NHC-heterocycle carbene gold catalyst intercalated in Layered Double Hydroxides as reusable hybrid catalyst in acidic medium

Abstract

NHC-heterocyclic carbene metal complexes are efficient catalysts used for the oxidation, amination, cycloaddition or hydration of alkynes. Immobilization of such complex is a strategy to avoid its decomposition/deactivation and allow its recycling. One limitation of the immobilization of carbene metal complexes is the acidic pH required to perform the catalysis of hydration of alkynes. The support used must be resistant in such conditions. Layered Double Hydroxides (LDH) is a widely used material as catalyst support due to their tunable chemical composition and flexible open structure. However, the LDH phases usually found as catalyst support in the literature are chemically instable under severe conditions. In particular, when operating in extreme acidic pH. This study describes the development of a pH-resistant support by screening of LDH with various composition and formulation in order to obtain a stable material at pH down to 3.0, conditions required for a model reaction, the hydration of propargyl alcohol. Then, the intercalation of a NHC-heterocyclic carbene gold anionic complex (NHC-Au) was optimized and fully characterized. The newly heterogeneous catalysts were evaluated for the targeted hydration catalysis reaction. The selected support, a LiAl2 phase obtained by urea method and dried at 80 °C, allowed to immobilized up to 75% of the gold complex. The intercalated catalyst was finally used for the hydration of several alkynes with yields between 84% and 100% and was recycled up to 12 cycles without any activity loss.