THE COMPLEX FORMATION OF PALLADIUM(II) WITH 1-HYDROXY-2-(3-PYRIDYL) ETHYLIDENEDIPHOSPHONIC (RISEDRONIC) ACID

The complex formation of K2[PdCl4] with 1-hydroxy-2-(3-pyridyl)ethylidene-diphos­phonic (risedronic, Ris, H4L) acid in aqueous solutions with physiological concentration of chloride ions (0.15 mol L-1KCl) was studied using pH-potentiometry, UV-Vis and NMR 31Р spectroscopy. It was found that in the system K2[PdCl4]:Ris=1:1, equimolar complexes are formed, for which the formation constants were calculated as follows: [PdH3LCl3]2- (lgβ=39.26 (4)), [PdH2LCl3]3- (lgβ=35.66 (4)), [PdHLCl3]4- (lgβ=30.25 (6)), and [PdLCl3]5- (lgβ=22.35 (6)). The monodentate mode of coordination of risedronic acid to the central metal ion via the nitrogen atom of the pyridine ring in equimolar complexes was determined. All equimolar complexes have the chromophore composition [Pd;Namine;3Cl]. The sequential formation of complexes [PdH2LCl3]3-, [PdHLCl3]4-, and [PdLCl3]5- occurs upon deprotonation of the phosphonic groups of the ligand, which are not directly coordinated to the Pd(II) ion. Based on the constructed diagram of the equilibrium concentration distribution of complexes, it was established that in the physiological pH range, the complex [PdHLCl3]4- predominates. The possibility of the formation the solution of complexes with coordination to Pd(II) from one to four molecules of risedronic acid, depending on the K2[PdCl4]:Ris ratio and pH values, is shown. In the systems K2[PdCl4]:Ris = 1:2, 1:3, and 1:4 at pH>5 comp­lexes are formed in which two, three, or four molecules of risedronic acid are coordinated to Pd(II) in a monodentate mode via the nitrogen atom of the pyridine ring. These complexes have chromophores of the composition [Pd;2Namine;2Cl], [Pd;3Namine;Cl], and [Pd;4Namine], respectively. The presence of only a single signal at δ³¹P~17.3–17.8 ppm in the ³¹P NMR spectra of the K₂[PdCl₄]:Ris systems with 1:1, 1:2, 1:3, and 1:4 ratios at pH>5, along with the absence of signals corresponding to phosphonate groups directly coordinated to Pd(II) (typically observed at δ³¹P~45–30 ppm), indicates that the phosphonate groups of the ligand do not participate in the coordination sphere of Pd(II).