Skeletal Electron Count Research Articles

Chlorinated polyhedral selenaboranes revisited by joint experimental/computational efforts: the formation of closo-1-SeB9Cl9 and the crystal structure of closo-SeB11Cl11.

The recent success in the formation of chlorinated telluraboranes and the reactivities of pnictogenaboranes prompted us to re-examine the vacuum co-pyrolysis of B2Cl4 with Se2Cl2 at various molar ratios and temperatures in order to search for the generation of other polyhedral selenaboranes than closo-SeB5Cl5 (1a) and closo-SeB11Cl11 (1b), the latter being observed earlier. Interestingly, a new compound with the elemental composition SeB9Cl9 (2) was detected, this time by high- and low-resolution mass spectrometry. Further characterization by 1- and 2-D 11B-NMR spectroscopy suggests that 2 should adopt a closed bicapped square-antiprismatic geometry with selenium at the apical position. Moreover, vacuum sublimation gave suitable crystals of 1b, which were subjected to single-crystal X-ray structure determination. Crystallographic data analysis confirmed that 1b, consistent with its 26 skeletal electron count, adopts a distorted icosahedral structure close to the symmetry of C5v. Computations at the DFT-D3 level have revealed that 33% of the total computed binding motifs in the grown 1b crystals are due to the very strong chalcogen bonding. Moreover, SAPT decomposition has shown that the bonding motifs in the crystals are stabilized mainly by dispersion and electrostatic terms. Homodecoupling and high resolution 11B NMR and 77Se NMR experiments have resolved both coupling constants 1J(11B11B) and 1J(77Se11B) as well as the 77Se chemical shift of 1a and 1b, which are in reasonable agreement with the corresponding computed values. The computed 11B chemical shifts of 2 were determined by the well-established DFT/GIAO/NMR structural tool based on its B3LYP/6-311+G** internal coordinates. They agree well with the experimental values and provide a good representation of the molecular structure of 2 in solution. The extraordinary downfield 11B NMR chemical shift of B(10) in 2 has been ascribed to the intensive paramagnetic contribution to the shielding tensor in this bicapped square-antiprismatic motif. Calculations of the synproportionation free energies of smaller (n - 1) closo-selenaboranes with larger-sized (n + 1) ones support the extraordinary stability of octahedral, bicapped square-antiprismatic and icosahedral closo motifs in the SeBnCln family (n = 4-12).

Bridges and Vertices in Heteroboranes.

A number of (hetero)boranes are known in which a main group atom X 'bridges' a B-B connectivity in the open face, and in such species X has previously been described as simply a bridge or, alternatively, as a vertex in a larger cluster. In this study we describe an approach to distinguish between these options based on identifying the best fit of the experimental {Bx} cluster fragment with alternate exemplar {Bx} fragments derived from DFT-optimized [BnHn]2- models. In most of the examples studied atom X is found to be better regarded as a vertex, having 'a 'verticity' of ca. 60-65%. Consideration of our results leads to the suggestion that the radial electron contribution from X to the overall skeletal electron count is more significant than the tangential contribution.

Kenneth Wade. 13 October 1932—16 March 2014

Much of Ken Wade's scientific career was devoted to developing and exploiting the chemistries of boron, aluminium, gallium and indium (the Group 13 elements). He made significant contributions to both experimental and theoretical aspects of the subject and was a highly regarded teacher and attentive research supervisor. Ken was a modest and self-critical man who had a rare ability to recognize patterns in large chemical and molecular structure data sets and express the resulting generalizations in the form of simple rules that experimental chemists could appreciate and use to predict new compounds. Ken was fascinated by the structures of cluster compounds that are based on regular polyhedral shapes. Cluster molecules generally contain groups of metal atoms linked by metal–metal bonds and located on one or more spherical shells. Since the 1960s they have attracted the attention of both academic and industrial chemists. Understanding and predicting their three-dimensional shapes raised a significant intellectual challenge for chemists. The clusters also attracted attention because they occupy an intermediate position between co-ordination compounds based on a single metal atom and the parent metal, which has an infinite number of linked metal atoms. This provided chemists with a unique opportunity to explore the evolution of their electronic and chemical properties as the cluster size increases. Ken was the first to recognize the electronic relationships connecting closo- , nido- , arachno- and hypho - clusters. These clusters are inter-related by the successive removal of vertices from a regular closo- deltahedron, i.e. a polyhedron with all the faces triangular. His ability to connect the structures of these molecules to the number of valence electrons involved in skeletal bonding represented a significant contribution to chemical bonding theories. Ken communicated his conclusions in very clearly written papers and books. This made the complexities of cluster chemistry amenable to many and especially those who did not have a strong background in quantum chemistry. The skeletal electron counting rules, or Wade's Rules, which Ken pioneered, are now very much part of the fabric of modern cluster chemistry and form an important component of modern undergraduate inorganic chemistry courses. Wade's Rules may be applied to main group and transition metal carbonyl clusters as well as clusters containing both main group and transition metal fragments. Besides providing an accessible entry into cluster chemistry for students, the rules have been used creatively by many leading synthetic chemists to discover new classes of compounds.

