Structure-property correlation in stabilizing axial magnetic anisotropy in octahedral Co(II) complexes

Large easy-axis magnetic anisotropy in a series of trigonal prismatic mononuclear cobalt(ii) complexes with zero-field hidden single-molecule magnet behaviour: the important role of the distortion of the coordination sphere and intermolecular interactions in the slow relaxation

“…Therefore, in order to clarify this appar-ent contradiction more examples of this type of complexes, like 5, where the sign of D is unambiguously supported by a combination of theoretical and experimental methods, such as HFEPR, PND and single-crystal torque anisotropy, are needed. 18,21 In light of the above considerations, the aim of this work is fourfold: (i) to confirm that complexes 1-4 show large axial magnetic anisotropy. The previous results of Ruiz and coworkers 10 and Costes 22 and co-workers for Co II and Co II Y III Co II complexes (the latter can be considered as formed by two almost isolated mononuclear complexes) with CoN 6 and CoN 3 O 3 trigonal prismatic coordination environments, respectively, bearing similar tripodal ligands to L, showed negative D values of −72 cm −1 and −38.8 cm −1 , respectively; (ii) to analyse how the change of the anion alters the Co II coordination sphere and, consequently, the axial magnetic anisotropy.…”

“…In view of this, we decided to prepare a series of trigonal prismatic Co II Although complexes 1-4 have the expected distorted trigonal prismatic geometry, complex 5 is an axially compressed octahedral complex. It is worth noting that dc measurements and ab initio theoretical calculations recently reported for the compressed octahedral mononuclear complex [Co(tu) 4 Cl 2 ] (tu = thiourea), 18 bearing sulfur donor atoms in the equatorial positions and chloride ligands in axial positions, have shown that it exhibits easy-axis anisotropy, whereas the elongated octahedral [Co( py) 4 Cl 2 ] complex, with nitrogen donor atoms in equatorial positions, possesses easy-plane anisotropy. 18,19 The type of magnetic anisotropy was unambiguously supported by polarized neutron diffraction method (PND) in the former complex and by high-frequency and -field (HFEPR) measurements in the latter.…”

“…Thus, Co II complexes combining strong axial and weak equatorial bonds (axially compressed octahedral geometry), as it occurs in [Co(tu) 4 Cl 2 ], stabilize a large axial anisotropy, whereas the combination of large axial bonds and short equatorial bonds, like in [Co(py) 4 Cl 2 ], favors large positive D values. 18,19 This qualitative magneto-structural correlation seems not to be of general applicability, because compressed octahedral CoN 6 complexes, such as [Co(py) 4 (NCS) 2 ] 19 and [Co(pto) 4 (NCS) 2 ] (pto = 2-(pyridine-2-yl)-5-thiophen-2-yl)-1,3,4-oxadiazole, 20 have a similar ratio between the axial and equatorial bond distances, however, they exhibit D values of opposite sign. Therefore, in order to clarify this appar-ent contradiction more examples of this type of complexes, like 5, where the sign of D is unambiguously supported by a combination of theoretical and experimental methods, such as HFEPR, PND and single-crystal torque anisotropy, are needed.…”

“…It is worth noting that dc measurements and ab initio theoretical calculations recently reported for the compressed octahedral mononuclear complex [Co(tu) 4 Cl 2 ] (tu = thiourea), 18 bearing sulfur donor atoms in the equatorial positions and chloride ligands in axial positions, have shown that it exhibits easy-axis anisotropy, whereas the elongated octahedral [Co( py) 4 Cl 2 ] complex, with nitrogen donor atoms in equatorial positions, possesses easy-plane anisotropy. 18,19 The type of magnetic anisotropy was unambiguously supported by polarized neutron diffraction method (PND) in the former complex and by high-frequency and -field (HFEPR) measurements in the latter. Moreover, from theoretical ab initio calculations on model compounds (constructed from the above indicated complexes by changing axial or equatorial distances and keeping other structural parameters the same), it was deduced that both easy-axis and easy-plane anisotropies can be attained by fine-tuning the ligand field.…”

Field-induced mononuclear cobalt(ii) single-molecule magnet (SMM) based on a benzothiadiazole-ortho-vanillin ligand

“…These features are responsible for the extensive study of its complexes as candidates to mononuclear SMMs. [35][36][37][38][39][40][41][42] Indeed, six-coordinate Co(II) mononuclear complexes showing octahedral (O h ), [43][44][45] trigonal prismatic (TP) 46,47 or trigonal antiprismatic 48 coordination geometries are very good candidates for single-molecule magnets (SMMs). Usually, the octahedral complexes show a uniaxial anisotropy (D < 0) and only occasionally an easy-plane anisotropy (D > 0), but in all cases, the zfs is large.…”