Directly Observed β-H Elimination of Unsaturated PCP-Based Rhodium(III)−Alkyl Complexes

The unsaturated PCP-type complexes Rh(L){2,6-(CH 2 P t Bu 2 ) 2 C 6 H 3 }X (L = Et, n Pr; X = Cl, I) complexes convert upon heating to the corresponding Rh(III)−hydride complexes Rh(H){2,6-(CH 2 P t Bu 2 ) 2 C 6 H 3 }X (X = Cl, I) and ethylene or propylene, products indicative of a β -H elimination process. The i Pr analogue is observed upon reaction of Rh( η 1 -N 2 ){2,6-(CH 2 P t Bu 2 ) 2 C 6 H 3 } with i PrI at −10 °C and decomposes readily at room temperature to give Rh(H){2,6-(CH 2 P t Bu 2 ) 2 C 6 H 3 }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 13 C and deuterium labeling of the alkyl ligand (L = Et- d 5, 13 CH 2 CH 3 ). 13 C labeling shows that the β -H elimination is irreversible. A deuterium isotope effect of k Et / k Et - d 5 = 1.4 and a rate order of Et < n Pr ≪ i Pr 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-(CH 2 P t Bu 2 ) 2 C 6 H 3 )I in toluene were determined: Δ H ⧧ = 21.2 kcal/mol, Δ S ⧧ = −21.1 eu, and Δ G ⧧ 298 K = 27.5 kcal/mol.