# 125Te and 57Fe nuclear resonance vibrational spectroscopic characterization of intermediate spin state mixed-valent dimers

## TL;DR

This paper explores how replacing sulfur with heavier elements in iron-sulfur clusters affects their spin states and vibrational properties.

## Contribution

The study reveals that heavier chalcogens reduce vibronic coupling, enabling intermediate spin states in mixed-valent iron complexes.

## Key findings

- Heavier chalcogens like Te decrease vibronic coupling in [2Fe-2S]+ clusters.
- Intermediate S = 3/2 spin states are unattainable with lighter Fe2Q2 cores.
- NRVS and DFT show the role of vibronic coupling in modulating electronic structures.

## Abstract

Iron-sulfur clusters fulfill numerous roles throughout biology. The reduced [2Fe-2S]+ cluster offers unique electronic and magnetic properties due to its mixed-valent nature and can serve as an essential model for understanding electron transfer, electron delocalization, and accessible spin states not only in mixed-valent dimers, but potentially larger iron sulfur  clusters. Recently a series of mixed-valent diiron dichalcogenide complexes [L2Fe2Q2]− (Q = S (1), Se (2), Te (3), L = 2,6-diisopropylphenyl β-diketiminate ligand) were synthesized and characterized, where complex 1 showed a typical S = 1/2 spin state, while complexes 2 and 3 exhibited intermediate S = 3/2 spin states, potentially enabled by the minimization of vibronic coupling. Here we studied the vibrational dynamics of the Fe and Te centers in these complexes using 57Fe and 125Te nuclear resonance vibrational spectroscopy (NRVS), coupled with DFT calculations. The findings suggest that heavy character of larger chalcogen atoms results in decreased vibronic coupling. The observation of an intermediate spin state is shown to be unattainable for lighter Fe2Q2 cores. This highlights the crucial role of vibronic coupling in modulating the electronic structure of mixed-valence systems and should enhance understanding of the electronic structure in more complex biological Fe-S clusters.

While most biological and biomimetic mixed-valent [2Fe-2S]+ clusters exhibit a S = 1/2 ground spin state, substitutions with Se and Te significantly perturb the electronic structure and yield clear S = 3/2 spin state signatures. Here, the authors probe the vibrational dynamics of the Fe and Te centers using 57Fe and 125Te nuclear resonance vibrational spectroscopy and DFT calculations.