Amine derivatives of alkoxides such as (3-aminopropyl)trimethoxysilane and (3-(2-aminoethyl)aminopropyl)trimethoxysilane are reagents in the sol−gel process and, at the same time, versatile ligands forming
complexes with transition metal ions (e.g., Cu(II)). One of the major advantages of the sol−gel process is
the possibility of preparing Cu(II) complexes in the interior of and on the surface of organically modified
silicate xerogels. This paper compares Cu(II) complexes prepared in the interior of silica xerogels with
those grafted and anchored onto the surface of such xerogels. Grafting and anchoring require prior
preparation of supports with functionalized surfaces. In the former process Cu(II) ions are chemisorbed,
and in the latter Cu(II) complexes are immobilized by a condensation reaction. The following spectroscopic
results are presented: FT-IR, optical absorption in the visible region, and electron paramagnetic resonance
(EPR). The EPR studies are summarized in three identified models of the coordination sites on the modified
silica xerogel surfaces. The materials studied are put forward as catalyst precursors.
Copper (11) complexes with N-2-aminoethyl-3-aminopropyltrimethoxysilane (N-N) as an ethylenediamine analogue were analysed prior to the sol-gel process, during the reaction (N-N + tetramethoxysilane) with water and in the gel products, to
Semiempirical AM1 and ab initio SCF STO-3G calculations with full geometry optimization were performed on aminoazobenzene (AAB) and its N-methyl (MAAB), N,N-dimethyl (DMAAB), and Nphenyl (PhAAB) derivatives, as well as their azonium and ammonium conjugated acids. AM1 calculations were also performed on hydrated cationic acids, in order to estimate the effect of amphiprotic solvents on tautomerization. In all the cases studied but DMAAB the AM1 and STO-3G proton affinity of the azo nitrogen was definitely higher than that of the amino nitrogen. For the amino nitrogen the calculated proton affinity was found to increase in the series AAB < MAAB < PhAAB < DMAAB. The calculated proton affinity of the azo nitrogen increased in the same order with the exception of the STO-3G results of PhAAB. The tautomerization energy/ enthalpy (i.e. difference of the gas-phase proton affinity of the azo and the amino nitrogen) was found to increase in the series DMAAB < MAAB < AAB, the position of PhAAB in the series depending on whether the AM1 or STO-3G method was used. These results contradict the experimental data regarding aminoazobenzene protonation and tautomerization constants determined in various solvents, which indicates a strong effect of solvation on the protonation and tautomerization equilibria of aminoazobenzenes. AM1 calculations on hydrated cationic acids showed that solvation effects can be satisfactorily accounted for by enthalpy contributions in the case of tautomerization, because the order of tautomerization constants determined in methanol and dioxane-water mixture generally conforms with the order of tautomerization enthalpies with hydration included. However, the estimated proton affinities in water are still ranked in the same order as in vacuo which may indicate that entropy contributions play a much greater role in the case of protonation than in the case of tautomerization phenomena.Aminoazobenzenes have three potential centres of protonation: the amino (NY), and two azo (N" and NB) nitrogen atoms (Scheme 1). In our previous papers'.' we investigated, by means of UV-spectroscopy, the proton-exchange phenomena of N-substituted aminoazobenzenes in various solvents, in order to evaluate the relative proton affinity of the possible protonation centres.163, 149.
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