Theoretical studies on the structures and absorption spectra of -Ph and t-Bu substituted 5-(2-pyridyl) pyrazolate boron complexes

We present a theoretical study using density functional theory (DFT) on molecular structures, electronic structures and absorption characters of -Ph and t-Bu substituted 5-(2-pyridyl) pyrazole boron complexes, namely, BPh2(2-(5-Phenyl-4H-pyrazole-3-yl)-pyridine) (2d), BPh2(2-[5-(2,2,2-Trifluoro-1,1-bis-trifluoromethyl-ethyl)-4H-pyrazole-3-yl]-pyridine) (2e), BPh2(4-Phenyl-2-(4H-pyrazole-3-yl)-pyridine) (2d-1) and BPh2(2-(4H-Pyrazole-3-yl)-4-(2,2,2-trifluoro-1,1-bis-trifluoromethyl-ethyl)-pyridine) (2e-1). The ground state structures of the title complexes are optimized at B3LYP/6-31G level of theory. In addition, time dependent density functional theory (TD-DFT) method is applied to investigate the properties of absorption spectra and electronic transition mechanism based on the ground state geometries. The results show that the chemical bond formed between nitrogen on the pyridyl ring and boron can be attributed to coordination effect and the coordinate bond in 2d-1 is the strongest among the four compounds. The calculated absorption wavelengths for 2d and 2e are in good agreement with the experimental ones. It can be detected that the main transitions of 2d, 2e and 2d-1 correspond to the intraligand π → π∗ character. As the case of 2e-1, the main transition can be assigned as a mixed ligand-to-ligand/interligand charge transfer.