Kinetics of nucleophilic attack on co-ordinated organic moieties. Part 7. Mechanisms of addition of tertiary phosphines and phosphites to tricarbonyl(dienyl)iron cations

Abstract

The reaction of tertiary phosphines and phosphites with dienyl complexes such as [Fe(C₆H₇)(CO)₃][BF₄](1) provides a general route to phosphonium adducts of the type [Fe(C₆H₇·PR₃)(CO)₃][BF₄](R = Ph, p-tolyl, Buⁿ, OPh, or OBuⁿ). An analogous reaction with 1,2-bis(diphenylphosphino)ethane leads to the dimeric salt [Fe₂{( C₆H₇·Ph₂PCH₂)₂}(CO)₆][BF₄]₂. The rate law, Rate =k[Fe][PR₃], is observed for addition of a range of phosphines and phosphites to (1) and the related cations [Fe(C₆H₆OMe)(CO)₃]⁺ and [Fe(C₇H₉)(CO)₃]⁺. Rate and activation parameters indicate direct addition at the dienyl rings. The marked dependence of k on the basicity of phosphorus nucleophiles suggests significant bond making in the transition states for adduct formation.