Photoreactivity and photopolymerization of silicon-bridged [1]ferrocenophanes in the presence of terpyridine initiators: unprecedented cleavage of both iron-cyclopentadienyl bonds in the presence of chlorosilanes.
The photopolymerisation of sila[1]ferrocenophane [Fe(eta-C5H4)2SiMe2] (3) with 4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine (tBu3terpy) as initiator has been explored. High-molecular-weight polyferrocenylsilane (PFS) [{Fe(eta-C5H4)2SiMe2}n] (5) was formed in high yield when a stoichiometric amount of tBu3terpy was used at 5 degrees C. Photopolymerisation of ferrocenophane 3 at higher temperatures gave PFS 5 in lower yield and with a reduced molecular weight as a result of a slower propagation rate. Remarkably, when Me3SiCl was added as a capping agent before photopolymerisation, subsequent photolysis of the reaction mixture resulted in the unprecedented cleavage of both iron-Cp bonds in ferrocenophane 3: iron(II) complex [Fe(tBu3terpy)2Cl2] (7Cl) was formed and the silane fragment (C5H4SiMe3)2SiMe2 (8) was released. The iron-Cp bond cleavage reaction also proceeded in ambient light, although longer reaction times were required. In addition, the unexpected cleavage chemistry in the presence of Me3SiCl was found to be applicable to other photoactive ferrocenes such as benzoylferrocene. For benzoylferrocene and ferrocenophane 3, the presence of metal-to-ligand charge transfer (MLCT) character in their low-energy transitions in the visible region probably facilitates photolytic iron-Cp bond cleavage, but this reactivity is suppressed when the strength of the iron-Cp bond is increased by the presence of electron-donating substituents on the cyclopentadienyl rings.
Chan
Wing Yan
WY
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, M5S 3H6, Canada.
Lough
Alan J
AJ
Manners
Ian
I