Interaction between [(η--cym)M(HO)] (M = Ru, Os) or [(η-Cp*)M(HO)] (M = 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--cym)M(HO)] (M = Ru, Os; -cym = 1-methyl-4-isopropylbenzene) or [(η-Cp*)M(HO)] (M = Rh, Ir; Cp* = pentamethylcyclopentadienyl anion) cations towards iminodiacetic acid (HIda) and its biorelevant mono- and diphosphonate derivatives N-(phosphonomethyl)-glycine (HIdaP) and iminodi(methylphosphonic acid) (HIda2P) 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 and Ida2P 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 by a phosphonate group (IdaP) resulted in a significant increase in the stability of the metal complexes; however, this increase vanished with Ida2P, 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.