Isotype 1D polymers of cobalt(II) or zinc(II) constructed with square-planar tetraaqua-metal(2+) units and the bis-zwitterionic form of the μ2-O,O′-trans-1,4-dihydrogen- cyclohexanediaminotetraacetate(2-) ligand

Article abstract of DOI:10.1016/j.poly.2011.09.037

An uncompleted reaction between equimolar amounts of Co₂CO ₃(OH)₂·2H₂O and trans-1,4-H ₄cyclohexanediaminotetraacetic acid in water affords the 'acid' complex {[Co^II(trans-1,4-H₂CDTA)(H₂O) ₄]·6H₂O}_n (1). Its IR spectrum does not show the expected ν(CO) band of carboxylic groups. Reactions in aqueous solution between Na(trans-1,4-H₃CDTA) and Zn(AcO)₂· 2H₂O or Na₂(trans-1,4-H₂CDTA) and Zn(NO ₃)₂·6H₂O yield {[Zn(trans-1,4-H ₂CDTA)(H₂O)₄]·6H₂O} _n (2) and {[Zn₂(trans-1,4-CDTA)(H₂O) ₂]·H₂O}_n (3) respectively. Crystal structures of compounds 1-3 and that of the trans-1,4-H₄CDTA· 2H₂O acid are reported. For steric reasons, in the four reported structures the 1,4-CDTA ligand has the two iminodiacetate moieties as equatorial groups in the 1,4-cyclohexanedi-yl chair. Compounds 1 and 2 are isotype 1D polymers constructed by square planar M^II(H₂O) ₄²⁺ knots (M^II = Co^II or Zn ^II) linked to bis-zwitterionic trans-1,4-H₂CDTA ^2- ligands that play a typical μ₂-O,O′- dicarboxylate bridging role. These 1D polymeric structures seem to be favoured by the H-bonded intra-stabilization of the bis-zwitterionic trans-1,4-H ₂CDTA^2- ligand. In the neutral complex (3), the trans-1,4-CDTA acts as a bridging bis-chelating ligand as well as a syn-anti carboxylate building a polymer where the zinc(II) centres exhibit a rough bipyramidal trigonal coordination.

Full text of DOI:10.1016/j.poly.2011.09.037

Polyhedron 31 (2012) 463–471
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Isotype 1D polymers of cobalt(II) or zinc(II) constructed with square-planar
tetraaqua-metal(2+) units and the bis-zwitterionic form of the l
2-O,O0-trans-
1,4-dihydrogen-cyclohexanediaminotetraacetate(2ꢀ) ligand
Hanan El Bakkali a, Duane Choquesillo-Lazarte b, Alicia Domínguez-Martín a,
María del Pilar Brandi-Blanco a, Alfonso Castiñeiras c, Juan 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, Avda. del Conocimiento s/n, Armilla, E-18100 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:
An uncompleted reaction between equimolar amounts of Co2CO3(OH)2ꢁ2H2O and trans-1,4-H4cyclohex-
anediaminotetraacetic acid in water affords the ‘acid’ complex {[CoII(trans-1,4-H2CDTA)(H2O)4]ꢁ6H2O}n
Received 13 June 2011
Accepted 30 September 2011
Available online 8 October 2011
(1). Its IR spectrum does not show the expected m(C@O) band of carboxylic groups. Reactions in aqueous
solution between Na(trans-1,4-H3CDTA) and Zn(AcO)2ꢁ2H2O or Na2(trans-1,4-H2CDTA) and Zn(NO3)2ꢁ6H2O
yield {[Zn(trans-1,4-H2CDTA)(H2O)4]ꢁ6H2O}n (2) and {[Zn2(trans-1,4-CDTA)(H2O)2]ꢁH2O}n (3) respectively.
Crystal structures of compounds 1–3 and that of the trans-1,4-H4CDTAꢁ2H2O acid are reported. For steric
reasons, in the four reported structures the 1,4-CDTA ligand has the two iminodiacetate moieties as equa-
torial groups in the 1,4-cyclohexanedi-yl chair. Compounds 1 and 2 are isotype 1D polymers constructed by
Keywords:
Crystal structure
Coordination
Polymer
Cobalt(II)
Zinc(II)
square planar MII(H2O)4 knots (MII = CoII or ZnII) linked to bis-zwitterionic trans-1,4-H2CDTA2ꢀ ligands
2+
that play a typical l
2-O,O0-dicarboxylate bridging role. These 1D polymeric structures seem to be favoured
by the H-bonded intra-stabilization of the bis-zwitterionic trans-1,4-H2CDTA2ꢀ ligand. In the neutral com-
plex (3), the trans-1,4-CDTA acts as a bridging bis-chelating ligand as well as a syn–anti carboxylate building
a polymer where the zinc(II) centres exhibit a rough bipyramidal trigonal coordination.
Ó 2011 Elsevier Ltd. All rights reserved.
Amino-polycarboxylate
1. Introduction
free acetic arms) in mixed-ligand Cu(II) complexes also having l2
-
(4,40-bipiridine) or
l
2-pyrazine ligands [9]. Cis-1,2-H4CDTA isomer
Trans-1,2-cyclohexanediaminotetraacetic
acid
(trans-1,2-
is also known. It has one axial and one equatorial IDA group. Hence
it gives less stable metal chelates than those of the trans-isomer [1].
