Density Functional Theoretical Study on the Oxidative Additions of Silyl Halides to the Rh Pincer Complex

Abstract

The oxidative addition reactions of C–X bonds in alkyl halides or aryl halides to transition-metal complexes are one of the fundamental reactions in organometallic chemistry. However, the reaction involving silicon receives less attention. Recently oxidative addition reactions involving silicon-halide bonds to a rhodium complex were reported. (PNP)Rh fragment 1 (where PNP is the bis(o-diisopropylphosphinophenyl)amide “pincer” ligand), generated from (PNP)Rh(SPr2), reacts with silyl halides compounds to undergo a silicon-halogen oxidative addition reaction. The fragment 1 has been considered to be convenient for the comparative studies because it is compatible with many reactions which provide several kinetic and thermodynamic parameters of the reactions. In this study the density functional theoretical (DFT) studies were conducted in order to determine the Gibbs free energy of reaction for the Si–X oxidative addition to 1 and hence to compare the relative stability between the initial adduct and the oxidative addition product. These kinetic and thermodynamic quantities are the key parameters for elucidating the reactivity and the preference toward the chemical species. B3LYP/LACVP** level calculations were chosen because the level of theory was successfully applied to the oxidative addition/reductive elimination reactions involving organometallic pincer complexes. First we examined the dependence of the reactivity and the thermodynamics of the reaction on the halide species. Figure 1 shows the Gibbs free energy diagrams for the reaction between 1 and SiMe3X, where X = Cl, Br, or I. The Gibbs activation energy, ΔG, were 10.8 kcal/mol for Cl, 8.7 kcal/ mol for Br, and 7.8 kcal/mol for I in the gas phase at 298 K. In addition the Gibbs free energy of reaction, ΔGreaction, were –9.1 kcal/mol for Cl, –17.6 kcal/mol for Br, and –20.9 kcal/ mol for I, indicating higher stability of the oxidative addition reaction products over the reactants. The reactivities of silyl halides toward (PNP)Rh fragments are potentially related to the bond dissociation enthalpies of the Si–X bond in the SiMe3X molecule. For instance weaker bonds in heavier silyl halides are less inert to the Rh fragment. Furthermore the thermodynamics of the reaction is