Nine isomorphous lanthanide–uranyl f–f bimetallic materials with 2-thiophenecarboxylic acid and terpyridine: structure and concomitant luminescent properties

Abstract

A series of nine new 4f–5f lanthanide–uranyl complexes, [(UO₂)₂(O)(L¹)(L²)₂]₂[UO₂(L¹)₃]₂[Ln₂(L¹)₆(L²)₂]·2H₂O, (where L¹ and L² are 2-thiophenecarboxylic acid (C₅H₃SO₂) and 2,2′:6′2′′-terpyridine (C₁₅H₁₁N₃), respectively, and Ln = Pr³⁺ (1), Nd³⁺ (2), Sm³⁺ (3), Eu³⁺ (4), Gd³⁺ (5), Tb³⁺ (6), Dy³⁺ (7), Ho³⁺ (8), Er³⁺ (9)), and a uranyl only complex (10), [(UO₂)₂(O)(L¹)(L²)₂]₂[UO₂(L¹)₃](NO₃)·(H₂O)₃, have been hydrothermally synthesized and characterized with single crystal and powder X-ray diffraction. The nine bimetallic complexes are isomorphous, where only the identity of the lanthanide metal center changes, and contain two distinct uranyl units: a monomer with a formal negative charge and a dimer with a formal positive charge, as well as a single neutral dimeric lanthanide unit. The uranyl only phase (10) contains two unique dimers and a monomer, both similar to the units observed in the heterometallic series. Comparative analyses of speciation and the first coordination spheres between of title complexes and building units in the literature reveal common motifs and a possible structural influence of uranyl units. Further, the units in all complexes are held together primarily through weak hydrogen and offset π-stacking interactions. Raman and visible luminescent spectroscopic techniques found concurrent and selective emission from the uranyl and lanthanide metal centers in certain compounds.