Abstract
Bridging groups on the rhodium trimer, [Rh3(μ3–O)(μ–OOCCH3)6(H2O)3]+, Rh3+, have been exchanged with other bridging carboxylates and the rates of substitution quantified. Exchange of bridging acetate groups has been performed by acetic acid–d4, 3–bromopropionic acid, iodoacetic acid and acrylic acid. Kinetic studies by VT–1H–NMR determined rates spanning one order of magnitude for the exchange of bridging acetates for acetic acid–d4, 1.97±0.05×10^(-5) s–1 to 3.48±0.31×10^(-4) s–1. Additionally, the activation parameters were determined by use of the Eyring equation. An activation enthalpy (ΔH≠) was calculated to be 104 ± 15 kJ∙mol–1, and the activation entropy (ΔS≠) was determined to be 14 ± 49 J∙mol–1∙K–1. These parameters imply an interchange dissociative (ID) mechanism for the exchange of bridging acetates by acetic acid–d4. Although exchange of bridging acetates for iodoacetic acid and acrylic acid were unsuccessful, kinetic data for the exchange of acetates for 3–bromopropionic acid was collected. Kinetic studies were performed by VT–UV–Vis by sampling the reaction ex–situ for the exchange of bridging acetates for 3–bromopropionic acid. Rates were determined to also span one order of magnitude, 3.02±0.29×10^(-6) s–1 to 2.24±0.15×10^(-5) s–1. There is a characteristic lag present in the kinetic plots, which we believe is due to the breakdown of 3–bromopropionic acid into 3–hydroxypropionic acid and acrylic acid. An activation enthalpy (ΔH≠) for the exchange of bridging acetates by 3–bromopropionic acid was determined to be 112 ± 7 kJ∙mol–1, and activation entropy (ΔS≠) was calculated to be –24 ± 19 J∙mol–1∙K–1. The activation parameters suggest an interchange (I) mechanism, although a distinction between interchange dissociative (ID) and interchange associative (IA) is unclear with the given parameters. A synthetic method for the synthesis of Rh3+ and 3–bromopropionic acid has been established and the products have been characterized by 1H–NMR, 13C–NMR, and UV–Vis. Based on the data, it is clear that substituted species with mixed bridging groups are present. Data show the presence of both 3–bromopropionic acid and 3–hydroxypropionic acid ligated to the complex. While some of the data is not completely apparent, it is evident that Rh3+ has reacted to exchange the bridging acetate groups for a mixture of bridging ligands, largely due to the breakdown of 3–bromopropionic acid in solution.