Synthesis and characterization of new heterocyclic Schiff base palladacycles: Ring activation through N-oxide formation

Reaction of the Schiff base ligand derived from 4-pyridinecarboxaldehyde NC5H4C(H)N[2′,4′,6′-(CH3)C6H2], (1), with palladium(II) acetate in toluene at 60 °C for 24 h gave [Pd{NC5H4C(H)N[2′,4′,6′-(CH3)C6H2]}2(OCOCH3)2], (2), with two ligands coordinated through the pyridine nitrogen. Treatment of the Schiff base ligand derived from 4-pyridinecarboxaldehyde N-oxide, 4-(O)NC5H4C(H)N[2′,4′,6′-(CH3)C6H2], (4), with palladium(II) acetate in toluene at 75 °C gave the dinuclear acetato-bridged complex [Pd{4-(O)NC5H3C(H)N[2′,4′,6′-(CH3)C6H2]}(OCOCH3)]2, (5) with metallation of an aromatic phenyl carbon. Reaction of complex 5 with sodium chloride or lithium bromide gave the dinuclear halogen-bridged complexes [Pd{4-(O)NC5H3C(H)N[2′,4′,6′-(CH3)C6H2]}(Cl)]2, (6) and [Pd{4-(O)NC5H3C(H)N[2′,4′,6′-(CH3)C6H2]}(Br)]2, (7), after the metathesis reaction. Reaction of 6 and 7 with triphenylphosphine gave the mononuclear species [Pd{4-(O)NC5H3C(H)N[2′,4′,6′-(CH3)C6H2]}(Cl)(PPh3)], (8) and [Pd{4-(O)NC5H3C(H)N[2′,4′,6′-(CH3)C6H2]}-(Br)(PPh3)], (9), as air stable solids. Treatment of 6 and 7 with Ph2P(CH2)2PPh2 (dppe) in a complex/diphosphine 1:2 molar ratio gave the mononuclear complexes [Pd{4-(O)NC5H3C(H)N[2′,4′,6′-(CH3)C6H2]}(PPh2(CH2)2PPh2)][Cl], (10), and [Pd{4-(O)NC5H3C(H)N[2′,4′,6′-(CH3)C6H2]}(PPh2(CH2)2PPh2)][PF6], (11), with a chelating diphosphine. The molecular structure of complex 9 was determined by X-ray single crystal diffraction analysis.

Schiff base ligands with heterocyclic nitrogen rings may be activated towards metallation to give the corresponding palladacycles through N-oxide formation.Figure optionsDownload as PowerPoint slide