Crystal structures of two ytterbium(III) complexes comprising alkynylamidinate ligands

Two ytterbium(III) complexes comprising alkynylamidinate ligands, Cp2Yb[(iPr2N)2C—C≡C—c-C3H5] (1) and Yb[(CyN)2C—C≡C—Ph]3 (Cy = cyclohexyl) (2) have been synthesized and structurally characterized. Both complexes are monomers without any coordinated solvent in the solid state.


Chemical context
Anionic amidinate ligands of the type [RC(NR 0 ) 2 ] À (R = H, alkyl, aryl; R 0 = alkyl, cycloalkyl, aryl, SiMe 3 ) are highly useful and versatile spectator ligands in organolanthanide chemistry. These readily available N-chelating ligands are generally regarded as sterically demanding cyclopentadienyl equivalents (Collins, 2011;Edelmann, 2013). Mono-, di-and trisubstituted lanthanide amidinate complexes are all accessible, in close analogy to the long known mono-, di-and tricyclopentadienyl complexes. Over the past ca 25 years, lanthanide amidinates have witnessed an impressive transformation from laboratory curiosities to homogeneous catalysts as well as valuable precursors in materials science. Rare-earth metal amidinates have been reported to be highly active homogeneous catalysts e.g. for ring-opening polymerization reactions of lactones, the guanylation of amines or the addition of terminal alkynes to carbodiimides (Edelmann, 2009(Edelmann, , 2012. In materials science, certain homoleptic alkyl-substituted lanthanide tris-(amidinate) complexes are highly volatile and can be used as precursors for ALD (atomic layer deposition) and MOCVD (metal-organic chemical vapor deposition) processes, e.g. for the deposition of lanthanide oxide (Ln 2 O 3 ) or lanthanide nitride (LnN) thin films (Devi, 2013).
We recently initiated a study of alkynylamidinates derived from cyclopropylacetylene (Sroor et al., 2015c). The cyclopropyl group was chosen because of the well-known electrondonating ability of this substituent to an adjacent electrondeficient atom or group. This would give us the rare chance to electronically influence the amidinate ligand system rather than altering only its steric demand. We now describe the synthesis and structural characterization of two new ytterbium(III) alkynylamidinate complexes, namely Cp 2 Yb[( i PrN) 2 C-C C-c-C 3 H 5 ] (1) and Yb[(CyN) 2 C-C C-Ph] 3 (Cy = cyclohexyl; 2), shown in Figs. 1 and 2.

Structural commentary
The structural analyses revealed that both title compounds are monomeric in the solid state, with the alkynylamidinate anion acting as an N,N 0 -chelating ligand. Compound 1 crystallizes in the orthorhombic space group Pbca with one complex molecule in the asymmetric unit. The two cyclopentadienyl ligands feature a typical symmetric 5 -coordination with Yb-centroid(Cp) distances of 2.315 and 2.321 Å . The Yb-Cp distances are therefore slightly larger than in the related chloride [Cp 2 YbCl] 2 [Yb-centroid(Cp) 2.300 and 2.307 Å ; Lamberts et al., 1987;Lueken et al., 1987Lueken et al., , 1989, possibly due to the steric demand of the two N-isopropyl groups close to the ytterbium atom. Probably for the same reason, the product does not contain coordinating THF even though the complex was prepared in THF solution. Accordingly the coordination geometry around Yb can be described as distorted pseudotetrahedral. At 131.1 , the Cp-Yb-Cp angle is close to that observed in [Cp 2 YbCl] 2 (Cp-Yb-Cp 130.0 ; Lamberts et al., 1987;Lueken et al., 1987Lueken et al., , 1989) and compound 1 is therefore a typical bent metallocene complex of trivalent ytterbium. Due to the low formal coordination number of four around the Yb atom, the Yb-N bond lengths of 2.274 (2) Sroor et al., 2016].
Compound 2 crystallizes in the trigonal space group R3c, with the Yb atom located on a threefold rotation axis along the crystallographic c axis. The complex molecule is therefore C3 symmetric. The Yb atom is coordinated by the three symmetry-equivalent chelating amidinate ligands in a distorted octahedral fashion with C1-Yb-C1 0 angles of 120 and an angle of 90AE3 between the YbN 2 C planes. The cyclohexyl group attached to N2 is disordered over two The molecular structure of compound 2. Displacement ellipsoids are at the 50% probability level and H atoms have been omitted for clarity. Only one orientation of the disordered cyclohexyl group at N2 is shown. The Yb atom is located on a threefold rotation axis parallel to the crystallographic c axis. [Symmetry operators to generate equivalent atoms: ( 0 ) 1 À y, À1 + x À y, z; ( 00 ) 2 À x + y, 1 À x, z.]

