Nickel(II) derivatives of N-benzyliminodiacetate(2-) ligands, with and without imidazole: Synthesis, crystal structure and properties

Article abstract of DOI:10.1016/j.poly.2009.09.034

In an attempt to prepare binary and ternary compounds, we have obtained two molecular complexes [Ni(MEBIDA or MOBIDA)(H₂O)₃]·nH₂O (n = 0 or 1) and two iso-type salts [Ni(Him)₆][Ni(MEBIDA or MOBIDA)₂]·

Full text of DOI:10.1016/j.poly.2009.09.034

Polyhedron 29 (2010) 683–690
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Nickel(II) derivatives of N-benzyliminodiacetate(2ꢀ) ligands, with and without
imidazole: Synthesis, crystal structure and properties
D.K. Patel a, D. Choquesillo-Lazarte b, J.M. González-Pérez a, A. Domínguez-Martín a, A. Matilla-Hernandez a,
A. Castiñeiras c, J. Niclós-Gutiérrez a,
*
a Department of Inorganic Chemistry, Faculty of Pharmacy, University of Granada, E-18071 Granada, Spain
b Laboratorio de Estudios Cristalográficos, IACT-CSIC, P.T. Ciencias de la Salud, Granada, Spain
c Department of Inorganic Chemistry, Faculty of Pharmacy, University of Santiago de Compostela, E-15782 Santiago de Compostela, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
In an attempt to prepare binary and ternary compounds, we have obtained two molecular complexes
[Ni(MEBIDA or MOBIDA)(H2O)3]ꢁnH2O (n = 0 or 1) and two iso-type salts [Ni(Him)6][Ni(MEBIDA or MOB-
IDA)2]ꢁ4H2O [MEBIDA = N-(p-methylbenzyl)iminodiacetate(2ꢀ) and MOBIDA = N-(p-methoxybenzyl)-
iminodiacetate(2ꢀ) ligands, Him = imidazole]. Our results are discussed with regard to related copper(II)
and nickel(II) compounds. The reasons for which these chelating ligands produce nickel(II) salts instead
of ternary compounds remain unclear since other iminodiacetate-like ligands give true ternary Ni(II)
compounds with imidazole and other N-heterocyclic ligands.
Received 20 June 2009
Accepted 30 September 2009
Available online 30 October 2009
Keywords:
Nickel(II) complexes
Iminodiacetate
Imidazole
Ó 2009 Elsevier Ltd. All rights reserved.
Crystal structure
1. Introduction
tural information available [1–16] and secondly because Ni(II)
allows ternary compounds to be obtained as well as salts with sim-
The iminodiacetate(2ꢀ) ion (IDA) is a tridentate chelating li-
gand, able to partially satisfy a metal ion’s coordination. That en-
ables the formation of a large variety of mixed-ligand metal
complexes with IDA and/or IDA-like derivatives, and aqua or other
ligands. Structural studies of these complexes, based on crystallo-
graphic data, have proved interesting changes of the IDA moiety’s
conformation that can be rationalized according to the metal ion’s
coordination requirements and the metal binding possibilities of
other ligands involved in the coordination polyhedron.
Our group has been contributing in this field for two decades,
working on metal-iminodiacetates, mainly with first row transi-
tion metal ions. Most metal-IDA-like derivatives are obtained from
aqueous solutions. Thus, in the absence of other ligands other than
from water, complexes of the general formula [M(IDA-like)(-
H2O)x]ꢁnH2O are formed. This kind of compounds will hereafter
be referred to as a binary compound, assuming that the xH2O li-
gands are metal linkers to satisfy the coordinating requirements
of the metal ion M. A general strategy to obtain ternary compounds
is to react the binary compound (in solution) with other ligands (L),
frequently so-called secondary or auxiliary ligands. Statistical re-
sults show that the formation of a ternary complex, for example
[MAB], is favored against a salt such as [MA2][MB2].
ilar composition, in addition to related binary compounds or closely
similar polymeric compounds. The available information strongly
suggest that the non-coordinating moiety of the N-R-IDA ligand
(R = H, alkyl, benzyl or phenyl) can influence the system towards
a true mixed-ligand complex or to the corresponding salt. Under
the same conditions, the behavior of these systems noticeably dif-
fers from that observed for the copper(II) analogues, for which salt
formation has not been observed [17–32]. In particular, it is known
that N-methyliminodiacetate (MIDA) and N-benzyliminodiacetate
(NBzIDA) ligands yield salts with Ni(II) and imidazole (Him), but
not the corresponding ternary complexes [4]. In this context, the
aim of the present work is to contribute to this field by means of
the study of binary and ternary systems having Ni(II), an IDA-like
para-substituted ligand (see MEBIDA and MOBIDA in the Scheme
1) and Him. The need to carry out crystallographic studies is
founded in: (1) to distinguish whether the obtained crystals are
salts or ternary compounds; (2) to appreciate if the iminodiacetate
moiety experiences conformational changes between the binary
compound and the IDA derivative also containing imidazole.
