A neutron diffraction study of hydrogen bonding in isostructural potassium and ammonium lanthanoidates

Abstract

Two isostructural series of compounds NH₄[Ln(cdm)₄(H₂O)₄]·18c6·3H₂O (1Ln) and [K(18c6)(H₂O)₂][Ln(cdm)₄(H₂O)₄]·H₂O (2Ln) have been synthesised and structurally characterised (Ln = Gd, Dy, Er, cdm = C(CN)₂(CONH₂)⁻). These two classes of compounds are shown to be essentially isostructural to each other despite the change in counter-cation (ammonium vs. potassium) and the ensuing changes in coordination and hydrogen-bonding of water molecules in the structure. Structural data for 1Dy and 2Dy have been obtained using single crystal Laue neutron diffraction, allowing the precise location of all hydrogen atoms to be determined using fully anisotropic models. Both 1 and 2 contain the anionic complex [Ln(cdm)₄(H₂O)₄]⁻ in which the 8-coordinate lanthanoid is coordinated by a ring of four O-bound cdm ligands supported by inter-ligand N–H⋯O hydrogen bonds. The structures pack with complicated hydrogen-bonding networks in which both coordinated and non-coordinated water molecules are hydrogen-bond donors and all of the nitrile groups of the cdm ligands are hydrogen-bond acceptors. The change in cation between ammonium and potassium affects two water molecule sites which are both coordinated in 2Ln but not in 1Ln; in 1Ln one of these water molecules forms a hydrogen bond with NH₄⁺ whilst the other has neither a coordination interaction nor a hydrogen bond to its oxygen atom.