The high lattice energy of lithium hydride drastically reduces its reactivity and is responsible for its insolubility in organic solvents. Consequently, any form of lithium hydride, whether complexed by ligands or uncomplexed, is an amorphous solid. Crystal structures of lithium hydride or of mixed aggregates containing lithium hydride have not been reported. We have recently provided evidence that the photochemical and thermal decomposition of tert-butyllithium/lithium tert-butoxide mixtures (molar ratio 3:7) in cyclopentane leads to highly reactive, hydrocarbon-soluble LiH [5] in the form of mixed LiH/LiOtBu aggregates. [6] Precipitation of amorphous lithium hydride from such mixtures is apparently prevented by the excess of lithium tertbutoxide, which assists in solubilizing the LiH by incorporating it in mixed aggregates. [7, Previous NMR spectroscopic studies revealed that the photochemical decomposition product is the decamer Li 10 H(OtBu) 9 , while the thermal decomposition product is dodecameric Li 12 H(OtBu) 11 . [6] It is noteworthy that these mixed aggregates form only at high alcoholate:alkyllithium ratios (ROLi:RLi ! 7:3), that is, when a large excess of tert-butoxide ligands is present. At lower ROLi:RLi ratios, amorphous lithium hydride precipitates from solution.
X-ray structural analyses of crystals grown from photolyzed tBuOLi/tBuLi mixtures (ca. 7:3) showed the presence of the hydride-rich superaggregate Li 33 H 17 (OtBu) 16 (1, Figure ). The synthesis is reproducible and, under the conditions chosen, exclusively yields crystals of 1. To our knowledge, this is the largest lithium-containing aggregate so far known. The structural framework consists of fused cubic subunits (Figure ), whose arrangement to form an infinite lattice is disrupted by the incorporation of the sterically demanding tert-butoxide groups. The inner core of the aggregate is hydride-rich, while in the outer shell tert-butoxide ligands are [