Coordination chemistry insights into the role of alkali metal promoters in dinitrogen reduction

Abstract

The Haber-Bosch process is a major contributor to fixed nitrogen that supports the world's nutritional needs and is one of the largest-scale industrial processes known. It has also served as a testing ground for chemists’ understanding of surface chemistry. Thus, it is significant that the most thoroughly developed catalysts for N2 reduction use potassium as an electronic promoter. In this review, we discuss the literature on alkali metal cations as promoters for N2 reduction, in the context of the growing knowledge about cooperative interactions between N2, transition metals, and alkali metals in coordination compounds. Because the structures and properties are easier to characterize in these compounds, they give useful information on alkali metal interactions with N2. Here, we review a variety of interactions, with emphasis on recent work on iron complexes by the authors. Finally, we draw conclusions about the nature of these interactions and areas for future research.

Reversible Ligand‐Centered Reduction in Low‐Coordinate Iron Formazanate Complexes

“…However, we were unable to isolate sufficient amounts of this unusual product for full characterization, and thus we sought a more stable analogue. Since the C-H bonds in CoCp* 2 and [CoCp* 2 ] + are susceptible to deprotonation or C-H activation, 23 and because of potential stabilizing effects of alkali cations, 24 we used an equivalent of elemental Na to reduce complex 3 in THF, which resulted in a green solution similar in color to 4 . Addition of 2 equiv of 12-crown-4 and cooling the THF mixture gave 5 as a green crystalline solid in 67% yield (Scheme 2).…”

“…Though we have not been able to determine the fate of the CoCp* 2 cation, it is possible that decomposition occurs through a pathway involving C-H activation through deprotonation of one of the Cp* ligands. 24 …”

Efficient Non‐dissociative Activation of Dinitrogen to Ammonia over Lithium‐Promoted Ruthenium Nanoparticles at Low Pressure

“…To further reduce the metal loading of the working catalyst, acatalyst with 0.5 wt %Ruand 0.76 wt %Li(Table 1, entry 6) was used, which shows the highest rate in terms of moles of ammonia produced per mole of Ru metal at 1MPa (127.7 mol NH 3 mol À1 Ru hour À1 ). We note the combination of Ru-Li is clearly more effective than Ru-Cs irrespective of the support material used (Table 1e ntries 9, 10,14,15). This combination seems to display as trong synergetic effect enabling asignificantly higher ammonia production rate than that of Li promoted 1st row transition-metal elements,such as Fe-Li (Table 1e ntries 11,13] As stated, the use of lower applied pressure in coupling with electrolyser in the new eHB process is more important provided that higher activity at higher temperature can be maintained at long lifetime.I nf act, the comparison of the turnover frequencies (TOFs) by normalizing the exposed metal surfaces makes Ba-Ru-Li favourable than those other reported catalyst systems at comparable Ru loading disregard the higher temperature used ( Table 1, entries 2a nd 19).…”

The Role of Organoferrates in Iron‐Catalyzed Cross‐Couplings

“…Co-catalytic assistance of counterions can involve diverse modes of substrate or catalyst activation. [20][21][22][23] Upon implementation of metal-based cations, bimetallic catalysis mechanisms can be engineered. The distinct reactivity of magnesium vs. lithium organoferrates in these studies mirror the dominance of Grignard reagents as nucleophilic components in ironcatalyzed cross-couplings.…”