Magnetic anisotropy in pentacoordinate 2,6-bis(arylazanylidene-1-chloromethyl)pyridine cobalt(II) complexes with chlorido co-ligands

Abstract

Static and dynamic magnetic properties of a series of four pentacoordinate [CoII(L3A-D)Cl2] complexes 1–4 (1, L3A=N,N′-bis(2,4,6-trimethylphenyl)pyridine-2,6-dicarboximidoyl dichloride; 2, L3B=N,N′-bis(2,6-dimethylphenyl)pyridine-2,6-dicarboximidoyl dichloride; 3, L3C=N,N′-bis(4-chloro-2,6-dimethylphenyl)pyridine-2,6-dicarboximidoyl dichloride; and 4, L3D=N,N′-bis(2,6-diisopropyl)pyridine-2,6-dicarboximidoyl dichloride) were thoroughly studied. Irrespective of the sign of the zero-field splitting parameter D, all the compounds 1–4 showed the uniaxial type of magnetic anisotropy and also slow relaxation of magnetization, hence belong to the class of field-induced single-ion magnets. The values of the D parameter range from large and positive in 1 and 2 (45.8cm−1, and 38.4cm−1, respectively) to large and negative in 3 and 4 (−43.9cm−1, and −41.3cm−1, respectively). In the cases of complexes 1–3, a very large rhombicity was revealed (E/D=from 0.24 to 0.31). Only in compound 4, which possesses large negative axial and practically zero rhombic magnetic anisotropy (D=−41.3cm−1, E/D=0.0), the extraction of spin reversal barrier, U=22.8K, from Argand diagram was feasible. The experimental findings were also supported by ab initio multi-reference CASSCF/NEVPT2 calculations. Finally, the magneto-structural correlation of D vs. τ was proposed, showing that the square-pyramidal Co(II) complexes adopt large and negative D values, and moreover, that the sign of D is changed to positive on transition to the chromophore geometries closer to the trigonal bipyramidal arrangement, with a crossing point at τ≈0.29.