Triplet Carbenes with Transition-Metal Substituents

Abstract The extraordinary advances in carbene (R¹–C–R²) chemistry over the past 30 years have been fueled by the introduction of efficient strategies to stabilize the electronic singlet state via p-bonding interactions with the substituents (R¹, R²).¹–⁴ In contrast, the lack of similarly efficient approaches to authentic triplet carbenes with appreciable lifetimes beyond cryogenic temperatures hampers their exploitation in synthesis and catalysis.⁵ Transition-metal substitution represents a potential strategy,⁶ but thus far metallocarbenes (M–C–R) usually represent high-lying excited electronic configurations of the well-established carbyne complexes (M ≡ C–R). We here report the photochemical synthesis and characterization of persistent triplet metallocarbenes (M–C–SiMe₃, M = PdII, PtII). The carbon-centered diradicals exhibit selective reactions in solution and in the solid state, such as carbene C–H insertion and carbonylation. Bond analysis revealed significant stabilization by spin-polarized push and pull interactions along both π-bonding planes, which fundamentally differs from bonding in push/pull singlet carbenes. This bonding model, thus, expands key strategies for stabilizing carbenes towards the open-shell analogues and closes a conceptual gap to carbyne complexes. We anticipate that the introduction of closed-shell transition-metal substituents to triplet carbenes will facilitate their exploitation in chemical synthesis based on the well-developed principles of coordination chemistry.