Synthesis and structures of dinickel(II) complexes with a binucleating ligand, N,N′-bis[3-(N,N-dimethyl)aminopropyl]-N,N′-dimethyl-1,3- diamino-2-hydroxypropane (HL1), and investigation of carboxylic acid capture assisted by intramolecular hydrogen bonding

Article abstract of DOI:10.1016/j.ica.2014.01.031

New carboxylic-acid-coordinated dinickel(II) complexes with binucleating ligands - HL¹, which contains dimethylaminopropyl arms, and its analog HL², which contains one dimethylaminopropyl arm and one bis(methoxyethyl) arm - both of which contain a 2-hydroxytrimethylene bridge between the two chelating sites, were synthesized. The structures of acetic acid coordinated complexes, [Ni₂L¹(μ-CH₃COO) ₂(CH₃COOH)]BPh₄ (1), and [Ni₂L ²(μ-CH₃COO)₂(CH₃COOH)]BPh ₄ (6), a propionic acid coordinated complex, [Ni₂L ¹(μ-C₂H₅COO)₂(C₂H ₅COOH)]BPh₄ (3), and a benzoic acid coordinated complex, [Ni₂L¹(μ-PhCOO)₂(PhCOOH)]BPh₄ (4) were revealed by X-ray crystallography, as well as those of the parent dinickel(II) complexes [Ni₂L¹(μ-CH₃COO) ₂]BPh₄ (2), and [Ni₂L²(μ-CH ₃COO)₂]BPh₄ (5). In complexes 1, 3, 4, and 6, one carboxylic acid molecule is coordinated to one of the dinuclear nickel(II) ions, forming an intramolecular hydrogen bond with a μ-alkoxo bridge. Spectrophotometric investigation revealed that complex 1 underwent partial dissociation of the acetic acid in acetonitrile to give an equilibrium between 1 and 2, and that 2 reversibly associated with an acetic acid molecule in acetonitrile to form 1. Intramolecular hydrogen bonding plays an important role in the stabilization of carboxylic acid coordination to the dinickel(II) complexes with binucleating ligands.

Full text of DOI:10.1016/j.ica.2014.01.031

Inorganica Chimica Acta 414 (2014) 27–32
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis and structures of dinickel(II) complexes
with a binucleating ligand, N,N0-bis[3-(N,N-dimethyl)aminopropyl]-
N,N0-dimethyl-1,3-diamino-2-hydroxypropane (HL1), and investigation
of carboxylic acid capture assisted by intramolecular hydrogen bonding
⇑
Katsura Mochizuki , Junpei Takahashi
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 carboxylic-acid-coordinated dinickel(II) complexes with binucleating ligands—HL1, which contains
dimethylaminopropyl arms, and its analog HL2, which contains one dimethylaminopropyl arm and one
bis(methoxyethyl) arm—both of which contain a 2-hydroxytrimethylene bridge between the two
Article history:
Received 12 November 2013
Received in revised form 14 January 2014
Accepted 22 January 2014
Available online 31 January 2014
chelating sites, were synthesized. The structures of acetic acid coordinated complexes, [Ni2L1(
l
-CH3
-CH3COO)2(CH3COOH)]BPh4 (6), a propionic acid coordinated
-C2H5COO)2(C2H5COOH)]BPh4 (3), and a benzoic acid coordinated complex, [Ni2L1
-PhCOO)2(PhCOOH)]BPh4 (4) were revealed by X-ray crystallography, as well as those of the parent
dinickel(II) complexes [Ni2L1( -CH3COO)2]BPh4 (2), and [Ni2L2(
-CH3COO)2]BPh4 (5). In complexes 1,
3, 4, and 6, one carboxylic acid molecule is coordinated to one of the dinuclear nickel(II) ions, forming
an intramolecular hydrogen bond with a -alkoxo bridge. Spectrophotometric investigation revealed that
COO)2(CH3COOH)]BPh4 (1), and [Ni2L2(
l
complex, [Ni2L1(
l
Keywords:
(l
Dinickel complexes
Binucleating ligands
Synthesis
l
l
l
Acetic acid
complex 1 underwent partial dissociation of the acetic acid in acetonitrile to give an equilibrium between
1 and 2, and that 2 reversibly associated with an acetic acid molecule in acetonitrile to form 1. Intramo-
lecular hydrogen bonding plays an important role in the stabilization of carboxylic acid coordination to
the dinickel(II) complexes with binucleating ligands.
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
important role in stabilizing the coordination of urea to a
dinickel(II) complex with the binucleating ligand L3 (Chart 1)
[22]. In addition, Volkmer et al. reported that the coordination of
a neutral acetic acid molecule was selectively stabilized by a
strong intramolecular hydrogen bond in [Ni2(ppepO)(C6H5COO)2
(CH3COOH)]+ (ppepOH: 1-[bis(2-pyridylmethyl)amino]-3-[2-(2-
pyridyl)ethoxy]-2-hydroxypropane), which has relevance to the
active site of urease [23]. Such stabilization of the coordination
of weakly binding ligands by intramolecular hydrogen bonding is
an attractive research topic.
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]. Binucleating ligands, which have two
chelating side arms linked by various functional groups to attract
two nickel(II) ions, have been widely used in the design of active
site models bearing two nickel(II) ions [3,4]. During these synthetic
studies of urea-coordinated complexes using binucleating ligands
[5–21], it has been revealed that intramolecular hydrogen bonds
around the binuclear nickel(II) site are important in supporting
the coordination of weakly coordinating substrates such as urea.