Figure 1
The molecular structure of compound 1. orientations by rotation around the N2-C16 vector. As a result of the higher coordination number, the Yb-N bonds [2.310 (2) and 2.320 (2) Å ] are slightly longer than in compound 1. However, in consequence of the small size of the Yb 3+ ion, the Yb-N bonds in compound 2 are significantly shorter than in corresponding hexacoordinated lanthanide(III) amidinates, e.g.

Supramolecular features
Compounds 1 and 2 do not exhibit any specific intermolecular interactions. In compound 1, the closest intermolecular C-C contacts are found between Cp ligands and cyclopropyl substituents, 3.510-3.625 Å . Compound 2 features one intermolecular phenyl-cyclohexyl contact where the shortest C-C distance is 3.567 Å , and various cyclohexyl-cyclohexyl contacts with C-C distances of 3.441-3.576 Å . The crystal structure of compound 2 comprises a large void of ca 220 Å 3 that is probably filled with a highly disordered toluene molecule. The content of the voids was corrected for using the SQUEEZE method (Spek, 2015), yielding a solvent-accessible volume of 1316 Å 3 and 138 electrons, or about 1.5 solvate molecules per unit cell. The composition of the crystal can therefore be assumed to be 2Á0.166 toluene.

Synthesis and crystallization
Synthesis of Cp 2 Yb[( i Pr 2 N) 2 C-C C-c-C 3 H 5 ] (1) This compound was prepared by treatment of Cp 2 YbCl (Maginn et al., 1963) with Li[( i Pr 2 N) 2 C-C C-c-C 3 H 5 ] (Sroor et al., 2013) in a molar ratio of 1:1. Treatment of Cp 2 YbCl (0.68 g, 2.0 mmol) with Li[( i Pr 2 N) 2 C-C Cc-C 3 H 5 ] (2.0 mmol, prepared in situ from Li-C C-c-C 3 H 5 and N,N 0 -diisopropylcarbodiimide) in 30 ml of THF produced a bright-orange solution and a white precipitate (LiCl). After filtration and evaporation to dryness, the product was extracted with n-pentane (2 Â 20 ml). The extract was filtered again and concentrated to a total volume of ca 10 ml. Crystallization at 253 K afforded 1 as orange air-and moisturesensitive crystals. Yield: 0.53 g, 73%. M.p.: 478 K. Analysis Anhydrous ytterbium(III) trichloride (1.40 g, 5.0 mmol) (Freeman & Smith, 1958) was suspended in THF (50 ml) and treated with a solution of Li[Ph-C C-C(NCy) 2 ] (4.72 g, 15.0 mmol) (prepared in situ by addition of lithium phenylacetylide to N,N 0 -dicyclohexylcarbodiimide) in THF (60 ml). The reaction mixture was refluxed for 3 h. After cooling to room temperature, the white precipitate (LiCl) was removed by filtration, and the clear filtrate was evaporated to dryness. Off-white air-and moisture-sensitive solid. Yield: 3.07 g, 56%. M.p.: 505 K. Single crystal suitable for X-ray structure determination were obtained from a saturated toluene solution at

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. In the case of compound 2, C atoms C17-C21 of the disordered cyclohexyl substituent have been split over two sites, with a freely refined occupancy ratio. The N-bonded C atom C16 was refined as not disordered using EXYZ and EADP commands but the different orientation of the corresponding H atom H17 was taken into account. The contribution to the scattering from the solvent molecule in compound 2 was removed with the SQUEEZE routine (Spek, 2015) in PLATON (Spek, 2009), yielding a solvent accessible volume of 1316 Å 3 and 138 electrons. H atoms were fixed geometrically and refined using a riding model with U(H) = 1.20U eq (C). Computer programs: X-AREA and X-RED (Stoe & Cie, 2002), SHELXS2013 (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015) and DIAMOND (Brandenburg, 1999 program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015).

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.