Theoretical Insights on Mandelic Acid and Its Fe(II) Co(II), Ni(II), and Cu(II) Mandelate Compounds

ABSTRACT B3LYP and M06‐L density functional calculations have been performed to evaluate the binding phenomena of the Fe(II), Co(II), Ni(II), and Cu(II) ions with the biologically important mandelic acid (MA) ligand in the gas phase and in the aqueous PCM model. The influence of the metal nature, spin states, coordination mode, chelate effects, and ring strain was explored. The direct interactions of the Fe(II), Co(II), Ni(II), and Cu(II) ions with the deprotonated MA − ligand resulted in the stabilization of twenty‐eight stable mandelate complexes. A similar chemical reactivity was observed for the Fe(II), Co(II), and Ni(II) ions towards the MA − form, with the highest Δ E Bond values calculated for the Cu(II) complexes. The AIM, FMO, and NBO results indicate that the nature of all MO chemical bonds is partly ionic and covalent, with a medium strength bond. The IR and UV‐Vis absorption spectra of MA − and mandelate complexes were calculated and compared to experimental spectra. The UV‐Vis absorption bands of mandelate complexes involve metal‐to‐ligand charge transfer through d ‐ π * electron transitions, particularly with relatively high intensities in the visible region of the spectrum for the five‐coordinated Cu(II) complexes.