B111, B112, B113, and B114: The most stable core-shell borospherenes with an icosahedral B12 core at the center exhibiting superatomic behaviors

Boron allotropes are known to be predominately constructed by icosahedral B12 cages, while icosahedral-B12 stuffing proves to effectively improve the stability of fullerene-like boron nanoclusters in the size range between B98–B102. However, the thermodynamically most stable core-shell borospherenes with a B12 icosahedron at the center still remains unknown. Based on the structural motif of D5h C70 and extensive first-principles theory calculations, we predict herein the high-symmetry C5v B111+ (3) which satisfies the Wade’s n+1 and n+2 skeletal electron counting rules exactly and the approximately electron sufficient Cs B111 (4), Cs B112 (5), Cs B113 (6), and Cs B114 (7) which are the most stable neutral core-shell borospherenes with a B12 icosahedron at the center reported to date in the size range between B68–B130, with Cs B112 (5) being the thermodynamically most favorite species in the series. Detailed orbital and bonding analyses indicate that these spherically aromatic species all contain a negatively charged icosahedral B122− core at the center which exhibits typical superatomic behaviors in the electronic configuration of 1S21P61D101F8, with its dangling valences saturated by twelve radial B-B 2c-2e σ bonds between the B12 inner core and the B70 outer shell. The infrared (IR) and Raman spectra of the concerned species are computationally simulated to facilitate their future characterizations.

Geometries of 11‐Vertex Carborane Monoanion Radicals with 2n+3 Skeletal Electron Counts

While geometries of 12‐ and 13‐vertex dicarbaborane (carborane) radicals with rare 2n+3 skeletal electrons (SEs) have been determined elsewhere, the geometries of the 11‐vertex dicarbaborane monoanion radicals with 2n+3 SEs are established here for the first time by computations in agreement with observed electrochemical and electron paramagnetic resonance (EPR) spectroscopy data.

Dimetallaborane analogues of the octaboranes of the type Cp2M2B6H10: structural variations with changes in the skeletal electron count.

The structures and energetics of the complete series of hydrogen-rich dimetallaboranes Cp2M2B6H10 and Cp*2M2B6H10 (Cp = η(5)-C5H5; Cp* = η(5)-Me5C5; M = Pd, Pt; Rh, Ir; Ru, Os; Re; Mo, W; Ta), including the experimentally known Cp*2Rh2B6H10 and Cp*2W2B6H10 (Cp* = η(5)-Me5C5), have been investigated by density functional theory. The lowest energy structures of the hyperelectronic Cp2M2B6H10 (M = Pd, Pt; Rh, Ir) systems have central M2B6 frameworks with a hexagonal open face similar to the B8 networks in arachno-B8H14 and nido-B8H12. The two lowest energy structures for Cp2Rh2B6H10 and Cp*2Rh2B6H10, lying within 1 kcal mol(-1) of energy, differ only in the locations of the bridging hydrogen atoms around the hexagonal hole consistent with the experimentally observed fluxionality of the hydrogen atoms in Cp*2Rh2B6H10. Most of the lowest energy Cp2M2B6H10 (M = Ru, Os) structures also have a central M2B6 framework similar to B8H12, typically with such additional features as an additional metal-metal bond or a formal metal-metal double bond. A common motif for the low-energy structures of the hypoelectronic Cp2M2B6H10 (M = Re; Mo, W; Ta) systems, including the experimentally known Cp*2W2B6H10, is a central M2B4 octahedron with its two M2B faces capped by the remaining boron atoms and with four M-B edges bridged by hydrogen atoms. Such structures can also be considered as oblatonido structures derived from the experimentally known 9-vertex oblatocloso Cp*2Re2B7H7 structure by removal of the unique degree 4 vertex atom.

