Interaction between [(η 6 - p -cym)M(H 2 O) 3 ] 2+ (M II = Ru, Os) or [(η 5 -Cp*)M(H 2 O) 3 ] 2+ (M III = Rh, Ir) and Phosphonate Derivatives of Iminodiacetic Acid: A Solution Equilibrium and DFT Study.

The pH-dependent binding strengths and modes of the organometallic [(η ⁶ - p -cym)M(H ₂ O) ₃ ] ²⁺ (M ^II = Ru, Os; p -cym = 1-methyl-4-isopropylbenzene) or [(η ⁵ -Cp*)M(H ₂ O) ₃ ] ²⁺ (M ^III = Rh, Ir; Cp* = pentamethylcyclopentadienyl anion) cations towards iminodiacetic acid (H ₂ Ida) and its biorelevant mono- and diphosphonate derivatives N-(phosphonomethyl)-glycine (H ₃ IdaP) and iminodi(methylphosphonic acid) (H ₄ Ida2P) was studied in an aqueous solution. The results showed that all three of the ligands form 1:1 complexes via the tridentate (O,N,O) donor set, for which the binding mode was further corroborated by the DFT method. Although with IdaP ^3- and Ida2P ^4- in mono- and bis-protonated species, where H ⁺ might also be located at the non-coordinating N atom, the theoretical calculations revealed the protonation of the phosphonate group(s) and the tridentate coordination of the phosphonate ligands. The replacement of one carboxylate in Ida ^2- by a phosphonate group (IdaP ^3- ) resulted in a significant increase in the stability of the metal complexes; however, this increase vanished with Ida2P ^4- , which was most likely due to some steric hindrance upon the coordination of the second large phosphonate group to form (5 + 5) joined chelates. In the phosphonate-containing systems, the neutral 1:1 complexes are the major species at pH 7.4 in the millimolar concentration range that is supported by both NMR and ESI-TOF-MS.