A heptadentate 15-membered pyridine-based macrocyclic ligand containing two pyridine-N-oxide pendant arms (L4 = 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(L4)](ClO4)2·1DMF (MII = Mn (1), Fe (2), Co (3), and Ni (4); DMF = N,N'-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 1-3 or 5 + 2 for Ni(II) complex 4 with a large Jahn-Teller distortion. Fe(II), Co(II), and Ni(II) complexes 2, 3, and 4 show pronounced magnetic anisotropy (D = 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 3 showed a field-induced single-molecule magnet behavior described best by the combination of direct (DHm = 145 K-1s-1) and Raman (C = 0.58 K-ns-1 for n = 5.76) relaxation processes. Magneto-structural correlation for Fe(II)/Co(II)/Ni(II) complexes with L4 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).

Department of Inorganic Chemistry Faculty of Chemical and Food Technology Slovak University of Technology in Bratislava Bratislava SK 81237 Slovakia

Department of Inorganic Chemistry Faculty of Science Palacký University Olomouc 17 Listopadu 12 Olomouc CZ 77146 Czech Republic

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