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...

In comparison to the omnipresent two- and three-dimensional allotropes of carbon, namely, graphite and diamond (as well as recent entries graphene, carbon nanotubes, and fullerenes), a detailed understanding of the one-dimensional carbon allotrope carbyne is not well established, and even the existence of carbyne has been a matter of controversy over the past decades. Composed of sp-hybridized carbon, carbyne could potentially exist in two forms, either as a polyyne (alternating single and triple bonds, expected to show a semiconducting behavior) or as a cumulene (all carbon atoms are connected via double bonds, predicted to show metallic behavior). Although a number of publications are available on the hypothetical structure and properties of carbyne, specific knowledge about its physical and spectroscopic characteristics is still unclear. In order to predict the properties of carbyne, the synthesis and study of model compounds, namely, polyynes and cumulenes, has been a promising avenue. The synthesis of polyynes has been extensively explored in the last decades, culminating with the isolation of a polyyne with 22 acetylene units, which allows extrapolation to the properties of carbyne. Extended cumulenes, on the other hand, have remained much less well-known, and specific studies of properties versus molecular length are quite limited. A limiting factor to the study of [n]cumulenes has been their dramatically increased reactivity, especially in comparison to polyynes of comparable length. For example, most known [7]cumulenes can only be handled in solution, while the polyynes of equivalent length (i.e., a triyne with three acetylene units) are quite stable. [9]Cumulenes are the longest derivatives studied to date. In this Account, we describe our efforts to design and synthesize odd [n]cumulenes (i.e., n = 3, 5, 7, 9) that are sufficiently persistent under ambient conditions to allow in depth characterization of physical and spectral properties. This goal has been achieved through modification of the end-capping groups by increasing the steric bulk and thereby shielding the reactive cumulene framework to provide stable [7]- and [9]cumulenes. An alternative route to stabilization is accomplished via encapsulation of the cumulene skeleton in a macrocycle, that is, formation of cumulene rotaxanes. The new sterically encumbered cumulenic products are reasonably stable under normal laboratory conditions, although some readily undergo cycloaddition reactions to give interesting products. We have explored preliminary trends for the reactivity of long [n]cumulenes. Finally, trends in the series of [n]cumulene model compounds are now discernible, including a thorough consideration of bond length alternation (BLA) in long [n]cumulenes using X-ray crystallographic analyses, as well as electronic properties via UV-vis spectroscopy and cyclic voltammetry.

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 thermal dimerization of three [n]cumulenes (n = 5, 7, and 9) has been investigated, and a common reactivity pattern is observed that gives radialenes and expanded radialenes through regioselective cycloaddition reactions; all three products are characterized using X-ray crystallography.Cycloaddition reactions of polyynes have been used to create conjugated molecules that are otherwise difficult to obtain, and the regioselectivity of these reactions can often be controlled by the electronic nature of endcapping groups. 1-3 On the other hand, this same level of control has not yet been achieved for reactions for the sp-carbon chain of long [n]cumulenes (n Z 5). 4 There have been a handful of reports in which [5]cumulenes react with electron-poor olefins or alkynes, but the regiochemistry of cycloaddition reactions is difficult to predict. 5-8 Dimerization reactions under thermal, photochemical, or metal-catalysed conditions that give cyclic oligoynes or radialenes are also known. 9 The regiochemistry observed for these cycloaddition reactions has been typically attributed to either steric requirements or to the electronic structure of endcapping groups. [10][11][12][13][14] 12 and the regiochemical outcome of these reactions centers on the reaction at either the b-or g-bond, and is dependent on the size and electronic makeup of the end groups. The reactivity of longer ''odd'' [n]cumulenes (i.e., n = 7 or 9) in cycloaddition reactions has not, to our knowledge, been reported to date.We describe herein the dimerization of [n]cumulenes, including the first reported reactions of [7]-and [9]cumulenes. These transformations follow a similar reactivity pattern, and the products provide a unique series of expanded radialenes that have been characterized by X-ray crystallography.Intrigued by the formation of 3 from [5]Ph as reported by Kawamura and coworkers, the thermal reaction of [5]tBuPh, recently synthesized in our laboratory, 18 was examined. After heating in toluene at reflux for 4 d (Scheme 2), the crude reaction mixture showed many products (by TLC analysis). The main product could be isolated by flash chromatography on silica gel, although NMR spectroscopy indicated the presence of several compounds. On the other hand, mass spectrometry showed a signal consistent with the mass expected for a dimer of