The Wade–Mingos rules in seven-vertex dimetallaborane chemistry: Hydrogen-rich Cp2M2B5H9 systems of the second and third row transition metals

The complete series of hydrogen-rich seven-vertex cyclopentadienyl dimetallaboranes Cp2M2B5H9 (Cp = η5-C5H5; M = Ir, Ru/Os, Re, Mo/W, Ta), including the experimentally known Mo and W derivatives, have been examined by density functional theory. The central M2B5 polyhedra depend on the skeletal electron count and, in most cases, can be related to the Wade–Mingos rules. Thus the six lowest-energy Cp2Ir2B5H9 structures with 18 Wadean skeletal electrons are all based on the expected nido polyhedron obtained by removal of a degree 5 vertex from a D2d bisdisphenoid. Similar geometries are also found for most of the lowest energy Cp2M2B5H9 structures (M = Ru, Os). The three lowest-energy Cp2Re2B5H9 structures all have central Re2B5 capped octahedra in accord with their 14 Wadean skeletal electrons. The low energy structures of the more hypoelectronic Cp2M2B5H9 (M = Mo, W, Ta) systems have bicapped trigonal bipyramid geometries in which the two MB2 faces of a central trigonal bipyramid are each capped by BH vertices. For Cp2M2B5H9 (M = Mo, W) the experimentally realized symmetrical D2h bicapped trigonal bipyramid structure of this type is obviously very favorable since it lies more than 23 kcal/mol in energy below the next lowest energy structure.

Six-Vertex Hydrogen-Rich Cp2M2B4H8 Dimetallaboranes of the Second- and Third-Row Transition Metals: Effects of Skeletal Electron Count on Preferred Polyhedra

The complete series of hydrogen-rich six-vertex cyclopentadienyl dimetallaboranes Cp2M2B4H8 (Cp = η5-C5H5; M = Ir, Ru/Os, Re, Mo/W, and Ta), including the experimentally known Ir, Ru, and Re derivatives synthesized by Fehlner and co-workers, have now been examined by density functional theory. The nature of the central M2B4 polyhedra in the lowest energy Cp2M2B4H8 structures relates to the skeletal electron count as determined by the Wade–Mingos rules. Thus, the lowest energy Cp2Ir2B4H8 structures with 16 Wadean skeletal electrons have central pentagonal-pyramidal Ir2B4 units similar to that of the known pentagonal-pyramidal B6H10. The lowest energy Cp2M2B4H8 (M = Ru, Os) structures with 14 Wadean skeletal electrons have central capped-tetragonal-pyramidal rather than octahedral M2B4 units. However, isomeric Cp2M2B4H8 (M = Ru, Os) structures with central M2B4 octahedra are found at energies starting at ∼15 kcal/mol (M = Ru) and ∼10 kcal/mol (M = Os) above the capped-tetragonal-pyramidal global minima. The...

Empty versus filled polyhedra: 11 vertex bare germanium clusters

The structures and energetics of centered 10-vertex Ge@Ge₁₀(z) (z = -4, -2, 0, +2, +4) clusters have been investigated by density functional theory (DFT) for comparison with the previously studied isomeric empty 11-vertex Ge₁₁(z) clusters. For the cationic species (z = +2, +4) such centered Ge@Ge₁₀(z) structures are shown to be energetically competitive (within ∼1 kcal mol⁻¹) to the lowest energy isomeric empty Ge₁₁(z) structures. These Ge@Ge₁₀(z) structures can be derived from the lowest energy empty 10-vertex Ge₁₀(z-4) structures by inserting a Ge⁴⁺ ion in the center. The outer 10-vertex polyhedron in the lowest energy Ge@Ge₁₀²⁺ dication structure is the most spherical D(4d) bicapped square antiprism, which is also the lowest energy structure of the empty Ge₁₀²⁻ dianion, as expected from the Wade-Mingos skeletal electron counting rules. For the tetracationic Ge₁₁⁴⁺ /Ge@Ge₁₀⁴⁺ system the lowest energy centered Ge@Ge₁₀⁴⁺ structure can be obtained by inserting a Ge⁴⁺ ion in the center of a C(3v) deltahedral empty Ge10 cluster. Centered 10-vertex polyhedral Ge@Ge₁₀(z) structures were also found for the neutral (z = 0) and dianionic (z = -2) systems but at significantly higher energies than the lowest energy isomeric empty Ge₁₁(z) structures.

