Structural Chemistry of Lithium Tetrahydroborate Ether Solvates⋆

LiBH4 · OEt2 (1) and LiBH4 · O(Me)CMe3 (2) form double-stranded chains in the solid state. While 3 hydrides of the BH4 group in 1 bind to three different Li atoms through Li···H–B interactions, the fourth H atom interacts with three adjacent Li centers. In contrast, in compound 2 there are two single Li···H–B bonds and two doubly bridging hydrogen atoms, the latter again interacting with three lithium centers. Moreover, the arrangements of the ether molecules in 1 and 2 are different. LiBH4 · 2 DME (3) (DME = dimethoxyethane) forms a molecular lattice, in which the BH4– anions are present in Li···H2BH2 bridges. The same structural feature is present in LiBH4 · TG (4) (TG = triglyme = triethylene glycol dimethyl ether), but the TG molecule coordinates to two Li centers through two of its four oxygen atoms in such a manner that a chain structure results. The compound LiBH4 · 3 THF (5) exists as discrete molecules in the lattice. Its BH4– anion is triply bridging to the Li center. In contrast, LiBH4·C3H6O2 (6) (1,3-dioxolane) is polymeric. Due to the presence of two μ22-BH4 groups, a chain of the type ···Li(H2BH2)Li(H2BH2)··· is formed, and the 1,3-dioxolane molecules connect the chains through Li–O coordination to form a three-dimensional array. In spite of the variations in the bonding of the BH4 group to Li centers, the Li atoms are hexacoordinated in 3 to 6 but are heptacoordinated in 1 and 2.