Biomimetic nitrogen fixation provides an attractive alternative for the century-old Haber-Bosch process; however, the performance of the currently available molecular biomimetic catalysts is very limited. In this work, we are aiming to understand the catalytic cycle of one of the most promising biomimetic complex families that can be the cornerstone of future computer-aided rational design of biomimetic complexes. We calculate the Gibbs free energy of all elementary reaction steps of homogeneous dinitrogen reduction to NH on single-site iron complexes with EPPP tetradentate ligands (E = B, Si). We examine all possible mechanisms and identify the dominant pathways and the critical elementary steps that can be rate-determining in the catalytic cycle of nitrogen fixation. We find that the catalytic mechanism depends on the applied ligand and that the distal pathway observed with E = B is the most favorable route regarding the catalytic performance. Our calculations also reveal the lack of thermodynamic driving force in the last steps of the catalytic cycle that can be responsible for the low catalytic activity of the studied biomimetic catalysts. Our results can serve as a starting point for the rational design of biomimetic complexes, which should focus on establishing a steadily decreasing Gibbs free energy profile, as suggested by the Sabatier principle.
Nitrogen
reduction reaction (N2RR) carried out on biomimetic
catalytic systems is considered to be a promising alternative for
the traditional Haber–Bosch ammonia synthesis. Unfortunately,
the selectivity of the currently known biomimetic catalysts is poor,
as they also catalyze the unproductive hydrogen evolution reaction
(HER). In the present computational study, we examine the HER activity
of early N2RR intermediates in EP3 (E = B, Si)
ligated single-site biomimetic iron complexes by calculating and comparing
the activation Gibbs free energies of HER and N2RR elementary
steps. We find that, in contrast to previous suggestions, early N2RR intermediates are not likely sources of HER under turnover
conditions, as the barriers of the competing N2RR steps
are significantly lower. Consequently, future research should focus
on preventing other potential HER mechanisms, e.g., hydride formation,
rather than accelerating the consumption of early N2RR
intermediates as proposed earlier to design more efficient biomimetic
catalysts.
Synthetic
Fe nitrogenases are promising catalysts for atmospheric pressure ammonia
synthesis. However, their catalytic efficiency is severely limited
by the accompanying hydrogen evolution reaction (HER) and fast catalyst
deactivation. In order to reveal the origin of these undesired transformations,
we study potential reaction routes of HER, catalyst deactivation,
and nitrogen reduction reaction (N2RR) by density functional
theory in combination with microkinetic modeling, using a triphosphino-silyl
ligated iron complex as model system. Our results show that the most
favorable HER cycle is initiated by H2 molecules originated
from the noncatalytic reaction of acid and reductant reagents, which
can coordinate to a vacant binding site of an Fe complex. Thus, H2 coordination competes with the N2 coordination
step of the desired N2RR catalytic cycle, and the resulting
Fe–H2 complex can be protonated at both hydrogen
atoms to release two H2 molecules. The proposed mechanism,
called autocatalytic hydrogen evolution reaction (aHER), explains
all experimentally observed results including catalyst deactivation,
as aHER intermediates can be easily converted into thermodynamically
stable, catalytically inactive monohydrides. Our results suggest that
improved efficacy of synthetic Fe nitrogenases can be achieved by
several ways: (i) proper ligand modifications hindering the formation
of Fe–H2 complexes, (ii) suppressing the noncatalytic
H2 formation in the catalytic mixture by different reagent
choice, and (iii) using flow or semiflow reactor setup instead of
batch reactors and keep the proton and electron reagent excess low.
The synthesis of variously substituted indolo[2,3c]quinolin-6(7H)-ones was developed via Pd-catalyzed intramolecular C−H arylation. This method highlights a strategy for preparing indoloquinoline precursors bearing versatile functional groups and provides a new approach for the synthesis of antimalarial isoneocryptolepine analogues. The plausible ring closure mechanism was examined with quantum chemical calculations, where a trigonal bipyramidal concerted metalation− deprotonation transition state is presumable.
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