Reactions of hydrated rare earth nitrates with formamide under mild solvothermal reaction conditions give three different rare earth formate structure types Ln(HCOO) 3 , Ln(HCOO) 3 •(HCONH 2 ) 2 , and [CH(NH 2 ) 2 ][Ln(HCOO) 4 ].The conditions for the formation of a specific structure type were determined as a function of the size of the rare earth cation, the reaction time, and the reaction temperature. Thirteen previously unreported rare earth formates were obtained; nine were isostructural compounds with the composition Ln(HCOO) 3 •(HCONH 2 ) 2 (denoted by LnFA, Ln = Y (1Y), Sm (2Sm), Eu (3Eu), Gd (4Gd), Tb (5Tb), Ho (6Ho), Tm (7Tm), Yb (8Yb), and Lu (9Lu)), and four were isostructural compounds with the composition [CH(NH 2 ) 2 ]-[Ln(HCOO) 4 ] (denoted by LnFMD, Ln = Sm (10Sm), Ho (11Ho), Tm (12Tm), and Lu (13Lu)). All compounds contain metal ions that are eight coordinated by oxygen atoms and are connected by anti−anti bridging formato ligands to form frameworks. The coordination geometry is bisdisphenoid in LnFA and square antiprismatic in LnFMD. In contrast, the larger rare earth ions that adopt the Ln(HCOO) 3 structure are nine coordinated by oxygen atoms in tricapped trigonal prismatic coordination geometry. The luminescence properties of 2Sm, 3Eu, 5Tb, and 10Sm were investigated by solid state photoluminescence spectroscopy in the range of 300−800 nm. Rare earth oxides were also found to react directly with formamide in the presence of a small amount of water to give the same compositions suggesting that the reaction chemistry can provide a basis for a process to recover rare earth metals from end-of-life products containing oxides such as the phosphors used in fluorescent light bulbs. This possibility was demonstrated by crystallizing rare earth formates starting with a commercially available trichromatic phosphor.

A series of layered divalent metal formate compounds, [M(HCOO)2(HCONH2)2] (M = Mn (1Mn), Ni (2Ni), Cu(3Cu), Zn(4Zn), Mg(5Mg)), have been prepared by solvothermal synthesis and their room temperature (RT) and low-temperature (LT) crystal structures, and thermal and magnetic properties determined. All the compounds contain octahedral metal ions connected by four anti-anti formato ligands to form (4,4) nets with the composition of M(HCOO)2. The oxygen atoms from two coordinating formamide ligands above and below the layer complete the MO6 distorted octahedral coordination. Order-disorder phase transformations involving the formamide ligands were observed in the 1Mn, 2Ni, and 4Zn compounds. Like transitions in related formate structures with perovskite like topology, the transitions correspond to the ordering of the amine groups of the terminating formamide ligands which are disordered at ambient temperature. The magnetic properties of the three magnetic members of the series 1Mn, 2Ni, and 3Cu were investigated using microcrystalline samples, over the temperature range of 2 K-300 K under different applied fields. All compounds belong to antiferromagnetic square lattices with S = 5/2, 1, and 1/2. Exchange constants for a nearest neighbor model are presented here. Specific heat measurements indicate magnetic long-range order at lower temperatures, S = 5/2 (antiferromagnetic) and S = 1 (ferrimagnetic).

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