versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.50 n.1 Concepción mar. 2005
J. Chil. Chem. Soc., 50, N 1 (2005)
Polymeric ligand-metal acetate interactions. Spectroscopic study and semi-empirical calculations
GLORIA V. SEGUEL, BERNABÉ L. RIVAS, CLAUDIA NOVAS
Facultad de Química, Universidad de Concepción, Casilla 160-C, Concepción, Chile E-mail: firstname.lastname@example.org
Semiempirical calculations on the dihydrated Zn (II) acetate and monohydrated Cu(II) acetate structures have been carried out. The optimized structures by the PM3 and AM1 methods were compared with crystallographic data in order to analyze the reliability of the data and its use in the calculations of the structures of the compounds obtained from poly (allylamine) with Zn(II) acetate and Cu(II) acetate.
These compounds were characterized by means of elemental analysis, magnetic moments, FT-IR and electronic spectroscopy in the d-d region of the spectrum.
The PM3 calculations of copper acetate monohydrate and the polychelate do not show differences. The Cu-Cu distance presents a difference of 0.04 A, which is in agreement with the high similarity, the electronic spectra and the magnetic measurements.
Keywords: Metal acetates, semi-empirical calculations, poly (ally lamine)
The metal acetates are very interesting compounds due to their coordination possibility through the carboxylate group. The carboxylate group can show monodentate(I), bidentate(II) or bridge type(III) coordination.
The most widely studied metal acetates are those of Zn(II) dihydrate and Cu(II) monohydrate. The latter is very important due to its important magnetic properties. In [Cu(CH3COO)2·H2 O]2 the observed magnetic moment of 1.4 B.M. is related to the close proximity of the two copper(II) ions (2.64 Å). The precise mechanism of this interaction is not clear; it may involve a direct copper-copper bond or a super-exchange mechanism via the bridging acetate ligands [1,2].
The zinc acetate may coordinate with monodentate ligands, keeping its bidentate coordination as in the complexes with o-phenanthroline  and 2,2'-bipyridine . However, in the complex with rhodamine B  and with imidazole , the acetate groups show monodentate coordination. This is confirmed by IR spectroscopy, where Du of the anti symmetric and symmetric vibration frequencies of the carboxylate groups is 256 cm-1, which is in agreement with this type of coordination. Cu2(CH3COO)4·2H 2O, where the carboxylate groups show a bridge coordination, generally coordinates with a monodentate ligand, replacing the water molecules and keeping the structural unity Cu2(CH3COO)4. The unaltered Cu2(CH3COO)4 unit reacts with nicotamide, 2-aminopyrimidine, pyridine, and urea [7-9], but with some polydentate ligands, one of the acetate groups may change to monodentate coordination, while, the rest of the ligand groups retain the bridge structure. For these compounds stretching vibrations of both groups are observed in the FT IR spectra. The aim of this paper is to report the synthesis of polymer-metal chelates of poly (allylamine) and Zn(II) acetate dihy drate and Cu(II) acetate monohydrate, and to study the influence of the polymeric ligand on the coordination type of the acetate groups.
Semi-empirical calculations of the acetate structures of the Zn(II) dihydrate and Cu(II) monohydrate were carried out. The optimized structures with the PM3 and AM1 methods were compared with the crystallografic data (10,11,12). This was done to analyze the certainty of the data obtained with these methods and then use them to calculate the structures of the compounds obtained from Zn(II) acetate dihydrate and Cu(II) acetate monohydrate with poly (allylamine).EXPERIMENTAL
Materials: The metal salts, Zn(CH3COO)2·2H2 O and Cu2(CH3COO)4·2H 2O, were analytical grade (Merck).
Poly (allylamine) hydrochloride Mw = 60.000, Polysciences.
Polymer-metal chelates: The polymer-metal chelates were prepared by dissolving the polymer in distilled water and mixing it with the metal acetate, keeping a polymer:metal molar ratio of 3:1. The pH was adjusted to 8 with 0.1 M HCl or 0.1 M NaOH, and the mixture was stirred for 5 h, yielding a precipitate which was filtered, washed with water, and dried at 50 C to constant weight.
Measurements: Elemental analyses were made on a Perkin Elmer Series II CHNS 10 analyzer 2400. FT IR spectra of the samples were recorded on a Magna Nicolet 550 and a Nicolet Nexsus spectropho tometers. Diffuse reflectance spectra were measured on a Perkin Elmer Lambda 20 spectrophotometer coupled to a Labsphere diffuse reflectance attachment and Spectralon as reference. Magnetic moments were determined by the Gouy method using [HgCo(SCN)4] as calibration standard according to the relation meff =2.84÷XMT.