As a part of our program on metal chelates with amino-poly-
carboxylic acids where two chelating moieties are separated by
different spacers, we have prepared the trans-1,4-H4CDTA acid.
This latter acid presents two IDA chelating moieties linked to the
1,4-cyclohexanedi-yl chair as axial (a,a) or equatorial (e,e) groups.
Again, due to sterical reasons, the (e,e)-conformer would represent
the most stable conformation (Scheme 1b). The trans-1,4-H4CDTA
acid was first referred by Schwarzenbach et al. [10]. More recently,
Fuger and co-workers [11] have reported multi-step syntheses of
the six isomers from 1,x-cyclohexanediaminotetraacetic acid,
starting from mixed isomers of 1,2-, 1,3- or 1,4-cyclohexanediols.
Acidity constants [10,12] and stability constants for Mg(II) and
Ca(II) [10] or lanthanide(III) ion [12] complexes derived from the
trans-1,4-H4CDTA acid in solution are also available. However,
until now, any molecular and/or crystal structure concerning this
ligand has been reported. Now, we report a novel synthesis of such
chelating ligand as well as the first structures of metal complexes
derived from it.
H4CDTA) is one of the most known and used amino-polycarboxylic
acids. The trans-1,2-CDTA ligand has two iminodiacetate (IDA) moi-
eties linked to the cyclohexanedi-yl chair ring as axial (a,a) or equa-
torial (e,e) groups giving rise to different isomers respectively. The
(e,e) one is the most stable isomer and the unique conformer
(trans-1,2-H4CDTA, Scheme 1a) able to bind both chelating groups
to the same metal ion to achieve a variety of rather stable metal che-
lates [1]. A search in the CSD database (v. 5.31, up-to-date February
2011) revealed nearly 50 different structures for trans-1,2-CDTA
complexes with first-row transition metal ions. In such complexes,
the full anion trans-1,2-CDTA4ꢀ acts as hexadentate chelating agent.
These compounds also include chelates where the divalent anion
trans-1,2-H2CDTA2ꢀ acts: (1) as N2O4-hexadentate in the hepta-
coordinated complex of [CoII(trans-1,2-H2CDTA)(H2O)]ꢁ3H2O [2];
(2) as N2O3-pentadentate (with a free acetic arm) in ‘acid’ complexes
of Ni(II) [3,4] or Cu(II) [4–8] or (3) as N2O2-tetradentate (with two
⇑
Corresponding author.
E-mail address: jniclos@ugr.es (J. Niclós-Gutiérrez).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.09.037

464
H. El Bakkali et al. / Polyhedron 31 (2012) 463–471
cobalt(II) salt still remained unreacted. Despite this fact, the pink-
2. Experimental
2.1. Materials
ish solution was filtered on a crystallization device and covered by
a plastic film. After several weeks of slow evaporation, well shaped
single crystals and many poly-crystals appeared. Several fractions
of product were collected. Yield: 110 mg, ꢂ75%. Anal. Calc. for
Trans-1,4-diaminocyclohexane and chloroacetic acid were pur-
chase from Aldrich and used as received. The salts Co2CO3(OH)2ꢁxH2O
(with x = 2 as determined by elemental and thermogravimetric anal-
yses, with Co 47.55%), Zn(AcO)2ꢁ2H2O and Zn(NO3)2ꢁ6H2O were sup-
plied by Acros. Other required reagents and solvents were of
analytical grade.
C14H40CoN2O18: C, 28.82; H, 6.91; N, 4.80. Found: C, 28.93; H,
6.83; N, 4.74%.
2.2.3. Poly-{(
l
2-O,O0-trans-1,4-dihydrogen-
cyclohexanediaminotetraacetate)(tetraaqua)zinc(II) hexahydrate},
{[Zn(trans-1,4-H2CDTA)(H2O)4]ꢁ6H2O}n (2)
Several attempts to prepare this compounds using basic zinc
carbonate as well as zinc nitrate hexahydrate (see Section 2.2.4)
failed. Finally, we proceeded as follows: Zn(AcO)2ꢁ2H2O (99 mg,
0.45 mmol), NaHCO3 (42 mg, 0.5 mmol) and trans-1,4-
H4CDTAꢁ5.5H2O (111 mg, 0.25 mmol) were stirred and heated
(50 °C) in 100 mL of water. The obtained clear solution was slowly
filtered without vacuum and left to evaporate in a crystallization
device, covered with a plastic film, at room temperature. After 2–
3 weeks well shaped crystals appeared. Several fractions of product
were collected. Yield: 186 mg, ꢂ70%. Anal. Calc. for C14H40N2O18Zn:
C, 28.51; H, 6.84; N, 4.75. Found: C, 28.45; H, 6.63; N, 4.80%.
