About the polymorphism of [Li(C4H8O)3]I: crystal structures of trigonal and tetragonal polymorphs

Two new polymorphs of the ion pair LiI(C4H8O)3 (trigonal, space group P ; tetragonal, space group I41 cd) show different three-dimensional arrangements in the crystal structure and co-exist at the same temperature.

Two new trigonal and tetragonal polymorphs of the title compound, iodidotris(tetrahydrofuran-O)lithium, are presented, which both include the isolated ion pair Li(THF) 3 + ÁI À . One Li-I ion contact and three tetrahydrofuran (THF) molecules complete the tetrahedral coordination of the lithium cation. The three-dimensional arrangement in the two polymorphs differs notably. In the trigonal structure, the ion pair is located on a threefold rotation axis of space group P3 and only one THF molecule is present in the asymmetric unit. In the crystal, strands of ion pairs parallel to [001] are observed with an eclipsed conformation of the THF molecules relative to the LiÁ Á ÁI axis of two adjacent ion pairs. In contrast, the tetragonal polymorph shows a much larger unit cell in which all atoms are located on general positions of the space group I4 1 cd. The resulting three-dimensional arrangement shows helical chains of ion pairs parallel to [001]. Apart from van der Waals contacts, no remarkable intermolecular forces are present between the isolated ion pairs in both structures.

Chemical context
The tetrahedral arrangement of the [Li(THF) 3 ] + ÁI À ion pair has already been reported in the monoclinic crystal structure (space group P2 1 /n) by Nö th & Waldhö r (1998). Crystals of this phase could be obtained during the reaction of tmp 2 AlI (tmp = tetramethylpiperidine) with LiHAsPh (Ph = phenyl) in toluene/tetrahydrofuran (THF) or, more conveniently, from LiH and iodine in THF. The applied crystallization temperature was 233 K and the data collection for structure analysis was performed at 193 K.
In our case, we obtained two new polymorphs of [Li(THF) 3 ] + ÁI À from a solution of (H 3 C) 2 CuLiÁLiI in diethyl ether covered with THF. The reaction mixture was stored at 193 K, and the measurements for the single-crystal structure ISSN 1600-5368 analysis were performed at 123 K. The observation of such contact ion pairs directly confirms the NMR spectroscopic findings (Henze et al., 2005) that upon addition of THF, the LiI units are separated from the cuprate by the coordination of Li + by three THF molecules (Fig. 1).

Structural commentary
The polymorphs reported herein are higher in symmetry compared to the known monoclinic phase as they crystallize in the trigonal space group P3 and the tetragonal space group I4 1 cd. In the asymmetric unit of the trigonal polymorph, the lithium and iodide ion pair is located on a threefold rotation axis (Wyckoff position 2d) and one THF molecule is located on a general position. This results in a symmetric coordination of the lithium cation by the three THF molecules. The unit cell of this polymorph is small and contains two formula units. In contrast, in the structure of the tetragonal polymorph, all atoms are located on general positions. The resultant unit cell is considerably larger and contains 16 formula units. Nevertheless, the molecular structures of the [Li(THF) 3 ] + ÁI À ion pair in all three polymorphs are very similar in terms of bond lengths and angles. Table 1 compiles Li-I and Li-O distances for all three structures.

Supramolecular features
The reasons for the same molecular [Li(THF) 3 ] + ÁI À unit crystallizing in three different crystal systems and space groups lies in the supramolecular assembly of these ion pairs. The three-dimensional arrangement of the [Li(THF) 3 ] + ÁI À ion pairs is different in all three known polymorphs. The differences in the supramolecular structures can best be demonstrated when taking the shortest supramolecular LiÁ Á ÁI distances ($5.7 Å ) into account. Although this is a formal procedure since at distances of more than 5 Å no chemically reasonable interactions are present, it allows for a better understanding of the packing of the ion pairs in the unit cell.
In the previously reported monoclinic structure, the formation of linear chains of individual ion pairs parallel to [101] is observed (Fig. 2, top), where the THF molecules form a staggered conformation relative to a fictive Li-I axis of the shortest supramolecular LiÁ Á ÁI distance (Fig. 2, bottom). The complete structure is characterized by antiparallel oriented chains. The resulting calculated density of the compound is 1.468 g cm À3 (Nö th & Waldhö r, 1998).
A similar supramolecular arrangement is found in the trigonal structure. Here, the ion pairs are likewise aligned in linear chains, in this case parallel to [001] (Fig. 3,  Linear chains in the monoclinic polymorph of [Li(THF) 3 ] + ÁI À (top) show a staggered arrangement of the THF molecules relative to the LiÁ Á ÁI axis (bottom). Displacement ellipsoids (except for hydrogen atoms) are drawn at the 50% probability level.

Figure 3
Linear chains extend parallel to [001] in the trigonal polymorph (top) and show an eclipsed conformation of the THF molecules relative to the LiÁ Á ÁI axis (bottom, left) in an antiparallel arrangement in the unit cell (bottom, right). Displacement ellipsoids (except for hydrogen atoms) are drawn at the 50% probability level.

Figure 1
Proposed by NMR in solution: THF addition to iodidocuprates in diethyl ether solutions yields predominantly iodine-free cuprates and solvated Li-I units.
of the shortest supramolecular LiÁ Á ÁI distance (Fig. 3, bottom left). These chains again are packed with an antiparallel orientation in the crystal structure ( Fig. 3, bottom right), and the calculated density is 1.516 g cm À3 .
Finally, in the tetragonal structure, the situation is completely different, as the ion pairs form helical chains along the 4 1 screw axis of space group I4 1 cd (Fig. 4, top and bottom left). This assembly in the unit cell ( Fig. 4, bottom right) results in a calculated density of 1.503 g cm À3 .
The higher temperature during synthesis/crystallization of the monoclinic polymorph compared to the conditions applied for the title compounds obviously caused the crystallization of the two new polymorphs. Both have a very similar density and co-exist in one reaction batch. At higher temperatures, the crystals became amorphous, indicating an irreversible phase transition.

Synthesis and crystallization
A Schlenk flask, equipped with a stirring bar and 0.5 mmol (1 eq) CuI, was dried four times in vacuo to remove residual moisture. Then 5 ml of diethyl ether was added and the Cu(I) salt was suspended. Upon addition of 2 eq (H 3 C)Li in diethyl ether, the mixture gave a colourless solution. After removal of the stirring bar, the solution was covered with THF. The flask was then stored at 193 K. After several days, clear colourless needles could be observed. Suitable crystals were isolated in nitrogen-cooled perfluoroether oil and mounted on the goniometer for data collection at 123 K.

Figure 4
Helical chains parallel to [001] (top and bottom, left) are present in the crystal structure of the tetragonal polymorph. Displacement ellipsoids (except for hydrogen atoms) are drawn at the 50% probability level.
compounds did not differ in their forms. For several crystals, the unit cell was determined, proving the presence of either the tetragonal or the trigonal polymorph.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The positions of the lithium cations were located in difference Fourier maps. H atoms were positioned with idealized geometry and were refined with C-H = 0.99 Å and U iso (H) = 1.2U eq (C).

Special details
Experimental. crystal mounting in perfluorether (T. Kottke, D. Stalke, J. Appl. Crystallogr. 26, 1993, p. 615) Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.  Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.