A study of the second coordination sphere in 8-azaxanthinato salts of divalent metal aquacomplexes

Article abstract of DOI:10.1016/j.ica.2008.07.029

The interaction of the 4,6-dimethyl and 4-monomethyl derivatives of 1,2,3-triazolo[4,5-d]pyrimidin-5,7-dione (which may be named also as purine derivatives, 1,3-dimethyl-8-azaxanthine, Hdmax and 3-methyl-8-azaxanthine, H3max) with the divalent cations of

Full text of DOI:10.1016/j.ica.2008.07.029

Inorganica Chimica Acta 362 (2009) 1553–1558
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
A study of the second coordination sphere in 8-azaxanthinato salts
of divalent metal aquacomplexes
*
Carmen R. Maldonado, Miguel Quirós , Juan M. Salas, Antonio Rodríguez-Diéguez
Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 May 2008
Received in revised form 16 July 2008
Accepted 24 July 2008
Available online 5 August 2008
The interaction of the 4,6-dimethyl and 4-monomethyl derivatives of 1,2,3-triazolo[4,5-d]pyrimidin-5,7-
dione (which may be named also as purine derivatives, 1,3-dimethyl-8-azaxanthine, Hdmax and
3-methyl-8-azaxanthine, H3max) with the divalent cations of Mn, Co, Ni, Zn and Cd in aqueous media
generate solids with formulation ML2 ꢀ 6H2O. The crystal structure of the Mn and Cd compounds with
dmax and the Cd compound with 3max have been determined by single-crystal X-ray diffraction reveal-
ing that the compounds are salts with [M(H2O)6]2+ as cations and dmaxꢁ or 3dmaxꢁ as anions. The sec-
ond-sphere interactions in these compounds have been analysed, consisting in a network of very well-
defined hydrogen bonds, with all available potential donor and acceptor positions involved. The topology
of the motifs generated by these hydrogen bonds has been characterised, adapting to the second coordi-
nation sphere concepts usually applied to the first (chelate, bridge, monodentate, . . .). Monodimensional
(tapes) superstructures with the building blocks rather tightly bounded appear in the compounds with
dmaxꢁ as anion, whereas the corresponding superstructure in the Cd compound with 3maxꢁ is bidimen-
sional. These superstructures further interact among them in a less tight fashion to generate the three
dimensional crystal structures. Powder X-ray diffraction strongly suggests that Mn, Co, Zn and Cd com-
pounds of each ligand are isostructural, so the results of the samples determined by single-crystal X-ray
diffraction may be extended to all these compounds. On the other hand, powder X-ray diffraction indi-
cates that the nickel compounds have a different structure and the spectroscopic data for these com-
pounds suggest that the ligand is directly attached to the metal for them.
Keywords:
Azapurines
Azaxanthines
Triazolopyrimidines
Hydrogen bonds
Second coordination sphere
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
chemical systems, which range from inorganic to biological chem-
istry [3]. One of the most clear examples of this is the molecular
The study of the interaction of metal ions with nucleic acids,
their constituents and their analogues has been a largely developed
research topic in the last decades. These interactions have impor-
tant biochemical implications in nucleotide biochemistry, genetic
information storage and transfer, and control of gene expression
[1]. The main interest of these studies uses to be the direct bonds
formed between cation and ligand, the so called first coordination
sphere, relegating the weakest interactions known as second coor-
dination sphere to a secondary role. Nevertheless, these interac-
tions are, in discrete compounds, the responsible for building the
crystalline structures.
Among these interactions, hydrogen bonding is probably the
most important one in many systems. Hydrogen bonds, when act-
ing together, become much stronger since they lean on each other,
phenomenon called cooperativity in scientific terms (1+1 > 2) [2].
Currently, the hydrogen bond is more than ever a topic of vital sci-
entific research because it plays a key role in a vast number of
recognition between the complementary bases in DNA, responsible
of the formation of the well-known double helix. Thus, hydrogen
bonding could, in principle, be used as the way of joining the build-
ing blocks in the rational design of supramolecular structures [4];
however, performing this crystal engineering task in a accurately
predictable manner using synthons with several different possible
donors and acceptors groups cannot be easily done.
