Field-induced slow magnetic relaxation behaviours in binuclear cobalt(II) metallocycles and exchange-coupled clusters

Precise control of the structures and magnetic properties of a molecular material constitutes an important challenge to realize tailor-made magnetic function. Herein, we report that the ligand-directed coordination self-assembly of a dianionic cobalt(II) mononuclear complex and selective organic linkers has led to two distinct dicobalt(II) complexes, [Co₂(pdms)₂(bpym)₃]·2MeCN (1) and [Co(pdms)(bipm)]₂·3DMF (2) (H₂pdms = 1,2-bis(methanesulfonamide)benzene; bpym = 2,2'-bipyrimidine; bimp = 1,4-bis[(1H-imidazol-1-yl)methyl]benzene). Structural analyses revealed that complexes 1 and 2 are discrete binuclear molecules containing two neutral {Co(pdms)} species bridged by bpym and bimp ligands, respectively, forming an exchange-coupled CoII2 dimer and a rare CoII2 metallocycle. Magnetic studies found that 1 exhibits considerable antiferromagnetic interactions transferred by the bpym bridge while negligible magnetic interactions in 2 due to the long bimp ligands. Interestingly, both the complexes show significant magnetic anisotropy and thus exhibit slow magnetic relaxation under an external dc field originating from spin-lattice relaxation. Detailed theoretical calculations were further applied to understand the magnetic interactions and magnetic anisotropy in 1 and 2. The foregoing results highlight that coordination solids with programmed structures and magnetic properties can be designed and prepared through a judicious selection of molecular complex building blocks and organic linkers.