Reduction of dinitrogen

Conversion of dinitrogen to ammonia is required for all life. This conversion is accomplished by metalloenzymes on a scale of ≈108 tons/year, a scale equivalent to that of the Haber–Bosch process (1) for making ammonia from dinitrogen and dihydrogen at high temperatures (350–550°C) and pressures [150–350 atmosphere (atm); 1 atm = 101.3 kPa]. Although the benefit of crop rotation has been known for centuries, not until 1930 was a metal (molybdenum) found to be required for efficient dinitrogen “fixation” in the soil (2), and not until the 1960s was it determined that an FeMo nitrogenase is responsible for fixation of dinitrogen (3–6). At approximately the same time (1965), the first dinitrogen complex (of Ru) was discovered by Allen and Senoff (7). In the 1960s, transition metals were beginning to reveal their prowess in catalytic transformations such as olefin hydrogenation. Therefore, a catalytic process for reducing dinitrogen at a low temperature and 1 atm appeared to be only a few years away. However, reduction of dinitrogen, probably the most stable diatomic known, by protons and electrons or by dihydrogen selectively to ammonia at room temperature and pressure proved to be much more difficult than anticipated.