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 PdCl 2( p- or m-psb)] 2. The cyclodimeric species of PdCl 2( p-psb)] 2 at the boiling temperature or via the sonication of the chloroform solution is completely converted to the cyclotrimer PdCl 2( p-psb)] 3. Direct reaction of (COD)PdCl 2 with p-psb in a mixture of acetone and ethanol at reflux temperature yields the same trimer, PdCl 2( 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 PdCl 2( m-psb)] 2 is retained under the same treatment conditions for 40 h; that is, the trimeric species, PdCl 2( m-psb)] 3, is not formed. Such a notable difference between PdCl 2( p-psb)] 2 and PdCl 2( m-psb)] 2 might be explained by their different angle constraints.