Cleavage of CO by Mo[N(R)Ar]₃ Complexes

Abstract

The reaction of MoL3 [L = NH2 and N(tBu)Ar] with CO was explored using DFT in order to rationalize why CO cleavage is not observed experimentally for this system in contrast to the corresponding N2 reaction which results in spontaneous cleavage of the N–N bond. The binding of CO to MoL3 was found to be both kinetically and thermodynamically favored over the binding of N2, with the formation of the encounter complex, L3Mo–CO, calculated to be without barrier and exothermic. While the overall reaction to form the C–MoL3 and O–MoL3 products was calculated to be energetically favorable, both the encounter complex and intermediate dimer, L3Mo–CO–MoL3, were found to be lower in energy than the products, with the final C–O cleavage step calculated to be endothermic by 169 kJ mol–1 and 163 kJ mol–1 for L = NH2 and N(tBu)Ar, respectively. The unfavorable CO cleavage step can be attributed to the fact that Mo does not possess the optimum d‐electron configuration to sufficiently stabilise the carbide and oxide products relative to the CO‐bridged intermediate dimer.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

A Comparison of N2 Cleavage in Schrock's Mo[N3N] and Laplaza–Cummins' Mo[N(R)Ar]3 Systems

“…[48,52] For the model system, the energies of the reactant and product species are reported for both the minimum energy structures and for structures in which the Mo À N ax distance was fixed to 2.4 in order to mimic the constraints of the ligand straps. Full DFT single point calculations were also carried out on the QM/MM optimised geometries of the full ligand system.…”

“…A similar trend was observed for the corresponding three-coordinate MoL 3 (L = NH 2 , NA C H T U N G T R E N N U N G (tBu)Ar) system when the model NH 2 ligands were replaced by the bulky NA C H T U N G T R E N N U N G (tBu)Ar groups in the full ligand system. [48,52] The effect of ligand bulk is especially evident for the intermediate dimer which is destabilized by 105 kJ mol À1 relative to the optimized model system and even further when the MoÀN ax bond is constrained, bringing it 14 kJ mol À1 above the products in energy. The small exothermicity of the N À N cleavage step differs from the calculations of Cui et al [22] for which the same step was calculated to be endothermic by 42 kJ mol…”

“…Consequently, it makes sense to attribute the additional destabilization of the dimer in the full ligand system to the steric crowding of the R groups between the metal centres. The encounter complex, intermediate dimer and product are all found to be more favourable relative to reactants for the single point full DFT calculations and the overall reaction is calculated to be exothermic by 323 kJ mol [48,52] …”

Dinitrogen Activation by Fryzuk’s [Nb(P2N2)] Complex and Comparison with the Laplaza–Cummins [Mo{N(R)Ar}3] and Schrock [Mo(N3N)] Systems

“…The results in Table 3 show a clear correlation between the overall enthalpy of reaction and the M À N bond strength: as the M À N bond in the product becomes stronger, the overall reaction enthalpy progressively increases. We have observed this correlation in our earlier studies of three-coordinate systems, [7] and it formed the premise on which we initially optimised the cleavage reactions associated with N 2 , CO, CS, CN À and NO, [37][38][39][40] but on the basis of the data in Table 3, this trend is clearly more general. Overall, greater enthalpies and M À N bond strengths are calculated for the Nb complexes, in agreement with the results of our earlier bond energy study, which showed that the M À N …”

Tuning the Laplaza-Cummins 3-coordinate M[N(R)Ph]3 catalyst to activate and cleave CO2

“…This particular system has been the subject of numerous experimental and mechanistic studies relating not only to N 2 but also to N 2 O, NO, NO 2 , CO, CS, CN -, CS 2 , OSMe, P 4 and SO 2 . [21][22][23][24][25][26][27][28][29][30][31][32][33][34] In all of these cases the catalyst shows high activity, but particular note was made in regard to carbon dioxide. It is remarkably inactive, in fact the Laplaza-Cummins catalyst will not even bind to CO 2 .…”