57Fe-Mössbauer spectroscopic study of monomeric and polymeric ferrocenyl- and octamethylferrocenyl-substituted ethynes

A series of four monomeric and four polymeric ferrocenyl- and octamethylferrocenyl-substituted 1-alkynes, ethynylferrocene (1) [1271-47-2], 2-ethynylphenylferrocene (2) [1292-14-4], [2-[ethynylphen-2-yl]ethen-1-yl]-1′,2,2′,3,3′,4,4′,5-octamethylferrocene (3), novel [2-[ethynylphen-2-yl]ethen-1-yl]-1′,2,2′,3,3′,4,4′,5-octamethylferricinium hexafluorophosphate (4) and poly(ethynylferrocene) (5) [33410-56-9], poly(2-ethynylphenylferrocene) (6) [205865-61-8], poly([2-[ethynylphen-2-yl]ethen-1-yl]-1′,2,2′,3,3′,4,4′,5-octamethylferrocene) (7), and novel poly([2-[ethynylphen-2-yl]ethen-1-yl]-1′,2,2′,3,3′,4,4′,5-octamethylferricinium hexafluorophosphate) (8), have been studied by temperature-dependent 57Fe-Mössbauer spectroscopy. The relevant Mössbauer parameters have been determined and are compared with the data obtained for the model compound octamethylferrocene (OMF). The data obtained by 57Fe-Mössbauer spectroscopy reflect a small decrease of s-electron density from the ferrocene moiety to the conjugated backbone, as is demonstrated by the smaller IS of the monomers 1–3 in comparison with the corresponding polymers 5–7. As may be deduced from the temperature dependence of the recoil-free fraction and the calculated lattice temperatures, the polymers are much ‘softer’ (within the definitions of Mössbauer spectroscopy) than the corresponding monomers. Finally, an unusual phase transition has been observed in OMF some 80°C below the melting point, which appears to lead to a large, sudden, and unexpected increase in the mean-square-amplitude of vibration of the metal atom in methyl substituted ring systems. This anomalous T-dependence of the recoil-free fraction, which is associated with the presence of the ring methyl groups, is not observed when one (or more) of the ring protons is replaced by a sterically bulky side chain, as in 1–3.