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

Abstract

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.

Recent Advances and Challenges of Electrocatalytic N2Reduction to Ammonia

“…In addition to the examples of Halmann 223 and Greenlee et al, 75 this is another example that demonstrates the importance of control experiments in the accurate determination of electrochemically produced ammonia. Katayama et al 300,301 reported electrocatalytic reduction of N 2 to ammonia using carbon paper coated with a transitionmetal complex ([Cp 2 TiCl 2 ] or [(Cp 2 TiCl) 2 ]) and an ionic liquid (Pyr 4 FAP or Pyr 4 OTf). The cell configuration is shown in Figure 38a.…”

“…Katayama et al , reported electrocatalytic reduction of N 2 to ammonia using carbon paper coated with a transition-metal complex ([Cp 2 TiCl 2 ] or [(Cp 2 TiCl) 2 ]) and an ionic liquid (Pyr 4 FAP or Pyr 4 OTf). The cell configuration is shown in Figure a.…”

Catalytic N2-to-NH3(or -N2H4) Conversion by Well-Defined Molecular Coordination Complexes

“…More recent work from Masuda and co-workers has demonstrated that the related dimeric species, {(Cp) 2 Ti(μ-Cl)} 2 , can provide higher yields and FEs for nitrogen fixation (up to 0.34 equiv of NH 3 per Ti and up to 1.44% FE). 278…”

“…Comparison of conditions and results for the electrosynthesis of NH 3 from N 2 by (Cp) 2 Ti (Cl) 2 . − (Pyr 4 FAP = 1-butyl-1-methylpyrrolidinium tris­(pentafluoroethyl)­trifluorophosphate)…”

“…These observations suggest that the use of ionic liquids may provide a promising strategy for electrocatalytic nitrogen fixation by helping to both polarize the N 2 molecule activating it for binding/reduction, and to limit proton reduction. More recent work from Masuda and co-workers has demonstrated that the related dimeric species, {(Cp) 2 Ti (μ-Cl)} 2 , can provide higher yields and FEs for nitrogen fixation (up to 0.34 equiv of NH 3 per Ti and up to 1.44% FE) …”