Understanding the reactivity difference of metal boryl complexes toward alkanes and arenes: A density functional study on the functionalizations of methane and benzene by CpM(CO) n(BO 2C 2H 2) (M = Fe, Ru, W)

Abstract

Reaction mechanisms of the methane and benzene functionalizations (borylation) by CpFe(CO)(BO 2C 2H 2) and CpW(CO) 2(BO 2C 2H 2) have been investigated with the aid of B3LYP density functional theory calculations. The results show that the functionalizations by the Fe boryl complex favor a one-step mechanism with an oxidatively added transition state, while the functionalizations by the tungsten boryl complex only proceed by a two-step mechanism. The significant barrier difference between the functionalizations of methane and benzene by the Fe complex and the small barrier difference between the functionalizations by the W complex from our calculations are in good agreement with the experimental observation in a series of photochemical reactions of the transition-metal boryl complexes with alkanes and arenes. Cp*W(CO) 3Bcat′ (Bcat′ = B-1,2-O 2C 6H 2-3,5-Me 2) has comparable reactivity toward both alkanes and arenes, while the iron boryl complexes Cp′Fe(CO) 2Bcat (Cp′ = Cp, Cp*; Bcat = B-1,2-O 2C 6H 4) are very reactive toward the aromatic C-H bonds of arenes and are unreactive toward the alkane C-H bonds. The distinct barriers between the functionalizations of methane and benzene by the Fe complex can be explained by the significant stabilization interaction between the "empty" boron p orbital of the boryl group and the π orbitals of the benzene ring in the oxidatively added transition state for the iron - benzene system. The types of B⋯C interactions become unimportant in the functionalization of benzene by the tungsten boryl complex because the functionalization proceeds via a two-step mechanism. For the purpose of comparison, a mechanistic study on the functionalizations of methane and benzene by the model complex CpRu(CO)(BO 2C 2H 2) has also been done.