Alkylaluminum Complexes Featuring Bridged Bis-Formylfluorenimide Ligands for Hydroboration of Aldehyde, Ketone, and Imines

Three bis-formylfluorenimide ligands with different bridging groups were designed and synthesized, leading to the successful preparation of six novel alkylaluminum complexes through their reaction with alkylaluminum reagents (AlMe₃ or AlEt₃). Complexes <b>1</b> and <b>2</b> were obtained by the reaction of 1,2-propylene-bridged diamine (<b>L</b>^<b>1</b>) with AlMe₃ or AlEt₃. By reacting 1,2-cyclohexylene-bridged diamine (<b>L</b>^<b>2</b>) with AlMe₃ or AlEt₃ to obtain complexes <b>3</b> and <b>4</b>. The above ligands formed a bidentate four-coordinate structure with alkylaluminum, which involved the elimination of one alkyl group as the ligand reacted with alkylaluminum. The complexes <b>5</b> and <b>6</b> were synthesized through the reaction of 1,2-phenylene-bridged diamine (<b>L</b>^<b>3</b>) with AlEt₃ in toluene or tetrahydrofuran. Notably, <b>L</b>^<b>3</b> exhibited unique reactivity compared with the other ligands, which formed a tridentate four-coordinated structure when reacting with alkylaluminum. The formation of the tridentate complex resulted from the introduction of a benzimidazole derivative or tetrahydrofuran (THF) molecule along with the elimination of two alkyl groups during its coordination with alkylaluminum. All complexes were characterized via ¹H NMR, ¹³C NMR, and elemental analysis, with structural determination confirmed through X-ray. Furthermore, the catalytic activity in the hydroboration reaction of aldehyde, ketone, and imines was investigated with these complexes as catalysts. Among them, complex <b>1</b> demonstrated excellent catalytic performance (up to 99% yield) and broad substrate compatibility (more than 30 substrates) at low catalyst loading (1 mol %) under mild reaction conditions.