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

Abstract

Abstract A 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 H 2 O ( 1 H) and NCS − ( 13 C, 15 N), and Al 3+ ( 27 Al) in each complex. The 1 H NMR spectra reveal six sets of signals for the complexes, [Al(H 2 O) 6 ] 3 + through [Al(H 2 O)(NCS) 5 ] 2− , including isomers for three of the species. Signal area measurements show a decrease in the Al 3 + hydration number with increasing NCS − concentration, as this anion replaces water in the solvation shell. In the 27 Al NMR spectra of these systems, signals for seven complexes, [Al(H 2 O) 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 13 C and 15 N NMR spectra also reveal coordinated NCS − signals for these complexes, including 27 AlN 13 CS J ‐coupling in [Al(NCS) 6 ] 3− . Area evaluations of the 15 N NMR signals provide an excellent complement to the 1 H 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.