Reaction of trans-dichloro(ethylene)pyridineplatinum(II) complexes with substituted pyridines and ethylene: mechanism of ligand exchange and attack of free base on the co-ordinated olefin. Preparation of two zwitterionic complexes containing platinum–carbon σ-bonds

Abstract

Two complexes of formula trans-[Pt(CH₂CH₂L)CI₂L](L = 4-methylpyridine or 3,5-dimethylpyridine) have been prepared by nucleophilic attack of the free amine on the co-ordinated ethylene of trans-[Pt(C₂H₄)Cl₂L]. Such a reaction occurs only with the more basic and less hindered pyridines. The free pyridines also exhibit nucleophilic attack on the metal centre leading to a pyridine-exchange reaction. The free energy of activation for the exchange process is <42 kJ mol^–1 for all the pyridines with the exception of 2,4,6-trimethylpyridine for which the free energy of activation is >80 kJ mol^–1. In the presence of free ethylene, exchange between free and complexed olefin occurs. Also in this case the exchange process takes place through attack of the free ethylene on the metal centre and formation of a five-co-ordinate bis(ethylene) transition state, The free energy of activation for this process is <43 kJ mol^–1 for unhindered pyridines and >76 kJ mol^–1 for pyridines with two methyl substituents in the ortho positions.