Coordination of imidazoles by Cu(II) and Zn(II) as studied by NMR relaxometry, EPR, far-FTIR vibrational spectroscopy and ab initio calculations: effect of methyl substitution

Synthetic imidazole ligands are typically substituted at the N₁ ((1)-Im) position while natural imidazole ligands are substituted at the C ₄ ((4)-Im) position. To outline the difference in coordination properties, the methyl-substituted imidazoles Me(4)-Im and Me(1)-Im were complexed with CuCl₂ and ZnCl₂ and investigated by NMR relaxometry, electron paramagnetic resonance, far-Fourier transform IR vibrational spectroscopy, and ab initio calculations. Me(4)-Im, Me(1)-Im, and Im in excess form the usual tetragonal D_4h [CuL₄X ₂] complexes with CuCl₂ whereas the methylated imidazoles form pseudotetrahedral C_2v complexes instead of the usual octahedral O_h [ZnIm₆]²⁺ complex. All imidazoles display a high degree of covalence in the M-L σ- and π-bonds and the π-interaction strength affects the relative energies of complexation. Opportunities to tailor complexes by the chemical properties of the substituents are envisaged due to the role of the inductive and hyperconjugative effects, rather than position.