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Nitrogen fixation and reduction at boron

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Science  23 Feb 2018:
Vol. 359, Issue 6378, pp. 896-900
DOI: 10.1126/science.aaq1684

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Boron learns to give back to nitrogen

Although diatomic nitrogen is famously inert, a variety of transition metals can bind to it through a process termed backbonding. As the nitrogen weakly shares its own electrons, some electrons from the metal reach back out to it. Nonmetals would not seem to have the capacity for this type of bonding, but now Légaré et al. show that conventionally electron-deficient boron can be coaxed into it (see the Perspective by Broere and Holland). The authors treated boron-based precursors with potassium under a nitrogen atmosphere to produce several compounds with sandwiched dinitrogen between two boron centers in reduced motifs reminiscent of metal complexes.

Science, this issue p. 896; see also p. 871

Abstract

Currently, the only compounds known to support fixation and functionalization of dinitrogen (N2) under nonmatrix conditions are based on metals. Here we present the observation of N2 binding and reduction by a nonmetal, specifically a dicoordinate borylene. Depending on the reaction conditions under which potassium graphite is introduced as a reductant, N2 binding to two borylene units results in either neutral (B2N2) or dianionic ([B2N2]2–) products that can be interconverted by respective exposure to further reductant or to air. The 15N isotopologues of the neutral and dianionic molecules were prepared with 15N-labeled dinitrogen, allowing observation of the nitrogen nuclei by 15N nuclear magnetic resonance spectroscopy. Protonation of the dianionic compound with distilled water furnishes a diradical product with a central hydrazido B2N2H2 unit. All three products were characterized spectroscopically and crystallographically.

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