Control of the geometry and anisotropy driven by the combination of steric and anion coordination effects in CoII complexes with N6-tripodal ligands: the impact of the size of the ligand on the magnetization relaxation time

Abstract

Four mononuclear Co^II complexes of formula [Co(L)(SCN)₂(CH₃OH)_0.5(H₂O)_0.5]·1.5H₂O·0.75CH₃OH (1), [Co(L1)Cl₂]·H₂O·2CH₃CN (2), [Co(L1)(SCN)₂]·1.5H₂O·CH₃OH (3) and [Co(L1)]ClO₄·2CH₃OH (4) were prepared from the N₆-tripodal Schiff base ligands (S)P[N(Me)NC(H)2-Q]₃ (L) and (S)P[N(Me)NC(H)1-ISOQ]₃ (L1), where Q and ISOQ represent quinolyl and isoquinolyl moieties, respectively. In 1, the L ligand does not coordinate to the Co^II ion in a tripodal manner but using a new N,N,S tridentate mode, which is due to the fact that the N₆-tripodal coordination promotes a strong steric hindrance between the quinolyl moieties. However, L1 can coordinate to the Co^II ions either in a tripodal manner using Co^II salts with poorly coordinating anions to give 4 or in a bisbidentate fashion using Co^II salt-containing medium to strongly coordinating anions to afford 2 and 3. In the case of L1, there is no steric hindrance between ISOQ moieties after coordination to the Co^II ion. The Co^II ion exhibits a distorted octahedral geometry for compounds 1–3, with the anions in cis positions for the former and in trans positions for the two latter compounds. Compound 4 shows an intermediate geometry between an octahedral and trigonal prism but closer to the latter one. DC magnetic properties, HFEPR and FIRMS measurements and ab initio calculations demonstrate that distorted octahedral complexes 1–3 exhibit easy-plane magnetic anisotropy (D > 0), whereas compound 4 shows large easy-axis magnetic anisotropy (D < 0). Comparative analysis of the magneto-structural data underlines the important role that is played not only by the coordination geometry but also the electronic effects in determining the anisotropy of the Co^II ions. Compounds 2–3 show a field-induced slow relaxation of magnetization. Despite its large easy-axis magnetic anisotropy, compound 4 does not show significant slow relaxation (SMR) above 2 K under zero applied magnetic fields, but its magnetic dilution with Zn^II triggers SMR at zero field. Finally, it is worth remarking that compounds 2–4 show smaller relaxation times than the analogous complexes with the tripodal ligand bearing in its arms pyridine instead of isoquinoline moieties, which is most likely due to the increase of the molecular size in the former one.