A new enantiomerization mechanism for tripodal penta-coordinate ZnII(nta) complexes. Theoretical clarification of the observed 1H NMR spectrum

Abstract

Based on DFT calculations (RB3LYP/LANL2DZp), the unexpected single-line ¹H NMR spectrum of Zn^II(nta), nta = 2,2′,2″-nitrilotriacetate, can be ascribed to a non-dissociative enantiomerization process (δδδ ⇌ λλλ) from C₃viaC_3v to C₃′ symmetry. The energy barrier is rather low and depends to a lesser extent on the nature of the co-ligand in [Zn(nta)X]²⁻ (X: H⁻, CH₃⁻ NH₂⁻, OCH₃⁻, F⁻, Cl⁻, Br⁻, I⁻) and [Zn(nta)Y]⁻ (Y: NCH, CO, N₂, O(CH₃)₂), but more so on the overall charge of the complex. The energy barrier for enantiomerization of [Zn(nta)X]²⁻ is between 5.7 and 6.7 kcal mol⁻¹, and for [Zn(nta)Y]⁻ between 2.2 and 3.1 kcal mol⁻¹.