Formation, reactivity, and photorelease of metal bound nitrosyl in [Ru(trpy)(L)(NO)]ⁿ⁺ (trpy = 2,2':6',2''-Terpyridine, L = 2-Phenylimidazo[4,5-f]1,10-phenanthroline)

Abstract

Nitrosyl complexes with {Ru-NO}⁶ and {Ru-NO}⁷ configurations have been isolated in the framework of [Ru(trpy)(L)(NO)]ⁿ⁺ [trpy = 2,2':6',2''-terpyridine, L = 2-phenylimidazo[4,5-f]1,10-phenanthroline] as the perchlorate salts [4](ClO₄)₃ and [4](ClO₄)₂, respectively. Single crystals of protonated material [4-H⁺](ClO₄)₄.2H₂O reveal a Ru-N-O bond angle of 176.1(7)° and triply bonded N-O with a 1.127(9) Å bond length. Structures were also determined for precursor compounds of [4]³⁺ in the form of [Ru(trpy)(L)(Cl)](ClO₄).4.5H₂O and [Ru(trpy)(L-H)(CH₃CN)](ClO₄)₃.H₂O. In agreement with largely NO centered reduction, a sizable shift in ν(NO) frequency was observed on moving from [4]3+ (1953 cm^-1) to [4]²⁺ (1654 cm^-1). The Ru^II-NO°in isolated or electrogenerated [4]²⁺ exhibits an EPR spectrum with g₁ = 2.020, g₂ = 1.995, and g₃ = 1.884 in CH₃CN at 110 κ, reflecting partial metal contribution to the singly occupied molecular orbital (SOMO); ¹⁴N (NO) hyperfine splitting (A₂ = 30 G) was also observed. The plot of ν(NO) versus E°({RuNO}₆ → {RuNO}₇) for 12 analogous complexes [Ru(trpy)(L')(NO)]ⁿ⁺ exhibits a linear trend. The electrophilic Ru-NO⁺ species [4]³⁺ is transformed to the corresponding Ru-NO₂^- system in the presence of OH^- with k = 2.02 × 10^-4 s^-1 at 303 κ. In the presence of a steady flow of dioxygen gas, the Ru^II-NO^·state in [4]²⁺ oxidizes to [4]³⁺ through an associatively activated pathway (ΔS ≠-190.4 J K^-1 M^-1) with a rate constant (k [s^-1]) of 5.33 × 10^-3. On irradiation with light (Xe lamp), the acetonitrile solution of paramagnetic [Ru(trpy)(L)(NO)]²⁺ ([4]²⁺) undergoes facile photorelease of NO (k_NO = 2.0 × 10^-1 min^-1 and t_½ ≈ 3.5 min) with the concomitant formation of the solvate [Ru^II(trpy)(L)(CH₃CN)]²⁺ [2']²⁺. The photoreleased NO can be trapped as an Mb-NO adduct.