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(BO2C2H2) (M = Fe, Ru, W)

Reaction mechanisms of the methane and benzene functionalizations (borylation) by CpFe(CO)(BO 2 C 2 H 2 ) and CpW(CO) 2 (BO 2 C 2 H 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) 3 Bcat‘ (Bcat‘ = B-1,2-O 2 C 6 H 2 -3,5-Me 2 ) has comparable reactivity toward both alkanes and arenes, while the iron boryl complexes Cp‘Fe(CO) 2 Bcat (Cp‘ = Cp, Cp*; Bcat = B-1,2-O 2 C 6 H 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 2 C 2 H 2 ) has also been done.