A Robust C-Symmetric Aluminate Hydride for CO₂ Hydroboration Catalysis: Mechanistic Insights and Countercation...

The present study details the synthesis and characterization of a robust, monomeric Al-H aluminate supported by a tridentate tris-phenolate ligand, isolated as [2][Li(THF)₄] and [2][N(Bu)₄] salts, which were then exploited as CO₂ hydroboration catalysts. As initial reactivity studies, it was observed that the nucleophilic Al-H anion in [2][C] (C = countercation [Li(THF)₄]⁺ or [N(Bu)₄]⁺) reacts fast with CO₂, to afford the corresponding Al-formate complexes [3][C], which were isolated and structurally characterized. Such anions were then exploited as potential CO₂ reduction catalysts. Salts [2-3][N(Bu)₄] are efficient and robust CO₂ hydroboration catalysts in the presence of pinBH or Me₂S-BH₃ as hydroborane sources to selectively afford formate-equivalent or methanol-equivalent products (TON up 1920), depending on reaction conditions and the nature of the countercation. As deduced from detailed DFT calculations, the Al-formate anion [3]^- acts as a nucleophilic catalyst (for borane activation) but also as an electrophile (through the AlOCO carbon) allowing CO₂ activation/functionalization and thus the reduction catalysis to occur, a process thermodynamically driven by the stability of the reduction products. The anionic nature of [2]^- and [3]^- aluminates, resulting in an enhanced nucleophilicity (vs neutral analogues), may thus be crucial for catalytic activity. In contrast, according to DFT calculations performed with a model anion of [3]^- and pinBH, a CO₂ reduction processing via an Al-O/B-H σ-bond metathesis appears to be kinetically unfavored. The proposed mechanism involving an electrophilic/nucleophilic dual-activation mode also rationalizes the importance of countercation [C]⁺ in [2-3][C] for catalytic activity and selectivity, as demonstrated by the higher performance of [2][N(Bu)₄] vs [2][Li(THF)₄].