New compounds in the chemistry of Group 4 transition-metal alkoxides. Part 4. Synthesis and molecular structures of two polymorphs of [Ti16O16(OEt)32] and refinement of the structure of [Ti7O4(OEt)20]

Two polymorphs of the condensed species [Ti16(µ4-O)4(µ3-O)8(µ-O)4(µ-OEt)16(OEt)16] have been obtained: a monoclinic form prepared in absolute ethanol and a tetragonal one obtained from 5% aqueous ethanol. The monoclinic form (compound 1) crystallizes in the space group C2/c with a= 23.392(7), b= 19.627(8), c= 24.130(14)Å and β= 96.4(4)°. The tetragonal variety (compound 2) crystallizes in the space group P42/n with a= 14.642(2) and c= 25.972(7)Å. Both structures have been solved by direct methods and refined to R= 0.078 for 5766 independent reflections (compound 1) and to R= 0.053 for 3130 independent reflections (compound 2). They are built from different packings of the same hexadecanuclear molecule. Both molecules show slight differences in the titanium–oxygen network and in the orientations of the ethoxy groups. The initial hydrolysis product of tetraethyltitanium(IV), [Ti7O4(OEt)20]3 has been obtained by a new route. Its structure has been solved by direct methods and refined to R= 0.069 for 6293 independent reflections. This redetermination resulted in location of all the ethyl groups.