In the search for new compounds as effective antiviral and
antitumour therapy, the synthesis of many modified nucleobases
has been reported. For example, replacement of the imidazole ring
in purines for 1,2,3-triazole to build 8-azapurines (1,2,3-triazolo-
[4,5-d]pyrimidines) [5–7] leads to significant changes in the chem-
ical and biological properties such as their activity as anti-tumour
drugs [8,9]. Scheme 1 compares these two bicyclic systems, depict-
ing the IUPAC numbering scheme used for the triazolopyrimidine
derivatives and the biochemical numbering scheme, more usual
when dealing with purines. IUPAC numbering scheme is used
throughout this article, though the terms ‘‘8-azapurine” or ‘‘8-aza-
xanthine”, etc. are still used since they are more compact than the
proper IUPAC name of the compounds.
* Corresponding author. Tel.: +34 958240441; fax: +34 958248526.
E-mail address: mquiros@ugr.es (M. Quirós).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.07.029

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C.R. Maldonado et al. / Inorganica Chimica Acta 362 (2009) 1553–1558
Na(dmax) ꢀ 2.5H2O, white powder. Anal. Calc. for C6H11Na-
N5O4.5: C, 29.0; H, 4.5; N, 28.2. Found: C, 28.8; H, 4.1; N, 28.4%.
1H NMR (300.20 MHz, DMSO-d6): d 3.21 (s, 3H, H41/H42/H43), d
3.44 (s, 3H, H61/H62/H63). 13C NMR (75.49 MHz, DMSO-d6): d
27.4 (CH3, C4), d 30.5 (CH3, C6), d 122.2 (C, C7a), d 149.9 (C, C3a),
d 151.4 (C, C5), d 157.1 (C, C7).
Na(3max) ꢀ 2.5H2O, white powder. Anal. Calc. for C5H9NaN5-
O4.5: C, 25.6; H, 3.9; N, 29.9. Found: C, 25.4; H, 3.3; N, 29.7%.
1H NMR (300.20 MHz, DMSO-d6): d 3.37 (s, 3H, H41/H42/H43),
d 10.33 (bs, H, H6). 13C NMR (75.49 MHz, DMSO-d6): d 29.6
(CH3, C4), d 122.9 (C, C7a), d 151.7 (C, C3a), d 152.0 (C, C5), d
157.6 (C, C7).
Scheme 1.
Aquacomplexes salts were obtained by mixing two aqueous
solutions, 15 mL each, one containing 2 mmol of the sodium salt
of the appropriate ligand and the other containing 1 mmol of the
metal salt (sulphate for Mn and nitrate for Co, Ni, Zn and Cd). From
the solutions containing Na(dmax) and manganese sulphate, Na(d-
max) and cadmium nitrate and Na(3max) and cadmium nitrate,
colourless crystals suitable for X-ray studies were isolated. The
other complexes were obtained as powders from their correspond-
ing solutions after several hours. In all cases, the solids were
washed with water and ethanol and dried with ether.
From elemental analyses, the following formulae were deduced
for the isolated compounds:
[Mn(H2O)6](dmax)2 (1), colourless crystals. Anal. Calc. for
C12H24MnN10O10: C, 27.5; H, 4.6; N, 26.8. Found: C, 27.8; H, 4.8;
N, 26.8%.
[Co(H2O)6](dmax)2 (2), pale pink microcrystalline powder. Anal.
Calc. for C12H24CoN10O10: C, 27.3; H, 4.6; N, 26.6. Found: C, 27.8; H,
4.7; N, 26.5%.
The interaction of transition metal ions with 8-azapurine deriv-
atives (8-azaadenines, 8-azaguanines, 8-azaxanthines, 8-azahy-
poxanthines, etc.) has been the subject of relatively few
chemical, spectroscopic and crystallographic studies. The small
amount of accumulated data indicate that these interactions are
quite different to the interactions with purines [10–24]. The only
previous example of a crystal structure of a metal complex con-
taining an 8-azaxanthine derivative was reported by our research
group in the eighties [17].
Following this research line, we have studied the interaction in
aqueous medium of a series of divalent metal ions (Mn, Co, Ni, Zn
and Cd) with the anionic form of two 8-azaxanthines: 1,3-di-
methyl-8-azaxanthine (Hdmax) and 3-methyl-8-azaxanthine
(H3max). The systematic IUPAC names of these compounds are
4,6-dimethyl-1,2,3-triazolo[4,5-d]pyrimidin-5,7-dione
and
4-
methyl-1,2,3-triazolo[4,5-d]pyrimidin-5,7(6H)-dione, respectively.
