Electrochemical Evaluation of Titanocenes in Ionic Liquids with Non‐coordinating and Coordinating Anions and Application for NH3 Synthesis

Redox behaviors of monomeric and dimeric titanocenes, [Cp2TiCl2] (1) and [(Cp2TiCl)2] (2), were investigated in two different types of ionic liquid (IL) with non-coordinating and coordinating anions, 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)- trifluorophosphate (Pyr4FAP) and 1-butyl-1-methylpyrrolidinium triflate (Pyr4OTf), respectively. UV-vis/NIR spectra obtained during the electrochemical reduction of 1 revealed that it was converted to 2 in the ILs through the formation of [Cp2TiCl] (3). In Pyr4OTf, 3 was coordinated with the OTf anion in the η1-O mode, during the electrochemical reductions of Ti(IV/III) process, whereas 3 was not coordinated by the FAP anion in Pyr4FAP. Additionally, electrochemical NH3 synthesis was carried out in a solid polymer electrolyte cell by using a gas diffusion electrode coated with Pyr4FAP or Pyr4OTf containing titanocenes. The electrochemical reduction using 2 in Pyr4FAP at −1.5 V (vs. Ag/AgCl) gave a high yield (34 % at 29.9 C) of NH3 per Ti ion. The electrochemical reduction using 1 in Pyr4OTf gave a 6.8 % yield of NH3 per Ti ion at 20 °C. These findings indicate that the use of an IL with a non-coordinating anion such as FAP contributes to the coordination and reduction of N2 to titanocenes.