Restricting the versatile metal-binding behaviour of adenine by using deaza-purine ligands in mixed-ligand copper(II) complexes

Article abstract of DOI:10.1016/j.poly.2009.06.054

In order to deepen our understanding of the versatile behaviour of adenine (Hade) as ligand, we have synthesized four novel ternary copper(II) complexes having two deazaadenine ligands, namely 4-azabenzimidazole (H4abim) or 7-azaindole (H7azain) as N1,N6-dideazaadenine or N1,N6,N7-trideazaadenine, respectively. The related compounds were studied by thermal, spectral and single crystal X-ray diffraction methods. In [Cu(NBzIDA)(H4abim)]_n (1) the recognition between H4abim and the (N-benzyliminodiacetate)-copper(II) chelate only displays the formation of the Cu-N7(purine-like) bond, in contrast to Hade behaviour in [Cu(NBzIDA-like)(Hade)(H₂O)]·H₂O (Cu-N3(Hade) bond reinforced by N9-H···O(IDA-like) interaction). In [Cu(EIDA)(H7azain)(H₂O)] (2, EIDA = N-ethyliminodiacetate ligand), [Cu(NBzIDA)(H7azain)(H₂O)] (3) and [Cu(μ₂-SO₄)(H7azain)₂(H₂O)₂]_n (4), H7azain binds Cu(II) centre by the Cu-N3(purine-like) bond, reinforced by a N9-H···O(IDA-like or sulfate) intra-molecular interligand interaction.

Full text of DOI:10.1016/j.poly.2009.06.054

Polyhedron 29 (2010) 170–177
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Restricting the versatile metal-binding behaviour of adenine
by using deaza-purine ligands in mixed-ligand copper(II) complexes
b
b
Duane Choquesillo-Lazarte a, , Alicia Domínguez-Martín , Antonio Matilla-Hernández ,
Celia Sánchez de Medina-Revilla b, Josefa María González-Pérez b, Alfonso Castiñeiras c,
Juan Niclós-Gutiérrez b
*
a Laboratorio de Estudios Cristalográficos, IACT-CSIC, Avda. del Conocimiento s/n, E-18100 Armilla (Granada), Spain
b Department of Inorganic Chemistry, Faculty of Pharmacy, University of Granada, E-18071 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:
Available online 27 June 2009
In order to deepen our understanding of the versatile behaviour of adenine (Hade) as ligand, we have syn-
thesized four novel ternary copper(II) complexes having two deazaadenine ligands, namely 4-azabenzim-
idazole (H4abim) or 7-azaindole (H7azain) as N1,N6-dideazaadenine or N1,N6,N7-trideazaadenine,
respectively. The related compounds were studied by thermal, spectral and single crystal X-ray diffrac-
tion methods. In [Cu(NBzIDA)(H4abim)]n (1) the recognition between H4abim and the (N-benzylimino-
diacetate)-copper(II) chelate only displays the formation of the Cu–N7(purine-like) bond, in contrast to
Hade behaviour in [Cu(NBzIDA-like)(Hade)(H2O)]ÁH2O (Cu–N3(Hade) bond reinforced by N9–HÁÁÁO(IDA-
like) interaction). In [Cu(EIDA)(H7azain)(H2O)] (2, EIDA = N-ethyliminodiacetate ligand), [Cu(NBz-
Keywords:
Copper(II)
Mixed-ligand
Iminodiacetate
Deaza-purines
Crystal structure
H-bonding interactions
IDA)(H7azain)(H2O)] (3) and [Cu(l2-SO4)(H7azain)2(H2O)2]n (4), H7azain binds Cu(II) centre by the Cu–
N3(purine-like) bond, reinforced by
interaction.