The spectroscopic and X-ray diffraction data studies of the isolated
solids reveal that they are, with the possible exception of the Ni
compounds, ionic salts of the corresponding hexaacuacomplexes
with the organic moieties as counteranions without a direct bond
between the metal and the azaxanthine derivative. Nevertheless,
these compounds exhibit an interesting and very well-defined net-
work of hydrogen bonds, with all potential acceptor and donor
sites involved, which build their three-dimensional architectures,
this second coordination sphere being the main subject of the pres-
ent study. Less extensive H-bond networks have been previously
observed in azapurine metal compounds for bis(8-azahypoxanthi-
nato)tetraaquacadmium(II) [12] and tetraaquabis(8-azahypox-
anthinato)mercury(II) [14] but have not been studied in detail.
Ni(dmax)2 ꢀ 6H2O (3), pale blue powder. Anal. Calc. for
C12H24N10NiO10: C, 27.3; H, 4.6; N, 26.6. Found: C, 26.8; H, 4.5; N,
26.3%.
[Zn(H2O)6](dmax)2 (4), pale yellow microcrystalline powder.
Anal. Calc. for C12H24N10O10Zn: C, 27.0; H, 4.5; N, 26.2. Found: C,
26.5; H, 4.4; N, 25.8%. 1H NMR (300.20 MHz, DMSO-d6): d 3.23 (s,
3H, H41/H42/H43),
d
3.47 (s, 3H, H61/H62/H63). 13C NMR
(75.49 MHz, DMSO-d6): d 27.8 (CH3, C4), d 30.7 (CH3, C6), d 123.4
(C, C7a), d 149.6 (C, C3a), d 151.2 (C, C5), d 156.6 (C, C7).
[Cd(H2O)6](dmax)2 (5), colourless crystals. Anal. Calc. for
C12H24CdN10O10: C, 24.8; H, 4.2; N, 24.1. Found: C, 25.0; H, 4.4;
N, 24.3%. 1H NMR (300.20 MHz, DMSO-d6): d 3.23 (s, 3H, H41/
H42/H43), d 3.46 (s, 3H, H61/H62/H63). 13C NMR (75.49 MHz,
DMSO-d6): d 27.6 (CH3, C4), d 30.6 (CH3, C6), d 122.8 (C, C7a), d
149.7 (C, C3a), d 151.3 (C, C5), d 156.9 (C, C7).
2. Experimental
2.1. Synthesis of the compounds
[Mn(H2O)6](3max)2 (6), pale yellow microcystalline powder.
Anal. Calc. for C10H20MnN10O10: C, 24.3; H, 4.1; N, 28.3. Found: C,
24.1; H, 4.6; N, 28.2%.
[Co(H2O)6](3max)2 (7), pale orange microcrystalline powder.
Anal. Calc. for C10H20CoN10O10: C, 24.1; H, 4.0; N, 28.1. Found: C,
23.8; H, 4.2; N, 27.5%.
Ni(3max)2 ꢀ 6.5H2O (8), mauve powder. Anal. Calc. for
C10H21N10NiO10.5: C, 23.6; H, 4.2; N, 27.6. Found: C, 24.0; H, 4.5;
N, 27.2%.
[Zn(H2O)6](3max)2 (9), pale yellow microcrystalline powder.
Anal. Calc. for C10H20N10O10Zn: C, 23.8; H, 4.0; N, 27.7. Found: C,
23.8; H, 4.0; N, 27.2%. 1H NMR (300.20 MHz, DMSO-d6): d 3.40 (s,
3H, H41/H42/H43), d 10.95 (s, H, H6). 13C NMR (75.49 MHz,
DMSO-d6): d 29.7 (CH3, C4), d 124.1 (C, C7a), d 151.2 (C, C3a), d
151.4 (C, C5), d 156.9 (C, C7).
The synthesis of ligands 1,3-dimethyl-8-azaxanthine (Hdmax)
and 3-methyl-8-azaxanthine (H3max) was carried out according
to the published method of Nübel & Pfleiderer [25]. Other reagents
were obtained from commercial sources and used without further
purification.
4,6-Dimethyl-1,2,3-triazolo[4,5-d]pyrimidin-5,7-dione (1,3-di-
methyl-8-azaxanthine, Hdmax). 1H NMR (300.20 MHz, DMSO-d6):
d 3.23 (s, 3H, H41/H42/H43), d 3.42 (s, 3H, H61/H62/H63). 13C
NMR (75.49 MHz, DMSO-d6): d 27.9 (CH3, C4), d 30.6 (CH3, C6), d
123.5 (C, C7a), d 148.5 (C, C3a), d 150.8 (C, C5), d 155.9 (C, C7).