Meyer and co-workers reported the presence of intramolecular
hydrogen bonds between the urea NH groups and the O atoms of
the bridging acetate in dinuclear nickel(II) complexes [18]. We
have also found that intramolecular hydrogen bonds play an
In the present study, we report the synthesis of new carboxylic-
acid-coordinated dinickel(II) complexes with binucleating ligands
HL1 and HL2 (a new ligand), in which two chelating sites are linked
with a 2-hydroxytrimethylene bridge (Chart 2). The structures of
isolated dinickel(II) complexes were revealed by X-ray analysis
and, in addition, the behavior of the acetic-acid-coordinated
complex with L1 in acetonitrile solution was investigated spectro-
scopically. In this way we concluded that the capture of carboxylic
acids by our synthesized dinickel(II) complexes is assisted by
strong intramolecular hydrogen bonding.
⇑
Corresponding author. Tel.: +81 45 787 2186; fax: +81 45 787 2413.
E-mail address: katsura@yokohama-cu.ac.jp (K. Mochizuki).
http://dx.doi.org/10.1016/j.ica.2014.01.031
0020-1693/Ó 2014 Elsevier B.V. All rights reserved.

28
K. Mochizuki, J. Takahashi / Inorganica Chimica Acta 414 (2014) 27–32
(300 MHz, CDCl3):
d
3.75 (multiplet, J = 3.6 Hz, 1H), 3.46
(t, J = 5.7 Hz, 4H), 3.34 (s, 6H), 2.80 (t, J = 6.0 Hz, 4H), 2.66–2.29
(m, 8H), 2.27 (s, 3H), 2.21 (s, 6H), 1.63 (quintet, J = 7.2 Hz, 2H).
13C NMR (75 MHz, CDCl3): d 71.1 (t), 66.3 (d), 61.9 (t), 59.8 (t),
58.7 (q), 57.5 (t), 55.8 (t), 54.6 (t), 45.4 (q), 43.7 (q), 25.3 (t).
HRMS-ESI: Found m/z = 306.27479 [M+H]+, Calcd m/z = 306.27512
(C15H36O3N3).
2.2. Synthesis of nickel complexes
[Ni2L1(
l-CH3COO)2(CH3COOH)]BPh4 (1): To a methanol solution
(15 ml) of HL1 (0.50 g, 1.73 mmol) was added Ni(CH3COO)2ꢀ4H2O
(0.86 g, 3.46 mmol). The solution was heated at 60 °C for 2 h. The
addition of NaBPh4 (3.00 g, 8.77 mmol) resulted in the formation
of a green precipitate, which was filtered and dried under vacuum.
The slow diffusion of ethyl acetate vapor into a nitromethane solu-
tion of the precipitate (nitromethane/ethyl acetate vapor-diffusion
method) yielded green rectangular crystals (yield 40%). Anal. Calc.
for C45H65N4O7B1Ni2: C, 59.91; H, 7.26; N, 6.21. Found: C, 59.90;
Chart 1. Structure formulae of ligands.
2. Experimental
2.1. Synthesis of binucleating ligands
H, 7.31; N, 6.32%. IR(KBr): 1685 cmꢁ1
Vis(diffuse reflectance): 404, 679 nm.
(m (ml-COO).
C@O), 1604 cmꢁ1
N,N0-bis[3-(N,N-dimethyl)aminopropyl]-N,N0-dimethyl-1,3-dia-
mino-2-hydroxypropane (HL1) was synthesized by our previously
reported method [24].
[Ni2L1(
l-CH3COO)2]BPh4 (2): To an acetonitrile solution (2 ml) of
complex 1 (0.19 g, 0.21 mmol) was added triethylamine (1.06 g,
10.48 mmol). Slow diffusion of ethanol vapor into the mixture
yielded green crystals (yield 88%). Anal. Calc. for C43H61N4O5B1Ni2:
C, 61.33; H, 7.30; N, 6.65. Found: C, 61.34; H, 7.33; N, 6.62%.
N-[3-(N,N-dimethyl)aminopropyl]-N-methyl-N’,N’-bis(2-methoxy-
ethyl)-1,3-diamino-2-hydroxypropane (HL2): To an ethanol (5 ml)
solution of epichlorohydrin (0.35 g, 3.78 mmol) was added drop-
wise an ethanol (5 ml) solution of bis(2-methoxyethyl)amine
(0.50 g, 3.75 mmol). After the mixture had been refluxed for 4 h,
an ethanol solution (4 ml) of N,N,N0-trimethyl-1,3-propanediamine
(0.44 g, 3.79 mmol) and triethylamine (0.38 g 3.76 mmol) was
added to the solution, which was further refluxed for 19 h. The
resultant solution was evaporated to give a brown oil, which
was purified by silica gel column chromatography (chloroform/
methanol 80:20 v/v) to give a yellow oil in ca. 30% yield. 1H NMR
IR(KBr): 1603 cmꢁ1
[Ni2L1(
(
ml-COO). Vis(diffuse reflectance): 399, 680 nm.
l
-C2H5COO)2(C2H5COOH)]BPh4 (3): To an acetonitrile
solution (2 ml) of complex 2 (0.10 g, 0.12 mmol) was added
propionic acid (1.82 g, 24.6 mmol). Slow diffusion of ethanol vapor
into the mixture gave green crystals (yield 54%). Anal. Calc. for
C
48H71N4O7B1Ni2: C, 61.05; H, 7.58; N, 5.97. Found: C, 61.48; H,
7.76; N, 5.97%. IR(KBr): 1684 cmꢁ1
Vis(diffuse reflectance): 400, 669 nm.
