Oxametallacycle formation via ring-opening of 1-epoxy-3-butene on Ag(1 1 0): a combined experimental/theoretical approach

Abstract

Temperature-programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT) calculations were used to study the interaction of 1-epoxy-3-butene (EpB) with the Ag(1 1 0) surface. The goal of this study was to better understand the nature of the strongly adsorbed EpB species reported for the silver-catalyzed butadiene epoxidation process. EpB dosed on the Ag(1 1 0) surface at 300 K ring-opens to form a stable surface oxametallacycle intermediate. DFT geometry optimizations were utilized to calculate the structure of the oxametallacycle; subsequent frequency calculations were used to predict the vibrational spectrum of this intermediate. Comparison of the predicted vibrational spectrum with the experimental HREEL spectrum of this intermediate conclusively identifies the oxametallacycle. This oxametallacycle undergoes 1,2- and 1,4-ring-closure reactions during TPD to form EpB and 2,5-dihydrofuran (2,5-DHF), respectively. This reaction represents the first demonstration of a surface oxametallacycle ring-closure to form an epoxide, and thus we suggest that the strongly adsorbed surface intermediate in the butadiene epoxidation process is a surface oxametallacycle. Furthermore, in light of the similarity between butadiene epoxidation and other epoxidation processes, surface oxametallacycles may be of general importance in silver-catalyzed olefin epoxidation.