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 versus SCE, 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^-1 s^-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 κ. 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_½ = 6.2 s) and 7.67 × 10^-3 (t_½ = 90.3 s), respectively. The photo-released NO can bind to the reduced myoglobin to yield the Mb-NO adduct.