Crystal structure of bis{(S)-1-[2-(diphenylphosphanyl)ferrocenyl]-(R)-ethyl}ammonium bromide dichloromethane monosolvate

The absolute structure of (R,R,S Fc,S Fc)-[Fe2(C5H5)2(C38H36BrNP2)]·Br·CH2Cl2 has been determined by X-ray single-crystal diffraction.


Chemical context
During the last decade, chiral non-racemic substituted ferrocene derivatives have found broad applications in a number of different fields, including asymmetric catalysis, and an increasing number of new catalysts and ligands have been reported progressively (Helmchen & Pfaltz, 2000;Dai et al., 2003;Sutcliffe & Bryce, 2003;McManus & Guiry, 2004;Miyake et al., 2008;Š tě pnička, 2008;Hargaden & Guiry, 2009). During the synthesis of chiral PNP pincer ligands [tridentate ligands coordinating to a central metal atom via P, N and P (Szabo & Wendt, 2014)] with a ferrocene scaffold and their Fe II complexes (Hargaden & Guiry, 2009), the salt 1HÁBr was crystallized as its CH 2 Cl 2 solvate ( Fig. 1) instead of the expected [Fe(PNP)Br 2 ] complex (Fig. 2). However, neither the crystal structure of any salt of 1H + , nor of any of its solvates, has been reported up to now. The crystal structure of 1HÁBrÁCH 2 Cl 2 is reported in this communication with the aim of contributing to a deeper understanding of its molecular structure and the crystal packing. ISSN 2056-9890

Structural commentary
The title salt 1HÁBr crystallizes with one dichloromethane molecule in space group P4 3 , with one formula unit in the asymmetric unit. The correct space-group assignment, and by consequence absolute configuration, was confirmed by resonant scattering [Flack parameter 0.002 (3) ;Flack, 1983]. It is in agreement with the expected absolute configuration as determined by the enantioselective synthesis (Zirakzadeh et al., 2016). In contrast to classical PNP complexes, where the lone pairs of the P and N atoms are directed towards the coordinated metal, the 1H + ion adopts a distinctly more twisted conformation (Fig. 1) [the angles of the C-N bonds to the least-squares planes of connected pentadienyl moieties are 61.2 (2) and 81.9 (10) ]. Whereas the lone pairs of the P atoms are approximately in a face-to-face orientation, the hydrogen atoms of the secondary ammonium group are directed in a different direction towards distinct channels in the structure (see below). The ferrocene moieties adopt staggered (Fe2: average C-G-G-C torsion angle 30.1 , where C stands for a C atom of the ferrocene and G for the center of gravity of the C atoms of the corresponding ring) and somewhat more eclipsed (Fe1: 14.9 ) conformations, respectively.

Supramolecular features
One of the two ammonium H atoms forms a hydrogen bond with the Br À ion ( Table 1). The second H atom is not involved in hydrogen bonding. Besides the hydrogen bonding, no further notable supramolecular interactions are apparent. The 1H + ions form a van der Waals-packed three-dimensional framework (Fig. 3). The CH 2 Cl 2 solvent molecules and Br À ions are located in channels of this network that extend along <100>. Without CH 2 Cl 2 molecules and Br À ions, the packing Reaction scheme towards the formation of the title salt 1HÁBr. Table 1 Hydrogen-bond geometry (Å , ).

Figure 3
The crystal structure of 1HÁBrÁCH 2 Cl 2 viewed down [010]. Atoms are as in Fig. 1. H atoms have been omitted for clarity.

Synthesis and crystallization
All reactions were performed under an inert atmosphere of argon using Schlenk techniques. The solvents were purified according to standard procedures. The synthesis of 1 and the [Fe(PNP)Br 2 ] complex was described in detail by our group (Zirakzadeh et al., 2016). Single crystals suitable for X-ray structure determination were grown by vapour diffusion of Et 2 O into a CH 2 Cl 2 solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms bonded to C atoms were placed in calculated positions and refined as riding atoms, with fixed bond lengths in the range 0.95-1.00 Å and U iso (H) = 1.2U eq (C) or 1.5U eq (C Me ). Ammonium H atoms were found in difference Fourier maps and were refined freely.