2.2. Syntheses
2.2.1. Trans-1,4-H4CDTA acid n-hydrate
The trans-1,4-H4CDTAꢁnH2O acid was prepared as follows: a
cooled KOH solution (108.94 g, 1.94 mol, in 180 mL of water) was
drop-wise added to a solution of chloroacetic acid (86.94 g,
0.92 mol, in 180 mL of water) maintaining the temperature below
ꢀ10 °C with the help of an ice-NaCl salt bath. To the resulting alka-
line solution, trans-1,4-diamino-cyclohexane (25 g, 0.22 mol) in
100 mL of water was added. This reaction mixture was heated at
80–85 °C during 4 h, following a similar procedure to that previ-
ously reported for N-benzyl-iminodiacetic acid and closely related
ligands [13]. The reaction mixture was cooled until room temper-
ature, acidified with HCl ꢂ6 N (pH ꢂ6) and repeatedly concen-
trated under vacuum and cooled in an ice-NaCl salt bath until a
half of the expected KCl (by-product) was removed by filtration
under vacuum. The mother liquors were then acidified (pH ꢂ2)
by addition of HCl ꢂ6 N and the white precipitate was removed
by filtration, washed (cool water and acetone) and air-dried. The
crude product was re-crystallized in 1.5 L of water, collected by fil-
tration and washed and dried as above. Powder samples of the acid
had variable amounts of water that were studied by elemental
analysis and/or thermogravimetry before to be used for the synthe-
ses of metal complexes. The collected powder sample agrees to an
average formula trans-1,4-H4CDTAꢁ5.5H2O and yields the following
analytical data (%): Anal. Calc. for C14H22N2O8ꢁ5.5H2O: C, 37.75; H,
7.47; N, 6.29. Found: C, 37.79; H, 7.04; N, 6.36%. Yield: 48 g, ꢂ50%.
A powder sample (1 g) was repeatedly re-crystallized in a water:-
DMSO mixture and then in bi-distilled water to yield parallelepi-
ped crystals suitable for X-ray crystallography giving rise the
following final formula trans-1,4-H4CDTAꢁ2H2O. For this crystalline
form (trans-1,4-H4CDTAꢁ2H2O) analytical data (%) are: Anal. Calc.
for C14H22N2O8ꢁ2H2O: C, 43.98; H, 6.85; N, 7.33. Found: C, 43.87;
H, 6.80; N, 7.29%. FT-IR spectrum and NMR spectra of trans-1,4-
H4CDTAꢁ2H2O acid are supplied as Supplementary Material S.1.
FT-IR data (cmꢀ1): for H2O, mas 3406, ms ꢂ3200 and d 1615; for –
COOH, m(O–H) 3541, m(C@O) 1676, m(C-O)+d(O-H) 1402, p(O–H) 701; for
2.2.4. Poly-{(l
2-O,O0-trans-1,4-
cyclohexanediaminotetraacetate)(diaqua)dizinc(II) dihydrate},
{[Zn2(trans-1,4-CDTA)(H2O)2]ꢁ2H2O}n (3)
An aqueous solution of the salt Na2[trans-1,4-H2CDTA] was pre-
pared by reaction of H4CDTAꢁ5.5H2O (111 mg, 0.25 mmol) and
NaHCO3 (42 mg, 0.5 mmol) in 50 mL of water. To this solution,
Zn(NO3)2ꢁ6H2O (74 mg, 0.25 mmol) was added with stirring. The
resulting solution was filtered without vacuum on a crystallization
device. Slow evaporation at room temperature gave rise to well
shaped crystals of 3 in a yield of ꢂ50% (35 mg, based on Zn). Anal.
Calc. for C14H26N2O12Zn2: C, 30.85; H, 4.81; N, 5.14. Found: C,
30.58; H, 4.72; N, 5.03%.
2.3. X-ray data collection and structure refinement
Suitable crystals from trans-1,4-H4CDTAꢁ2H2O and compounds
1–3 were mounted on a glass fibre and these samples were used
for data collection. Data were collected with a Bruker SMART
CCD APEX diffractometer at 298 K (1) or 100 K (other crystals).
The data were processed with SAINT [14] and corrected for absorp-
tion using SADABS [15]. The structures were solved by direct meth-
ods [16], which revealed the position of all non-hydrogen atoms.
These atoms were refined on F2 by a full-matrix least-squares pro-
cedure using anisotropic displacement parameters [16]. All hydro-
gen atoms were located in difference Fourier maps and included as
fixed contributions riding on attached atoms with isotropic ther-
mal displacement parameters 1.2 times those of the respective
atom. Atom scattering factors were taken from the International
Tables of Crystallography [17]. Geometric calculations and molec-
ular graphics were performed with PLATON [18]. Additional crystal
data are shown in Table 1 and Supplementary Material S.3, S.4,
S.5, S.6.
m
as(COO)ꢁ ꢁ ꢁ(H–N+–) 1615, 3067 (1717 + 1350, strong combination
band); for –CH2–, mas 2957 and 2930, ms ꢂ2821; for C–H,
m 2980.
1H NMR (300 MHz, D2O + NaOD; d, ppm): two m centred at 1.05
and 1.68 from 8H of –CH2– (ring) groups, broad s at 2.31 from
2H of C–H (ring) groups and s at 2.98 from 8H of –CH2–COOꢀ arms.
13C NMR (75 MHz, D2O + NaOD; d, ppm): 29.44 from –CH2– (ring),
57.84 from –CH2–(COO), 62.48 from C–H (ring) and 182.57 from –
COOꢀ.