4-Methyl-1,2,3-triazolo[4,5-d]pyrimidin-5,7(6H)-dione
(3-
methyl-8-azaxanthine, H3max). 1H NMR (300.20 MHz, DMSO-d6):
d 3.37 (s, 3H, H41/ H42/H43), d 11.41 (s, 1H, H6). 13C NMR
(75.49 MHz, DMSO-d6): d 29.6 (CH3, C4), d 124.2 (C, C7a), d 150.3
(C, C3a), d 150.8 (C, C5), d 156.3 (C, C7).
[Cd(H2O)6](3max)2 (10), colourless crystals. Anal. Calc. for
C10H20CdN10O10: C, 21.7; H, 3.6; N, 25.3. Found: C, 22.0; H, 3.9;
N, 25.3%. 1H NMR (300.20 MHz, DMSO-d6): d 3.39 (s, 3H, H41/
H42/H43), d 10.68 (s, H, H6). 13C NMR (75.49 MHz, DMSO-d6): d
29.6 (CH3, C4), d 123.4 (C, C7a), d 151.3 (C, C3a), d 151.5 (C, C5), d
157.3 (C, C7).
The sodium salts of these compounds were obtained by mixing
aqueous solutions of each of them (1 mmol, 30 mL) with NaOH
solutions (1.5 mmol, 5 mL). Elemental analysis data reveal that
the salts crystallize as hydrates.

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1555
2.2. Instrumentation
equipped with a zero background plate. Le Bail fits of the diffracto-
grams acquired were performed with the aid of TOPAS [27].
Microanalysis of C, H and N were performed in a Fisons Instru-
ments EA-1008 analyser. Thermal behaviour was studied under an
air flow in Shimadzu TGA-50 and Shimadzu DSC-50 equipment, at
heating rates of 20 °C minꢁ1 and 10 °C minꢁ1, respectively. Reflec-
tance diffuse spectra were made on a VARIAN model CARY-5E
spectrophotometer. NMR spectra were recorded in D6[dmso] on
Varian Inova 300 MHz instrument, solvent signals (1H, 13C) were
used as internal references (all this equipment is sited at the Centre
of Scientific Instrumentation of the University of Granada). IR spec-
tra were recorded on a ThermoNicolet IR 200 spectrometer using
KBr pellets.
3. Results and discussion
3.1. X-ray structural studies: the building blocks
Three of the isolated compounds (1, 5 and 10) have been ob-
tained as crystals suitable for monocrystal X-ray analysis and their
crystal structures have been determined. The three compounds are
ionic salts with [M(H2O)6]2+ as the cation (M = Mn for 1 and M = Cd
for 5 and 10) and the deprotonated form of the 8-azaxanthine
derivative as counteranion (dmaxꢁ for 1 and 5 and 3maxꢁ for
10). Fig. 1 displays a view of the basic ionic units in 5, those in
the other two compounds are very similar. The metal atoms lie
in crystallographic inversion centres and the respective octahedra
are appreciably distorted, surely due to the hydrogen bonding
interactions, this distortion being higher for the compounds with
dmax, metal–water distances ranging from 2.1413(15) to
2.1983(16) Å and from 2.2472(14) to 2.2974(15) Å for 1 and 5,
respectively, whereas the maximum deviations from perpendicu-
larity of cis bond angles are 2.01° and 2.68°, respectively. The octa-
hedron in 10 is somewhat more regular with distances in the
2.2545(18)–2.2658(18) Å range and deviations from perpendicu-
larity below 1.2°. The ionization of the acidic protons of Hdmax
and 3Hmax, located at the nitrogen atom at position 2 in the crys-
tal structures of the free ligands [6],[17], leads to a ꢂ5° closing of
the endocyclic angle at that atom and the opening of the adjacent
endocyclic angles.
2.3. Crystallography
Data were collected, at room temperature for 1 and 10 and at
100 K for 5, in a Bruker SMART APEX CCD system with Mo Ka radi-
ation (k = 0.7107 Å). Data were corrected for absorption (multi-
scan, transmission ranges, 0.904–0.780, 0.967–0.814 and 0.954–
0.807 for 1, 5 and 10, respectively). The structures were solved
by the heavy atom method and anisotropically refined in F2 using
SHELXL-97 [26]. Hydrogen atoms of the heterocycles were placed in
ideal positions whereas those of water molecules were clearly
spotted in the
DF maps and refined with restrained O–H distance
(0.85(1) Å). Isotropic thermal parameters of all H atoms fixed to
1.2 times the equivalent isotropic thermal parameter of their par-
ent atoms. Table 1 summarizes the crystallographic data for these
compounds.
