Chelating ligand conformation driving the hypoxanthine metal binding patterns

Article abstract of DOI:10.1021/ic201918y

The X-ray diffraction structural results of 23 ternary compounds, type M^II(iminodiacetate-like)(hypoxanthine) [M = Co, Ni, Cu, or Zn], show that the iminodiacetate moiety conformation (mer-NO₂ or fac-NO ₂) is able to drive the M-hypoxanthine binding patterns displaying the M-N9 or M-N3 bond, cooperating with a N9-H???O intramolecular interaction, respectively.

Full text of DOI:10.1021/ic201918y

COMMUNICATION
pubs.acs.org/IC
Chelating Ligand Conformation Driving the Hypoxanthine Metal
Binding Patterns
Dheerendra K. Patel,† Duane Choquesillo-Lazarte,‡ Alicia Domínguez-Martín,† M. Pilar Brandi-Blanco,†
Josefa María Gonzꢀalez-Pꢀerez,† Alfonso Casti~neiras,§ and Juan Niclꢀos-Gutiꢀerrez*,†
†Department of Inorganic Chemistry, Faculty of Pharmacy, University of Granada, 18071 Granada, Spain
‡Laboratorio de Estudios Cristalogrꢀagficos, IACT, CSIC-Universidad de Granada, Avenida de las Palmeras 4, 18100 Armilla,
Granada, Spain
§Department of Inorganic Chemistry, Faculty of Pharmacy, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain
S Supporting Information
b
The binary compounds were obtained by a stoichiometric
ABSTRACT: The X-ray diffraction structural results of 23
ternary compounds, type MII(iminodiacetate-like)(hypoxan-
thine) [M = Co, Ni, Cu, or Zn], show that the iminodiacet-
ate moiety conformation (mer-NO2 or fac-NO2) is able to
drive the M-hypoxanthine binding patterns displaying the
reaction of the appropriate iminodiacetic acid and basic metal
carbonate.7 The reaction of the chelating agents and the basic
metal carbonates yields CO2 as the main byproduct, which can be
easily removed by means of stirring, heating, and moderate
vacuum. The syntheses of the ternary compounds were carried
out by a stoichiometic reaction between hypoxanthine and a solution
of the corresponding metal chelate, prepared following the pre-
viously reported procedure.7 These latter syntheses were pre-
pared in a range of 1À0.5 mmol of reactants in 100 mL of an
aqueous solution. In some cases, crystallization of the ternary com-
plexes needs the addition of ethanol to the partially concentrated
mother liquors.
MÀN9 or MÀN3 bond, cooperating with a N9ÀH
O
3 3 3
intramolecular interaction, respectively.
n the past decade, a large number of papers based on the metal
binding patterns of natural nucleobases and closely related
I
ligands have been published.1,2 As a result, adenine is now con-
sidered a fairly versatile ligand able to bind different metal centers
in a variety of cationic, neutral, or anionic forms.2 Furthermore,
adenine is currently being used to build a new class of interesting
metalÀorganic frameworks, the so-called MBioFs.3 However,
natural oxopurines are poorly studied. Prior to the present study,
the structural information concerning the metal binding patterns
of hypoxanthine was limited to a few compounds. Two poly-
