Most favorable cumulenic structures in iron-capped linear carbon chains are short singlet odd-carbon dications: a theoretical view

Abstract

Iron-capped, linear-carbon-chain dications have been investigated at the M06L/DZP level of quantum chemistry in order to determine their structural and electronic properties, focusing on differences between chains containing odd and even numbers of carbon atoms. Such differences result from changes in the electronic states and the acetylenic or cumulenic nature of the carbon chain. Interestingly, the short even-carbon chains exhibit distinct properties, but upon chain lengthening undergo a transition to structures similar to those of odd-carbon chains, with a turning point around [FeC10Fe]2+. On the other hand, the less extensively investigated short odd-carbon chains, such as [FeC5Fe]2+, [FeC7Fe]2+ and [FeC9Fe]2+, due to synthetic difficulty, are predicted to exhibit more exceptional properties than the short even-carbon chains in every aspect, such as excellent back bonding as well as more cumulenic and more nearly linear structures. This theoretical study suggests that more experimental work should be considered on metal-capped, short linear odd-carbon chains as potential building blocks for novel electronic and optical materials.

Unimolecular Double Photoionization-Induced Processes in Iron Pentacarbonyl

“…Our calculations show that the FeCCO ++ ion has the cumulenic structure, Fe=C=C=O, perfectly compatible with the above hypothesis. Our computed Fe–C bond distance is close to the computed values found by Pu et al 43 for the related cumulenic structures [FeC m Fe] ++ . Although this reaction is observed at high energy in the gas phase, we speculate that a similar mechanism might be relevant in clusters or in solution where ions are highly stabilized relative to their free gas phase forms.…”

Reverse Bond-Length Alternation in Cumulenes: Candidates for Increasing Electronic Transmission with Length

“…Finally, we should mention a class of cumulenes we have not yet touched upon in this paper. Research in metallacumulenes (cumulenylidene complexes) has seen great progress and may hold promise for making longer cumulenic oligomers. − From the perspective of the cumulene these show reverse bond-length alternation; − some approach fully reversed alkyne bond lengths . As seen in polyynediyl complexes, metal centers may allow for stabilization of longer chains .…”

Iron Versus Ruthenium: Evidence for the Distinct Differences in the Electronic Structures of Hexa-1,3,5-triyn-1,6-diyl-bridged Complexes [Cp*(dppe)M}{μ-(C≡C)3}{M(dppe)Cp*}]+ (M = Fe, Ru)

“…Homo- and hetero-bimetallic polyyndiyl complexes of the general form [{L n M}­{μ-(CC) m }­{ML n }] n + have proved to be objects of sustained interest over the past decades. , The metal end-capping groups have allowed for stabilization of carbon chains of various lengths and formal valence structures, the longest members of the series serving as models for the putative linear carbon allotrope carbyne. − Polyyndiyl complexes have allowed the reactivity profile of the stabilized carbon fragments to be explored and, in addition to serving as “carbon in a bottle” reagents, − the combination of redox-active metal fragments {ML n } with the extended π-conjugated electronic structure of the bridging polyyndiyl chain has made these all-carbon bridged bimetallic complexes popular systems through which to study intramolecular electron-transfer phenomena in mixed-valence derivatives. ,− …”