Crystal structure of (2-acetylferrocen-1-yl)boronic acid

In the crystal structure of (2-acetylferrocen-1-yl)boronic acid, centrosymmetric dimers held together by –B(OH)⋯O hydrogen bonds are present.


Chemical context
The synthesis of 1,2-functionalized ferrocenes is a striking topic in ferrocene chemistry (Schaarschmidt & Lang, 2013;Korb et al., 2014a) and is mostly realized via ortho-directed metalation and subsequent reaction with electrophiles (Schaarschmidt & Lang, 2013) or intramolecular rearrangement (Werner & Butenschö n, 2013;Korb et al., 2017). The resulting ferrocenes are predominantly used as ligands in C,C cross-coupling catalysis (Schaarschmidt et al., 2014;Jensen & Johannsen, 2003;Vinci et al., 2009;Debono et al., 2010;Karpus et al., 2016), but also the introduction of ferrocenyl substituents by catalytic conversions is of rising interest (Hildebrandt et al., 2011a,b;Speck et al., 2015;Korb et al., 2014b). The introduction of electronically and sterically modified substrates requires the synthesis of the respective ferrocenes that bear groups suitable for oxidative additions or transmetalation reactions (Lehrich et al., 2015;Speck et al., 2014). In case of substrates that are sensitive towards a nucleophilic attack, e.g. acyl groups, the Suzuki-Miyaura instead of a Negishi reaction is commonly used, and hence requires the presence of a boronic acid functionality (Speck et al., 2015). However, the acidic protons prevent a straightforward ortho-directed metalation, and additional reaction steps for the introduction and removal of protecting groups are required. Electrophilic aromatic substitution (S E Ar) reactions are also not suitable, since they usually give 1 0 -or 3-functionalized products (Rosenblum & Woodward, 1958).
Within our attempts to synthesize new electronically modified ferrocenes as substrates for Suzuki-Miyaura reactions, we herein present the synthesis and crystal structure of an ortho-functionalized ferrocenylboronic acid, obtained via S E Ar without using a protection group strategy for the acidic protons.

Structural commentary
The title compound crystallizes in the centrosymmetric space group P2 1 /n with one molecule in the asymmetric unit (Fig. 1). An intramolecular hydrogen bond between the oxygen atom of the acetyl group (O1) and the neighbouring hydroxy group (O2) of the boronic acid functionality of 2.650 (2) Å (Table 1) is present. Therefore, both substituents are co-planar with each other [BO 2 Á Á ÁC 2 O = 2.9 (4) ]. The C O distance of 1.233 (2) Å is neither affected by the involvement into this hydrogen bond, nor the presence of an ortho substituent and is therefore similar to unsubstituted acetyl ferrocene (Sato et al., 1984).
With regard to the C 5 H 3 plane of the ferrocenyl backbone, both substituents reveal a slight endo-bending of 7.0 (3) (C 5 H 3 Á Á ÁC 2 O) and 9.5 (3) (C 5 H 3 Á Á ÁBO 2 ). The ferrocenyl backbone exhibits an eclipsed conformation (C1-Cg-Cg-C8 = 8.21 (14) ; Cg is the centroid of the respective cyclopentadienyl ring) and a tilt angle of 179.28 (2) . The hydrogen atom at O3 is directed away from the ferrocenyl backbone and points to an adjacent molecule.

Supramolecular features
In addition to the aforementioned intramolecular hydrogen bond between O1 and O2, the latter atom is also involved as an acceptor of an intermolecular hydrogen bond with the second hydroxy group (O3) of an adjacent boronic acid functionality of 2.744 (2) Å (Fig. 2, Table 1). The resulting dimer is centrosymmetric with the inversion center located at the middle of the eight-membered ring formed by the two boronic acid functionalities. Therefore, both ferrocenyl moieties are positioned anti with regard to the central B 2 O 4 plane. Hence, a racemic mixture of both enantiomers crystallized, giving the R p /S p -configured, i.e. meso diastereomer if the dimer is considered as one supramolecular entity. The respective racem configuration (R p /R p or S p /S p ) is not present within the packing (Fig. 3).
A short contact of 4.6807 (14) Å between a C 5 H 3 and a C 5 H 5 ring does not show a perpendicular positioning of the two groups ( = 25 ) and therefore does not fit the criteria for a T-shapedinteraction (Sinnokrot et al., 2002). However, weak C-HÁ Á ÁO interactions between aromatic H atoms and the carbonyl O1 atom and a boronic acid O atom (O3) consolidate the crystal packing (Table 1).

