Polyyne

Polyynes are organic compounds built from alternating single and triple bonds—a chain of consecutive alkynes, (−C≡C−)n, where n is greater than 1. In natural products and chemical ecology, they are often called polyacetylenes, though that term more accurately describes polymers with alternating single and double bonds (polyenes). They are also referred to as oligoynes or carbinoids, the latter after "carbyne" (−C≡C−)∞, a hypothetical carbon allotrope that would be the chain’s ultimate member. Claims of synthesizing carbyne have appeared since the 1960s but remain disputed; what earlier researchers identified as short carbyne chains would now be called polyynes.

The simplest polyyne is diacetylene (butadiyne), H−C≡C−C≡C−H. Like cumulenes, polyynes stand out for their rigidity and high electrical conductivity, which make them candidates for molecular wires in nanotechnology. They have also been detected in interstellar molecular clouds where hydrogen is scarce.

**Synthesis** The first reported polyyne synthesis came in 1869, when Carl Andreas Glaser found that copper phenylacetylide (CuC≡C−C6H5) oxidatively dimerizes in air to give diphenylbutadiyne (C6H5−C≡C−C≡C−C6H5). Subsequent research has developed several general routes, most relying on acetylene homocoupling reactions such as Glaser coupling, the related Elinton and Hay protocols, or Cadiot–Chodkiewicz coupling to join separate alkyne building blocks. Alkylation of pre-formed polyyne units is also used. The longest known polyyne (C44) was made using a Fritsch–Buttenberg–Wiechell rearrangement as a key step, while the longest phenyl end-capped polyynes were finished by eliminating chlorovinylsilanes.

**Organic and Organosilicon Polyynes** In the 1950s, polyynes H(−C≡C−)nH with n up to 4 or 5 were synthesized. Around 1971, T. R. Johnson and D. R. M. Walton used –SiR3 end-caps (usually with ethyl groups) to protect the polyyne chain during chain-doubling with Hay’s catalyst (a copper(I)–TMEDA complex). This yielded polyynes like (CH3CH2)3Si(−C≡C−)nSi(CH2CH3)3 with n up to 8 in pure form and n up to 16 in solution. Later, Tykwinski and co-workers obtained ((CH3)2CH)3Si(−C≡C−)nSi(CH(CH3)2)3 chains up to C20. In 2002, a polyyne with 10 acetylenic units (20 atoms) capped by Fréchet-type aromatic polyether dendrimers was isolated and characterized. Dicyanopolyynes with up to 8 acetylenic units have also been reported, and in 2007 Cox and co-workers reported the longest phenyl end-capped polyynes. As of 2010, the longest isolated polyyne chain had 22 acetylenic units (44 carbon atoms), end-capped with tris(3,5-di-t-butylphenyl)methyl groups. Alkynes H(−C≡C−)nH with n from 2 to 6 appear in the decomposition products of partially oxidized copper(I) acetylide when treated with hydrochloric acid; a carbonaceous residue from this decomposition also shows spectral signatures of (−C≡C−)n chains.

**Organometallics** Organometallic polyynes capped with metal complexes are well characterized. By the mid-2010s, the most studied examples included rhenium (Re(−C≡C−)nRe, n = 3–10), ruthenium (RuRu(−C≡C−)nRuRu, n = 4–10), iron (Fe(−C≡C−)6Fe), platinum (Pt(−C≡C−)nPt, n = 8–14), palladium (Ar(−C≡C−)nPd, n = 3–5, Ar = aryl), and cobalt (Co3C(−C≡C−)nCCo3, n = 7–13) complexes.

**Stability** Long polyyne chains are inherently unstable in bulk because they can cross-link exothermically, posing an explosion hazard. They become fairly stable, even against moisture and oxygen, when terminal hydrogen atoms are replaced with inert end-groups like tert-butyl or trifluoromethyl. Bulky end-groups that keep chains apart are especially effective. In 1995, the preparation of carbyne chains with over 300 carbon atoms was reported using this approach, but the claim was contested on the grounds that the detected molecules were fullerene-like structures, not long polyynes. Polyyne chains have also been stabilized to heating by co-deposition with silver nanoparticles or by complexation with a mercury-containing tridentate Lewis acid to form layered adducts. Long chains encapsulated in double-walled carbon nanotubes or as rotaxanes are also stable. Despite the low stability of longer polyynes, some have been used as synthetic precursors in organic and organometallic chemistry.

**Structure** Synthetic polyynes of the form R(−C≡C−)nR with n about 8 or more often show a smoothly curved or helical backbone in the crystalline solid state, due to crystal packing effects. For example, when the cap R is triisopropylsilyl and n is 8, X-ray crystallography reveals the backbone bent by about 25–30 degrees in a broad arch, with each C−C≡C angle deviating by 3.1 degrees from linearity. This geometry allows denser packing: the bulky cap of a neighboring molecule nests into the concave side of the backbone, reducing the distance between backbones to about 0.35–0.5 nm—close to the range where spontaneous cross-linking is expected.