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

Abstract

The two palladium(II) complexes, [PdL²2222][ClO₄]₂·H₂O 1 and [Pd(H₋₂L²2222)]·H₂O 2 with 1,11-bis(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane-5,7-dione, L²2222, were prepared and characterized by X-ray crystallography and UV-visible spectroscopy. In 1 the palladium centre is co-ordinated by the tertiary amines of the macrocycle and the two pyridylmethyl nitrogens, forming a square-planar geometry in which two amide nitrogen groups remain protonated and do not take part in co-ordination. Moreover, there is a weak interaction between palladium and H(–N(8)) [d(Pd^II ⋯ H) = 2.90 Å]. In contrast, 2 isolated under basic conditions has a square-planar geometry but the palladium ion is co-ordinated by two deprotonated amides and the two tertiary amines of the macrocycle. In solution the reversible interconversion between complexes 1 and 2 easily occurs through pH-dependent deprotonation of the amide group.