Complexes of divalent transition metal ions with bis(aminomethyl)phosphinic acid in aqueous solution and in the solid state

Abstract

Bis(aminomethyl)phosphinic acid, (NH₂CH₂)₂PO₂H (HL), was synthesized using a new procedure. Its coordination ability towards Co(II)/(III), Ni(II), Cu(II) and Zn(II) was studied both in solution and in the solid state. Because of the presence of two nitrogen atoms the ligand exhibits a higher overall basicity than common (aminoalkyl)phosphinic acids. Consequently, the values of the determined stability constants are comparable with those found for (aminoalkyl)phosphonic acids. NMR titrations of Zn(II) point to the interaction of phosphinate with the metal ion in a strong acid solution. The X-ray structures show several coordination modes in the solid state. All-trans-[MCl₂(H₂L-O)₂(H₂O)₂]Cl₂, where M(II) are Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Ca(II), and Cd(II), crystallized from acid solutions. The central ion is octahedrally coordinated with two phosphinate oxygen atoms, two molecules of water and two chlorides in all-trans arrangement. Amine groups are protonated and non-coordinated. Participation of the donor groups in crystals isolated from neutral solutions depends on the metal ion. All donor atoms are coordinated in monomeric fac-N,N,O-trans-O,O′-[Co(L-N,N,O)₂]⁺. On the other hand, in the zinc(II) complex, two phosphinate oxygen atoms and two amine nitrogen atoms (trans to each other) of two different ligand molecules are coordinated in an equatorial plane and two amino groups of the two other ligand molecules are bound in axial positions. Thus, each molecule of the amino acid forms a five-membered N,O-chelate to one zinc(II) ion and the other amino group is bound to the neighbouring ion creating an infinite chain. Nickel(II) forms a trans-O,O′-[Ni(H₂O)₂(L-N,N)₂] complex in which the metal ion is chelated by four amine nitrogen atoms forming two six-membered chelates in an equatorial plane and the octahedron is completed with two water molecules at the apical positions. The phosphinate group is not coordinated. The above results point to a relatively low coordination ability of the phosphinate group; however, due to its low pK_A, it is able to bind metal ions at lower pH than other coordinating groups do.