Influence of the morphology and impurities of Ni(OH)2 on the synthesis of neutral Ni(II)–amino acid complexes

Synthesis of neutral complexes of Ni2+ with amino acids has often been reported on a qualitative basis, with a lack of information on the parameters involved in the dissolution of the nickel-containing solid precursor. This paper reports on a systematic study of the reactivity of Ni(OH)2 toward glycine in aqueous solution. The crystallinity and size of hydroxide particles are found to be key parameters in the rapid glycine-promoted dissolution of the hydroxide and synthesis of [Ni(glycinate)2(H2O)2]. These parameters derive from the nature of the salt used to prepare the hydroxide. Ni(II) chloride leads to the most reactive solid precursor, because of the presence of defects in the Ni(OH)2 sheets arrangements, assigned to the substitution of Cl− ions to OH− ions at the edges of the particles. The reaction between this hydroxide and glycine at 80 °C is quantitative after 7 min and similar rates of dissolution are obtained with other amino acids, alanine or histidine, the reaction with serine being slower. When the hydroxide contains nitrate or carbonate ions, a glycinato complex with composition similar to [Ni(glycinate)2(H2O)2], but with a different crystal structure, is also formed. Spectroscopic results may suggest a structure involving bridging ligands.