Crystal structure of dimethyl N,N′-[(ethyne-1,2-diyl)bis(1,4-phenylenecarbonyl)]bis(l-alaninate)

The title compound shows a molecular framework with the diphenylethyne unit slightly deviating from planarity and the l-alanine moieties adopting a distorted helical conformation. The crystal structure features a two-dimensional network supported by N—H⋯O and C—H⋯O hydrogen bonding.

The diphenylethyne unit of the title molecule, C 24 H 24 N 2 O 6 , deviates slightly from planarity. The l-alanine moieties adopt distorted helical conformations of opposite winding direction. Infinite ribbons of N-HÁ Á ÁO C-connected molecules represent the basic supramolecular entities of the crystal structure. These aggregates are linked by C-HÁ Á ÁO hydrogen bonds involving the oxygen atoms of the methyl carboxylate units. The crystal studied was refined as an inversion twin.

Chemical context
Currently, the design of solid porous framework materials has developed into a very significant aspect of supramolecular crystal engineering (Desiraju et al., 2011). In connection with it, molecules frequently featuring a linear rigid structure and having coordinating or otherwise binding active functions as terminal groups are a desired structural unit in building such systems (Lin et al., 2006;Hausdorf et al., 2009;Zheng et al., 2010). For this reason, the corresponding structural units are called 'linker molecules'. A particular type of linker molecule consisting of a rod-like central unit and peptide terminal groups are promising in the assembly of bio-inspired framework materials including the subject chirality. Examples are the coordination polymers put together by N,N 0 -terephthalatoylbis(glycinate) (Eissmann et al., 2010) and Cu II (Kostakis et al., 2005) or equivalent bis(l-phenylalaninate) and Cu II (Wisser et al., 2008). In view of this applicability, the structural extension of this compound type is probably a future-oriented design. Precursor substances concerning this project have been prepared and structurally described in considerable numbers (Eissmann & Weber, 2011a,b). Here, we report for the first time the synthesis and crystal structure of a corresponding linker molecule. ISSN 2056-9890

Figure 2
Structure of the molecular ribbon including the mode of intermolecular bonding in the crystal of the title compound. Dashed lines represent hydrogen bonds (Table 1). Ring motifs [graph sets R 2 2 (30),R 2 2 (10)] are marked by colour highlighting.

Figure 3
Packing diagram of the title compound. The intermolecular contacts are shown as dashed lines.

Figure 1
Perspective view of the molecular structure of the title compound with the atom labeling. Displacement ellipsoids of non-H atoms are shown at the 50% probability level. described by the graph sets R 2 2 (30) and R 2 2 (10) (Etter et al., 1990;Bernstein et al., 1995). The ester groups participate to a different degree in molecular association along the stacking direction (c axis) of the molecular tapes. With the exception of O6, all ester oxygen atoms are involved in C-HÁ Á ÁO interactions with methoxy hydrogen atoms acting as donors. The analysis of these intertape interactions reveals another two ring motifs of graph set R 2 2 (8) and R 4 4 (26) (Fig. 3). According to the given pattern of hydrogen bonding, the crystal structure is composed of two-dimensional hydrogen-bonded layers connected by the linker molecules in a zigzag pattern. The presence of the bulky headgroups prevents areneÁ Á Áarene interactions.

Database survey
A search in the Cambridge Structural Database (CSD, Version 5.38, update February 2017; Groom et al., 2016) revealed six hits for crystal structures of methyl N-benzoyl-l-alaninate and its para-substituted derivatives. Of particular interest are the structures of methyl N-(4-bromolbenzoyl)-l-alaninate (IVOKIO; Eissmann & Weber, 2011a) and methyl N-(4ethynylbenzoyl)-l-alaninate (PAHMIN; Eissmann & Weber, 2011b). Their crystal packings are composed of structurally similar strands of N-HÁ Á ÁO C-bonded molecules in which the amide N-H group acts as a donor and the amide O atom as an acceptor site. Unlike in the title compound, this interaction is assisted by a C-HÁ Á ÁO contact involving the l-alanine C methyl group as a donor and the sp 3 -hybridized ester oxygen atom as an acceptor. In contrast, the crystal structure of methyl N-benzoyl-l-alaninate (XAZZON; Coghlan et al., 2000) is composed of zigzag strands of N-HÁ Á ÁO C-bonded molecules. The ester group of the molecule participates in interstand association via C-HÁ Á ÁC O-type hydrogen bonds, giving rise to two-dimensional supramolecular networks.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The hydrogen atoms were positioned geometrically and refined isotropically using a riding model with C-H = 0.98 Å and U iso (H) = 1.5U eq (C) for methyl and C-H = 0.95 Å and U iso (H) = 1.2U eq (C) for aryl H atoms. The crystal studied was refined as an inversion twin.

N,N′-[(Ethyne-1,2-diyl)bis(1,4-phenylenecarbonyl)]bis(L-alaninate)
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. Refinement. Refined as a 2-component inversion twin.