Crystal Structure of a Lithium Chloride Cubane Cluster Solvated by Diethyl Ether

Lithium chloride is frequently a by-product of reactions w here organolithium com pounds or o ther reagents prepared by deprotonation with butyl-lithium are em ployed in syntheses. Extensive investigations in the structural chemistry of com ­ plexes of lithium salts were carried out and the results have recently been compiled in a review by Snaith and W right [1]. The structures adopted by aggregates of ion pairs Li+X (scheme 1) include rhom bic dim ers (a), ladder-like polymers (b) as in [(THF)LiCl]a [2], cyclic trim ers (c), drum-like hexam ers (d) and cubic tetram ers (e), which can fur­ ther aggregate into polymers (f). All of these structural motifs are verified with various donor molecules bonded to the lithium centres. Frag­ m ents of the polymeric structure (b) have been stabilised by azetidine based ligands [3]. Structure (a) w ithout donor stabilisation is also present in the vapour phase of [LiCl]2 [4]. A m ong all lithium halides and donor solvent molecules, the com bination of lithium chloride and diethyl e ther is probably the m ost relevant with regard to preparative chem istry and thus the knowledge of the structure of a lithium chloride diethyl ether solvate is desirable. Earlier investiga­ tions have established the structure of a lithium chloride solvate with HM PA [5], which consists of a Li4Cl4 cube with the lithium centres coordinated to the oxygen atom of 0 = P (N M e2)3 molecules. A nother solvate of LiCl was observed with TM ED A [6], which adopts a bicyclic structure of the com position (LiCl)4tm eda35, consisting of a six-m em bered LiCl trim er fused to a four-membered ring which adds another LiCl unit. A diethyl e ther solvate of lithium brom ide is also known, which is again a cube of (L iB r)4 coordinated by four E t20 units [7]. We got hold of a crystal of Li4Cl4-4 E t20 during our attem pts to synthesise ethers a-m etallated with group 13 m etals from the therm ally unstable carbenoid L iC H 2OM e. The preparation required continuous m anipulation at low tem perature and afforded a crystalline m aterial, which itself turned out to be unstable and visibly decom posed with gas evolution at tem peratures higher than -3 0 °C. We m anaged to transfer such a crystal to the dif­ fractom eter under perm anent cooling and were thus able to determ ine its structure. M olecules of Li4Cl4 -4 E t20 are present as m onom ers in the orthorhom bic crystal structure (space group P 2 12121). The structure is shown in Fig. 1, the figure caption gives ranges of bond lengths and angles. No crystallographic symmetry is im posed on the heterocubane structure. In this respect Li4Cl4-4 E t20 and its brom ine analogue Li4Br4-4 E t20 behave differently. The latter crys­ tallises in the monoclinic space group C2/c and the molecules obey crystallographic C2 symmetry [7], This m akes the skeleton of Li4B r4-4 E t20 more regular than that of Li4Cl4 -4 E t20 .