2.4. Other physical methods
2.2.2. Poly-{(
l
2-O,O0-trans-1,4-dihydrogen-
Analytical data were obtained in a Fisons–Carlo Erba EA 1108
elemental micro-analyzer. NMR spectra were recorded in a Varian
Inova Unity instrument (300 MHz). Infrared spectra were recorded
by using KBr pellets on a Jasco FT-IR 410 or Jasco FT-IR 6300 spec-
trometer. TG analysis (pyrolysis) of the studied Co(II) compound
(295–950 °C, 10 °C/min) was carried out in air flow (100 mL/min)
by a Shimadzu Thermobalance TGA–50H instrument, and a series
of 20–30 time-spaced FT-IR spectra were recorded for identify
cyclohexanediaminotetraacetate)(tetraaqua)cobalt(II) hexahydrate},
{[CoII(trans-1,4-H2CDTA)(H2O)4]ꢁ6H2O}n (1)
Co2CO3(OH)2ꢁ2H2O (62.0 mg, 0.25 mmol) was added to a warm
water solution (50 mL, 50 °C) of trans-1,4-H4CDTAꢁ5.5H2O (111 mg,
0.25 mmol). The reaction mixture was stirred and heated during
1.5 h. After cooling, the slow filtration, without vacuum, of the
aforementioned solution revealed that approximately half of the

H. El Bakkali et al. / Polyhedron 31 (2012) 463–471
465
Table 1
Summary of crystal data, structure solution and refinement.
Compound ?
trans-1,4-H4CDTAꢁ2H2O
1
2
3
Empirical
Formula weight
T (K)
C7H13NO5
191.18
100(2)
C14H40CoN2O18
583.41
298(2)
C14H40N2O18Zn
589.85
100(2)
C7H13NO6Zn
272.55
100(2)
Crystal system
Space group
Wavelength (Å)
Unit cell dimensions
a (Å)
triclinic
P1
0.71073
triclinic
P1
0.71073
triclinic
P1
0.71073
orthorhombic
Pbca
ꢀ
ꢀ
ꢀ
0.71073
6.884(3)
7.907(3)
8.566(3)
88.300(9)
86.079(8)
64.227/8)
2
7.603(1)
8.192(1)
10.557(1)
77.885(1)
83.097(19
72.134(1)
1
7.618(1)
8.074(1)
10.482(1)
78.500(1)
82.742(1)
71.938)1)
1
9.311(1)
14.323(2)
14.999(2)
90
90
90
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
Z
8
Data/restraints/parameters
Absorption correction
R1 [I < 2r(I)]
1448/0/119
semiempirical
0.040
2184/0/160
semiempirical
0.028
2098/0/160
semiempirical
0.026
1754/0/136
semiempirical
0.033
wR2
0.106
1.094
0.075
1.014
0.066
1.121
0.036
0961
Goodness-of-fit (GOF) on F2
the evolved gasses using a coupled FT-IR Nicolet Magma 550 spec-
trometer. Electronic (diffuse reflectance) spectrum was obtained in
a Varian Cary-5E spectrophotometer.
Finally, slow evaporationof the solutionaccomplishes manycrystals
of compound 1, some of them suitable for X-ray diffraction studies.
In this context, we considered the possibility to prepare a re-
lated compound with Zn(II) instead of Co(II). This hypothesis is
sustained on the well-known similar stability of metal chelates
with amino-polycarboxylate ligands and these metal ions (accord-
ing to the Irving-Williams series: CoII < Ni(II) < Cu(II) > Zn(II) and
Co(II) ꢂ Zn(II)). An attempt to react the disodium salt of the chelat-
ing agent with zinc nitrate yields the neutral bimetallic complex
(see Section 3.6). Alternatively, the monosodium salt of the chela-
tor was reacted with Zn(AcO)2 in water, finally achieving good
crystals of compound 2.
3. Results and discussion
3.1. Crystal structure of trans-1,4-H4CDTAꢁ2H2O acid
As a typical diamino-tetracarboxylic acid, the crystal of trans-1,4-
H4CDTAꢁ2H2O acid consists of bis-zwitterions and water molecules
in a 3D H-bonded network. These zwitterions are centro-symmetric.
Each half of the acid molecule has a proton located on the N1 atom
whereas the other is delocated between O9 and O5 carboxy atoms
in a ratio 0.72:0.28, respectively. The internal stability of the com-
pound is reinforced by two unequal H-bonds involving each N+–H
group with two carboxy acceptors of the same half of the molecule:
N1–H1ꢁ ꢁ ꢁO8 (2.761(2) Å, 112.9°) and N1–H1ꢁ ꢁ ꢁO4 (2.784(2) Å,
85.8°). Each N+–H group further cooperates in an intermolecular
interaction, N1–H1ꢁ ꢁ ꢁO8(#2) (2.89 Å, 151.0°; #2 = ꢀx + 1, ꢀy + 1,
ꢀz + 2). Hence, each ‘oniumH1-atom’interacts with four neighbour-
ing ‘acceptors’ in a distorted tetrahedral surrounding.
