New phosphorescent supramolecular Re-rectangles and their mixed-valent anions

We report on the synthesis and characterization of two new macrocyclic, rectangular tetrarhenium complexes resulting from the {Re(CO) 3 } 2 (2,2′-bis(benzimidazolate)) precursor {Re(CO) 3 } 2 (BiBzIm) and the core-rigidified 4,4′-bipyridine-type bridging ligands thieno[2,3-c:5,4-c']dipyridine (complex 2a ) or 9-(4-methylthiophenyl)-9H-pyrrolo[2,3-c:5,4-c']dipyridine (complex 2b ). Both complexes undergo four consecutive one-electron reductions which are all centered on the diimine ligands. Redox splittings of the first pair of reductions are traced to electronic coupling in the mixed-valent state where the charges at the two diimine ligands, 0 and −1, (formally) differ. IR and UV/vis/NIR spectroscopic investigations on the neutral, the one-, and the two-electron reduced forms of these complexes characterize the singly reduced monoanions as moderately strongly coupled mixed-valent systems of Class II. Interestingly, the charge distribution parameter Δ ρ derived from IR spectroscopy of ca. 0.25 is roughly twice as large as the optically derived values of Hush´s delocalization parameter α. Complexes 2a,b are strongly phosphorescent at 77 K, with quantum yields of 62% for 2a and 91% for 2b . Phosphorescence is retained at r. t., albeit with significantly reduced quantum yields of 1.1% or 2.0%, respectively.