Solid-State NMR Studies on Ionic closo-Dodecaborates

Abstract

Alkali metal dodecahydro-closo-dodecaborates M2[B12H12] (M = K, Rb, Cs, NH4, N(CH3)4) and the perhalogenated cesium salts Cs2[B12X12] (X = Cl, Br, I) are studied by solid-state 11B nuclear magnetic resonance (NMR) spectroscopy as well as X-ray diffraction (XRD) and differential scanning calorimetry. The present work addresses the molecular dynamics of the anionic [B12X12]2− icosahedra which is examined by variable-temperature 11B NMR line shape studies between 120 and 370 K. Characteristic line shape effects are observed which strongly depend on the actual substituent X and the counterion M+. All alkali metal dodecahydro-closo-dodecaborates M2 [B12H12] exhibit at elevated temperatures 11B NMR spectra with a single isotropic line which proves the presence of an efficient molecular process, resulting in dynamic (rotational) disorder along with vanishing dipolar and quadrupolar interactions. The positional order of the boron clusters, however, remains unaffected, as shown by the XRD data. At lower temperatures, the underlying motions are frozen on the NMR timescale resulting in characteristic 11B NMR spectra with a dominant homonuclear 11B–11B dipolar splitting. The per-halogenated cesium salts Cs2[B12X12] behave differently. Hence, from the experimental 11B NMR spectra at room temperature a substantial mobility is only seen for the [B12Cl12]2− anion. Obviously, the degree of anion mobility depends on the size of the substituent X in the [B12X12]2− clusters (X = H, Cl, Br, I). A quantitative analysis of the experimental 11B NMR spectra of the alkali metal dodecahydro-closo-dodecaborates M2 [B12H12] is achieved by line shape simulations, considering [B12H12]2− ions undergoing reorientational jumps between icosahedral sites. From the motional correlation times the activation energies are derived. It is found that a correlation exists between the activation energies, the motional correlation times and the lattice constant. Hence, the activation energies and correlation times strongly increase with decreasing size of the cation M+, which reflects an increasing sterical hindrance due to a decreasing crystallo-graphic lattice constant in the same direction.