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, Fc2PyrDIH (1), and its trinuclear complexes with rare earth ions(Fc2PyrDI)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 (Fc2PyrDI)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]+.