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Molecular structure of the compound [Rh2(O2CCH3)3{(C6H4)P(BrC6H4-1,2)(C6H5)} · (HO2CCH3)2].Kinetic study of the exchange reaction of acetate groups with CD3CO2D

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Abstract The compound [Rh 2 (O 2 CCH 3 ) 3 {(C 6 H 4 )P(BrC 6 H 4 -1,2) (C 6 H 5 )} · (HO 2 CCH 3 ) 2 ] has been isolated in high yield from the thermal reaction of dirhodium tetraacetate and the phosphine P(BrC 6 H 4 -1,2) (C 6 H 5 ) 2 in acetic acid. The structure of this compound has been determined by X-ray diffraction; it crystallizes in the P 1 (triclinic) space group and contains three acetate groups bridging a Rh 2 4+ unit the RhRh distance being 2.432(1)A; the fourth bridging ligand is an ortho -bromophenyldiphenylphosphine metallated at one of the ortho positions of the unsubstituted phenyl rings. Two molecules of acetic acid occupy the axial coordination positions. Stepwise exchange of CH 3 CO 2 − by CD 3 CO 2 − is observed in CDCl 3 /CD 3 CO 2 D mixtures. The first step involves a fast exchange of the acetate group trans to the metallated phosphine and exchange of the two axial molecules of acetic acid. In a second step, the two acetate groups cis to the metallated phosphine slowly exchange with CD 3 CO 2 D at two different rates. The kinetics of the two slow processes have been studied by 1 H NMR spectroscopy using different CDCl 3 /CD 3 CO 2 D mixtures as solvent. The kinetic data follow rate laws of the type, v = k [CD 3 CO 2 D] 1 2 [Rh 2 ]. At 298 K, the rate constants are k 2a (1.86 ± 0.02) × 10 −6 s −1 M- 1 2 (Δ H ‡ = 103 ± 3 kJ mol −1 ; Δ S ‡ = −7 ± 9 J K −1 mol −1 ) and k 2b = (0.77 ± 0.01) × 10 −6 s −1 M- 1 2 (Δ H ‡ = 100 ± 6 kJ mol −1 ; Δ S ‡ = −20 ± 20 J K −1 mol −1 ). Electrophilic attack at one oxygen atom of the bridging acetate by a protonated acetic acid species is assumed to be the first and rate-determining step. The unimolecular rate constant observed for the analogous compound with triphenylphosphine is negligible in this case, probably due to the low lability of the single axial acetic acid.