a
N9–HÁÁÁO(IDA-like or sulfate) intra-molecular interligand
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
forced by N6–HÁÁÁO and N3–HÁÁÁO interactions. No
p,p-stacking
interactions are observed [6]. A recent paper [9] has reported a un-
In recent years, much effort has been made to rationalize the me-
tal-binding modes of nucleobases and their derivatives [1]. In this
context, adenine (Hade) has proved to display a remarkable versatile
behaviour in ternary copper(II) complexes having iminodiacetate
(IDA) or N-substituted-IDA ligands [2–6], as well as glycyl–glycinate
and other dipeptide ligands [7,8]. In these complexes, the molecular
recognition between Hade and different copper(II) chelates are fea-
tured by a Cu–N(Hade) bond and an intra-molecular interligand
(Hade)N–HÁÁÁO(coordinated, IDA-like or glygly) interaction. For
example, Cu(N-alkyl-IDA) chelates bind Hade by the Cu–N7 bond
reinforced by a N6–HÁÁÁO interaction; in these crystals, pairs of com-
ique compound where the adeninate(1À) anion plays a l4
-
N1,N3,N7,N9-ade role as bridging ligand between four Cu(II)
centres.
As a part of our research, we have been working on ternary
Cu(II) complexes having deaza-adenines as ligands (see Fig. 1). In
this regard, we have reported our results on [Cu2(
H4abim)4(SO4)2]Á11H2O [10] (a molecular compound closely re-
lated to the salts [Cu2(
l-N3,N9-
l
-N3,N9-H4abim)4X2]ÁX2 (X = Cl, Br) [11])
and [Cu(IDA)(H7azain)]n [12], a polymer where the Cu–N3 bond
is reinforced by the N9–HÁÁÁO(coordinated, IDA) interaction.
The information about H4abim is rather limited but clearly
reveals that it is able to bind dinuclear Cu(II) complexes with four
plex molecules stack Hade bases by a p,p-interaction [2,3]. On the
other hand, Cu(N-benzylIDA) chelates and Hade exhibits a Cu–N3
bridging
(Hade) role in [Cu2(
-N3,N9-ade in [Cu2(
2-ade)4(H2O)2]Á7H2O [14,15] and a related
hexa-copper(II) derivative [15]. However,
is known in polymers of Fe(II) [16] and Zn(II) [17]. We have not
found structural information of metal complexes with monoden-
tate H4abim ligands.
l
-N3,N9-(H4abim) moieties, very similar to
l-N3,N9-
bond and a N9–HÁÁÁO interaction. In this time, crystals build multi-
l-Hade)4(H2O)2] (ClO4)4ÁH2O [13] or
stacked chains by means of benzyl-(adjacent Hade)
p
,p-stacking
l
l
[2,4]. Curiously, the reaction of Cu(NBzIDA) chelate and the ade-
nine:thymine base-pair yields the binuclear compound [Cu2(NBz-
l2-N7,N9-abim(1À) role
IDA)2(H2O)2(l-N7,N9-Hade)] with the unusual tautomer H(N3)ade
acting as a 2-bridging ligand, with Cu–N7 and Cu–N9 bonds, rein-
l
It has been referred plenty of Cu(II) complexes with Cu2
(l-N3,N9-7azain) bridges [18–20] and also with Cu–N3(H7azain)
* Corresponding author.
bonds [21,22] or only having Cu–N3(H7azain) bonds [23–25]
E-mail address: duanec@ugr.es (D. Choquesillo-Lazarte).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.06.054

D. Choquesillo-Lazarte et al. / Polyhedron 29 (2010) 170–177
171
Table 2
Selected interatomic bonds lengths (A) and trans angles (°) in compounds 1–4.
Lengths
Angles
Lengths
Angles
1
Cu1–O11
Cu1–O21
Cu1–N7
Cu1–N10
Cu1–O22#1
1.9537(14)
1.9601(14)
1.9624(17)
2.0060(16)
2.2798(14)
O11–Cu1–O21
N7–Cu1–N10
168.11(6)
168.00(7)
#1 Àx + 1, y + 1/2, Àz + 1/2
2
Cu1–O14
Cu1–O18
Cu1–N3
Cu1–N11
Cu1–O1
1.9447(16)
N3–Cu1–N11
O14–Cu1–O18
160.43(7)
167.49(6)
1.9680(15)
1.9945(19)
2.0124(16)
2.2805(15)
3
Cu1–O18
Cu1–O14
Cu1–N3
Cu1–N11
Cu1–O1
1.9403(16)
1.9663(16)
1.996(2)
2.0194(19)
2.2748(17)
O18–Cu1–O14
N3–Cu1–N11
168.54(7)
157.05(8)
Fig. 1. Deaza-adenines used in this work, with the notation (a) as 4-azabenzimi-
dazole or 7-azaindole and (b) as purine-like conventional numbering related to the
adenine.
