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

## TL;DR

This paper describes the synthesis and study of transition metal complexes with a specific macrocyclic ligand, focusing on their structural and magnetic properties.

## Contribution

The study introduces a new macrocyclic ligand with pyridine-N-oxide pendant arms and reveals its impact on magnetic anisotropy in transition metal complexes.

## Key findings

- The complexes exhibit pentagonal bipyramidal geometry with magnetic anisotropy values of D = 4.47, 30.10, and −7.58 cm–1 for Fe(II), Co(II), and Ni(II) complexes.
- The Co(II) complex shows field-induced single-molecule magnet behavior with specific relaxation processes.
- Coordination ability of pendant arms significantly influences magnetic anisotropy, favoring π-acceptor groups.

## Abstract

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).