Pentagonal Bipyramidal First-Row Transition Metal Complexes with Macrocyclic Ligand Containing Two Pyridine- <i>N</i> -Oxide Pendant Arms: Structural, Magnetic, and Theoretical Studies

A heptadentate 15-membered pyridine-based macrocyclic ligand containing two pyridine-<i>N</i>-oxide pendant arms (<b>L4</b> = 3,12-bis((pyridine-1-oxide-2-yl)methyl)-6,9-dioxa-3,12,18-triazabicyclo[12.3.1]octadeca-1(18),14,16-triene) was synthesized together with its first-row transition metal complexes with the general formula [M(<b>L4</b>)](ClO₄)₂·1DMF (M^II = Mn (<b>1</b>), Fe (<b>2</b>), Co (<b>3</b>), and Ni (<b>4</b>); DMF = <i>N</i>,<i>N'</i>-dimethylformamide), which were thoroughly investigated. According to the obtained X-ray crystal structures, all complexes possess axially compressed pentagonal bipyramidal geometry with a coordination number of 7 for <b>1</b>-<b>3</b> or 5 + 2 for Ni(II) complex <b>4</b> with a large Jahn-Teller distortion. Fe(II), Co(II), and Ni(II) complexes <b>2</b>, <b>3</b>, and <b>4</b> show pronounced magnetic anisotropy (<i>D</i> = 4.47, 30.10, -7.58 cm^-1, respectively). The magnetic properties of the studied complexes were supported by theoretical calculations, which corresponded very well to the experimental data for magnetic anisotropy. Furthermore, complex <b>3</b> showed a field-induced single-molecule magnet behavior described best by the combination of direct (<i>DH^m</i> = 145 K^-1s^-1) and Raman (<i>C</i> = 0.58 K^-ns^-1 for <i>n</i> = 5.76) relaxation processes. Magneto-structural correlation for Fe(II)/Co(II)/Ni(II) complexes with <b>L4</b> and previously studied structurally similar ligands revealed a significant impact of the coordination ability of the functional group in pendant arms on the final magnetic anisotropy (π-acceptors appear to be more suitable).