(m (ml-COO).
C@O), 1599 cmꢁ1
[Ni2L1(
l-PhCOO)2(PhCOOH)]BPh4 (4): To an acetonitrile solution
(2 ml) of complex 2 (0.10 g, 0.12 mmol) was added benzoic acid
(0.29 g, 2.37 mmol). Slow diffusion of ethyl acetate vapor into the
mixture gave green crystals (yield 70%). Anal. Calc. for C61H74N4O7-
B1Ni2: C, 66.40; H, 6.76; N, 5.08. Found: C, 66.03; H, 6.68; N, 5.11%.
IR(KBr): 1685 cmꢁ1
tance): 401, 679 nm.
[Ni2L2(
-CH3COO)2]BPh4 (5): To a methanol solution (8 ml) of
(m (ml-COO). Vis(diffuse reflec-
C@O), 1613 cmꢁ1
l
HL2 (0.11 g, 0.36 mmol) and NaOH (0.02 g, 0.50 mmol) was added
Ni(CH3COO)2ꢀ4H2O (0.18 g, 0.72 mmol). The mixture was heated at
60 °C for 2 h. The addition of NaBPh4 (0.12 g, 0.35 mmol) gave a
green precipitate, which was filtered, washed with methanol, and
dried in vacuum. The obtained precipitate was recrystallized from
acetonitrile/diethyl ether by the vapor-diffusion method to afford
green crystals (yield: 39%). Anal. Calc. for C43H60N3O7B1Ni2: C,
60.11; H, 7.04; N, 4.89. Found: C, 60.13; H, 7.15; N, 4.89%. IR(KBr):
1604 cmꢁ1
669 nm.
(ml-COO). Vis(diffuse reflectance): 398, 459(shoulder),
[Ni2L2(
l-CH3COO)2(CH3COOH)]BPh4 (6): Complex 6 was pre-
pared by the same procedure as described for 1 using HL2 instead
of HL1. The crude product was recrystallized from acetonitrile/
ethyl acetate by the vapor-diffusion method to give green crystals
(yield: 52%). Anal. Calc. for C45H64N3O9B1Ni2: C, 58.80; H, 7.02; N,
4.57. Found: C, 58.78; H, 7.07; N, 4.47%. IR(KBr): 1685 cmꢁ1
(mC@O),
1610 cmꢁ1
(ml-COO). Vis(diffuse reflectance): 393, 647 nm.
2.3. Physical measurements
Elemental analysis was performed by the Center for Instrumen-
tal Analysis of Hokkaido University. Visible absorption spectra
were measured using a VARIAN Cary 500 spectrophotometer. IR
Chart 2. Structure formulae of complexes.

K. Mochizuki, J. Takahashi / Inorganica Chimica Acta 414 (2014) 27–32
29
spectra were measured using
spectrophotometer.
a
Shimadzu FT-IR-8300
2.4. Crystallographic studies
X-ray crystallography of single crystals was carried out on MAC
Science MXC3k four-circle (1–3) and Rigaku XtaLAB-mini (4–6)
diffractometers with graphite-monochromatized Mo K
a radiation
(k = 0.71073 or 0.71075 Å). The structures were solved by the
direct method (SIR92, SIR97, SIR2004, SIR2008 [25]) and refined
on F2 by the full-matrix least-squares method SHELXL 97 [26]).
The
u-scan was applied for absorption correction [27] of complexes
1–3, as well as numerical absorption correction for 4–6. All non-
hydrogen atoms were refined using anisotropic thermal parame-
ters. Hydrogen atoms were placed by SHELXL [26] (riding model
refinement). Calculations were carried out using the MAXUS program
system provided by MAC Science (SiliconGraphics O2 work station)
and the CRYSTALSTRUCTURE program (ver. 4.0) provided by Rigaku
(personal computer with Windows XP operating system). Struc-
tural diagrams were drawn using ORTEP-3 for Windows [28]. Crys-
tallographic data for the complexes are listed in Table 1.
Fig. 1. ORTEP drawing of the cation part of 1, [Ni2L1(
l
-CH3COO)2(CH3COOH)]+, with
50% probability ellipsoids. Hydrogen atoms have been omitted for clarity. Selected
bond lengths (Å) and angles (°): Ni(1)–N(1) 2.164(7), Ni(1)–N(5) 2.142(6), Ni(1)–
O(20) 2.074(5), Ni(1)–O(32) 2.057(5), Ni(1)–O(42) 2.046(5), Ni(1)–O(53) 2.113(6),
Ni(2)–N(9) 2.088(6), Ni(2)–N(13) 2.126(6), Ni(2)–O(20) 2.039(5), Ni(2)–O(33)
1.972(5), Ni(2)–O(43) 1.984(6), N(1)–Ni(1)–N(5) 95.5(3), O(20)–Ni(1)–O(32)
92.4(2), O(20)–Ni(1)–O(53) 89.7(2), O(32)–Ni(1)–O(42) 95.0(2), N(9)–Ni(2)–N(13)
94.9(2), O(20)–Ni(2)–O(33) 91.4(2), O(33)–Ni(2)–O(43) 109.1(3), Ni(1)–O(20)–
Ni(2) 114.3(2).
