Two four-coordinate and seven-coordinate Co(II) complexes based on the bidentate ligand 1, 8-naphthyridine showing slow magnetic relaxation behavior.

For a long time, the cobalt(II) complex ([Co(napy) 4 ](ClO 4 ) 2 ) (napy = 1, 8-naphthyridine) has been considered as an eight-coordinate complex without any structural proof. After carefully studied, two complexes [Co(napy) 2 Cl 2 ] ( 1 ) and [Co(napy) 4 ](ClO 4 ) 2 ( 2 ) based on the bidentate ligand napy were synthesized and structurally characterized. X-ray single crystal structural determination showed that the cobalt(II) center in [Co(napy) 2 Cl 2 ] ( 1 ) is four-coordinate with tetrahedral geometry ( T d ), while [Co(napy) 4 ](ClO 4 ) 2 ( 2 ) is seven-coordinate rather than eight-coordinate with capped trigonal prism geometry ( C 2v ). Direct-current (dc) magnetic data revealed that complexes 1 and 2 possess positive zero-field splitting (ZFS) parameters of 11.08 and 25.30 cm -1 , respectively, with easy-plane magnetic anisotropy. Alternating current(ac) susceptibility measurements revealed that both the complexes showed slow magnetic relaxation behaviour. Theoretical calculations demonstrated that the presence of easy-plane magnetic anisotropy ( D ˃ 0) for complexes 1 and 2 is in agreement with the experimental data. Furthermore, these results can pave a way to obtain the four-coordinate and seven-coordinate cobalt(II) single-ion magnets(SIMs) by using bidentate ligand.