Physico-chemical investigation of new μ3-oxo-hetero (s, d, f)-nuclear iron complexes with anions of di- and trichloroacetic acids
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TURTA, Constantin, FILOTI, George, MERIAKRE, V., PRODIUS, Denis, KUNCSER, Victor Eugen, BOBKOVA, Svetlana. Physico-chemical investigation of new μ3-oxo-hetero (s, d, f)-nuclear iron complexes with anions of di- and trichloroacetic acids. In: Physical Methods in Coordination and Supramolecular Chemistry, 27 septembrie - 1 octombrie 2006, Chişinău. Chisinau, Republic of Moldova: 2006, XVII, pp. 49-51. ISBN 978-9975-62-066-6.
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Physical Methods in Coordination and Supramolecular Chemistry
XVII, 2006
Conferința "The XV-th International Conference Physical Methods in Coordination and Supramolecular Chemistry : The XVII-th Reading in memory of Acad. A.Ablov"
Chişinău, Moldova, 27 septembrie - 1 octombrie 2006

Physico-chemical investigation of new μ3-oxo-hetero (s, d, f)-nuclear iron complexes with anions of di- and trichloroacetic acids


Pag. 49-51

Turta Constantin1, Filoti George2, Meriakre V.1, Prodius Denis1, Kuncser Victor Eugen2, Bobkova Svetlana3
 
1 Institute of Chemistry,
2 National Institute of Materials Physics Bucharest-Magurele,
3 ”Nicolae Testemițanu” State University of Medicine and Pharmacy
 
