Crystal structure of the formal 20 electron zirconocene pentafulvene complex Cp2Zr(η5,η1-adamantylidenepentafulvene):toluene:n-hexane = 1:0.125:0.125

The molecular and crystal structure of a formal 20 electron zirconium(IV) complex bearing two cyclopentadienyl and one sterically demanding pentafulvene ligand is reported in which the pentafulvene is bound in an η5:η1 manner. The complex crystallizes together with toluene and n-hexane in a ratio of 1:0.125:0.125.

The crystal structure of a solvated zirconocene pentafulvene complex with a bulky adamantylidene substitution pattern, namely ( 5 , 1 -adamantylidenepentafulvene)bis( 5 -cyclopentadienyl)zirconium(IV)-toluene-n-hexane (8/1/ 1), [Zr(C 15 H 18 )(C 5 H 5 ) 2 ]Á0.125C 7 H 8 Á0.125C 6 H 14 , is reported. Reducing zirconocene dichloride with magnesium results in the formation of a low-valent zirconocene reagent that reacts readily with adamantylidenepentafulvene to give the aforementioned complex. Single crystal X-ray diffraction proves the dianion-like 5 : 1 binding mode of the fulvene ligand to the central Zr IV atom. The asymmetric unit contains four independent molecules of [ 5 : 1 -adamantylidenepentafulvene]bis[( 5 )-cyclopentadienyl]zirconium(IV), together with half a molecule of toluene disordered with half a molecule of n-hexane (the solvent molecules have no direct influence on the complex). In each of the four complex molecules, the central Zr IV atom has a distorted tetrahedral coordination environment. The measured crystal consisted of two domains with a refined ratio of 0.77:0.23.

Chemical context
Over the last few decades, pentafulvenes have found plenty of applications in organometallic chemistry (Preethalayam et al., 2017;Neuenschwander, 1989), one of which is their use as versatile ligands for a variety of early and late transition metals featuring a multitude of coordination modes and reactivity patterns (Preethalayam et al., 2017;Kreindlin & Rybinskaya, 2004). Whereas for late transition metals 2 -and 4 -bindng modes are known (Kim et al., 2000;Rais & Bergman, 2004), most metals are bound in an 6 -manner, either in a neutral olefinic 2 : 2 : 2 (Konietzny et al., 2010) or in a dianionic 5 : 1 fashion (Ebert et al., 2014). The change of polarity at the exocyclic carbon atom of the pentafulvene ligand, resulting from its bonding to the central metal atom, enables a multitude of insertion reactions and C-H-activation reactions that are of great interest to our research group (Ebert et al., 2014;Manssen et al., 2015Manssen et al., , 2017Oswald et al., 2016) . In this context we have recently reported the syntheses of the first zirconocene-based pentafulvene complexes and their reactivities (Jaroschik et al., 2017). Here we report the synthesis and crystal structure of the solvated title compound, ( 5 , 1 -adamantylidenepentafulvene)bis( 5 -cyclopentadienyl)zirconium(IV), 1. ISSN 2056-9890

