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

The reaction of [CoIII(4,4′dmsalen)(CH2Cl)(S)], where 4,4′dmsalen = 4,4′-dimethylsalen and S = solvent, with pyridine led to the formation of [CoIII(4,4′dmsalen)(CH2py)(Cl)], containing a nonstabilized pyridinium ylide as axial ligand. The complex has been unambiguously characterized by single-crystal X-ray diffraction analysis. Time-resolved 1H NMR spectra showed that the formation of [CoIII(4,4′dmsalen)(CH2py)(Cl)] occurs in a two-step process involving a metallacyclized intermediate, cis-β-[CoIII(4,4′dmsalenCH2)(py)(S)]+. A similar experiment carried out in the presence of different nitrogen bases having higher pKa values (4-Me-py or 4-t-Bu-py) allowed a better separation of the two consecutive reactions. The almost complete conversion of [CoIII(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 [CoIII(4,4′dmsalen)(CH2Cl)(S)] may be ruled out.