morphs of neutral hypoxanthine (Hhyp) have been reported for
the same tautomer H(N9)hyp.4 In metal complexes, the tauto-
mer H(N9)hyp gives rise to the MÀN7 or M2(μ2-N3,N7)
modes, whereas the tautomer H(N7)hyp displays the mode
M2(μ2-N3,N9).5 Hence, these patterns do not show examples in
which Hhyp plays unidentate coordination roles by its most basic
N9 or less basic N3 donor atoms. In addition, in the CSD database
are also reported some structures of the hypoxanthinium(1+)
cation, which exhibits MÀN3 coordination as the H2(N7,
N9)hyp+ tautomer, or the hypoxanthinate(1À) anion, which
binds the metal center through the bond MÀN9 or as the
bridging μ2-N7,N9 mode. Very recently, because of increasing
interest on metal complexes with oxopurines, a CuIIMBioF assisted
by the unprecedented μ2-N7,O6ÀH(N9)hyp role (according to
the conventional numbering of purines) has been reported.6
This Communication deals with the structures of 22 novel
(1À22) ternary complexes having one MII first-row metal center
and iminodiacetate-like (IDA-like) chelators and Hhyp as main
coligands. Moreover, the molecular and crystal structure of
The formulas of the ternary compounds are [Cu(IDA)(Hhyp)-
(H2O)] H2O (1), [Cu(MIDA)(Hhyp)(H2O)] H2O (2), [Cu-
3
3
(μ2-NBzIDA)(Hhyp)]n (3), [Cu(μ2-MEBIDA)(Hhyp)]n (4),
[Cu(FBIDA)(Hhyp)(H2O)] 1.5H2O (5), {[Cu(μ2-CBIDA)-
3
(Hhyp) Cu(CBIDA)(Hhyp)(H2O)]4 2DEPh 21H2O}n (6),
3
3
3
[Cu(pheida)(Hhyp)(H2O)] 2H2O (7), [Cu2(p-XDTA)(Hhyp)2-
(H2O)2] 2H2O (8), [Co(NBzIDA)(Hhyp)(H2O)2] H2O (9),
[Co(MEBIDA)(Hhyp) (H2O)2] H2O (10), [Co(MOBIDA)-
(Hhyp)(H2O)2] H2O (11), [Co(FBIDA)(Hhyp)(H2O)2] H2O
(12), [Co(CBIDA)(Hhyp) (H2O)2] H2O (13), [Ni(NBzIDA)-
(Hhyp)(H2O)2] H2O (14), [Ni(MEBIDA)(Hhyp)(H2O)2]
H2O (15), [Ni(MOBIDA)(Hhyp)(H2O)2] H2O (16), [Ni-
(FBIDA)(Hhyp)(H2O)2] H2O (17), [Ni(CBIDA)(Hhyp)-
(H2O)2] H2O (18), [Cu(pdc)(Hhyp)(H2O)] (19), [Co(pdc)-
(Hhyp)(H2O)2] H2O (20), [Ni(pdc)(Hhyp)(H2O)2] (21), [Zn-
(pdc)(Hhyp)(H2O)2] (22), and [Cu(tda)(Hhyp)(H2O)] 2H2O
(23). The reported binary compounds agree with the formulas
3
3
3
3
3
3
3
3
3
3
3
3
3
3
[Co(MOBIDA)(H2O)3] H2O (11b), [Ni(NBzIDA)(H2O)3]
3
(14b), [Ni(FBIDA)(H2O)3] H2O (17b), and [Ni(CBIDA)-
3
(H2O)3] H2O (18b).
3
The ternary copper(II) compounds from 1 to 8 exhibit a dis-
torted square-pyramidal-based coordination, type 4 + 1. The four
basal donor atoms are N9(Hhyp) and the three donors of the
iminodiacetate moiety in mer-NO2 conformation (Figure 1, top).
The referred compounds are mostly molecular, except for the poly-
mers (3 and 4) and the crystal of 6, where molecules and 1D
polymers coexist. The formula of 6 also reveals cocrystallization
[Cu(tda)(Hhyp)(H2O)] 2H2O (23) is also reported. Besides,
3
four nickel(II) or cobalt(II) binary chelate compounds were also
Received: September 5, 2011
Published: October 13, 2011
studied (11b, 14b, 17b, and 18b).
r
2011 American Chemical Society
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|

