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

The present article describes ruthenium nitrosyl complexes with the {RuNO}(6) and {RuNO}(7) notations in the selective molecular frameworks of [Ru-II([9]aneS(3))(bpy)(NO+)](3+) (4(3+)), [Ru-II([9] aneS(3))(pap) (NO+)](3+) (8(3+)) and [Ru-II([9] aneS(3))(bpy)(NOS)](2+) (4(2+)), [Ru-II([9]aneS(3))(pap)(NO center dot)](2+) (8(2+)) ([9] aneS(3) = 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 center dot}. The single-crystal X-ray structure of 4(3+) and DFT optimised structures of 4(3+), 8(3+) and 4(2+), 8(2+) establish the localised linear and bent geometries for {Ru-NO+} and {Ru-NO center dot} complexes, respectively. The crystal structures and H-1/C-13 NMR suggest the [333] conformation of the coordinated macrocyclic ligand ([9] aneS(3)) in the complexes. The difference in pi-accepting strength of the co-ligands, bpy in 4(3+) and pap in 8(3+) (bpy < pap) has been reflected in the nu(NO) frequencies of 1945 cm(-1) (DFT: 1943 cm(-1)) and 1964 cm(-1) (DFT: 1966 cm(-1)) and E degrees ({Ru-II-NO+}/{Ru-II-NO center dot}) of 0.49 and 0.67 V versus SCE, respectively. The nu(NO) frequency of the reduced {Ru-NO center dot} state in 4(2+) or 8(2+) 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 4(3+), 8(3+) to bent {Ru-II-NO center dot} geometry in 4(2+), 8(2+). The preferential stabilisation of the eclipsed conformation of the bent NO in 4(2+) and 8(2+) has been supported by the DFT calculations. The reduced {Ru-II-NO center dot} exhibits free-radical EPR with partial metal contribution revealing the resonance formulation of {Ru-II-NO center dot}(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 (4(3+)) of 1.39 x 10(-1) is half of that determined for the pap derivative (8(3+)), 2.84 x 10(-1) in CH3CN at 298 K. The Ru-NO bond in 4(3+) or 8(3+) undergoes facile photolytic cleavage to form the corresponding solvent species {Ru-II-CH3CN}, 2(2+) or 6(2+) 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.