Ruthenium mononitro and mononitroso terpyridine complexes incorporating azoimine based ancillary ligands. Synthesis, crystal structure, spectroelectrochemical properties and kinetic aspects

Abstract

Ruthenium mononitro and mononitroso terpyridine complexes incorporating strong π-acidic azopyridine ligands of the types [Ru^II(trpy)(L)(NO₂)]ClO₄6–10 and [Ru^II(trpy)(L)(NO)][ClO₄]₃11–15 [trpy = 2,2′∶6′,2″-terpyridine; L = NC₅H₄NNC₆H₄(R), R = H, m-Me/Cl or p-Me/Cl] respectively have been synthesized starting from [Ru^II(trpy)L(H₂O)]²⁺1–5. The single crystal X-ray structure of the nitro complex 7 has been determined. In acetonitrile solvent the nitro complexes exhibit strong MLCT transitions near 500 nm and the same MLCT bands are observed to be blue shifted near 370 nm in the case of the nitroso derivatives. The nitrosyl complexes exhibit ν_NO in the range 1960–1950 cm⁻¹. The nitro (6–10) and nitroso (11–15) complexes exhibit ruthenium(II)–ruthenium(III) oxidation in the ranges 1.33–1.47 V and 1.62–1.72 V versus SCE respectively. The nitroso complexes exhibit two successive one-electron reductions near 0.7 and 0.0 V due to reductions of coordinated NO⁺ → NO and NO → NO⁻ respectively. All the complexes (6–15) systematically display three ligand based (trpy and L) reductions to negative potentials of the SCE. The nitro complexes (6–10) display weak emissions near 700 nm (Φ = 0.83 × 10⁻²–2.98 × 10⁻²). The electrochemically generated 6⁺++++–10⁺++++ show LMCT transitions near 490 nm and rhombic EPR spectra in acetonitrile at 77 K. The pseudo first order rate constants (k) and the thermodynamic parameters (ΔH^‡, ΔS ^‡ and K) for formation of the nitro complexes, [Ru^II(trpy)(L)(H₂O)]²⁺ → [Ru^II(trpy)(L)(NO₂)]⁺, and the conversion [Ru^II(trpy)(L)(NO)]³⁺ → [Ru^II(trpy)(L)(NO₂)]⁺ have been determined.