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

We report solid-state ²⁷Al NMR spectroscopic results for the sulfate salt of the γ-Al₁₃ Keggin cluster, γ-[AlO₄Al₁₂(OH)₂₅(OH₂)₁₁][SO₄]₃·[H₂O]₁₄, that provide a spectroscopic signature for partial hydrolysis of this Keggin-type cluster. In ²⁷Al 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 ²⁷Al 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_Q; 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 η-OH group, causes a large reduction in the ²⁷Al C_Q, allowing assignment of a distinct, narrow peak for octahedral Al to this hydroxyl-terminated site. This result suggests a relationship between octahedral ²⁷Al NMR line width and hydrolysis for solids prepared from Keggin-type clusters.