(R)-N-(Ferrocenylmethyl)-1-hydroxy-3-phenylpropan-2-aminium (E)-but-2-enoate

The crystal structure of the title salt, [Fe(C5H5)(C15H19NO)](C4H5O2), consists of discrete ammonium and carboxylate ions, which associate into infinite chains parallel to [100] by means of N—H⋯O and O—H⋯O interactions. These chains are further cross-linked into a three-dimensional network by additional C—H⋯O contacts and by offset π–π stacking interactions of inversion-related aromatic rings [centroid–centroid distance = 3.7040 (14) Å]. The molecular parameters of the ionic components are in no way unexpected, the geometry of the ammonium cation being similar to that found in other structurally characterized salts obtained from N-ferrocenylmethyl β-aminoalcohols. The (E)-but-2-enoate anion consists of two approximately planar subunits, viz the delocalized carboxylate unit and the butenyl group (the latter being planar within ca. 0.002 Å), which are mutually rotated by 30.3 (4)°.

The crystal structure of the title salt, [Fe(C 5 H 5 )(C 15 H 19 NO)]-(C 4 H 5 O 2 ), consists of discrete ammonium and carboxylate ions, which associate into infinite chains parallel to [100] by means of N-HÁ Á ÁO and O-HÁ Á ÁO interactions. These chains are further cross-linked into a three-dimensional network by additional C-HÁ Á ÁO contacts and by offsetstacking interactions of inversion-related aromatic rings [centroidcentroid distance = 3.7040 (14) Å ]. The molecular parameters of the ionic components are in no way unexpected, the geometry of the ammonium cation being similar to that found in other structurally characterized salts obtained from Nferrocenylmethyl -aminoalcohols. The (E)-but-2-enoate anion consists of two approximately planar subunits, viz the delocalized carboxylate unit and the butenyl group (the latter being planar within ca. 0.002 Å ), which are mutually rotated by 30.3 (4) .

Comment
With our recent work (Štěpnička et al., 2004 and 2008a,b), we demonstrated that N-ferrocenylmethyl β-aminoalcohols with general formula FcCH 2 NHCR 1 R 2 CR 3 R 4 OH (Fc = ferrocenyl) are potentially useful building blocks for organometallic crystal engineering (Braga et al., 2008 and references therein). These compounds possess a balanced number of conventional H-bond donors and acceptors (OH and NH) in their native form. When reacted with protonic acids, they are readily converted to the corresponding ammonium salts, the crystal assembly of which receives additional support from charge interactions. This contribution reports the crystal structure of a salt obtained from a chiral aminoalcohol, namely Several crystals of the title compound were isolated unexpectedly during attempted crystallization of (R)-2-[(ferrocenylmethyl)amino]-3-phenylpropan-1-ol from ethyl acetate/hexane, apparently resulting from the reaction of the free amine with (E)-but-2-enoic acid present as a trace impurity in reagent grade ethyl acetate. A view of the molecular structure is presented in Fig. 1.
The geometry of the cation is rather unexceptional, and compares well with those reported earlier for salts obtained from Atom C13 is directed towards the ferrocenyl group and the C13-O1 bond extends away from the N1-C12 bond (cf. torsion angle N1-C12-C13-O1).
Although the Fe-C distances between Fe1 and individual carbon atoms in the substituted cyclopentadienyl ring C(1-5) differ by less than 0.03 Å [2.0271 (16)-2.054 (2) Å], there is a clear trend with the Fe-C distance gradually increasing from the C ipso to the opposite edge of the five-membered ring (ipso < α < β). The Fe-C distances observed for the unsubstituted cyclopentadienyl ring vary significantly less [2.040 (2)-2.058 (2) Å].
The C═C double bond within the (E)-but-2-enoate anion has an almost ideal trans configuration with the torsion angle C21-C22-C23-C24 of -179.6 (3)°, which renders the whole CH 3 -CH═CH-C group nearly perfectly planar (within ca. 0.002 Å). The terminal carboxyl group (C21, O2, O3) is symmetrically rotated from the plane of the CH 3 -CH═ CH-C moiety by as much as 30.3 (4)° and shows a delocalized character. The individual C-O distances differ by only ca. 0.02 Å and even this relatively small difference may come mainly from crystal packing effects as the longer C-O distance is associated with O3 acting as a double H-bond acceptor, while the shorter one involves O2 atom for which only one strong H-bond was detected.
supplementary materials sup-2 The ions constituting the crystal of the title compound assemble by means of H-bonds between OH and NH groups and carboxylate oxygen atoms to form infinite chains in the [1 0 0] direction (Fig. 2). Distances between the H-bond donors and acceptors are close to 2.7 Å while the H-bond angles fall into the range 170-173° (Table 2). In addition, these conventional H-bonds are supported by the softer C-H···O interactions formed by CH groups at the terminal phenyl ring and proximal oxygen atoms O1 and O2 and further by π···π stacking interactions between the unsubstituted cyclopentadienyl ring C(6-10) and phenyl ring in a molecule related by unit-cell translation along the c-axis (Fig. 3). Least-squares planes of the interacting aromatic rings make a dihedral angle of 6.22 (12)°, and the distance of their respective centroids is 3.7040 (14) Å.
Ferrocene carboxaldehyde (428 mg, 2.00 mmol) and (R)-2-amino-3-phenylpropan-1-ol (318 mg, 2.1 mmol) were dissolved in dry chloroform (20 ml). The resulting solution was refluxed under argon for 90 min and then evaporated under vacuum. The residue was immediately re-dissolved in dry methanol (20 ml) and the solution was cooled in ice. An amount of NaBH 4 (380 mg, 10 mmol) was added over 30 min causing the colour of the reaction mixture to change from initial orange red to orange yellow. After the addition, the reaction mixture was stirred at 0 °C for 1 h and at room temperature for 90 min before being quenched by addition of 10% aqueous NaOH and extracted with dichloromethane (2×20 ml). The combined organic extracts were washed with brine (2×20 ml) and dried over MgSO 4 overnight.
The drying agent was filtered off and the filtrate was evaporated under vacuum leaving a residue which was purified by column chromatography over silica gel. Elution with dichloromethane-methanol (10:1 v/v) led to the development of two yellow bands. The first one containing ferrocenylmethanol was discarded. The second one was collected and evaporated under vacuum to afford (R)-2-[(ferrocenylmethyl)amino]-3-phenylpropan-1-ol (371 mg, 53%) as an orange amorphous solid.

Refinement
N-and O-bonded H atoms were identified on the difference electron maps and isotropically refined as riding atoms. The remaining H-atoms were included in their calculated positions and refined as riding atoms with U iso (H) assigned to a multiple of U eq (C) of their bonding carbon atoms.