Schrock cycle

Fig. 2. The Schrock cycle with its intermediates, based on the molybdenum(III) complex [Mo(HIPTN3N)] containing the triamidoa-mine ligand HIPTN3N (hexaisopropylterphenyl-triamidoamine) (2).

The alternative mechanistic scenario for the protonation and reduction of end-on terminally coordinated N2 through the Schrock cycle is represented by the Chatt cycle which has been developed many years earlier (5). This system is based on Mo(0) and W(0) dinitrogen complexes with phosphine coligands (Fig. 3). As expected, the intermediates of the dinitrogen reduction scheme are very similar to those of the Schrock cycle. Moreover, a cyclic generation of NH3 from N2 has been demonstrated on the basis of this system, however, with very small yields (3,4a). In order to obtain general insight into the mechanism of the Chatt cycle we have studied most of the intermediates of Fig. 3 with... 

Top row the three components of the Yandulov-Schrock cycle for catalytic dinitrogen reduction. Left the catalyst with the substrate Nj coordinated to Mo (Ar refers to the aryl substituent drawn in detail for one of the nitrogens). Centre the reductant bis(pentamethylcyclopentadienyl)chromium, CpjCr. Right the proton source 2,6-lutidinium borate. Bottom row vanadium complexes with intermediates of nitrogen reduction activated dinitrogen or diazenido(2—) (46a), imide (46b) and ammonia (46c). 

Figure 6.5 Schrock cycle with Mo catalyst, acid and reductant.

In addition to our study, a number of other theoretical treatments of the energetics of the Schrock cycle and related systems appeared, which, in part, considered larger models of the catalysts. Based on a model of the [Mo( N3N)] complex where the HIPT residues are replaced by phenyl groups (Figure 6.2b), Gao et al. treated all intermediates of the Schrock cycle with DFT, using the BLYP functional. Ammonium (NH4 ) was considered as... 

Table 6.1 Energetics of the Schrock cycle in kcal mol from Thimm et al and other authors (as indicated). ...

Figure 6.6 Free energy profile of the Schrock cycle by Studt 2005 (gray) and Thimm et al 2015 (red). Reproduced with permission from Thimm et aL, 2015. Copyright 2015 American Chemical Society.

In analogy to the Schrock cycle, Nishibayashi et aL postulated a Chatt-like reaction mechanism, where the bimetallic complex breaks into two monometallic fragments in solution, followed by catalytic reduction at a single metal center. However, the proposed intermediates could not be observed, and the reaction mechanism remained unclear. In order to address this problem, DFT calculations on the mechanism of the ammonia synthesis catalyzed by the Nishibayashi system were performed. Importantly, Batista and coworkers found that the bimetallic complex is the effective catalyst instead of the monometallic species that was originally postulated to play this role. Moreover, the dinitrogen-bridged dinuclear structure remains intact throughout... 

The resulting mechanistic cycle is shown in Figure 6.9. Analogous to the Schrock cycle, a strictly alternating sequence of protonation and reduction steps is derived. In the first half of the cycle the N-N bond is weakened step by step, leading to cleavage of the N-N bond and release of the first molecule of ammonia after formation of the hydrazidium complex. In the second half of the cycle, the Ru-N bond is progressively weakened and the... 

N2 atmosphere yields 60% of NH3 (relative to [(r -C5Me5)2Cr]), that corresponds to a TON = 8. Furthermore, several of the intermediates of the proposed catalytic cycle were successfully isolated and demonstrated to be catalytically competent. The Schrock cycle is shown in Scheme 33 and isolated and fuUy characterized intermediates are marked. Further support of this mechanistic cycle also stems from DFT computations. Nevertheless, to achieve an optimal catalytic performance the choice of the proton source, solvent, and reducing reagent is absolutely crucial. [LutH] [BAr 4] is sparingly soluble in the reaction solvent (heptane) and therefore ensures slower concurrent proton reduction to H2 than NH3 extrusion from [(HITPN3N)Mo(NH3)] (124). For the same reason, the reducing reagent is also added slowly to the suspension of catalyst and solid acid in heptane. However, despite optimized reaction conditions the limited catalytic TONs are attributed to catalyst decomposition induced by the protonation of the Mo—amide bond. 

Scheme 33 Schrock cycle for the reduction of N2 to NH3 catalyzed by [(mTPN3N)Mo N2)] (complexes marked in grey represent characterized intermediates). ...

Studt F, Tuczek F. Energetic and mechanism of a room-temperature catalytic process for ammonia synthesis (Schrock cycle) comparison with biological nitrogen fixation. Angew Chem Int Ed. 2005 44 5639-5642. 

Guha AK, Phukan AK. Why vanadium complexes perform poorly in comparison to related molybdenum complexes in the catalytic reduction of dinitrogen to ammonia (Schrock cycle) a theoretical study. Inorg Chem. 2011 50 8826-8833. 

Thimm W, Gradert C, Broda H, Wennmohs F, Neese F, Tuczek F. Free reaction enthalpy profile of the Schrock cycle derived from density functional theory calculations on the fUl [M0HIPTN3N] catalyst. Inorg Chem. 2015 54 9248-9255. 

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