Preparation, characterization, X-ray crystal structures, and isomerization equilibria and kinetics of cis- and trans-bis(5-imino-2,2-dimethyl-3-hexanonato-N, O)palladium(II)

摘要

The palladium(II) beta-ketoenaminate complexes cis- and trans-bis(5-imino-2,2-dimethy1-3-hexanonato-N,O)palladium(II), Pd(admh)(2) , have been prepared by reaction of PdCl2 and 5-amino-2,2-dimethylhex-4-en-3-one, Hadmh, in aqueous NaOH and have been separated by column chromatography on Florisil. The square-planar complexes were characterized in solution by H-1-INMR chemical shifts, the nuclear Overhauser effect, and electric dipole moments, which showed that the first-eluted isomer from Florisil is trans and the second-eluted isomer is cis. X-ray crystal structures of the solid complexes confirm the isomer assignments and reveal close NH center dot center dot center dot HN contacts between the two ligands in the cis isomer and close NH center dot center dot center dot O contacts in the trans isomer. Isomerization equilibria favor the trans isomer in chlorobenzene and favor the cis isomer in pyridine. The kinetics of trans-to-cis isomerization in pyridine was studied in the range 55.8-100.6 degrees C: Delta H-double dagger = 100 +/- 2 kJ/mol and Delta S-double dagger = - 50 +/- 6 J/(K mol). H-1 NMR spectra of Ni(admh)(2) in CHClF2 down to -170 degrees C suggests that only one isomer (probably trans) exists in solution.