In the crystal, zwitterions build chains along the a axis linked by
the O9–H9ꢁ ꢁ ꢁO5#5 (2.46 Å, 172.8°; #5 = x + 1, y, z) and its symmet-
rical yielding graph sets R22(26). These chains are further connected,
one side and another, by the above referred N1–H1ꢁ ꢁ ꢁO8(#2) inter-
action building graph sets R22(10) in a 2D H-bonded framework par-
allel to the ab plane. In addition, water molecules act only as H-
donors [O1-H1Aꢁ ꢁ ꢁO4#3 (2.81 Å, 172.7°; #3 x, y ꢀ 1, z) and O1–
H1Bꢁ ꢁ ꢁO5#4 (2.87 Å, 154.8°; #4 ꢀx, ꢀy + 1, ꢀz + 1)] building frame-
works in a 3D crystal.
3.3. Crystal structure of {[MII(trans-1,4-H2CDTA)(H2O)4]ꢁ6H2O}n with
MII = Co (1) or Zn (2)
The referred compounds are isotype. These crystals consist of a
coordination 1D-polymer and non-bonded to the metal water mol-
ecules. In the polymer, MII(H2O)42+ units are connected by bridging
l
2-O,O0-trans-1,4-H2CDTA2ꢀ ligands which play a typical 2-O,O0-
l
dicarboxylate role via two acetate arms belonging to two IDA
groups separated by the 1,4-cyclohexanedi-yl spacer. The centro-
symmetric coordination polyhedra of both MII atoms are slightly
distorted octahedra, type 2 + 2 + 2. The shortest distances corre-
spond to the MII–O4(carboxylate) bonds [2.052(1) Å (1),
2.039(1) Å (2)]. The aqua ligands give Co-O(aqua) distances of
2.089(1) or 2.161(1) Å and Zn–O(aqua) distances of 2.088(1) or
2.200(2) Å. It is noteworthy that the trans-1,4-H2CDTA2ꢀ ligand
adopts a bis-zwitterionic form.
In the crystals, polymeric chains extend parallel to the vector
ꢀ
[111]. The internal stability of the 1D-polymer is reinforced by four
relevant H-bonding interactions located in the linking point of the
zwitterionic ligand and the metallic centre (Fig. 1). Two of these
interactions relate the N1–H1 bond with O(carboxy) acceptors of
the acetate arms N1–H1ꢁ ꢁ ꢁO4(coord.) [2.662(2) Å, 94.2° (1);
2.657(2) Å, 97.9° (2)] and N1–H1ꢁ ꢁ ꢁO8(noncoord.) [2.662(2) Å,
119.5° (1); 2.667(2) Å, 117.9° (2)]. In addition, the aqua-O1 ligand
builds H-bonding interactions with O-carboxylate acceptors, con-
necting two different zwitterionic ligands. These interactions are:
O1–H1Bꢁ ꢁ ꢁO5 [2.811(2) Å, 123.6° (1); 2.781(2) Å, 143.7° (2)] and
O1–H1Aꢁ ꢁ ꢁO8#1 [2.768(2) Å, 168.4° (1); 2.739(2) Å, 133.3° (2); #1
ꢀx + 1, ꢀy + 1, ꢀz + 1]. Additional H-bonding interactions involving
the N(1)–H(1) group, O-carboxylate acceptors, aqua ligands and
water molecules build a 3D-network.
3.2. About the syntheses of {[MII(trans-1,4-H2CDTA)(H2O)4]ꢁ6H2O}n
with M = Co (1) and Zn (2)
In a programmed task to obtain a compound type Co2(trans-1,4-
CDTA)ꢁnH2O, a reaction of equimolar amounts of Co2CO3(OH)2 and
trans-1,4-H4CDTA acid in water was carried out. In this reaction,
CO2 is expected to be the main by-product (which can be easily re-
moved with heating and stirring under moderate vacuum). As a con-
sequence of the uncompleted reaction (see Section 2.2.2), it could be
expected the formation of an ‘acid’ complex (Co2(trans-1,4-
H2CDTA)ꢁnH2O) or a disproportion between the binary compound
(Co2(trans-1,4-CDTA)ꢁnH2O) and the free acid (trans-1,4-H4CDTA).

466
H. El Bakkali et al. / Polyhedron 31 (2012) 463–471
Fig. 1. Fragment of the 1D-polymer {[MII(trans-H21,4-CDTA)(H2O)4]ꢁ6H2O}n with MII = Co (1) or Zn (2), showing H-bonds involved in intra-stabilization. Noncoordinated
water molecules omitted for clarity.
Fig. 2. FT-IR spectra of the isotype compounds 1 and 2 (up – green and blue lines, respectively) and the zinc(II) 3D-coordination polymer 3 (down – dark red line). (For
interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 2
Wavenumbers (cmꢀ1) and assignments for relevant infra-red absorptions for FT-IR spectra of isotype compounds 1 and 2 (solid samples).
