MOLECULAR-STRUCTURE OF THE COMPOUND H2(O2CCH3)3((C6H4)P(BRC6H4-1,2)(C6H5)).(HO2CCH3)2] - KINETIC-STUDY OF THE EXCHANGE-REACTION OF ACETATE GROUPS WITH CD3CO2D

The compound [Rh2(O2CCH3)3((C6H4)P(BrC6H4-1,2)(C6H5)) . (HO2CCH3)2] ha s been isolated in high yield from the thermal reaction of dirhodium t etraacetate and the phosphine P(BrC6H4-1,2) (C6H5)2 in acetic acid. Th e structure of this compound has been determined by X-ray diffraction; it crystallizes in the P1BAR (triclinic) space group and contains thr ee acetate groups bridging a Rh2(4+) unit the Rh-Rh distance being 2.4 32(1)angstrom; the fourth bridging ligand is an ortho-bromophenyldiphe nylphosphine metallated at one of the ortho positions of the unsubstit uted phenyl rings. Two molecules of acetic acid occupy the axial coord ination positions. Stepwise exchange of CH3CO2- by CD3CO2- is observed in CDCl3/CD3CO2D mixtures. The first step involves a fast exchange of the acetate group trans to the metallated phosphine and exchange of t he two axial molecules of acetic acid. In a second step, the two aceta te groups cis to the metallated phosphine slowly exchange with CD3CO2D at two different rates. The kinetics of the two slow processes have b een studied by H-1 NMR spectroscopy using different CDCl3/CD3CO2D mixt ures as solvent. The kinetic data follow rate laws of the type, v = k[ CD3CO2D]1/2[Rh2]. At 298 K, the rate constants are k2a (1.86 +/- 0.02) X 10(-6) S-1 M-1/2 (DELTAH(double dagger) = 103 +/- 3 kJ mol-1; DELTA S(double dagger) = -7 +/- 9 J K-1 mol-1) and k2b = (0.77 +/- 0.01) x 1 0(-6) S-1 M-1/2 (DELTAH(double dagger) = 100 +/- 6 kJ mol-1; DELTAS(do uble dagger) = -20 +/- 20 J K-1 mol-1). Electrophilic attack at one ox ygen 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 triphenylphosp hine is negligible in this case, probably due to the low lability of t he single axial acetic acid.