ꢀNi (Å)
Ni–O–Ni (°)
Urea Ni–O (Å)
O@C (Å)
C–Nav (Å)a
1
2
3
4
5
6
7
3.5479(7)
3.545(1)
2.969(1)
3.237(2)
3.647(1)
3.470(1)
3.330(1)
123.89(10)
124.53(11)
97.13(13)
110.07(9)
133.11(7)
122.49(8)
113.3(1)
2.062(3)
2.041(3)
1.250(4)
1.242(6)
1.332
1.331
2.1779(16)
2.116(3)
1.253(3)
1.247(5)
1.327
1.340
a
Averaged values.
Fig. 2. (a) ORTEP Drawing of the cation section of 2, [Ni2L1( -CH3COO)(CH3COO)(urea)(EtOH)]+, with 50% probability ellipsoids. Hydrogen atoms have been omitted for clarity.
l
Selected bond lengths (Å) and angles (°): Ni(1)–O(20) 1.987(2), Ni(1)–O(32) 2.042(3), Ni(1)–N(4) 2.126(3), Ni(1)–N(1) 2.128(3), Ni(1)–O(42) 2.139(3), Ni(1)–O(43) 2.219(3),
Ni(2)–O(20) 2.018(2), Ni(2)–O(33) 2.029(3), Ni(2)–O(53) 2.062(3), Ni(2)–N(8) 2.114(4), Ni(2)–O(62) 2.140(3), Ni(2)–N(11) 2.158(4), N(52)–C(50) 1.335(5), N(51)–C(50)
1.329(5), O(53)–C(50) 1.250(4); O(20)–Ni(1)–O(32) 96.28(10), O(42)–Ni(1)–O(43) 59.95(10), O(53)–Ni(2)–O(62) 85.61(11), Ni(1)–O(20)–Ni(2) 124.53(11), O(53)–C(50)–
N(51) 122.8(4), O(53)–C(50)–N(52) 119.9(4), N(51)–C(50)–N(52) 117.3(4). (b) Hydrogen bond network of 2.
2.721(4) Å). Similar hydrogen bonding between urea NH and the O
atoms of the bridging acetate has been observed in other model
systems [18]. Thus, it was suggested that hydrogen bonds play
an important role in the stabilization of urea binding to the dinu-
clear nickel(II) site in the crystal. The formation of 2 essentially
corresponded to the substitution of the coordinated water mole-
cules of 1 by one urea and one ethanol. The reaction of 1 with urea
in a nitromethane solution containing a small amount of ethanol
(ca. 10%) followed by recrystallization from nitromethane/ethanol
(vapor diffusion method) gave the urea-coordinated complex 2.
coordinated nickel(II) ions were doubly linked by two chloride
anions, which resulted in a considerably shortened Ni(1)ꢀ ꢀ ꢀNi(2)
distance (2.969(1) Å) and
a sharper Ni(1)–O(20)–Ni(2) angle
(97.13(13)°) compared to the singly l-acetato-bridged dinickel(II)
complexes 1 and 2. One urea molecule was coordinated to Ni(2)
through the O(60) oxygen atom, also forming a hydrogen bond be-
tween N(62) and Cl(40) of the
3.177(5) Å).
