Facile and environmentally friendly synthesis of six heterometallic dumbbell-shaped MLn (M = Co, Ni; Ln = Eu, Gd, Dy) clusters as cryogenic magnetic coolants and molecular magnets.

Six analogous dumbbell-shaped 3d-4f complexes MLn were obtained by the reaction of Ln(NO3)3 (Ln = Eu, Gd, Dy) and M(OAc)2 (M = Co, Ni) without any other organic ligands, and M5Gd4 exhibited a large magnetocaloric effect (-ΔSm = 31.0, 37.3 J kg-1 K-1 for Co5Gd4 and Ni5Gd4 respectively), while M5Dy4 and Co5Eu4 showed single molecule magnet behaviour. To the best of our knowledge, Co5Eu4 is the first Eu-based cluster to display single molecule magnet behaviour (Ueff = 16.4 K).