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Energetics and Mechanism of Ammonia Synthesis through the Chatt Cycle: Conditions for a Catalytic Mode and Comparison with the Schrock Cycle
Author(s) -
Stephan Gerald C.,
Sivasankar Chinnappan,
Studt Felix,
Tuczek Felix
Publication year - 2008
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200700849
Subject(s) - catalytic cycle , protonation , disproportionation , chemistry , catalysis , ammonia , diethyl ether , medicinal chemistry , exergonic reaction , reaction mechanism , computational chemistry , organic chemistry , ion
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 HBF 4 /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 N 2 complex and the most exergonic reaction is the cleavage of the NN bond. If lutidinium is employed as acid and ${{\rm Cp}{{{\ast}\hfill \atop 2\hfill}}}$ Cr as reductant, the reaction course involves steps that are not thermally allowed. For HBF 4 /diethyl ether as the acid and ${{\rm Cp}{{{\ast}\hfill \atop 2\hfill}}}$ 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.