Complexing properties of phosphinic analogues of glycine

Abstract

A series of aminomethylphosphinic acids H₂NCH₂PRO₂H (R = H, Me, Bu^t or Ph) was synthesized and the acid–base and complexing properties with Co^II, Ni^II and Cu^II were determined pH-metrically (25 °C, 0.1 mol dm^–3 KNO₃). The pK_a values were found to increase from R = H to Ph, Me and Bu^t, as the –I effect decreases. The complex stability constants lie in the same order, except for R = H. The compound H₂NCH₂PHO₂H (HL¹) exhibits higher values than the ligands with methyl or phenyl substituents, probably due to better deformability of the PHO₂^– group and its better ability to form chelate rings. The complexes of H₂NCH₂P(Ph)O₂H (HL⁴), H₂NCH₂P(Me)O₂H (HL²) and H₂NCH₂P(Bu^t)O₂H (HL³) with several transition metals were prepared and the crystal structures of [CuL⁴₂]1 and [Cu₃L²₄][ClO₄]₂2 were determined. The formation of chelate rings was observed in both structures; 1 forms dimers and 2 polymeric layers via the phosphinates.