On the unprecedented level of dinitrogen activation in the calixarene complex of Nb(III)
Terrett, R and Cavigliasso, G and Stranger, R and Yates, BF, On the unprecedented level of dinitrogen activation in the calixarene complex of Nb(III), Dalton Transactions, 40, (42) pp. 11267-11275. ISSN 1477-9226 (2011) [Refereed Article]
The calixarene niobium(III) complex ([L]Nb–NN–Nb[L] where [L] = p-tert-butylcalixarene), 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 calixarene ligand with hydroxide or methoxide ligands reveals that the organic backbone structure of the calixarene 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 metallacalixarene 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.