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 13+–53+ display
broad and intense absorptions between 1000 and 3000 nm. Complexes 13+ and 23+ 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 33+–53+. Three-state
theory has been used to explain this difference. The one-electron
oxidized forms, 13+–53+, 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 12+–52+ to characterize their electronic structures and rationalize
the absorption spectra in a wide energy range. DFT computations on 13+–53+ show that
both ruthenium ions and the bridging ligand have comparable spin densities.
TDDFT computations on 13+ and 43+ have been performed to complement the experimental
results.