Activation and cleavage of dinitrogen by three-coordinate metal complexes involving Mo(III) and Nb(II/III)

Density functional calculations have been employed to rationalize why the heteronuclear N₂-bridged Mo^IIINb^III dimer, [Ar(^tBu)N]₃Mo(μ-N₂)Nb[N(ⁱPr)Ar] ₃ (Ar = 3,5-C₆H₃Me₂), does not undergo cleavage of the dinitrogen bridge in contrast to the analogous Mo ^IIIMo^III complex which, although having a less activated N-N bond, undergoes spontaneous dinitrogen cleavage at room temperature. The calculations reveal that although the overall reaction is exothermic for both systems, the actual cleavage step is endothermic by 144 kJ mol^-1 for the Mo^IIINb^III complex whereas the Mo^IIIMo ^III system is exothermic by 94 kJ mol^-1. The reluctance of the Mo^IIINb^III system to undergo N₂ cleavage is attributed to its d³d² metal configuration which is one electron short of the d³d³ configuration necessary to reductively cleave the dinitrogen bridge. This is confirmed by additional calculations on the related d³d³ Mo^IIINb ^II and Nb^IINb^II systems for which the cleavage step is calculated to be substantially exothermic, accounting for why in the presence of the reductant KC₈, the [Ar(^tBu)N] ₃Mo(μ-N2)Nb[N('Pr)Ar]3 complex was observed to undergo spontaneous cleavage of the dinitrogen bridge. On the basis of these results, it can be concluded that the level of activation of the N-N bond does not necessarily correlate with the ease of cleavage of the dinitrogen bridge.