Quinonoid-Bridged Chair-Shaped Dirhenium(I) Metallacycles: Synthesis, Characterization, and Spectroelectrochemical Studies

Self-assembled, chair-shaped dirhenium(I) macrocyclic compounds featuring the two different bis-chelating quinone dianions (1, L = dhnq(2-); 2, L = dhaq(2-); H(2)dhnq = 6,11-dihydroxy-5,12-naphthacenedione; H(2)dhaq = 1,4-dihydroxy-9,10-anthraquinone) that interface with two fac-Re(CO)(3) cores and a ditopic semirigid N-donor 1,4-bis(5,6-dimethylbenzimidazol-1-ylmethyl)naphthalene (L' = p-NBimM) ligand coordinated to the remaining orthogonal site were prepared in high yields. Their structures were confirmed by single-crystal X-ray diffraction analysis. Electrochemical assessments, using cyclic voltammetry (CV) and UV-vis-NIR spectroelectrochemistry (SEC), revealed the existence of two well-separated, single-electron quinone ligand-centered, reversibly accessible 0, -1, and -2 redox states. Among the two singly reduced radical complexes, the symmetrically bridged quinone complex 1(center dot-), showed a strong absorption in the NIR regions, which was not observed for the neutral and doubly reduced states, analogous to that of the free dhnq(3 center dot-) quinone. In contrast, when 2 was reduced to 2(center dot-), a broad signal at 866 nm was observed, very similar to the reduced dhaq(3 center dot-) quinone. This difference in spectral behavior in the singly reduced states is likely due to the annealed benzene ring in 1 and dhnq(2-) because of its symmetrical pi-electron system, which is perturbed to a lesser degree compared to asymmetric 2 and dhaq(2-). Reduction to 1(center dot-) produces a small but not negligible g factor anisotropy (Delta g = 0.024) in the electron spin resonance (ESR) signal, indicative of a very small metal-centered spin (5%), but 2(center dot-) shows a g value in the expected range for organic radicals (no detectable Delta g). Thus, the combined investigations reveal that the singly reduced metallacycles are best described as being highly stable, noncommunicating, localized, quinonoid-centered radical complexes, [(CO)(3)Re-I(mu-L3 center dot-)(mu-L')Re-I(CO)(3)](center dot-).