A Mixed-Valent Low-Spin Triiron Fe0FeIIFe0 Complex Stabilized by a Bis-Radical Platform

The synthesis, characterization, and electronic structure of a mixed-valent triiron complex of redox-active tppz ligands [{Fe0(CO)2(μ-tppz•-)}2FeII] are discussed. This complex bears a low-spin FeII center, bound to two tppz radical anions, that displays no exchange coupling at room temperature between the unpaired electrons of the ligands. The assignment of this electronic structure is supported by Mössbauer, EPR, and IR spectroscopies, solid-state geometric parameters, SQUID magnetometry, multireference calculations, and cyclic voltammetry. The electronic structure of the title compound is compared to previously reported FeII complexes of polyazine radicals, which bear high-spin FeII centers with strong antiferromagnetic coupling with the ligand-centered unpaired electrons. This difference sparks a discussion on the relative importance of ligand-metal exchange coupling and ligand field strength in controlling the configuration of metal complexes of redox-active ligands.