Synthesis and crystal structures of 2-(ferrocenylcarbonyl)benzoic acid and 3-ferrocenylphthalide

2-Ferrocenylcarbonylbenzoic acid (C18H14FeO3) was synthesized from the Friedel–Crafts acylation of ferrocene with phthalic anhydride, and the product was reduced to 3-ferrocenylphthalide (C18H14FeO2) using Zn(Cu) in aqueous sodium hydroxide. Both compounds were characterized using IR, NMR, and single-crystal X-ray analysis.


Chemical context
Our research group has been interested in developing methodologies to synthesize metallocene-fused quinones as synthetic precursors of -extended metallocenes. These are of interest because an integration of the redox-active metal center with the polycyclic aromatic hydrocarbons could alter their properties for organic semiconducting applications (Anthony, 2006). Previously, we synthesized metallocenefused quinones via the double Friedel-Crafts acylation reaction between 1 0 ,2 0 ,3 0 ,4 0 ,5 0 -pentamethylruthenocene-1,2-diacyl chloride with organic aromatics (Pokharel et al., 2011). Later, we realized that switching the functionality of two reaction partners allows us to obtain quinones in a much simpler synthetic scheme. Ferrocene being a close analog of ruthenocene, we decided to pursue the synthesis of ferrocene-fused quinones (Nesmeyanov et al., 1966;Pokharel, 2012), starting from ferrocene itself as the aromatic reagent. As the first step of this synthetic route, we prepared 2-ferrocenylcarbonyl benzoic acid, 1, following a previously reported procedure (Shen et al., 2012;Xu et al., 2017). The published procedure uses dichloromethane as the reaction solvent. However, using this solvent, we obtained consistently low reaction yields. On switching to dichloroethane from dichloromethane, the yield of the reaction was improved from 13% to a more satisfactory 51% even at room temperature, possibly due to higher solubility of the reaction mixture. The crystal structure of the ISSN 2056-9890 complex has been reported at room temperature (Qin, 2019). Our redetermination of its crystal structure at 90 K has improved the precision by a factor of about three.
With an easy route towards 2-ferrocenylcarbonyl benzoic acid, 1, at hand, we investigated the reduction of its keto group to methylene using a large excess of zinc powder (ca 48 equivalents) with aqueous sodium hydroxide as the solvent (Lee & Harvey, 1986). Under these reaction conditions, we were able to reduce complex 1 to 2-carboxybenzylferrocene in 89% yield (Pokharel, 2012). Following this successful transformation, we investigated the reaction outcome in the presence of a smaller amount (5 equivalents) of Zn. Under these reaction conditions, the reaction mixture changed color from red to light orange. However, on acidification, the reaction yielded the title compound 2 in a 77% yield. We assume that the limited amount of zinc leads to incomplete reduction of the ketone to a secondary alcohol, 1 0 0 0 ( Fig. 1), similar to the reduction of aryl ketones reported by Zhang and co-workers (Zhang et al., 2007). Upon acidification during reaction workup, alcohol 1 0 0 0 undergoes solvolysis to give the carbocation, which is electronically stabilized by the ferrocenyl group (Goodman et al., 2019). The nucleophilic attack of the carboxylic O atom leads to the formation of the cyclic lactone, 2. Although the title compound 2 was reported long ago as a major product from the reaction of 3,3 0 -diferrocenyl-3,3 0 -diphthalide with KOH (Nesmeyanov et al., 1961) and as a byproduct from the polycondensation reaction of ferrocene with o-carboxybenzaldehyde (Neuse & Koda, 1966), to our knowledge, this is the first report of the conversion of keto carboxylic acid, 1, to cyclic lactone 2 in a reasonably high yield. Here we report the synthesis, spectroscopic characterization, and single-crystal X-ray analysis of the title compounds 1 and 2.

