Transition metal complexes in aminated silicate xerogels with Ni and Co ions

Two series (N and N-N) of aminated silicate xerogels complexed with Ni2+ an d Co2+ ions have been synthesized by a two-step reaction procedure: coordin ation and sol-gel process. The N series consists of unidentate ligands H2N( CH2)(3)SiO3/2 and the N-N series of bidentate ligands H2N(CH2)(2)NH(CH2)(3) SiO3/2. The coordination environments of the transition metal ions in the x erogels were analysed by UV-Vis absorption, luminescence, IR absorption and EPR spectroscopies. Moreover. the spectral data are complemented with ther moanalytical methods such as differential thermal analysis (DTA), different ial thermogravimetry (DTG) and thermogravimetry (TG). The UV-Vis absorption and EPR spectra show that Ni and Co ions are in the +2 valence state in bo th xerogel series studied. For the Ni2+-and Co2+-doped xerogels the positio ns of the vibration bands related to the potential ligand groups and molecu les (OH and H2O) are shifted according to the Irving-Williams order. In the case of Co2+, the tendency to transit from a tetrahedral to an octahedral environment increases with water and/or OH group content in the xerogels. W hen the temperature is increased, amino groups in the coordination sphere d issociate first and only then do amino groups not involved in coordination dissociate. The modified silicate xerogels can be considered as catalyst pr ecursors. (C) 1999 Elsevier Science B.V. All rights reserved.