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

Abstract

The nitrosyl complexes, [Ru^II(trpy)(L)(NO⁺)Cl]BF₄, [1]BF₄, and [Ru^II(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(CO) frequencies as well as in their NO based two-step reduction processes, {Ru^II–NO⁺} → {Ru^II–NO^•} and {Ru^II–NO^•}→{Ru^II–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 k_NO (s⁻¹) and t_1/2 (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, [Ru^II(trpy)(L)(NO₂)Cl]⁻.