Molecules composed of a contiguous sequence of double bonds, the [n]cumulenes, share structural similarities to both of their conjugated relatives, the polyenes and polyynes. The synthesis and properties of [n]cumulenes are, however, quite different from those of either polyenes or polyynes. At an infinite length, [n]cumulenes would provide one structural form of the hypothetical sp-hybridized carbon allotrope carbyne, while shorter derivatives offer model compounds to help to predict the properties of carbyne. Finally, derivatization of the π-electron framework of [n]cumulenes provides a number of different synthetic transformations, with cycloaddition reactions being the most common. In this review, both historical and recent synthetic achievements toward long [n]cumulenes (n ≥ 5) are discussed. This is followed by a description of our current understanding of the physical and electronic structure of [n]cumulenes based on UV/vis spectroscopy and X-ray crystallography. Finally, the reactivity of long [n]cumulenes is described.

Dedicated to Professor FranÅois Diederich on the occasion of his 60 th birthday Chains composed of sp-hybridized carbon atoms have been explored for decades because of their unique linear structure and interesting physical properties. [1] More recently, the wirelike nature of sp-carbon oligomers has inspired a variety of studies that aim to evaluate these structures as components in nanometer-sized devices. [2] Particularly interesting is the reported formation of such wires linking graphene nanoribbons, offering the prospect of all-carbon-based devices. [2d-f, 3] Molecules composed of a skeleton of sp-hybridized carbon atoms can be constructed from a framework of either polyynes (alternating single and triple bonds) or cumulenes (cumulated double bonds). The chemistry of polyynes has been advanced to systems as long as 44 consecutive carbon atoms (22 acetylene units), [4] and studies have shed considerable light on the physical and optoelectronic properties of polyynes. [5] However, the study of cumulenes has lain essentially dormant since early work [6,7] reported by Kuhn [8,9] and Bohlmann. [10,11] Thus, there remain many unanswered questions about the physical properties of this intriguing class of linear molecules. To date, UV/Vis spectroscopy has been the most useful method for the characterization of cumulenes, [12] and analyses of cumulenes show a lowering of the lowest-energy electronic absorption (l max ) as a function of length, such as that for [n]Ph and [n]Cy (n = 3, 5, 7, 9, Figure 1). [9][10][11] Obviously, changes in l max versus molecular length are intricately dependent on structure and on the degree of bond-length alternation (BLA, defined as the bondlength difference between the two central-most double bonds of the cumulene chain). Recent theoretical studies predict that the BLA for cumulenes will rapidly approach zero (BLA 0.01), [13][14][15] that is, Peierls distortion is essentially absent. [16] Experimentally, X-ray crystallographic analysis would provide an opportunity to confirm or refute theoretical trends in BLA as a function of cumulene length. Unfortunately, few solid-state structures have been reported for cumulenes, and data for [n]cumulenes with n > 5 are not available. The results presented herein offer an answer to the important question of BLA in long cumulenes.It was clear from the onset of the study that the synthesis and solid-state analysis of long [n]cumulenes (n > 5) would be challenging, because available reports emphasized that these species were not typically stable enough for isolation. [6] In order to stabilize the cumulene core through steric shielding, initial efforts targeted formation of the [n]tBuPh series of cumulenes ( Figure 1). It quickly became clear, however, that the di(tert-butyl)phenyl (R = tBu 2 C 6 H 3 ) groups do not afford a sufficient stabilizing force to easily isolate the [7]-and [9]tBuPh cumulenes, and our attention then switched to the [n]Mes series.Synthesis of [3]tBuPh began with the formation of 1 a through reaction of the Li-acetylide of...

The stabilization of long [n]cumulenes has traditionally been achieved by placing sterically bulky "protecting groups" at the termini, which shield the reactive carbon chain from unwanted reactions. Herein, we present an alternative strategy: stabilization through threading the sp-hybridized carbon chain through a phenanthroline-based macrocycle. The result is stable [9]cumulene rotaxanes that enable the study of properties as a function of length for [n]cumulenes in unprecedented detail, including by quantitative UV/Vis spectroscopy, cyclic voltammetry, and differential scanning calorimetry. The experimental results are supported by DFT calculations.

We have investigated the structure and spectroscopic properties of cumulenic carbon chains, focusing on the peculiar π-conjugation properties and end-group effects that influence their behavior. With support from Density Functional Theory (DFT) calculations, we have analyzed the IR and Raman spectra of cumulenes characterized by different end-capping groups and we have related them to the bond length alternation (BLA) pattern and local spectroscopic parameters associated with the CC bonds along the sp-carbon chain. For cumulenes we observe a breakdown of the correlation existing in polyynes among frequencies, Raman intensities of the Ʀ line (longitudinal CC stretching modes), and BLA. While the low Ʀ line frequency and equalized CC bonds would indicate the “metallic” character of cumulenic species, we obtain an unusually strong Raman intensity, which is typical of bond-alternated (semiconductive) structures. DFT calculations reveal that this is a consequence of π-electron conjugation, which markedly extends from the sp-carbon chain to the aryl rings belonging to the end groups. These findings suggest the existence of a strong electronic, vibrational and structural coupling between sp-carbon chains and sp²-carbon species, which could play a key role in nanostructured sp/sp²-hybrid carbon materials (e.g., linear carbon chains coupled to graphene domains). Within this context, Raman spectroscopy is a valuable tool for the detailed characterization of the molecular properties of this kind of materials

The use of cumulenes in synthetic transformations offers the possibility to form structurally interesting and potentially useful conjugated molecules. The cycloaddition reaction of a tetraaryl[5]cumulene with the electron-deficient olefin tetracyanoethylene affords unusual products, including functionalized dendralenes and alkylidene cyclobutanes, as well as a symmetric [4]radialene that shows unique solvatochromism, with λ(max) values approaching the near-IR region. These carbon-rich products have been investigated spectroscopically and by X-ray crystallographic analysis (five structures). The cycloaddition reaction sequence has also been explored by mechanistic and theoretical studies. The obtained results clearly demonstrate the potential of [5]cumulenes to serve as precursors for unprecedented conjugated structures.