H2and N2Binding Affinities Are Coupled in Synthetic Fe Nitrogenases Limiting N2Fixation

Abstract

Synthetic nitrogenases, that are transition metal complexes capable of reducing N2to NH3at atmospheric pressure, are considered as alternatives for the Haber-Bosch process; however, the currently studied complexes deactivate rapidly. Experimental studies on different types of artificial nitrogenases suggest that there exists a universal deactivation mechanism via the formation of catalytically inactive metal hydrides. In the present computational study, we examine whether the coordination of H2, which leads to hydride formation, can be suppressed by the proper tuning of the ligand field. Using the trisphosphino-E (E = borate, alkyl, or silyl) ligated iron nitrogenases as model systems, we investigate the effect of introducing common substituents on the relation between the binding affinity of N2and that of H2. We find that the Gibbs free energy of H2and N2coordination strongly correlate, as the desired decrease of H2affinity can only be achieved at the cost of an undesired decrease in N2affinity. This interdependence can be interpreted by orbital analysis, which reveals that the coordination of N2and H2comes with similar interactions toward the d orbitals of the Fe center. We conclude that the continuous removal of H2from the reaction mixture─rather than the redesign of the catalyst─is the effective way of eliminating H2-induced catalyst poisoning.