Nevertheless, the main interest of these structures does not lie
in the first coordination sphere but in the second, which generates
a three-dimensional hydrogen bonded network, with all potential
donor and acceptor sites involved. The networks are quite well
characterized, since all hydrogen atoms have been clearly spotted
in the crystallographic analysis. The structures are a good example
of how concepts usually applied to the first coordination sphere
find clear analogies in the second coordination sphere, so we
may use terms like ‘‘monodentate”, ‘‘bridging ligand”, ‘‘chelate”
and so on. The results for the isostructural compounds 1 and 5
are presented firstly and those of compound 10 are described after-
wards. Table 2 lists hydrogen bond parameters for the three
compounds.
Powder X-ray diffraction data have been collected on a h:h Bru-
ker AXS D8 vertical scan diffractometer with Cu K
a radiation
(k = 1.5418 Å). The powders were gently ground in an agate mortar
and then deposited with care in the hollow of an aluminium holder
Table 1
Crystal data
Compound
1
5
10
Chemical formula
Formula weight
Crystal size (mm)
Crystal system
Space group
Unit cell dimensions
a (Å)
C12H24MnN10O10
523.35
C12H24CdN10O10
580.81
C10H20CdN10O10
552.76
0.47 ꢃ 0.34 ꢃ 0.15 0.38 ꢃ 0.26 ꢃ 0.03 0.37 ꢃ 0.07 ꢃ 0.04
triclinic
triclinic
triclinic
ꢀ
ꢀ
ꢀ
P1
P1
P1
3.2. Hydrogen bonding network in compounds 1 and 5
7.7462(14)
8.0979(15)
9.9361(18)
74.563(3)
80.102(3)
63.533(3)
536.79(17)
1
7.7334(5)
7.8437(5)
9.9055(6)
75.722(1)
80.977(1)
65.012(1)
526.84(6)
1
6.8110(8)
8.7316(10)
8.9234(10)
86.413(2)
67.813(2)
87.315(2)
490.27(10)
1
b (Å)
c (Å)
Compounds 1 and 5 are isostructural and therefore the topology
of the hydrogen bonding network is identical for both. The ligand
has five potential hydrogen acceptor sites (N1, N2, N3, O5 and
O7), while the water molecules provide twelve donor sites for
the cation, all of these sites are involved in hydrogen bonding
building the supramolecular structure of these compounds. There-
fore, the second coordination sphere number of the metal is twelve
(twelve atoms from eight anions) and that of the anion is six (O5
accepting two H-bonds).
a
(°)
b (°)
(°)
c
Volume (Å3)
Z
Dcalc (calculated, g/
1.619
1.831
1.872
cm3)
Absorption
coefficient
0.688
1.111
1.189
(mmꢁ1
)
N2 and N3 interact with water molecules belonging to the same
hexaaquacation, this closes a seven-atom ring (the metal, two oxy-
gens, two hydrogens and two nitrogens) that may be defined as a
‘‘chelate”, this pattern may be represented in a condensed way as
R22(7) according to the graph set notation proposed by Etter,
Bernstein et al. [28,29]: R indicates a ring, the degree (in parenthe-
ses) is the number of atoms in the ring, and the super- and
subscripts represent the number of hydrogen bond acceptors and
donors in the motif, respectively. N1 and O7 define another chelate
(in this case R22(9)) with another [M(H2O)6]2+ cation. These chelat-
ing interactions build tape structures that grow in the (1,ꢁ1,0)
direction (Fig. 2); a tape is defined as an infinite monodimensional
h Range for data
collection (°)
Reflections
collected/unique
Data/restrains/
parameters
2.13–28.27
6253/2425
2425/6/171
1.031
2.13–28.26
6126/2384
2384/6/171
1.087
2.34–28.40
5761/2223
2223/6/161
1.042
Goodness-of-fit on
F2
R (I P 2
r
(I))
0.0372
0.1126
0.309 and ꢁ0.326
0.0242
0.0624
0.0250
0.0640
wR2 (all data)
Largest difference
peak and hole
0.754 and ꢁ0.512 0.443 and ꢁ0.304
(e Åꢁ3
)

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C.R. Maldonado et al. / Inorganica Chimica Acta 362 (2009) 1553–1558
C4
O3W
O5
N4
C3a
Cd
C5
N6
N3
N2
C7a
C7
O1W
N1
O2W
C6
O7
Fig. 1. View of the basic ionic unit of compound 5.
