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

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.

The construction of dynamic dysprosium-carboxylate ribbons by utilizing the hybrid-ligand conception

By utilizing the hybrid-ligand conception, three novel dysprosium complexes Dy(2-py-4-pmc)(L)(H2O) (H2-py-4-pmc = 2-(2-pyridyl)pyrimidine-4-carboxylic acid; L = fumarate (fum, 1), succinate (suc, 2), or pimelate (pim, 3)) have been successfully synthesized. Structural analysis reveals that the dicarboxylate ligands connect 2-py-4-pmc--protected Dy3+ to form one-dimensional molecular ribbons. Magnetic measurements indicate that the three complexes exhibit typical slow magnetic relaxation under a zero dc field with effective reversal barriers Ueff of 180 K, 145 K and 137 K for 1-3, respectively, which is mainly attributed to the strong Ising anisotropy of dysprosium ions induced by the appropriate arrangement of carboxylate groups. Ab initio calculations demonstrate that the charge distribution around dysprosium ions and the magnetic interactions between them are key contributions to their different dynamic behaviour.

Regulating the structural dimensionality and dynamic properties of a porous dysprosium coordination polymer through solvent molecules

The solvent molecules can regulate the structural dimensionality of a porous dysprosium coordination polymer from three to two and its dynamic behavior, as evidenced by the increase of the effective energy barriers and slowdown of quantum tunneling.

Rational construction of a porous lanthanide coordination polymer featuring reversible guest-dependent magnetic relaxation behavior

The adjustment of a magnetic relaxation pathway by guest molecules is found in a porous dysprosium complex constructed by hybrid ligands.

Two hybrid lanthanide complexes exhibiting a large magnetocaloric effect and slow magnetic relaxation

Two isostructural lanthanide (Ln) hybrid complexes co-bridged by organic oxalate and inorganic hypophosphite, [Ln(oxa)(H₂PO₂)(H₂O)₂] (oxa = oxalate; Ln = Gd (1), Dy (2)), were solvothermally prepared with the goal of elucidating the role of a hybrid framework in the generation of novel molecular magnetic materials. The title compounds feature a two dimensional (2D) hybrid layer. The Ln^III ions are octa-coordinated with distorted square antiprism geometry. The adjacent Ln^III ions are co-bridged by hypophosphite and one type of oxalate ligand to form 1D hybrid chains, which are further linked by a second type of oxalate ligand to generate the resulting 2D framework. Magnetic investigations reveal that compound 1 features a large magnetocaloric effect with -ΔS = 46.60 J kg^-1 K^-1 (134.39 mJ cm^-3 K^-1), due to the combined advantages of organic oxalate and inorganic hypophosphite ligands, while compound 2 displays slow magnetic relaxation.

3D oxalato-bridged lanthanide(iii) MOFs with magnetocaloric, magnetic and photoluminescence properties

Four isostructural 3D lanthanide(iii) metal–organic frameworks with the general formula (H₆edte)_0.5[Ln^III(ox)₂(H₂O)] (Ln = Gd (1), Tb (2), Dy (3) and Ho (4); H₄edte = N,N,N′,N′-tetrakis(2-hydroxyethyl)ethylenediamine) and ox = oxalate have been synthesized from oxalate.

Field-Induced Relaxation of Magnetization in a Three-Dimensional LnMOF with the Second Bridging Ligand Squarate

A three-dimensional (3D) dysprosium(III) metal-organic framework with nicotinate N-oxide (NNO^-) and squarate (C₄O₄ ^2-) mixed bridging ligands, [Dy(NNO)(C₄O₄)(H₂O)] _n (1), has been hydrothermally synthesized. The dysprosium(III) ions are linked to each other by the squarate anions to form a unique dysprosium(III) squarate double-layered network; the NNO^- anions then bridge such layers to complete the 3D framework. Complex 1 exhibits a two-step relaxation of magnetization under a dc field of 1000 Oe, with effective energy barrier values of 8.5 and 14.3 K, respectively.

Two-step warming solvothermal syntheses, luminescence and slow magnetic relaxation of isostructural dense LnMOFs based on nanoscale 3-connected linkers

Two isostructural 3D dense LnMOFs exhibiting either photoluminescence or slow magnetic relaxation are assembled under two-step warming solvothermal conditions.

A 3D MOF constructed from dysprosium(iii) oxalate and capping ligands: ferromagnetic coupling and field-induced two-step magnetic relaxation

Field-induced two-step magnetic relaxation was observed in the first ferromagnetic 3D lanthanide(iii) oxalate framework with a capping ligand.

Multiple thermal magnetic relaxation in a two-dimensional ferromagnetic dysprosium(iii) metal–organic framework

Intramolecular ferromagnetic interaction exists in a dysprosium(iii) MOF with a highly ordered 2D (4, 4) grid network, showing two-step thermal magnetic relaxation.

Magnetic dynamics of two salen type Dy2 complexes modulated by coordination geometry

Two salen type dinuclear dysprosium complexes bridged by two ligands displaying broken hula hoop-like and hula hoop-like coordination geometries were isolated and showed the distinct magnetic properties of single and double magnetic relaxation.

Luminescence and slow magnetic relaxation of isostructural 2D lanthanide metal–organic frameworks derived from both nicotinate N-oxide and glutarate

The isostructural 2D lanthanide coordination polymers may exhibit either luminescence or slow magnetic relaxation, depending on the lanthanide ions.