Cyanoethylaminopropyl Silica As Initial Material For Construction Of Bonded Supramolecular Structures
CYANOETHYLAMINOPROPYL SILICA AS INITIAL MATERIAL FOR CONSTRUCTION OF BONDED SUPRAMOLECULAR STRUCTURES
V. D. Oleynic, V. N. Zaitsev
Kiev National T. Shevchenko University 64 Vladimirskaya str. Kiev Ukraine
It is known that properties of chemically modified silicas depend not only from a nature of the immobilized groups, but also from their concentration and character of distribution on surface (topography). Immobilization of silanes with aliphatic amines as functional groups, results in formation of the immobilized layer with cluster topography, irrespective of concentration of modifying agent and nature of a carrier. The reason for that can be stronger extramolecular interaction between adsorbing molecules than interaction between adsorbate with adsorbent. If association of silanes before immobilization could be avoided or diminished, it would be possible to change the immobilization scheme and to affect topography on surface groups. For achieving this purpose, it was proposed to immobilize silane, which in addition to amino-group contains electron acceptor cyanoethyl fragment. This fragment will lower donor activity. Besides the introduction of cyanoethyl fragment into a ligand molecule create the backgrounds for forming of supramolecular structure since the specified fragment can act as a bridging ligand only. The analysis of topography of the immobilized groups was studied through properties of chemically-modified silicas (CMS), namely through their protolytic and complexing properties. With the purpose of comparison, some CMS synthesized were differed by concentration of the immobilized groups, nature of a silica matrix (Aerosil (A), silica gel (Sg)). A complexing properties of immobilized N-cyanoethylaminopropyl silica (SiO2-NHCN) was studied with Cu(BF4)2·6H2O and with PdCl2 in non-aqueous bipolar solvents – dimethylformamide (DMF) and acetonitrile.
The adsorption isotherms of palladium chloride on A-NHСN and CГ-NHСN from a DMF are similar in limits of an experimental error. It testifies that the porosity of a silica matrix does not influence on their complexing
properties of the learned CMS. In exes of metal salt in solution, each two moles of the immobilized ligand react with one mole of metal. It confirms formation of palladium immobilized complex with composition PdL2 (where L – is functional group of the immobilized silane) on a surface of a CMS. The results obtained from the adsorption capacity of a CMS, were proved by the calculation of hetero-phase equilibrium in the reaction of palladium chloride and A-NHСN in DMF. In named system one complex with composition ML2 and stability constant lgβ2=7,29 was only revealed.
Composition of internal coordination sphere of the immobilized palladium complexes were studied from IR and diffuse reflection electron spectroscopy. From IR spectrum, the invariance of stretching vibrations n(СºN) at 2250 cm-1 were observed for AC-NHСN and their immobilized complexes with Pd. It means that nitrile fragment of immobilized ligand is not coordinate to metal. In electronic spectrum of palladium complexes on A-NHСN surface, two bands with maximums at 25000 cm-1 and 30000 cm-1 are observed. The peak at 25000 cm-1 should be assign to d-d transition in chromophore PdN2Cl2. With the purpose to increase a correctness of UV/VIS spectra interpretation and to control possible formation of other Pd bonded complexes, namely PdL3Cl, replacement of chloride ions in internal sphere of immobilized complexes with bromide and iodine ions were performed according to reaction: LnPdCl4-n + NaX = LnPdX4-n + NaCl, where Х - halide ions. In UV/VIS spectra of immobilized palladium complexes that were obtained in DMF and treated with sodium bromide and iodide, the relevant shift of absorption maximum at 25000 cm-1 toward low energy is observed. This is a result of replacement chloride ligand in PdL2Cl2 on bromide and iodide. Thus, irrespective to silica matrix type and concentration of immobilized groups, interaction of PdCl2 with SiO2-NHСN in DMF is took place with formation of single bonded complex with PdL2Cl2 composition. In this complex, immobilized cyanoethylaminopropyl ligand reacts with palladium through amine fragment only, reveals itself as mono biting ligand. Two atoms of chlorine enhance the coordination number of palladium up to four.
The adsorption of palladium ions on the Sg-NHСN from acetonitrile is much more intensive than from DMF. Capacity of CMS to palladium from CH3CN is as twice as from DMF. Absence of the band at 22000 см-1 (d-d transition in chromophore [PdCl4]2-) in UV/VIS spectrum, indicates absence of immobilized complex with composition [PdL2][PdCl4]. Therefore, increased capacity of Sg-NHСN to palladium ions in acetonitrile we assign to formation of surface complexes with composition [PdLCl2]. As it was pointed above, double-biting chelating coordination is impossible for N-cyanoethylaminopropyl ligands. Alternatively, next scheme was proposed where surface complex is formed from one bonded ligand and two chloride ions, where CN- functional group of ligand acts as a bridge, forming supramolecular structure on surface, see scheme.
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