Inorganic Chemistry
COMMUNICATION
Scheme 1. Formulas of the Chelating Ligands Used in the
Present Work
Figure 1. Complex molecules of 2 (top) and 9 (bottom).
of diethyl phthalate (DEph, additive of the partially denatured
ethanol). A few related examples of this phenomenon have been
previously recorded in CSD.8 The crystal of 7 has two indepen-
dent molecules in the asymmetric unit, mainly because of the
distinct conformations of the N-phenethyl arm in pheida ligands.
The binuclear nature of 8 is related to the bridging role of the
p-XDTA ligand (see Scheme 1). Noteworthy, because copper(II)
binds to the most basic N9 donor of Hhyp, this implies the use of
the tautomer H(N7)hyp. The structures of compounds 1À8 are
in accordance with a large number of ternary compounds displaying
IDA-like chelators and one N-heterocyclic donor atom supplied
by an appropriate coligand. As can be checked in the CSD,
related binary copper(II) complexes have the chelating IDA-like
ligand in fac-NO+O(apical) or mer-NO2 conformation, whereas
ternary complexes with two N-heterocyclic donors atoms from
other coligands show the IDA-like chelator in an imposed
fac-NO+O(apical) conformation.
Figure 2. Complex molecule of 21.
moiety. It should stand out that the reported compounds (1À18)
represent two new metal binding patterns for neutral Hhyp: by
N9 or by N3 in cooperation with an intramolecular interligand
N9ÀH O interaction.
3 3 3
In order to discern whether the two latter molecular recogni-
tion modes of hypoxanthine, with iminodiacetateÀmetal che-
lates, are driven by the metal coordination polyhedron or the
iminodiacetate moiety conformation, compounds 19À22 were
synthesized using 2,6-pyridinedicarboxylate (pdc) as the chelat-
ing agent, which imposes mer-NO2 conformation (Figure 2). Inter-
estingly, compound 19 exhibits a 4 + 1 copper(II) coordination,
whereas the cobalt(II), nickel(II), or zinc(II) complexes (20À22)
show a typical octahedral coordination. In these four molecular
complexes, hypoxanthine is bonded to the metal via MÀN9
using the H(N7) tautomer. Then, we conclude that the IDA
moiety conformation is the main driving factor that determines
the molecular recognition between the metal chelate and Hhyp.
Likewise, we wonder if the scope of our conclusion could be
extended to other chelating ligands; hence, we prepared com-
pound 23. In this case, the metal exhibits a 4 + 1 coordination, but
the tridentate thiodiacetate (tda) ligand adopts a rather unusual
fac-SO+O(apical) conformation (Figure 3). Moreover, the mo-
lecular recognition pattern involves the formation of CuÀ
N9[H(N7)hyp]. The aqua ligand is trans to the CuÀS bond.
The reported ternary compounds with cobalt(II) (9À13)
or nickel(II) (14À18) have in common the presence of one
N-benzyl-like iminodiacetate chelator. All of these compounds are
molecular complexes where the metal exhibits a typical octahe-
dral coordination (Figure 1, bottom). In these cases, the imino-
diacetate moiety adopts a fac-NO2 conformation with the
MÀN(IDA-like) distance as the longest coordination bond. This
family of compounds shows the metal binding of Hhyp by means
of the bond MÀN3 reinforced by an intramolecular interligand
N9ÀH O(coordinated carboxyl) interaction. The MÀN3-
3 3 3
[H(N9)hyp] bond falls trans to the MÀN(IDA-like) bond; thus,
these molecules are cis-diaqua complexes. Indeed, in addition to
the four binary complexes reported herein (11b, 14b, 17b, and 18b),
all of the binary MÀ(IDA-like) chelates (M = Co, Ni, or Zn)
reported in the CSD have the iminodiacetate moiety in fac-NO2
conformation, irrespective of the presence of any other coligands.
Therefore, the available structural information strongly suggests
that the octahedral chemistry of cobalt(II) and nickel(II), and prob-
ably of zinc(II), induces fac-NO2 conformation to the iminodiacetate
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Inorganic Chemistry
COMMUNICATION
’ ACKNOWLEDGMENT
We gratefully acknowledge “Factoría Espa~nola de Cristal-
izaciꢀon” CONSOLIDER INGENIO-2010 and MICINN-Spain
(Project MAT2010-15594) for financial support on this research.
A.D.-M. also thanks ME-Spain for a predoctoral FPU fellowship.
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S
Supporting Information. Formula and structural plots of
b
all of the compounds reported in this work (S1 and S2) as well as
tables with relevant spectral properties and thermal stability
information (S3). This material is available free of charge via
the Internet at http://pubs.acs.org. The atomic coordinates for
these structures have been deposited with the Cambridge Crys-
tallographic Data Centre (CCDC 695340À695345 and 834987À
835007). The coordinates can be obtained, upon request, from the
Director, Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge, CB2 1EZ, U.K. Additional spectroscopic (FT-IR and
UVÀvis) and thermal (TGA) analyses for the reported compounds
have been carried out by the authors. For further information, please
contact the corresponding author.
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: jniclos@ugr.es.
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dx.doi.org/10.1021/ic201918y |Inorg. Chem. 2011, 50, 10549–10551