Quantum Chemical Calculations: PM3 and AM1 semi-empirical calculations were carried out using a MOPAC program that is included in HyperChem 7. The compounds were created graphically with the support of the HyperChem Program. It was optimized by molecular mechanical and subsequently by PM3 and AM1 calculation methods.RESULTS AND DISCUSSION
Spectroscopy and analyses
Table 1. Analytical data of polychelates of poly(allylamine) (L) with Cu(II) and Zn(II).acetates
The Zn(II) acetate dihydrate coordinates with polyallylamine through two neighbor nitrogen atoms of the polymer chain which replace the water molecules, keeping an octahedral
Fig. 1 : Coordination of polyallylamine with Zn(CH3COO)2The antisymmetric and symmetric stretching vibrations of the carboxylate groups of Zn(CH3COO)2·2H2 O are seen at 1559 and 1447 cm-1 respectively, with Du = 112 cm-1. For the corresponding polychelate it appeared at 1562 and 1407 cm-1, with Du = 155 cm-1. In this compound the Zn-O bonds have a highly ionic character which would explain the large value of Du, corresponding to bidentate coordination. For the reported poly(maleic acid ) with Zn+2, where the carboxylate groups show bidentate coordination, the antisymmetric and symmetric stretching vibrations are seen in the same frequency range .
The N-H stretching vibration modes are seen at 3436 and 3283 cm-1.
The far-IR spectrum shows the loss of water molecules in the spectrum of Zn(CH3COO)2·2H2 O, the antisymmetric stretching vibration (Zn-OH2) is seen at 394 cm-1 , but in the polychelate this band was not seen, but a band at 525 cm-1 is present which can be assigned to the Zn-N stretching vibration. The active vibration frequencies of ZnO4, in Zn(CH3COO)2·2H2 O are seen at 280, 194, and 109 cm-1, and for the polychelate at 284 cm-1, and the absorption band at 194 cm-1 is split into two bands at 181 and 154 cm-1 due to a lower symmetry.
The Cu2(CH3COO)4·2H 2O coordinates with polyallylamine with nitrogen atoms coming from three repeated units, one of which does not participate in the coordination.
Fig. 3: Coordination of poly (allylamine) with Cu2(CH3COO)4
The FTIR spectrum of Cu2(CH3COO)4·2H 2O shows the antisymmetric and symmetric stretching vibrations of the acetate group at 1608 cm-1 and 1440 cm-1, and the polychelate at 1560 cm-1 and 1416 cm-1, with Du = 145 cm-1. For the reported poly(acrylic acid-co-maleic acid ) with Cu+2, where the carboxylate groups show bidentate coordination, the anti symmetric and symmetric stretching vibrations are seen at 1568 cm-1 and 1401 cm-1 .
The electronic spectrum of Cu2(CH3COO)4·2H 2O shows absorption bands at 411, 588, 706, and 892 nm, and the corresponding polychelate at 404, 569, 703, and 892 nm. The high similarity of both spectra would indicate that the Cu2(CH3COO)4 group does not suffer important changes. This is supported by the magnetic measurements, since for copper acetate monohydrate a magnetic moment of 1.4 B.M. is reported , which is similar to the experimental value of 1.58 B.M. for the polychelate.
Semiempirical calculations were carried out for the metal acetates and polychelates. To analyze the values it is necessary to consider that these are made for an isolated unit and that interactions in the solid state are not considered.
Fig. 4: PM3 calculated structure of Zn(CH3COO)2·2H2 O
The PM3 calculations for Zn(CH3COO)2·2H2 O present a distortion for the length of the Zn-O bond which does not occur with the AM1 method. The Nara equation  correlates some structural parameters with Du (difference between the antisymmetric and symmetric stretching vibrations of the carboxylate groups).
Du = 1818.1dr + 16.47(qOCO - 120) + 66.8
where dr is the difference between the two CO bond lengths (Å) and qOCO is the OCO angle (°).
To apply this equation for the calculation of Du using the AM1 data, a negative value was obtained, however with the PM3 data a value of 127 cm-1 was obtained, which is close to the experimental value. In agreement with this result, the PM3 method was used for the calculations of the polychelate.
The structural calculations were carried out for the proposed structures according to the experimental data.
The structural data of Zn(II) acetate dihydrate and the polychelate do not present important differences. For the charge distribution, differences were found only for the Zn atom, but not in the carboxylate groups.
Fig. 5: PM3 calculated structure of Cu2(CH3COO)4·2H 2O
The structural calculations for copper compounds were carried out only by the PM3 method, as copper is not parameterized for the AM1 method.
These calculations also do not show significant differences with those reported by the data of the crystalline structure, except for the size of the angle of the carboxylate group and the Cu-Cu distance, which is due to the parameterization of copper.
The PM3 calculations of copper acetate monohydrate and the polychelate do not show differences. The Cu-Cu distance hass a difference of 0.04 Å, which is in close agreement with the electronic spectra and magnetic measurements. In this polychelate, charge distribution differences were seen on the metal ion and carboxylate groups.
The polymeric ligands showed no important effects on the metal acetates. This can be attributed to two factors: that the coordinated nitrogen atoms are less electronegative than the oxygen atoms, and the flexibility of the polymer ligand.
The semiempirical calculations allowed a structure to be postulated for these compounds, which are not crystalline, and because of the large size, ab initio calculations can not be carried out.ACKNOWLEDGEMENTS
The authors thank Universidad de Concepción (Grant No 202.021.016-0) for financial support.
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