Influence of Rare-Earth Ion Radius on Metal-Metal Charge Transfer in Trinuclear Mixed-Valent Complexes

We report the synthesis and characterization of a highly conjugated bisferrocenyl pyrrolediimine ligand, Fc(2)PyrDIH (1), and its trinuclear complexes with rare earth ions-(Fc(2)PyrDI)M(N(TMS)(2))(2) (2-M, M = Sc, Y, Lu, La). Crystal structures, nuclear magnetic resonance (NMR) spectra, and ultraviolet/visible/near-infrared (UV/vis-NIR) data are presented. The latter are in good agreement with DFT calculations, illuminating the impact of the rare earth ionic radius on NIR charge transfer excitations. For [2-Sc](+), the charge transfer is at 11,500 cm(-1), while for [2-Y](+), only a d-d transition at 8000 cm(-1) is observed. Lu has an ionic radius in between Sc and Y, and the [2-Lu](+) complex exhibits both transitions. From time-dependent density functional theory (TDDFT) analysis, we assign the 11,500 cm(-1) transition as a mixture of metal-to-ligand charge transfer (MLCT) and metal-to-metal charge transfer (MMCT), rather than pure metal-to-metal CT because it has significant ligand character. Typically, the ferrocenes moieties have high rotational freedom in bis-ferrocenyl mixed valent complexes. However, in the present (Fc(2)PyrDI)M(N(TMS)(2))(2) complexes, ligand-ligand repulsions lock the rotational freedom so that rare-earth ionic radius-dependent geometric differences increasingly influence orbital overlap as the ionic radius falls. The Marcus-Hush coupling constant HAB trends as [2-Sc](+) > [2-Lu](+) > [2-Y](+).