Reaction mechanism of regioisomerization in binuclear (diaminocarbene)Pd^II complexes

A series of binuclear PdII carbene complexes were synthesized via the treatment of cis-[PdCl2(CNXyl)2] (1) with benzo-1,3-thiazol-2-amines (2–6) and structurally characterized. In every case the reaction leads to the mixture of two regioisomers, which are able to interconvert. The study of the regioisomerization of the binuclear diaminocarbene species showed that it is a first-order reaction, that is, it occurs intramolecularly, and was analyzed with the Hammett function. Electron-withdrawing substituents in the benzothiazole moiety of the complexes as well as increasing the solvent polarity accelerate the reaction. The solvent donor strength correlates less well with the isomerization rates. Based on the obtained activation parameters the studied regioisomerization could be defined as the interchange/dissociative process type. A combined approach including kinetic and mass spectrometric studies allowed the conclusion that the rate-determining step of the isomerization is breaking the carbon–nitrogen bond in the carbene fragment of the binuclear complex.