EXCHANGE-REACTION OF BRIDGING ACETATE LIGANDS AND REVERSIBLE ELECTROPHILIC RHODIUM-CARBON ACTIVATION IN ORTHO-METALATED RHODIUM(II) COMPOUNDS WITH ACETIC-ACID - KINETIC-STUDY OF THE EXCHANGE-REACTION OF ACETATES WITH CD3CO2D IN THE COMPOUND RH2(O2CCH3)3[(C6H4)P(C6H5)2].2HO2CCH3

Stepwise exchange reactions of CH3CO2- groups by CD3CO2- groups are observed for Rh2(O2CCH3)3-[(C6H4)P(C6H5)2].2HO2CCH3 in CHCl3/CD3CO2D mixtures. The first reaction step involves a fast exchange of the acetate group trans to the metalated phosphine as well as exchange of the two axial molecules of acetic acid. In a second and slower step the exchange of the acetate groups cis to the metalated phosphine occurs. The kinetics of the second process has been studied by H-1 NMR spectroscopy using different CHCl3/CD3CO2D mixtures as reaction solvents. The kinetic data follow a two-term rate law, upsilon = [k1 + k2[CD3CO2D]1/2][Rh2]. The first-order constant k1 at 298 K is (2.08 +/- 0.02) X 10(-6) s-1 (DELTAH(double dagger) = 98 +/- 5 kJ mol-1; DELTAS(double dagger) = 45 +/- 20 J K-1 mol-1), and the second-order rate constant at the same temperature is k2 = (3.83 +/- 0.01) X 10(-6) s-1 M-1/2 (DELTAH(double dagger) = 103 +/- 5 kJ mol-1; DELTAS(double dagger) = -15 +/- 15 J K-1 mol-1). Electrophilic attack at one oxygen atom of the bridging acetate group by a proton of acetic acid is concluded to be the first and rate-determining step of the exchange process. The axially coordinated acetic acid is likely responsible for the intramolecular attack. Very fast exchange of all the CH3CO2- groups by CD3CO2- is observed for the compound Rh2(O2CCH3)2[(C6H4)P(C6H5)2]2.2HO2CCH3 in the presence of CD3CO2D. The partially deuterated compound Rh2(O2CCH3)3[(C6D4)P(C6D5)2].2HO2CCH3 undergoes 90% D-H exchange at the ortho position of the phenyl rings of the phosphine after 3 h of reflux in acetic acid. Much slower exchange is observed for Rh2(O2CCH3)2[(C6D4)P(C6D5)2]2.2HO2CCH3 in the same conditions. The observed D-H exchange is explained by an electrophilic attack at the rhodium-carbon bond by acetic acid, which produces the protonation of the ortho aromatic carbon atom, followed by a cyclometalation at one of the ortho C-H bonds.