Estimation of Solvent Effects for the Complexing Reaction of Propylene and Nickel Dithiolene

抄録

The formation of olefin complexes is of potential importance in the separation of olefins. The solvents affect the activation energies, and hence the rates and equilibrium constants of the complexing reactions, which performance should be well estimated for the purpose of industrial practice. The solvent effects on the complexing reaction of propylene and nickel dithiolene Ni(S₂C₂H₂)₂ + C₂H₄=CH₂ -> Ni(S₂C₂H₂)₂.C₂H₄=CH₂ are studied in this work, using density functional theory with B3LYP and an Onsager model. Complete optimizations of all the stagnation points are performed in benzene, toluene, tetrahydrofuran, dichloromethane, 1,2-dichloroethane, acetone, ethanol, methanol, 1,2,3-propanetriol, dimethylsulfoxide and water, respectively. The reaction of complexing nickel dithiolene with propylene is a two-step process: the first step coordinates the propylene to S atoms in dithiolene, forming a trans-structural intermediate. The second step then yields the cis-structural product. The activation energy of the first step is higher than that of the second, indicating that the first step is the rate-determining step. The solvents make slight changes in the geometries of the reactants, transition states, intermediates and products. However, the corresponding molecular dipole moment becomes large with increase of the solvent polarity, which is beneficial to accelerate the reaction. Furthermore, the activation energies of the first (or second) step will exponentially decrease from 125.0 to 113.0 kJ mol^-1 (or from 101.8 to 83.43 kJ mol^-1) when the dielectric constants of solvents increase from 1.00 to 78.39, while the reaction rates of the first (or second) step exponentially increase from 0.7673x10^-9 to 96.20x10^-9 s^-1 (or from 0.5503 to 1.038 s^-1), and the equilibrium constants rapidly increase from 0.5066 to 343.4 lmol^-1. The sharp variations of activation energies, rate constants, and equilibrium constants appear when the value of the dielectric constant of solvents lies between 1 and approximately 20, while these variations become mild when the dielectric constant of solvents is larger than 20. All of these results demonstrate that the complexing reaction of propylene and nickel dithiolene become much easier and faster to occur in polar solvents. The relationship between the equilibrium constants of the complexing reaction K_eq and the dielectric constants of solvents ε can be presented mathematically as K_eq=A exp(-ε/t) with correlation parameters A = 378.4 lmol^-1, B = 350.7 lmol^-1 and t = 21.17. This relationship may be seen as a reference for solvent selection in olefin separation practice.