Captured in a Crystal: Photogeneration and Reactivity of an Aura-Nitrene for Gold-Mediated Nitrene Transfer

The challenge of making C-N bonds

Nitrogen-containing compounds are ubiquitous in drugs and agrochemicals. Therefore, efficient ways to create C–N bonds remain a central goal for the chemical industry. Nitrenes¹—subvalent nitrogen species with only 6 electrons—offer a complementary route to traditional C–N coupling methods via nitrogen-atom transfer (NAT) reactions.² Harnessing their potential, however, is challenging as their highly reactive nature makes it difficult to control the selectivity of these transformations. Late transition metals surrounded by carefully designed chelating ligands provide a promising strategy to stabilize and tune the reactivity of these elusive species, but detailed characterization of such systems remains scarce.³

Towards gold-nitrenes

Building on our group's expertise studying highly reactive gold(III) complexes,^4,5 we designed a project to generate aura-nitrene species (Au–N) supported by a tridentate (P^N^C) ligand from the corresponding azide precursor (Au–N₃) in crystallo. This chemical reaction is triggered by light inside a single crystal, promoting the extrusion of N₂ and the in situ formation of the desired Au–N fragment. Carrying out the reaction in a monocrystal enables direct structural characterization by X-ray diffraction, providing a molecular snapshot of this elusive species.

To achieve the necessary resolution, we employed synchrotron radiation to generate the X-rays. This technology played a key role in studies that led to the Nobel Prize in Chemistry 2009, awarded for elucidating the structure of the ribosome, and is now routinely used to solve structures ranging from small molecules to large biomacromolecules. In cases where conventional X-ray sources fall short, synchrotron methods are indispensable for obtaining precise structural information.

A major challenge when performing in crystallo reactions lies in obtaining high-quality single crystals. We systematically screened a wide range of conditions, including solvent mixtures and temperatures, to crystallize the azide precursor (Au–N₃) in sufficient quality and quantity.⁶ Crystallization is highly sensitive to experimental parameters and can take from days to weeks. Through extensive optimization, we prepared more than 60 batches of suitable crystals, which were pre-screened using in-house X-ray facilities at the University of Zurich and stored prior to synchrotron measurements.

Following extensive optimization at the beamline, we successfully captured the elusive gold-nitrene intermediate at the Swiss Light Source (PSI). Complementary theoretical studies revealed the electrophilic and diradical character of the metallonitrene and provided mechanistic insight into its reactivity.

The spontaneous O₂ activation

Arguably even more intriguing were the observations made during the preparation stage. Aged batch of crystals gradually changed colour over several months, before any planned experiment could be carried out. This prompted us to re-examine their structure. Remarkably, the original Au–N₃ complex had undergone a spontaneous transformation into Au–NO₂ species. After confirming the identity of this new compound, we hypothesized that ambient laboratory light had promoted the release of N₂ from the Au–N₃ crystals, generating a Au–N intermediate capable of activating O₂ from the air.

We were able to reproduce this transformation by exposing our Au–N₃ crystals directly to an atmosphere of pure O₂ in the presence of white light, obtaining a 1:1 mixture of Au–NO₂ and unreacted Au–N₃. However, these experiments proved difficult to control as the crystals were sensitive to the reaction conditions and frequently fractured, losing the crystallinity before the transformation could be completed.⁷

Reactivity landscape and future horizons

After numerous unsuccessful attempts, we adopted a different strategy that ultimately reshaped the project: rather than preserving crystallinity, we deliberately ground the crystals and carried out the reactions in the solid state using bulk powder. This approach circumvented the issue entirely and, perhaps unexpectedly, led to higher conversions over shorter timescales. It also enabled a broader exploration of the reactivity of the photogenerated gold–nitrene with reagents in the gas phase.

The resulting reactivity profile includes carbon monoxide fixation, intra- and intermolecular C–H bond activation, and alkyne addition followed by a sigmatropic rearrangement, alongside the previously observed conversion to the Au–NO₂ species.

Together, these findings highlight the largely untapped potential of gold—long regarded as an inert metal—in nitrogen-atom transfer (NAT) chemistry. Although other “gold-catalyzed nitrene transfer reactions” have previously been explored,⁸ our results point to a distinct pathway that directly involves gold–nitrene intermediates. By capturing, characterizing, and interrogating the reactivity of these elusive species, we reveal their central role in enabling new modes of C–N bond formation and uncover reactivity patterns that expand the scope of gold-mediated transformations.

References

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