Do B3LYP and CCSD(T) Predict Different Hydrosilylation Mechanisms? Influences of Theoretical Methods and Basis Sets on Relative Energies in Ruthenium−Silylene-Catalyzed Ethylene Hydrosilylation

Abstract

A series of density functional theory (DFT) and wave function theory (WFT) methods were used in conjunction with a series of basis sets to investigate the influence of the computational methodology on the relative energies of key intermediates and transition states in potential reaction pathways in ruthenium-silylene-catalyzed hydrosilylation reactions. A variety of DFT methods in a modest basis set and B3LYP calculations in a variety of basis sets calculated the key transition in the Glaser-Tilley (GT) pathway to be energetically favored. In contrast, with the smaller basis sets, the CCSD(T) method calculated the Chalk-Harrod (CH) pathway to be favored; however, CCSD(T) results extrapolated to larger basis sets favored the GT pathway.

Efficient and selective alkene hydrosilation promoted by weak, double Si–H activation at an iron center

“…1) indicates that the transition state for Si-H addition to the olen (TS) is at a higher energy than that for the Ru system (DG ‡ ¼ 19.1 kcal mol À1 for Fe vs. 11.7 kcal mol À1 for Ru; previous computations indicated a 8-15 kcal mol À1 barrier for Ru). 28,29 Importantly, this barrier height is comparable to that of silane exchange (vide infra), and a consequence is that the ratedetermining step for catalysis is substrate dependent. Thus, in at least some cases the Si-H addition is slow enough (e.g., due to steric factors) to render this step rate-determining.…”

“…This selectivity toward primary silanes is reminiscent of the cationic Ru silylene hydrosilation catalyst, but other selectivities (vide infra) suggest that there are fundamental differences in the mechanistic pathways. 24,26,28,29 For this reaction the iron catalyst is more active, as the ruthenium catalyst required a higher loading (1 mol%) and elevated temperature (80 C) to achieve a comparable conversion. 26 This activity is in the range of the most active iron catalysts for the hydrosilation of olens by primary silanes (Table S1 †).…”

Does the Preferred Mechanism of a Catalytic Transformation Depend on the Density Functional? Ethylene Hydrosilylation by a Metal Complex as a Case Study

“…13 They examined two key reaction steps (including four intermediates and two transition states, TS), comparing a series of DFT and "ab initio" methods, in conjunction with basis sets of various degrees of flexibility along the pathways of the GT and CH mechanisms. 13 They based their discussion on single-point calculations of structures optimized at the B3LYP level in a preceding mechanistic study. 14 With the "ab initio" methods MP2 and MP4SDQ they determined the CH mechanism as preferred.…”

“…Their CCSD and CCSD(T) calculations predicted a change in the preferred mechanism from CH to GT when the basis sets were increased. 13 The current work, addressing the catalytic system based on the Rh(I)−bis-NHC complex, is based on our previous mechanistic study 11 of olefin hydrosilylation where two characteristics of this system were revealed that render the present case more complicated than the case just described. 13 In our case, first, the pathways of the four mechanisms are interrelated at several junctures.…”

“…13 The current work, addressing the catalytic system based on the Rh(I)−bis-NHC complex, is based on our previous mechanistic study 11 of olefin hydrosilylation where two characteristics of this system were revealed that render the present case more complicated than the case just described. 13 In our case, first, the pathways of the four mechanisms are interrelated at several junctures. Second, the potential energy profiles of the SBM and CH mechanisms are overall flat ( Figure S2 of the Supporting Information).…”