Dianionic silicate derivatives of transition metals: synthesis by a facile ion exchange route, characterization and thermolysis studies

Abstract

A new series of transition metal complexes [M = Cr(III) ( 1), Mn(II) ( 2), Co(III) ( 3), Ni(II) ( 4a–d), and Cu(II) ( 5a–b)] with the dianionic hypervalent, tris(catecholato)silicate, [Si(C 6H 4O 2) 3] 2−, have been synthesized by an ion exchange method under both aqueous and non-aqueous conditions using bis(triethylammonium) tris(catecholato)silicate and a cationic transition metal complex, [ML n ]X [L = ethylenediamine (en), diethylenetriamine (dien), ammonia or water and X = Cl −, SO 4 2− ]. Different ammonium tris(catecholato)silicates [viz. morpholinium, diisopropylammonium and diethylammonium] also serve as appropriate for the ion exchange reaction. Ligands like H 2O and NH 3 afford less stable complexes compared to the ligands en and dien. The powdered form of Ni(II) complexes, [NiL 6][Si(C 6H 4O 2) 3] (L = H 2O, NH 3), readily convert to [Ni(en) 3][Si(C 6H 4O 2) 3] quantitatively when treated with en. The new derivatives have been characterized on the basis of analytical, spectroscopic (IR, UV–Vis, solid state NMR, EPR and MS) and magnetic data. The results of TG and EG analysis of the complexes 1– 5 reveal (i) different decomposition behaviour compared to ammonium catecholatosilicates and (ii) the influence of the bulky tris(catecholato)silicate anion on the thermal stability of these complexes. The bulk pyrolysis study of the above complexes provides a stoichiometric mixture of the corresponding metal oxide and silica except in case of Co(III) which gave the orthosilicate, Co 2SiO 4. Variable temperature magnetic susceptibility measurements indicate a weak antiferromagnetic interaction in case of [Ni(en) 3][Si(C 6H 4O 2) 3] and [Cu(en) 2][Si(C 6H 4O 2) 3].