Synthesis, crystal structure and Hirshfeld surface analysis of 1-ferrocenylundecane-1,11-diol

The molecular and crystal structure of a ferrocenyl derivative with an undecyl-1,11-diol chain on one cyclopentadienyl ring is reported: O—H⋯O, C—H⋯O and C—H⋯π(ring) contacts occur in the extended structure.


Chemical context
The title compound, 1, is a rare example of a ferrocene molecule substituted with an extended, in this instance 11-membered, alkane chain. It was synthesized to provide a ferrocenyl-substituted diol for the preparation of polyesters with regular pendant electroactive groups. Similar ferrocenyl neo-pentyl diol-derived terephthalate polymers have been shown to display interesting electrochemical properties (McAdam et al., 2008a,b). Friedel-Crafts methodology (Saji et al., 1991) provided the 1-ferrocenyl-undec-10-en-1-one precursor. This was reduced to the racemic alcohol 1-ferrocenyl-undec-10-en-1-ol (2) using LiAlH 4 . Enantiomeric selection of the individual chiral forms should be possible using more complex synthetic methodology (Ursini et al., 2006;Schwink et al., 1998), but was deemed unnecessary for our purposes. Hydroboration of ferrocenylalkenes has been previously reported (Lo Sterzo et al., 1984) using borane generated in situ from NaBH 4 /BF 3 ÁOEt 2 . Predictably, this method was unsuitable as a means of preparing 1 from 2, the ferrocenylmethanol moiety being susceptible to attack by BF 3 , and the resultant loss of OH À abetted by the formation of the stable -ferrocenyl carbenium ion. This prediction was borne out by experiment, the Lewis acid attack resulting in synthesis of 1-ferrocenyl-undec-10-ene and 1-ferrocenyl-undec-11-ol. Instead, a successful synthesis of 1 was achieved using hydroboration of 2 with 9-BBN. ISSN 2056-9890

Structural commentary
The title compound, [Fe(C 5 H 5 )(C 16 H 27 O 2 )], comprises a ferrocene unit that carries a well-ordered undecane chain (atoms C11-C21) with hydroxyl substituents at the 1 and 11 positions along the chain (Fig. 1). The C13-C12-C11-O11 and C19-C20-C21-O21 torsion angles are 60.9 (3) and 173.9 (2) , respectively. Atom C11 is a stereogenic centre: in the arbitrarily chosen asymmetric molecule it has an R configuration, but crystal symmetry generates a racemic mixture. The alkane chain is almost planar with the r.m.s. deviation from the best fit plane through all 11 C atoms being 0.129 Å . This plane is nearly orthogonal to the substituted ferrocene ring with an angle of 84.22 (13) between them. The C 11 undecyl chain in 1 is conformationally extended with the typical antiperiplanar (Kane & Hersh, 2000) arrangement for C n -C n+3 groupings and a C11Á Á ÁC21 separation of 12.627 (4) Å . The C1-C5 and C6-C10 cyclopentadienyl rings of the ferrocenyl group are approximately 3 from being eclipsed and are almost coplanar with a dihedral angle of 1.7 (2) between them; the separation of the ring centroids is is 3.298 (2) Å .

Figure 2
Inversion dimers of 1 in the ab plane with O-HÁ Á ÁO hydrogen bonds shown as blue lines.   Double chains of molecules of 1 along c. Cg2 is the centroid of the C6-C10 cyclopentadienyl ring, shown here as red spheres, with the C-HÁ Á Á contacts drawn as dashed red lines.
where Cg2 is the centroid of the C6-C10 cyclopentadienyl ring. Overall these various contacts combine to stack the molecules of 1 along the c-axis direction in two discrete, parallel and well-separated columns (Fig. 5).

Hirshfeld surface analysis
Further details of the intermolecular interactions in 1 were obtained using Hirshfeld surface analysis (Spackman & Jayatilaka, 2009) with Hirshfeld surfaces and two-dimensional fingerprint plots generated with Crystal Explorer (Turner et al., 2017). Hirshfeld surfaces for opposite faces of 1 are shown in Fig. 6(a) and (b). Bold red areas on the Hirshfeld surfaces correspond to the classical O-HÁ Á ÁO hydrogen bonds while the weaker C-HÁ Á ÁO and C-HÁ Á Á contacts appear as faint red circles. Fingerprint plots (Fig. 7) reveal that HÁ Á ÁH interactions dominate the surface contacts, as would be expected for a molecule with such a predominance of H atoms, with HÁ Á ÁC/CÁ Á ÁH and HÁ Á ÁO/OÁ Á ÁH contacts also making significant contributions to the surface (Table 2).  Table 2 Percentage contributions to the Hirshfeld surface of 1.

Contents
Included surface area

Figure 5
Overall packing of 1 viewed along the c-axis direction.

Synthesis and crystallization
The title compound 1 was prepared in two steps from 1-ferrocenyl-undec-10-en-1-one (Evans et al., 2008) via a lithium aluminium hydride reduction followed by hydroboration with 9-borabicyclo[3.3.1]nonane (9-BBN) (Aristoff et al., 1985), Fig. 8. LiAlH 4 (0.10 g, 2.6 mmol) was added to 1-ferrocenyl-undec-10-en-1-one (0.615 g, 1.75 mmol) in Et 2 O (10 mL) at 273 K and stirred for 1 h before quenching with a few drops of water. The ether fraction was rinsed with saturated NaCl solution and dried over MgSO 4 . The solvent was removed under vacuum to give 0.61 g (99%) of the yellow oil 1-ferrocenyl-undec-10-en-1-ol. To this oil, without further purification, in THF (10 ml) was added a solution of 9-BBN (0.5 M in hexane, 3.5 mmol), the mixture stirred at room temperature for 18 h before quenching with a few drops of water. The pH was raised to 8.5 with NaOH, then hydrogen peroxide (30% in H 2 O, 7 ml) was added and the mixture allowed to stir for another 2 h. The organic layer was rinsed with saturated NaCl solution and dried over MgSO 4 . Column chromatography on SiO 2 with CH 2 Cl 2 eluted a trace of the unreacted alcohol. Further elution with EtOAc/CH 2 Cl 2 gave the title compound 1 as a yellow solid (0.60 g, 94%). X-ray quality crystals were grown from the mixed solvents of CH 2 Cl 2 layered with hexane.   Data collection: APEX2 (Bruker, 2011); cell refinement: APEX2 (Bruker, 2011) and SAINT (Bruker, 2011); data reduction: SAINT (Bruker, 2011); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/1 (Sheldrick, 2015b) and TITAN (Hunter & Simpson, 1999); molecular graphics: Mercury (Macrae et al., 2020); software used to prepare material for publication: SHELXL2018/1 (Sheldrick, 2015b), enCIFer (Allen et al., 2004), PLATON (Spek, 2020), publCIF (Westrip 2010) and WinGX (Farrugia 2012). 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 reflection effected by the beamstop and two reflections with Fo >>> Fc were omitted from the final refinement cycles.