Electrochemical and spectroelectrochemical studies of C-benzodiazaborolyl-ortho-carboranes

Fifteen C-diazaborolyl-ortho-carboranes, 1-R'-2-R''-1,2-C(2)B(10)H(10), where R' represents the groups 2-(1,3-Et(2)-1,3,2-N(2)BC(6)H(4))-, 2-(1,3-Ph(2)-1,3,2-N(2)BC(6)H(4))-, 2-(1,3-Ph(2)-5,6-Me(2)-1,3,2-N(2)BC(6)H(2))-, 2-(1,3-(i)Pr(2)-1,3,2-N(2)BC(6)H(4))-, and 2-(1,3,2-N(2)BC(6)H(6))- and where R'' is H, Me, Ph, (t)Bu or SiMe(3), were synthesized. Cyclic voltammetry studies of the compounds showed irreversible oxidation waves which are caused by the oxidation of the heterocycle. Those C-diazaborolyl-ortho-carboranes with Ph, tBu and SiMe(3) substituents at the adjacent C-atom of the cage displayed two one-electron reduction waves reflecting the formation of stable radical monoanions with unusual (2n + 3) skeletal electron counts. The geometries of these anions were determined by combinations of infrared, UV-visible spectroelectrochemical and computational studies. Additionally the structures of seven new C-diazaborolyl-ortho-carboranes and one new 2-bromo-1,3,2-benzodiazaborole were determined by X-ray crystallography and compared with previously obtained structures.

Iron-Centered Ten-Vertex Germanium Clusters: The Ubiquity of Low Energy Pentagonal Prismatic Structures with Various Skeletal Electron Counts

One of the most significant recent developments (in 2009) is the discovery of the clusters M@Ge10(3-) (M = Fe, Co) in which the outer Ge10 polyhedron is a pentagonal prism rather than a deltahedral structure of the type predicted by the Wade-Mingos rules. Consistent with this experimental observation, density functional theory shows the lowest energy structures to be pentagonal prisms for the iron-centered clusters Fe@Ge10(z) in all nine charge states ranging from -5 to +3. This contrasts with the previously studied cobalt-centered germanium clusters Co@Ge10(z) for which the lowest energy structures are pentagonal prisms only for the electron richest systems where z ranges from -3 to -5. The C3v structures derived from the tetracapped trigonal prism found as lowest energy structures of the electron poorer Co@Ge10(z) (z = 0, -1, -2) systems are higher energy structures for the iron-centered germanium clusters Fe@Ge10(z) (z = 0, -1, -2). The strong energetic preference for pentagonal prismatic structures in the Fe@Ge10(z) clusters can be attributed to the need for the larger volume of the pentagonal prism relative to other 10-vertex closed polyhedra to accommodate the interstitial iron atom.

Chemical bonding in oblatonido ditantalaboranes and related compounds

The recently discovered ditantalaboranes Cp2Ta2B n H n+6 (n = 4, 5) are isoelectronic with the previously discovered dimetallaboranes Cp2M2B n H n+4 of the group 6 metals Cr, Mo, and W where Cp = η5-cyclopentadienyl or substituted cyclopentadienyl. Their oblatonido polyhedral structures can be derived from the oblate (flattened) deltahedra of the oblatocloso dirhenaboranes Cp2Re2B n+1H n+1 by removal of an equatorial BH vertex with adjustment of the skeletal electron count by changing the metal atoms and adding hydrogen atoms. In these oblatocloso dirhenaborane deltahedra, the approximately antipodal rhenium atoms are close enough together to form a formal Re=Re double bond with lengths in the range 2.69–2.82 A. Similarly, short M=M distances are maintained in the related oblatonido derivatives Cp2Ta2B n H n+6 (n = 4, 5) and Cp2M2B n H n+4 (M=Cr, Mo, W). However, the synthesis of Cp2Ta2B n H n+6 (n = 4, 5) from CpTaCl4 + LiBH4/BH3 also gives a less-reduced product Cp2Ta2Cl2B5H11 with a longer Ta–Ta distance of ~3.2 A. This may be regarded as a formal single bond bridged by one of the hydrogen atoms. Vertices of degree 5 (excluding terminal atoms/groups but not edge-bridging hydrogens) are sites of highest stability/lowest chemical reactivity not only in metal-free boranes but also in the dimetallaboranes discussed in this paper. For example, all four boron vertices in Cp2Ta2B4H10 have the favorable degree or 5. The recently discovered ditantalaboranes Cp2Ta2B n H n+6 (n = 4, 5) are isoelectronic with the previously discovered dimetallaboranes Cp2M2B n H n+4 (M=Cr, Mo, W). They have oblatonido structures derived by removal of one BH vertex from the flattened oblatocloso polyhedra of the dirhenaboranes Cp2Re2B n H n . All of these structures have internal M=M formal double bonds inside the deltahedron.

