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]2BiBzIm)2-μ,μ‘-(LL)2, where BiBzIm 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, Hab2, 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.