Catalytic Hydrogenation of Metal Nitrides to Generate Ammonia Using Transition Metal Hydrides

Abstract

Ammonia (NH3) is one of the most crucial chemical compounds in human civilization due to its wide range of applications and demonstrated ability to transform global food production. The traditional, but still contemporary method for the synthesis of ammonia on industrial scale is the Haber-Bosch process, but high temperature and pressure conditions (>300 atm, >300 ºC) result 1-2% of the global energy consumption and 7% of the global CO2 emissions solely to produce ammonia. The nitrogenase family of enzymes converts dinitrogen to ammonia under mild reaction conditions by proton-coupled electron transfer (PCET). By utilizing this concept, chemists have achieved the catalytic ammonia formation from N2 and H2 under mild reaction conditions using well-defined transition metal catalysts. However, all these examples used stoichiometric amounts of acids and reductants as the source of hydrogen, which incurs a tremendous chemical overpotential and stochiometric waste. Consequently, these problems raise this question. Can we develop a system to convert dinitrogen into ammonia with well-defined metal complexes 1) using dihydrogen as the terminal reductant just like the Haber-Bosch process, 2) under milder reaction conditions by PCET just like nitrogenase? To develop the corresponding system, metal hydrides that can undergo PCET or HAT (hydrogen atom transfer) using H2 as the terminal reductant were explored along with photoactivated metal hydrides to energetically access weak N–H bonds in the N2-derived complexes to make ammonia. In this thesis, four major research projects were discussed: 1) evaluation of the strategy using the rhodium hydride catalysts, identification of undesired side reactions and utilizing the side reaction for an application to catalytic hydrogenation of N-heteroarenes, 2) development of a thermally stable rhodium hydride catalyst to generate ammonia from an NH3-derived manganese nitride using H2, 3) development of a thermally- and photochemically stable ruthenium hydride HAT catalyst, and 4) photodriven ammonia formation from an N2-derived molybdenum nitride using iridium hydride catalysts and H2 as the terminal reductant and completion of the synthetic cycle for N2 hydrogenation.