Species
Group
Chromophore
Mode
Compound 1 (Co)
Compound 2 (Zn)
1,4-H2CDTA2ꢀ
–COOH
–COOH
–COOH
–COOH
–COOH
–COOꢀ
–COOꢀ
–N+–H
@CH–
–O–H
–C@O
–C–O and O–H
–O–H
m
m
m
(O–H)
(C@O)
(C–O) + d(O–H)
3584 (sh)
1730 (sh)
1405
3572 (sh)
1732 (sh)
1405
d(O–H)
p
mas
ms
d
1232
1257
–O–H
(O–H)
742
742
–COOꢀ
–COOꢀ
–N+–H
@CH–
1617a
1624a
1396
1507
2980
1397
1508
2980
m
–CH2–
–CH2–
–CH2–
–CH2–
mas
ms
2946, 2914
2873, 2852
2949, 2912
2873, 2856
H2O
H2O
H2O
H2O
H2O
mas
ms
d
3411
2257
1617a
3416
2257
1624a
Additional comments
Combination band
3055 (1730 + 1325)
3021 (ꢂ2 ꢃ 1508)
3056 (1730 + 1326)
3024 (ꢂ2 ꢃ 1508)
Overtone of d(N+–H) band
Symbols:
m
as, anti-symmetric stretching;
m
s, symmetric stretching;
m
, stretching; d, ‘in plane’ bending;
p
, ‘out of plane’ bending; sh, shoulder.
a
Band dues to overlapped modes.

H. El Bakkali et al. / Polyhedron 31 (2012) 463–471
467
Fig. 3. Thermogravimetric study of 1. (A) TG-plot against temperature (°C). (B) Two selected FT-IR spectra (at 31.68 and 34.65 min) for identification of evolved gases during
the third step (first pyrolitic step).
3.4. Other metal complexes with zwitterionic amino-polycarboxylate
ligands
(H6-1,6-HDTA) [28], diethylenetriaminepentaacetic acid (H5DTPA)
[45] or triethylenetetraaminehexaacetic acid (H6TTHA) [46–50].
The rather common HIDAꢀ zwitterion always acts as bridging
polydentate ligand, using two or more O-carboxylate atoms to bind
There are reported plenty of structures for metal complexes
where the amino-polycarboxylate ligand, partially protonated,
has at least one N+–H group. Interesting cases concern to deriva-
tives of iminodiacetic acid (H2IDA) [19–41], nitrilotriacetic acid
(H3NTA) [42–44], 1,6-hexamethylenediaminotetraacetic acid
two or more metallic centres. Indeed the simplest
l
2-O,O0-dicar-
l2-HIDA)2(H2O)2]n
boxylate role seems to be restricted to [MnII(
(a 2D polymer) [22–25] and [ZnII(HIDA)2]ꢁ4H2O (a 3D polymer)
[26,27]. Therefore the novel compound 1 (1D polymer) should be

468
H. El Bakkali et al. / Polyhedron 31 (2012) 463–471
m
(C@O) for free carboxylic acids. Only a smooth shoulder is re-
corded near 1730. Alternatively, the –N+–H group (typical for salts
of ternary amines) is expected to give a series of weak peaks in the
region between 2800 and 2250 (including one peak related to its
stretching mode) and a weak peak of the ‘in-plane’ deformation
at 1500–1550. Only this weak latter peak is observed in these
spectra.
As expected for compound 1, the electronic spectrum is typical
for an high-spin octahedral Co(II) complex (see Supplementary
Material S.2), with a Do value close to that of the hexaaquaco-
balt(II) ion (ꢂ8200 cmꢀ1).
Fig. 4. Fragment of compound 3 {[Zn2(trans-1,4-CDTA)(H2O)2]ꢁ2H2O}n showing the
bipyramidal trigonal coordination of the metal. Noncoordinated water molecules
omitted for clarity.
Thermogravimetric study of
1 (Fig. 3) revealed that the
H-bonded 3D-netwok is rather stable. Under air-dry flow, 1 only
loses a little amount of water and the sample weight become stable
for a TG-formula {[CoII(H2-trans-1,4-CDTA)(H2O)4]ꢁ5.4H2O}n. Dur-
ing the TG-experiment, the evolved gases were identified by means
of a series of 23 time-spaced FT-IR spectra. First, all remaining
water molecules and aqua ligands are lost in an overall process
which corresponds to the first step (40–180 °C, exp. 29.544%, calc.
29.575% for 9.4H2O). Two pyrolytic steps (180–275 and 275–
390 °C) produce, respectively, H2O, CO2 and CO or these gases plus
four different N-derivatives (NH3, N2O, NO and NO2) as well as
trace amounts of CH4 and ethylene. The residue at 450 °C (exp.
15.349%) should correspond to a sample of the rather stable
mixed-valence Co3O4 oxide (calc. 14.018%) as estimated by ele-
mental analysis and within an acceptable error (<1.5%). According
to that, Fig. 3 reveals an additional small weight loss near 930 °C
(as expected to the process 2 Co3O4 ? 6 CoO + O2") yielding a
CoO residue at 950 °C of 14.652% (calc. 15.349%).
structurally related to those previously reported for
gand with Mn(II) [22] or Zn(II) [29], but differing to each other in
the polymeric dimensionality.
l
2-HIDAꢀ li-
As far as concern the type of metal centres, the hard behaviour
of O-carboxylate donors in hydrogen-amino-polycarboxylate zwit-
terions explains the clear trend to bind Pearson’s hard acid ions as
alkaline(I) [19,20,42–44], alkaline earth(II) [37], manganese(II)
[22–25], bismuth(III) [31], uranium(VI) [36] or lanthanide(III)
[32–35,38,41,46–50] derivatives. However examples have been
also reported for borderline acids such as zinc(II) [26,27] and lea-
d(II) [28,29] or the soft acid silver(I) [30]. Interestingly, similar
complexes have never been reported for copper(II), which is the
softest acid metal ion among borderline M(II) ions (M = Fe–Zn) of
the second half of the first row transition metal ions. Likewise, to
the best of our knowledge, none of previously referred compounds
[19–50] have intra-stabilised H-bonded zwitterionic ligands as
observed twice in 1 and 2.
