Masked alkynes for synthesis of threaded carbon chains

Abstract

Polyynes are chains of sp1 carbon atoms with alternating single and triple bonds. As they become longer, they evolve towards carbyne, the 1D allotrope of carbon, and they become increasingly unstable. It has been anticipated that long polyynes could be stabilized by supramolecular encapsulation, by threading them through macrocycles to form polyrotaxanes, but, until now, polyyne polyrotaxanes with many threaded macrocycles have been synthetically inaccessible. Here we show that masked alkynes, in which the CC triple bond is temporarily coordinated to cobalt, can be used to synthesize polyrotaxanes, up to the C68 [5]rotaxane with 34 contiguous triple bonds and four threaded macrocycles. This is the length regime at which the electronic properties of polyynes converge to those of carbyne. Cyclocarbons constitute a related family of molecular carbon allotropes, and cobalt-masked alkynes also provide a route to [3] and [5]catenanes built around cyclo[40]carbon and cyclo[80]carbon, respectively.

Monodisperse Molecular Models for the sp Carbon Allotrope Carbyne; Syntheses, Structures, and Properties of Diplatinum Polyynediyl Complexes with PtC20Pt to PtC52Pt Linkages

“…Also, some of the above issues can be avoided by the use of “masked alkyne equivalents”. As developed by Anderson, π adducts of polyynes and Co 2 (CO) 4 L 2 fragments are used to assemble long C x moieties and the dicobalt moieties removed in the final step …”

“…By use of the labile building block HC 8 Si , the sp carbon chains of PtC x Si could be extended in fewer steps than in previous efforts, thereby allowing access to the new diplatinum polyynediyl complexes PtC 28 Pt , PtC 32 Pt , PtC 36 Pt , PtC 40 Pt , PtC 44 Pt , PtC 48 Pt , and PtC 52 Pt . Excluding supramolecular systems, , this series terminates with the longest polyyne isolated in pure form to date. NMR, IR, Raman, and UV–visible data as well as 18 crystal structures have been carefully analyzed as a function of chain length.…”

“…There has been a school of thought that bulkier and more electropositive endgroups enhance stabilities, and the former design element is particularly evident in [D3]­C 20 [D3] , Tr*C x Tr* ( x = 44, 48), and Py*C 48 Py* . Steric protection is likely a stabilizing factor in the very long sp carbon chains that have been sequestered within carbon nanotubes as well as in supramolecular systems in which C 68 segments are shielded in rotaxanes …”

“…Samples of t -BuC 24 t -Bu , which can be isolated as red needles that decompose within minutes at room temperature, have been similarly characterized, but pure samples of t -BuC 20 t -Bu have been reported. , The remaining compounds in Figures and were isolated in pure form and characterized by a full array of modern spectroscopies. The longest carbyne models realized to date have been reported by Tykwinski and Anderson and include (1) C 44 and C 48 systems with “supertrityl” endgroups ( Tr* ) featuring two meta disposed t -butyl groups on each aryl ring , and (2) a C 48 system with a diarylated C -pyridyl endgroup ( Py* ) with meta disposed t -butyl groups on each aryl ring …”

“…For more than 60 years, chemists have approached the study of carbyne by preparing monodisperse conjugated polyynes of ever-increasing lengths. − Many of these efforts have sought to define asymptotic limits for physical properties capable of providing insight into the macromolecule. In addition, there is the purely synthetic challenge of the length of an sp carbon chain that can be accessed.…”