Toward a Neutral Single-Component Amidinate Iodide Aluminum Catalyst for the CO₂ Fixation into Cyclic Carbonates

A new iodide aluminum complex ({AlI(κ⁴-naphbam)}, 3) supported by a tetradentate amidinate ligand derived from a naphthalene-1,8-bisamidine precursor (naphbamH, 1) was obtained in quantitative yield via reaction of the corresponding methyl aluminum complex ({AlMe(κ⁴-naphbam)}, 2) with 1 equiv of I₂ in CH₂Cl₂ at room temperature. Complexes 2 and 3 were tested and found to be active as catalysts for the cyclic carbonate formation from epoxides at 80 °C and 1 bar of CO₂ pressure. A first series of experiments were carried out with 1.5 mol % of the alkyl complex 2 and 1.5 mol % of tetrabutylammonium iodide (TBAI) as a cocatalyst; subsequently, the reactions were carried out with 1.5 mol % of iodide complex 3 as a single-component catalyst. Compound 3 is one of the first examples of a nonzwitterionic halide single-component aluminum catalyst producing cyclic carbonates. The full catalytic cycle with characterization of all minima and transition states was characterized by quantum chemistry calculations (QCCs) using density functional theory. QCCs on the reaction mechanism support a reaction pathway based on the exchange of the iodine contained in the catalyst by 1 equiv of epoxide, with subsequent attack of I^- to the epoxide moiety producing the ring opening of the epoxide. QCCs triggered new insights for the design of more active halide catalysts in future explorations of the field.