3.3. Hydrogen bonding network in compound 10
Table 2
Hydrogen-bond geometry (Å, °)
The replacement of a methyl group by an hydrogen at position 6
generates a donor site (H6) for the ligand 3maxꢁ in addition to the
five acceptor sites (N1, N2, N3, O5 and O7) present in dmaxꢁ, this is
the reason why the supramolecular structure in compound 10 is
different to the described above for compounds 1 and 5. The pres-
ence of this new donor site makes the anions to associate in pairs
due to the hydrogen bonding motif DA–AD (D = N6–H and A = O5,
R22(8)) across an inversion centre, association of the same kind of
that present between guanine and cytosine in DNA structure. An
alternative way of describing the structure is to consider that the
anionic species in the structure is in fact the couple ð3maxÞ22ꢁ. This
association is also observed between neutral 3max molecules in
the crystal structure of the free ligand [17].
Compound 1
d(Dꢀ ꢀ ꢀA)
2.799(2)
2.789(2)
2.779(2)
2.855(2)
2.756(2)
2.7998(19)
Compound 5
d(Dꢀ ꢀ ꢀA)
2.792(2)
2.783(2)
2.757(2)
2.839(2)
2.744(2)
2.771(2)
\(DHA)
\(DHA)
O(1W)–H(11W)ꢀ ꢀ ꢀN(2)
O(1W)–H(12W)ꢀ ꢀ ꢀN(1)#2
O(2W)–H(21W)ꢀ ꢀ ꢀO(5)#3
O(2W)–H(22W)ꢀ ꢀ ꢀO(5)#4
O(3W)–H(31W)ꢀ ꢀ ꢀN(3)
O(3W)–H(32W)ꢀ ꢀ ꢀO(7)#5
167(3)
172(3)
164(3)
168(3)
172(3)
161(3)
162(3)
172(3)
168(3)
171(3)
174(3)
161(3)
Symmetry transformations used to generate equivalent
atoms: #2 ꢁx + 2, ꢁy, ꢁz + 1; #3 x ꢁ 1, y, z + 1; #4
ꢁx + 1, ꢁy + 1, ꢁz; #5 x ꢁ 1, y + 1, z
Compound 10
d(Dꢀ ꢀ ꢀA)
\(DHA)
O(1W)–H(12W)ꢀ ꢀ ꢀN(1)
O(1W)–H(11W)ꢀ ꢀ ꢀO(7)#2
O(2W)–H(21W)ꢀ ꢀ ꢀO(5)#3
O(2W)–H(22W)ꢀ ꢀ ꢀO(7)#4
O(3W)–H(32W)ꢀ ꢀ ꢀN(2)
O(3W)–H(31W)ꢀ ꢀ ꢀN(3)#5
N(6)–H(6)ꢀ ꢀ ꢀO(5)#6
2.758(2)
2.783(2)
2.778(2)
2.817(2)
2.845(2)
2.831(3)
2.896(2)
168(3)
163(3)
173(3)
162(3)
156(3)
174(3)
168.9
The three imidaloze nitrogen atoms accept H-bonds from two
cations, N1 and N2 defining a R22(7) ‘‘chelate” ring and N2 and N3
forming a centrosymmetric R44(10) motif involving two cations.
These interactions lead to the formation of a bidimensional array
in the ab plane (Fig. 3), with alternating rows of cations and rows
of ð3maxÞ22ꢁ entities. Additional H-bonds links these layers to each
other, the main of which is, possibly the formation of a R23(10) ring
involving two water molecules of the same cation, the O5 atom of
Symmetry transformation used to generate equivalent atoms: #2 ꢁx + 1, ꢁy + 1, ꢁz
#3 xꢁ1, y + 1, z + 1; #4 x ꢁ 1, y, z + 1; #5 ꢁx + 1, ꢁy, ꢁz + 1; #6 ꢁx + 2, ꢁy, ꢁz ꢁ 1
2ꢁ
one of the components of a ð3maxÞ2 entity and the O7 atom of
the other, which may be considered also as a chelate if the dimeric
entity is considered as if it were an unique ligand. O7 further inter-
acts with another cation, thus acting as a bridge. Alternate sheets
structure in which the components are linked by more than one
hydrogen bond, with the hydrogen bonds approximately coplanar
[4]. A third ring motif R44(10) involving two cations and two anions
appears in the tape around crystallographic inversion centres. The
anion is roughly coplanar with the tape.
The tapes are linked by the monodentate interactions between
the ‘‘out-of-tape” water molecule (O2W) and the O5 atom of neigh-
bouring tapes, thus generating the three-dimensional network.
