Employing Lewis Acidity to Generate Bimetallic Lanthanide Complexes

With the advent of lanthanide-based technologies, there is a clear need to advance the fundamental understanding of 4f-element chelation chemistry. Herein, we contribute to a growing body of lanthanide chelation chemistry and report the synthesis of bimetallic 4f-element complexes within an imine/hemiacetalate framework, Ln(2)TPT(OMe) [Ln = lanthanide; TPTOMe = tris(pyridineimine)(Tren)tris(methoxyhemiacetalate); Tren = tris(2-aminoethylamine)]. These products are generated from hydrolysis and methanolysis of the cage ligand tris(pyridinediimine)bis(Tren) (TPT; Tadanobu et al. Chem. Lett. 1993, 22 (5), 859-862) likely facilitated by inductive effects stemming from the Lewis acidic lanthanide cations. These complexes are interesting because they result from imine cleavage to generate two metal binding sites: one pocketed site within the macrocycle and the other terminal site capping a hemiacetalate moiety. A clear demarcation in reactivity is observed between samarium and europium, where the lighter and larger lanthanides generate a mixture of products, Ln(2)TPT(OMe) and LnTPT. Meanwhile, the heavier and smaller lanthanides generate exclusively bimetallic Ln(2)TPT(OMe). The cleavage reactivity to form Ln(2)TPT(OMe) was extended beyond methanol to include other primary alcohols.