László Korecz

Nickel(II)-, copper(II)- and zinc(II)-complexes of some substituted imidazole ligands

Nickel(II)-, copper(II)- and zinc(II)-complexes of four imidazole derivatives (4(5)-aminoimidazole-5(4)-carboxamide (aic), 2,2′-biimidazole (biim), bis(1,1′-imidazol-2-yl)(4-imidazol-4(5)-yl)-2-aza-butane (biib) and imidazole-4-acetic acid (iaa)), having different coordination environments around the imidazole ring(s), have been studied by potentiometric, UV–VIS and EPR spectroscopic methods. The data revealed very strong bidentate coordination of biim, in spite of the low basicity of the donor sites. Three dominant species (CuLH, Cu2L2, Cu2LH−1) are formed in the Cu(II)-biib system. Since the biib offers extremely stable tri- and tetradentate coordination to copper(II) (in CuLH and Cu2L2, respectively), the bis-imidazolyl like coordination, which is predominant in metal complexes of any other bis-imidazolyl like ligand, does not appear in its original form in the copper(II)-biib system. The EPR spectra of above dimer species show coupling between the copper(II) centers. In the M(II)-iaa systems only parent complexes are formed up to pH 9, while at higher pH further deprotonations were observed, which resulted in a formation of dimer complexes in case of copper(II), having antiferromagnetically coupled metal centers.

Magnetic Fullerenes inside Single-Wall Carbon Nanotubes

C(59)N magnetic fullerenes were formed inside single-wall carbon nanotubes by vacuum annealing functionalized C(59)N molecules encapsulated inside the tubes. A hindered, anisotropic rotation of C(59)N was deduced from the temperature dependence of the electron spin resonance spectra near room temperature. Shortening of the spin-lattice relaxation time T(1) of C(59)N indicates a reversible charge transfer toward the host nanotubes above approximately 350 K. Bound C(59)N-C(60) heterodimers are formed at lower temperatures when C(60) is coencapsulated with the functionalized C(59)N. In the 10-300 K range, T(1) of the heterodimer shows a relaxation dominated by the conduction electrons on the nanotubes.

Catalytic oxidation of 2-aminophenol to questiomycin a by dioxygen in the presence of cobaloxime derivatives. Free radical intermediates

The multistep oxidative dehydrogenation of 2-aminophenol to Questiomycin A, catalyzed by cobaloxime(II) derivatives, involves ESR-detectable 2-aminophenoxyl type free radical intermediates.

Isomorphously substituted Fe-ZSM-5 zeolites as catalysts: Causes of catalyst ageing as revealed by X-band EPR, Mössbauer and 29Si MAS NMR spectra

Abstract An unconstrained curve fitting/parameter estimation program adapted to PC was applied for the deconvolution of X-band EPR spectra of Fe(III) in ZSM-5 (MFI) zeolites. The sub-spectra of framework (FW) and extra-framework (EFW) Fe(III) ions sited in environments of different (ligand) symmetry could be identified. The EPR transition probability, r , of Fe(III) in the little oxide clusters (diads, triads) was 13.75 times larger than that incorporated into the lattice. Increase of the cluster size and ordering of the random structure led to reduction of r . When the co-ordination of Fe(III) ions in the oxide clusters approached the cubic symmetry of Fe(III) in the lattice, r converged to 1.0. The state: r =1.0 is equivalent to the development of a crystalline phase. The experimental determination of r revealed important details of catalyst action. The initial activity in the direct oxidation of benzene to phenol (oxidant: N 2 O) is due to oxide clusters of random structure (known as “ferrihydrite”). Partial and eventually (almost) complete loss of activity occurs when the clusters get ordered to hematite (or in reducing atmosphere to magnetite, as well). Contrary to fully inactive hematite, magnetite retains about 50% of the original activity because it exposes [Fe(III)] Th –O–[FeIII)] Oh linkages supposed to be responsible for oxidation activity.

