On the unprecedented level of dinitrogen activation in the calix[4]arene complex of Nb(iii)

The calix[4]arene niobium(iii) complex ([L]Nb-NN-Nb[L] where [L] = p-tert-butylcalix[4]arene), reported to bind N₂ in a μ₂-linear dimeric capacity and to activate the N₂ triple bond to 1.39 Å, corresponding to the longest N₂ bond known in the end-on coordination mode, was subjected to a computational investigation involving both density functional and wavefunction based methods to establish the basis for the unprecedented level of activation. Replacement of the calix[4]arene ligand with hydroxide or methoxide ligands reveals that the organic backbone structure of the calix[4]arene ligand exerts negligible electronic influence over the metal centre, serving only to geometrically constrain the coordinating phenoxide groups. A fragment bonding analysis shows that metal-to-dinitrogen π^* backbonding is the principal Nb-N interaction, providing a strong electronic basis for analogy with other well-characterised three- and four-coordinate complexes which bind N₂ end-on. While the calculated structure of the metallacalix[4]arene unit is reproduced with high accuracy, as is also the Nb-Nb separation, the calculated equilibrium geometry of the complex under a variety of conditions consistently indicates against a 1.39 Å activation of the N₂ bond. Instead, the calculated N-N distances fall within the range 1.26-1.30 Å, a result concordant with closely related three- and four-coordinate μ₂- N₂ complexes as well as predictions derived from trends in N-N stretching frequency for a number of crystallographically characterized linear N₂ activators. A number of potential causes for this bond length discrepancy are explored.