Interconversion between cyclodimer and cyclotrimer: Synthesis and characterization of cyclo-[Pd(II)Cl-2(N-N)] complexes

Abstract

The reaction of (COD)PdCl2 (COD = 1,5-cyclooctadiene) with (3-Py)(2)SiR1R2 (3-Py = 3-pyridyl; R-1 = Ph, R-2 = Ph (m-pdps); R-1 = Ph, R-2 = Me (m-pmps)) in acetone affords single crystals consisting of cyclodimers, [PdCl2((3-Py)(2)SiR1R2)](2), whereas the same reaction in a mixture of dichloromethane and ethanol yields amorphous spheres consisting of cyclotrimers, [PdCl2((3-Py)(2)SiR1R2)](3). In a boiling chloroform solution, the cyclodimers are completely converted to cyclotrimers. These cyclotrimers, in the 10-60 degrees C range, are partly returned to cyclodimers. By contrast, the reaction of (COD) PdCl2 with (3-Py)(2)SiR1R2 (R-1 = Bu, R-2 = Me (m-pbms); R-1 = dodecyl, R-2 = Me (m-pddms)) yields amorphous spheres consisting of cyclotrimers irrespective of solvents. Both [PdCl2(m-pbms)](3) and [PdCl2(m-pddms)](3) are initially cyclotrimers in chloroform, but they exist as a mixture of cyclodimers and cyclotrimers in solution in the 10-60 degrees C range. The metallacycles tend to form cyclodimers in the order m-pdps > m-pmps > m-pbms > m-pddms. The equilibrium between cyclodimers and the cyclotrimers is sensitive to solvent, temperature, and concentration as well as molecular structure. (c) 2008 Published by Elsevier B.V.