4
Cu1–O10
Cu1–O10#1
Cu1–N3
Cu1–N3#1
Cu1–O1
Cu1–O1#1
#1 Àx, y, Àz + 3/2
1.985(2)
1.985(2)
2.009(3)
2.009(3)
2.478(2)
2.478(2)
O10#1–Cu1–N3
O10–Cu1–N3#1
O1–Cu1–O1#1
173.49(11)
173.49(11)
166.00(8)
reinforced by N9–HÁÁÁX(acceptor) interactions (X = O [21,22], F [24]
or Cl [23–25]). A noticeable and recent example of this latter
behaviour is found in [Cu(H7azain)4F]BF4ÁH2O [24], where all four
H7azain ligands reinforce the Cu–N3 bond with a intra-molecular
N9–HÁÁÁF interactions (2.69 Å, 147.5°).
Biological and therapeutic activities of a large variety of purine
ligands or some purine-like metal complexes are well known and
extensively documented. Most of them are as metallo-enzyme
inhibitors as well as anti-viral and/or anti-cancer drugs or pre-drugs
[1]. In recent years, biological activities of some deaza-purines have
focused a lot of attention, among them, 4-azabenzimidazole deriv-
atives [26] as well as 7-azaindole derivatives [27,28]. For example,
some 7-azaindole-nucleosides have anti-viral properties [27] and
other ones have been studied as synthetic cytokinin analogues [28].
2.2. Synthesis of compounds
[Cu(NBzIDA)(H4abim)]n (1), [Cu(EIDA)(H7azain)(H2O)] (2) and
[Cu(NBzIDA)(H7azain)(H2O)] (3) were prepared following a gen-
eral procedure which could be described as: Cu2CO3(OH)2
(0.25 mmol, 0.0553 g) was reacted in 80 mL of distilled water
with the appropriate N-substituted iminodiacetic acid (0.5 mmol)
in a Kitasato flask, by heating (50 °C) and stirring under vacuum
(water pump) in order to remove CO2, a by-product of the reac-
2. Experimental
2.1. Materials
tion. Once obtain a clear blue solution, H4abim (0.5 mmol,
0.06 g) (1) or H7azain (0.5 mmol, 0.059 g) (2 and 3) were added,
respectively. The reacting mixtures were stirred until the ligands
were completely dissolved, approximately half an hour in case of
All reagents are commercially available and were purchased
from Sigma–Aldrich.
Table 1
Crystal and structure refinement data.
Compound
1
2
3
4
Empirical formula
Formula weight
Crystal system
Space group
C17H16CuN4O4
403.88
monoclinic
P21/c
C13H17CuN3O5
358.84
orthorhombic
Pbca
C18H19CuN3O5
420.90
monoclinic
P21/c
C14H16CuN4O6S
431.91
orthorhombic
Pbcn
Unit cell dimensions
a (Å)
b (Å)
c (Å)
12.8288(4)
8.1099(3)
18.0022(7)
90
10.44350(10)
10.64250(10)
27.1157(3)
90
11.503(2)
8.2259(16)
18.337(4)
90
17.4233(4)
14.7918(4)
6.8851(2)
90
a
(°)
b (°)
103.4870(10)
90
1821.30(11)
4
1.473
828
90
90
94.484(3)
90
1729.8(6)
4
1.616
868
90
90
c
(°)
V (Å3)
3013.74(5)
8
1.582
1774.44(8)
4
1.617
Z
qcalc (g cmÀ3
F(0 0 0)
)
1480
884
Crystal size (mm)
h Range (°)
Reflections collected
Independent reflections
Maximum/minimum transmission
Goodness-of-fit (GOF) on F2
0.34 Â 0.20 Â 0.04
1.63–26.02
24 542
3585
0.9525–0.6801
1.068
0.42 Â 0.40 Â 0.08
8.49–66.24
36 778
2448
0.8370–0.4442
1.028
0.36 Â 0.18 Â 0.10
1.78–26.37
3535
3535
0.8810–0.6519
1.057
0.10 Â 0.06 Â 0.04
7.86–66.25
21 722
1544
0.8824–0.7398
1.058
Final R indices [I > 2
r(I)]
R1 = 0.0290
wR2 = 0.699
R1 = 0.0369
wR2 = 0.1057
R1 = 0.0314
wR2 = 0.0748
R1 = 0.0464
wR2 = 0.1193

172
D. Choquesillo-Lazarte et al. / Polyhedron 29 (2010) 170–177
Fig. 2. Structure of [Cu(NBzIDA)(H4abim)]n (1). Left: coordination polymer running along the b axis. Right: detail of the numeration scheme of the coordination environment
in 1.
