Synthesis, Characterization, and Reactivity of Palladium(II) Salen and Oxazoline Complexes

Two methods for the syntheses of palladium(II) salen complexes are described. The first involves template synthesis of the ligand in which bis(salicylaldehydato)palladium(II), 2, is initially synthesized and reacted with the appropriate diamine bridge to yield the desired palladium salen. The template synthesis approach suffers, however, from low overall yields (17-30%). Alternatively, treatment of bis(acetonitrile)palladium(II) chloride with the appropriate salen ligand under inert atmosphere yields the palladium salen complex in high yields (80-85%). [2-(2'-Hydroxyphenyl)-2-oxazolinato]palladium(II), 7, was also synthesized. All the palladium complexes were fully characterized spectroscopically. A single-crystal X-ray structure of 7 has been solved. In contrast to previous reports, complex 2 was found to be stable over weeks and remained suitable for the template syntheses. The catalytic activity of complex 2 in cyclopropanation of alkenes with ethyl diazoacetate (EDA) was investigated. The catalyst is not functional group tolerant and works best for styrene; turnover numbers (TON's) of 45 were achieved. The availability of an open coordination site seems to be a prerequisite for catalytic decomposition of EDA.