l
-chloro bridge (N(62)ꢀ ꢀ ꢀCl(40):
The reaction of another binucleating ligand, HL2, which contains
two bis(methoxyethyl)amino arms, with Ni(II)(CH3COO)2ꢀ4H2O
gave the dinuclear nickel(II) complex [Ni2L2(
l-CH3COO)2]BPh4-
Complex 1 þ urea þ EtOH ! complex 2 þ 2H2O
ꢀCH3CN (4), in which L2 acted as a O,O,N,O,N,O,O-heptadentate li-
The reaction of HL1 with Ni(II)Cl2ꢀ6H2O instead of Ni(II)(CH3-
COO)2ꢀ4H2O in the presence of excess urea, followed by recrystal-
lization from acetonitrile/ethyl acetate, afforded the urea-
gand and two 6-coordinate nickel(II) ions were linked by an
alkoxo group and two
distance (3.237(2) Å) was shorter than those of the singly
l
-acetato bridges (Fig. 4). The Ni(1)ꢀ ꢀ ꢀNi(2)
l-aceta-
coordinated dinickel(II) complex [Ni2L1(
l
-Cl)2(urea)(CH3CN)]BPh4ꢀ
to-bridged complexes 1 and 2 (av. 3.55 Å), and accordingly, the
Ni(1)–O(4)–Ni(2) angle (110.07(9)°) was smaller than those of 1
and 2 (av. 124°). Because 4—which had no coordinated solvent
H2O (3) (Fig. 3). The coordination behavior of the pentadentate
ligand L1 was similar to its behavior in 1 and 2, but the two six-

156
K. Mochizuki et al. / Inorganica Chimica Acta 400 (2013) 151–158
(BPh4)2ꢀH2O (5) (Fig. 5). In 5, L2 acted as a O,O,N,O,N,O,O-heptaden-
tate ligand, in which only an alkoxo group linked the two nickel(II)
ions, without any other bridging ligands. In this way it differed
from 2 and 4, which contained
l-acetato bridges. The Ni(II)ꢀ ꢀ ꢀNi(II)
distance was 3.647(1) Å and the Ni(1)–O(4)–Ni(2) angle was
133.11(7)°; these values were the longest and the largest, respec-
tively, among the dinickel(II) complexes synthesized in this study.
One urea was also coordinated to Ni(1) through the oxygen atom
O(40), forming a hydrogen bond between the nitrogen atom
N(42) of the urea and Cl(30) coordinated to Ni(1) (N(42)ꢀ ꢀ ꢀCl(30):
3.235(3) Å). The Cl(30) atom further interacted through hydrogen
bonding with O(60) of the water molecule coordinated with
Ni(2) (Cl(30)ꢀ ꢀ ꢀO(60): 3.043(2) Å). A significant feature of this com-
plex was the positional relationship between the urea molecule
coordinated with Ni(1) and the water molecule coordinated with
Ni(2). The distance between O(50) of the water molecule and
O(40) of the coordinated urea was quite short (2.747(2) Å), sug-
gesting the existence of a hydrogen bonding interaction between
them. Because of this, O(50) of the water molecule was located clo-
sely to the carbonyl carbon C(41) of the urea molecule
(O(50)ꢀ ꢀ ꢀC(41): 3.473(3) Å). We consider this positional relation-
ship to be interesting, because it resembles a proposed intermedi-
ate for a urea hydrolysis mechanism in which one terminal
hydroxide ion attacks the carbonyl carbon of urea in binuclear
nickel(II) sites [3,4]. The possibility of using 5 in urea hydrolysis
reactions is currently under investigation.
Fig. 3. ORTEP Drawing of the cation section of 3, [Ni2L1( -Cl)2(urea)(CH3CN)]+, with
l
50% probability ellipsoids. Hydrogen atoms have been omitted for clarity. Selected
bond lengths (Å) and angles (°): Ni(1)–O(20) 1.976(3), Ni(1)–N(32) 2.071(4), Ni(1)–
N(4) 2.079(4), Ni(1)–N(1) 2.115(4), Ni(1)–Cl(40) 2.4579(14), Ni(1)–Cl(50)
2.6271(16), Ni(2)–O(20) 1.983(3), Ni(2)–O(60) 2.041(3), Ni(2)–N(8) 2.102(4),
Ni(2)–N(11) 2.138(4), Ni(2)–Cl(40) 2.5065(15), Ni(2)–Cl(50) 2.5542(16), O(60)–
C(61) 1.242(6), N(62)–C(61) 1.332(7), N(63)–C(61) 1.330(6); Ni(1)–O(20)–Ni(2)
97.13(13), Ni(1)–Cl(40)–Ni(2) 73.44(4), Ni(2)–Cl(50)–Ni(1) 69.89(4), O(60)–C(61)–
N(63) 120.3(5), O(60)–C(61)–N(62) 123.1(5), N(63)–C(61)–N(62) 116.5(5).