H4abim while H7azain needed nearly an hour. Only with H4abim,
an intensification of the colour was observed when the ligand
was added. Then, the blue solutions were filtered, without vac-
uum, on a crystallization device. These solutions were allowed
to stand at room temperature covered with a plastic film to con-
trol the evaporation. After two weeks, blue crystals appeared.
Typical yields in these syntheses are ca. 60–75%. Anal. Calc. for
C17H16CuN4O4 (1): C, 50.56; H, 3.99; N, 13.87. Found: C, 50.26;
H, 4.05; N, 13.67%. Anal. Calc. for C13H17CuN3O5 (2): C, 43.51; H,
4.78; N, 11.71. Found: C, 43.48; H, 4.35; N, 11.66%. Anal. Calc.
for C18H19CuN4O5 (3): C, 51.36; H, 4.55; N, 9.98. Found: C,
51.10; H, 4.99; N, 10.28%.
Fig. 3. View in the bc plane of the 2D layer in 1 obtained by hydrogen bonding interactions.

D. Choquesillo-Lazarte et al. / Polyhedron 29 (2010) 170–177
173
Compound 4 was synthesized adding a water solution of 10 mL
of CuSO4Á5H2O (0.25 mmol, 0.0624 g) to 40 mL of another solution
of H7azain (0.5 mmol, 0.059 g) in distilled water. The reaction did
not carry out properly and after 1 h stirring and heating (50 °C), the
reacting mixture was not a completely clear solution. Therefore the
solution was filtered on a crystallization device obtaining a very
pale greenish solution. The residue retained on the filtration device
was eliminated. The solution was left evaporating at room temper-
ature till the presence of green crystals one month later. Yield was
65%. Anal. Calc. for C14H16CuN4O6S (4): C, 38.93; H, 3.73; N, 12.97,
S, 7.42. Found: C, 38.79; H, 3.79; N, 12.82; S, 7.22%.
2.3. Physical methods
Analytical data were obtained in a Fisons–Carlo Erba EA 1108
elemental micro-analyser. TG analysis (pyrolysis) of the studied
compounds (295–800 °C) were carried out in air flow (100 mL/
min) by a Shimadzu Thermobalance TGA–DTG–50H instrument,
and a series of FT-IR spectra (20–30 per sample) of evolved gasses
were recorded for the studied compounds, using a coupled FT-IR
Nicolet Magma 550 spectrometer. Infrared spectra were recorded
by using KBr pellets on a Jasco FT-IR 410 spectrometer. Electronic
(diffuse reflectance) spectra were obtained in a Varian Cary-5E
spectrophotometer. Electron spin resonance spectra were recorded
in X band at room temperature in a Bruker ERP 300E spectrometer.
2.4. X-ray crystallography
Suitable crystals were mounted on glass fibers and these sam-
ples were used for data collection. Data were collected with Bruker
SMART CCD 1000 (1 and 3, 100 K) or Bruker X8 Proteum (2 and 4,
293 K) diffractometers. The data were processed with SAINT (1 and
3) [29] or APEX2 (2 and 4) [30] and corrected for absorption using
SADABS [31]. These structures were solved by direct methods [32],
which revealed the position of all non-hydrogen atoms. These
atoms were refined on F2 by a full-matrix least-squares procedure
using anisotropic displacement parameters [33]. All hydrogen
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. Geometric calculations were carried out with PLATON [34]
and drawings were produced with PLATON [34] and MERCURY [35].
Fig. 4. Molecular structure of [Cu(EIDA)(H7azain)(H2O)] (2, top) and [Cu(NBz-
IDA)(H7azain)(H2O)] (3, bottom).
