Abstracts

The development of green synthetic methods for organic compounds, especially with biological activity, is relevant to academia and industry. Mechanochemistry is considered by IUPAC as one of the Top 10 emerging technologies for green synthesis of organic and inorganic compounds and materials. Heterogeneous catalysis has also a strong appeal in the green synthesis of organic compounds. Thus, our research group has interest in combining mechanochemistry and catalysis with lanthanide-based compounds. Due to its peculiar and unique luminescent properties, Eu(III) was chosen to probe the structural reorganization of the Lewis-acid catalyst during organic reactions. In this context, cyanohydrins are important intermediates for the synthesis of beta-amino-alcohols, alpha-hydroxy-acids, and alpha-hydroxy-ketones, which present biological activities and are building blocks for drugs and agrochemicals. So, cyanosilylation reactions catalyzed by resistant Lewis-acid based on trivalent lanthanide ions were investigated. Eu(III)-based MOFs, namely frameworks of Eu(III)-fumarate and Eu(III)-mandelate, presented the best catalytic activities for cyanosilylation reactions involving ketones, which are much less explored than aldehydes due to their lower reactivity. The Eu(III)-mandelate catalyst showed better performance regarding recycling, because even after eight cycles, the catalyst kept its activity. The luminescent properties of the Eu(III)-mandelate catalyst were investigated before and after the eight recycling processes. Both excitation and emission spectra showed significant differences before and after the catalysis. After the Eu(III)-mandelate acting as catalyst, the excitation spectrum changed from a well-structured 4f-4f transitions with intense ⁷F₀→⁷L₆ excitation to a broader (intense band from 300 to 390 nm) and less resolved spectrum, indicating amorphization of the structure. The emission spectra presented the typical 4f-4f ⁵D₀→⁷F_J (J = 1, 2, 3 e 4) transitions of Eu(III). However, after the catalysis, emission spectrum changed significantly, for instance, all bands became broader and that associated with the ⁵D₀→⁷F₀ transition became very intense. These combined results indicate that the Eu(III)-mandelate catalyst interacted strongly with the reagents and products causing significant structural changes. Most likely, the environment around the Eu(III) sites becomes less symmetric by coordination to different species and modifications of the surfaces. This shows the feasibility of using the Eu(III) luminescent properties as a structural probe of catalysts. So, further investigation should establish the number of cycles required to loss of catalytic activity as well as the gradual changes in the catalyst structure during each cycle. These investigations should provide some insights into the reaction mechanism and the action of the catalyst. They could also aid in designing lanthanide-based catalysts with enhanced performance for mechanochemistry.