Mono-and Dinuclear Asymmetric Aluminum Guanidinates for the Catalytic CO2 Fixation into Cyclic Carbonates

Abstract

A set of trisubstituted guanidine ligands L1H2-L4H2 with general formula (PrHN)(2)CNR (R = Ph (L1H2), R = 2,4,6-Me3C6H2(L2H2), R = p-BrC6H4(L3H2), R = (C5H4)Fe(C5H5), Fc (L4H2)) was employed to synthesize a family of mono- and dinuclear asymmetric methyl aluminum guanidinato compounds ((L2H)AlMe2 (1), (L4H)AlMe2 (2), (L-1)Al2Me4 (3), (L-2)Al2Me4 (4), (L-3)Al2Me4 (5), (L-4)Al2Me4 (6), (L1H)(2)AlMe (7), (L2H)(2)AlMe (8), and (L4H)(2)AlMe (9)) that were characterized by NMR spectroscopy (1-9) and single-crystal X-ray diffraction (4 and 8). These compounds were tested as catalysts for the fixation of carbon dioxide with epoxides to give cyclic carbonates, using tetrabutylammonium iodide (TBAI) as cocatalyst. The reactions were performed under solvent-free conditions at 70 degrees C and 1 bar CO2 pressure. Complexes 1-9 were more active than their respective free guanidines under the same experimental conditions for the synthesis of styrene carbonate (11a). The dinuclear complex 6 was the most efficient and active catalyst for the synthesis of several monosubstituted carbonates (11a-1) with excellent conversions and selectivities. Furthermore, the formation of some disubstituted cyclic carbonates (13a-c) using this dinuclear aluminum catalyst was also studied.