Tuning the Coordination Geometry and Magnetic Relaxation of Co(II) Single-Ion Magnets by Varying the Ligand Substitutions

Three mononuclear Co(II) complexes with the formulas of Co(L1)2 (1), Co(L2)2(CH3OH)2 (2), and Co(L3)2(CH3OH)2 (3) (HL1 = 4-nitro-2-((E)- (propylimino)methyl)phenol, HL2 = 2,4-dinitro-6-((E)-(propylimino) methyl)phenol, HL3 = 2-(methoxymethyl)-4-nitro-6-((E)-(propylimino)methyl)phenol) have been synthesized and structurally characterized. The -CH2OCH3 group in the ligand of complex 3 was in situ formed during the reaction. The Co(II) ion of complex 1 is in a distorted tetrahedral environment, while the Co(II) centers in complexes 2 and 3 adopt a deformed octahedral geometry. The static magnetic data can be well fitted by the spin (1) or Griffith-Figgis (2 and 3) Hamiltonian and negative D and B20 values were obtained. Quantum chemical calculations confirm the presence of significant easy-axial magnetic anisotropy with non-negligible transversal contributions in all the three complexes. All the three complexes show field-induced slow magnetic relaxation with one (2) or two (1 and 3) relaxation processes. Interestingly, their coordination geometry and magnetic relaxation behaviors can be tuned by ligand substitutions.