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)PdCl2 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, [PdCl2(p-psb)]2, and the thermodynamic product, [PdCl2(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 [PdCl2(p-psb)]2 and [PdCl2(m-psb)]2 might be explained by their different angle constraints.