A Robust C 3 -Symmetric Aluminate Hydride for CO 2 Hydroboration Catalysis: Mechanistic Insights and Countercation Influence on Catalytic Performance

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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( n Bu) 4 ] salts, which were then exploited as CO 2 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( n Bu) 4 ] + ) reacts fast with CO 2, to afford the corresponding Al-formate complexes [ 3 ][C], which were isolated and structurally characterized. Such anions were then exploited as potential CO 2 reduction catalysts. Salts [ 2 – 3 ][N( n Bu) 4 ] are efficient and robust CO 2 hydroboration catalysts in the presence of pinBH or Me 2 S-BH 3 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 CO 2 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 CO 2 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( n Bu) 4 ] vs [ 2 ][Li(THF) 4 ].