Modular Design enables rational access to π-Complexes of elusive Phosphacyclic Biradicaloids

Four-membered biradicaloids exhibit unique coordination behavior, yet no rational synthetic strategy has enabled their modular modification and systematic tuning. Herein, we report a modular approach enabling the isolation of half-sandwich complexes featuring elusive P-centered biradicaloids. Specifically, cationic manganese and cobalt complexes featuring η⁴-coordinated RR’C₂P₂ ligands [R, R’ = PPh₃, IPr (1,3-bis(2,6-diisopropylphenyl)imidazolium-2-ylidene)] were accessed via reaction of halogenated phosphorus cycles RR’C₂(PX)₂ (X = Cl, Br) with the corresponding carbonyl metalates. In these complexes, the biradicaloid ligands behave as zwitterionic homologues of the cyclobutadiene-diide anion [C4H4]2−. Notably, the choice of the substituents R,R’ allows for a gradual control of the steric environment while retaining the exceptional π-donating properties of the biradicaloids. Furthermore, the CO ligands in the cationic complexes allow for facile ligand exchange as is demonstrated by the light-driven synthesis of the 2,3 dimethylbutadiene (DMB) complex [Co{η4 DMB}{η4 (PPh3)2C2P2}][OTf].