Probing the Effect of Axial Ligands on Easy-Plane Anisotropy of Pentagonal-Bipyramidal Cobalt(II) Single-Ion Magnets

We herein reported
the synthetic, structural, computational, and
magnetic studies of four air-stable heptacoordinated mononuclear cobalt­(II)
complexes, namely, [CoII(tdmmb)­(H2O)2]­[BF4]2 (1), [CoII(tdmmb)­(CN)2]·2H2O (2), [CoII(tdmmb)­(NCS)2] (3), and [CoII(tdmmb)­(SPh)2] (4) (tdmmb = 1,3,10,12-tetramethyl-1,2,11,12-tetraaza[[3]­(2,6)­pyridino[3]­(2,9)-1,10-phenanthrolinophane-2,10-diene;
SPh– = thiophenol anion). Constrained by the rigid
pentadentate macrocyclic ligand tdmmb, the CoII centers
in all of these complexes are in the heptacoordinated pentagonal-bipyramidal
geometry. While the equatorial environments of these complexes remain
very similar to each other, the axial ligands are systematically modified
from C to N to O to S atoms. Analyses of the magnetic data and the
ab initio calculations both reveal large easy-plane magnetic anisotropy
(D > 0) for all four complexes. While the experimentally
obtained D values do not show any clear tendency
when the axial coordinated atoms change from C to N to O atoms (complexes 1–3), the largest value is for the heavier
and softer S-atom-coordinated complex 4. Because of significant
magnetic anisotropy, all four complexes are field-induced single-ion
magnets. This work represents a delicate modification of the magnetic
anisotropy by tuning the chemical environment of the metal centers.