Structure and Magnetism of Seven‐Coordinate FeIII, FeII, CoII and NiII Complexes Containing a Heptadentate 15‐Membered Pyridine‐Based Macrocyclic Ligand

Abstract

A heptadentate macrocyclic ligand, H2L (3,12,18‐triaza‐6,9‐dioxabicyclo[12.3.1]octadeca‐1,14,16‐triene‐3,12‐diacetic acid) with two acetate pendant arms, and its complexes [FeIIIL]ClO4 (1), [FeIIL]·H2O (2), [CoIIL]·H2O (3), and [NiIIL]·H2O (4) were synthesized. The complexes possess an axially compressed pentagonal bipyramidal geometry with the coordination numbers of 7 for 2 and 3, and 6 + 1 for 4. The magnetic susceptibility measurements revealed magnetic anisotropy for compounds 1–4 expressed by axial zero‐field splitting (ZFS) parameters (D = –1.3 cm–1 for 1, –9.6 cm–1 for 2, 29.1 cm–1 for 3, and –8.5 cm–1 for 4), with a rhombic ZFS (E/D = 0, 0.006, 0, and 0.193, respectively). Field‐induced slow relaxation of magnetization was observed for the CoII complex 3 only. The structural and magnetic features of 1–4 were compared with those of similar complexes containing ligand with two 2‐pyridylmethyl pendant arms. Magnetic features of the complexes were also evaluated using theoretical calculations (DFT, CASSCF/NEVPT2).