2. Experimental
Currently, we are interested in the study of nickel(II)-iminodiac-
etates because of two main reasons, firstly due to the limited struc-
2.1. Materials
The chelating ligands were synthesized in the acid form
(H2MEBIDA or H2MOBIDA) as reported for N-benzyliminodiacetic
acid (H2NBzIDA) [26], but using 4-methylbenzylamine or
* Corresponding author.
E-mail address: jniclos@ugr.es (J. Niclós-Gutiérrez).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.09.034

684
D.K. Patel et al. / Polyhedron 29 (2010) 683–690
[Ni(Him)6][Ni(MOBIDA)2]ꢁ4H2O (C42H58N14Ni2O14, hereafter 4): C,
45.84; H, 5.32; N, 17.82. Found: C, 45.63; H, 5.17; N, 17.71%.
2.3. Physical measurements
Analytical data were obtained in a Fisons–Carlo Erba EA 1108
elemental micro-analyzer. TG analysis (pyrolysis) of the studied
compounds (295–800 °C) were carried out in CO2-free dry-air flow
(100 mL/min) by
a Shimadzu Thermobalance TGA-DTG-50H
instrument, meanwhile a series of FT-IR spectra (20–22 per sam-
ple) were recorded in order to identify the evolved gases, using a
coupled FT-IR Nicolet Magma 550 spectrometer. Infrared spectra
(KBr pellets, 4000–400 cmꢀ1) were recorded on a Jasco FT-IR 410
spectrometer. Electronic (diffuse reflectance) spectra were ob-
tained in a Varian Cary-5E spectrophotometer.
Scheme 1.
4-methoxybenzylamine instead of benzylamine. These latter
amines and imidazole were purchased from Aldrich, whereas nick-
el(II) basic carbonate [NiCO3ꢁ2Ni(OH)2ꢁ4H2O] was purchased from
Merck. Related salts are also available from Aldrich, such as 2Ni-
CO3ꢁ3Ni(OH)2ꢁ4H2O, or from various suppliers as NiCO3ꢁ2Ni(OH)2ꢁ
xH2O. In order to accurately determine the amount of this Ni(II)
source required for each synthesis, elemental and thermo-gravi-
metric analysis of this reagent was encouraged. These reagents
were used as received.
2.4. X-ray data collection and structure refinement
Suitable crystals were mounted on glass fibers and these sam-
ples were used for data collection. Data were collected with Bruker
X8 Kappa APEXII (1, 100 K), Bruker SMART CCD 1000 (2, 100 K) or
Bruker X8 Proteum (3 and 4, 293 K) diffractometers. The data were
processed with SAINT (2) [33] or APEX2 (1, 3 and 4) [34] and corrected
for absorption using SADABS [35]. The structures were solved by di-
rect methods [36], which revealed the position of all the non-
hydrogen atoms. These atoms were refined on F2 by a full-matrix
least-squares procedure using anisotropic displacement parame-
ters [37]. All hydrogen atoms were located in difference Fourier
maps and included as fixed contributions riding on their attached
atoms with isotropic thermal displacement parameters 1.2 times
those of the respective atoms. Geometric calculations were carried
out with PLATON [38] and drawings were produced with PLATON [38]
and MERCURY [39]. A summary of crystal data is reported in Table 1.
Selected coordination bonds and angles are listed in Table 2.
