Side-on End-on Bound Dinitrogen: An Activated Bonding Mode That Facilitates Functionalizing Molecular Nitrogen

Molecular nitrogen is the source of all of the nitrogen necessary to sustain life on this planet. How it is incorporated into the biosphere is complicated by its intrinsic inertness. For example, biological nitrogen fixation takes N(2) and converts it into ammonia using various nitrogenase enzymes, whereas industrial nitrogen fixation converts N(2) and H(2) to NH(3) using heterogeneous iron or ruthenium surfaces. In both cases, the processes are energy-intensive. Is it possible to discover a homogeneous catalyst that can convert molecular nitrogen into higher-value organonitrogen compounds using a less energy-intensive pathway? If this could be achieved, it would be considered a major breakthrough in this area. In contrast to carbon monoxide, which is reactive and an important feedstock in many homogeneous catalytic reactions, the ischelectronic but inert N(2) molecule is a very poor ligand and not a common industrial feedstock, except for the above-mentioned industrial production of NH(3). Because N(2) is readily available from the atmosphere and because nitrogen is an essential element for the biosphere, attempts to discover new processes involving this simple small molecule have occupied chemists for over a century. Since the first discovery of a dinitrogen complex in 1965, inorganic chemists have been key players in this area and have contributed much fundamental knowledge on structures, binding modes, and reactivity patterns. For the most part, the synthesis of dinitrogen complexes relies on the use of reducing agents to generate an electron-rich intermediate that can interact with this rather inert molecule. In this Account, a facile reaction of dinitrogen with a ditantalum tetrahydride species to generate the unusual side-on end-on bound N(2) moiety is described. This particular process is one of a growing number of new, milder ways to generate dinitrogen complexes. Furthermore, the resulting dinitrogen complex undergoes a number of reactions that expand the known patterns of reactivity for coordinated N(2). This Account reviews the reactions of ([NPN]Ta)(2)(mu-H)(2)(mu-eta(1):eta(2)-N(2)), 2 (where NPN = PhP(CH(2)SiMe(2)NPh)(2)), with a variety of simple hydride reagents, E-H (where E-H = R(2)BH, R(2)AlH, RSiH(3), and Cp(2)ZrCl(H)), each of which results in the cleavage of the N-N bond to form various functionalized imide and nitride moieties. This work is described in the context of a possible catalytic cycle that in principle could generate higher-value nitrogen-containing materials and regenerate the starting ditantalum tetrahydride. How this fails for each particular reagent is discussed and evaluated.