Research Article | Open Access
The Use of a Flexible Calixarene Template to Stabilize a Cyclooctatetraindiyl Samarium-Potassium Complex
A sandwich compound of cyclooctatetraendiyl (COT)2− samarium-potassium was synthesized and analyzed using a flexible calixarene dianion. This compound, [p-tBu-calix-(OMe)2(O)2]arenediyl-samarium-(η8-cyclooctatetraendiyl)-potassium (tetrahydrofurane)3, is constructed as a linear sequence L-Sm--K-, where L, , and are specific ligands with L = O,O-dimethyl-calixarene2−, = cyclo-octatetraendiyl, and = tetrahydrofurane templates.
For a long time, the organometallic chemistry of lanthanides (Ln)  has been dominated by the use of polycyclic hydrocarbons including the cyclopentadienyl anion or the cyclooctatetraendiyl dianion, as exemplified by the well-known lanthanocene  and complexes . In the last two decades, the organometallic and redox chemistry of these hard metal ions has been extensively developed by the introduction of amido, thiolato, and aryloxo ligands as well as macrocyclic ligands, such as octa-alkyl-porphyrinogens and calixarenes [4, 5], and a number of such examples of complexes have recently been published [6–10]. Calixarenes are useful building blocks in supramolecular chemistry for complexing metallic cations ; the use of calixarene anions  brings significant peculiarities: the electron-rich environment provided by the oxygen donor atoms forms an ideal platform for developing the organometallic and redox chemistry of lanthanides, the quasi-planar arrangement of the four oxygen atoms resulting from the cone conformation of the calix allows the formation of “half-sandwich”-type metallic complexes, and it can accommodate up to 12 electrons to the metal centre depending on the degree of -donation. Therefore, it came out that an interesting analogy can be drawn between the quasi-planar calixarene polyanion and its electronically similar cyclooctatetraendiyl dianion  in the attempt to develop new scaffoldings in organometallic chemistry of Lanthanides. In the present study we report on the synthesis and crystallographic analysis of the mixed cyclooctatetraendiyl samarium-potassium complex: arenediyl samarium-(8-cyclo-octatetraendiyl) potassium-, stabilized by a flexible calixarene dianion template, a compound that widens the ability of unsymmetrical calixarene as a useful and adaptable ligand in organometallic chemistry.
A solution of cyclooctatetraendiide dipotassium in THF  (8.8 mL, 0.275 M, 2.42 mmol) was added dropwise to a suspension of 2 (4.6 g, 2.47 mmol)  in THF (150 mL) at −30°C. The reaction mixture was stirred overnight while slowly warming to room temperature, leading to a suspension of a white solid in a red solution. The solid was filtered off, the solution was concentrated to ca. 30 mL, and subsequently kept at 3°C. The orange precipitate was collected and dried in vacuo. Yield: 2.1 g (34%) of 3.
Elemental analysis calcd (%) for C70H98KO8Sm (1257.04): C 66.88, H 7.86; found: C 66.49, H 7.60. Crystals suitable for Xray crystallography were grown at 3°C from a THF solution. Crystal data: Triclinic, with parameters ; ; ; ; ; , and . Crystals were very sensitive to air and moisture. They were fished out under neutral atmosphere or under oil using a nylon loop and immediately cryocooled in liquid nitrogen.
Diffraction data were collected at K on a Mar345 Image Plate detector () using the Mo K wavelength in the rotation method (180 steps of one degree per frame). Diffraction data were processed with the HKL package  (22696 recorded reflections; redundancy ~ 2; = 0.028; completeness 92%). No absorption correction was applied. The final data set was reformatted for structure determination and refinement using the SHELXS/L programs .