0.13 Å out of the basal plane towards the apex. A close similar
low distortion towards the bi-pyramidal coordination (with
s
= 1.00) occurs in the binuclear compound [Cu2(
H4bim)4(SO4)2]Á11H2O [10] and the salts [Cu2(
H4bim)4X2]ÁX2 (X = Cl, Br) [11], where the
ing role of H4abim approaches to 2-N3,N9-Hade [13] or l2
l
-N3,N9-
l
-N3,N9-
l
-N3,N9-H4abim bridg-
3. Results and discussion
l
-
N3,N9-ade [14,15] modes. The formation of the Cu–N7(H4abim)
bond in 1 is according to the basicity order (N7 > N3) in purines.
However, it should be noted that such coordination cannot be rein-
forced by an intra-molecular N–HÁÁÁO interaction. Consider that the
H(N9)4abim tautomer has the dissociable hydrogen atom on N9. A
reason of this finding can be related to the crystal packing, with
polymeric chains extending along 21 screw axis (intra-chain short-
est CuÁÁÁCu separations are 5.699(1) Å). The chains connect to each
other by means of symmetry related H-bonding interactions, N9–
H9ÁÁÁO12#3 (2.73 Å, 173.6°) with O-acceptors from the non-bridg-
Crystal data for compounds [Cu(NBzIDA)(H4abim)]n (1), [Cu(EI-
DA)(H7azain)(H2O)] (2), [Cu(NBzIDA)(H7azain)(H2O)] (3) and
[Cu(2-SO4)(H7azain)2(H2O)2]n (4) (NBzIDA and EIDA are N-benzyl-
and N-ethyl-iminodiacetate ligands, respectively) are shown in
Table 1.
3.1. Crystal structure and properties of 1
Selected interatomic distances and angles are listed in Table 2.
The coordination environment of the copper atom is shown in
Fig. 2, together with the atom-numbering scheme used.
The crystal structure of 1 is based on a coordination polymer
built by means of the syn–anti bridging role of a carboxylate group
of the NBzIDA ligand. The copper(II) atom exhibits a square-base
pyramidal coordination, type 4+1, with a rather planar base (Addi-
ing carboxylate group of
(#3 = Àx + 1, Ày + 1, Àz + 1), building a 2D layer (Fig. 3) parallel
to the bc plane. No -stacking interactions between aromatic
a neighbouring NBzIDA ligands
p,p
rings were observed in 1. Hydrophobic contacts contribute to the
crystal structure connecting layers along the a axis resulting in a
3D architecture. It seems clear that the crystal packing in the coor-
dination polymer 1 strongly influences the observed recognition
patterns between the Cu(NBzIDA) chelate and the H4abim
(N1,N6-dideazaadenine) base.
son parameter
s = 0.002). The tridentate mer-NO2-NBzIDA chelat-
ing agent and the N7 atom from the H4abim ligand are the
closest Cu donors. The apical/distal donor is the O#1 (carboxyl)
from the bridging carboxylate group of an adjacent NBzIDA
(#1 = Àx + 1, y + 1/2, Àz + 1/2) and the Cu(II) atom is moved
Compound 1 is stable until 180 °C and decomposes in dry-air
(three steps between 180 and 435 °C, with production of H2O,

174
D. Choquesillo-Lazarte et al. / Polyhedron 29 (2010) 170–177
CO2, CO, H2CO and three N-oxides) to give
a
residue of
additional inter-molecular N9–HÁÁÁO(12)#2 interaction (2.87 Å,
163.5°; #2 = Àx + 1, Ày + 1, Àz). In this regard, the dihedral angle
between the basal coordination plane and H7azain plane is
25.45° in 2 and 41.29° in 3. In both compounds, apical aqua ligands
uses their H atoms to build inter-molecular O–HÁÁÁO(noncoordinat-
ed) interactions with O-carboxylate acceptor of two adjacent mol-
ꢀCuOÁCu(NO3)3 (Found 22.99%, Calc. 21.72%). The FT-IR spectrum
(cmÀ1) shows typical bands of carboxylate groups (mas 1600, ms
1387), N–H (m 3134, d 1500) and C–H(arom) (p 750 and 707) chro-
mophores. A large series of peaks are observed in this spectrum be-
tween 2500 and 3250 cmÀ1, including N–H and C–H bands (and
overtones and/or combination bands). The electronic (diffuse
reflectance) spectrum exhibits an asymmetric d–d band (mmax
14 560 cmÀ1) whereas the ESR spectrum has a typical axial shape
ecules forming chains running along the b axis (Fig. 5).