3. Results and discussion
The reaction of the binucleating ligand HL3, which has dimeth-
ylaminoethyl arms, with Ni(CH3COO)2ꢀ4H2O did not result in the
formation of a complex with an acetic acid molecule but only a
Table 2
Structural data.
l
-acetato bridged dinuclear nickel(II) complex, which had been re-
Complex
Niꢀ ꢀ ꢀNi (Å)
Ni–O–Ni (°)
Acetic acid
Ni–O (Å)
2.113(6)
ported previously [22]. In contrast, the reaction of the ligand HL1,
which had longer arms, with Ni(CH3COO)2ꢀ4H2O in methanol at a
molar ratio of HL1:Ni = 1:2 resulted the formation of an acetic-
O@C (Å)
C–OH (Å)
1.277(10)
1
2
3
4
5
6
3.455(2)
3.282(1)
3.396(1)
3.4629(9)
3.242(1)
3.411(1)
114.3(2)
1.216(10)
111.92(13)
112.08(13)
114.10(10)
108.96(7)
111.44(7)
acid-coordinated dinuclear nickel(II) complex, [Ni2L1(
l-CH3-
2.146(3)
2.111(3)
1.211(6)
1.229(4)
1.286(6)
1.289(5)
COO)2(CH3COOH)]BPh4 (1), in which the deprotonated species L1
acted as a N,N,O,N,N-pentadentate ligand and the two nickel(II)
2.126(2)
1.237(4)
1.279(4)
ions were linked by a
l-alkoxo and two l-acetato bridges; the
Niꢀ ꢀ ꢀNi distance was 3.455(2) Å and the Ni(1)–O(20)–Ni(2) angle
was 114.3(2)° (Fig. 1 and Table 2). Although HL1 was a binucleating
ligand with symmetrical chelating side arms with respect to the
central 2-hydroxytrimethylene bridge, the coordination environ-
ments of the two nickel(II) ions were different; one nickel(II) ion
was in a six-coordinate geometry and the other nickel(II) ion was
in a pseudo-pyramidal five-coordinate geometry (
s
= 0.38; top:
O(43), bottom: N(13), N(9), O(20), and O(33)). Each nickel(II) ion
was encircled by a side arm containing N(1) or N(13), so as to form
a distorted plane comprised of N(1), N(5), O(20), and O(32), or N(9),
Table 1
Crystallographic data of complexes.
Complex
1
2
3
4
5
6
Empirical formula
Formula weight
T (K)
B1C45H65N4Ni2O7
902.24
298(2)
B1C43 H61N4Ni2O5
842.17
298(2)
B1C48H71N4Ni2O7
944.32
298(2)
B1C60H71N4Ni2O7
1088.45
298(2)
B1C43H60N3Ni2O7
859.17
298(2)
B1C45H64N3Ni2O9
919.23
298(2)
k (Å)
0.71073
0.71073
0.71073
0.71075
0.71075
0.71075
Crystal system
Space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/c
14.569(10)
9.687(4)
triclinic
P1
triclinic
P1
monoclinic
P21/n
19.727(6)
12.982(4)
22.528(8)
triclinic
P1
triclinic
P1
9.981(5)
14.789(8)
16.927(8)
105.908(8)
90.358(4)
106.792(5)
2290.6(19)
2
ꢀ
ꢀ
ꢀ
ꢀ
9.534(4)
15.236(3)
16.480(4)
74.607(15)
77.69(3)
83.93(2)
2251.9(11)
2
10.9320(14)
13.070(6)
18.723(5)
72.14(3)
76.146(15)
82.73(2)
2468.1(14)
2
1.271
12.192(7)
12.855(7)
15.418(9)
87.75(3)
86.166(19)
67.343(9)
2225(2)
33.624(4)
a
(°)
b (°)
106.47(2)
99.673(3)
c
(°)
V (Å3)
4551(4)
4
1.317
0.880
5688(3)
4
1.271
0.717
Z
2
Dcalc (Mg mꢁ3
)
1.242
0.881
1.282
0.896
1.333
0.878
l
(mmꢁ1
)
0.814
F(000)
1920
896
1008
2304
912
976
Crystal size (mm)
0.35 ꢂ 0.30 ꢂ 0.25
0.35 ꢂ 0.30 ꢂ 0.20
0.70 ꢂ 0.40 ꢂ 0.15
0.70 ꢂ 0.60 ꢂ 0.40
0.76 ꢂ 0.41 ꢂ 0.15
0.38 ꢂ 0.31 ꢂ 0.28
Number of reflections
10451
10345
11317
13035
10012
10413
Goodness-of-fit (GOF) on F2
1.082
1.100
1.010
1.044
1.040
1.047
Final R indices [I > 2
r
(I)]
R1 = 0.0840,
wR2 = 0.2752
0.712 and ꢁ0.598
R1 = 0.0611,
wR2 = 0.2098
2.006 and ꢁ0.555
R1 = 0.0637,
wR2 = 0.1906
0.865 and ꢁ0.388
R1 = 0.0560,
wR2 = 0.1685
1.540 and ꢁ1.030
R1 = 0.0425,
wR2 = 0.1180
0.61 and ꢁ0.700
R1 = 0.0449,
wR2 = 0.1269
1.060 and ꢁ0.700
Largest difference in peak and
hole (e Å3)

30
K. Mochizuki, J. Takahashi / Inorganica Chimica Acta 414 (2014) 27–32
N(13), O(20), and O(33), respectively. Four methyl groups on the
nitrogen donor atoms were elongated toward the same side with
respect to these N2O2 planes. Although possible steric hindrance
of the 6th coordination to the nickel(II) ions by the methyl groups
was anticipated, we were surprised to find that one acetic acid
molecule, acting as a monodentate ligand, was coordinated to
one of the nickel(II) ions, accompanied by a strong intramolecular
hydrogen bond between O(52)–H(52) of the acetic acid and the
oxygen atom O(20) of the
2.486(8) Å). The C–O bond length of the coordinated oxygen atom,
l
-alkoxo bridge (O(52)ꢀ ꢀ ꢀO(20)
C(51)–O(53), was notably short (1.216(10) Å), and was also shorter
than the average value for C–O bonds (av. 1.26 Å) in
l-bridged
acetato ligands, while the C–O distance of the non-coordinated
oxygen atom, C(51)–O(52), was rather long (1.277(10) Å). A similar
acetic-acid-coordinated dinuclear nickel(II) complex prepared
previously, [Ni2(ppepO)(C6H5COO)(CH3COOH)]+ [23], had an anal-
ogous binucleating ligand with a 2-hydroxytrimethylene linker.
