This thesis concerns itself with the proton-coupled reduction of dinitrogen (N2) to ammonia (NH3). Throughout the following chapters you will see this process referred to as nitrogen fixation, N2-to-NH3 conversion, and the nitrogen reduction reaction (N2RR). This reaction has motivated this work because of its critical importance to human and environmental health. Although dinitrogen is the largest component of our atmosphere (~80%) and is a key component of many biomolecules (i.e., DNA, RNA, amino acids, etc.), it cannot be directly incorporated into these biomolecules but must first be fixed into a chemically reactive form, NH3.1 Biologically this fixation is achieved by a highly conserved family of nitrogenase enzymes which feature, I would argue, the most complicated transition metal architectures known in biology (Figure 1.1). Metalloenzymes typically operate at low overpotential and with high selectivity, and thus the significant excess energetic demand (fixation of 1 N2 requires the hydrolysis of 14 ATP) and the poor selectivity (under typical conditions ~2 equivalents of H2 are produced for every NH3) demonstrates both the significant challenge presented by this reaction and its critical importance to the organism(s).2 Energy demand and selectivity of N2RR will form key threads through this work.
Figure 1.1: Depiction of the active site of the FeMo nitrogenase (PDB: 1M1N)3 with the idealized stoichiometry for biological nitrogen fixation.2
Industrially, nitrogen fixation is (almost) exclusively achieved through the Haber- Bosch process in which NH3 is formed via the high-temperature and high-pressure combination of H2 with N2 on an Fe surface in the presence of promoters. The increased access to fixed nitrogen provided by the discovery of Haber-Bosch in the early 20th century truly transformed the world.4 It resulted not only in us fundamentally altering the global biogeochemical nitrogen cycle, but also, via the population boom it enabled, the carbon cycle. The environmental consequences of the disruption of these key biogeochemical cycles will continue to play out for millennia.5
The importance of nitrogen fixation would easily motivate study of either of these two globally-important NH3-producing systems, biological or industrial. The nitrogen fixing system that we have chosen to study is far removed from both of these in both its atomistic structure and its practicality, although certain design elements are drawn from the active site
cluster in biological nitrogenases. These design elements include the metal identity (Fe), the symmetry (threefold), and the flexible anchoring atom (Figure 1.2). Nonetheless, the catalyst is clearly bio-inspired rather than bio-mimetic. One of the key analogies to the biological system is our choice to use protons and electrons (rather than H2) to reduce N2. Indeed, the work presented here is largely focused on the mechanisms by which protons and electrons can be combined to form N–H (or O–H) bonds.
Figure 1.2: Molecular nitrogen fixation catalysts used in this thesis. They feature three in- plane phosphine donors and a variable anchoring atom, E (E = B, C, Si). The ratio of NH3
to H2 depends on the exact conditions (i.e., temperature, solvent, acid, reductant).
We have chosen to study the mechanism of X–H bond formation, because we anticipate that combining protons with electrons for X–H bond formation will be one of the defining chemical challenges of the coming decades.6 Humans have long used X–H bonds to store, transport, and release energy. Indeed, it is the abundance of energy-rich C–H bonds in biomass such as wood, coal, or oil that makes them such an excellent source of energy:
dense and easily-controlled. As energy is increasingly derived from renewable sources such as solar, wind, and hydro, the relevant energy currency will increasingly be electrons. Only by pairing these electrons with protons to form new energy-rich X–H bonds will we be able
to match the benefits in terms of storage, transportation, and controlled release that characterize our current carbon-based fuel cycle.7
The conversion of N2 to NH3 is thus a fertile training ground for the chemist, as it requires the coordination of 6 protons and 6 electrons to form the energy-rich N–H bonds.
For reactions of biogeochemical relevance, this is matched only by sulfite (SO32−) reduction to sulfide (S2−)8 and nitrite (NO2−) reduction to ammonia.9 Furthermore, no substrate is as resistant to initial functionalization as N2. N2 has an electron affinity lower than that of the noble gases. It has a proton affinity less than methane.10 N2 is so unreactive that the addition of a free hydrogen radical to form NNH, the first intermediate of nitrogen fixation, is unfavorable.11 Thus understanding the mechanism by which we can use transition metals in the presence of protons and electrons to functionalize N2 is a useful whetstone for the chemist interested in the problem of X–H bond formation. Furthermore, NH3 is a promising fuel candidate with high energy-density, well-developed infrastructure for transportation, and the potential to be carbon-free.12
Rather than reviewing a history of molecular species for nitrogen fixation, I will now attempt to sketch out the ideas that have animated my thesis work. In doing so, I hope to pull together some threads that might otherwise seem unrelated to the reader. Naturally, this logic is being applied with the benefit of hindsight and I will inevitably engage in some degree of ex post facto rationalization, in order to improve coherence. However, I hope that this exercise will help to inform anyone who would endeavor to read this thesis in its entirety.