Structural commentary
A view of the molecular structures of the title compounds 1 and 2, with their atom labeling, is shown in Fig. 2. The ferrocenyl moieties adopt typical sandwich structures with Fe-C distances in the range 2.0287 (17)-2.0498 (15) Å in compound 1 and of 2.032 (2)-2.055 (2) Å in 2. In both structures, the Fe-C bond lengths towards the substituted carbon are shorter [Fe-C1 2.031 (1) Å in 1; 2.032 (2) Å in 2] than the remaining Fe-C bond lengths, similar to literature reports (Pé rez et al., 2015;Wu et al., 2011). The C-C distances within the Cp rings fall in the range 1.412 (2)-1.429 (2) Å in compound 1 and 1.414 (3)-1.431 (3) Å in 2. Similar to its carboxylate salts (Li et al., 2003;Li, Liu et al., 2008;Xu et al., 2016), the two Cp rings of the ferrocene residue in complex 1 are close to an eclipsed conformation (mean of five C-Cg-Cg-C torsion angles = 12.68 ; Cg is the centroid of the respective cyclopentadienyl ring). The analogous angle in complex 2 is 3.31 . The Cp rings are essentially parallel in both complexes, making a dihedral angle of 2.45 (12) in compound 1 and 1.14 (10) in 2. The FeÁ Á ÁCg distances in both compounds are in a similar range [substituted and unsubstituted Cp in 1: 1.6436 (7) and 1.6458 (7) Å ; 2: 1.6455 (10) and 1.6510 (10) Å , respectively]. The Cg-Fe-Cg angle in both structures is ca 178 . The carbonyl carbon, C11 in compound 1 bends toward the iron center with a distance of 0.163 (3) from the leastsquares plane of the substituted Cp while the corresponding C11 atom in compound 2 bends slightly outward with a distance of 0.117 (4) Å from the plane of Cp. Similar bending can be seen in the N-imidazolyl derivative of compound 2 (Simenel et al., 2008). The carbonyl carbon in compound 1 lies roughly in the same plane as the substituted Cp with a torsional angle C2-C1-C11-O1 of 2.9 (2) . The phenyl ring in compound 1 is twisted away from the plane of the carbonyl (C O) plane with a torsional angle O1-C11-C12-C13 of À112.41 (16) . The aromatic ring of the phthalide moiety in compound 2 bends away from ferrocene and orients roughly perpendicular to the ferrocene backbone. The nine-atom phthalide plane of compound 2 inclines with the substituted Cp at a dihedral angle of 77.31 (7) . This molecule contains a single asymmetric center at the C11 position in this racemic structure. The synthetic scheme to the formation of unexpected title compound 2 from the title compound 1 with proposed intermediate.

Figure 2
Molecular structure of the title compounds 1 and 2 showing the atomnumbering schemes. Displacement ellipsoids are drawn at the 50% probability level.

Supramolecular features
The molecules in compound 1 are associated via classical hydrogen-bonding interactions between the carboxylic OH group of one molecule with the carbonyl oxygen of an adjacent molecule. The carboxylic acid groups are related via a crystallographic inversion center to form hydrogen bonds [O3-H3OÁ Á ÁO2 i [symmetry code: (i) Àx, Ày, 1 À z] with an R 2 2 (8) dimer (Etter et al., 1990) motif (Table 1 and Fig. 3). This centrosymmetric pairwise hydrogen-bonding dimer formation results in short hydrogen-bond distances of 2.6073 (15). In the crystal packing of title compound 2 (Fig. 4), the unsubstituted Cp orients towards the substituted Cp of a molecule at x, 1 À y, z À 1 2 with a CgÁ Á ÁCg separation of 3.929 (1) Å . There is a weak hydrogen-bonding interaction between the carbonyl oxygen O2 of the phthalide ring, and hydrogen H6 of the unsubstituted Cp with an H6Á Á ÁO2 (x, 2 À y, z À 1 2 ) distance of 2.58 Å ( Table 2). The phthalide moieties in the two molecules are oriented at an angle of 73.49 and exhibit a weak C-HÁ Á Á interaction as evidenced by the distance of 3.044 Å between H16 and the centroid of the aromatic ring of a phthalide moiety at 3 2 À x, y À 1 2 , 3 2 À z.

Database survey
The structure of title compound 1 (CSD refcode JOJGOH) at room temperature has been recently reported as a CSD Communication (Qin, 2019) (Li et al., 2003) have been reported. The structure of a compound analogous to the title compound 2 but with an N-imidazolyl group at C11 has also been reported (VIYTIH; Simenel et al., 2008). That structure has a disorder of the ferrocenyl substituent involving both eclipsed and staggered conformations.

Figure 3
The hydrogen-bonded dimer of title compound 1. Unlabeled atoms are related to their labeled counterparts by a crystallographic inversion center [Symmetry code: (i) Àx, Ày, 1 À z]. Displacement ellipsoids are drawn at the 50% probability level.

Figure 4
The crystal packing of title compound 2, viewed along the b axis. Displacement ellipsoids are drawn at the 50% probability level. Table 2 Hydrogen-bond geometry (Å , ) for 2. (3)  148 10 minutes. The solution was decanted, and the residue was washed with water (50 mL). To the activated zinc, keto-acid 1 (5.0 g, 0.015 mol) in NaOH solution (4.80 g in 30 mL of water) was added. The reaction mixture was allowed to reflux for 5 h, and then cooled to room temperature. The reaction mixture was filtered, and the filtrate acidified with conc. HCl. The resulting precipitate was collected, washed with water, and dried to give a viscous mass. The crude product was redissolved in dichloromethane (100 mL) and the acidic impurities extracted with 1 M NaOH (2 Â 10 mL). The organic layer was collected, dried with anhydrous MgSO 4 , filtered, and the filtrate evaporated to dryness to give the title compound 2 (3.65 g, 77%) as a pale-yellow solid. Crystals suitable for single-crystal X-ray diffraction were grown by slow evapora-

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All H atoms were located in difference maps and then treated as riding in geometrically idealized positions with C-H distances of 1.00 Å (0.95 Å phenyl) and with U iso (H) =1.2U eq for the attached C atom. The coordinates of the OH hydrogen atom in 1 were refined with the O-H distance restrained to 0.88 (2) Å , and its U iso value was assigned as 1.5U eq of the O atom.

2-(Ferrocenylcarbonyl)benzoic acid (1)
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.