IR spectroscopy also supported the presence of a coordinated
acetic acid molecule in 1, as well as spectrophotometric studies
on the solution behavior of 1 in acetonitrile. The IR spectrum of 1
showed peaks at 1685 and 1604 cmꢁ1 ascribable to the mC@O
of the coordinated acetic acid and to antisymmetric stretching
Fig. 3. Visible absorption spectral change of complex
acetonitrile upon addition of 0- (1), 1- (2), 2- (3), 5- (4), 10- (5), and 20- (6) fold
stoichiometric amounts of acetic acid against 2.
2 ) in
(2.01 mmol dmꢁ3
2 changed depending on the concentration of acetic acid added
in acetonitrile (Fig. 3). The absorption spectrum of 2 in the absence
of acetic acid showed absorption bands at 396 nm (
e
: 160 cmꢁ1
(
ml-COO) of the
able to C@O were also observed for the propionic-acid-coordinated
complex
l-acetato bridge, respectively [29,30]. Peaks ascrib-
molꢁ1 dm3) and 675 nm ( : 48 cmꢁ1 molꢁ1 dm3). As the concentra-
e
m
3
(1684 cmꢁ1
) and the benzoic-acid-coordinated 4
tion of the added acetic acid increased, the intensities of these
absorption bands decreased to reach almost constant values at a
20-fold stoichiometric excess of acetic acid against 2, accompanied
by a blue shift of the longer absorption band from 675 to 654 nm.
This spectral change corresponded to the change from complex 2
to complex 1 upon coordination of an acetic acid molecule. The
(1685 cmꢁ1), as well as complex 6 (1685 cmꢁ1), whereas no peaks
due to mC@O were observed for complexes 2 and 5, which did not
have coordinated acetic acid molecules (see below).
Treatment of 1 with triethylamine as a base in acetonitrile fol-
lowed by recrystallization released the acetic acid molecule from 1
to afford [Ni2L1(
l-CH3COO)2]BPh4 (2) (Fig. 2). In 2, the coordination
final absorption spectrum, with bands at 396 nm (
e
: 54 cmꢁ1
mode of the binucleating ligand L1 was almost the same as had
been observed in 1; L1 was coordinated as a pentadentate ligand,
and the two square-pyramidal five-coordinate nickel(II) ions
molꢁ1 dm3) and 654 nm ( : 22 cmꢁ1 molꢁ1 dm3), was considered
e
to correspond to that of complex 1 in acetonitrile. Although a sim-
ilar absorption spectrum was actually observed for an acetonitrile
(Ni(1):
s
= 0.23, Ni(2):
s = 0.33) were also linked by a l-alkoxo
solution of complex 1, the observed
larger than those of the final spectrum obtained in the acetic acid
addition experiments (396 nm
106 cmꢁ1 molꢁ1 dm3) and
669 nm ( values were ascrib-
: 36 cmꢁ1 molꢁ1 dm3)). These larger
e values of the two bands were
and two
l-acetato bridges. The Niꢀ ꢀ ꢀNi distance was 3.282(1) Å
and the Ni(1)–O(20)–Ni(2) angle was 111.92(13)°. The dissociation
of the acetic acid molecule slightly shortened the Niꢀ ꢀ ꢀNi distance
and decreased the Ni(1)–O(20)–Ni(2) angle. Thus, intramolecular
hydrogen bonding plays an important role in stabilizing the coor-
dination of the acetic acid molecule to the dinuclear nickel(II) site.
The reaction of 2 with acetic acid in acetonitrile was investi-
gated spectrophotometrically. The visible absorption spectrum of
(e:
e
e
able to the partial dissociation of the acetic acid molecule from 1 in
acetonitrile solution,1 because the addition of acetic acid to an ace-
tonitrile solution of 1 decreased these
e values to afford a spectrum
the same as the final one observed in the above experiments.
Fig. 4 shows the temperature dependence of the absorption
spectrum of 1 in acetonitrile. As the temperature increased, the
absorption bands around 400 and 670 nm increased, indicating
that dissociation of the acetic acid molecule proceeded at higher
temperatures. The temperature dependence of the spectral change
was reversible in acetonitrile, suggesting the presence of the equi-
librium formulated as Eq. (1).
