Stepwise synthesis of [Ru(trpy)(L)(X)]ⁿ⁺ (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

Abstract

Ruthenium-terpyridine complexes incorporating a 2,2'-dipyridylamine ancillary ligand [Ru^II(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 Ru^III(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 ρH 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 [Ru^IV(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 Ru^II-NO⁺ → Ru^II-NO· and Ru^II-NO° → Ru^II-NO^-, respectively. The one-electron-reduced Ru^II-NO° species exhibited a free-radical electron paramagnetic resonance signal at g = 1.990 with nitrogen hyperfine structures at 77 κ . The NO stretching frequency of 4 (1945 cm^-1) was shifted to 1830 cm^-1 in the case of [Ru^II(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.