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 (Pyr(4)FAP) and 1-butyl-1-methylpyrrolidinium triflate (Pyr(4)OTf), 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 Pyr(4)OTf, 3 was coordinated with the OTf anion in the eta(1)-O mode, during the electrochemical reductions of Ti(IV/III) process, whereas 3 was not coordinated by the FAP anion in Pyr(4)FAP. Additionally, electrochemical NH3 synthesis was carried out in a solid polymer electrolyte cell by using a gas diffusion electrode coated with Pyr(4)FAP or Pyr(4)OTf containing titanocenes. The electrochemical reduction using 2 in Pyr(4)FAP 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 Pyr(4)OTf gave a 6.8% yield of NH3 per Ti ion at 20 degrees 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 N-2 to titanocenes.