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

The unsaturated PCP-type complexes Rh(L){2,6-((CH2PBu2)-Bu-t)(2)C6H3}X (L = Et, Pr-n; X = Cl, I) complexes convert upon heating to the corresponding R h(III)-hydride complexes Rh(H)(2,6-((CH2PBu2)-Bu-t)(2)C6H3)X (X = Cl, I) an d ethylene or propylene, products indicative of a beta-H elimination proces s. The Pr-i analogue is observed upon reaction of Rh(eta(1)-N-2){2,6-((CH2P Bu2)-Bu-t)(2)C6H3} With (PrI)-Pr-i at -10 degrees C and decomposes readily at room temperature to give Rh(H){2,6-((CH2PBu2)-Bu-t)(2)C6H3}I and propyle ne. Analogous alkyl complexes - lacking beta-hydrogens - are stable under t he applied reaction conditions. The mechanism of this process has been stud ied by NMR, using C-13 and deuterium labeling of the alkyl ligand (L = Et-d (5), (CH2)-C-13- CH3). 13C labeling shows that the beta-H elimination is ir reversible. A deuterium isotope effect of k(Et)/k(Et-d5) = 1.4 and a rate o rder of Et < Pr-n much less than Pr-i were observed. The overall process fo llows first-order kinetics in the Rh(III)-alkyl complexes. The activation p arameters for the thermolysis of Rh(Et)(2,6-((CH2PBu2)-Bu-t)(2)C6H3)I in to luene were determined: Delta H double dagger = 21.2 kcal/mol, Delta S-doubl e dagger = -21.1 eu, and Delta G(298K)(double dagger) = 27.5 kcal/mol.