p,p-
Stacking interactions between H7azain ligands (2) or between
H7azain and the aryl ring of NBzIDA ligands (3) from adjacent
chains contribute to the formation of 2D layers. In 2, five-member
(g// 2.23 and g\ 2.05) corresponding to Cu(II) centres in dx2
Ày2
ground state and misaligned CuN2O2 + O chromophores.
rings from H7azain ligands are involved in the p,p-stacking (cen-
troid to centroid distance of 3.46 Å and an interplanar dihedral an-
gle of 0.0°, Fig. 6) meanwhile in 3, C6-ring from NBzIDA ligands and
both five- and six-member rings from H7azain ligands are involved
(centroid to centroid distances of 3.57/3.52 Å and an interplanar
3.2. Molecular and crystal structure and properties of 2 and 3
Compounds 2 and 3 have the same type of formula, [Cu(IDA-
like)(H7azain)(H2O)] and consist of ternary complex molecules,
with the Cu(II) atom in a distorted square-base pyramidal coordi-
nation. Drawings of the asymmetric units 2 and 3 are shown in
Fig. 4, together with the atom-numbering schemes. The basal plane
of the four closest donors is defined by the mer-NO2(IDA-like) li-
gand and N3-purine-like donor of H7azain. The coordination bond
Cu–N3(H7azain) is reinforced by the intra-molecular interligand
N9–HÁÁÁO(coordinated, IDA-like) interaction (2.77 Å and 133.7° in
2, 2.99 Å and 108.8° in 3). The weakness of the intra-molecular
H-bond in 3 is due to the implication of its H7azain ligand in one
dihedral angle of 1.1°, Fig. 7). Thus, p,p-stacking results in a face-
to-face mode with a parallel disposition of the aromatic rings.
Hydrophobic interactions connect 2D layers in a 3D architecture
of crystals 2 and 3.
Unlike the molecular nature of 2 and 3, the closely related com-
pound [Cu(IDA)(H7azain)]n [12] is a coordination polymer. How-
ever, it is interesting to note that these three ternary complexes
exhibit the same Cu(IDA-like) chelate-H7azain recognition pattern,
with the Cu–N3(purine-like) coordination bond reinforced by an
intra-molecular N9–HÁÁÁO(coordinated IDA-like) interaction. It
Fig. 5. View in the ab plane of chains obtained from hydrogen bonding interactions, running along the b axis in 2 (top) and 3 (bottom).

D. Choquesillo-Lazarte et al. / Polyhedron 29 (2010) 170–177
175
Fig. 6. View in the ab plane of the 3D architecture in 2, showing hydrogen bonding and
p,p
-stacking interactions. Right: detail of the
p
,p-stacking between H7azain ligands.
Fig. 7. View in the ab plane of the 3D architecture in 3, showing hydrogen bonding and
p,p-stacking interactions. Right: detail of the p,p-stacking between NBzIDA and
H7azain ligands.
seems clear that the absence or presence of an alkyl or benzyl N-
arm in the IDA-like chelating ligand does not alter the chelate-
H7azain recognition pattern in the three referred complexes. That
is in clear contrast to the different recognition patterns reported
for ternary Cu(IDA-like)-Hade complexes [3,4,6].
