Accessing Lanthanide Tricyanomethanide Coordination Polymers Using Ionic Liquids

In contrast to alkaline, alkaline earth, and transition metals, tricyanomethanide complexes of f-elements are largely underexplored mainly due to synthetic challenges originating from the harder Lewis acidic character of the trivalent cations; only one crystal structure of an f-element-tricyanomethanide complex has ever been reported. Here, we report that by using the tricyanomethanide-based ionic liquids (ILs) 1-butyl-4-methylpyridinium tricyanomethanide ([C4mpyrid][TCM]) or 1-ethyl-3-methylimidazolium tricyanomethanide ([C2mim][TCM]), we have identified a way to begin building a library of 1D and 2D lanthanide/tricyanomethanide coordination polymers, although not yet in an optimized, controllable fashion. Saturation of NdX3·6H2O (X = Cl– or [NO3]−) over 100 °C in [C4mpyrid][TCM] or [C2mim][TCM] followed by slow cooling allowed for the crystallization of [C2mim]n[Nd(NO3)2(μ3-TCM)(μ2-TCM)(OH2)]n (1), [C4mpyrid]2n[Nd(μ2-Cl)Cl2(μ2-TCM)(TCM)(OH2)]n (2), [C4mpyrid]n[Nd(μ2-TCM)2(TCM)2(OH2)3]n (3), and [C4mpyrid]n[NdCl2(μ2-TCM)2(OH2)2]n (4) as anionic chains (2, 4) or layers (1, 3). Compound 3 could be isolated either by saturation and heating of Nd(NO3)3·6H2O or by reheating the reaction mixture leading to 2. The use of [TCM]−-based ILs as solvent and sources of the coordinating ligand provides a method to promote f-element-[TCM]− complexation without the use of competing O-donor solvents or high temperatures, potentially opening a new route to structurally characterize [TCM]− complexes of the entire lanthanide series. Although these products were not synthesized in high yields, their isolation provides clues on how to modify and optimize the syntheses to obtain specific f-element-[TCM]− targets. This, in turn, should ultimately lead to dramatic diversification of the body of f-element-[TCM]− complexes available for study.