A Revisit of Co(PMe ₃ ) ₄ -Catalyzed Hydrophosphination of Phenylacetylene with HPPh ₂

The low-valent cobalt complex Co(PMe3)4 was previously reported to catalyze the hydrophosphination of phenylacetylene with HPPh2 to yield (E)-PhCH═CH(PPh2) exclusively. This work provides a more in-depth analysis of the catalytic system showing that Co(PMe3)4 is contaminated with HCo(PMe3)4. The impurity undergoes a reversible ligand exchange reaction with HPPh2 to form a series of new cobalt hydrides with the formula HCo(PMe3)4–x(HPPh2)x (x = 1–4). One of these hydride complexes, namely, HCo(PMe3)3(HPPh2), features a doublet of quartets for the hydride resonance, which was previously misassigned to the paramagnetic Co(II) complex, HCo(PMe3)3(PPh2). The contaminated Co(PMe3)4 reacts with HPPh2, under heating, to generate a dinuclear cobalt complex, (Me3P)2Co(μ-PPh2)2Co(PMe3)2. In addition to “Co(PMe3)4”, the independently synthesized HCo(PMe3)4, HCo(HPPh2)4, and (Me3P)2Co(μ-PPh2)2Co(PMe3)2 all prove to be catalytically competent. Regardless of the cobalt catalysts examined herein, both (Ph2P)CPh═CH2 and (Z)-PhCH═CH(PPh2) are present as minor hydrophosphination products.