Rhenium-based molecular rectangles as frameworks for ligand-centered mixed valency and optical electron transfer

A series of six neutral, tetrametallic, molecular rectangles has been synthesized that have the form ([Re(CO)₃]₂BiBzlm) ₂-μ,μ′(LL)₂, where BiBzlm is 2,2′-bisbenzimidazolate and LL is a reducible, dipyridyl or diazine ligand. X-ray crystallographic studies of the six show that the rectangle frameworks, as defined by the metal atoms, range in size from 5.7 Å × 7.2 Å to 5.7 Å × 19.8 Å. The singly reduced rectangles are members of an unusual category of mixed-valence compounds in which the ligands themselves are the redox centers and interligand electronic communication is controlled by direct ligand orbital overlap rather than by superexchange through the metal ions. Despite nominally identical coordination-defined ligand positioning, the spectrally determined electronic strengths, H_ab², vary by roughly 100-fold. As shown by X-ray crystallography and computational modeling, the observed differences largely reflect detailed geometric configurational differences that can either facilitate or frustrate productive direct orbital overlap.