Complex 2 þ CH3COOH ꢀ Complex 1
ð1Þ
On the basis of the above spectrophotometric experiments, it
was concluded that the synthetic treatment of 2 with acetic acid
in acetonitrile actually produced complex 1. Similar spectral
changes were observed upon addition of propionic acid or benzoic
acid to an acetonitrile solution of 2. The treatment of 2 with an
excess of propionic acid instead of acetic acid resulted not only
in the coordination of a propionic acid molecule but also in the
substitution of
l-acetato bridges with l-propionato bridges to
Fig. 2. ORTEP drawing of the cation part of 2, [Ni2L1( -CH3COO)2]+, with 50%
l
afford [Ni2L1(
l
-C2H5COO)2(C2H5COOH)]BPh4 (3) (Fig. 5). In com-
probability ellipsoids. Hydrogen atoms have been omitted for clarity. Selected bond
lengths (Å) and angles (°): Ni(1)–N(1) 2.127(4), Ni(1)–N(5) 2.092(3), Ni(1)–O(20)
1.978(3), Ni(1)–O(32) 2.004(3), Ni(1)–O(42) 1.996(3), Ni(2)–N(9) 2.085(4), Ni(2)–
N(13) 2.116(4), Ni(2)–O(20)–1.983(3), Ni(2)–O(33) 1.989(3), Ni(2)–O(43) 2.001(3),
N(1)–Ni(1)–N(5) 93.79(14), O(20)–Ni(1)–O(42) 97.76(12), O(32)–Ni(1)–O(42)
102.48(14), O(20)–Ni(2)–O(43) 96.28(13), O(33)–Ni(2)–O(43) 107.26(16), N(9)–
Ni(2)–N(13) 94.99(17), Ni(1)–O(20)–Ni(2) 111.92(13).
plex 3, the coordination geometry of ligand L1 was very similar
to that found in 1; one nickel(II) had six-coordinate geometry
1
The value of the equilibrium constant K for the Eq. (1) was estimated as ca.
1 ꢂ 103 molꢁ1 dm3.

K. Mochizuki, J. Takahashi / Inorganica Chimica Acta 414 (2014) 27–32
31
Fig. 4. Temperature dependence of vis-absorption spectrum of complex
1
(1.98 mmol dmꢁ3) in acetonitrile measured at 10.8 (1), 19.8 (2), 29.2 (3) and
38.7 °C (4).
and the other pseudo-pyramidal five-coordinate geometry (top:
O(43);
s = 0.31). It was notable that the coordination of the propi-
Fig. 6. ORTEP drawing of the cation part of 4, [Ni2L1(
l
-PhCOO)2(PhCOOH)]+, with
onic acid molecule was supported by a strong intramolecular
hydrogen bond between O(53)–H(53) of the propionic acid and
O(20) of the
to complex 1, with
complex 3, with
(3.396(1) Å).
The treatment of 2 with excess benzoic acid afforded similar re-
sults. A benzoic-acid-coordinated complex with two -benzoato
bridges, [Ni2L1(
-PhCOO)2(PhCOOH)]BPh4 (4) (Fig. 6), was ob-
50% probability ellipsoids. Hydrogen atoms have been omitted for clarity. Selected
bond lengths (Å) and angles (°): Ni(1)–N(1) 2.177(3), Ni(1)–N(5) 2.152(3), Ni(1)–
O(20) 2.083(3), Ni(1)–O(37) 2.043(3), Ni(1)–O(47) 2.042(3), Ni(1)–O(57) 2.111(3),
Ni(2)–N(9) 2.088(3), Ni(2)–N(13) 2.125(3), Ni(2)–O(20) 2.043(2), Ni(2)–O(38)
1.980(2), Ni(2)–O(48) 1.982(3), N(1)–Ni(1)–N(5) 97.19(11), O(20)–Ni(1)–O(37)
89.85(9), O(20)–Ni(1)–O(57) 90.81(9), O(37)–Ni(1)–O(47) 94.83(10), N(9)–Ni(2)–
N(13) 95.23(10), O(20)–Ni(2)–O(38) 95.09(8), O(38)–Ni(2)–O(48) 106.27(10),
Ni(1)–O(20)–Ni(2) 114.10(10).
l
-alkoxo bridge (O(53)ꢀ ꢀ ꢀO(20) 2.496(7) Å). Compared
l
-acetato bridges, the Niꢀ ꢀ ꢀNi distance of
l-propionato bridges, was slightly shorter
l
l
Thus, ligand L1, with symmetric arms, acted as a N,N,O,N,N-
pentadentate ligand in the dinuclear nickel(II) complexes 1, 3,
and 4, in which one of the nickel(II) ions had a six-coordinate
geometry associated with a coordinated carboxylic acid molecule
and the other nickel(II) was five-coordinate; the adjacent coordina-
tion site with respect to the coordinated carboxylic acid was
unoccupied. A new non-symmetric binucleating ligand L2, with a
dimethylaminopropyl arm and a bis(methoxyethyl) arm, was
expected to act as a N,N,O,N,O,O-hexadentate ligand. The reaction
of HL2 with Ni(CH3COO)2ꢀ4H2O in the presence of NaOH gave a
tained (Niꢀ ꢀ ꢀNi: 3.4629(9) Å); one nickel(II) had a six-coordinate
geometry and the other a pseudo-pyramidal five-coordinate geom-
etry (top: O(38),
s = 0.30). One benzoic acid molecule was also
coordinated to one of the nickel(II) ions, accompanied by a
significant intramolecular hydrogen bond between O(58)–H(58)
of the benzoic acid and O(20) of the
l-alkoxo bridge
(O(58)ꢀ ꢀ ꢀO(20) 2.445(4) Å).
