Complexation behaviour of aza-phosphinic acids
An alkylphosphinate is an attractive isostere for a carboxylate. The acid is more acidic than a carboxylic acid, is amenable to (^31)P NMR analysis and is subject to easy structural modification by variation of the phosphorus alkyl or aryl substituent. Amino acid complexing agents incorporating carboxymethyl groups are ubiquitous, but the corresponding phosphinic acid analogues have been much less studied. On chelation of a metal ion to a ligand nitrogen and a phosphorus oxygen atom, a new stereogenic centre at phosphorus is created. In polydentate ligands diastereoisomeric complexes may form. A new family of acyclic ligands with phosphinic acid binding groups have been synthesised. The protonation constants have been determined and used for the determination of the stability constants for a range of metal complexes. A complexing agent based on a 9N(_3) macrocyclic skeleton incorporating pendant arm phosphinic acid donors, provides an octahedral site for metal complexation. The structures of seven complexes with a ligand of this type, have been determined by X-ray crystallography. They fall into two groups, those with a C(_3) axis (Cu (II), Co (II), Zn (II) and Ni (II)) and those with an approximate C(_3) axis (Ga (III), Fe (III) and In (III)).