Tetra-rhenium molecular rectangles as organizational motifs for the investigation of ligand-centered mixed valency: Three examples of full delocalization

Molecular rectangles having the form {[Re(CO)₃] ₂(X)₂}₂-μ,μ′-(LL)₂, where X is either a bridging alkoxide or phenylthiolate group and LL is 4,4′-bipyridine or pyrazine, are characterized by cofacial LL pairs that are in van der Waals contact across the "long" side of the rectangle. Cyclic voltammetry shows that the redox-accessible bridging ligands, LL, are reduced in sequential, one-electron reactions. The singly reduced rectangles represent an unusual type of mixed-valence compound in which the LL ligands themselves are the redox centers. Spectroelectrochemical measurements for mixed-valence forms of these rectangles reveal intense, asymmetric absorption bands in the near-infrared region, assigned as intervalence transitions. Electroabsorption (Stark spectroscopy) measurements reveal minute changes in dipole moment and therefore a lack of significant charge transfer upon intervalence excitation. Thus, the rectangles are unusual examples of class III (fully valence delocalized) molecular mixed-valence species that employ direct donor-orbital/acceptor-orbital overlap rather than covalent-bond-mediated superexchange to achieve the large electronic coupling strengths required for delocalization.