PNNL

Abstract

The reaction enthalpy and rate of reduction of 1,1-diphenylethylene (DPE) by the Frustrated Lewis pair formed between tris-pentafluorophenylborane (BCF) and diethyl ether (Et2O) in dichloromethane have been determined by mixing calorimetry. At 50 °C and 13.6 atm hydrogen a 0.08 M solution of DPE is reduced to 1,1-diphenylethane in the presence of 1 equivalent BCF and 0.8 equivalents Et2O in 40 minutes. NMR spectroscopy showed >99% conversion to the reduced product. The rate of conversion of the olefin to the alkane as monitored by the time dependent heat flow showed a linear dependence on the free Et2O and BCF concentration. Integration of the heat flux provides a measure of the reaction enthalpy of ?H ca. -116 ± 4 kJ/mol for the reaction Ph2C=CH2 + H2 ? Ph2CHCH3. The equilibrium constant for dative adduct formation, Et2O + BCF ? Et2O—BCF, was determined as a function of temperature by 19F NMR spectroscopy and provided an experimental measure of the enthalpy, ?H = -54.6 ± 3.3 kJ/mol and entropy, ?S = -154 ± 13 J/mol/K, for dative bond formation in DCM. Extrapolation of the van’t Hoff plot to 50 ºC provides Keq that is used to estimate the concentration of free BCF and Et2O available to activate hydrogen. This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle.