Amino-Modified Silicate Xerogels Complexed with Cu(II) as Catalyst Precursors. Coordination State and Thermal Decomposition

The sol-gel process is a useful method for preparing two series of organically and co-ordinately modified xerogels of the types [CuN _n ]·N _5–n ·5xSiO_4/2 (n < 4) and [Cu(N–N)_n]·(N–N)_2–n ·2x SiO_4/2(n 2), where N = NH₂(CH₂)₃ SiO_3/2, N–N = NH₂(CH₂)₂NH·(CH₂)₃SiO_3/2 and x = [SiO_4/2]/[N] or [SiO_4/2]/[N–N]. The amino groups in the materials are coordinately active and participate partly in the coordination sphere of Cu(II) ions. The composition of the coordination sphere can be varied with the SiO_4/2 content and also as a result of the thermal decomposition of the organic residues at higher temperatures.Because the xerogel materials are considered to be catalyst precursors, this study is focused on their coordination and thermal properties. The prepared xerogels, such as silica, aminated silicates with N and N–N, as well as those entities complexed with Cu(II), were characterised by FT-IR spectroscopy. During gelation and thermal decomposition the materials were analysed by electron paramagnetic resonance (EPR) spectroscopy. The xerogels were additionally studied by UV-Vis absorption spectroscopy. The gaseous products of the thermal decomposition of these materials in an Ar atmosphere were investigated by the use of FT-IR spectroscopy coupled with TG and DTG thermal analysis. These data were complemented by temperature-programmed decomposition (TPDec) in a 2% O₂ + 98% Ar stream coupled with quadrupole mass spectroscopy.     

Influence of inorganic anions on the tautomeric equilibria of N-substituted aminoazobenzenes — The evidence for ionic association

The influence of inorganic anions: perchlorate, chloride, sulphate, and hydrogen sulphate on the acid-base and tautomeric equilibria of 4-aminoazobenzene (AAB) and its N-ethyl (EAAB), N,N-diethyl (DEAAB), and N-phenyl (PhAAB) derivatives was investigated by UV-spectrophotometry in 80%:20% vol. ethanol:water system. The possible equilibrium models were evaluated by means of the nonlinear confluence analysis program STOICHIO in order to explain the observed dependence of the electronic spectra on acid and salt concentration. It was found that all the anions considered form 1:1 associates with both ammonium and azonium tautomers of the protonated aminoazobenzenes. The corresponding association constants were generally about one order of magnitude greater for SO ₄ ^2– than for the univalent ions. The azonium species were found to form more stable associates in all the cases except SO ₄ ^2– , which explains the observed shift of the tautomeric equilibrium towards the azonium form on increasing the mineral acid or salt concentration. Based on the obtained values of the molar absorption coefficients all but the PhAAB associates were found to be of the ionic pair type, with proton transfer taking place only in the last case. This was due to the greater basicity SO ₄ ^2– , compared with PhAAB. The association constant values obtained are discussed in terms of the tendency of the anions studied to associate, as well as in terms of the effect of solvation on the stability of the ionic pairs formed.