3.6. Crystal structure and properties of compound {[Zn2(trans-1,4-
3.5. FT-IR spectra and thermal stability of compounds 1 and 2
CDTA)(H2O)2]ꢁ2H2O}n (3)
FT-IR spectra of solid samples of compounds 1 and 2 are extre-
mely similar (Fig. 2), as corresponds to isotype crystals. Table 2
summarizes the wavenumber (cmꢀ1) of the main absorptions re-
lated to the chromophores and/or groups present in the samples.
The broad absorption in the range 2500–4000 and that of the band
near 1600 enhances the relevance of H-bonding interactions in
these compounds. An important consequence of these latter H-
bonds is the absence of a band corresponding to the mode
This compound 3 was unexpectedly obtained from an experi-
ment initially designed to obtain compound 2. The crystal of com-
pound 3 is a 3D coordination polymer. The metal surrounding is
better described as a trigonal bipyramidal coordination, with the
N1(amino) and O1(aqua) as trans-apical donors (N1–Zn–O1
167.4°) (Fig. 4). The Zn1–N1 bond is the largest coordination bond
(2.197(2) Å). The Zn1–O1 bond distance is 2.018(2) Å which is
close similar to those of the three Zn–O(carboxylate) equatorial
Fig. 5. Role of the water molecules in the crystal of 3. (Left) Fragment of the 3D polymer without non-coordinated water. Yellowish balls indicate the space of channel
cavities. (Right) Fragment showing the H-bonded aqua ligands and non-coordinated water inside the channels.

H. El Bakkali et al. / Polyhedron 31 (2012) 463–471
469
Fig. 6. Thermogravimetric study of 3. (A) TG-plots against temperature (°C) left under air-dry flow and right under N2 flow. (B) Selected FT-IR spectra for identification of
evolved gases. Up: recorded FT-IR spectra in the TG experiment under air-dry flow (at 41.86 and 44.92 min) during to third step (first pyrolitic step). Down: FT-IR spectrum in
the TG experiment under N2 flow (at 44.91 min) during to third step (first pyrolitic step).

470
H. El Bakkali et al. / Polyhedron 31 (2012) 463–471
Scheme 1. (a) trans-1,2-H4CDTA acid. (b) trans-H41,4-CDTA acid.
bonds (Zn1–O4 2.047(2) Å; Zn1–O8 1.992 Å; Zn1–O5 2.035(2) Å).
N1, O4 and O8 donors belong to the same iminodiacetate moiety
from one half of the trans-1,4-CDTA ligand, whereas the O5 atom
pertains to an adjacent chelating ligand. The sum of the three equa-
torial coordination angles (O4–Zn1–O5, 105.8°; O4–Zn1–O8,
114.1°; O5–Zn1–O8, 139.2°) is 359.1°. The metal is displaced
0.108 Å from the equatorial plane [P(O4,O5,O8)] towards the apical
aqua ligand. The sixth candidate donor atom is the O4 atom from
the –C3O4O5 carboxylate group of the adjacent chelator, but such
long distance [Zn1–O4 (2.718(2) Å] indicates that it is a weak inter-
action (indeed, the sum of van der Waals radii of Zn and O is 2.9 Å
and the distance of this interaction Znꢁ ꢁ ꢁO4 represents a 33% ex-
cess versus the averaged Zn–O4 and Zn1–O8 bond distances).
The bipyramidal trigonal coordination as well as other five- and
six-coordination polyhedra are widely documented for the border-
line Pearson acid Zn(II), according to its [Ar]3d10 filled electronic
configuration and ionic radius (0.88 Å for six-coordination polyhe-
dra). In the crystal, the aqua ligand and non-coordinated water
molecules are involved in different H-bonds. In particular, the 3D
coordination polymer builds channels parallel to the a axis with
contribution of coordination bonds and the (aqua)O1–H1Bꢁ ꢁ ꢁO#5
interaction [2.692(3) Å, 170.9°, #5 = x, ꢀy + 3/2, z ꢀ 1/2]. The non
coordinated water molecule acts as acceptor in the (aqua)O1–
H1Aꢁ ꢁ ꢁO2#4 [2.655(3) Å, 171.1°, #4 = x + 1/2, ꢀy + 3/2, z ꢀ 1/2] as
well as twice H-donor for O-carboxylate acceptors [O2–
H2ꢁ ꢁ ꢁO5#6 2.843(3) Å, 176.0°, #6 = ꢀx + 1, ꢀy + 2, z + 1] and O2–
H2Dꢁ ꢁ ꢁO5#7 [2.811(3) Å, 171.9°, #7 = xꢀ1, y + 3/2, zꢀ1/2]. The
channels represent a 40.62% of the crystal volume (Fig. 5 – left)
and have the apical aqua ligands oriented inward. The noncoordi-
nated water molecules are placed inside the above referred chan-
nels H-bonded with the three aforementioned interactions (Fig. 5
– right).
