Thorium-nitrogen multiple bonds provide evidence for pushing-from-below for early actinides

Stephen T Liddle,Ashley J Wooles,Laurent Maron,Elizabeth P Wildman,Jingzhen Du,Carlos Alvarez-Lamsfus

doi:10.1038/s41467-019-12206-5

Abstract

Although the chemistry of uranium-ligand multiple bonding is burgeoning, analogous complexes involving other actinides such as thorium remain rare and there are not yet any terminal thorium nitrides outside of cryogenic matrix isolation conditions. Here, we report evidence that reduction of a thorium-azide produces a transient Th≡N triple bond, but this activates C-H bonds to produce isolable parent imido derivatives or it can be trapped in an N-heterocycle amine. Computational studies on these thorium-nitrogen multiple bonds consistently evidences a σ > π energy ordering. This suggests pushing-from-below for thorium, where 6p-orbitals principally interact with filled f-orbitals raising the σ-bond energy. Previously this was dismissed for thorium, being the preserve of uranium-nitrides or the uranyl dication. Recognising that pushing-from-below perhaps occurs with thorium as well as uranium, and with imido ligands as well as nitrides, suggests this phenomenon may be more widespread than previously thought.

Highlights

The chemistry of uranium-ligand multiple bonding is burgeoning, analogous complexes involving other actinides such as thorium remain rare and there are not yet any terminal thorium nitrides outside of cryogenic matrix isolation conditions
This phenomenon can be partly explained by anti-bonding interactions raising the energy of the σ-bond at short actinide-nitrogen distances
All characterisation, including a single-crystal cell check, are in accord with published data, and the formation of KN3 was evidenced by the presence of an azide asymmetric stretch at 2032 cm−1 in the ATR-IR spectrum, which is identical to that found for an authentic sample of KN3

Summary

Introduction

The chemistry of uranium-ligand multiple bonding is burgeoning, analogous complexes involving other actinides such as thorium remain rare and there are not yet any terminal thorium nitrides outside of cryogenic matrix isolation conditions. Diuranium-nitrides [{U (TrenDMBS)}2(μ-N)]0/1− in two different charge states could be prepared and isolated and were found to be robust, the corresponding dithorium-nitride [{Th(TrenDMBS)}2(μ-N)]1− proved elusive and bridging parent imido complexes, formed from C–H bond activation reactions, were the only isolable species Given that these diactinide-nitride species derive from azide precursors and are produced in concerted reactivity the formation of any Th≡N triple bond seems quite unlikely. An interesting feature of these bridging [{An(TrenDMBS)}2(μ-N)]1− and terminal uranium-nitrides is that the σ-component of their An-N multiple bonds is higher in energy than the π-bonds, and this is found for the uranyl dication but not bis(imido) analogues or other actinide-ligand multiple bonds more generally[74,75] This phenomenon can be partly explained by anti-bonding interactions raising the energy of the σ-bond at short actinide-nitrogen distances. We focussed our attention on the more sterically demanding ligand {N(CH2CH2NSiPri3)3}3− (TrenTIPS) to extend our search for a thorium-nitride since this ligand is the only ligand so far to support isolable terminal nitride linkages at uranium

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Results

Conclusion