Subtle ring-constraint effects on the formation of metallacycles. Cyclodimer versus cyclotrimer of Bis(chloro)-1,4-bis(dimethyl-3 or 4-pyridylsilyl)benzenepalladium(II) complexes

Stepwise modulation of the ring size of metallacyclic compounds via subtle ring-constraint effects has been established. The reactions of (COD)PdCl ₂ with p- or m-psb ligand [COD = 1,5-cyclooctadiene; p-psb = 1,4-bis(dimethyl-4-pyridylsilyl)benzene; m-psb = 1,4-bis(dimethyl-3- pyridylsilyl)benzene] in a mixture of acetone and ethanol at room temperature produce the metallacyclodimers [PdCl₂(p- or m-psb)]₂. The cyclodimeric species of [PdCl₂(p-psb)]₂ at the boiling temperature or via the sonication of the chloroform solution is completely converted to the cyclotrimer [PdCl₂(p-psb)]₃. Direct reaction of (COD)PdCl₂ with p-psb in a mixture of acetone and ethanol at reflux temperature yields the same trimer, [PdCl₂(p-psb)] ₃. Furthermore, equilibria between the kinetic product, [PdCl ₂(p-psb)]₂, and the thermodynamic product, [PdCl ₂(p-psb)]₃, have been observed in N,N-dimethylformamide as well as in dimethyl sulfoxide. In contrast, the cyclodimeric structure of [PdCl₂(m-psb)]₂ is retained under the same treatment conditions for 40 h; that is, the trimeric species, [PdCl₂(m-psb)] ₃, is not formed. Such a notable difference between [PdCl ₂(p-psb)]₂ and [PdCl₂(m-psb)]₂ might be explained by their different angle constraints.