Electrochemical investigations on the sol–gel polymerization of transition-metal alkoxides H. Cattey,a P. Audebert,*a C. Sanchezb and P. Hapiotc aL aboratoire de Chimie et Electrochimie Mole ´culaire, Universite ´ de Franche Comte ´, L a Bouloie, Route de Gray, 25030, Besanc ¸on, France bL aboratoire de Chimie de la Matie `re Condense ´e, URA CNRS no. 1466, Universite ´ Pierre et Marie Curie, 2 Place Jussieu, 75005, Paris, France cL aboratoire d’Electrochimie Mole ´culaire, URA CNRS no. 438, Universite ´ Denis Diderot, 2 Place Jussieu, 75005, Paris, France The kinetics of polymerization of zirconium propoxide has been studied under various conditions using an original electrochemical method. The alkoxides were complexed with bidentate ligands some of which were functionalized by ferrocene electroactive moieties. The bound electroactive ferrocene component diusion is used as a probe for determining the mass variation of the oligomers formed through the hydrolysis–condensation process. Chronoamperometry provided a means for studying diusion kinetics. In this way polymerization kinetics can be followed with a fair precision on a real timescale, provided that the electroactive probe binds irreversibly to the polymerizing zirconium species. Two organic complexants, acetylacetone and ethyl acetoacetate, have been used in this study, in order to obtain gels from the highly reactive zirconium n-propoxide. The binding part of the electroactive probe was the strongly complexant salicylate ligand. The electrochemical results reveal the kinetics of polymerization, and give in addition an estimation of the mobile species present once the system has reached a stable state. The results have been confirmed by several other techniques including SEM, elemental analysis, light diusion and BET specific surface determination on the resulting xerogels. While the polymerization mechanisms of silicon alkoxides are average size of the polymers or aggregates to which the functionalized complexant has bound. relatively well known, the more complex case of transition- For zirconium, the complexing power is in the following metal alkoxides has only been investigated to a limited extent,1 order: salicylate&acac>alkyl acetoacetate, and therefore sali- especially with organic complexants.1a Most of the previously cylic acid is one of the best complexants for zirconium and performed in situ studies rely on NMR experiments.2 NMR is titanium alkoxides.5 We have also prepared the functionalized an adequate tool to follow kinetics when they are slow but complexant Fc-sal, where a ferrocenyl group is covalently NMR studies are expensive and the equipment is not always bound to a salicylate moiety. available. As a consequence, most NMR data has been devoted to study the hydrolysis–condensation behaviour of silicon alkoxides. The hydrolysis of transition-metal alkoxides displays much faster kinetics, due to their usually higher reactivity, and their study is thus more dicult. We recently introduced a new electrochemical approach to investigate the microviscosity of the sols and gels obtained This irreversably bound5b electrochemical probe has been through the hydrolysis of silicon and titanium alkoxides, based used to investigate the polymerization of zirconium n-propox- on the in situ determination of diusion coecients of a ide in the presence of either acetylacetone or ethyl acetoacetate electroactive species embedded in zirconia or silica gels,3a or as the main (weaker) complexant, with various hydrolysis anchored to the molecular precursors of zirconia, silica or ratios. Scheme 1 illustrates our working hypothesis. titania gels.3b This work addresses a further development of this method, using ferrocene species strongly bonded to a zirconium alkoxide. Experimental Since transition-metal alkoxides are very reactive, com- Synthesis of ligands plexation of the metal is commonly used to tune the reactivity. In this way, a polymer network can be generated instead of a The Fc-sal ligand is made from 1-(6-bromohexyl)ferrocene, precipitate.4 Exchange of one propylate group by a bidentate which was produced in two steps from ferrocene, according to ligand such as acetylacetone increases the coordination of a the following sequence. zirconium species and diminishes the reactivity4b to enable the formation of transparent sols and gels. The propylate moiety (a) Preparation of 1-(6-bromohexanoyl )ferrocene. AlCl 3 is always hydrolysed before a coordinated bidentate ligand ( 8.72 g, 6.54×10-2 mol ) was added at 5 °C under N 2 to 13.37 g such as acetylacetone. Therefore it is possible to bind irreversi- (7.19×10-2 mol ) of ferrocene dissolved in CH 2 Cl 2 ( ca. 200 ml). bly electroactive moieties to zirconium alkoxides and to the 10 ml (6.53×10-2 mol ) of 6-bromohexanoyl chloride dissolved resulting condensed species after the hydrolysis–condensation in CH 2 Cl 2 (about 50 ml) was then added dropwise to the process has begun, using a strong bidentate complexant func- above mixture. The solution was stirred for 10 min, and then tionalized with a ferrocenic electroactive probe. The diusion allowed to warm to ambient temperature. 1-(6- kinetics of the functionalized complexant can be followed by Bromohexanoyl )ferrocene is obtained as an orange oil after hydrolysis in an ice/water mixture. Extraction with CH 2 Cl 2 chronoamperometry, and information can be extracted on the J. Mater. Chem., 1997, 7 (8), 1461–1466 1461 Downloaded on 05 December 2011 Published on 01 January 1997 on http://pubs.rsc.org | doi:10.1039/A608085E View Online / Journal Homepage / Table of Contents for this issue