Crystal Engineering of 1-Halopolyynes by End-Group Manipulation

Abstract

Polyynes are compounds with two or more conjugated carbon–carbon triple bonds. Such molecules are usually regarded as models of carbynea hypothetical one-dimensional allotropic form of carbon. 1-Halopolyynes are rare representatives of such species, but few interesting crystal-to-crystal reactions have been recently reported for them. Herein, we present 11 new X-ray structures of 1-halopolyynes, nearly doubling the number of such compounds characterized with the use of single-crystal X-ray diffraction. The influence of an end-group on packing motifs in crystal structures of the 1-halopolyynes were thoroughly examined. Especially the strength of halogen bond as a function of polyyne chain length was analyzed with the support of theoretical calculations. Moreover, the compound CNC 8 I is the longest 1-halopolyyne characterized to date with the use of rentgenostructural analysis. In addition to that, the compounds NO 2 C 6 Br and MeC­(O)­C 6 Br are the first 1-bromohexatriynes characterized by means of X-ray single-crystal diffraction.

Impact of Halogen Termination and Chain Length on π-Electron Conjugation and Vibrational Properties of Halogen-Terminated Polyynes

“…Single crystals suitable for measurements were obtained for all 1-halopolyynes except very unstable C 8 Br. Structures of C n I compounds were reported before, but those of C 4 Cl, C 6 Cl, C 4 Br, and C 6 Br are new and their packing motifs are presented in the Supporting Information (Figures S16–S19). The solid-state structure of C 4 Br is a typical example of the most common packing motif observed for 1-halopolyynes bearing the 4-cyanophenyl end group (Figure a).…”

“…To analyze a possible push–pull behavior, we evaluated the charge distribution using atomic or group charges. In a previous work, charges calculated from electrostatic potentials using a grid-based method (CHELPG) allowed us to investigate the halogen bond formation in several halopolyynes, demonstrating that the halogen atoms can donate electrons to the π-system. Another effective description of the charge distribution in molecules, based on point partial charges on individual atoms (IR charges), can be obtained from DFT IR atomic polar tensors (APTs). − The APT, P α (where α labels the atoms) is a 3 × 3 tensor, which collects the three Cartesian components of the derivative of the molecular dipole moment with respect to the Cartesian displacements of the atom α, namely: false( P α ) u w = true( ∂ M u ∂ w α true) 0 (“0” indicates the equilibrium geometry).…”

“Triply Activated” Phenyl 3-Iodopropiolates: Halogen-Bond Donors with Remarkable σ-Hole Potentials

“…With this strategy, we developed our target library (Figure ) consisting of the doubly activated unsubstituted parent molecule ( US ) as a benchmark, along with five triply activated esters: 2,4-difluoro ( 24DF ), 3-cyano ( 3CN ), 4-cyano ( 4CN ), 3-nitro ( 3N ), and 4-nitro ( 4N ). The σ-hole potential of 227 kJ/mol for 4N far exceeds those reported for other strong halogen-bond donors such as 1,3,5-triiodo-2,4,6-trinitrobenzene ( TITNB , 207.0 kJ/mol), or doubly activated 4-(iodoocta-1,3,5,7-tetrayn-1-yl)­benzonitrile ( CNC 8 I , 208.4 kJ/mol), , and 1-(iodoethynyl)-3,5-dinitrobenzene ( IEDNB , 217.7 kJ/mol) . These known molecules have also been included as benchmark comparisons.…”

“…MEP surfaces computed at the B3LYP/6-311++G** level of theory at iso = 0.002 showing the σ-hole potential (top, in kJ/mol) on the iodine atoms of targets and benchmark compounds TITNB , CNC 8 I , and IEDNB explored in this study.…”

“…Library of target molecules explored in this study, along with the benchmark molecules (red box) from literature TITNB , CNC 8 I , and IEDNB …”