Tunable valence tautomerism in lanthanide-organic alloys

The electronic structures of the lanthanide (Ln) ions are inimitable and key to advanced materials and technologies. The trivalent ions are ubiquitous and dwarf the use of di- and tetra-valent analogues, which, however, possess vastly different optical and magnetic properties. Hence, alteration of the valence electron count by external stimuli would lead to dramatic changes in materials properties. Compounds exhibiting a temperature-induced, complete Ln(III) ⇄ Ln(II) switch, referred to as a valence tautomeric (VT) transition, are virtually absent. Herein, we present an abrupt and hysteretic VT transition in a lanthanide-based coordination polymer, SmI2(pyrazine)3, driven by the interconversion of closed-shell Sm(II)-pyrazine(0) and paramagnetic Sm(III)-pyrazine(●–) redox pairs. Alloying SmI2(pyrazine)3 with Yb(II) yields isomorphous Sm(1–x)Yb(x)I2(pyrazine)3 solid solutions with VT transition critical temperatures ranging widely from 200 K to ~50 K at ambient pressure. These findings demonstrate a simple strategy to realize thermally switchable magnetic materials with chemically tunable transition temperatures.