Fluorine/phenyl chelated boron complexes: Synthesis, fluorescence properties and catalyst for transfer hydrogenation of aromatic ketones

The synthesis of salen/salan ligands (L1 and L2) and their fluorine/phenyl chelated boron complexes [L(1,2)BF2] or [L(1,2)BPh2] is described in this paper. The fluorine/phenyl chelated boron complexes were synthesized from the reaction of BF3·OEt2 or BPh3 with the corresponding ligands in different solvent. The boron complexes display high stability and can be handled in air due to the presence of coordinative B ← N and covalent BO bonds in their structures. The salen/salan ligands (designated as salan, a saturated version of the corresponding salen ligands) and their fluorine/phenyl chelated boron complexes have been characterized by 1H, 13C NMR and 19F NMR spectra, elemental analysis, FT-IR spectra, UV–vis spectra, LC–MS spectra, melting point and fluorescence spectroscopy. The fluorescence efficiencies of BF2–chelate boron complexes are greatly improved compared to those of the BPh2–chelate boron analogs based on the same salen/salan ligands, probably due to the enhanced conjugation degree of the diphenyl boron chelation, which can effectively prevent molecular aggregation. The boron complexes [L(1,2)BF2] or [L(1,2)BPh2] were also applied to the transfer hydrogenation of aromatic ketones to the corresponding alcohol derivatives in the presence of iso-PrOH as the hydrogen source. Catalytic studies showed that all complexes are good catalytic precursors for transfer hydrogenation of aryl alkyl ketones in 0.1 M iso-PrOH solution. This transfer hydrogenation is characterized by low reversibility under these conditions.