2.2. Synthesis of the nickel(II) derivatives
2.2.1. Molecular nickel(II)-chelates
In a Kitasato flask, 237 mg of H2MEBIDA or 253 mg of H2MOB-
IDA (1 mmol) was added to a slurry of Ni(II) basic carbonate
(124 mg, 0.33 mmol) in 120 mL of distilled water. The reacting
mixture was heated (70 °C) and stirred well under reduced pres-
sure (20 min) to remove CO2 (by-product) for ca. 2 h until a clear
solution seemed to be obtained. To remove a small amount of
unreacted metal basic carbonate, the solution was filtered without
vacuum through a sintered disk filter funnel (Ø 10–16 lm) onto a
crystallization device. Evaporation of the solvent was controlled by
covering the device with a plastic film. After a few days, well
shaped light bluish crystals appeared. Some of these were used
for single-crystal X-ray diffraction purposes. Additional samples
were collected, washed with cool water and air dried for analytical
studies. Typical yields are ca. 50–60%. Anal. Calc. for [Ni(MEB-
IDA)(H2O)3] (C12H19NNiO7, hereafter 1): C, 41.42; H, 5.50; N,
4.03. Found: C, 41.07; H, 5.70; N, 4.14%. Anal. Calc. for [Ni(MOB-
IDA)(H2O)3]ꢁH2O (C12H21NNiO9, hereafter 2): C, 37.73; H, 5.54; N,
3.67. Found: C, 37.87; H, 5.64; N, 3.60%.
3. Results and discussion
3.1. Structure and properties of binary compounds 1 and 2
As a part of the present study, we have made an effort to obtain
the structures of appropriate binary compounds of Ni(II) and MEB-
IDA (Fig. 1, left) or MOBIDA (Fig. 2, left), compounds 1 and 2,
respectively. These compounds do not have the same type of for-
mula but consist of [Ni(MEBIDA or MOBIDA)(H2O)3] molecules,
without (1) or with (2) non-coordinated water molecules. In these
complex molecules, the Ni(II) atom exhibits the expected octahe-
dral coordination and the IDA-like ligand only plays a tridentate
chelating role, adopting a fac-NO2 conformation. All Ni–O coordi-
nation bond length are in the range 2.00–2.07 Å, whereas the Ni–
N(MEBIDA or MOBIDA) bond distance is near 2.14 Å. It is interest-
ing to note that the lowest value of the trans-angle is the N(MEB-
IDA or MOBIDA)–Ni–O(aqua) one (ꢂ170°), whereas the trans O–
Ni–O(aqua) angle values are 175–177°.
2.2.2. Novel salts with the [Ni(Him)6]2+ ion
In a series of attempts to obtain ternary complexes of the for-
mula [Ni(IDA-like)(Him)3]ꢁnH2O, we proceeded as follows: to a
slurry of Ni(II) basic carbonate (124 mg, 0.33 mmol) in 120 mL of
distilled water in a Kitasato flask, 237 mg of H2MEBIDA or
253 mg of H2MOBIDA (1 mmol) was added. The reacting mixture
was heated (70 °C) and stirred well under reduced pressure
(20 min) to remove CO2 (by-product) over ca. 2 h until a clear solu-
tion was obtained, then Him (204 mg, 3 mmol) was added to each
binary complex solution. After a few minutes, the intensity of the
blue color of the solutions increased. These solutions were filtered
without vacuum on the appropriate crystallization device and left
to evaporate at room temperature, controlling the process with the
aid of a plastic film. In these cases, no unreacted residue of nicke-
l(II) basic carbonate was observed on the disk filter funnels. After
3 weeks, well shaped crystals were formed, many of them suitable
for X-ray crystallographic purposes. Additional samples were also
collected to give a yield of ca. 70%. Anal. Calc. for [Ni(Him)6]-
[Ni(MEBIDA)2]ꢁ4H2O (C42H58N14Ni2O12, hereafter 3): C, 47.22; H,
5.47; N, 18.35. Found: C, 46.99; H, 5.48; N, 18.27%. Anal. Calc. for
The large difference between compounds 1 and 2 refers to crys-
tal packing forces. There are no p,p-stacking interactions between
the aromatic rings of neighboring MEBIDA or MOBIDA ligands in
these compounds. In the crystal of 1, all O–H(aqua) bonds are in-
volved in (aqua)O–Hꢁ ꢁ ꢁO(carboxylate) interactions between four
different neighboring molecules, forming 2D frameworks parallel
to the ab plane, where the MEBIDA hydrophobic arms fall towards
the external faces (Fig. 1, right). These layers are connected by
hydrophobic contacts along the c-axis of the crystal. Again in the
crystal of 2, (aqua or water)O–Hꢁ ꢁ ꢁO(aqua, water or carboxylate)
interactions build 2D frameworks parallel to the ab plane, with
the MOBIDA non-coordinating arms oriented towards the external

D.K. Patel et al. / Polyhedron 29 (2010) 683–690
685
Table 1
Crystal and structure refinement data.
