Neutral N-O chelated palladium(II) complexes: Syntheses, characterization, and reactivity

A series of neutral salicylaldiminato Pd(II) complexes, Pd(Me)(Ph-CH=N-R′)[3- ^tBu-2-(O)-C ₆H ₃-CH=N-2,6-di ⁱPr-C ₆H ₃] (R = CH ₃, n-Pr, tert-Bu, Ph, benzyl) (3a-3e) and Pd(Me)[2-(CH=N-t-Bu)-Py][3- ^tBu-2-(O)C ₆H ₃-CH=N-2,6-di- ⁱPr-C ₆H ₃] (4), have been synthesized and characterized. The structures of complexes 3a, 3c-3e, and 4 have been confirmed by X-ray analysis. NMR studies utilizing 3c indicate that the insertion of CO into the Pd-Me bond occurs through a five-coordinate intermediate. The acyl compounds in turn undergo imine insertion. The neutral complexes 3a-3e show moderate catalytic activity for the polymerization of methyl acrylate at ambient temperature. However, a radical rather than vinyl insertion mechanism appears to be operative.