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RESEARCH PRODUCT
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
Eduardo PerisJulio LatorreSantiago García-grandaPascual LahuertaMercedes Sanaúsubject
StereochemistryOrganic ChemistryBridging ligandProtonationBiochemistryMedicinal chemistryInorganic Chemistrychemistry.chemical_compoundAcetic acidReaction rate constantchemistryYield (chemistry)Materials ChemistryCarboxylatePhysical and Theoretical ChemistryTriphenylphosphinePhosphinedescription
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 RhRh 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.
year | journal | country | edition | language |
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1993-09-01 | Journal of Organometallic Chemistry |