Stilbene analogues incorporating the Co(η 4-C 4Ph 4)(η 5-C 5H 4-) end group

Abstract

A number of organometallic stilbenes of the general type [Co(η 4-C 4Ph 4)(η 5-C 5H 4CH CHR] are reported where R is C 6H 4X-4 (X = H, OMe, Br, NO 2), 1-naphthyl, 9-anthryl, 1-pyrenyl, (η 5-C 5H 4)Co(η 4-C 4Ph 4), and (η 5-C 5H 4)Fe(η 5-C 5H 4Y) {Y = CHO, CH C(CN) 2 and CH CHC 5H 4-η 5)Co(η 4-C 4Ph 4)}. They were prepared by Wittig or Horner–Wadsworth–Emmons reactions which yield both E and Z or only E products respectively. The isomers were separated and all compounds characterised by standard spectroscopic techniques as well as by X-ray diffraction methods in many cases. The electrochemistry of the stilbene analogues in dichloromethane solution is also reported. In most, the (η 5-C 5H 4)Co(η 4-C 4Ph 4) functional group undergoes a reversible one-electron oxidation. For those molecules that also include (η 5-C 5H 4)Fe(η 5-C 5H 4Y), this is preceded by the reversible oxidation of the ferrocenyl group. Spectroscopic and structural data suggests that for most compounds there is little electronic interaction between Co(η 4-C 4Ph 4)(η 5-C 5H 4) and the R end groups which are effectively independent of one another. The only exceptions to this are Z and E-[Co(η 4-C 4Ph 4)(η 5-C 5H 4CH CHC 6H 4NO 2-4], and [Co(η 4-C 4Ph 4)(η 5-C 5H 4CH CHC 5H 4-η 5)Fe{η 5-C 5H 4CH C(CN) 2}] where the electronic spectra are respectively consistent with a significant Co(η 4-C 4Ph 4)(η 5-C 5H 4)/NO 2 donor/acceptor interaction and a less significant Co(η 4-C 4Ph 4)(η 5-C 5H 4)/C(CN) 2 one. However, OTTLE studies show that in the electronic spectra of [Co(η 4-C 4Ph 4)(η 5-C 5H 4CH CHR] + there are low energy absorption bands (950–1800 nm) which are attributed to R → Co(η 4-C 4Ph 4)(η 5-C 5H 4) + or, when R is a ferrocenyl-base group, Co(η 4-C 4Ph 4)(η 5-C 5H 4) → (η 5-C 5H 4)Fe(η 5-C 5H 4Y) + charge transfer transitions. The ferrocenyl compounds undergo cis/trans isomerisation on the OTTLE experiment timescale.