Lanthanide complexes with zwitterionic amidoximes stabilized by noncoordinating water molecules

We present the first structural report of lanthanides complexed with free amidoxime ligands as part of our ongoing research effort to understand the interactions of amidoximes with metal ions in seawater. Three isomorphous lanthanide complexes with acetamidoxime (AcAO) having the formula Ln2(NO3)6(AcAO)3(OH2)3·3H2O (Ln = Pr3+, Nd3+, Gd3+) have been crystallized and structurally characterised. Notably, the AcAO ligands coordinate in a bridging mode as zwitterions, which creates pre-organised hydrogen bonding cavities into which water molecules are incorporated. Charge-assisted hydrogen bonds to these water molecules appear to stabilize the zwitterionic form and thus the overall complex, underscoring the importance of supramolecular phenomena and overall complex geometry on the relative stability of amidoxime complexes. This has implications for controlling their selectivity towards metal ions.