Reaction of metal ion complexes with hydrocarbons. Part I. ‘Palladation’ and some other new electrophilic substitution reactions. The preparation of palladium(I)

Abstract

A survey has been made to identify those metal ions that will displace hydrogen ions from benzene with formation of arylmetal complexes. Certain stable complexes were isolated while the formation of thermally unstable complexes was detected by the appearance of biphenyl. An isotopic tracer method was used to seek hydrolytically unstable complexes. Gold(III), mercury(II), and thallium(III) react with benzene in aqueous acid solution while gold(III), mercury(II), palladium(II), platinum(II), and thallium(III) react in acetic acid solution. Reactivity is associated with ‘class b’ character. A more detailed study of the reaction of diacetatopalladium with benzene and toluene in 0·5N-perchloric acid in acetic acid was made. It appears to be an electrophilic substitution that yields arylpalladium complexes. These decompose by a concerted mechanism to give biaryls and either palladium metal or a univalent palladium complex depending on temperature. An explosive purple complex, [Pd(C₆H₆)(H₂O)ClO₄]_n has been isolated. The ultraviolet absorption of this compound at 380 and 550 mµ has been used to determine its rate of appearance in solution during the reaction of benzene with diacetato-palladium in perchloric acid–acetic acid at 50°. The kinetic data are consistent with a second-order reaction between benzene and monomeric palladium(II)(k₂= 4·6 × 10^–4l.mole^–1sec.^–1) followed by fast decomposition. For deuteriobenzene, a large primary isotope effect (k_H/k_D= 5·0) was found.