Stabilization of intermediate spin states in mixed-valent diiron dichalcogenide complexes

The electronic structure and ground spin states, S, observed for mixed-valent iron-sulfur dimers (Fe-II-Fe-III) are typically determined by the Heisenberg exchange interaction, J, that couples the magnetic interaction of the two metal centres either ferromagnetically (J > 0, S = 9/2) or antiferromagnetically (J < 0, S = 1/2). In the case of antiferromagnetically coupled iron centres, stabilization of the high-spin S = 9/2 ground state is also feasible through a Heisenberg double-exchange interaction, B, which lifts the degeneracy of the Heisenberg spin states. This theorem also predicts intermediate spin states for mixed-valent dimers, but those have so far remained elusive. Herein, we describe the structural, electron paramagnetic resonance and Mossbauer spectroscopic, and magnetic characterization of a series of mixed-valent complexes featuring [Fe(2)Q(2)](+) (Q = S2-, Se2-, Te2-), where the Se and Te complexes favour S = 3/2 spin states. The incorporation of heavier chalcogenides in this series reveals a delicate balance of antiferromagnetic coupling, Heisenberg double-exchange and vibronic coupling.

Automated summary

This article investigates the electronic structure and spin states of mixed-valent iron-sulfur dimers and discovers a new interaction that can stabilize high spin states. By studying a series of mixed-valent complexes, complex interactions between antiferromagnetic coupling, Heisenberg double-exchange, and vibronic coupling are revealed.