Terminal uranium(V)-nitride hydrogenations involving direct addition or Frustrated Lewis Pair mechanisms

Lucile Chatelain,Iskander Douair,Laurent Maron,Floriana Tuna,Benedict M Gardner,Stephen T Liddle,Erli Lu,Ashley J Wooles,Elisa Louyriac

doi:10.1038/s41467-019-14221-y

Abstract

Despite their importance as mechanistic models for heterogeneous Haber Bosch ammonia synthesis from dinitrogen and dihydrogen, homogeneous molecular terminal metal-nitrides are notoriously unreactive towards dihydrogen, and only a few electron-rich, low-coordinate variants demonstrate any hydrogenolysis chemistry. Here, we report hydrogenolysis of a terminal uranium(V)-nitride under mild conditions even though it is electron-poor and not low-coordinate. Two divergent hydrogenolysis mechanisms are found; direct 1,2-dihydrogen addition across the uranium(V)-nitride then H-atom 1,1-migratory insertion to give a uranium(III)-amide, or with trimesitylborane a Frustrated Lewis Pair (FLP) route that produces a uranium(IV)-amide with sacrificial trimesitylborane radical anion. An isostructural uranium(VI)-nitride is inert to hydrogenolysis, suggesting the 5f1 electron of the uranium(V)-nitride is not purely non-bonding. Further FLP reactivity between the uranium(IV)-amide, dihydrogen, and triphenylborane is suggested by the formation of ammonia-triphenylborane. A reactivity cycle for ammonia synthesis is demonstrated, and this work establishes a unique marriage of actinide and FLP chemistries.

Highlights

Despite their importance as mechanistic models for heterogeneous Haber Bosch ammonia synthesis from dinitrogen and dihydrogen, homogeneous molecular terminal metal-nitrides are notoriously unreactive towards dihydrogen, and only a few electron-rich, low-coordinate variants demonstrate any hydrogenolysis chemistry
As part of our studies investigating actinide-ligand multiple bonding supported by triamidoamine ancillary ligands[29,30,31,32,33,34,35], we have reported two closely related terminal uranium-nitrides [UV (TrenTIPS)(N)][K(B15C5)2] (1) and [UVI(TrenTIPS)(N)] (2) [TrenTIPS = N(CH2CH2NSiPri3)33−; B15C5 = benzo-15-crown-5 ether]36–38 that, unusually[1,5,39], permit examination of the electronic structure and reactivity of the same isostructural terminal nitride linkage with more than one metal oxidation state
Further motivation to study this fundamental reaction stems from the fact that bridging and terminal uranium-nitride reactivity with H2 is implicated in Haber Bosch NH3 synthesis when uranium is used as the catalyst[49], and uranium-nitrides have been proposed as accident tolerant fuels (ATFs) for nuclear fission, but likely reactivity with H2 formed from radiolysis under extreme conditions or when stored as spent fuel remains poorly understood

Summary

Introduction

Despite their importance as mechanistic models for heterogeneous Haber Bosch ammonia synthesis from dinitrogen and dihydrogen, homogeneous molecular terminal metal-nitrides are notoriously unreactive towards dihydrogen, and only a few electron-rich, low-coordinate variants demonstrate any hydrogenolysis chemistry. As part of our studies investigating actinide-ligand multiple bonding supported by triamidoamine ancillary ligands[29,30,31,32,33,34,35], we have reported two closely related terminal uranium-nitrides [UV (TrenTIPS)(N)][K(B15C5)2] (1) and [UVI(TrenTIPS)(N)] (2) [TrenTIPS = N(CH2CH2NSiPri3)33−; B15C5 = benzo-15-crown-5 ether]36–38 that, unusually[1,5,39], permit examination of the electronic structure and reactivity of the same isostructural terminal nitride linkage with more than one metal oxidation state Both react with the small molecules CO, CO2, and CS240,41, but since only the protonolysis of 1 with H2O to give NH3 had been previously examined[36] the ability of 1 and 2 to react with H2 has remained an open question. We demonstrate an azide to nitride to amide to ammonia reaction cycle, supported by overall hydrogenation involving hydrogenolysis and electrophilic quenching steps

Methods

Results

Conclusion