Dinitrogen Binding at a Trititanium Chloride Complex and Its Conversion to Ammonia Under Ambient Conditions

The production of ammonia is important for many reasons, primarily because it is a key component in the production of fertilizers, which are essential for modern agriculture. Ammonia is also used in a variety of industrial processes, such as the production of plastics, fibers, and explosives. Large scale methods for the production of ammonia typically involve the Haber-Bosch process, which was developed in the early 20th century. The Haber-Bosch process involves the reaction of nitrogen gas and hydrogen gas under high temperature and pressure conditions to produce ammonia. The Haber-Bosch process is highly efficient and can produce large quantities of ammonia, making it the dominant method for ammonia production today. However, the process is also energy-intensive and produces significant amounts of greenhouse gas emissions, primarily in the form of carbon dioxide that contributes to climate change. This is because the process relies heavily on fossil fuels for the production of hydrogen gas, which is one of the main components of ammonia. Furthermore, the Haber-Bosch process is also energy-intensive, requiring high temperatures and pressures in the presence of a heterogeneous catalyst to produce the reaction between nitrogen and hydrogen. This translates into high energy costs, which can make ammonia production expensive, particularly in regions with high energy prices. Moreover, the Haber-Bosch process also relies on the availability of finite natural resources such as natural gas, which is the primary source of hydrogen for ammonia production. This dependence on non-renewable resources raises concerns about long-term sustainability and the need to develop alternative methods for ammonia production. As a result, there is ongoing research into developing more sustainable and environmentally-friendly methods for ammonia production.

Some microorganisms use electrons and protons to perform natural nitrogen fixation under ambient conditions, inspiring the development of biological models and homogenous catalysts for artificial ammonia production in mild conditions. This requires a thorough knowledge of the coordination and activation of dinitrogen molecules at metal atoms in well-defined complexes. To this end, various catalytic homogenous systems have been developed to facilitate the conversion of nitrogen to ammonia under mild conditions. In addition to traditional metal complexes of molybdenum and iron inspired on nitrogenase enzymes, these newly developed catalytic systems also use other metals like titanium.

Dinitrogen fixation using titanium species is well-known and documented in the literature. In addition to the end-on binding mode, structurally characterized end-on/end-on, end-on/side-on and side-on/side-on modes have been reported for titanium. However, studies on the related coordination and activation of nitrogen on trinuclear systems with low valent titanium centers are still limited.

To this note, Dr. Estefanía del Horno, Professor Miguel Mena, Professor Adrián Pérez-Redondo, and led by Professor Carlos Yélamos from Universidad de Alcalá together with Professor Jesús Jover from Universitat de Barcelona in Spain investigated the dinitrogen binding at a trititanium chloride complex and its conversion to ammonia under ambient conditions. Their work is currently published in the peer-reviewed journal, Angewandte Chemie International Edition.

The authors obtained a paramagnetic trinuclear complex [{TiCp*(µ-Cl)}₃(µ₃-Cl)] when [TiCp*Cl₃] (Cp* = η⁵-C₅Me₅) was reacted with one equivalent of magnesium in tetrahydrofuran at room temperature. The resulting complex had an oxidation state of +2.33 for individual metals and based on the analysis of its reactivity with unsaturated molecules, the mixed valence species behaved like a source of titanium(II) and titanium(III) fragments. Thus, the trinuclear complex reacted with dinitrogen under ambient conditions to produce a diamagnetic derivative [{TiCp*(µ-Cl)}₃(µ₃-η¹:η²:η²-N₂)] and the titanium(III) dimer [{TiCp*Cl(µ-Cl)}₂]. The diamagnetic compound represents the first electronic and structural characterization of a well-defined µ₃-η¹:η²:η² coordination mode for a dinitrogen molecule in a stable trimetallic complex. Based on the theoretical and experimental study results, the dinitrogen complex can be described as a mixed-valence compound with a [N₂]^4- unit bonded to two titanium atoms in a side-on fashion and through an important donor-acceptor interaction between the lone pair of one nitrogen and an empty d-orbital of the third titanium atom.

The reaction of the dinitrogen complex with excess HCl in tetrahydrofuran resulted in the high-yield production of clean NH₄Cl with regeneration of the [TiCp*Cl₃] precursor. Based on the recyclability of this precursor, a catalytic synthesis of ammonia mediated by the titanium complex [TiCp*Cl₃] can be envisaged. The system involves reduction of N₂ with magnesium in tetrahydrofuran solution and protonolysis with HCl in presence of a commercially available, or easily prepared in multigram quantities, titanium catalyst to give two equivalents of NH₄Cl. Thus, the treatment of a tetrahydrofuran solution of [TiCp*Cl₃] with an excess of Mg under alternating N₂ / HCl atmospheres for 9 cycles afforded NH₄Cl (0.20 g) in 72% isolated yield (based on nine completed reactions of the titanium complex).

Overall, the development of a new method for ammonia production is important for the future of agriculture and industry and could have significant implications for global food security, energy security, and climate change mitigation. Professor Carlos Yélamos and colleagues reported successful production of ammonia by a trinuclear complex due to the reduction of [TiCp*Cl₃] with magnesium in tetrahydrofuran at ambient temperature. In a statement to Advances in Engineering, Professor Carlos Yélamos, the corresponding author, explained their findings contribute to the use of titanium complexes for sustainable ammonia production via reduction of nitrogen with magnesium and protonation with HCl under ambient conditions.