Síntese, caracterização e formação de monocamadas sobre ouro de complexos do tipo trans-[Ru(NH₃)₄(pyS)(L)]²⁺

Abstract

The trans-tetra(amine) ruthenium complexes have equatorial (NH₃) ligands with π-innocent characteristics, which do not significantly affect the π-backdonation of the axial ligands. The preparation of a series of such compounds allows for the analysis of the trans effect on the electronic properties of the complexes, as well as on the films formed by them. In this context, the present work initially aims at the synthesis, characterization, and formation of monolayers on gold surfaces of complexes of the form trans-Ru(NH₃)₄(pyS)(L)₂, where pyS corresponds to 4-mercaptopyridine and L can be pyrazine (pz), 4,4’-bipyridine (4,4’-bpy), or 4,4’-dithiodipyridine (pySSpy). First, the precursor complexes were prepared and characterized. Then, the target complexes were synthesized and characterized using infrared vibrational spectroscopy, ultraviolet and visible electronic spectroscopy (UV-Vis), and cyclic voltammetry. Calculations using DFT and TD-DFT were done to help identify the vibrational modes and electronic transitions. The half-wave potential (E₁/₂) values corresponding to the Ru²⁺/³⁺ redox pair were observed at 590, 446, and 460 mV for the complexes with L equal to pz (RuPZ), 4,4’-bpy (RuBT), and pySSpy (RuBP), respectively. Based on these values, it was found that the RuPZ complex is the most stable in the reduced state, followed by the RuBT and RuBP complexes. The formation of self-assembled monolayers (SAM) of the complexes was monitored in real time via surface plasmon resonance (SPR). The results indicated that the SAMs of the complexes exhibited surface coverage (Γ) values of 8.03 × 10⁻¹⁰, 5.56 × 10⁻¹⁰, and 6.36 × 10⁻¹⁰ mol cm⁻² for RuPZ, RuBT, and RuBP, respectively, confirming monolayer formation. The interaction strength with the surface was assessed indirectly through reductive desorption in 0.5 mol L⁻¹ KOH alkaline medium. The desorption potentials (Edr) were observed at -0.93, -0.86, and -0.87 V for the SAM of RuPZ, RuBT, and RuBP, respectively. The values suggest that the enhanced π-acceptor nature of the ligand trans to pyS leads to a stronger interaction with the surface. The attempt to correlate these results with data from the initially selected theoretical calculations was unsatisfactory.