Compounds 2 and 3 also have very similar properties. The FT-IR
spectra show bands of the aqua ligand, carboxylate, chromophores
of IDA-like ligands and N–H bond of H7azain. In the spectrum of 3,
two deformation out-of-plane
p(C–H)arom are easily recognised, as
well defined absorptions, at 754 and 738 cmÀ1. On the other hand,

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D. Choquesillo-Lazarte et al. / Polyhedron 29 (2010) 170–177
the spectrum of 2 only shows a
p
(C–H)arom band at 723 cmÀ1, as
exhibits an elongated octahedral coordination, type 4+2. The four
closest donors are two sets of symmetry related donors (#1 = Àx,
y, Àz + 3/2), cis-O10(aqua), O10#1 and cis-N3(H7azain), N3#1
‘the most intense and defined absorption among 800–700 cmÀ1
spectral range’. It is also interesting to point out the wave-number
(cmÀ1) of the stretching mode
m
(N–H) band, observed at 3322 for 2
atoms. Apical donors belong to two trans-l2-sulfate bridges, result-
(with a single N–HÁÁÁO interaction) and 3160 for 3 (with a ‘bifur-
ing polymeric chains. The internal stability of these chains is rein-
forced by two kind of H-bonding interactions: N9–
H9ÁÁÁO(1)#1(coordinated sulfate; 2.82 Å, 153.61°) and O10–
H10AÁÁÁO2(noncoordinated sulfate; 2.65 Å, 174.3°). Pairs of chains
are linked by H-bonding interactions O10–H10BÁÁÁO(2)#3(noncoor-
dinated sulfate; 2.72 Å, 172.1°; #3 = x, Ày, z + 1/2), giving double-
stranded chains with H7azain ligands on the outer positions and
a paddle–wheel shape (Fig. 9). The cis-coordination of pairs of aqua
and H7azain ligands seems to enable the referred H-bonding stabil-
ization of pairs of polymeric chains. In the crystal, each double-
stranded chain is interconnected to other four through symmetrical
cated’ H-bonding interaction).
The thermal behaviour of these molecular complexes exhibits
two closely related features. The first step corresponds to the loss
weight of H2O + CO2 (these gasses are identified in the correspond-
ing FT-IR spectra). Pyrolytic steps yield a residue of CuO (Found
19.65% and Calc. 18.90% for 2; Found 23.00% and Calc. 22.17% for
3). The electronic spectrum of 3 shows an asymmetric d–d band
with mmax at 14560 cmÀ1, with an intensity barycentre near
13 000 cmÀ1
.
3.3. Crystal structure and properties of 4
C–HÁÁÁ
p interactions, involving the –C8–H8 bond and the five-mem-
ber ring of H7azain, allowing a 3D network (Fig. 9). The C–HÁÁÁcen-
The stoichiometric reaction of copper(II) sulfate and H7azain
afforded green crystals. The structure of 4 is based on a coordina-
tion polymer extending along the c axis (Fig. 8). The copper(II) atom
troid distance is 2.74 Å and the C–HÁÁÁcentroid angle is 146.0°.
The structure of compound [Cu(l2-SO4)(H7azain)2(H2O)2]n (4)
strongly differs from that reported to [Cu2(l2-N3,N9-H4bim)4
Fig. 8. Structure of [Cu(
l2-SO4)(H7azain)2(H2O)2]n (4). Left: double-stranded chain obtained by hydrogen bonding, running along the c axis. Right: detail of the numeration
scheme of the coordination environment in 4.
Fig. 9. View of the 3D architecture in 4 (left). Right: retail of CHÁÁÁp interactions involving H7azain ligand.

D. Choquesillo-Lazarte et al. / Polyhedron 29 (2010) 170–177
177
(SO4)2]Á11H2O [11]. This feature will be rationalized on the basis of
contract. The project ‘‘Factoría de Cristalización, CONSOLIDER
INGENIO-2010’’ provided X-ray structural facilities for this work.
the impossibility of neutral H7azain to use both N-atoms for me-
tal-binding. It is well proved that the 7azain anion build l2
-
N3,N9-bridges. The bridging
l
2-N3,N9-H4abim role uses the
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4. Concluding remarks
In contrast to Hade-like behaviour and the one reported to
H4abim in [Cu2(
l
-N3,N9-H4abim)4(SO4)2]Á11H2O and related salts,
having the tautomer H(N7)4abim, this 1,6-dideazaadenine binds
the Cu(N-benzyliminodiacetate) chelate by the N7-donor, using
the H(N9)4abim tautomer, without intra-molecular interligand
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5. Supplementary data
CCDC 730725, 730726, 730727 and 730728 contain contains
the supplementary crystallographic data for 1, 2, 3 and 4, respec-
tively. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements
Financial support from ERDF-EC, MEC-Spain (Project CTQ2006-
15329-C02/BQU) is acknowledged. ADM thanks MICINN-Spain for
a FPU Grant. DChL thanks CSIC-EU for a I3P postdoctoral research