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 N-2 complex and the most exergonic reaction is the cleavage of the N-N bond. If lutidinium is employed as acid and Cp-2*Cr as reductant, 2 the reaction course involves steps that are not thermally allowed. For HBF4/diethyl ether as the acid and Cp-2*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.