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, [La 2 (ox) 2 (H 6 L)]·2H 2 O ( 1, H 8 L = (H 2 O 3 PCH 2 ) 2 NCH 2 CH 2 CH 2 CH 2 N(CH 2 PO 3 H 2 ) 2 ) and Ln 2 (ox) 2 (H 6 L)(H 2 O) 2 ·4H 2 O(Ln = Nd, 2; Eu, 3 ), have been prepared and structurally characterized. The interconnection of La(III) ions in 1 by bridging H 6 L 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 H 6 L 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(CH 2 PO 3 H) 2 moieties of H 6 L 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 H 6 L 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.