Macrocycles based on magnetically functionalized zirconium oxide clusters

In the presence of 4,4′-bipyridine and Co(II) or Ni(II) groups, which are coordinatively constrained by bis-alkoxyamine chelating groups, dimeric [Zr6IVO4(OH)4(RCOO)12]2 clusters rearrange to {Zr12O11(OH)11(RCOO)16} frameworks featuring highly condensed zirconium oxide cores. These {Zr12} groups subsequently each coordinate to three M(II) groups and self-assemble via two bridging 4,4′-bpy into 2.6 nm {M6Zr24}-type macrocycles of composition [M6Zr24O22(OH)22(C2H5COO)40(Hmdea)2(H2mdea)4(4,4′-bpy)2]. Surprisingly, two M(II) sites peripherally attached to the {Zr12} frameworks adopt a rare eightfold coordination mode where the M2+ ions reside in a distorted square antiprismatic O7N environment. Magnetic measurements reveal that the zirconium oxide fragments efficiently mediate antiferromagnetic exchange between Ni2+ spin centers, whereas in case of the virtually isostructural Co2+ derivative pronounced ligand field asymmetry causes single-ion effects to significantly influence the low-field susceptibility data.