Energetics and Mechanism of Ammonia Synthesis through the Chatt Cycle: Conditions for a Catalytic Mode and Comparison with the Schrock Cycle

Abstract

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.

(Dinitrogen)molybdenum Complexes Supported by Asymmetric Silicon‐Centered Tripod Ligands: Steric and Electronic Influences on the Coordination of Mono‐ and Diphosphine Coligands

“…On the basis of the latter systems, the first complete mechanistic scenario of N 2 reduction has been established by Chatt and co-workers. [26,27] To this 3564 (dmpm), or two PMe 2 Ph] and [Mo(N 2 ) 2 (SiPPPЈ)(PMe 2 Ph)]. [11][12][13][14][15][16][17][18] More recently we focused our attention on the design of new ligand environments.…”

Dinitrogen Splitting Coupled to Protonation

“…[3] These are proposed to operate via successive protonation/reduction mechanisms, as expressed in the Chatt and Schrock cycles. [4] Molecular complexes that split N 2 into nitrides offer an alternative mechanistic entry. [5,6] For both pathways, efficient charge storage and transfer to N 2 define key challenges, particularly for the development of electrocatalytic methods.…”

Nitrogen Fixation Catalyzed by Transition Metal Complexes: Recent Developments

“…Given that the protonating agent and thus also the triflate anion are present in vast excess, the free reaction energy for ligand coordination of OTf – should be even more negative. This scenario also seems likely in view of the similarity of the proposed pathway to the classic Chatt cycle, which indeed involves the coordination of anions at the position trans to the dinitrogen ligand at the Mo center …”

“…The classic Chatt cycle, however, suffers from a couple of drawbacks that have limited its performance in a catalytic reaction mode , . In particular, the presence of anionic coligands causes disproportionation at the Mo I stage . To address this problem new multidentate phosphine ligands have been developed …”