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Influence of nitrosyl coordination on the binding mode of quinaldate in selective ruthenium frameworks. Electronic structure and reactivity aspects

Chowdhury, Abhishek Dutta, De, Prinaka, Mobin, Shaikh M., Lahiri, Goutam Kumar
RSC advances 2012 v.2 no.8 pp. 3437-3446
X-radiation, electron paramagnetic resonance spectroscopy, geometry, nitric oxide, nitrogen dioxide, oxygen, pH, ruthenium, solvents
The nitrosyl complexes, [Ruᴵᴵ(trpy)(L)(NO⁺)Cl]BF₄, [1]BF₄, and [Ruᴵᴵ(trpy)(L)(NO⁺)](BF₄)₂, [2](BF₄)₂, (trpy = 2,2′:6′,2′′-terpyridine, L⁻ = deprotonated form of unsymmetrical quinaldic acid) have been synthesized. Single crystal X-ray structures of [1]BF₄ and [2](BF₄)₂ reveal that in the former L⁻ binds to the ruthenium ion selectively in a monodentate fashion through the O⁻ donor whereas the usual bidentate mode of L⁻ (O⁻, N donors) has been retained in [2](BF₄)₂ with the same meridional configuration of trpy being seen in both. The Ru–NO group in [1]BF₄ or [2](BF₄)₂, exhibits almost linear (sp-hybridized form of NO⁺) geometry. The difference in bonding mode of the unsymmetrical quinaldate in [1]BF₄ and [2](BF₄)₂ has been reflected in their corresponding ν(NO)/ν(CO) frequencies as well as in their NO based two-step reduction processes, {Ruᴵᴵ–NO⁺} → {Ruᴵᴵ–NO•} and {Ruᴵᴵ–NO•}→{Ruᴵᴵ–NO⁻}. The close to bent geometry (sp²-hybridized form of NO•) of the one-electron reduced 1 or [2]⁺ is been reflected in their DFT optimized structures. The spin density plot of the reduced species reveals that NO is the primary spin-bearing center with slight delocalization onto the metal ion which has been reflected in its radical EPR spectrum. [1]⁺ and [2]²⁺ undergo facile photorelease of NO with significantly different kNO (s⁻¹) and t₁/₂ (s) values which eventually lead to the concomitant formation of the corresponding solvent species. The photoreleased NO• can be trapped as an Mb–NO adduct. The reduced species 1 selectively reacts with the molecular oxygen (O₂) at pH ∼ 1 to yield the corresponding nitro species, [Ruᴵᴵ(trpy)(L)(NO₂)Cl]⁻.