Tetranuclear complexes of PdII with tridentate [C,N,O] and [O,N,O] ligands:: Synthesis, reactivity and structural isomerism

Treatment of the Schiff base ligand 2,3,4-(MeO)(3-)C6H2C(H)=N[2-(OH)-5-tBuC(6)H(3)] (a) with palladium(II) acetate in toluene gave the cyclometallated complex [Pd{2,3,4-(MeO)(3)C6HC(H)=N(2-O-5-t BuC6H3)}](4) (1a) with the tridentate ligand [C,N,O]. The complex showed a tetranuclear arrangement of the cyclometallated monomers with a P4O4 core. Reaction of 2,6-(Me)(2)C6H3C(H)=N[2-(OH)C6H4] (b) and 2,5-(Me)(2)C6H3C(H)=N[2-(OH)C6H4] (c) with palladium(II) acetate under similar conditions yielded the tetranuclear complexes [Pd{6-Me-2-CH2C6H3C(H)=N(2-OC6H4)}](4) (1b) and [Pd{5-Me-2-CH2C6H3C(H)=N(2-OC6H4)}](4) (1c), respectively, in which the tridentate [C,N,O] ligand is bound through the aliphatic methyl carbon. Treatment of 2,4-(Me)(2)C6H3C(H)=N[2-(OH)C6H4] (d) with palladium(II) acetate yielded a mixture of isomers, [Pd{4-Me-2-CH2C6H3C(H)=N(2-OC6H4)}](n)[Pd{2,4-(Me)(2)C6H2C(H)=N(2-OC6H4)}](4-n) (n = 0-4), also bearing a tetranuclear structure. However, the tetramer was assembled by two types of mononuclear Pd(ligand) building blocks: the ligand is coordinated through the aliphatic methyl carbon in one and through the aromatic ortho carbon in the other. The crystal structure of one such isomer confirmed the spectroscopic data. Treatment of 2-(OH)-4,6-(MeO)(2)C6H2C(H)=N[2-(OH)C6H4] (e) and 2-(OH)C6H4C(H)=N[2-(OH)C6H4] (f) with palladium(II) acetate yielded the tetranuclear complexes [Pd{2-O-4,6-(MeO)(2)C6H2C(H)=N(2-OC6H4)}](4) (1e) and [Pd{2-OC6H4C(H)=N(2-OC6H4)}](4) (If), respectively, after double deprotonation of the ligand, which coordinates to the metal through the two phenoxy oxygen atoms and the imine nitrogen atom in these complexes. Reaction of compounds 1a-1f with tertiary phosphanes in acetone gave mono- and dinuclear complexes, after cleavage of the tetranuclear structure. In these complexes, the tertiary phosphane is bonded trans to the nitrogen atom. The crystal structures for 1a, 1c, 1e, 1d/1d' are reported; they show the characteristic Pd4O4 core. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)