Multinuclear magnetic resonance study of aluminum(III)‐isothiocyanate complexes in water‐acetone mixtures

Abstract

AbstractA multinuclear magnetic resonance (NMR) study of the complexes of aluminum(III) with isothiocyanate ion in water‐acetone mixtures has been completed. At temperatures low enough to slow proton and ligand exchange, separate resonance signals are observed for coordinated and bulk H2O (1H) and NCS− (13C, 15N), and Al3+ (27Al) in each complex. The 1H NMR spectra reveal six sets of signals for the complexes, [Al(H2O)6]3+ through [Al(H2O)(NCS)5]2−, including isomers for three of the species. Signal area measurements show a decrease in the Al3+ hydration number with increasing NCS− concentration, as this anion replaces water in the solvation shell. In the 27Al NMR spectra of these systems, signals for seven complexes, [Al(H2O)6]3+ through [Al(NCS)6]3−, are observed, with chemical shifts increasing by about 6 ppm with each additional NCS−. Although broadened somewhat by the Al(III) quadrupole, the 13C and 15N NMR spectra also reveal coordinated NCS− signals for these complexes, including 27AlN13CS J‐coupling in [Al(NCS)6]3−. Area evaluations of the 15N NMR signals provide an excellent complement to the 1H hydration number data. These NMR results demonstrate that a multinuclear approach to the study of solution complexes can provide detailed structural information about the species being formed.