Ruthenium nitrosyl complexes with 1,4,7-trithiacyclononane and 2,2′-bipyridine (bpy) or 2-phenylazopyridine (pap) coligands. Electronic structure and reactivity aspects

Abstract

The present article describes ruthenium nitrosyl complexes with the {RuNO}⁶ and {RuNO}⁷ notations in the selective molecular frameworks of [Ru^II([9]aneS₃)(bpy)(NO⁺)]³⁺ (4³⁺), [Ru^II([9]aneS₃)(pap) (NO⁺)]³⁺ (8³⁺) and [Ru^II([9]aneS₃)(bpy)(NO˙)]²⁺ (4²⁺), [Ru^II([9]aneS₃)(pap)(NO˙)]²⁺ (8²⁺) ([9]aneS₃ = 1,4,7-trithiacyclononane, bpy = 2,2′-bipyridine, pap = 2-phenylazopyridine), respectively. The nitrosyl complexes have been synthesized by following a stepwise synthetic procedure: {Ru^II–Cl} → {Ru^II–CH₃CN} → {Ru^II–NO₂} → {Ru^II–NO⁺} → {Ru^II–NO˙}. The single-crystal X-ray structure of 4³⁺ and DFT optimised structures of 4³⁺, 8³⁺ and 4²⁺, 8²⁺ establish the localised linear and bent geometries for {Ru–NO⁺} and {Ru–NO˙} complexes, respectively. The crystal structures and ¹H/¹³C NMR suggest the [333] conformation of the coordinated macrocyclic ligand ([9]aneS₃) in the complexes. The difference in π-accepting strength of the co-ligands, bpy in 4³⁺ and pap in 8³⁺ (bpy < pap) has been reflected in the ν(NO) frequencies of 1945 cm⁻¹ (DFT: 1943 cm⁻¹) and 1964 cm⁻¹ (DFT: 1966 cm⁻¹) and E°({Ru^II–NO⁺}/{Ru^II–NO˙}) of 0.49 and 0.67 V versusSCE, respectively. The ν(NO) frequency of the reduced {Ru–NO˙} state in 4²⁺ or 8²⁺ however decreases to 1632 cm⁻¹ (DFT: 1637 cm⁻¹) or 1634 cm⁻¹ (DFT: 1632 cm⁻¹), respectively, with the change of the linear {Ru^II–NO⁺} geometry in 4³⁺, 8³⁺ to bent {Ru^II–NO˙} geometry in 4²⁺, 8²⁺. The preferential stabilisation of the eclipsed conformation of the bent NO in 4²⁺ and 8²⁺ has been supported by the DFT calculations. The reduced {Ru^II–NO˙} exhibits free-radical EPR with partial metal contribution revealing the resonance formulation of {Ru^II–NO˙}(major)↔{Ru^I–NO⁺}(minor). The electronic transitions of the complexes have been assigned based on the TD-DFT calculations on their DFT optimised structures. The estimated second-order rate constant (k, M⁻¹s⁻¹) of the reaction of the nucleophile, OH⁻ with the electrophilic {Ru^II–NO⁺} for the bpy derivative (4³⁺) of 1.39 × 10⁻¹ is half of that determined for the pap derivative (8³⁺), 2.84 × 10⁻¹ in CH₃CN at 298 K. The Ru–NO bond in 4³⁺ or 8³⁺ undergoes facile photolytic cleavage to form the corresponding solvent species {Ru^II–CH₃CN}, 2²⁺ or 6²⁺ with widely varying rate constant values, (k_NO, s⁻¹) of 1.12 × 10⁻¹ (t_1/2 = 6.2 s) and 7.67 × 10⁻³ (t_1/2 = 90.3 s), respectively. The photo-released NO can bind to the reduced myoglobin to yield the Mb-NO adduct.