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On the unprecedented level of dinitrogen activation in the calix[4]arene complex of Nb(iii)Electronic supplementary information (ESI) available. See DOI: 10.1039/c1dt11091h.

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Title: On the unprecedented level of dinitrogen activation in the calix[4]arene complex of Nb(iii)Electronic supplementary information (ESI) available. See DOI: 10.1039/c1dt11091h.
Authors: Terrett, R.1, Cavigliasso, G.1, Stranger, R.1, Yates, B. F.2
Source: Dalton Transactions: An International Journal of Inorganic Chemistry. Nov2011, Vol. 40 Issue 42, p11267-11275. 9p.
Subjects: Niobium, Aromatic compounds, Hydroxides, Chemical bonds, Crystallography, Metal complexes
Abstract: The calix[4]arene niobium(iii) complex ([L]Nb–NN–Nb[L] where [L] = p-tert-butylcalix[4]arene), reported to bind N2in a μ2-linear dimeric capacity and to activate the N2triple bond to 1.39 Å, corresponding to the longest N2bond 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 N2end-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 N2bond. 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–N2complexes as well as predictions derived from trends in N–N stretching frequency for a number of crystallographically characterized linear N2activators. A number of potential causes for this bond length discrepancy are explored. [ABSTRACT FROM AUTHOR]
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Abstract:The calix[4]arene niobium(iii) complex ([L]Nb–NN–Nb[L] where [L] = p-tert-butylcalix[4]arene), reported to bind N2in a μ2-linear dimeric capacity and to activate the N2triple bond to 1.39 Å, corresponding to the longest N2bond 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 N2end-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 N2bond. 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–N2complexes as well as predictions derived from trends in N–N stretching frequency for a number of crystallographically characterized linear N2activators. A number of potential causes for this bond length discrepancy are explored. [ABSTRACT FROM AUTHOR]
ISSN:14779226
DOI:10.1039/c1dt11091h