Different ways of coordination for aminoalkyldiphosphonic acids in palladium(II) complexes

Complexation in the systems K₂PdCl₄-3-amino-1-hydroxypropane-1,1-diyldiphosphonic acid (AHPDP) and K₂PdCl₄-1-aminoethane-1,1-diyldiphosphonic acid (AEDP) was studied using pH-potentiometry, spectrophotometry, and ³¹P NMR spectroscopy. It was found that AHPDP in equimolar complexes is coordinated by Pd(II) in a bidentate fashion through two phosphonate O atoms forming a six-membered chelate [O,O]-ring, while AEDP is coordinated through the N atom and one phosphonate O atom forming a five-membered chelate [N,O]-ring. In 1: 2 complexes of Pd(II) with AEDP, the second AEDP molecule is coordinated by Pd(II) in a similar way to form a second five-membered [N,O]-ring. The latter complexes show cis-trans and syn-anti isomerism. The structural characteristics of the corresponding isomers were calculated at the DFT level of theory and their stabilities were compared.