Energetics and mechanism of ammonia synthesis through the Chatt Cycle: conditions for a catalytic mode and comparison with the Schrock Cycle.
Through a series of DFT calculations the energy profile of the Chatt cycle is evaluated. This is the counterpiece of our earlier investigations of the Schrock cycle (Angew. Chem. 2005, 117, 5783; Angew. Chem. Int. Ed. 2005, 44, 5639), applying the same quantumchemical methodology and approximations. As for the Schrock cycle, decamethylchromocene acts as reductant. The protonation reactions are considered to be mediated by HBF4/diethyl ether or lutidinium. For all protonation and reduction steps the corresponding free reaction enthalpy changes are calculated. The derived energy profile and corresponding reaction mechanism bear strong similarities to the Schrock cycle. In particular, the most endergonic reaction is the first protonation of the N2 complex and the most exergonic reaction is the cleavage of the N--N bond. If lutidinium is employed as acid and Cp2*Cr as reductant, the reaction course involves steps that are not thermally allowed. For HBF4/diethyl ether as the acid and Cp2*Cr as reducant, however, a catalytic cycle consisting of thermally allowed reactions is principally feasible. This cycle involves a Mo I-fluoro complex as dinitrogen intermediate. It is shown that regeneration to the Mo 0-bis(dinitrogen) complex is thermally not accessible in this system. Moreover, the Mo I fluoro-dinitrogen complex is labile towards disproportionation. The implications of these results with respect to the realization of a catalytic system on the basis of Mo and W phosphine complexes are discussed.
Stephan
Gerald C
GC
Institut für Anorganische Chemie, der Christian Albrechts Universität Kiel, Otto Hahn Platz 6/7, 24098 Kiel, Germany.
Sivasankar
Chinnappan
C
Studt
Felix
F
Tuczek
Felix
F
D000641:Ammonia / Q000138:chemical synthesis* / Q000737:chemistry; D002384:Catalysis; D003198:Computer Simulation; D008956:Models, Chemical; D015394:Molecular Structure; D008982:Molybdenum / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D011522:Protons; D011789:Quantum Theory; D013816:Thermodynamics
D009942:Organometallic Compounds; D011522:Protons; D000641:Ammonia; D008982:Molybdenum