# Redox and Spin States Series of an Organometallic Heme Analogue Based on a Non-Innocent NHC/N-Donor Hybrid Macrocycle

## TL;DR

A new organometallic heme analogue with redox-active ligands is developed, enabling tunable iron spin states and potential bioinspired reactivity.

## Contribution

A novel NHC/N-donor hybrid macrocycle with redox noninnocence is introduced, enabling distinct iron spin states and tunable reactivity.

## Key findings

- The FeIII complexes are antiferromagnetically coupled to a carbazolide-based π-radical.
- Axial ligands like MeCN induce low-spin configurations, while solvents and triflate anions lead to intermediate-spin states.
- The system restricts iron to low-spin or intermediate-spin states due to strong equatorial σ-donor effects.

## Abstract

Iron complexes of tetradentate macrocyclic ligands containing N-heterocyclic carbene (NHC) donors have been referred to
as organometallic heme analogues, but they usually lack the redox
noninnocence under oxidizing conditions that is characteristic of
porphyrins. Here we report a novel NHC/N-donor hybrid macrocyclic
ligand containing two trans NHC moieties, a pyridine
and a redox active carbazolide fragment. Its FeII, FeIII and formal FeIV complexes have been isolated
and comprehensively characterized, where UV/vis and 57Fe
Mössbauer spectroscopies, SQUID magnetometry and density functional
theory (DFT) calculations reveal that the latter are best described
as FeIII systems antiferromagnetically coupled to a carbazolide-based
organic π-radical. Two different redox series are obtained depending
on the axial ligands: nitriles such as MeCN give low-spin (LS) configurations
of the metal ion, while in case of weakly coordinating solvents and
triflate anions the iron adopts an intermediate-spin (IS) configuration;
MeCN binding constants have been determined. As in other heme analogues
with NHC-based macrocycles, the strong equatorial σ-donor character
raises the energy of the Fe­(d
x
2–y
2
) orbital, making high-spin (HS) iron
species inaccessible. The combined features of equatorial ligand redox
noninnocence, restriction to LS/IS surfaces and tunability via the
axial coligands makes this a promising platform for bioinspired reactivity
such as the generation of reactive Fe/O
x
intermediates.