Computational Studies of Ambient Catalytic Dinitrogen Reduction by Electropositive Metal Tetraphenolate Complexes

The worldwide industrial catalytic conversion of nitrogen to millions of tons of ammonia per annum is a starting point for the production of pharmaceuticals, plastics, fine chemicals and fertiliser. The latter has enabled the lives of around 3 billion people and addresses the United Nations global sustainable development goal #2: zero hunger. The high pressure/temperature Haber-Bosch (HB) process that converts nitrogen and hydrogen to ammonia, known as the nitrogen reduction reaction (N2RR), is perfectly optimised but still very energy intensive. Small scale, low-energy N2RR reactions, including to products other than ammonia, would be complementary to the HB process. They would also improve energy justice by allowing isolated communities to generate their own fertilisers or amines, and potentially facilitate off-world food production. Furthermore, ammonia has the potential to replace fossil fuels as an energy carrier, as it is energy dense and compatible with current infrastructures and fuel cell technologies. However, its incorporation into renewable technologies demands further understanding and better catalysts.

Professor Polly Arnold, from the University of California at Berkeley and the Project Partner on this proposal, has recently reported the synthesis and characterisation of molecular uranium and thorium complexes that can convert nitrogen to ammonia at room temperature and pressure, and the first catalytic conversion of dinitrogen into a secondary silylamine by any metal. She has now extended this work to cerium and samarium analogues - the first non-radioactive f-block N2RR catalysts. All of these molecules feature two metal atoms, held in place by two tetraphenol-arene (mTP) ligands. Arnold has also synthesised d-block analogues using titanium and zirconium, which again can effect catalytic conversion of nitrogen to secondary silylamines, and uranium and lanthanide compounds containing two metals but only one mTP ligand, some of which are also effective catalysts. Work is ongoing in Arnold's laboratories to optimise this chemistry, and extend it to other metals, including the very abundant s-block elements calcium, strontium and barium.

The proposed research is a comprehensive programme of computational quantum chemistry in the laboratory of Principal Investigator Kaltsoyannis to link synergistically with, and guide, Arnold's ongoing experimental development of new bimetallic homogeneous catalysts for the conversion of nitrogen to ammonia, and secondary or tertiary amines, with particular emphasis on furnishing detailed mechanistic and electronic structural insight. Kaltsoyannis and Arnold have collaborated on many previous projects, and have made important and well-received contributions to f and d block chemistry. Arnold's earlier report of uranium and thorium N2RR catalysts included quantum chemical analysis of the reaction mechanism, and the combination of experiment and computation was essential to the success of that work. The proposed combination of experiment and theory will yield new N2RR chemistry and catalysts, and deep insight into both mechanism and electronic structure. It will deconvolute the roles of the alkali metal reductant, mTP ligand and the Lewis acidic f-, d- and s-block metals to understand the path of the electrons to the metal bound nitrogen, and their subsequent behaviour. In so doing, it will also make fundamental contributions to understanding the role of d and f orbitals in bonding, including the interplay of these orbitals in f block chemistry. It is also anticipated that significant success in catalysis arising from the target compounds will stimulate further advances in the field of electropositive d block catalysis.