“Carbo-aromaticity” and novel carbo-aromatic compounds

Abstract

While the concept of aromaticity is being more and more precisely delineated, the category of "aromatic compounds" is being more and more expanded. This is illustrated by an introductory highlight of the various types of "aromaticity" previously invoked, and by a focus on the recently proposed "aromatic character" of the "two-membered rings" of the acetylene and butatriene molecules. This serves as a general foundation for the definition of "carbo-aromaticity", the relevance of which is surveyed through recent results in the synthetic, physical, and theoretical chemistry of carbo-mers and in particular macrocyclic-polycyclic representatives constituting a natural family of "novel aromatic compounds". With respect to their parent molecules, carbo-mers are constitutionally defined as "carbon-enriched", and can also be functionally regarded as "π-electron-enriched". This is exemplified by recent experimental and theoretical results on functional, aromatic, rigid, σ,π-macrocyclic carbo-benzene archetypes of various substitution patterns, with emphasis on the quadrupolar pattern. For the purpose of comparison, several types of non-aromatic references of carbo-benzenes are then considered, i.e. freely rotating σ,π-acyclic carbo-n-butadienes and flexible σ-cyclic, π-acyclic carbo-cyclohexadienes, and to "pro-aromatic" congeners, i.e. rigid σ,π-macrocyclic carbo-quinoids. It is shown that functional carbo-mers are entering the field of "molecular materials" for properties such as linear or nonlinear optical properties (e.g. dichromism and two-photon absorption) and single molecule conductivity. Since total or partial carbo-mers of aromatic carbon-allotropes of infinite size such as graphene (graphynes and graphdiynes) and graphite ("graphitynes") have long been addressed at the theoretical or conceptual level, recent predictive advances on the electrical, optical and mechanical properties of such carbo-materials are surveyed. Very preliminary experimental results on a carbo-benzenoid fragment are finally disclosed.

How carbo-benzenes fit molecules in their inner core as do biologic ion carriers?

“…fullerenes [11] and CNTs [12,13], have addressed the fundamental description of their electronic properties with a focus on the analysis of electron delocalization and aromaticity [14]. Computational studies of a-graphyne instigated efforts for the synthesis and experimental study of the aggregation and assembly of carbo-benzene rings [15][16][17]. Within this spirit, Oleg Shishkin et al addressed the crystallographic packing mode of generic carbo-benzenes or their [6] pericyclic precursors [18], in particular by using the hedgehog model [19].…”

On the nature of the dative bond: Coordination to metals and beyond. The carbon case

“…This is presented in three respective sections, from the complementary standpoints of (i) theoretical analysis of dative bonding [16], (ii) organometallic chemistry of phospho-carbon ligands [17], and (iii) organometallic chemistry of alkynyl and alkyne ligands [18].…”

carbo‐Naphthalene: A Polycyclic carbo‐Benzenoid Fragment of α‐Graphyne

“…The last step is aS nCl 2 / HCl-mediated reduction of ad ecaoxy-carbo-decalin, whichi s prepared through successive [8+ +10] macrocyclization steps. [5] Them ost homogeneous variant is agraphyne (with only two types of CÀC bonds), that is,t he total carbo-mer of graphene (Figure 1), [6] or al ayer of agraphityne. The stability and aromaticity of this smallest fused molecular fragment of a-graphyne allows prediction of the same properties for the carbon allotrope itself.…”

“…Tw oc arbo-benzene references are also described, C 18 Ar 6 and o-C 18 Ar 4 (CC-SiiPr 3 ) 2 .T he carbo-naphthalene bicycle is locally aromatic according to structural and magnetic criteria, as revealed by strong diatropic ring current effects on the deshielding of 1 Hn uclei of the Ar groups and on the negative value of the DFT-calculated NICS at the center of the C 18 rings (À12.8 ppm). [2, 3b, 7] Whereas acyclic and unicyclic molecular fragments of agraphyne have been exemplified by carbo-oligoacetylenes [8] and carbo-benzenes, [6,9] afirst fused bicyclicfragment is envisaged in a carbo-naphthalene.With av iew to securing both stability and solubility,t he selected target was octa(p-n-pentylphenyl)-carbo-naphthalene (1). [3] Besides putative variants (a-, b-, 6,6,12-graphynes), the existence of graphdiyne is today demonstrated, [4] and g-graphyne has been approached through several polycyclic molecular fragments.…”