A Robust C3‑Symmetric Aluminate Hydride for CO2 Hydroboration Catalysis: Mechanistic Insights and Countercation Influence on Catalytic Performance

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(nBu)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(nBu)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(nBu)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(nBu)4] vs [2]­[Li­(THF)4].