Intervalence Charge Transfer in an Osmium(IV) Tetra(ferrocenylaryl) Complex

The functional properties of tetraaryl compounds, M(aryl)₄ (M = transition metal or group 14 element), are dictated not only by their common tetrahedral geometry but also by their central atom. The identity of this atom may serve to modulate the reactivity, electrochemical, magnetic, and optical behavior of the molecular species, or of extended materials built from appropriate tetraaryl building blocks, but this has not yet been systematically evaluated. Toward this goal, here we probe the influence of Os(IV), C, and Si central atoms on the spectroelectrochemical properties of a series of redox-active tetra(ferrocenylaryl) complexes. We prepare these compounds through Negishi cross-coupling from brominated precursors and confirm, through single-crystal X-ray diffraction, they exhibit comparable molecular structures with only small differences attributable to their dissimilar M-aryl bond lengths. Solution voltammetric and spectroelectrochemical studies reveal that access to mixed-valence states is uniquely provided by the Os(IV) species, which also exhibits a near-IR absorption band that is characteristic of intervalence charge transfer processes. Together, this work serves to highlight the remarkable electrochemical stability and distinct electronic properties of the Os(aryl)₄ unit, as well as the potential utility of these and other M(aryl)₄ species as modular building blocks for three-dimensional polymers or functional molecular devices.