Reactivity of the Latent 12-Electron Fragment [Rh(PiBu₃)₂]⁺ with Aryl Bromides: ArylBr and Phosphine Ligand CH Activation

Abstract

Oxidative addition of aryl bromides to 12-electron [Rh(PiBu₃)₂][BAr^F₄] (Ar^F=3,5-(CF₃)₂C₆H₃) forms a variety of products. With p-tolyl bromides, Rh^III dimeric complexes result [Rh(PiBu₃)₂(o/p-MeC₆H₄)(μ-Br)]₂[BAr^F₄]₂. Similarly, reaction with p-ClC₆H₄Br gives [Rh(PiBu₃)₂(p-ClC₆H₄)(μ-Br)]₂[BAr^F₄]₂. In contrast, the use of o-BrC₆H₄Me leads to a product in which toluene has been eliminated and an isobutyl phosphine has undergone CH activation: [Rh{PiBu₂(CH₂CHCH₃CH₂)}(PiBu₃)(μ-Br)]₂[BAr^F₄]₂. Trapping experiments with ortho-bromo anisole or ortho-bromo thioanisole indicate that a possible intermediate for this process is a low-coordinate Rh^III complex that then undergoes CH activation. The anisole and thioanisole complexes have been isolated and their structures show OMe or SMe interactions with the metal centre alongside supporting agostic interactions, [Rh(PiBu₃)₂(C₆H₄OMe)Br][BAr^F₄] (the solid-state structure of the 5-methyl substituted analogue is reported) and [Rh(PiBu₃)₂(C₆H₄SMe)Br][BAr^F₄]. The anisole-derived complex proceeds to give [Rh{PiBu₂(CH₂CHCH₃CH₂)}(PiBu₃)(μ-Br)]₂[BAr^F₄]₂, whereas the thioanisole complex is unreactive. The isolation of [Rh(PiBu₃)₂(C₆H₄OMe)Br][BAr^F₄] and its onward reactivity to give the products of CH activation and aryl elimination suggest that it is implicated on the pathway of a σ-bond metathesis reaction, a hypothesis strengthened by DFT calculations. Calculations also suggest that CH bond cleavage through phosphine-assisted deprotonation of a non-agostic bond is also competitive, although the subsequent protonation of the aryl ligand is too high in energy to account for product formation. CH activation through oxidative addition is also ruled out on the basis of these calculations. These new complexes have been characterised by solution NMR/ESIMS techniques and in the solid-state by X-ray crystallography.