The impact of counterion on the metastable state properties of nitrosyl ruthenium complexes

Abstract

In the present study, four new complexes of the trans-[RuNO(NH₃)₄F]²⁺ cation with noble-metal anions [PtCl₆]²⁻, [PdCl₄]²⁻, and [PtCl₄]²⁻, and perchlorate anion ClO₄⁻ were synthesized and characterized by spectroscopic (IR, UV-Vis), calorimetric (DSC), and single-crystal X-ray diffraction methods. All three complexes with platinum and palladium cations crystallize in a non-centrosymmetric structure, determined by N–H⋯Halogen intermolecular interactions. The irradiation of complexes by light in the spectral range of 300–420 nm induces Ru–ON isomers MS1 and the subsequent irradiation of MS1 in trans-[RuNO(NH₃)₄F](ClO₄)₂ by 940 nm light results in the formation of the MS2 Ru–(η²-(NO)) isomer, which was detected by IR and UV-Vis-spectroscopy. For trans-[RuNO(NH₃)₄F](ClO₄)₂ and trans-[RuNO(NH₃)₄F][PdCl₄], the activation energy (E_a) and frequency factors (lg k₀) of the MS1 → GS transformation were determined as 104(1) kJ mol⁻¹ and 15.0(2), and 112(6) kJ mol⁻¹ and 16.0(9), respectively. It is shown that in these complexes, the MS1 isomers exhibit excellent thermal stability, and trans-[RuNO(NH₃)₄F][PdCl₄] exhibits the highest thermal stability known to date. The spectroscopic analysis of the ground and metastable states allows us to underline two important features: (i) the absorption of a counterion in the excitation range results in a decreased efficiency of light absorption of the nitrosyl complex cation and consequently leads to a decrease in the metastable state population; (ii) the interactions with the counterion can influence the position of the absorption bands of MS1 and MS2, which requires a change in the excitation wavelength for the achievement of the maximum metastable state population. The obtained results can be expanded and applied for other metastable systems, such as complexes containing nitrite (NO₂⁻) or sulfoxide-like (SO) ligands.