The utilization of solid state chemistry reaction routes as new syntheses strategies for the coordination chemistry of rare earth amides

Solvent free high-temperature reactions in melts are well known procedures in Solid-State Chemistry. Although the reaction conditions are extreme considering the properties of organic ligands they can also be utilized for Coordination Chemistry and offer a fruitful alternative to usual solvent treatments. This includes the chemistry of organic amides of the rare earth elements. The avoidance of any solvent renders novel homoleptic complexes accessible but also implies difficulties bound to the solid state of the reaction mixtures. The high chemical affinity of the rare earth elements towards halides and especially oxygen limits known homoleptic amides obtained via solvent treatments mostly to multi-chelating ligands like porphyrines, calix-pyrroles etc. With no special conditions met like a high steric demand, solvent molecules as cocoordinating partners enforce the formation of heteroleptic species. This influence can be avoided by the use of completely solvent free reactions, such as melt reactions in which a solid is reacted directly with a melt or with a substance under solvothermal conditions. The high reactivity of the rare earth metals allows the direct oxidation with amines and thus to use high-temperature reactions for the formation of rare earth amides. This includes homoleptic compounds from simple ligands. Crystallization under reaction conditions is possible; no re-crystallization step is necessary preventing the risk of a change of the chemical character of the products. Additionally, the solubility of rare earth elements in liquid ammonia under formation of an electride solution enlarges the temperature range of these oxidation reactions down to the melting point of ammonia. It further enhances the reactivity of the metals and less N-H acidic and thermally less stable amines can be introduced into these syntheses enabling the formation of meta stable products. The crystal structures and hence the properties of the products of both high- and low-temperature oxidation of rare earth metals with amines strongly differ from reactions carried out in classic solvents. Thus reaction routes frequently used in Solid State Chemistry can well be utilized for Coordination Chemistry and offer alternatives to classic solvent based synthesis, particularly if certain properties like homoleptic character or the coordination of elements with a low chemical affinity are aimed for.