Cleavage of a P=P Double Bond Mediated by N‐Heterocyclic Carbenes

The reaction of the bulky diphosphenes (Rind)P=P(Rind) (1; Rind=1,1,3,3,5,5,7,7‐octa‐R‐substituted s‐hydrindacen‐4‐yl) with two molecules of N‐heterocyclic carbene (NHC; 1,3,4,5‐tetramethylimidazol‐2‐ylidene) resulted in the quantitative formation of the NHC‐bound phosphinidenes NHC→P(Rind) (2), along with the cleavage of the P=P double bond. The reaction times are dependent on the steric size of the Rind groups (11 days for 2 a (R=Et) and 2 h for 2 b (R=Et, Me) at room temperature). The mechanism for the double bond‐breaking is proposed to proceed via the formation of the NHC‐coordinated, highly polarized diphospehenes 3 as an intermediate. Approach of a second NHC to 3 induces P−P bond cleavage and P−C bond formation, which proceeds through a transition state with a large negative Gibbs energy change to afford the two molecules of 2, thus being the rate‐determining step of the overall reaction with the activation barriers of 80.4 for 2 a and 29.1 kJ mol−1 for 2 b. P=P bond‐breaking: Bulky R8‐s‐hydrindacen‐4‐yl (Rind)‐substituted diphosphenes (R=Me, Et) react with two equivalents of N‐heterocyclic carbene (NHC) to produce the NHC‐coordinated phosphinidene adducts. The mechanism for P=P cleavage through formation and breaking of NHC‐coordinated, highly polarized diphosphene intermediates is proposed.