Directly observed β-H elimination of unsaturated PCP-based rhodium(III)-alkyl complexes

The unsaturated PCP-type complexes Rh(L){2,6-(CH₂P^tBu₂)₂C ₆H₃}X (L = Et, ⁿPr; X = Cl, I) complexes convert upon heating to the corresponding Rh(III)-hydride complexes Rh(H)-{2,6-(CH₂P^tBu₂)₂C ₆H₃}X (X = Cl, I) and ethylene or propylene, products indicative of a β-H elimination process. The ⁱPr analogue is observed upon reaction of Rh(η¹-N₂){2,6-(CH₂P^t-Bu ₂)₂C₆H₃} with ⁱPrI at -10°C and decomposes readily at room temperature to give Rh-(H){2,6-(CH₂P^tBu₂)₂C ₆H₃}I and propylene. Analogous alkyl complexes - lacking β-hydrogens - are stable under the applied reaction conditions. The mechanism of this process has been studied by NMR, using ¹³C and deuterium labeling of the alkyl ligand (L = Et-d₅, ¹³CH₂-CH₃). ¹³C labeling shows that the β-H elimination is irreversible. A deuterium isotope effect of k_Et/k_Et-d5 = 1.4 and a rate order of Et <ⁿPr ≪ ⁱPr were observed. The overall process follows first-order kinetics in the Rh(III)-alkyl complexes. The activation parameters for the thermolysis of Rh(Et)(2,6-(CH₂P^tBu₂)₂C ₆H₃)I in toluene were determined: ΔH‡ = 21.2 kcal/mol, ΔS‡ = -21.1 eu, and ΔG‡_298K = 27.5 kcal/mol.