A non-symmetric binucleating ligand, HL3, which contained one
N,N-dimethylaminoethyl and one bis(methoxyethyl)amino arm (a
mixture of the arms of HL1 and those of HL2) also afforded a binu-
clear nickel(II) complex and the corresponding urea-coordinated
complex. This ligand was anticipated to show mixed coordination
behavior, between that of HL1 and that HL2. Complex formation be-
tween HL3 and Ni(II)(CH3COO)2ꢀ4H2O followed by recrystallization
from acetonitrile/ethanol gave a dinuclear nickel(II) complex, [Ni2-
L3(
two six-coordinate nickel(II) ions linked by an alkoxo group and
one
-acetato bridge, with a Ni(1)ꢀ ꢀ ꢀNi(2) distance of 3.470(1) Å
and a Ni(1)–O(20)–Ni(2) angle of 122.49(8)°; these values were
close to those of the singly -acetato-bridged complexes 1 and 2.
l-CH3COO)(CH3COO)(EtOH)]BPh4 (6) (Fig. 6), which comprised
l
l
Fig. 4. ORTEP Drawing of the cation section of 4, [Ni2L2( -CH3COO)2]+, with 50%
l
probability ellipsoids. Hydrogen atoms have been omitted for clarity. Selected bond
lengths (Å) and angles (°): Ni(1)–O(32) 2.035(3), Ni(1)–O(6) 2.170(3), Ni(1)–O(22)
1.974(3), Ni(1)–O(42) 2.013(3), Ni(1)–O(2) 2.138(3), Ni(1)–N(9) 2.094(4), Ni(2)–
O(33) 2.059(3), Ni(2)–O(43) 1.980(3), Ni(2)–O(22) 1.975(3), Ni(2)–O(20) 2.196(3),
Ni(2)–O(16) 2.156(4), Ni(2)–N(13) 2.051(3); Ni(1)–O(22)–Ni(2) 110.07(9).
molecules, as the coordination sites of the two nickel(II) ions were
fully occupied by L2 and the bridging acetate ligands—was rela-
tively stable, the reaction of 4 with excess urea did not afford
urea-coordinated complexes. In addition, complex formation be-
tween Ni(II)(CH3COO)2ꢀ4H2O and HL2, even in the presence of urea,
did not afford urea-coordinated complexes but gave 4. However,
changing the counter anion from acetate to chloride resulted in a
considerable alteration of the bridging mode between the two
nickel(II) ions and enabled the coordination of urea to the dinick-
el(II) site formed by L2.
Fig. 5. ORTEP Drawing of the cation section of 5, [Ni2L2( -CH3COO)2]+, with 50%
l
probability ellipsoids. Hydrogen atoms have been omitted for clarity. Selected bond
lengths (Å) and angles (°): Ni(1)–O(4) 1.9771(13), Ni(1)–N(9) 2.0785(18), Ni(1)–
O(2) 2.1303(16), Ni(1)–O(6) 2.1449(18), Ni(1)–O(40) 2.1779(16), Ni(1)–Cl(30)
2.3135(8), Ni(2)–O(4) 1.9978(13), Ni(2)–O(60) 2.0348(17), Ni(2)–N(13)
2.0436(18), Ni(2)–O(50) 2.0741(16), Ni(2)–O(20) 2.1146(15), Ni(2)–O(16)
2.1162(16), O(40)–C(41) 1.253(3), C(41)–N(42) 1.318(4), C(41)–N(43) 1.335(4);
Ni(1)–O(4)–Ni(2) 133.11(7), O(40)–C(41)–N(42) 122.6(3), O(40)–C(41)–N(43)
119.8(3), N(42)–C(41)–N(43) 117.5(3).
