Isolable Phosphonioacetylides as Strong Neutral Carbon Donors with Variable Steric Demand and Tunable Donor Properties.

Phosphonioacetylides are rod-shaped, neutral carbon donors with ambiphilic character at the terminal carbon. Despite their analogy to CO and isocyanides, they remain underexplored due to their high reactivity, and only one isolable free phosphonioacetylide has been reported. Here, we describe two new isolable, room-temperature-persistent examples stabilized either by two N-heterocyclic imines (NHIs) bearing bulky, flexible tert-octyl groups or by a single dipp-substituted NHI combined with alkyl substituents. Comprehensive characterization (SCXRD, NMR, IR, MS, DFT) confirms their strong donor properties and the accessibility of the reactive C2 unit. DFT analysis quantify how the substituents at phosphorus modulate the frontier molecular orbital energies, spanning a wide range of σ and π-donor strengths and π-acceptor abilities, with trends that mirror those of the corresponding phosphines. Both phosphonioacetylides readily form AuI complexes, whereas the less electron-rich compound shows the tendency to eliminate its C2 moiety thermally or upon hydrolysis. These results establish design principles for stabilizing and tuning phosphonioacetylides and open avenues for their use in coordination chemistry and catalysis.