Structural commentary
Compound 1 crystallizes in the triclinic space group P1 with four formula units per asymmetric unit together with one disordered solvent molecule (ratio toluene:n-hexane = 1:1). Fig. 1 shows one of the complex molecules present in the crystal of 1. As a result of the high similarities with respect to structural parameters (bond lengths and angles) of the four complexes in the asymmetric unit, only this complex (Zr1) is discussed in detail. The molecular structure shows the zirconium(IV) atom to be in a distorted tetrahedral coordination environment. The zirconium atom lies 0.21 Å above the plane defined by the three centroids of the pentafulvene and cyclopentadienyl ligands, which is in good agreement with related complexes, e.g. 0.20 Å for the analogous complex with a 6,6 0 -di-para-tolylfulvene substitution pattern (Jaroschik et al., 2017) and 0.20 Å for Cp 3 ZrH (Edelbach et al., 1999). The molecular structure of 1 in the solid state clearly confirms the -5 :-1 binding mode of the fulvene moiety to the central metal atom. Characteristic parameters for this coordination mode are the deviation (bend angle ) of the C exo -C ipso bond toward the central zirconium(IV) atom (29.4 ) as well as the ring slippage (Á) toward the C ipso atom of the five-membered ring of the pentafulvene ligand (0.318 Å ). The bond between the zirconium(IV) atom and the exocyclic carbon atom [Zr1-C16 = 2.605 (3) Å ] is considerably longer than those of other zirconium complexes [Kraft et al., 2002 (2.37 Å );Novarino et al., 2011 (2.37 Å )], indicating a weak Zr-C exo contact, but in good agreement with [-5 :-1 -C 5 H 4 C(para-tolyl) 2 ]-Zr(THF) (2.70 and 2.71 Å ) reported previously by our group (Ebert et al., 2014). Regarding the fulvene moiety, the coordination to the zirconocene fragment leads to the loss of the alternating single-and double-bond pattern of free pentafulvene. This is indicated by the narrow range of the C-C bond lengths within the five-membered ring of the fulvene ligand [1.406 (4) to 1.437 (4) Å ] in comparison with free fulvene [1.327 (3) to 1.459 (2) Å ] (Garcia et al., 1989). Hence, the hybridization of the exocyclic carbon atom lies between sp 2 and sp 3 , which is further confirmed by the sum of angles around the C16 carbon atom [C11-C16-C17 = 116.9 (2) , C17-C16-C21 = 109.4 (2) , C11-C16-C21 = 118.7 (3) = 345 ].

Supramolecular features
No significant supramolecular features between the complex molecules or between the complex molecules and the solvent molecules are observed. Hence the intermolecular forces appear to be dominated by van der Waals interactions only. In the crystal structure of 1, the solvent molecules are located in the voids resulting from the packing arrangements of the complex molecules. Fig. 2 shows the packing without solvent molecules and Fig. 3 the packing with the contribution of the solvents.

Synthesis and crystallization
All reactions were carried out under a dry nitrogen atmosphere using Schlenk techniques or in a glove box. Zirconocene dichloride was purchased from Strem Chemicals and used as received. Adamantylidenepentafulvene was prepared according to a published procedure (Miller & Bercaw, 2006). Solvents were dried according to standard procedures over Na/K alloy with benzophenone as indicator and distilled under a nitrogen atmosphere.
Zirconocene dichloride (1.000 g, 3.421 mmol), magnesium (0.083 g, 3.421 mmol) and adamantylidenefulvene (0.884 g, 3.421 mmol) were added to a Schlenk tube under argon. THF (40 ml) was added, and the reaction was stirred for 16 h at room temperature. THF was evaporated under vacuum and 40 ml of toluene were added to the crude product. After filtration, toluene was evaporated under vacuum to give 1 as a yellow solid in 81% yield.
Crystals suitable for single crystal X-ray diffraction were obtained from a saturated solution of 1 in toluene, layered with n-hexane at room temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1.
The measured crystal consisted of two domains. TWINABS was therefore used to model the absorption correction and to generate a reflection file in the HKLF5 format. The refined ratio of the two domains was 0.77:0.23. Hydrogen atoms bonded to the carbon atoms were located from difference-Fourier maps but were subsequently fixed to idealized positions using appropriate riding models with U iso (H) = 1.2U eq (C). Reflections (001) and (001) were obstructed from the primary beam stop and consequently omitted from the refinement. The solvent molecules toluene and n-hexane were located from difference maps and refined with RIGU commands, with site occupancies fixed to 0.50 each. Computer programs: APEX2 and SAINT (Bruker, 2013), SHELXS2013 (Sheldrick, 2015a), SHELXL2014 (Sheldrick, 2015b), DIAMOND (Brandenburg & Putz, 2006) and publCIF (Westrip, 2010).

Figure 3
A view along the a axis showing the packing of molecules in the asymmetric unit. Color code: C grey, H white, Zr plum spheres. Solvent molecules are highlighted in black.

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.