Direct Determination of the Reaction Volume of an Organometallic Reaction at Very High Dilution

In n-hexane, under Ar (1atm) and 298.15 K, Rh4(CO)12 and PPh3 react to form a monosubstituted Rh4(CO)11PPh3 and gaseous CO. This stoichiometric reaction was followed using in-situ FTIR. The concentrations of the organometallics were in the 10s of ppm range. Simultaneously, very precise online density measurements were made. Based on mass injection, partial molar volumes of reactive species were directly determined from multi-component reaction mixtures. These partial molar volumes were in good agreement compared to those of separate binary system study. Subsequently, the reaction volume was determined. From spectral analysis, not only the progress of reaction can be monitored, but also the pure component spectra of each existing species in the reaction can be recovered. The results from spectral analysis were used to confirm the reaction. 1.0 Introduction It has been shown for some inorganic, organometallic, and organic reactions, application of pressure enable the control not only of the yield of the reaction but also selectivity for the competitive and consecutive reaction(s) which can lead to an improvement of chemo-, regio-, and stereoselectivity. While the sensitivity of the reaction rate dependency on pressure is characterized by activation volume, the volume of reaction is significant for the reaction equilibrium. Reactions accompanied by a decrease in volume are accelerated by pressure and the equilibria are shifted toward the side of products, while those accompanied by an increase of volume are retarded and the equilibria are shifted toward the side of reactants. Typically, determinations of both parameters, namely activation volume and volumes of reactions are measured by kinetic studies at several pressures. Asano 2 and van Eldik 1 have reported the values of activation and reaction volumes of many reactions. In general, this method would require a wide pressure range (ca. 1~1000 bar). Alternatively, volume of reaction can also be determined by a relatively simple method by calculating the differences of partial molar volume at infinite dilution of products and reactants that are measured at atmospheric pressure. It had already been shown that both approaches are in agreement 3 . In this study, a well-known ligand substituted reaction in organometallics was selected and shown in Equation 1. Rh4(CO)12 + PPh3 → Rh4PPh3(CO)11 + CO (T= 298.15 K, 1 atm) (1) In n-hexane, under Ar (1 bar) and 298.15 K, Rh4(CO)12 and triphenylphosphine, PPh3 react to form a mono-substituted Rh4(CO)11PPh3 and released gaseous CO . This reaction was followed using in-situ FTIR. The concentrations of the organometallics were in the 10s of ppm range. Simultaneously, very precise online density measurements were made. In this work, the determination of partial molar volume at infinite dilution i V ∞ and hence reaction volume rV ∞ ∆ were achieved using density measurements at very high dilution (xi ≈ 0.00001-0.0001) of the multi-component reaction system. 2.0 Results and Discussion The ligand substitution reaction of Rh4(CO)12 and PPh3 was done by adding several titration steps of PPh3 into the reactor containing the Rh4(CO)12 with n-hexane as a solvent medium. All concentrations were in 10s ppm level (mole fraction). While doing the reaction, both density and IR spectra of every step of injection were taken. The IR reaction spectra taken in the wavenumber range1800-2200 cm are shown in Figure 1. 180