Attempts to produce uniform Fe(III) siting in various Fe-content ZSM-5 zeolites: Determination of framework/extra-framework ratio of Fe(III) in zeolites by EPR and Mössbauer spectroscopy

Abstract Fe(III)-content ZSM-5 catalysts play important role in selective redox reactions, like biomimetic oxidations, or selective catalytic reactions (SCR) of pollutants, dangerous for the environment. There is a dispute in the literature whether the catalytic activity of these materials is due to isomorphously substituting framework (FW) iron, or extra-framework (EFW) oxide/hydroxide material of high dispersity, not incorporated into the lattice during the synthesis, or became ejected from the framework following the post-synthesis treatments (calcination, heat-treatment). In either case it is a must to know the FW/EFW ratio in order to correlate the catalytic activity with the number of active sites, or active surface area of the catalyst ingredient. In the case of EFW iron only (introduced, e.g. by ion-exchange) the chemical identity, particle size, location and thereby the probability of attainment from the fluid phase(s) are uncertain. Isomorphic substitution is more complicated, because in addition to these even the FW/EFW ratio is questionable. From both practical and academic interest it would be advantageous (e.g. from the point of view of above correlation) to have uniform FW, or EFW Fe(III) siting, or when it is mixed, to know the FW/EFW ratio. The authors approach this problem from several directions: they elaborate various synthesis methods to minimise EFW iron; extract unwanted EFW iron by reducing/complexing agents (SO 2 , hydroxylamine), or, if necessary, reintroduce EFW iron by (liquid and solid) ion-exchange and monitor the changes thus produced in the FW and EFW iron contents by X-band EPR and Mossbauer spectroscopy. Using mathematical methods and sophisticated analysis of the sub-spectra (intensity dependence on the iron content and temperature), the EPR spectrum-deconvolution made possible to reinterpret the X-band EPR Fe(III) spectra in weak ligand-field media, like zeolites. If the total iron contents of samples are known (e.g. from XRF spectra) the FW/EFW ratio can be computed on the basis of the deconvoluted EPR spectra alone. These values are in very good agreement with similar estimates obtained from the Mossbauer spectra, proving that the Mossbauer “loudness” for the various iron components is nearly equal.

An electron spin resonance study of coordination modes in the copper(II)-histamine and copper(II)-L-histidine systems in fluid aqueous solution

Abstract Copper(II)–histamine and copper(II)– l -histidine equilibrium systems were studied in fluid aqueous solution by ESR spectroscopy. Eighty-seven spectra taken in a circulating system at various ligand-to-metal concentration ratios and pH were analysed. The experimental curves were decomposed to one to four component spectra which were built up from the hyperfine lines of 63Cu and 65Cu, and a maximum of four non-equivalent 14N nuclei. The isotropic ESR parameters (g-factors, hyperfine coupling constants and relaxation parameters) and the relative concentrations of the different species were optimized. New, pH-potentiometrically non-identified species were also considered in the equilibrium models. In the copper(II)–histamine system the complex [CuLH−2] was added to the species [CuLH]3+, [CuL]2+, [CuLH−1]+, [Cu2L2H−2], [CuL2H]3+ and [CuL2]2+. In the copper(II)– l -histidine system, in addition to the complexes [CuLH]2+, [CuL]+, [CuLH−1], [Cu2L2H−2], [CuL2H2]2+, [CuL2H]+ and [CuL2], the new species [CuLH2]3+ and [CuLH−2]− were found. The relative concentrations obtained from the ESR spectra are in good accordance with the concentrations calculated from the literature pH-potentiometric formation constants. The two ligands in their ‘LH’ states are bound differently: the histamine by the imidazole, and the l -histidine through the amino and the carboxylate groups in equatorial positions (complexes [CuLH], [CuL2H] and [CuL2H2]). For the [CuL], [CuL2H] and [CuL2] complexes of both histamine and l -histidine, the first ‘L’ ligand is coordinated equatorially by the amino and imidazole nitrogens. The second ‘L’ ligand in the [CuL2] complexes is either bound in the former way, or its imidazole group occupies an axial site. The carboxylate group of l -histidine is coordinated to the metal ion in each complex, in either an equatorial or an axial position. The deprotonation of the [CuL] complex takes place from the imidazole ring, which is followed by the proton loss of the equatorial water molecule in the highly alkaline region.

