Effect of 2-(2-pyridyl)azole-based ancillary ligands (L^1-4) on the electrophilicity of the nitrosyl Function in [Ru^II(trpy)(L^1-4)(NO)]³⁺ [trpy = 2,2':6',2' '-terpyridine]. Synthesis, structures, and spectroscopic, electrochemical, and kinetic aspects

Abstract

Ruthenium nitrosyl complexes [Ru(trpy)(L^1-4)(NO)]³⁺ (13-16) [trpy = 2,2':6',2''-terpyridine, L¹ = 2-(2-pyridyl)benzoxazole, L² = 2-(2-pyridyl)benzthiazole, L³ = 2-(2-pyridyl)benzimidazole, L⁴ = 1-methyl-2-(2-pyridyl)-1H-benzimidazole] were obtained in a stepwise manner starting from [Ru^II(trpy)(L^1-4)(Cl)]ClO₄ (1-4) → [Ru^II(trpy)(L1-4)(H₂O)](ClO₄)₂ (5-8) → [Ru^II(trpy)(L^1-4) (NO₂)]ClO₄ (9-12) → [Ru^II(trpy)(L^1,2,4)(NO)](ClO₄)₃ (13, 14, 16)/[Ru^II(trpy)(L³)(NO)](ClO₄)₂(NO₃) (15). Crystal structures of 1, 2, 4, 9, 12, 13, 15, and 16 established the stereoretentive nature of the transformation processes. Though the complexes of L¹, L³, and L⁴ were isolated in the isomeric form A (π -acceptor trpy and azole ring in the equatorial plane and the pyridine and chloride donors in the axial positions), complexes of L2 preferentially stabilized in form B (trpy and pyridine in the equatorial plane and the azole ring and chloride donors in the axial positions). The ν(NO) stretching frequency varied in the range of 1957-1932 cm^-1, 13 » 14 ≈15 > 16, primarily depending on the electronic aspects of L as well as the isomeric structural forms. The coordinated nitrosyl function underwent successive reductions of [Ru^II-NO^+]3+ → [Ru^II-NO^° ]²⁺ and [Ru^II-NO^° ]²⁺ → [Ru^II-NO^-]⁺, and the first reduction potential follows the order 14 > 13 »15 ≈ 16. The nearly axial EPR spectra having nitrogen hyperfine splittings (A ≈ 26 G) at 77 κ of 13^--16^- with g ≈ 2.0 established that the reduction process is largely centered around the nitrosyl function. Despite an appreciably high ν (NO), the complexes were found to be unusually stable even in the aqueous medium. They transformed slowly and only partially into the corresponding nitro derivatives in H₂O (k ≈ 10^-4 s^-1 and K = 0.4-3.8). The chloro (1-4), aqua (5-8), and nitro (9-12) derivatives displayed reasonably strong emissions near 700 nm at 77κ (φ = 10^-1-10^-2). The aqua derivative 7 was found to interact with the calf thymus and the circular form of p-Bluescript SK DNA.