Optisch aktive übergangsmetall-komplexe. XXV. Substituenteneinflüsse auf die intramolekulare epimerisierung von tetragonal-pyramidalen molybdanund wolfram-komplexen

The synthesis, IR, ¹H NMR and UV spectra of optically active square pyramidal cyclopentadienyldicarbonyl-molybdenum and -tungsten complexes with Schiff bases, derived from methyl-substituted pyridine carbaldehyde(2) and S-(-)-α-methylbenzylamine, are described. The epimerisation of these complexes, measured polarimetrically at 75-97°C in DMF, follows a 1^st order rate law and represents a regular pentatopal skeletal rearrangement. The inductive effect of a methyl group in the para position of a pyridine ring leads to a retardation of the epimerisation compared to unsubstituted complexes. The rate constants decrease even more if an ortho methyl group is introduced into the pyridine system, since additional steric effects are introduced. If the metal complex contains a 4,6-dimethyl-substituted Schiff base as a chelating ligand, the contributions of a 4- and a 6-methyl group are additive. A benzene ring fused to the 5,6-positions of the pyridine system also leads to stabilisation of the configuration.