High-Resolution ¹⁷O Solid-State NMR as a Unique Probe for Investigating Oxalate Binding Modes in Materials: The Case...

Oxalate ligands are found in many classes of materials, including energy storage materials and biominerals. Determining their local environments at the atomic scale is thus paramount to establishing the structure and properties of numerous phases. Here, we show that high-resolution ¹⁷O solid-state NMR is a valuable asset for investigating the structure of crystalline oxalate systems. First, an efficient ¹⁷O-enrichment procedure of oxalate ligands is demonstrated using mechanochemistry. Then, ¹⁷O-enriched oxalates were used for the synthesis of the biologically relevant calcium oxalate monohydrate (COM) phase, enabling the analysis of its structure and heat-induced phase transitions by high-resolution ¹⁷O NMR. Studies of the low-temperature COM form (LT-COM), using magnetic fields from 9.4 to 35.2 T, as well as ¹³C-¹⁷O MQ/D-RINEPT and ¹⁷O{¹H} MQ/REDOR experiments, enabled the 8 inequivalent oxygen sites of the oxalates to be resolved, and tentatively assigned. The structural changes upon heat treatment of COM were also followed by high-resolution ¹⁷O NMR, providing new insight into the structures of the high-temperature form (HT-COM) and anhydrous calcium oxalate α-phase (α-COA), including the presence of structural disorder in the latter case. Overall, this work highlights the ease associated with ¹⁷O-enrichment of oxalate oxygens, and how it enables high-resolution solid-state NMR, for "NMR crystallography" investigations.