Direct hydrogen-1, carbon-13, and nitrogen-15 NMR study of magnesium(II)-isothiocyanate complexing

A hydrogen-1, carbon-13, and nitrogen-15 NMR study of magnesium(II)-isothiocyanate complexation in aqueous mixtures has been completed. At temperatures low enough to slow proton and ligand exchange, separate H-1, C-13, and N-15 NMR signals are observed for coordinated and bulk water molecules and anions. The H-1 NMR spectra reveal signals for the hexahydrate and the mono-through triisothiocyanato complexes, as well as two small signals attributed to [Mg(H2O)(5)(OH)](1+) and [Mg(H2O)(4)(OH)(NCS)]. Accurate hydration numbers were obtained from signal area integrations at each NCS- concentration. in the N-15 NMR spectra, signals also were observed for the mono-through triisothiocyanato complexes, and a small signal believed to be due to [Mg(H2O)(4)(OH)(NCS)]. Coordination number contributions for NCS- were measured from these spectra and when combined with the hydration numbers they totalled essentially six at each anion concentration. Signals for [Mg(H2O)(5)(NCS)](1+) through [Mg(H2O)(3)(NCS)(3)](1-) also were observed in the C-13 NMR spectra and the area evaluations were comparable to the N-15 NMR results. An analysis of the magnitude and sign of the coordinated NCS- chemical shifts identified the nitrogen atom as the anion binding site. All spectra indicated [Mg(H2O)(5)(NCs)](1+) and [Mg(H2O)(4)(NCS)(2)] were the dominant isothiocyanato complexes over the entire range of anion concentrations. The inability to detect evidence for complexes higher than the triisothiocyanato reflects the competitive binding ability of water molecules and perhaps the decreased electrostatic interaction between NCS- and negatively charged higher complexes.