Compound
1
2
3
4
Empirical formula
Formula weight
Crystal system
Space group
Unit cell dimensions
a (Å)
C12H19NNiO7
347.99
monoclinic
P21
C12H21NNiO9
382.01
monoclinic
P21
C42H58N14Ni2O12
1068.40
C42H58N14Ni2O14
1100.44
triclinic
triclinic
ꢀ
ꢀ
P1
P1
7.1474(16)
7.4206(16)
14.346(3)
90
94.011(4)
90
759(3)
2
1.523
364
7.5845(4)
6.9195(3)
15.3709(6)
90
101.903(2)
90
789.33(6)
2
1.607
8.9079(18)
11.051(2)
12.953(3)
87.781(7)
74.283(7)
81.694(7)
1214.6(4)
1
9.0570(4)
11.0641(5)
12.8792(5)
88.1960(10)
74.4480(10)
82.2480(10)
1231.96(9)
1
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
qcalc (g cmꢀ3
F(0 0 0)
)
1.461
560
1.483
576
400
Crystal size (mm)
h Range (°)
Reflections collected
Independent reflections
Maximum/minimum transmission
Goodness-of-fit on F2
0.45 ꢃ 0.22 ꢃ 0.13
2.85–26.36
8705
3084
0.8483–0.5904
1.458
0.15 ꢃ 0.15 ꢃ 0.04
2.71–25.65
13383
3002
0.9508–0.8317
1.035
0.35 ꢃ 0.30 ꢃ 0.12
3.54–66.13
15616
3985
0.8314–0.6048
1.063
0.30 ꢃ 0.27 ꢃ 0.20
3.56–66.49
16685
3903
0.7374–0.6419
1.023
Final R indices [I > 2
r
(I)]
R1 = 0.0305
wR2 = 0.0735
R1 = 0.0225
wR2 = 0.0455
R1 = 0.0544
wR2 = 0.1488
R1 = 0.0496
wR2 = 0.1313
Table 2
Selected interatomic bonds lengths (Å) and trans angles (°) in compounds 1–4.
Lengths (Å)
Angles (°)
1
Ni1–O11
Ni1–O21
Ni1–O3
Ni1–O2
Ni1–O1
Ni1–N1
2.024(2)
2.032(2)
2.033(2)
2.046(2)
2.0682(18)
2.137(2)
O11–Ni1–O3
O1–Ni1–N1
O21–Ni1–O2
175.38(9)
172.21(12)
175.74(9)
2
Ni1–O11
Ni1–O3
Ni1–O21
Ni1–O2
Ni1–O1
Ni1–N10
2.0099(15)
2.0467(15)
2.0526(14)
2.0598(15)
2.0646(13)
2.1375(15)
O11–Ni1–O3
O21–Ni1–O2
O1–Ni1–N10
177.50(6)
175.53(6)
170.27(7)
3
Ni1–O4#1
Ni1–O4
Ni1–O8#1
Ni1–O8
Ni1–N1
Ni1–N1#1
2.043(2)
2.043(2)
2.0745(19)
2.0745(19)
2.108(2)
Ni2–N18#2
Ni2–N18
Ni2–N23#2
Ni2–N23
Ni2–N28
Ni2–N28#2
2.121(2)
2.121(2)
2.138(2)
2.138(2)
2.149(2)
2.149(2)
O4#1–Ni1–O4
N1–Ni1–N1#1
O8#1–Ni1–O8
N28–Ni2–N28#2
N23#2–Ni2–N23
N18#2–Ni2–N18
180.0
180.0
180.0
180.0
179.998(1)
180.0
2.108(2)
#1 ꢀx + 1, ꢀy, ꢀz; #2 ꢀx, ꢀy + 1, ꢀz + 1
4
Ni1–O4#1
Ni1–O4
Ni1–O8
Ni1–O8#1
Ni1–N1
Ni1–N1#1
2.0470(16)
2.0470(16)
2.0682(16)
2.0683(16)
2.1073(17)
2.1073(17)
Ni2–N29#2
Ni2–N29
Ni2–N19#2
Ni2–N19
Ni2–N24#2
Ni2–N24
2.128(2)
2.128(2)
2.1396(19)
2.1396(19)
2.1428(18)
2.1428(18)
O4#1–Ni1–O4
O8–Ni1–O8#1
N1–Ni1–N1#1
N29#2–Ni2–N29
N19#2–Ni2–N19
N24#2–Ni2–N24
180.00(6)
179.999(1)
179.999(1)
180.0
180.0
180.0
#1 ꢀx + 1, ꢀy + 1, ꢀz + 1; #2 ꢀx, ꢀy, ꢀz
faces (Fig. 2, right). However, these layers are connected by C–H/
p
with the production of H2O, CO2, CO, CH3OH, CH4 and probably
N2O, to give a residue of NiO (exp. 22.480%, calc. 21.461%). Com-
pound 2 seems to be less stable under dry-air flow, and loses water
before the start of the TG analysis to give Ni(MOBIDA)ꢁ3.16H2O.
