A stable rhodium-coordinated carbene with a σ ⁰ π ² electronic configuration

Abstract

Isolable singlet carbenes have universally adopted a σ 2 π 0 electronic state, making them σ-donors and π-acceptors. We present a rhodium-coordinated, cationic cyclic diphosphinocarbene with a σ 0 π 2 ground state configuration. Nuclear magnetic resonance spectroscopy studies show a carbene carbon chemical shift below −30.0 parts per million. X-ray crystallography reveals a planar RhP 2 C configuration. Quantum chemical calculations rationalize how σ-electron delocalization/donation and π-electron negative hyperconjugation together stabilize the formally vacant σ orbital and the filled π orbital at the carbene center. In contrast to traditional carbene counterparts this carbene can undergo synthetic transformations with both a Lewis base and a silver salt, producing a Lewis acid/base adduct and a silver π-complex, respectively. Exhibiting ambiphilic reactivity, it can also form a ketenimine through reaction with an isocyanide.

Recent advances in the chemistry of isolable carbene analogues with group 13–15 elements

“…Pursuing the isolation of the heavier carbyne anion analogues, whether as free entities or within complexes, clusters, and polymers, continues to be a formidable challenge. Furthermore, with the recent revelation of a singlet carbene having an inverted electronic state, 17 similar advancements in group 14 metallylenes with a reversed electronic configuration could be forthcoming.…”

“…15 More recently, the field of carbenes has been further diversified with the discovery of novel structures, such as a doubly oxidized carbene reported by Loh and Bertrand in 2023 16 and a stable rhodium-coordinated carbene with a reversed electronic configuration by our team in 2024. 17…”

Designing a σ0π2 singlet ground state carbene from dicationic carbones

“…This result reveals that negative hyperconjugation to phosphonium groups is far from sufficient to stabilize the σ 0 π 2 state and re-emphasizes the role of the metal in 4 . 18 A comparable geometric and electronic structure is observed for the methylated diphosphonium ion 6 .…”

“…1C). 18–20 While donation of electron density from the metal centre into the σ orbital stabilizes this carbene, negative hyperconjugation of the π electrons was also discussed to play a role. The importance of the latter interaction suggests that a similar electronic structure could be achieved in carbodiphosphoranes by removal of two electrons to obtain a dicationic carbene (Fig.…”

Molecular-strain induced phosphinidene reactivity of a phosphanorcaradiene

“…Phosphinidenes are neutral monocoordinate and monovalent phosphorus species possessing six valence electrons. Such species are isoelectronic to well-explored carbenes 6 – 14 and tetrylenes 15 – 17 . In contrast, most phosphinidenes are transient species because of their exceptionally high reactivity 18 – 21 .…”