and pyrolysis of organic ligands to yield the appropriate metal
oxide as final residue. On the other hand, the TG-analysis under
dry-N2 flow should reflect the water loss followed by a total or par-
tial decomposition of organic ligands. Our results revealed that, in
air-dry flow, compound 3 loses all water molecules (with trace
amounts of CO2) in two unequal steps (45–210 °C, experimental
weight loss 12.924% and 210–310 °C, experimental weight loss
0.430%; overall experimental loss 13.649%; calculated weight loss
for 4H2O per formula 13.219%). In air flow, the anhydrous sample
undergoes the pyrolysis of the organic ligands in two very different
steps [310–520 °C, 54.543% and 520–870 °C, 1.945%] with produc-
tion of H2O, CO2, CO, H2@CH2, NH3, N2O and NO or mainly H2O and
CO2. The complete pyrolysis yields ZnO as final residue (experi-
mental 30.821%, calculated 29.855%). Under N2 flow, the two pyro-
lytic steps are less dissimilar (220–495 °C, experimental loss
42.381%, 495–730 °C, experimental loss 13.783%) with production
of H2O, CO2, CO, CH4, H2@CH2 and H3C–CHO or H2O, CO2, CO, CH4,
H2@CH2 and NO, respectively. Again, ZnO is the final residue
(experimental 29.855%, calculated 29.860%). It is particularly
noticeable that thermal treatments under air or nitrogen flows rep-
resents the nonbonded to the metal and coordinated water loss
(from room temperature to 210 or 180 °C, respectively) followed
by two pyrolytic or decomposition steps for organic ligands. These
results dismiss the possibility of just removing the non-coordi-
nated water and thus preserving the structural integrity of the
3D coordination polymer. For this reason, no further studies of 3
as guesting MOF were carried out.
4. Concluding remarks
The separation of two iminodiacetate chelating moieties by
trans-1,2- or trans-1,4-cyclohexanedi-yl spacers noticeably modi-
fies the ligand structure and metal binding function of the trans-
1,2- or trans-1,4-CDTA ligands. Hence, while the compound [CoII(-
trans-1,2-H2CDTA)(H2O)]ꢁ3H2O [2] has the trans-1,2-H2CDTA2ꢀ an-
ion (not zwitterion) as N2O4-hexadentate chelating ligand in a
hepta-coordinated molecular complex, both isotype 1D-polymeric
compounds {[MII(trans-1,4-H2CDTA)(H2O)4]ꢁ6H2O}n [MII = Co (1) or
Zn (2)] have the trans-1,4-H2CDTA2ꢀ bis-zwitterion playing a sim-
The FT-IR spectrum of 3 (see Fig. 2) is in accordance to the water
content of the sample and to the presence of the full anion trans-1,4-
CDTA4ꢀ. Some tentative assignments need additional comments.
Firstly, it should be noted two bands due to the
masH2O mode
(3504 and 3394 cmꢀ1) near to a broad absorption of the correspond-
ing symmetric mode centred at 3182 cmꢀ1. Around 1600 cmꢀ1, a
broad band with peaks at 1652, 1619 and 1574 cmꢀ1 was assigned
ple l
2-O,O0-dicarboxylate ligand role. The dimensionality of these
to dH2O deformation and
masCOO mode. The assignment of
masCOO
band is supported by the fact that the difference
D(COO) in Zn(II)
two polymers falls below those of related compounds formed by
the zwitterion HIDAꢀ with Mn(II) (2D frameworks) [22–25] or
Zn(II) (3D-polymer) [26,27]. In addition, the structure of the 3D-
polymeric compound 3 also reflects a higher dimensionality than
carboxylates is expected to be around 190 cmꢀ1 (with a lower value
than those corresponding to Cu(II) and Co(II) carboxylates [51]). On
this basis, the band at 1574 cmꢀ1 is assigned to the
m
asCOO mode
whereas the peaks at 1652 and 1619 cmꢀ1 were assigned to the scis-
that of the tetranuclear complex of [Zn(H2O)4ꢁZn(
l2-trans-1,2-
soring deformation mode of aqua ligands and water. The msCOO
CDTA)(H2O)]2ꢁ8H2O [52].
mode is well defined at 1384 cmꢀ1. This band does not differentiate
two kinds of carboxylate groups.
Acknowledgements
Compound 3 agrees with the profile of many other molecular
organic frameworks (MOFs). For this reason, it was decided to carry
out its thermogravimetric analysis under dry-N2 and CO2-free dry-
air flows (Fig. 6). Air-flow TG-analyses inform about the water loss
Financial support from Research Group FQM-283 (Junta de
Andalucía) and Project MAT2010-15594 (MICINN) is acknowl-
edged. The project ‘‘Factoría de Cristalización, CONSOLIDER

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