Effects of weak intramolecular interactions and distortions from trigonal prismatic coordination on the magnetic properties of zero-field Co() single-ion magnets

The mononuclear cobalt( ii ) complexes [Co(L N8 )](BPh 4 ) 2 ·C 4 H 10 O ( 1-BPh 4 ) and [Co(L N8 )](NO 3 ) 2 ·CH 3 CN ( 2-NO 3 ) (L N8 = 1,4,7,10-tetrakis(2-pyridinemethyl)-1,4,7,10-tetraaza-cyclododecane) have been synthesized and fully characterized. They differ well beyond a formal replacement of the counter anions, with their Co( ii ) centers in significantly different coordination geometries. In 1-BPh 4 , the Co( ii ) ion is not only coordinated by six N atoms from the L N8 ligand but also associated with the two remaining N atoms of L N8 via weak interactions. In contrast, the Co( ii ) ion in 2-NO 3 is only six-coordinated in a distorted trigonal prismatic geometry. Magnetic anisotropy and slow magnetic dynamics are drastically affected by these different environments around Co( ii ). The combination of dc magnetic data, high-frequency and -field electron paramagnetic resonance (HFEPR) and theoretical calculations unambiguously reveals large negative zero-field split (ZFS) parameters D for both complexes and a large difference between the D values. Both 1-BPh 4 and 2-NO 3 show slow magnetic relaxation at zero field. Magnetic dilution experiments reveal effective energy barriers of 54(4) cm −1 for 1-BPh 4 @Zn and 95(5) cm −1 for 2-NO 3 @Zn and confirm that the slow magnetic relaxation for both complexes originates from single molecule behaviour. Ab initio computational studies explain their electronic structures and the origin of the large negative magnetic anisotropy; they support the corresponding experimental observations. Further magneto-structural analyses reveal that different distortions from the ideal trigonal prismatic geometry exert drastic effects on D values and slow relaxation, and that the additional weak intramolecular interactions between Co and N reduce the axial anisotropy. The change of counter anions significantly affects the environment around Co( ii ) ions, leading to two zero-field single-ion magnets possessing different uniaxial anisotropies and dynamic magnetic properties.