Synthesis, structure and bonding of the hypoelectronic cluster compound Sr3Tl5

The Sr3Tl5 phase was prepared by high temperature synthesis techniques through reaction of the high purity elements in the welded Nb tubes. The structure established through X-ray structural analysis shows the compound is a good hypoelectronic trielide example of the Pu3Pd5 structural type in which skeletal electron count is lower than in a traditional Zintl phase (Cmcm, Z = 4; a = 10.604(2) A, b = 8.675(2) A, and c = 10.985(2) A; V = 1010.5(3) A3). The strontium size and the compound's polarity appear responsible for this thallium phase crystallization in the Pu3Pd5 family type rather than the isoelectronic Sr3In5 version. A first-principle electronic structure calculation (LMTO) demonstrates that the strontium atoms participate substantially in Sr-Tl bonding in the structure.

ChemInform Abstract: Homoatomic Polyanions of Post-Transition Elements. Synthesis, Structure, and Characterization of the Paramagnetic Nonagermanide (3-) Ion Ge3- 9, a C2v Cluster with an Odd Skeletal Electron Count.

The salt K3(2,2,2-cryptand)3(PPh3)3Ge9 has been obtained by complexation of the KGe1.8 alloy by 2,2,2-cryptand in ethylenediamine with solute triphenylphosphine. The structure of the anion has been determined crystallographically and the results are supported by EPR and extended Huckel calculations.

Stuffed fullerenelike boron carbide nanoclusters

Viable stuffed fullerenelike boron carbide nanoclusters, C50B34, C48B36−2, and their isomers based on an icosahedral B84 fragment of elemental β-rhombohedral boron have been investigated using density functional theory calculations. The structure and the stability of these clusters are rationalized using the polyhedral skeletal electron counting and ring-cap orbital overlap compatibility rules. The curvature of the fullerene was found to play a vital role in achieving the most stable isomer C50B34(3B). The large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps, three dimensional aromaticity, and electron detachment energies support their high stability. Further, the IR and Raman active modes were recognized.

Syntheses and structural characterizations ofcloso and nido 10-vertex metallatricarbanonaboranes

Deboronation of the 11-vertex metallatricarbadecaboranes 1-(η5-C5H5)-2-Ph-closo-1,2,3,4-M-C3B7H9 [M = Ru, Fe] and 1,1,1-(CO)3-2-Ph-closo-1,2,3,4-M-C3B7H9 [M = Re] with tetrabutylammonium fluoride (TBAF) produced the first examples of the 10-vertex families of closo and nido metallatricarbanonaborane complexes, 2-(η5-C5H5)-10-Ph-closo-2,1,6,10-M-C3B6H8 [M = Ru (1), Fe (2)] and 5,5,5-(CO)3-10-Ph-nido-5,6,9,10-Re-C3B6H9– (3). Crystallographic characterizations of 1 and 3 confirmed geometries, based on a bicapped square antiprism for 1 and a bicapped square antiprism with one missing vertex for 3, that are consistent with their respective 22 (closo) and 24 (nido) skeletal-electron counts.