dinuclear complex, [Ni2L2( -CH3COO)2]BPh4 (5), in which L2 acted
l
as a hexadentate ligand (Fig. 7). Complex 5 contained a six-coordi-
nate nickel(II) ion and a square pyramidal five-coordinate nickel(II)
ion (top: O(42),
s
= 0.15); the Niꢀ ꢀ ꢀNi distance was 3.242(1) Å and
the Ni(1)–O(21)–Ni(2) angle was 108.96(6)°. The conformation of
the five-membered chelate ring comprised of N(9), C(10), C(11),
O(12), and Ni(2) on the bis(methoxyethyl) arm adopted a gauche
form, with the C(11) atom located toward the center of the com-
plex. Judging from this, it was anticipated that the coordination
of a carboxylic acid molecule to the five-coordinate nickel(II) ion
with a vacant coordination site would be sterically hindered by
the bis(methoxyethyl) arm coordinated to the adjacent nickel(II)
ion. On the contrary, an acetic acid molecule was coordinated to
the dinickel(II) complex to give [Ni2L2(
COOH)]BPh4 (6) (Fig. 8), in which L2 also acted as a hexadentate li-
gand and the two six-coordinate nickel(II) ions were linked by a
alkoxo and two
-acetato bridges. The Niꢀ ꢀ ꢀNi distance was
l-CH3COO)2(CH3-
l
-
l
Fig. 5. ORTEP drawing of the cation part of 3, [Ni2L1( -C2H5COO)2(C2H5COOH]+,
l
3.411(1) Å and the Ni(1)–O(21)–Ni(2) angle was 111.44(7)°; these
were longer and wider, respectively, than those of complex 5. A
similar relationship was also observed between 1 and 2. One acetic
acid molecule coordinated to the nickel(II) ion on the dimethylami-
nopropyl arm site, accompanied by a strong intramolecular hydro-
gen bond (O(53)–O(21): 2.453(3) Å). The six-membered chelate
ring comprised of N(1), C(2), C(3), C(4), N(5), and Ni(1) adopted a
with 50% probability ellipsoids. Hydrogen atoms have been omitted for clarity.
Selected bond lengths (Å) and angles (°): Ni(1)–N(1) 2.130(4), Ni(1)–N(5) 2.086(4),
Ni(1)–O(20) 2.024(3), Ni(1)–O(33) 1.979(3), Ni(1)–O(43) 1.973(3), Ni(2)–N(9)
2.130(4), Ni(2)–N(13) 2.151(4), Ni(2)–O(20) 2.070(3), Ni(2)–O(34) 2.059(3),
Ni(2)–O(44) 2.047(3), Ni(2)–O(54) 2.146(3), N(1)–Ni(1)–N(5) 95.25(16), O(20)–
Ni(1)–O(43) 97.62(13), O(33)–Ni(1)–O(43) 109.72(16), N(9)–Ni(2)–N(13)
95.80(17), O(20)–Ni(2)–O(34) 93.17(13), O(20)–Ni(2)–O(54) 89.75(13), O(34)–
Ni(2)–O(44) 92.35(15), Ni(1)–O(20)–Ni(2) 112.08(13).

32
K. Mochizuki, J. Takahashi / Inorganica Chimica Acta 414 (2014) 27–32
the coordinated acetic acid on the adjacent coordination site, as
expected.
Thus, new carboxylic-acid-coordinated dinickel(II) complexes
with binucleating ligands HL1 and HL2 were synthesized and their
X-ray structures were determined. The solution behavior of com-
plex 2 in capturing the acetic acid molecule in acetonitrile was
elucidated spectrophotometrically. The results revealed that intra-
molecular hydrogen bonding plays an important role in stabilizing
the coordination of carboxylic acid to the dinickel(II) complex with
binucleating ligands.
Appendix A. Supplementary material
CCDC 968596–968600 and 968595 contain the supplementary
crystallographic data for compounds 1–5 and 6, respectively. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic 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.2014.01.031.
Fig. 7. ORTEP drawing of the cation part 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)–N(1) 2.154(3), Ni(1)–N(5) 2.134(2), Ni(1)–O(21)
1.9746(17), Ni(1)–O(32) 2.0163(17), Ni(1)–O(42) 1.9972(17), Ni(2)–N(9)
2.0891(19), Ni(2)–O(12) 2.2121(19), Ni(2)–O(15) 2.136(3), Ni(2)–O(21)
2.0085(18), Ni(2)–O(33) 1.9908(16), Ni(2)–O(43) 2.0587(18), N(1)–Ni(1)–N(5)
94.18(8), O(21)–Ni(1)–O(32) 92.78(7), O(32)–Ni(1)–O(42) 104.70(7), O(12)–Ni(2)–
N(9) 79.23(7), O(15)–Ni(2)–N(9) 81.65(9), O(21)–Ni(2)–O(33) 103.77(8), O(33)–
Ni(2)–O(43) 95.04(7). Ni(1)–O(21)–Ni(2) 108.96(7).
References
[1] E. Jabri, M.B. Carr, R.P. Hausinger, P.A. Karplus, Science 268 (1995) 998.
[2] S. Benini, W.R. Rypniewski, K.S. Wilson, S. Miletti, S. Ciurli, S. Mangani,
Structure 7 (1999) 205.
[3] F. Meyer, Prog. Inorg. Chem. 56 (2009) 487.
[4] A. Sigel, H. Sigel, R.K.O. Sigel, Nickel and Its Surprising Impact in Nature, Metal
Ions in Life Sciences, vol. 2, Wiley, 2007.