This hydrate loses all remaining water in two overlapping steps
(75–170 and 170–265 °C, exp. 15.539%, calc. for 3.16H2O
15.519%). Two pyrolytic steps (265–410 °C and 410–490 °C) of
Ni(MOBIDA) give H2O, CO2, CO, CH3OH, CH4, N2O and NO, leaving
a residue of NiO (exp. 19.228%, calc. 20.360%).
interactions [C(33)–H(33)ꢁ ꢁ ꢁ(Cg, centroid of the involved ring)
0
154.0°, C(33)ꢁ ꢁ ꢁCg 3.70 ÅA; symmetry code 1ꢀx, 1/2 + y, ꢀz] (see de-
tail in Fig. 2, right). It is noteworthy that in this way the O-methoxy
atom of the MOBIDA ligand cannot participate in H-bonding
interactions.
Compound 1 is stable up to ꢂ100 °C and then shows the loss of
the three aqua ligands (100–200 °C, exp. 14.907%, calc. 15.532%).
Anhydrous Ni(MEBIDA) exhibits a pyrolytic process (300–400 °C)

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D.K. Patel et al. / Polyhedron 29 (2010) 683–690
Fig. 1. Structure of [Ni(MEBIDA)(H2O)3] (1). Left: asymmetric unit, with the numbering of the coordination atoms. Right: fragment of a H-bonded layer, parallel to the ab
plane, with the benzyl arms of MEBIDA towards its external faces.
Fig. 2. Structure of [Ni(MOBIDA)(H2O)3]ꢁH2O (2). Left: asymmetric unit, with the numbering of the coordination atoms. Right: fragment of two H-bonded layers, parallel to
the ab plane, with the benzyl arms of MOBIDA towards its external faces. Detail of one of the C–H/p interactions connecting the MOBIDA ligands of the external faces from
two layers.
The FT-IR spectra of compounds 1 and 2 show a broad band
near 3350 cmꢀ1 due to the stretching absorption of water and/aqua
ligands. In the range 3000–2800 cmꢀ1, several peaks are assignable
to stretching modes of the chromophores C–Harom, –CH2– and –
CH3. An skeletal absorption is recorded near 1675 cmꢀ1 followed
by a broad band at ꢂ1595 cmꢀ1, due to overlapped contributions
of d(H2O) and mas(COO) modes. The band of ms(COO) is clearly rec-
ognized at 1414 (1) or 1412 cmꢀ1 (2). A comparison of both spectra
enables the valuable assignation of a defined and intense band at
1182 cmꢀ1 corresponding to
mas(C–O–C) due to the alkyl–aryl ether
chromophores in the spectrum of (2). This band is also observed in
the spectrum of 4 (see below). FT-IR spectra of both binary com-

D.K. Patel et al. / Polyhedron 29 (2010) 683–690
687
pounds also show a defined band for the ‘out-of-plane’ deforma-
tion mode,
(C–Harom), at 741 (1) or 742 cmꢀ1 (2).
6.244%, calc. 6.239%) and [Ni(Him)6] [Ni(MOBIDA)2]ꢁ3.13H2O
(exp. 5.209%, calc. 5.198%) and the corresponding pyrolyses give
a residue of NiO (exp. 13.800% at 430 °C, calc. 14.058% for 3; exp.
14.464% at 450 °C, calc. 13.810% for 4). If the beginning of the TG
experiment is retarded until the sample weight remains stable,
the water content (n) of the compounds decreases to n = 1.35
and 0 for 3 and 4, respectively. The anhydrous salt [Ni(Him)6]-
[Ni(MEBIDA or MOBIDA)2] decomposes (>250 °C) in three over-
lapped steps to give NiO as the residue at ca. 450 °C. After the loss
of the non coordinated water, pyrolysis of the organic ligands from
3 yields CO2, H2O, CO, CH4, NH3 and probably N2O, and trace
amounts of NO. The three pyrolytic steps (a–c) of 4 enable the
FT-IR identification of CO2 and H2O (a), CO2, H2O, CO, CH4 and
CH3OH (b) or CO2, H2O, CO, CH4, NH3, N2O, NO and trace amounts
of NO2 (c).
p
Octahedral symmetry is well reflected in the shape of electronic
spectra of the compounds with the m1 band at 1050 or 1060 nm
(ꢂDo 9535 cmꢀ1). As expected for a NiNO5 chromophore, the esti-
mated Do values of these compounds exceed that of the
[Ni(H2O)6]2+ ion (8500 cmꢀ1).
