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

Abstract

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(ⁱPr)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.