[5] W.-Z. Lee, H.-S. Teng, M.-Y. Ku, T.-S. Kuo, Dalton Trans. (2008) 2538.
[6] H. Adams, S. Chinas, D.E. Fenton, S.E. Spey, Dalton Trans. (2003) 625.
[7] T. Koga, H. Furutachi, T. Nakamura, N. Fukita, M. Ohba, K. Takahashi, H. Okawa,
Inorg. Chem. 37 (1998) 989.
[8] R. Amase, H. Shiraishi, Y. Miyasato, M. Ohba, H. Okawa, Bull. Chem. Soc. Jpn. 78
(2005) 1814.
[9] S. Uozumi, H. Furutachi, M. Ohba, H. Okawa, D.E. Fenton, K. Shindo, S. Murata,
D.J. Kitko, Inorg. Chem. 37 (1998) 6281.
[10] H. Carlsson, M. Haukka, A. Bousseksou, J.-M. Latour, E. Nordlander, Inorg.
Chem. 43 (2004) 8252.
[11] H. Carlsson, M. Haukka, E. Nordlander, Inorg. Chem. 41 (2002) 4981.
[12] S. Mukherjee, T. Weyhermüller, E. Bothe, K. Wieghardt, P. Chaudhuri, Eur. J.
Inorg. Chem. (2003) 863.
[13] K. Yamaguchi, S. Koshino, F. Akagi, M. Suzuki, A. Uehara, S. Suzuki, J. Am. Chem.
Soc. 119 (1997) 5752.
[14] H. Yamane, Y. Nakao, S. Kawabe, Y. Xie, N. Kanehisa, Y. Kai, M. Kinoshita,
W. Mori, Y. Hayashi, Bull. Chem. Soc. Jpn. 74 (2001) 2107.
[15] S.V. Kryatov, E.V. Rybak-Akimova, F. Meyer, H. Pritzkow, Eur. J. Inorg. Chem.
(2003) 1581.
[16] F. Meyer, H. Pritzkow, Chem. Commun. (1998) 1555.
[17] F. Meyer, E. Kaifer, P. Kircher, K. Heinze, H. Pritzkow, Chem. Eur. J. 5 (1999)
1617.
[18] M. Konrad, F. Meyer, A. Jacobi, P. Kircher, P. Rutsch, L. Zsolnai, Inorg. Chem. 38
(1999) 4559.
[19] S. Buchler, F. Meyer, E. Kaifer, H. Pritzkow, Inorg. Chim. Acta 337 (2002) 371.
[20] F. Meyer, M. Konrad, E. Kaifer, Eur. J. Inorg. Chem. (1999) 1851.
[21] A.M. Barrios, S.J. Lippard, J. Am. Chem. Soc. 122 (2000) 9172.
[22] K. Mochizuki, J. Takahashi, Y. Ishima, T. Shindo, Inorg. Chem. Acta 400 (2013)
151.
Fig. 8. ORTEP drawing of the cation part of 6, [Ni2L2( -CH3COO)2(CH3COOH)]+, with
l
50% probability ellipsoids. Hydrogen atoms have been omitted for clarity. Selected
bond lengths (Å) and angles (°): Ni(1)–N(1) 2.162(3), Ni(1)–N(5) 2.1482(18), Ni(1)–
O(21) 2.0847(19), Ni(1)–O(32) 2.0496(17), Ni(1)–O(42) 2.0545(18), Ni(1)–O(52)
2.126(2), Ni(2)–N(9) 2.081(2), Ni(2)–O(12) 2.172(3), Ni(2)–O(15) 2.144(2), Ni(2)–
O(21) 2.0434(16), Ni(2)–O(33) 1.9822(19), Ni(2)–O(43) 2.032(2), N(1)–Ni(1)–N(5)
96.96(9), O(21)–Ni(1)–O(32) 91.44(8), O(21)–Ni(1)–O(52) 88.95(8), O(32)–Ni(1)–
O(42) 92.83(7), O(12)–Ni(2)–N(9) 80.00(8), O(15)–Ni(2)–N(9) 81.27(8), O(21)–
Ni(2)–O(33) 105.07(8), O(33)–Ni(2)–O(43) 93.35(9), Ni(1)–O(21)–Ni(2) 111.44(7).
[23] D. Volkmer, A. Hörstmann, K. Griesar, W. Haase, B. Krebs, Inorg. Chem. 35
(1996) 1132.
[24] K. Mochizuki, Y. Ishima, K. Hayano, Inorg. Chim. Acta 362 (2009) 2722.
[25] A. Altomare, G. Cascarno, C. Giacovazzo, A. Gualiardi, J. Appl. Crystallogr. 27
(1994) 435.
[26] G.M. Sheldrick, University of Göttingen, Germany, 1997.
[27] A.C.T. North, D.C. Phillips, F.S. Mathews, Acta Crystallogr., Sect. A 24 (1968)
351.
[28] L.J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565.
[29] S. Mandal, V. Balamurugan, F. Lloret, R. Mukherjee, Inorg. Chem. 48 (2009)
7544.
[30] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination
Compounds, fourth ed., Wiley, New York, 1986.
chair-like conformation similar to that observed for 5. However,
the complex was different from 5 in the gauche five-membered
chelate ring comprised of N(9), C(10), C(11), O(12), and Ni(2) in
the bis(methoxyethyl) arm site, in which C(11) was located as far
as possible from the O(53) of the coordinated acetic acid. This
was thought to be due to the reduction in steric hindrance from