Aluminum complexes with new non-symmetric ferrocenyl amidine ligands and their application in CO2 transformation into cyclic carbonates

Abstract

A set of alkyl aluminum complexes supported by non-symmetric ferrocenyl amidine ligands were used as catalysts for the preparation of cyclic carbonates from epoxides and carbon dioxide using Bu₄NI as a co-catalyst. A modified method for the synthesis of aminoferrocene allowed us to obtain this precursor in quantitative yield. Treatment of aminoferrocene with the corresponding acetimidoyl chloride afforded the desired ferrocenyl amidine ligands L₁H, (E)-N-(2,6-diisopropylphenyl)-N’-(ferrocenyl)acetimidamide, and L₂H, (E)-N-(2,6-dimethylphenyl)-N’-(ferrocenyl)acetimidamide. The reaction of these ligands with 1.0 or 0.5 equiv. of AlMe₃ led to the synthesis of aminoferrocene based aluminum complexes ((L₁)AlMe₂ (1), (L₂)AlMe₂ (2), (L₁)₂AlMe (3), and (L₂)₂AlMe (4)) in excellent yields, which were characterized by spectroscopic and X-ray diffraction methods. In addition, we have studied their electrochemical properties and complex 1 was found to be the most active catalyst for the formation of cyclic carbonates 6a–j from their corresponding epoxides 5a–j and CO₂.