On the Lack of Ring-Current Aromaticity of (Heteroatom) [N]Radialenes and their Dianions

Hexakis(3,6-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)cyclohexane: Closed-Shell [6]Radialene or Open-Shell Hexa-Radicaloid?

A star-shaped hexaquinocyclohexane molecule 4 c is reported, which turns out to be a closed-shell extended [6]radialene with a twisted-boat conformation according to X-ray crystallographic analysis. It was formed by an unusually slow decay of its in situ generated open-shell valence isomer, the hexa-radicaloid 4 o, with a half-life time of about 156 min at room temperature. Reaction progress kinetic analysis revealed a large energy barrier of about 95.5±4.3 kJ mol^-1 at room temperature from the hexa-radical form 4 o to the contorted [6]radialene form 4 c, because the transformation needs to overcome large steric repulsion between the neighboring phenoxyl units. Compound 4 c can be chemically reduced to radical anion and dianion, and the dianion is actually a diradical dianion, with a calculated diradical character of 71.9 %. This study demonstrates the unique chemical bonding nature of contorted quinoidal π-conjugated molecules and a very unusual valence isomerization process.

A quantitative metric for conjugation in polyene hydrocarbons having a single classical structure

A quantitative measure of the extent of conjugation is introduced. The number of Dewar resonance structures (DS) in a conjugated hydrocarbon quantitatively determines its extent of conjugation interaction. The greater the degree of conjugation the more stable is the conjugated hydrocarbon. The number of single bonds joining the exo-double bonds of a purely cross-conjugated hydrocarbon is equal to its number of Dewar resonance structures (DS). Polyene cross-conjugated hydrocarbons are minimally conjugated and possess only one classical structure with a succession of fixed single and exo-double bonds. There are three increasing levels of conjugation for systems with a single classical structure according to whether they are cross-conjugated, linear extended two-way conjugated, and ring-enhanced conjugated. Numerous analytical expressions have been derived. Our valence-bond (VB) approach involves studying the interrelationship of several well-known series of conjugated hydrocarbons having only a single classical structure, i.e., the number of Kekulé resonance structures is one (K = 1). This work provides a totally new perspective to the concept of conjugation.

Synthesis and Diels-Alder reactions of a benzo[5]radialene derivative

Apart from their exotic structure, radialenes have been employed as precursors of more complex polycyclic molecules. In this work we report the synthesis of the first compound having the benzo[5]radialene substructure, starting from simple materials. Such a compound proved to be a convenient diene in Diels-Alder reactions, for the preparation of highly functionalized fluorenes and benzo[b]fluorenes in a quimio- and stereocontrolled fashion.

Writing with ring currents: selectively hydrogenated polycyclic aromatics as finite models of graphene and graphane

Alternating partial hydrogenation of the interior region of a polycyclic aromatic hydrocarbon gives a finite model system representing systems on the pathway from graphene to the graphane modification of the graphene sheet. Calculations at the DFT and coupled Hartree-Fock levels confirm that sp² cycles of bare carbon centres isolated by selective hydrogenation retain the essentially planar geometry and electron delocalization of the annulene that they mimic. Delocalization is diagnosed by the presence of ring currents, as detected by ipsocentric calculation and visualization of the current density induced in the π system by a perpendicular external magnetic field. These induced 'ring' currents have essentially the same sense, strength and orbital origin as in the free hydrocarbon. Subjected to the important experimental proviso of the need for atomic-scale control of hydrogenation, this finding predicts the possibility of writing single, multiple and concentric diatropic and/or paratropic ring currents on the graphene/graphane sheet. The implication is that pathways for free flow of ballistic current can be modelled in the same way.

Coordination chemistry of polyoxo-carbocyclic compounds containing one or more neighboring oxo-groups

This is a review of publications concerning the results of X-ray analysis of single crystals and spectrochemical characteristics of metal complexes containing polyoxo-carbocyclic compounds with one or more neighboring oxo-groups. The features of lattice characteristics and coordination modes, depending on the nature of the organic dianion, are shown here. The squarate anions, ${{\rm{C}}_{\rm{4}}}{\rm{O}}_{\rm{4}}^{{\rm{2 - }}}$ are predominantly involved in monodentate bridging coordination, and the derivatives containing five or six C-atoms in the cycle (croconate and rhodizonate anions) usually form bidentate chelating bridged modes. The stabilization of the crystal lattices of the metal complexes is due to the formation of hydrogen bonds and π-π stackings, which produce self-assembled three-dimensional (3D) structures with cavities and intercalation of some small molecules. The strong interlayer interactions lead to appearance of specific conductometric and magnetic properties, which together with their high thermal stabilities, make the oxocarbon complexes new and perspective materials for electronics and related areas.

Fluorescent hexaaryl- and hexa-heteroaryl[3]radialenes: Synthesis, structures, and properties

The syntheses of three new [3]radialenes – hexakis(3,5-dimethylpyrazolyl)-, hexakis(3-cyanophenyl)-, and hexakis(3,4-dicyanophenyl)[3]radialene (1–3) – are reported. Compound 3 is obtained in five steps with an excellent yield of 76% in the key step. Compared to that, the respective steps of the syntheses of 1 and 2 result in lower yields. All compounds adopt a double bladed propeller conformation in solution. Compound 3 is considerably more electron deficient than previously reported hexaaryl[3]radialenes, with reduction potentials of −0.06 and −0.45 V in CH₂Cl₂. The compounds mostly display red fluorescence with large Stokes shifts.

1,3,4,6,7,9-Hexamethylbenzo[1,2-c:3,4-c':5,6-c'']trithiophene: a twisted heteroarene

1,3,4,6,7,9-Hexamethylbenzo[1,2-c:3,4-c':5,6-c'']trithiophene (C-Me) was prepared by palladium-catalyzed methylations of the corresponding hexabromide C-Br. The twisted structure of C-Me has been confirmed by X-ray crystal analysis. The physical properties of twisted C-Me and planar benzo[1,2-c:3,4-c':5,6-c'']trithiophene (C-H) were studied and compared. Crystal structures are compared to computational structures determined using density functional theory, with both the M06-2X and B3PW91 functionals.

Aromaticity, π-electron delocalization, and ring currents

The magnetic criterion of aromaticity is based on the ability of a cyclic system to support a ring current. If the ring current has the diatropic sense, the system is aromatic; if the sense is paratropic, the system is antiaromatic. Attribution of aromaticity ab initio therefore reduces to the calculation and visualization of induced current density. This can be achieved at modest computational cost within the ipsocentric approach, where current density at any point in space is calculated with that point as origin of vector potential. This choice leads to accurate maps of current density and a uniquely simple interpretation in terms of nonredundant orbital contributions governed by translational and rotational selection rules. For example, four-electron diatropicity and two-electron paratropicity are predicted for delocalized forms of [4n+2]- and [4n]-annulenes, respectively. A new application to homoaromatic systems is described. Two model tris-homoaromatics, both neutral molecules, are shown to exhibit a through-space, highest occupied molecular orbital (HOMO)-dominated, four-electron ring current, in a direct analogy with the conventional "aromatic" benzene π-current.