Structure and Magnetism of Seven‐Coordinate Fe III , Fe II , Co II and Ni II Complexes Containing a Heptadentate 15‐Membered Pyridine‐Based Macrocyclic Ligand

A heptadentate macrocyclic ligand, H 2 L (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 [Fe III L ]ClO 4 ( 1 ), [Fe II L ] · H 2 O ( 2 ), [Co II L ] · H 2 O ( 3 ), and [Ni II L ] · H 2 O ( 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 Co II 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).