Synthesis, Characterization and Catalytic Activity of V-Zsm-5 Zeolites

Abstract A V-ZSM-5 sample with a Si/V = 42 was synthetized outgoing from VO(COO)2 and Q-brand sodium silicate using TPA-Br as template. ESR spectroscopy proved that vanadium(IV) ions in the zeolitic framework exhibit a distorted square planar symmetry. Upon heat treatment a part of the framework vanadium ions migrate to extra-framework positions. After dehydration no Bronsted acidity was found. Treatment in oxygen and hydrogen above 570 K revealed the redox character of the V-ZSM-5 sample. In oxidation of n-butane (as catalytic test reaction) the V-ZSM-5 zeolite exhibits selective dehydrogenation and aromatization activity.

Superoxide dismutase activity of a Cu–Zn complex—bare and immobilised

A binuclear, imidazolato-bridged complex (Cu(II)-diethylenetriamino-μ-imidazolato-Zn(II)-tris(aminoethyl)amine perchlorate) was prepared and immobilised on silica gel or among the layers of montmorillonite. Immobilisation occurred via hydrogen bonding for the silica gel and through electrostatic forces for the montmorillonite. The obtained substances were characterised by EPR and FT-IR spectroscopies and their thermal behaviour was studied by thermogravimetry. The superoxide dismutase (SOD) activity of the complex before and after immobilisation was studied by a SOD assay. It was found that the SOD activity of the host-free complex increased by more than an order of magnitude and approached the efficiency of the real enzyme when silica gel was used as host. The enhanced activity could be assigned to the formation of magnetically isolated centers in the silica pores. On the other hand, the immobilisation with montmorillonite slightly reduced the SOD activity.

Attempts to produce uniform Fe(III) siting in Fe content SOD and LTA zeolites: An EPR and Mössbauer study

The authors’ studies dealing with Fe(III) content sodalites (SOD) and NaA (LTA) zeolites [Appl. Catal. A 223 (2002) 147] revealed a lesser degree of framework (FW) aluminium substitution by Fe(III) than anticipated on the basis of previous publications [Solid State Ionics 53–56 (1992) 1282]. The fairly high stability of SOD structure “withstood” the attack by diluted mineral acids to some degree in that amounts in about 30–40% excess compared to the ion-exchange capacity of the zeolite did not result in complete dissolution, but in amorphisation only, while the solid habit was retained. Fe content LTA samples exhibited lower tolerance against mineral acids. The acid treatment of SOD’s (or industrial by-products with high SOD content) combined with addition of reducing and/or complexing agents (like sulphur dioxide, hydroxylamine, EDTA, “phen”, “dipy”, etc.) permits extraction of extra-framework (EFW), or EFW and FW iron, if necessary. The recrystallisation of fully deferrated, amorphised material makes the synthesis of various zeolites possible with acceptable white colour. X-band EPR and Mossbauer spectra of SOD’s and LTA samples and those enriched in 57Fe tracer, revealed uniform Fe(III) siting in the (partially) deferrated and recrystallised samples, although at low Fe(III) levels. It is an interesting observation that this uniform siting is dominantly FW in SOD’s and EFW in LTA’s with unexpected low symmetry, unfilled co-ordinations. Besides that, these investigations may contribute to the understanding of Fe(III) ion substitution in three-dimensional tectosilicate frameworks, the LTA preparations can be attractive as selective oxidation catalysts for reactants not excluded from the structure by sieving effects.

Metastability in EuBa2(Cu1-xSnx)3O7-y studied by 119Sn and 151Eu Mössbauer spectroscopy.

The metallic oxide compound system EuBa/sub 2/(Cu/sub 1-//sub x/Sn/sub x/)/sub 3/O/sub 7-y/ (x = 0.05, 0.10, 0.20) was prepared for investigation with /sup 151/Eu and /sup 119/Sn Moessbauer spectroscopy. The best fits of the /sup 119/Sn Moessbauer lines reveal at least two different Sn/sup IV/ sites. The /sup 151/Eu spectra are characteristic for Eu/sup III/ state in these compounds. We have found a considerable difference between the shapes of /sup 119/Sn Moessbauer spectra recorded at room temperature and those recorded at the temperature of liquid nitrogen in the case of the sample having the highest tin content. Nevertheless, there is no evidence that this difference has a direct connection with the superconducting behavior.