Crystal structures of [(N,N-dimethylamino)methyl]ferrocene and (R p,R p)-bis{2-[(dimethylamino)methyl]ferrocenyl}dimethylsilane

The cyclopentadienyl rings are eclipsed in both of the title compounds and their packing is dominated by H⋯H (van der Waals) contacts, as determined by Hirshfeld surface analyses.


Chemical context
In 1951, ferrocene was synthesized serendipitously (Kealy & Pauson, 1951) and one year later it was examined by X-ray crystallography (Fischer & Pfab, 1952). N,N-Dimethylaminomethylferrocene (C 13 H 17 FeN,1) was first synthesized by Hauser & Lindsay (1956) by the reaction of ferrocene with paraformaldehyde and N,N,N 0 ,N 0 -tetramethyldiaminomethane. The derivatization of ferrocene to planar chiral ferrocene makes it an important ligand for catalytic asymmetric transformations, both for scientific and industrial applications (Schaarschmidt & Lang, 2013). In particular, 1 is appropriate for the formation of 1,2-disubstituted ferrocenes because of the free electron pair at the nitrogen atom: the lithiation of the ortho-position is preferred due to the DoM effect (Directed ortho Metalation) and can be converted by a further step using an electrophile (Marr et al., 1967). The ortho-lithiation can be carried out both racemically or with a high degree of enantiomeric control. The best known example for ortho-lithiation with high stereoselectivity is the (R)-N,Ndimethyl-1-ferrocenylethylamine, or Ugi's amine with a chiral directing group (Marquarding et al., 1970).
Many applications based on 1 have been established by our research group: it is an inexpensive non-chiral analogue of Ugi's amine, therefore the desymmetrization must be implemented by the chiral auxiliary (R,R)-tetramethyl-1,2-cyclohexanediamine (TMCDA) with yields in high stereoselectivity (Steffen et al., 2013). One application of the 1,2-disubstituted ferrocenes based on 1 is the formation of racemic and enantiomerically pure siloxides of zinc, whereby disiloxanes ISSN 2056-9890 can be synthesized while avoiding condensation reactions . Another application is the kinetically controlled asymmetric synthesis of silicon-stereogenic methoxy silanes using a planar chiral ferrocene backbone based on 1. Here, silicon-stereogenic methoxy silanes could be prepared with excellent stereoinduction (d.r. > 99:1) and the mechanistic course of the reaction can be described by quantumchemical calculations (Barth et al., 2019). Nayyar et al. (2018) reported 1,2-disubstituted ferrocenes based on 1 and their use as precursors for the diastereoselective synthesis of divalentelement chlorides and an unprecedented organolithiuminduced carbon-carbon single-bond cleavage. Furthermore, Gawron et al. (2019) were able to synthesize N,N-dimethylaminomethylferrocene-backboned unsymmetrical pincer-type proligands, which are interesting as ligands for transitionmetal complexes as catalysts for a variety of reactions in organic chemistry. The (R,S)-meso-compound of bis[dimethyl(aminomethyl)ferrocenyl]dimethylsilane was characterized by Roewer and co-workers using X-ray diffraction analysis and formed during the synthesis of dimethyldichlorosilane with two equivalents of the racemic lithiated N,N-dimethylaminomethylferrocene (Palitzsch et al., 1999).
In this paper, we report the crystal structures of 1 and enantiomerically pure (R p ,R p )-bis[dimethyl(aminomethyl)-ferrocenyl]dimethylsilane (2) and analyze their intermolecular interactions using Hirshfeld surfaces and two-dimensional fingerprint plots.

Structural commentary
Compound 1 crystallizes from n-pentane at 243 K as orange needles with monoclinic (P2 1 /n) symmetry. There are no noticeable irregularities in the bond lengths or bond angles found: the aminomethyl side chain is oriented above its attached cyclopentadienyl ring, and the Cp rings are eclipsed, the dihedral angle between their mean planes being 1.53 (15) . The molecular structure of 1 is presented in Fig. 1.
Compound 2 is an orange-red crystalline solid and occurs in enantiomerically pure form in the orthorhombic space group P2 1 2 1 2 1 . The structure is illustrated in Fig. 2. Using Cahn-Ingold-Prelog (CIP) prioritization, compound 2 can be assigned the (R p ,R p )-configuration; furthermore the cyclopentadienyl rings are also in an eclipsed conformation for both iron atoms [dihedral angles = 4.89 (17) and 1.34 (18) for the Fe1 and Fe2 rings, respectively]. The Si-C bonds span the range of 1.869 (3) to 1.874 (3) Å , which is consistent with the literature (Allen et al., 1987). The silicon centre of compound 2 adopts a slightly distorted tetrahedral geometry, as shown by the angles of 105.43 (14) (C14-Si1-C10) as the smallest and The molecular structure of 2 showing 50% displacement ellipsoids.