3.2. Structure and properties of salts 3 and 4 with the
hexakis(imidazole)nickel(II) ion
The attempt to obtain ternary compounds have yielded two iso-
type salts [Ni(Him)6][Ni(MEBIDA or MOBIDA)2]ꢁ4H2O. They consist
of centro-symmetric anions and cations, and non-coordinated
water molecules (see Fig. 3 for the MOBIDA salt). The Ni–N(Him)
bond lengths are similar both within each cation and between
the compounds (ꢂ2.14 Å), with average values of 2.136 and
2.139 Å for compounds 3 and 4, respectively. Again, the bond dis-
tances in the bis-chelate anions are rather similar and have values
for Ni–O of 2.05 and 2.07 Å, shorter than the Ni–N distance of
2.11 Å. There are no important differences in the corresponding
bond lengths of the neutral binary compounds and the bis-chelate
anions of the studied salts, and all IDA moieties revealed a fac-NO2
conformation.
The FT-IR spectra for solid samples of 3 and 4 are remarkably
similar, according to the iso-type nature of these crystals and the
small structural differences between the MEBIDA and MOBIDA li-
gands. It is easy to identify bands due to the chromophores of
H2O (mas ꢂ3450, d 1615 cmꢀ1), MEBIDA or MOBIDA (
mas(COO)
1588 cmꢀ1 s(COO) 1382 cmꢀ1 (C–H)arom 747 or 745 cmꢀ1) and
, m , p
Him (m , p(C–H)arom
(N–H) 3133 cmꢀ1, d(N–H) 1542 or 1515 cmꢀ1
668 cmꢀ1). It is interesting to point out that the stretching band
of C–O(ether)–Carom at 1181 cmꢀ1 (defined absorption of medium
intensity) is present in the spectrum of 4, but not in that of 3.
The electronic spectra of these compounds exhibit the typical
bands of octahedral nickel(II) complexes, which represent the
averaged contribution of NiN6 and NiN2O4 chromophores of the
cation and anion, respectively. However, the maximum of the m1
band (ꢂDo 10417 cmꢀ1) has a higher value than that observed
for the corresponding binary compounds (ꢂ9500 cmꢀ1) and the
corresponding hexaaqua-cation (8500 cmꢀ1), according to the
strength of the ligand field from the chromophores: NiN2O4
(averaged) > NiNO5 > NiO6.
In these crystals,
p,p-stacking interactions between the aro-
matic rings of MEBIDA or MOBIDA are missing. These crystals con-
sist of 2D layers parallel to the ab plane where four Him ligands of
each cation acts as H-donors to four N–Hꢁ ꢁ ꢁO(carboxylate) accep-
tors of four different neighboring anions. This builds two kinds of
channels (I and II, in Fig. 4, top). These layers are connected along
the c-axis by additional H-bonding interactions involving N–H
bonds of the remaining Him ligands and the water molecules, in
a H-bonding 3D framework (Fig. 4, bottom). Interestingly, the p-
methoxy-benzyl arms of the MOBIDA ligands in 4 only participate
in hydrophobic interactions. This role is also displayed by the p-
methyl-benzyl arms of the MEBIDA ligand in 3. The hydrophobic
behavior of the benzyl-like arms of MEBIDA and MOBIDA in these
salts is consistent with the iso-type structures of their crystals.
These compounds lose water rather easily under dry-air flow,
giving TG-formulas of [Ni(Him)6][Ni(MEBIDA)2]ꢁ3.68H2O (exp.
3.3. Ni(II) and Cu(II) iminodiacetates: a structural and conformational
comparison
We found it instructive to make a comparison of the structural
and ligand conformational information available for the copper(II)
Fig. 3. Asymmetric unit of [Ni(Him)6][Ni(MEBIDA)2]ꢁ4H2O (3), with the numbering of the coordination atoms.

688
D.K. Patel et al. / Polyhedron 29 (2010) 683–690
Fig. 4. Crystal packing of [Ni(Him)6][Ni(MOBIDA)2]ꢁ4H2O (4). Top: H-bonded layer parallel to the ab plane, with two sizes of channels (I and II). Bottom: H-bonded layers of 4
along the c-axis, building the 3D crystal. Note that the O-methoxy atom of the MOBIDA ligands is not involved in H-bonding interactions.
Table 3
Iminodiacetate moiety conformation in copper(II) complexes. Selected references are given in brackets. Most complexes exhibit a 4 + 1 coordination polyhedron, whereas the
others exhibit an elongated octahedral coordination, type ꢂ4 + 2.
Type of complex
Binary
IDA-moiety conformation
IDA-like ligandsa
IDA [17] 2-1,6-HDTA [18]
TEBIDA [21], BCAA [22], N-
N-heterocyclic ligands
Denticity of the N-heterocycle
fac-NO + O(apical)
mer-NO2
l
None
None
phenyl-IDA-like [19], l2
-
EDTA [20]
All used tridentate IDA-like
ligands [17,21–28]
Ternary
mer-NO2
Imidazole (various). Also NH3. 4,40-
Monodentate
Bipyridine (in
2,40-bipyridine (as N4-monodentate),
pyrazine (as 2-bridge), creatinine
adenine (as N3 or N7-monodentate)
or as 2-N3, N7- and 2-N7, N9-bridge
Aromatic
a0-diimines (such as 2,20-
l2-bridging mode),
l
l
l
fac-NO + O (apical)
fac-O2 + N(apical)
All used IDA-like [21,22,29–
32] excepting those with a
bulky N-substituent (cited
below)
a
,
Bidentate
(or two monodentate Him)
bipyridine or 1,10-phenanthroline
and related ligands), 2-aminomethyl-
benzimidazole, 2-pyridine-
carboxamide imidazole (two per CuII
atom)
TEBIDA [21], BCAA [22], N-
(benzyl-like)-IDA [28]
Aromatic
a
,
a0-diimines (such as 2,20-
Bidentate
bipyridine or 1,10-phenanthroline)
a
Chelating ligands: IDA = iminodiacetate(2ꢀ), IDA-like = any tridentate N-substituted-IDA(2ꢀ), 1,6-HDTA = hexamethylene-1,6-diamino tetraacetate(4ꢀ), TEBIDA = N-
tert-butyl-IDA(2ꢀ), BCAA = N-(1-adamantyl)-IDA(2ꢀ), N-phenyl-IDA-like = several ligands with one or two IDA groups directly linked to a benzene ring (2ꢀ or 4ꢀ charged),
EDTA = ethylenediaminotetraacetate(4ꢀ), N-(benzyl-like)-IDA = the own N-benzyl-IDA(2ꢀ) or various N-(p-substituted-benzyl)-IDA(2ꢀ).
and nickel(II) complexes. With this purpose in mind, Table 3 sum-
marized the data concerning Cu(II) iminodiacetates. Binary com-
pounds can be mononuclear or binuclear (1,6-HDTA [18] or EDTA
[20]). In both kinds of complex, the IDA moiety group exhibits a
fac-NO + O(apical) conformation, as seen for the polymeric com-
pound [Cu(IDA)(H2O)2] [17] and the binuclear compound with a
bridging 1,6-HDTA ligand [18]; therefore fac-NO + O(apical) con-
formation seem to be proved for iminodiacetate-chelating groups
where the N-amino atom is linked to a hydrogen atom (IDA) or a
long hexamethylene chain (1,6-HDTA). In contrast, when N-R-
IDA has a bulky R-group, such as tert-butyl, adamantyl or phenyl,
the binary complex exhibits a mer-NO2 conformation for the IDA
chelating group. This latter conformation is also observed in the
binuclear binary complex of EDTA, with a short ethylene chain
bridging the two IDA-like chelating groups [20]. Three kinds of ter-
nary Cu(II)-iminodiacetates are known, the most numerous group
refers to all ternary complexes where the Cu(IDA-like) chelate
binds one monodentate N-heterocyclic donor ligand (or one NH3 li-
gand). All these complexes exhibit a mer-NO2 conformation for the
IDA moiety group in a five-coordination polyhedron. In ternary
complexes where the Cu(IDA-like) chelate binds two Him ligands
per copper(II), atom or one aromatic a,
a0-diimine or closely related
bidentate ligand, the IDA chelating conformation is fac-NO + O(api-
cal), except for those compounds where the IDA moiety has a bulky
N-R arm. In these latter cases, the IDA moiety exhibits a less com-
mon conformation, fac-O2 + N(apical). Thus, two main features
should be observed in Cu(II) iminodiacetates. First, the IDA moiety
conformation is different in ternary compounds where the auxil-
iary ligand is monodentate (ammonia, imidazoles, etc.) or biden-
tate (aromatic
a,
a0-diimines and related bidentate ligands or two
Him ligands). Second, all the ternary systems reported in Table 3
are mixed ligand complexes; it means that none of these systems
yield the corresponding salts.
The available information concerning Ni(II) iminodiacetates is
summarized in Table 4, including the compounds reported in this
paper. In this context, we found binary and ternary complexes as

D.K. Patel et al. / Polyhedron 29 (2010) 683–690
689
Table 4
Conformation of the iminodiacetate chelating moiety in octahedral nickel(II) complexes. References are given in brackets.
Type of
IDA moiety
IDA-like ligandsa
N-heterocyclic ligands
Denticity of N-
compound
conformation
heterocyclic ligand
Binary
fac-NO2
IDA [1–3], MIDA [14], HNTA (as
tridentate!)
None
[15], p-Cph-IDA [9], MEBIDA or MOBIDA
(this work)
Ternary
fac-NO2
IDA [4–6], N-ph-IDA, p-PhDTA [11,12],
ADA (as tridentate!) [15]
3 Him or 3 benzimidazole or
Monodentate or
bidentate
one bis-thiazole ligands per NiII atom
2,20-bipyridine, 2,20-bipyridine or 1,10-phenanthroline, Bidentate
2,20-bipypiridine
mer-NO2
fac-NO2
p-Cph-IDA [9]
MIDA [4], NBzIDA [3], MEBIDA or
MOBIDA (this work)
l
2-4,40-bipyridine (thus, one N donor per NiII) atom
Monodentate
Monodentate
Salt
Imidazole in [Ni(Him)6]2+ [4,this work] or
[Ni(Him)2(H2O)4]2+ [3] cations
a
Chelating ligands: IDA = iminodiacetate(2ꢀ), MIDA = N-methyl-IDA(2ꢀ), HNTA = hydrogen-nitrilotriacetate(2ꢀ) or N-carboxymethyl-IDA(2ꢀ), p-Cph-IDA = N-(p-car-
boxyphenyl)-IDA(2ꢀ), N-ph-IDA = N-phenyl-IDA(2ꢀ), p-PhDTA = p-phenylenediaminotetraacetate(4ꢀ), ADA = N-(2-amidomethyl)-IDA(2ꢀ), NBzIDA = N-benzyl-IDA(2ꢀ).
well as some salts. All these complexes exhibit octahedral coordi-
nation polyhedra, as is always found for the structures of Ni(II)
derivatives with amino-polycarboxylate ligands. The limited infor-
mation available for Ni(IDA-like) complexes shows remarkable dif-
ferences with regard to that which is observed for the Cu(II)
complexes. First, all binary Ni(IDA-like) compounds have the
IDA-like moiety in a fac-NO2 conformation (including the HNTA2ꢀ
ligand acting only as a tridentate ligand [15]). Second, with just one
exception [9], the ternary Ni(II) complexes also have a fac-NO2 con-
formation for the IDA chelating group. It is interesting to note that
this exception, with the mer-NO2 IDA moiety conformation, corre-
sponds to a compound where the bridging 4,40-bipyridine ligand
supplies only one heterocyclic donor atom to each Ni(II) centre.
In contrast, all other ternary compounds referred to in Table 4 cor-
respond to examples where each Ni(IDA-like) chelate adds two or
three N-heterocyclic donor atoms. A few Ni(II) iminodiacetates are
salts where the bis-chelate anion (IDA moiety) exhibits a fac-NO2
conformation. These compounds have one methyl or benzyl-like
non-coordinating arm in the IDA-like chelating agent. Moreover,
although with one exception, these salts contain the [Ni(Him)6]2+
counter-cation. It is interesting to point out that the salt
[Ni(Him)2(H2O)4][Ni(NBzIDA)2] [4] has a Ni/Him/NBzIDA equimo-
lar ratio. Thus, we can observe that Ni(II) iminodiacetates where
the IDA-like ligand has a methyl or benzyl-like non-coordinating
arm are salts and not ternary complexes, irrespectively of the num-
ber of N-heterocyclic donor atoms (one or three) per nickel(II)
centre.
Appendix A. Supplementary data
CCDC 732816, 732817, 732818 and 732819 contain the supple-
mentary crystallographic data for 1, 2, 3 and 4. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-
033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data asso-
ciated with this article can be found, in the online version, at
doi:10.1016/j.poly.2009.09.034.
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Acknowledgements
Financial support from ERDF-EC, MEC-Spain (Project CTQ2006-
15329-C02/BQU) is acknowledged. DKP thanks MAE-AECID (Spain)
for a PhD Grant. DChL thanks SIC-EU for a I3P postdoctoral research
contract. The Project ‘‘Factoría de Cristalización, CONSOLIDER
INGENIO-2010” provided X-ray structural facilities for this work.
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