This interaction between two neighbouring O2W water molecules
and two O5 carbonyl oxygens defines another centrosymmetric
ring (O4H4, R2(8)). Apart from H-bonding, another interaction that
4
links tapes is the stacking between pairs of anions, the two compo-
nents of the couple are related by an inversion centre and dispose
parallel to each other, with the imidazole ring of one of the anions
over the pyrimidine ring of the other and separated 3.31 Å.
Fig. 2. Tape along the (1,ꢁ1,0) direction built by the hydrogen bond interactions in
compound 5.
Fig. 3. Layer built by the hydrogen bond interactions in compound 10.

C.R. Maldonado et al. / Inorganica Chimica Acta 362 (2009) 1553–1558
1557
dispose the rows of cadmium atoms over the rows of middle points
of the dimeric anionic units. Stacking along the c-axis is observed
also between anions of neighbouring sheets (distance, 3.39 Å) but
the overlapping between the aromatic rings is worse than in the
compounds with dmax.
In resume, these structures are good examples of supramolecu-
lar networks whose main glue is hydrogen bonding. H-bonds that
assemble cations and anions are very well-defined displaying a
great variability in their topology. These compounds are a demon-
stration of the capability of second-sphere hydrogen bonding as a
potential crystal engineering tool for building polymeric metal-
containing materials.
slightly displaced to lower wavenumbers in the complexes. A
new band around 1544 cmꢁ1 appears in all compounds with the li-
gands in anionic form (sodium salts and complexes). An intense
characteristic band close to 1310 cmꢁ1 is present in all compounds
of both ligands in the region expected for vibration modes of the
aromatic ring.
The spectra of compounds involving Hdmax show broad bands
in the 3400–3100 cmꢁ1 region, assignable to
m(OH) of water,
broadened and lower wavenumber displaced by the hydrogen
bonds. The free ligand also displays a broad absorption around
2800 cmꢁ1, which disappears in the sodium salt and in the com-
plexes, probably related with the acidic proton of the imidazole
ring, extensively associated in H-bonding. For H3max compounds,
3.4. Powder X-ray diffraction analysis
the region where m
(OH) bands appear is wider (3500–3000 cmꢁ1),
with a higher number of better defined bands. An additional in-
Powder X-ray diffraction diagrams have been recorded for all
samples in order to check whether the structural results found
for 1, 5 and 10 may be extended or not to the remaining com-
pounds. Calculated powder diagrams from the single-crystal re-
sults have been also performed. A surprising result is found for
the cadmium compounds 5 and 10, the simulated diagram being
different to the experimental one: for 5, there are many additional
peaks together with the expected ones whereas for 10, the exper-
imental pattern is quite different to the calculated. This indicates
that a different phase appears in the powder diagrams, partially
(5) or fully (10) replacing the single-crystal phase. It seems that a
structure change occurs in the necessary grinding process prior
to the recording of the powder diagram, perhaps because of the
(partial?) dehydration of the sample. On the other hand, for com-
pound 1, the simulation reproduces quite faithfully the experimen-
tal diagram, indicating that, for this compound, the above
described structure survives the grinding process.
The diagrams of the Co and Zn compounds with dmax (2 and 4)
are very similar to that of 1, strongly suggesting that the three
compounds are isostructural (and also to 5 prior to grinding). Look-
ing at 3max compounds, we find that the diagrams of 6, 7 and 9 are
very similar and also similar to the simulated diagram of 10 (but
not to the experimental one), this let us to propose that the Mn,
Co and Zn compounds with 3max present the same structure than
that of 10 prior to grinding and above described. Le Bail refine-
ments, using the X-ray single-crystal cell parameters of 1 as start-
ing point for dmax compounds and those of 10 for 3dmax
compounds, yielded the cell parameters shown in Table 3.
tense and well-defined band appears at 2855 (free ligand), 2816
(Na salt) or 2840 cmꢁ1 (complexes), that we assign to
m(N6–H),
well defined but considerably displaced to lower wavenumbers
due to the formation of the above mentioned DA–AD H-bond motif.
The metal complexes also present wide absorptions in the 400–
600 cmꢁ1 region, attributable to rocking frequencies of water.
