Combined Experimental and Computational Study of Pyren-2,7-diyl-Bridged Diruthenium Complexes with Various Terminal Ligands

Cyclometalated diruthenium complexes 1(PF6)2-5(PF6)2 bridged by 1,3,6,8-tetra(pyrid-2-yl)-pyrene have been prepared, with the terminal ligand bis(N-methylbenzimidazolyl)pyridine (1(PF6)2), 4'-di-(p-methoxyphenyl)amino-2,2':6',2″-terpyridine (2(PF6)2), 4'-p-methoxyphenyl-2,2':6',2″-terpyridine (3(PF6)2), 2,2':6',2″-terpyridine (4(PF6)2), and trimethyl-4,4',4″-tricarboxylate-2,2':6',2″-terpyridine (5(PF6)2). The single-crystal X-ray structure of 4(PF6)2 is presented. These complexes show two stepwise anodic redox pairs, and the potentials progressively increase from 1(PF6)2 to 5(PF6)2. Complexes 1(PF6)2-4(PF6)2 have comparable electrochemical potential splitting of 200-210 mV, while complex 5(PF6)2 has a splitting of 170 mV. Upon one-electron oxidation by chemical oxidation or electrolysis, the resulting mixed-valent complexes 1(3+)-5(3+) display broad and intense absorptions between 1000 and 3000 nm. Complexes 1(3+) and 2(3+) show the presence of a higher-energy shoulder band in addition to the main near-infrared absorption band. This shoulder band is less distinguished for 3(3+)-5(3+). Three-state theory has been used to explain this difference. The one-electron oxidized forms, 1(3+)-5(3+), exhibit rhombic EPR signals at 77 K with the isotropic g values in the range of 2.18-2.24. Density functional theory (DFT) and time-dependent DFT (TDDFT) computations have been performed on 1(2+)-5(2+) to characterize their electronic structures and rationalize the absorption spectra in a wide energy range. DFT computations on 1(3+)-5(3+) show that both ruthenium ions and the bridging ligand have comparable spin densities. TDDFT computations on 1(3+) and 4(3+) have been performed to complement the experimental results.