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.