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.