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) 32 (2, 2′-bis (benzimidazolate) ) precursor Re (CO) 32 (BiBzIm) and the core-rigidified 4, 4′-bipyridine-type bridging ligands thieno2, 3-c: 5, 4-c'dipyridine (complex 2a) or 9- (4-methylthiophenyl) -9H-pyrrolo2, 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 Δρ 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% and 2. 0%, respectively.