Polyhedral Structures with Three‐, Four‐, and Five Fold Symmetry in Metal‐Centered Ten‐Vertex Germanium Clusters

Studies using density functional theory (DFT) at the hybrid B3LYP level indicate that the relative energies of structures with three-fold, four-fold, and five-fold symmetry for centered 10-vertex bare germanium clusters of the general type M@Ge(10) (z) depend on the central metal atom M and the skeletal electron count. For M@Ge(10) clusters with 20 skeletal electrons the DFT results agree with experimental data on the isoelectronic centered 10-vertex bare metal clusters. Thus the lowest energy structure for Ni@Ge(10), isoelectronic with the known Ni@In(10) (10-), is a C(3v) polyhedron derived from the tetracapped trigonal prism. However, Zn@Ge(10) (2+) is isoelectronic with the known cluster Zn@In(10) (8-), which has the lowest energy structure, a D(4d) bicapped square antiprism. For the clusters Ni@Ge(10) (2-), Cu@Ge(10) (-), and Zn@Ge(10) that have 22 skeletal electrons the lowest energy structures are the D(4d) bicapped square antiprism predicted by the Wade-Mingos rules. For the clusters Ni@Ge(10) (4-), Cu@Ge(10) (3-), and Zn@Ge(10) (2-) that have 24 skeletal electrons the lowest energy structures are C(3v) polyhedra with 10 triangular faces and 3 quadrilateral faces derived from a tetracapped trigonal prism by extreme lengthening of the edges of the capped triangular face of the underlying trigonal prism. For the clusters Cu@Ge(10) (5-) and Zn@Ge(10) (4-) that have 26 skeletal electrons the lowest energy structures are the D(5d) pentagonal antiprisms predicted by the Wade-Mingos rules and the C(3v) tetracapped trigonal prism as a somewhat higher energy structure. However, for the isoelectronic Ni@Ge(10) (6-) the relative energies of these two structure types are reversed so that the C(3v) tetracapped trigonal prism becomes the global minimum. The effects of electron count on the geometries of the D(5d) pentagonal prism and D(4d) bicapped square antiprism centered metal cluster structures are consistent with the bonding/antibonding characteristics of the corresponding HOMO and LUMO frontier molecular orbitals.

Beyond the Wade-Mingos Rules in Bare 10- and 12-Vertex Germanium Clusters: Transition States for Symmetry Breaking Processes.

The lowest energy structures of bare Gen(z) clusters (n = 10, 12; z = -6, 0, +2, +4) obtained using density functional theory (DFT) at the hybrid B3LYP level often are relatively low-symmetry polyhedra not readily recognizable by the Wade-Mingos rules. However, such optimized structures may arise from higher symmetry transition states through symmetry breaking processes. Thus the lowest energy structures for the Ge10(6)(-) and Ge12(6)(-) clusters with hyperelectronic arachno 2n + 6 skeletal electron counts are derived from pentagonal and hexagonal prism transition states, respectively, and retain the pentagonal and hexagonal faces of the prisms upon symmetry-breaking optimization. In addition, a variety of capped cube, prism, and antiprism transition states are found for the hypoelectronic Ge10(4+), Ge12, and Ge12(4+) clusters, which go to low-energy low-symmetry optimized structures, typically Cs or Ci, upon following the normal modes of the imaginary vibrational frequencies.

Kinetic Stability of Gold Fullerenes and the 2(N+1)2 Rule

Abstract Gold fullerenes conform to the 2(N+1)2 skeletal electron counting rule, namely, they attain the maximum degree of kinetic stability when they have 2(N+1)2 mobile electrons. Icosahedral Au32 (N=3) is predicted to possess the highest kinetic stability with the largest minimum bond resonance energy. Thus, gold fullerenes are much less sensitive to local ring structure than carbon fullerenes. In such a situation, the 2(N+1)2 rule works as a rule of kinetic stability rather than as that of spherical aromaticity.

Synthesis and Crystal Structure of a 13-Vertex Carborane Radical Anion with 2n + 3 Framework Electrons

Boron clusters with 2n + 3 framework electrons are long-sought species. This work reports the synthesis and structural characterization of a 13-vertex carborane radical anion with a 2n + 3 skeletal electron count, [·{1,2-(CH2)3-1,2-C2B11H11}][Na(18-crown-6)(THF)2]. It was isolated as brown crystals by treatment of 1,2-(CH2)3-1,2-C2B11H11 with 1 equiv of finely cut Na metal in THF, followed by recrystallization from a THF/hexane solution of 18-crown-6.