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

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