Synthesis and crystal structures of palladium(II) complexes of 1,11-bis(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane-5,7-dione

The two palladium(II) complexes, [PdL2][ClO4](2).H2O 1 and [Pd(H-2L2)].H2O 2 with 1,11-bis(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane-5,7-dion e, L-2, were prepared and characterized by X-ray crystallography and UV-vis ible spectroscopy. In 1 the palladium centre is co-ordinated by the tertiar y amines of the macrocycle and the two pyridylmethyl nitrogens, forming a s quare-planar geometry in which two amide nitrogen groups remain protonated and do not take part in co-ordination. Moreover, there is a weak interactio n between palladium and H(-N(8)) [d(Pd-II. . .H)=2.90 Angstrom]. In contras t, 2 isolated under basic conditions has a square-planar geometry but the p alladium ion is co-ordinated by two deprotonated amides and the two tertiar y amines of the macrocycle. In solution the reversible interconversion betw een complexes 1 and 2 easily occurs through pH-dependent deprotonation of t he amide group.