Alkyl zinc complexes derived from formylfluorenimide ligands: synthesis, characterization and catalysis for hydroboration of aldehydes and ketones

Abstract

Five new alkyl zinc complexes supported by different formylfluorenimide ligands were prepared and characterized. Complex 1 was obtained by the reaction of 9-[N(CH₃)₂–Cy–NCH]Fl (Cy = 2-cyclohexyl) (Fl = fluorenyl) (L¹) with diethylzinc (ZnEt₂) in tetrahydrofuran. Reacting 9-[2-pyridyl–CH–NCH]Fl (L²) with ZnEt₂ in tetrahydrofuran yielded complex 2. Under analogous conditions, complexes 3 and 4 were obtained through the reaction of 9-[2-pyridyl–CH(CH₃)–NCH]Fl (L³) and 9-[8-quinoline–CH–NCH]Fl (L⁴) with ZnEt₂ in tetrahydrofuran, respectively. The above ligands formed a tridentate four-coordinate structure with the introduction of a THF molecule along with the elimination of one ethyl group during its coordination with zinc. Notably, the formylfluorenimide ligand 9-[2-phenoxy–Ph–NCH]Fl (Ph = phenyl) (L⁵) bearing a phenoxy group exhibited unique reactivity compared to the other ligands and formed a bidentate three-coordinated structure when reacting with ZnEt₂ in THF. All complexes were characterized via¹H NMR, ¹³C NMR spectroscopy, and elemental analysis, with structural determination confirmed through single-crystal X-ray diffraction. Furthermore, the catalytic properties of these complexes were investigated. All these complexes exhibited excellent catalytic activities for the hydroboration of benzaldehyde, among which complex 5 demonstrated excellent catalytic performance (up to 99% yield) with a versatile substrate scope and high tolerance to functional groups (27 substrates) at a low catalyst loading (0.8 mol%) under mild reaction conditions.