Assembly of cyclopalladated units: synthesis, characterisation, X-ray crystal structure and study of the reactivity of the tetrametallic cyclopalladated complex [Pd{C6H4CHN(C6H42-O)}]4·2CHCl3

The reaction of the equimolar amounts of the Schiff base C6H4CHN(C6H42-OH) (1) and palladium(II) acetate in refluxing methanol for 2 h produces [Pd{C6H4CHN(C6H42-O)}]4 (2)·2CHCl3. The X-ray crystal structure of 2·2CHCl3 reveals that it contains a central non-planar eight-membered ring “Pd4O4” formed by the self-assembly of four cyclopalladated fragments in which the ligand behaves as a [C, N, O]2− terdentate ligand and the oxygen atoms act as bridges between the monomeric units. The reaction of 2 with triphenylphosphine or 1,1′-bis(diphenylphosphino)ferrocene (dppf) produces the opening of the central “Pd4O4” core and the formation of the monomeric derivative [Pd{C6H4CHN(C6H42-O)}(PPh3)] (3)·CH2Cl2 and the trimetallic complex [{Pd[C6H4CHN(C6H42-O)]}2(μ-dppf)] (4), respectively. In 3 and 4, the ligand also behaves as a dianionic and terdentate [C, N, O]2− group, thus indicating that the PdO bond exhibits remarkable stability. The X-ray crystal structure of 3·CH2Cl2 confirmed the mode of binding of the Schiff base and a trans arrangement between the imine nitrogen and the PPh3 ligand. A comparative study of the spectrochemical properties of compounds 2–4 is also reported.