The preparation, spectroscopy, structure and electrochemistry of some [Co(η4-C4Ph4)(η5-C5H4CC–CHY)], [Fe(η5-C5H5)(η5-C5H4CC–CHY)], and related complexes

[Co(η4-C4Ph4)(η5-C5H4CC–CHO)], 4a, has been prepared from [Co(η4-C4Ph4)(η5-C5H4CHO)], 1a, by reactions similar to those used previously to convert [Fe(η5-C5H5)(η5-C5H4CHO)], 1b, to [Fe(η5-C5H5)(η5-C5H4CC–CHO)], 4b, i.e. via the dibromoethene [Co(η4-C4Ph4)(η5-C5H4CHCBr2)], 2a, and alkyne [Co(η4-C4Ph4)(η5-C5H4CCH)], 3a. Both 4a and 4b undergo the normal aldehyde reactions with malononitrile and 2,4-dinitrophenylhydrazines to give their respective condensation products [Co(η4-C4Ph4){η5-C5H4CC–CHC(CN)2}], 5a, and [Fe(η5-C5H5){η5-C5H4CC–CHC(CN)2}], 5b; [Co(η4-C4Ph4){η5-C5H4CC–CHN–N(H)C6H3(NO2)2-2,4}], 6a, and [Fe(η5-C5H5){η5-C5H4CC–CHN–N(H)C6H3(NO2)2-2,4}], 6b, as separable mixtures of syn- and anti-isomers; and the anti-isomers of [Co(η4-C4Ph4){η5-C5H4CC–CHN–N(Me)C6H3(NO2)2-2,4}], 7a, and [Fe(η5-C5H5){η5-C5H4CC–CHN–N(Me)C6H3(NO2)2-2,4}], 7b. With [Fe2(η-C5H5)2(CO)2(μ-CO)(μ-CMe)][BF4] 4a and 4b form blue-green [Fe2(η-C5H5)2(CO)2(μ-CO)(μ-C–CHCH-CC–C5H4-η5)Co(η4-C4Ph4)][BF4], [8a][BF4], and black [Fe2(η-C5H5)2(CO)2(μ-CO)(μ-C–CHCH–CC–C5H4-η5)Fe(η5-C5H5)][BF4], [8b][BF4] salts. 4a and 4b also react with the Wittig reagent [Fe(η5-C5H5)(η5-C5H4–CH2PPh3)][I]/nBuLi to give mixtures of Z and E-[Co(η4-C4Ph4)(η5-C5H4–CC–CHCH–C5H4-η5)Fe(η5-C5H5)], 9a, and [Fe(η5-C5H5)(η5-C5H4–CC–CHCH–C5H4-η5)Fe(η5-C5H5)], 9b, which are separable for 9b but not 9a. The Wittig reagent obtained from [Co(η4-C4Ph4)(η5-C5H4–CH2PPh3)][Cl]/nBuLi failed to react with 4a or 4b, but the reaction of 4a with [ClCH2PPh3][Cl]/tBuOK gives very low yields of one expected product, E-[Co(η4-C4Ph4)(η5-C5H4–CC–CHCH–C5H4-η5)Co(η4-C4Ph4)], E-10a, together with a number of other unidentified compounds. The IR, 1H NMR and 13C NMR, and UV/Vis spectra of 4–9 are reported, assigned and discussed. They confirm that 4–9 are Donor–π–Acceptor complexes in which Co(η4-C4Ph4)(η5-C5H4–) is a weaker donor than Fe(η5-C5H5)(η5-C5H4–), the acceptor strength increases for Acceptor  CHCHFc < CHN–N(R)C6H3(NO2)2-2,4 < CHO < CHC(CN)2 < (–CHCH–Cμ+)(μ-CO)(CO)2Fe2(η5-C5H5)2, and that an ethyne linker, π = CC, is less effective than an ethene linker, π = CHCH, in promoting electronic communication between the Donor and Acceptor. The molecular structures of 2a, 3a, 4a, 4b, 5b, syn-6a, syn-6b (two crystal forms) and anti-7b have been determined by X-ray diffraction. They have normal molecular dimensions, and the C5H4–CC–CHY moiety does not deviate greatly from planarity with angles between the C5H4 and C–C(H)Y planes of 4.2–19.6°. This contrasts with the structures of Fc–CC–R (R = aryl) complexes where the C5H4 and aryl planes are orthogonal or close to it. The electrochemistry of 3a/3b, 4a/4b, 5a/5b, syn-6a/6b, anti-7a/7b, E/Z-9a, Z-9b, E-9b and [Co(η4-C4Ph4)(η5-C5H4–CC–C6H5)] has been studied. The Co(η4-C4Ph4)(η5-C5H4–) complexes undergo reversible 1e oxidations at higher E° than their Fe(η5-C5H5)(η5-C5H4–) counterparts with E° increasing as the electron-withdrawing ability of the acceptor group increases. Furthermore, like their ferrocenyl counterparts, the alkyne derivatives [Co(η4-C4Ph4)(η5-C5H4–CC–X)] are oxidised at a more positive E° than the alkene complexes [Co(η4-C4Ph4)(η5-C5H4–CHCH–X)]. The UV/Visible spectrum of the oxidized species [Co(η4-C4Ph4)(η5-C5H4–CC–C6H5)]+ shows an absorption band at 960 nm due to a C6H5 → Co(η4-C4Ph4)(η5-C5H4–) charge transfer transition; its equivalent in the spectrum of [Fe(η5-C5H5)(η5-C5H4–CC–C6H5)]+ is found at 797 nm. This implies that {Co(η4-C4Ph4)(η5-C5H4–)}+ is a stronger acceptor than {Fe(η5-C5H5)(η5-C5H4–)}+.

[Co(η4-C4Ph4)(η5-C5H4CCCHO)] and [Fe(η5-C5H5)(η5-C5H4CCCHO)] are suitable precursors for a variety of D-spacer-A compounds containing the Co(η4-C4Ph4)(η5-C5H4–) and ferrocenyl alkyne moieties.The complexes are characterised by spectroscopy, X-ray diffraction and electrochemistry.Figure optionsDownload as PowerPoint slideHighlights
► Preparation of new metallocene D–π–A systems and precursor aldehydes.
► Solid state structural parameters of 4 cobalt compounds and 5 ferrocenes.
► Electrochemistry of several cobalt and ferrocenyl D–π–A systems.