Formation and Structure of a Cobalt(III) Complex Containing a Nonstabilized Pyridinium Ylide Ligand

The reaction of [Co-III(4,4'dmsalen)(CH2Cl)(S)], where 4,4'dmsalen = 4,4'-dimethylsalen and S = solvent, with pyridine led to the formation of [Co-III(4,4'dmsalen)(CH(2)py)(Cl)], containing a nonstabilized pyridinium ylide as axial ligand. The complex has been unambiguously characterized by single-crystal X-ray diffraction analysis. Time-resolved H-1 NMR spectra showed that the formation of [CoIII(4,4'dmsalen)(CH(2)py)(Cl)] occurs in a two-step process involving a metallacyclized intermediate, cis-beta-[Co-III(4,4'dmsalenCH(2))(py)(S)](+). A similar experiment carried out in the presence of different nitrogen bases having higher pK(a) values (4-Me-py or 4-t-Bu-py) allowed a better separation of the two consecutive reactions. The almost complete conversion of [Co-III(4,4'dmsalen)(CH2Cl)(S)] in the cyclized intermediate before the formation of the ylide indicates that the ylide complex forms exclusively through the nucleophilic attack of the nitrogen base at the -CH2O carbon of the cyclized species, whereas a parallel direct conversion through the displacement of Cl from the axial CH2Cl group of [Co-III(4,4'dmsalen)(CH2Cl)(S)] may be ruled out.