Oxametallacycle formation via ring-opening of 1-epoxy-3-butene on Ag(110):a combined experimental/theoretical approach

Temperature-programmed desorption (TPD), high resolution electron energy lo ss spectroscopy (HREELS), and density functional theory (DFT) calculations were used to study the interaction of 1-epoxy-3-butene (EpB) with the Ag(11 0) surface. The goal of this study was to better understand the nature of t he strongly adsorbed EpB species reported for the silver-catalyzed butadien e epoxidation process. EpB dosed on the Ag(110) surface at 300 K ring-opens to form a stable surface oxametallacycle intermediate. DFT geometry optimi zations were utilized to calculate the structure of the oxametallacycle; su bsequent frequency calculations were used to predict the vibrational spectr um of this intermediate. Comparison of the predicted vibrational spectrum w ith the experimental HREEL spectrum of this intermediate conclusively ident ifies 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), r espectively. This reaction represents the first demonstration of a surface oxametallacycle ring-closure to form an epoxide, and thus we suggest that t he strongly adsorbed surface intermediate in the butadiene epoxidation proc ess is a surface oxametallacycle. Furthermore, in light of the similarity b etween butadiene epoxidation and other epoxidation processes, surface oxame tallacycles may be of general importance in silver-catalyzed olefin epoxida tion. (C) 2000 Elsevier Science B.V. All rights reserved.