Interconversion of Molybdenum or Tungsten d2 Styrene Complexes with d0 1‑Phenethylidene Analogues

Upon addition of 5–15% PhNMe2H+X– (X = B­(3,5-(CF3)2C6H3)4 or B­(C6F5)4) to Mo­(NAr)­(styrene)­(OSiPh3)2 (Ar = N-2,6-i-Pr2C6H3) in C6D6 an equilibrium mixture of Mo­(NAr)­(styrene)­(OSiPh3)2 and Mo­(NAr)­(CMePh)­(OSiPh3)2 is formed over 36 h at 45 °C (K eq = 0.36). A plausible intermediate in the interconversion of the styrene and 1-phenethylidene complexes is the 1-phenethyl cation, [Mo­(NAr)­(CHMePh)­(OSiPh3)2]+, which can be generated using [(Et2O)2H]­[B­(C6F5)4] as the acid. The interconversion can be modeled as two equilibria involving protonation of Mo­(NAr)­(styrene)­(OSiPh3)2 or Mo­(NAr)­(CMePh)­(OSiPh3)2 and deprotonation of the α or β phenethyl carbon atom in [Mo­(NAr)­(CHMePh)­(OSiPh3)2]+. The ratio of the rate of deprotonation of [Mo­(NAr)­(CHMePh)­(OSiPh3)2]+ by PhNMe2 in the α position versus the β position is ∼10, or ∼30 per Hβ. The slow step is protonation of Mo­(NAr)­(styrene)­(OSiPh3)2 (k 1 = 0.158(4) L/(mol·min)). Proton sources such as (CF3)3COH or Ph3SiOH do not catalyze the interconversion of Mo­(NAr)­(styrene)­(OSiPh3)2 and Mo­(NAr)­(CMePh)­(OSiPh3)2, while the reaction of Mo­(NAr)­(styrene)­(OSiPh3)2 with pyridinium salts generates only a trace (∼2%) of Mo­(NAr)­(CMePh)­(OSiPh3)2 and forms a monopyridine adduct, [Mo­(NAr)­(CHMePh)­(OSiPh3)2(py)]+ (two diastereomers). The structure of [Mo­(NAr)­(CHMePh)­(OSiPh3)2]+ has been confirmed in an X-ray study; there is no structural indication that a β proton is activated through a CHβ interaction with the metal. W­(NAr)­(CMePh)­(OSiPh3)2 is also converted into a mixture of W­(NAr)­(CMePh)­(OSiPh3)2 and W­(NAr)­(styrene)­(OSiPh3)2 (K eq = 0.47 at 45 °C in favor of the styrene complex) with 10% [PhNMe2H]­[B­(C6F5)4] as the catalyst; the time required to reach equilibrium is approximately the same as in the Mo system.