Stilbene analogues incorporating the Co(η4-C4Ph4)(η5-C5H4-) end group

A number of organometallic stilbenes of the general type [Co(η4-C4Ph4)(η5-C5H4CHdouble bond; length as m-dashCHR] are reported where R is C6H4X-4 (X = H, OMe, Br, NO2), 1-naphthyl, 9-anthryl, 1-pyrenyl, (η5-C5H4)Co(η4-C4Ph4), and (η5-C5H4)Fe(η5-C5H4Y) {Y = CHO, CHdouble bond; length as m-dashC(CN)2 and CHdouble bond; length as m-dashCHC5H4-η5)Co(η4-C4Ph4)}. 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 (η5-C5H4)Co(η4-C4Ph4) functional group undergoes a reversible one-electron oxidation. For those molecules that also include (η5-C5H4)Fe(η5-C5H4Y), 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(η4-C4Ph4)(η5-C5H4) and the R end groups which are effectively independent of one another. The only exceptions to this are Z and E-[Co(η4-C4Ph4)(η5-C5H4CHdouble bond; length as m-dashCHC6H4NO2-4], and [Co(η4-C4Ph4)(η5-C5H4CHdouble bond; length as m-dashCHC5H4-η5)Fe{η5-C5H4CHdouble bond; length as m-dashC(CN)2}] where the electronic spectra are respectively consistent with a significant Co(η4-C4Ph4)(η5-C5H4)/NO2 donor/acceptor interaction and a less significant Co(η4-C4Ph4)(η5-C5H4)/C(CN)2 one. However, OTTLE studies show that in the electronic spectra of [Co(η4-C4Ph4)(η5-C5H4CHdouble bond; length as m-dashCHR]+ there are low energy absorption bands (950–1800 nm) which are attributed to R → Co(η4-C4Ph4)(η5-C5H4)+ or, when R is a ferrocenyl-base group, Co(η4-C4Ph4)(η5-C5H4) → (η5-C5H4)Fe(η5-C5H4Y)+ charge transfer transitions. The ferrocenyl compounds undergo cis/trans isomerisation on the OTTLE experiment timescale.