Borane-catalysed dinitrogen borylation by 1,3-B–H bond addition

Abstract

The borylation of ligated dinitrogen by 1,3-B–H bond addition over a W–NN unit using various hydroboranes has been examined. In a previous study, we have shown that Piers’ borane (1) reacted with the tungsten dinitrogen complex 2 to afford a boryldiazenido–hydrido–tungsten species. The ease and mildness of this reaction have encouraged us to extend its scope, with the working hypothesis that 1 could potentially catalyse the 1,3-B–H bond addition of other hydroboranes. Under productive reaction conditions, dicyclohexylborane (HBCy₂) and diisopinocampheylborane (HBIpc₂) underwent retro-hydroboration to give cyclohexylborane (H₂BCy) or isopinocampheylborane (H₂BIpc), respectively; these monoalkylboranes act as N₂-borylating agents in the presence of a catalytic amount of 1. Under similar conditions, 9-borabicyclononane (9-BBN) slowly adds over the W–NN unit without rearrangement to a monoalkylborane. Catecholborane (HBcat) undergoes the 1,3-B–H bond addition without the need for a catalyst. We were not able to build more than one covalent B–N bond between the terminal N of the N₂ ligand and the boron reagent with this methodology.