The first step of the thermal decomposition of the isolated com-
plexes, as shown by their TG and DSC diagrams, is their dehydra-
tion. All compounds except 3 and 8 display a well-defined
weight loss effect in the range 70–152 °C which agree well with
the elimination of the water molecules indicated by their elemen-
tal analysis. Dehydration takes place in a single step (endothermic
peaks in the range 100–140 °C) except for 1 for which two well-de-
fined stages are observed (peaks at 105 and 140 °C). Dehydration
energy is very similar for all compounds, ranging from 44.2 to
51.3 kJ/mol of water. For 3 and 8, the experimental values for
weight losses are lower than the theoretical and dehydration
enthalpies cannot be reliably obtained from the integration of
the peaks in the DSC diagrams, suggesting that dehydration par-
tially occurs at room temperature. Dehydrated complexes are sta-
ble in an wide temperature range (up to 300 °C for Mn(II), Co(II)
and Ni(II), 330 °C for Zn(II) and Cd(II)-3max complexes and
400 °C for Zn(II) and Cd(II)-dmax complexes). In most cases, and
previously to pyrolytic decomposition, a small exothermic peak
appears that suggests an structural change before the pyrolysis of
the organic moiety. Decomposition processes normally occurs in
one step (300–500 °C for Mn(II), Co(II) and Ni(II)) or several steps
(330–700 °C for Zn(II) and Cd(II)-3max complexes and 400–
700 °C for Zn(II) and Cd(II)-dmax complexes), giving the corre-
sponding metal oxide as residue.
On the other hand, the nickel compounds (3 and 8) exhibit pow-
der patterns very different to the other compounds, indicating that
they present different crystal structures, this is in agreement with
the electronic spectra of these compounds (see below) that point to
direct metal coordination to the ligand for nickel.
The diffuse reflectance spectra of 2, 3, 7 and 8 are typical of
octahedral species [30]. Cobalt compounds exhibit two main
absorptions: a very broad one centred around 8500 cmꢁ1, assigned
to the 4T2g(F) 4T1g(F) (
m1) transition and another one at 20790 (2)
3.5. Spectral and thermal studies
and 21834 (7) cmꢁ1 assigned to the 4T1g(P) 4T1g(F) (
m3)transition.
The lack of precision in the reading of the first band do not let us to
perform a calculation of the ligand field parameters. The spectra for
nickel compounds show two of the three typical bands expected
The 1500–1700 cmꢁ1 region of the infrared spectra of the free
heterocycles displays three bands (at 1717, 1681 and 1604 cmꢁ1
for Hdmax and 1713, 1686 and 1593 cmꢁ1 for H3max), assignable
to carbonyl stretching and possibly to d (H2O), these bands being
for octahedral Ni(II) complexes at 10471 (3T2g (F) 3A2g(F), m1
)
and 17361 (3T1g (F) 3A2g(F),
m
2) cmꢁ1 for 3 and 10417 and
17007 cmꢁ1 for 8, from which the parameters Do = 10471 cmꢁ1
and B = 1106 cmꢁ1 for 3 and Do = 10417 cmꢁ1 and B = 964 cmꢁ1
for 8 were calculated using Dou equations [31]. These values are
closer to those of cromophores with N atoms involved in the coor-
dination sphere than to that of the [Ni(H2O)6]2+ cation [30], sug-
gesting that there is a direct bond between the metal and the
heterocycle for the nickel compounds.
Table 3
Unit cell parameters determined from Le Bail refinement of powder data
a (Å)
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
1
2
4
6
7
9
7.743
8.1109
8.1676
8.1722
8.6395
8.7423
8.7798
9.9388
9.9166
9.9313
8.9465
8.9861
8.9781
74.4956
73.6854
73.4742
86.529
86.7625
86.909
79.973
79.4962
79.3993
67.636
67.6771
68.214
63.5775
63.418
63.4401
87.577
87.7892
88.185
537.54
530.08
533.64
484.07
499.86
497.37
7.6419
7.6845
6.786
6.8906
6.8054
We have measured 1H and 13C NMR spectra for the ligands,
their sodium salts and the Zn(II) and Cd(II) complexes in dmso-
d6 (see the data in the experimental section). The spectra for the
compounds with the same ligand are virtually identical, without

1558
C.R. Maldonado et al. / Inorganica Chimica Acta 362 (2009) 1553–1558
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ˇ
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CCDC 687074, 687075 and 687076 contain the supplementary
crystallographic data for this paper. These data can be obtained
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via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
This work was supported by a project of the Ministry of Educa-
tion and Science of Spain (Project: CTQ-2005-00329/BQU) and the
Junta de Andalucía (FQM00425). C.R. Maldonado is grateful for a
FPU Grant from the Ministry of Education and Science of Spain.
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