Synthesis, characterisation and dynamic behaviour of palladium complexes containing the novel terdentate nitrogen ligand 2,6-bis(pyrimidin-2-yl)pyridine

Abstract

The ionic methylpalladium complexes [Pd(Me)(bppy)]X {X = Cl 1a, SO₃CF₃ 1b, [3,5-(CF₃)₂C₆H₃]₄B 1c}, containing the novel flexible terdentate nitrogen ligand 2,6-bis(pyrimidin-2-yl)pyridine, have been synthesised. Complexes 1a–1c reacted quantitatively with carbon monoxide, resulting in formation of the acylpalladium complexes [Pd{C(O)Me}(bppy)]X {X = Cl 2a, SO₃CF₃ 2b, [3,5-(CF₃)₂C₆H₃]₄B 2c}. Complexes 1a–1c and 2a–2c have been isolated and fully characterised. In chloroform as well as in methanol, all the complexes show dynamic behaviour due to exchange of the co-ordinated and unco-ordinated pyrimidinyl nitrogen atoms. The influence of solvent, methyl and acyl ligand, counter ion and the presence of free Cl^– and CO on the dynamic behaviour has been studied and the thermodynamic parameters have been determined. A mechanism for the observed exchange process has been proposed. This mechanism involves (a) dissociation of one of the pyrimidinyl nitrogen atoms, which may be initiated by chloride, solvent or CO co-ordination, (b) rotation about the pyrimidinyl–pyridine bond and (c) reformation of the nitrogen–palladium bond.