Cobalt-Promoted Tandem C(sp2)–H Activation and C–C Coupling of Vinyldiphenylphosphine: σ- vs π‑Coordination Modes within Cyclometalation

We have studied the reactivity of a series of terminal alkenyldiphenylphosphines {Ph2P­(CH2)n-CHCH2), n = 0, 1, 2, 3} under mild conditions (−70 °C) in reaction with univalent Co­(I). Methane and ethane elimination occurs when different olefinic P,C-coordination modes are established. Subsequent C–H activation and C–C coupling take place in the reaction of Co­(CH3)­(PMe3)4 with vinyldiphenylphosphine (1:2 molar ratio) to form a novel anionic P,C,P-type ligand with asymmetric ring sizes attached to the cobalt center of (PMe3)2Co­{κ3-(P,C,P)-(Ph2)­CH2CHCH-CH­(PPh2)} (1). X-ray study of 1 revealed that the complex has a unique cyclometalated structure of a three-membered ring within the newly assembled bis-phosphine ligand. Cyclometalation via selective vinyl-group C­(sp2)–H activation of stoichiometric amounts allyl-diphenylphosphine with Co­(CH3)­(PMe3)4 afforded for the first time σ-allyl coordination incorporated in a five-membered cobaltacycle of (PMe3)2Co­{κ2(P,C)-HCCHCH2PPh2} (2); subsequent C–C coupling is not observed. By contrast, equimolar mixtures of pentenyl-diphenylphosphine and Co­(CH3)­(PMe3)4 afforded a η3-coordinated π-allyl complex of composition (PMe3)2Co­{η3-κ4-(P,C,C,C)-H2CCH-CH­(CH2)2PPh2} (3). All unprecedented complexes were characterized by multinuclear NMR spectroscopy and X-ray diffraction, and their mechanisms of formation are discussed.