Introducing a Second Ligand: New Route to Luminescent Lanthanide Polyphosphonates

By introduction of oxalate as the second ligand, three novel luminescent lanthanide tetraphosphonates with two
types of structures, namely, [La2(ox)2(H6L)]·2H2O (1, H8L = (H2O3PCH2)2NCH2CH2CH2CH2N(CH2PO3H2)2) and Ln2(ox)2(H6L)(H2O)2·4H2O(Ln = Nd, 2; Eu, 3), have been prepared and structurally characterized. The interconnection of La(III) ions in 1 by
bridging H6L anions resulted in a 3D network with tunnels running along the a-axis, the oxalate anions are located inside the above
tunnels, and each forms two five-membered chelating rings with two La(III) ions. All four phosphonate groups of the H6L anion are
bidentate and singly protonated; its two amine groups are also protonated. Compounds 2 and 3 are isostructural, and their structures
feature a pillared layered structure. The layer is formed by Ln(III) ions interconnected by oxalate and −NH(CH2PO3H)2 moieties
of H6L anions, and the organic groups of the tetraphosphonate ligand act as the pillars. Alternatively, their structures can be described
as the lanthanide tetraphosphonate layers being cross-linked by oxalate anions (pillars). The H6L anions in 2 and 3 are hexadentate,
and each bridges with six Ln(III) ions; two phosphonate groups are bidentate, and the other two are unidentate. Compounds 1, 2,
and 3 exhibit strong emission bands in the blue, red light, and near-IR region, respectively.