Synthesis, structural characterization, and catalytic performance of Pd(II) complexes with fluorine- and methyl-substituted Schiff bases: Experimental and theoretical insights

Abstract

Two meta-substituted Schiff bases were synthesized and characterized using physicochemical and spectroscopic analysis. These Schiff bases were complexed with palladium(II) to form [Pd(AD1F)] and [Pd(AD1Me)], both characterized through single-crystal X-ray diffraction, elemental analysis, molar conductivity, FTIR, ¹H NMR, and UV–Vis spectroscopy. Pd(AD1F) crystallized in a monoclinic system (P21/c space group), with the phenolic oxygen and azomethine nitrogen chelating Pd(II) to form a distorted square planar geometry. The formation of the Pd(II) complexes was observed through the shifting of ν(C = N) peak and δ(HC = N) in IR and ¹H NMR spectra, respectively. Additionally, the shift of the n–π*(C = N) band in UV–Vis spectra corroborated the involvement of azomethine nitrogen in the complexation. Pd(AD1Me) has been previously reported in our earlier work. DFT calculations revealed similar electronic transitions at 259 nm (π → π*) and 333 nm (n → π*), dominated by specific HOMO to LUMO transitions for [Pd(AD1Me)] and [Pd(AD1F)] with minor effects from substituents. [Pd(AD1F)] demonstrated catalytic activity in copper-free Sonogashira coupling, achieving a TOF of 2.4 h⁻¹ and a TON of 7.1 at 120 °C. Its higher ionization potential and lower electron affinity, as determined by DFT, support its superior catalytic performance compared to [Pd(AD1Me)].