First quadruple-glycine bridging mono-lanthanide-substituted borotungstate hybrids

A class of novel organic-inorganic hybrid lanthanide (Ln)-substituted Keggin-type borotungstates K₄Na₄H₄[Ln₂(gly)₄(α-BW₁₁O₃₉)₂]·23H₂O [Ln = Ce³⁺ (1), Pr³⁺ (2), Nd³⁺ (3), Sm³⁺ (4), Eu³⁺ (5), Tm³⁺ (6); gly = glycine] have been synthesized from the reaction of K₈[BW₁₁O₃₉H]·13H₂O, NaAc·6H₂O and Ln(NO₃)₃·6H₂O by employing gly ligands as structure-stabilizing agents in the conventional aqueous solution system and structurally characterized by elemental analyses, IR spectroscopy, thermogravimetric (TG) analyses, powder X-ray diffraction (PXRD) and single-crystal X-ray diffraction. The common prominent structural feature of isomorphic 1-6 is that all of them consist of two mono-Ln-substituted Keggin [Ln(α-BW₁₁O₃₉)]^6- fragments linked by four gly ligands, furnishing an intriguing dimeric assembly of the quadruple-gly-connective mono-Ln-substituted borotungstate, in which each carboxylic oxygen atom from gly ligands is bound to Ln³⁺ cations in the μ₂-O or μ₃-O mode. To the best of our knowledge, 1-6 represent the first examples of inorganic-organic hybrid Ln-substituted borotungstates functionalized by quadruple amino acid bridges. The solid-state photoluminescence properties of 3-5 have been determined at ambient temperature and the photoluminescence emission spectra exhibit the characteristic emission bands derived from Ln³⁺ centers. The thermostability of 1-6 has been studied and the thermal decomposition procedure of 3 has been comprehensively investigated with the assistance of variable-temperature PXRD patterns and variable-temperature IR spectra. Furthermore, magnetic susceptibility measurements of 1, 2 and 4 have been conducted.