pH – Specific synthesis, spectroscopic, and structural characterization of an assembly of species between Co(II) and N,N-bis(phosphonomethyl)glycine. Gaining insight into metal-ion phosphonate interactions in aqueous Co(II)–organophosphonate systems

Cobalt involvement in chemical and metallobiological processes entails largely unknown reactivity pathways with a variety of ligands. Such ligands include phosphonate and carboxylate-containing metal ion binders. In an attempt to investigate the nature and properties of species arising from aqueous interactions between Co(II) and N,N-bis(phosphonomethyl)-glycine (H5NTA2P), reactions between the two led to an assembly of species in (NH4)4[Co(H2O)6][(H2O)2Co(HNTA2P)Co(NH3)2(H2O)3]2[Co(NTA2P)(H2O)2]2 · 10H2O · 1.36CH3CH2OH (1) at pH ∼ 5.5. The analytical, spectroscopic and X-ray data on 1 reveal mononuclear and dinuclear complexes of Co(II) surrounded by oxygens, belonging to terminal carboxylates, phosphonates and bound water molecules, and nitrogen atoms from coordinated ammonia and HxNTA2Pq− (x = 1, q = 4; x = 0, q = 5) ligands. Worth noting is the variable protonation state of the bound diphosphonate ligand and its ability to bridge two Co(II) centers with ostensibly differing coordination spheres. The assembly of three Co(II) species of variable nuclearity and composition attests to the importance of pH-specific conditions, under which “capturing” of more than one species can be achieved for a given Co(II):H5NTA2P stoichiometry in the presence of ammonia. Collectively, 1 provides a rare glimpse of a “slice” of the aqueous speciation of the binary Co(II)–H5NTA2P system, while its lattice composition projects key structural features in Co(II)-carboxyphosphonate materials.