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]·2CF3CO2H

Abstract

Stepwise exchange reactions of acetate groups by trifluoroacetate groups have been observed for [Rh₂(O₂CMe)₃{(C₆H₄)PPh₂}]·2MeCO₂H 1, in CDCl₃–CF₃CO₂H at room temperature. The first reaction involves a fast exchange 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 exchange of the second cis acetate group yielding [Rh₂(O₂CCF₃)₃{(C₆H₄)PPh₂}]·2CF₃CO₂H 3, has been studied by ¹H NMR spectroscopy. This exchange follows a rate equation which is dependent on the concentration of trifluoroacetic acid used, k_obs=(k₁+k₂[CF₃CO₂H]^½[Rh₂]. The partially 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 has 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, and subsequent demetallation followed by a reverse metallation reaction. An X-ray crystal structure determination of compound 3 has been carried out: space group P2₁/n(monoclinic), a= 12.369(2), b= 21.711 (6), c= 13.367(2), β= 90.11 (2)°, Z= 4 and R= 0.044. It contains three trifluoroacetate groups bridging a Rh₂⁴⁺ unit which has a Rh–Rh bond distance of 2.438(1)Å; the fourth bridging ligand is a triphenylphosphine metallated at one of the ortho positions. Two molecules of trifluoroacetic acid occupy the axial positions.