Solid-State 27Al NMR Spectroscopy of the γ-Al13 Keggin Containing Al Coordinated by a Terminal Hydroxyl Ligand

We report solid-state Al-27 NMR spectroscopic results for the sulfate salt of the gamma-Al-13 Keggin cluster, gamma-[AlO4Al12(OH)(25)(OH2)(11)] [SO4](3)center dot[H2O](14), that provide a spectroscopic signature for partial hydrolysis of this Keggin-type cluster. In Al-27 multiple-quantum magic-angle spinning NMR spectra, all 13 Al positions of the cluster are at least partially resolved and assigned with the aid of density functional theory (DFT) calculations of the Al-27 electric field gradients. The isotropic chemical shift of the single tetrahedral site, 75.7 ppm, is nearly identical to that reported for solutions from which the cluster crystallizes. Reflecting broadly similar coordination environments, the octahedral Al show mostly small variations in isotropic chemical shift (+7 to +11 ppm) and quadrupolar coupling constant (C-Qi 6-7.5 MHz), except for one resonance that exhibits a much smaller C-Q and another site with a larger value. DFT calculations show that deprotonation of a terminal water ligand, to form an eta-OH group, causes a large reduction in the Al-27 C-Q, allowing assignment of a distinct, narrow peak for octahedral Al to this hydroxyl-termipated site. This result suggests a relationship between octahedral Al-27 NMR line width and hydrolysis for solids prepared from Keggin-type clusters.