Eight-membered chelate-ring complexes of cobalt(III)-polyamine complexes of aminopolyphosphonates in aqueous solution

The complex formation of N-methyliminodimethylenebis(phosphonic acid)[MeN(CH2PO3H2)2, H4midmp] and nitrilotrimethylenetris(phosphonic acid)[N(CH2PO3H2)3, H6ntmp] with cobalt(III)–polyamine complexes has been investigated by 31P NMR and UV/VIS spectroscopies. The 31P NMR spectra of the reaction mixtures were measured at 0 °C as a function of pH. The analysis of the pH dependence of the 31P NMR signals revealed that two types of complexes were formed in the reaction of cis-[Co(en)2Cl2]+(en = ethane-1,2-diamine) with midmp: O-monodentate and O,O-bidentate having an eight-membered chelate ring. The N atom is not coordinated. The two types of complexes were separated by HPLC and characterized by 31P NMR and UV/VIS spectrometry. The protonation constants and 31P NMR chemical shifts of each protonated complex were determined and structures are proposed. The first protonation occurs on the imino nitrogen of midmp of the Co(en)2 complexes. An intramolecular hydrogen bond plays an important role in the stability of the monoprotonated O-monodentate ligand complex. The complex cis-[Co(en)2(NH3)Cl]Cl2 forms an O-monodentate ligand complex on reaction with midmp. The protonation constants and 31P NMR chemical shifts of this complex corroborate the structure of the Co(en)2 O-monodentate midmp complex. The complexes of the cobalt(III)–polyamines with ntmp have the same structures as those of the corresponding midmp complexes.