Toward an Inverse Approach for the Design of Small-Molecule Fixating Catalysts
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Toward an Inverse Approach for the Design of Small-Molecule Fixating Catalysts Thomas Weymuth1 and Markus Reiher1,* 1 ETH Zurich, Laboratory for Physical Chemistry, Wolfgang-Pauli-Strasse 10, 8093 Zurich, Switzerland * email: [email protected] ABSTRACT Within an inverse design approach applied to a nitrogen-fixation catalyst we discuss options for calculating “jacket” potentials that fulfill a purpose-oriented target requirement. As a target requirement we choose the vanishing geometric gradients on all atoms of a subsystem consisting of a metal center binding the small molecule to be activated - in our case dinitrogen. The additional potential can be represented within a full quantum model or by a sequence of approximations of which a field of electrostatic point charges is the simplest. In order to analyze the feasibility of this approach, we dissect a known dinitrogen-fixating complex and analyze its ligand environment expressed by the “jacket” potential. It is discussed how this ligand-bypotential replacement can be generalized for future applications that eventually allow us to find a competitive synthetic nitrogen-fixation transition metal complex. It can be expected that such a ligand-by-potential replacement approach will be applicable to any type of host-guest chemical process. INTRODUCTION Inverse quantum chemical approaches [1-3] are particularly appealing for the solution of chemical problems that have remained elusive despite decades of research. It is a fact that the inversion of a typical quantum chemical study, running from pre-defined molecular structures to the calculation of their electronic properties, to a definition of a target property that is in search of a molecular structure which features this property is a truly complex computational challenge because of the combinatorial possibilities that represent the dimensional curse of chemical compound space. However, we believe that a problem-specific design of inverse quantum chemical methods could be very beneficial for the above-mentioned type of elusive chemical problems. A challenge to synthetic chemistry is still the task of finding a transition metal catalyst that reduces molecular dinitrogen to ammonia under ambient conditions at high turnover number. It is important to stress that ambient, i.e., gentle, sustainable conditions are one of the challenges, while the stability of the catalyst under strong reduction conditions is another one. Note that the process is carried out in industry by an heterogeneous iron catalyst at elevated temperature and pressure [4]. However, it is clear that it must be possible to achieve this chemical transformation under mild conditions as the enzyme nitrogenase accomplishes exactly this task. So far, only two synthetic catalysts have achieved the goal [5, 6], but the turn-over number is still extremely low.
It is therefore a goal of intense research efforts [7-10] to increase the stability of the catalyst. We have studied this problem with quantum chemical methods [10-22] and identified the mo
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