Preparation and isomerisation of carbonyl(η-cyclohepta-1,4-diene)(η-cyclohexa-1,3-diene) iron complexes

Abstract

The reduction of [Fe(C₆H₈)(C₇H₉)(CO)]⁺, (I), with Na[BH₄] in water produces [Fe(η-1,3-C₆H₈)(η-1,4-C₇H₁₀)-(CO)], (II), in good yield together with some [Fe(η-1,3-C₇H₁₀)(CO)], (III). On heating, (II)(as a solid or in solution) undergoes isomerisation to (III) and some [Fe(η-1,3-C₆H₈)(1-σ, 3–5-η-C₇H₁₀)(CO)]. The kinetics of this first-order reaction have been studied in n-heptane over the temperature range 40–70 °C and the results compared with those for related systems. Reduction of (I) with Na[BD₄] afforded the monodeuteriated η-cyclohepta-1,4-diene complex. Reactions of, and spectroscopic studies (particularly ¹H and ¹³C n.m.r.) on, this complex indicate that nucleophilic addition has taken place at the exo-3-position, and that the endo-hydrogen atom migrates during the isomerisation process. Addition of cyanide ion to the complex cation also takes place at the exo-3-position, and the product rearranges to the η-2-substituted-1,3-diene complex ca. 5 times more rapidly than the unsubstituted complex. Complex (II) reacts with CO to give [Fe(η-1,3-C₆H₈)(CO)₃] and with PPh₃ the complex [Fe(η-1,3-C₆H₈)(CO)(PPh₃)₂] is formed via a second-order process.