Disponibil în IBN: 4 iunie 2020


Rezumat

The universality of carboxylic acids as ligands and the extraordinary wide binding facilities of their acid residues favor the existence of a great variety of carboxylate-based complexes. This class of compounds is active with respect to alkane activation, the catalytic activity depending on the specific carboxylate and metal identities. A number of heterotrinuclear iron(III) carboxylates with the general formula [Fe2MO(RCOO)6(L)3]·nSolv is known for a variety of bivalent 3d metals (M), carboxylate ligands (R-CO2), monodentate ligands (L), and solvate molecules (Solv). Polynuclear carboxylates of 3d transition metals are of a renewed attracting interest because of their intramolecular magnetic exchange interactions including Single-Molecule-Magnets (SMM) and their application as the simple models of oligonuclear active sites in metalloproteins. Our previous investigations on iron coordination compounds with trifluoro- and trichloroacetate as a ligand revealed that the nature of carboxylate renders essential influence on some dynamic processes (for example, electron de-localization) in trinuclear mixed-valence clusters. From another point of view the remarkable results derived from the study of these compounds in solid state (single crystal X-ray crystallography) has opened new opportunities for deeper research. Finally, halogenoacetates of some d-metals have also demonstrated interesting biological activity properties. The unambiguous assignation of the individual site to one or another metal (M) in these trimer complexes is hindered by at least two factors: i) the close atomic numbers of the Fe and M metals and ii) the statistical distribution of orientation of the clusters’ apex in the crystal structure. For instance: in the compound [Fe2MgO(CCl3COO)6(Py)3]·CH3C6H5 one iron atom occupies a special position on the mirror plane, while another iron and magnesium atoms of the Fe2MgO core are located in the known general position with 50% probability. This arrangement occurs in spite of different mode of interaction of the co-ordination environment with d (Fe) and s (Mg) metals. The negligible difference between the ionic radii of Fe(III) and Mg(II), being respectively, 0.65 and 0.72 Å does not permit to determine unambigously the position of each atom, and the complex conserves practically equilateral triangle of the three metal atoms. The positions of paramagnetic (Fe) and diamagnetic s–metal (Ca, Sr, Ba) anions in other three new μ3-oxo heterotrinuclear iron carboxylates of the composition [Fe2M(Сa, Sr, Ba)O(RCOO)6(L)n], where R= CCl3, L = tetrahydrofuran (THF), have been assigned unambiguously (Fig 1).figureFigure 1. Crystal structures of [Fe2M(μ3-O)(CCl3COO)6(THF)n], where M=Mg(II), n=3 (left); M=Ca(II), n=4 (centre); M=Ba(II), n=6 (right).Due to the larger difference in radii and coordination number for other s-metals:7 for Ca and Sr, 9 for Ba, it was possible to refine the structure, which is also a result of their specific electronic shell population. The temperature dependence of magnetic susceptibility for these complexes was described using the HDVV model. Between paramagnetic ions of the triangles the antiferromagnetic interaction is present. The values of the exchange interaction between iron(III) ions are: -58.9 ± 0.5 cm-1 (for Ca), -75.4 ± 0.8 cm-1 (for Sr), -60.4 ± 0.4 cm-1 (for Ba). These values are higher than analogous parameters in homo- iron carboxylates, but are practically the same for heteronuclear acetates with {Fe2MO} cores, where M - 3d-element. Another possibility to distinguish the positions of d- and s- elements in μ3- oxo carboxylates is the difference of the affinity of these metals to monodentate ligand in comparison with iron ions. If monodentate ligands, coordinated to each M ion in two trinuclear clusters, form mutual hydrogen or other type of intermolecular interactions, the nominal hexanuclear unit is present in the crystal. In this case the statistical distribution of triangle apex orientation disappears and it is possible to localize the position of M. The same situation was depicted for the clusters of the compositions [Fe2MnO(μ3-OOCCHCl2)6(THF)2(H2O)]2 (Fig.2) and [Fe4Ca2O2(μ2-HCCl2COO)10(μ3-HCCl2COO)2(THF)6], where water molecules for Mn-complex and two tridentate dichloracetate anions for Ca–complex ions fasten together two triangular complexes.figureFigure 2. Association of two H-bonded μ3-oxo-trinuclear complexes in {Fe4M2O2} supramolecular aggregate (left); projection of the crystal structure with {Fe2MnO} core (centre); magnetic properties of {Fe4Ca2O2} hexanuclear cluster (right). Magnetochemical studies of {Fe4Ca2O2} complex (Fig.2, right) reveal the presence of antiferromagnetic exchange interaction in the two isosceles triangular skeletons. Using the HDVV approximation, the better fit of experimental and theoretical magnetic susceptibility data was found for the value of JFe-Fe = - 65.8(5) cm-1, θCurie-Weiss = 2.46 K. The mentioned complexes where utilized as initial structural blocks for building-up new other polynuclear constructions. For the first time, in agreement with the Scheme 1 it was realized the synthesis of hetero tetra- {Fe3LnO2}- and dodeca{Fe2Mn10O12}nuclear clusters.schemeScheme1All tetranuclear complexes present butterfly structure with FeO6 coordination core, with the oxygen related to different molecules or ions, as follows: Fe1Oc(Ocarb)3OaqOTHF; Fe2Oc (Ocarb)4OTHF; Fe3Oc 2(Ocarb)4. The Mossbauer spectra (MS) of clusters with {Fe3LnO2} core were measured at 300, 80 and 15 K. The RT 57Fe MS at RT of all complexes show two asymmetrical central absorption lines (Fig.3,left,top). According to the molecular structure it is normal to expect three un-equivalent positions in the MS in that butterfly structure. But a good fit of the experimental data was obtained by the superposition of only two doublets with hyperfine parameters characteristic for Fe(III) positions in a high spin state (S=5/2): T = 300 K: Doublet1 (~66 %) δFe = 0.31 - 0.40; ΔEq = 1.13 - 1.51 mm/s; Doublet 2 (~33 %) δFe = 0.13 - 0.38; ΔEq = 1.13 - 1.51 mm/s; T = 15 K: Doublet 1 (~66 %) δFe = 0.48 - 0.49; ΔEq = 1.51.- 1.72 mm/s; Doublet 2 (~33 %) δFe = 0.42 - 0.52; ΔEq = 0.90 1.07 mm/s. At 80 K and 15 K (for Eu – complex) and at 15 K (for La -, and Y – complex) the Mossbauer spectra are characterized by the superposition of a central paramagnetic doublet 1 and a broad Zeeman split magnetic pattern, simulated via a hyperfine magnetic field distribution. The relative areas of the two Mossbauer components are in the ratio ~ 2:1. The assignment of doublets and electronic environment to positions Fe1, Fe2 and Fe3 is discussed. The nature of lanthanide ion in the complexes has an insignificant effect on the electronic state of the Fe(III) (doublet 1) and more influence on magnetic ordering of Fe(III) (doublet 2). At room temperature the χMT value of “Fe3Eu” cluster (5.8 cm3 K mol-1) is much lower than expected for three uncoupled Fe(III) and one Eu(III) (3*4.375+1*1.575 =14.7 cm3 K mol-1) then suggesting the existence of strong intramolecular antiferromagnetic interactions. The lowering of χMT observed between 300 and 20 K may then be attributed to two concomitant effects, i.e. the progressive depopulation of the paramagnetic excited 7F1 state of Eu(III) and the exchange coupling between the three Fe(III) centers. Given the topology of the system, antiferromagnetic interactions in the latter sub-unit will result in a non-zero spin ground state, explaining the quite high χMT value observed at low temperature. For the Eu complex the M vs H curve recorded at 1.4 K clearly indicates that the ground state is an S=5/2, the magnetization curve saturating at 5.00 μB. In this framework, the increase in χMT observed below 10 K should be attributed to intermolecular ferromagnetic interactions. The magnetic and MS of dodecanuclear complex with {Mn10Fe2O12} core is analysed too.figureFigure 3. MS of: {Fe3EuO2} (left) and some {Fe3LnO2} complexes at 40, and 4.2 K(right) 4.2K.(right).