Figure 2
Intra-and intermolecular hydrogen bonds within the dimer, with displacement ellipsoids drawn at the 50% probability level. C-bonded hydrogen atoms have been omitted for clarity. [Symmetry code: (A) 1 À x, 1 À y, 1 À z.]

Figure 1
The molecular structure of the title compound showing the intramolecular hydrogen bond between the acetyl and the boronic acid functionalities. Displacement ellipsoids are drawn at the 50% probability level; C-bonded hydrogen atoms have been omitted for clarity.
general, other ortho-substituted analogues are sparsely described.
In case of non-ferrocenyl-based aromatics, the 2-C(O)CH 3 -1-B(OH) 2 substitution pattern is solely reported for the benzene core (Ganguly et al., 2003). In contrast to the title compound, the boronic acid functionality is rotated out of coplanarity with the benzene core and the acetyl group by 78.2 and 77.7 , respectively.
For ortho-carbonyl groups in general, the involvement of the boronic acid functionality in inter-and intramolecular hydrogen bonds, similar to the title compound, is a common feature (Yan et al., 2003;Luliń ski et al., 2007;Durka et al., 2014;Madura et al., 2015).

Synthesis and crystallization
Ferroceneboronic acid (0.5 g, 2.175 mmol) was suspended in acetic anhydride (10 ml). To this suspension BF 3 ÁOEt 2 (0.40 ml, 3.15 mmol) was added in a single portion. The reaction mixture was stirred for 30 min at ambient temperature. Afterwards, the mixture was poured into ice and was stirred for 10 minutes. A KOH solution (9 M, 10 ml) was added in a single portion following a neutralization with K 2 CO 3 until the CO 2 evolution stopped. The mixture was extracted with dichloromethane (3Â20 ml) and the organic phase was dried over MgSO 4 . The volatiles were removed in vacuum (1 mbar). The crude material obtained was purified by flash chromatography on silica using a 4/1 (v/v) diethyl ether/ dichloromethane mixture. The title compound was isolated as a brown solid. Yield: 75 mg (0.28 mmol, 13% based on ferroceneboronic acid).

Figure 3
Unit cell of the title compound in a view along [100]. Hydrogen bonds are shown as pale-blue dashed lines; displacement ellipsoids are drawn at the 50% probability level. C-bonded hydrogen atoms have been omitted for clarity.
(w), 668 (m), 642 (w Crystals suitable for X-ray crystallography were obtained from evaporation of a saturated dichloromethane solution at ambient temperature.

Refinement
Crystal data, data collection and structure refinement detail are summarized in Table 2. C-bound H atoms were placed in calculated positions and constrained to ride on their parent atoms with U iso (H) = 1.2U eq (C) and a C-H distance of 0.93 Å for aromatic and U iso (H) = 1.5U eq (C) and a C-H distance of 0.96 Å for methyl H atoms, with their torsion angle derived from the residual electron density. The hydroxy hydrogen atoms were located from difference-Fourier maps but were treated with idealized geometry with U iso (H) = 1.5U eq (O), an O-H distance of 0.82 Å and a torsion angle derived from the residual electron density.

Funding information
We are grateful to the Federal Cluster of Excellence EXC 1075 "MERGE Technologies for Multifunctional Lightweight Structures". This project has received funding from the European Social Fund (ESF). The publication costs of this article were funded by the German Research Foundation/ DFG-392676956 and the Technische Universitä t Chemnitz in the funding program Open Access Publishing.
Windows (Farrugia, 2012) and SHELXTL (Sheldrick, 2008); software used to prepare material for publication: WinGX (Farrugia, 2012) and 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. Refinement. Refinement of F 2 against ALL reflections. The weighted R factor wR and goodness of fit S are based on F 2 , conventional R factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating R factors(gt) etc. and is not relevant to the choice of reflections for refinement. R factors based on F 2 are statistically about twice as large as those based on F, and R factors based on ALL data will be even larger.