High-throughput investigation of metal carboxyarylphosphonate hybrid compounds

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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–HO 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–HO 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.