Complex formation between HL2 and Ni(II)Cl2ꢀ6H2O gave the
urea-coordinated dinickel(II) complex [Ni2L2(H2O)2(urea)Cl]

K. Mochizuki et al. / Inorganica Chimica Acta 400 (2013) 151–158
157
Table 3
Absorption spectrum data.
a
Complex
kmax/nm (
e
/molꢁ1 dm3 cmꢁ1
)
kmax/nmb
1
2
3
4
5
6
7
387(83) 646(27) 1045(23)
391(95) 652(31) 1051(25)
405(76) 686(30) 1090(26)
395(34) 660(15) 997(34)
394(42) 700(14) 1017(25)
386(69) 647(22) 1021(30)
388(51) 646(18) 1023(27)
392 651
391 648
395 676
395 652
405 692
394 651
392 661
a
Nitromethane solution, 298 K.
Diffuse reflectance spectral data (350–800 nm) for solid samples.
b
Fig. 6. ORTEP Drawing of the cation section of 6, [Ni2L3(
l-CH3COO)(CH3-
COO)(EtOH)]+, with 50% probability ellipsoids. Hydrogen atoms have been omitted
for clarity. Selected bond lengths (Å) and angles (°): Ni(1)–O(2) 2.122(2), Ni(1)–O(6)
2.155(3), Ni(1)–O(20) 1.9738(18), Ni(1)–O(32) 1.963(3), Ni(1)–O(52) 2.124(2),
Ni(1)–N(9) 2.052(3), Ni(2)–O(20) 1.9847(18), Ni(2)–O(33) 2.060(2), Ni(2)–O(42)
2.217(2), Ni(2)–O(43) 2.1061(19), Ni(2)–N(13) 2.127(3), Ni(2)–N(17) 2.093(2);
Ni(1)–O(20)–Ni(2) 122.50(9).
Fig. 7. ORTEP Drawing of the cation section of 7, [Ni2L3( -CH3COO)2(urea)]+, with
l
Fig. 8. Temperature dependence of molar magnetic susceptibility (s) and effective
magnetic moment (d) per nickel(II) for 1 (a), and 2 (b). The solid lines represent the
best-fit calculated curves corresponding to the theoretical equation.
50% probability ellipsoids. Hydrogen atoms have been omitted for clarity. Selected
bond lengths (Å) and angles (°): Ni(1)–O(2) 2.167(3), Ni(1)–O(6) 2.156(3), Ni(1)–
O(20) 1.977(3), Ni(1)–O(32) 1.993(3), Ni(1)–O(42) 2.083(3), Ni(1)–N(9) 2.080(3),
Ni(2)–O(20) 2.009(3), Ni(2)–O(33) 2.096(3), Ni(2)–O(43) 2.054(3), Ni(2)–O(50)
2.116(3), Ni(2)–N(13) 2.122(4), Ni(2)–N(17) 2.147(4), O(50)–C(51) 1.247(5), N(52)–
C(51) 1.340(6), N(53)–C(51) 1.340(6); Ni(1)–O(20)–Ni(2) 113.32(11), O(50)–C(51)–
N(52) 121.7(4), O(50)–C(51)–N(53) 121.2(4), N(52)–C(51)–N(53) 117.1(4).
for IR data) and subsequent release of the ethanol molecule from
the Ni(1) ion. The resulting double linkage of the two nickel(II) ions
by
l
-acetato bridges resulted in a smaller Ni(1)ꢀ ꢀ ꢀNi(2) distance
(3.330(1) Å) and Ni(1)–O(20)–Ni(2) angle (113.3(1)°) compared
with the parent complex 6. The urea molecule, which was coordi-
nated to Ni(2) through the O(50) oxygen atom, formed an intramo-
The Ni(1) and the Ni(2) atoms were associated with an ethanol
molecule and a non-bridging bidentate acetato ligand, respec-
tively; they interacted with each other through a hydrogen bond
(O(52)ꢀ ꢀ ꢀO(42) (2.739(3) Å)).
lecular hydrogen bond with the
l
-acetato bridge (N(52)ꢀ ꢀ ꢀO(33)
(2.823(5) Å)) and an intermolecular hydrogen bond with the meth-
oxy oxygen atom (N(53)ꢀ ꢀ ꢀO(6) (1 ꢁ x, ꢁ1/2 + y, 1/2 ꢁ z)
(3.018(5) Å)). The following short contacts were also observed
around the coordinated urea: N(52)ꢀ ꢀ ꢀO(43) (3.319(5) Å) and
N(53)ꢀ ꢀ ꢀO(42) (1 ꢁ x, ꢁ1/2 + y, 1/2 ꢁ z) (3.150(5) Å). These results
suggest that the urea molecule may favor coordination to the nick-
el(II) ion on the dimethylaminoethyl arm over coordination with
the corresponding ion on the bis(methoxyethyl) arm, which results
in the carboxylate shift.
