Crystal structure of the [(THF)Cs(μ-η5:η5-Cp′)3Yb]n oligomer

Reduction of (C5H4SiMe3)3YbIII in THF using excess Cs metal forms the oligomeric complex [(THF)Cs(μ-η5:η5-Cp′)3YbII]n. The complex has hexagonal layers of Cs3Yb3 with THF ligands and Me3Si groups in between the layers.


Chemical context
The new +2 oxidation states for the rare-earth metals Y, La, Ce, Pr, Gd, Tb, Ho, Er, and Lu were recently discovered by reduction of Cp x 3 Ln (Cp x = C 5 H 4 SiMe 3 , C 5 H 3 (SiMe 3 ) 2 ; Ln = rare-earth metal) using alkali metal reductants Li, Na, K, and KC 8 (Fig. 1) (Hitchcock et al., 2008;MacDonald et al., 2013;Fieser et al., 2015;Evans, 2016;Palumbo et al., 2018). In each of these cases, 2.2.2-cryptand was added in these reactions to encapsulate the alkali metal. It was thought that chelating agents were necessary to sequester the alkali metal to prevent interactions with cyclopentadienide ligands and subsequent ligand dissociation leading to product decomposition. This idea was challenged by examining reduction reactions of Cp 00 3 M (Cp 00 = C 5 H 3 (SiMe 3 ) 2 ; M = La, Ce, U) with Li and Cs in the absence of chelating agents (Huh et al., 2018). The reaction resulted in the isolation of the first chelate-free synthesis of La II , Ce II , and U II complexes. The [Li(THF) 4 ] 1+ cation of the Synthesis of (Cp x 3 Ln II ) 1À complexes by alkali metal reduction of Cp x 3 Ln III precursors; Cp x = C 5 H 4 SiMe 3 , C 5 H 3 (SiMe 3 ) 2 .
Li salts in these chelate-free M II complexes were wellseparated from the (Cp 00 3 M) 1À anion. However, the Cs reductions yielded polymeric complexes of general formula [Cp 00 M(-Cp 00 ) 2 Cs(THF) 2 ] n where the Cs cation has coordinated THF and cyclopentadienide ligands. Attempts to extend this chemistry to smaller rare-earth metals by reduction of Cp 0 3 Ln (Cp 0 = C 5 H 4 SiMe 3 ; Ln = Y, Tb, Dy) showed evidence of Ln II in solution; however, the reduction products were highly unstable and decomposed even at 238 K.
In this study, we were interested in examining the reduction of Cp 0 3 Yb III with Cs metal. Unlike Y II , Tb II , and Dy II ions, Yb II complexes are more easily obtainable, as reflected by their less negative reduction potentials (Morss, 1976). A crystal containing the oligomeric compound; [(THF)Cs(-5 : 5 -Cp 0 ) 3 Yb] n , 1 (Cp 0 = C 5 H 4 SiMe 3 ) was isolated by reduction of the Cp 0 3 Yb III complex (Fieser et al., 2015) in THF using Cs metal (Figs. 2 and 3).

Structural commentary
All three Cp 0 rings remain coordinated to the Yb metal center after reduction and are coordinated in a trigonal-planar fashion. The Yb atom is within 0.107 Å of the plane of the three ring centroids. Each ring bridges Yb to Cs, which also is surrounded by three cyclopentadienyl ligands as well as a coordinated molecule of THF. The three ring centroids and the oxygen of THF are arranged in a pseudo-tetrahedral geometry around Cs with a calculated four-coordinate Cs 0 4 value of 0.76 ( 0 4 = 1 for tetrahedral; 0 4 = 0 for square planar; Rosiak et al., 2018). The Cs metal center has a pseudo-tetrahedral geometry with Cp 0 (centroid)Á Á ÁCsÁ Á ÁCp 0 (centroid) angles of 109.0, 114.3, and 121.4 and Cp 0 (centroid)Á Á Á CsÁ Á ÁO(THF) angles of 88.8, 94.1, and 127.8 .

Supramolecular features
In 1, all of the cyclopentadienyl ligands are bridging. The threefold symmetry of three bridging Cp 0 ligands on each metal generates a hexagonal pattern as shown in Fig. 5. The YbÁ Á ÁCp 0 (centroid)Á Á ÁCs angles are 172.5-176.7 such that each side of the hexagon is nearly linear. The 112.4-117.3 YbÁ Á ÁCsÁ Á ÁYb angles are smaller than the 120.8-125.6 CsÁ Á ÁYbÁ Á ÁCs angles, which makes the hexagon slightly irregular. This could be of interest to quantum scientists trying to make thin-film layers of magnetic materials since the hexagonal pattern could lead to spin frustration with a paramagnetic lanthanide. The side view of these layers in Fig. 6 shows how the space in between them is filled with THF and Me 3 Si substituent groups. The 116.6-122.8 Cp 0 (centroid)Á Á ÁYbÁ Á ÁCp 0 (centroid) and 109.0-121.4 Cp 0 (centroid)Á Á ÁCsÁ Á ÁCp 0 (centroid) angles generate the undulation of the hexagons shown in Fig. 6.

Synthesis and crystallization
In an argon-filled glovebox, addition of a red solution of Cp 0 3 Yb (50 mg, 0.085 mmol) in THF (2 mL) to excess Cs as a smear produced a green solution. This was stirred for 15 min at room temperature and then layered at the bottom of a vial below an Et 2 O (10 mL) layer for crystallization at À35 C. After 1 d, X-ray quality dark-green crystals of [(THF)Cs(-5 : 5 -Cp 0 ) 3 Yb II ] n were isolated. A small number of crystals were obtained and used for crystallographic analysis. Too little sample was available for other characterization.

Poly[(tetrahydrofuran)tris[µ-η 5 :η 5 -1-(trimethylsilyl)cyclopentadienyl]caesium(I)ytterbium(II)]
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. A green crystal of approximate dimensions 0.079 x 0.086 x 0.148 mm was mounted in a cryoloop and transferred to a Bruker SMART APEX II diffractometer. The APEX2 program package was used to determine the unitcell parameters and for data collection (90 sec/frame scan time for a sphere of diffraction data). The raw frame data was processed using SAINT and SADABS to yield the reflection data file. Subsequent calculations were carried out using the SHELXTL program. The diffraction symmetry was 2/m and the systematic absences were consistent with the monoclinic space group P21/n that was later determined to be correct. The structure was solved by dual space methods and refined on F2 by full-matrix least-squares techniques. The analytical scattering factors for neutral atoms were used throughout the analysis. Hydrogen atoms were included using a riding model. The structure is polymeric. Least-squares analysis yielded wR2 = 0.0562 and Goof = 1.017 for 316 variables refined against 8223 data (0.75 Å), R1 = 0.0315 for those 6580 data with I > 2.0sigma(I).