Activation of C()-X and C()-X σ-Bonds (X = F, Cl, Br, and OMe) by Rhodium Complex with Pincer-Type Aluminyl Ligands: Theoretical Insight into High Reactivity and Flexible Reaction Mechanism.

C()-X (X = F, Cl, Br, and OMe) σ-bond activation of halo- and methoxybenzenes and C()-X (X = F and Cl) σ-bond activation of fluoro- and chloromethanes by a rhodium complex with pincer-type aluminyl ligand (named Rh(PAlP)) are theoretically studied. Rh(PAlP) exhibits high reactivity in difficult σ-bond activations and flexible dependence of reaction mechanism on substrate and solvent. In the gas phase and cyclohexane solvent, all the C()-X σ-bond activations occur via cooperative activation by the Rh-Al moiety (Pathway 1) with moderate Gibbs activation energy (Δ°). In polar THF solvent, C()-Cl and C()-Br σ-bond activations occur via nucleophilic attack of Rh to the phenyl group (Pathway 3) with a moderate Δ° value, whereas even in THF solvent, C()-F and C()-OMe σ-bond activations occur via Pathway 1. Concerted oxidative addition to the Rh atom (Pathway 2) is not preferred in these substrates. C()-F σ-bond activation of fluoromethane occurs via Pathway 1 in the THF solvent, while C()-Cl σ-bond activation occurs via Pathway 2. The transition state of Pathway 3 could not be optimized in the halomethane case. The reactivity of Rh(PAlP) and flexible dependence of the reaction mechanism on the substrate and solvent result from the presence of a Rh-Al direct bond, Al···X attractive interaction, and the strongly donating nature of the aluminyl ligand.