The reaction of 6 with urea in a mixed nitromethane/ethanol
solvent gave the urea-coordinated dinickel(II) complex [Ni2L3(
l-
CH3COO)2(urea)]BPh4 (7) (Fig. 7). In this reaction, surprisingly,
there was no direct substitution of the coordinated ethanol mole-
cule by a urea molecule, in contrast to the formation of the urea
complexes of 1 and 2, in which direct substitution of a water mol-
ecule by a urea molecule was observed (see above). The coordina-
tion of urea with the Ni(2) ion was associated with a coordination
change (carboxylate shift) of the non-bridging acetato ligand on
Table 3 shows visible absorption spectral data for our synthe-
sized dinickel(II) complexes. The absorption spectra measured in
Ni(2) to become a
l-acetato bridge (see the Experimental section

158
K. Mochizuki et al. / Inorganica Chimica Acta 400 (2013) 151–158
nitromethane essentially corresponded to those measured for the
solid state samples (diffuse reflectance spectra). Each complex in
nitromethane showed three absorption bands, around 400, 700,
and 1000 nm, which were ascribed to the spin-allowed transitions
by the binucleating ligand L1 resulted in a weak antiferromagnetic
interaction between the nickel(II) ions, as observed for 1 and 2.
Appendix A. Supplementary material
3
3
from A2g(F) (the ground state of the octahedral d8 ion) to T1g(P),
3
3T1g(F), and T2g(F), respectively [27]. The urea-coordinated nicke-
CCDC 908550–908556, contain the supplementary crystallo-
graphic data for complexes 1–7, respectively. 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 http://dx.doi.org/10.1016/j.ica.2013.01.036.
l(II) complexes 2, 3, 5, and 7 showed absorption bands at 388–405,
646–700, and 1017–1090 nm, which were similar to those (376,
425, 745, and 1060 nm) observed for jack-bean urease [28].
The close linking of two nickel(II) ions by a binucleating ligand
leads to a significant magnetic interaction between the nickel(II)
ions. The effective magnetic moment per nickel(II) of 1 in the solid
state was found to be dependent on temperature in the range 5–
300 K (Fig. 8a), decreasing gradually with decreasing temperature
from 2.8 BM at 300 K. A similar temperature dependence was also
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2
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These behaviors indicate the existence of antiferromagnetic
coupling between the two nickel(II) centers. The coupling
constants J = ꢁ16.7 cmꢁ1 (g = 1.99,
q
= 0.011, R =
R
(
v
exptl(i) ꢁ
v
calcd(i))2/
R
(v
exptl(i))2 = 4.32 ꢂ 10ꢁ3) (1) and ꢁ15.0 cmꢁ1 (g = 2.08,
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vm ¼ ð1 ꢁ
q
Þ
vdim
þ
q
vmono þ TIP
vdim ¼ ð2NAg2
l
B2=kTÞðexpð2J=kTÞ þ 5 expð6J=kTÞÞ=ð1 þ 3 exp
ð2J=kTÞ þ 5 expð6J=kTÞÞ;
vmono ¼ 2NAg2
H ¼ ꢁ2JS1S2;
S1 ¼ S2 ¼ 1:
lB2=3kT;
The fittings were appropriate for each dinickel(II) complex. The
observation of similar J values for these complexes regardless of
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whether the bridging ligand was a
l-acetato or l-chloro bridge.
The use of ligand L3, which had one arm from each of L1 and L2,
showed that the coordination of the urea molecule took place pref-
erentially at the dimethylaminoethyl site rather than at the
bis(methoxyethyl) site. The close linking of the two nickel(II) ions