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)(2)C6H3)(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 degrees 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)(2)H][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 alpha or beta 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 a position versus the beta position is similar to 10, or similar to 30 per H beta. The slow step is protonation of Mo(NAr)(styrene)(OSiPh3)(2) (k(1) = 0.158(4) L/(mol.min)). Proton sources such as (CF3)(3)COH 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 (similar to 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 beta proton is activated through a CH beta 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 degrees 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.

Automated summary

In a series of reactions involving transition metal complexes, the addition of PhNMe2H+X- can facilitate the interconversion between Mo(NAr)(styrene)(OSiPh3)(2) and Mo(NAr)(CMePh)(OSiPh3)(2), leading to the formation of the intermediate 1-phenethyl cation, and this conversion process may involve multiple intermediate steps of protonation and deprotonation.