Chapter 12
Sol—Gel Synthesis of Heterometallic Oxopolymers F. Babonneau, S. Diré, L. Bonhomme-Coury, and J. Livage
Downloaded by FUDAN UNIV on January 15, 2017 | http://pubs.acs.org Publication Date: November 18, 1994 | doi: 10.1021/bk-1994-0572.ch012
Chimie de la Matière Condenseé, Université Peirre et Marie Curie, 4 place Jussieu, 75252 Paris, France
Heterometallic oxo-alkoxides provide convenient "building blocks" for the sol-gel synthesis of multicomponent ceramics. Heterometallic M-O -M' oxo bonds can be obtained when two alkoxides are mixed in solution. They are formed via Lewis acid-base reactions and are favored by a large electronegativity difference between both metal atoms and their tendency to increase their coordination number. Silicon has a rather high electronegativity and does not show any tendency toward coordination expansion. It is therefore difficult to prepare homogeneous multicomponent gels in silicate systems because of the large difference in hydrolysis and condensation rates between silicon alkoxide and other alkoxides. Several approaches are described to overcome this problem, including partial prehydrolysis of silicon alkoxides and matching of hydrolysis rates by chemical modification of other alkoxides with chelating ligands.
Sol-gel chemistry is based on the hydrolysis and condensation of metal alkoxides M(OR) . These reactions can be described as nucleophilic substitutions or additions. The reactivity of metal alkoxides toward hydrolysis therefore increases as the electronegativity of the metal atom decreases and its size increases. Silicon alkoxides are not very sensitive toward hydrolysis and condensation. Reaction rates have to be increased via acid or base catalysis. Most other alkoxides are highly sensitive to z
0097-6156/94/0572-0134S08.00/0 © 1994 American Chemical Society Wisian-Neilson et al.; Inorganic and Organometallic Polymers II ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
Downloaded by FUDAN UNIV on January 15, 2017 | http://pubs.acs.org Publication Date: November 18, 1994 | doi: 10.1021/bk-1994-0572.ch012
12.
BABONNEAU ET AL.
Sol-Gel Synthesis of Heterometallic Oxopolymers
moisture and their reactivity has to be tailored by chemical modification with complexing hydroxylated species ΧΟΗ. These reagents decrease the effective functionality of alkoxide precursors leading to less condensed species. The formation of oxopolymers can then be chemically controlled with two parameters; the hydrolysis ratio h=H20/M and the amount of complexing ligand x=X/M (2). For low hydrolysis ratio (h
