КООРДИНАЦИОННАЯ ХИМИЯ, 2010, том 36, № 12, с. 913-917
УДК 541.49
THE FIRST BARBITURATE-BRIDGED MANGANESE(II) POLYMER: SYNTHESIS, CRYSTAL STRUCTURE, AND MAGNETIC PROPERTIES
© 2010 J. Chu, Z. Y. Liu, X. J. Zhao, and E. C. Yang*
College of Chemistry, Tianjin Key Laboratory of Structure and Performance for Functional Molecule, Tianjin Normal University, Tianjin 300387, P.R. China *E-mail: encui__yang@yahoo.com.cn Received March 10, 2010
Self-assembly of barbituric acid with Mn(II) salt in slightly basic medium generates the first barbiturate-bridged polymer, [Mn(Barb)2(H2O)2]n (I) (Barb = barbiturate), which was fully characterized by single-crystal X-ray diffraction, elemental analysis, FT-IR, TG—DTA, and variable-temperature magnetic susceptibility. In the crystal, the unique octahedral Mn2+ ion is periodically linked into a one-dimensional linear chain by pairs of monodentate-bridging Barb anions, which is further aggregated into a three-dimensional su-pramolecular architecture by popular hydrogen-bonding interactions. Additionally, complex I with considerably higher thermal stability displayed weak antiferromagnetic coupling interactions between the Barb-bridged paramagnetic Mn(II) centers.
INTRODUCTION
Bearing a pyrimidine heterocyclic skeleton, barbituric acid, and its diverse derivatives has already become the parent compound ofa large class ofbarbiturates that have central nervous system depressant properties. Therefore, they have extensively been used either as sedative hypnotic drugs or as efficient zincbinding groups for the design of matrix metalloproteinase inhibitors in medicinal chemistry [1, 2]. Additionally, owing to the important roles in clinical detection and identification ofthe specific drug, a variety of transition metal complexes with barbituric acid as ligand have also been prepared and developed. However, only limited solid structures of these complexes have been reported up to date [3—8]. Although the barbituric acid contains five potential metal binding sites (three O and two N donors), it has been known that it is generally in an anionic form in the complexes and acts as a monodentate ligand with the hydrox-yl O or a deprotonated ring N donor to coordinate with metal ions, resulting in discrete mono- or binuclear structures [3—8]. On the other hand, in addition to presenting the metal binding site, barbituric acid is also good hydrogen-bond synthon with multiple hydrogen bonding acceptors and donors by carbonyl and/or imido groups, so it can also be used as a building block to construct ordered organic supramolecular assemblies with desired hydrogen-bonding network. Indeed, diverse binary and/or ternary cocrystals involving barbituric acid have been obtained [9—14], which have exhibited different packing topology (chain, sheet, tape, and so on) in the polymorphs. Herein, to continue to explore the coordination behavior of the barbituric acid as well as to establish the structure—function relationship of the Barb-based metal complexes, the self-assembly reaction of barbituric acid with inorganic Mn(II) salt was carried out
in a weak basic medium. Unexpectedly, a novel chainlike polymer with barbiturate as a bridging ligand, [Mn(Barb)2(H2O)2]B (I), was successfully obtained, and fully characterized by single-crystal X-ray diffraction, elemental analysis, FT-IR, TG—DTA, and variable-temperature magnetic susceptibility. Notably, to the best of our knowledge, complex I is the first example that the barbiturate anion behaves as a bridging ligand to connect adjacent metal ions. Additionally, complex I also exhibits considerably higher thermal stability and weak antiferro-magnetic coupling interactions. Such a coupling interaction by the bridging barbiturate ligand is slightly weaker than those of typical dicyanamide, azide, or carboxylate group bridges.
EXPERIMENTAL
Reagents and instruments. All the starting materials employed herein were commercially purchased from Acros and used as received without further purification. Doubly deionized water was used for the conventional synthesis. Elemental analyses for C, H, and N were carried out with a CE-440 (Leeman-Labs) analyzer. FT-IR spectra (KBr pellets) were taken on an Avatar-370 spectrometer (Nicolet) in the range 4000—400 cm-1. TG—DTA experiment was carried out on a Shimadzu simultaneous DTG-60A compositional analysis instrument from room temperature to 800°C under a N2 atmosphere at a heating rate of 5°C min-1. Variable temperature magnetic susceptibility measurement of I was carried out at an applied DC field of1000 Oe from 2 to 300 K on a Quantum Design (SQUID) magnetometer MPMS-XL-7. Dia-magnetic corrections were calculated using Pascal's constants, and an experimental correction for the sample holder was applied.
