EXCHANGE-REACTIONS OF ACETATE LIGANDS AND ELECTROPHILIC RHODIUM CARBON BOND ACTIVATION IN ORTHOMETALLATED RHODIUM(II) COMPOUNDS WITH TRIFLUOROACETIC-ACID - CRYSTAL-STRUCTURE OF [RH2(O2CCF3)3((C6H4)PPH2)]CENTER-DOT-2CF3CO2H

Stepwise exchange reactions of acetate groups by trifluoroacetate grou ps have been observed for [Rh2(O2CMe)3{(C6H4)PPh2}].2MeCO2H 1, in CDCl 3-CF3CO2H at room temperature. The first reaction involves a fast exch ange of the two axial molecules of acetic acid as well as the acetate group trans to the metallated phosphine. In a second step the exchange of one acetate group cis to the metallated phosphine occurs. The exch ange of the second cis acetate group yielding [Rh2(O2CCF3)3{(C6H4)PPh2 }].2CF3CO2H 3, has been studied by H-1 N MR spectroscopy. This exchang e follows a rate equation which is dependent on the concentration of t rifluoroacetic acid used, k(obs) = (k1 + k2[CF3CO2H]1/2[Rh2]. The part ially deuteriated compound [Rh2(O2CCF3)3{(C6D4)P(C6D5)2}].2CF3CO2H 3D in the presence of trifluoroacetic acid undergoes D-H exchange at the ortho positions of the phenyl rings of the phosphine. This exchange ha s been studied at 318 K. The observed D-H exchange is explained by an electrophilic attack at the rhodium-carbon bond by the trifluoroacetic acid, which produces protonation of the ortho aromatic carbon atom, a nd subsequent demetallation followed by a reverse metallation reaction . An X-ray crystal structure determination of compound 3 has been carr ied out: space group P2(1)/n (monoclinic), a = 12.369(2), b = 21.711(6 ), c = 13.367(2), beta = 90.11(2)degrees, Z = 4 and R = 0.044. It cont ains three trifluoroacetate groups bridging a Rh2(4+) unit which has a Rh-Rh bond distance of 2.438(1) angstrom; the fourth bridging ligand is a triphenylphosphine metallated at one of the ortho positions. Two molecules of trifluoroacetic acid occupy the axial positions.