Abstract
A μ-η1:η1-N2-bridged Mo dimer, {(η5-C5Me5)[N(Et)C(Ph)N(Et)]Mo}2(μ-N2), cleaves dinitrogen thermally resulting in a crystallographically characterized bis-μ-N-bridged dimer, {(η5-C5Me5)[N(Et)C(Ph)N(Et)]Mo}2(μ-N)2. A structurally related Mo dimer with a bulkier amidinate ligand, ([N(iPr)C(Me)N(iPr)]), is only capable of photochemical dinitrogen activation. These opposing reactivities were rationalized as steric switching between the thermally and photochemically active species. A computational analysis of the geometric and electronic structures of intermediates along the isomerization pathway from Mo2(μ-η1:η1-N2) to Mo2(μ-η2:η1-N2) and Mo2(μ-η2:η2-N2), and finally Mo2(μ-N)2, is presented here. The extent to which dispersion affects the thermodynamics of the isomers is evaluated, and it is found that dispersion interactions play a significant role in stabilizing the product and making the reaction exergonic. The concept of steric switching is further explored with theoretical models with sterically even less demanding ligands, indicating that systematic ligand modifications could be used to rationally design the N2 activation energy landscape. An analysis of electronic excitations in the computed UV-vis spectra of the two complexes shows that a particular type of asymmetric excitations is only present in the photoactive complex.
Highlights
Catalytic nitrogen fixation with well-defined molecular complexes remains a grand challenge despite decades of research in this field
The research field is driven by the vision that a molecular catalyst capable of catalytically transforming nitrogen atoms from the dinitrogen molecule into ammonia or chemicals of higher economic value would contribute to a more sustainable chemical industry not dependent on fossil resources (Crossland and Tyler, 2010; Broda et al, 2013; Tanabe and Nishibayashi, 2013; Lee et al, 2014; Burford and Fryzuk, 2017; Burford et al, 2017; Connor and Holland, 2017; Creutz and Peters, 2017; Eizawa and Nishibayashi, 2017; Kuriyama and Nishibayashi, 2017; Roux et al, 2017)
Catalysts that produce ammonia from dinitrogen are based on molybdenum, iron and cobalt (Roux et al, 2017), with many more elements known to be capable of binding N2 and activating the strong N-N bond (Burford and Fryzuk, 2017; Klopsch et al, 2017)
Summary
Catalytic nitrogen fixation with well-defined molecular complexes remains a grand challenge despite decades of research in this field. While a molecular catalyst for NH3 production may never be efficient enough to replace the highly optimized Haber-Bosch process, research in this area results in valuable insights into the fundamental principles and electronic structure requirements for N2 activation. This in turn may be relevant for fertilizer production, and for alternative fuels that are based on nitrogen instead of carbon (Schlögl, 2010) (Grinberg et al, 2016; Chen et al, 2018). The complexes will either fully cleave the N2 molecule or activate the bond sufficiently that the nitrogen atoms are prepared for subsequent chemical reactions, and be part of a complete catalytic cycle with reasonable turnover numbers and turnover frequencies
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