3 КООРДИНАЦИОННАЯ ХИМИЯ том 36 № 12 2010
Table 1. Crystallographic data and experimental details for I
Parameter Value
Formula weight 172.57
Crystal size, mm 0.17 x 0.15 x 0.13
Crystal system Orthorhombic
Space group Ibam
a, A 13.8076(5)
b, A 12.4754(5)
c, A 6.7084(3)
V, A3 1155.56(8)
Z 8
9 range, deg 2.20-24.98
Range of reflection indices -11 < h <16, -14 < k < 14, -7 < l < 7
P calcd g/cm3 1.984
^Mo, mm-1 1.197
Reflections collected/unique 2816/557
Data/restraints/parameters 557/0/64
Rint 0.0121
GOOF on F2 1.067
F(000) 700
R (I > 2ct(I)) R1 = 0.0186, wR2 = 0.0573
R (all reflections) 0.0188/0.0574
A Pmax/A Pmin e A~3 0.203/-0.249
Table 2. Selected bond distance (A) and bond angles (deg) in I*
Bond d, A Angle ro, deg
Mn(1)-O(4) 2.1405(17) O(4)Mn(1)O(1) 87.03(4)
Mn(1)-O(1) 2.1786(9) O(1)Mn(1)O(1)' 174.05(8)
O(1)'Mn(1)O(1)" 79.33(6)
O(1)Mn(1)O(1)" 100.99(6)
* Symmetry codes: ' -x, y, z + 1/2, " x, —y, -z + 1/2.
Table 3. Parameters of hydrogen bond in structure I*
Contact D- H-A Distance, A Angle D-H-A,
D-H H A D- • A deg
O(4)-H(4)'- •O(3y 0.85 2.00 2.825(1) 163
N(1)-H(1). •O(3)" 0.86 2.08 2.906(2) 161
N(2)-H(2). •O(2)'" 0.86 2.00 2.865(1) 179
* Symmetry codes: ' x - 1/2, y - 1/2, z + 1/2; " 1/2 - x, y - 1/2, z; 1 - x, -y, -z.
Synthesis of I. To an aqueous solution (5 ml) containing barbituric acid (12.8 mg, 0.1 mmol) was slowly added a methanol solution (10 ml) of Mn(ClO4)2 • 6H2O (72.2 mg, 0.2 mmol) with constant stirring. And then the pH value of the mixture was adjusted to 8 with a NaOH solution (0.2 mol l-1). After further stirring for about one hour, the resulting light brown mixture was filtered. Pink block-shaped crystals suitable for X-ray structure analysis were obtained by the slow evaporation ofthe filtrate within two weeks (60% yield based on barbituric acid).
For C4H5N2O4Mn05
anal. calcd, %: C, 27.84; H, 2.92; N, 16.23. Found, %: C, 27.98; H, 2.77; N, 16.40.
IR spectrum (v, cm-1): 3395 v(O-H), 1708 vas(CO), 1596 vs(CO).
X-ray diffraction analysis. Diffraction intensities of I were collected on a Bruker APEX-II CCD diffractome-ter equipped with graphite-monochromated MoZa radiation with the wavelength 0.71073 A by using the 9-® scan technique at 296(2) K. There was no evidence of crystal decay during data collection. Semiempirical absorption corrections were applied, and the program SAINT was used for the integration of the diffraction profiles [15]. The structures were solved by a direct method and refined with the full-matrix least-squares technique using the SHELXS-97 and SHELXL-97 programs [16]. Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The crystallographic data are given in Tables 1-3. Hydrogen-bonding parameters are listed in Table 3. Supplementary material has been deposited with the Cambridge Crystallographic Data Centre (no. 763514; deposit@ccdc.cam.ac.uk or http://www. ccdc.cam.ac.uk).
RESULTS AND DISCUSSION
Complex I was generated by the self-assembly of the inorganic Mn(II) salt and barbituric acid in a basic medium. Notably, the basic medium is essentially important for both the tautomerism and the coordination process of the barbituric acid ligand. In the IR spectra, a broad absorption centered around 3395 cm-1 should be ascribed to the stretching vibrations ofO-H, confirming the presence ofa water molecule in I. The strong bands for the carbonyl group were observed at 1708 and 1596 cm-1 in I, which correspond to the carbonyl groups in free barbituric acid (1700 and 1558 cm-1). The obvious red-shift (38 cm-1) for the low frequency of the carbonyl group is indirectly indicative of the keto-enol tautomerism and the disappearance of one of the carbonyl group in I [4].
Complex I exhibiting an infinite one-dimensional (1D) linear chain bridged by pairs of symmetry-related barbiturate anions. As shown in Fig. 1, the sole crystallo-graphically unique Mn2+ ion resides at the inversion center, assuming axially compressed octahedral coordina-
KOOP,3HHAUHOHHAH XHMH3 tom 36 № 12 2010
z
Fig. 1. View of the 1D linear chain of I with the atomic labels around the Mn2+ ion.
tion geometry (Table 2). The equatorial plane is defined by four hydroxyl O atoms from four symmetry-related barbiturate anions with the Mn-O distance of being 2.1786(9) A, and the axial positions are occupied by two aqua ligands with an Mn-O separation of 2.1405(17) A. Barbituric acid in I is in a singly deprotonated anion with the negative charge from C(4) migrating to O(1) upon coordination with the Mn2+ ion, which means that the C(1) carbonyl group became a hydroxyl anion in I. Similar tautomerism of the anion has also been observed in the barbituric acid-involving cocrystals [9-11], as well as its metal complexes [3-8]. Surprisingly, pairs of barbiturate anions present hydroxyl O atoms as a double bridge connecting the adjacent Mn(II) atoms into an infinite 1D linear chain running along the crystallographic z axis (Fig. 1). The nearest Mn-Mn separation within the linear chain is 3.3542(1) A. Notably, in contrast to the previously barbiturate-based Zn(II), Cu(II), Pd(II), Ca(II), and Co(II) complexes [3-8], this is the first example that the barbitura
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