Mo‐Coordinated Phosphinophosphonium Cations in Catalytic P–H Activation

ABSTRACT Phosphinophosphonium complexes with terminal P–H bonds, [Mo(CO) 4 (PR 2 H)(PR 2 –PR 2 H)] + ( 3 , R = Ph, Tol p ), form spontaneously from the phosphenium precursors [Mo(CO) 4 (PR 2 H)(PR 2 )] + ( 1 ) in the presence of secondary phosphines. These new complexes were isolated to investigate the lability of their P–P bonds in the context of possible catalytic, phosphenium‐mediated, electrophilic (hydro)phosphination of unsaturated substrates, a process we previously showed can be mediated in a stepwise fashion by 1 . However, reactivity studies show that alkene and ketone substrates cannot sufficiently displace the phosphenium‐bound phosphine in 3 to allow turnover. On the other hand, the Mo‐coordinated phosphinophosphonium ligand in 3 exhibits high Bronsted acidity that allows its participation, via P–H proton transfer chemistry, in two other catalytic reactions of secondary phosphines. Complex 3 initiates unusual, acid‐catalyzed Markovnikov hydrophosphination of 1,1‐diphenylethylene and 1‐phenyl‐1‐trimethylsiloxyethylene. It is also a key intermediate in phosphine dehydrocoupling via a non‐traditional, electrophilic mechanism, catalyzed by 1 . An analogous, more electron‐rich phosphinophosphonium complex, [[Mo(CO) 3 (PPh 2 H) 2 (PPh 2 –PPh 2 H)] + (4 a ), also participates in these reactions that rely on the P–H acidity of the phosphinophosphonium ligand. It gives comparable activity for the Markovnikov hydrophosphination and higher activity for phosphine dehydrocoupling, consistent with proposed mechanisms for both processes.