Crystal structure of 1,1′,2,2′,4,4′-hexaisopropylmagnesocene

The title compound was synthesized from the corresponding triisopropylcyclopentadiene by treatment with n-butyl-sec-butylmagnesium and crystallizes in the triclinic space group P with half a molecule per asymmetric unit and a staggered arrangement of the cyclopentadienide ligands.

Each 3 Cp 2 Mg moiety has eight neighboring molecules within the bc and ac planes ( Fig. 4a and 4b), but only six neighboring molecules within the ab plane, forming an almost hexagonal layer ( = 63.00 ), but with the layers being congruent to each other (Fig. 4c).

Database survey
A search in the Cambridge Structural Database (CSD, Version 5.42, update of September 2021; Groom et al., 2016) showed that 14 crystal structures of magnesocenes of the type (C 5 R 5 ) 2 Mg had previously been reported. In this search, any type of donor complexes of magnesocenes of the form (C 5 R 5 ) 2 MgÁD n are not counted. The MgÁ Á ÁCp centroid bonding distances in these structures lie between 1.9562 (1) and 2.0628 (11) Å and the dihedral angles between the Cp planes are between 0 (co-planar geometry) and 17.892 . Thus, the bond distances and angles in the title compound are within normal ranges of known magnesocenes. Asymmetric unit of the title compound (displacement ellipsoids are drawn at the 50% probability level).   (a) Supramolecular coordination geometry of the title compound in the crystal and (b) the corresponding polyhedron. Symmetry codes: (i) À1 + x, y, z; (ii) x, À1 + y, z; (iii) 1 + x, À1 + y, z; (iv) x, y, 1 + z. H atoms and isopropyl groups are omitted for clarity.

Figure 4
Arrangement of the layers of the title compound along the crystallographic a, b and c axes (H atoms and isopropyl groups omitted for clarity).
To a solution of 4.00 g (20.8 mmol) of an isomeric mixture of triisopropylcyclopentadiene in 100 mL of hexane were added 15.0 mL of a 0.7 M solution of n-butyl-sec-butylmagnesium in hexane (10.5 mmol). The light-yellow reaction solution was stirred at 333 K overnight. Subsequently, all volatiles were removed in vacuo and a yellow oil was obtained, from which the title compound crystallized over the course of one day at ambient temperature. The crystallized material was washed with small portions of hexane and dried in vacuo to obtain the title compound as a pure, colorless, crystalline solid in 43% yield (1.83 g; 4.50 mmol).
In addition to a structural characterization by single-crystal X-ray diffraction, the title compound was also characterized by 1 H and 13 C NMR spectroscopy: 1 H NMR (400 MHz

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. All non H-atoms were located in the electron density maps and refined anisotropically. Cbound H atoms were placed in positions of optimized geometry and treated as riding atoms: C-H = 1.00 Å (CH), 0.98 Å (CH 3 ), and with U iso (H) = kU eq (C), where k = 1.2 for CH and 1.5 for CH 3 .

Figure 5
Reaction scheme for the formation of the title compound 3 Cp 2 Mg. program(s) used to solve structure: SHELXT2018/2 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b), shelXle (Hübschle et al., 2011); software used to prepare material for publication: publCIF (Westrip, 2010). 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.