Figure 1
The molecular structure of 1 showing 50% displacement ellipsoids.

Figure 3
A view along the a-axis direction of the crystal packing of 1.
112.17 (13) (C14-Si1-C16) as the largest. This flexibility is often observed for Si-C bonds (Otte et al., 2017). Compared to compound 1, the aminomethyl side chains are oriented in the direction of the silicon atom, but the NÁ Á ÁSi contact distances of 3.552 (3) for N2 and 3.584 (3) Å for N1 are too long to be regarded as coordinate bonds to Si from the N lone pairs.

Supramolecular features
The crystal packing of compound 1 is shown in Fig. 3. To further investigate close contacts and intermolecular interactions, a Hirshfeld surface analysis was carried out: Fig. 4 illustrates the Hirshfeld surface mapped over d norm in the range from À0.072 to 1.201 (arbitrary units) and the related fingerprint plots generated by CrystalExplorer (Turner et al., 2017;McKinnon et al., 2007). On the Hirshfeld surface, weak van der Waals HÁ Á ÁH contacts appear as by far the largest region (83.9%) and show significant red spots on the Hirshfeld surface. CÁ Á ÁH/HÁ Á ÁC contacts contribute to 13.2% of the Hirshfeld area and appear as two spikes and also show a slight colouration, which indicates that the cyclopentadienyl ring interacts with adjacent molecules. The NÁ Á ÁH/HÁ Á ÁN interactions occupy the smallest region (2.9%) and display no noticeable interactions.

Figure 5
A view along the b-axis direction of the crystal packing of 2.  of intermolecular interactions are shown in Fig. 6 in the twodimensional fingerprint plot. The Hirshfeld surface of compound 2 mapped over d norm in the range from À0.149 to 1.497 a.u. shows significant intermolecular interactions, indicated by the red spots. Both the van der Waals HÁ Á ÁH contacts (88.4%) and the CÁ Á ÁH/HÁ Á ÁC contacts (11.6%) contribute to the packing arrangement of the crystal. Intermolecular interactions of the cyclopentadienyl rings with neighbouring molecules can also be visualized.

Database survey
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Synthesis and crystallization
N,N-Dimethylaminomethylferrocene was purchased from ABCR and used without further purification. A solution of N,N-dimethylaminomethylferrocene (1.00 mmol) in n-pentane (1 ml) was made up and stored at 243 K and compound 1 crystallized in the form of orange needles. The reaction scheme for the synthesis of compound 2 is illustrated in Fig. 7. To a solution of (S p )-[2-(dimethylaminomethyl)ferrocenyl]lithium (4.00 mmol) ( Reaction scheme for the synthesis of 2.  dropwise at 195 K. The reaction was slowly warmed up to room temperature and stirred overnight. Afterwards the reaction was quenched by the addition of water. The aqueous phase was extracted three times with diethyl ether and the combined organic phases were dried with MgSO 4 . After the volatile components were removed and purified by column chromatography (n-pentane:diethyl ether + triethylamine; 100:1 + 5 Vol.-%), the product (46%) could be obtained as yellowish plates.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. For both compounds, the H atoms were positioned geometrically (C-H = 0.95-1.00 Å ) and refined using a riding model, with U iso (H) = 1.2U eq (C) for CH 2 and CH hydrogen atoms and U iso (H) = 1.5U eq (C) for CH 3 hydrogen atoms. For both structures, data collection: APEX2 (Bruker, 2018); cell refinement: SAINT (Bruker, 2016); data reduction:

[(N,N-Dimethylamino)methyl]ferrocene (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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Fe1 0.86749 (6) (17) C7 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq