Emergent Spin-Glass Behavior in an Iron(II)-based Metal-Organic Framework Glass

Metal-organic framework (MOF) glasses combine the structural tunability of crystalline MOFs with the processability of amorphous materials, offering exciting opportunities for functional hybrid materials. Here, we report a one-pot, solvent-free synthesis of an Fe2+-based MOF glass, gFe-tBubipy, with the composition [Fe2(im)3.12(bim)0.88(tBubipy)0.11]·[Fe(Cp)2]0.09 (im– = imidazolate, bim– = benzimidazolate, tBubipy = 4,4’-di-tert-butyl-2,2’-bipyridine, Cp– = cyclopentadienyl anion). This material forms a continuous random network structure of four-connected tetrahedral and octahedral Fe2+ nodes and exhibits an exceptionally low glass transition temperature (Tg = 87 °C). Despite its amorphous nature and complex composition, gFe-tBubipy exhibits a high degree of local structural order that enables strong antiferromagnetic exchange interactions between Fe2+ centers. Remarkably, it exhibits clear signatures of spin-glass behaviour, with a well-defined magnetic freezing transition around 14 K. This combination of a MOF glass exhibiting a distinct glass transition with spin-glass magnetism arising from topological disorder and frustrated, short-range magnetic interactions represent a significant advance. This discovery underscores the transformative potential of MOF glasses as a versatile platform for exploring the interplay between structural disorder and cooperative magnetic phenomena in hybrid materials.