Stepwise Synthesis of [Ru(trpy)(L)(X)]<i>ⁿ</i>⁺ (trpy = 2,2‘:6‘,2‘ ‘-Terpyridine; L = 2,2‘-Dipyridylamine; X = Cl^-, H₂O, NO₂^-, NO⁺, O^2-). Crystal Structure, Spectral, Electron-Transfer, and Photophysical Aspects

Ruthenium−terpyridine complexes incorporating a 2,2‘-dipyridylamine ancillary ligand [RuII(trpy)(L)(X)](ClO4)n [trpy = 2,2‘:6‘,2‘ ‘-terpyridine; L = 2,2‘-dipyridylamine; and X = Cl-, n = 1 (1); X = H2O, n = 2 (2); X = NO2-, n = 1 (3); X = NO+, n = 3 (4)] were synthesized in a stepwise manner starting from RuIII(trpy)(Cl)3. The single-crystal X-ray structures of all of the four members (1−4) were determined. The Ru(III)/Ru(II) couple of 1 and 3 appeared at 0.64 and 0.88 V versus the saturated calomel electrode in acetonitrile. The aqua complex 2 exhibited a metal-based couple at 0.48 V in water, and the potential increased linearly with the decrease in pH. The electron−proton content of the redox process over the pH range of 6.8−1.0 was calculated to be a 2e-/1H+ process. However, the chemical oxidation of 2 by an aq Ce(IV) solution in 1 N H2SO4 led to the direct formation of corresponding oxo species [RuIV(trpy)(L)(O)]2+ via the concerted 2e-/2H+ oxidation process. The two successive reductions of the coordinated nitrosyl function of 4 appeared at +0.34 and −0.34 V corresponding to RuII−NO+ → RuII−NO• and RuII−NO• → RuII−NO-, respectively. The one-electron-reduced RuII−NO• species exhibited a free-radical electron paramagnetic resonance signal at g = 1.990 with nitrogen hyperfine structures at 77 K. The NO stretching frequency of 4 (1945 cm-1) was shifted to 1830 cm-1 in the case of [RuII(trpy)(L)(NO•)]2+. In aqueous solution, the nitrosyl complex 4 slowly transformed to the nitro derivative 3 with the pseudo-first-order rate constant of k298/s-1 = 1.7 × 10-4. The chloro complex 1 exhibited a dual luminescence at 650 and 715 nm with excited-state lifetimes of 6 and 1 μs, respectively.