Reactivity of co-ordinated ligands. Part XXI. Ligand substitution and nucleophilic reactivity of cyclohexa-, cyclohepta-, and cyclo-octa-dienylium complexes of iron

Abstract

Cations [L(C₇H₉)Fe(CO)₂]⁺[IV; L = PPh₃, AsPh₃, SbPh₃, pyridine (py), MeCN, NH₃, or CH₂CHCN] have been prepared from dicarbonyl(1–5-η-cycloheptadienylium)iodoiron. In the cases where L = PPh₃ or AsPh₃, nucleophilic attack generates solely 3–5-η,1-σ-cycloheptenyl products, (VI), resulting from attack at the 2-position of the dienylium system. Studies on the nucleophilic reactivity of tricarbonyl(1–5-η-cycloheptadienylium)iron, (II), show that mixtures of η-cyclohepta-1,3-diene and 3–5-η,1-σ-cycloheptenyl products [(IX) and (X)] result, with the proportions being dependent on both the conditions and the nucleophile. Tricarbonyl(1–5-η-cyclohexadienylium)iron, (I), and its dicarbonyl(triphenylphosphine) analogue, (VIII), both react with nucleophiles to generate only the η-cyclohexa-1,3-diene product, (VII). Tricarbonyl(1-5-η-cyclo-octadienylium) iron, (III), gives only the η-cyclo-octa-1,3-diene product on treatment with tetrahydroborate, but evidence is presented suggesting that attack at the 2-position also occurs. These differences in reactivity are discussed in terms of steric, kinetic, and electronic factors.