Stabilization of {RuNO}⁶ and {RuNO}⁷ states in [Ru^II(trpy)(bik)(NO)]ⁿ⁺ {trpy = 2,2':6',2" -terpyridine, bik = 2,2'-bis(1-methylimidazolyl) ketone} - formation, reactivity, and photorelease of metal-bound nitrosyl

Abstract

Ruthenium nitrosyl complexes have been isolated in the {RuNO}⁶ and {RuNO}⁷ configurations, employing the following reaction pathway for [Ru(trpy)(bik)(X)]ⁿ⁺: X= Cl^-, [1](ClO₄) → X= CH₃CN, [2](ClO₄)₂ → X= NO₂^-, [3](ClO₄) → X= NO⁺, [4](ClO₄)₃ → X= NO^., [4](ClO₄)₂. The single-crystal X-ray structures of [1](ClO₄).(C₆H₆).H₂O, [2](ClO₄)₂.H₂O, and [3](ClO₄).H₂O have been determined. The successive NO⁺/NO^. (reversible) and NO^./NO^- (irreversible) reduction processes of [4]³⁺ appear at +0.36 and -0.40 V vs. SCE, respectively. While the ν(C=O) frequency of the bik ligand at about 1630 cm^-1 is largely invariant on complexation and reduction, the ν(NO) frequency for the {RuNO}⁶ state in [4]³⁺at 1950 cm^-1 shifts to about 1640 cm^-1 on one-electron reduction to the {RuNO}⁷ form in [4]²⁺, reflecting the predominant NO⁺ → NO^. character of this electron transfer. However, a sizeable contribution from ruthenium with its high spin-orbit coupling constant to the singly occupied molecular orbital (SOMO) is apparent from the enhanced g anisotropy in the EPR spectrum [4]²⁺ (g₁ = 2.015, g₂= 1.995, g₃ = 1.881; g_av = 1.965; Δg = 0.134). The {RuNO}⁶ unit in [4]³⁺reacts with OH^- via an associatively activated process (ΔS^# = -126.5 ± 2 J K^-1 mol^-1) with a second-order rate constant of k = 3.3 × 10^-2M^-1 s^-1, leading to the corresponding nitro complex [3]⁺. On exposure to light both {RuNO}⁶ and {RuNO}⁷in [4]³⁺ and [4]²⁺ undergo Ru-NO photocleavage in CH₃CN via the formation of [Ru(trpy)(bik)(CH₃CN)]²⁺, [2]²⁺. The rate of photocleavage of the Ru^II-NO⁺ bond in [4]³⁺ (k_NO, 8.57 × 10^-1 s^-1, t_½= 0.80 s) is found to be much faster than that of the Ru^II-NO^. bond in [4]²⁺, [k_NO^., 5.45 × 10^-4 s^-1, t_½ = 21.2 min (= 1272 s)]. The photoreleased nitrosyl can be trapped as an Mb-NO adduct.