Hydrogen and Nitrogen

The hydrogen economy represents a way of fulfilling our need for energy by using molecular hydrogen as an energy carrier and using reactions in which polluting products like greenhouse gases are avoided. The prospects of such an economy require the development of clean and efficient ways of producing and storing molecular hydrogen, or, in an extended sense, of molecules by which energy is stored in their chemical bonds. We have investigated a catalytically active nickel-bis-dithiolate complex by theory and spectroscopy and conclusively identified the energetically most favourable energetic pathway.

Figure 1. Overview of reorganization steps with thermodynamic parameters (green box) and schematic overviews including ligand field of the intermediates and transition states (blue boxes).[1]

In addition, we have performed similar investigation for the naturally occurring enzymes, [NiFe] hydrogenases. A recurring theme is the binding of the substrate, electron donation into empty metal d orbitals, and backdonation into antibonding orbitals of the ligand, thus doubly weakening the chemical bond in the substrate.

Figure 2. (left) Schematic overview of the orbitals that play a role in hydrogen oxidation in [NiFe] hydrogenases; (right) orbital mixing scheme after McGrady and Guilera leading to weakening of the H-H bond.[1]

Nitrogen

Nitrogen is one of the most inert molecules on the planet. It features a triple bond. Presently, nitrogen is turned into ammonia in the Haber-Bosch process, which requires high temperature and pressure. In nature, nitrogenase enzymes, featuring an iron-molybdenum (FeMoCo) active site are able to catalytically turnover N2 into ammonia in an impressive 8-electron reduction process, in which additionally an H2 molecule is produced. We investigated a simplified molecular complex that features some of the structural elements of the FeMoCo, bound NO+, which is iso-electronic to N2, and derived on the electronic level how the weakening of the NN triple bond occurs.

Figure 3. 3d orbital manifolds of the irons atoms as well as the 5 valence orbitals of the NO+ ligand for pictorially representing the resonance structure of NO+ with one s and two p orbitals. The orbital structure reflects s donation of NO+ into multiple iron 3d orbitals as well as a spectacular quadruple π backdonation into the π* orbitals of the substrate analogue. All 6 phenomena weaken the triple bond.[2]

 

References

[1] Das, R.; Neese, F.; van Gastel, M. Phys. Chem. Chem. Phys. 2016, 18, 24681-24692.

[2] Kalläne, S.I.; Hahn, A.; Weyhermüller, T.; Bill, E.; Neese, F.; DeBeer, S.; van Gastel, M. Inorg. Chem. 2019, 58, 5111-5125.

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