Multielectron Redox Chemistry of a Neutral, NIR-Active, Indigo-Pillared ReI-Based Triangular Metalloprism

Self-assembled, hexarhenium(I), triangular metalloprism compound [{(CO)3Re(mu-2)Re(CO)3}3(mu 3-1)2] (3) featuring three bis-chelating pillarlike indigo dianions (mu-2), each of which connects two fac-Re(CO)3 cores, which are interconnected by a tritopic N donor, that is, a 2,4,6-tris(4-pyridyl)-1,3,5-triazine (mu 3-1, tPyTz) ligand, has been synthesized in high yield and characterized. Metalloprism 3 exhibits a strong absorption in the near-infrared (NIR) region. The reversible, multielectron redox properties of the electrogenerated 3n species, where n=3+, 0, 3-, 4-, 5-, 8-, in the visible and especially in the NIR region were investigated in THF solution by cyclic voltammetry (CV), chronocoulometry, EPR spectroscopy, and thin-layer UV/Vis/NIR spectroelectrochemistry (SEC). Stepwise, site-specific electrochemical reductions lead to the formation of a series of highly stable ion (radical) species in which electrons associated with mu-2 or mu 3-1 components of the molecule can be clearly distinguished. An EPR investigation revealed interaction of unpaired electrons with the metal nuclei (185,187Re, I=5/2) in the reduced intermediates. The framework has C2 symmetry, and accidental degeneracies suffice. Detailed theoretical calculations by structure-based DFT confirm that the triply degenerate HOMO has =70?% indigo character with a sizable dp-Re character, while the LUMO is dominated by the triply degenerate indigo ligands, and the LUMO+1 by doubly degenerate tPyTz ligands. A comparison of 3 and previously reported 2,2'-bis-benzimidazolate- (BiBzlm) or alkoxy-pillared ReI metalloprisms indicates a very low switching potential with a potential window of less than 1 V and reversibly accessible optical properties with higher stability of the intermediates. The properties exhibited by 3 appear to be due to the slight tuning of the bridging ligand from N,N- to N,O-.