A New Chiral N,N',O-Donor Heteroscorpionate Ligand. Structures of Ni2+, Cu2+, Zn2+ Complexes and Study of Solution Equilibria by Means of 1H NMR/UV-Vis Titrations and EXSY NMR Spectroscopy

The N,N′,O-heteroscorpionate ligand 1-(4-methoxy-3,5-dimethyl-pyridin-2-yl)-2-methyl-1-pyrazol- 1-yl-propan-2-ol (LOH) was prepared in two high-yield steps. Complexes [M(LOH)2][MCl4] (M2+ ) Cu2+ and Zn2+) and [M(LOH)2]Cl2 (M2+ ) Ni2+ and Cu2+) were prepared and characterized by X-ray crystallography. The speciation in solution (methanol: water 95:5) of the M2+/LOH systems was investigated by means of spectrophotometric (Ni2+ and Cu2+) and 1H NMR (Zn2+) titrations. The β1 and β2 global formation constants for the [M(LOH)]2+ and [M(LOH)2]2+ species were obtained and are in agreement with the Irving−Williams series: Ni2+< Cu2+> Zn2+. The Zn2+/LOH system was studied by means of quantitative 1H−1H EXSY spectroscopy (300 K, mixing time ) 0.2−0.8 s), which allows the description of the equilibria occurring between five octahedral [Zn(LOH)2]2+ structural isomers and tetrahedral [Zn(LOH)Cl]Cl species. Exchange constants kijex and associated rate constants kij suggest that two types of interconversion occur: octahedral−octahedral (faster) and octahedral−tetrahedral (slower). DFT calculations (B3LYP/6-311+G(d)) were employed to evaluate the relative stability of the [Zn(LOH)2]2+ isomers, which are comparable for the five complexes with a maximum energy difference of 6.3 kJ/mol.