Synthesis, crystal structures, fluorescent properties and DFT investigation of some new bis-boranil complexes based on a phenylene backbone

Abstract

Four new bis-boranil complexes based on a phenylene backbone B₂L1–B₂L4 have been synthesized by a very simple synthetic procedure and structurally characterized by ESI MS, ¹H NMR spectra and X-ray diffraction analysis. The results show that each complex exhibits an internal inversion center at the center of the central phenyl ring. Complexes B₂L1-B₂L3 differ from complex B₂L4 in two important respects: (1) the boron-containing six-membered ring shows an envelope conformation for the former, but a screw-boat conformation for the latter, (2) the dihedral angle between the central ring and the boranil ring is ∼53° for the former, and ∼58° for the latter. In toluene, tetrahydrofuran and acetonitrile, the complexes B₂L1–B₂L4 exhibit moderate fluorescence in the wavelength range 439–596 nm with a large Stokes shift of 71–250 nm. Electron-donating substituents (OMe and Me) at the 4-position in the anil group enhanced the fluorescence efficiency of the complexes both in solution and in the solid state. Quantum chemical calculations were also performed to study the molecular structures in the ground and excited states as well as the relationship between the substituent positions on the phenylene backbone and the luminescent properties of these complexes. The results contribute to a deeper understanding of the structure-property relationships of bis-boranil complexes.