EPR study of arene-solvated cobalt clusters as catalysts in the co-cyclization of α,ω-diynes and nitriles

Abstract

The thermal evolution of a Co-arene co-condensate, obtained by direct evaporation of both the metal and the organic substrate at −196°C, has been studied by EPR spectroscopy. On warming at −40°C, the solid matrix melts and the solution obtained has been shown to contain large and small metal clusters, a trimeric species being recognizable. At room temperature, a precipitate containing microcrystals and large clusters separates, while the presence of small and microclusters persists in solution. The reaction between 1,7-octadiyne and acetonitrile using the solution or the precipitate as precursor catalysts can provide completely different products, affording the annelated pyridine ( 1) or alkynyl pyridine derivatives ( 2). A mononuclear Co 0 species has been identified by EPR spectroscopy during the reaction affording 1, while the formation of 2, which occurs in the presence of microcrystals or large clusters, may be ascribed to a polynuclear catalyst. Transmission electron microscopy has been employed to evaluate the dimensions and distribution of the clusters.