High-throughput investigation of metal carboxyarylphosphonate hybrid compounds

Abstract

The phosphonocarboxylic acid H₂O₃PCH₂–C₆H₅–COOH has been employed in the synthesis of several metal carboxyarylphosphonates under hydrothermal conditions. The system MnCl₂/H₂O₃PCH₂–C₆H₄–COOH/NaOH was investigated in detail using high-throughput methods and reaction trends as well as the fields of formation could be identified. Three new compounds Mn[HO₃PCH₂–C₆H₄–COOH]₂·2H₂O (1) Mn[O₃PCH₂–C₆H₄–COOH]·H₂O (2) and Mn₂(OH)[O₃PCH₂–C₆H₄–COO]·2H₂O (3) were obtained and compounds 1 and 2 could be isolated as single crystals suitable for single crystal X-ray diffraction. Using similar reaction conditions the corresponding Cu and Cd compounds of 1 and 2 (Cu[HO₃PCH₂–C₆H₄–COOH]₂·2H₂O (4), Cd[O₃PCH₂–C₆H₄–COOH]·H₂O (5)) were obtained and structurally characterized. The results of the high-throughput study demonstrate the effect of the gradual deprotonation of the phosphonocarboxylic acid on the structure of the resulting compounds. Compounds 1 and 4 are formed under more acidic conditions and therefore the phosphonic acid group is mono-protonated. The structure is built of four-membered rings of alternating corner-sharing MO₆ and O₃PC polyhedra that are linked to form chains. These chains are connected through O–H⋯O hydrogen bonds to form a three-dimensional framework. In compounds 2 and 5 formed under more basic conditions the phosphonic acid group is fully deprotonated and, due to the additional coordinating O atom, a layered structure composed of MO₆ and O₃PC polyhedra is formed. These layers are held together by O–H⋯O hydrogen bonds involving the carboxylic acid groups. Based on spectroscopic, thermogravimetric and EDX-analysis data, compound 3 has been characterized as Mn₂(OH)[O₃PCH₂–C₆H₄–COO]·2H₂O where the phosphonocarboxylic acid is fully deprotonated.