Exploiting Unsaturated Carbon-Iodine Compounds for the Preparation of Carbon-Rich Materials

Conjugated carbon-rich materials have drawn much academic and industrial attention in recent years, due to their intriguing electronic and optical properties and potential applications including organic photovoltaics, flexible and wearable electronics, and chemical and biological sensors. Unsaturated carbon-iodine compounds, mainly the derivatives of iodoalkenes and iodoalkynes, are a class of molecules in which iodine atoms are directly connected to unsaturated carbons. These compounds provide unique advantages in the pursuit of carbon-rich materials, largely due to the Lewis acidity of iodine atoms and the lability of the carbon-iodine bonds. The Lewis acidity and electrophilicity of iodine in unsaturated carbon-iodine compounds make them excellent donors of halogen bonding, which is an attractive interaction between the electrophilic halogen atoms and Lewis basic species. Halogen bonding has emerged as a reliable building block in crystal engineering and supramolecular architectures. In this Account, we illustrate examples of the controlled assembly of diiodopolyynes within host-guest cocrystals that contain oxalamide or urea hosts with appropriate Lewis basic end groups and diiodobutadiyne or diiodohexatriyne guests. Halogen bonding interactions between the host and guest result in an ordered alignment of the diiodopolyynes that allows for a solid-state topochemical polymerization. We have used this approach to prepare poly(diiododiacetylene), PIDA, and poly(iodoethynyliododiacetylene), PIEDA, two conjugated polymers composed only of carbon and iodine. In addition, the polarity of the carbon-iodine bond gives unsaturated carbon-iodine compounds an electron-rich π-system, permitting electrophilic addition reactions with molecular halogens. The halogenated products of these additions can then serve as precursors to other conjugated carbon-rich systems. The lability of the carbon-iodine bond, together with the polarizability of iodine and the higher electronegativity of sp- and sp²-hybridized carbons, open up further possibilities in pursuing novel carbon nanomaterials from unsaturated carbon-iodine compounds. For example, we have developed an iterative method for the synthesis of longer symmetric polyynes from shorter diiodopolyynes, using Stille coupling to the iodine-capped polyynes. The iodination/coupling cycle symmetrically lengthens the polyyne chain by two carbon-carbon triple bonds. This method is particularly helpful for preparing polyynes with an odd number of carbon-carbon triple bonds. In addition, the lability of the carbon-iodine bonds of PIDA leads to facile carbonization by pyrolysis or laser irradiation. More strikingly, diiodoalkenes undergo quantitative elimination of iodine in the presence of Lewis bases. This reaction can be used to eliminate iodine at room temperature from PIDA, in which the carbon-iodine bonds are much more easily broken than in the diiodopolyynes, resulting in graphitic carbon materials.

Polyyne Rotaxanes: Stabilization by Encapsulation

Active metal template Glaser coupling has been used to synthesize a series of rotaxanes consisting of a polyyne, with up to 24 contiguous sp-hybridized carbon atoms, threaded through a variety of macrocycles. Cadiot–Chodkiewicz cross-coupling affords higher yields of rotaxanes than homocoupling. This methodology has been used to prepare [3]rotaxanes with two polyyne chains locked through the same macrocycle. The crystal structure of one of these [3]rotaxanes shows that there is extremely close contact between the central carbon atoms of the threaded hexayne chains (C···C distance 3.29 Å vs 3.4 Å for the sum of van der Waals radii) and that the bond-length-alternation is perturbed in the vicinity of this contact. However, despite the close interaction between the hexayne chains, the [3]rotaxane is remarkably stable under ambient conditions, probably because the two polyynes adopt a crossed geometry. In the solid state, the angle between the two polyyne chains is 74°, and this crossed geometry appears to be dictated by the bulk of the “supertrityl” end groups. Several rotaxanes have been synthesized to explore gem-dibromoethene moieties as “masked” polyynes. However, the reductive Fritsch–Buttenberg–Wiechell rearrangement to form the desired polyyne rotaxanes has not yet been achieved. X-ray crystallographic analysis on six [2]rotaxanes and two [3]rotaxanes provides insight into the noncovalent interactions in these systems. Differential scanning calorimetry (DSC) reveals that the longer polyyne rotaxanes (C16, C18, and C24) decompose at higher temperatures than the corresponding unthreaded polyyne axles. The stability enhancement increases as the polyyne becomes longer, reaching 60 °C in the C24 rotaxane.

Differential scanning calorimetry (DSC) as a tool for probing the reactivity of polyynes relevant to hexadehydro-Diels-Alder (HDDA) cascades

The differential scanning calorimetry (DSC) behavior of a number of alkyne-rich compounds is described. The DSC trace for each compound exhibits an exothermic event at a characteristic onset temperature. For the tri- and tetraynes whose [4 + 2] HDDA reactivity in solution has been determined, these onset temperatures show a strong correlation with the cyclization activation energy. The studies reported here exemplify how the data available through this operationally simple analytical technique can give valuable insights into the thermal behavior of small molecules.