Syntheses, structures and properties of ruthenium complexes of tridentate ligands: isolation and characterization of a rare example of ruthenium nitrosyl complex containing {RuNO}5 moiety

Abstract

Novel ruthenium complexes [Ru(L¹)(NO)Cl₂] (1), [Ru(L²)(PPh₃)Cl₂] (2), [Ru(L²)(PPh₃)(NO₂)Cl] (3) and [Ru(L²)(PPh₃)(NO)Cl](ClO₄)₂ (4) (where L¹H = N′-phenyl-N′-(pyridin-2-yl)picolinohydrazide and L² = (1-phenyl-1-(pyridin-2-yl)-2-(pyridin-2-ylmethylene)hydrazine) were synthesized. These complexes were characterized by using IR, UV-Vis, elemental analysis, electrochemical and NMR spectral studies. The molecular structures of nitrosyl complexes 1 and 4 were determined by X-ray crystallographic studies. Complexes 1 and 4 readily released NO under visible and ultraviolet light and free NO was transferred to reduced myoglobin. The amount of photoreleased NO was estimated using Griess reagent assay. During photolysis of NO, the generation of reactive nitrogen or/and reactive oxygen species was determined by DPPH (2,2-diphenyl-1-picrylhydrazine) radical quenching studies under aerobic conditions. A paramagnetic complex [Ru(L²)(PPh₃)(NO)Cl](NO₃)₃ (5) was synthesized via chemical oxidation of 4 with an excess of ceric ammonium nitrate (CAN) in acetonitrile. Complex 5 was characterized by UV-Vis, IR, elemental analysis and EPR spectral studies which authenticated the presence of the {RuNO}⁵ moiety in 5. Theoretical investigation by DFT calculation supported the oxidation of complex 4 having {RuNO}⁶ species and the formation of 5 containing {RuNO}⁵.