The structure was solved in centrosymmetric P-1 space group by direct methods. During refinements, a solvated THF molecule was located highly disordered in the packing with high temperature factors. Hydrogen atoms were kept at their theoretical places and refined as riding atoms to their bonded carbon. The four terbutyls of the calixarene ligand known to still rotate at low temperature were refined as restrained groups. The final was 0.0514 (10196 observed data with ) and (calculated with the 11873 unique squared reflections having ). The coordinates were deposited with the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, ref. 880827.
The starting materials we devised in our previous work on lanthanides chemistry are displayed in Scheme 1. The use of the bis-O-methylated calixarene allows tuning of the macrocyclic charge  to a value appropriate for entering the chemistry of lanthanides. The reaction of LnCl3(THF)2 with the lithium or sodium salt of  led to the formation of dimeric complexes 2 in which the metal ions are bridged by two chloride anions. Under dinitrogen, the reduction of these lanthanide complexes (Ln = Pr, Sm) by sodium naphthalenide has been reported to carry out the four-electron reduction of dinitrogen to a moiety supported by a trinuclear complex and exhibiting a very high degree of d(N–N) bond elongation . The analogy between and dianions led us to investigate the exchange reaction between and C8H8K2. This gave the mixed sandwich compound 3.
The content of the asymmetric unit is shown in Figure 1. The samarium-potassium complex adopts a linear structure with a cylindrical shape of 15 Å in length and forms a sandwich scaffold with the samarium and potassium cations interspersed with the three different ligands, the calixarene dianion on one side of the samarium (ligand L), the cyclooctatetraendiyl dianion (ligand L′) in between samarium and potassium, and three coordinating tetrahydrofuranes (THF) on the other side of the potassium (ligand L′′).
Ligand L. On one side, a dissymmetric calixarene is used with alternate protected (methoxy) and free (oxygen anion) polar atoms. It interacts with the samarium cation at short distance with only two oxygens, forcing the skeleton to assume a very constrained elliptical conformation with the Sm1-O and Sm1-O distances, particularly long ( Å and Å) compared to the two others, Sm1-O and Sm1-O ( Å and Å). Table 1 reports the macrocycle ring eccentricity compared with two other nonsymmetric calixarene compounds, the parent dimer (L-SmCl)2 2, and the corresponding (L3Sm2N2) complex 4 (Scheme 1), all using the same flexible ligand. The calix ligand in the asymmetric unit of 3 has a much higher value than those observed in the structures of 2 and 4 . It is interesting to note that, in the dinitrogen complex 4, the three calixarene ligands adapt themselves to the central Sm2-N2 cluster, through their high plasticity. This is evidenced by the different values of each parameter (Table 1).
|Da, Db, and are the long and short distances and eccentricity of the ellipse which encompasses the four para aromatic carbons of the ligand.|
-The Central Ligand. The cyclooctatetraindiyl dianion adopts a perfect planar configuration due to its aromaticity and supports a full coordination with both the potassium and samarium cations located on opposite sides. Within the dianion, the average bond distance is with s.d. ~ 0.03 Å, while the mean angle is around the ideal value of 3/4 with s.d. ~1.0°. These values correspond to a nearly perfect, unconstrained octagon. The two samarium and potassium cations at 4.552 Å from each other are perpendicular to the center of the ring with individual distances = 2.09 and 2.46 Å from the COT plane, respectively. These distances are relevant to a similar partition of charges on both sides according to the respective ionic radii of and (~ 1.1 and 1.33 Å).
-Potassium Template. The potassium ion completes on the other side its coordination sphere with three molecules of THF.
Solvation. In addition to the title molecule in the asymmetric unit, there is an additional solvated THF molecule (not shown in Figure 1) with very large thermal parameters. This molecule lies in the vicinity of the L′′ ligand.
The present structure of a mixed rare earth-potassium complex of cyclooctatetraindiyl dianion, together with the two other crystal structures of 2 and 4, illustrates and generalizes how the plasticity of a calixarene dianion with alternate MeO/O-polar groups can be used in the building of stable scaffolds in reactive organometallic systems of lanthanides.
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