Isomorphous crystal structures of chlorodiacetylene and iododiacetylene derivatives: simultaneous hydrogen and halogen bonds on carbonyl

tert-Butyl (5-chloropenta-2,4-diyn-1-yl)carbamate and tert-butyl (5-iodopenta-2,4-diyn-1-yl)carbamate are new members of the isostructural family of compounds with the general formula BocNHCH2CCCCX (X = H, Cl, Br, I). In the crystals of all these diacetylenes, molecules are linked via a bifurcated N—H⋯O hydrogen bond and C—X⋯O halogen bond involving the same carbonyl group.

The crystal structures of tert-butyl (5-chloropenta-2,4-diyn-1-yl)carbamate, C 10 H 12 ClNO 2 (II), and tert-butyl (5-iodopenta-2,4-diyn-1-yl)carbamate, C 10 H 12 INO 2 (IV), are isomorphous to previously reported structures and accordingly their molecular and supramolecular structures are similar. In the crystals of (II) and (IV), molecules are linked into very similar two-dimensional wall organizations with antiparallel carbamate groups involved in a combination of hydrogen and halogen bonds (bifurcated N-HÁ Á ÁO C and C C-XÁ Á ÁO C interactions on the same carbonyl group). There is no long-range parallel stacking of diynes, so the topochemical polymerization of diacetylene is prevented. A Cambridge Structural Database search revealed that C C-XÁ Á ÁO C contacts shorter than the sum of the van der Waals radii are scarce (only one structure for the C C-ClÁ Á ÁO C interaction and 13 structures for the similar C C-IÁ Á ÁO C interaction).

Chemical context
Hydrogen bonds (HBs) and halogen bonds (XBs) are considered to be useful noncovalent synthetic tools in crystal engineering (Aakerö y et al., 2015;Grabowski, 2016;Resnati et al., 2015;Cinčić et al., 2008). Indeed, these directional intermolecular interactions facilitate the preparation of the desired solid-state motifs and architectures (Gilday et al., 2015;Cavallo et al., 2016;Priimagi et al., 2013;Mukherjee et al., 2014;Shirman et al., 2015;Mukherjee et al., 2017). For example, using HBs and XBs, the specific organization of terminal diacetylenes (Li et al., 2009;Ouyang et al., 2003), bromodiacetylenes (Jin et al., 2015) and iododiacetylenes (Jin et al., 2013;Sun et al., 2006) has been obtained to achieve the solidstate topochemical polymerization of diacetylenes. On the other hand, to the best of our knowledge, no chlorodiacetylene topochemical polymerizations have been reported. Our results show that chlorodiacetylene (II) is isostructural to iododiacetylene (IV) and the previously reported bromodiacetylene (III) and terminal diacetylene (I) (Baillargeon et al., 2016) (see Scheme). Although the arrangement of diynes in the present article stands no chance of undergoing topochemical polymerization, we suggest that in other systems prone to polymerization, replacing Br, I or H atoms by Cl atoms in their diyne groups might result in successful PolyChloroDiAcetylene (PCDA) formation as well. This work also contributes to an emerging research theme, namely the concept of orthogonal molecular interactions such as HBs and XBs (Kratzer et al., 2015;Takemura et al., 2014;Voth et al., 2009), which may find applications in medicinal chemistry and chemical biology (Wilcken et al., 2013).

Structural commentary
The molecular structures of compounds (II) and (IV) are shown in Fig. 1. All bond lengths and angles are within normal ranges. For example, the internal diyne C2-C3 bonds lengths [1.376 (3) Å for (II) and 1.385 (4) Å for (IV)] follow the useful rule of thumb describing a C-C single-bond distance (1.54 Å ) decreasing by 0.04 Å each time one of the participating C atoms changes hybridization from sp 3 to sp 2 or from sp 2 to sp (Bent, 1961). Moreover, the observed distances are almost identical to those found recently in the literature for similar halodiynes (Hoheisel et al., 2013;Baillargeon et al., 2016). The relative orientation between the diacetylenic moiety and the carbamate functional group can be established by the absolute value of the torsion angles C4-C5-N1- C6 [111.07 (19) ] for (II) and [103.8 (3) ] for (IV).

Supramolecular features
In the crystals of compounds (II) and (IV), molecules are linked via an N-HÁ Á ÁO C hydrogen bond between their respective carbamate functionalities [N1-H1Á Á ÁO1 i (Table 1) and N1-H1Á Á ÁO2 i (Table 2)], generating an antiparallel stacking pattern which orients the diacetylene skeleton on each side of the one-dimensional carbamate tape (parts B and D in Fig. 2    Halogen (green lines) and/or hydrogen bonds (blue lines) inside the supramolecular walls of (A) diyne (I), (B) chlorodiyne (II), (C) bromodiyne (III) and (D) iododiyne (IV). The nonpolar H atoms have been omitted for clarity. Table 1 Hydrogen-bond and halogen-bond geometries (Å , ) for (II).  Fig. 2) and the bromodiacetylene (III) (part C in Fig. 2). In fact, diynes (I)-(IV) (Fig. 2) constitute a complete set of truly isomorphous crystals that can be carefully examined to evaluate the differences and similarities that exist between halogen and hydrogen bonds. Thus, the XÁ Á ÁOÁ Á ÁH angle increases as the size of the halogen atom becomes larger. This angle, which is pretty open in the chlorine crystal (II) (Cl1Á Á ÁO1Á Á ÁH1; part B in Fig. 2; 69 ) adopts a near orthogonal geometry with the iodine (I1Á Á ÁO2Á Á ÁH1; part D in Fig. 2; 83 ). It is not a surprise that the bromine crystal (III) represents an intermediate case (part C in Fig. 2; 72 ). The value for the terminal diacetylene (I) X = H (part A in Fig. 2 Table 3). For the iodoalkyne, results are limited to monovalent iodine and for a structure in which the carbonyl group is not involved in an organometallic complex.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4.

tert-Butyl (5-chloropenta-2,4-diyn-1-yl)carbamate (II)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.22 e Å −3 Δρ min = −0.21 e Å −3 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. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.32 e Å −3 Δρ min = −0.69 e Å −3 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.