Terminal, Open-Shell Mo Carbide and Carbyne Complexes: Spin Delocalization and Ligand Noninnocence.

Open-shell compounds bearing metal-carbon triple bonds, such as carbides and carbynes, are of significant interest as plausible intermediates in the reductive catenation of C oxygenates. Despite the abundance of closed-shell carbynes reported, open-shell variants are very limited, and an open-shell carbide has yet to be reported. Herein, we report the synthesis of the first terminal, open-shell carbide complexes, and ( = P2Mo(≡C:)(CO), P2 = a terphenyl diphosphine ligand), which differ by two redox states, as well as a series of related open-shell carbyne complexes. The complexes are characterized by single-crystal X-ray diffraction and NMR, EPR, and IR spectroscopies, while the electronic structures are probed by EPR studies and DFT calculations to assess spin delocalization. In the d complexes, the spin is primarily localized on the metal (∼55-77% Mo d) with delocalization on the triply bonded carbon of ∼0.05-0.09 e. In the reduced carbide , a direct metal-arene interaction enables ancillary ligand reduction, resulting in reduced radical character on the terminal carbide (⩽0.02 e). Reactivity studies with reveal the formation of mixed-valent C-C coupled products at -40 °C, illustrating how productive reactivity manifolds can be engendered through the manipulation of redox states. Combined, the results inform on the electronic structure and reactivity of a new and underrepresented class of compounds with potential significance to a wide array of reactions involving open-shell species.