Internal ferrocenylalkynes—a comparative electrochemical and mass spectrometric study

A series of ferrocenylalkynes FcCCY, where Fc is ferrocenyl and Y=H (1), Me (2), Ph (3), EMe3 [E=C (4), Si (5), and Ge (6)], I (8), CPh2(OR) [R=H (9), Me (10)], CHO (11), C(O)Me (12), and CO2R [R=H (13), Et (14)] was synthesized (some representatives for the first time) and subjected to spectroscopic (IR, NMR, UV–vis, MS) and electrochemical study. In electron-impact mass spectra, all alkynes fragment via processes typical for the ferrocene skeleton and the substituent Y. Besides, the molecular ions of alkynes 1–3 decompose by a loss of {FeH2} affording likely a Y-substituted fulvene as the result of a cyclopentadienyl-ring transfer. The carbonyl group containing alkynes 11–14 tend to fragment so that ions isobaric with 1radical dot+ are produced. Similarly, the fragmentation of alcohol 10 appears as a superposition of fragmentation pathways due to Ph2COradical dot+ and 1radical dot+. In cyclic voltammograms, all alkynes exhibit one-electron reversible wave of the ferrocene/ferrocenium couple, whose redox potential correlates linearly with Hammett σp constants. A similar correlation with inductive σI constants is less pronounced due to neglecting resonance effects. An influence of the triple bond spacer between the substituent Y and the ferrocene unit is discussed.